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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..0d96444 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #62693 (https://www.gutenberg.org/ebooks/62693) diff --git a/old/62693-0.txt b/old/62693-0.txt deleted file mode 100644 index 99d8c19..0000000 --- a/old/62693-0.txt +++ /dev/null @@ -1,23632 +0,0 @@ -The Project Gutenberg EBook of Climate and Time in their Geological -Relations, by James Croll - -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll -have to check the laws of the country where you are located before using -this ebook. - - - -Title: Climate and Time in their Geological Relations - A Theory of Secular Changes of the Earth's Climate - -Author: James Croll - -Release Date: July 18, 2020 [EBook #62693] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK CLIMATE AND TIME *** - - - - -Produced by WebRover, MWS, Robert Tonsing, and the Online -Distributed Proofreading Team at https://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive/American Libraries.) - - - - - - - - - - CLIMATE AND TIME - - - [Illustration: FRONTISPIECE - - N. WINTER SOLSTICE IN APHELION. - - N. WINTER SOLSTICE IN PERIHELION. - - W. & A. K. Johnston, Edinb^r. and London.] - - - - - CLIMATE AND TIME - _IN THEIR GEOLOGICAL RELATIONS_ - - A THEORY OF - SECULAR CHANGES OF THE EARTH’S CLIMATE - - BY JAMES CROLL - OF H.M. GEOLOGICAL SURVEY OF SCOTLAND - - LONDON - DALDY, ISBISTER, & CO. - 56, LUDGATE HILL - 1875 - - - LONDON: - PRINTED BY VIRTUE AND CO., - CITY ROAD. - - - - - PREFACE. - - -In the following pages I have endeavoured to give a full and concise -statement of the facts and arguments adduced in support of the theory -of Secular Changes of the Earth’s Climate. Considerable portions of -the volume have already appeared in substance as separate papers in -the Philosophical Magazine and other journals during the past ten or -twelve years. The theory, especially in as far as it relates to the -cause of the glacial epoch, appears to be gradually gaining acceptance -with geologists. This, doubtless, is owing to the greatly increased -and constantly increasing knowledge of the drift-phenomena, which has -induced the almost general conviction that a climate such as that of -the glacial epoch could only have resulted from cosmical causes. - -Considerable attention has been devoted to objections, and to the -removal of slight misapprehensions, which have naturally arisen in -regard to a subject comparatively new and, in many respects, complex, -and beset with formidable difficulties. - -I have studiously avoided introducing anything of a hypothetical -character. All the conclusions are based either on known facts or -admitted physical principles. In short, the aim of the work, as will be -shown in the introductory chapter, is to prove that secular changes of -climate follow, as a necessary effect, from admitted physical agencies, -and that these changes, in as far as the past climatic condition of -the globe is concerned, fully meet the demand of the geologist. - -The volume, though not intended as a popular treatise, will be found, -I trust, to be perfectly plain and intelligible even to readers not -familiar with physical science. - -I avail myself of this opportunity of expressing my obligations to my -colleagues, Mr. James Geikie, Mr. Robert L. Jack, Mr. Robert Etheridge, -jun., and also to Mr. James Paton, of the Edinburgh Museum of Science -and Art, for their valuable assistance rendered while these pages were -passing through the press. To the kindness of Mr. James Bennie I am -indebted for the copious index at the end of the volume, as well as -for many of the facts relating to the glacial deposits of the West of -Scotland. - - JAMES CROLL. - -EDINBURGH, _March, 1875_. - - - - - CONTENTS. - - - CHAPTER I. - - INTRODUCTION. - - PAGE - The Fundamental Problem of Geology.—Geology a Dynamical - Science.—The Nature of a Geological Principle.—Theories - of Geological Climate.—Geological Climate dependent - on Astronomical Causes.—An Important Consideration - overlooked.—Abstract of the Line of Argument pursued in the - Volume 1 - - - CHAPTER II. - - OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER - THE GLOBE. - - The absolute Heating-power of Ocean-currents.—Volume of - the Gulf-stream.—Absolute Amount of Heat conveyed by - it.—Greater Portion of Moisture in Inter-tropical Regions - falls as Rain in those Regions.—Land along the Equator - tends to lower the Temperature of the Globe.—Influence - of Gulf-stream on Climate of Europe.—Temperature of - Space.—Radiation of a Particle.—Professor Dove on Normal - Temperature.—Temperature of Equator and Poles in the Absence - of Ocean-currents.—Temperature of London, how much due to - Ocean-currents 23 - - - CHAPTER III. - - OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER THE - GLOBE.—(_Continued._) - - Influence of the Gulf-stream on the Climate of the Arctic - Regions.—Absolute Amount of Heat received by the Arctic - Regions from the Sun.—Influence of Ocean-currents shown by - another Method.—Temperature of a Globe all Water or all - Land according to Professor J. D. Forbes.—An important - Consideration overlooked.—Without Ocean-currents the - Globe would not be habitable.—Conclusions not affected by - Imperfection of Data 45 - - - CHAPTER IV. - - OUTLINE OE THE PHYSICAL AGENCIES WHICH LEAD TO SECULAR - CHANGES OF CLIMATE. - - Eccentricity of the Earth’s Orbit; its Effect on - Climate.—Glacial Epoch not the direct Result of an - Increase of Eccentricity.—An important Consideration - overlooked.—Change of Eccentricity affects Climate only - indirectly.—Agencies which are brought into Operation - by an Increase of Eccentricity.—How an Accumulation - of Snow is produced.—The Effect of Snow on the Summer - Temperature.—Reason of the Low Summer Temperature of Polar - Regions.—Deflection of Ocean-currents the chief Cause of - Secular Changes of Climate.—How the foregoing Causes deflect - Ocean-currents.—Nearness of the Sun in Perigee a Cause of - the Accumulation of Ice.—A remarkable Circumstance regarding - the Causes which lead to Secular Changes of Climate.—The - primary Cause an Increase of Eccentricity.—Mean Temperature - of whole Earth should be greater in Aphelion than in - Perihelion.—Professor Tyndall on the Glacial Epoch.—A - general Reduction of Temperature will not produce a Glacial - Epoch.—Objection from the present Condition of the Planet - Mars 54 - - - CHAPTER V. - - REASON WHY THE SOUTHERN HEMISPHERE IS COLDER THAN THE NORTHERN. - - Adhémar’s Explanation.—Adhémar’s Theory founded upon a physical - Mistake in regard to Radiation.—Professor J. D. Forbes on - Underground Temperature.—Generally accepted Explanation.—Low - Temperature of Southern Hemisphere attributed to - Preponderance of Sea.—Heat transferred from Southern to - Northern Hemisphere by Ocean-current the true Explanation.—A - large Portion of the Heat of the Gulf-stream derived from the - Southern Hemisphere 81 - - - CHAPTER VI. - - EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC - CIRCULATION.—LIEUT. MAURY’S THEORY. - - Introduction.—Ocean-currents, according to Maury, due to - Difference of Specific Gravity.—Difference of Specific - Gravity resulting from Difference of Temperature.—Difference - of Specific Gravity resulting from Difference of - Saltness.—Maury’s two Causes neutralize each other.—How, - according to him, Difference in Saltness acts as a Cause 95 - - - CHAPTER VII. - - EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC - CIRCULATION.—LIEUT. MAURY’S THEORY.—(_Continued._) - - Methods of determining the Question.—The Force resulting from - Difference of Specific Gravity.—Sir John Herschel’s Estimate - of the Force.—Maximum Density of Sea-Water.—Rate of Decrease - of Temperature of Ocean at Equator.—The actual Amount of - Force resulting from Difference of Specific Gravity.—M. - Dubuat’s Experiments 115 - - - CHAPTER VIII. - - EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC - CIRCULATION.—DR. CARPENTER’S THEORY. - - Gulf-stream according to Dr. Carpenter not due to Difference of - Specific Gravity.—Facts to be Explained.—The Explanation of - the Facts.—The Explanation hypothetical.—The Cause assigned - for the hypothetical Mode of Circulation.—Under Currents - account for all the Facts better than the Gravitation - Hypothesis.—Known Condition of the Ocean inconsistent with - that Hypothesis 122 - - - CHAPTER IX. - - EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC - CIRCULATION.—THE MECHANICS OF DR. CARPENTER’S THEORY. - - Experimental Illustration of the Theory.—The Force exerted by - Gravity.—Work performed by Gravity.—Circulation not by - Convection.—Circulation depends on Difference in Density - of the Equatorial and Polar Columns.—Absolute Amount of - Work which can be performed by Gravity.—How Underflow is - produced.—How Vertical Descent at the Poles and Ascent at - the Equator is produced.—The Gibraltar Current.—Mistake in - Mechanics concerning it.—The Baltic Current 145 - - - CHAPTER X. - - EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC - CIRCULATION.—DR. CARPENTER’S THEORY.—OBJECTIONS CONSIDERED. - - _Modus Operandi_ of the Matter.—Polar Cold considered by Dr. - Carpenter the _Primum Mobile_.—Supposed Influence of - Heat derived from the Earth’s Crust.—Circulation without - Difference of Level.—A Confusion of Ideas in Reference to the - supposed Agency of Polar Cold.—M. Dubuat’s Experiments.—A - Begging of the Question at Issue.—Pressure as a Cause of - Circulation 172 - - - CHAPTER XI. - - THE INADEQUACY OF THE GRAVITATION THEORY PROVED BY ANOTHER - METHOD. - - Quantity of Heat which can be conveyed by the General Oceanic - Circulation trifling.—Tendency in the Advocates of the - Gravitation Theory to under-estimate the Volume of the - Gulf-stream.—Volume of the Stream as determined by the - _Challenger_.—Immense Volume of Warm Water discovered by - Captain Nares.—Condition of North Atlantic inconsistent with - the Gravitation Theory.—Dr. Carpenter’s Estimate of the - Thermal Work of the Gulf-stream 191 - - - CHAPTER XII. - - MR. A. G. FINDLAY’S OBJECTIONS CONSIDERED. - - Mr. Findlay’s Estimate of the Volume of the Gulf-stream.—Mean - Temperature of a Cross Section less than Mean Temperature - of Stream.—Reason of such Diversity of Opinion regarding - Ocean-currents.—More rigid Method of Investigation necessary 203 - - - CHAPTER XIII. - - THE WIND THEORY OF OCEANIC CIRCULATION. - - Ocean-Currents not due alone to the Trade-winds.—An Objection - by Maury.—Trade-winds do not explain the Great Antarctic - Current.—Ocean-currents due to the System of Winds.—The - System of Currents agrees with the System of the - Winds.—Chart showing the Agreement between the System - of Currents and System of Winds.—Cause of the Gibraltar - Current.—North Atlantic an immense Whirlpool.—Theory of Under - Currents.—Difficulty regarding Under Currents obviated.—Work - performed by the Wind in impelling the Water forward.—The - _Challenger’s_ crucial Test of the Wind and Gravitation - Theories.—North Atlantic above the Level of Equator.—Thermal - Condition of the Southern Ocean irreconcilable with the - Gravitation Theory 210 - - - CHAPTER XIV. - - THE WIND THEORY OF OCEANIC CIRCULATION IN RELATION TO CHANGE - OF CLIMATE. - - Direction of Currents depends on Direction of the Winds.—Causes - which affect the Direction of Currents will affect - Climate.—How Change of Eccentricity affects the Mode - of Distribution of the Winds.—Mutual Reaction of Cause - and Effect.—Displacement of the Great Equatorial - Current.—Displacement of the Median Line between the Trades, - and its Effect on Currents.—Ocean-currents in Relation to the - Distribution of Plants and Animals.—Alternate Cold and Warm - Periods in North and South.—Mr. Darwin’s Views quoted.—How - Glaciers at the Equator may be accounted for.—Migration - across the Equator 226 - - - CHAPTER XV. - - WARM INTER-GLACIAL PERIODS. - - Alternate Cold and Warm Periods.—Warm Inter-glacial Periods - a Test of Theories.—Reason why their Occurrence has not - been hitherto recognised.—Instances of Warm Inter-glacial - Periods.—Dranse, Dürnten, Hoxne, Chapelhall, Craiglockhart, - Leith Walk, Redhall Quarry, Beith, Crofthead, Kilmaurs, - Sweden, Ohio, Cromer, Mundesley, &c., &c.—Cave and River - Deposits.—Occurrence of Arctic and Warm Animals in some Beds - accounted for.—Mr. Boyd Dawkins’s Objections.—Occurrence - of Southern Shells in Glacial Deposits.—Evidence of Warm - Inter-glacial Periods from Mineral Borings.—Striated - Pavements.—Reason why Inter-glacial Land-surfaces are so rare 236 - - - CHAPTER XVI. - - WARM INTER-GLACIAL PERIODS IN ARCTIC REGIONS. - - Cold Periods best marked in Temperate, and Warm Periods - in Arctic, Regions.—State of Arctic Regions during - Glacial Period.—Effects of Removal of Ice from Arctic - Regions.—Ocean-currents; Influence on Arctic Climate.—Reason - why Remains of Inter-glacial Period are rare in Arctic - Regions.—Remains of Ancient Forests in Banks’s Land, Prince - Patrick’s Island, &c.—Opinions of Sir R. Murchison, Captain - Osborn, and Professor Haughton.—Tree dug up by Sir E. Belcher - in lat. 75° N. 258 - - - CHAPTER XVII. - - FORMER GLACIAL EPOCHS.—REASON OF THE IMPERFECTION OF - GEOLOGICAL RECORDS IN REFERENCE TO THEM. - - Two Reasons why so little is known of Glacial Epochs.—Evidence - of Glaciation to be found on Land-surfaces.—Where are all - our ancient Land-surfaces?—The stratified Rocks consist - of a Series of old Sea-bottoms.—Transformation of a - Land-surface into a Sea-bottom obliterates all Traces of - Glaciation.—Why so little remains of the Boulder Clays of - former Glacial Epochs.—Records of the Glacial Epoch are fast - disappearing.—Icebergs do not striate the Sea-bottom.—Mr. - Campbell’s Observations on the Coast of Labrador.—Amount - of Material transported by Icebergs much exaggerated.—Mr. - Packard on the Glacial Phenomena of Labrador.—Boulder Clay - the Product of Land-ice.—Palæontological Evidence.—Paucity of - Life characteristic of a Glacial Period.—Warm Periods better - represented by Organic Remains than cold.—Why the Climate - of the Tertiary Period was supposed to be warmer than the - present.—Mr. James Geikie on the Defects of Palæontological - Evidence.—Conclusion 266 - - - CHAPTER XVIII. - - FORMER GLACIAL EPOCHS; GEOLOGICAL EVIDENCE OF. - - Cambrian Conglomerate of Islay and North-west of - Scotland.—Ice-action in Ayrshire and Wigtownshire - during Silurian Period.—Silurian Limestones in Arctic - Legions.—Professor Ramsay on Ice-action during Old - Red Sandstone Period.—Warm Climate in Arctic Regions - during Old Red Sandstone Period.—Professor Geikie and - Mr. James Geikie on a Glacial Conglomerate of Lower - Carboniferous Age.—Professor Haughton and Professor Dawson - on Evidence of Ice-action during Coal Period.—Mr. W. T. - Blanford on Glaciation in India during Carboniferous - Period.—Carboniferous Formations of Arctic Regions.—Professor - Ramsay on Permian Glaciers.—Permian Conglomerate in - Arran.—Professor Hull on Boulder Clay of Permian Age.—Permian - Boulder Clay of Natal.—Oolitic Boulder Conglomerate in - Sutherlandshire.—-Warm Climate in North Greenland during - Oolitic Period.—Mr. Godwin-Austen on Ice-action during - Cretaceous Period.—Glacial Conglomerates of Eocene Age in the - Alps.—M. Gastaldi on the Ice-transported Limestone Blocks of - the Superga.—Professor Heer on the Climate of North Greenland - during Miocene Period 292 - - - CHAPTER XIX. - - GEOLOGICAL TIME.—PROBABLE DATE OF THE GLACIAL EPOCH. - - Geological Time measurable from Astronomical Data.—M. Leverrier’s - Formulæ.—Tables of Eccentricity for 3,000,000 Years in the - Past and 1,000,000 Years in the Future.—How the Tables have - been computed.—Why the Glacial Epoch is more recent than had - been supposed.—Figures convey a very inadequate Conception - of immense Duration.—Mode of representing a Million of - Years.—Probable Date of the Glacial Epoch 311 - - - CHAPTER XX. - - GEOLOGICAL TIME.—METHOD OF MEASURING THE RATE OF SUBAËRIAL - DENUDATION. - - Rate of Subaërial Denudation a Measure of Time.—Rate determined - from Sediment of the Mississippi.—Amount of Sediment carried - down by the Mississippi; by the Ganges.—Professor Geikie on - Modern Denudation.—Professor Geikie on the Amount of Sediment - conveyed by European Rivers.—Rate at which the Surface of - the Globe is being denuded.—Alfred Tylor on the Sediment - of the Mississippi.—The Law which determines the Rate of - Denudation.—The Globe becoming less oblate.—Carrying Power - of our River Systems the true Measure of Denudation.—Marine - Denudation, trifling in comparison to Subaërial.—Previous - Methods of measuring Geological Time.—Circumstances which - show the recent Date of the Glacial Epoch.—Professor Ramsay - on Geological Time 329 - - - CHAPTER XXI. - - THE PROBABLE AGE AND ORIGIN OF THE SUN. - - Gravitation Theory.—Amount of Heat emitted by the Sun.—Meteoric - Theory.—Helmholtz’s Condensation Theory.—Confusion of - Ideas.—Gravitation not the chief Source of the Sun’s - Heat.—Original Heat.—Source of Original Heat.—Original Heat - derived from Motion in Space.—Conclusion as to Date of - Glacial Epoch.—False Analogy.—Probable Date of Eocene and - Miocene Periods 346 - - - CHAPTER XXII. - - A METHOD OF DETERMINING THE MEAN THICKNESS OF THE SEDIMENTARY - ROCKS OF THE GLOBE. - - Prevailing Methods defective.—Maximum Thickness of British - Rocks.—Three Elements in the Question.—Professor Huxley - on the Rate of Deposition.—Thickness of Sedimentary Rocks - enormously over-estimated.—Observed Thickness no Measure of - mean Thickness.—Deposition of Sediment principally along - Sea-margin.—Mistaken Inference regarding the Absence of a - Formation.—Immense Antiquity of existing Oceans 360 - - - CHAPTER XXIII. - - THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE - OF THE LAND DURING THE GLACIAL EPOCH. - - Displacement of the Earth’s Centre of Gravity by Polar - Ice-cap.—Simple Method of estimating Amount of - Displacement.—Note by Sir W. Thomson on foregoing - Method.—Difference between Continental Ice and - a Glacier.—Probable Thickness of the Antarctic - Ice-cap.—Probable Thickness of Greenland Ice-sheet.—The - Icebergs of the Southern Ocean.—Inadequate Conceptions - regarding the Magnitude of Continental Ice 368 - - - CHAPTER XXIV. - - THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE - LAND DURING THE GLACIAL EPOCH.—(_Continued._) - - Extent of Submergence from Displacement of Earth’s Centre - of Gravity.—Circumstances which show that the Glacial - Submergence resulted from Displacement of the Earth’s - Centre of Gravity.—Agreement between Theory and Observed - Facts.—Sir Charles Lyell on submerged Areas during - Tertiary Period.—Oscillations of Sea-Level in Relation to - Distribution.—Extent of Submergence on the Hypothesis that - the Earth is fluid in the Interior 387 - - - CHAPTER XXV. - - THE INFLUENCE OF THE OBLIQUITY OF THE ECLIPTIC ON CLIMATE - AND ON THE LEVEL OF THE SEA. - - The direct Effect of Change of Obliquity on Climate.—Mr. - Stockwell on the maximum Change of Obliquity.—How Obliquity - affects the Distribution of Heat over the Globe.—Increase of - Obliquity diminishes the Heat at the Equator and increases - that at the Poles.—Influence of Change of Obliquity on the - Level of the Sea.—When the Obliquity was last at its superior - Limit.—Probable Date of the 25-foot raised Beach.—Probable - Extent of Rise of Sea-level resulting from Increase of - Obliquity.—Lieutenant-Colonel Drayson’s and Mr. Belt’s - Theories.—Sir Charles Lyell on Influence of Obliquity 398 - - - CHAPTER XXVI. - - COAL AN INTER-GLACIAL FORMATION. - - Climate of Coal Period Inter-glacial in Character.—Coal Plants - indicate an Equable, not a Tropical Climate.—Conditions - necessary for Preservation of Coal Plants.—Oscillations - of Sea-level necessarily implied.—Why our Coal-fields - contain more than One Coal-seam.—Time required to form a - Bed of Coal.—Why Coal Strata contain so little evidence of - Ice-action.—Land Flat during Coal Period.—Leading Idea of the - Theory.—Carboniferous Limestones 420 - - - CHAPTER XXVII. - - PATH OF THE ICE-SHEET IN NORTH-WESTERN EUROPE AND ITS RELATIONS - TO THE BOULDER CLAY OF CAITHNESS. - - Character of Caithness Boulder Clay.—Theories of the Origin - of the Caithness Clay.—Mr. Jamieson’s Theory.—Mr. C. W. - Peach’s Theory.—The proposed Theory.—Thickness of Scottish - Ice-sheet.—Pentlands striated on their Summits.—Scandinavian - Ice-sheet.—North Sea filled with Land-ice.—Great Baltic - Glacier.—Jutland and Denmark crossed by Ice.—Sir R. - Murchison’s Observations.—Orkney, Shetland, and Faroe Islands - striated across.—Loess accounted for.—Professor Geikie’s - Suggestion.—Professor Geikie and B. N. Peach’s Observations - on East Coast of Caithness.—Evidence from Chalk Flints and - Oolitic Fossils in Boulder Clay 435 - - - CHAPTER XXVIII. - - NORTH OF ENGLAND ICE-SHEET, AND TRANSPORT OF WASTDALE CRAG - BLOCKS. - - Transport of Blocks; Theories of.—Evidence of Continental - Ice.—Pennine Range probably striated on Summit.—Glacial - Drift in Centre of England.—Mr. Lacy on Drift of Cotteswold - Hills.—England probably crossed by Land-ice.—Mr. Jack’s - Suggestion.—Shedding of Ice North and South.—South of England - Ice-sheet.—Glaciation of West Somerset.—Why Ice-markings are - so rare in South of England.—Form of Contortion produced by - Land-ice 456 - - - CHAPTER XXIX. - - EVIDENCE FROM BURIED RIVER CHANNELS OF A CONTINENTAL PERIOD - IN BRITAIN. - - Remarks on the Drift Deposits.—Examination of Drift - by Borings.—Buried River Channel from Kilsyth to - Grangemouth.—Channels not excavated by Sea nor by - Ice.—Section of buried Channel at Grangemouth.—Mr. Milne - Home’s Theory.—German Ocean dry Land.—Buried River Channel - from Kilsyth to the Clyde.—Journal of Borings.—Marine - Origin of the Drift Deposits.—Evidence of Inter-glacial - Periods.—Oscillations of Sea-Level.—Other buried River - Channels 466 - - - CHAPTER XXX. - - THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THEORIES OF - GLACIER-MOTION. - - Why the Question of Glacier-motion has been found to be so - difficult.—The Regelation Theory.—It accounts for the - Continuity of a Glacier, but not for its Motion.—Gravitation - proved by Canon Moseley insufficient to shear the Ice - of a Glacier.—Mr. Matthew’s Experiment.—No Parallel - between the bending of an Ice Plank and the shearing - of a Glacier.—Mr. Ball’s Objection to Canon Moseley’s - Experiment.—Canon Moseley’s Method of determining the Unit - of Shear.—Defect of Method.—Motion of a Glacier in some - Way dependent on Heat.—Canon Moseley’s Theory.—Objections - to his Theory.—Professor James Thomson’s Theory.—This - Theory fails to explain Glacier-motion.—De Saussure and - Hopkins’s “Sliding” Theories.—M. Charpentier’s “Dilatation” - Theory.—Important Element in the Theory 495 - - - CHAPTER XXXI. - - THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THE MOLECULAR - THEORY. - - Present State of the Question.—Heat necessary to the Motion of - a Glacier.—Ice does not shear in the Solid State.—Motion - of a Glacier _molecular_.—How Heat is transmitted through - Ice.—Momentary Loss of Shearing Force.—The _Rationale_ - of Regelation.—The Origin of “Crevasses.”—Effects of - Tension.—Modification of Theory.—Fluid Molecules crystallize - in Interstices.—Expansive Force of crystallizing Molecules - a Cause of Motion.—Internal molecular Pressure the chief - Moving Power.—How Ice can excavate a Rock Basin.—How Ice can - ascend a Slope.—How deep River Valleys are striated across.—A - remarkable Example in the Valley of the Tay.—How Boulders can - he carried from a lower to a higher Level 514 - - - APPENDIX. - - I. Opinions expressed previous to 1864 regarding the - Influence of the Eccentricity of the Earth’s Orbit on - Climate 528 - - II. On the Nature of Heat-Vibrations 544 - - III. On the Reason why the Difference of Reading between a - Thermometer exposed to direct Sunshine and One Shaded - diminishes as we ascend in the Atmosphere 547 - - IV. Remarks on Mr. J. Y. Buchanan’s Theory of the Vertical - Distribution of Temperature of the Ocean 550 - - V. On the Cause of the Cooling Effect produced on Solids by - Tension 552 - - VI. The Cause of Regelation 554 - - VII. List of Papers which have appeared in Dr. A. Petermann’s - _Geographische Mittheilungen_ relating to the Gulf-stream - and Thermal Condition of the Arctic Regions 556 - - VIII. List of Papers by the Author to which Reference is made in - this Volume 560 - - - INDEX 563 - - - - - LIST OF PLATES. - - - EARTH’S ORBIT WHEN ECCENTRICITY IS AT ITS SUPERIOR - LIMIT _Frontispiece._ - - PLATE - - I. SHOWING AGREEMENT BETWEEN THE SYSTEM OF OCEAN-CURRENTS - AND WINDS _To face page_ 212 - - II. SHOWING HOW OPPOSING CURRENTS INTERSECT EACH OTHER 219 - - III. SECTION OF MID-ATLANTIC 222 - - IV. DIAGRAM REPRESENTING THE VARIATIONS OF ECCENTRICITY OF THE - EARTH’S ORBIT 313 - - V. SHOWING PROBABLE PATH OF THE ICE IN NORTH-WESTERN EUROPE 449 - - VI. SHOWING PATH OF ICE ACROSS CAITHNESS 453 - - VII. MAP OF THE MIDLAND VALLEY (SCOTLAND), SHOWING BURIED RIVER - CHANNELS 471 - - - - - CHAPTER I. - - INTRODUCTION. - - The Fundamental Problem of Geology.—Geology a Dynamical - Science.—The Nature of a Geological Principle.—Theories - of Geological Climate.—Geological Climate dependent - on Astronomical Causes.—An Important Consideration - overlooked.—Abstract of the Line of Argument pursued in the - Volume. - - -_The Fundamental Problem of Geology._—The investigation of the -successive changes and modifications which the earth’s crust has -undergone during past ages is the province of geology. It will be -at once admitted that an acquaintance with the agencies by means of -which those successive changes and modifications were effected, is of -paramount importance to the geologist. What, then, are those agencies? -Although volcanic and other subterranean eruptions, earthquakes, -upheavals, and subsidences of the land have taken place in all ages, -yet no truth is now better established than that it is not by these -convulsions and cataclysms of nature that those great changes were -effected. It was rather by the ordinary agencies that we see every day -at work around us, such as rain, rivers, heat and cold, frost and snow. -The valleys were not produced by violent dislocations, nor the hills -by sudden upheavals, but were actually carved out of the solid rock, -silently and gently, by the agencies to which we have referred. “The -tools,” to quote the words of Professor Geikie, “by which this great -work has been done are of the simplest and most every-day order—the -air, rain, frosts, springs, brooks, rivers, glaciers, icebergs, and the -sea. These tools have been at work from the earliest times of which -any geological record has been preserved. Indeed, it is out of the -accumulated chips and dust which they have made, afterwards hardened -into solid rock and upheaved, that the very framework of our continents -has been formed.”[1] - -It will be observed—and this is the point requiring particular -attention—that the agencies referred to are the ordinary meteorological -or climatic agencies. In fact, it is these agencies which constitute -climate. The various peculiarities or modifications of climate result -from a preponderance of one or more of these agencies over the rest. -When heat, for example, predominates, we have a hot or tropical -climate. When cold and frost predominate, we have a rigorous or arctic -climate. With moisture in excess, we have a damp and rainy climate; -and so on. But this is not all. These climatic agencies are not only -the factors which carved out the rocky face of the globe into hill -and dale, and spread over the whole a mantle of soil; but by them are -determined the character of the _flora_ and _fauna_ which exist on -that soil. The flora and fauna of a district are determined mainly by -the character of the climate, and not by the nature of the soil, or -the conformation of the ground. It is from difference of climate that -tropical life differs so much from arctic, and both these from the life -of temperate regions. It is climate, and climate alone, that causes -the orange and the vine to blossom, and the olive to flourish, in the -south, but denies them to the north, of Europe. It is climate, and -climate alone, that enables the forest tree to grow on the plain, but -not on the mountain top; that causes wheat and barley to flourish on -the mainland of Scotland, but not on the steppes of Siberia. - -Again, if we compare flat countries with mountainous, highlands with -lowlands, or islands with continents, we shall find that difference of -climatic conditions is the chief reason why life in the one differs -so much from life in the other. And if we turn to the sea we find -that organic life is there as much under the domain of climate as on -the land, only the conditions are much less complex. For in the case -of the sea, difference in the temperature of the water may be said -to constitute almost the only difference of climatic conditions. -If there is one fact more clearly brought out than another by the -recent deep-sea explorations, it is this, that nothing exercises so -much influence on organic life in the ocean as the temperature of the -water. In fact, so much is this the case, that warm zones were found -to be almost equivalent to zones of life. It was found that even the -enormous pressure at the bottom of the ocean does not exercise so much -influence on life as the temperature of the water. There are few, I -presume, who reflect on the subject that will not readily admit that, -whether as regards the great physical changes which are taking place on -the surface of our globe, or as regards the growth and distribution of -plant and animal life, the ordinary climatic agents are the real agents -at work, and that, compared with them, all other agencies sink into -insignificance. - -It will also be admitted that what holds true of the present holds -equally true of the past. Climatic agents are not only now the most -important and influential; they have been so during all past geological -ages. They were so during the Cainozoic as much as during the present; -and there is no reason for supposing they were otherwise during the -remoter Mesozoic and Palæozoic epochs. They have been the principal -factors concerned in that long succession of events and changes which -have taken place since the time of the solidification of the earth’s -crust. The stratified rocks of the globe contain all the records which -now remain of their action, and it is the special duty of the geologist -to investigate and read those records. It will be at once admitted that -in order to a proper understanding of the events embodied in these -records, an acquaintance with the agencies by which they were produced -is of the utmost importance. In fact, it is only by this means that we -can hope to arrive at their rational explanation. A knowledge of the -agents, and of the laws of their operations, is, in all the physical -sciences, the means by which we arrive at a rational comprehension -of the effects produced. If we have before us some complex and -intricate effects which have been produced by heat, or by light, or by -electricity, &c., in order to understand them we must make ourselves -acquainted with the agents by which they were produced and the laws of -their action. If the effects to be considered be, for example, those of -heat, then we must make ourselves acquainted with this agent and its -laws. If they be of electricity, then a knowledge of electricity and -its laws becomes requisite. - -This is no mere arbitrary mode of procedure which may be adopted in -one science and rejected in another. It is in reality a necessity of -thought arising out of the very constitution of our intellect; for the -objective law of the agent is the conception by means of which the -effects are subjectively united in a rational unity. We may describe, -arrange, and classify the effects as we may, but without a knowledge of -the laws of the agent we can have no rational unity. We have not got -the higher conception by which they can be _comprehended_. It is this -relationship between the effects and the laws of the agent, a knowledge -of which really constitutes a science. We might examine, arrange, and -describe for a thousand years the effects produced by heat, and still -we should have no science of heat unless we had a knowledge of the -laws of that agent. The effects would never be seen to be necessarily -connected with anything known to us; we could not connect them with -any rational principle from which they could be deduced _à priori_. -The same remarks hold, of course, equally true of all sciences, in -which the things to be considered stand in the relationship of cause -and effect. Geology is no exception. It is not like systematic botany, -a mere science of classification. It has to explain and account for -effects produced; and these effects can no more be explained without -a knowledge of the laws of the agents which produced them, than can -the effects of heat without a knowledge of the laws of heat. The only -distinction between geology and heat, light, electricity, &c., is, -that in geology the effects to be explained have almost all occurred -already, whereas in these other sciences effects actually taking place -have to be explained. But this distinction is of no importance to -our present purpose, for effects which have already occurred can no -more be explained without a knowledge of the laws of the agent which -produced them than can effects which are in the act of occurring. It -is, moreover, not strictly true that all the effects to be explained -by the geologist are already past. It falls within the scope of his -science to account for the changes which are at present taking place on -the earth’s crust. - -No amount of description, arrangement, and classification, however -perfect or accurate, of the facts which come under the eye of the -geologist can ever constitute a science of geology any more than a -description and classification of the effects of heat could constitute -a science of heat. This will, no doubt, be admitted by every one who -reflects upon the subject, and it will be maintained that geology, -like every other science, must possess principles applicable to the -facts. But here confusion and misconception will arise unless there be -distinct and definite ideas as to what ought to constitute a geological -principle. It is not every statement or rule that may apply to a great -many facts, which will constitute a geological principle. A geological -principle must bear the same characteristics as the principles of those -sciences to which we have referred. What, then, is the nature of the -principles of light, heat, electricity, &c.? The principles of heat -are the laws of heat. The principles of electricity are the laws of -electricity. And these laws are nothing more nor less than the ways -according to which these agents produce their effects. The principles -of geology are therefore the laws of geology. But the laws of geology -must be simply the laws of the geological agents, or, in other words, -the methods by which they produce their effects. Any other so-called -principle can be nothing more than an empirical rule, adopted for -convenience. Possessing no rationality in itself, it cannot be justly -regarded as a principle. In order to rationality the principle must be -either resolvable into, or logically deducible from, the laws of the -agents. Unless it possess this quality we cannot give the explanation -_à priori_. - -The reason of all this is perfectly obvious. The things to be explained -are effects; and the relationship between cause and effect affords the -subjective connection between the principle and the explanation. The -explanation follows from the principle simply as the effect results -from the laws of the agent or cause. - -_Theories of Geological Climate._—We have already seen that the -geological agents are chiefly the ordinary climatic agents. -Consequently, the main principles of geology must be the laws of the -climatic agents, or some logical deductions from them. It therefore -follows that, in order to a purely scientific geology, the grand -problem must be one of geological climate. It is through geological -climate that we can hope to arrive ultimately at principles which will -afford a rational explanation of the multifarious facts which have -been accumulating during the past century. The facts of geology are -as essential to the establishment of the principles, as the facts of -heat, light, and electricity are essential to the establishment of the -principles of these sciences. A theory of geological climate devised -without reference to the facts would be about as worthless as a theory -of heat or of electricity devised without reference to the facts of -these sciences. - -It has all along been an admitted opinion among geologists that the -climatic condition of our globe has not, during past ages, been -uniformly the same as at present. For a long time it was supposed that -during the Cambrian, Silurian, and other early geological periods, the -climate of our globe was much hotter than now, and that ever since -it has been gradually becoming cooler. And this high temperature of -Palæozoic ages was generally referred to the influence of the earth’s -internal heat. It has, however, been proved by Sir William Thomson[2] -that the general climate of our globe could not have been sensibly -affected by internal heat at any time more than ten thousand years -after the commencement of the solidification of the surface. This -physicist has proved that the present influence of internal heat on -the temperature amounts to about only 1/75th of a degree. Not only -is the theory of internal heat now generally abandoned, but it is -admitted that we have no good geological evidence that climate was much -hotter during Palæozoic ages than now; and much less, that it has been -becoming _uniformly_ colder. - -The great discovery of the glacial epoch, and more lately that of a -mild and temperate condition of climate extending during the Miocene -and other periods to North Greenland, have introduced a complete -revolution of ideas in reference to geological climate. Those -discoveries showed that our globe has not only undergone changes of -climate, but changes of the most extraordinary character. They showed -that at one time not only an arctic condition of climate prevailed in -our island, but that the greater part of the temperate region down -to comparatively low latitudes was buried under ice, while at other -periods Greenland and the Arctic regions, probably up to the North -Pole, were not only free from ice, but were covered with a rich and -luxuriant vegetation. - -To account for these extraordinary changes of climate has generally -been regarded as the most difficult and perplexing problem which has -fallen to the lot of the geologist. Some have attempted to explain -them by assuming a displacement of the earth’s axis of rotation in -consequence of the uprising of large mountain masses on some part -of the earth’s surface. But it has been shown by Professor Airy,[3] -Sir William Thomson,[4] and others, that the earth’s equatorial -protuberance is such that no geological change on its surface could -ever possibly alter the position of the axis of rotation to an extent -which could at all sensibly affect climate. Others, again, have tried -to explain the change of climate by supposing, with Poisson, that the -earth during its past geological history may have passed through hotter -and colder parts of space. This is not a very satisfactory hypothesis. -There is no doubt a difference in the quantity of force in the form of -heat passing through different parts of space; but space itself is not -a substance which can possibly be either cold or hot. If, therefore, -we were to adopt this hypothesis, we must assume that the earth during -the hot periods must have been in the vicinity of some other great -source of heat and light besides the sun. But the proximity of a -mass of such magnitude as would be sufficient to affect to any great -extent the earth’s climate would, by its gravity, seriously disarrange -the mechanism of our solar system. Consequently, if our solar system -had ever, during any former period of its history, really come into -the vicinity of such a mass, the orbits of the planets ought at the -present day to afford some evidence of it. But again, in order to -account for a cold period, such as the glacial epoch, we have to assume -that the earth must have come into the vicinity of a cold body.[5] -But recent discoveries in regard to inter-glacial periods are wholly -irreconcilable with this theory. - -A change in the obliquity of the ecliptic has frequently been, and -still is, appealed to as an explanation of geological climate. This -theory appears, however, to be beset by a twofold objection: (1), it -can be shown from celestial mechanics, that the variations in the -obliquity of the ecliptic must always have been so small that they -could not materially affect the climatic condition of the globe; and -(2), even admitting that the obliquity could change to an indefinite -extent, it can be shown[6] that no increase or decrease, however great, -could possibly account for either the glacial epoch or a warm temperate -condition of climate in polar regions. - -The theory that the sun is a variable star, and that the glacial -epochs of the geologists may correspond to periods of decrease in the -sun’s heat, has lately been advanced. This theory is also open to two -objections: (1), a general diminution of heat[7] never could produce -a glacial epoch; and (2), even if it could, it would not explain -inter-glacial periods. - -The only other theory on the subject worthy of notice is that - -of Sir Charles Lyell. Those extraordinary changes of climate are, -according to his theory, attributed to differences in the distribution -of land and water. Sir Charles concludes that, were the land all -collected round the poles, while the equatorial zones were occupied by -the ocean, the general temperature would be lowered to an extent that -would account for the glacial epoch. And, on the other hand, were the -land all collected along the equator, while the polar regions were -covered with sea, this would raise the temperature of the globe to an -enormous extent. It will be shown in subsequent chapters that this -theory does not duly take into account the prodigious influence exerted -on climate by means of the heat conveyed from equatorial to temperate -and polar regions by means of ocean-currents. In Chapters II. and III. -I have endeavoured to prove (1), that were it not for the heat conveyed -from equatorial to temperate and polar regions by this means, the -thermal condition of the globe would be totally different from what it -is at present; and (2), that the effect of placing all the land along -the equator would be diametrically the opposite of that which Sir -Charles supposes. - -But supposing that difference in the distribution of land and water -would produce the effects attributed to it, nevertheless it would not -account for those extraordinary changes of climate which have occurred -during geological epochs. Take, for example, the glacial epoch. -Geologists almost all agree that little or no change has taken place -in the relative distribution of sea and land since that _epoch_. All -our main continents and islands not only existed then as they do now, -but every year is adding to the amount of evidence which goes to show -that so recent, geologically considered, is the glacial epoch that the -very contour of the surface was pretty much the same then as it is at -the present day. But this is not all; for even should we assume (1), -that a difference in the distribution of sea and land would produce the -effects referred to, and (2), that we had good geological evidence to -show that at a very recent period a form of distribution existed which -would produce the necessary glacial conditions, still the glacial -epoch would not be explained, for the phenomena of warm inter-glacial -periods would completely upset the theory. - -_Geological Climate depending on Astronomical Causes._—For a good many -years past, an impression has been gradually gaining ground amongst -geologists that the glacial epoch, as well as the extraordinary -condition of climate which prevailed in arctic regions during the -Miocene and other periods, must some way or other have resulted from -a cosmical cause; but all seemed at a loss to conjecture what that -cause could possibly be. It was apparent that the cosmical cause must -be sought for in the relations of our earth to the sun; but a change -in the obliquity of the ecliptic and the eccentricity of the earth’s -orbit are the only changes from which any sensible effect on climate -could possibly be expected to result. It was shown, however, by Laplace -that the change of obliquity was confined within so narrow limits that -it has scarcely ever been appealed to as a cause seriously affecting -climate. The only remaining cause to which appeal could be made was -the change in the eccentricity of the earth’s orbit—precession of the -equinoxes without eccentricity producing, of course, no effect whatever -on climate. Upwards of forty years ago Sir John Herschel and a few -other astronomers directed their attention to the consideration of this -cause, but the result arrived at was adverse to the supposition that -change of eccentricity could greatly affect the climate of our globe. - -As some misapprehension seems to prevail with reference to this, I -would take the liberty of briefly adverting to the history of the -matter,—referring the reader to the Appendix for fuller details. - -About the beginning of the century some writers attributed the lower -temperature of the southern hemisphere to the fact that the sun remains -about seven days less on that hemisphere than on the northern; their -view being that the southern hemisphere on this account receives -seven days less heat than the northern. Sir Charles Lyell, in the -first edition of his “Principles,” published in 1830, refers to this -as a cause which might produce some slight effect on climate. Sir -Charles’s remarks seem to have directed Sir John Herschel’s attention -to the subject, for in the latter part of the same year he read a -paper before the Geological Society on the astronomical causes which -may influence geological phenomena, in which, after pointing out the -mistake into which Sir Charles had been led in concluding that the -southern hemisphere receives less heat than the northern, he considers -the question as to whether geological climate could be influenced by -changes in the eccentricity of the earth’s orbit. He did not appear at -the time to have been aware of the conclusions arrived at by Lagrange -regarding the superior limit of the eccentricity of the earth’s orbit; -but he came to the conclusion that possibly the climate of our globe -may have been affected by variations in the eccentricity of its orbit. -“An amount of variation,” he says, “which we need not hesitate to -admit (at least provisionally) as a possible one, may be productive -of considerable diversity of climate, and may operate during great -periods of time either to mitigate or to exaggerate the difference of -winter and summer temperatures, so as to produce alternately in the -same latitude of either hemisphere a perpetual spring, or the extreme -vicissitudes of a burning summer and a rigorous winter.” - -This opinion, however, was unfortunately to a great extent nullified -by the statement which shortly afterwards appeared in his “Treatise -on Astronomy,” and also in the “Outlines of Astronomy,” to the effect -that the elliptic form of the earth’s orbit has but a very trifling -influence in producing variation of temperature corresponding to the -sun’s distance; the reason being that whatever may be the ellipticity -of the orbit, it follows that equal amounts of heat are received -from the sun in passing over equal angles round it, in whatever part -of the ellipse those angles may be situated. Those angles will of -course be described in unequal times, but the greater proximity of -the sun exactly compensates for the more rapid description, and thus -an equilibrium of heat is maintained. The sun, for example, is much -nearer the earth when he is over the southern hemisphere than he is -when over the northern; but the southern hemisphere does not on this -account receive more heat than the northern; for, owing to the greater -velocity of the earth when nearest the sun, the sun does not remain -so long on the southern hemisphere as he does on the northern. These -two effects so exactly counterbalance each other that, whatever be -the extent of the eccentricity, the total amount of heat reaching -both hemispheres is the same. And he considered that this beautiful -compensating principle would protect the climate of our globe from -being seriously affected by an increase in the eccentricity of its -orbit, unless the extent of that increase was very great. - -“Were it not,” he says, “for this, the eccentricity of the orbit -would materially influence the transition of seasons. The fluctuation -of distance amounts to nearly 1/30th of its mean quantity, and -consequently the fluctuation in the sun’s direct heating power to -double this, or 1/15th of the whole. Now the perihelion of the orbit is -situated nearly at the place of the northern winter solstice; so that, -were it not for the compensation we have just described, the effect -would be to exaggerate the difference of summer and winter in the -southern hemisphere, and to moderate it in the northern; thus producing -a more violent alternation of climate in the one hemisphere, and an -approach to perpetual spring in the other. As it is, however, no such -inequality subsists, but an equal and impartial distribution of heat -and light is accorded to both.”[8] - -Herschel’s opinion was shortly afterwards adopted and advocated by -Arago[9] and by Humboldt.[10] - -Arago, for example, states that so little is the climate of our globe -affected by the eccentricity of its orbit, that even were the orbit to -become as eccentric as that of the planet Pallas (that is, as great as -0·24), “still this would not alter in any appreciable manner the mean -thermometrical state of the globe.” - -This idea, supported by these great authorities, got possession of the -public mind; and ever since it has been almost universally regarded -as settled that the great changes of climate indicated by geological -phenomena could not have resulted from any change in the relation of -the earth to the sun. - -There is, however, one effect that was not regarded as compensated. The -total amount of heat received by the earth is inversely proportional -to the minor axis of its orbit; and it follows, therefore, that the -greater the eccentricity, the greater is the total amount of heat -received by the earth. On this account it was concluded that an -increase of eccentricity would tend to a certain extent to produce a -warmer climate. - -All those conclusions to which I refer, arrived at by astronomers, are -perfectly legitimate so far as the direct effects of eccentricity are -concerned; and it was quite natural, and, in fact, proper to conclude -that there was nothing in the mere increase of eccentricity that could -produce a glacial epoch. How unnatural would it have been to have -concluded that an increase in the quantity of heat received from the -sun should lower the temperature, and cover the country with snow and -ice! Neither would excessively cold winters, followed by excessively -hot summers, produce a glacial epoch. To assert, therefore, that the -purely astronomical causes could produce such an effect would be simply -absurd. - -_Important Consideration overlooked._—The important fact, however, was -overlooked that, although the glacial epoch could not result _directly_ -from an increase of eccentricity, it might nevertheless do so -_indirectly_. Although an increase of eccentricity could have no direct -tendency to lower the temperature and cover our country with ice, yet -it might bring into operation physical agents which would produce this -effect. - -If, instead of endeavouring to trace a direct connection between a high -condition of eccentricity and a glacial condition of climate, we turn -our attention to the consideration of what are the physical effects -which result from an increase of eccentricity, we shall find that a -host of physical agencies are brought into operation, the combined -effect of which is to lower to a very great extent the temperature of -the hemisphere whose winters occur in aphelion, and to raise to nearly -as great an extent the temperature of the opposite hemisphere, whose -winters of course occur in perihelion. Until attention was directed to -those physical circumstances to which I refer, it was impossible that -the true cause of the glacial epoch could have been discovered; and, -moreover, many of the indirect and physical effects, which in reality -were those that brought about the glacial epoch, could not, in the -nature of things, have been known previously to recent discoveries in -the science of heat. - -The consideration and discussion of those various physical agencies are -the chief aim of the following pages. - -_Abstract of the Line of Argument pursued in this Volume._—I shall -now proceed to give a brief abstract of the line of argument pursued -in this volume. But as a considerable portion of it is devoted to the -consideration of objections and difficulties bearing either directly -or indirectly on the theory, it will be necessary to point out what -those difficulties are, how they arose, and the methods which have been -adopted to overcome them. - -Chapter IV. contains an outline of the physical agencies affecting -climate which are brought into operation by an increase of -eccentricity. By far the most important of all those agencies, and the -one which mainly brought about the glacial epoch, is the _Deflection_ -of Ocean-Currents. The consideration of the indirect physical -connection between a high state of eccentricity and the deflection -of ocean-currents, and also the enormous influence on climate which -results from this deflection constitute not only the most important -part of the subject, but the one beset with the greatest amount of -difficulties. - -The difficulties besetting this part of the theory arise mainly from -the imperfect state of our knowledge, (1st) with reference to the -absolute amount of heat transferred from equatorial to temperate and -polar regions by means of ocean-currents and the influence which the -heat thus transferred has on the distribution of temperature on the -earth’s surface; and (2nd) in connection with the physical cause of -ocean circulation. - -In Chapters II. and III. I have entered at considerable length into -the consideration of the effects of ocean currents on the distribution -of heat over the globe. The only current of which anything like -an accurate estimate of volume and temperature has been made is -the Gulf-stream. In reference to this stream we have a means of -determining in absolute measure the quantity of heat conveyed by it. -On the necessary computation being made, it is found that the amount -transferred by the Gulf-stream from equatorial regions into the North -Atlantic is enormously greater than was ever anticipated, amounting -to no less than one-fifth part of the entire heat possessed by the -North Atlantic. This striking fact casts a new light on the question -of the distribution of heat over the globe. It will be seen that to -such an extent is the temperature of the equatorial regions lowered, -and that of high temperate, and polar regions raised, by means of ocean -currents, that were they to cease, and each latitude to depend solely -on the heat received directly from the sun, only a very small portion -of the globe would be habitable by the present order of beings. This -being the case, it becomes obvious to what an extent the deflection -of ocean currents must affect temperature. For example, were the -Gulf-stream stopped, and the heat conveyed by it deflected into the -Southern Ocean, how enormously would this tend to lower the temperature -of the northern hemisphere, and raise the temperature south of the -equator. - -Chapters VI., VII., VIII., IX., X., and XIII., are devoted to the -consideration of the physical cause of oceanic circulation. This has -been found to be the most difficult and perplexing part of the whole -inquiry. The difficulties mainly arise from the great diversity of -opinion and confusion of ideas prevailing in regard to the mechanics -of the subject. There are two theories propounded to account for -oceanic circulation; the one which may be called the _Wind_ theory, and -the other the _Gravitation_ theory; and this diversity of opinion and -confusion of ideas prevail in connection with both theories. As the -question of the cause of oceanic circulation has not only a direct and -important bearing on the subject of the present volume, but is further -one of much general interest, I have entered somewhat fully into the -matter. - -The Gravitation theories may be divided into two classes. The first of -these attributes the Gulf-stream and other sensible currents of the -ocean to difference of specific gravity, resulting from difference -of temperature between the sea in equatorial and polar regions. The -leading advocate of this theory was the late Lieutenant Maury, who -brought it so much into prominence in his interesting book on the -“Physical Geography of the Sea.” The other class does not admit that -the sensible currents of the ocean can be produced by difference of -specific gravity; but they maintain that difference of temperature -between the sea in equatorial and polar regions produces a general -movement of the upper portion of the sea from the equator to the -poles, and a counter-movement of the under portion from the poles -to the equator. This form of the gravitation theory has been ably -and zealously advocated by Dr. Carpenter, who may be regarded as -its representative. The Wind theories also divide into two classes. -According to the one ocean currents are caused and maintained by the -impulse of the trade-winds, while according to the other they are -due not to the impulse of the trade-winds alone, but to that of the -prevailing winds of the globe, regarded as a general system. The former -of these is the one generally accepted; the latter is that advocated in -the present volume. - -The relations which these theories bear to the question of secular -change of climate, will be found stated at length in Chapter VI. It -will, however, be better to state here in a few words what those -relations are. When the eccentricity of the earth’s orbit attains a -high value, the hemisphere, whose winter solstice occurs in aphelion, -has, for reasons which are explained in Chapter IV., its temperature -lowered, while that of the opposite hemisphere is raised. Let us -suppose the northern hemisphere to be the cold one, and the southern -the warm one. The difference of temperature between the equator and -the North Pole will then be greater than between the equator and the -South Pole; according, therefore, to theory, the trades of the northern -hemisphere will be stronger than those of the southern, and will -consequently blow across the equator to some distance on the southern -hemisphere. This state of things will tend to deflect equatorial -currents southwards, impelling the warm water of the equatorial regions -more into the southern or warm hemisphere than into the northern or -cold hemisphere. The tendency of all this will be to exaggerate the -difference of temperature already existing between the two hemispheres. -If, on the other hand, the great ocean currents which convey the warm -equatorial waters to temperate and polar regions be not produced by -the impulse of the winds, but by difference of temperature, as Maury -maintains, then in the case above supposed the equatorial waters would -be deflected more into the northern or cold hemisphere than into the -southern or warm hemisphere, because the difference of temperature -between the equator and the poles would be greater on the cold than -on the warm hemisphere. This, of course, would tend to neutralize or -counteract that difference of temperature between the two hemispheres -which had been previously produced by eccentricity. In short, this -theory of circulation would effectually prevent eccentricity from -seriously affecting climate. - -Chapters VI. and VII. have been devoted to an examination of this form -of the gravitation theory. - -The above remarks apply equally to Dr. Carpenter’s form of the theory; -for according to a doctrine of General Oceanic Circulation resulting -from difference of specific gravity between the water at the equator -and at the poles, the equatorial water will be carried more to the -cold than to the warm hemisphere. It is perfectly true that a belief -in a general oceanic circulation may be held quite consistently with -the theory of secular changes of climate, provided it be admitted -that not this general circulation but ocean currents are the great -agency employed in distributing heat over the globe. The advocates of -the theory, however, admit no such thing, but regard ocean currents -as of secondary importance. It may be stated that the existence of -this general ocean circulation has never been detected by actual -observation. It is simply assumed in order to account for certain -facts, and it is asserted that such a circulation must take place as -a physical necessity. I freely admit that were it not that the warm -water of equatorial regions is being constantly carried off by means -of ocean currents such as the Gulf-stream, it would accumulate till, -in order to restoration of equilibrium, such a general movement as is -supposed would be generated. But it will be shown that the warm water -in equatorial regions is being drained off so rapidly by ocean currents -that the actual density of an equatorial column differs so little -from that of a polar column that the force of gravity resulting from -that difference is so infinitesimal that it is doubtful whether it is -sufficient to produce sensible motion. I have also shown in Chapter -VIII. that all the facts which this theory is designed to explain are -not only explained by the wind theory, but are deducible from it as -necessary consequences. In Chapter XI. it is proved, by contrasting -the quantity of heat conveyed by ocean currents from inter-tropical to -temperate and polar regions with such an amount as could possibly be -conveyed by means of a general oceanic circulation, that the latter -sinks into insignificance before the former. In Chapters X. and XII. -the various objections which have been advanced by Dr. Carpenter and -Mr. Findlay are discussed at considerable length, and in Chapter IX. -I have entered somewhat minutely into an examination of the mechanics -of the gravitation theory. A statement of the wind theory is given in -Chapter XIII.; and in Chapter XIV. is shown the relation of this theory -to the theory of Secular changes of climate. This terminates the part -of the inquiry relating to oceanic circulation. - -We now come to the _crucial test_ of the theories respecting the cause -of the glacial epoch, viz., Warm Inter-glacial Periods. In Chapters -XV. and XVI. I have given a statement of the geological facts which -go to prove that that long epoch known as the Glacial was not one -of continuous cold, but consisted of a succession of cold and warm -periods. This condition of things is utterly inexplicable on every -theory of the cause of the glacial epoch which has hitherto been -advanced; but, according to the physical theory of secular changes of -climate under consideration, it follows as a necessary consequence. -In fact, the amount of geological evidence which has already been -accumulated in reference to inter-glacial periods may now be regarded -as perfectly sufficient to establish the truth of that theory. - -If the glacial epoch resulted from some accidental distribution of sea -and land, then there may or may not have been more than one glacial -epoch, but if it resulted from the cause which we have assigned, then -there must have been during the geological history of the globe a -succession of glacial epochs corresponding to the secular variations -in the eccentricity of the earth’s orbit. A belief in the existence -of recurring glacial epochs has been steadily gaining ground for many -years past. I have, in Chapter XVIII., given at some length the facts -on which this belief rests. It is true that the geological evidence of -glacial epochs in prior ages is meagre in comparison with that of the -glacial epoch of Post-tertiary times; but there is a reason for this in -the nature of geological evidence itself. Chapter XVII. deals with the -geological records of former glacial epochs, showing that they are not -only imperfect, but that there is good reason why they should be so, -and that the imperfection of the records in reference to them cannot be -advanced as an argument against their existence. - -If the glacial epoch resulted from a high condition of eccentricity, we -have not only a means of determining the positive date of that epoch, -but we have also a means of determining geological time in absolute -measure. For if the glacial epochs of prior ages correspond to periods -of high eccentricity, then the intervals between those periods of high -eccentricity become the measure of the intervals between the glacial -epochs. The researches of Lagrange and Leverrier into the secular -variations of the elements of the orbits of the planets enable us -to determine with tolerable accuracy the values of the eccentricity -of the earth’s orbit for, at least, four millions of years past and -future. With the view of determining those values, I several years -ago computed from Leverrier’s formula the eccentricity of the earth’s -orbit and longitude of the perihelion, at intervals of ten thousand and -fifty thousand years during a period of three millions of years in the -past, and one million of years in the future. The tables containing -these values will be found in Chapter XIX. These tables not only give -us the date of the glacial epoch, but they afford, as will be seen -from Chapter XXI., evidence as to the probable date of the Eocene and -Miocene periods. - -Ten years ago, when the theory was first advanced, it was beset by -a very formidable difficulty, arising from the opinions which then -prevailed in reference to geological time. One or two glacial epochs in -the course of a million of years was a conclusion which at that time -scarcely any geologist would admit, and most would have felt inclined -to have placed the last glacial epoch at least one million of years -back. But then if we assume that the glacial epoch was due to a high -state of eccentricity, we should be compelled to admit of at least two -glacial epochs during that lapse of time. It was the modern doctrine -that the great changes undergone by the earth’s crust were produced, -not by convulsions of nature, but by the slow and almost imperceptible -action, of rain, rivers, snow, frost, ice, &c., which impressed so -strongly on the mind of the geologist the vast duration of geological -periods. When it was considered that the rocky face of our globe had -been carved into hills and dales, and ultimately worn down to the -sea-level by means of those apparently trifling agents, not only once -or twice, but many times, during past ages, it was not surprising that -the views entertained by geologists regarding the immense antiquity of -our globe should not have harmonised with the deductions of physical -science on the subject. It had been shown by Sir William Thomson and -others, from physical considerations relating to the age of the sun’s -heat and the secular cooling of our globe, that the geological history -of our earth’s crust must be limited to a period of something like -one hundred millions of years. But these speculations had but little -weight when pitted against the stern and undeniable facts of subaërial -denudation. How, then, were the two to be reconciled? Was it the -physicist who had under-estimated geological time, or the geologist -who had over-estimated it? Few familiar with modern physics, and who -have given special attention to the subject, would admit that the sun -could have been dissipating his heat at the present enormous rate for -a period much beyond one hundred millions of years. The probability -was that the amount of work performed on the earth’s crust by the -denuding agents in a period so immense as a million of years was, for -reasons stated in Chapter XX., very much under-estimated. But the -difficulty was how to prove this. How was it possible to measure the -rate of operation of agents so numerous and diversified acting with -such extreme slowness and irregularity over so immense areas? In other -words, how was it possible to measure the rate of subaërial denudation? -Pondering over this problem about ten years ago, an extremely simple -and obvious method of solving it suggested itself to my mind. This -method—the details of which will be found in Chapter XX.—showed that -the rate of subaërial denudation is enormously greater than had been -supposed. The method is now pretty generally accepted, and the result -has already been to bring about a complete reconciliation between -physics and geology in reference to time. - -Chapter XXI. contains an account of the gravitation theories of the -origin of the sun’s heat. The energy possessed by the sun is generally -supposed to have been derived from gravitation, combustion being -totally inadequate as a source. But something more than gravitation -is required before we can account for even one hundred millions of -years’ heat. Gravitation could not supply even one-half that amount. -There must be some other and greater source than that of gravitation. -There is, however, as is indicated, an obvious source from which far -more energy may have been derived than could have been obtained from -gravitation. - -The method of determining the rate of subaërial denudation enables us -also to arrive at a rough estimate of the actual mean thickness of the -stratified rocks of the globe. It will be seen from Chapter XXII. that -the mean thickness is far less than is generally supposed. - -The physical cause of the submergence of the land during the glacial -epoch, and the influence of change in the obliquity of the ecliptic on -climate, are next considered. In Chapter XXVI. I have given the reasons -which induce me to believe that coal is an inter-glacial formation. - -The next two chapters—the one on the path of the ice in north-western -Europe, the other on the north of England ice-sheet—are reprints of -papers which appeared a few years ago in the _Geological Magazine_. -Recent observations have confirmed the truth of the views advanced -in these two chapters, and they are rapidly gaining acceptance among -geologists. - -I have given, at the conclusion, a statement of the molecular theory of -glacier motion—a theory which I have been led to modify considerably on -one particular point. - -There is one point to which I wish particularly to direct -attention—viz., that I have studiously avoided introducing into the -theories propounded anything of a hypothetical nature. There is not, -so far as I am aware, from beginning to end of this volume, a single -hypothetical element: nowhere have I attempted to give a hypothetical -explanation. The conclusions are in every case derived either from -facts or from what I believe to be admitted principles. In short, I -have aimed to prove that the theory of secular changes of climate -follows, as a necessary consequence, from the admitted principles of -physical science. - - - - - CHAPTER II. - - OCEANS-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER - THE GLOBE. - - The absolute Heating-power of Ocean-currents.—Volume of - the Gulf-stream.—Absolute Amount of Heat conveyed by - it.—Greater Portion of Moisture in inter-tropical Regions - falls as Rain in those Regions.—Land along the Equator - tends to lower the Temperature of the Globe.—Influence - of Gulf-stream on Climate of Europe.—Temperature of - Space.—Radiation of a Particle.—Professor Dove on Normal - Temperature.—Temperature of Equator and Poles in the Absence - of Ocean-currents.—Temperature of London, how much due to - Ocean-currents. - - -_The absolute Heating-power of Ocean-currents._—There is perhaps no -physical agent concerned in the distribution of heat over the surface -of the globe the influence of which has been so much underrated as that -of ocean-currents. This is, no doubt, owing to the fact that although -their surface-temperature, direction, and general influence have -obtained considerable attention, yet little or nothing has been done -towards determining the absolute amount of heat or of cold conveyed by -them or the resulting absolute increase or decrease of temperature. - -The modern method of determining the amount of heat-effects in absolute -measure is, doubtless, destined to cast new light on all questions -connected with climate, as it has done, and is still doing, in every -department of physics where energy, under the form of heat, is being -studied. But this method has hardly as yet been attempted in questions -of meteorology; and owing to the complicated nature of the phenomena -with which the meteorologist has generally to deal, its application -will very often prove practically impossible. Nevertheless, it is -particularly suitable to all questions relating to the direct thermal -effects of currents, whatever the nature of these currents may happen -to be. - -In the application of the method to an ocean-current, the two most -important elements required as data are the volume of the stream and -its mean temperature. But although we know something of the temperature -of most of the great ocean-currents, yet, with the exception of the -Gulf-stream, little has been ascertained regarding their volume. - -The breadth, depth, and temperature of the Gulf-stream have formed the -subject of extensive and accurate observations by the United States -Coast Survey. In the memoirs and charts of that survey cross-sections -of the stream at various places are given, showing its breadth and -depth, and also the temperature of the water from the surface to the -bottom. We are thus enabled to determine with some precision the -mean temperature of the stream. And knowing its mean velocity at any -given section, we have likewise a means of determining the number of -cubic feet of water passing through that section in a given time. But -although we can obtain with tolerable accuracy the mean temperature, -yet observations regarding the velocity of the water at all depths have -unfortunately not been made at any particular section. Consequently we -have no means of estimating as accurately as we could wish the volume -of the current. Nevertheless, since we know the surface-velocity of the -water at places where some of the sections were taken, we are enabled -to make at least a rough estimate of the volume. - -From an examination of the published sections, I came to the conclusion -some years ago[11] that the total quantity of water conveyed by the -stream is probably equal to that of a stream fifty miles broad and -1,000 feet deep,[12] flowing at the rate of four miles an hour, -and that the mean temperature of the entire mass of moving water is -not under 65° at the moment of leaving the Gulf. But to prevent the -possibility of any objections being raised on the grounds that I may -have over-estimated the volume of the stream, I shall take the velocity -to be _two_ miles instead of four miles an hour. We are warranted, -I think, in concluding that the stream before it returns from its -northern journey is on an average cooled down to at least 40°,[13] -consequently it loses 25° of heat. Each cubic foot of water, therefore, -in this case carries from the tropics for distribution upwards of -1,158,000 foot-pounds of heat. According to the above estimate of the -size and velocity of the stream, which in Chapter XI. will be shown -to be an under-estimate, 2,787,840,000,000 cubic feet of water are -conveyed from the Gulf per hour, or 66,908,160,000,000 cubic feet -daily. Consequently the total quantity of heat thus transferred per day -amounts to 77,479,650,000,000,000,000 foot-pounds. - -This estimate of the volume of the stream is considerably less by -one-half than that given both by Captain Maury and by Sir John -Herschel. Captain Maury considers the Gulf-stream equal to a stream -thirty-two miles broad and 1,200 feet deep, flowing at the rate of five -knots an hour.[14] This gives 6,165,700,000,000 cubic feet per hour -as the quantity of water conveyed by this stream. Sir John Herschel’s -estimate is still greater. He considers it equal to a stream thirty -miles broad and 2,200 feet deep, flowing at the rate of four miles -an hour.[15] This makes the quantity 7,359,900,000,000 cubic feet -per hour. Dr. Colding, in his elaborate memoir on the Gulf-stream, -estimates the volume at 5,760,000,000,000 cubic feet per hour, while -Mr. Laughton’s estimate is nearly double that of mine. - -From observations made by Sir John Herschel and by M. Pouillet on the -direct heat of the sun, it is found that, were no heat absorbed by the -atmosphere, about eighty-three foot-pounds per second would fall upon -a square foot of surface placed at right angles to the sun’s rays.[16] -Mr. Meech estimates that the quantity of heat cut off by the atmosphere -is equal to about twenty-two per cent. of the total amount received -from the sun. M. Pouillet estimates the loss at twenty-four per cent. -Taking the former estimate, 64·74 foot-pounds per second will therefore -be the quantity of heat falling on a square foot of the earth’s surface -when the sun is in the zenith. And were the sun to remain stationary in -the zenith for twelve hours, 2,796,768 foot-pounds would fall upon the -surface. - -It can be shown that the total amount of heat received upon a unit -surface on the equator, during the twelve hours from sunrise till -sunset at the time of the equinoxes, is to the total amount which -would be received upon that surface, were the sun to remain in the -zenith during those twelve hours, as the diameter of a circle to half -its circumference, or as 1 to 1·5708. It follows, therefore, that -a square foot of surface on the equator receives from the sun at -the time of the equinoxes 1,780,474 foot-pounds daily, and a square -mile 49,636,750,000,000 foot-pounds daily. But this amounts to only -1/1560935th part of the quantity of heat daily conveyed from the -tropics by the Gulf-stream. In other words, the Gulf-stream conveys as -much heat as is received from the sun by 1,560,935 square miles at the -equator. The amount thus conveyed is equal to all the heat which falls -upon the globe within thirty-two miles on each side of the equator. -According to calculations made by Mr. Meech,[17] the annual quantity -of heat received by a unit surface on the frigid zone, taking the -mean of the whole zone, is 5·45/12th of that received at the equator; -consequently the quantity of heat conveyed by the Gulf-stream in one -year is equal to the heat which falls on an average on 3,436,900 -square miles of the arctic regions. The frigid zone or arctic regions -contain 8,130,000 square miles. There is actually, therefore, nearly -one-half as much heat transferred from tropical regions by the -Gulf-stream as is received from the sun by the entire arctic regions, -the quantity conveyed from the tropics by the stream to that received -from the sun by the arctic regions being nearly as two to five. - -But we have been assuming in our calculations that the percentage of -heat absorbed by the atmosphere is no greater in polar regions than -it is at the equator, which is not the case. If we make due allowance -for the extra amount absorbed in polar regions in consequence of the -obliqueness of the sun’s rays, the total quantity of heat conveyed by -the Gulf-stream will probably be nearly equal to one-half the amount -received from the sun by the entire arctic regions. - -If we compare the quantity of heat conveyed by the Gulf-stream with -that conveyed by means of aërial currents, the result is equally -startling. The density of air to that of water is as 1 to 770, and -its specific heat to that of water is as 1 to 4·2; consequently the -same amount of heat that would raise 1 cubic foot of water 1° would -raise 770 cubic feet of air 4°·2, or 3,234 cubic feet 1°. The quantity -of heat conveyed by the Gulf-stream is therefore equal to that which -would be conveyed by a current of air 3,234 times the volume of the -Gulf-stream, at the same temperature and moving with the same velocity. -Taking, as before, the width of the stream at fifty miles, and its -depth at 1,000 feet, and its velocity at two miles an hour, it follows -that, in order to convey an equal amount of heat from the tropics by -means of an aërial current, it would be necessary to have a current -about 1¼ mile deep, and at the temperature of 65°, blowing at the -rate of two miles an hour from every part of the equator over the -northern hemisphere towards the pole. If its velocity were equal to -that of a good sailing-breeze, which Sir John Herschel states to be -about twenty-one miles an hour, the current would require to be above -600 feet deep. A greater quantity of heat is probably conveyed by the -Gulf-stream alone from the tropical to the temperate and arctic regions -than by all the aërial currents which flow from the equator. - -We are apt, on the other hand, to over-estimate the amount of the heat -conveyed from tropical regions to us by means of aërial currents. The -only currents which flow from the equatorial regions are the upper -currents, or anti-trades as they are called. But it is not possible -that much heat can be conveyed directly by them. The upper currents of -the trade-winds, even at the equator, are nowhere below the snow-line; -they must therefore lie in a region of which the temperature is -actually below the freezing-point. In fact, if those currents were -warm, they would elevate the snow-line above themselves. The heated air -rising off the hot burning ground at the equator, after ascending a -few miles, becomes exposed to the intense cold of the upper regions of -the atmosphere; it then very soon loses all its heat, and returns from -the equator much colder than it went thither. It is impossible that -we can receive any heat directly from the equatorial regions by means -of aërial currents. It is perfectly true that the south-west wind, to -which we owe so much of our warmth in this country, is a continuation -of the anti-trade; but the heat which this wind brings to us is not -derived from the equatorial regions. This will appear evident, if we -but reflect that, before the upper current descends to the snow-line -after leaving the equator, it must traverse a space of at least 2,000 -miles; and to perform this long journey several days will be required. -During all this time the air is in a region below the freezing-point; -and it is perfectly obvious that by the time it begins to descend it -must have acquired the temperature of the region in which it has been -travelling. - -If such be the case, it is evident that a wind whose temperature -is below 32° could never warm a country such as ours, where the -temperature does not fall below 38° or 39°. The heat of our south-west -winds is derived, not directly from the equator, but from the warm -water of the Atlantic—in fact, from the Gulf-stream. The upper current -acquires its heat after it descends to the earth. There is one way, -however, whereby heat is indirectly conveyed from the equator by the -anti-trades; that is, in the form of aqueous vapour. In the formation -of one pound of water from aqueous vapour, as Professor Tyndall -strikingly remarks, a quantity of heat is given out sufficient to melt -five pounds of cast iron.[18] It must, however, be borne in mind that -the greater part of the moisture of the south-west and west winds is -derived from the ocean in temperate regions. The upper current receives -the greater part of its moisture after it descends to the earth, whilst -the moisture received at the equator is in great part condensed, and -falls as rain in those regions. - -This latter assertion has been so frequently called in question -that I shall give my reasons for making it. According to Dr. Keith -Johnston (“Physical Atlas”) the mean rainfall of the torrid regions -is ninety-six inches per annum, while that of the temperate regions -amounts to only thirty-seven inches. If the greater part of the -moisture of the torrid regions does not fall as rain in those regions, -it must fall as such beyond them. Now the area of the torrid to that -of the two temperate regions is about as 39·3 to 51. Consequently -ninety-six inches of rain spread over the temperate regions would give -seventy-four inches; but this is double the actual rainfall of the -temperate regions. If, again, it were spread over both temperate and -polar regions this would yield sixty-four inches, which, however, is -nearly double the mean rainfall of the temperate and polar regions. If -we add to this the amount of moisture derived from the ocean within -temperate and polar regions, we should have a far greater rainfall for -these latitudes than for the torrid region, and we know, of course, -that it is actually far less. This proves the truth of the assertion -that by far the greater part of the moisture of the torrid regions -falls in those regions as rain. It will hardly do to object that the -above may probably be an over-estimate of the amount of rainfall in -the torrid zone, for it is not at all likely that any error will ever -be found which will affect the general conclusion at which we have -arrived. - -Dr. Carpenter, in proof of the small rainfall of the torrid zone, -adduces the case of the Red Sea, where, although evaporation is -excessive, almost no rain falls. But the reason why the vapour raised -from the Red Sea does not fall in that region as rain, is no doubt -owing to the fact that this sea is only a narrow strip of water in a -dry and parched land, the air above which is too greedy of moisture -to admit of the vapour being deposited as rain. Over a wide expanse -of ocean, however, where the air above is kept to a great extent in a -constant state of saturation, the case is totally different. - -_Land at the Equator tends to Lower the Temperature of the Globe._—The -foregoing considerations, as well as many others which might be stated, -lead to the conclusion that, in order to raise the mean temperature of -the whole earth, _water_ should be placed along the equator, and not -_land_, as is supposed by Sir Charles Lyell and others. For if land is -placed at the equator, the possibility of conveying the sun’s heat from -the equatorial regions by means of ocean-currents is prevented. The -transference of heat could then be effected only by means of the upper -currents of the trades; for the heat conveyed by _conduction_ along the -solid crust, if any, can have no sensible effect on climate. But these -currents, as we have just seen, are ill-adapted for conveying heat. - -The surface of the ground at the equator becomes intensely heated by -the sun’s rays. This causes it to radiate its heat more rapidly into -space than a surface of water heated under the same conditions. Again, -the air in contact with the hot ground becomes also more rapidly -heated than in contact with water, and consequently the ascending -current of air carries off a greater amount of heat. But were the -heat thus carried away transferred by means of the upper currents to -high latitudes and there employed to warm the earth, then it might to -a considerable extent compensate for the absence of ocean-currents, -and in this case land at the equator might be nearly as well adapted -as water for raising the temperature of the whole earth. But such is -not the case; for the heat carried up by the ascending current at the -equator is not employed in warming the earth, but is thrown off into -the cold stellar space above. This ascending current, instead of being -employed in warming the globe, is in reality one of the most effectual -means that the earth has of getting quit of the heat received from the -sun, and of thus maintaining a much lower temperature than it would -otherwise possess. It is in the equatorial regions that the earth loses -as well as gains the greater part of its heat; so that, of all places, -here ought to be placed the substance best adapted for preventing the -dissipation of the earth’s heat into space, in order to raise the -general temperature of the earth. Water, of all substances in nature, -seems to possess this quality to the greatest extent; and, besides, it -is a fluid, and therefore adapted by means of currents to carry the -heat which it receives from the sun to every region of the globe. - -These results show (although they have reference to only one stream) -that the general influence of ocean-currents on the distribution of -heat over the surface of the globe must be very great. If the quantity -of heat transferred from equatorial regions by the Gulf-stream -alone is nearly equal to all the heat received from the sun by the -arctic regions, then how enormous must be the quantity conveyed from -equatorial regions by all the ocean-currents together! - -_Influence of the Gulf-stream on the Climate of Europe._—In a paper -read before the British Association at Exeter, Mr. A. G. Findlay -objects to the conclusions at which I have arrived in former papers -on the subject, that I have not taken into account the great length -of time that the water requires in order to circulate, and the -interference it has to encounter in its passage. - -The objection is, that a stream so comparatively small as the -Gulf-stream, after spreading out over such a large area of the -Atlantic, and moving so slowly across to the shores of Europe, losing -heat all the way, would not be able to produce any very sensible -influence on the climate of Europe. - -I am unable to perceive the force of this objection. Why, the very -efficiency of the stream as a heating agent necessarily depends upon -the slowness of its motion. Did the Gulf-stream move as rapidly along -its whole course as it does in the Straits of Florida, it could produce -no sensible effect on the climate of Europe. It does not require much -consideration to perceive this. (1) If the stream during its course -continued narrow, deep, and rapid, it would have little opportunity of -losing its heat, and the water would carry back to the tropics the heat -which it ought to have given off in the temperate and polar regions. -(2) The Gulf-stream does not heat the shores of Europe by direct -radiation. Our island, for example, is not heated by radiation from a -stream of warm water flowing along its shores. The Gulf-stream heats -our island _indirectly_ by heating the winds which blow over it to our -shores. - -The anti-trades, or upper return-currents, as we have seen, bring no -heat from the tropical regions. After traversing some 2,000 miles -in a region of extreme cold they descend on the Atlantic as a cold -current, and there absorb the heat and moisture which they carry to -north-eastern Europe. Those aërial currents derive their heat from the -Gulf-stream, or if it is preferred, from the warm water poured into the -Atlantic by the Gulf-stream. - -How, then, are these winds heated by the warm water? The air is heated -in two ways, viz., by direct _radiation_ from the water, and by -_contact_ with the water. Now, if the Gulf-stream continued a narrow -and deep current during its entire course similar to what it is at -the Straits of Florida, it could have little or no opportunity of -communicating its heat to the air either by radiation or by contact. If -the stream were only about forty or fifty miles in breadth, the aërial -particles in their passage across it would not be in contact with the -warm water more than an hour or two. Moreover, the number of particles -in contact with the water, owing to the narrowness of the stream, -would be small, and there would therefore be little opportunity for -the air becoming heated by contact. The same also holds true in regard -to radiation. The more we widen the stream and increase its area, the -more we increase its radiating surface; and the greater the radiating -surface, the greater is the quantity of heat thrown off. But this is -not all; the number of aërial particles heated by radiation increases -in proportion to the area of the radiating surface; consequently, the -wider the area over which the waters of the Gulf-stream are spread, -the more effectual will the stream be as a heating agent. And, again, -in order that a very wide area of the Atlantic may be covered with the -warm waters of the stream, slowness of motion is essential. - -Mr. Findlay supposes that fully one-half of the Gulf-stream passes into -the south-eastern branch, and that it is only the north-eastern branch -of the current that can be effectual in raising the temperature of -Europe. But it appears to me that it is to this south-eastern portion -of the current, and not to the north-eastern, that we, in this country, -are chiefly indebted for our heat. The south-west winds, to which we -owe our heat, derive their temperature from this south-eastern portion -which flows away in the direction of the Azores. The south-west winds -which blow over the northern portion of the current which flows past -our island up into the arctic seas cannot possibly cross this country, -but will go to heat Norway and northern Europe. The north-eastern -portion of the stream, no doubt, protects us from the ice of Greenland -by warming the north-west winds which come to us from that cold region. - -Mr. Buchan, Secretary of the Scottish Meteorological Society, has -shown[19] that in a large tract of the Atlantic between latitudes 20° -and 40° N., the mean pressure of the atmosphere is greater than in any -other place on the globe. To the west of Madeira, between longitude -10° and 40° W., the mean annual pressure amounts to 30·2 inches, while -between Iceland and Spitzbergen it is only 29·6, a lower mean pressure -than is found in any other place on the northern hemisphere. There -must consequently, he concludes, be a general tendency in the air to -flow from the former to the latter place along the earth’s surface. -Now, the air in moving from the lower to the higher latitudes tends -to take a north-easterly direction, and in this case will pass over -our island in its course. This region of high pressure, however, -is situated in the very path of the south-eastern branch of the -Gulf-stream, and consequently the winds blowing therefrom will carry -directly to Britain the heat of the Gulf-stream. - -As we shall presently see, it is as essential to the heating of our -island as to that of the southern portion of Europe, that a very large -proportion of the waters of the Gulf-stream should spread over the -surface of the Atlantic and never pass up into the arctic regions. - -Even according to Mr. Findlay’s own theory, it is to the south-west -wind, heated by the warm waters of the Atlantic, that we are indebted -for the high temperature of our climate. But he seems to be under the -impression that the Atlantic would be able to supply the necessary -heat independently of the Gulf-stream. This, it seems to me, is the -fundamental error of all those who doubt the efficiency of the stream. -It is a mistake, however, into which one is very apt to fall who does -not adopt the more rigid method of determining heat-results in absolute -measure. When we apply this method, we find that the Atlantic, without -the aid of such a current as the Gulf-stream, would be wholly unable to -supply the necessary amount of heat to the south-west winds. - -The quantity of heat conveyed by the Gulf-stream, as we have seen, -is equal to all the heat received from the sun by 1,560,935 square -miles at the equator. The mean annual quantity of heat received from -the sun by the temperate regions per unit surface is to that received -by the equator as 9·08 to 12.[20] Consequently, the quantity of heat -conveyed by the stream is equal to all the heat received from the sun -by 2,062,960 square miles of the temperate regions. The total area of -the Atlantic from the latitude of the Straits of Florida, 200 miles -north of the tropic of Cancer, up to the Arctic Circle, including also -the German Ocean, is about 8,500,000 square miles. In this case the -quantity of heat carried by the Gulf-stream into the Atlantic through -the Straits of Florida, is to that received by this entire area from -the sun as 1 to 4·12, or in round numbers as 1 to 4. It therefore -follows that one-fifth of all the heat possessed by the waters of the -Atlantic over that area, even supposing that they absorb every ray that -falls upon them, is derived from the Gulf-stream. Would those who call -in question the efficiency of the Gulf-stream be willing to admit that -a decrease of one-fourth in the total amount of heat received from the -sun, over the entire area of the Atlantic from within 200 miles of -the tropical zone up to the arctic regions, would not sensibly affect -the climate of northern Europe? If they would not willingly admit -this, why, then, contend that the Gulf-stream does not affect climate? -for the stoppage of the Gulf-stream would deprive the Atlantic of -77,479,650,000,000,000,000 foot-pounds of energy in the form of heat -per day, a quantity equal to one-fourth of all the heat received from -the sun by that area. - -How much, then, of the temperature of the south-west winds derived from -the water of the Atlantic is due to the Gulf-stream? - -Were the sun extinguished, the temperature over the whole earth -would sink to _nearly_ that of stellar space, which, according to -the investigations of Sir John Herschel[21] and of M. Pouillet,[22] -is not above −239° F. Were the earth possessed of no atmosphere, the -temperature of its surface would sink to exactly that of space, or to -that indicated by a thermometer exposed to no other heat-influence than -that of radiation from the stars. But the presence of the atmospheric -envelope would slightly modify the conditions of things; for the -heat from the stars (which of course constitutes what is called the -temperature of space) would, like the sun’s heat, pass more freely -through the atmosphere than the heat radiated back from the earth, and -there would in consequence of this be an accumulation of heat on the -earth’s surface. The temperature would therefore stand a little higher -than that of space; or, in other words, it would stand a little higher -than it would otherwise do were the earth exposed in space to the -direct radiation of the stars without the atmospheric envelope. But, -for reasons which will presently be stated, we may in the meantime, -till further light is cast upon this matter, take -239° F. as probably -not far from what would be the temperature of the earth’s surface were -the sun extinguished. - -Suppose now that we take the mean annual temperature of the Atlantic -at, say, 56°.[23] Then 239° + 56° = 295° represents the number of -degrees of rise due to the heat which it receives. In other words, -it takes all the heat that the Atlantic receives to maintain its -temperature 295° above the temperature of space. Stop the Gulf-stream, -and the Atlantic would be deprived of one-fifth of the heat which -it possesses. Then, if it takes five parts of heat to maintain a -temperature of 295° above that of space, the four parts which would -remain after the stream was stopped would only be able to maintain a -temperature of four-fifths of 295°, or 236° above that of space: the -stoppage of the Gulf-stream would therefore deprive the Atlantic of an -amount of heat which would be sufficient to maintain its temperature -59° above what it would otherwise be, did it depend alone upon the heat -received directly from the sun. It does not, of course, follow that -the Gulf-stream actually maintains the temperature 59° above what it -would otherwise be were there no ocean-currents, because the actual -heating-effect of the stream is neutralized to a very considerable -extent by cold currents from the arctic regions. But 59° of rise -represents its actual power; consequently 59°, minus the lowering -effect of the cold currents, represents the actual rise. What the rise -may amount to at any particular place must be determined by other means. - -This method of calculating how much the temperature of the earth’s -surface would rise or fall from an increase or a decrease in the -absolute amount of heat received is that adopted by Sir John Herschel -in his “Outlines of Astronomy,” § 369^a. - -About three years ago, in an article in the _Reader_, I endeavoured -to show that this method is not rigidly correct. It has been shown -from the experiments of Dulong and Petit, Dr. Balfour Stewart, -Professor Draper, and others, that the rate at which a body radiates -its heat off into space is not directly proportionate to its absolute -temperature. The rate at which a body loses its heat as its temperature -rises increases more rapidly than the temperature. As a body rises -in temperature the rate at which it radiates off its heat increases; -the _rate_ of this increase, however, is not uniform, but increases -with the temperature. Consequently the temperature is not lowered in -proportion to the decrease of the sun’s heat. But at the comparatively -low temperature with which we have at present to deal, the error -resulting from assuming the decrease of temperature to be proportionate -to the decrease of heat would not be great. - -It may be remarked, however, that the experiments referred to were -made on solids; but, from certain results arrived at by Dr. Balfour -Stewart, it would seem that the radiation of a material particle may -be proportionate to its absolute temperature.[24] This physicist found -that the radiation of a thick plate of glass increases more rapidly -than that of a thin plate as the temperature rises, and that, if we go -on continually diminishing the thickness of the plate whose radiation -at different temperatures we are ascertaining, we find that as it grows -thinner and thinner the rate at which it radiates off its heat as its -temperature rises becomes less and less. In other words, as the plate -grows thinner and thinner its rate of radiation becomes more and more -proportionate to its absolute temperature. And we can hardly resist the -conviction that if we could possibly go on diminishing the thickness -of the plate till we reached a film so thin as to embrace but only one -particle in its thickness, its rate of radiation would be proportionate -to its temperature. Dr. Balfour Stewart has very ingeniously suggested -the probable reason why the rate of radiation of thick plates increases -with rise of temperature more rapidly than that of thin. It is this: -all substances are more diathermanous for heat of high temperatures -than for heat of low temperatures. When a body is at a low temperature, -we may suppose that only the exterior rows of particles supply the -radiation, the heat from the interior particles being all stopped by -the exterior ones, the substance being very opaque for heat of low -temperature; while at a high temperature we may imagine that part -of the heat from the interior particles is allowed to pass, thereby -swelling the total radiation. But as the plate becomes thinner and -thinner, the obstructions to interior radiation become less and less, -and as these obstructions are greater for radiation at low temperatures -than for radiation at high temperatures, it necessarily follows that, -by reducing the thickness of the plate, we assist radiation at low -temperatures more than we do at high. - -In a gas, where each particle may be assumed to radiate by itself, and -where the particles stand at a considerable distance from one another, -the obstruction to interior radiation must be far less than in a -solid. In this case the rate at which a gas radiates off its heat as -its temperature rises must increase more slowly than that of a solid -substance. In other words, its rate of radiation must correspond more -nearly to its absolute temperature than that of a solid. If this be the -case, a reduction in the amount of heat received from the sun, owing to -an increase of his distance, should tend to produce a greater lowering -effect on the temperature of the air than it does on the temperature of -the solid ground. But as the temperature of our climate is determined -by the temperature of the air, it must follow that the error of -assuming that the decrease of temperature would be proportionate to the -decrease in the intensity of the sun’s heat may not be great. - -It may be observed here, although it does not bear directly on this -point, that although the air in a room, for example, or at the earth’s -surface is principally cooled by convection rather than by radiation, -yet it is by radiation alone that the earth’s atmosphere parts with its -heat to stellar space; and this is the chief matter with which we are -at present concerned. Air, like all other gases, is a bad radiator; -and this tends to protect it from being cooled to such an extent as it -would otherwise be, were it a good radiator like solids. True, it is -also a bad absorber; but as it is cooled by radiation into space, and -heated, not altogether by absorption, but to a very large extent by -convection, it on the whole gains its heat more easily than it loses -it, and consequently must stand at a higher temperature than it would -do were it heated by absorption alone. - -But, to return; the error of regarding the decrease of temperature -as proportionate to the decrease in the amount of heat received, is -probably neutralized by one of an opposite nature, viz., that of taking -space at too high a temperature; for by so doing we make the result too -small. - -We know that absolute zero is at least 493° below the melting-point -of ice. This is 222° below that of space. Consequently, if the heat -derived from the stars is able to maintain a temperature of −239°, -or 222° of absolute temperature, then nearly as much heat is derived -from the stars as from the sun. But if so, why do the stars give so -much heat and so very little light? If the radiation from the stars -could maintain a thermometer 222° above absolute zero, then space must -be far more transparent to heat-rays than to light-rays, or else the -stars give out a great amount of heat, but very little light, neither -of which suppositions is probably true. The probability is, I venture -to presume, that the temperature of space is not very much above -absolute zero. At the time when these investigations into the probable -temperature of space were made, at least as regards the labours of -Pouillet, the modern science of heat had no existence, and little or -nothing was then known with certainty regarding absolute zero. In this -case the whole matter would require to be reconsidered. The result of -such an investigation in all probability would be to assign a lower -temperature to stellar space than −239°. - -Taking all these various considerations into account, it is probable -that if we adopt −239° as the temperature of space, we shall not be far -from the truth in assuming that the absolute temperature of a place -above that of space is proportionate to the amount of heat received -from the sun. - -We may, therefore, in this case conclude that 59° of rise is probably -not very far from the truth, as representing the influence of the -Gulf-stream. The Gulf-stream, instead of producing little or no effect, -produces an effect far greater than is generally supposed. - -Our island has a mean annual temperature of about 12° above the normal -due to its latitude. This excess of temperature has been justly -attributed to the influence of the Gulf-stream. But it is singular -how this excess should have been taken as the measure of the _rise -resulting from the influence of the stream_. These figures only -represent the number of degrees that the mean normal temperature of -our island stands above what is called the normal temperature of the -latitude. - -The mode in which Professor Dove constructed his Tables of normal -temperature was as follows:—He took the temperature of thirty-six -equidistant points on every ten degrees of latitude. The mean -temperature of these thirty-six points he calls in each case the -_normal_ temperature of the parallel. The excess above the normal -merely represents how much the stream raises our temperature above -the mean of all places on the same latitude, but it affords us no -information regarding the absolute rise produced. In the Pacific, as -well as in the Atlantic, there are immense masses of water flowing -from the tropical to the temperate regions. Now, unless we know how -much of the normal temperature of a latitude is due to ocean-currents, -and how much to the direct heat of the sun, we could not possibly, -from Professor Dove’s Tables, form the most distant conjecture as -to how much of our temperature is derived from the Gulf-stream. The -overlooking of this fact has led to a general misconception regarding -the positive influence of the Gulf-stream on temperature. The 12° -marked in Tables of normal temperature do not represent the absolute -effect of the stream, but merely show how much the stream raises the -temperature of our country above the mean of all places on the same -latitude. Other places have their temperature raised by ocean-currents -as well as this country; only the Gulf-stream produces a rise of -several degrees over and above that produced by other streams in the -same latitude. - -At present there is a difference merely of 80° between the mean -temperature of the equator and the poles;[25] but were each part of the -globe’s surface to depend only upon the direct heat which it receives -from the sun, there ought, according to theory, to be a difference of -more than 200°. The annual quantity of heat received at the equator is -to that received at the poles (supposing the proportionate quantity -absorbed by the atmosphere to be the same in both cases) as 12 to 4·98, -or, say, as 12 to 5. Consequently, if the temperatures of the equator -and the poles be taken as proportionate to the absolute amount of heat -received from the sun, then the temperature of the equator above that -of space must be to that of the poles above that of space as 12 to 5. -What ought, therefore, to be the temperatures of the equator and the -poles, did each place depend solely upon the heat which it receives -directly from the sun? Were all ocean and aërial currents stopped, -so that there could be no transference of heat from one part of the -earth’s surface to another, what ought to be the temperatures of the -equator and the poles? We can at least arrive at a rough estimate -on this point. If we diminish the quantity of warm water conveyed -from the equatorial regions to the temperate and arctic regions, the -temperature of the equator will begin to rise, and that of the poles -to sink. It is probable, however, that this process would affect the -temperature of the poles more than it would that of the equator; for as -the warm water flows from the equator to the poles, the area over which -it is spread becomes less and less. But as the water from the tropics -has to raise the temperature of the temperate regions as well as the -polar, the difference of effect at the equator and poles might not, on -that account, be so very great. Let us take a rough estimate. Say that, -as the temperature of the equator rises one degree, the temperature of -the poles sinks one degree and a half. The mean annual temperature of -the globe is about 58°. The mean temperature of the equator is 80°, and -that of the poles 0°. Let ocean and aërial currents now begin to cease, -the temperature of the equator commences to rise and the temperature -of the poles to sink. For every degree that the temperature of the -equator rises, that of the poles sinks 1½°; and when the currents are -all stopped and each place becomes dependent solely upon the direct -rays of the sun, the mean annual temperature of the equator above that -of space will be to that of the poles, above that of space, as 12 to -5. When this proportion is reached, the equator will be 374° above -that of space, and the poles 156°; for 374 is to 156 as 12 is to 5. -The temperature of space we have seen to be −239°, consequently the -temperature of the equator will in this case be 135°, reckoned from the -zero of the Fahrenheit thermometer, and the poles 83° below zero. The -equator would therefore be 55° warmer than at present, and the poles -83° colder. The difference between the temperature of the equator and -the poles will in this case amount to 218°. - -Now, if we take into account the quantity of positive energy in the -form of heat carried by warm currents from the equator to the temperate -and polar regions, and also the quantity of negative energy (cold) -carried by cold currents from the polar regions to the equator, we -shall find that they are sufficient to reduce the difference of -temperature between the poles and the equator from 218° to 80°. - -The quantity of heat received in the latitude of London, for example, -is to that received at the equator nearly as 12 to 8. This, according -to theory, should produce a difference of about 125°. The temperature -of the equator above that of space, as we have seen, would be 374°. -Therefore 249° above that of space would represent the temperature -of the latitude of London. This would give 10° as its temperature. -The stoppage of all ocean and aërial currents would thus increase the -difference between the equator and the latitude of London by about -85°. The stoppage of ocean-currents would not be nearly so much felt, -of course, in the latitude of London as at the equator and the poles, -because, as has been already noticed, in all latitudes midway between -the equator and the poles the two sets of currents to a considerable -extent compensate each other—the warm currents from the equator -raise the temperature, while the cold ones from the poles lower it; -but as the warm currents chiefly keep on the surface and the cold -return-currents are principally under-currents, the heating effect very -greatly exceeds the cooling effect. Now, as we have seen, the stoppage -of all currents would raise the temperature of the equator 55°; that -is to say, the rise at the equator alone would increase the difference -of temperature between the equator and that of London by 55°. But the -actual difference, as we have seen, ought to be 85°; consequently the -temperature of London would be lowered 30° by the stoppage of the -currents. For if we raise the temperature of the equator 55° and lower -the temperature of London 30°, we then increase the difference by -85°. The normal temperature of the latitude of London being 40°, the -stoppage of all ocean and aërial currents would thus reduce it to 10°. -But the Gulf-stream raises the actual mean temperature of London 10° -above the normal. Consequently 30° + 10° = 40° represents the actual -rise at London due to the influence of the Gulf-stream over and above -all the lowering effects resulting from arctic currents. On some parts -of the American shores on the latitude of London, the temperature is -10° below the normal. The stoppage of all ocean and aërial currents -would therefore lower the temperature there only 20°. - -It is at the equator and the poles that the great system of ocean and -aërial currents produces its maximum effects. The influence becomes -less and less as we recede from those places, and between them there -is a point where the influence of warm currents from the equator and -of cold currents from the poles exactly neutralize each other. At -this point the stoppage of ocean-currents would not sensibly affect -temperature. This point, of course, is not situated on the same -latitude in all meridians, but varies according to the position of the -meridian in relation to land, and ocean-currents, whether cold or hot, -and other circumstances. A line drawn round the globe through these -various points would be very irregular. At one place, such as on the -western side of the Atlantic, where the arctic current predominates, -the neutral line would be deflected towards the equator, while on -the eastern side, where warm currents predominate, the line would be -deflected towards the north. It is a difficult problem to determine the -mean position of this line; it probably lies somewhere not far north of -the tropics. - - - - - CHAPTER III. - - OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER THE - GLOBE.—(_Continued._) - - Influence of the Gulf-stream on the Climate of the Arctic - Regions.—Absolute Amount of Heat received by the Arctic - Regions from the Sun.—Influence of Ocean-currents shown by - another Method.—Temperature of a Globe all Water or all - Land according to Professor J. D. Forbes.—An important - Consideration overlooked.—Without Ocean-currents the - Globe would not be habitable.—Conclusions not affected by - Imperfection of Data. - - -_Influence of the Gulf-stream on the Climate of the Arctic -Regions._—Does the Gulf-stream pass into the arctic regions? Are the -seas around Spitzbergen and North Greenland heated by the warm water of -the stream? - -Those who deny this nevertheless admit the existence of an arctic -current. They admit that an immense mass of cold water is continually -flowing south from the polar regions around Greenland into the -Atlantic. If it be admitted, then, that a mass of water flows across -the arctic circle from north to south, it must also be admitted that an -equal mass flows across from south to north. It is also evident that -the water crossing from south to north must be warmer than the water -crossing from north to south; for the temperate regions are warmer than -the arctic, and the ocean in temperate regions warmer than the ocean in -the arctic; consequently the current which flows into the arctic seas, -to compensate for the cold arctic current, must be a warmer current. - -Is the Gulf-stream this warm current? Does this compensating warm -current proceed from the Atlantic or from the Pacific? If it proceeds -from the Atlantic, it is simply the warm water of the Gulf-stream. -We may call it the warm water of the Atlantic if we choose; but this -cannot materially affect the question at issue, for the heat which -the waters of the Atlantic possess is derived, as we have seen, to -an enormous extent from the water brought from the tropics by the -Gulf-stream. If we deny that the warm compensating current comes from -the Atlantic, then we must assume that it comes from the Pacific. But -if the cold current flows from the arctic regions into the Atlantic, -and the warm compensating current from the Pacific into the arctic -regions, the highest temperature should be found on the Pacific side of -the arctic regions and not on the Atlantic side; the reverse, however, -is the case. In the Atlantic, for example, the 41° isothermal line -reaches to latitude 65°30′, while in the Pacific it nowhere goes beyond -latitude 57°. The 27° isotherm reaches to latitude 75° in the Atlantic, -but in the Pacific it does not pass beyond 64°. And the 14° isotherm -reaches the north of Spitzbergen in latitude 80°, whereas on the -Pacific side of the arctic regions it does not reach to latitude 72°. - -On no point of the earth’s surface does the mean annual temperature -rise so high above the normal as in the northern Atlantic, just at -the arctic circle, at a spot believed to be in the middle of the -Gulf-stream. This place is no less than 22°·5 above the normal, while -in the northern Pacific the temperature does not anywhere rise more -than 9° above the normal. These facts prove that the warm current -passes up the Atlantic into the arctic regions and not up the Pacific, -or at least that the larger amount of warm water must pass into the -arctic regions through the Atlantic. In other words, the Gulf-stream is -the warm compensating current. Not only must there be a warm stream, -but one of very considerable magnitude, in order to compensate for the -great amount of cold water that is constantly flowing from the arctic -regions, and also to maintain the temperature of those regions so much -above the temperature of space as they actually are. - -No doubt, when the results of the late dredging expedition are -published, they will cast much additional light on the direction and -character of the currents forming the north-eastern branch of the -Gulf-stream. - -The average quantity of heat received by the arctic regions as a whole -per unit surface to that received at the equator, as we have already -seen, is as 5·45 to 12, assuming that the percentage of rays cut off by -the atmosphere is the same at both places. In this case the mean annual -temperature of the arctic regions, taken as a whole, would be about -−69°, did those regions depend entirely for their temperature upon the -heat received directly from the sun. But the temperature would not even -reach to this; for the percentage of rays cut off by the atmosphere in -arctic regions is generally believed to be greater than at the equator, -and consequently the actual mean quantity of heat received by the -arctic regions will be less than 5·45−12ths of what is received at the -equator. - -In the article on Climate in the “Encyclopædia Britannica” there is -a Table calculated upon the principle that the quantity of heat cut -off is proportionate to the number of aërial particles which the rays -have to encounter before reaching the surface of the earth—that, as -a general rule, if the tracts of the rays follow an arithmetical -progression, the diminished force with which the rays reach the ground -will form a decreasing geometrical progression. According to this Table -about 75 per cent. of the sun’s rays are cut off by the atmosphere -in arctic regions. If 75 per cent. of the rays were cut off by the -atmosphere in arctic regions, then the direct rays of the sun could -not maintain a mean temperature 100° above that of space. But this is -no doubt much too high a percentage for the quantity of heat cut off; -for recent discoveries in regard to the absorption of radiant heat by -gases and vapours prove that Tables computed on this principle must be -incorrect. The researches of Tyndall and Melloni show that when rays -pass through any substance, the absorption is rapid at first: but the -rays are soon “sifted,” as it is called, and they then pass onwards -with but little further obstruction. Still, however, owing to the dense -fogs which prevail in arctic regions, the quantity of heat cut off -must be considerable. If as much as 50 per cent. of the sun’s rays -are cut off by the atmosphere in arctic regions, the amount of heat -received directly from the sun would not be sufficient to maintain a -mean annual temperature of −100°. Consequently the arctic regions must -depend to an enormous extent upon ocean-currents for their temperature. - -_Influence of Ocean-currents shown by another Method._—That the -temperature of the arctic regions would sink enormously, and the -temperature of the equator rise enormously, were all ocean-currents -stopped, can be shown by another method—viz., by taking the mean annual -temperature from the equator to the pole along a meridian passing -through the ocean, say, the Atlantic, and comparing it with the mean -annual temperature taken along a meridian passing through a great -continent, say, the Asiatic. - -Professor J. D. Forbes, in an interesting memoir,[26] has endeavoured -by this method to determine what would be the temperature of the -equator and the poles were the globe all water or all land. He has -taken the temperature of the two meridians from the tables and charts -of Professor Dove, and ascertained the exact proportion of land and -water on every 10° of latitude from the equator to the poles, with the -view of determining what proportion of the average temperature of the -globe in each parallel is due to the land, and what to the water which -respectively belongs to it. He next endeavours to obtain a formula for -expressing the mean temperature of a given parallel, and thence arrives -at “an approximate answer to the inquiry as to what would have been the -equatorial or polar temperature of the globe, or that of any latitude, -had its surface been entirely composed of land or of water.” - -The result at which he arrived is this: that, were the surface of -the globe all water, 71°·7 would be the temperature of the equator, -and 12°·5 the temperature of the poles; and were the surface all -land, 109°·8 would be the temperature of the equator, and −25°·6 the -temperature of the poles. - -But in Professor Forbes’s calculations no account whatever is taken -of the influence of currents, whether of water or of air, and the -difference of temperature is attributed wholly to difference of -latitude and the physical properties of land and water in relation to -their powers in absorbing and detaining the sun’s rays, and to the laws -of conduction and of convection which regulate the internal motion of -heat in the one and in the other. He considers that the effects of -currents are all compensatory. - -“If a current of hot water,” he says, “moderates the cold of a Lapland -winter, the counter-current, which brings the cold of Greenland to the -shores of the United States, in a great measure restores the balance of -temperature, so far as it is disturbed by this particular influence. -The prevalent winds, in like manner, including the trade-winds, though -they render some portions of continents, on the average, hotter or -colder than others, produce just the contrary effect elsewhere. Each -continent, if it has a cold eastern shore, has likewise a warm western -one; and even local winds have for the most part established laws of -compensation. In a given parallel of latitude all these secondary -causes of local climate may be imagined to be mutually compensatory, -and the outstanding gradation of mean or normal temperature will -mainly depend, 1st, upon the effect of latitude simply; 2nd, on -the distribution of land and water considered in their primary or -_statical_ effect.” - -It is singular that a physicist so acute as Professor Forbes should, -in a question such as this, leave out of account the influence of -currents, under the impression that their effects were compensatory. - -If there is a constant transference of hot water from the equatorial -regions to the polar, and of cold water from the polar regions to the -equatorial (a thing which Professor Forbes admitted), then there can -only be one place between the equator and the pole where the two sets -of currents compensate each other. At all places on the equatorial -side of this point a cooling effect is the result. Starting from this -neutral point, the preponderance of the cooling effect over the heating -increases as we approach towards the equator, and the preponderance of -the heating effect over the cooling increases as we recede from this -point towards the pole—the cooling effect reaching a maximum at the -equator, and the heating effect a maximum at the pole. - -Had Professor Forbes observed this important fact, he would have -seen at once that the low temperature of the land in high latitudes, -in comparison with that of the sea, was no index whatever as to -how much the temperature of those regions would sink were the sea -entirely removed and the surface to become land; for the present -high temperature of the sea is not due wholly to the mere physical -properties of water, but to a great extent is due to the heat brought -by currents from the equator. Now, unless it is known how much of -the absolute temperature of the ocean in those latitudes is due to -currents, we cannot tell how much the removal of the sea would lower -the absolute temperature of those places. Were the sea removed, -the continents in high latitudes would not simply lose the heating -advantages which they presently derive from the mere fact of their -proximity to so much sea, but the removal would, in addition to this, -deprive them of an enormous amount of heat which they at present -receive from the tropics by means of ocean-currents. And, on the other -hand, at the equator, were the sea removed, the continents there -would not simply lose the cooling influences which result from their -proximity to so much water, but, in addition to this, they would have -to endure the scorching effects which would result from the heat which -is at present carried away from the tropics by ocean-currents. - -We have already seen that Professor Forbes concluded that the -removal of the sea would raise the mean temperature of the equator -30°, and lower the temperature of the poles 28°; it is therefore -perfectly certain that, had he added to his result the effect due to -ocean-currents, and had he been aware that about one-fifth of all the -heat possessed by the Atlantic is actually derived from the equator by -means of the Gulf-stream, he would have assigned a temperature to the -equator and the poles, of a globe all land, differing not very far from -what I have concluded would be the temperature of those places were all -ocean and aërial currents stopped, and each place to depend solely upon -the heat which it received directly from the sun. - -_Without Ocean-currents the Globe would not be habitable._—All these -foregoing considerations show to what an extent the climatic condition -of our globe is due to the thermal influences of ocean-currents. - -As regards the northern hemisphere, we have two immense oceans, the -Pacific and the Atlantic, extending from the equator to near the north -pole, or perhaps to the pole altogether. Between these two oceans lie -two great continents, the eastern and the western. Owing to the earth’s -spherical form, far too much heat is received at the equator and far -too little at high latitudes to make the earth a suitable habitation -for sentient beings. The function of these two great oceans is to -remove the heat from the equator and carry it to temperate and polar -regions. Aërial currents could not do this. They might remove the heat -from the equator, but they could not, as we have already seen, carry -it to the temperate and polar regions; for the greater portion of the -heat which aërial currents remove from the equator is dissipated into -stellar space: the ocean alone can convey the heat to distant shores. -But aërial currents have a most important function; for of what avail -would it be, though ocean-currents should carry heat to high latitudes, -if there were no means of distributing the heat thus conveyed over the -land? The function of aërial currents is to do this. Upon this twofold -arrangement depends the thermal condition of the globe. Exclude the -waters of the Pacific and the Atlantic from temperate and polar regions -and place them at the equator, and nothing now existing on the globe -could live in high latitudes. - -Were these two great oceans placed beside each other on one side of the -globe, and the two great continents placed beside each other on the -other side, the northern hemisphere would not then be suitable for the -present order of things: the land on the central and on the eastern -side of the united continent would be far too cold. - -_The foregoing Conclusions not affected by the Imperfection of the -Data._—The general results at which we have arrived in reference to the -influence of ocean-currents on the climatic condition of the globe are -not affected by the imperfection of the data employed. It is perfectly -true that considerable uncertainty prevails regarding some of the data; -but, after making the fullest allowance for every possible error, the -influence of currents is so enormous that the general conclusion cannot -be materially affected. I can hardly imagine that any one familiar -with the physics of the subject will be likely to think that, owing to -possible errors in the data, the effects have probably been doubled. -Even admitting, however, that this were proved to be the case, still -that would not materially alter the general conclusion at which we -have arrived. The influence of ocean-currents in the distribution of -heat over the surface of the globe would still be admittedly enormous, -whether we concluded that owing to them the present temperature of the -equator is 55° or 27° colder than it would otherwise be, or the poles -83° or 41° hotter than they would be did no currents exist. - -Nay, more, suppose we should again halve the result; even in that case -we should have to admit that, owing to ocean-currents, the equator -is about 14° colder and the poles about 21° hotter than they would -otherwise be; in other words, we should have to admit that, were it not -for ocean-currents, the mean temperature of the equator would be about -100° and the mean temperature of the poles about −21°. - -If the influence of ocean-currents in reducing the difference between -the temperature of the equator and poles amounted to only a few -degrees, it would of course be needless to put much weight on any -results arrived at by the method of calculation which I have adopted; -but when it is a matter of two hundred degrees, it is not at all likely -that the general results will be very much affected by any errors which -may ever be found in the data. - -Objections of a palæontological nature have frequently been urged -against the opinion that our island is much indebted for its mild -climate to the influence of the Gulf-stream; but, from what has already -been stated, it must be apparent that all objections of that nature -are of little avail. The palæontologist may detect, from the character -of the flora and fauna brought up from the sea-bottom by dredging and -other means, the presence of a warm or of a cold current; but this can -never enable him to prove that the temperate and polar regions are -not affected to an enormous extent by warm water conveyed from the -equatorial regions. For anything that palæontology can show to the -contrary, were ocean-currents to cease, the mean annual temperature -of our island might sink below the present midwinter temperature of -Siberia. What would be the thermal condition of our globe were there no -ocean-currents is a question for the physicist; not for the naturalist. - - - - - CHAPTER IV. - - OUTLINE OF THE PHYSICAL AGENCIES WHICH LEAD TO SECULAR CHANGES - OF CLIMATE. - - Eccentricity of the Earth’s Orbit; its Effect on - Climate.—Glacial Epoch not the direct Result of an - Increase of Eccentricity.—An important Consideration - overlooked.—Change of Eccentricity affects Climate only - indirectly.—Agencies which are brought into Operation - by an Increase of Eccentricity.—How an Accumulation - of Snow is produced.—The Effect of Snow on the Summer - Temperature.—Reason of the low Summer Temperature of Polar - Regions.—Deflection of Ocean-currents the chief Cause of - secular Changes of Climate.—How the foregoing Causes deflect - Ocean-currents.—Nearness of the Sun in Perigee a Cause of - the Accumulation of Ice.—A remarkable Circumstance regarding - the Causes which lead to secular Changes of Climate.—The - primary Cause an Increase of Eccentricity.—Mean Temperature - of whole Earth should be greater in Aphelion than in - Perihelion.—Professor Tyndall on the Glacial Epoch.—A - general Reduction of Temperature will not produce a Glacial - Epoch.—Objection from the present Condition of the Planet - Mars. - - -_Primary cause of Change of Eccentricity of the Earth’s Orbit._—There -are two causes affecting the position of the earth in relation to -the sun, which must, to a very large extent, influence the earth’s -climate; viz., the precession of the equinoxes and the change in the -eccentricity of the earth’s orbit. If we duly examine the combined -influence of these two causes, we shall find that the northern and -southern portions of the globe are subject to an excessively slow -secular change of climate, consisting in a slow periodic change of -alternate warmer and colder cycles. - -According to the calculations of Leverrier, the superior limit of the -earth’s eccentricity is 0·07775.[27] The eccentricity is at present -diminishing, and will continue to do so during 23,980 years, from the -year 1800 A.D., when its value will be then ·00314. - -The change in the eccentricity of the earth’s orbit may affect -the climate in two different ways; viz., by either increasing or -diminishing the mean annual amount of heat received from the sun, or -by increasing or diminishing the difference between summer and winter -temperature. - -Let us consider the former case first. The total quantity of heat -received from the sun during one revolution is inversely proportional -to the minor axis. - -The difference of the minor axis of the orbit when at its maximum and -its minimum state of eccentricity is as 997 to 1000. This small amount -of difference cannot therefore sensibly affect the climate. Hence we -must seek for our cause in the second case under consideration. - -There is of course as yet some little uncertainty in regard to the -exact mean distance of the sun. I shall, however, in the present volume -assume it to be 91,400,000 miles. When the eccentricity is at its -superior limit, the distance of the sun from the earth, when the latter -is in the aphelion of its orbit, is no less than 98,506,350 miles; -and when in the perihelion it is only 84,293,650 miles. The earth is -therefore 14,212,700 miles further from the sun in the former position -than in the latter. The direct heat of the sun being inversely as the -square of the distance, it follows that the amount of heat received -by the earth when in these two positions will be as 19 to 26. Taking -the present eccentricity to be ·0168, the earth’s distance during -winter, when nearest to the sun, is 89,864,480 miles. Suppose now that, -according to the precession of the equinoxes, winter in our northern -hemisphere should happen when the earth is in the aphelion of its -orbit, at the time when the orbit is at its greatest eccentricity; the -earth would then be 8,641,870 miles further from the sun in winter than -at present. The direct heat of the sun would therefore be one-fifth -less during that season than at present; and in summer one-fifth -greater. This enormous difference would affect the climate to a very -great extent. But if winter under these circumstances should happen -when the earth is in the perihelion of its orbit, the earth would then -be 14,212,700 miles nearer the sun in winter than in summer. In this -case the difference between winter and summer in the latitude of this -country would be almost annihilated. But as the winter in the one -hemisphere corresponds with the summer in the other, it follows that -while the one hemisphere would be enduring the greatest extremes of -summer heat and winter cold, the other would be enjoying a perpetual -summer. - -It is quite true that whatever may be the eccentricity of the earth’s -orbit, the two hemispheres must receive equal quantities of heat per -annum; for proximity to the sun is exactly compensated by the effect of -swifter motion—the total amount of heat received from the sun between -the two equinoxes is the same in both halves of the year, whatever the -eccentricity of the earth’s orbit may be. For example, whatever extra -heat the southern hemisphere may at present receive from the sun during -its summer months owing to greater proximity to the sun, is exactly -compensated by a corresponding loss arising from the shortness of the -season; and, on the other hand, whatever deficiency of heat we in the -northern hemisphere may at present have during our summer half year -in consequence of the earth’s distance from the sun, is also exactly -compensated by a corresponding length of season. - -It has been shown in the introductory chapter that a simple change in -the sun’s distance would not alone produce a glacial epoch, and that -those physicists who confined their attention to purely astronomical -effects were perfectly correct in affirming that no increase of -eccentricity of the earth’s orbit could account for that epoch. But -the important fact was overlooked that although the glacial epoch -could not result directly from an increase of eccentricity, it might -nevertheless do so indirectly. The glacial epoch, as I hope to show, -was not due directly to an increase in the eccentricity of the earth’s -orbit, but to a number of physical agents that were brought into -operation as a result of an increase. - -I shall now proceed to give an outline of what these physical agents -were, how they were brought into operation, and the way in which they -led to the glacial epoch. - -When the eccentricity is about its superior limit, the combined -effect of all those causes to which I allude is to lower to a very -great extent the temperature of the hemisphere whose winters occur in -aphelion, and to raise to nearly as great an extent the temperature of -the opposite hemisphere, where winter of course occurs in perihelion. - -With the eccentricity at its superior limit and the winter occurring -in the aphelion, the earth would be 8,641,870 miles further from the -sun during that season than at present. The reduction in the amount -of heat received from the sun owing to this increased distance would, -upon the principle we have stated in Chapter II., lower the midwinter -temperature to an enormous extent. In temperate regions the greater -portion of the moisture of the air is at present precipitated in the -form of rain, and the very small portion which falls as snow disappears -in the course of a few weeks at most. But in the circumstances under -consideration, the mean winter temperature would be lowered so much -below the freezing-point that what now falls as rain during that season -would then fall as snow. This is not all; the winters would then not -only be colder than now, but they would also be much longer. At present -the winters are nearly eight days shorter than the summers; but with -the eccentricity at its superior limit and the winter solstice in -aphelion, the length of the winters would exceed that of the summers by -no fewer than thirty-six days. The lowering of the temperature and the -lengthening of the winter would both tend to the same effect, viz., to -increase the amount of snow accumulated during the winter; for, other -things being equal, the larger the snow-accumulating period the greater -the accumulation. I may remark, however, that the absolute quantity -of heat received during winter is not affected by the decrease in the -sun’s heat,[28] for the additional length of the season compensates -for this decrease. As regards the absolute amount of heat received, -increase of the sun’s distance and lengthening of the winter are -compensatory, but not so in regard to the amount of snow accumulated. - -The consequence of this state of things would be that, at the -commencement of the short summer, the ground would be covered with the -winter’s accumulation of snow. - -Again, the presence of so much snow would lower the summer temperature, -and prevent to a great extent the melting of the snow. - -There are three separate ways whereby accumulated masses of snow and -ice tend to lower the summer temperature, viz.:— - -_First._ By means of direct radiation. No matter what the intensity of -the sun’s rays may be, the temperature of snow and ice can never rise -above 32°. Hence the presence of snow and ice tends by direct radiation -to lower the temperature of all surrounding bodies to 32°. - -In Greenland, a country covered with snow and ice, the pitch has been -seen to melt on the side of a ship exposed to the direct rays of the -sun, while at the same time the surrounding air was far below the -freezing-point; a thermometer exposed to the direct radiation of the -sun has been observed to stand above 100°, while the air surrounding -the instrument was actually 12° below the freezing-point.[29] A similar -experience has been recorded by travellers on the snow-fields of the -Alps.[30] - -These results, surprising as they no doubt appear, are what we ought -to expect under the circumstances. The diathermancy of air has been -well established by the researches of Professor Tyndall on radiant -heat. Perfectly dry air seems to be nearly incapable of absorbing -radiant heat. The entire radiation passes through it almost without any -sensible absorption. Consequently the pitch on the side of the ship may -be melted, or the bulb of the thermometer raised to a high temperature -by the direct rays of the sun, while the surrounding air remains -intensely cold. “A joint of meat,” says Professor Tyndall, “might be -roasted before a fire, the air around the joint being cold as ice.”[31] -The air is cooled by _contact_ with the snow-covered ground, but is not -heated by the radiation from the sun. - -When the air is humid and charged with aqueous vapour, a similar -cooling effect also takes place, but in a slightly different way. Air -charged with aqueous vapour is a good absorber of radiant heat, but -it can only absorb those rays which agree with it in _period_. It so -happens that rays from snow and ice are, of all others, those which it -absorbs best. The humid air will absorb the total radiation from the -snow and ice, but it will allow the greater part of, if not nearly all, -the sun’s rays to pass unabsorbed. But during the day, when the sun is -shining, the radiation from the snow and ice to the air is negative; -that is, the snow and ice cool the air by radiation. The result is, the -air is cooled by radiation from the snow and ice (or rather, we should -say, _to_ the snow and ice) more rapidly than it is heated by the sun; -and, as a consequence, in a country like Greenland, covered with an -icy mantle, the temperature of the air, even during summer, seldom -rises above the freezing-point. Snow is a good reflector, but as simple -reflection does not change the character of the rays they would not be -absorbed by the air, but would pass into stellar space. - -Were it not for the ice, the summers of North Greenland, owing to the -continuance of the sun above the horizon, would be as warm as those of -England; but, instead of this, the Greenland summers are colder than -our winters. Cover India with an ice sheet, and its summers would be -colder than those of England. - -_Second._ Another cause of the cooling effect is that the rays which -fall on snow and ice are to a great extent reflected back into -space.[32] But those that are not reflected, but absorbed, do not raise -the temperature, for they disappear in the mechanical work of melting -the ice. The latent heat of ice is about 142° F.; consequently in the -melting of every pound of ice a quantity of heat sufficient to raise -one pound of water 142° disappears, and is completely lost, so far -as temperature is concerned. This quantity of heat is consumed, not -in raising the temperature of the ice, but in the mechanical work of -tearing the molecules separate against the forces of cohesion binding -them together into the solid form. No matter what the intensity of the -sun’s heat may be, the surface of the ground will remain permanently at -32° so long as the snow and ice continue unmelted. [**P1:missing page -number] - -_Third._ Snow and ice lower the temperature by chilling the air and -condensing the vapour into thick fogs. The great strength of the sun’s -rays during summer, due to his nearness at that season, would, in the -first place, tend to produce an increased amount of evaporation. But -the presence of snow-clad mountains and an icy sea would chill the -atmosphere and condense the vapour into thick fogs. The thick fogs -and cloudy sky would effectually prevent the sun’s rays from reaching -the earth, and the snow, in consequence, would remain unmelted during -the entire summer. In fact, we have this very condition of things -exemplified in some of the islands of the Southern Ocean at the present -day. Sandwich Land, which is in the same parallel of latitude as the -north of Scotland, is covered with ice and snow the entire summer; -and in the island of South Georgia, which is in the same parallel -as the centre of England, the perpetual snow descends to the very -sea-beach. The following is Captain Cook’s description of this dismal -place:—“We thought it very extraordinary,” he says, “that an island -between the latitudes of 54° and 55° should, in the very height of -summer, be almost wholly covered with frozen snow, in some places many -fathoms deep.... The head of the bay was terminated by ice-cliffs of -considerable height; pieces of which were continually breaking off, -which made a noise like a cannon. Nor were the interior parts of the -country less horrible. The savage rocks raised their lofty summits till -lost in the clouds, and valleys were covered with seemingly perpetual -snow. Not a tree nor a shrub of any size were to be seen. The only -signs of vegetation were a strong-bladed grass growing in tufts, wild -burnet, and a plant-like moss seen on the rocks.... We are inclined to -think that the interior parts, on account of their elevation, never -enjoy heat enough to melt the snow in such quantities as to produce -a river, nor did we find even a stream of fresh water on the whole -coast.”[33] - -Captain Sir James Ross found the perpetual snow at the sea-level at -Admiralty Inlet, South Shetland, in lat. 64°; and while near this -place the thermometer in the very middle of summer fell at night to -23° F.; and so rapidly was the young ice forming around the ship that -he began, he says, “to have serious apprehensions of the ships being -frozen in.”[34] At the comparatively low latitude of 59° S., in long. -171° E. (the corresponding latitude of our Orkney Islands), snow was -falling on the longest day, and the surface of the sea at 32°.[35] And -during the month of February (the month corresponding to August in our -hemisphere) there were only three days in which they were not assailed -by snow-showers.[36] - -In the Straits of Magellan, in 53° S. lat., where the direct heat of -the sun ought to be as great as in the centre of England, MM. Churrca -and Galcano have seen snow fall in the middle of summer; and though the -day was eighteen hours long, the thermometer seldom rose above 42° or -44°, and never above 51°.[37] - -This rigorous condition of climate chiefly results from the rays -of the sun being intercepted by the dense fogs which envelope those -regions during the entire summer; and the fogs again are due to the -air being chilled by the presence of the snow-clad mountains and the -immense masses of floating ice which come from the antarctic seas. The -reduction of the sun’s heat and lengthening of the winter, which would -take place when the eccentricity is near to its superior limit and the -winter in aphelion, would in this country produce a state of things -perhaps as bad as, if not worse than, that which at present exists in -South Georgia and South Shetland. - -If we turn our attention to the polar regions, we shall find that -the cooling effects of snow and ice are even still more marked. The -coldness of the summers in polar regions is owing almost solely to this -cause. Captain Scoresby states that, in regard to the arctic regions, -the general obscurity of the atmosphere arising from fogs or clouds is -such that the sun is frequently invisible during several successive -days. At such times, when the sun is near the northern tropic, there is -scarcely any sensible quantity of light from noon till midnight.[38] -“And snow,” he says, “is so common in the arctic regions, that it may -be boldly stated that in nine days out of ten during the months of -April, May, and June more or less falls.”[39] - -On the north side of Hudson’s Bay, for example, where the quantity of -floating ice during summer is enormous, and dense fogs prevail, the -mean temperature of June does not rise above the freezing-point, being -actually 13°·5 below the normal temperature; while in some parts of -Asia under the same latitude, where there is comparatively little ice, -the mean temperature of June is as high as 60°. - -The mean temperature of Van Rensselaer Harbour, in lat. 78° 37′ N., -long. 70° 53′ W., was accurately determined from hourly observations -made day and night over a period of two years by Dr. Kane. It was found -to be as follows:— - - ° - Winter −28·59 - Spring −10·59 - Summer +33·38 - Autumn - 4·03 - -But although the quantity of heat received from the sun at that -latitude ought to have been greater during the summer than in -England,[40] yet nevertheless the temperature is only 1°·38 above the -freezing-point. - -The temperature of Port Bowen, lat. 73° 14′ N., was found to be as -follows:— - - ° - Winter −25·09 - Spring - 5·77 - Summer +34·40 - Autumn +10·58 - -Here the summer is only 2°·4 above the freezing-point. - -The condition of things in the antarctic regions is even still worse -than in the arctic. Captain Sir James Ross, when between lat. 66° S. -and 77° 5′ S., during the months of January and February, 1841, found -the mean temperature to be only 26°·5; and there were only two days -when it rose even to the freezing-point. When near the ice-barrier on -the 8th of February, 1841, a season of the year equivalent to August -in England, he had the thermometer at 12° at noon; and so rapidly was -the young ice forming around the ships, that it was with difficulty -that he escaped being frozen in for the winter. “Three days later,” -he says, “the thick falling snow prevented our seeing to any distance -before us; the waves as they broke over the ships froze as they fell -on the decks and rigging, and covered our clothes with a thick coating -of ice.”[41] On visiting the barrier next year about the same season, -he again ran the risk of being frozen in. He states that the surface -of the sea presented one unbroken sheet of young ice as far as the eye -could discover from the masthead. - -Lieutenant Wilkes, of the American Exploring Expedition, says that the -temperature they experienced in the antarctic regions surprised him, -for they seldom, if ever, had it above 30°, even at midday. Captain -Nares, when in latitude 64°S., between the 13th and 25th February last -(1874), found the mean temperature of the air to be 31°·5; a lower -temperature than is met with in the arctic regions, in August, ten -degrees nearer the pole.[42] - -These extraordinarily low temperatures during summer, which we have -just been detailing, were due solely to the presence of snow and ice. -In South Georgia, Sandwich Land, and some other places which we have -noticed, the summers ought to be about as warm as those of England; yet -to such an extent is the air cooled by means of floating ice coming -from the antarctic regions, and the rays of the sun enfeebled by the -dense fogs which prevail, that there is actually not heat sufficient -even in the very middle of summer to melt the snow lying on the -sea-beach. - -We read with astonishment that a country in the latitude of England -should in the very middle of summer be covered with snow down to the -sea-shore—the thermometer seldom rising much above the freezing-point. -But we do not consider it so surprising that the summer temperature of -the polar regions should be low, for we are accustomed to regard a low -temperature as the normal condition of things there. We are, however, -mistaken if we suppose that the influence of ice on climate is less -marked at the poles than at such places as South Georgia or Sandwich -Land. - -It is true that a low summer temperature is the normal state of -matters in very high latitudes, but it is so only in consequence of -the perpetual presence of snow and ice. When we speak of the normal -temperature of a place we mean, of course, as we have already seen, -the normal temperature under the present condition of things. But -were the ice removed from those regions, our present Tables of normal -summer temperature would be valueless. These Tables give us the normal -June temperature while the ice remains, but they do not afford us the -least idea as to what that temperature would be were the ice removed. -The mere removal of the ice, all things else remaining the same, would -raise the summer temperature enormously. The actual June temperature of -Melville Island, for example, is 37°, and Port Franklin, Nova Zembla, -36°·5; but were the ice removed from the arctic regions, we should -then find that the summer temperature of those places would be about -as high as that of England. This will be evident from the following -considerations:— - -The temperature of a place, other things being equal, is proportionate -to the quantity of heat received from the sun. If Greenland receives -per given surface as much heat from the sun as England, its temperature -ought to be as high as that of England. Now, from May 10 till August -3, a period of eighty-five days, the quantity of heat received from -the sun in consequence of his remaining above the horizon is actually -greater at the north pole than at the equator. - -Column II. of the following Table, calculated by Mr. Meech,[43] -represents the quantity of heat received from the sun on the 15th of -June at every 10° of latitude. To simplify the Table, I have taken 100 -as the unit quantity received at the equator on that day instead of the -unit adopted by Mr. Meech:— - - +-----------+---------+-----------+-------------+ - | | I. | II. | III. | - | | | | | - | |Latitude.| Quantity | June | - | | | of heat. | temperature.| - +-----------+---------+-----------+-------------+ - | | ° | | ° | - |Equator | 0 | 100 | 80·0 | - | | 10 | 111 | 81·1 | - | | 20 | 118 | 81·1 | - | | 30 | 123 | 77·3 | - | | 40 | 125 | 68·0 | - | | 50 | 125 | 58·8 | - | | 60 | 123 | 51·4 | - | | 70 | 127 | 39·2 | - | | 80 | 133 | 30·2 | - |North Pole | 90 | 136 | 27·4 | - +-----------+---------+-----------+-------------+ - - -The calculations are, of course, made upon the supposition that the -quantity of rays cut off in passing through the atmosphere is the -same at the poles as at the equator, which, as we know, is not exactly -the case. But, notwithstanding the extra loss of solar heat in high -latitudes caused by the greater amount of rays that are cut off, still, -if the temperature of the arctic summers were at all proportionate to -the quantity of heat received from the sun, it ought to be very much -higher than it actually is. Column III. represents the actual mean June -temperature, according to Prof. Dove, at the corresponding latitudes. -A comparison of these two columns will show the very great deficiency -of temperature in high latitudes during summer. At the equator, for -example, the quantity of heat received is represented by 100 and the -temperature 80°; while at the pole the temperature is only 27°·4, -although the amount of heat received is 136. This low temperature -during summer, from what has been already shown, is due chiefly to the -presence of snow and ice. If by some means or other we could remove -the snow and ice from the arctic regions, they would then enjoy a -temperate, if not a hot, summer. In Greenland, as we have already seen, -snow falls even in the very middle of summer, more or less, nine days -out of ten; but remove the snow from the northern hemisphere, and a -snow-shower in Greenland during summer would be as great a rarity as it -would be on the plains of India. - -Other things being equal, the quantity of solar heat received in -Greenland during summer is considerably greater than in England. -Consequently, were it not for snow and ice, it would enjoy as warm a -climate during summer as that of England. Conversely, let the polar -snow and ice extend to the latitude of England, and the summers of that -country would be as cold as those of Greenland. Our summers would then -be as cold as our winters are at present, and snow in the very middle -of summer would perhaps be as common as rain. - -_Mr. Murphy’s Theory._—In a paper read before the Geological Society -by Mr. Murphy[44] he admits that the glacial climate was due to an -increase of eccentricity, but maintains in opposition to me that the -glaciated hemisphere must be that in which the _summer_ occurs in -_aphelion_ during the greatest eccentricity of the earth’s orbit. - -I fear that Mr. Murphy must be resting his theory on the mistaken idea -that a summer in aphelion ought to melt less snow and ice than one in -perihelion. It is quite true that the longer summer in aphelion—other -things being equal—is colder than the shorter one in perihelion, but -the quantity of heat received from the sun is the same in both cases. -Consequently the quantity of snow and ice melted ought also to be the -same; for the amount melted is in proportion to the quantity of energy -in the form of heat received. - -It is true that with us at present less snow and ice are melted during -a cold summer than during a warm one. But this is not a case in point, -for during a cold summer we have less heat than during a warm summer, -the length of both being the same. The coldness of the summers in -this case is owing chiefly to a portion of the heat which we ought to -receive from the sun being cut off by some obstructing cause. - -The reason why we have so little snow, and consequently so little ice, -in temperate regions, is not, as Mr. Murphy seems to suppose, that -the heat of summer melts it all, but that there is so little to melt. -And the reason why we have so little to melt is that, owing to the -warmth of our winters, we have generally rain instead of snow. But -if you increase the eccentricity very much, and place the winter in -perihelion, we should probably have no snow whatever, and, as far as -glaciation is concerned, it would then matter very little what sort of -summer we had. - -But it is not correct to say that the perihelion summer of the glacial -epoch must have been hot. There are physical reasons, as we have just -seen, which go to prove that, notwithstanding the nearness of the sun -at that season, the temperature would seldom, if ever, rise much above -the freezing-point. - -Besides, Mr. Murphy overlooks the fact that the nearness of the sun -during summer was nearly as essential to the production of the ice, as -we shall shortly see, as his great distance during winter. - -We must now proceed to the consideration of an agency which is brought -into operation by the foregoing condition of things, an agency far -more potent than any which has yet come under our notice, viz., the -_Deflection of Ocean-currents_. - -_Deflection of Ocean-currents the chief Cause of secular Changes -of Climate._—The enormous extent to which the thermal condition of -the globe is affected by ocean-currents seems to cast new light on -the mystery of geological climate. What, for example, would be the -condition of Europe were the Gulf-stream stopped, and the Atlantic thus -deprived of one-fifth of the absolute amount of heat which it is now -receiving above what it has in virtue of the temperature of space? If -the results just arrived at be at all justifiable, it follows that the -stoppage of the stream would lower the temperature of northern Europe -to an extent that would induce a condition of climate as severe as that -of North Greenland; and were the warm currents of the North Pacific -also at the same time to be stopped, the northern hemisphere would -assuredly be subjected to a state of general glaciation. - -Suppose also that the warm currents, having been withdrawn from the -northern hemisphere, should flow into the Southern Ocean: what then -would be the condition of the southern hemisphere? Such a transference -of heat would raise the temperature of the latter hemisphere about -as much as it would lower the temperature of the former. It would -consequently raise the mean temperature of the antarctic regions much -above the freezing-point, and the ice under which those regions are -at present buried would, to a great extent at least, disappear. The -northern hemisphere, thus deprived of the heat from the equator, would -be under a condition of things similar to that which prevailed during -the glacial epoch; while the other hemisphere, receiving the heat from -the equator, would be under a condition of climate similar to what we -know prevailed in the northern hemisphere during a part of the Upper -Miocene period, when North Greenland enjoyed a climate as mild as that -of England at the present day. - -This is no mere picture of the imagination, no mere hypothesis devised -to meet a difficult case; for if what has already been stated be not -completely erroneous, all this follows as a necessary consequence from -physical principles. If the warm currents of the equatorial regions -be all deflected into one hemisphere, such must be the condition of -things. How then do the agencies which we have been considering deflect -ocean-currents? - -_How the foregoing Causes deflect Ocean-currents._—A high condition -of eccentricity tends, we have seen, to produce an accumulation of -snow and ice on the hemisphere whose winters occur in aphelion. This -accumulation tends in turn to lower the summer temperature, to cut -off the sun’s rays, and so to retard the melting of the snow. In -short, it tends to produce on that hemisphere a state of glaciation. -Exactly opposite effects take place on the other hemisphere, which -has its winter in perihelion. There the shortness of the winters and -the highness of the temperature, owing to the sun’s nearness, combine -to prevent the accumulation of snow. The general result is that the -one hemisphere is cooled and the other heated. This state of things -now brings into play the agencies which lead to the deflection of the -Gulf-stream and other great ocean-currents. - -Owing to the great difference between the temperature of the equator -and the poles, there is a constant flow of air from the poles to the -equator. It is to this that the trade-winds owe their existence. Now as -the strength of these winds, as a general rule, will depend upon the -difference of temperature that may exist between the equator and higher -latitudes, it follows that the trades on the cold hemisphere will be -stronger than those on the warm. When the polar and temperate regions -of the one hemisphere are covered to a large extent with snow and ice, -the air, as we have just seen, is kept almost at the freezing-point -during both summer and winter. The trades on that hemisphere will, of -necessity, be exceedingly powerful; while on the other hemisphere, -where there is comparatively little snow and ice, and the air is warm, -the trades will, as a consequence, be weak. Suppose now the northern -hemisphere to be the cold one. The north-east trade-winds of this -hemisphere will far exceed in strength the south-east trade-winds of -the southern hemisphere. The _median-line_ between the trades will -consequently lie to a very considerable distance to the south of -the equator. We have a good example of this at the present day. The -difference of temperature between the two hemispheres at present is -but trifling to what it would be in the case under consideration; yet -we find that the south-east trades of the Atlantic blow with greater -force than the north-east trades, and the result is that the south-east -trades sometimes extend to 10° or 15° N. lat., whereas the north-east -trades seldom blow south of the equator. The effect of the northern -trades blowing across the equator to a great distance will be to impel -the warm water of the tropics over into the Southern Ocean. But this -is not all; not only would the median-line of the trades be shifted -southwards, but the great equatorial currents of the globe would also -be shifted southwards. - -Let us now consider how this would affect the Gulf-stream. The South -American continent is shaped somewhat in the form of a triangle, with -one of its angular corners, called Cape St. Roque, pointing eastwards. -The equatorial current of the Atlantic impinges against this corner; -but as the greater portion of the current lies a little to the north -of the corner, it flows westward into the Gulf of Mexico and forms the -Gulf-stream. A considerable portion of the water, however, strikes the -land to the south of the Cape and is deflected along the shores of -Brazil into the Southern Ocean, forming what is known as the Brazilian -current. - -Now it is perfectly obvious that the shifting of the equatorial -current of the Atlantic only a few degrees to the south of its present -position—a thing which would certainly take place under the conditions -which we have been detailing—would turn the entire current into the -Brazilian branch, and instead of flowing chiefly into the Gulf of -Mexico as at present, it would all flow into the Southern Ocean, and -the Gulf-stream would consequently be stopped. The stoppage of the -Gulf-stream, combined with all those causes which we have just been -considering, would place Europe under glacial conditions; while, at the -same time, the temperature of the Southern Ocean would, in consequence -of the enormous quantity of warm water received, have its temperature -(already high from other causes) raised enormously. - -_Deflection of the Gulf-stream during the Glacial Epoch indicated by -the Difference between the Clyde and Canadian Shell-beds._—That the -glaciation of north-western Europe resulted to a great extent from -the stoppage of the Gulf-stream may, I think, be inferred from a -circumstance pointed out by the Rev. Mr. Crosskey, several years ago, -in a paper read before the Glasgow Geological Society.[45] He showed -that the difference between the glacial shells of Canada and those -now existing in the Gulf of St. Lawrence is much less marked than the -difference between the glacial shells of the Clyde beds and those now -existing in the Firth. And from this he justly infers that the change -of climate in Canada since the glacial epoch has been far less complete -than in Scotland. - -The return of the Gulf-stream has raised the mean annual temperature of -our island no less than 15° above the normal, while Canada, deprived of -its influence and exposed to a cold stream from polar regions, has been -kept nearly as much below the normal. - -Let us compare the present temperature of the two countries. In making -our comparison we must, of course, compare places on the same latitude. -It will not do, for example, to compare Glasgow with Montreal or -Quebec, places on the latitude of the south of France and north of -Italy. It will be found that the difference of temperature between -the two countries is so enormous as to appear scarcely credible to -those who have not examined the matter. The temperatures have all been -taken from Professor Dove’s work on the “Distribution of Heat over the -Surface of the Globe,” and his Tables published in the Report of the -British Association for 1847. - -The mean temperature of Scotland for January is about 38° F., while -in some parts of Labrador, on the same latitude, and all along the -central parts of North America lying to the north of Upper Canada, -it is actually 10°, and in many places 13° below zero. The January -temperature at the Cumberland House, which is situated on the latitude -of the centre of England, is more than 13° below zero. Here is a -difference of no less than 51°. The normal temperature for the month -of January in the latitude of Glasgow, according to Professor Dove, is -10°. Consequently, owing to the influence of the Gulf-stream, we are -28° warmer during that month than we would otherwise be, while vast -tracts of country in America are 23° colder than they should be. - -The July temperature of Glasgow is 61°, while on the same latitude -in Labrador and places to the west it is only 49°. Glasgow during -that month is 3° above the normal temperature, while America, owing -to the influence of the cold polar stream, is 9° below it. The mean -annual temperature of Glasgow is nearly 50°, while in America, on the -same latitude, it is only 30°, and in many places as low as 23°. The -mean normal temperature for the whole year is 35°. Our mean annual -temperature is therefore 15° above the normal, and that of America from -5° to 12° below it. The American winters are excessively cold, owing -to the continental character of the climate, and the absence of any -benefit from the Gulf-stream, while the summers, which would otherwise -be warm, are, in the latitude of Glasgow, cooled down to a great extent -by the cold ice from Greenland; and the consequence is, that the mean -annual temperature is about 20° or 27° below that of ours. The mean -annual temperature of the Gulf of St. Lawrence is as low as that of -Lapland or Iceland. It is no wonder, then, that the shells which -flourished in Canada during the glacial epoch have not left the gulf -and the neighbouring seas. - -We have good reason to believe that the climate of America during the -glacial epoch was even then somewhat more severe than that of Western -Europe, for the erratics of America extend as far south as latitude -40°, while on the old continent they are not found much beyond latitude -50°. This difference may have resulted from the fact that the western -side of a continent is always warmer than the eastern. - -In order to determine whether the cold was as great in America during -the glacial epoch as in Western Europe, we must not compare the fossils -found in the glacial beds about Montreal, for example, with those found -in the Clyde beds, for Montreal lies much further to the south than the -Clyde. The Clyde beds must be compared with those of Labrador, while -the beds of Montreal must be compared with those of the south of France -and the north of Italy, if any are to be found there. - -On the whole, it may be concluded that had the Gulf-stream not returned -to our shores at the close of the glacial epoch, and had its place -been supplied by a cold stream from the polar regions, similar to that -which washes the shores of North America, it is highly probable that -nearly every species found in our glacial beds would have had their -representatives flourishing in the British seas at the present day. - -It is no doubt true that when we compare the places in which the -Canadian shell-beds referred to by Mr. Crosskey are situated with -places on the same latitude in Europe, the difference of climate -resulting from the influence of the Gulf-stream is not so great as -between Scotland and those places which we have been considering; but -still the difference is sufficiently great to account for why the -change of climate in Canada has been less complete than in Scotland. - -And what holds true in regard to the currents of the Atlantic holds -also true, though perhaps not to the same extent, of the currents of -the Pacific. - -_Nearness of the Sun in Perigee a Cause of the Accumulation of -Ice._—But there is still another cause which must be noticed:—A strong -under current of air _from_ the north implies an equally strong upper -current _to_ the north. Now if the effect of the under current would -be to impel the warm water at the equator to the south, the effect -of the upper current would be to carry the aqueous vapour formed at -the equator to the north; the upper current, on reaching the snow and -ice of temperate regions, would deposit its moisture in the form of -snow; so that, notwithstanding the great cold of the glacial epoch, -it is probable that the quantity of snow falling in the northern -regions would be enormous. This would be particularly the case during -summer, when the earth would be in the perihelion and the heat at the -equator great. The equator would be the furnace where evaporation would -take place, and the snow and ice of temperate regions would act as a -condenser. - -Heat to produce _evaporation_ is just as essential to the accumulation -of snow and ice as cold to produce _condensation_. Now at Midsummer, -on the supposition of the eccentricity being at its superior limit, -the sun would be 8,641,870 miles nearer than at present during that -season. The effect would be that the intensity of the sun’s rays would -be one-fifth greater than now. That is to say, for every five rays -received by the ocean at present, six rays would be received then, -consequently the evaporation during summer would be excessive. But the -ice-covered land would condense the vapour into snow. It would, no -doubt, be during summer that the greatest snowfall would take place. In -fact, the nearness of the sun during that season was as essential to -the production of the glacial epoch as was his distance during winter. - -The direct effect of eccentricity is to produce on one of the -hemispheres a long and cold winter. This alone would not lead to a -condition of things so severe as that which we know prevailed during -the glacial epoch. But the snow and ice thus produced would bring into -operation, as we have seen, a host of physical agencies whose combined -efforts would be quite sufficient to do this. - -_A remarkable Circumstance regarding those Causes which lead to Secular -Changes of Climate._—There is one remarkable circumstance connected -with those physical causes which deserves special notice. They not only -all lead to one result, viz., an accumulation of snow and ice, but -they react on one another. It is quite a common thing in physics for -the effect to react on the cause. In electricity and magnetism, for -example, cause and effect in almost every case mutually act and react -upon each other. But it is usually, if not universally, the case that -the reaction of the effect tends to weaken the cause. The weakening -influences of this reaction tend to impose a limit on the efficiency -of the cause. But, strange to say, in regard to the physical causes -concerned in the bringing about of the glacial condition of climate, -cause and effect mutually reacted so as to strengthen each other. And -this circumstance had a great deal to do with the extraordinary results -produced. - -We have seen that the accumulation of snow and ice on the ground -resulting from the long and cold winters tended to cool the air -and produce fogs which cut off the sun’s rays. The rays thus cut -off diminished the melting power of the sun, and so increased the -accumulation. As the snow and ice continued to accumulate, more and -more of the rays were cut off; and on the other hand, as the rays -continued to be cut off, the _rate_ of accumulation increased, because -the quantity of snow and ice melted became thus annually less and less. - -Again, during the long and dreary winters of the glacial epoch the -earth would be radiating off its heat into space. Had the heat thus -lost simply gone to lower the temperature, the lowering of the -temperature would have tended to diminish the rate of loss; but the -necessary result of this was the formation of snow and ice rather than -the lowering of temperature. - -And, again, the formation of snow and ice facilitated the rate at which -the earth lost its heat; and on the other hand, the more rapidly the -earth parted with its heat, the more rapidly were the snow and ice -formed. - -Further, as the snow and ice accumulated on the one hemisphere, they -at the same time continued to diminish on the other. This tended to -increase the strength of the trade-winds on the cold hemisphere, and -to weaken those on the warm. The effect of this on ocean currents -would be to impel the warm water of the tropics more to the warm -hemisphere than to the cold. Suppose the northern hemisphere to be -the cold one, then as the snow and ice began gradually to accumulate -there, the ocean currents of that hemisphere would begin to decrease in -volume, while those on the southern, or warm, hemisphere, would _pari -passu_ increase. This withdrawal of heat from the northern hemisphere -would tend, of course, to lower the temperature of that hemisphere -and thus favour the accumulation of snow and ice. As the snow and ice -accumulated the ocean currents would decrease, and, on the other hand, -as the ocean currents diminished the snow and ice would accumulate,—the -two effects mutually strengthening each other. - -The same must have held true in regard to aërial currents. The more -the polar and temperate regions became covered with snow and ice, the -stronger would become the trades and anti-trades of the hemisphere; and -the stronger those winds became, the greater would be the amount of -moisture transferred from the tropical regions by the anti-trades to -the temperate regions; and on the other hand, the more moisture those -winds brought to temperate regions, the greater would be the quantity -of snow produced. - -The same process of mutual action and reaction would take place among -the agencies in operation on the warm hemisphere, only the result -produced would be diametrically opposite of that produced in the cold -hemisphere. On this warm hemisphere action and reaction would tend to -raise the mean temperature and diminish the quantity of snow and ice -existing in temperate and polar regions. - -Had it been possible for each of those various physical agents which we -have been considering to produce its direct effects without influencing -the other agents or being influenced by them, its real efficiency in -bringing about either the glacial condition of climate or the warm -condition of climate would not have been so great. - -The primary cause that set all those various physical agencies in -operation which brought about the glacial epoch, was a high state of -eccentricity of the earth’s orbit. When the eccentricity is at a -high value, snow and ice begin to accumulate, owing to the increasing -length and coldness of the winter on that hemisphere whose winter -solstice is approaching toward the aphelion. The accumulating snow -then begins to bring into operation all the various agencies which -we have been describing; and, as we have just seen, these, when once -in full operation, mutually aid one another. As the eccentricity -increases century by century, the temperate regions become more and -more covered with snow and ice, first by reason of the continued -increase in the coldness and length of the winters, and secondly, -and chiefly, owing to the continued increase in the potency of those -physical agents which have been called into operation. This glacial -state of things goes on at an increasing rate, and reaches a maximum -when the solstice-point arrives at the aphelion. After the solstice -passes the aphelion, a contrary process commences. The snow and ice -gradually begin to diminish on the cold hemisphere and to make their -appearance on the other hemisphere. The glaciated hemisphere turns, by -degrees, warmer and the warm hemisphere colder, and this continues to -go on for a period of ten or twelve thousand years, until the winter -solstice reaches the perihelion. By this time the conditions of the two -hemispheres have been reversed; the formerly glaciated hemisphere has -now become the warm one, and the warm hemisphere the glaciated. The -transference of the ice from the one hemisphere to the other continues -as long as the eccentricity remains at a high value. This will, -perhaps, be better understood from an inspection of the frontispiece. - -_The Mean Temperature of the whole Earth should be greater in Aphelion -than in Perihelion._—When the eccentricity becomes reduced to about -its present value, its influence on climate is but little felt. -It is, however, probable that the present extension of ice on the -southern hemisphere may, to a considerable extent, be the result of -eccentricity. The difference in the climatic conditions of the two -hemispheres is just what should be according to theory:—(1) The mean -temperature of that hemisphere is less than that of the northern. -(2) The winters of the southern hemisphere are colder than those of -the northern. (3) The summers, though occurring in perihelion, are -also comparatively cold; this, as we have seen, is what ought to be -according to theory. (4) The mean temperature of the whole earth is -greater in June, when the earth is in aphelion, than in December, when -it is in perihelion. This, I venture to affirm, is also what ought to -follow according to theory, although this very fact has been adduced -as a proof that eccentricity has at present but little effect on the -climatic condition of our globe. - -That the mean temperature of the whole earth would, during the -glacial epoch, be greater when the earth was in aphelion than -when in perihelion will, I think, be apparent from the following -considerations:—When the earth was in the perihelion, the sun would -be over the hemisphere nearly covered with snow and ice. The great -strength of the sun’s rays would in this case have little effect in -raising the temperature; it would be spent in melting the snow and -ice. But when the earth was in the aphelion, the sun would be over the -hemisphere comparatively free, or perhaps wholly free, from snow and -ice. Consequently, though the intensity of the sun’s rays would be less -than when the earth was in perihelion, still it ought to have produced -a higher temperature, because it would be chiefly employed in heating -the ground and not consumed in melting snow and ice. - -_Professor Tyndall on the Glacial Epoch._—“So natural,” says Professor -Tyndall, “was the association of ice and cold, that even celebrated -men assumed that all that is needed to produce a great extension of -our glaciers is a diminution of the sun’s temperature. Had they gone -through the foregoing reflections and calculations, they would probably -have demanded _more_ heat instead of less for the production of a -glacial epoch. What they really needed were _condensers_ sufficiently -powerful to congeal the vapour generated by the heat of the sun.” (_The -Forms of Water_, p. 154. See also, to the same effect, _Heat Considered -as a Mode of Motion_, chap. vi.) - -I do not know to whom Professor Tyndall here refers, but certainly his -remarks have no application to the theory under consideration, for -according to it, as we have just seen, the ice of the glacial epoch was -about as much due to the nearness of the sun in perigee as to his great -distance in apogee. - -There is one theory, however, to which his remarks justly apply, viz., -the theory that the great changes of climate during geological ages -resulted from the passage of our globe through different temperatures -of space. What Professor Tyndall says shows plainly that the glacial -epoch was not brought about by our earth passing through a cold part -of space. A general reduction of temperature over the whole globe -certainly would not produce a glacial epoch. Suppose the sun were -extinguished and our globe exposed to the temperature of stellar space -(−239° F.), this would certainly freeze the ocean solid from its -surface to its bottom, but it would not cover the land with ice. - -Professor Tyndall’s conclusions are, of course, equally conclusive -against Professor Balfour Stewart’s theory, that the glacial epoch may -have resulted from a general diminution in the intensity of the sun’s -heat. - -Nevertheless it would be in direct opposition to the well-established -facts of geology to assume that the ice periods of the glacial epoch -were warm periods. We are as certain from palæontological evidence -that the cold was then much greater than now, as we are from physical -evidence that the accumulation of ice was greater than now. Our glacial -shell-beds and remains of the mammoth, the reindeer, and musk-ox, tell -of cold as truly as the markings on the rocks do of ice. - -_Objection from the Present Condition of the Planet Mars._—It has been -urged as an objection by Professor Charles Martins[46] and others, -that if a high state of eccentricity could produce a glacial epoch, -the planet Mars ought to be at present under a glacial condition. The -eccentricity of its orbit amounts to 0·09322, and one of its southern -winter solstices is, according to Dr. Oudemans, of Batavia,[47] within -17° 41′ 8″ of aphelion. Consequently, it is supposed that one of the -hemispheres should be in a glacial state and the other free from snow -and ice. But it is believed that the snow accumulates around each pole -during its winter and disappears to a great extent during its summer. - -There would be force in this objection were it maintained that -eccentricity alone can produce a glacial condition of climate, but -such is not the case, and there is no good ground for concluding that -those physical agencies which led to the glacial epoch of our globe -exist in the planet Mars. It is perfectly certain that either water -must be different in constitution in that planet from what it is in our -earth, or else its atmospheric envelope must be totally different from -ours. For it is evident from what has been stated in Chapter II., that -were our globe to be removed to the distance of Mars from the sun, the -lowering of the temperature resulting from the decrease in the sun’s -heat would not only destroy every living thing, but would convert the -ocean into solid ice. - -But it must be observed that the eccentricity of Mars’ orbit is at -present far from its superior limit of 0·14224, and it may so happen in -the economy of nature that when it approaches to that limit a glacial -condition of things may supervene. - -The truth is, however, that very little seems to be known with -certainty regarding the climatic condition of Mars. This is obvious -from the fact that some astronomers believe that the planet possesses -a dense atmosphere which protects it from cold; while others maintain -that its atmosphere is so exceedingly thin that its mean temperature is -below the freezing-point. Some assert that the climatic condition of -Mars resembles very much that of our earth, while others affirm that -its seas are actually frozen solid to the bottom, and the poles covered -with ice thirty or forty miles in thickness. For reasons which will be -explained in the Appendix, Mars, notwithstanding its greater distance -from the sun, may enjoy a climate as warm as that of our earth. - - - - - CHAPTER V. - - REASON WHY THE SOUTHERN HEMISPHERE IS COLDER THAN THE - NORTHERN. - - Adhémar’s Explanation.—Adhémar’s Theory founded upon a physical - Mistake in regard to Radiation.—Professor J. D. Forbes on - Underground Temperature.—Generally accepted Explanation.—Low - Temperature of Southern Hemisphere attributed to - Preponderance of Sea.—Heat transferred from Southern to - Northern Hemisphere by Ocean-current the true Explanation.—A - large Portion of the Heat of the Gulf-stream derived from the - Southern Hemisphere. - - -_Adhémar’s Explanation._—It has long been known that on the southern -hemisphere the temperature is lower and the accumulation of ice greater -than on the northern. This difference has usually been attributed to -the great preponderance of sea on the southern hemisphere. M. Adhémar, -on the other hand, attempts to explain this difference by referring it -to the difference in the amount of heat lost by the two hemispheres -in consequence of the difference of seven days in the length of their -respective winters. As the northern winter is shorter than the summer, -he concludes that there is an accumulation of heat on that hemisphere, -while, on the other hand, the southern winter being longer than the -summer, there is therefore a loss of heat on the southern hemisphere. -“The south pole,” he says, “loses in one year more heat than it -receives, because the total duration of its night surpasses that of -its day by 168 hours; and the contrary takes place for the north pole. -If, for example, we take for unity the mean quantity of heat which the -sun sends off in one hour, the heat accumulated at the end of the year -at the north pole will be expressed by 168, while the heat lost by the -south pole will be equal to 168 times what the radiation lessens it by -in one hour, so that at the end of the year the difference in the heat -of the two hemispheres will be represented by 336 times what the earth -receives from the sun or loses in an hour by radiation.”[48] - -Adhémar supposes that about 10,000 years hence, when our northern -winter will occur in aphelion and the southern in perihelion, the -climatic conditions of the two hemispheres will be reversed; the -ice will melt at the south pole, and the northern hemisphere will -become enveloped in one continuous mass of ice, leagues in thickness, -extending down to temperate regions. - -This theory seems to be based upon an erroneous interpretation of a -principle, first pointed out, so far as I am aware, by Humboldt in -his memoir “On Isothermal Lines and Distribution of Heat over the -Globe.”[49] This principle may be stated as follows:— - -Although the total quantity of heat received by the earth from the -sun in one revolution is inversely proportional to the minor axis of -the orbit, yet this amount, as was proved by D’Alembert, is equally -distributed between the two hemispheres, whatever the eccentricity may -be. Whatever extra heat the southern hemisphere may at present receive -from the sun daily during its summer months owing to greater proximity -to the sun, is exactly compensated by a corresponding loss arising from -the shortness of the season; and, on the other hand, whatever daily -deficiency of heat we in the northern hemisphere may at present have -during our summer half-year, in consequence of the earth’s distance -from the sun, is also exactly compensated by a corresponding length of -season. - -But the surface temperature of our globe depends as much upon the -amount of heat radiated into space as upon the amount derived from the -sun, and it has been thought by some that this compensating principle -holds true only in regard to the latter. In the case of the heat -lost by radiation the reverse is supposed to take place. The southern -hemisphere, it is asserted, has not only a colder winter than the -northern in consequence of the sun’s greater distance, but it has also -a longer winter; and the extra loss of heat from radiation during -winter is not compensated by its nearness to the sun during summer, for -it gains no additional heat from this proximity. And in the same way it -is argued that as our winter in the northern hemisphere, owing to the -less distance of the sun, is not only warmer than that of the southern -hemisphere, but is also at the same time shorter, so our hemisphere -is not cooled to such an extent as the southern. And thus the mean -temperature of the winter half-year, as well as the intensity of the -sun’s heat, is affected by a change in the sun’s distance. - -Although I always regarded this cause of Humboldt’s to be utterly -inadequate to produce such effects as those attributed to it by -Adhémar, still, in my earlier papers[50] I stated it to be a _vera -causa_ which ought to produce some sensible effect on climate. But -shortly afterwards on a more careful consideration of the whole -subject, I was led to suspect that the circumstance in question can, -according to theory, produce little or no effect on the climatic -condition of our globe. - -As there appears to be a considerable amount of misapprehension in -reference to this point, which forms the basis of Adhémar’s theory, I -may here give it a brief consideration.[51] - -The rate at which the earth radiates into space the heat received -from the sun depends upon the temperature of its surface; and the -temperature of its surface (other things being equal) depends upon -the rate at which the heat is received. The greater the rate at which -the earth receives heat from the sun, the greater will therefore be -the rate at which it will lose that heat by radiation. Now the total -quantity of heat received during winter by the southern hemisphere is -exactly equal to that received during winter by the northern. But as -the southern winter is longer than the northern, the rate at which the -heat is received, and consequently the rate of radiation, during that -season must be less on the southern hemisphere than on the northern. -Thus the southern hemisphere loses heat during a longer period than the -northern, and therefore the less rate of radiation (were it not for a -circumstance presently to be noticed) would wholly compensate for the -longer period, and the total quantity of heat lost during winter would -be the same on both hemispheres. The southern summer is shorter than -the northern, but the heat is more intense, and the surface of the -ground kept at a higher temperature; consequently the rate of radiation -into space is greater. - -When the rate at which a body receives heat is increased, the -temperature of the body rises till the rate of radiation equals the -rate of absorption, after which equilibrium is restored; and when the -rate of absorption is diminished, the temperature falls till the rate -of radiation equals that of absorption. - -But notwithstanding all this, owing to the slow conductivity of the -ground for heat, more heat will pass into it during the longer summer -of aphelion than during the shorter one of perihelion; for the amount -of heat which passes into the ground depends on the length of time -during which the earth is receiving heat, as well as upon the amount -received. In like manner, more heat will pass out of the ground -during the longer winter in aphelion than during the shorter one in -perihelion. Suppose the length of the days on the one hemisphere (say -the northern) to be 23 hours, and the length of the nights, say one -hour; while on the other hemisphere the days are one hour and the -nights 23 hours. Suppose also that the quantity of heat received from -the sun by the southern hemisphere during the day of one hour to be -equal to that received by the northern hemisphere during the day of -23 hours. It is evident that although the surface of the ground on -the southern hemisphere would receive as much heat from the sun during -the short day of one hour as the surface of the northern hemisphere -during the long day of 23 hours, yet, owing to the slow conductivity -of the ground for heat, the amount absorbed would not be nearly so -much on the southern hemisphere as on the northern. The temperature -of the surface during the day, it is true, would be far higher on the -southern hemisphere than on the northern, and consequently the rate -at which the heat would pass into the ground would be greater on that -hemisphere than on the northern; but, notwithstanding the greater rate -of absorption resulting from the high temperature of the surface, it -would not compensate for the shortness of the day. On the other hand, -the surface of the ground on the southern hemisphere would be colder -during the long night of 23 hours than it would be on the northern -during the short night of only one hour; and the low temperature of the -ground would tend to lessen the rate of radiation into space. But the -decrease in the rate of radiation would not compensate fully for the -great length of the night. The general and combined result of all those -causes would be that a slight accumulation of heat would take place on -the northern hemisphere and a slight loss on the southern. But this -loss of heat on the one hemisphere and gain on the other would not go -on accumulating at a uniform rate year by year, as Adhémar supposes. - -Of course we are at present simply considering the earth as an absorber -and radiator of heat, without taking into account the effects of -distribution of sea and land and other modifying causes, and are -assuming that everything is the same in both hemispheres, with the -exception that the winter of the one hemisphere is longer than that of -the other. - -What, then, is the amount of heat stored up by the one hemisphere and -lost by the other? Is it such an amount as to sensibly affect climate? - -The experiments and observations which have been made on underground -temperature afford us a means of making at least a rough estimate of -the amount. And from these it will be seen that the influence of an -excess of seven or eight days in the length of the southern winter over -the northern could hardly produce an effect that would be sensible. - -Observations were made at Edinburgh by Professor J. D. Forbes on -three different substances; viz., sandstone, sand, and trap-rock. By -calculation, we find from the data afforded by those observations that -the total quantity of heat accumulated in the ground during the summer -above the mean temperature was as follows:—In the sandstone-rock, a -quantity sufficient to raise the temperature of the rock 1° C. to a -depth of 85 feet 6 inches; in the sand a quantity sufficient to raise -the temperature 1° C. to a depth of 72 feet 6 inches; and in the -trap-rock a quantity only sufficient to raise the temperature 1° C. to -a depth of 61 feet 6 inches. - -Taking the specific heat of the sandstone per unit volume, as -determined by Regnault, at ·4623, and that of sand at ·3006, and -trap at ·5283, and reducing all the results to one standard, viz., -that of water, we find that the quantity of heat stored up in the -sandstone would, if applied to water, raise its temperature 1° C. to -a depth of 39 feet 6 inches; that stored up in the sand would raise -the temperature of the water 1° C. to a depth of 21 feet 8 inches, and -that stored up in the trap would raise the water 1° C. to the depth -of 32 feet 6 inches. We may take the mean of these three results as -representing pretty accurately the quantity stored up in the general -surface of the country. This would be equal to 31 feet 3 inches depth -of water raised 1° C. The quantity of heat lost by radiation during -winter below the mean was found to be about equal to that stored up -during summer. - -The total quantity of heat per square foot of surface received by the -equator from sunrise till sunset at the time of the equinoxes, allowing -22 per cent. for the amount cut off in passing through the atmosphere, -is 1,780,474 foot-pounds. In the latitude of Edinburgh about 938,460 -foot-pounds per square foot of surface is received, assuming that not -more than 22 per cent. is cut off by the atmosphere. At this rate a -quantity of heat would be received from the sun in two days ten hours -(say, three days) sufficient to raise the temperature of the water 1° -C. to the required depth of 31 feet 3 inches. Consequently the total -quantity of heat stored up during summer in the latitude of Edinburgh -is only equal to what we receive from the sun during three days at the -time of the equinoxes. Three days’ sunshine during the middle of March -or September, if applied to raise the temperature of the ground, would -restore all the heat lost during the entire winter; and another three -days’ sunshine would confer on the ground as much heat as is stored -up during the entire summer. But it must be observed that the total -duration of sunshine in winter is to that of summer in the latitude of -Edinburgh only about as 4 to 7. Here is a difference of two months. -But this is not all; the quantity of heat received during winter is -scarcely one-third of that received during summer; yet, notwithstanding -this enormous difference between summer and winter, the ground during -winter loses only about six days’ sun-heat below the maximum amount -possessed by it in summer. - -But if what has already been stated is correct, this loss of heat -sustained by the earth during winter is not chiefly owing to radiation -during the longer absence of the sun, but to the decrease in the -quantity of heat received in consequence of his longer absence combined -with the obliquity of his rays during that season. Now in the case -of the two hemispheres, although the southern winter is longer than -the northern, yet the quantity of heat received by each is the same. -But supposing it held true, which it does not, that the loss of -heat sustained by the earth in winter is as much owing to radiation -resulting from the excess in the length of the winter nights over those -of the summer as to the deficiency of heat received in winter from that -received in summer, three days’ heat would then in this case be the -amount lost by radiation in consequence of this excess in the length of -the winter nights. The total length of the winter nights to those of -the summer is, as we have seen, about as 7 to 4. This is a difference -of nearly 1200 hours. But the excess of the south polar winter over the -north amounts to only about 184 hours. Now if 1200 hours give a loss of -three days’ sun-heat, 184 hours will give a loss of scarcely 5½ hours. - -It is no doubt true that the two cases are not exactly analogous; but -it is obvious that any error which can possibly arise from regarding -them as such cannot materially alter the conclusion to which we have -arrived. Supposing the effect were double, or even quadruple, what -we have concluded it to be, still it would not amount to a loss of -two days’ heat, which could certainly have little or no influence on -climate. - -But even assuming all the preceding reasoning to be incorrect, and that -the southern hemisphere, in consequence of its longer winter, loses -heat to the extravagant extent of 168 hours, supposed by Adhémar, still -this could not materially affect climate. The climate is influenced -by the mere _temperature_ of the _surface_ of the ground, and not by -the quantity of heat or cold that may be stored up under the surface. -The climate is determined, so far as the ground is concerned, by -the temperature of the surface, and is wholly independent of the -temperature which may exist under the surface. Underground temperature -can only affect climate through the surface. If the surface could, -for example, be kept covered with perpetual snow, we should have a -cold and sterile climate, although the temperature of the ground under -the snow was actually at the boiling-point. Let the ground to a depth -of, say 40 or 50 feet, be deprived of an amount of heat equal to that -received from the sun in 168 hours. This could produce little or no -sensible effect on climate; for, owing to the slow conductivity of the -ground for heat, this loss would not sensibly affect the temperature -of the surface, as it would take several months for the sun’s heat -to penetrate to that depth and restore the lost heat. The cold, if I -may be allowed to use the expression, would come so slowly out to the -surface that its effect in lowering the temperature of the surface -would scarcely be sensible. And, again, if we suppose the 168 hours’ -heat to be lost by the mere surface of the ground, the effect would -certainly be sensible, but it would only be so for a few days. We -might in this case have a week’s frozen soil, but that would be all. -Before the air had time to become very sensibly affected by the low -temperature of the surface the frozen soil would be thawed. - -The storing up of heat or cold in the ground has in reality very little -to do with climate. Some physicists explain, for example, why the month -of July is warmer than June by referring it to the fact that by the -month of July the ground has become possessed of a larger accumulation -of heat than it possessed in June. This explanation is evidently -erroneous. The ground in July certainly possesses a greater store of -heat than it did in June; but this is not the reason why the former -month is hotter than the latter. July is hotter than June because the -_air_ (not the _ground_) has become possessed of a larger store of -heat than it had in June. Now the air is warmer in July than in June -because, receiving little increase of temperature from the direct rays -of the sun, it is heated chiefly by radiation from the earth and by -contact with its warm surface. Consequently, although the sun’s heat -is greater in June than it is in July, it is near the middle of July -before the air becomes possessed of its maximum store of heat. We -therefore say that July is hotter than June because the air is hotter, -and consequently the temperature in the shade is greater in the former -month than in the latter. - -It is therefore, I presume, quite apparent that Adhémar’s theory fails -to explain why the southern hemisphere is colder than the northern. - -_The generally accepted Explanation._—The difference in the mean -temperature of the two hemispheres is usually attributed to the -proportion of sea to land in the southern hemisphere and of land to -sea in the northern hemisphere. This, no doubt, will account for the -greater _annual range_ of temperature on the northern hemisphere, -but it seems to me that it will not account for the excess of _mean_ -temperature possessed by that hemisphere over the southern. - -The general influence of land on climate is to exaggerate the -variation of temperature due to the seasons. On continents the summers -are hotter and the winters colder than on the ocean. The days are -also hotter and the nights colder on land than on sea. This is a -result which follows from the mere physical properties of land and -water, independently of currents, whether of ocean or of air. But it -nevertheless follows, according to theory (and this is a point which -has been overlooked), that the mean annual temperature of the ocean -ought to be greater than that of the land in equatorial regions as -well as in temperate and polar regions. This will appear obvious for -the following reasons:—(1) The ground stores up heat only by the slow -process of conduction, whereas water, by the mobility of its particles -and its transparency for heat-rays, especially those from the sun, -becomes heated to a considerable depth rapidly. The quantity of heat -stored up in the ground is thus comparatively small, while the quantity -stored up in the ocean is great. (2) The air is probably heated more -rapidly by contact with the ground than with the ocean; but, on the -other hand, it is heated far more rapidly by radiation from the ocean -than from the land. The aqueous vapour of the air is to a great extent -diathermanous to radiation from the ground, while it absorbs the -rays from water and thus becomes heated. (3) The air radiates back a -considerable portion of its heat, and the ocean absorbs this radiation -from the air more readily than the ground does. The ocean will not -reflect the heat from the aqueous vapour of the air, but absorbs it, -while the ground does the opposite. Radiation from the air, therefore, -tends more readily to heat the ocean than it does the land. (4) The -aqueous vapour of the air acts as a screen to prevent the loss by -radiation from water, while it allows radiation from the ground to pass -more freely into space; the atmosphere over the ocean consequently -throws back a greater amount of heat than is thrown back by the -atmosphere over the land. The sea in this case has a much greater -difficulty than the land has in getting quit of the heat received from -the sun; in other words, the land tends to lose its heat more rapidly -than the sea. The consequence of all these circumstances is that the -ocean must stand at a higher mean temperature than the land. A state of -equilibrium is never gained until the rate at which a body is receiving -heat is equal to the rate at which it is losing it; but as equal -surfaces of sea and land receive from the sun the same amount of heat, -it therefore follows that, in order that the sea may get quit of its -heat as rapidly as the land, it _must stand at a higher temperature_ -than the land. The temperature of the sea must continue to rise till -the amount of heat thrown off into space equals that received from the -sun; when this point is reached, equilibrium is established and the -temperature remains stationary. But, owing to the greater difficulty -that the sea has in getting rid of its heat, the mean temperature -of equilibrium of the ocean must be higher than that of the land; -consequently the mean temperature of the ocean, and also of the air -immediately over it, in tropical regions should be higher than the mean -temperature of the land and the air over it. - -The greater portion of the southern hemisphere, however, is occupied by -water, and why then, it may be asked, is this water hemisphere colder -than the land hemisphere? Ought it not also to follow that the sea in -inter-tropical regions should be warmer than the land under the same -parallels; yet, as we know, the reverse is actually found to be the -case. How then is all this to be explained, if the foregoing reasoning -be correct? We find when we examine Professor Dove’s charts of mean -annual temperature, that the ocean in inter-tropical regions has a -mean annual temperature below the normal, and the land a mean annual -temperature above the normal. Both in the Pacific and in the Atlantic -the mean temperature sinks to 2°·3 below the normal, while on the -land it rises 4°·6 above the normal. The explanation in this case is -obviously this: the temperature of the ocean in inter-tropical regions, -as we have already seen, is kept much lower than it would otherwise be -by the enormous amount of _heat_ that is being constantly carried away -from those regions into temperate and polar regions, and of _cold_ that -is being constantly carried from temperate and polar regions to the -tropical regions by means of ocean-currents. The same principle which -explains why the sea in inter-tropical regions has a lower mean annual -temperature than the land, explains also why the southern hemisphere -has a lower mean annual temperature than the northern. The temperature -of the southern hemisphere is lowered by the transference of heat by -means of ocean-currents. - -_Heat transferred from the Southern to the Northern Hemisphere by -Ocean-currents the true Explanation._—The great ocean-currents of -the globe take their rise in three immense streams from the Southern -Ocean, which, on reaching the tropical regions, become deflected in -a westerly direction and flow along the southern side of the equator -for thousands of miles. Perhaps more than one half of this mass of -moving water returns into the Southern Ocean without ever crossing the -equator, but the quantity which crosses over to the northern hemisphere -is enormous. This constant flow of water from the southern hemisphere -to the northern in the form of surface currents must be compensated by -_under currents_ of equal magnitude from the northern hemisphere to the -southern. The currents, however, which cross the equator are far higher -in temperature than their compensating under currents; consequently -there is a constant transference of heat from the southern hemisphere -to the northern. Any currents taking their rise in the northern -hemisphere and flowing across into the southern are comparatively -trifling, and the amount of heat transferred by them is also trifling. -There are one or two currents of considerable size, such as the -Brazilian branch of the great equatorial current of the Atlantic, and -a part of the South Equatorial Drift-current of the Pacific, which -cross the equator from north to south; but these cannot be regarded as -northern currents; they are simply southern currents deflected back -after crossing over to the northern hemisphere. The heat which these -currents possess is chiefly obtained on the southern hemisphere before -crossing over to the northern; and although the northern hemisphere may -not gain much heat by means of them, it, on the other hand, does not -lose much, for the heat which they give out in their progress along the -southern hemisphere does not belong to the northern hemisphere. - -But, after making the fullest allowance for the amount of heat carried -across the equator from the northern hemisphere to the southern, we -shall find, if we compare the mean temperature of the currents from -south to north with that of the great compensating under currents and -the one or two small surface currents, that the former is very much -higher than the latter. The mean temperature of the water crossing the -equator from south to north is probably not under 65°, that of the -under currents is probably not over 39°. But to the under currents -we must add the surface currents from north to south; and assuming -that this will raise the mean temperature of the entire mass of water -flowing south to, say, 45°, we have still a difference of 20° between -the temperature of the masses flowing north and south. Each cubic -foot of water which crosses the equator will in this case transfer -about 965,000 foot-pounds of heat from the southern hemisphere to the -northern. If we had any means of ascertaining the volume of those great -currents crossing the equator, we should then be able to make a rough -estimate of the total amount of heat transferred from the southern -hemisphere to the northern; but as yet no accurate estimate has been -made on this point. Let us assume, what is probably below the truth, -that the total amount of water crossing the equator is at least double -that of the Gulf-stream as it passes through the Straits of Florida, -which amount we have already found to be equal to 66,908,160,000,000 -cubic feet daily. Taking the quantity of heat conveyed by each cubic -foot of water of the Gulf-stream as 1,158,000 foot-pounds, it is -found, as we have seen, that an amount of heat is conveyed by this -current equal to all the heat that falls within 32 miles on each -side of the equator. Then, if each cubic foot of water crossing the -equator transfers 965,000 foot-pounds, and the quantity of water be -double that of the Gulf-stream, it follows that the amount of heat -transferred from the southern hemisphere to the northern is equal to -all the heat falling within 52 miles on each side of the equator, or -equal to all the heat falling on the southern hemisphere within 104 -miles of the equator. This quantity taken from the southern hemisphere -and added to the northern will therefore make a difference in the -amount of heat possessed by the two hemispheres equal to all the heat -which falls on the southern hemisphere within somewhat more than 208 -miles of the equator. - -_A large Portion of the Heat of the Gulf-stream derived from the -Southern Hemisphere._—It can be proved that a very large portion of the -heat conveyed by the Gulf-stream comes from the southern hemisphere. -The proof is as follows:— - -If all the heat came from the northern hemisphere, it could only come -from that portion of the Atlantic, Caribbean Sea, and Gulf of Mexico -which lies to the north of the equator. The entire area of these seas, -extending to the Tropic of Cancer, is about 7,700,000 square miles. -But this area is not sufficient to supply the current passing through -the “Narrows” with the necessary heat. Were the heat which passes -through the Straits of Florida derived exclusively from this area, the -following table would then represent the relative quantity per unit -surface possessed by the Atlantic in the three zones, assuming that one -half of the heat of the Gulf-stream passes into the arctic regions and -the other half remains to warm the temperate regions[52]:— - - From the equator to the Tropic of Cancer 773 - From the Tropic of Cancer to the Arctic Circle 848 - From the Arctic Circle to the North Pole 610 - -These figures show that the Atlantic, from the equator to the Tropic -of Cancer, would be as cold as from the Tropic of Cancer to the North -Pole, were it not that a large proportion of the heat possessed by the -Gulf-stream is derived from the southern hemisphere. - - - - - CHAPTER VI. - - EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC - CIRCULATION.—LIEUT. MAURY’S THEORY. - - Introduction.—Ocean-currents, according to Maury, due to - Difference of Specific Gravity.—Difference of Specific - Gravity resulting from Difference of Temperature.—Difference - of Specific Gravity resulting from Difference of - Saltness.—Maury’s two Causes neutralize each other.—How, - according to him, Difference in Saltness acts as a Cause. - - -_Introduction._—Few subjects have excited more interest and attention -than the cause of ocean circulation; and yet few are in a more -imperfect and unsatisfactory condition, nor is there any question -regarding which a greater diversity of opinion has prevailed. Our -incomplete acquaintance with the facts relating to the currents of the -ocean and the modes of circulation actually in operation, is no doubt -one reason for this state of things. But doubtless the principal cause -of such diversity of opinion lies in the fact that the question is one -which properly belongs to the domain of physics and mechanics, while -as yet no physicist of note (if we except Dr. Colding, of Copenhagen) -has given, as far as I know, any special attention to the subject. It -is true that in works of meteorology and physical geography reference -is continually made to such eminent physicists as Herschel, Pouillet, -Buff, and others; but when we turn to the writings of these authors we -find merely a few remarks expressive of their opinions on the subject, -and no special discussion or investigation of the matter, nor anything -which could warrant us in concluding that such investigations have ever -been made. At present the question cannot be decided by a reference to -authorities. - -The various theories on the subject may be classed under two divisions; -the first of these attributes the motion of the water to the _impulse -of the wind_, and the second to the _force of gravity_ resulting from -difference of density. But even amongst those who adopt the former -theory, it is generally held that the winds are not the sole cause, -but that, to a certain extent at least, difference of specific gravity -contributes to produce motion of the waters. This is a very natural -conclusion; and in the present state of physical geography on this -subject one can hardly be expected to hold any other view. - -The supporters of the latter theory may be subdivided into two -classes. The first of these (of which Maury may be regarded as the -representative) attributes the Gulf-stream, and other sensible currents -of the ocean, to difference of specific gravity. The other class (at -present the more popular of the two, and of which Dr. Carpenter may be -considered the representative) denies altogether that such currents can -be produced by difference of specific gravity,[53] and affirms that -there is a general movement of the upper portion of the ocean from the -equator to the poles, and a counter-movement of the under portion from -the poles to the equator. This movement is attributed to difference of -specific gravity between equatorial and polar water, resulting from -difference of temperature. - -The widespread popularity of the gravitation theory is no doubt, to a -great extent, owing to the very great prominence given to it by Lieut. -Maury in his interesting and popular work, “The Physical Geography of -the Sea.” Another cause which must have favoured the reception of this -theory is the ease with which it is perceived how, according to it, -circulation of the waters of the ocean is supposed to follow. One has -no difficulty, for example, in perceiving that if the inter-tropical -waters of the ocean are expanded by heat, and the waters around the -poles contracted by cold, the surface of the ocean will stand at a -higher level at the equator than at the poles. Equilibrium being -thus disturbed, the water at the equator will tend to flow towards -the poles as a surface current, and the water at the poles towards -the equator as an under current. This, at first sight, looks well, -especially to those who take but a superficial view of the matter. - -We shall examine this theory at some length, for two reasons: 1, -because it lies at the root of a great deal of the confusion and -misconception which have prevailed in regard to the whole subject of -ocean-currents: 2, because, if the theory is correct, it militates -strongly against the physical theory of secular changes of climate -advanced in this volume. We have already seen (Chapter IV.) that -when the eccentricity of the earth’s orbit reaches a high value, a -combination of physical circumstances tends to lower the temperature of -the hemisphere which has its winter solstice in aphelion, and to raise -the temperature of the opposite hemisphere, whose winter solstice will, -of course, be in perihelion. The direct result of this state of things, -as was shown, is to strengthen the force of the trade-winds on the -cold hemisphere, and to weaken their strength on the warm hemisphere: -and this, in turn, we also saw, tends to impel the warm water of -the inter-tropical region on to the warm hemisphere, and to prevent -it, in a very large degree, from passing into the cold hemisphere. -This deflection of the ocean-currents tends to an enormous extent to -increase the difference of temperature previously existing between the -two hemispheres. In other words, the warm and equable condition of the -one hemisphere, and the cold and glacial condition of the other, are, -to a great extent, due to this deflection of ocean-currents. But if -the theory be correct which attributes the motion of ocean-currents to -a difference in density between the sea in inter-tropical and polar -regions, then it follows that these currents (other things being -equal) ought to be stronger on the cold hemisphere than on the warm, -because there is a greater difference of temperature and, consequently, -a greater difference of density, between the polar seas of the cold -hemisphere and the equatorial seas, than between the polar seas of the -warm hemisphere and the equatorial seas. And this being the case, -notwithstanding the influence of the trade-winds of the cold hemisphere -blowing over upon the warm, the currents will, in all probability, -be stronger on the cold hemisphere than on the warm. In other words, -the influence of the powerful trade-winds of the cold hemisphere to -transfer the warm water of the equator to the warm hemisphere will -probably be more than counterbalanced by the tendency of the warm -and buoyant waters of the equator to flow towards the dense and cold -waters around the pole of the cold hemisphere. But if ocean-currents -are due not to difference in specific gravity, but to the influence of -the winds, then it is evident that the waters at the equator will be -impelled, not into the cold hemisphere, but into the warm. - -For this reason I have been the more anxious to prove that -inter-tropical heat is conveyed to temperate and polar regions by -ocean-currents, and not by means of any general movement of the ocean -resulting from difference of gravity. I shall therefore on this account -enter more fully into this part of the subject than I otherwise would -have done. Irrespective of all this, however, the important nature of -the whole question, and the very general interest it excites, warrant a -full consideration of the subject. - -I shall consider first that form of the gravitation theory advocated -by Maury in his work on the “Physical Geography of the Sea,” which -attributes the motion of the Gulf-stream and other sensible currents -of the ocean to differences of specific gravity. One reason which has -induced me to select Maury’s work is, that it not only contains a much -fuller discussion on the cause of the motion of ocean-currents than is -to be found anywhere else, but also that it has probably passed through -a greater number of editions than any other book of a scientific -character in the English language in the same length of time. - -_Examination of Lieut. Maury’s Gravitation Theory._—Although Lieut. -Maury has expounded his views on the cause of ocean-currents at -great length in the various editions of his work, yet it is somewhat -difficult to discover what they really are. This arises chiefly -from the generally confused and sometimes contradictory nature of -his hydrodynamical conceptions. After a repeated perusal of several -editions of his book, the following, I trust, will be found to be a -pretty accurate representation of his theory:— - -_Ocean-currents, according to Maury, due to Difference of Specific -Gravity._—Although Maury alludes to a number of causes which, he -thinks, tend to produce currents, yet he deems their influence so -small that, practically, all currents may be referred to difference of -specific gravity. - -“If we except,” he says, “the tides, and the partial currents of the -sea, such as those that may be created by the wind, we may lay it down -as a rule that all the currents of the ocean owe their origin to the -differences of specific gravity between sea-water at one place and -sea-water at another; for wherever there is such a difference, whether -it be owing to difference of temperature or to difference of saltness, -&c., it is a difference that disturbs equilibrium, and currents are the -consequence” (§ 467)[54]. To the same effect see §§ 896, 37, 512, 520, -and 537. - -Notwithstanding the fact that he is continually referring to difference -of specific gravity as the great cause of currents, it is difficult to -understand in what way he conceives this difference to act as a cause. - -Difference of specific gravity between the waters of the ocean at one -place and another can give rise to currents only through the influence -of the earth’s gravity. All currents resulting from difference of -specific gravity can be ultimately resolved into the general principle -that the molecules that are specifically heavier _descend_ and displace -those that are specifically lighter. If, for example, the ocean at the -equator be expanded by heat or by any other cause, it will be forced by -the denser waters in temperate and polar regions to rise so that its -surface shall stand at a higher level than the surface of the ocean in -these regions. The surface of the ocean will become an inclined plane, -sloping from the equator to the poles. Hydro-statically, the ocean, -considered as a mass, will then be in a state of equilibrium; but the -individual molecules will not be in equilibrium. The molecules at the -surface in this case may be regarded as lying on an inclined plane -sloping from the equator down to the poles, and as these molecules -are at liberty to move they will not remain at rest, but will descend -the incline towards the poles. When the waters at the equator are -expanded, or the waters at the poles contracted, gravitation makes, as -it were, a twofold effort to restore equilibrium. It in the first place -sinks the waters at the poles, and raises the waters at the equator, -in order that the two masses may balance each other; but this very -effort of gravitation to restore equilibrium to the mass destroys the -equilibrium of the molecules by disturbing the level of the ocean. It -then, in the second place, endeavours to restore equilibrium to the -molecules by pulling the lighter surface water at the equator down the -incline towards the poles. This tends not only to restore the level -of the ocean, but to bring the lighter water to occupy the surface -and the denser water the bottom of the ocean; and when this is done, -complete equilibrium is restored, both to the mass of the ocean and -to its individual molecules, and all further motion ceases. But if -heat be constantly applied to the waters of the equatorial regions, -and cold to those of the polar regions, and a permanent disturbance of -equilibrium maintained, then the continual effort of gravitation to -restore equilibrium will give rise to a constant current. In this case, -the heat and the cold (the agents which disturb the equilibrium of the -ocean) may be regarded as causes of the current, inasmuch as without -them the current would not exist; but the real efficient cause, that -which impels the water forward, is the force of gravity. But the force -of gravity, as has already been noticed, cannot produce motion (perform -work) unless the thing acted upon _descend_. Descent is implied in -the very conception of a current produced by difference of specific -gravity. - -But Maury speaks as if difference of specific gravity could give rise -to a current without any descent. - -“It is not necessary,” he says, “to associate with oceanic currents -the idea that they must of necessity, as on land, run from a higher to -a lower level. So far from this being the case, some currents of the -sea actually run up hill, while others run on a level. The Gulf-stream -is of the first class” (§ 403). “The top of the Gulf-stream runs on a -level with the ocean; therefore we know it is not a descending current” -(§ 18). And in § 9 he says that between the Straits of Florida and -Cape Hatteras the waters of the Gulf-stream “are actually forced up an -inclined plane, whose submarine ascent is not less than 10 inches to -the mile.” To the same effect see §§ 25, 59. - -It is perfectly true that “it is not necessary to associate with -ocean-currents the idea that they must of necessity, as on land, -run from a higher to a lower level.” But the reason of this is that -ocean-currents do not, like the currents on land, owe their motion to -the force of gravitation. If ocean-currents result from difference of -specific gravity between the waters in tropical and polar regions, -as Maury maintains, then it is necessary to assume that they are -descending currents. Whatever be the cause which may give rise to a -difference of specific gravity, the motion which results from this -difference is due wholly to the force of gravity; but gravity can -produce no motion unless the water _descend_. - -This fact must be particularly borne in mind while we are considering -Maury’s theory that currents are the result of difference of specific -gravity. - -Ocean-currents, then, according to that writer, owe their existence to -the difference of specific gravity between the waters of inter-tropical -and polar regions. This difference of specific gravity he attributes to -two causes—(1) to difference as to _temperature_, (2) to difference as -to saltness. There are one or two causes of a minor nature affecting -the specific gravity of the sea, to which he alludes; but these two -determine the general result. Let us begin with the consideration of -the first of these two causes, viz.:— - -_Difference of Specific Gravity resulting from Difference of -Temperature._—Maury explains his views on this point by means of an -illustration. “Let us now suppose,” he says, “that all the water within -the tropics, to the depth of one hundred fathoms, suddenly becomes oil. -The aqueous equilibrium of the planet would thereby be disturbed, and -a general system of currents and counter currents would be immediately -commenced—the oil, in an unbroken sheet on the surface, running toward -the poles, and the water, in an under current, toward the equator. The -oil is supposed, as it reaches the polar basin, to be reconverted into -water, and the water to become oil as it crosses Cancer and Capricorn, -rising to the surface in inter-tropical regions, and returning as -before” (§ 20). “Now,” he says (§ 22), “do not the cold waters of the -north, and the warm waters of the Gulf, made specifically lighter by -tropical heat, and which we see actually preserving such a system of -counter currents, hold, at least in some degree, the relation of the -supposed water and oil?” - -In § 24 he calculates that at the Narrows of Bemini the difference in -weight between the volume of the Gulf-water that crosses a section of -the stream in one second, and an equal volume of water at the ocean -temperature of the latitude, supposing the two volumes to be equally -salt, is fifteen millions of pounds. Consequently the force per second -operating to propel the waters of the Gulf towards the pole would in -this case, he concludes, be the “equilibrating tendency due to fifteen -millions of pounds of water in the latitude of Bemini.” In §§ 511 and -512 he states that the effect of expanding the waters at the torrid -zone by heat, and of contracting the waters at the frigid zone by cold, -is to produce a set of surface-currents of warm and light water from -the equator towards the poles, and another set of under currents of -cooler and heavy water from the poles towards the equator. (See also to -the same effect §§ 513, 514, 896.) - -There can be no doubt that his conclusion is that the waters in -inter-tropical regions are expanded by heat, while those in polar -regions are contracted by cold, and that this tends to produce a -surface current from the equator to the poles, and an under current -from the poles to the equator. - -“We shall now consider his second great cause of ocean currents, viz.:— - -_Difference of Specific Gravity resulting from Difference in Degree of -Saltness._—Maury maintains, and that correctly, that saltness increases -the density of water—that, other things being equal, the saltest water -is the densest. He suggests “that one of the purposes which, in the -grand design, it was probably intended to accomplish by having the sea -salt and not fresh, was to impart to its waters the forces and powers -necessary to make their circulation complete” (§ 495). - -Now it is perfectly obvious that if difference in saltness is to -co-operate with difference in temperature in the production of -ocean-currents, the saltest waters, and consequently the densest, must -be in the polar regions, and the waters least salt, and consequently -lightest, must be in equatorial and inter-tropical regions. Were the -saltest waters at the equator, and the freshest at the poles, it would -tend to neutralize the effect due to heat, and, instead of producing -a current, would simply tend to prevent the existence of the currents -which otherwise would result from difference of temperature. - -A very considerable portion of his work, however, is devoted to proving -that the waters of equatorial and inter-tropical regions are salter -and heavier than those of the polar regions; and yet, notwithstanding -this, he endeavours to show that this difference in respect to saltness -between the waters of the equatorial and the polar regions is one of -the chief causes, if not the chief cause, of ocean-currents. In fact, -it is for this special end that so much labour is bestowed in proving -that the saltest water is in the equatorial and inter-tropical regions, -and the freshest in the polar. - -“In the present state of our knowledge,” he says, “concerning this -wonderful phenomenon (for the Gulf-stream is one of the most marvellous -things in the ocean) we can do little more than conjecture. But we have -two causes in operation which we may safely assume are among those -concerned in producing the Gulf-stream. One of these is the increased -saltness of its water after the trade-winds have been supplied with -vapour from it, be it much or little; and the other is the diminished -quantum of salt which the Baltic and the Northern Seas contain” (§ 37). -“Now here we have, on one side, the Caribbean Sea and Gulf of Mexico, -with their waters of brine; on the other, the great Polar Basin, the -Baltic, and the North Sea, the two latter with waters that are but -little more than brackish. In one set of these sea-basins the water is -heavy, in the other it is light. Between them the ocean intervenes; but -water is bound to seek and to maintain its level; and here, therefore, -we unmask one of the agents concerned in causing the Gulf-stream” (§ -38). To the same effect see §§ 52, 522, 523, 524, 525, 526, 528, 530, -554, 556. - -Lieut. Maury’s _two causes neutralize each other_. Here we have two -theories put forth regarding the cause of ocean-currents, the one -in direct opposition to the other. According to the one theory, -ocean-currents exist because the waters of equatorial regions, in -consequence of their higher temperature, are _less dense_ than the -waters of the polar regions; but according to the other theory, -ocean-currents exist because the waters of equatorial regions, in -consequence of their greater saltness, are _more dense_ than the -waters of the polar regions. If the one cause be assigned as a reason -why ocean-currents exist, then the other can be equally assigned as -a reason why they should not exist. According to both theories it is -the difference of density between the equatorial and polar waters that -gives rise to currents; but while the one theory maintains that the -equatorial waters are _lighter_ than the polar, the other holds that -they are _heavier_. Either the one theory or the other may be true, -or neither; but it is logically impossible that both of them can. Let -it be observed that it is not two currents, the one contrary to the -other, with which we have at present to do; it is not temperature -producing currents in one direction, and saltness producing currents -in the contrary direction. We have two theories regarding the origin -of currents, the one diametrically opposed to the other. The tendency -of the one cause assigned is to prevent the action of the other. If -temperature is allowed to act, it will make the inter-tropical waters -lighter than the polar, and then, according to theory, a current will -result. But if we bring saltness into play (the other cause) it will -do the reverse: it will increase the density of the inter-tropical -waters and diminish the density of the polar; and so far as it acts it -will diminish the currents produced by temperature, because it will -diminish the difference of specific gravity between the inter-tropical -and polar regions which had been previously caused by temperature. And -when the effects of saltness are as powerful as those of temperature, -the difference of specific gravity produced by temperature will be -completely effaced, or, in other words, the waters of the equatorial -and polar seas will be of the same density, and consequently no current -will exist. And so long as the two causes continue in action, no -current can arise, unless the energy of the one cause should happen to -exceed that of the other; and even then a current will only exist to -the extent by which the strength of the one exceeds that of the other. - -The contrary nature of the two theories will be better seen by -considering the way in which it is supposed that difference in saltness -is produced and acts as a cause. - -If there is a constant current resulting from the difference in -saltness between the equatorial and polar waters, then there must be a -cause which maintains this difference. The current is simply the effort -to restore the equilibrium lost by the difference; and the current -would very soon do this, and then all motion would cease, were there -not a constantly operating cause maintaining the disturbance. What, -then, according to Maury, is the cause of this disturbance, or, in -other words, what is it that keeps the equatorial waters salter than -the polar? - -The agencies in operation are stated by him to be heat, radiation, -evaporation, precipitation, and secretion of solid matter in the form -of shells, &c. The two most important, however, are evaporation and -precipitation. - -The trade-winds enter the equatorial regions as relatively dry winds -thirsting for vapour; consequently they absorb far more moisture than -they give out; and the result is that in inter-tropical regions, -evaporation is much in excess of precipitation; and as fresh water only -is taken up, the salt being left behind, the process, of course, tends -to increase the saltness of the inter-tropical seas. Again, in polar -and extra-tropical regions the reverse is the case; precipitation is in -excess of evaporation. This tends in turn to diminish the saltness of -the waters of those regions. (See on these points §§ 31, 33, 34, 37, -179, 517, 526, and 552.) - -In the system of circulation produced by difference of temperature, -as we have already seen, the surface-currents flow from the equator -to the poles, and the under or return currents from the poles to the -equator; but in the system produced by difference of saltness, the -surface currents flow from the poles to the equator, and the return -under currents from the equator to the poles. That the surface currents -produced by difference of saltness flow from the poles to the equator, -Maury thinks is evident for the two following reasons:— - -(1) As evaporation is in excess of precipitation in inter-tropical -regions, more water is taken off the surface of the ocean in those -regions than falls upon it in the form of rain. This excess of water -falls in the form of rain on temperate and polar regions, where, -consequently, precipitation is in excess of evaporation. The lifting -of the water off the equatorial regions and its deposit on the polar -tend to lower the level of the ocean in equatorial regions and to raise -the level in polar; consequently, in order to restore the level of -the ocean, the surface water at the polar regions flows towards the -equatorial regions. - -(2) As the water taken up at the equator is fresh, and the salt -is left behind, the ocean, in inter-tropical regions, is thus made -saltier and consequently denser. This dense water, therefore, sinks -and passes away as an under current. This water, evaporated from -inter-tropical regions, falls as fresh and lighter water in temperate -and polar regions; and therefore not only is the level of the ocean -raised, but the waters are made lighter. Hence, in order to restore -equilibrium, the waters in temperate and polar regions will flow as -a surface current towards the equator. Under currents will flow from -the equator to the poles, and surface or upper currents from the poles -to the equator. Difference in temperature and difference in saltness, -therefore, in every respect tend to produce opposite effects. - -That the above is a fair representation of the way in which Maury -supposes difference in saltness to act as a cause in the production of -ocean-currents will appear from the following quotations:— - -“In those regions, as in the trade-wind region, where evaporation is -in excess of precipitation, the general level of this supposed sea -would be altered, and immediately as much water as is carried off by -evaporation would commence to flow in from north and south toward the -trade-wind or evaporation region, to restore the level” (§ 509). “On -the other hand, the winds have taken this vapour, borne it off to the -extra-tropical regions, and precipitated it, we will suppose, where -precipitation is in excess of evaporation. Here is another alteration -of sea-level, by elevation instead of by depression; and hence we -have the motive power for a _surface current from each pole towards -the equator_, the object of which is only to supply the demand for -evaporation in the trade-wind regions” (§ 510). - -The above result would follow, supposing the ocean to be fresh. He then -proceeds to consider an additional result that follows in consequence -of the saltness of the ocean. - -“Let evaporation now commence in the trade-wind region, as it was -supposed to do in the case of the freshwater seas, and as it actually -goes on in nature—and what takes place? Why a lowering of the sea-level -as before. But as the vapour of salt water is fresh, or nearly so, -fresh water only is taken up from the ocean; that which remains behind -is therefore more salt. Thus, while the level is lowered in the salt -sea, the equilibrium is destroyed because of the saltness of the water; -for the water that remains after evaporation takes place is, on account -of the solid matter held in solution, specifically heavier than it was -before any portion of it was converted into vapour” (§ 517). - -“The vapour is taken from the surface-water; the surface-water thereby -becomes more salt, and, under certain conditions, heavier. When it -becomes heavier, it sinks; and hence we have, due to the salts of the -sea, a vertical circulation, namely, a descent of heavier—because -salter and cooler—water from the surface, and an ascent of water that -is lighter—because it is not so salt—from the depths below” (§ 518). - -In section 519 he goes on to show that this vapour removed from the -inter-tropical region is precipitated in the polar regions, where -precipitation is in excess of evaporation. “In the precipitating -regions, therefore, the level is destroyed, as before explained, by -elevation, and in the evaporating regions by depression; which, as -already stated, gives rise to a system of _surface_ currents, moved by -gravity alone, from the _poles towards the equator_” (§ 520). - -“This fresh water being emptied into the Polar Sea and agitated by the -winds, becomes mixed with the salt; but as the agitation of the sea by -the winds is supposed to extend to no great depth, it is only the upper -layer of salt water, and that to a moderate depth, which becomes mixed -with the fresh. The specific gravity of this upper layer, therefore, is -diminished just as much as the specific gravity of the sea-water in the -evaporating regions was increased. _And thus we have a surface current -of saltish water from the poles towards the equator, and an under -current of water salter and heavier from the equator to the poles_” (§ -522). - -“This property of saltness imparts to the waters of the ocean another -peculiarity, by which the sea is still better adapted for the -regulation of climates, and it is this: by evaporating fresh water from -the salt in the tropics, the surface water becomes heavier than the -average of sea-water. This heavy water is also warm water; it sinks, -and being a good retainer, but a bad conductor, of heat, this water -is employed in transporting through _under currents_ heat for the -mitigation of climates in far distant regions” (§ 526). - -“For instance, let us suppose the waters in a certain part of the -torrid zone to be 90°, but by reason of the fresh water which has been -taken from them in a state of vapour, and consequently, by reason of -the proportionate increase of salts, these waters are heavier than -waters that may be cooler, but not so salt. This being the case, the -tendency would be for this warm but salt and heavy water to flow off as -an _under current towards the polar or some other regions of lighter -water_” (§ 554). - -That Maury supposes the warm water at the equator to flow to the polar -regions as an under current is further evident from the fact that he -maintains that the climate of the arctic regions is mitigated by a warm -under current, which comes from the equatorial regions, and passes up -through Davis Straits (see §§ 534−544). - -The question now suggests itself: to which of these two antagonistic -causes does Maury really suppose ocean-currents must be referred? -Whether does he suppose, difference in temperature or difference in -saltness, to be the real cause? I have been unable to find anything -from which we can reasonably conclude that he prefers the one cause -to the other. It would seem that he regards both as real causes, and -that he has failed to perceive that the one is destructive of the -other. But it is difficult to conceive how he could believe that the -sea in equatorial regions, by virtue of its higher temperature, is -lighter than the sea in polar regions, while at the same time it _is -not_ lighter but heavier, in consequence of its greater saltness—how -he could believe that the warm water at the equator flows to the poles -as an upper current, and the cold water at the poles to the equator -as an _under_ current, while at the same time the warm water at the -equator does not flow to the poles as a surface current, nor the cold -water at the poles to the equator as an under current, but the reverse. -And yet, unless these absolute impossibilities be possible, how can an -ocean-current be the result of both causes? - -The only explanation of the matter appears to be that Maury has failed -to perceive the contradictory nature of his two theories. This fact is -particularly seen when he comes to apply his two theories to the case -of the Gulf-stream. He maintains, as has already been stated, that -the waters of the Gulf-stream are salter than the waters of the sea -through which they flow (see §§ 3, 28, 29, 30, 34, and several other -places). And he states, as we have already seen (see p. 104), that the -existence of the Gulf-stream is due principally to the difference of -density of the water of the Caribbean Sea and the Gulf of Mexico as -compared with that of the great Polar Basin and the North Sea. There -can be no doubt whatever that it is the _density_ of the waters of the -Gulf-stream at its fountain-head, the Gulf of Mexico, resulting from -its superior saltness, and the deficiency of density of the waters in -polar regions and the North Sea, &c., that is here considered to be -unmasked as one of the agents. If this be a cause of the motion of the -Gulf-stream, how then can the difference of temperature between the -waters of inter-tropical and polar regions assist as a cause? This -difference of temperature will simply tend to undo all that has been -done by difference of saltness: for it will tend to make the waters -of the Gulf of Mexico lighter, and the waters of the polar regions -heavier. But Maury maintains, as we have seen, that this difference of -temperature is also a cause, which shows that he does not perceive the -contradiction. - -This is still further apparent. He holds, as stated, that “the waters -of the Gulf-stream are salter than the waters of the sea through which -they flow,” and that this excess in saltness, by making the water -heavier, is a cause of the motion of the stream. But he maintains that, -notwithstanding the effect which greater saltness has in increasing -the density of the waters of the Gulf-stream, yet, owing to their -higher temperature, they are actually lighter than the water through -which they flow; and as a proof that this is the case, he adduces the -fact that the surface of the Gulf-stream is roof-shaped (§§ 39−41), -which it could not be were its waters not actually lighter than the -waters through which the stream flows. So it turns out that, in -contradiction to what he had already stated, it is the lesser density -of the waters of the Gulf-stream that is the real cause of their -motion. The greater saltness of the waters, to which he attributes so -much, can in no way be regarded as a cause of motion. Its effect, so -far as it goes, is to stop the motion of the stream rather than to -assist it. - -But, again, although he asserts that difference of saltness and -difference of temperature are both causes of ocean-currents, yet he -appears actually to admit that temperature and saltness neutralize each -other so as to prevent change in the specific gravity of the ocean, as -will be seen from the following quotation:— - -“It is the trade-winds, then, which prevent the thermal and specific -gravity curves from conforming with each other in inter-tropical seas. -The water they suck up is fresh water; and the salt it contained, being -left behind, is just sufficient to counterbalance, by its weight, the -effect of thermal dilatation upon the specific gravity of sea-water -between the parallels of 34° north and south. As we go from 34° to the -equator, the water grows warmer and expands. It would become lighter; -but the trade-winds, by taking up vapour without salt, make the water -salter, and therefore heavier. The conclusion is, the proportion of -salt in sea-water, its expansibility between 62° and 82°, and the -thirst of the trade-winds for vapour are, where they blow, so balanced -as to produce _perfect compensation_; and a more beautiful compensation -cannot, it appears to me, be found in the mechanism of the universe -than that which we have here stumbled upon. It is a triple adjustment; -the power of the sun to expand, the power of the winds to evaporate, -and the quantity of salts in the sea—these are so proportioned and -adjusted that when both the wind and the sun have each played with -its forces upon the inter-tropical waters of the ocean, _the residuum -of heat and of salt should be just such as to balance each other in -their effects; and so the aqueous equilibrium of the torrid zone is -preserved_” (§ 436, eleventh edition). - -“Between 35° or 40° and the equator evaporation is in excess of -precipitation; and though, as we approach the equator on either side -from these parallels, the solar ray warms and expands the surface-water -of the sea, the winds, by the vapour they carry off, and the salt they -leave behind, _prevent it from making that water lighter_” (§ 437, -eleventh edition). - -“Philosophers have admired the relations between the size of the earth, -the force of gravity, and the strength of fibre in the flower-stalks of -plants; but how much more exquisite is the system of counterpoises and -adjustments here presented between the sea and its salts, the winds and -the heat of the sun!” (§ 438, eleventh edition). - -How can this be reconciled with all that precedes regarding -ocean-currents being the result of difference of specific gravity -caused by a difference of temperature and difference of saltness? Here -is a distinct recognition of the fact that difference in saltness, -instead of producing currents, tends rather to prevent the existence of -currents, by counteracting the effects of difference in temperature. -And so effectually does it do this, that for 40°, or nearly 3,000 -miles, on each side of the equator there is absolutely no difference in -the specific gravity of the ocean, and consequently nothing, either as -regards difference of temperature or difference of saltness, that can -possibly give rise to a current. - -But it is evident that, if between the equator and latitude 40° the -two effects completely neutralize each other, it is not at all likely -that between latitude 40° and the poles they will not to a large extent -do the same thing. And if so, how can ocean-currents be due either -to difference in temperature or to difference in saltness, far less -to both. If there be any difference of specific gravity of the ocean -between latitude 40° and the poles, it must be only to the extent -by which the one cause has failed to neutralize the other. If, for -example, the waters in latitude 40°, by virtue of higher temperature, -are less dense than the waters in the polar regions, they can be so -only to the extent that difference in saltness has failed to neutralize -the effect of difference in temperature. And if currents result, they -can do so only to the extent that difference in saltness has thus -fallen short of being able to produce complete compensation. Maury, -after stating his views on compensation, seems to become aware of -this; but, strangely, he does not appear to perceive, or, at least, he -does not make any allusion to the fact, that all this is fatal to his -theories about ocean-currents being the combined result of differences -of temperature and of saltness. For, in opposition to all that he -had previously advanced regarding the difficulty of finding a cause -sufficiently powerful to account for such currents as the Gulf-stream, -and the great importance that difference in saltness had in their -production, he now begins to maintain that so great is the influence -of difference in temperature that difference in saltness, and a number -of other compensating causes are actually necessary to prevent the -ocean-currents from becoming too powerful. - -“If all the inter-tropical heat of the sun,” he says, “were to pass -into the seas upon which it falls, simply raising the temperature of -their waters, it would create a thermo-dynamical force in the ocean -capable of transporting water scalding hot from the torrid zone, and -spreading it while still in the tepid state around the poles.... Now, -suppose there were no trade-winds to evaporate and to counteract the -dynamical force of the sun, this hot and light water, by becoming -hotter and lighter, would flow off in currents with almost mill-tail -velocity towards the poles, covering the intervening sea with a mantle -of warmth as a garment. The cool and heavy water of the polar basin, -coming out as under currents, would flow equatorially with equal -velocity.” - -“Thus two antagonistic forces are unmasked, and, being unmasked, we -discover in them a most exquisite adjustment—a compensation—by which -the dynamical forces that reside in the sunbeam and the trade-wind -are made to counterbalance each other, by which the climates of -inter-tropical seas are regulated, and by which the set, force, and -volume of oceanic currents are measured” (§§ 437 and 438, eleventh -edition). - - - - - CHAPTER VII. - - EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC - CIRCULATION.—LIEUT. MAURY’S THEORY (_continued_). - - Methods of determining the Question.—The Force resulting from - Difference of Specific Gravity.—Sir John Herschel’s Estimate - of the Force.—Maximum Density of Sea-Water.—Rate of Decrease - of Temperature of Ocean at Equator.—-The actual Amount of - Force resulting from Difference of Specific Gravity.—M. - Dubuat’s Experiments. - - -_How the Question may be Determined._—Whether the circulation of the -ocean is due to difference in specific gravity or not may be determined -in three ways: viz. (1) by direct experiment; (2) by ascertaining the -absolute amount of _force_ acting on the water to produce motion, in -virtue of difference of specific gravity, and thereafter comparing it -with the force which has been shown by experiment to be necessary to -the production of sensible motion; or (3) by determining the greatest -possible amount of _work_ which gravity can perform on the waters in -virtue of difference of specific gravity, and then ascertaining if the -work of gravity does or does not equal the work of the resistances in -the required motion. But Maury has not adopted either of these methods. - -_The Force resulting from Difference of Specific Gravity._—I shall -consider first whether the force resulting from difference of specific -gravity be sufficient to account for the motion of ocean-currents. - -The inadequacy of this cause has been so clearly shown by Sir John -Herschel, that one might expect that little else would be required than -simply to quote his words on the subject, which are as follows:— - -“First, then, if there were no atmosphere, there would be no -Gulf-stream, or any other considerable ocean-current (as distinguished -from a mere surface-drift) whatever. By the action of the sun’s rays, -the _surface_ of the ocean becomes _most_ heated, and the heated water -will, therefore, neither directly tend to _ascend_ (which it could -not do without leaving the sea) nor to _descend_, which it cannot do, -being rendered buoyant, nor to move laterally, no lateral impulse being -given, and which it could only do by reason of a general declivity -of surface, the dilated portion occupying a higher level. Let us see -what this declivity would amount to. The equatorial surface-water -has a temperature of 84°. At 7,200 feet deep the temperature is 39°, -the level of which temperature rises to the surface in latitude 56°. -Taking the dilatability of sea-water to be the same as that of fresh, a -uniformly progressive increase of temperature, from 39° to 84° Fahr., -would dilate a column of 7,200 feet by 10 feet, to which height, -therefore, above the spheroid of equilibrium (or above the sea-level in -lat. 56°), the equatorial surface is actually raised by dilatation. An -arc of 56° on the earth’s surface measures 3,360 geographical miles; -so that we have a slope of 1/28th of an inch per geographical mile, or -1/32nd of an inch per statute mile for the water so raised to run down. -As the accelerating force corresponding to such a slope (of 1/10th of -a second, 0″·1) is less than one two-millionth part of gravity, we -may dismiss this as a cause capable of creating only a very trifling -surface-drift, and not worth considering, even were it in the proper -direction to form, by concentration, a current from east to west, -_which it could not be, but the very reverse_.”[55] - -It is singular how any one, even though he regarded this conclusion as -but a rough approximation to the truth, could entertain the idea that -ocean-currents can be the result of difference in specific gravity. -There are one or two reasons, however, which may be given for the -above not having been generally received as conclusive. Herschel’s -calculations refer to the difference of gravity resulting from -difference of temperature; but this is only one of the causes to which -Maury appeals, and even not the one to which he most frequently refers. -He insists so strongly on the effects of difference of saltness, that -many might think that, although Herschel may have shown that difference -in specific gravity arising from difference of temperature could not -account for the motion of ocean-currents, yet nevertheless that this, -combined with the effects resulting from difference in saltness, might -be a sufficient explanation of the phenomena. Such, of course, would -not be the case with those who perceived the contradictory nature of -Maury’s two causes; but probably many read the “Physical Geography of -the Sea” without being aware that the one cause is destructive of the -other. Again, a few plausible objections, which have never received due -consideration, have been strongly urged by Maury and others against the -theory that ocean-currents can be caused by the impulses of the winds; -and probably these objections appear to militate as strongly against -this theory as Herschel’s arguments against Maury’s. - -There is one trifling objection to Herschel’s result: he takes 39° as -the temperature of maximum density. This, however, as we shall see, -does not materially affect his conclusions. - -Observations on the temperature of the maximum density of sea-water -have been made by Erman, Despretz, Rossetti, Neumann, Marcet, Hubbard, -Horner, and others. No two of them have arrived at exactly the same -conclusion. This probably arises from the fact that the temperature -of maximum density depends upon the amount of salt held in solution. -No two seas, unless they are equal as to saltness, have the same -temperature of maximum density. The following Table of Despretz will -show how rapidly the temperature of both the freezing-point and of -maximum density is lowered by additional amounts of salt:— - - +-----------+-----------------+------------------+ - | Amount | Temperature of | Temperature of | - | of salt. | freezing-point. | Maximum density. | - +-----------+-----------------+------------------+ - | | ° | ° | - | 0·000123 | −1·21 C. | + 1·19 C. | - | 0·0246 | −2·24 | − 1·69 | - | 0·0371 | −2·77 | − 4·75 | - | 0·0741 | −5·28 | −16·00 | - +-----------+-----------------+------------------+ - -He found the temperature of maximum density of sea-water, whose density -at 20°C. was 1·0273, to be −3°·67C. (25°·4F.), and the temperature of -freezing-point −2°·55C. (27°·4F.).[56] Somewhere between 25° and 26° -F. may therefore be regarded as the temperature of maximum density -of sea-water of average saltness. We have no reason to believe that -the ocean, from the surface to the bottom, even at the poles, is at -27°·4F., the freezing-point. - -The actual slope resulting from difference of specific gravity, -as we shall presently see, does not amount to 10 feet. Herschel’s -estimate was, however, made on insufficient data, both as to the rate -of expansion of sea-water and that at which the temperature of the -ocean at the equator decreases from the surface downwards. We are -happily now in the possession of data for determining with tolerable -accuracy the amount of slope due to difference of temperature between -the equatorial and polar seas. The rate of expansion of sea-water -from 0°C. to 100°C. has been experimentally determined by Professor -Muncke, of Heidelberg.[57] The valuable reports of Captain Nares, of -H.M.S. _Challenger_, lately published by the Admiralty, give the rate -at which the temperature of the Atlantic at the equator decreases -from the surface downwards. These observations show clearly that the -super-heating effect of the sun’s rays does not extend to any great -depth. They also prove that at the equator the temperature decreases -as the depth increases so rapidly that at 60 fathoms from the surface -the temperature is 62°·4, the same as at Madeira at the same depth; -while at the depth of 150 fathoms it is only 51°, about the same as -that in the Bay of Biscay (Reports, p. 11). Here at the very outset -we have broad and important facts hostile to the theory of a flow of -water resulting from difference of temperature between the ocean in -equatorial and temperate and polar regions. - -Through the kindness of Staff-Captain Evans, Hydrographer of the -Admiralty, I have been favoured with a most valuable set of serial -temperature soundings made by Captain Nares of the _Challenger_, close -to the equator, between long. 14° 49′ W. and 32° 16′ W. The following -Table represents the mean of the whole of these observations:— - - +----------+-------------+ - | Fathoms. | Temperature.| - +----------+-------------+ - | | ° | - | Surface. | 77·9 | - | 10 | 77·2 | - | 20 | 77·1 | - | 30 | 76·9 | - | 40 | 71·7 | - | 50 | 64·0 | - | 60 | 60·4 | - | 70 | 59·4 | - | 80 | 58·0 | - | 90 | 58·0 | - | 100 | 55·6 | - | 150 | 51·0 | - | 200 | 46·6 | - | 300 | 42·2 | - | 400 | 40·3 | - | 500 | 38·9 | - | 600 | 39·2 | - | 700 | 39·0 | - | 800 | 39·1 | - | 900 | 38·2 | - | 1000 | 36·9 | - | 1100 | 37·6 | - | 1200 | 36·7 | - | 1300 | 35·8 | - | 1400 | 36·4 | - | 1500 | 36·1 | - | Bottom. | 34·7 | - +----------+-------------+ - -We have in this Table data for determining the height at which the -surface of the ocean at the equator ought to stand above that of the -poles. Assuming 32°F. to be the temperature of the ocean at the poles -from the surface to the bottom and the foregoing to be the rate at -which the temperature of the ocean at the equator decreases from the -surface downwards, and then calculating according to Muncke’s Table of -the expansion of sea-water, we have only 4 feet 6 inches as the height -to which the level of the ocean at the equator ought to stand above -that at the poles in order that the ocean may be in static equilibrium. -In other words, the equatorial column requires to be only 4 feet 6 -inches higher than the polar in order that the two may balance each -other. - -Taking the distance from the equator to the poles at 6,200 miles, the -force resulting from the slope of 4½ feet in 6,200 will amount to only -1/7,340,000th that of gravity, or about 1/1000th of a grain on a pound -of water. But, as we shall shortly see, there can be no permanent -current resulting from difference of temperature while the two columns -remain in equilibrium, for the current is simply an effort to the -retardation of equilibrium. In order to have permanent circulation -there must be a permanent disturbance of equilibrium. Or, in other -words, the weight of the polar column must be kept in excess of that -of the equatorial. Suppose, then, that the weight of the polar column -exceeds that of the equatorial by 2 feet of water, the difference of -level between the two columns will, in that case, amount to only 2 -feet 6 inches. This would give a force of only 1/13,200,000th that of -gravity, or not much over 1/1,900th of a grain on a pound of water, -tending to draw the water down the slope from the equator to the poles, -a force which does not much exceed the weight of a grain on a ton of -water. But it must be observed that this force of a grain per ton would -affect only the water at the surface; a very short distance below the -surface the force, small as it is, would be enormously reduced. If -water were a perfect fluid, and offered no resistance to motion, it -would not only flow down an incline, however small it might be, but -would flow down with an accelerated motion. But water is not a perfect -fluid, and its molecules do offer considerable resistance to motion. -Water flowing down an incline, however steep it may be, soon acquires -a uniform motion. There must therefore be a certain inclination below -which no motion can take place. Experiments were made by M. Dubuat -with the view of determining this limit.[58] He found that when the -inclination was 1 in 500,000, the motion of the water was barely -perceptible; and he came to the conclusion that when the inclination -is reduced to 1 in 1,000,000, all motion ceases. But the inclination -afforded by the difference of temperature between the sea in equatorial -and polar regions does not amount to one-seventh of this, and -consequently it can hardly produce even that “trifling surface-drift” -which Sir John Herschel is willing to attribute to it. - -There is an error into which some writers appear to fall to which I -may here refer. Suppose that at the equator we have to descend 10,000 -feet before water equal in density to that at the poles is reached. We -have in this case a plain with a slope of 10,000 feet in 6,200 miles, -forming the upper surface of the water of maximum density. Now this -slope exercises no influence in the way of producing a current, as some -seem to think; for it is not a case of disturbed equilibrium, but the -reverse. It is the condition of static equilibrium resulting from a -difference between the temperature of the water at the equator and the -poles. The only slope that has any tendency to produce motion is that -which is formed by the surface of the ocean in the equatorial regions -being higher than the surface at the poles; but this is an inclination -of only 4 feet 6 inches, and is therefore wholly inadequate to produce -such currents as the Gulf-stream. - - - - - CHAPTER VIII. - - EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC - CIRCULATION.—DR. CARPENTER’S THEORY. - - Gulf-stream according to Dr. Carpenter not due to Difference of - Specific Gravity.—Facts to be Explained.—The Explanation of - the Facts.—The Explanation hypothetical.—The Cause assigned - for the hypothetical Mode of Circulation.—Under currents - account for all the Facts better than the Gravitation - Hypothesis.—Known Condition of the Ocean inconsistent with - that Hypothesis. - - -Dr. Carpenter does not suppose, with Lieut. Maury, that the difference -of temperature between the ocean in equatorial and polar regions can -account for the Gulf-stream and other great currents of the ocean. -He maintains, however, that this difference is quite sufficient to -bring about a slow general interchange of water between the polar and -inter-tropical areas—to induce a general movement of the upper portion -of the ocean from the equator to the poles and a counter-movement of -the under portion in a contrary direction. It is this general movement -which, according to that author, is the great agent by which heat is -distributed over the globe.[59] - -In attempting to estimate the adequacy of this hypothesis as an -explanation of the phenomena involved, there are obviously two -questions to be considered: namely, (1) is the difference of -temperature between the sea in inter-tropical and polar regions -sufficiently great to produce the required movement? and (2) assuming -that there is such a movement, does it convey the amount of heat which -Dr. Carpenter supposes? I shall begin with the consideration of the -first of these two points. - -But before doing so let us see what the facts are which this -gravitation theory is intended to explain. - -_The Facts to be Explained._—Dr. Carpenter considers that the great -mass of warm water proved during recent dredging expeditions to -occupy the depths of the North Atlantic, must be referred, not to the -Gulf-stream, but to a general movement of water from the equator. “The -inference seems inevitable,” he says, “that the bulk of the water in -the warm area must have come thither from the south-west. The influence -of the Gulf-stream proper (meaning by this the body of super-heated -water which issues through the ‘Narrows’ from the Gulf of Mexico), if -it reaches this locality at all (which is very doubtful), could only -affect the _most superficial_ stratum; and the same may be said of -the surface-drift caused by the prevalence of south-westerly winds, -to which some have attributed the phenomena usually accounted for by -the extension of the Gulf-stream to these regions. And the presence -of the body of water which lies between 100 and 600 fathoms deep, and -the range of whose temperature is from 48° to 42°, can scarcely be -accounted for on any other hypothesis than that of a _great general -movement of equatorial water towards the polar area_, of which -movement the Gulf-stream constitutes a peculiar case modified by local -conditions. In like manner the Arctic stream which underlies the warm -superficial stratum in our cold area constitutes a peculiar case, -modified by the local conditions to be presently explained, of _a great -general movement of polar water towards the equatorial area_, which -depresses the temperature of the deepest parts of the great oceanic -basins nearly to the freezing-point.” - -It is well-known that, wherever temperature-observations have been -made in the Atlantic, the bottom of that ocean has been found to be -occupied by water of an ice-cold temperature. And this holds true -not merely of the Atlantic, but also of the ocean in inter-tropical -regions—a fact which has been proved by repeated observations, and more -particularly of late by those of Commander Chimmo in the China Sea and -Indian Ocean, where a temperature as low as 32° Fahr. was found at a -depth of 2,656 fathoms. In short, the North Atlantic, and probably the -inter-tropical seas also, may be regarded, Dr. Carpenter considers, as -divided horizontally into two great layers or strata—an upper warm, and -a lower cold stratum. All these facts I, of course, freely admit; nor -am I aware that their truth has been called in question by any one, no -matter what his views may have been as to the mode in which they are to -be explained. - -_The Explanation of the Facts._—We have next the explanation of the -facts, which is simply this:—The cold water occupying the bottom of -the Atlantic and of inter-tropical seas is to be accounted for by the -supposition that _it came from the polar regions_. This is obvious, -because the cold possessed by the water could not have been derived -from the crust of the earth beneath: neither could it have come from -the surface; for the temperature of the bottom water is far below the -normal temperature of the latitude in which it is found. Consequently -“the inference seems irresistible that this depression must be produced -and maintained by the convection of cold from the polar towards the -equatorial area.” Of course, if we suppose a flow of water from the -poles towards the equator, we must necessarily infer a counter flow -from the equator towards the poles; and while the water flowing from -equatorial to polar regions will be _warm_, that flowing from polar to -equatorial regions will be _cold_. The doctrine of a mutual interchange -of equatorial and polar water is therefore a _necessary consequence_ -from the admission of the foregoing facts. With this _explanation -of the facts_ I need hardly say that I fully agree; nor am I aware -that its correctness has ever been disputed. Dr. Carpenter surely -cannot charge me with overlooking the fact of a mutual interchange of -equatorial and polar water, seeing that my estimate of the thermal -power of the Gulf-stream, from which it is proved that the amount -of heat conveyed from equatorial to temperate and polar regions -is enormously greater than had ever been anticipated, was made a -considerable time before he began to write on the subject of oceanic -circulation.[60] And in my paper “On Ocean-currents in relation to the -Distribution of Heat over the Globe”[61] (the substance of which is -reproduced in Chapters II. and III. of this volume), I have endeavoured -to show that, were it not for the raising of the temperature of polar -and high temperate regions and the lowering of the temperature of -inter-tropical regions by means of this interchange of water, these -portions of the globe would not be habitable by the present existing -orders of beings. - -The explanation goes further:—“It is along the surface and upper -portion of the ocean that the equatorial waters flow towards the -poles, and it is along the bottom and under portion of the ocean that -polar waters flow towards the equator; or, in other words, the warm -water keeps the _upper_ portion of the ocean and the cold water the -_under_ portion.” With this explanation I to a great extent agree. It -is evident that, in reference to the northern hemisphere at least, the -most of the water which flows from inter-tropical to polar regions -(as, for example, the Gulf-stream) keeps to the surface and upper -portion of the ocean; but for reasons which I have already stated, a -very large proportion of this water must return in the form of _under_ -currents; or, which is the same thing, the return compensating current, -whether it consist of the identical water which originally came from -the equator or not, must flow towards the equator as an under current. -That the cold water which is found at the bottom of the Atlantic and -of inter-tropical seas must have come as under currents is perfectly -obvious, because water which should come along the surface of the ocean -from the polar regions would not be cold when it reached inter-tropical -regions. - -_The Explanation hypothetical._—Here the general agreement between -us in a great measure terminates, for Dr. Carpenter is not satisfied -with the explanation generally adopted by the advocates of the -_wind theory_, viz., that the cold water found in temperate and -inter-tropical areas comes from polar regions as compensating under -currents, but advances a _hypothetical_ form of circulation to account -for the phenomenon. He assumes that there is a _general set_ or flow of -the surface and upper portion of the ocean from the equator to polar -regions, and a _general set_ or flow of the bottom and under portion of -the ocean from polar regions to the equator. Mr. Ferrel (_Nature_, June -13, 1872) speaks of that “interchanging motion of the water between the -equator and the pole _discovered_ by Dr. Carpenter.” In this, however, -Mr. Ferrel is mistaken; for Dr. Carpenter not only makes no claim to -any discovery of the kind, but distinctly admits that none such has -yet been made. Although in some of his papers he speaks of a “_set_ of -warm surface-water in the southern oceans toward the Antarctic pole” -as being well known to navigators, yet he nowhere affirms, as far as I -know, that the existence of such a general oceanic circulation as he -advocates has ever been directly determined from observations. This -mode of circulation is _simply inferred_ or _assumed_ in order to -account for the facts referred to above. “At present,” Dr. Carpenter -says, “I claim for it no higher character than that of a good working -_hypothesis_ to be used as a guide in further inquiry” (§ 16); and lest -there should be any misapprehension on this point, he closes his memoir -thus:—“At present, as I have already said, I claim for the doctrine of -a general oceanic circulation no higher a character than that of a good -working _hypothesis_ consistent with our present knowledge of facts, -and therefore entitled to be _provisionally_ adopted for the purpose of -stimulating and directing further inquiry.” - -I am unable to agree with him, however, on this latter point. It -seems to me that there is no necessity for adopting any hypothetical -mode of circulation to account for the facts, as they can be quite -well accounted for by means of that mode of circulation which does -_actually exist_. It has been determined from direct observation that -surface-currents flow from equatorial to polar regions, and their -paths have been actually mapped out. But if it is established that -currents flow from equatorial to polar regions, it is equally so that -return currents flow from polar to equatorial regions; for if the one -_actually_ exists, the other of necessity _must_ exist. We know also -on physical grounds, to which I have already referred, and which fall -to be considered more fully in a subsequent chapter, that a very large -portion of the water flowing from polar to equatorial regions must -be in the form of under currents. If there are cold under currents, -therefore, flowing from polar to temperate and equatorial regions, -this is all that we really require to account for the cold water which -is found to occupy the bed of the ocean in those regions. It does not -necessarily follow, because cold water may be found at the bottom of -the ocean all along the equator, that there must be a direct flow -from the polar regions to every point of the equator. Water brought -constantly from the polar regions to various points along the equator -by means of under currents will necessarily accumulate, and in course -of time spread over the bottom of the inter-tropical seas. It must -either do this, or the currents on reaching the equator must bend -upwards and flow to the surface in an unbroken mass. Considerable -portions of some of those currents may no doubt do so and join -surface-currents; but probably the greater portion of the water coming -from polar regions extends itself over the floor of the equatorial -seas. In a letter in _Nature_, January 11, 1872, I endeavoured to show -that the surface-currents of the ocean are not separate and independent -of one another, but form one grand system of circulation, and that -the impelling cause keeping up this system of circulation is not the -_trade-winds_ alone, as is generally supposed, but the _prevailing -winds of the entire globe considered also as one grand system_. The -evidence for this opinion, however, will be considered more fully in -the sequel. - -Although the under currents are parts of one general system of oceanic -circulation produced by the impulse of the system of prevailing winds, -yet their direction and position are nevertheless, to a large extent, -determined by different laws. The water at the surface, being moved -by the force of the wind, will follow the path of _greatest pressure -and traction_,—the effects resulting from the general contour of the -land, which to a great extent are common to both sets of currents, not -being taken into account; while, on the other hand, the under currents -from polar regions (which to a great extent are simply “indraughts” -compensating for the water drained from equatorial regions by the -Gulf-stream and other surface currents) will follow, as a general rule, -the path of _least resistance_. - -_The Cause assigned for the Hypothetical Mode of Circulation._—Dr. -Carpenter assigns a cause for his mode of circulation; and that cause -he finds in the difference of specific gravity between equatorial -and polar waters, resulting from the difference of temperature -between these two regions. “Two separate questions,” he says, “have -to be considered, which have not, perhaps, been kept sufficiently -distinct, either by Mr. Croll or by myself;—_first_, whether there -is adequate evidence of the existence of a general vertical oceanic -circulation; and _second_, whether, supposing its existence to be -provisionally admitted, a _vera causa_ can be found for it in the -difference of temperature between the oceanic waters of the polar and -equatorial areas” (§ 17). It seems to me that the facts adduced by -Dr. Carpenter do not necessarily require the assumption of any such -mode of circulation as that advanced by him. The phenomena can be -satisfactorily accounted for otherwise; and therefore there does not -appear to be any necessity for considering whether his hypothesis be -sufficient to produce the required effect or not. - -_An important Consideration overlooked._—But there is one important -consideration which seems to have been overlooked—namely, the fact -that the sea is salter in inter-tropical than in polar regions, and -that this circumstance, so far as it goes, must tend to neutralize -the effect of difference of temperature. It is probable, indeed, that -the effect produced by difference of temperature is thus entirely -neutralized, and that no difference of density whatever exists between -the sea in inter-tropical and polar regions, and consequently that -there is no difference of level nor anything to produce such a general -motion as Dr. Carpenter supposes. This, I am glad to find, is the -opinion of Professor Wyville Thomson. - -“I am greatly mistaken,” says that author, “if the low specific gravity -of the polar sea, the result of the condensation and precipitation -of vapour evaporated from the inter-tropical area, do not fully -counterbalance the contraction of the superficial film by arctic -cold.... Speaking in the total absence of all reliable data, it is my -general impression that if we were to set aside all other agencies, and -to trust for an oceanic circulation to those conditions only which are -relied upon by Dr. Carpenter, if there were any general circulation at -all, which seems very problematical, the odds are rather in favour of -a warm under current travelling northwards by virtue of its excess of -salt, balanced by a surface return current of fresher though colder -arctic water.”[62] - -This is what actually takes place on the west and north-west of -Spitzbergen. There the warm water of the Gulf-stream flows underneath -the cold polar current. And it is the opinion of Dr. Scoresby, Mr. -Clements Markham, and Lieut. Maury that this warm water, in virtue -of its greater saltness, is denser than the polar water. Mr. Leigh -Smith found on the north-west of Spitzbergen the temperature at 500 -fathoms to be 52°, and once even 64°, while the water on the surface -was only a degree or two above freezing.[63] Mr. Aitken, of Darroch, -in a paper lately read before the Royal Scottish Society of Arts, -showed experimentally that the polar water in regions where the ice is -melting is actually less dense than the warm and more salt tropical -waters. Nor will it help the matter in the least to maintain that -difference of specific gravity is not the reason why the warm water of -the Gulf-stream passes under the polar stream—because if difference -of specific gravity be not the cause of the warm water underlying the -cold water in polar regions, then difference of specific gravity may -likewise not be the cause of the cold water underlying the warm at -the equator; and if so, then there is no necessity for the gravitation -hypothesis of oceanic circulation. - -There is little doubt that the super-heated stratum at the surface of -the inter-tropical seas, which stratum, according to Dr. Carpenter, -is of no great thickness, is less dense than the polar water: but if -we take a column extending from the surface down to the bottom of the -ocean, this column at the equator will be found to be as heavy as one -of equal length in the polar area. And if this be the case, then there -can be no difference of level between the equator and the poles, and -no disturbance of static equilibrium nor anything else to produce -circulation. - -_Under Currents account for all the Facts better than Dr. Carpenter’s -Hypothesis._—Assuming, for the present, the system of prevailing winds -to be the true cause of oceanic currents, it necessarily follows (as -will be shown hereafter) that a large quantity of Atlantic water must -be propelled into the Arctic Ocean; and such, as we know, is actually -the case. The Arctic Ocean, however, as Professor Wyville Thomson -remarks, is a well-nigh closed basin, not permitting of a free outflow -into the Pacific Ocean of the water impelled into it. - -But it is evident that the water which is thus being constantly -carried from the inter-tropical to the arctic regions must somehow -or other find its way back to the equator; in other words, there -must be a return current equal in magnitude to the direct current. -Now the question to be determined is, what path must this return -current take? It appears to me that it will take the _path of least -resistance_, whether that path may happen to be at the surface or under -the surface. But that the path of least resistance will, as a general -rule, lie at a very considerable distance below the surface is, I -think, evident from the following considerations. At the surface the -general direction of the currents is opposite to that of the return -current. The surface motion of the water in the Atlantic is from the -equator to the pole; but the return current must be from the pole to -the equator. Consequently the surface currents will oppose the motion -of any return current unless that current lie at a considerable depth -below the surface currents. Again, the winds, as a general rule, blow -in an opposite direction to the course of the return current, because, -according to supposition, the winds blow in the direction of the -surface currents. From all these causes the path of least resistance to -the return current will, as a general rule, not be at the surface, but -at a very considerable depth below it. - -A large portion of the water from the polar regions no doubt leaves -those regions as surface currents; but a surface current of this kind, -on meeting with some resistance to its onward progress along the -surface, will dip down and continue its course as an under current. We -have an example of this in the case of the polar current, which upon -meeting the Gulf-stream on the banks of Newfoundland divides—a portion -of it dipping down and pursuing its course underneath that stream into -the Gulf of Mexico and the Caribbean Sea. And that this under current -is a real and tangible current, in the proper sense of the term, and -not an imperceptible movement of the water, is proved by the fact that -large icebergs deeply immersed in it are often carried southward with -considerable velocity against the united force of the wind and the -Gulf-stream. - -Dr. Carpenter refers at considerable length (§ 134) to Mr. Mitchell’s -opinion as to the origin of the polar current, which is the same as -that advanced by Maury, viz., that the impelling cause is difference -of specific gravity. But although Dr. Carpenter quotes Mr. Mitchell’s -opinion, he nevertheless does not appear to adopt it: for in §§ 90−93 -and various other places he distinctly states that he does not agree -with Lieut. Maury’s view that the Gulf-stream and polar current -are caused by difference of density. In fact, Dr. Carpenter seems -particularly anxious that it should be clearly understood that he -dissents from the theory maintained by Maury. But he does not merely -deny that the Gulf-stream and polar current can be caused by difference -of density; he even goes so far as to affirm that no sensible current -whatever can be due to that cause, and adduces the authority of Sir -John Herschel in support of that opinion:—“The doctrine of Lieut. -Maury,” he says, “was powerfully and convincingly opposed by Sir -John Herschel; who showed, beyond all reasonable doubt, first, that -the Gulf-stream really has its origin in the propulsive force of the -trade-winds, and secondly, that the greatest disturbance of equilibrium -which can be supposed to result from the agencies invoked by Lieut. -Maury would be utterly inadequate to generate and maintain either the -Gulf-stream or any other sensible current” (§ 92). This being Dr. -Carpenter’s belief, it is somewhat singular that he should advance the -case of the polar current passing under the Gulf-stream as evidence -in favour of his theory; for in reality he could hardly have selected -a case more hostile to that theory. In short, it is evident that, if -a polar current impelled by a force other than that of gravity can -pass from the banks of Newfoundland to the Gulf of Mexico (a distance -of some thousands of miles) under a current flowing in the opposite -direction and, at the same time, so powerful as the Gulf-stream, it -could pass much more easily under comparatively still water, or water -flowing in the same direction as itself. And if this be so, then all -our difficulties disappear, and we satisfactorily explain the presence -of cold polar water at the bottom of inter-tropical seas without having -recourse to the hypothesis advanced by Dr. Carpenter. - -But we have an example of an under current more inexplicable on the -gravitation hypothesis than even that of the polar current, viz., the -warm under current of Davis Strait. - -There is a strong current flowing north from the Atlantic through Davis -Strait into the Arctic Ocean underneath a surface current passing -southwards in an opposite direction. Large icebergs have been seen to -be carried northwards by this under current at the rate of four knots -an hour against both the wind and the surface current, ripping and -tearing their way with terrific force through surface ice of great -thickness.[64] A current so powerful and rapid as this cannot, as Dr. -Carpenter admits, be referred to difference of specific gravity. But -even supposing that it could, still difference of temperature between -the equatorial and polar seas would not account for it; for the current -in question flows in the _wrong direction_. Nor will it help the matter -the least to adopt Maury’s explanation, viz., that the warm under -current from the south, in consequence of its greater saltness, is -denser than the cold one from the polar regions. For if the water of -the Atlantic, notwithstanding its higher temperature, is in consequence -of its greater saltness so much denser than the polar water on the -west of Greenland as to produce an under current of four knots an hour -in the direction of the pole, then surely the same thing to a certain -extent will hold true in reference to the ocean on the east side of -Greenland. Thus instead of there being, as Dr. Carpenter supposes, -an underflow of polar water south into the Atlantic in virtue of its -_greater_ density, there ought, on the contrary, to be a surface flow -in consequence of its lesser density. - -The true explanation no doubt is, that the warm under current from -the south and the cold upper current from the north are both parts -of one grand system of circulation produced by the winds, difference -of specific gravity having no share whatever either in impelling the -currents, or in determining which shall be the upper and which the -lower. - -The wind in Baffin’s Bay and Davis Strait blows nearly always in one -direction, viz. from the north. The tendency of this is to produce a -surface or upper current from the north down into the Atlantic, and to -prevent or retard any surface current from the south. The warm current -from the Atlantic, taking the path of least resistance, dips under the -polar current and pursues its course as an under current. - -Mr. Clements Markham, in his “Threshold of the Unknown Region,” is -inclined to attribute the motion of the icebergs to tidal action or -to counter under currents. That the motion of the icebergs cannot -reasonably be attributed to the tides is, I think, evident from the -descriptions given both by Midshipman Griffin and by Captain Duncan, -who distinctly saw the icebergs moving at the rate of about four knots -an hour against a surface current flowing southwards. And Captain -Duncan states that the bergs continued their course northwards for -several days, till they ultimately disappeared. The probability is that -this northward current is composed partly of Gulf-stream water and -partly of that portion of polar water which is supposed to flow round -Cape Farewell from the east coast of Greenland. This stream, composed -of both warm and cold water, on reaching to about latitude 65°N., where -it encounters the strong northerly winds, dips down under the polar -current and continues its northward course as an under current. - -We have on the west of Spitzbergen, as has already been noticed, a -similar example of a warm current from the south passing under a polar -current. A portion of the Gulf-stream which passes round the west -coast of Spitzbergen flows under an arctic current coming down from -the north; and it does so no doubt because it is here in the region of -prevailing northerly winds, which favour the polar current but oppose -the Gulf-stream. Again, we have a cold and rapid current sweeping -round the east and south of Spitzbergen, a current of which Mr. Lamont -asserts that he is positive he has seen it running at the rate of seven -or eight miles an hour. This current, on meeting the Gulf-stream about -the northern entrance to the German Ocean, dips down under that stream -and pursues its course southwards as an under current. - -Several other cases of under currents might be adduced which cannot -be explained on the gravitation theory, and which must be referred to -a system of oceanic circulation produced by the impulse of the wind; -but these will suffice to show that the assumption that the winds can -produce only a mere surface-drift is directly opposed to facts. And -it will not do to affirm that a current which forms part of a general -system of circulation produced by the impulse of the winds cannot -possibly be an under current; for in the case referred to we have -proof that the thing is not only possible but actually exists. This -point, however, will be better understood after we have considered the -evidence in favour of a general system of oceanic currents. - -Much of the difficulty experienced in comprehending how under currents -can be produced by the wind, or how an impulse imparted to the surface -of the ocean can ever be transmitted to the bottom, appears to me to -result, to a considerable extent at least, from a slight deception -of the imagination. The thing which impresses us most forcibly in -regard to the ocean is its profound depth. A mean depth of, say, three -miles produces a striking impression; but if we could represent to -the mind the vast area of the ocean as correctly as we can its depth, -_shallowness_ rather than _depth_ would be the impression produced. If -in crossing a meadow we found a sheet of water one hundred yards in -diameter and only an inch in depth, we should not call that a _deep_, -but a very _shallow_ pool. The probability is that we should speak of -it as simply a piece of ground covered with a thin layer of water. -Yet such a thin layer of water would be a correct representation in -miniature of the ocean; for the ocean in relation to its superficial -area is as shallow as the pool of our illustration. In reference to -such a pool or thin film of water, we have no difficulty in conceiving -how a disturbance on its surface would be transmitted to its bottom. -In fact our difficulty is in conceiving how any disturbance extending -over its entire surface should not extend to the bottom. Now if we -could form as accurate a sensuous impression of the vast area of the -ocean as we do of such a pool, all our difficulty in understanding how -the impulses of the wind acting on the vast area of the ocean should -communicate motion down to its bottom would disappear. It is certainly -true that sudden commotions caused by storms do not generally extend to -great depths. Neither will winds of short continuance produce a current -extending far below the surface. But prevailing winds which can produce -such immense surface-flow as that of the great equatorial currents of -the globe and the Gulf-stream, which follow definite directions, must -communicate their motion to great depths, unless water be frictionless, -a thing which it is not. Suppose the upper layer of the ocean to be -forced on by the direct action of the winds with a constant velocity -of, say, four miles an hour, the layer immediately below will be -dragged along with a constant velocity somewhat less than four miles -an hour. The layer immediately below this second layer will in turn be -also dragged along with a constant velocity somewhat less than the one -above it. The same will take place in regard to each succeeding layer, -the constant velocity of each layer being somewhat less than the one -immediately above it, and greater than the one below it. The question -to be determined is, at what depth will all motion cease? I presume -that at present we have not sufficient data for properly determining -this point. The depth will depend, other things being equal, upon the -amount of molecular resistance offered by the water to motion—in other -words, on the amount of the shearing-force of the one layer over the -other. The fact, however, that motion imparted to the surface will -extend to great depths can be easily shown by direct experiment. If a -constant motion be imparted to the surface of water, say, in a vessel, -motion will ultimately be communicated to the bottom, no matter how -wide or how deep the vessel may be. The same effect will take place -whether the vessel be 5 feet deep or 500 feet deep. - -_The known Condition of the Ocean inconsistent with Dr. Carpenter’s -Hypothesis._—Dr. Carpenter says that he looks forward with great -satisfaction to the results of the inquiries which are being prosecuted -by the Circumnavigation Expedition, in the hope that the facts brought -to light may establish his theory of a general oceanic circulation; and -he specifies certain of these facts which, if found to be correct, will -establish his theory. It seems to me, however, that the facts to which -he refers are just as explicable on the theory of under currents as on -the theory of a general oceanic circulation. He begins by saying, “If -the views I have propounded be correct, it may be expected that near -the border of the great antarctic ice-barrier a temperature below 30° -will be met with (as it has been by Parry, Martens, and Weyprecht near -Spitzbergen) at no great depth beneath the surface, and that instead of -rising at still greater depths, the thermometer will fall to near the -freezing-point of salt water” (§ 39). - -Dr. Carpenter can hardly claim this as evidence in favour of his -theory; for near the borders of the ice-barrier the water, as a matter -of course, could not be expected to have a much higher temperature than -the ice itself. And if the observations be made during summer months, -the temperature of the water at the surface will no doubt be found to -be higher than that of the bottom; but if they be carried on during -winter, the surface-temperature will doubtless be found to be as low as -the bottom-temperature. These are results which do not depend upon any -particular theory of oceanic circulation. - -“The bottom temperature of the North Pacific,” he continues, “will -afford a crucial test of the truth of the doctrine. For since the sole -communication of this vast oceanic area with the arctic basin is a -strait so shallow as only to permit an inflow of warm surface water, -its deep cold stratum must be entirely derived from the antarctic area; -and if its bottom temperature is not actually higher than that of the -South Pacific, the glacial stratum ought to be found at a greater depth -north of the equator than south of it” (§ 39). - -This may probably show that the water came from the antarctic regions, -but cannot possibly prove that it came in the manner which he supposes. - -“In the North Atlantic, again, the comparative limitation of -communication with the arctic area may be expected to prevent its -bottom temperature from being reduced as low as that of the Southern -Atlantic” (§ 39). Supposing the bottom temperature of the South -Atlantic should be found to be lower than the bottom temperature of the -North Atlantic, this fact will be just as consistent with the theory of -under currents as with his theory of a general movement of the ocean. - -I am also wholly unable to comprehend how he should imagine, because -the bottom temperature of the South Atlantic happens to be lower, and -the polar water to lie nearer to the surface in this ocean than in the -North Atlantic, that therefore this proves the truth of his theory. -This condition of matters is just as consistent, and even more so, as -will be shown in Chapter XIII., with my theory as with his. When we -consider the immense quantity of warm surface water which, as has been -shown (Chapter V.), is being constantly transferred from the South into -the North Atlantic, we readily understand how the polar water comes -nearer to the surface in the former ocean than in the latter. Every -pound of water, of course, passing from the southern to the northern -hemisphere must be compensated by an equal amount passing from the -northern to the southern hemisphere. But nevertheless the warm water -drained off the South Atlantic is not replaced directly by water from -the north, but by that cold antarctic current, the existence of which -is, unfortunately, too well known to navigators from the immense masses -of icebergs which it brings along with it. In fact, the whole of the -phenomena are just as easily explained upon the principle of under -currents as upon Dr. Carpenter’s theory. But we shall have to return to -this point in Chapter XIII., when we come to discuss a class of facts -which appear to be wholly irreconcilable with the gravitation theory. - -Indeed I fear that even although Dr. Carpenter’s expectations should -eventually be realised in the results of the Circumnavigation -Expedition, yet the advocates of the wind theory will still remain -unconverted. In fact the Director of this Expedition has already, on -the wind theory, offered an explanation of nearly all the phenomena -on which Dr. Carpenter relies;[65] and the same has also been done by -Dr. Petermann,[66] who, as is well known, is equally opposed to Dr. -Carpenter’s theory. Dr. Carpenter directs attention to the necessity of -examining the broad and deep channel separating Iceland from Greenland. -The observations which have already been made, however, show that -nearly the entire channel is occupied, on the surface at least, by -water flowing southward from the polar area—a direction the opposite of -what it ought to be according to the gravitation theory. In fact the -surface of one half of the entire area of the ocean, extending from -Greenland to the North Cape, is moving in a direction the opposite of -that which it ought to take according to the theory under review. The -western half of this area is occupied by water which at the surface is -flowing southwards; while the eastern half, which has hitherto been -regarded by almost everybody but Dr. Carpenter himself and Mr. Findlay -as an extension of the Gulf-stream, is moving polewards. The motion of -the western half must be attributed to the winds and not to gravity; -for it is moving in the wrong direction to be accounted for by the -latter cause; but had it been moving in the opposite direction, no -doubt its motion would have been referred to gravitation. To this cause -the motion of the eastern half, which is in the proper direction, is -attributed;[67] but why not assign this motion also to the impulse of -the winds, more especially since the direction of the prevailing winds -blowing over that area coincides with that of the water? If the wind -can produce the motion of the water in the western half, why may not it -do the same in the eastern half? - -If there be such a difference of density between equatorial and polar -waters as to produce a general flow of the upper portion of the ocean -poleward, how does it happen that one half of the water in the above -area is moving in opposition to gravity? How is it that in a wide -open sea gravitation should act so powerfully in the one half of it -and with so little effect in the other half? There is probably little -doubt that the ice-cold water of the western half extends from the -surface down to the bottom. And it is also probable that the bottom -water is moving southwards in the same direction as the surface water. -The bottom water in such a case would be moving in harmony with the -gravitation theory; but would Dr. Carpenter on this account attribute -its motion to gravity? Would he attribute the motion of the lower half -to gravity and the upper half to the wind? He could not in consistency -with his theory attribute the motion of the upper half to gravity: for -although the ice-cold water extended to the surface, this could not -explain how gravity should move it southward instead of polewards, as -according to theory it ought to move. He might affirm, if he chose, -that the surface water moves southwards because it is dragged forward -by the bottom water; but if this view be held, he is not entitled to -affirm, as he does, that the winds can only produce a mere surface -drift. If the viscosity and molecular resistance of water be such that, -when the lower strata of the ocean are impelled forward by gravity or -by any other cause, the superincumbent strata extending to the surface -are perforce dragged after them, then, for the same reason, when the -upper strata are impelled forward by the wind or any other cause, the -underlying strata must also be dragged along after them. - -If the condition of the ocean between Greenland and the north-western -shore of Europe is irreconcilable with the gravitation theory, we find -the case even worse for that theory when we direct our attention to -the condition of the ocean on the southern hemisphere; for according -to the researches of Captain Duperrey and others on the currents of -the Southern Ocean, a very large portion of the area of that ocean is -occupied by water moving on the surface more in a northward than a -poleward direction. Referring to the deep trough between the Shetland -and the Faroe Islands, called by him the “Lightning Channel,” Dr. -Carpenter says, “If my view be correct, a current-drag suspended in -the _upper_ stratum ought to have a perceptible movement in the N.E. -direction; whilst another, suspended in the _lower_ stratum, should -move S.W.” (§ 40). - -Any one believing in the north-eastern extension of the Gulf-stream -and in the Spitsbergen polar under current, to which I have already -referred, would not feel surprised to learn that the surface strata -have a perceptible north-eastward motion, and the bottom strata a -perceptible south-westward motion. North-east and east of Iceland -there is a general flow of cold polar water in a south-east direction -towards the left edge of the Gulf-stream. This water, as Professor Mohn -concludes, “descends beneath the Gulf-stream and partially finds an -outlet in the lower half of the Faroe-Shetland channel.”[68] - -_An Objection Considered._—In Nature, vol. ix. p. 423, Dr. Carpenter -has advanced the following objection to the foregoing theory of -under-currents:—“According to Mr. Croll’s doctrine, the whole of that -vast mass of water in the North Atlantic, averaging, say, 1,500 fathoms -in thickness and 3,600 miles in breadth, the temperature of which -(from 40° downwards), as ascertained by the _Challenger_ soundings, -clearly shows it to be mainly derived from a polar source, is nothing -else than _the reflux of the Gulf-stream_. Now, even if we suppose -that the whole of this stream, as it passes Sandy Hook, were to go on -into the closed arctic basin, it would only force out an equivalent -body of water. And as, on comparing the sectional areas of the two, -I find that of the Gulf-stream to be about 1/900th that of the North -Atlantic underflow; and as it is admitted that a large part of the -Gulf-stream returns into the Mid-Atlantic circulation, only a branch of -it going on to the north-east, the extreme improbability (may I not say -impossibility?) that so vast a mass of water can be put in motion by -what is by comparison a mere rivulet (the north-east motion of which, -as a distinct current, has not been traced eastward of 30° W. long.) -seems still more obvious.” - -In this objection three things are assumed: (1) that the mass of cold -water 1,500 fathoms deep and 3,600 miles in breadth is in a state of -motion towards the equator; (2) that it cannot be the reflux of the -Gulf-stream, because its sectional area is 900 times as great as that -of the Gulf-stream; (3) that the immense mass of water is, according to -my views, set in motion by the Gulf-stream. - -As this objection has an important bearing on the question under -consideration, I shall consider these three assumptions separately -and in their order: (1) That this immense mass of cold water came -originally from the polar regions I, of course, admit, but that the -whole is in a state of motion I certainly do not admit. There is no -warrant whatever for any such assumption. According to Dr. Carpenter -himself, the heating-power of the sun does not extend to any great -depth below the surface; consequently there is nothing whatever to -heat this mass but the heat coming through the earth’s crust. But -the amount of heat derived from this source is so trifling, that an -under current from the arctic regions far less in volume than that -of the Gulf-stream would be quite sufficient to keep the mass at an -ice-cold temperature. Taking the area of the North Atlantic between -the equator and the Tropic of Cancer, including also the Caribbean -Sea and the Gulf of Mexico, to be 7,700,000 square miles, and the -rate at which internal heat passes through the earth’s surface to be -that assigned by Sir William Thomson, we find that the total quantity -of heat derived from the earth’s crust by the above area is equal to -about 88 × 10^{15} foot-pounds per day. But this amount is equal to -only 1/894th that conveyed by the Gulf-stream, on the supposition that -each pound of water carries 19,300 foot-pounds of heat. Consequently -an under current from the polar regions of not more than 1/35th the -volume of the Gulf-stream would suffice to keep the entire mass of -water of that area within 1° of what it would be were there no heat -derived from the crust of the earth; that is to say, were the water -conveyed by the under current at 32°, internal heat would not maintain -the mass of the ocean in the above area at more than 33°. The entire -area of the North Atlantic from the equator to the arctic circle is -somewhere about 16,000,000 square miles. An under current of less than -1/17th that of the Gulf-stream coming from the arctic regions would -therefore suffice to keep the entire North Atlantic basin filled with -ice-cold water. In short, whatever theory we adopt regarding oceanic -circulation, it follows equally as a necessary consequence that the -entire mass of the ocean below the stratum heated by the sun’s rays -must consist of cold water. For if cold water be continually coming -from the polar regions either in the form of under currents, or in the -form of a general underflow as Dr. Carpenter supposes, the entire under -portion of the ocean must ultimately become occupied by cold water; for -there is no source from which this influx of water can derive heat, -save from the earth’s crust. But the amount thus derived is so trifling -as to produce no sensible effect. For example, a polar under current -one half the size of the Gulf-stream would be sufficient to keep the -entire water of the globe (below the stratum heated by the sun’s rays) -at an ice-cold temperature. Internal heat would not be sufficient under -such circumstances to maintain the mass 1° Fahr. above the temperature -it possessed when it left the polar regions. - -It follows therefore that the presence of the immense mass of ice-cold -water in the great depths of the ocean is completely accounted for by -under currents, and there is no necessity for supposing it to be all -in a state of motion towards the equator. In fact, this very state of -things, which the general oceanic circulation hypothesis was devised to -explain, results as a necessary consequence of polar under currents. -Unless these were entirely stopped it is physically impossible that the -ocean could be in any other condition. - -But suppose that this immense mass of cold water occupying the great -depths of the ocean were, as Dr. Carpenter assumes it to be, in a -state of constant motion towards the equator, and that its sectional -area were 900 times that of the Gulf-stream, it would not therefore -follow that the quantity of water passing through this large sectional -area must be greater than that flowing through a sectional area of -the Gulf-stream; for the quantity of water flowing through this large -sectional area depends entirely on the rate of motion. - -I am wholly unable to understand how it could be supposed that this -underflow, according to my view, is set in motion by the Gulf-stream, -seeing that I have shown that the return under current is as much due -to the impulse of the wind as the Gulf-stream itself. - -Dr. Carpenter lays considerable stress on the important fact -established by the _Challenger_ expedition, that the great depths of -the sea in equatorial regions are occupied by ice-cold water, while -the portion heated by the sun’s rays is simply a thin stratum at the -surface. It seems to me that it would be difficult to find a fact more -hostile to his theory than this. Were it not for this upper stratum -of heated water there would be no difference between the equatorial -and polar columns, and consequently nothing to produce motion. But the -thinner this stratum is the less is the difference, and the less there -is to produce motion. - - - - - CHAPTER IX. - - EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC - CIRCULATION.—THE MECHANICS OF DR. CARPENTER’S THEORY. - - Experimental Illustration of the Theory.—The Force exerted by - Gravity.—Work performed by Gravity.—Circulation not by - Convection.—Circulation depends on Difference in Density - of the Equatorial and Polar Columns.—Absolute Amount of - Work which can be performed by Gravity.—How Underflow is - produced.—How Vertical Descent at the Poles and Ascent at - the Equator is produced.—The Gibraltar Current.—Mistake in - Mechanics concerning it.—The Baltic Current. - - -_Experiment to illustrate Theory._—In support of the theory of a -general movement of water between equatorial and polar regions, Dr. -Carpenter adduces the authority of Humboldt and of Prof. Buff.[69] -I have been unable to find anything in the writings of either from -which it can be inferred that they have given this matter special -consideration. Humboldt merely alludes to the theory, and that in the -most casual manner; and that Prof. Buff has not carefully investigated -the subject is apparent from the very illustration quoted by Dr. -Carpenter from the “Physics of the Earth.” “The water of the ocean at -great depths,” says Prof. Buff, “has a temperature, even under the -equator, nearly approaching to the freezing-point. This low temperature -cannot depend on any influence of the sea-bottom.... The fact, however, -is explained by a continual current of cold water flowing from the -polar regions towards the equator. The following well-known experiment -clearly illustrates the manner of this movement. A glass vessel is to -be filled with water with which some powder has been mixed, and is then -to be heated _at bottom_. It will soon be seen, from the motion of the -particles of powder, that currents are set up in opposite directions -through the water. Warm water rises from the bottom up through the -middle of the vessel, and spreads over the surface, while the colder -and therefore heavier liquid falls down at the sides of the glass.” - -This illustration is evidently intended to show not merely the form -and direction of the great system of oceanic circulation, but also the -mode in which the circulation is induced by heat. It is no doubt true -that if we apply heat (say that of a spirit-lamp) to the bottom of a -vessel filled with water, the water at the bottom of the vessel will -become heated and rise to the surface; and if the heat be continued -an ascending current of warm water will be generated; and this, of -course, will give rise to a compensating under current of colder water -from all sides. In like manner it is also true that, if heat were -applied to the bottom of the ocean in equatorial regions, an ascending -current of hot water would be also generated, giving rise to an under -current of cold water from the polar regions. But all this is the -diametrically opposite of what actually takes place in nature. The heat -is not applied to the bottom of the ocean, so as to make the water -there lighter than the water at the surface, and thus to generate an -ascending current; but the heat is applied to the surface of the ocean, -and the effect of this is to prevent an ascending current rather than -to produce one, for it tends to keep the water at the surface lighter -than the water at the bottom. In order to show how the heat of the sun -produces currents in the ocean, Prof. Buff should have applied the -heat, not to the bottom of his vessel, but to the upper surface of the -water. But this is not all, the form of the vessel has something to -do with the matter. The wider we make the vessel in proportion to its -depth, the more difficult it is to produce currents by means of heat. -But in order to represent what takes place in nature, we ought to have -the same proportion between the depth and the superficial area of the -water in our vessel as there is between the depth and the superficial -area of the sea. The mean depth of the sea may be taken roughly to be -about three miles.[70] The distance between pole and pole we shall take -in round numbers to be 12,000 miles. The sun may therefore be regarded -as shining upon a circular sea 12,000 miles in diameter and three miles -deep. The depth of the sea to its diameter is therefore as 1 to 4,000. -Suppose, now, that in our experiment we make the depth of our vessel -one inch, we shall require to make its diameter 4,000 inches, or 333 -feet, say, in round numbers, 100 yards in diameter. Let us, then, take -a pool of water 100 yards in diameter, and one inch deep. Suppose the -water to be at 32°. Apply heat to the upper surface of the pool, so -as to raise the temperature of the surface of the water to 80° at the -centre of the pool, the temperature diminishing towards the edge, where -it is at 32°. It is found that at a depth of two miles the temperature -of the water at the equator is about as low as that of the poles. We -must therefore suppose the water at the centre of our pool to diminish -in temperature from the surface downwards, so that at a depth of half -an inch the water is at 32°. We have in this case a thin layer of warm -water half an inch thick at the centre, and gradually thinning off to -nothing at the edge of the pool. The lightest water, be it observed, -is at the surface, so that an ascending or a descending current -is impossible. The only way whereby the heat applied can have any -tendency to produce motion is this:—The heating of the water expands -it, consequently the surface of the pool must stand at a little higher -level at its centre than at its edge, where no expansion takes place; -and therefore, in order to restore the level of the pool, the water at -the centre will tend to flow towards the sides. But what is the amount -of this tendency? Its amount will depend upon the amount of slope, -but the slope in the case under consideration amounts to only 1 in -7,340,000. - -_Dr. Carpenter’s Experiment._—In order to obviate the objection to -Professor Buff’s experiment Dr. Carpenter has devised another mode. -But I presume his experiment was intended rather to illustrate the way -in which the circulation of the ocean, according to his theory, takes -place, than to prove that it actually does take place. At any rate, all -that can be claimed for the experiment is the proof that water will -circulate in consequence of difference of specific gravity resulting -from difference of temperature. But this does not require proof, for no -physicist denies it. The point which requires to be proved is this. Is -the difference of specific gravity which exists in the ocean sufficient -to produce the supposed circulation? Now his mode of experimenting -will not prove this, unless he makes his experiment agree with the -conditions already stated. - -But I decidedly object to the water being heated in the way in which it -has been done by him in his experiment before the Royal Geographical -Society; for I feel somewhat confident that in this experiment the -circulation resulted not from difference of specific gravity, as was -supposed, but rather from the way in which the heat was applied. In -that experiment the one half of a thick metallic plate was placed in -contact with the upper surface of the water at one end of the trough; -the other half, projecting over the end of the trough, was heated -by means of a spirit-lamp. It is perfectly obvious that though the -temperature of the great mass of the water under the plate might not -be raised over 80° or so, yet the molecules in contact with the metal -would have a very high temperature. These molecules, in consequence of -their expansion, would be unable to sink into the cooler and denser -water underneath, and thus escape the heat which was being constantly -communicated to them from the heated plate. But escape they must, or -their temperature would continue to rise until they would ultimately -burst into vapour. They cannot ascend, neither can they descend: they -therefore must be expelled by the heat from the plate in a horizontal -direction. The next layer of molecules from beneath would take their -place and would be expelled in a similar manner, and this process would -continue so long as the heat was applied to the plate. A circulation -would thus be established by the direct expansive force of vapour, and -not in any way due to difference of specific gravity, as Dr. Carpenter -supposes. - -But supposing the heated bar to be replaced by a piece of ice, -circulation would no doubt take place; but this proves nothing more -than that difference of density will produce circulation, which is what -no one calls in question. - -The case referred to by Dr. Carpenter of the heating apparatus in -London University is also unsatisfactory. The water leaves the boiler -at 120° and returns to it at 80°. The difference of specific gravity -between the water leaving the boiler and the water returning to it -is supposed to produce the circulation. It seems to me that this -difference of specific gravity has nothing whatever to do with the -matter. The cause of the circulation must be sought for in the boiler -itself, and not in the pipes. The heat is applied to the bottom of -the boiler, not to the top. What is the temperature of the molecules -in contact with the bottom of the boiler directly over the fire, is -a question which must be considered before we can arrive at a just -determination of the causes which produce circulation in the pipes -of a heating apparatus such as that to which Dr. Carpenter refers. -But, in addition to this, as the heat is applied to the bottom of the -boiler and not to the top, convection comes into play, a cause which, -as we shall find, does not come into play in the theory of oceanic -circulation at present under our consideration. - -_The Force exerted by Gravity._—Dr. Carpenter speaks of his doctrine of -a general oceanic circulation sustained by difference of temperature -alone, “as one of which physical geographers could not recognise the -importance, so long as they remained under the dominant idea that -the temperature of the deep sea is everywhere 39°.” And he affirms -that “until it is clearly apprehended that sea-water becomes more and -more dense as its temperature is reduced, the immense motive power of -polar cold cannot be understood.” But in chap. vii. and also in the -Phil. Mag. for October, 1870 and 1871, I proved that if we take 39° -as the temperature of maximum density the force exerted by gravity -tending to produce circulation is just as great as when we take 32°. -The reason for this is that when we take 32° as the temperature of -maximum density, although we have, it is true, a greater elevation of -the ocean above the place of maximum density, yet this latter occurs at -the poles; while on the other hand, when we take 39°, the difference -of level is less—the place not being at the poles but in about lat. -56°. Now the shorter slope from the equator to lat. 56° is as steep as -the larger one from the equator to the poles, and consequently gravity -exerts as much force in the production of motion in the one case as in -the other. Sir John Herschel, taking 39° as the temperature of maximum -density, estimated the slope at 1/32nd of an inch per mile, whereas -we, taking 32° as the actual temperature of maximum density of the -polar seas and calculating from modern data, find that the slope is not -one-half that amount, and that the force of gravity tending to produce -circulation is much less than Herschel concluded it to be. The reason, -therefore, why physical geographers did not adopt the theory that -oceanic circulation is the result of difference of temperature could -not possibly be the one assigned by Dr. Carpenter, viz., that they had -under-estimated the force of gravity by taking 39° instead of 32° as -the temperature of maximum density. - -_The Work performed by Gravity._—But in order clearly to understand -this point, it will be better to treat the matter according to the -third method, and consider not the mere _force_ of gravity impelling -the waters, but the amount of _work_ which gravitation is capable of -performing. - -Let us then assume the correctness of my estimate, that the height of -the surface of the ocean at the equator above that at the poles is 4 -feet 6 inches, for in representing the mode in which difference of -specific gravity produces circulation it is of no importance what we -may fix upon as the amount of the slope. In order, therefore, to avoid -fractions of a foot, I shall take the slope at 4 feet instead of 4½ -feet, which it actually is. A pound of water in flowing down this slope -from the equator to either of the poles will perform 4 foot-pounds of -work; or, more properly speaking, gravitation will. Now it is evident -that when this pound of water has reached the pole, it is at the -bottom of the slope, and consequently cannot descend further. Gravity, -therefore, cannot perform any more work upon it; as it can only do so -while the thing acted upon continues to descend—that is, moves under -the force exerted. But the water will not move under the influence -of gravity unless it move downward; it being in this direction only -that gravity acts on the water. “But,” says Dr. Carpenter, “the effect -of surface-cold upon the water of the polar basin will be to reduce -the temperature of its whole mass below the freezing-point of fresh -water, the surface-stratum _sinking_ as it is cooled in virtue of its -diminished bulk and increased density, and being replaced by water not -yet cooled to the same degree.”[71] By the cooling of the whole mass -of polar water by cold and the heating of the water at the equator by -the sun’s rays the polar column of water, as we have seen, is rendered -denser than the equatorial one, and in order that the two may balance -each other, the polar column is necessarily shorter than the equatorial -by 4 feet; and thus it is that the slope of 4 feet is formed. It is -perfectly true that the water which leaves the equator warm and light, -becomes by the time it reaches the pole cold and dense. But unless -it be denser than the underlying polar water it will not sink down -_through_ it.[72] We are not told, however, why it should be colder -than the whole mass underneath, which, according to Dr. Carpenter, -is cooled by polar cold. But that he does suppose it to sink to the -bottom in consequence of its contraction by cold would appear from the -following quotation:— - -“Until it is clearly apprehended that sea-water becomes more and more -dense as its temperature is reduced, and that it consequently continues -to sink until it freezes, the immense motor power of polar cold cannot -be apprehended. But when this has been clearly recognised, it is seen -that the application of _cold at the surface_ is precisely equivalent -as a moving power to that application of _heat at the bottom_ by which -the circulation of water is sustained in every heating apparatus that -makes use of it” (§ 25). - -The application of cold at the surface is thus held to be equivalent -as a motor power to the application of heat at the bottom. But heat -applied to the bottom of a vessel produces circulation by _convection_. -It makes the molecules at the bottom expand, and they, in consequence -of buoyancy, rise _through_ the water in the vessel. Consequently if -the action of cold at the surface in polar regions is equivalent to -that of heat, the cold must contract the molecules at the surface and -make them sink _through_ the mass of polar water beneath. But assuming -this to be the meaning in the passage just quoted, how much colder is -the surface water than the water beneath? Let us suppose the difference -to be one degree. How much work, then, will gravity perform upon this -one pound of water which is one degree colder than the mass beneath -supposed to be at 32°? The force with which the pound of water will -sink will not be proportional to its weight, but to the difference -of weight between it and a similar bulk of the water through which -it sinks. The difference between the weight of a pound of water at -31° and an equal volume of water at 32° is 1/29,000th of a pound. Now -this pound of water in sinking to a depth of 10,000 feet, which is -about the depth at which a polar temperature is found at the equator, -would perform only one-third of a foot-pound of work. And supposing -it were three degrees colder than the water beneath, it would in -sinking perform only one foot-pound. This would give us only 4 + 1 = 5 -foot-pounds as the total amount that could be performed by gravitation -on the pound of water from the time that it left the equator till -it returned to the point from which it started. The amount of work -performed in descending the slope from the equator to the pole and in -sinking to a depth of 10,000 feet or so through the polar water assumed -to be warmer than the surface water, comprehends the total amount of -work that gravitation can possibly perform; so that the amount of force -gained by such a supposition over and above that derived from the slope -is trifling. - -It would appear, however, that this is not what is meant after all. -What Dr. Carpenter apparently means is this: when a quantity of water, -say a layer one foot thick, flows down from the equator to the pole, -the polar column becomes then heavier than the equatorial by the -weight of this additional layer. A layer of water equal in quantity -is therefore pressed away from the bottom of the column and flows off -in the direction of the equator as an under current, the polar column -at the same time sinking down one foot until equilibrium of the polar -and equatorial columns is restored. Another foot of water now flows -down upon the polar column and another foot of water is displaced -from below, causing, of course, the column to descend an additional -foot. The same process being continually repeated, a constant downward -motion of the polar column is the result. Or, perhaps, to express the -matter more accurately, owing to the constant flow of water from the -equatorial regions down the slope, the weight of the polar column is -kept always in excess of that of the equatorial; therefore the polar -column in the effort to restore equilibrium is kept in a constant state -of descent. Hence he terms it a “vertical” circulation. The following -will show Dr. Carpenter’s theory in his own words:— - -“The action of cold on the surface water of each polar area will be -exerted as follows:— - -“(_a_) In diminishing the height of the polar column as compared with -that of the equatorial, so that a lowering of its _level_ is produced, -which can only be made good by a surface-flow from the latter towards -the former. - -“(_b_) In producing an excess in the downward _pressure_ of the column -when this inflow has restored its level, in virtue of the increase of -specific gravity it has gained by its reduction in volume; whereby a -portion of its heavy bottom-water is displaced laterally, causing a -further reduction of level, which draws in a further supply of the -warmer and lighter water flowing towards its surface. - -“(_c_) In imparting a downward _movement_ to each new surface-stratum -as its temperature undergoes reduction; so that the _entire column_ may -be said to be in a state of constant descent, like that which exists in -the water of a tall jar when an opening is made at its bottom, and the -water which flows away through it is replaced by an equivalent supply -poured into the top of the jar” (§ 23). - -But if this be his theory, as it evidently is, then the 4 foot-pounds -(the amount of work performed by the descent of the water down the -slope) comprehends all the work that gravitation can perform on a pound -of water in making a complete circuit from the equator to the pole and -from the pole back to the equator. - -This, I trust, will be evident from the following considerations. When -a pound of water has flowed down from the equator to the pole, it has -descended 4 feet, and is then at the foot of the slope. Gravity has -therefore no more power to pull it down to a lower level. It will not -sink through the polar water, for it is not denser than the water -beneath on which it rests. But it may be replied that although it will -not sink through the polar water, it has nevertheless made the polar -column heavier than the equatorial, and this excess of pressure forces -a pound of water out from beneath and allows the column to descend. -Suppose it may be argued that a quantity of water flows down from the -equator, so as to raise the level of the polar water by, say, one foot. -The polar column will now be rendered heavier than the equatorial by -the weight of one foot of water. The pressure of the one foot will -thus force a quantity of water laterally from the bottom and cause the -entire column to descend till the level of equilibrium is restored. In -other words, the polar column will sink one foot. Now in the sinking of -this column work is performed by gravity. A certain amount of work is -performed by gravity in causing the water to flow down the slope from -the equator to the pole, and, in addition to this, a certain amount is -performed by gravity in the vertical descent of the column. - -I freely admit this to be sound reasoning, and admit that so much is -due to the slope and so much to the vertical descent of the water. But -here we come to the most important point, viz., is there the full slope -of 4 feet and an additional vertical movement? Dr. Carpenter seems -to conclude that there is, and that this vertical force is something -in addition to the force which I derive from the slope. And here, I -venture to think, is a radical error into which he has fallen in regard -to the whole matter. Let it be observed that, when water circulates -from difference of specific gravity, this vertical movement is just as -real a part of the process as the flow down the slope; but the point -which I maintain is that _there is no additional power derived from -this vertical movement over and above what is derived from the full -slope_—or, in other words, that this _primum mobile_, which he says I -have overlooked, has in reality no existence. - -Perhaps the following diagram will help to make the point still -clearer:— - - [Illustration: Fig. 1.] - -Let P (fig. 1) be the surface of the ocean at the pole, and E the -surface at the equator; P O a column of water at the pole, and E Q a -column at the equator. The two columns are of equal weight, and balance -each other; but as the polar water is colder, and consequently denser -than the equatorial, the polar column is shorter than the equatorial, -the difference in the length of the two columns being 4 feet. The -surface of the ocean at the equator E is 4 feet higher than the surface -of the ocean at the pole P; there is therefore a slope of 4 feet from E -to P. The molecules of water at E tend to flow down this slope towards -P. The amount of work performed by gravity in the descent of a pound of -water down this slope from E to P is therefore 4 foot-pounds. - -But of course there can be no permanent circulation while the full -slope remains. In order to have circulation the polar column must be -heavier than the equatorial. But any addition to the weight of the -polar column is at the expense of the slope. In proportion as the -weight of the polar column increases the less becomes the slope. This, -however, makes no difference in the amount of work performed by gravity. - -Suppose now that water has flowed down till an addition of one foot -of water is made to the polar column, and the difference of level, -of course, diminished by one foot. The surface of the ocean in this -case will now be represented by the dotted line P′ E, and the slope -reduced from 4 feet to 3 feet. Let us then suppose a pound of water to -leave E and flow down to P′; 3 foot-pounds will be the amount of work -performed. The polar column being now too heavy by the extent of the -mass of water P′ P one foot thick, its extra pressure causes a mass of -water equal to P′ P to flow off laterally from the bottom of the column. -The column therefore sinks down one foot till P′ reaches P. Now the -pound of water in this vertical descent from P′ to P has one foot-pound -of work performed on it by gravity; this added to the 3 foot-pounds -derived from the slope, gives a total of 4 foot-pounds in passing from -E to P′ and then from P′ to P. This is the same amount of work that -would have been performed had it descended directly from E to P. In -like manner it can be proved that 4 foot-pounds is the amount of work -performed in the descent of every pound of water of the mass P′ P. The -first pound which left E flowed down the slope directly to P, and -performed 4 foot-pounds of work. The last pound flowed down the slope E -P′, and performed only 3 foot-pounds; but in descending from P′ to P it -performed the other one foot-pound. A pound leaving at a period exactly -intermediate between the two flowed down 3½ feet of slope and descended -vertically half a foot. Whatever path a pound of water might take, by -the time that it reached P, 4 foot-pounds of work would be performed. -But no further work can be performed after it reaches P. - -But some will ask, in regard to the vertical movement, is it only in -the descent of the water from P′ to P that work is performed? Water -cannot descend from P′ to P, it will be urged, unless the entire column -P O underneath descend also. But the column P O descends by means of -gravity. Why, then, it will be asked, is not the descent of the column -a motive power as real as the descent of the mass of water P′ P? - -That neither force nor energy can be derived from the mere descent of -the polar column P O is demonstrable thus:—The reason why the column P -O descends is because, in consequence of the mass of water P′ P resting -on it, its weight is in excess of the equatorial column E Q. But the -force with which the column descends is equal, not to the weight of -the column, but to the weight of the mass P′ P; consequently as much -work would be performed by gravity in the descent of the mass P′ P (the -one foot of water) alone as in the descent of the entire column P′ O, -10,000 feet in height. Suppose a ton weight is placed in each scale of -a balance: the two scales balance each other. Place a pound weight in -one of the scales along with the ton weight and the scale will descend. -But it descends, not with the pressure of a ton and a pound, but with -the pressure of the pound weight only. In the descent of the scale, -say, one foot, gravity can perform only one foot-pound of work. In like -manner, in the descent of the polar column, the only work available is -the work of the mass P′ P laid on the top of the column. But it must be -observed that in the descent of the column from P′ to P, a distance of -one foot, each pound of water of the mass P′ P does not perform one -foot-pound of work; for the moment that a molecule of water reaches P, -it then ceases to perform further work. The molecules at the surface P′ -descend one foot before reaching P; the molecules midway between P′ and -P descend only half a foot before reaching P, and the molecules at the -bottom of the mass are already at P, and therefore cannot perform any -work. The mean distance through which the entire mass performs work is -therefore half a foot. One foot-pound per pound of water represents in -this case the amount of work derived from the vertical movement. - -That such is the case is further evident from the following -considerations. Before the polar column begins to descend, it is -heavier than the equatorial by the weight of one foot of water; but -when the column has descended half a foot, the polar column is heavier -than the equatorial by the weight of only half a foot of water; and, -as the column continues to descend, the force with which it descends -continues to diminish, and when it has sunk to P the force is zero. -Consequently the mean pressure or weight with which the one foot of -water P′ P descended was equal to that of a layer of half a foot of -water; in other words, each pound of water, taking the mass as a whole, -descended with the pressure or weight of half a pound. But a half -pound descending one foot performs half a foot-pound; so that whether -we consider the _full pressure acting through the mean distance, or -the mean pressure acting through the full distance, we get the same -result_, viz. a half foot-pound as the work of vertical descent. - -Now it will be found, as we shall presently see, that if we calculate -the mean amount of work performed in descending the slope from the -equator to the pole, 3½ foot-pounds per pound of water is the amount. -The water at the bottom of the mass P P′ moved, of course, down the -full slope E P 4 feet. The water at the top of the mass which descended -from E to P′ descended a slope of only 3 feet. The mean descent of the -whole mass is therefore 3½ feet. And this gives 3½ foot-pounds as the -mean amount of work per pound of water in descending the slope; this, -added to the half foot-pound derived from vertical descent, gives 4 -foot-pounds as the total amount of work per pound of the mass. - -I have in the above reasoning supposed one foot of water accumulated -on the polar column before any vertical descent takes place. It is -needless to remark that the same conclusion would have been arrived -at, viz., that the total amount of work performed is 4 foot-pounds per -pound of water, supposing we had considered 2 feet, or 3 feet, or even -4 feet of water to have accumulated on the polar column before vertical -motion took place. - -I have also, in agreement with Dr. Carpenter’s mode of representing -the operation, been considering the two effects, viz., the flowing of -the water down the slope and the vertical descent of the polar column -as taking place alternately. In nature, however, the two effects take -place simultaneously; but it is needless to add that the amount of work -performed would be the same whether the effects took place alternately -or simultaneously. - -I have also represented the level of the ocean at the equator as -remaining permanent while the alterations of level were taking place at -the pole. But in representing the operation as it would actually take -place in nature, we should consider the equatorial column to be lowered -as the polar one is being raised. We should, for example, consider the -one foot of water P′ P put upon the polar column as so much taken off -the equatorial column. But in viewing the problem thus we arrive at -exactly the same results as before. - -Let P (Fig. 2), as in Fig. 1, be the surface of the ocean at the pole, -and E the surface at the equator, there being a slope of 4 feet from E -to P. Suppose now a quantity of water, E E′, say, one foot thick, to -flow from off the equatorial regions down upon the polar. It will thus -lower the level of the equatorial column by one foot, and raise the -level of the polar column by the same amount. I may, however, observe -that the one foot of water in passing from E to P would have its -temperature reduced from 80° to 32°, and this would produce a slight -contraction. But as the weight of the mass would not be affected, in -order to simplify our reasoning we may leave this contraction out of -consideration. Any one can easily satisfy himself that the assumption -that E E′ is equal to P′ P does not in any way affect the question at -issue—the only effect of the contraction being to _increase_ by an -infinitesimal amount the work done in descending the slope, and to -_diminish_ by an equally infinitesimal amount the work done in the -vertical descent. If, for example, 3 foot-pounds represent the amount -of work performed in descending the slope, and one foot-pound the -amount performed in the vertical descent, on the supposition that E′ E -does not contract in passing to the pole, then 3·0024 foot-pounds will -represent the work of the slope, and 0·9976 foot-pounds the work of -vertical descent when allowance is made for the contraction. But the -total amount of work performed is the same in both cases. Consequently, -to simplify our reasoning, we may be allowed to assume P′ P to be equal -to E E′. - - [Illustration: Fig. 2.] - -The slope E P being 4 feet, the slope E′ P′ is consequently 2 feet; -the mean slope for the entire mass is therefore 3 feet. The mean -amount of work performed by the descent of the mass will of course -be 3 foot-pounds per pound of water. The amount of work performed by -the vertical descent of P′ P ought therefore to be one foot-pound per -pound. That this is the amount will be evident thus:—The transference -of the one foot of water from the equatorial column to the polar -disturbs the equilibrium by making the equatorial column too light by -one foot of water and the polar column too heavy by the same amount of -water. The polar column will therefore tend to sink, and the equatorial -to rise till equilibrium is restored. The difference of weight of -the two columns being equal to 2 feet of water, the polar column -will begin to descend with a pressure of 2 feet of water; and the -equatorial column will begin to rise with an equal amount of pressure. -When the polar column has descended half a foot the equatorial column -will have risen half a foot. The pressure of the descending polar -column will now be reduced to one foot of water. And when the polar -column has descended another foot, P′ will have reached P, and E′ -will have reached E; the two columns will then be in equilibrium. It -therefore follows that the mean pressure with which the polar column -descended the one foot was equal to the pressure of one foot of water. -Consequently the mean amount of work performed by the descent of the -mass was equal to one foot-pound per pound of water; this, added to the -3 foot-pounds derived from the slope, gives a total of 4 foot-pounds. - -In whatever way we view the question, we are led to the conclusion that -if 4 feet represent the amount of slope between the equatorial and -polar columns when the two are in equilibrium, then 4 foot-pounds is -the total amount of work that gravity can perform upon a pound of water -in overcoming the resistance to motion in its passage from the equator -to the pole down the slope, and then in its vertical descent to the -bottom of the ocean. - -But it will be replied, not only does the one foot of water P′ P -descend, but the entire column P O, 10,000 feet in length, descends -also. What, then, it will be asked, becomes of the force which gravity -exerts in the descent of this column? We shall shortly see that this -force is entirely applied in work against gravity in other parts of -the circuit; so that not a single foot-pound of this force goes to -overcome cohesion, friction, and other resistances; it is all spent in -counteracting the efforts which gravity exerts to stop the current in -another part of the circuit. - -I shall now consider the next part of the movement, viz., the under -or return current from the bottom of the polar to the bottom of the -equatorial column. What produces this current? It is needless to say -that it cannot be caused directly by gravity. Gravitation cannot -directly draw any body horizontally along the earth’s surface. The -water that forms this current is pressed out laterally by the weight -of the polar column, and flows, or rather is pushed, towards the -equator to supply the vacancy caused by the ascent of the equatorial -column. There is a constant flow of water from the equator to the poles -along the surface, and this draining of the water from the equator is -supplied by the under or return current from the poles. But the only -power which can impel the water from the bottom of the polar column -to the bottom of the equatorial column is the pressure of the polar -column. But whence does the polar column derive its pressure? It can -only press to the extent that its weight exceeds that of the equatorial -column. That which exerts the pressure is therefore the mass of water -which has flowed down the slope from the equator upon the polar column. -It is in this case the vertical movement that causes this under -current. The energy which produces this current must consequently be -derived from the 4 foot-pounds resulting from the slope; for the energy -of the vertical movement, as has already been proved, is derived from -this source; or, in other words, whatever power this vertical movement -may exert is so much deducted from the 4 foot-pounds derived from the -full slope. - -Let us now consider the fourth and last movement, viz., the ascent of -the under current to the surface of the ocean at the equator. When -this cold under current reaches the equatorial regions, it ascends -to the surface to the point whence it originally started on its -circuit. What, then, lifts the water from the bottom of the equatorial -column to its top? This cannot be done directly, either by heat or -by gravity. When heat, for example, is applied to the bottom of a -vessel, the heated water at the bottom expands and, becoming lighter -than the water above, rises through it to the surface; but if the -heat be applied to the surface of the water instead of to the bottom, -the heat will not produce an ascending current. It will tend rather -to prevent such a current than to produce one—the reason being that -each successive layer of water will, on account of the heat applied, -become hotter and consequently lighter than the layer below it, and -colder and consequently heavier than the layer above it. It therefore -cannot ascend, because it is too heavy; nor can it descend, because -it is too light. But the sea in equatorial regions is heated from -above, and not from below; consequently the water at the bottom does -not rise to the surface at the equator in virtue of any heat which it -receives. A layer of water can never raise the temperature of a layer -below it to a higher temperature than itself; and since it cannot do -this, it cannot make the layer under it lighter than itself. That which -raises the water at the equator, according to Dr. Carpenter’s theory, -must be the downward pressure of the polar column. When water flows -down the slope from the equator to the pole, the polar column, as we -have seen, becomes too heavy and the equatorial column too light; -the former then sinks and the latter rises. It is the sinking of the -polar column which raises the equatorial one. When the polar column -descends, as much water is pressed in underneath the equatorial column -as is pressed from underneath the polar column. If one foot of water -is pressed from under the polar column, a foot of water is pressed in -under the equatorial column. Thus, when the polar column sinks a foot, -the equatorial column rises to the same extent. The equatorial water -continuing to flow down the slope, the polar column descends: a foot -of water is again pressed from underneath the polar column and a foot -pressed in under the equatorial. As foot after foot is thus removed -from the bottom of the polar column while it sinks, foot after foot is -pushed in under the equatorial column while it rises; so by this means -the water at the surface of the ocean in polar regions descends to -the bottom, and the water at the bottom in equatorial regions ascends -to the surface—the effect of solar heat and polar cold continuing, of -course, to maintain the surface of the ocean in equatorial regions at a -higher level than at the poles, and thus keeping up a constant state of -disturbed equilibrium. Or, to state the matter in Dr. Carpenter’s own -words, “The cold and dense polar water, as it flows in at the bottom of -the equatorial column, will not directly take the place of that which -has been drafted off from the surface; but this place will be filled -by the rising of the whole superincumbent column, which, being warmer, -is also lighter than the cold stratum beneath. Every new arrival from -the poles will take its place below that which precedes it, since its -temperature will have been less affected by contact with the warmer -water above it. In this way an ascending movement will be imparted to -the whole equatorial column, and in due course every portion of it will -come under the influence of the surface-heat of the sun.”[73] - -But the agency which raises up the water of the under current to the -surface is the pressure of the polar column. The equatorial column -cannot rise directly by means of gravity. Gravity, instead of raising -the column, exerts all its powers to prevent its rising. Gravity -here is a force acting against the current. It is the descent of -the polar column, as has been stated, that raises the equatorial -column. Consequently the entire amount of work performed by gravity -in pulling down the polar column is spent in raising the equatorial -column. Gravity performs exactly as much work in preventing motion -in the equatorial column as it performs in producing motion in the -polar column; so that, so far as the vertical parts of Dr. Carpenter’s -circulation are concerned, gravity may be said neither to produce -motion nor to prevent it. And this remark, be it observed, applies not -only to P O and E Q, but also to the parts P′ P and E E′ of the two -columns. When a mass of water E E′, say one foot deep, is removed off -the equatorial column and placed upon the polar column, the latter -column is then heavier than the former by the weight of two feet of -water. Gravity then exerts more force in pulling the polar column down -than it does in preventing the equatorial column from rising; and -the consequence is that the polar column begins to descend and the -equatorial column to rise. But as the polar column continues to descend -and the equatorial to rise, the power of gravity to produce motion in -the polar column diminishes, and the power of gravity to prevent motion -in the equatorial column increases; and when P′ descends to P and E′ -rises to E, the power of gravity to prevent motion in the equatorial -column is exactly equal to the power of gravity to produce motion in -the polar column, and consequently motion ceases. It therefore follows -that the entire amount of work performed by the descent of P′ P is -spent in raising E′ E against gravity. - -It follows also that inequalities in the sea-bottom cannot in any -way aid the circulation; for although the cold under current should -in its progress come to a deep trough filled with water less dense -than itself, it would no doubt sink to the bottom of the hollow; yet -before it could get out again as much work would have to be performed -against gravity as was performed by gravity in sinking it. But whilst -inequalities in the bed of the ocean would not aid the current, they -would nevertheless very considerably retard it by the obstructions -which they would offer to the motion of the water. - -We have been assuming that the weight of P′ P is equal to that of E E′; -but the mass P′ P must be greater than E E′ because P′ P has not only -to raise E E′, but to impel the under current—to push the water along -the sea-bottom from the pole to the equator. So we must have a mass of -water, in addition to P′ P, placed on the polar column to enable it to -produce the under current in addition to the raising of the equatorial -column. - -It follows also that the amount of work which can be performed by -gravity depends entirely on the _difference_ of temperature between -the equatorial and the polar waters, and is wholly independent of the -way in which the temperature may decrease from the equator to the -poles. Suppose, in agreement with Dr. Carpenter’s idea,[74] that the -equatorial heat and polar cold should be confined to limited areas, and -that through the intermediate space no great difference of temperature -should prevail. Such an arrangement as this would not increase the -amount of work which gravity could perform; it would simply make the -slope steeper at the two extremes and flatter in the intervening space. -It would no doubt aid the surface-flow of the water near the equator -and the poles, but it would retard in a corresponding degree the flow -of the water in the intermediate regions. In short, it would merely -destroy the uniformity of the slope without aiding in the least degree -the general motion of the water. - -It is therefore demonstrable that _the energy derived from the full -slope, whatever that slope may be, comprehends all that can possibly be -obtained from gravity_. - -It cannot be urged as an objection to what has been advanced that I -have determined simply the amount of the force acting on the water at -the surface of the ocean and not that on the water at all depths—that I -have estimated the amount of work which gravity can perform on a given -quantity of water at the surface, but not the total amount of work -which gravity can perform on the entire ocean. This objection will not -stand, because it is at the surface of the ocean where the greatest -difference of temperature, and consequently of density, exists between -the equatorial and polar waters, and therefore there that gravity -exerts its greatest force. And if gravity be unable to move the water -at the surface, it is much less able to do so under the surface. So -far as the question at issue is concerned, any calculations as to the -amount of force exerted by gravity at various depths are needless. - -It is maintained also that the winds cannot produce a vertical current -except under some very peculiar conditions. We have already seen that, -according to Dr. Carpenter’s theory, the vertical motion is caused -by the water flowing off the equatorial column, down the slope, upon -the polar column, thus destroying the equilibrium between the two by -diminishing the weight of the equatorial column and increasing that of -the polar column. In order that equilibrium may be restored, the polar -column sinks and the equatorial one rises. Now must not the same effect -occur, supposing the water to be transferred from the one column to -the other, by the influence of the winds instead of by the influence -of gravity? The vertical descent and ascent of these columns depend -entirely upon the difference in their weights, and not upon the nature -of the agency which makes this difference. So far as difference of -weight is concerned, 2 feet of water, propelled down the slope from the -equatorial column to the polar by the winds, will produce just the same -effect as though it had been propelled by gravity. If vertical motion -follows as a necessary consequence from a transference of water from -the equator to the poles by gravity, it follows equally as a necessary -consequence from the same transference by the winds; so that one is not -at liberty to advocate a vertical circulation in the one case and to -deny it in the other. - -_Gravitation Theory of the Gibraltar Current._—If difference of -specific gravity fails to account for the currents of the ocean in -general, it certainly fails in a still more decided manner to account -for the Gibraltar current. The existence of the submarine ridge -between Capes Trafalgar and Spartel, as was shown in the Phil. Mag. -for October, 1871, p. 269, affects currents resulting from difference -of specific gravity in a manner which does not seem to have suggested -itself to Dr. Carpenter. The pressure of water and other fluids is -not like that of a solid—not like that of the weight in the scale of -a balance, simply a downward pressure. Fluids press downwards like -the solids, but they also press laterally. The pressure of water is -hydrostatic. If we fill a basin with water or any other fluid, the -fluid remains in perfect equilibrium, provided the sides of the basin -be sufficiently strong to resist the pressure. The Mediterranean and -Atlantic, up to the level of the submarine ridge referred to, may be -regarded as huge basins, the sides of which are sufficiently strong to -resist all pressure. It follows that, however much denser the water -of the Mediterranean may be than that of the Atlantic, it is only the -water above the level of the ridge that can possibly exercise any -influence in the way of disturbing equilibrium, so as to cause the -level of the Mediterranean to stand lower than that of the Atlantic. -The water of the Atlantic below the level of this ridge might be as -light as air, and that of the Mediterranean as heavy as molten lead, -but this could produce no disturbance of equilibrium; and if there be -no difference of density between the Atlantic and the Mediterranean -waters from the surface down to the level of the top of the ridge, then -there can be nothing to produce the circulation which Dr. Carpenter -infers. Suppose both basins empty, and dense water to be poured into -the Mediterranean, and water less dense into the Atlantic, until they -are both filled up to the level of the ridge, it is evident that the -heavier water in the one basin can exercise no influence in raising -the level of the lighter water in the other basin, the entire pressure -being borne by the sides of the basins. But if we continue to pour in -water till the surface is raised, say one foot, above the level of the -ridge, then there is nothing to resist the lateral pressure of this one -foot of water in the Mediterranean but the counter pressure of the one -foot in the Atlantic. But as the Mediterranean water is denser than the -Atlantic, this one foot of water will consequently exert more pressure -than the one foot of water of the Atlantic. We must therefore continue -to pour more water into the Atlantic until its lateral pressure equals -that of the Mediterranean. The two seas will then be in equilibrium, -but the surface of the Atlantic will of course be at a higher level -than the surface of the Mediterranean. The difference of level will be -proportionate to the difference in density of the waters of the two -seas. But here we come to the point of importance. In determining the -difference of level between the two seas, or, which is the same thing, -the difference of level between a column of the Atlantic and a column -of the Mediterranean, we must take into consideration _only the water -which lies above the level of the ridge_. If there be one foot of water -above the ridge, then there is a difference of level proportionate to -the difference of pressure between the one foot of water of the two -seas. If there be 2 feet, 3 feet, or any number of feet of water above -the level of the ridge, the difference of level is proportionate to -the 2 feet, 3 feet, or whatever number of feet there may be of water -above the ridge. If, for example, 13 should represent the density of -the Mediterranean water and 12 the density of the Atlantic water, then -if there were one foot of water in the Mediterranean above the level of -the ridge, there would require to be one foot one inch of water in the -Atlantic above the ridge in order that the two might be in equilibrium. -The difference of level would therefore be one inch. If there were 2 -feet of water, the difference of level would be 2 inches; if 3 feet, -the difference would be 3 inches, and so on. And this would follow, -no matter what the actual depth of the two basins might be; the water -below the level of the ridge exercising no influence whatever on the -level of the surface. - -Taking Dr. Carpenter’s own data as to the density of the Mediterranean -and Atlantic waters, what, then, is the difference of density? The -submarine ridge comes to within 167 fathoms of the surface; say, in -round numbers, to within 1,000 feet. What are the densities of the two -basins down to the depth of 1,000 feet? According to Dr. Carpenter -there is little, if any, difference. His own words on this point are -these:—“A comparison of these results leaves no doubt that there is -an excess of salinity in the water of the Mediterranean above that of -the Atlantic; but that this excess _is_ slight in the surface-water, -whilst somewhat greater in the deeper water” (§ 7). “Again, it was -found by examining samples of water taken from the surface, from 100 -fathoms, from 250 fathoms, and from 400 fathoms respectively, that -whilst the _first two_ had the _characteristic temperature and density -of Atlantic water_, the last two had the characteristics and density of -Mediterranean water” (§ 13). Here, at least to the depth of 100 fathoms -or 600 feet, there is little difference of density between the waters -of the two basins. Consequently down to the depth of 600 feet, there is -nothing to produce any sensible disturbance of equilibrium. If there -be any sensible disturbance of equilibrium, it must be in consequence -of difference of density which may exist between the depths of 600 -feet and the surface of the ridge. We have nothing to do with any -difference which may exist between the water of the Mediterranean and -the Atlantic below the ridge; the water in the Mediterranean basin may -be as heavy as mercury below 1,000 feet: but this can have no effect -in disturbing equilibrium. The water to the depth of 600 feet being of -the same density in both seas, the length of the two columns acting on -each other is therefore reduced to 400 feet—that is, to that stratum of -water lying at a depth of from 600 to the surface of the ridge 1,000 -feet below the surface. But, to give the theory full justice, we shall -take the Mediterranean stratum at the density of the deep water of -the Mediterranean, which he found to be about 1·029, and the density -of the Atlantic stratum at 1·026. The difference of density between -the two columns is therefore ·003. Consequently, if the height of the -Mediterranean column be 400 feet, it will be balanced by the Atlantic -column of 401·2 feet; the difference of level between the Mediterranean -and the Atlantic cannot therefore be more than 1·2 foot. The amount -of work that can be performed by gravity in the case of the Gibraltar -current is little more than one foot-pound per pound of water, an -amount of energy evidently inadequate to produce the current. - -It is true that in his last expedition Dr. Carpenter found the -bottom-water on the ridge somewhat denser than Atlantic water at the -same depth, the former being 1·0292 and the latter 1·0265; but it -also proved to be denser than Mediterranean water at the same depth. -He found, for example, that “the dense Mediterranean water lies about -100 fathoms nearer the surface over a 300-fathoms bottom, than it -does where the bottom sinks to more than 500 fathoms” (§ 51). But any -excess of density which might exist at the ridge could have no tendency -whatever to make the Mediterranean column preponderate over the -Atlantic column, any more than could a weight placed over the fulcrum -of a balance have a tendency to make the one scale weigh down the other. - -If the objection referred to be sound, it shows the mechanical -impossibility of the theory. It proves that whether there be an under -current or not, or whether the dense water lying in the deep trough of -the Mediterranean be carried over the submarine ridge into the Atlantic -or not, the explanation offered by Dr. Carpenter is one which cannot be -admitted. It is incumbent on him to explain either (1) how the almost -infinitesimal difference of density which exists between the Atlantic -and Mediterranean columns down to the level of the ridge can produce -the upper and under currents carrying the deep and dense water of -the Mediterranean over the ridge, or (2) how all this can be done by -means of the difference of density which exists below the level of the -ridge.[75] What the true cause of the Gibraltar current really is will -be considered in Chap. XIII. - -_The Baltic Current._—The entrance to the Baltic Sea is in some -places not over 50 or 60 feet deep. It follows, therefore, from what -has already been proved in regard to the Gibraltar current, that the -influence of gravity must be even still less in causing a current in -the Baltic strait than in the Gibraltar strait. - - - - - CHAPTER X. - - EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.—DR. - CARPENTER’S THEORY.—OBJECTIONS CONSIDERED. - - _Modus Operandi_ of the Matter.—Polar Cold considered by Dr. - Carpenter the _Primum Mobile_.—Supposed Influence of - Heat derived from the Earth’s Crust.—Circulation without - Difference of Level.—A Confusion of Ideas in Reference to the - supposed Agency of Polar Cold.—M. Dubuat’s Experiments.—A - Begging of the Question at Issue.—Pressure as a Cause of - Circulation. - - -In the foregoing chapter, the substance of which appeared in the -Phil. Mag. for October, 1871, I have represented the manner in which -difference of specific gravity produces circulation. But Dr. Carpenter -appears to think that there are some important points which I have -overlooked. These I shall now proceed to consider in detail. - -“Mr. Croll’s whole manner of treating the subject,” he says, “is so -different from that which it appears to me to require, and he has so -completely misapprehended my own view of the question, that I feel it -requisite to present this in fuller detail in order that physicists and -mathematicians, having both sides fully before them, may judge between -us” (§ 26).[76] - -He then refers to a point so obvious as hardly to require -consideration, viz., the effect which results when the surface of -the entire area of a lake or pond of water is cooled. The whole of -the surface-film, being chilled at the same time, sinks through the -subjacent water, and a new film from the warmer layer immediately -beneath the surface rises into its place. This being cooled in its -turn, sinks, and so on. He next considers what takes place when only -a portion of the surface of the pond is cooled, and shows that in this -case the surface-film which descends is replaced not from beneath, but -by an inflow from the neighbouring area. - -“That such must be the case,” says Dr. Carpenter, “appears to me so -self-evident that I am surprised that any person conversant with the -principles of physical science should hesitate in admitting it, still -more that he should explicitly deny it. But since others may feel the -same difficulty as Mr. Croll, it may be worth while for me to present -the case in a form of yet more elementary simplicity” (§ 29). - -Then, in order to show the mode in which the general oceanic -circulation takes place, he supposes two cylindrical vessels, W and -C, of equal size, to be filled with sea-water. Cylinder W represents -the equatorial column, and the water contained in it has its -temperature maintained at 60°; whilst the water in the other cylinder -C, representing the polar column, has its temperature maintained at -30° by means of the constant application of cold at the top. Free -communication is maintained between the two cylinders at top and -bottom; and the water in the cold cylinder being, in virtue of its -low temperature, denser than the water in the warm cylinder, the two -columns are therefore not in static equilibrium. The cold, and hence -heavier column tends to produce an outflow of water from its bottom to -the bottom of the warm column, which outflow is replaced by an inflow -from the top of the warm column to the top of the cold column. In fact, -we have just a simple repetition of what he has given over and over -again in his various memoirs on the subject. But why so repeatedly -enter into the _modus operandi_ of the matter? Who feels any difficulty -in understanding how the circulation is produced? - -_Polar Cold considered by Dr. Carpenter the Primum Mobile._—It is -evident that Dr. Carpenter believes that he has found in polar _cold_ -an agency the potency of which, in producing a general oceanic -circulation, has been overlooked by physicists; and it is with the view -of developing his ideas on this subject that he has entered so fully -and so frequently into the exposition of his theory. “If I have myself -done anything,” he says, “to strengthen the doctrine, it has been by -showing that polar cold, rather than equatorial heat, is the _primum -mobile_ of this circulation.”[77] - -The influence of the sun in heating the waters of the inter-tropical -seas is, in Dr. Carpenter’s manner of viewing the problem, of no -great importance. The efficient cause of motion he considers resides -in _cold_ rather than in _heat_. In fact, he even goes the length -of maintaining that, as a power in the production of the general -interchange of equatorial and polar water, the effect of polar cold is -so much superior to that of inter-tropical heat, that the influence of -the latter may be _practically disregarded_. - -“Suppose two basins of ocean-water,” he says, “connected by a strait to -be placed under such different climatic conditions that the surface of -one is exposed to the heating influence of tropical sunshine, whilst -the surface of the other is subjected to the extreme cold of the -sunless polar winter. The effect of the surface-heat upon the water -of the tropical basin will be for the most part limited (as I shall -presently show) to its uppermost stratum, and may here be _practically -disregarded_.”[78] - -Dr. Carpenter’s idea regarding the efficiency of cold in producing -motion seems to me to be not only opposed to the generally received -views on the subject, but wholly irreconcilable with the ordinary -principles of mechanics. In fact, there are so many points on which -Dr. Carpenter’s theory of a “General _Vertical_ Oceanic Circulation” -differs from the generally received views on the subject of circulation -by means of difference of specific gravity, that I have thought it -advisable to enter somewhat minutely into the consideration of the -mechanics of that theory, the more so as he has so repeatedly asserted -that eminent physicists agree with what he has advanced on the subject. - -According to the generally received theory, the circulation is due to -the _difference of density_ between the sea in equatorial and polar -regions. The real efficient cause is gravity; but gravity cannot act -when there is no difference of specific gravity. If the sea were of -equal density from the poles to the equator, gravity could exercise no -influence in the production of circulation; and the influence which it -does possess is in proportion to the difference of density. But the -difference of density between equatorial and polar waters is in turn -due not absolutely either to polar cold or to tropical heat, but to -both—or, in other words, to the _difference_ of temperature between -the polar and equatorial seas. This difference, in the very nature of -things, must be as much the result of equatorial heat as of polar cold. -If the sea in equatorial regions were not being heated by the sun as -rapidly as the sea in polar regions is being cooled, the difference of -temperature between them, and consequently the difference of density, -would be diminishing, and in course of time would disappear altogether. -As has already been shown, it is a necessary consequence that the -water flowing from equatorial to polar regions must be compensated by -an equal amount flowing from polar to equatorial regions. Now, if the -water flowing from polar to equatorial regions were not being heated -as rapidly as the water flowing from equatorial to polar regions is -being cooled, the equatorial seas would gradually become colder and -colder until no sensible difference of temperature existed between -them and the polar oceans. In fact, _equality of the two rates_ is -necessary to the very existence of such a general circulation as that -advocated by Dr. Carpenter. If he admits that the general interchange -of equatorial and polar water advocated by him is caused by the -difference of density between the water at the equator and the poles, -resulting from difference of temperature, then he must admit also that -this difference of density is just as much due to the heating of the -equatorial water by the sun as it is to the cooling of the polar water -by radiation and other means—or, in other words, that it is as much due -to equatorial heat as to polar cold. And if so, it cannot be true that -polar cold rather than equatorial heat is the “_primum mobile_” of -this circulation; and far less can it be true that the heating of the -equatorial water by the sun is of so little importance that it may be -“practically disregarded.” - -_Supposed Influence of Heat derived from the Earth’s Crust._—There is, -according to Dr. Carpenter, another agent concerned in the production -of the general oceanic circulation, viz., the heat derived by the -bottom of the ocean from the crust of the earth.[79] We have no reason -to believe that the quantity of internal heat coming through the -earth’s crust is greater in one part of the globe than in another; nor -have we any grounds for concluding that the bottom of inter-tropical -seas receives more heat from the earth’s crust than the bottom of those -in polar regions. But if the polar seas receive as much heat from this -source as the seas within the tropics, then the difference of density -between the two cannot possibly be due to heat received from the -earth’s crust; and this being so, it is mechanically impossible that -internal heat can be a cause in the production of the general oceanic -circulation. - -_Circulation without Difference of Level._—There is another part of -the theory which appears to me irreconcilable with mechanics. It is -maintained that this general circulation takes place without any -difference of level between the equator and the poles. Referring to the -case of the two cylinders W and C, which represent the equatorial and -polar columns respectively, Dr. Carpenter says:— - -“The force which will thus lift up the entire column of water in W -is that which causes the descent of the entire column in C, namely, -the excess of gravity constantly acting in C,—the levels of the -two columns, and consequently their heights, being maintained at a -_constant equality_ by the free passage of surface-water from W to C.” - -“The whole of Mr. Croll’s discussion of this question, however,” he -continues, “proceeds upon the assumption that the levels of the polar -and equatorial columns are _not kept at an_ _equality_, &c.” (§ 30.) -And again, “Now, so far from asserting (as Captain Maury has done) that -the trifling difference of level arising from inequality of temperature -is adequate to the production of ocean-currents, I simply affirm that -as fast as the level is disturbed by change of temperature it will be -restored by gravity.” (§ 23.)[80] - - [Illustration: Fig. 3.] - -In order to understand more clearly how the circulation under -consideration cannot take place without a difference of level, let W E -(Fig. 3) represent the equatorial column, and C P the polar column. The -equatorial column is warmer than the polar column because it receives -_more_ heat from the sun than the latter; and the polar is colder -than the equatorial column because it receives _less_. The difference -in the density of the two columns results from their difference of -temperature; and the difference of temperature results in turn from the -difference in the quantity of heat received from the sun by each. Or, -to express the matter in other words, the difference of density (and -consequently the circulation under consideration) is due to the excess -of heat received from the sun by the equatorial over that received by -the polar column; so that to leave out of account the super-heating of -the inter-tropical waters by the sun is to leave out of account the -very thing of all others that is absolutely essential to the existence -of the circulation. The water being assumed to be the same in both -columns and differing only as regards temperature, and the equatorial -column possessing more heat than the polar, and being therefore less -dense than the latter, it follows, in order that the two columns may -be in static equilibrium, that the surface of the equatorial column -must stand at a higher level than that of the polar. This produces the -slope W C from the equator to the pole. The extent of the slope will of -course depend upon the extent of the difference of their temperatures. -But, as was shown on a former occasion,[81] it is impossible that -static equilibrium can ever be fully obtained, because the slope -occasioned by the elevation of the equatorial column above the polar -produces what we may be allowed to call a _molecular_ disturbance of -equilibrium. The surface of the ocean, or the molecules of water lying -on the slope, are not in a position of equilibrium, but tend, in virtue -of gravity, to roll down the slope in the direction of the polar column -C. It will be observed that the more we gain of static equilibrium -of the entire ocean the greater is the slope, and consequently the -greater is the disturbance of molecular equilibrium; and, _vice versâ_, -the more molecular equilibrium is restored by the reduction of the -slope, the greater is the disturbance of static equilibrium. _It is -therefore absolutely impossible that both conditions of equilibrium can -be fulfilled at the same time so long as a difference of temperature -exists between the two columns._ And this conclusion holds true even -though we should assume water to be a perfect fluid absolutely devoid -of viscosity. It follows, therefore, that a general oceanic circulation -without a difference of level is a _mechanical impossibility_. - -In a case of actual circulation due to difference of gravity, there -is always a constant disturbance of both _static_ and molecular -equilibrium. Column C is always higher and column W always lower than -it ought to be were the two in equilibrium; but they never can be at -the same level. - -It is quite conceivable, of course, that the two conditions of -equilibrium may be fulfilled alternately. We can conceive column C -remaining stationary till the water flowing from column W has restored -the level. And after the level is restored we can conceive the polar -column C sinking and the equatorial column W rising till the two -perfectly balance each other. Such a mode of circulation, consisting -of an alternate surface-flow and vertical descent and ascent of the -columns, though conceivable, is in reality impossible in nature; for -there are no means by which the polar column C could be supported -from sinking till the level had been restored. But Dr. Carpenter does -not assume that the general oceanic circulation takes place in this -intermitting manner; according to him, the circulation is _constant_. -He asserts that there is a “_continual_ transference of water from the -bottom of C to the bottom of W, and from the top of W to the top of C, -with a _constant_ descending movement in C and a _constant_ ascending -movement in W” (§ 29). But such a condition of things is irreconcilable -with the idea of “the levels of the two columns, and consequently their -heights, being maintained at a _constant_ equality” (§ 29). - -Although Dr. Carpenter does not admit the existence of a permanent -difference of level between the equator and the pole, he nevertheless -speaks of a depression of level in the polar basin resulting from the -contraction by cooling of the water flowing into it. This reduction of -level induces an inflow of water from the surrounding area; “and since -what is drawn away,” to quote his own words, “is supplied from a yet -greater distance, the continued cooling of the surface-stratum in the -polar basin will cause a ‘set’ of waters towards it, to be propagated -backwards through the whole intervening ocean in communication with -it until it reaches the tropical area.” The slope produced between -the polar basin and the surrounding area, if sufficiently great, will -enable the water in the surrounding area to flow polewards; but unless -this slope extend to the equator, it will not enable the tropical -waters also to flow polewards. One of two things necessarily follows: -either the slope extends from the equator to the pole, or water can -flow from the equator to the pole without a slope. If Dr. Carpenter -maintains the former, he contradicts himself; and if he adopts the -latter, he contradicts an obvious principle of mechanics. - -_A Confusion of Ideas in Reference to the supposed Agency of Polar -Cold._—It seems to me that Dr. Carpenter has been somewhat misled by a -slight confusion of ideas in reference to the supposed agency of polar -cold. This is brought out forcibly in the following passage from his -memoir in the Proceedings of the Royal Geographical Society, vol. xv. - -“Mr. Croll, in arguing against the doctrine of a general oceanic -circulation sustained by difference of temperature, and _justly -maintaining_ that such a circulation cannot be produced by the -application of heat at the surface, has entirely ignored the agency of -cold.” - -It is here supposed that there are two agents at work in the production -of the general oceanic circulation. The one agent is _heat_, acting -at the equatorial regions; and the other agent is _cold_, acting at -the polar regions. It is supposed that the agency of cold is far more -powerful than that of heat. In fact so trifling is the agency of -equatorial heat in comparison with that of polar cold that it may be -“practically disregarded”—left out of account altogether,—polar cold -being the _primum mobile_ of the circulation. It is supposed also that -I have considered the efficiency of one of the agents, viz., heat, and -found it totally inadequate to produce the circulation in question; and -it is admitted also that my conclusions are perfectly correct. But then -I am supposed to have left out of account the other agent, viz., polar -cold, the only agent possessing real potency. Had I taken into account -polar cold, it is supposed that I should have found at once a cause -perfectly adequate to produce the required effect. - -This is a fair statement of Dr. Carpenter’s views on the subject; I am -unable, at least, to attach any other meaning to his words. And I have -no doubt they are also the views which have been adopted by those who -have accepted his theory. - -It must be sufficiently evident from what has already been stated, -that the notion of there being two separate agents at work producing -circulation, namely heat and cold, the one of which is assumed to have -much more potency than the other, is not only opposed to the views -entertained by physicists, but is also wholly irreconcilable with the -ordinary principles of mechanics. But more than this, if we analyze the -subject a little so as to remove some of the confusion of ideas which -besets it, we shall find that these views are irreconcilable with even -Dr. Carpenter’s own explanation of the cause of the general oceanic -circulation. - -_Cold_ is not a something positive imparted to the polar waters giving -them motion, and of which the tropical waters are deprived. If, dipping -one hand into a basin filled with tropical water at 80° and the other -into one filled with polar water at 32°, we refer to our _sensations_, -we call the water in the one _hot_ and that in the other _cold_; but -so far as the water itself is concerned heat and cold simply mean -difference in the amounts of heat possessed. Both the polar and the -tropical water possess a certain amount of energy in the form of heat, -only the polar water does not possess so much of it as the tropical. - -How, then, according to Dr. Carpenter, does polar cold impart motion -to the water? The warm water flowing in upon the polar column becomes -chilled by cold, but it is not cooled below that of the water -underneath; for, according to Dr. Carpenter, the ocean in polar regions -is as cold and as dense underneath as at the surface. The cooled -surface-water does not sink through the water underneath, like the -surface-water of a pond chilled during a frosty night. “The descending -motion in column C will not consist,” he says, “in a successional -descent of surface-films from above downwards, but it will be a -downward movement of the _entire mass_, as if water in a tall jar -were being drawn off through an orifice at the bottom” (§ 29). There -is a downward motion of the entire column, producing an outflow of -water at the bottom towards the equatorial column W, which outflow is -compensated by an inflow from the top of the equatorial column to the -top of the polar column C. But what causes column C to descend? The -cause of the descent is its excess of weight over that of column W. -Column C descends and column W ascends, for the same reason that in -a balance the heavy scale descends and the light scale rises. Column -C descends not simply because it is cold, but because it is _colder_ -than column W. Column C descends not simply because in consequence of -being cold it is dense and therefore heavy, but because in consequence -of being cold it is _denser_ and therefore _heavier_ than column W. -It might be as cold as frozen mercury and as heavy as lead; but it -would not on that account descend unless it were heavier than column -W. The descent of column C and ascent of column W, and consequently -the general oceanic circulation, results, therefore, according to Dr. -Carpenter’s explanation, from the _difference_ in the weights of the -two columns; and the difference in the weights of the two columns -results from their _difference_ of density; and the difference of -density of the two columns in turn results from their _difference_ of -temperature. But it has already been proved that the difference of -temperature between the polar and equatorial columns depends wholly on -the difference in the amount of heat received by each from the sun. The -equatorial column W possesses more heat than the polar column C, solely -because it receives more heat from the sun than column C. Consequently -Dr. Carpenter’s statement that the circulation is produced by polar -cold rather than by equatorial heat, is just as much in contradiction -to his own theory as it is to the principles of mechanics. Again, his -admission that the general oceanic circulation “cannot be produced by -the application of heat to the surface,” is virtually a giving up the -whole point in debate; for according to his gravitation theory, and -every form of that theory, the circulation results from _difference_ of -temperature between equatorial and polar seas; but this difference, as -we have seen, is entirely owing to the difference in the amount of heat -received from the sun at these two places. The heat received, however, -is “surface-heat;” for it is at the surface that the ocean receives all -its heat from the sun; and consequently if surface-heat cannot produce -the effect required, nothing else can. - -_M. Dubuat’s Experiments._—Referring to the experiments of M. Dubuat -adduced by me to show that water would not run down a slope of 1 -in 1,820,000,[82] he says, “Now the experiments of M. Dubuat had -reference, not to the slow restoration of level produced by the motion -of water on itself, but to the sensible movement of water flowing over -solid surfaces and retarded by its friction against them” (§ 22). -Dr. Carpenter’s meaning, I presume, is that if the incline consist -of any solid substance, water will not flow down it; but if it be -made of _water_ itself, _water_ will flow down it. But in M. Dubuat’s -experiments it was only the molecules in actual _contact_ with the -solid incline that could possibly be retarded by friction against it. -The molecules not in contact with the solid incline evidently rested -upon an _incline of water_, and were at perfect liberty to roll down -that incline if they chose; but they did not do so; and consequently M. -Dubuat’s experiment proved that water will not flow over itself on an -incline of 1 in 1,000,000. - -_A Begging of the Question at Issue._—“It is to be remembered,” says -Dr. Carpenter, “that, however small the original amount of movement -may be, a _momentum_ tending to its continuance _must_ be generated -from the instant of its commencement; so that if the initiating force -be in constant action, there will be a _progressive acceleration_ of -its rate, until the increase of resistance equalises the tendency to -further acceleration. Now, if it be admitted that the propagation of -the disturbance of equilibrium from one column to another is simply -_retarded_, _not_ prevented, by the viscosity of the liquid, I cannot -see how the conclusion can be resisted, that the constantly maintained -difference of gravity between the polar and equatorial columns really -acts as a _vis viva_ in maintaining a circulation between them” (§ 35). - -If it be true, as Dr. Carpenter asserts, that in the case of the -general oceanic circulation advocated by him “viscosity” simply -_retards_ motion, but does not _prevent_ it, I certainly agree with him -“that the constantly maintained difference of gravity between the polar -and equatorial columns really acts as a _vis viva_ in maintaining a -circulation between them.” But to assert that it merely retards, but -does not prevent, motion, is simply _begging the question at issue_. -It is an established principle that if the _force_ resisting motion be -greater than the force tending to produce it, then no motion can take -place and no work can be performed. The experiments of M. Dubuat prove -that the _force_ of the molecular resistance of water to motion is -_greater_ than the _force_ derived from a slope of 1 in 1,000,000; and -therefore it is simply begging the question at issue to assert that it -is _less_. The experiments of MM. Barlow, Rainey, and others, to which -he alludes, are scarcely worthy of consideration in relation to the -present question, because we know nothing whatever regarding the actual -amount of force producing motion of the water in these experiments, -further than that it must have been enormously greater than that -derived from a slope of 1 in 1,000,000. - -_Supposed Argument from the Tides._—Dr. Carpenter advances Mr. -Ferrel’s argument in regard to the tides. The power of the moon to -disturb the earth’s water, he asserts, is, according to Herschel, -only 1/11,400,000th part of gravity, and that of the sun not over -1/25,736,400th part of gravity; yet the moon’s attractive force, even -when counteracted by the sun, will produce a rise of the ocean. But as -the disturbance of gravity produced by difference of temperature is far -greater than the above, it ought to produce circulation. - -It is here supposed that the force exerted by gravity on the ocean, -resulting from difference of temperature, tending to produce the -general oceanic circulation, is much greater than the force exerted -on the ocean by the moon in the production of the tides. But if we -examine the subject we shall find that the opposite is the case. The -attraction of the moon tending to lift the waters of the ocean acts -directly on every molecule from the surface to the bottom; but the -force of gravity tending to produce the circulation in question acts -directly on only a portion of the ocean. Gravity can exercise no direct -force in impelling the underflow from the polar to the equatorial -regions, nor in raising the water to the surface when it reaches the -equatorial regions. Gravity can exercise no direct influence in pulling -the water horizontally along the earth’s surface, nor in raising it -up to the surface. The pull of gravity is always _downwards_, never -_horizontally_ nor upwards. Gravity will tend to pull the surface-water -from the equator to the poles because here we have _descent_. Gravity -will tend to sink the polar column because here also we have _descent_. -But these are the only parts of the circuit where gravity has any -tendency to produce motion. Motion in the other parts of the circuit, -viz., along the bottom of the ocean from the poles to the equator and -in raising the equatorial column, is produced by the _pressure_ of the -polar column; and consequently it is only _indirectly_ that gravity may -be said to produce motion in those parts. It is true that on certain -portions of the ocean the force of gravity tending to produce motion is -greater than the force of the moon’s attraction, tending to produce the -tides; but this portion of the ocean is of inconsiderable extent. The -total force of gravity acting on the entire ocean tending to produce -circulation is in reality prodigiously less than the total force of the -moon tending to produce the tides. - -It is no doubt a somewhat difficult problem to determine accurately -the total amount of force exercised by gravity on the ocean; but for -our present purpose this is not necessary. All that we require at -present is a very rough estimate indeed. And this can be attained by -very simple considerations. Suppose we assume the mean depth of the -sea to be, say, three miles. The mean depth may yet be found to be -somewhat less than this, or it may be found to be somewhat greater; -a slight mistake, however, in regard to the mass of the ocean will -not materially affect our conclusions. Taking the depth at 3 miles, -the force or direct pull of gravity on the entire waters of the ocean -tending to the production of the general circulation will not amount to -more than 1/24,000,000,000th that of gravity, or only about 1/2,100th -that of the attraction of the moon in the production of the tides. Let -it be observed that I am referring to the force or pull of gravity, -and not to hydrostatic pressure. - -The moon, by raising the waters of the ocean, will produce a slope of 2 -feet in a quadrant; and because the raised water sinks and the level is -restored, Mr. Ferrel concludes that a similar slope of 2 feet produced -by difference of temperature will therefore be sufficient to produce -motion and restore level. But it is overlooked that the restoration of -level in the case of the tides is as truly the work of the moon as the -disturbance of that level is. For the water raised by the attraction of -the moon at one time is again, six hours afterwards, pulled down by the -moon when the earth has turned round a quadrant. - -No doubt the earth’s gravity alone would in course of time restore -the level; but this does not follow as a logical consequence from Mr. -Ferrel’s premises. If we suppose a slope to be produced in the ocean by -the moon and the moon’s attraction withdrawn so as to allow the water -to sink to its original level, the raised side will be the heaviest and -the depressed side the lightest; consequently the raised side will tend -to sink and the depressed side will tend to rise, in order that the -ocean may regain its static equilibrium. But when a difference of level -is produced by difference of temperature, the raised side is always the -lightest and the depressed side is always the heaviest; consequently -the very effort which the ocean makes to maintain its equilibrium -tends to prevent the level being restored. The moon produces the tides -chiefly by means of a simple yielding of the entire ocean considered as -a mass; whereas in the case of a general oceanic circulation the level -is restored by a _flow_ of water at or near the surface. Consequently -the amount of friction and molecular resistance to be overcome in the -restoration of level in the latter case is much greater than in the -former. The moon, as the researches of Sir William Thomson show, will -produce a tide in a globe composed of a substance where no currents or -general flow of the materials could possibly take place. - -_Pressure as a Cause of Circulation._—We shall now briefly refer to -the influence of pressure (the indirect effects of gravity) in the -production of the circulation under consideration. That which causes -the polar column C to descend and the equatorial column W to ascend, -as has repeatedly been remarked, is the difference in the weight of -the two columns. The efficient cause in the production of the movement -is, properly speaking, gravity; _cold_ at the poles and _heat_ at the -equator, or, what is the same thing, the _excess_ of heat received -by the equator over that received by the poles is what maintains the -difference of temperature between the two columns, and consequently is -that also which maintains the difference of weight between them. In -other words, difference of temperature is the cause which maintains -the _state of disturbed equilibrium_. But the efficient cause of -the circulation in question is gravity. Gravity, however, could not -act without this state of disturbed equilibrium; and difference of -temperature may therefore be called, in relation to the circulation, -a necessary _condition_, while gravity may be termed the _cause_. -Gravity sinks column C _directly_, but it raises column W _indirectly_ -by means of pressure. The same holds true in regard to the motion of -the bottom-waters from C to W, which is likewise due to pressure. The -pressure of the excess of the weight of column C over that of column W -impels the bottom-water equatorwards and lifts the equatorial column. -But on this point I need not dwell, as I have in the preceding chapter -entered into a full discussion as to how this takes place. - -We come now to the most important part of the inquiry, viz., how is -the surface-water impelled from the equator to the poles? Is pressure -from behind the impelling force here as in the case of the bottom-water -of the ocean? It seems to me that, in attempting to account for the -surface-flow from the equator to the poles, Dr. Carpenter’s theory -signally fails. The force to which he appeals appears to be wholly -inadequate to produce the required effect. - -The experiments of M. Dubuat, as already noticed, prove that, any slope -which can possibly result from the difference of temperature between -the equator and the poles is wholly insufficient to enable gravity to -move the waters; but it does not necessarily prove that the _pressure_ -resulting from the raised water at the equator may not be sufficient to -produce motion. This point will be better understood from the following -figure, where, as before, P C represents the polar column and E W the -equatorial column. - - [Illustration: Fig. 4.] - -It will be observed that the water in that wedge-shaped portion W C -W′ forming the incline cannot be in a state of static equilibrium. -A molecule of water at O, for example, will be pressed more in the -direction of C than in the direction of W′, and the amount of this -excess of pressure towards C will depend upon the height of W above -the line C W′. It is evident that the pressure tending to move the -molecule at O towards C will be far greater than the direct pull of -gravity tending to draw a molecule at O′ lying on the surface of the -incline towards C. The experiments of M. Dubuat prove that the direct -force of gravity will not move the molecule at O′—that is, cause it to -roll down the incline W C; but they do not prove that it may not yield -to pressure from above, or that the pressure of the column W W′ will -not move the molecule at O. The pressure is caused by gravity, and -cannot, of course, enable gravity to perform more work than what is -derived from the energy of gravity; it will enable gravity, however, -to overcome resistance, which it could not do by direct action. But -whether the pressure resulting from the greater height of the water -at the equator due to its higher temperature be actually sufficient -to produce displacement of the water is a question which I am wholly -unable to answer. - -If we suppose 4 feet 6 inches to be the height of the equatorial -surface above the polar required to make the two columns balance -each other, the actual difference of level between the two columns -will certainly not be more than one-half that amount, because, if a -circulation exist, the weight of the polar column must always be in -excess of that of the equatorial. But this excess can only be obtained -at the expense of the surface-slope, as has already been shown at -length. The surface-slope probably will not be more than 2 feet or 2 -feet 6 inches. Suppose the ocean to be of equal density from the poles -to the equator, and that by some means or other the surface of the -ocean at the equator is raised, say, 2 feet above that of the poles, -then there can be little doubt that in such a case the water would -soon regain its level; for the ocean at the equator being heavier than -at the poles by the weight of a layer 2 feet in thickness, it would -sink at the former place and rise at the latter until equilibrium was -restored, producing, of course, a very slight displacement of the -bottom-waters towards the poles. It will be observed, however, that -restoration of level in this case takes place by a simple yielding, as -it were, of the entire mass of the ocean without displacement of the -molecules of the water over each other to any great extent. In the case -of a slope produced by difference of temperature, however, the raised -portion of the ocean is not heavier but lighter than the depressed -portion, and consequently has no tendency to sink. Any movement which -the ocean as a mass makes in order to regain equilibrium tends, as we -have seen, rather to increase the difference of level than to reduce -it. Restoration of level can only be produced by the forces which are -in operation in the wedge-shaped mass W C W′, constituting the slope -itself. But it will be observed by a glance at the Figure that, in -order to the restoration of level, a large portion of the water W W′ at -the equator will require to flow to C, the pole. - -According to the general _vertical_ oceanic circulation theory, -pressure from behind is not one of the forces employed in the -production of the flow from the equator to the poles. This is evident; -for there can be no pressure from behind acting on the water if there -be no slope existing between the equator and the poles. Dr. Carpenter -not only denies the actual existence of a slope, but denies the -necessity for its existence. But to deny the existence of a slope is to -deny the existence of pressure, and to deny the necessity for a slope -is to deny the necessity for pressure. That in Dr. Carpenter’s theory -the surface-water is supposed to be _drawn_ from the equator to the -poles, and not _pressed_ forward by a force from behind, is further -evident from the fact that he maintains that the force employed is not -_vis a tergo_ but _vis a fronte_.[83] - - - - - CHAPTER XI. - - THE INADEQUACY OF THE GRAVITATION THEORY PROVED BY ANOTHER - METHOD. - - Quantity of Heat which can be conveyed by the General Oceanic - Circulation trifling.—Tendency in the Advocates of the - Gravitation Theory to under-estimate the Volume of the - Gulf-stream.—Volume of the Stream as determined by the - _Challenger_.—Immense Volume of Warm Water discovered by - Captain Nares.—Condition of North Atlantic inconsistent with - the Gravitation Theory.—Dr. Carpenter’s Estimate of the - Thermal Work of the Gulf-stream. - - -I shall now proceed by another method to prove the inadequacy of such -a general oceanic circulation as that which Dr. Carpenter advocates. -By contrasting the quantity of heat carried by the Gulf-stream from -inter-tropical to temperate and polar regions with such amount as -can possibly be conveyed in the same direction by means of a general -oceanic circulation, it will become evident that the latter sinks into -utter insignificance before the former. - -In my earlier papers on the amount of heat conveyed by the -Gulf-stream,[84] I estimated the volume of that stream as _equal -to that_ of a current 50 miles broad and 1,000 feet deep, flowing -(from the surface to the bottom) at 4 miles an hour. Of course I did -not mean, as Dr. Carpenter seems to suppose, that the stream at any -particular place is 50 miles broad and 1,000 feet deep, or that it -actually flows at the uniform rate of 4 miles an hour at surface and -bottom. All I meant was, that the Gulf-stream is _equal to that_ of -a current of the above size and velocity. But in my recent papers on -Ocean-currents, the substance of which appears in the present volume, -to obviate any objections on the grounds of having over-estimated the -volume, I have taken that at one half this estimate, viz., equal to -a current 50 miles broad and 1,000 feet deep flowing at the rate of -2 miles an hour. I have estimated the mean temperature of the stream -as it passes the Straits of Florida to be 65°, and have supposed that -the water in its course becomes ultimately cooled down on an average -to 40°. In this case each pound of water conveys 19,300 foot-pounds of -heat from the Gulf of Mexico, to be employed in warming temperate and -polar regions. Assuming these data to be correct, it follows that the -amount of heat transferred from the Gulf of Mexico by this stream per -day amounts to 77,479,650,000,000,000,000 foot-pounds. This enormous -quantity of heat is equal to one-fourth of all that is received from -the sun by the whole of the Atlantic Ocean from the Tropic of Cancer up -to the Arctic Circle. - -This is the amount of heat conveyed from inter-tropical to temperate -and polar regions by the Gulf-stream. What now is the amount conveyed -by means of the General Oceanic Circulation? - -According to this theory there ought to be as much warm water flowing -from inter-tropical regions towards the Antarctic as towards the Arctic -Circle. We may, therefore, in our calculations, consider that the heat -which is received in tropical regions to the south of the equator goes -to warm the southern hemisphere, and that received on the north side -of the equator to warm the northern hemisphere. The warm currents -found in the North Atlantic in temperate regions we may conclude came -from the regions lying to the north of the equator,—or, in other -words, from that part of the Atlantic lying between the equator and -the Tropic of Cancer. At least, according to the gravitation theory, -we have no reason to believe that the quantity of warm water flowing -from tropical to temperate and polar regions in the Atlantic is -greater than the area between the equator and the Tropic of Cancer -can supply—because it is affirmed that a very large proportion of the -cold water found in the North Atlantic comes, not from the arctic, but -from the antarctic regions. But if the North Atlantic is cooled by a -cold stream from the southern hemisphere, the southern hemisphere in -turn must be heated by a warm current from the North Atlantic—unless -we assume that the compensating current flowing from the Atlantic into -the southern hemisphere is as cold as the antarctic current, which is -very improbable. But Dr. Carpenter admits that the quantity of warm -water flowing from the Atlantic in equatorial regions towards the -south is even greater than that flowing northwards. “The unrestricted -communication,” he says, “which exists between the antarctic area and -the great Southern Ocean-basins would involve, if the doctrine of a -general oceanic circulation be admitted, a much more considerable -interchange of waters between the antarctic and the equatorial areas -than is possible in the northern hemisphere.”[85] - -We have already seen that, were it not for the great mass of warm water -which finds its way to the polar regions, the temperature of these -regions would be enormously lower than they really are. It has been -shown likewise that the comparatively high temperature of north-western -Europe is due to the same cause. But if it be doubtful whether the -Gulf-stream reaches our shores, and if it be true that, even supposing -it did, it “could only affect the _most superficial_ stratum,” and that -the great mass of warm water found by Dr. Carpenter in his dredging -expeditions came directly from the equatorial regions, and not from -the Gulf-stream, then the principal part of the heating-effect must be -attributed, not to the Gulf-stream, but to the general flow of water -from the equatorial regions. It surely would not, then, be too much to -assume that the quantity of heat conveyed from equatorial regions by -this general flow of water into the North Atlantic is at least equal to -that conveyed by the Gulf-stream. If we assume this to be the amount of -heat conveyed by the two agencies into the Atlantic from inter-tropical -regions, it will, of course, be equal to twice that conveyed by the -Gulf-stream alone. - -We shall now consider whether the area of the Atlantic to the north of -the equator is sufficient to supply the amount of heat demanded by Dr. -Carpenter’s theory. - -The entire area of the Atlantic, extending from the equator to the -Tropic of Cancer, including the Caribbean Sea and the Gulf of Mexico, -is about 7,700,000 square miles. - -The quantity of heat conveyed by the Gulf-stream through the Straits of -Florida is, as we have already endeavoured to show, equal to all the -heat received from the sun by 1,560,935 square miles at the equator. -The annual quantity of heat received from the sun by the torrid zone -per unit surface, taking the mean of the whole zone, is to that -received by the equator as 39 to 40, consequently the quantity of -heat conveyed by the Gulf-stream is equal to all the heat received by -1,600,960 square miles of the Atlantic in the torrid zone. - -But if, according to Dr. Carpenter’s views, the quantity of heat -conveyed from the tropical regions is double that conveyed by the -Gulf-stream, the amount of heat in this case conveyed into the Atlantic -in temperate regions will be equal to all the heat received from the -sun by 3,201,920 square miles of the Atlantic between the equator and -the Tropic of Cancer. This is 32/77ths of all the heat received from -the sun by that area. - -Taking the annual quantity received per unit surface at the equator at -1,000, the quantities received by the three zones would be respectively -as follows:— - - Equator 1000 - Torrid zone 975 - Temperate zone 757 - Frigid zone 454 - -Now, if we remove from the Atlantic in tropical regions 32/77ths of the -heat received from the sun, we remove 405 parts from every 975 received -from the sun, and consequently only 570 parts per unit surface remain. - -It has been shown[86] that the quantity of heat conveyed by the -Gulf-stream from the equatorial regions into the temperate regions -is equal to 100/412ths of all the heat received by the Atlantic in -temperate regions. But according to the theory under consideration the -quantity removed is double this, or equal to 100/206ths of all the heat -received from the sun. But the amount received from the sun is equal -to 757 parts per unit surface; add then to this 100/206ths of 757, or -367, and we have 1,124 parts of heat per unit surface as the amount -possessed by the Atlantic in temperate regions. The Atlantic should in -this case be much warmer in temperate than in tropical regions; for -in temperate regions it would possess 1,124 parts of heat per unit -surface, whereas in tropical regions it would possess only 570 parts -per unit surface. Of course the heat conveyed from tropical regions -does not all remain in temperate regions; a very considerable portion -of it must pass into the arctic regions. Let us, then, assume that -one half goes to warm the Arctic Ocean, and the other half remains -in the temperate regions. In this case 183·5 parts would remain, and -consequently 757 + 183·5 = 940·5 parts would be the quantity possessed -by the Atlantic in temperate regions, a quantity which still exceeds by -no less than 370·5 parts the heat possessed by the Atlantic in tropical -regions. - -As one half of the amount of heat conveyed from the tropical regions -is assumed to go into the Arctic Ocean, the quantity passing into -that ocean would therefore be equal to that which passes through the -Straits of Florida, an amount which, as we have found, is equal to all -the heat received from the sun by 3,436,900 square miles of the Arctic -Ocean.[87] The entire area covered by sea beyond the Arctic Circle is -under 5,000,000 square miles; but taking the Arctic Ocean in round -numbers at 5,000,000 square miles, the quantity of heat conveyed into -it by currents to that received from the sun would therefore be as -3,436,900 to 5,000,000. - -The amount received on the unit surface of the arctic regions we have -seen to be 454 parts. The amount received from the currents would -therefore be 312 parts. This gives 766 parts of heat per unit surface -as the quantity possessed by the Arctic Ocean. Thus the Arctic Ocean -also would contain more heat than the Atlantic in tropical regions; for -the Atlantic in these regions would, in the case under consideration, -possess only 570 parts, while the Arctic Ocean would possess 766 -parts. It is true that more rays are cut off in arctic regions than in -tropical; but still, after making due allowance for this, the Arctic -Ocean, if the theory we are considering were true, ought to be as warm -as, if not warmer than, the Atlantic in tropical regions. The relative -quantities of heat possessed by the three zones would therefore be as -follows:— - - Atlantic, in torrid zone 570 - 〃 in temperate zone 940 - 〃 in frigid zone 766 - -It is here assumed, however, that none of the heat possessed by the -Gulf-stream is derived from the southern hemisphere, which, we know, -is not the case. But supposing that as much as one half of the heat -possessed by the stream came from the southern hemisphere, and that the -other half was obtained from the seas lying between the equator and the -Tropic of Cancer, the relative proportions of heat possessed by the -three zones per given area would be as follows:— - - Atlantic, in torrid zone 671 - 〃 in temperate zone 940 - 〃 in frigid zone 766 - -This proves incontestably that, supposing there is such a general -oceanic circulation as is maintained, the quantity of heat conveyed by -means of it into the North Atlantic and Arctic Oceans must be trifling -in comparison with that conveyed by the Gulf-stream; for if it nearly -equalled that conveyed by the Gulf-stream, then not only the North -Atlantic in temperate regions, but even the Arctic Ocean itself would -be much warmer than the inter-tropical seas. In fact, so far as the -distribution of heat over the globe is concerned, it is a matter of -indifference whether there really is or is not such a thing as this -general oceanic circulation. The enormous amount of heat conveyed by -the Gulf-stream alone puts it beyond all doubt that ocean-currents are -the great agents employed in distributing over the globe the excess of -heat received by the sea in inter-tropical regions. - -It is therefore, so far as concerns the theory of a General Oceanic -Circulation, of the utmost importance that the advocates of that -theory should prove that I have over-estimated the thermal power of -the Gulf-stream. This, however, can only be done by detecting some -error either in my computation or in the data on which it is based; -yet neither Dr. Carpenter nor any one else, as far as I know, has -challenged the accuracy of my figures. The question at issue is the -correctness of the data; but the only part of the data which can -possibly admit of being questioned is my estimate of the _volume_ -and _temperature_ of the stream. Dr. Carpenter, however, does not -maintain that I have over-estimated the temperature of the stream; on -the contrary, he affirms that I have really under-estimated it. “If we -assume,” he remarks, “the limit of the stratum above 60° as that of -the real Gulf-stream current, we shall find its average temperature to -be somewhat higher than it has been stated by Mr. Croll, who seems to -have taken 65° as the average of the water flowing through the entire -channel. The average surface temperature of the Florida channel for -the whole year is 80°; and we may fairly set the average of the entire -outgoing stream, down to the plane of 60°, at 70°, instead of 65° as -estimated by Mr. Croll” (§ 141). It follows, then, that every pound of -water of the Gulf-stream actually conveys 5 units of heat more than -I have estimated it to do—the amount conveyed being 30 units instead -of 25 units as estimated by me. Consequently, if the Gulf-stream be -equal to that of a current of merely 41½ miles broad and 1,000 feet -deep, flowing at the rate of 2 miles an hour, it will still convey the -estimated quantity of heat. But this estimate of the volume of the -stream, let it be observed, barely exceeds _one-third_ of that given -by Herschel, Maury, and Colding,[88] and is little more than one-half -that assigned to it by Mr. Laughton, while it very little exceeds that -given by Mr. Findlay,[89] an author whom few will consider likely to -overrate either the volume or heating-power of the stream. - -The important results obtained during the _Challenger_ expedition have -clearly proved that I have neither over-estimated the temperature nor -the volume of the Gulf-stream. Between Bermuda and Sandy Hook the -stream is 60 miles broad and 600 feet deep, with a maximum velocity of -from 3½ to 4 miles an hour. If the mean velocity of the entire section -amounts to 2¼ miles an hour, which it probably does, the volume of the -stream must equal that given in my estimate. But we have no evidence -that all the water flowing through the Straits of Florida passes -through the section examined by the officers of the _Challenger_. Be -this, however, as it may, the observations made between St. Thomas -and Sandy Hook reveal the existence of an immense flow of warm water, -2,300 feet deep, entirely distinct from the water included in the above -section of the Gulf-stream proper. As the thickest portion of this -immense body of water joins the warm water of the Gulf-stream, Captain -Nares considers that “it is evidently connected with it, and probably -as an offshoot.” At Sandy Hook, according to him, it extends 1,200 -feet deeper than the Gulf-stream itself, but off Charleston, 600 miles -nearer the source, the same temperature is found at the same depth. -But whether it be an offshoot of the Gulf-stream or not, one thing is -certain, it can only come from the Gulf of Mexico or from the Caribbean -Sea. This mass of water, after flowing northwards for about 1,000 -miles, turns to the right and crosses the Atlantic in the direction of -the Azores, where it appears to thin out. - -If, therefore, we take into account the combined heat conveyed by -both streams, my estimate of the heat transferred from inter-tropical -regions into the North Atlantic will be found rather under than above -the truth. - -_Dr. Carpenter’s Estimate of the Thermal Work of the Gulf-stream._—In -the appendix to an elaborate memoir on Oceanic Circulation lately -read before the Geographical Society, Dr. Carpenter endeavours to -show that I have over-estimated the thermal work of the Gulf-stream. -In that memoir[90] he has also favoured us with his own estimate of -the sectional area, rate of flow, and temperature of the stream. Even -adopting his data, however, I find myself unable to arrive at his -conclusions. - -Let us consider first his estimate of the sectional area of the -stream. He admits that “it is impossible, in the present state of our -knowledge, to arrive at any exact estimate of the sectional area of the -stream; since it is for the most part only from the temperatures of -its different strata that we can judge whether they are, or are not, -in movement, and what is the direction of their movement.” Now it is -perfectly evident that our estimate of the sectional area of the stream -will depend upon what we assume to be its bottom temperature. If, for -example, we assume 70° to be the bottom temperature, we shall have a -small sectional area. Taking the temperature at 60°, the sectional -area will be larger, and if 50° be assumed to be the temperature, the -sectional area will be larger still, and so on. Now the small sectional -area obtained by Dr. Carpenter arises from the fact of his having -assumed the high temperature of 60° to be that of the bottom of the -stream. He concludes that all the water below 60° has an inward flow, -and that it is only that portion from 60° and upwards which constitutes -the Gulf-stream. I have been unable to find any satisfactory evidence -for assuming so high a temperature for the bottom of the stream. It -must be observed that the water underlying the Gulf-stream is not -the ordinary water of the Atlantic, but the cold current from the -arctic regions. In fact, it is the same water which reaches the -equator at almost every point with a temperature not much above the -freezing-point. It is therefore highly improbable that the under -surface of the Gulf-stream has a temperature so high as 60°. - -Dr. Carpenter’s method of measuring the mean velocity of the -Gulf-stream is equally objectionable. He takes the mean annual rate at -the surface in the “Narrows” to be two miles an hour and the rate at -the bottom to be zero, and he concludes from this that the average rate -of the whole is one mile an hour—the arithmetical mean between these -two extremes. Now it will be observed that this conclusion only holds -true on the supposition that the breadth of the stream is as great at -the bottom as at the surface, which of course it is not. All admit that -the sides of the Gulf-stream are not perpendicular, but slope somewhat -in the manner of the banks of a river. The stream is broad at the -surface and narrows towards the bottom. It is therefore evident that -the upper half of the section has a much larger area than the lower; -the quantity of water flowing through the upper half with a greater -velocity than one mile an hour must be much larger than the quantity -flowing through the lower half with a less velocity than one mile an -hour. - -His method of estimating the mean temperature of the stream is even -more objectionable. He says, “The average surface temperature of the -Florida Channel for the whole year is 80°, and we may set the average -of the entire outgoing stream down to the plane of 60° at 70°, instead -of 65°, as estimated by Mr. Croll.” If 80° be the surface and 60° be -the bottom temperature, temperature and rate of velocity being assumed -of course to decrease uniformly from the surface downwards, how is it -possible that 70° can be the average temperature? The amount of water -flowing through the upper half of the section, with a temperature above -70°, is far more than the amount flowing through the under half of the -section, with a temperature below 70°. Supposing the lower half of the -section to be as large as the upper half, which it is not, still the -quantity of water flowing through it would only equal one-third of -that flowing through the upper half, because the mean velocity of the -water in the lower half would be only half a mile per hour, whereas -the mean velocity of that in the upper half would be a mile and a half -an hour. But the area of the lower half is much less than that of the -upper half, consequently the amount of water whose temperature is under -70° must be even much under one-third of that, the temperature of which -is above 70°. - -Had Dr. Carpenter taken the proper method of estimating the mean -temperature, he would have found that 75°, even according to his own -data, was much nearer the truth than 70°. I pointed out, several years -ago,[91] the fallacy of estimating the mean temperature of a stream in -this way. - -So high a mean temperature as 75° for the Gulf-stream, even in the -Florida Channel, is manifestly absurd, but if 60° be the bottom -temperature of the stream, the mean temperature cannot possibly be much -under that amount. It is, of course, by under-estimating the sectional -area of the stream that its mean temperature is over-estimated. We -cannot reduce the mean temperature without increasing the sectional -area. If my estimate of 65° be taken as the mean temperature, which I -have little doubt will yet be found to be not far from the truth, Dr. -Carpenter’s estimate of the sectional area must be abandoned. For if -65° be the mean temperature of the stream, its bottom temperature must -be far under 60°, and if the bottom temperature be much under 60°, then -the sectional area must be greater than he estimates it to be. - -Be this, however, as it may; even if we suppose that 60° will -eventually be found to be the actual bottom temperature of the -Gulf-stream, nevertheless, if the total quantity of heat conveyed by -the stream from inter-tropical regions be estimated in the proper way, -we shall still find that amount to be so enormous, that there is not -sufficient heat remaining in those regions to supply Dr. Carpenter’s -oceanic circulation with a quantity as great for distribution in the -North Atlantic. - -It therefore follows (and so far as regards the theory of Secular -changes of climate, this is all that is worth contending for) that -Ocean-currents and not a General Oceanic Circulation resulting from -gravity, are the great agents employed in the distribution of heat over -the globe. - - - - - CHAPTER XII. - - MR. A. G. FINDLAY’S OBJECTIONS CONSIDERED. - - Mr. Findlay’s Estimate of the Volume of the Gulf-stream.—Mean - Temperature of a Cross Section less than Mean Temperature - of Stream.—Reason of such Diversity of Opinion regarding - Ocean-currents.—More rigid Method of Investigation necessary. - - -At the conclusion of the reading of Dr. Carpenter’s paper before the -Royal Geographical Society, on January 9th, 1871, Mr. Findlay made the -following remarks:— - -“When, by the direction of the United States Government, ten or eleven -years ago, the narrowest part of the Gulf-stream was examined, figures -were obtained which shut out all idea of its ever reaching our shores -as a heat-bearing current. In the narrowest part, certainly not more -than from 250 to 300 cubic miles of water pass per diem. Six months -afterwards that water reaches the banks of Newfoundland, and nine or -twelve months afterwards the coast of England, by which time it is -popularly supposed to cover an area of 1,500,000 square miles. The -proportion of the water that passes through the Gulf of Florida will -not make a layer of water more than 6 inches thick per diem over such -a space. Every one knows how soon a cup of tea cools; and yet it is -commonly imagined that a film of only a few inches in depth, after the -lapse of so long a time, has an effect upon our climate. There is no -need for calculations; the thing is self-evident.”[92] - -About five years ago, Mr. Findlay objected to the conclusions which I -had arrived at regarding the enormous heating-power of the Gulf-stream -on the ground that I had over-estimated the volume of the stream. He -stated that its volume was only about the half of what I had estimated -it to be. To obviate this objection, I subsequently reduced the volume -to one-half of my former estimate.[93] But taking the volume at this -low estimate, it was nevertheless found that the quantity of heat -conveyed into the Atlantic through the Straits of Florida by means of -the stream was equal to about _one-fourth_ of all the heat received -from the sun by the Atlantic from the latitude of the Strait of Florida -up to the Arctic Circle. - -Mr. Findlay, in his paper read before the British Association, affirmed -that the volume of the stream is somewhere from 294 to 333 cubic miles -per day; but in his remarks at the close of Dr. Carpenter’s address, he -stated it to be not greater than from 250 to 300 cubic miles per day. I -am unable to reconcile any of those figures with the data from which he -appears to have derived them. In his paper to the British Association, -he remarks that “the Gulf-stream at its outset is not more than 39½ -miles wide, and 1,200 feet deep.” From all attainable data, he computes -the mean annual rate of motion to be 65·4 miles per day; but as the -rate decreases with the depth, the mean velocity of the whole mass does -not exceed 49·4 miles per day. When he speaks of the mean velocity of -the Gulf-stream being so and so, he must refer to the mean velocity at -some particular place. This is evident; for the mean velocity entirely -depends upon the sectional area of the stream. The place where the -mean velocity is 49·4 miles per day must be the place where it is 39½ -miles broad and 1,200 feet deep; for he is here endeavouring to show us -how small the volume of the stream actually is. Now, unless the mean -velocity refers to the place where he gives us the breadth and depth -of the stream, his figures have no bearing on the point in question. -But a stream 39½ miles broad and 1,200 feet deep has a sectional area -of 8·97 square miles, and this, with a mean velocity of 49·4 miles -per day, will give 443 cubic miles of water. The amount, according to -my estimate, is 459 cubic miles per day; it therefore exceeds Mr. -Findlay’s estimate by only 16 cubic miles. - -Mr. Findlay does not, as far as I know, consider that I have -over-estimated the mean temperature of the stream. He states[94] that -between Sand Key and Havana the Gulf-stream is about 1,200 feet deep, -and that it does not reach the summit of a submarine ridge, which he -states has a temperature of 60°. It is evident, then, that the bottom -of the stream has a temperature of at least 60°, which is within 5° of -what I regard as the mean temperature of the mass. But the surface of -the stream is at least 17° above this mean. Now, when we consider that -it is at the upper parts of the stream, the place where the temperature -is so much above 65°, that the motion is greatest, it is evident that -the mean temperature of the entire moving mass must, according to Mr. -Findlay, be considerably over 65°. It therefore follows, according -to his own data, that the Gulf-stream conveys into the Atlantic an -amount of heat equal to one-fourth of all the heat which the Atlantic, -from the latitude of the Straits of Florida up to the arctic regions, -derives from the sun. - -But it must be borne in mind that although the mean temperature of the -cross section should be below 65°, it does not therefore follow that -the mean temperature of the _water flowing through this cross section_ -must be below that temperature, for it is perfectly obvious that the -mean temperature of the mass of water flowing through the cross section -in a given time must be much higher than that of the cross section -itself. The reason is very simple. It is in the upper half of the -section where the high temperature exists; but as the velocity of the -stream is much greater in its upper than in its lower half, the greater -portion of the water passing through this cross section is water of -high temperature. - -But even supposing we were to halve Mr. Findlay’s own estimate, and -assume that the volume of the stream is equal to only 222 cubic miles -of water per day instead of 443, still the amount of heat conveyed -would be equal to one-eighth part of the heat received from the sun by -the Atlantic. But would not the withdrawal of an amount of heat equal -to one-eighth of that received from the sun greatly affect the climate -of the Atlantic? Supposing we take the mean temperature of the Atlantic -at, say, 56°; this will make its temperature 295° above that of space. -Extinguish the sun and stop the Gulf-stream, and the temperature ought -to sink 295°. How far, then, ought the temperature to sink, supposing -the sun to remain and the Gulf-stream to stop? Would not the withdrawal -of the stream cause the temperature to sink some 30°? Of course, if -the Gulf-stream were withdrawn and everything else were to remain the -same, the temperature of the Atlantic would not actually remain 30° -lower than at present; for heat would flow in from all sides and partly -make up for the loss of the stream. But nevertheless 30° represents the -amount of temperature maintained by means of the heat from the stream. -And this, be it observed, is taking the volume of the stream at a lower -estimate than even Mr. Findlay himself would be willing to admit. Mr. -Findlay says that, by the time the Gulf-stream reaches the shores of -England, it is supposed to cover a space of 1,500,000 square miles. -“The proportion of water that passes through the Straits of Florida -will not make,” according to him, “a layer of water more than 6 inches -thick per diem over such a space.” But a layer of water 6 inches thick -cooling 25° will give out 579,000 foot-pounds of heat per square foot. -If, therefore, the Gulf-stream, as he asserts, supplies 6 inches per -day to that area, then every square foot of the area gives off per -day 579,000 foot-pounds of heat. The amount of heat received from the -sun per square foot in latitude 55°, which is not much above the mean -latitude of Great Britain, is 1,047,730 foot-pounds per day, taking, of -course, the mean of the whole year; _consequently this layer of water -gives out an amount of heat equal to more than_ one-half _of all that -is received from the sun_. But assuming that the stream should leave -the half of its heat on the American shores and carry to the shores of -Britain only 12½° of heat, still we should have 289,500 foot-pounds per -square foot, which notwithstanding _is more than equal to_ one-fourth -_of that received from the sun_. If an amount of heat so enormous -cannot affect climate, what can? - -I shall just allude to one other erroneous notion which prevails in -regard to the Gulf-stream; but it is an error which I by no means -attribute either to Mr. Findlay or to Dr. Carpenter. The error to which -I refer is that of supposing that when the Gulf-stream widens out to -hundreds of miles, as it does before it reaches our shores, its depth -must on this account be much less than when it issues from the Gulf of -Mexico. Although the stream may be hundreds of miles in breadth, there -is no necessity why it should be only 6 inches, or 6 feet, or 60 feet, -or even 600 feet in depth. It may just as likely be 6,000 feet deep as -6 inches. - -_The Reason why such Diversity of Opinion prevails in Regard to -Ocean-currents._—In conclusion I venture to remark that more than -nine-tenths of all the error and uncertainty which prevail, both -in regard to the cause of ocean-currents and to their influence on -climate, is due, not, as is generally supposed, to the intrinsic -difficulties of the subject, but rather to the defective methods -which have hitherto been employed in its investigation—that is, in -not treating the subject according to the rigid methods adopted in -other departments of physics. What I most particularly allude to is -the disregard paid to the modern method of determining the amount of -effects in _absolute measure_. - -But let me not be misunderstood on this point. I by no means suppose -that the _absolute quantity_ is the thing always required for its -own sake. It is in most cases required simply as a means to an end; -and very often that end is the knowledge of the _relative_ quantity. -Take, for example, the Gulf-stream. Suppose the question is asked, -to what extent does the heat conveyed by that stream influence the -climate of the North Atlantic? In order to the proper answering of this -question, the principal thing required is to know what proportion the -amount of heat conveyed by the stream into the Atlantic bears to that -received from the sun by that area. We want the _relative proportions_ -of these two quantities. But how are we to obtain them? We can only -do so by determining first the _absolute_ quantity of each. We must -first measure each before we can know how much the one is greater -than the other, or, in other words, before we can know their relative -proportions. We have the means of determining the absolute amount -of heat received from the sun by a given area at any latitude with -tolerable accuracy; but the same cannot be done with equal accuracy in -regard to the amount of heat conveyed by the Gulf-stream, because the -volume and mean temperature of the stream are not known with certainty. -Nevertheless we have sufficient data to enable us to fix upon such a -maximum and minimum value to these quantities as will induce us to -admit that the truth must lie somewhere between them. In order to give -full justice to those who maintain that the Gulf-stream exercises -but little influence on climate, and to put an end to all further -objections as to the uncertainty of my data, I shall take a minimum -to which none of them surely can reasonably object, viz. that the -volume of the stream is not over 230 cubic miles per day, and the heat -conveyed per pound of water not over 12½ units. Calculating from these -data, we find that the amount of heat carried into the North Atlantic -is equal to one-sixteenth of all the heat received from the sun by that -area. There are, I presume, few who will not admit that the actual -proportion is much higher than this, probably as high as 1 to 3, or 1 -to 4. But, who, without adopting the method I have pursued, could ever -have come to the conclusion that the proportion was even 1 to 16? He -might have guessed it to be 1 to 100 or 1 to 1000, but he never would -have guessed it to be 1 to 16. Hence the reason why the great influence -of the Gulf-stream as a heating agent has been so much under-estimated. - -The same remarks apply to the gravitation theory of the cause of -currents. Viewed simply as a theory it looks very reasonable. There is -no one acquainted with physics but will admit that the tendency of the -difference of temperature between the equator and the poles is to cause -a surface current from the equator towards the poles, and an under -current from the poles to the equator. But before we can prove that -this tendency does actually produce such currents, another question -must be settled, viz. is this force sufficiently great to produce -the required motion? Now when we apply the method to which I refer, -and determine the absolute amount of the force resulting from the -difference of specific gravity, we discover that not to be the powerful -agent which the advocates of the gravitation theory suppose, but a -force so infinitesimal as not to be worthy of being taken into account -when considering the causes by which currents are produced. - - - - - CHAPTER XIII. - - THE WIND THEORY OF OCEANIC CIRCULATION. - - Ocean-currents not due alone to the Trade-winds.—An Objection - by Maury.—Trade-winds do not explain the Great Antarctic - Current.—Ocean-currents due to the System of Winds.—The - System of Currents agrees with the System of the - Winds.—Chart showing the Agreement between the System - of Currents and System of Winds.—Cause of the Gibraltar - Current.—North Atlantic an immense Whirlpool.—Theory of Under - Currents.—Difficulty regarding Under Currents obviated.—Work - performed by the Wind in impelling the Water forward.—The - _Challenger’s_ crucial Test of the Wind and Gravitation - Theories.—North Atlantic above the Level of Equator.—Thermal - Condition of the Southern Ocean irreconcilable with the - Gravitation Theory. - - -_Ocean-currents not due alone to the Trade-winds._—The generally -received opinion amongst the advocates of the wind theory of oceanic -circulation is that the Gulf-stream and other currents of the ocean are -due to the impulse of the trade-winds. The tendency of the trade-winds -is to impel the inter-tropical waters along the line of the equator -from east to west; and were those regions not occupied in some places -by land, this equatorial current would flow directly round the -globe. Its westward progress, however, is arrested by the two great -continents, the old and the new. On approaching the land the current -bifurcates, one portion trending northwards and the other southwards. -The northern branch of the equatorial current of the Atlantic passes -into the Caribbean Sea, and after making a circuit of the Gulf of -Mexico, flows northward and continues its course into the Arctic -Ocean. The southern branch, on the other hand, is deflected along the -South-American coast, constituting what is known as the Brazilian -current. In the Pacific a similar deflection occurs against the -Asiatic coast, forming a current somewhat resembling the Gulf-stream, -a portion of which (Kamtschatka current) in like manner passes into -the arctic regions. In reference to all these various currents, the -impelling cause is supposed to be the force of the trade-winds. - -It is, however, urged as an objection by Maury and other advocates of -the gravitation theory, that a current like the Gulf-stream, extending -as far as the arctic regions, could not possibly be impelled and -maintained by a force acting at the equatorial regions. But this is -a somewhat weak objection. It seems to be based upon a misconception -of the magnitude of the force in operation. It does not take into -account that this force acts on nearly the whole area of the ocean in -inter-tropical regions. If, in a basin of water, say three feet in -diameter, a force is applied sufficient to produce a surface-flow one -foot broad across the centre of the basin, the water impelled against -the side will be deflected to the extremes of the vessel. And this -result does not in any way depend upon the size of the basin. The -same effect which occurs in a small basin will occur in a large one, -provided the proportion between the breadth of the belt of water put in -motion and the size of the vessel be the same in both cases. It does -not matter, therefore, whether the diameter of the basin be supposed to -be three feet, or three thousand miles, or ten thousand miles. - -There is a more formidable objection, however, to the theory. -The trade-winds will account for the Gulf-stream, Brazil, Japan, -Mozambique, and many other currents; but there are currents, such as -some of the polar currents, which cannot be so accounted for. Take, -for example, the great antarctic current flowing northward into the -Pacific. This current does not bend to the left under the influence -of the earth’s rotation and continue its course in a north-westerly -direction, but actually bends round to the right and flows eastward -against the South-American coast, in direct opposition both to the -influence of rotation and to the trade-winds. The trade-wind theory, -therefore, is insufficient to account for all the facts. But there is -yet another explanation, which satisfactorily solves our difficulties. -The currents of the ocean owe their origin, not to the trade-winds -alone, but to the _prevailing_ winds of the globe (including, of -course, the trade-winds). - -_Ocean-currents due to the System of Winds._—If we leave out of account -a few small inland sheets of water, the globe may be said to have but -one sea, just as it possesses only one atmosphere. We have accustomed -ourselves, however, to speak of parts or geographical divisions of -the one great ocean, such as the Atlantic and the Pacific, as if they -were so many separate oceans. And we have likewise come to regard the -currents of the ocean as separate and independent of one another. This -notion has no doubt to a considerable extent militated against the -acceptance of the theory that the currents are caused by the winds, and -not by difference of specific gravity; for it leads to the conclusion -that currents in a sea must flow in the direction of the prevailing -winds blowing over that particular sea. The proper view of the matter, -as I hope to be able to show, is that which regards the various -currents merely as members of one grand system of circulation produced, -not by the trade-winds alone, nor by the prevailing winds proper alone, -but by the combined action of all the prevailing winds of the globe, -regarded as one system of circulation. - -If the winds be the impelling cause of currents, the _direction_ of the -currents will depend upon two circumstances, viz.:—(1) the direction -of the prevailing winds of the globe, including, of course, under this -term the prevailing winds proper and the trade-winds; and (2) the -conformation of land and sea. It follows, therefore, that as a current -in any given sea is but a member of a general system of circulation, -its direction is determined, not alone by the prevailing winds blowing -over the sea in question, but by the general system of prevailing -winds. It may consequently sometimes happen that the general system -of winds may produce a current directly opposite to the prevailing -wind blowing over the current. The accompanying Chart (Plate I.) shows -how exactly the system of ocean-currents agrees with the system of -the prevailing winds. The fine lines indicate the paths of the -prevailing winds, and the fine arrows the direction in which the wind -blows along those paths. The large arrows show the direction of the -principal ocean-currents. - - [Illustration: PLATE I. - - CHART SHOWING THE GENERAL AGREEMENT BETWEEN THE SYSTEM OF OCEAN - CURRENTS AND WINDS. - - W. & A. K. Johnston, Edinb^r. and London.] - -The directions and paths of the prevailing winds have been taken from -Messrs. Johnston’s small physical Atlas, which, I find, agrees exactly -with the direction of the prevailing winds as deduced from the four -quarterly wind charts lately published by the Hydrographic Department -of the Admiralty. The direction of the ocean-currents has been taken -from the Current-chart published by the Admiralty. - -In every case, without exception, the direction of the main currents of -the globe agrees exactly with the direction of the prevailing winds. -There could not possibly be a more convincing proof that those winds -are the cause of the ocean-currents than this general agreement of the -two systems as indicated by the chart. Take, for example, the North -Atlantic. The Gulf-stream follows exactly the path of the prevailing -winds. The Gulf-stream bifurcates in mid-Atlantic; so does the wind. -The left branch of the stream passes north-eastwards into the arctic -regions, and the right branch south-eastwards by the Azores; so does -the wind. The south-eastern branch of the stream, after passing the -Canaries, re-enters the equatorial current and flows into the Gulf -of Mexico; the same, it will be observed, holds true of the wind. A -like remarkable agreement exists in reference to all the other leading -currents of the ocean. This is particularly seen in the case of the -great antarctic current between long. 140° W. and 160° W. This current, -flowing northwards from the antarctic regions, instead of bending to -the left under the influence of rotation, turns to the right when it -enters the regions of the westerly winds, and flows eastwards towards -the South-American shores. In fact, all the currents in this region of -strong westerly winds flow in an easterly or north-easterly direction. - -Taking into account the effects resulting from the conformation of -sea and land, the system of ocean-currents agrees precisely with -the system of the winds. All the principal currents of the globe are -in fact moving in the exact direction in which they ought to move, -assuming the winds to be the sole impelling cause. In short, so perfect -is the agreement between the two systems, that, given the system of -winds and the conformation of sea and land, and the direction of all -the currents of the ocean, or more properly the system of oceanic -circulation, might be determined _à priori_. Or given the system of the -ocean-currents together with the conformation of sea and land, and the -direction of the prevailing winds could also be determined _à priori_. -Or, thirdly, given the system of winds and the system of currents, -and the conformation of sea and land might be roughly determined. For -example, it can be shown by this means that the antarctic regions -are probably occupied by a continent and not by a number of separate -islands, nor by sea. - -While holding that the currents of the ocean form one system of -circulation, we must not be supposed to mean that the various currents -are connected end to end, having the same water flowing through them -all in succession like that in a heating apparatus. All that is -maintained is simply this, that the currents are so mutually related -that any great change in one would modify the conditions of all the -others. For example, a great increase or decrease in the easterly flow -of antarctic water in the Southern Ocean would decrease or increase, -as the case might be, the strength of the West Australian current; -and this change would modify the equatorial current of the Indian -Ocean, a modification which in like manner would affect the Agulhas -current and the Southern Atlantic current—this last leading in turn -to a modification of the equatorial current of the Atlantic, and -consequently of the Brazilian current and the Gulf-stream. Furthermore, -since a current impelled by the winds, as Mr. Laughton in his excellent -paper on Ocean-currents justly remarks, tends to leave a vacancy -behind, it follows that a decrease or increase in the Gulf-stream would -affect the equatorial current, the Agulhas current, and all the other -currents back to the antarctic currents. Again, a large modification -in the great antarctic drift-current would in like manner affect all -the currents of the Pacific. On the other hand, any great change in -the currents of the Pacific would ultimately affect the currents of -the Atlantic and Indian Oceans, through its influence on the Cape Horn -current, the South Australian current, and the current passing through -the Asiatic archipelago; and _vice versâ_, any changes in the currents -of the Atlantic or Indian Oceans would modify the currents of the -Pacific. - -_Cause of Gibraltar Current._—I may now consider the cause of the -Gibraltar current. There can be little doubt that this current owes its -origin (as Mr. Laughton points out) to the Gulf-stream. “I conceive,” -that author remarks, “that the Gibraltar current is distinctly a stream -formed by easterly drift of the North Atlantic, which, although it -forms a southerly current on the coast of Portugal, is still strongly -pressed to the eastward and seeks the first escape it can find. So -great indeed does this pressure seem to be, that more water is forced -through the Straits than the Mediterranean can receive, and a part -of it is ejected in reverse currents, some as lateral currents on -the surface, some, it appears, as an under current at a considerable -depth.”[95] The funnel-shaped nature of the strait through which the -water is impelled helps to explain the existence of the under current. -The water being pressed into the narrow neck of the channel tends to -produce a slight banking up; and as the pressure urging the water -forward is greatest at the surface and diminishes rapidly downwards, -the tendency to the restoration of level will cause an underflow -towards the Atlantic, because below the surface the water will find the -path of least resistance. It is evident indeed that this underflow will -not take place toward the Mediterranean, from the fact that that sea is -already filled to overflowing by the current received from the outside -ocean. - -If we examine the Current-chart published by the Hydrographic -Department of the Admiralty, we shall find the Gibraltar current -represented as merely a continuation of the S.E. flow of Gulf-stream -water. Now, if the arrows shown upon this chart indicate correctly the -direction of the flow, we must become convinced that the Gulf-stream -water cannot possibly avoid passing through the Gibraltar Strait. Of -course the excess of evaporation over that of precipitation within -the Mediterranean area would alone suffice to produce a considerable -current through the Strait; but this of itself would not fill that -inland sea to overflowing.[96] - -The Atlantic may, in fact, be regarded as an immense whirlpool with the -Saragossa Sea as its vortex; and although it is true, as will be seen -from an inspection of the Chart, that the wind blows round the Atlantic -along the very path taken by the water, impelling the water forward -along every inch of its course, yet nevertheless it must hold equally -true that the water has a tendency to flow off in a straight line at -a tangent to the circular course in which it is moving. But the water -is so hemmed in on all sides that it cannot leave this circular path -except only at two points; and at these two points it actually does -flow outwards. On the east and west sides the land prevents any such -outflow. Similarly, in the south the escape of the water is frustrated -by the pressure of the opposing currents flowing from that quarter; -while in the north it is prevented by the pressure exerted by polar -currents from Davis Strait and the Arctic Ocean. But in the Strait of -Gibraltar and in the north-eastern portion of the Atlantic between -Iceland and the north-eastern shores of Europe there is no resistance -offered: and at these two points an outflow does actually take place. -In both cases, however, especially the latter, the outflow is greatly -aided by the impulse of the prevailing winds. - -No one, who will glance at the accompanying chart (Plate I.) showing -how the north-eastern branch of the Gulf-stream bends round and, of -course, necessarily presses against the coast, can fail to understand -how the Atlantic water should be impelled into the Gibraltar Strait, -even although the loss sustained by the Mediterranean from evaporation -did not exceed the gain from rain and rivers. - -_Theory of Under Currents._—The consideration that ocean-currents are -simply parts of a system of circulation produced by the system of -prevailing winds, and not by the impulse of the trade-winds alone, -helps to remove the difficulty which some have in accounting for the -existence of under currents without referring them to difference of -specific gravity. Take the case of the Gulf-stream, which passes -under the polar stream on the west of Spitzbergen, this latter stream -passing in turn under the Gulf-stream a little beyond Bear Island. The -polar streams have their origin in the region of prevailing northerly -winds, which no doubt extends to the pole. The current flowing past -the western shores of Spitzbergen, throughout its entire course up -to near the point where it disappears under the warm waters of the -Gulf-stream, lies in the region of these same northerly winds. Now why -should this current cease to be a surface current as soon as it passes -out of the region of northerly into that of south-westerly winds? The -explanation seems to be this: when the stream enters the region of -prevailing south-westerly winds, its progress southwards along the -surface of the ocean is retarded both by the wind and by the surface -water moving in opposition to its course; but being continually pressed -forward by the impulse of the northerly winds acting along its whole -course back almost to the pole, perhaps, or as far north at least as -the sea is not wholly covered with ice, the polar current cannot stop -when it enters the region of opposing winds and currents; it must move -forward. But the water thus pressed from behind will naturally take -the _path of least resistance_. Now in the present case this path will -necessarily lie at a considerable distance below the surface. Had the -polar stream simply to contend with the Gulf-stream flowing in the -opposite direction, it would probably keep the surface and continue its -course along the side of that stream; but it is opposed by the winds, -from which it cannot escape except by dipping down under the surface; -and the depth to which it will descend will depend upon the depth of -the surface current flowing in the opposite direction. There is no -necessity for supposing a heaping up of the water in order to produce -by pressure a force sufficient to impel the under current. The pressure -of the water from behind is of itself enough. The same explanation, of -course, applies to the case of the Gulf-stream passing under the polar -stream. And if we reflect that these under currents are but parts of -the general system of circulation, and that in most cases they are -currents compensating for water drained off at some other quarter, we -need not wonder at the distance which they may in some cases flow, as, -for example, from the banks of Newfoundland to the Gulf of Mexico. -The under currents of the Gulf-stream are necessary to compensate for -the water impelled southwards by the northerly winds; and again, the -polar under currents are necessary to compensate for the water impelled -northward by the south and south-westerly winds. - -But it may be asked, how do the opposing currents succeed in crossing -each other? It is evident that the Gulf-stream must plunge through -the whole thickness of the polar stream before it can become an -under current, and so likewise must the cold water of the polar-flow -pass through the genial water of the Gulf-stream in order to get -underneath it and continue on its course towards the south. The -accompanying diagram (Plate II., Fig. 1) will render this sufficiently -intelligible. - - [Illustration: _Fig. 3_ PLATE II. - - _Map shewing meeting of the Gulf-stream and Polar Current (from - D^r. Petermann’s Geographische Mittheilungen._) _The curved lines - are Isotherms; temperatures are in Fahrenheit._] - - [Illustration: _Fig. 1_ - - _Diagram to shew how two opposing currents intersect each other_] - - [Illustration: _Surface Plan to shew how two opposing currents meet - each other_ - - W. & A. K. Johnston, Edinb^r. and London. - - _Fig. 2_] - -Now these two great ocean-currents are so compelled to intersect each -other for the simple reason that they cannot turn aside, the one to the -left and the other to the right. When two broad streams like those in -question are pressed up against each other, they succeed in mutually -intersecting each other’s path by breaking up into bands or belts—the -cold water being invaded and pierced as it were by long tongues of -warm water, while at the same time the latter is similarly intersected -by corresponding protrusions of cold water. The two streams become -in a manner interlocked, and the one passes through the other very -much as we pass the fingers of one hand between the fingers of the -other. The diagram (Plate II., Fig. 2), representing the surface of -the ocean at the place of meeting of two opposing currents, will show -this better than description. At the surface the bands necessarily -assume the tongue-shaped appearance represented in the diagram, but -when they have succeeded in mutually passing down through the whole -thickness of the opposing currents, they then unite and form two -definite under currents, flowing in opposite directions. The polar -bands, after penetrating the Gulf-stream, unite below to form a -southward-flowing under current, and in the same way the Gulf-stream -bands, uniting underneath the polar current, continue in their -northerly course as a broad under current of warm water. That this is -a correct representation of what actually occurs in nature becomes -evident from an inspection of the current charts. Thus in the chart -of the North Atlantic which accompanies Dr. Petermann’s Memoir on the -Gulf-stream, we observe that south of Spitzbergen the polar current and -the Gulf-stream are mutually interpenetrated—long tongues invading and -dipping down underneath the Gulf-stream, while in like manner the polar -current becomes similarly intersected by well-marked protrusions of -warm water flowing from the south. (See Plate II., Fig. 3.) - -No accurate observations, as far as I know, have been made regarding -the amount of work performed by the wind in impelling the water -forward; but when we consider the great retarding effect of objects -on the earth’s surface, it is quite apparent that the amount of work -performed on the surface of the ocean must be far greater than is -generally supposed. For example, Mr. Buchan, Secretary to the Scottish -Meteorological Society, has shown[97] that a fence made of slabs of -wood three inches in width and three inches apart from each other is a -protection even during high winds to objects on the lee side of it, and -that a wire screen with meshes about an inch apart affords protection -during a gale to flower-pots. The same writer was informed by Mr. Addie -that such a screen put up at Rockville was torn to pieces by a storm of -wind, the wire screen giving way much in the same way as sails during a -hurricane at sea. - -_The “Challenger’s” Crucial Test of the Wind and Gravitation Theories -of Oceanic Circulation._—It has been shown in former chapters that all -the facts which have been adduced in support of the gravitation theory -are equally well explained by the wind theory. We may now consider a -class of facts which do not appear to harmonize with either theory. The -recent investigations of the _Challenger_ Expedition into the thermal -state of the ocean reveal a condition of things which appears to me -utterly irreconcilable with the gravitation theory. - -It is a condition absolutely essential to the gravitation theory that -the surface of the ocean should be highest in equatorial regions and -slope downwards to either pole. Were water absolutely frictionless, an -incline, however small, would be sufficient to produce a surface-flow -from the equator to the poles, but to induce such an effect some slope -there must be, or gravitation could exercise no power in drawing the -surface-water polewards. - -The researches of the _Challenger_ Expedition bring to light the -striking and important fact that the general surface of the North -Atlantic in order to produce equilibrium must stand at a higher level -than at the equator. In other words the surface of the Atlantic is -lowest at the equator, and rises with a gentle slope to well-nigh the -latitude of England. If this be the case, then it is mechanically -impossible that, as far as the North Atlantic is concerned, there can -be any such general movement as Dr. Carpenter believes. Gravitation can -no more cause the surface-water of the Atlantic to flow towards the -arctic regions than it can compel the waters of the Gulf of Mexico up -the Mississippi into the Missouri. The impossibility is equally great -in both cases. - -In order to prove what has been stated, let us take a section of the -mid-Atlantic, north and south, across the equator; and, to give the -gravitation theory every advantage, let us select that particular -section adopted by Dr. Carpenter as the one of all others most -favourable to his theory, viz., Section marked No. VIII. in his memoir -lately read before the Royal Geographical Society.[98] - -The fact that the polar cold water comes so near the surface at the -equator is regarded by Dr. Carpenter as evidence in favour of the -gravitation theory. On first looking at Dr. Carpenter’s section it -forcibly struck me that if it was accurately drawn, the ocean to be -in equilibrium would require to stand at a higher level in the North -Atlantic than at the equator. In order, therefore, to determine -whether this is the case or not I asked the hydrographer of the -Admiralty to favour me with the temperature soundings indicated in the -section, a favour which was most obligingly granted. The following -are the temperature soundings at the three stations A, B, and C. The -temperature of C are the mean of six soundings taken along near the -equator:— - - +--------+----------------+----------------+----------------------+ - | | A | B | C | - | | | | | - | Depth |Lat. 37° 54′ N. |Lat. 23° 10′ N. | Mean of six | - | in |Long. 41° 44′ W.|Long. 38° 42′ W.|temperature soundings | - |Fathoms.| | | near equator. | - | | | +---------+------------+ - | | Temperature. | Temperature. | Depth in|Temperature.| - | | | | Fathoms.| | - +--------+----------------+----------------+---------+------------+ - | | ° | ° | | ° | - |Surface.| 70·0 | 72·0 |Surface. | 77·9 | - | 100 | 63·5 | 67·0 | 10 | 77·2 | - | 200 | 60·6 | 57·6 | 20 | 77·1 | - | 300 | 60·0 | 52·5 | 30 | 76·9 | - | 400 | 54·8 | 47·7 | 40 | 71·7 | - | 500 | 46·7 | 43·7 | 50 | 64·0 | - | 600 | 41·6 | 41·7 | 60 | 60·4 | - | 700 | 40·6 | 40·6 | 70 | 59·4 | - | 800 | 38·1 | 39·4 | 80 | 58·0 | - | 900 | 37·8 | 39·2 | 90 | 58·0 | - | 1000 | 37·9 | 38·3 | 100 | 55·6 | - | 1100 | 37·1 | 38·0 | 150 | 51·0 | - | 1200 | 37·1 | 37·6 | 200 | 46·6 | - | 1300 | 37·2 | 36·7 | 300 | 42·2 | - | 1400 | 37·1 | 36·9 | 400 | 40·3 | - | 1500 | .. | 36·7 | 500 | 38·9 | - | 2700 | 35·2 | .. | 600 | 39·2 | - | 2720 | .. | 35·4 | 700 | 39·0 | - | | | | 800 | 39·1 | - | | | | 900 | 38·2 | - | | | | 1000 | 36·9 | - | | | | 1100 | 37·6 | - | | | | 1200 | 36·7 | - | | | | 1300 | 35·8 | - | | | | 1400 | 36·4 | - | | | | 1500 | 36·1 | - | | | | Bottom. | 34·7 | - +--------+----------------+----------------+---------+------------+ - -On computing the extent to which the three columns A, B, and C are each -expanded by heat according to Muncke’s table of the expansion of sea -water for every degree Fahrenheit, I found that column B, in order to -be in equilibrium with C (the equatorial column), would require to have -its surface standing fully 2 feet 6 inches above the level of column C, -and column A fully 3 feet 6 inches above that column. In short, it is -evident that there must be a gradual rise from the equator to latitude -38° N. of 3½ feet. Any one can verify the accuracy of these results by -making the necessary computations for himself.[99] - - [Illustration: PLATE III. - - W. & A. K. Johnston, Edinb^r. and London. - - SECTION OF THE ATLANTIC nearly North and South, between LAT. 38° N. - & LAT. 38° S.] - -I may observe that, had column C extended to the same depth as columns -A and B, the difference of level would be considerably greater, for -column C requires to balance only that portion of columns A and B -which lies above the level of its base. Suppose a depth of ocean equal -to that of column C to extend to the north pole, and the polar water -to have a uniform temperature of 32° from the surface to the bottom, -then, in order to produce equilibrium, the surface of the ocean at -the equator would require to be 4 feet 6 inches above that at the -pole. But the surface of the ocean at B would be 7 feet, and at A 8 -feet, above the poles. Gravitation never could have caused the ocean -to assume this form. It is impossible that this immense mass of warm -water, extending to such a depth in the North Atlantic, could have been -brought from equatorial regions by means of gravitation. And, even -if we suppose this accumulation of warm water can be accounted for -by some other means, still its presence precludes the possibility of -any such surface-flow as that advocated by Dr. Carpenter. For so long -as the North Atlantic stands 3½ feet above the level of the equator, -gravitation can never move the equatorial waters polewards. - -There is another feature of this section irreconcilable with the -gravitation theory. It will be observed that the accumulation of warm -water is all in the North Atlantic, and that there is little or none -in the south. But according to the gravitation theory it ought to -have been the reverse. For owing to the unrestricted communication -between the equatorial and antarctic regions, the general flow of -water towards the south pole is, according to that theory, supposed to -be greater than towards the north, and consequently the quantity of -warm equatorial water in the South Atlantic ought also to be greater. -Dr. Carpenter himself seems to be aware of this difficulty besetting -the theory, and meets it by stating that “the upper stratum of the -North Atlantic is not nearly as much cooled down by its limited polar -underflow, as that of the South Atlantic is by the vast movement of -antarctic water which is constantly taking place towards the equator.” -But this “vast movement of antarctic water” necessarily implies a vast -counter-movement of warm surface-water. So that if there is more polar -water in the South Atlantic to produce the cooling effect, there should -likewise be more warm water to be cooled. - -According to the wind theory of oceanic circulation the explanation of -the whole phenomena is simple and obvious. It has already been shown -that owing to the fact that the S. E. trades are stronger than the N. -E., and blow constantly over upon the northern hemisphere, the warm -surface-water of the South Atlantic is drifted across the equator. It -is then carried by the equatorial current into the Gulf of Mexico, and -afterwards of course forms a part of the Gulf-stream. - -The North Atlantic, on the other hand, not only does not lose its -surface heat like the equatorial and South Atlantic, but it receives -from the Gulf-stream in the form of warm water an amount of heat, as we -have seen, equal to one-fourth of all the heat which it receives from -the sun. The reason why the warm surface strata are so much thicker -on the North Atlantic than on the equatorial regions is perfectly -obvious. The surface-water at the equator is swept into the Gulf of -Mexico by the trade-winds and the equatorial current, as rapidly as it -is heated by the sun, so that it has not time to gather to any great -depth. But all this warm water is carried by the Gulf-stream into the -North Atlantic, where it accumulates. That this great depth of warm -water in the North Atlantic, represented in the section, is derived -from the Gulf-stream, and not from a direct flow from the equator due -to gravitation, is further evident from the fact that temperature -sounding A in latitude 38° N. is made through that immense body of warm -water, upwards of 300 fathoms thick, extending from Bermuda to near the -Azores, discovered by the _Challenger_ Expedition, and justly regarded -by Captain Nares as an offshoot of the Gulf-stream. This, in Captain -Nares’s Report, is No. 8 “temperature sounding,” between Bermuda and -the Azores; sounding B is No. 6 “temperature curve,” between Teneriffe -and St. Thomas. - -There is an additional reason to the one already stated why the -surface temperature of the South Atlantic should be so much below -that of the North. It is perfectly true that whatever amount of water -is transferred from the southern hemisphere to the northern must be -compensated by an equal amount from the northern to the southern -hemisphere, nevertheless the warm water which is carried off the South -Atlantic by the winds is not directly compensated by water from the -north, but by that cold antarctic current whose existence is so well -known to mariners from the immense masses of ice which it brings from -the Southern Ocean. - -_Thermal Condition of Southern Ocean._——The thermal condition of the -Southern Ocean, as ascertained by the _Challenger_ Expedition, appears -to me to be also irreconcilable with the gravitation theory. Between -the parallels of latitude 65° 42′ S. and 50° 1′ S., the ocean, with -the exception of a thin stratum at the surface heated by the sun’s -rays, was found, down to the depth of about 200 fathoms, to be several -degrees colder than the water underneath.[100] The cold upper stratum -is evidently an antarctic current, and the warm underlying water an -equatorial under current. But, according to the gravitation theory, the -colder water should be underneath. - -The very fact of a mass of water, 200 fathoms deep and extending over -fifteen degrees of latitude, remaining above water of three or four -degrees higher temperature shows how little influence difference of -temperature has in producing motion. If it had the potency which some -attribute to it, one would suppose that this cold stratum should sink -down and displace the warm water underneath. If difference of density -is sufficient to move the water horizontally, surely it must be more -than sufficient to cause it to sink vertically. - - - - - CHAPTER XIV. - - THE WIND THEORY OF OCEANIC CIRCULATION IN RELATION TO CHANGE - OF CLIMATE. - - Direction of Currents depends on Direction of the Winds.—Causes - which affect the Direction of Currents will affect - Climate.—How Change of Eccentricity affects the Mode - of Distribution of the Winds.—Mutual Reaction of Cause - and Effect.—Displacement of the Great Equatorial - Current.—Displacement of the Median Line between the Trades, - and its Effect on Currents.—Ocean-currents in Relation to the - Distribution of Plants and Animals.—Alternate Cold and Warm - Periods in North and South.—Mr. Darwin’s Views quoted.—How - Glaciers at the Equator may be accounted for.—Migration - across the Equator. - - -_Ocean-currents in Relation to Change of Climate._—In my attempts to -prove that oceanic circulation is produced by the winds and not by -difference of specific gravity, and that ocean-currents are the great -distributors of heat over the globe, my chief aim has been to show -the bearing which these points have on the grand question of secular -changes of climate during geological epochs, more particularly in -reference to that mystery the cause of the glacial epoch. - -In concluding this discussion regarding oceanic circulation, I may -therefore be allowed briefly to recapitulate those points connected -with the subject which seem to shed most light on the question of -changes of climate. - -The complete agreement between the systems of ocean-currents and -winds not only shows that the winds are the impelling cause of the -currents, but it also indicates to what an extent the _directions_ of -the currents are determined by the winds, or, more properly, to what an -extent their directions are determined by the _direction_ of the winds. - -We have seen in Chapter II. to what an enormous extent the climatic -conditions of the globe are dependent on the distribution of heat -effected by means of ocean-currents. It has been there pointed out -that, if the heat conveyed from inter-tropical to temperate and polar -regions by oceanic circulation were restored to the former, the -equatorial regions would then have a temperature about 55° warmer, -and the high polar regions a climate 83° colder than at present. It -follows, therefore, that any cause which will greatly affect the -currents or greatly change their paths and mode of distribution, will -of necessity seriously affect the climatic condition of the globe. But -as the existence of these currents depends on the winds, and their -direction and form of distribution depend upon the direction and form -of distribution of the winds, any cause which will greatly affect the -winds will also greatly affect the currents, and consequently will -influence the climatic condition of the globe. Again, as the existence -of the winds depends mainly on the difference of temperature between -equatorial and polar regions, any cause which will greatly affect this -difference of temperature will likewise greatly affect the winds; and -these will just as surely react on the currents and climatic conditions -of the globe. A simple increase or decrease in the difference of -temperature between equatorial and polar regions, though it would -certainly produce an increase or a decrease, as the case might be, in -the strength of the winds, and consequently in the strength of the -currents, would not, however, greatly affect the mode of _distribution_ -of the winds, nor, as a consequence, the mode of _distribution_ of -the currents. But although a simple change in the difference of -temperature between the equator and the poles would not produce a -different _distribution_ of aërial, and consequently of ocean-currents, -nevertheless a _difference in the difference_ of temperature between -the equator and the two poles would do so; that is to say, any cause -that should increase the difference of temperature between the equator -and the pole on the one hemisphere, and decrease that difference on -the other, would effect a change in the distribution of the aërial -currents, which change would in turn produce a corresponding change in -the distribution of ocean-currents. - -It has been shown[101] that an increase in the eccentricity of the -earth’s orbit tends to lower the temperature of the one hemisphere and -to raise the temperature of the other. It is true that an increase of -eccentricity does not afford more heat to the one hemisphere than to -the other; nevertheless it brings about a condition of things which -tends to lower the temperature of the one hemisphere and to raise the -temperature of the other. Let us imagine the eccentricity to be at its -superior limit, 0·07775, and the winter solstice in the aphelion. The -midwinter temperature, owing to the increased distance of the sun, -would be lowered enormously; and the effect of this would be to cause -all the moisture which now falls as rain during winter in temperate -regions to fall as snow. Nor is this all; the winters would not merely -be colder than now, but they would also be much longer. At present the -summer half-year exceeds the winter half year by nearly eight days; but -at the period in question the winters would be longer than the summers -by upwards of thirty-six days. The heat of the sun during the short -summer, for reasons which have already been explained, would not be -sufficient to melt the snow of winter; so that gradually, year by year, -the snow would continue to accumulate on the ground. - -On the southern hemisphere the opposite condition of things would -obtain. Owing to the nearness of the sun during the winter of that -hemisphere, the moisture of the air would be precipitated as rain in -regions where at present it falls as snow. This and the shortness of -the winter would tend to produce a decrease in the quantity of snow. -The difference of temperature between the equatorial and the temperate -and polar regions would therefore be greater on the northern than on -the southern hemisphere; and, as a consequence, the aërial currents -of the former hemisphere would be stronger than those of the latter. -This would be more especially the case with the trade-winds. The -N.E. trades being stronger than the S.E. trades would blow across the -equator, and the median line between them would therefore be at some -distance to the south of the equator. Thus the equatorial waters would -be impelled more to the southern than to the northern hemisphere; and -the warm water carried over in this manner to the southern hemisphere -would tend to increase the difference of temperature between the two -hemispheres. This change, again, would in turn tend to strengthen the -N.E. and to weaken the S.E. trades, and would thus induce a still -greater flow of equatorial waters into the southern hemisphere—a -result which would still more increase the difference of temperature -between the northern and southern hemisphere, and so on—the one cause -so reacting on the other as to increase its effects, as was shown at -length in Chapter IV. - -It was this mutual reaction of those physical agents which led, as was -pointed out in Chapter IV., to that extraordinary condition of climate -which prevailed during the glacial epoch. - -There is another circumstance to be considered which perhaps more -than any thing else would tend to lower the temperature of the one -hemisphere and to raise the temperature of the other; and this is -the _displacement of the great equatorial current_. During a glacial -period in the northern hemisphere the median line between the trades -would be shifted very considerably south of the equator; and the same -would necessarily be the case with the great equatorial currents, the -only difference being that the equatorial currents, other things being -equal, would be deflected farther south than the median line. For the -water impelled by the strong N.E. trades would be moving with greater -velocity than the waters impelled by the weaker S.E. trades, and, of -course, would cross the median line of the trades before its progress -southwards could be arrested by the counteracting influence of the S.E. -trades. Let us glance briefly at the results which would follow from -such a condition of things. In the first place, as was shown on former -occasions,[102] were the equatorial current of the Atlantic (the feeder -of the Gulf-stream) shifted considerably south of its present position, -it would not bifurcate, as it now does, off Cape St. Roque, owing to -the fact that the whole of the waters would strike obliquely against -the Brazilian coast and thus be deflected into the Southern Ocean. The -effect produced on the climate of the North Atlantic and North-Western -Europe by the withdrawal of the water forming the Gulf-stream, may be -conceived from what has already been stated concerning the amount of -heat conveyed by that stream. The heat thus withdrawn from the North -Atlantic would go to raise the temperature of the Southern Ocean and -antarctic regions. A similar result would take place in the Pacific -Ocean. Were the equatorial current of that ocean removed greatly to -the south of its present position, it would not then impinge and be -deflected upon the Asiatic coast, but upon the continent of Australia; -and the greater portion of its waters would then pass southward into -the Southern Ocean, while that portion passing round the north of -Australia (owing to the great strength of the N.E. trades) would rather -flow into the Indian Ocean than turn round, as now, along the east -coast of Asia by the Japan Islands. The stoppage of the Japan current, -combined with the displacement of the equatorial current to the south -of the equator, would greatly lower the temperature of the whole of the -North Pacific and adjoining continents, and raise to a corresponding -degree the temperature of the South Pacific and Southern Ocean. Again, -the waters of the equatorial current of the Indian Ocean (owing to the -opposing N.E. trades), would not, as at present, find their way round -the Cape of Good Hope into the North Atlantic, but would be deflected -southwards into the Antarctic Sea. - -We have in the present state of things a striking example of the extent -to which the median line between the two trades may be shifted, and the -position of the great equatorial currents of the ocean may be affected, -by a slight difference in the relative strength of the two aërial -currents. The S.E. trades are at present a little stronger than the -N.E.; and the consequence is that they blow across the equator into the -northern hemisphere to a distance sometimes of 10 or 15°, so that the -mean position of the median line lies at least 6 or 7 degrees north of -the equator. - -And it is doubtless owing to the superior strength of the S.E. trades -that so much warm water crosses the equator from the South to the North -Atlantic, and that the main portion of the equatorial current flows -into the Caribbean Sea rather than along the Brazilian coast. Were the -two trades of equal strength, the transference of heat into the North -Atlantic from the southern hemisphere by means of the Southern Atlantic -and equatorial currents would be much less than at present. The same -would also hold true in regard to the Pacific. - -_Ocean-currents in Relation to the Distribution of Plants and -Animals._—In the fifth and last editions of the “Origin of Species,” -Mr. Darwin has done me the honour to express his belief that the -foregoing view regarding alternate cold and warm periods in north -and south during the glacial epoch explains a great many facts in -connection with the distribution of plants and animals which have -always been regarded as exceedingly puzzling. - -There are certain species of plants which occur alike in the temperate -regions of the southern and northern hemispheres. At the equator these -same temperate forms are found on elevated mountains, but not on the -lowlands. How, then, did these temperate forms manage to cross the -equator from the northern temperate regions to the southern, and _vice -versâ_? Mr. Darwin’s solution of the problem is (in his own words) as -follows:— - -“As the cold became more and more intense, we know that arctic forms -invaded the temperate regions; and from the facts just given, there -can hardly be a doubt that some of the more vigorous, dominant, and -widest-spreading temperate forms invaded the equatorial lowlands. -The inhabitants of these hot lowlands would at the same time have -migrated to the tropical and subtropical regions of the south; for the -southern hemisphere was at this period warmer. On the decline of the -glacial period, as both hemispheres gradually recovered their former -temperatures, the northern temperate forms living on the lowlands under -the equator would have been driven to their former homes or have been -destroyed, being replaced by the equatorial forms returning from the -south. Some, however, of the northern temperate forms would almost -certainly have ascended any adjoining high land, where, if sufficiently -lofty, they would have long survived like the arctic forms on the -mountains of Europe.” - -“In the regular course of events the southern hemisphere would in -its turn be subjected to a severe glacial period, with the northern -hemisphere rendered warmer; and then the southern temperate forms -would invade the equatorial lowlands. The northern forms which had -before been left on the mountains would now descend and mingle with the -southern forms. These latter, when the warmth returned, would return -to their former homes, leaving some few species on the mountains, and -carrying southward with them some of the northern temperate forms which -had descended from their mountain fastnesses. Thus we should have some -few species identically the same in the northern and southern temperate -zones and on the mountains of the intermediate tropical regions” (p. -339, sixth edition). - -Additional light is cast on this subject by the results already stated -in regard to the enormous extent to which the temperature of the -equator is affected by ocean-currents. Were there no transferrence of -heat from equatorial to temperate and polar regions, the temperature -of the equator, as has been remarked, would probably be about 55° -warmer than at present. In such a case no plant existing on the face of -the globe could live at the equator unless on some elevated mountain -region. On the other hand, were the quantity of warm water which is -being transferred from the equator to be very much increased, the -temperature of inter-tropical latitudes might be so lowered as easily -to admit of temperate species of plants growing at the equator. A -lowering of the temperature at the equator some 20° or 30° is all that -would be required; and only a moderate increase in the volume of the -currents proceeding from the equator, taken in connection with the -effects flowing from the following considerations, might suffice to -produce that result. During the glacial epoch, when the one hemisphere -was under ice and the other enjoying a warm and equable climate, the -median line between the trades may have been shifted to almost the -tropical line of the warm hemisphere. Under such a condition of things -the warmest part would probably be somewhere about the tropic of the -warm hemisphere, and not, as now, at the equator; for since all, or -nearly all, the surface-water of the equator would then be impelled -over to the warm hemisphere, the tropical regions of that hemisphere -would be receiving nearly double their present amount of warm water. - -Again, as the equatorial current at this time would be shifted towards -the tropic of the warm hemisphere, the surface-water would not, as at -present, be flowing in equatorial regions parallel to the equator, -but obliquely across it from the cold to the warm hemisphere. This of -itself would tend greatly to lower the temperature of the equator. - -It follows, therefore, as a necessary consequence, that during the -glacial epoch, when the one hemisphere was under snow and ice and -the other enjoying a warm and equable climate, the temperature of -the equator would be lower than at present. But when the glaciated -hemisphere (which we may assume to be the northern) began to grow -warmer and the climate of the southern or warm hemisphere to get -colder, the median line of the trades and the equatorial currents -of the ocean also would begin to move back from the southern tropic -towards the equator. This would cause the temperature of the equator -to rise and to continue rising until the equatorial currents reached -their normal position. When the snow began to accumulate on the -southern hemisphere and to disappear on the northern, the median line -of the trades and the equatorial currents of the ocean would then -begin to move towards the northern tropic as they had formerly towards -the southern. The temperature of the equator would then again begin -to sink, and continue to do so until the glaciation of the southern -hemisphere reached its maximum. This oscillation of the thermal equator -to and fro across the geographical equator would continue so long as -the alternate glaciation of the two hemispheres continued. - -This lowering of the temperature of the equator during the severest -part of the glacial epoch will help to explain the former existence of -glaciers in inter-tropical regions at no very great elevation above the -sea-level, evidence of which appears recently to have been found by -Professor Agassiz, Mr. Belt, and others. - -The glacial _epoch_ may be considered as contemporaneous in both -hemispheres. But the epoch consisted of a succession of cold and warm -_periods_, the cold periods of one hemisphere coinciding with the warm -periods of the other, and _vice versâ_. - -_Migration across the Equator._—Mr. Belt[103] and others have felt -some difficulty in understanding how, according to theory, the plants -and animals of temperate regions could manage to migrate from one -hemisphere to the other, seeing that in their passage they would have -to cross the thermal equator. The oscillation to and fro of the thermal -equator across the geographical, removes every difficulty in regard to -how the migration takes place. When, for example, a cold period on the -northern hemisphere and the corresponding warm one on the southern were -at their maximum, the thermal equator would by this time have probably -passed beyond the Tropic of Capricorn. The geographical equator would -then be enjoying a subtropical, if not a temperate condition of -climate, and the plants and animals of the northern hemisphere would -manage then to reach the equator. When the cold began to abate on -the northern and to increase on the southern hemisphere, the thermal -equator would commence its retreat towards the geographical. The plants -and animals from the north, in order to escape the increasing heat as -the thermal equator approached them, would begin to ascend the mountain -heights; and when that equator had passed to its northern limit, and -the geographical equator was again enjoying a subtropical condition of -climate, the plants and animals would begin to descend and pursue their -journey southwards as the cold abated on the southern hemisphere. - - - - - CHAPTER XV. - - WARM INTER-GLACIAL PERIODS. - - Alternate Cold and Warm Periods.—Warm Inter-glacial Periods - a Test of Theories.—Reason why their Occurrence has not - been hitherto recognised.—Instances of Warm Inter-glacial - Periods.—Dranse, Dürnten, Hoxne, Chapelhall, Craiglockhart, - Leith Walk, Redhall Quarry, Beith, Crofthead, Kilmaurs, - Sweden, Ohio, Cromer, Mundesley, &c., &c.—Cave and River - Deposits.—Occurrence of Arctic and Warm Animals in some Beds - accounted for.—Mr. Boyd Dawkins’s Objections.—Occurrence - of Southern Shells in Glacial Deposits.—Evidence of Warm - Inter-glacial Periods from Mineral Borings.—Striated - Pavements.—Reason why Inter-glacial Land-surfaces are so rare. - - -_Alternate Cold and Warm Periods._—If the theory developed in the -foregoing chapters in reference to the cause of secular changes of -climate be correct, it follows that that long age known as the glacial -epoch did not, as has hitherto been generally supposed, consist of one -long unbroken period of cold and ice. Neither did it consist, as some -have concluded, of two long periods of ice with an intervening mild -period, but it must have consisted of a long succession of cold and -warm periods; the warm periods of the one hemisphere corresponding in -time with the cold periods of the other and _vice versâ_. It follows -also from theory that as the cold periods became more and more severe, -the warm intervening periods would become more and more warm and -equable. As the ice began to accumulate during the cold periods in -subarctic and temperate regions in places where it previously did not -exist, so in like manner during the corresponding warm periods it would -begin to disappear in arctic regions where it had held enduring sway -throughout the now closing cycle. As the cold periods in the southern -hemisphere became more and more severe, the ice would continue to -advance northwards in the temperate regions; but at that very same -time the intervening warm periods in the northern hemisphere would -become warmer and warmer and more equable, and the ice of the arctic -regions would continue to disappear farther and farther to the north, -till by the time that the ice had reached a maximum during the cold -antarctic periods, Greenland and the arctic regions would, during the -warm intervening periods, be probably free of ice and enjoying a mild -and equable climate. Or we may say that as the one hemisphere became -cold the other became warm, and when the cold reached a maximum in the -one hemisphere, the warmth would reach a maximum in the other. The time -when the ice had reached its greatest extension on the one hemisphere -would be the time when it had disappeared from the other. - -_Inter-glacial Periods a Test of Theories._—Here we have the grand -crucial test of the truth of the foregoing theory of the cause of -the glacial epoch. That the glacial epoch should have consisted of a -succession of cold and warm periods is utterly inconsistent with all -previous theories which have been advanced to account for it. What, -then, is the evidence of geology on this subject? If the glacial epoch -can be proved from geological evidence to have consisted of such a -succession of cold and warm periods, then I have little doubt but the -theory will soon be generally accepted. But at the very outset an -objection meets us, viz., why call an epoch, which consisted as much of -warm periods as of cold, a glacial epoch, or an “Ice Age,” as Mr. James -Geikie tersely expresses it? Why not as well call it a warm epoch as a -cold one, seeing that, according to theory, it was just as much a warm -as a cold epoch? The answer to this objection will be fully discussed -in the chapter on the Reason of the Imperfection of Geological Records. -But in the meantime, I may remark that it will be shown that the epoch -known as the glacial has been justly called the glacial epoch or “Ice -Age,” because the geological evidences of the cold periods remain in -a remarkably perfect state, whilst the evidences of the warm periods -have to a great extent disappeared. The reason of this difference -in the two cases will be discussed in the chapter to which I have -referred. Besides, the condition of things during the cold periods was -so extraordinary, so exceptional, so totally different from those now -prevailing, that even supposing the geological records of the warm -periods had been as well preserved as those of the cold, nevertheless -we should have termed the epoch in question a glacial epoch. There -is yet another reason, however, for our limited knowledge of warm -inter-glacial periods. Till very lately, little or no attention was -paid by geologists to this part of the subject in the way of keeping -records of cases of inter-glacial deposits which, from time to time, -have been observed. Few geologists ever dreamt of such a thing as -warm periods during the age of ice, so that when intercalated beds of -sand and gravel, beds of peat, roots, branches, trunks, leaves, and -fruits of trees were found in the boulder clay, no physical importance -was attached to them, and consequently no description or record of -them ever kept. In fact, all such examples were regarded as purely -accidental and exceptional, and were considered not worthy of any -special attention. A case which came under my own observation will -illustrate my meaning. An intelligent geologist, some years ago, read a -paper before one of our local geological societies, giving an account -of a fossiliferous bed of clay found intercalated between two distinct -beds of till. In this intercalated bed were found rootlets and stems of -trees, nuts, and other remains, showing that it had evidently been an -old inter-glacial land surface. In the transactions of the society a -description of the two beds of till was given, but no mention whatever -was made of the intercalated bed containing the organic remains, -although this was the only point of any real importance. - -Since the theory that the glacial epoch resulted from a high state -of eccentricity of the earth’s orbit began to receive some little -acceptance, geologists have paid a good deal of attention to cases of -intercalated beds in the till containing organic remains, and the -result is that we have already a great body of evidence of a geological -nature in favour of warm inter-glacial periods, and I have little -doubt that in the course of a few years the former occurrence of warm -inter-glacial periods will be universally admitted. - -I shall now proceed to give a very brief outline of the evidence -bearing on the subject. But the cases to which I shall have to refer -are much too numerous to allow me to enter into details. - -_Inter-glacial Beds of Switzerland._—The first geologist, so far as I -am aware, who directed attention to evidence of a break in the cold of -the glacial epoch was M. Morlot. It is now twenty years ago since he -announced the existence of a warm period during the glacial epoch from -geological evidence connected with the glacial drift of the Alps.[104] - -The rivers of Switzerland, he found, show on their banks three -well-marked terraces of regularly stratified and well-rounded shingle, -identical with the modern deposits of the rivers. They stand at 50, -100, and 150 feet above the present level of the rivers. These terraces -were evidently formed by the present system of rivers when these flowed -at a higher level, and extend up the Alps to a height of from 3,000 to -4,000 feet above the level of the sea. There is a terrace bordering the -Rhine at Camischollas, above Disentis, 4,400 feet above the level of -the sea, proving that during the period of its formation the Alps were -free of ice up to the height of 4,400 feet above the sea-level. It is -well known that a glacial period must have succeeded the formation of -these drifts, for they are in many places covered with erratics. At -Geneva, for example, an erratic drift nearly 50 feet thick is seen to -rest on the drift of the middle terrace, which rises 100 feet above the -level of the lake. But it is also evident that a glacial period must -have preceded the formation of the drift beds, for they are found to -lie in many places upon the unstratified boulder clay or _till_. M. -Morlot observed in the neighbourhood of Clareus, from 7 to 9 feet of -drift resting upon a bed of true till 40 feet thick; the latter was -composed of a compact blue clay, containing worn and scratched alpine -boulders and without any trace of stratification. In the gorge of -Dranse, near Thoron, M. Morlot found the whole three formations in a -direct superimposed series. At the bottom was a mass of compact till or -boulder clay, 12 feet thick, containing boulders of alpine limestone. -Over this mass came regularly stratified beds 150 feet thick, made -up of rounded pebbles in horizontal beds. Above this again lay a -second formation of unstratified boulder clay, with erratic blocks and -striated pebbles, which constituted the left lateral moraine of the -great glacier of the Rhone, when it advanced for the second time to the -Lake of Geneva. A condition of things somewhat similar was observed by -M. Ischer in the neighbourhood of Berne. - -These facts, M. Morlot justly considers, prove the existence of two -glacial periods separated by an intermediate one, during which the -ice, which had not only covered Switzerland, but the greater part of -Europe, disappeared even in the principal valleys of the Alps to a -height of more than 4,400 feet above the present level of the sea. This -warm period, after continuing for long ages, was succeeded by a second -glacial period, during which the country was again covered with ice as -before. M. Morlot even suggests the possibility of these alternations -of cold and warm periods depending upon a cosmical cause. “Wild as it -may have appeared,” he says, “when first started, the idea of general -and periodical eras of refrigeration for our planet, connected perhaps -with some cosmic agency, may eventually prove correct.”[105] - -Shortly afterwards, evidence of a far more remarkable character was -found in the glacial drift of Switzerland, namely, the famous lignite -beds of Dürnten. In the vicinity of Utznach and Dürnten, on the Lake of -Zurich, and near Mörschwyl, on the Lake of Constance, there are beds of -coal or lignite, nearly 12 feet thick, lying directly on the boulder -clay. Overlying these beds is another mass of drift and clay 30 feet -in thickness, with rounded blocks, and on the top of this upper drift -lie long angular erratics, which evidently have been transported on -the back of glaciers.[106] Professor Vogt attributes their transport -to floating ice; but he evidently does so to avoid the hypothesis of a -warm period during the glacial epoch. - -Here we have proof not merely of the disappearance of the ice during -the glacial epoch, but of its absence during a period of sufficient -length to allow of the growth of 10 or 12 feet of coal. Professor Heer -thinks that this coal-bed, when in the condition of peat, must have -been 60 feet thick; and assuming that one foot of peat would be formed -in a century, he concludes that 6,000 years must have been required -for the growth of the coal plants. According to Liebig, 9,600 years -would be required. This, as we have already seen, is about the average -duration of a warm period. - -In these beds have been found the bones of the elephant (_E. Merkii_), -stag, cave-bear, and other animals. Numerous insects have also been met -with, which further prove the warm, mild condition of climate which -must have prevailed at the time of the formation of the lignite. - -At Hoxne, near Diss, in Suffolk, a black peaty mass several feet thick, -containing fragments of wood of the oak, yew, and fir, was found, -overlying the boulder clay.[107] Professor Vogt believes that this peat -bed is of the same age as the lignite beds of Switzerland. - -In the glacial drift of North America, particularly about Lake -Champlain and the valley of the St. Lawrence, there is similar evidence -of two glacial periods with an intervening non-glacial or warm -period.[108] - -_Glacial and Inter-glacial Periods of the Southern Hemisphere_—(_South -Africa_).—Mr. G. W. Stow, in a paper on the “Geology of South -Africa,”[109] describes a recent glaciation extending over a large -portion of Natal, British Kaffraria, the Kaga and Krome mountains, -which he attributes to the action of land-ice. He sums up the phenomena -as follows:—“The rounding off of the hills in the interiors of the -ancient basins; the numerous dome-shaped (_roches moutonnée_) rocks; -the enormous erratic boulders in positions where water could not have -carried them; the frequency of unstratified clays—clays with imbedded -angular boulders; drift and lofty mounds of boulders; large tracts of -country thickly spread over with unstratified clays and superimposed -fragments of rock; the Oliphant’s-Hoek clay, and the vast piles of Enon -conglomerate.” In addition to these results of ice-action, he records -the discovery by himself of distinct ice-scratches or groovings on the -surface of the rocks at Reit-Poort in the Tarka, and subsequently[110] -the discovery by Mr. G. Gilfillan of a large boulder at Pniel with -_striæ_ distinctly marked upon it, and also that the same observer -found that almost every boulder in the gravel at “Moonlight Rush” had -unmistakable striæ on one or more sides. - -In South Africa there is evidence not only of a glacial condition -during the Pliocene period, but also of a warmer climate than now -prevails in that region. “The evidence,” says Mr. Stow, “of the -Pliocene shells of the superficial limestone of the Zwartkops heights, -and elsewhere, leads us to believe that the climate of South Africa -must have been of a far more tropical character than at present. - -“Take, for instance, the characteristic _Venericardia_ of that -limestone. This has migrated along the coast some 29° or 30° and is now -found within a few degrees of the equator, near Zanzibar, gradually -driven, as I presume it must have been, further and further north by a -gradual lowering of the temperature of the more southern parts of this -coast since the limestone was deposited.” - -“During the formation of the shell-banks in the Zwartkops estuary, -younger than the Pliocene limestone, the immense number of certain -species of shells, which have as yet been found living only in -latitudes nearer the equator, point to a somewhat similar though a more -modified change of temperature.” - -_Inter-glacial Beds of Scotland._—Upwards of a dozen years ago, -Professor Geikie arrived, from his own observations of the glacial -drift of Scotland, at a similar conclusion to that of M. Morlot -regarding the intercalation of warm periods during the glacial epoch; -and the facts on which Professor Geikie’s conclusions were based are -briefly as follows. In a cliff of boulder clay on the banks of the -Slitrig Water, near the town of Hawick, he observed a bed of stones -or shingle. Over the lower stratum of stones lay a few inches of -well-stratified sand, silt, and clay, some of the layers being black -and peaty, _with enclosed vegetable fibres_ in a crumbling state.[111] -There were some 30 or 40 feet of boulder clay above these stratified -beds, and 15 or 20 feet under them. The stones in the shingle band -were identical with those of the boulder clay, but they showed no -striations, and were more rounded and water-worn, and resembled in -every respect the stones now lying in the bed of the Slitrig. The -section of the cliff stood as under:— - - 1. Vegetable soil. - 2. Boulder clay, thirty to forty feet. - { 3. Yellowish gravelly sand. - Stratified beds { 4. Peaty silt and clay. - { 5. Fine ferruginous sand. - { 6. Coarse shingle, two to three feet. - 7. Coarse, stiff boulder clay, fifteen to twenty feet. - -A few more cases of intercalation of stratified materials in the true -till were also found in the same valley. - -In a cliff of stiff brown boulder clay, about 20 feet high, on the -banks of the Carmichael Water, Lanarkshire, Professor Geikie observed -a stratified bed of clay about 3 or 4 inches in thickness. About a mile -higher up the stream, he found a series of beds of gravel, sand, and -clay in the true _till_. “A thin seam of _peaty matter_,” he says, “was -observed to run for a few inches along the bottom of a bed of clay and -then disappear, while in a band of fine laminated clay with thin sandy -partings occasional _fragments of mouldering wood_ were found.”[112] - -At Chapelhall, near Airdrie, a sand-bed has been extensively mined -under about 114 feet of till. This bed of finely stratified sand -is about 20 feet thick. In it were found lenticular beds of fine -pale-coloured clay containing layers of peat and decaying twigs and -branches. Professor Geikie found the vegetable fibres, though much -decayed, still distinct, and the substance when put into the fire -burned with a dull lambent flame. Underlying these stratified beds, and -forming the floor of the mine, is a deposit of _the true till_ about -24 feet in thickness. In another pit adjoining, the till forming the -floor is 30 feet thick, but it is sometimes absent altogether, so as to -leave the sand beds resting directly on the sandstone and shale of the -coal-measures. At some distance from this sand-pit an old buried river -channel was met with in one of the pit workings. This channel was found -to contain a coating of boulder clay, on which the laminated sands and -clays reposed, showing, as Professor Geikie has pointed out, that this -old channel had been filled with boulder clay, and then re-excavated -to allow of the deposition of the stratified deposits. Over all lay a -thick mantle of boulder clay which buried the whole. - -A case somewhat similar was found by Professor Nicol in a cutting on -the Edinburgh and Leith Railway. In many places the till had been -worn into hollows as if part of it had been removed by the action of -running water.[113] One of these hollows, about 5 or 6 feet wide by 3 -or 4 feet deep, closely resembled the channel of a small stream. It -was also filled with gravel and sand, in all respects like that found -in such a stream at the present day. It was seen to exhibit the same -characters on both sides of the cutting, but Professor Nicol was unable -to determine how far it may have extended beyond; but he had no doubt -whatever that it had been formed by a stream of water. Over this old -watercourse was a thick deposit of true till. - -In reference to the foregoing cases, Professor Geikie makes the -following pertinent remarks:—“Here it is evident that the scooping out -of this channel belongs to the era of the boulder clay. It must have -been effected during a pause in the deposition of the clay, when a run -of water could find its way along the inequalities of the surface of -the clay. This pause must have been of sufficient duration to enable -the runnel to excavate a capacious channel for itself, and leave in it -a quantity of sand and shingle. We can scarcely doubt that when this -process was going on the ground must have been a land surface, and -could not have been under the sea. And lastly, we see from the upper -boulder clay that the old conditions returned, the watercourse was -choked up, and another mass of chaotic boulder clay was tumbled down -upon the face of the country. This indicates that the boulder clay is -not the result of one great catastrophe, but of slow and silent, yet -mighty, forces acting sometimes with long pauses throughout a vast -cycle of time.”[114] - -At Craiglockhart Hill, about a mile south of Edinburgh, an extensive -bed of fine sand of from one to three feet in thickness was found -between two distinct masses of true boulder clay or till. The sand was -extensively used for building purposes during the erection of the city -poorhouse a few years ago. In this sand-bed I found a great many tree -roots in the position in which they had grown. During the time of the -excavations I visited the place almost daily, and had every opportunity -of satisfying myself that this sand-bed, prior to the time of the -formation of the upper boulder clay, must have been a land surface -on which the roots had grown. In no case did I find them penetrating -into the upper boulder clay, and in several places I found stones of -the upper clay resting directly on the broken ends of the roots. These -roots were examined by Professor Balfour, but they were so decayed that -he was unable to determine their character. - -In digging a foundation for a building in Leith Walk, Edinburgh, a few -years ago, two distinct beds of sand were passed through, the upper, -about 10 feet in thickness, rested upon what appeared to be a denuded -surface of the lower bed. In this lower bed, which evidently had been -a land surface, numbers of tree roots were found. I had the pleasure -of examining them along with my friend Mr. C. W. Peach, who first -directed my attention to them. In no instance were the roots found -in the upper bed. That these roots did not belong to trees which had -grown on the present surface and penetrated to that depth, was further -evident from the fact that in one or two cases we found the roots -broken off at the place where they had been joined to the trunk, and -there the upper sand-bed over them was more than 10 feet in thickness. -If we assume that the roots belonged to trees which had grown on the -present surface, then we must also assume, what no one would be willing -to admit, that the trunks of the trees had grown downwards into the -earth to a depth of upwards of ten feet. I have shown these roots to -several botanists, but none of them could determine to what trees they -belonged. The surface of the ground at the spot in question is 45 feet -above sea-level. Mr. Peach and I have found similar roots in the under -sand-bed at several other places in the same neighbourhood. That they -belong to an inter-glacial period appears probable for the following -reasons:—(1.) This upper sand-bed is overlaid by a tough clay, which -in all respects appears to be the same as the Portobello clay, which -we know belongs to the glacial series. In company with Mr. Bennie, -I found the clay in some places to be contorted in a similar manner -to the Portobello clays. (2.) In a sand-pit about one or two hundred -yards to the west of where the roots were found, the sand-bed was -found contorted in the most extraordinary manner to a depth of about 15 -feet. In fact, for a space of more than 30 feet, the bedding had been -completely turned up on end without the fine layers being in the least -degree broken or disarranged, showing that they had been upturned by -some enormous powers acting on a large mass of the sand. - -One of the best examples of true till to be met with in the -neighbourhood of Edinburgh is at Redhall Quarry, about three miles to -the south-west of the city. In recently opening up a new quarry near -the old one a bed of peat was found intercalated in the thick mass of -till overlying the rock. The clay overlying and underlying the peat-bed -was carefully examined by Mr. John Henderson,[115] and found to be true -till. - -In a quarry at Overtown, near Beith, Ayrshire, a sedimentary bed of -clay, intercalated between two boulder clays, was some years ago -observed by Mr. Robert Craig, of the Glasgow Geological Society. This -bed filled an elliptical basin about 130 yards long, and about 30 yards -broad. Its thickness averaged from one to two feet. This sedimentary -bed rested on the till on the north-east end of the basin, and was -itself overlaid on the south-west end by the upper bed of till. The -clay bed was found to be full of roots and stems of the common hazel. -That these roots had grown in the position in which they were found -was evident from the fact that they were in many places found to pass -into the “cutters” or fissures of the limestone, and were here found -in a flattened form, having in growing accommodated themselves to the -size and shape of the fissures. Nuts of the hazel were plentifully -found.[116] - -At Hillhead, some distance from Overtown, there is a similar -intercalated bed full of hazel remains, and a species of freshwater -_Ostracoda_ was detected by Mr. David Robertson. - -In a railway cutting a short distance from Beith, Mr. Craig pointed out -to my colleague, Mr. Jack, and myself, a thin layer of peaty matter, -extending for a considerable distance between an upper and lower mass -of till; and at one place we found a piece of oak about four feet in -length and about seven or eight inches in thickness. This oak boulder -was well polished and striated. - -Not far from this place is the famous Crofthead inter-glacial bed, so -well known from the description given by Mr. James Geikie and others -that I need not here describe it. I had the pleasure of visiting the -section twice while it was well exposed, once, in company with Mr. -James Geikie, and I do not entertain the shadow of a doubt as to its -true inter-glacial character. - -In the silt, evidently the mud of an inter-glacial lake, were found the -upper portion of the skull of the great extinct ox (_Bos primigenius_), -horns of the Irish elk or deer, and bones of the horse. In the detailed -list of the lesser organic remains found in the intercalated peat-bed -by Mr. J. A. Mahony,[117] are the following, viz., three species of -_Desmidaceæ_, thirty-one species of _Diatomaceæ_, eleven species of -mosses, nine species of phanerogamous plants, and several species of -annelids, crustacea, and insects. This list clearly shows that the -inter-glacial period, represented by these remains, was not only mild -and warm, but of considerable duration. Mr. David Robertson found in -the clay under the peat several species of _Ostracoda_. - -The well-known Kilmaurs bed of peaty matter in which the remains of -the mammoth and reindeer were found, has now by the researches of the -Geological Survey been proved to be of inter-glacial age.[118] - -In Ireland, as shown by Professors Hull and Harkness, the inter-glacial -beds, called by them the “manure gravels,” contain numerous fragments -of shells indicating a more genial climate than prevailed when the -boulder clays lying above and below them were formed.[119] - -In Sweden inter-glacial beds of freshwater origin, containing plants, -have been met with by Herr Nathorst and also by Herr Holmström.[120] - -In North America Mr. Whittlesey describes inter-glacial beds of blue -clay enclosing pieces of wood, intercalated with beds of hard pan -(till). Professor Newberry found at Germantown, Ohio, an immense bed of -peat, from 12 to 20 feet in thickness, underlying, in some places 30 -feet, and in other places as much as 80 feet, of till, and overlying -drift beds. The uppermost layers of the peat contain undecomposed -sphagnous mosses, grasses, and sedges, but in the other portions of -the bed abundant fragments of coniferous wood, identified as red cedar -(_Juniperus virginiana_), have been found. Ash, hickory, sycamore, -together with grape-vines and beech-leaves, were also met with, and -with these the remains of the mastodon and great extinct beaver.[121] - -_Inter-glacial Beds of England._—Scotland has been so much denuded by -the ice sheet with which it was covered during the period of maximum -glaciation that little can be learned in this part of the island -regarding the early history of the glacial epoch. But in England, -and more especially in the south-eastern portion of it, matters are -somewhat different. We have, in the Norwich Crag and Chillesford beds, -a formation pretty well developed, which is now generally regarded as -lying at the base of the Glacial Series. That this formation is of a -glacial character is evident from the fact of its containing shells of -a northern type, such as _Leda lanceolata_, _Cardium Groènlandicum_, -_Lucina borealis_, _Cyprina Islandica_, _Panopæa Norvegica_, and -_Mya truncata_. But the glacial character of the formation is -more strikingly brought out, as Sir Charles Lyell remarks, by the -predominance of such species as _Rhynchonella psittacea_, _Tellina -calcarea_, _Astarte borealis_, _Scalaria Groènlandica_, and _Fusus -carinatus_. - -_The “Forest Beds.”_—Immediately following this in the order of -time comes the famous “Forest Bed” of Cromer. This buried forest has -been traced for more than forty miles along the coast from Cromer to -near Kessengland, and consists of stumps of trees standing erect, -attached to their roots, penetrating the original soil in which they -grew. Here and in the overlying fluvio-marine beds we have the first -evidence of at least a temperate, if not a warm, inter-glacial period. -This is evident from the character of the flora and fauna belonging -to these beds. Among the trees we have, for example, the Scotch and -spruce fir, the yew, the oak, birch, the alder, and the common sloe. -There have also been found the white and yellow water-lilies, the -pond-weed, and others. Amongst the mammalia have been met with the -_Elephas meridionalis_, also found in the Lower Pliocene beds of the -Val d’Arno, near Florence; _Elephas antiquus_, _Hippopotamus major_, -_Rhinoceros Etruscus_, the two latter Val d’Arno species, the roebuck, -the horse, the stag, the Irish elk, the _Cervus Polignacus_, found -also at Mont Perrier, France, _C. verticornis_, and _C. carnutorum_, -the latter also found in Pliocene strata of St. Prest, France. In -the fluvio-marine series have been found the _Cyclas omnica_ and the -_Paludina marginata_, a species of mollusc still found in the South of -France, but no longer inhabiting the British Isles. - -Above the forest bed and fluvio-marine series comes the well-known -unstratified Norwich boulder till, containing immense blocks 6 or 8 -feet in diameter, many of which must have come from Scandinavia, and -above the unstratified till are a series of contorted beds of sand and -gravel. This series may be considered to represent a period of intense -glaciation. Above this again comes the middle drift of Mr. Searles -Wood, junior, yielding shells which indicate, as is now generally -admitted, a comparatively mild condition of climate. Upon this middle -drift lies the upper boulder clay, which is well developed in South -Norfolk and Suffolk, and which is of unmistakable glacial origin. Newer -than all these are the Mundesley freshwater beds, which lie in a hollow -denuded out of the foregoing series. In this formation a black peaty -deposit containing seeds of plants, insects, shells, and scales and -bones of fishes, has been found, all indicating a mild and temperate -condition of climate. Among the shells there is, as in the forest bed, -the _Paludina marginata_. And that an arctic condition of things in -England followed is believed by Mr. Fisher and others, on the evidence -of the “Trail” described by the former observer. - -_Cave and River Deposits._—Evidence of the existence of warm periods -during the glacial epoch is derived from a class of facts which -have long been regarded by geologists as very puzzling, namely, the -occurrence of mollusca and mammalia of a southern type associated -in England and on the continent with those of an extremely arctic -character. For example, _Cyrena fluminalis_ is a shell which does not -live at present in any European river, but inhabits the Nile and parts -of Asia, especially Cashmere. _Unio littoralis_, extinct in Britain, -is still abundant in the Loire; _Paludina marginata_ does not exist -in this country. These shells of a southern type have been found in -post-tertiary deposits at Gray’s Thurrock, in Essex; in the valley -of the Ouse, near Bedford; and at Hoxne, in Suffolk, associated with -a _Hippopotamus_ closely allied to that now inhabiting the Nile, and -_Elephas antiquus_, an animal remarkable for its southern range. -Amongst other forms of a southern type which have been met with in -the cave and river deposits, are the spotted hyæna from Africa, -an animal, says Mr. Dawkins, identical, except in size, with the -cave hyæna, the African elephant (_E. Africanus_), and the _Elephas -meridionalis_, the great beaver (_Trogontherium_), the cave hyæna -(_Hyæna spelæa_), the cave lion (_Felis leo_, var. _spelæa_), the lynx -(_Felis lynx_), the sabre-toothed tiger (_Machairodus latidens_), the -rhinoceros (_Rhinoceros megarhinus_ and _R. leptorhinus_). But the -most extraordinary thing is that along with these, associated in the -same beds, have been found the remains of such animals of an arctic -type as the glutton (_Gulo luscus_), the ermine (_Mustela erminea_), -the reindeer (_Cervus tarandus_), the musk-ox or musk-sheep (_Ovibos -moschatus_), the aurochs (_Bison priscus_), the woolly rhinoceros -(_Rhinoceros tichorhinus_), the mammoth (_Elephas primigenius_), and -others of a like character. According to Mr. Boyd Dawkins, these -southern animals extended as far north as Yorkshire in England, and -the northern animals as far south as the latitude of the Alps and -Pyrenees.[122] - -_The Explanation of the Difficulty._—As an explanation of these -puzzling phenomena, I suggested, in the Philosophical Magazine for -November, 1868, that these southern animals lived in our island during -the warm periods of the glacial epoch, while the northern animals -lived during the cold periods. This view I am happy to find has lately -been supported by Sir John Lubbock; further, Mr. James Geikie, in his -“Great Ice Age,” and also in the Geological Magazine, has entered so -fully into the subject and brought forward such a body of evidence -in support of it, that, in all probability, it will, ere long, be -generally accepted. The only objection which has been advanced, so far -as I am aware, deserving of serious consideration, is that by Mr. Boyd -Dawkins, who holds that if these migrations had been _secular_ instead -of seasonal, as is supposed by Sir Charles Lyell and himself, the -arctic and southern animals would now be found in separate deposits. -It is perfectly true that if there had been only one cold and one warm -period, each of geologically immense duration, the remains might, of -course, be expected to have been found in separate beds; but when -we consider that the glacial epoch consisted of a long _succession -of alternate cold and warm periods_, of not more than ten or twelve -thousand years each, we can hardly expect that in the river deposits -belonging to this long cycle we should be able to distinguish the -deposits of the cold periods from those of the warm. - -_Shell Beds._—Evidence of warm inter-glacial periods may be justly -inferred from the presence of shells of a southern type which have been -found in glacial beds, of which some illustrations follow. - -In the southern parts of Norway, from the present sea-level up to 500 -feet, are found glacial shell beds, similar to those of Scotland. In -these beds _Trochus magus_, _Tapes decussata_, and _Pholas candida_ -have been found, shells which are distributed between the Mediterranean -and the shores of England, but no longer live round the coasts of -Norway. - -At Capellbacken, near Udevalla, in Sweden, there is an extensive bed of -shells 20 to 30 feet in thickness. This formation has been described -by Mr. Gwyn Jeffreys.[123] It consists of several distinct layers, -apparently representing many epochs and conditions. Its shells are of a -highly arctic character, and several of the species have not been found -living south of the arctic circle. But the remarkable circumstance -is that it contains _Cypræa lurida_, a Mediterranean shell, which -Mr. Jeffreys, after some hesitation, believed to belong to the bed. -Again, at Lilleherstehagen, a short distance from Capellbacken, -another extensive deposit is exposed. “Here the upper layer,” says Mr. -Jeffreys, “gives a singular result. Mixed with the universal _Trophon -clathratus_ (which is a high northern species, and found living only -within the arctic circle) are many shells of a southern type, such are -_Ostrea edulis_, _Tapes pullastra_, _Corbula gibba_, and _Aporrhais -pes-pelicani_.” - -At Kempsey, near Worcester, a shell bed is described by Sir R. -Murchison in his “Silurian System” (p. 533), in which _Bulla ampulla_ -and a species of _Oliva_, shells of a southern type, have been found. - -A case somewhat similar to the above is recorded by the Rev. Mr. -Crosskey as having been met with in Scotland at the Kyles of Bute. -“Among the Clyde beds, I have found,” he says, “a layer containing -shells, in which those of a more southern type appear to exist in -greater profusion and perfection than even in our present seas. It is -an open question,” he continues, “whether our climate was not slightly -warmer than it is now between the glacial epoch and the present -day.”[124] - -In a glacial bed near Greenock, Mr. A. Bell found the fry of living -Mediterranean forms, viz., _Conus Mediterraneus_ and _Cardita trapezia_. - -Although deposits containing shells of a temperate or of a southern -type in glacial beds have not been often recorded, it by no means -follows that such deposits are actually of rare occurrence. That -glacial beds should contain deposits indicating a temperate or a -warm condition of climate is a thing so contrary to all preconceived -opinions regarding the sequence of events during the glacial epoch, -that most geologists, were they to meet with a shell of a southern -type in one of those beds, would instantly come to the conclusion that -its occurrence there was purely accidental, and would pay no special -attention to the matter. - -_Evidence derived from “Borings.”_—With the view of ascertaining if -additional light would be cast on the sequence of events, during the -formation of the boulder clay, by an examination of the journals of -bores made through a great depth of surface deposits, I collected, -during the summer of 1867, about two hundred and fifty such records, -put down in all parts of the mining districts of Scotland. An -examination of these bores shows most conclusively that the opinion -that the boulder clay, or lower till, is one great undivided formation, -is wholly erroneous. - -These two hundred and fifty bores represent a total thickness of 21,348 -feet, giving 86 feet as the mean thickness of the deposits passed -through. Twenty of these have one boulder clay, with beds of stratified -sand or gravel beneath the clay; twenty-five have _two_ boulder clays, -with stratified beds of sand and gravel between; ten have _three_ -boulder clays; one has _four_ boulder clays; two have _five_ boulder -clays; and no one has fewer than _six_ separate masses of boulder -clay, with stratified beds of sand and gravel between; sixteen have -two or three separate boulder clays, differing altogether in colour -and hardness, without any stratified beds between. We have, therefore, -out of two hundred and fifty bores, seventy-five of them representing -a condition of things wholly different from that exhibited to the -geologist in ordinary sections. - -The full details of the character of the deposits passed through by -these bores, and their bearing on the history of the glacial epoch, -have been given by Mr. James Bennie, in an interesting paper read -before the Glasgow Geological Society,[125] to which I would refer all -those interested in the subject of surface geology. - -The evidence afforded by these bores of the existence of warm -inter-glacial periods will, however, fall to be considered in a -subsequent chapter.[126] - -Another important and unexpected result obtained from these bores to -which we shall have occasion to refer, was the evidence which they -afforded of a Continental Period. - -_Striated Pavements._—It has been sometimes observed that in horizontal -sections of the boulder clay, the stones and boulders are all striated -in one uniform direction, and this has been effected over the original -markings on the boulders. It has been inferred from this that a pause -of long duration must have taken place in the formation of the boulder -clay, during which the ice disappeared and the clay became hardened -into a solid mass. After which the old condition of things returned, -glaciers again appeared, passed over the surface of the hardened clay -with its imbedded boulders, and ground it down in the same way as they -had formerly done the solid rocks underneath the clay. - -An instance of striated pavements in the boulder clay was observed by -Mr. Robert Chambers in a cliff between Portobello and Fisherrow. At -several places a narrow train of blocks was observed crossing the line -of the beach, somewhat like a quay or mole, but not more than a foot -above the general level. All the blocks _had flat sides uppermost, -and all the flat sides were striated in the same direction_ as that -of the rocky surface throughout the country. A similar instance was -also observed between Leith and Portobello. “There is, in short,” says -Mr. Chambers, “a surface of the boulder clay, deep down in the entire -bed, which, to appearance, has been in precisely the same circumstances -as the fast rock surface below had previously been. It has had in its -turn to sustain the weight and abrading force of the glacial agent, -in whatever form it was applied; and the additional deposits of the -boulder clay left over this surface may be presumed to have been formed -by the agent on that occasion.”[127] - -Several cases of a similar character were observed by Mr. James -Smith, of Jordanhill, on the beach at Row, and on the shore of the -Gareloch.[128] Between Dunbar and Cockburnspath, Professor Geikie found -along the beach, for a space of 30 or 40 square yards, numbers of large -blocks of limestone with flattened upper sides, imbedded in a stiff red -clay, and all striated in one direction. On the shores of the Solway he -found another example.[129] - -The cases of striated pavements recorded are, however, not very -numerous. But this by no means shows that they are of rare occurrence -in the boulder clay. These pavements, of course, are to be found only -in the interior of the mass, and even there they can only be seen -along a horizontal section. But sections of this kind are rarely to be -met with, for river channels, quarries, railway cuttings, and other -excavations of a similar character which usually lay open the boulder -clay, exhibit vertical sections only. It is therefore only along the -sea-shore, as Professor Geikie remarks, where the surface of the clay -has been worn away by the action of the waves, that opportunities have -hitherto been presented to the geologist for observing them. - -There can be little doubt that during the warm periods of the glacial -epoch our island would be clothed with a luxuriant flora. At the end -of a cold period, when the ice had disappeared, the whole face of the -country would be covered over to a considerable depth with a confused -mass of stones and boulder clay. A surface thus wholly destitute of -every seed and germ would probably remain for years without vegetation. -But through course of time life would begin to appear, and during -the thousands of years of perpetual summer which would follow, the -soil, uncongenial as it no doubt must have been, would be forced to -sustain a luxuriant vegetation. But although this was the case, we -need not wonder that now scarcely a single vestige of it remains; for -when the ice sheet again crept over the island everything animate and -inanimate would be ground down to powder. We are certain that prior -to the glacial epoch our island must have been covered with life and -vegetation. But not a single vestige of these are now to be found; -no, not even of the very soil on which the vegetation grew. The solid -rock itself upon which the soil lay has been ground down to mud by the -ice sheet, and, to a large extent, as Professor Geikie remarks, swept -away into the adjoining seas.[130] It is now even more difficult to -find a trace of the ancient soil _under_ the boulder clay than it is -to find remains of the soil of the warm periods _in_ that clay. As -regards Scotland, cases of old land surfaces under the boulder clay are -as seldom recorded as cases of old land surfaces in it. In so far as -geology is concerned, there is as much evidence to show that our island -was clothed with vegetation during the glacial epoch as there is that -it was so clothed prior to that epoch. - - - - - CHAPTER XVI. - - WARM INTER-GLACIAL PERIODS IN ARCTIC REGIONS. - - Cold Periods best marked in Temperate, and Warm Periods - in Arctic, Regions.—State of Arctic Regions during - Glacial Period.—Effects of Removal of Ice from Arctic - Regions.—Ocean-Currents; Influence on Arctic Climate.—Reason - why Remains of Inter-glacial Period are rare in Arctic - Regions.—Remains of Ancient Forests in Banks’s Land, Prince - Patrick’s Island, &c.—Opinions of Sir R. Murchison, Captain - Osborn, and Professor Haughton.—Tree dug up by Sir E. Belcher - in lat. 75° N. - - -In the temperate regions the cold periods of the glacial epoch would be -far more marked than the warm inter-glacial periods. The condition of -things which prevailed during the cold periods would differ far more -widely from that which now prevails than would the condition of things -during the warm periods. But as regards the polar regions the reverse -would be the case; there the warm inter-glacial periods would be far -more marked than the cold periods. The condition of things prevailing -in those regions during the warm periods would be in strongest contrast -to what now obtains, but this would not hold true in reference to the -cold periods; for during the latter, matters there would be pretty -much the same as at present, only a good deal more severe. The reason -of this may be seen from what has already been stated in Chapter IV.; -but as it is a point of considerable importance in order to a proper -understanding of the physical state of things prevailing in polar -regions during the glacial epoch, I shall consider this part of the -subject more fully. - -During the cold periods, our island, and nearly all places in the -northern temperate regions down to about the same latitude, would be -covered with snow and ice, and all animal and vegetable life within -the glaciated area would to a great extent be destroyed. The presence -of the ice would of itself, for reasons already explained, lower the -mean annual temperature to near the freezing-point. The summers, -notwithstanding the proximity of the sun, would not be warm, on the -contrary their temperature would rise little above the freezing-point. -An excess of evaporation would no doubt take place, owing to the -increase in the intensity of the sun’s rays, but this result would only -tend to increase the snowfall.[131] - -During the warm periods our country and the regions under consideration -would experience conditions not differing much from those of the -present, but the climate would probably be somewhat warmer and more -equable. The proximity of the sun during winter would prevent snow -from falling. The summers, owing to the greater distance of the sun, -would probably be somewhat colder than they are now. But the loss of -heat during summer would be to a large extent compensated for by two -causes to which we must here refer. (1.) The much greater amount of -heat conveyed by ocean-currents than at present. (2.) Our summers are -now cooled to a considerable extent by cold aërial currents from the -ice-covered regions of the north. But during the period in question -there would be little or no ice in arctic regions, consequently the -winds would be comparatively warm, whatever direction they came from. - -Let us next direct our attention to the state of things in the arctic -regions during the glacial epoch. At present Greenland and other parts -of the arctic regions occupied by land are almost wholly covered -with ice, and as a consequence nearly destitute of vegetable life. -During the cold periods of the glacial epoch the quantity of snow -falling would doubtless be greater and the ice thicker, but as regards -organic life, matters would not probably be much worse than they are -at present. In fact, so far as Greenland and the antarctic continent -are concerned, they are about as destitute of plant life as they can -be. Although an increase in the thickness of the arctic ice would not -greatly alter the present state of matters in those regions, yet what -a transformation would ensue upon the disappearance of the ice! This -would not only raise the summer temperature some twenty degrees or so, -but would afford the necessary conditions for the existence of abundant -animal and plant life. The severity of the climate of Greenland is -due to a very considerable extent, as we have already seen, to the -presence of ice. Get rid of the permanent ice, and the temperature of -the country, _cæteris paribus_, would instantly rise. That Greenland -should ever have enjoyed a temperate climate, capable of supporting -abundant vegetation, has often been matter of astonishment, but this -wonder diminishes when we reflect that during the warm periods it would -be in the arctic regions that the greatest heating effect would take -place, this being due mainly to the transference of nearly all the warm -inter-tropical waters to one hemisphere. - -It has been shown in Chapter II. that the heating effects at present -resulting from the transference of heat by ocean-currents increase as -we approach the poles. As a consequence of this it follows that during -the warm periods, when the quantity of warm water transferred would be -nearly doubled, the _increase of heat resulting from this cause would -itself increase_ as the warm pole was approached. This effect, combined -with the shortness of the winter in perihelion and the nearness of the -sun during that season, would prevent the accumulation of snow. During -summer, the sun, it is true, would be at a much greater distance from -the earth than at present, but it must be borne in mind that for a -period of three months the quantity of heat received from the sun at -the north pole would be greater than that received at the equator. -Consequently, after the winter’s snow was melted, this great amount of -heat would go to raise the temperature, and the arctic summer could -not be otherwise than hot. It is not hot at present, but this, be it -observed, is because of the presence of the ice. When we take all these -facts into consideration we need not be surprised that Greenland once -enjoyed a condition of climate totally different from that which now -obtains in that region. - -It is, therefore, in the arctic and antarctic regions where we ought -to find the most marked and decided evidence of warm inter-glacial -periods. And doubtless such evidence would be abundantly forthcoming -had these regions not been subjected to such intense denudation since -the glacial epoch, and were so large a portion of the land not still -buried beneath an icy covering, and therefore beyond the geologist’s -reach. Only on islands and such outlying places as are not shrouded in -snow and ice can we hope to meet with any trace of the warm periods of -the glacial epoch: and we may now proceed to consider what relics of -these warm periods have actually been discovered in arctic regions. - -_Evidence of Warm Periods in Arctic Regions._—The fact that stumps, -&c., of full-grown trees have been found in places where at present -nothing is to be met with but fields of snow and ice, and where the -mean annual temperature scarcely rises above the zero of the Fahrenheit -thermometer, is good evidence to show that the climate of the arctic -regions was once much warmer than now. The remains of an ancient forest -were discovered by Captain McClure, in Banks’s Land, in latitude 74° -48′. He found a great accumulation of trees, from the sea-level to an -elevation of upwards of 300 feet. “I entered a ravine,” says Captain -McClure, “some miles inland, and found the north side of it, for a -depth of 40 feet from the surface, composed of one mass of wood similar -to what I had before seen.”[132] In the ravine he observed a tree -protruding about 8 feet, and 3 feet in circumference. And he further -states that, “_From the perfect state of the bark_, and the position of -the trees so far from the sea, there can be but little doubt that they -grew originally in the country.” A cone of one of these fir-trees was -brought home, and was found to belong apparently to the genus _Abies_, -resembling _A. (Pinus) alba_. - -In Prince Patrick’s Island, in latitude 76° 12′ N., longitude 122° -W., near the head of Walker Inlet, and a considerable distance in the -interior in one of the ravines, a tree protruding about 10 feet from -a bank was discovered by Lieutenant Mecham. It proved to be 4 feet -in circumference. In its neighbourhood several others were seen, all -of them similar to some he had found at Cape Manning; each of them -measured 4 feet round and 30 feet in length. The carpenter stated that -the trees resembled larch. Lieutenant Mecham, from their appearance and -position, concluded that they must have grown in the country.[133] - -Trees under similar conditions were also found by Lieutenant Pim on -Prince Patrick’s Island, and by Captain Parry on Melville Island, all -considerably above the present sea-level and at a distance from the -shore. On the coast of New Siberia, Lieutenant Anjou found a cliff of -clay containing stems of trees still capable of being used for fuel. - -“This remarkable phenomenon,” says Captain Osborn, “opens a vast field -for conjecture, and the imagination becomes bewildered in trying to -realise that period of the world’s history when the absence of ice and -a milder climate allowed forest trees to grow in a region where now the -ground-willow and dwarf-birch have to struggle for existence.” - -Sir Roderick Murchison came to the conclusion that all those trees -were drifted to their present position when the islands of the arctic -archipelago were submerged. But it was the difficulty of accounting -for the growth of trees in such a region which led him to adopt this -hypothesis. His argument is this: “If we imagine,” he says, “that the -timber found in those latitudes grew on the spot we should be driven -to adopt the anomalous hypothesis that, notwithstanding physical -relations of land and water similar to those which now prevail, trees -of large size grew on such _terra firma_ within a few degrees of the -north pole!—a supposition which I consider to be wholly incompatible -with the data in our possession, and at variance with the laws of the -isothermal lines.”[134] This reasoning of Sir Roderick’s may be quite -correct, on the supposition that changes of climate are due to changes -in the distribution of sea and land, as advocated by Sir Charles Lyell. -But these difficulties disappear if we adopt the views advocated in -the foregoing chapters. As Captain Osborn has pointed out, however, -Sir Roderick’s hypothesis leaves the real difficulty untouched. “A -very different climate,” he says, “must then have existed in those -regions to allow driftwood so perfect as to retain its bark to reach -such great distances; and perhaps it may be argued that if that sea was -sufficiently clear of ice to allow such timber to drift unscathed to -Prince Patrick’s Land, that that _very absence of a frozen sea would -allow fir-trees to grow in a soil naturally fertile_.”[135] - -As has been already stated, all who have seen those trees in arctic -regions agree in thinking that they grew _in situ_. And Professor -Haughton, in his excellent account of the arctic archipelago appended -to McClintock’s “Narrative of Arctic Discoveries,” after a careful -examination of the entire evidence on the subject, is distinctly of -the same opinion; while the recent researches of Professor Heer put it -beyond doubt that the drift theory must be abandoned. - -Undoubtedly the arctic archipelago was submerged to an extent that -could have admitted of those trees being floated to their present -positions. This, as we shall see, follows from theory; but submergence, -without a warmer condition of climate, would not enable trees to reach -those regions with their bark entire. - -But in reality we are not left to theorise on the subject, for we -have a well-authenticated case of one of those trees being got by -Captain Belcher standing erect in the position in which it grew. It was -found immediately to the northward of the narrow strait opening into -Wellington Sound, in lat. 75° 32′ N. long. 92° W., and about a mile and -a half inland. The tree was dug up out of the frozen ground, and along -with it a portion of the soil which was immediately in contact with the -roots. The whole was packed in canvas and brought to England. Near to -the spot several knolls of peat mosses about nine inches in depth were -found, containing the bones of the lemming in great numbers. The tree -in question was examined by Sir William Hooker, who gave the following -report concerning it, which bears out strongly the fact of its having -grown _in situ_. - -“The piece of wood brought by Sir Edward Belcher from the shores of -Wellington Channel belongs to a species of pine, probably to the _Pinus -(Abies) alba_, the most northern conifer. The structure of the wood -of the specimen brought home differs remarkably in its anatomical -character from that of any other conifer with which I am acquainted. -Each concentric ring (or annual growth) consists of two zones of -tissue; one, the outer, that towards the circumference, is broader, of -a pale colour, and consists of ordinary tubes of fibres of wood, marked -with discs common to all coniferæ. These discs are usually opposite -one another when more than one row of them occur in the direction of -the length of the fibre; and, what is very unusual, present radiating -lines from the central depression to the circumference. Secondly, -the inner zone of each annual ring of wood is narrower, of a dark -colour, and formed of more slender woody fibres, with thicker walls in -proportion to their diameter. These tubes have few or no discs upon -them, but are covered with spiral striæ, giving the appearance of each -tube being formed of a twisted band. The above characters prevail in -all parts of the wood, but are slightly modified in different rings. -Thus the outer zone is broader in some than in others, the disc-bearing -fibres of the outer zone are sometimes faintly marked with spiral -striæ, and the spirally marked fibres of the inner zone sometimes bear -discs. These appearances suggest the annual recurrence of some special -cause that shall thus modify the first and last formed fibres of each -year’s deposit, so that that first formed may differ in amount as -well as in kind from that last formed; and the peculiar conditions of -an arctic climate appear to afford an adequate solution. The inner, -or first-formed zone, must be regarded as imperfectly developed, -being deposited at a season when the functions of the plant are very -intermittently exercised, and when a few short hours of sunshine are -daily succeeded by many of extreme cold. As the season advances the -sun’s heat and light are continuous during the greater part of the -twenty-four hours, and the newly formed wood fibres are hence more -perfectly developed, they are much longer, present no signs of striæ, -but are studded with discs of a more highly organized structure than -are usual in the natural order to which this tree belongs.”[136] - -Another circumstance which shows that the tree had grown where it was -found is the fact that in digging up the roots portions of the leaves -were obtained. It may also be mentioned that near this place was found -an old river channel cut deeply into the rock, which, at some remote -period, when the climate must have been less rigorous than at present, -had been occupied by a river of considerable size. - -Now, it is evident that if a tree could have grown at Wellington Sound, -there is no reason why one might not have grown at Banks’s Land, or -at Prince Patrick’s Island. And, if the climatic condition of the -country would allow one tree to grow, it would equally as well allow -a hundred, a thousand, or a whole forest. If this, then, be the case, -Sir Roderick’s objection to the theory of growth _in situ_ falls to the -ground. - -Another circumstance which favours the idea that those trees grew -during the glacial epoch is the fact that although they are recent, -geologically speaking, and belong to the drift series, yet they are, -historically speaking, very old. The wood, though not fossilized, is so -hardened and changed by age that it will scarcely burn. - - - - - CHAPTER XVII. - - FORMER GLACIAL EPOCHS.—REASON OF THE IMPERFECTION OF - GEOLOGICAL RECORDS IN REFERENCE TO THEM. - - Two Reasons why so little is known of Glacial Epochs.—Evidence - of Glaciation to be found on Land-surfaces.—Where are all - our ancient Land-surfaces?—The stratified Rocks consist - of a Series of old Sea-bottoms.—Transformation of a - Land-surface into a Sea-bottom obliterates all Traces of - Glaciation.—Why so little remains of the Boulder Clays of - former Glacial Epochs.—Records of the Glacial Epoch are fast - disappearing.—Icebergs do not striate the Sea-bottom.—Mr. - Campbell’s Observations on the Coast of Labrador.—Amount - of Material transported by Icebergs much exaggerated.—Mr. - Packard on the Glacial Phenomena of Labrador.—Boulder Clay - the Product of Land-ice.—Palæontological Evidence.—Paucity of - Life characteristic of a Glacial Period.—Warm Periods better - represented by Organic Remains than cold.—Why the Climate - of the Tertiary Period was supposed to be warmer than the - present.—Mr. James Geikie on the Defects of Palæontological - Evidence.—Conclusion. - - -_Two Reasons why so little is known of former Glacial Epochs._—If the -glacial epoch resulted from the causes discussed in the foregoing -chapters, then such epochs must have frequently supervened. We may, -therefore, now proceed to consider what evidence there is for the -former occurrence of excessive conditions of climate during previous -geological ages. When we begin our inquiry, however, we soon find -that the facts which have been recorded as evidence in favour of the -action of ice in former geological epochs are very scanty indeed. Two -obvious reasons for this may be given, namely, (1) The imperfection -of the geological records themselves, and (2) the little attention -hitherto paid toward researches of this kind. The notion, once so -prevalent, that the climate of our earth was much warmer in the earlier -geological ages than it is now, and that it has ever since been -gradually becoming cooler, was wholly at variance with the idea of -former ice-periods. And this conviction of the _à priori_ improbability -of cold periods having obtained during Palæozoic and Mesozoic ages -tended to prevent due attention being paid to such facts as seemed to -bear upon the subject. But our limited knowledge of former glacial -epochs must no doubt be attributed chiefly to the actual imperfection -of the geological records. So great is this imperfection that the mere -absence of direct geological evidence cannot reasonably be regarded as -sufficient proof that the conclusions derived from astronomical and -physical considerations regarding former ice-periods are improbable. -Nor is this all. The geological records of ancient glacial conditions -are not only imperfect, but, as I shall endeavour to show, this -imperfection _follows as a natural consequence from the principles of -geology itself_. There are not merely so many blanks or gaps in the -records, but a reason exists in the very nature of geological evidence -why such breaks in the record might reasonably be expected to occur. - -_Evidence of Glaciation to be found chiefly on Land-surfaces._—It is on -a land-surface that the principal traces of the action of ice during -a glacial epoch are left, for it is there that the stones are chiefly -striated, the rocks ground down, and the boulder clay formed. But where -are all our ancient land-surfaces? They are not to be found. The total -thickness of the stratified rocks of Great Britain is, according to -Professor Ramsay, nearly fourteen miles. But from the top to the bottom -of this enormous pile of deposits there is hardly a single land-surface -to be detected. True patches of old land-surfaces of a local character -exist, such, for example, as the dirt-beds of Portland; but, with the -exception of coal-seams, every general formation from top to bottom -has been accumulated under water, and none but the under-clays _ever -existed as a land_-surface. And it is here, in such a formation, -that the geologist has to collect all his information regarding the -existence of former glacial epochs. The entire stratified rocks of the -globe, with the exception of the coal-beds and under-clays (in neither -of which would one expect to find traces of ice-action), consist almost -entirely of a _series of old sea-bottoms_, with here and there an -occasional freshwater deposit. Bearing this in mind, what is the sort -of evidence which we can now hope to find in these old sea-bottoms of -the existence of former ice-periods? - -Every geologist of course admits that the stratified rocks are not -old land-surfaces, but a series of old sea-bottoms formed out of -the accumulated material derived from the degradation of primeval -land-surfaces. And it is true that all land-surfaces once existed -as sea-bottoms; but the stratified rocks consist of a series of old -sea-bottoms which never were land-surfaces. Many of them no doubt -have been repeatedly above the sea-level, and may once have possessed -land-surfaces; but these, with the exception of the under-clays of the -various coal measures, the dirt-beds of Portland, and one or two more -patches, have all been denuded away. The important bearing which this -consideration has on the nature of the evidence which we can now expect -to find of the existence of former glacial epochs has certainly been -very much overlooked. - -If we examine the matter fully we shall be led to conclude that the -_transformation of a land-surface into a sea-bottom_ will probably -completely obliterate every trace of glaciation which that land-surface -may once have presented. We cannot, for example, expect to meet with -polished and striated stones belonging to a former land glaciation; for -such stones are not carried down bodily and unchanged by our rivers -and deposited in the sea. They become broken up by subaërial agencies -into gravel, sand, and clay, and in this condition are transported -seawards. Nor even if we supposed it possible that the stones and -boulders derived from a mass of till could be carried down to sea by -river-action, could we at the same time fail to admit that such stones -would be deprived of all their ice-markings, and become water-worn and -rounded on the way.[137] - -Nor can we expect to find boulder clay among the stratified rocks, for -boulder clay is not carried down as such and deposited in the sea, but -under the influence of the denuding agents becomes broken up into soft -mud, clay, sand, and gravel, as it is gradually peeled off the land and -swept seawards. Patches of boulder clay may have been now and again -forced into the sea by ice and eventually become covered up; but such -cases are wholly exceptional, and their absence in any formation cannot -fairly be adduced as a proof that that formation does not belong to a -glacial period. - -The only evidence of the existence of land-ice during former periods -which we can reasonably expect to meet with in the stratified rocks, -consists of erratic blocks which may have been transported by icebergs -and dropped into the sea. But unless the glaciers of such epochs -reached the sea, we could not possibly possess even this evidence. -Traces in the stratified rocks of the effects of land-ice during former -epochs must, in the very nature of things, be rare indeed. The only -sort of evidence which, as a general rule, we may expect to detect, -is the presence of large erratic blocks imbedded in strata which from -their constitution have evidently been formed in still water. But this -is quite enough; for it proves the existence of ice at the time the -strata were being deposited as conclusively as though we saw the ice -floating with the blocks upon it. This sort of evidence, when found in -low latitudes, ought to be received as conclusive of the existence of -former glacial epochs; and, no doubt, would have been so received had -it not been for the idea that, if these blocks had been transported -by ice, there ought in addition to have been found striated stones, -boulder clay, and other indications of the agency of land-ice. - -Of course all erratics are not necessarily transported by masses of -ice broken from the terminal front of glaciers. The “ice foot,” formed -by the freezing of the sea along the coasts of the higher latitudes of -Greenland, carries seawards immense quantities of blocks and _débris_. -And again stones and boulders are frequently frozen into river-ice, -and when the ice breaks up in spring are swept out to sea, and may be -carried some little distance before they are dropped. But both these -cases can occur only in regions where the winters are excessive; nor -is it at all likely that such ice-rafts will succeed in making a long -voyage. If, therefore, we could assure ourselves that the erratics -occasionally met with in certain old geological formations in low -latitudes were really transported from the land by an ice-foot or a -raft of river-ice, we should be forced to conclude that very severe -climatic conditions must have obtained in such latitudes at the time -the erratics were dispersed. - -The reason why we now have, comparatively speaking, so little direct -evidence of the existence of former glacial periods will be more -forcibly impressed upon the mind, if we reflect on how difficult it -would be in a million or so of years hence to find any trace of what -we now call the glacial epoch. The striated stones would by that time -be all, or nearly all, disintegrated, and the till washed away and -deposited in the bottom of the sea as stratified sands and clays. And -when these became consolidated into rock and were raised into dry land, -the only evidence that we should probably then have that there ever -had been a glacial epoch would be the presence of large blocks of the -older rocks, which would be found imbedded in the upraised formation. -We could only infer that there had been ice at work from the fact that -by no other known agency could we conceive such blocks to have been -transported and dropped in a still sea. - -Probably few geologists believe that during the Middle Eocene and -the Upper Miocene periods our country passed through a condition of -glaciation as severe as it has done during the Post-pliocene period; -yet when we examine the subject carefully, we find that there is -actually no just ground to conclude that it has not. For, in all -probability, throughout the strata to be eventually formed out of the -destruction of the now existing land-surfaces, evidence of ice-action -will be as scarce as in Eocene or Miocene strata. - -If the stratified rocks forming the earth’s crust consisted of a series -of old land-surfaces instead (as they actually do) of a series of old -sea-bottoms, then probably traces of many glacial periods might be -detected. - -Nearly all the evidence which we have regarding the glacial epoch -has been derived from what we find on the now existing land-surfaces -of the globe. But probably not a vestige of this will exist in the -stratified beds of future ages, formed out of the destruction of the -present land-surfaces. Even the very arctic shell-beds themselves, -which have afforded to the geologist such clear proofs of a frozen sea -during the glacial epoch, will not be found in those stratified rocks; -for they must suffer destruction along with everything else which now -exists above the sea-level. There is probably not a single relic of -the glacial epoch which has ever been seen by the eye of man that will -be treasured up in the stratified rocks of future ages. Nothing that -does not lie buried in the deeper recesses of the ocean will escape -complete disintegration and appear imbedded in those formations. It -is only those objects which lie in our existing sea-bottoms that will -remain as monuments of the glacial epoch of the Post-tertiary period. -And, moreover, it will only be those portions of the sea-bottoms that -may happen to be upraised into dry land that will be available to the -geologist of future ages. The point to be determined now is this:—_Is -it probable that the geologist of the future will find in the rocks -formed out of the now existing sea-bottoms more evidence of a glacial -epoch during Post-tertiary times than we now do of one during, say, the -Miocene, the Eocene, or the Permian period?_ Unless this can be proved -to be the case, we have no ground whatever to conclude that the cold -periods of the Miocene, Eocene, and Permian periods were not as severe -as that of the glacial epoch. This is evident, for the only relics -which now remain of the glacial epochs of those periods are simply -what happened to be protected in the then existing sea-bottoms. Every -vestige that lay on the land would in all probability be destroyed by -subaërial agency and carried into the sea in a sedimentary form. But -before we can determine whether or not there is more evidence of the -glacial epoch in our now existing sea-bottoms than there is of former -glacial epochs in the stratified rocks (which are in reality the -sea-bottoms belonging to ancient epochs), we must first ascertain what -is the nature of those marks of glaciation which are to be found in a -sea-bottom. - -_Icebergs do not striate the Sea-bottom._—We know that the rocky face -of the country was ground down and striated during the glacial epoch; -and this is now generally believed to have been done by land-ice. But -we have no direct evidence that the floor of the ocean, beyond where it -may have been covered with land-ice, was striated. Beyond the limits -of the land-ice it could be striated only by means of icebergs. But -do icebergs striate the rocky bed of the ocean? Are they adapted for -such work? It seems to be often assumed that they are. But I have been -totally unable to find any rational grounds for such a belief. Clean -ice can have but little or no erosive power, and never could scratch a -rock. To do this it must have grinding materials in the form of sand, -mud, or stones. But the bottoms of icebergs are devoid of all such -materials. Icebergs carry the grinding materials on their backs, not on -their bottoms. No doubt, when the iceberg is launched into the deep, -great masses of sand, mud, and stones will be adhering to its bottom. -But no sooner is the berg immersed, than a melting process commences -at its sides and lower surface in contact with the water; and the -consequence is, the materials adhering to the lower surface soon drop -off and sink to the bottom of the sea. The iceberg, divested of these -materials, can now do very little harm to the rocky sea-bottom over -which it floats. It is true that an iceberg moving with a velocity -of a few miles an hour, if it came in contact with the sea-bottom, -would, by the mere force of concussion, tear up loose and disjointed -rocks, and hurl some of the loose materials to a distance; but it would -do but little in the way of grinding down the rock against which it -struck. But even supposing the bottom of the iceberg were properly -shod with the necessary grinding materials, still it would be but a -very inefficient grinding agent; for a _floating_ iceberg would not -be in contact with the sea-bottom. And if it were in contact with the -sea-bottom, it would soon become stranded and, of course, motionless, -and under such conditions could produce no effect. - -It is perfectly true that although the bottom of the berg may be devoid -of grinding materials, yet these may be found lying on the surface -of the submarine rock over which the ice moves. But it must be borne -in mind that the same current which will move the icebergs over the -surface of the rock will move the sand, mud, and other materials -over it also; so that the markings effected by the ice would in all -probability be erased by the current. In the deep recesses of the -ocean the water has been found to have but little or no motion. But -icebergs always follow the path of currents; and it is very evident -that at the comparatively small depth of a thousand feet or so reached -by icebergs the motion of the water will be considerable; and the -continual shifting of the small particles of the mud and sand will in -all probability efface the markings which may be made now and again by -a passing berg. - -Much has been said regarding the superiority of icebergs as grinding -and striating agents in consequence of the great velocity of their -motion in comparison with that of land-ice. But it must be remembered -that it is while the iceberg is floating, and before it touches the -rock, that it possesses high velocity. When the iceberg runs aground, -its motion is suddenly arrested or greatly reduced. But if the iceberg -advancing upon a sloping sea-bottom is raised up so as to exert great -pressure, it will on this account be the more suddenly arrested, -the motion will be slow, and the distance passed over short, before -the berg becomes stranded. If it exerts but little pressure on the -sea-bottom, it may retain a considerable amount of motion and advance -to a considerable distance before it is brought to a stand; but, -exerting little pressure, it can perform but little work. Land-ice -moves slowly, but then it exerts enormous pressure. A glacier 1,000 -feet in thickness has a pressure on its rocky bed equal to about 25 -tons on the square foot; but an iceberg a mile in thickness, forced up -on a sloping sea-bottom to an elevation of 20 feet (and this is perhaps -more than any ocean-current could effect), would only exert a pressure -of about half a ton on the square foot, or about 1/50th part of the -pressure of the glacier 1,000 feet in thickness. A great deal has been -said about the erosive and crushing power of icebergs of enormous -thickness, as if their thickness gave them any additional pressure. An -iceberg 100 feet in thickness will exert just as much pressure as one -a mile in thickness. The pressure of an iceberg is not like that of a -glacier, in proportion to its thickness, but to the height to which it -is raised out of the water. An iceberg 100 feet in thickness raised 10 -feet will exert exactly the same pressure as one a mile in thickness -raised to an equal height. - -To be an efficient grinding agent, steadiness of motion, as well as -pressure, is essential. A rolling or rocking motion is ill-adapted -for grinding down and striating a rock. A steady rubbing motion under -pressure is the thing required. But an iceberg is not only deficient in -pressure, but also deficient in steadiness of motion. When an iceberg -moving with considerable velocity comes on an elevated portion of the -sea-bottom, it does not move steadily onwards over the rock, unless -the pressure of the berg on the rock be trifling. The resistance being -entirely at the bottom of the iceberg, its momentum, combined with the -pressure of the current, applied wholly above the point of resistance, -tends to make the berg bend forward, and in some cases upset (when -it is of a cubical form). The momentum of the moving berg, instead -of being applied in forcing it over the rock against which it comes -in contact, is probably all consumed in work against gravitation in -raising the berg upon its front edge. After the momentum is consumed, -unless the berg be completely upset, it will fall back under the force -of gravitation to its original position. But the momentum which it -acquires from gravitation in falling backwards carries it beyond its -position of repose in an opposite direction. It will thus continue to -rock backwards and forwards until the friction of the water brings it -to rest. The momentum of the berg, instead of being applied to the work -of grinding and striating the sea-bottom, will chiefly be consumed in -heat in the agitation of the water. But if the berg does advance, it -will do so with a rocking unsteady motion, which, as Mr. Couthouy[138] -and Professor Dana[139] observe, will tend rather to obliterate -striations than produce them. - -A floating berg moves with great steadiness; but a berg that has run -aground cannot advance with a steady motion. If the rock over which the -berg moves offers little resistance, it may do so; but in such a case -the berg could produce but little effect on the rock. - -Dr. Sutherland, who has had good opportunities to witness the effects -of icebergs, makes some most judicious remarks on the subject. “It -will be well” he says, “to bear in mind that when an iceberg _touches -the ground, if that ground be hard and resisting, it must come to a -stand_, and the propelling power continuing, a slight leaning over in -the water, or yielding motion of the whole mass, may compensate readily -for being so suddenly arrested. If, however, the ground be soft, so -as not to arrest the motion of the iceberg at once, a moraine will be -the result; but the moraine thus raised will tend to bring it to a -stand.”[140] - -There is another cause referred to by Professor Dana, which, to a -great extent, must prevent the iceberg from having an opportunity of -striating the sea-bottom, even though it were otherwise well adapted -for so doing. It is this: the bed of the ocean in the track of -icebergs must be pretty much covered with stones and rubbish dropped -from the melting bergs. And this mass of rubbish will tend to protect -the rock.[141] - -If icebergs cannot be shown _à priori_, from mechanical considerations, -to be well adapted for striating the sea-bottom, one would naturally -expect, from the confident way in which it is asserted that they are -so adapted, that the fact has been at least established by actual -observation. But, strange as it may appear, we seem to have little or -no proof that icebergs actually striate the bed of the ocean. This can -be proved from the direct testimony of the advocates of the iceberg -theory themselves. - -We shall take the testimony of Mr. Campbell, the author of two -well-known works in defence of the iceberg theory, viz., “Frost and -Fire,” and “A Short American Tramp.” Mr. Campbell went in the fall of -the year 1864 to the coast of Labrador, the Straits of Belle Isle, and -the Gulf of St. Lawrence, for the express purpose of witnessing the -effects of icebergs, and testing the theory which he had formed, that -the ice-markings of the glacial epoch were caused by floating ice and -not by land-ice, as is now generally believed. - -The following is the result of his observations on the coast of -Labrador. - -Hanly Harbour, Strait of Belle Isle:—“The water is 37° F. in July.... -As fast as one island of ice grounds and bursts, another takes its -place; and in winter the whole strait is blocked up by a mass which -swings bodily up and down, grating along the bottom at all depths.... -Examined the beaches and rocks at the water-line, especially in sounds. -Found the rocks ground smooth, _but not striated_, in the sounds” -(_Short American Tramp_, pp. 68, 107). - -Cape Charles and Battle Harbour:—“But though these harbours are all -frozen every winter, the _rocks at the water-line are not striated_” -(p. 68). - -At St. Francis Harbour:—“The water-line is much rubbed, smooth, _but -not striated_” (p. 72). - -Cape Bluff:—“Watched the rocks with a telescope, and _failed to make -out striæ anywhere_; but the water-line is everywhere rubbed smooth” -(p. 75). - -Seal Islands:—“_No striæ are to be seen at the land-wash in these -sounds or on open sea-coasts near the present water-line_” (p. 76). - -He only mentions having here found striations in the three following -places along the entire coast of Labrador visited by him; and in regard -to two of these, it seems very doubtful that the markings were made by -modern icebergs. - -Murray’s Harbour:—“This harbour was blocked up with ice on the 20th of -July. The water-line is rubbed, and in _some places_ striated” (p. 69). - -Pack Island:—“The water-line in a narrow sound was polished and -striated in the direction of the sound, about N.N.W. This seems to be -fresh work done by heavy ice drifting from Sandwich Bay; _but, on the -other hand, stages with their legs in the sea, and resting on these -very rocks, are not swept away by the ice_” (p. 96). If these markings -were modern, why did not the “heavy ice” remove the small fir poles -supporting the fishing-stages? - -Red Bay:—“Landed half-dressed, and found some striæ perfectly fresh at -the water-level, but weathered out a short distance _inland_” (p. 107). -The striations “inland” could not have been made by modern icebergs; -and it does not follow that because the markings at the water-level -were not weathered they were produced by modern ice. - -These are the evidences which he found that icebergs striate rocks, -on a coast of which he says that, during the year he visited it, “the -winter-drift was one vast solid raft of floes and bergs more than 150 -miles wide, and perhaps 3,000 feet thick at spots, driven by a whole -current bodily over one definite course, year after year, since this -land was found” (p. 85). - -But Mr. Campbell himself freely admits that the floating ice which -comes aground along the shores does not produce striæ. “It is -sufficiently evident,” he says, “_that glacial striæ are not produced -by thin bay-ice_” (p. 76). And in “Frost and Fire,” vol. ii., p. 237, -he states that, “from a careful examination of the water-line at many -spots, it appears that bay-ice grinds rocks, _but does not produce -striation_.” - -“It is impossible,” he continues, “to get at rocks over which heavy -icebergs now move; but a mass 150 miles wide, perhaps 3,000 feet thick -in some parts, and moving at the rate of a mile an hour, or more, -_appears to be an engine amply sufficient_ to account for striæ on -rising rocks.” And in “American Tramp,” p. 76, he says, “_striæ must be -made_ in deep water by the large masses which seem to pursue the even -tenor of their way in the steady current which flows down the coast.” - -Mr. Campbell, from a careful examination of the sea-bottom along the -coast, finds that the small icebergs do not produce striæ, but the -large ones, which move over rocks impossible to be got at, “must” -produce them. They “appear” to be amply sufficient to do so. If the -smaller bergs cannot striate the sea-bottom, why must the larger ones -do so? There is no reason why the smaller bergs should not move as -swiftly and exert as much pressure on the sea-bottom as the larger -ones. And even supposing that they did not, one would expect that the -light bergs would effect on a smaller scale what the heavy ones would -do on a larger. - -I have no doubt that when Mr. Campbell visited Labrador he expected to -find the sea-coast under the water-line striated by means of icebergs, -and was probably not a little surprised to find that it actually was -not. And I have no doubt that were the sea-bottom in the tracks of the -large icebergs elevated into view, he would find to his surprise that -it was free from striations also. - -So far as observation is concerned, we have no grounds from what Mr. -Campbell witnessed to conclude that icebergs striate the sea-bottom. - -The testimony of Dr. Sutherland, who has had opportunities of seeing -the effects of icebergs in arctic regions, leads us to the same -conclusion. “Except,” he says, “from the evidence afforded by plants -and animals at the bottom, we have _no means whatever_ to ascertain -the effect produced by icebergs upon the rocks.[142] In the Malegat -and Waigat I have seen whole clusters of these floating islands, -drawing from 100 to 250 fathoms, moving to and fro with every return -and recession of the tides. I looked very earnestly for grooves and -scratches left by icebergs and glaciers in the rocks, but always failed -to discover any.”[143] - -We shall now see whether river-ice actually produces striations or not. -If floating ice under any form can striate rocks, one would expect that -it ought to be done by river-ice, seeing that such ice is obliged to -follow one narrow definite track. - -St. John’s River, New Brunswick:—“This river,” says Mr. Campbell, -“is obstructed by ice during five months of the year. When the ice -goes, there is wild work on the bank. Arrived at St. John, drove -to the suspension-bridge.... At this spot, if _anywhere in the -world_, river-ice ought to produce striation. The whole drainage of -a wide basin and one of the strongest tides in the world, here work -continually in one rock-groove; and in winter this water-power is armed -with heavy ice. _There are no striæ_ about the water-line.”[144] - -River St. Lawrence:—“In winter the power of ice-floats driven by -water-power is tremendous. The river freezes and packs ice till the -flow of water is obstructed. The rock-pass at Quebec is like the -Narrows at St. John’s, Newfoundland. The whole pass, about a mile -wide, was paved with great broken slabs and round boulders of worn ice -as big as small shacks, piled and tossed, and heaped and scattered -upon the level water below and frozen solid.... This kind of ice does -NOT _produce striation_ at the water-margin at Quebec. At Montreal, -when the river ‘goes,’ the ice goes with it with a vengeance.... The -_piers are not yet striated_ by river-ice at Montreal.... The rocks -at the high-water level have _no trace_ of glacial striæ.... The rock -at Ottawa is rubbed by river-ice every spring, and _always in one -direction, but it is not striated_.... The surfaces are all rubbed -smooth, and the edges of broken beds are rounded where exposed to the -ice; _but there are no striæ_.”[145] - -When Sir Charles Lyell visited the St. Lawrence in 1842, at Quebec he -went along with Colonel Codrington “and searched carefully below the -city in the channel of the St. Lawrence, at low water, near the shore, -for the signs of glacial action at the precise point where the chief -pressure and friction of packed ice are exerted every year,” but found -none. - -“At the bridge above the Falls of Montmorenci, over which a large -quantity of ice passes every year, the gneiss is polished, and kept -perfectly free from lichens, but not more so than rocks similarly -situated at waterfalls in Scotland. In none of these places were any -long straight grooves observable.”[146] - -The only thing in the shape of modern ice-markings which he seems to -have met with in North America was a few straight furrows half an inch -broad in soft sandstone, at the base of a cliff at Cape Blomidon in the -Bay of Fundy, at a place where during the preceding winter “packed” -ice 15 feet thick had been pushed along when the tide rose over the -sandstone ledges.[147] - -The very fact that a geologist so eminent as Sir Charles Lyell, after -having twice visited North America, and searched specially for modern -ice-markings, was able to find only two or three scratches, upon a soft -sandstone rock, which he could reasonably attribute to floating ice, -ought to have aroused the suspicion of the advocates of the iceberg -theory that they had really formed too extravagant notions regarding -the potency of floating ice as a striating agent. - -There is no reason to believe that the grooves and markings noticed -by M. Weibye and others on the Scandinavian coast and other parts of -northern Europe were made by icebergs. - -Professor Geikie has clearly shown, from the character and direction -of the markings, that they are the production of land-ice.[148] If -the floating ice of the St. Lawrence and the icebergs of Labrador are -unable to striate and groove the rocks, it is not likely that those of -northern Europe will be able to do so. - -It will not do for the advocates of the iceberg theory to assume, as -they have hitherto done, that, as a matter of course, the sea-bottom is -being striated and grooved by means of icebergs. They must prove that. -They must either show that, as a matter of fact, icebergs are actually -efficient agents in striating the sea-bottom, or prove from mechanical -principles that they must be so. The question must be settled either by -observation or by reason; mere opinion will not do. - -_The Amount of Material transported by Icebergs much exaggerated._—The -transporting of boulders and rubbish, and not the grinding and -striating of rocks, is evidently the proper function of the iceberg. -But even in this respect I fear too much has been attributed to it. - -In reading the details of voyages in the arctic regions one cannot help -feeling surprised how seldom reference is made to stones and rubbish -being seen on icebergs. Arctic voyagers, like other people, when they -are alluding to the geological effects of icebergs, speak of enormous -quantities of stones being transported by them; but in reading the -details of their voyages, the impression conveyed is that icebergs with -stones and blocks of rock upon them are the exceptions. The greater -portion of the narratives of voyages in arctic regions consists of -interesting and detailed accounts of the voyager’s adventures among the -ice. The general appearance of the icebergs, their shape, their size, -their height, their colour, are all noticed; but rarely is mention -made of stones being seen. That the greater number of icebergs have -no stones or rubbish on them is borne out by the positive evidence of -geologists who have had opportunities of seeing icebergs. - -Mr. Campbell says:—“It is remarkable that up to this time we have only -seen a few doubtful stones on bergs which we have passed.... Though -no bergs with stones _on them or in them_ have been approached during -this voyage, many on board the _Ariel_ have been close to bergs heavily -laden.... A man who has had some experience of ice has _never seen a -stone on a berg_ in these latitudes. Captain Anderson, of the _Europa_, -who is a geologist, has _never seen a stone on a berg_ in crossing the -Atlantic. _No stones were clearly seen on this trip._”[149] Captain Sir -James Anderson (who has long been familiar with geology, has spent a -considerable part of his life on the Atlantic, and has been accustomed -to view the iceberg as a geologist as well as a seaman) has never seen -a stone on an iceberg in the Atlantic. This is rather a significant -fact. - -Sir Charles Lyell states that, when passing icebergs on the Atlantic, -he “was most anxious to ascertain whether there was any mud, stones, -or fragments of rocks on any one of these floating masses; but after -examining about forty of them without perceiving any signs of frozen -matter, I left the deck when it was growing dusk.”[150] After he had -gone below, one was said to be seen with something like stones upon it. -The captain and officers of the ship assured him that they had _never -seen a stone upon a berg_. - -The following extract from Mr. Packard’s “Memoir on the Glacial -Phenomena of Labrador and Maine,” will show how little is effected by -the great masses of floating ice on the Labrador coast either in the -way of grinding and striating the rocks, or of transporting stones, -clay, and other materials. - -“Upon this coast, which during the summer of 1864 was lined with a -belt of floe-ice and bergs probably two hundred miles broad, and which -extended from the Gulf of St. Lawrence at Belles Amours to the arctic -seas, this immense body of floating ice seemed _directly_ to produce -but little alteration in its physical features. If we were to ascribe -the grooving and polishing of rocks to the action of floating ice-floes -and bergs, how is it that the present shores far above (500), and at -least 250 feet below, the water-line are often jagged and angular, -though constantly stopping the course of masses of ice impelled four to -six miles an hour by the joint action of tides, currents, and winds? No -boulders, or gravel, or mud were seen upon any of the bergs or masses -of shore-ice. They had dropped all burdens of this nature nearer their -points of detachment in the high arctic regions.” ... - -“This huge area of floating ice, embracing so many thousands of square -miles, was of greater extent, and remained longer upon the coast, in -1864, than for forty years previous. It was not only pressed upon the -coast by the normal action of the Labrador and Greenland currents, -which, in consequence of the rotatory motion of the earth, tended to -force the ice in a south-westerly direction, but the presence of the -ice caused the constant passage of cooler currents of air from the -sea over the ice upon the heated land, giving rise during the present -season to a constant succession of north-easterly winds from March -until early in August, which further served to crowd the ice into every -harbour and recess upon the coast. It was the universal complaint -of the inhabitants that the easterly winds were more prevalent, and -the ice ‘held’ later in the harbours this year than for many seasons -previous. Thus the fisheries were nearly a failure, and vegetation -greatly retarded in its development. But so far as polishing and -striating the rocks, depositing drift material, and thus modifying -the contour of the surface of the present coast, this modern mass of -bergs and floating ice effected comparatively little. Single icebergs, -when small enough, entered the harbours, and there stranding, soon -pounded to pieces upon the rocks, melted, and disappeared. From Cape -Harrison, in lat. 55°, to Caribo Island, was an interrupted line of -bergs stranded in 80 to 100 or more fathoms, often miles apart, while -others passed to the seaward down by the eastern coast of Newfoundland, -or through the Straits of Belle Isle.”[151] - -_Boulder Clay the Product of Land-ice._—There is still another point -connected with icebergs to which we must allude, viz., the opinion -that great masses of the boulder clay of the glacial epoch were formed -from the droppings of icebergs. If boulder clay is at present being -accumulated in this manner, then traces of the boulder clay deposits of -former epochs might be expected to occur. It is perfectly obvious that -_unstratified_ boulder clay could not have been formed in this way. -Stones, gravel, sand, clay, and mud, the ingredients of boulder clay, -tumbled all together from the back of an iceberg, could not sink to the -bottom of the sea without separating. The stones would reach the bottom -first, then the gravel, then the sand, then the clay, and last of all -the mud, and the whole would settle down in a stratified form. But, -besides, how could the _clay_ be derived from icebergs? Icebergs derive -their materials from the land before they are launched into the deep, -and while they are in the form of land-ice. The materials which are -found on the backs of icebergs are what fell upon the ice from mountain -tops and crags projecting above the ice. Icebergs are chiefly derived -from continental ice, such as that of Greenland, where the whole -country is buried under one continuous mass, with only a lofty mountain -peak here and there rising above the surface. And this is no doubt -the chief reason why so few icebergs have stones upon their backs. -The continental ice of Greenland is not, like the glaciers of the -Alps, covered with loose stones. Dr. Robert Brown informs me that no -moraine matter has ever been seen on the inland ice of Greenland. It is -perfectly plain that clay does not fall upon the ice. What falls upon -the ice is stones, blocks of rocks, and the loose _débris_. Clay and -mud we know, from the accounts given by arctic voyagers, are sometimes -washed down upon the coast-ice; but certainly very little of either can -possibly get upon an iceberg. Arctic voyagers sometimes speak of seeing -clay and mud upon bergs; but it is probable that if they had been near -enough they would have found that what they took for clay and mud were -merely dust and rubbish. - -Undoubtedly the boulder clay of many places bears unmistakable evidence -of having been formed under water; but it does not on that account -follow that it was formed from the droppings of icebergs. The fact -that the boulder clay in every case _is chiefly composed of materials -derived from the country on which the clay lies_, proves that it was -not formed from matter transported by icebergs. The clay, no doubt, -contains stones and boulders belonging to other countries, which in -some cases may have been transported by icebergs; but the clay itself -has not come from another country. But if the clay itself has been -derived from the country on which it lies, then it is absurd to suppose -that it was deposited from icebergs. The clay and materials which are -found on icebergs are derived from the land on which the iceberg is -formed; but to suppose that icebergs, after floating about upon the -ocean, should always return to the country which gave them birth, and -there deposit their loads, is rather an extravagant supposition. - -From the facts and considerations adduced we are, I would venture to -presume, warranted to conclude that, with the exception of what may -have been produced by land-ice, very little in the shape of boulder -clay or striated rocks belonging to the glacial epoch lies buried -under the ocean—and that when the now existing land-surfaces are all -denuded, probably scarcely a trace of the glacial epoch will then be -found, except the huge blocks that were transported by icebergs and -dropped into the sea. It is therefore probable that we have as much -evidence of the existence of a glacial epoch during former periods as -the geologists of future ages will have of the existence of a glacial -epoch during the Post-tertiary period, and that consequently we are not -warranted in concluding that the glacial epoch was something unique in -the geological history of our globe. - -_Palæontological Evidence._—It might be thought that if glacial epochs -have been numerous, we ought to have abundance of palæontological -evidence of their existence. I do not know if this necessarily follows. -Let us take the glacial epoch itself for example, which is quite a -modern affair. Here we do not require to go and search in the bottom -of the sea for the evidence of its existence; for we have the surface -of the land in almost identically the same state in which it was when -the ice left it, with the boulder clay and all the wreck of the ice -lying upon it. But what geologist, with all these materials before him, -would be able to find out from palæontological evidence alone that -there had been such an epoch? He might search the whole, but would not -be able to find fossil evidence from which he could warrantably infer -that the country had ever been covered with ice. We have evidence -in the fossils of the Crag and other deposits of the existence of a -colder condition of climate prior to the true glacial period, and in -the shell-beds of the Clyde and other places of a similar state of -matters after the great ice-sheets had vanished away. But in regard -to the period of the true boulder clay or till, when the country was -enveloped in ice, palæontology has almost nothing whatever to tell -us. “Whatever may be the cause,” says Sir Charles Lyell, “the fact is -certain that over large areas in Scotland, Ireland, and Wales, I might -add throughout the northern hemisphere on both sides of the Atlantic, -the stratified drift of the glacial period is very commonly devoid of -fossils.”[152] - -In the “flysch” of the Eocene of the Alps, to which we shall have -occasion to refer in the next chapter, in which the huge blocks are -found which prove the existence of ice-action during that period, few -or no fossils have been found. So devoid of organic remains is that -formation, that it is only from its position, says Sir Charles, that -it is known to belong to the middle or “nummulitic” portion of the -great Eocene series. Again, in the conglomerates at Turin, belonging -to the Upper Miocene period, in which the angular blocks of limestone -are found which prove that during that period Alpine glaciers reached -the sea-level in the latitude of Italy, not a single organic remain has -been found. It would seem that an extreme paucity of organic life is a -characteristic of a glacial period, which warrants us in concluding -that the absence of organic remains in any formation otherwise -indicative of a cold climate cannot be regarded as sufficient evidence -that that formation does not belong to a cold period. - -In the last chapter it was shown why so little evidence of the warm -periods of the glacial epoch is now forthcoming. The remains of the -_faunas_ and _floras_ of those periods were nearly wholly destroyed and -swept into the adjoining seas by the ice-sheet that covered the land. -It is upon the present land-surface that we find the chief evidence -of the last glacial epoch, but the traces of the warm periods of that -epoch are hardly now to be met with in that position since they have -nearly all been obliterated or carried into the sea. - -In regard to former glacial epochs, however, ice-marked rocks, -scratched stones, moraines, till, &c., no longer exist; the -land-surfaces of those old times have been utterly swept away. The only -evidence, therefore, of such ancient glacial epochs, that we can hope -to detect, must be sought for in the deposits that were laid down upon -the sea-bottom; where also we may expect to find traces of the warm -periods that alternated during such epochs with glacial conditions. It -is plain, moreover, that the palæontological evidence in favour of warm -periods will always be the most abundant and satisfactory. - -Judging from geological evidence alone, we naturally conclude that, as -a general rule, the climate of former periods was somewhat warmer than -it is at the present day. It is from fossil remains that the geologist -principally forms his estimate of the character of the climate during -any period. Now, in regard to fossil remains, the warm periods will -always be far better represented than the cold; for we find that, as -a _general rule, those formations which geologists are inclined to -believe indicate a cold condition of climate are remarkably devoid of -fossil remains_. If a geologist does not keep this principle in view, -he will be very apt to form a wrong estimate of the general character -of the climate of a period of such enormous length as say the Tertiary. - -Suppose that the presently existing sea-bottoms, which have been -forming since the commencement of the glacial epoch, were to become -consolidated into rock and thereafter to be elevated into dry land, we -should then have a formation which might be properly designated the -Post-pliocene. It would represent the time which has elapsed from the -beginning of the glacial epoch to the present day. Suppose one to be -called upon as a geologist to determine from that formation what was -the general character of the climate during the period in question, -what would probably be the conclusion at which he would arrive? He -would probably find here and there patches of boulder clay containing -striated and ice-worn stones. Now and again he would meet with bones -of the mammoth and the reindeer, and shells of an arctic type. He -would likewise stumble upon huge blocks of the older rocks imbedded -in the formation, from which he would infer the existence of icebergs -and glaciers reaching the sea-level. But, on the whole, he would -perceive that the greater portion of the fossil remains met with in -this formation implied a warm and temperate condition of climate. At -the lower part of the formation, corresponding to the time of the true -boulder clay, there would be such a scarcity of organic remains that -he would probably feel at a loss to say whether the climate at that -time was cold or hot. But if the intense cold of the glacial epoch -was not continuous, but broken up by intervening warm periods during -which the ice, to a considerable extent at least, disappeared for a -long period of time (and there are few geologists who have properly -studied the subject who will positively deny that such was the case), -then the country would no doubt during those warm periods possess an -abundance of plant and animal life. It is quite true that we may almost -search in vain on the present land-surface for the organic remains -which belonged to those inter-glacial periods; for they were nearly -all swept away by the ice which followed. But no doubt in the deep -recesses of the ocean, buried under hundreds of feet of sand, mud, -clay, and gravel, lie multitudes of the plants and animals which then -flourished on the land, and were carried down by rivers into the sea. -And along with these lie the skeletons, shells, and other exuviæ of -the creatures which flourished in the warm seas of those periods. Now -looking at the great abundance of fossils indicative of warm and genial -conditions which the lower portions of this formation would contain, -the geologist might be in danger of inferring that the earlier part -of the Post-pliocene period was a warmer period, whereas we, at the -present day, looking at the matter from a different standpoint, declare -that part to have been characterized by cold or glacial conditions. No -doubt, if the beds formed during the cold periods of the glacial epoch -could be distinguished from those formed during the warm periods, the -fossil remains of the one would indicate a cold condition of climate, -and those of the other a warm condition; but still, taking the entire -epoch as a whole, the percentage of fossil remains indicative of a -warm condition would probably so much exceed that indicative of a cold -condition, that we should come to the conclusion that the character -of the climate, as a whole, during the epoch in question was warm and -equable. - -As geologists we have, as a rule, no means of arriving at a knowledge -of the character of the climate of any given period but through an -examination of the sea-bottoms belonging to that period; for these -contain all the evidence upon the subject. But unless we exercise -caution, we shall be very apt, in judging of the climate of such -a period, to fall into the same error that we have just now seen -one might naturally fall into were he called upon to determine the -character of the climate during the glacial epoch from the nature of -the organic remains which lie buried in our adjoining seas. On this -point Mr. J. Geikie’s observations are so appropriate, that I cannot -do better than introduce them here. “When we are dealing,” says this -writer, “with formations so far removed from us in time, and in which -the animal and plant remains depart so widely from existing forms of -life, we can hardly expect to derive much aid from the fossils in our -attempts to detect traces of cold climatic conditions. The arctic -shells in our Post-tertiary clays are convincing proofs of the former -existence in our latitude of a severe climate; but when we go so far -back as Palæozoic ages, we have no such clear evidence to guide us. -All that palæontologists can say regarding the fossils belonging to -these old times is simply this, that they seem to indicate, generally -speaking, mild, temperate, or genial, and even sometimes tropical, -conditions of climate. Many of the fossils, indeed, if we are to reason -from analogy at all, could not possibly have lived in cold seas. But, -for aught that we know, there may have been alternations of climate -during the deposition of each particular formation; and these changes -may be marked by the presence or absence, or by the greater or less -abundant development, of certain organisms at various horizons in -the strata. Notwithstanding all that has been done, our knowledge of -the natural history of these ancient seas is still very imperfect; -and therefore, in the present state of our information, we are not -entitled to argue, from the general aspect of the fossils in our older -formations, that the temperature of the ancient seas was never other -than mild and genial.”[153] - -_Conclusion._—From what has already been stated it will, I trust, be -apparent that, assuming glacial epochs during past geological ages to -have been as numerous and as severe as the Secular theory demands, -still it would be unreasonable to expect to meet with abundant traces -of them. The imperfection of the geological record is such that we -ought not to be astonished that so few relics of former ice ages have -come down to us. It will also be apparent that the palæontological -evidence of a warm condition of climate having obtained during any -particular age, is no proof that a glacial epoch did not also supervene -during the same cycle of time. Indeed it is quite the reverse; for -the warm conditions of which we have proof may indicate merely the -existence of an inter-glacial period. Furthermore, if the Secular -theory of changes of climate be admitted, then evidence of a warm -condition of climate having prevailed in arctic regions during any -past geological age may be regarded as presumptive proof of the -existence of a glacial epoch; that is to say, of an epoch during -which cold and warm conditions of climate alternated. Keeping these -considerations in view, we shall now proceed to examine briefly what -evidence we at present have of the former existence of glacial epochs. - - - - - CHAPTER XVIII. - - FORMER GLACIAL EPOCHS; GEOLOGICAL EVIDENCE OF. - - Cambrian Conglomerate of Islay and North-west of - Scotland.—Ice-action in Ayrshire and Wigtownshire - during Silurian Period.—Silurian Limestones in Arctic - Regions.—Professor Ramsay on Ice-action during Old - Red Sandstone Period.—Warm Climate in Arctic Regions - during Old Red Sandstone Period.—Professor Geikie and - Mr. James Geikie on a Glacial Conglomerate of Lower - Carboniferous Age.—Professor Haughton and Professor Dawson - on Evidence of Ice-action during Coal Period.—Mr. W. T. - Blanford on Glaciation in India during Carboniferous - Period.—Carboniferous Formations of Arctic Regions.—Professor - Ramsay on Permian Glaciers.—Permian Conglomerate in - Arran.—Professor Hull on Boulder Clay of Permian Age.—Permian - Boulder Clay of Natal.—Oolitic Boulder Conglomerate in - Sutherlandshire.—Warm Climate in North Greenland during - Oolitic Period.—Mr. Godwin-Austen on Ice-action during - Cretaceous Period.—Glacial Conglomerates of Eocene Age in the - Alps.—M. Gastaldi on the Ice-transported Limestone Blocks of - the Superga.—Professor Heer on the Climate of North Greenland - during Miocene Period. - - - CAMBRIAN PERIOD. - -_Island of Islay._—Good evidence of ice-action has been observed by -Mr. James Thomson, F.G.S.,[154] in strata which he believes to be of -Cambrian age. At Port Askaig, Island of Islay, below a precipitous -cliff of quartzite 70 feet in height, there is a mass of arenaceous -talcose schist containing fragments of granite, some angular, but -most of them rounded, and of all sizes, from mere particles to -large boulders. As there is no granite in the island from which -these boulders could have been derived, he justly infers that they -must have been transported by the agency of ice. The probability -of his conclusion is strengthened by the almost total absence of -stratification in the deposit in question. - -_North-west of Scotland._—Mr. J. Geikie tells me that much of the -Cambrian conglomerate in the north-west of Scotland strongly reminds -him of the coarse shingle beds (Alpine diluvium) which so often crowd -the old glacial valleys of Switzerland and Northern Italy. In many -places the stones of the Cambrian conglomerate have a subangular, -blunted shape, like those of the re-arranged moraine débris of Alpine -countries. - - - SILURIAN PERIOD. - -_Wigtownshire._—The possibility of glacial action so far back as -the Silurian age has been suggested. In beds of slate and shales in -Wigtownshire of Lower Silurian age Mr. J. Carrick Moore found beds of -conglomerate of a remarkable character. The fragments generally vary -from the size of one inch to a foot in diameter, but in some of the -beds, boulders of 3, 4, and even 5 feet in diameter occur. There are -no rocks in the neighbourhood from which any of these fragments could -have been derived. The matrix of this conglomerate is sometimes a green -trappean-looking sandstone of exceeding toughness, and sometimes an -indurated sandstone indistinguishable from many common varieties of -greywacke.[155] - -_Ayrshire._—Mr. James Geikie states that in Glenapp, and near -Dalmellington, he found embedded in Lower Silurian strata blocks -and boulders from one foot to 5 feet in diameter of gneiss, -syenite, granite, &c., none of which belong to rocks of those -neighbourhoods.[156] Similar cases have been found in Galway, Ireland, -and at Lisbellaw, south of Enniskillen.[157] In America, Professor -Dawson describes Silurian conglomerates with boulders 2 feet in -diameter. - -_Arctic Regions._—The existence of warm inter-glacial periods -during that age may be inferred from the fact that in the arctic -regions we find widespread masses of Silurian limestones containing -encrinites, corals, and mollusca, and other fossil remains, for an -account of which see Professor Haughton’s geological account of the -Arctic Archipelago appended to McClintock’s “Narrative of Arctic -Discoveries.”[158] - - - OLD RED SANDSTONE. - -_North of England._—According to Professor Ramsay and some other -geologists the brecciated, subangular conglomerates and boulder beds -of the Old Red Sandstone of Scotland and the North of England are of -glacial origin. When these conglomerates and the recent boulder clay -come together it is difficult to draw the line of demarcation between -them. - -Professor Ramsay observed some very remarkable facts in connection -with the Old Red Sandstone conglomerates of Kirkby Lonsdale, and -Sedburgh, in Westmoreland and Yorkshire. I shall give the results of -his observations in his own words. - -“The result is, that we have found many stones and blocks distinctly -scratched, and on others the ghosts of scratches nearly obliterated -by age and chemical action, probably aided by pressure at a time when -these rocks were buried under thousands of feet of carboniferous -strata. In some cases, however, the markings were probably produced -within the body of the rock itself by pressure, accompanied by -disturbance of the strata; but in others the longitudinal and cross -striations convey the idea of glacial action. The shapes of the stones -of these conglomerates, many of which are from 2 to 3 feet long, their -flattened sides and subangular edges, together with the confused manner -in which they are often arranged (like stones in the drift), have -long been enough to convince me of their ice-borne character; and the -scratched specimens, when properly investigated, may possibly convince -others.”[159] - -_Isle of Man._—The conglomerate of the Old Red Sandstone in the Isle of -Man has been compared by Mr. Cumming to “a consolidated ancient boulder -clay.” And he remarks, “Was it so that those strange trilobitic-looking -fishes of that era had to endure the buffeting of ice-waves, and to -struggle amidst the wreck of ice-floes and the crush of bergs?”[160] - -_Australia._—A conglomerate similar to that of Scotland has been found -in Victoria, Australia, by Mr. Selwyn, at several localities. Along -the Wild Duck Creek, near Heathcote, and also near the Mia-Mia, Spring -Plains, Redesdale, localities in the Colony of Victoria, where it was -examined by Messrs. Taylor and Etheridge, Junior, this conglomerate -consists of a mixture of granite pebbles and boulders of various -colours and textures, porphyries, indurated sandstone, quartz, and -a peculiar flint-coloured rock in a matrix of bluish-grey very hard -mud-cement.[161] Rocks similar to the pebbles and blocks composing the -conglomerate do not occur in the immediate neighbourhood; and from the -curious mixture of large and small angular and water-worn fragments -it was conjectured that it might possibly be of glacial origin. -Scratched stones were not observed, although a careful examination was -made. From similar mud-pebble beds on the Lerderderg River, Victoria, -Mr. P. Daintree obtained a few pebbles grooved after the manner of -ice-scratched blocks.[162] - -And the existence of a warm condition of climate during the Old Red -Sandstone period is evidenced by the fossiliferous limestones of -England, Russia, and America. On the banks of the Athabasca River, -Rupert-Land, Sir John Richardson found beds of limestone containing -_Producti_, _Spiriferi_, an _Orthis_ resembling _O. resupinata_, -_Terebratula reticularis_,[163] and a _Pleurotomaria_, which, in the -opinion of the late Dr. Woodward, who examined the specimens, are -characteristic of Devonian rocks of Devonshire. - - - CARBONIFEROUS PERIOD. - -_France._—It is now a good many years since Mr. Godwin-Austen directed -attention to what he considered evidence of ice-action during the coal -period. This geologist found in the carboniferous strata of France -large angular blocks which he could not account for without inferring -the former action of ice. “Whether from local elevation,” he says, -“or from climatic conditions, there are certain appearances over the -whole which imply that at one time the temperature must have been very -low, as glacier-action can alone account for the presence of the large -angular blocks which occur in the lowest detrital beds of many of the -southern coal-basins.”[164] - -_Scotland._—In Scotland great beds of conglomerate are met with in -various parts, which are now considered by Professor Geikie, Mr. -James Geikie, and other officers of the Geological Survey who have -had opportunities of examining them, to be of glacial origin. “They -are,” says Mr. James Geikie, “quite unstratified, and the stones often -show that peculiar blunted form which is so characteristic of glacial -work.”[165] Many of the stones found by Professor Geikie, several of -which I have had an opportunity of seeing, are well striated. - -In 1851 Professor Haughton brought forward at the Geological Society -of Dublin, a case of angular fragments of granite occurring in the -carboniferous limestone of the county of Dublin; and he explained the -phenomena by the supposition of the transporting power of ice. - -_North America._—In one of the North American coal-fields Professor -Newberry found a boulder of quartzite 17 inches by 12 inches, imbedded -in a seam of coal. Similar facts have also been recorded both in the -United States, and in Nova Scotia. Professor Dawson describes what he -calls a gigantic esker of Carboniferous age, on the outside of which -large travelled boulders were deposited, probably by drift-ice; while -in the swamps within, the coal flora flourished.[166] - -_India._—Mr. W. T. Blanford, of the Geological Survey of India, states -that in beds considered to be of Carboniferous age are found large -boulders, some of them as much as 15 feet in diameter. The bed in -which these occur is a fine silt, and he refers the deposition of the -boulders to ice-action. Within the last three years his views have -received singular confirmation in another part of India, where beds -of limestone were found striated below certain overlying strata. The -probability that these appearances are due, as Mr. Blanford says, to -the action of ice, is strengthened by the consideration that about five -degrees farther to the north of the district in question rises the -cold and high table-land of Thibet, which during a glacial epoch would -undoubtedly be covered with ice that might well descend over the plains -of India.[167] - -_Arctic Regions._—A glacial epoch during the Carboniferous age may be -indirectly inferred from the probable existence of warm inter-glacial -periods, as indicated by the limestones with fossil remains found in -arctic regions. - -That an equable condition of climate extended to near the north pole -is proved by the fact that in the arctic regions vast masses of -carboniferous limestone, having all the characters of the mountain -limestone of England, have been found. “These limestones,” says -Mr. Isbister, “are most extensively developed in the north-east -extremity of the continent, where they occupy the greater part of -the coast-line, from the north side of the Kotzebue Sound to within -a few miles of Point Barrow, and form the chief constituent of the -lofty and conspicuous headlands of Cape Thomson, Cape Lisburn, and -Cape Sabine.”[168] Limestone of the same age occurs extensively -along the Mackenzie River. The following fossils have been found -in these limestones:—_Terebratula resupinata_,[169] _Lithostrotion -basaltiforme_, _Cyathophyllum dianthum_, _C. flexuosum_, _Turbinolia -mitrata_, _Productus Martini_,[170] _Dentalium Sarcinula_, -_Spiriferi_, _Orthidæ_, and encrinital fragments in the greatest -abundance. - -Among the fossils brought home from Depôt Point, Albert Land, by -Sir E. Belcher, Mr. Salter found the following, belonging to the -Carboniferous period:—_Fusulina hyperborea_, _Stylastrea inconferta_, -_Zaphrentis ovibos_, _Clisiophyllum tumulus_, _Syringopora (Aulopora)_, -_Fenestella Arctica_, _Spirifera Keilhavii_, _Productus cora_, _P. -semireticulatus_.[171] - -Coal-beds of Carboniferous age are extensively developed in arctic -regions. The fuel is of a highly bituminous character, resembling, says -Professor Haughton, the gas coals of Scotland. The occurrence of coal -in such high latitudes indicates beyond doubt that a mild and temperate -condition of climate must, during some part of the Carboniferous age, -have prevailed up to the very pole. - -“In the coal of Jameson’s Land, on the east side of Greenland, lying -in latitude 71°, and in that of Melville Island, in latitude 75° N., -Professor Jameson found plants resembling fossils of the coal-fields of -Britain.”[172] - - - PERMIAN PERIOD. - -_England._—From the researches of Professor Ramsay in the Permian -breccias, we have every reason to believe that during a part of the -Permian age our country was probably covered with glaciers reaching -to the sea. These brecciated stones, he states, are mostly angular -or subangular, with flattened sides and but very slightly rounded at -the edges, and are imbedded in a deep red marly paste. At Abberley -Hill some of the masses are from 2 to 3 feet in diameter, and in one -of the quarries, near the base of Woodbury Hill, Professor Ramsay -saw one 2 feet in diameter. Another was observed at Woodbury Rock, 4 -feet long, 3 feet broad, and 1½ feet thick. The boulders were found -in South Staffordshire, Enville, in Abberley and Malvern Hills, -and other places. “They seem,” he says, “to have been derived from -the conglomerate and green, grey, and purple Cambrian grits of the -Longmynd, and from the Silurian quartz-rocks, slates, felstones, -felspathic ashes, greenstones, and Upper Caradoc rocks of the country -between the Longmynd and Chirbury. But then,” he continues, “the south -end of the Malvern Hills is from forty to fifty miles, the Abberleys -from twenty-five to thirty-five miles, Enville from twenty to thirty -miles, and South Staffordshire from thirty-five to forty miles distant -from that country.”[173] - -It is physically impossible, Professor Ramsay remarks, that these -blocks could have been transported to such distances by any other -agency than that of ice. Had they been transported by water, supposing -such a thing possible, they would have been rounded and water-worn, -whereas many of these stones are flat slabs, and most of them have -their edges but little rounded. And besides many of them are highly -polished, and others grooved and finely striated, exactly like those of -the ancient glaciers of Scotland and Wales. Some of these specimens are -to be seen in the Museum of Practical Geology, Jermyn Street. - -_Scotland._—In the Island of Arran, Mr. E. A. Wunsch and Mr. James -Thomson found a bed of conglomerate which they considered of Permian -age, and probably of glacial origin. This conglomerate enclosed angular -fragments of various schistose, volcanic, and limestone rocks, and -contained carboniferous fossils. - -_Ireland._—At Armagh, Ireland, Professor Hull found boulder beds of -Permian age, containing pebbles and boulders, sometimes 2 feet in -diameter. Some of the boulders must have been transported from a -region lying about 30 miles to the north-west of the locality in which -they now occur. It is difficult to conceive, says Professor Hull, -how rock fragments of such a size could have been carried to their -present position by any other agency than that of floating ice. This -boulder-bed is overlaid by a recent bed of boulder clay. Professor -Ramsay, who also examined the section, agrees with Professor Hull that -the bed is of Permian age, and unquestionably of ice-formation.[174] - -Professor Ramsay feels convinced that the same conclusions which he has -drawn in regard to the Permian breccia of England will probably yet be -found to hold good in regard to much of that of North Germany.[175] And -there appears to be some ground for concluding that the cold of that -period even reached to India.[176] - -_South Africa._—An ancient boulder clay, supposed to be either -of Permian or Jurassic age, has been extensively found in Natal, -South Africa. This deposit, discovered by Dr. Sutherland, the -Surveyor-General of the colony, is thus described by Dr. Mann:— - -“The deposit itself consists of a greyish-blue argillaceous matrix, -containing fragments of granite, gneiss, graphite, quartzite, -greenstone, and clay-slate. These imbedded fragments are of various -size, from the minute dimensions of sand-grains up to vast blocks -measuring 6 feet across, and weighing from 5 to 10 tons. They are -smoothed, as if they had been subject to a certain amount of attrition -in a muddy sediment; but they are not rounded like boulders that -have been subjected to sea-breakers. The fracture of the rock is not -conchoidal, and there is manifest, in its substance, a rude disposition -towards wavy stratification.” - -“Dr. Sutherland inclines to think that the transport of vast massive -blocks of several tons’ weight, the scoring of the subjacent surfaces -of sandstone, and the simultaneous deposition of minute sand-grains -and large boulders in the same matrix, all point to one agency as the -only one which can be rationally admitted to account satisfactorily -for the presence of this remarkable formation in the situations in -which it is found. He believes that the boulder-bearing clay of Natal -is of analogous nature to the great Scandinavian drift, to which it -is certainly intimately allied in intrinsic mineralogical character; -that it is virtually a vast moraine of olden time; and that ice, in -some form or other, has had to do with its formation, at least so far -as the deposition of the imbedded fragments in the amorphous matrix are -concerned.”[177] - -In the discussion which followed the reading of Dr. Sutherland’s paper, -Professor Ramsay pointed out that in the Natal beds enormous blocks of -rock occurred, which were 60 or 80 miles from their original home, and -still remained angular; and there was a difficulty in accounting for -the phenomena on any other hypothesis than that suggested. - -Mr. Stow, in his paper on the Karoo beds, has expressed a similar -opinion regarding the glacial character of the formation.[178] - -But we have in the Karoo beds evidence not only of glaciation, but of a -much warmer condition of things than presently exists in that latitude. -This is shown from the fact that the shells of the _Trigona_-beds -indicate a tropical or subtropical condition of climate. - -_Arctic Regions._—The evidence which we have of the existence of a -warm climate during the Permian period is equally conclusive. The -close resemblance of the _flora_ of the Permian period to that of -Carboniferous times evidently points to the former prevalence of a warm -and equable climate. And the existence of the magnesian limestone in -high latitudes seems to indicate that during at least a part of the -Permian period, just as during the accumulation of the carboniferous -limestone, a warm sea must have obtained in those latitudes. - - - OOLITIC PERIOD. - -_North of Scotland._—There is not wanting evidence of something like -the action of ice during the Oolitic period.[179] - -In the North of Scotland Mr. James Geikie says there is a coarse -boulder conglomerate associated with the Jurassic strata in the east -of Sutherland, the possibly glacial origin of which long ago suggested -itself to Professor Ramsay and other observers. Mr. Judd believes the -boulders to have been floated down by ice from the Highland mountains -at the time the Jurassic strata were being accumulated. - -_North Greenland._—During the Oolitic period a warm condition of -climate extended to North Greenland. For example, in Prince Patrick’s -Island, at Wilkie Point, in lat. 76° 20′ N., and long. 117° 20′ -W., Oolitic rocks containing an ammonite (_Ammonites McClintocki_, -Haughton), like _A. concavus_ and other shells of Oolitic species, -were found by Captain McClintock.[180] In Katmai Bay, near Behring’s -Straits, the following Oolitic fossils were discovered—_Ammonites -Wasnessenskii_, _A. biplex_, _Belemnites paxillosus_, and _Unio -liassinus_.[181] Captain McClintock found at Point Wilkie, in Prince -Patrick’s Island, lat. 76° 20′, a bone of _Ichthyosaurus_, and Sir E. -Belcher found in Exmouth Island, lat. 76° 16′ N., and long. 96° W., at -an elevation of 570 feet above the level of the sea, bones which were -examined by Professor Owen, and pronounced to be those of the same -animal.[182] Mr. Salter remarks that at the time that these fossils -were deposited, “a condition of climate something like that of our own -shores was prevailing in latitudes not far short of 80° N.”[183] And -Mr. Jukes says that during the Oolitic period, “in latitudes where -now sea and land are bound in ice and snow throughout the year, there -formerly flourished animals and plants similar to those living in our -own province at that time. The questions thus raised,” continues Mr. -Jukes, “as to the climate of the globe when cephalopods and reptiles -such as we should expect to find only in warm or temperate seas, -could live in such high latitudes, are not easy to answer.”[184] And -Professor Haughton remarks, that he thinks it highly improbable that -any change in the position of land and water could ever have produced a -temperature in the sea at 76° north latitude which would allow of the -existence of ammonites, especially species so like those that lived -at the same time in the tropical warm seas of the South of England -and France at the close of the Liassic, and commencement of the Lower -Oolitic period.[185] - -The great abundance of the limestone and coal of the Oolitic system -shows also the warm and equable condition of the climate which must -have then prevailed. - - - CRETACEOUS PERIOD. - -_Croydon._—A large block of crystalline rock resembling granite was -found imbedded in a pit, on the side of the old London and Brighton -road near Purley, about two miles south of Croydon. Mr. Godwin-Austen -has shown conclusively that it must have been transported there by -means of floating ice. This boulder was associated with loose sea-sand, -coarse shingle, and a smaller boulder weighing twenty or twenty-five -pounds, and all water-worn. These had all sunk together without -separating. Hence they must have been firmly held together, both during -the time that they were being floated away, and also whilst sinking to -the bottom of the cretaceous sea. Mr. Godwin-Austen supposes the whole -to have been carried away frozen to the bottom of a mass of ground-ice. -When the ice from melting became unable to float the mass attached to -it, the whole would then sink to the bottom together.[186] - -_Dover._—While the workmen were employed in cutting the tunnel on -the London, Chatham, and Dover Railway, between Lydden Hill and -Shepherdswell, a few miles from Dover, they came upon a mass of coal -imbedded in chalk, at a depth of 180 feet. It was about 4 feet square, -and from 4 to 10 inches thick. The coal was friable and highly -bituminous. It resembled some of the Wealden or Jurassic coal, and -was unlike the true coal of the coal-measures. The specific gravity -of the coal precluded the supposition that it could have floated away -of itself into the cretaceous sea. “Considering its friability,” says -Mr. Godwin-Austen, “I do not think that the agency of a floating tree -could have been engaged in its transport; but, looking at its flat, -angular form, it seems to me that its history may agree with what I -have already suggested with reference to the boulder in the chalk -at Croydon. We may suppose that during the Cretaceous period some -bituminous beds of the preceding Oolitic period lay so as to be covered -with water near the sea-margin, or along some river-bank, and from -which portions could be carried off by ice, and so drifted away, until -the ice was no longer able to support its load.”[187] - -Mr. Godwin-Austen then mentions a number of other cases of blocks -being found in the chalk. In regard to those cases he appropriately -remarks that, as the cases where the occurrence of such blocks has -been observed are likely to be far less numerous than those which have -escaped observation, or failed to have been recorded, and as the chalk -exposed in pits and quarries bears only a most trifling proportion to -the whole horizontal extent of the formation, we have no grounds to -conclude that the above are exceptional cases. - -Boulders have also been found in the cretaceous strata of the Alps by -Escher von der Linth.[188] - -The existence of warm periods during the Cretaceous age is plainly -shown by the character of the flora and fauna of that age. The fact -that chalk is of organic origin implies that the climate must have -been warm and genial, and otherwise favourable to animal life. This is -further manifested by such plants as _Cycas_ and _Zamia_, which betoken -a warm climate, and by the corals and huge sauroid reptiles which then -inhabited our waters. - -It is, in fact, the tropical character of the fauna of that period -which induced Sir Charles Lyell to reject Mr. Godwin-Austen’s idea that -the boulders found in the chalk had been transported by floating ice. -Such a supposition, implying a cold climate, “is,” Sir Charles says, -“inconsistent with the luxuriant growth of large chambered univalves, -numerous corals, and many fish, and other fossils of tropical forms.” - -The recent discovery of the Cretaceous formation in Greenland shows -that during that period a mild and temperate condition of climate -must have prevailed in that continent up to high latitudes. “This -formation in Greenland,” says Dr. Robert Brown, “has only been recently -separated from the Miocene formation, with which it is associated and -was supposed to be a part of. It is, as far as we yet know, only found -in the vicinity of Kome or Koke, near the shores of Omenak Fjord, in -about 70° north latitude, though traces have been found elsewhere -on Disco, &c. The fossils hitherto brought to Europe have been very -few, and consist of plants which are now preserved in the Stockholm -and Copenhagen Museums. From these there seems little doubt that the -age assigned to this limited deposit (so far as we yet know) by the -celebrated palæontologist, Professor Oswald Heer, of Zurich, is the -correct one.”[189] Dr. Brown gives a list of the Cretaceous flora found -in Greenland. - - - EOCENE PERIOD. - -_Switzerland._—In a coarse conglomerate belonging to the “_flysch_” -of Switzerland, an Eocene formation, there are found certain immense -blocks, some of which consist of a variety of granite which is not -known to occur _in situ_ in any part of the Alps. Some of the blocks -are 10 feet and upwards in length, and one at Halekeren, at the Lake of -Thun, is 105 feet in length, 90 feet in breadth, and 45 feet in height. -Similar blocks are found in the Apennines. These unmistakably indicate -the presence of glaciers or floating ice. This conclusion is further -borne out by the fact that the “_flysch_” is destitute of organic -remains. But the hypothesis that these huge masses were transported -to their present sites by glaciers or floating ice has been always -objected to, says Sir Charles Lyell, “on the ground that the Eocene -strata of Nummulitic age in Switzerland, as well as in other parts of -Europe, contain genera of fossil plants and animals characteristic of a -warm climate. And it has been particularly remarked,” he continues, “by -M. Desor that the strata most nearly associated with the ‘_flysch_’ in -the Alps are rich in echinoderms of the _Spatangus_ family which have a -decided tropical aspect.”[190] - -But according to the theory of Secular Changes of Climate, the very -fact that the “_flysch_” is immediately associated with beds indicating -a warm or even tropical condition of climate, is one of the strongest -proofs which could be adduced in favour of its glacial character, for -the more severe a cold period of a glacial epoch is, the warmer will be -the periods which immediately precede and succeed. These crocodiles, -tortoises, and tropical flora probably belong to a warm Eocene -inter-glacial period. - - - MIOCENE PERIOD. - -_Italy._—We have strong evidence in favour of the opinion that a -glacial epoch existed during the Miocene period. It has been shown -by M. Gastaldi, that during that age Alpine glaciers extended to the -sea-level. - -Near Turin there is a series of hills, rising about 500 or 600 feet -above the valleys, composed of beds of Miocene sandstone, marl, and -gravel, and loose conglomerate. These beds have been carefully examined -and described by M. Gastaldi.[191] The hill of the Luperga has been -particularly noticed by him. Many of the stones in these beds are -striated in a manner similar to those found in the true till or boulder -clay of this country. But what is most remarkable is the fact that -large erratic blocks of limestone, many of them from 10 to 15 feet in -diameter, are found in abundance in these beds. It has been shown by -Gastaldi that these blocks have all been derived from the outer ridge -of the Alps on the Italian side, namely, from the range extending from -Ivrea to the Lago Maggiore, and consequently they must have travelled -from twenty to eighty miles. So abundant are these large blocks, that -extensive quarries have been opened in the hills for the sake of -procuring them. These facts prove not only the existence of glaciers -on the Alps during the Miocene period, but of glaciers extending to -the sea and breaking up into icebergs; the stratification of the beds -amongst which the blocks occur sufficiently indicating aqueous action -and the former presence of the sea. - -That the glaciers of the Southern Alps actually reached to the sea, -and sent their icebergs adrift over what are now the sunny plains of -Northern Italy, is sufficient proof that during the cold period of -Miocene times the climate must have been very severe. Indeed, it may -well have been as severe as, if not even more excessive than, the -intensest severity of climate experienced during the last great glacial -epoch. - -_Greenland._—Of the existence of warm conditions during Miocene times, -geology affords us abundant evidence. I shall quote the opinion of Sir -Charles Lyell on this point:— - -“We know,” says Sir Charles, “that Greenland was not always covered -with snow and ice; for when we examine the tertiary strata of Disco -Island (of the Upper Miocene period), we discover there a multitude -of fossil plants which demonstrate that, like many other parts of the -arctic regions, it formerly enjoyed a mild and genial climate. Among -the fossils brought from that island, lat. 70° N., Professor Heer has -recognised _Sequoia Landsdorfii_, a coniferous species which flourished -throughout a great part of Europe in the Miocene period. The same -plant has been found fossil by Sir John Richardson within the Arctic -Circle, far to the west on the Mackenzie River, near the entrance of -Bear River; also by some Danish naturalists in Iceland, to the east. -The Icelandic surturband or lignite, of this age, has also yielded a -rich harvest of plants, more than thirty-one of them, according to -Steenstrup and Heer, in a good state of preservation, and no less than -fifteen specifically identical with Miocene plants of Europe. Thirteen -of the number are arborescent; and amongst others is a tulip-tree -(_Liriodendron_), with its fruit and characteristic leaves, a plane -(_Platanus_), a walnut, and a vine, affording unmistakable evidence -of a climate in the parallel of the Arctic Circle which precludes the -supposition of glaciers then existing in the neighbourhood, still less -any general crust of continental ice like that of Greenland.”[192] - -At a meeting of the British Association, held at Nottingham in August -1866, Professor Heer read a valuable paper on the “Miocene Flora of -North Greenland.” In this paper some remarkable conclusions as to the -probable temperature of Greenland during the Miocene period were given. - -Upwards of sixty different species brought from Atanekerdluk, a place -on the Waigat opposite Disco, in lat. 70° N., have been examined by him. - -A steep hill rises on the coast to a height of 1,080 feet, and at -this level the fossil plants are found. Large quantities of wood in -a fossilized or carbonized condition lie about. Captain Inglefield -observed one trunk thicker than a man’s body standing upright. The -leaves, however, are the most important portion of the deposit. The -rock in which they are found is a sparry iron ore, which turns reddish -brown on exposure to the weather. In this rock the leaves are found, in -places packed closely together, and many of them are in a very perfect -condition. They give us a most valuable insight into the nature of the -vegetation which formed this primeval forest. - -He arrives at the following conclusions:— - -1. _The fossilized plants of Atanekerdluk cannot have been drifted from -any great distance. They must have grown on the spot where they were -found._ - -This is shown— - -(_a_) By the fact that Captain Inglefield and Dr. Ruik observed trunks -of trees standing upright. - -(_b_) By the great abundance of the leaves, and the perfect state of -preservation in which they are found. - -(_c_) By the fact that we find in the stone both fruits and seeds of -the trees whose leaves are also found there. - -(_d_) By the occurrence of insect remains along with the leaves. - -2. _The flora of Atanekerdluk is Miocene._ - -3. _The flora is rich in species._ - -4. _The flora proves without a doubt that North Greenland, in the -Miocene epoch, had a climate much warmer than its present one. The -difference must be at least_ 29° F. - -Professor Heer discusses at considerable length this proposition. He -says that the evidence from Greenland gives a final answer to those -who objected to the conclusions as to the Miocene climate of Europe -drawn by him on a former occasion. It is quite impossible that the -trees found at Atanekerdluk could ever have flourished there if -the temperature were not far higher than it is at present. This is -clear from many of the species, of which we find the nearest living -representative 10° or even 20° of latitude to the south of the locality -in question. - -The trees of Atanekerdluk were not, he says, all at the extreme -northern limit of their range, for in the Miocene flora of Spitzbergen, -lat. 78° N., we find the beech, plane, hazelnut, and some other species -identical with those from Greenland, and we may conclude, he thinks, -that the firs and poplars which we meet at Atanekerdluk and Bell Sound, -Spitzbergen, must have reached up to the North Pole if land existed -there in the tertiary period. - -“The hills of fossilized wood,” he adds, “found by McClure and his -companions in Banks’s Land (lat. 74° 27′ N.), are therefore discoveries -which should not astonish us, they only confirm the evidence as to the -original vegetation of the polar regions which we have derived from -other sources.” - -The _Sequoia landsdorfii_ is the most abundant of the trees of -Atanekerdluk. The _Sequoia sempervirens_ is its present representative. -This tree has its extreme northern limit about lat. 53° N. For its -existence it requires a summer temperature of 59° or 61° F. Its fruit -requires a temperature of 64° for ripening. The winter temperature must -not fall below 34°, and that of the whole year must be at least 49°. -The temperature of Atanekerdluk during the time that the Miocene flora -grew could not have been under the above.[193] - -Professor Heer concludes his paper as follows:— - -“I think these facts are convincing, and the more so that they are not -insulated, but confirmed by the evidence derivable from the Miocene -flora of Iceland, Spitzbergen, and Northern Canada. These conclusions, -too, are only links in the grand chain of evidence obtained from the -examination of the Miocene flora of the whole of Europe. They prove to -us that we could not by any re-arrangement of the relative positions -of land and water produce for the northern hemisphere a climate which -would explain the phenomena in a satisfactory manner. We must only -admit that we are face to face with a problem, whose solution in all -probability must be attempted, and, we doubt not, completed by the -astronomer.” - - - - - CHAPTER XIX. - - GEOLOGICAL TIME.—PROBABLE DATE OF THE GLACIAL EPOCH. - - Geological Time measurable from Astronomical Data.—M. Leverrier’s - Formulæ.—Tables of Eccentricity for 3,000,000 Years in the - Past and 1,000,000 Years in the Future.—How the Tables have - been computed.—Why the Glacial Epoch is more recent than had - been supposed.—Figures convey a very inadequate Conception - of immense Duration.—Mode of representing a Million of - Years.—Probable Date of the Glacial Epoch. - - -If those great Secular variations of climate which we have been -considering be indirectly the result of changes in the eccentricity -of the earth’s orbit, then we have a means of determining, at least -so far as regards recent epochs, when these variations took place. -If the glacial epoch be due to the causes assigned, we have a means -of ascertaining, with tolerable accuracy, not merely the date of its -commencement, but the length of its duration. M. Leverrier has not -only determined the superior limit of the eccentricity of the earth’s -orbit, but has also given formulæ by means of which the extent of the -eccentricity for any period, past or future, may be computed. - -A well-known astronomer and mathematician, who has specially -investigated the subject, is of opinion that these formulæ give results -which may be depended upon as approximately correct for _four millions -of years_ past and future. An eminent physicist has, however, expressed -to me his doubts as to whether the results can be depended on for a -period so enormous. M. Leverrier in his Memoir has given a table of the -eccentricity for 100,000 years before and after 1800 A.D., computed -for intervals of 10,000 years. This table, no doubt, embraces a period -sufficiently great for ordinary astronomical purposes, but it is by far -too limited to afford information in regard to geological epochs. - -With the view of ascertaining the probable date of the glacial epoch, -as well as the character of the climate for a long course of ages, -Table I. was computed from M. Leverrier’s formulæ.[194] It shows the -eccentricity of the earth’s orbit and longitude of the perihelion for -3,000,000 of years back, and 1,000,000 of years to come, at periods -50,000 years apart. - -On looking over the table it will be seen that there are three -principal periods when the eccentricity rose to a very high value, -with a few subordinate maxima between. It will be perceived also that -during each of those periods the eccentricity does not remain at the -same uniform value, but rises and falls, in one case twice, and in the -other two cases three times. About 2,650,000 years back we have the -eccentricity almost at its inferior limit. It then begins to increase, -and fifty thousand years afterwards, namely at 2,600,000 years ago, it -reaches ·0660; fifty thousand years after this period it has diminished -to ·0167, which is about its present value. It then begins to increase, -and in another fifty thousand years, namely at 2,500,000 years ago, it -approaches to almost the superior limit, its value being then ·0721. It -then begins to diminish, and at 2,450,000 years ago it has diminished -to ·0252. These two maxima, separated by a minimum and extending over a -period of 200,000 years, constitute the first great period of high -eccentricity. We then pass onwards for upwards of a million and a half -years, and we come to the second great period. It consists of three -maxima separated by two minima. The first maximum occurred at 950,000 -years ago, the second or middle one at 850,000 years ago, and the -third and last at 750,000 years ago—the whole extending over a period -of nearly 300,000 years. Passing onwards for another million and half -years, or to about 800,000 years in the future, we come to the third -great period. It also consists of three maxima one hundred thousand -years apart. Those occur at the periods 800,000, 900,000, and 1,000,000 -years to come, respectively, separated also by two minima. Those three -great periods, two of them in the past and one of them in the future, -included in the Table, are therefore separated from each other by an -interval of upwards of 1,700,000 years. - - [Illustration: PLATE IV - - W. & A. K. Johnston, Edinb^r. and London. - - DIAGRAM REPRESENTING THE VARIATIONS IN THE ECCENTRICITY OF THE - EARTH’S ORBIT FOR THREE MILLION OF YEARS BEFORE 1800 A.D. ONE - MILLION OF YEARS AFTER IT. - - _The Ordinates are joined by straight lines where the values, at - intervals of 10,000 years, between them have not been determined._] - -In this Table there are seven periods when the earth’s orbit becomes -nearly circular, four in the past and three in the future. - -The Table shows also four or five subordinate periods of high -eccentricity, the principal one occurring 200,000 years ago. - -The variations of eccentricity during the four millions of years, are -represented to the eye diagrammatically in Plate IV. - -In order to determine with more accuracy the condition of the earth’s -orbit during the three periods of great eccentricity included in Table -I., I computed the values for periods of ten thousand years apart, and -the results are embodied in Tables II., III., and IV. - -There are still eminent astronomers and physicists who are of opinion -that the climate of the globe never could have been seriously affected -by changes in the eccentricity of its orbit. This opinion results, no -doubt, from viewing the question as a purely astronomical one. Viewed -from an astronomical standpoint, as has been already remarked, there -is actually nothing from which any one could reasonably conclude with -certainty whether a change of eccentricity would seriously affect -climate or not. By means of astronomy we ascertain the extent of the -eccentricity at any given period, how much the winter may exceed -the summer in length (or the reverse), how much the sun’s heat is -increased or decreased by a decrease or an increase of distance, -and so forth; but we obtain no information whatever regarding how -these will actually affect climate. This, as we have already seen, -must be determined wholly from physical considerations, and it is -an exceedingly complicated problem. An astronomer, unless he has -given special attention to the physics of the question, is just as -apt to come to a wrong conclusion as any one else. The question -involves certain astronomical elements; but when these are determined -everything then connected with the matter is purely physical. Nearly -all the astronomical elements of the question are comprehended in the -accompanying Tables. - - TABLE I. - - THE ECCENTRICITY AND LONGITUDE OF THE PERIHELION OF THE EARTH’S - ORBIT FOR 3,000,000 YEARS IN THE PAST AND 1,000,000 YEARS IN - THE FUTURE, COMPUTED FOR INTERVALS OF 50,000 YEARS. - - +---------------------------------------------+ - | PAST TIME. | - +------------------+-------------+------------+ - | Number of years |Eccentricity.|Longitude of| - |before epoch 1800.| |perihelion. | - +------------------+-------------+------------+ - | | | ° ′ | - | −3,000,000 | 0·0365 | 39 30 | - | −2,950,000 | 0·0170 | 210 39 | - | −2,900,000 | 0·0442 | 200 52 | - | −2,850,000 | 0·0416 | 0 18 | - | −2,800,000 | 0·0352 | 339 14 | - | −2,750,000 | 0·0326 | 161 22 | - | −2,700,000 | 0·0330 | 65 37 | - | −2,650,000 | 0·0053 | 318 40 | - | −2,600,000 | 0·0660 | 190 4 | - | −2,550,000 | 0·0167 | 298 34 | - | −2,500,000 | 0·0721 | 338 36 | - | −2,450,000 | 0·0252 | 109 33 | - | −2,400,000 | 0·0415 | 116 40 | - | −2,350,000 | 0·0281 | 308 23 | - | −2,300,000 | 0·0238 | 195 25 | - | −2,250,000 | 0·0328 | 141 18 | - | −2,200,000 | 0·0352 | 307 6 | - | −2,150,000 | 0·0183 | 307 5 | - | −2,100,000 | 0·0304 | 98 40 | - | −2,050,000 | 0·0170 | 334 46 | - | −2,000,000 | 0·0138 | 324 4 | - | −1,950,000 | 0·0427 | 120 32 | - | −1,900,000 | 0·0336 | 188 31 | - | −1,850,000 | 0·0503 | 272 14 | - | −1,800,000 | 0·0334 | 354 52 | - | −1,750,000 | 0·0350 | 65 25 | - | −1,700,000 | 0·0085 | 95 13 | - | −1,650,000 | 0·0035 | 168 23 | - | −1,600,000 | 0·0305 | 158 42 | - | −1,550,000 | 0·0239 | 225 57 | - | −1,500,000 | 0·0430 | 303 29 | - | −1,450,000 | 0·0195 | 57 11 | - | −1,400,000 | 0·0315 | 97 35 | - | −1,350,000 | 0·0322 | 293 38 | - | −1,300,000 | 0·0022 | 0 48 | - | −1,250,000 | 0·0475 | 105 50 | - | −1,200,000 | 0·0289 | 239 34 | - | −1,150,000 | 0·0473 | 250 27 | - | −1,100,000 | 0·0311 | 55 24 | - | −1,050,000 | 0·0326 | 4 8 | - | −1,000,000 | 0·0151 | 248 22 | - | −950,000 | 0·0517 | 97 51 | - | −900,000 | 0·0102 | 135 2 | - | −850,000 | 0·0747 | 239 28 | - | −800,000 | 0·0132 | 343 49 | - | −750,000 | 0·0575 | 27 18 | - | −700,000 | 0·0220 | 208 13 | - | −650,000 | 0·0226 | 141 29 | - | −600,000 | 0·0417 | 32 34 | - | −550,000 | 0·0166 | 251 50 | - | −500,000 | 0·0388 | 193 56 | - | −450,000 | 0·0308 | 356 52 | - | −400,000 | 0·0170 | 290 7 | - | −350,000 | 0·0195 | 182 50 | - | −300,000 | 0·0424 | 23 29 | - | −250,000 | 0·0258 | 59 39 | - | −200,000 | 0·0569 | 168 18 | - | −150,000 | 0·0332 | 242 56 | - | −100,000 | 0·0473 | 316 18 | - | −50,000 | 0·0131 | 50 14 | - +------------------+-------------+------------+ - - +---------------------------------------------+ - | FUTURE TIME. | - +------------------+-------------+------------+ - | Number of years |Eccentricity.|Longitude of| - |before epoch 1800.| |perihelion. | - +------------------+-------------+------------+ - | | | ° ′ | - | A.D. 1800 | 0·0168 | 99 30 | - | +50,000 | 0·0173 | 38 12 | - | +100,000 | 0·0191 | 114 50 | - | +150,000 | 0·0353 | 201 57 | - | +200,000 | 0·0246 | 279 41 | - | +250,000 | 0·0286 | 350 54 | - | +300,000 | 0·0158 | 172 29 | - | +350,000 | 0·0098 | 201 40 | - | +400,000 | 0·0429 | 6 9 | - | +450,000 | 0·0231 | 98 37 | - | +500,000 | 0·0534 | 157 26 | - | +550,000 | 0·0259 | 287 31 | - | +600,000 | 0·0395 | 285 43 | - | +650,000 | 0·0169 | 144 3 | - | +700,000 | 0·0357 | 17 12 | - | +750,000 | 0·0195 | 0 53 | - | +800,000 | 0·0639 | 140 38 | - | +850,000 | 0·0144 | 176 41 | - | +900,000 | 0·0659 | 291 16 | - | +950,000 | 0·0086 | 115 13 | - | +1,000,000 | 0·0528 | 57 31 | - +------------------+-------------+------------+ - - TABLE II. - - ECCENTRICITY, LONGITUDE OF THE PERIHELION, &C., &C., FOR INTERVALS OF - 10,000 YEARS, FROM 2,650,000 TO 2,450,000 YEARS AGO. - - THE GLACIAL EPOCH OF THE _Eocene period_ IS PROBABLY COMPREHENDED - WITHIN THIS TABLE. - - +---------+------------+-----------+-----------+-------------------------------------------+ - | I. | II. | III. | IV. | Winter occurring in aphelion. | - | | | | +---------+---------+-----------+-----------+ - | | | | | V. | VI. | VII. | VIII. | - |Number of|Eccentricity|Longitude | Number of |Excess of|Midwinter|Number of | Midwinter | - | years | of orbit. | of | degrees | winter |intensity|degrees by |temperature| - | before | |perihelion.| passed | over | of the | which the | of Great | - | A.D. | | |over by the| summer, | sun’s | midwinter | Britain. | - | 1800. | | |perihelion.|in days. | heat. |temperature| | - | | | | Motion | | Present |is lowered | | - | | | |retrograde | |intensity| | | - | | | |at periods | | =1000. | | | - | | | | marked R. | | | | | - +---------+------------+-----------+-----------+---------+---------+-----------+-----------+ - | | | ° | | | | | | - |2,650,000| 0·0053 | 318 40 | ° ′ | | | F. | F. | - |2,640,000| 0·0173 | 54 25 | 95 45 | | | ° | ° | - |2,630,000| 0·0331 | 93 37 | 39 12 | 15·4 | 906 | 26·2 | 12·8 | - |2,620,000| 0·0479 | 127 12 | 33 35 | 22·2 | 884 | 33·3 | 5·7 | - |2,610,000| 0·0591 | 158 36 | 31 24 | 27·4 | 862 | 38·3 | 0·7 | - |2,600,000| 0·0660 | 190 4 | 31 28 | 30·6 | 851 | 41·5 | −2·5 | - |2,590,000| 0·0666 | 220 28 | 30 24 | 30·9 | 850 | 41·8 | −2·8 | - |2,580,000| 0·0609 | 249 56 | 29 28 | 28·3 | 859 | 39·2 | −0·2 | - +---------+------------+-----------+-----------+---------+---------+-----------+-----------+ - |2,570,000| 0·0492 | 277 24 | 27 28 | 22·9 | 878 | 33·9 | 5·1 | - |2,560,000| 0·0350 | 305 2 | 27 38 | 16·2 | 902 | 27·1 | 11·9 | - |2,550,000| 0·0167 | 298 34 | R 6 28 | | | | | - |2,540,000| 0·0192 | 253 58 | R 44 36 | | | | | - |2,530,000| 0·0369 | 259 19 | 5 21 | 17·1 | 899 | 28·0 | 11·0 | - |2,520,000| 0·0537 | 283 7 | 23 48 | 25·0 | 871 | 35·9 | 3·1 | - |2,510,000| 0·0660 | 310 4 | 26 57 | 30·6 | 851 | 41·5 | −2·5 | - |2,500,000| 0·0721 | 338 36 | 28 32 | 33·5 | 841 | 44·2 | −5·2 | - |2,490,000| 0·0722 | 7 36 | 29 0 | 33·6 | 841 | 44·3 | −5·3 | - |2,480,000| 0·0662 | 35 46 | 28 10 | 30·8 | 850 | 41·7 | −2·7 | - |2,470,000| 0·0553 | 63 26 | 27 40 | 25·7 | 868 | 36·6 | 2·4 | - |2,460,000| 0·0410 | 89 13 | 25 47 | 19·1 | 892 | 30·0 | 9·0 | - |2,450,000| 0·0252 | 109 33 | 20 20 | 11·7 | | | | - +---------+------------+-----------+-----------+---------+---------+-----------+-----------+ - - TABLE III. - - ECCENTRICITY, LONGITUDE OF THE PERIHELION, &C., &C., FOR INTERVALS OF - 10,000 YEARS, FROM 1,000,000 TO 750,000 YEARS AGO. - - THE GLACIAL EPOCH OF THE _Miocene period_ IS PROBABLY COMPREHENDED - WITHIN THIS TABLE. - - +---------+------------+-----------+-----------+-------------------------------------------+ - | I. | II. | III. | IV. | Winter occurring in aphelion. | - | | | | +---------+---------+-----------+-----------+ - | | | | | V. | VI. | VII. | VIII. | - |Number of|Eccentricity|Longitude | Number of |Excess of|Midwinter|Number of | Midwinter | - | years | of orbit. | of | degrees | winter |intensity|degrees by |temperature| - | before | |perihelion.| passed | over | of the | which the | of Great | - | A.D. | | |over by the| summer, | sun’s | midwinter | Britain. | - | 1800. | | |perihelion.|in days. | heat. |temperature| | - | | | | Motion | | Present |is lowered | | - | | | |retrograde | |intensity| | | - | | | |at periods | | =1000. | | | - | | | | marked R. | | | | | - +---------+------------+-----------+-----------+---------+---------+-----------+-----------+ - | | | ° ′ | | | | | | - |1,000,000| 0·0151 | 248 22 | ° ′ | | | F. | F. | - | 990,000| 0·0224 | 313 50 | 65 28 | | | ° | ° | - | 980,000| 0·0329 | 358 2 | 44 12 | 15·3 | 906 | 26·1 | 12·9 | - | 970,000| 0·0441 | 32 40 | 34 38 | 20·5 | 887 | 31·5 | 7·5 | - | 960,000| 0·0491 | 66 49 | 34 9 | 22·8 | 878 | 33·8 | 5·2 | - | 950,000| 0·0517 | 97 51 | 31 2 | 24·0 | 874 | 35·0 | 4·0 | - | 940,000| 0·0495 | 127 42 | 29 51 | 23·0 | 878 | 34·0 | 5·0 | - | 930,000| 0·0423 | 156 11 | 28 29 | 19·7 | 890 | 30·6 | 8·4 | - | 920,000| 0·0305 | 181 40 | 25 29 | 14·2 | 910 | 25·0 | 14·0 | - | 910,000| 0·0156 | 194 15 | 12 35 | | | | | - | 900,000| 0·0102 | 135 2 | R 59 13 | | | | | - | 890,000| 0·0285 | 127 1 | R 8 1 | | | | | - | 880,000| 0·0456 | 152 33 | 25 32 | 21·2 | 884 | 32·2 | 6·8 | - | 870,000| 0·0607 | 180 23 | 27 50 | 28·2 | 859 | 39·0 | 0·0 | - | 860,000| 0·0708 | 209 41 | 29 18 | 32·9 | 843 | 43·6 | −4·6 | - | 850,000| 0·0747 | 239 28 | 29 47 | 34·7 | 837 | 45·3 | −6·3 | - | 840,000| 0·0698 | 269 14 | 29 46 | 32·4 | 845 | 43·2 | −4·2 | - | 830,000| 0·0623 | 298 28 | 29 14 | 29·0 | 857 | 40·0 | −1·0 | - | 820,000| 0·0476 | 326 4 | 27 36 | 22·1 | 881 | 33·1 | 5·9 | - | 810,000| 0·0296 | 348 30 | 22 26 | | | | | - | 800,000| 0·0132 | 343 49 | R 4 41 | | | | | - | 790,000| 0·0171 | 293 19 | R 50 30 | | | | | - | 780,000| 0·0325 | 303 37 | 10 18 | 15·2 | 907 | 26·0 | 13·0 | - | 770,000| 0·0455 | 328 38 | 25 1 | 21·2 | 884 | 32·2 | 6·8 | - | 760,000| 0·0540 | 357 12 | 28 34 | 25·1 | 870 | 36·0 | 3·0 | - | 750,000| 0·0575 | 27 18 | 30 6 | 26·7 | 864 | 37·7 | 1·3 | - | 740,000| 0·0561 | 58 30 | 31 12 | 26·1 | 867 | 37·0 | 2·0 | - | 730,000| 0·0507 | 90 55 | 32 25 | 23·6 | 876 | 34·6 | 4·4 | - | 720,000| 0·0422 | 125 14 | 34 19 | 19·6 | 890 | 30·6 | 8·4 | - | 710,000| 0·0307 | 177 26 | 52 12 | 14·3 | 910 | 25·0 | 14·0 | - | 700,000| 0·0220 | 208 13 | 30 47 | | | | | - +---------+------------+-----------+-----------+---------+---------+-----------+-----------+ - - TABLE IV. - - ECCENTRICITY, LONGITUDE OF THE PERIHELION, &C., &C., FOR INTERVALS OF - 10,000 YEARS, FROM 250,000 YEARS AGO TO THE PRESENT DATE. - - THE _Glacial epoch_ IS PROBABLY COMPREHENDED WITHIN THIS TABLE. - - +---------+------------+-----------+-----------+-------------------------------------------+ - | I. | II. | III. | IV. | Winter occurring in aphelion. | - | | | | +---------+---------+-----------+-----------+ - | | | | | V. | VI. | VII. | VIII. | - |Number of|Eccentricity|Longitude | Number of |Excess of|Midwinter|Number of | Midwinter | - | years | of orbit. | of | degrees | winter |intensity|degrees by |temperature| - | before | |perihelion.| passed | over | of the | which the | of Great | - | A.D. | | |over by the| summer, | sun’s | midwinter | Britain. | - | 1800. | | |perihelion.|in days. | heat. |temperature| | - | | | | Motion | | Present |is lowered | | - | | | |retrograde | |intensity| | | - | | | |at periods | | =1000. | | | - | | | | marked R. | | | | | - +---------+------------+-----------+-----------+---------+---------+-----------+-----------+ - | | | ° ′ | | | | F. | F. | - | 250,000| 0·0258 | 59 39 | ° ′ | | | ° | ° | - | 240,000| 0·0374 | 74 58 | 15 19 | 17·4 | 898 | 28·3 | 10·7 | - |S 230,000| 0·0477 | 102 49 | 27 51 | 22·2 | 885 | 33·2 | 5·8 | - |S 220,000| 0·0497 | 124 33 | 21 44 | 23·2 | 877 | 34·1 | 4·9 | - |S 210,000| 0·0575 | 144 55 | 20 22 | 26·7 | 864 | 37·7 | 1·3 | - | 200,000| 0·0569 | 168 18 | 23 23 | 26·5 | 865 | 37·4 | 1·6 | - |S 190,000| 0·0532 | 190 4 | 21 46 | 24·7 | 871 | 35·7 | 3·3 | - |S 180,000| 0·0476 | 209 22 | 19 18 | 22·1 | 881 | 33·1 | 5·9 | - |S 170,000| 0·0437 | 228 7 | 18 45 | 20·3 | 887 | 31·3 | 7·7 | - | 160,000| 0·0364 | 236 38 | 8 31 | 16·9 | 900 | 27·8 | 11·2 | - | 150,000| 0·0332 | 242 56 | 6 18 | 15·4 | 905 | 26·2 | 12·8 | - | 140,000| 0·0346 | 246 29 | 3 33 | 16·1 | 903 | 26·9 | 12·1 | - | 130,000| 0·0384 | 259 34 | 13 5 | 17·8 | 896 | 28·8 | 10·2 | - | 120,000| 0·0431 | 274 47 | 15 13 | 20·1 | 888 | 31·0 | 8·0 | - | 110,000| 0·0460 | 293 48 | 19 1 | 21·4 | 883 | 32·4 | 6·6 | - | 100,000| 0·0473 | 316 18 | 22 30 | 22·0 | 881 | 33·0 | 6·0 | - |L 90,000| 0·0452 | 340 2 | 23 44 | 21·0 | 885 | 32·0 | 7·0 | - |L 80,000| 0·0398 | 4 13 | 24 11 | 18·5 | 894 | 29·4 | 9·6 | - |L 70,000| 0·0316 | 27 22 | 23 9 | 14·7 | 908 | 25·5 | 13·5 | - |L 60,000| 0·0218 | 46 8 | 18 46 | | | | | - | 50,000| 0·0131 | 50 14 | 4 6 | | | | | - |L 40,000| 0·0109 | 28 36 | R 21 38 | | | | | - |L 30,000| 0·0151 | 5 50 | R 22 46 | | | | | - |L 20,000| 0·0188 | 44 0 | 38 10 | | | | | - |L 10,000| 0·0187 | 78 28 | 34 28 | | | | | - |A.D. 1800| 0·0168 | 99 30 | 21 2 | | | | | - +---------+------------+-----------+-----------+---------+---------+-----------+-----------+ - -In Tables II., III., and IV., column I. represents the dates of the -periods, column II. the eccentricity, column III. the longitude of -the perihelion. In Table IV. the eccentricity and the longitude of -the perihelion of the six periods marked with an S are copied from a -letter of Mr. Stone to Sir Charles Lyell, published in the Supplement -of the Phil. Mag. for June, 1865; the eight periods marked L are copied -from M. Leverrier’s Table, to which reference has been made. For the -correctness of everything else, both in this Table and in the other -three, I alone am responsible. - -Column IV. gives the number of degrees passed over by the perihelion -during each 10,000 years. From this column it will be seen how -irregular is the motion of the perihelion. At four different periods -it had a retrograde motion for 20,000 years. Column V. shows the -number of days by which the winter exceeds the summer when the winter -occurs in aphelion. Column VI. shows the intensity of the sun’s heat -during midwinter, when the winter occurs in aphelion, the present -midwinter intensity being taken at 1,000. These six columns comprehend -all the astronomical part of the Tables. Regarding the correctness of -the principles upon which these columns are constructed, there is no -diversity of opinion. But these columns afford no direct information -as to the character of the climate, or how much the temperature is -increased or diminished. To find this we pass on to columns VII. and -VIII., calculated on physical principles. Now, unless the physical -principles upon which these three columns are calculated be wholly -erroneous, change of eccentricity must undoubtedly very seriously -affect climate. Column VII. shows how many degrees Fahrenheit the -temperature is lowered by a decrease in the intensity of the sun’s heat -corresponding to column VI. For example, 850,000 years ago, if the -winters occurred then in aphelion, the direct heat of the sun during -midwinter would be only 837/1000 of what it is at present at the same -season of the year, and column VII. shows that this decrease in the -intensity of the sun’s heat would lower the temperature 45°·3 F. - -The principle upon which this result is arrived at is this:--The -temperature of space, as determined by Sir John Herschel, is −239° -F. M. Pouillet, by a different method, arrived at almost the same -result. If we take the midwinter temperature of Great Britain at -39°, then 239° + 39° = 278° will represent the number of degrees of -rise due to the sun’s heat at midwinter; in other words, it takes a -quantity of sun-heat which we have represented by 1000 to maintain the -temperature of the earth’s surface in Great Britain 278° above the -temperature of space. Were the sun extinguished, the temperature of -our island would sink 278° below its present midwinter temperature, -or to the temperature of space. But 850,000 years ago, as will be -seen from Table III., if the winters occurred in aphelion, the heat -of the sun at midwinter would only equal 837 instead of 1000 as at -present. Consequently, if it takes 1,000 parts of heat to maintain the -temperature 278° above the temperature of space, 837 parts of heat will -only be able to maintain the temperature 232°·7 above the temperature -of space; for 232°·7 is to 278 as 837 is to 1,000. Therefore, if the -temperature was then only 232°·7 above that of space, it would be -45°·3 below what it is at present. This is what the temperature would -be on the supposition, of course, that it depended wholly on the -sun’s intensity and was not modified by other causes. This method has -already been discussed at some length in Chapter II. But whether these -values be too high or too low, one thing is certain, that a very slight -increase or a very slight decrease in the quantity of heat received -from the sun must affect temperature to a considerable extent. The -direct heat of the moon, for example, cannot be detected by the finest -instruments which we possess; yet from 238,000 observations made at -Prague during 1840−66, it would seem that the temperature is sensibly -affected by the mere change in the lunar perigee and inclination of the -moon’s orbit.[195] - -Column VIII. gives the midwinter temperature. It is found by -subtracting the numbers in column VII. from 39°, the present midwinter -temperature. - -I have not given a Table showing the temperature of the summers at -the corresponding periods. This could not well be done; for there is -no relation at the periods in question between the intensity of the -sun’s heat and the temperature of the summers. One is apt to suppose, -without due consideration, that the summers ought to be then as much -warmer than they are at present, as the winters were then colder than -now. Sir Charles Lyell, in his “Principles,” has given a column of -summer temperatures calculated from my table upon this principle. -Astronomically the principle is correct, but physically, as was shown -in Chapter IV., it is totally erroneous, and calculated to convey a -wrong impression regarding the whole subject of geological climate. -The summers at those periods, instead of being much warmer than they -are at present, would in reality be much colder, notwithstanding the -great increase in the intensity of the sun’s heat resulting from the -diminished distance of the sun. - -What, then, is the date of the glacial epoch? It is perfectly obvious -that if the glacial epoch resulted from a high state of eccentricity, -it must be referred either to the period included in Table III. or -to the one in Table IV. In Table III. we have a period extending from -about 980,000 to about 720,000 years ago, and in Table IV. we have a -period beginning about 240,000 years ago, and extending down to about -80,000 years ago. As the former period was of greater duration than -the latter, and the eccentricity also attained to a higher value, I at -first felt disposed to refer the glacial epoch proper (the time of the -till and boulder clay) to the former period; and the latter period, I -was inclined to believe, must have corresponded to the time of local -glaciers towards the close of the glacial epoch, the evidence for which -(moraines) is to be found in almost every one of our Highland glens. -On this point I consulted several eminent geologists, and they all -agreed in referring the glacial epoch to the former period; the reason -assigned being that they considered the latter period to be much too -recent and of too short duration to represent that epoch. - -Pondering over the subject during the early part of 1866, reasons soon -suggested themselves which convinced me that the glacial epoch must -be referred to the latter and not to the former period. Those reasons -I shall now proceed to state at some length, since they have a direct -bearing, as will be seen, on the whole question of geological time. - -It is the modern and philosophic doctrine of uniformity that has -chiefly led geologists to over-estimate the length of geological -periods. This philosophic school teaches, and that truly, that the -great changes undergone by the earth’s crust must have been produced, -not by convulsions and cataclysms of nature, but by those ordinary -agencies that we see at work every day around us, such as rain, snow, -frost, ice, and chemical action, &c. It teaches that the valleys -were not produced by violent dislocations, nor the hills by sudden -upheavals, but that they were actually carved out of the solid rock -by the silent and gentle agency of chemical action, frost, rain, ice, -and running water. It teaches, in short, that the rocky face of our -globe has been carved into hill and dale, and ultimately worn down -to the sea-level, by means of these apparently trifling agents, not -only once or twice, but probably dozens of times over during past -ages. Now, when we reflect that with such extreme slowness do these -agents perform their work, that we might watch their operations from -year to year, and from century to century, if we could, without being -able to perceive that they make any very sensible advance, we are -necessitated to conclude that geological periods must be enormous. And -the conclusion at which we thus arrive is undoubtedly correct. It is, -in fact, impossible to form an adequate conception of the length of -geological time. It is something too vast to be fully grasped by our -minds. But here we come to the point where the fundamental mistake -arises; Geologists do not err in forming too great a conception of the -extent of geological periods, _but in the mode in which they represent -the length of these periods in numbers_. When we speak of units, tens, -hundreds, thousands, we can form some notion of what these quantities -represent; but when we come to millions, tens of millions, hundreds -of millions, thousands of millions, the mind is then totally unable -to follow, and we can only use these numbers as representations of -quantities that turn up in calculation. We know, from the way in which -they do turn up in our process of calculation, whether they are correct -representations of things in actual nature or not; but we could not, -from a mere comparison of these quantities with the thing represented -by them, say whether they were actually too small or too great. - -At present, geological estimates of time are little else than mere -conjectures. Geological science has hitherto afforded no trustworthy -means of estimating the positive length of geological epochs. -Geological phenomena tell us most emphatically that these periods -must be long; but how long they have hitherto failed to inform us. -Geological phenomena represent time to the mind under a most striking -and imposing form. They present to the eye, as it were, a sensuous -representation of time; the mind thus becomes deeply impressed with -a sense of immense duration; and when one under these feelings is -called upon to put down in figures what he believes will represent that -duration, he is very apt to be deceived. If, for example, a million of -years as represented by geological phenomena and a million of years as -represented by figures were placed before our eyes, we should certainly -feel startled. We should probably feel that a unit with six ciphers -after it was really something far more formidable than we had hitherto -supposed it to be. Could we stand upon the edge of a gorge a mile and -a half in depth that had been cut out of the solid rock by a tiny -stream, scarcely visible at the bottom of this fearful abyss, and were -we informed that this little streamlet was able to wear off annually -only 1/10 of an inch from its rocky bed, what would our conceptions be -of the prodigious length of time that this stream must have taken to -excavate the gorge? We should certainly feel startled when, on making -the necessary calculations, we found that the stream had performed this -enormous amount of work in something less than a million of years. - -If, for example, we could possibly form some adequate conception of a -period so prodigious as one hundred millions of years, we should not -then feel so dissatisfied with Sir W. Thomson’s estimate that the age -of the earth’s crust is not greater than that. - -Here is one way of conveying to the mind some idea of what a million -of years really is. Take a narrow strip of paper an inch broad, or -more, and 83 feet 4 inches in length, and stretch it along the wall of -a large hall, or round the walls of an apartment somewhat over 20 feet -square. Recall to memory the days of your boyhood, so as to get some -adequate conception of what a period of a hundred years is. Then mark -off from one of the ends of the strip 1/10 of an inch. The 1/10 of the -inch will then represent one hundred years, and the entire length of -the strip a million of years. It is well worth making the experiment, -just in order to feel the striking impression that it produces on the -mind. - -The latter period, which we have concluded to be that of the glacial -epoch, extended, as we have seen, over a period of 160,000 years. But -as the glaciation was only on one hemisphere at a time, 80,000 years -or so would represent the united length of the cold periods. In order -to satisfy ourselves that this period is sufficiently long to account -for all the amount of denudation effected during the glacial epoch, -let us make some rough estimate of the probable rate at which the -surface of the country would be ground down by the ice. Suppose the -ice to grind off only one-tenth of an inch annually this would give -upwards of 650 feet as the quantity of rock removed during the time. -But it is probable that it did not amount to one-fourth part of that -quantity. Whether one-tenth of an inch per annum be an over-estimate or -an under-estimate of the rate of denudation by the ice, it is perfectly -evident that the period in question is sufficiently long, so far as -denudation is concerned, to account for the phenomena of the glacial -epoch. - -But admitting that the period under consideration is sufficiently -_long_ to account for all the denudation which took place _during_ -the glacial epoch, we have yet to satisfy ourselves that it is also -sufficiently _remote_ to account for all the denudation which has taken -place _since_ the glacial epoch. Are the facts of geology consistent -with the idea that the close of the glacial epoch does not date back -beyond 80,000 years? - -This question could be answered if we knew the present rate of -subaërial denudation, for the present rate evidently does not differ -greatly from that which has obtained since the close of the glacial -epoch. - - - - - CHAPTER XX. - - GEOLOGICAL TIME.—METHOD OF MEASURING THE RATE OF SUBAËRIAL - DENUDATION. - - Rate of Subaërial Denudation a Measure of Time.—Rate determined - from Sediment of the Mississippi.—Amount of Sediment carried - down by the Mississippi; by the Ganges.—Professor Geikie on - Modern Denudation.—Professor Geikie on the Amount of Sediment - conveyed by European Rivers.—Rate at which the Surface of - the Globe is being denuded.—Alfred Tylor on the Sediment - of the Mississippi.—The Law which determines the Rate of - Denudation.—The Globe becoming less oblate.—Carrying Power - of our River Systems the true Measure of Denudation.—Marine - Denudation trifling in comparison to Subaërial.—Previous - Methods of measuring Geological Time.—Circumstances which - show the recent Date of the Glacial Epoch.—Professor Ramsay - on Geological Time. - - -It is almost self-evident that the rate of subaërial denudation must -be equal to the rate at which the materials are carried off the land -into the sea, but the rate at which the materials are carried off the -land is measured by the rate at which sediment is carried down by our -river systems. _Consequently, in order to determine the present rate -of subaërial denudation, we have only to ascertain the quantity of -sediment annually carried down by the river systems._ - -Knowing the quantity of sediment transported by a river, say annually, -and the area of its drainage, we have the means of determining the -rate at which the surface of this area is being lowered by subaërial -denudation. And if we know this in reference to a few of the great -continental rivers draining immense areas in various latitudes, we -could then ascertain with tolerable correctness the rate at which the -surface of the globe is being lowered by subaërial denudation, and -also the length of time which our present continents can remain above -the sea-level. Explaining this to Professor Ramsay during the winter -of 1865, I learned from him that accurate measurements had been made -of the amount of sediment annually carried down by the Mississippi -River, full particulars of which investigations were to be found -in the Proceedings of the American Association for the Advancement -of Science for 1848. These proceedings contain a report by Messrs. -Brown and Dickeson, which unfortunately over-estimated the amount of -sediment transported by the Mississippi by nearly four times what -was afterwards found by Messrs. Humphreys and Abbot to be the actual -amount. From this estimate, I was led to the conclusion that if the -Mississippi is a fair representative of rivers in general, our existing -continents would not remain longer than one million and a half years -above the sea-level.[196] This was a conclusion so startling as to -excite suspicion that there must have been some mistake in reference -to Messrs. Brown and Dickeson’s data. It showed beyond doubt, however, -that the rate of subaërial denudation, when accurately determined by -this method, would be found to be enormously greater than had been -supposed. Shortly afterwards, on estimating the rate from the data -furnished by Humphreys and Abbot, I found the rate of denudation to -be about one foot in 6,000 years. Taking the mean elevation of all -the land as ascertained by Humboldt to be 1,000 feet, the whole would -therefore be carried down into the ocean by our river systems in about -6,000,000 of years if no elevation of the land took place.[197] The -following are the data and mode of computation by which this conclusion -was arrived at. It was found by Messrs. Humphreys and Abbot that the -average amount of sediment held in suspension in the waters of the -Mississippi is about 1/1500 of the weight of the water, or 1/2900 -by bulk. The annual discharge of the river is 19,500,000,000,000 -cubic feet of water. The quantity of sediment carried down into the -Gulf of Mexico amounts to 6,724,000,000 cubic feet. But besides -that which is held in suspension, the river pushes down into the -sea about 750,000,000 cubic feet of earthy matter, making in all a -total of 7,474,000,000 cubic feet transferred from the land to the -sea annually. Where does this enormous mass of material come from? -Unquestionably it comes from the ground drained by the Mississippi. The -area drained by the river is 1,244,000 square miles. Now 7,474,000,000 -cubic feet removed off 1,224,000 square miles of surface is equal to -1/4566 of a foot off that surface per annum, or one foot in 4,566 -years. The specific gravity of the sediment is taken at 1·9, that of -rock is about 2·5; consequently the amount removed is equal to one foot -of rock in about 6,000 years. The average height of the North American -continent above the sea-level, according to Humboldt, is 748 feet; -consequently, at the present rate of denudation, the whole area of -drainage will be brought down to the sea-level in less than 4,500,000 -years, if no elevation of the land takes place. - -Referring to the above, Sir Charles Lyell makes the following -appropriate remarks:—“There seems no danger of our overrating the -mean rate of waste by selecting the Mississippi as our example, for -that river drains a country equal to more than half the continent of -Europe, extends through twenty degrees of latitude, and therefore -through regions enjoying a great variety of climate, and some of its -tributaries descend from mountains of great height. The Mississippi -is also more likely to afford us a fair test of ordinary denudation, -because, unlike the St. Lawrence and its tributaries, there are no -great lakes in which the fluviatile sediment is thrown down and -arrested on its way to the sea.”[198] - -The rate of denudation of the area drained by the river Ganges is much -greater than that of the Mississippi. The annual discharge of that -river is 6,523,000,000,000 cubic feet of water. The sediment held in -suspension is equal to 1/510 by weight; area of drainage 432,480 square -miles. This gives one foot of rock in 2,358 years as the amount removed. - -Rough estimates have been made of the amount of sediment carried down -by some eight or ten European rivers; and although those estimates -cannot be depended upon as being anything like perfectly accurate, -still they show (what there is very little reason to doubt) that it is -extremely probable that the European continent is being denuded about -as rapidly as the American. - -For a full account of all that is known on this subject I must -refer to Professor Geikie’s valuable memoir on Modern Denudation -(Transactions of Geological Society of Glasgow, vol. iii.; also Jukes -and Geikie’s “Manual of Geology,” chap. xxv.) It is mainly through the -instrumentality of this luminous and exhaustive memoir that the method -under consideration has gained such wide acceptance amongst geologists. - -Professor Geikie finds that at the present rate of erosion the -following is the number of years required by the undermentioned rivers -to remove one foot of rock from the general surface of their basins. -Professor Geikie thus shows that the rate of denudation, as determined -from the amount of sediment carried down the Mississippi, is certainly -not too high. - - Danube 6,846 years. - Mississippi 6,000 〃 - Nith 4,723 〃 - Ganges 2,358 〃 - Rhone 1,528 〃 - Hoang Ho 1,464 〃 - Po 729 〃 - -By means of subaërial agencies continents are being cut up into -islands, the islands into smaller islands, and so on till the whole -ultimately disappears. - -No proper estimate has been made of the quantity of sediment carried -down into the sea by our British rivers. But, from the principles just -stated, we may infer that it must be as great in proportion to the area -of drainage as that carried down by the Mississippi. For example, the -river Tay, which drains a great portion of the central Highlands of -Scotland, carries to the sea three times as much water in proportion -to its area of drainage as is carried by the Mississippi. And any one -who has seen this rapidly running river during a flood, red and turbid -with sediment, will easily be convinced that the quantity of solid -material carried down by it into the German Ocean must be very great. -Mr. John Dougall has found that the waters of the Clyde during a flood -hold in suspension 1/800 by bulk of sediment. The observations were -made about a mile above the city of Glasgow. But even supposing the -amount of sediment held in suspension by the waters of the Tay to be -only one-third (which is certainly an under-estimate) of that of the -Mississippi, viz. 1/4500 by weight, still this would give the rate of -denudation of the central Highlands at one foot in 6,000 years, or -1,000 feet in 6 millions of years. - -It is remarkable that although so many measurements have been made of -the amount of fluviatile sediment being transported seawards, yet that -the bearing which this has on the broad questions of geological time -and the rate of subaërial denudation should have been overlooked. One -reason for this, no doubt, is that the measurements were made, not -with a view to determine the rate at which the river basins are being -lowered, but mainly to ascertain the age of the river deltas and the -rate at which these are being formed.[199] - -_The Law which determines the Rate at which any Country is being -denuded._—By means of subaërial agencies continents are being cut up -into islands, the islands into smaller islands, and so on till the -whole ultimately disappears. - -So long as the present order of things remains, the rate of denudation -will continue while land remains above the sea-level; and we have no -warrant for supposing that the rate was during past ages less than it -is at the present day. It will not do to object that, as a considerable -amount of the sediment carried down by rivers is boulder clay and -other materials belonging to the Ice age, the total amount removed -by the rivers is on that account greater than it would otherwise be. -Were this objection true, it would follow that, prior to the glacial -period, when it is assumed that there was no boulder clay, the face of -the country must have consisted of bare rock; for in this case no soil -could have accumulated from the disintegration and decomposition of the -rocks, _since, unless the rocks of a country disintegrate more rapidly -than the river systems are able to carry the disintegrated materials -to the sea, no surface soil can form on that country_. The rate at -which rivers carry down sediment is evidently not determined by the -rate at which the rocks are disintegrated and decomposed, but by the -quantity of rain falling, and the velocity with which it moves off the -face of the country. Every river system possesses a definite amount of -carrying-power, depending upon the slope of the ground, the quantity of -rain falling per annum, the manner in which the rain falls, whether it -falls gradually or in torrents, and a few other circumstances. When it -so happens, as it generally does, that the amount of rock disintegrated -on the face of the country is greater than the carrying-power of the -river systems can remove, then a soil necessarily forms. But when the -reverse is the case no soil can form on that country, and it will -present nothing but barren rock. This is no doubt the reason why in -places like the Island of Skye, for example, where the rocks are -exceedingly hard and difficult to decompose and separate, the ground -steep, and the quantity of rain falling very great, there is so much -bare rock to be seen. If, prior to the glacial epoch, the rocks of -the area drained by the Mississippi did not produce annually more -material from their destruction under atmospheric agency than was being -carried down by that river, then it follows that the country must have -presented nothing but bare rock, if the amount of rain falling then was -as great as at present. - -But, after all, one foot removed off the general level of the country -since the creation of man, according to Mosaic chronology, is certainly -not a very great quantity. No person but one who had some preconceived -opinions to maintain, would ever think of concluding that one foot of -soil during 6,000 years was an extravagant quantity to be washed off -the face of the country by rain and floods during that long period. -Those who reside in the country and are eye-witnesses of the actual -effects of heavy rains upon the soil, our soft country roads, ditches, -brooks, and rivers, will have considerable difficulty in actually -believing that only one foot has been washed away during the past 6,000 -years. - -Some may probably admit that a foot of soil may be washed off during -a period so long as 6,000 years, and may tell us that what they deny -is not that a foot of loose and soft soil, but a foot of solid rock -can be washed away during that period. But a moment’s reflection must -convince them that, unless the rocks of the country were disintegrating -and decomposing as rapidly into soil as the rain is carrying the soil -away, the surface of the country would ultimately become bare rock. It -is true that the surface of our country in many places is protected by -a thick covering of boulder clay; but when this has once been removed, -the rocks will then disintegrate far more rapidly than they are doing -at present. - -But slow as is the rate at which the country is being denuded, yet -when we take into consideration a period so enormous as 6 millions of -years, we find that the results of denudation are really startling. -One thousand feet of solid rock during that period would be removed -from off the face of the country. But if the mean level of the country -would be lowered 1,000 feet in 6 millions of years, how much would our -valleys and glens be deepened during that period? This is a problem -well worthy of the consideration of those who treat with ridicule the -idea that the general features of our country have been carved out by -subaërial agency. - -In consequence of the retardation of the earth’s rotation, occasioned -by the friction of the tidal wave, the sea-level must be slowly sinking -at the equator and rising at the poles. But it is probable that the -land at the equator is being lowered by denudation as rapidly as -the sea-level is sinking. _Nearly one mile must have been worn off -the equator during the past 12 millions of years_, if the rate of -denudation all along the equator be equal to that of the basin of the -Ganges. It therefore follows that we cannot infer from the present -shape of our globe what was its form, or the rate at which it was -rotating, at the time when its crust became solidified. Although it -had been as oblate as the planet Jupiter, denudation must in time have -given it its present form. - -There is another effect which would result from the denudation of the -equator and the sinking of the ocean at the equator and its rise at -the poles. This, namely, that it would tend to increase the rate of -rotation; or, more properly, it would tend to _lessen_ the rate of -tidal retardation. - -But if the rate of denudation be at present so great, what must it have -been during the glacial epoch? It must have been something enormous. -At present, denudation is greatly retarded by the limited power of -our river systems to remove the loose materials resulting from the -destruction of the rocks. These materials accumulate and form a thick -soil over the surface of the rocks, which protects them, to a great -extent, from the weathering effects of atmospheric agents. So long as -the amount of rock disintegrated exceeds that which is being removed -by the river systems, the soil will continue to accumulate till the -amount of rock destroyed per annum is brought to equal that which -is being removed. It therefore follows from this principle that the -CARRYING-POWER OF OUR RIVER SYSTEMS IS THE TRUE MEASURE OF DENUDATION. -But during the glacial epoch the thickness of the soil would have but -little effect in diminishing the waste of the rocks; for at that period -the rocks were not decomposed by atmospheric agency, but were ground -down by the mechanical friction of the ice. But the presence of a thick -soil at this period, instead of retarding the rate of denudation, -would tend to increase it tenfold, for the soil would then be used as -grinding-material for the ice-sheet. In places where the ice was, say, -2,000 feet in thickness, the soil would be forced along over the rocky -face of the country, exerting a pressure on the rocks equal to 50 tons -on the square foot. - -It is true that the rate at which many kinds of rocks decompose and -disintegrate is far less than what has been concluded to be the mean -rate of denudation of the whole country. This is evident from the fact -which has been adduced by some writers, that inscriptions on stones -which have been exposed to atmospheric agency for a period of 2,000 -years or so, have not been obliterated. But in most cases epitaphs on -monuments and tombstones, and inscriptions on the walls of buildings, -200 years old, can hardly be read. And this is not all: the stone on -which the letters were cut has during that time rotted in probably to -the depth of several inches; and during the course of a few centuries -more the whole mass will crumble into dust. - -The facts which we have been considering show also how trifling is the -amount of denudation effected by the sea in comparison with that by -subaërial agents. The entire sea-coast of the globe, according to Dr. -A. Keith Johnston, is 116,531 miles. Suppose we take the average height -of the coast-line at 25 feet, and take also the rate at which the sea -is advancing on the land at one foot in 100 years, then this gives -15,382,500,000 cubic feet of rock as the total amount removed in 100 -years by the action of the sea. The total amount of land is 57,600,000 -square miles, or 1,605,750,000,000,000 square feet; and if one foot is -removed off the surface in 6,000 years, then 26,763,000,000,000 cubic -feet is removed by subaërial agency in 100 years, or about 1,740 times -as much as that removed by the sea. Before the sea could denude the -globe as rapidly as the subaërial agents, it would have to advance on -the land at the rate of upwards of 17 feet annually. - -It will not do, however, to measure marine denudation by the rate at -which the sea is advancing on the land. There is no relation whatever -between the rate at which the sea is _advancing_ on the land and the -rate at which the sea is _denuding_ the land. For it is evident that as -the subaërial agents bring the coast down to the sea-level, all that -the sea has got to do is simply to advance, or at most to remove the -loose materials which may lie in its path. The amount of denudation -which has been effected by the sea during past geological ages, -compared with what has been effected by subaërial agency, is evidently -but trifling. Denudation is not the proper function of the sea. The -great denuding agents are land-ice, frost, rain, running-water, -chemical agency, &c. The proper work which belongs to the sea is the -transporting of the loose materials carried down by the rivers, and the -spreading of these out so as to form the stratified beds of future ages. - -_Previous Methods of measuring Geological Time unreliable._—The method -which has just been detailed of estimating the rate of subaërial -denudation seems to afford the only reliable means of a geological -character of determining geological time in absolute measure. The -methods which have hitherto been adopted not only fail to give the -positive length of geological periods, but some of them are actually -calculated to mislead. - -The common method of calculating the length of a period from the -thickness of the stratified rocks belonging to that period is one of -that class. Nothing whatever can be inferred from the thickness of a -deposit as to the length of time which was required to form it. The -thickness of a deposit will depend upon a great many circumstances, -such as whether the deposition took place near to land or far away in -the deep recesses of the ocean, whether it occurred at the mouth of a -great river or along the sea-shore, or at a time when the sea-bottom -was rising, subsiding, or remaining stationary. Stratified formations -10,000 feet in thickness, for example, may, under some conditions, have -been formed in as many years, while under other conditions it may have -required as many centuries. Nothing whatever can be safely inferred as -to the absolute length of a period from the thickness of the stratified -formations belonging to that period. Neither will this method give us a -trustworthy estimate of the _relative_ lengths of geological periods. -Suppose we find the average thickness of the Cambrian rocks to be, -say, 26,000 feet, the Silurian to be 28,000 feet, the Devonian to be -6,000 feet, and the Tertiary to be 10,000 feet, it would not be safe -to assume, as is sometimes done, that the relative duration of those -periods must have corresponded to these numbers. Were we sure that we -had got the correct average thickness of all the rocks belonging to -each of those formations, we might probably be able to arrive at the -relative lengths of those periods; but we can never be sure of this. -Those formations all, at one time, formed sea-bottoms; and we can only -measure such deposits as are now raised above the sea-level. But is -not it probable that the relative positions of sea and land during the -Cambrian, Silurian, Old Red Sandstone, Carboniferous, and other early -periods of the earth’s history, differed more from the present than the -distribution of sea and land during the Tertiary period differed from -that which obtains now? May not the greater portion of the Tertiary -deposits be still under the sea-bottom? And if this be the case, it may -yet be found at some day in the distant future, when these deposits -are elevated into dry land, that they are much thicker than we now -conclude them to be. Of course, it is by no means asserted that this -is so, but only that they _may_ be thicker for anything we know to the -contrary; and the possibility that they may, destroys our confidence -in the accuracy of this method of determining the relative lengths of -geological periods. - -Neither does palæontology afford any better mode of measuring -geological time. In fact, the palæontological method of estimating -geological time, either absolute or relative, from the rate at which -species change, appears to be even still more unsatisfactory. If we -could ascertain by some means or other the time that has elapsed from -some given epoch (say, for example, the glacial) till the present -day, and were we sure at the same time that species have changed at a -uniform rate during all past ages, then, by ascertaining the percentage -of change that has taken place since the glacial epoch, we should -have a means of making something like a rough estimate of the length -of the various periods. But without some such period to start with, -the palæontological method is useless. It will not do to take the -historic period as a base-line. It is far too short to be used with -safety in determining the distance of periods so remote as those which -concern the geologist. But even supposing the palæontologist had a -period of sufficient length measured off correctly to begin with, his -results would still be unsatisfactory; for it is perfectly obvious, -that unless the climatic conditions of the globe during the various -periods were nearly the same, the rate at which the species change -would certainly not be uniform; but such has not been the case, as an -examination of the Tables of eccentricity will show. Take, for example, -that long epoch of 260,000 years, beginning about 980,000 years ago -and terminating about 720,000 years ago. During that long period the -changes from cold to warm conditions of climate every 10,000 or 12,000 -years must have been of the most extreme character. Compare that -period with the period beginning, say, 80,000 years ago, and extending -to nearly 150,000 years into the future, during which there will be -no extreme variations of climate, and how great is the contrast! How -extensive the changes in species must have been during the first period -as compared with those which are likely to take place during the latter! - -Besides, it must also be taken into consideration that organization was -of a far more simple type in the earlier Palæozoic ages than during the -Tertiary period, and would probably on this account change much more -slowly in the former than in the latter. - -The foregoing considerations render it highly probable, if not -certain, that the rate at which the general surface of the globe is -being lowered by subaërial denudation cannot be much under one foot -in 6,000 years. How, if we assign the glacial epoch to that period of -high eccentricity beginning 980,000 years ago, and terminating 720,000 -years ago, then we must conclude that as much as 120 feet must have -been denuded off the face of the country since the close of the glacial -epoch. But if as much as this had been carried down by our rivers into -the sea, hardly a patch of boulder clay, or any trace of the glacial -epoch, should be now remaining on the land. It is therefore evident -that the glacial epoch cannot be assigned to that remote period, but -ought to be referred to the period terminating about 80,000 years ago. -We have, in this latter case, 13 feet, equal to about 18 feet of drift, -as the amount removed from the general surface of the country since -the glacial epoch. This amount harmonizes very well with the direct -evidence of geology on this point. Had the amount of denudation since -the close of the glacial epoch been much greater than this, the drift -deposits would not only have been far less complete, but the general -appearance and outline of the surface of all glaciated countries would -have been very different from what they really are. - -_Circumstances which show the Recent Date of the Glacial Epoch._—One -of the circumstances to which I refer is this. When we examine the -surface of any glaciated country, such as Scotland, we can easily -satisfy ourselves that the upper surface of the ground differs very -much from what it would have been had its external features been due -to the action of rain and rivers and the ordinary agencies which have -been at work since the close of the Ice period. Go where one will in -the Lowlands of Scotland, and he shall hardly find a single acre whose -upper surface bears the marks of being formed by the denuding agents -which are presently in operation. He will observe everywhere mounds -and hollows, the existence of which cannot be accounted for by the -present agencies at work. In fact these agencies are slowly denuding -pre-existing heights and silting up pre-existing hollows. Everywhere -one comes upon patches of alluvium which upon examination prove to be -simply old glacially formed hollows silted up. True, the main rivers, -streams, and even brooks, occupy channels which have been formed by -running water, either since or prior to the glacial epoch, but, in -regard to the general surface of the country, the present agencies may -be said to be just beginning to carve a new line of features out of -the old glacially formed surface. But so little progress has yet been -made, that the kames, gravel mounds, knolls of boulder clay, &c., still -retain in most cases their original form. Now, when we reflect that -more than a foot of drift is being removed from the general surface of -the country every 5,000 years or so, it becomes perfectly obvious that -the close of the glacial epoch must be of comparatively recent date. - -There is another circumstance which shows that the glacial epoch must -be referred to the latest period of great eccentricity. If we refer the -glacial epoch to the penultimate period of extreme eccentricity, and -place its commencement one million of years back, then we must also -lengthen out to a corresponding extent the entire geological history -of the globe. Sir Charles Lyell, who is inclined to assign the glacial -epoch to this penultimate period, considers that when we go back as far -as the Lower Miocene formations, we arrive at a period when the marine -shells differed as a whole from those now existing. But only 5 per -cent. of the shells existing at the commencement of the glacial epoch -have since died out. Hence, assuming the rate at which the species -change to be uniform, it follows that the Lower Miocene period must -be twenty times as remote as the commencement of the glacial epoch. -Consequently, if it be one million of years since the commencement -of the glacial epoch, 20 millions of years, Sir Charles concludes, -must have elapsed since the time of the Lower Miocene period, and -60 millions of years since the beginning of the Eocene period, and -about 160 millions of years since the Carboniferous period, and about -240 millions of years must be the time which has elapsed since the -beginning of the Cambrian period. But, on the other hand, if we refer -the glacial epoch to the latest period of great eccentricity, and take -250,000 years ago as the beginning of that period, then, according -to the same mode of calculation, we have 15 millions of years since -the beginning of the Eocene period, and 40 millions of years since -the Carboniferous period, and 60 millions of years in all since the -beginning of the Cambrian period. - -If the beginning of the glacial epoch be carried back a million years, -then it is probable, as Sir Charles Lyell concludes, that the beginning -of the Cambrian period will require to be placed 240 millions of years -back. But it is very probable that the length of time embraced by the -pre-Cambrian ages of geological history may be as great as that which -has elapsed since the close of the Cambrian period, and, if this be -so, then we shall be compelled to admit that nearly 500 millions of -years have passed away since the beginning of the earth’s geological -history. But we have evidence of a physical nature which proves that it -is absolutely impossible that the existing order of things, as regards -our globe, can date so far back as anything like 500 millions of years. -The arguments to which I refer are those which have been advanced by -Professor Sir William Thomson at various times. These arguments are -well known, and to all who have really given due attention to them must -be felt to be conclusive. It would be superfluous to state them here; I -shall, however, for reasons which will presently appear, refer briefly -to one of them, and that one which seems to be the most conclusive of -all, viz., the argument derived from the limit to the age of the sun’s -heat. - -_Professor Ramsay on Geological Time._—In an interesting suggestive -memoir, “On Geological Ages as items of Geological Time,”[200] -Professor Ramsay discusses the comparative values of certain groups of -formations as representative of geological time, and arrives at the -following general conclusion, viz., “That the local continental era -which began with the Old Red Sandstone and closed with the New Red Marl -is comparable, in point of geological time, to that occupied in the -deposition of the whole of the Mesozoic, or Secondary series, later -than the New Red Marl and all the Cainozoic or Tertiary formations, -and indeed of all the time that has elapsed since the beginning of -the deposition of the Lias down to the present day.” This conclusion -is derived partly from a comparison of the physical character of -the formations constituting each group, but principally from the -zoological changes which took place during the time represented by them. - -The earlier period represented by the Cambrian and Silurian rocks he -also, from the same considerations, considers to have been very long, -but he does not attempt to fix its relative length. Of the absolute -length of any or all of these great eras of geological time no -estimate or guess is given. He believes, however, that the whole time -represented by all the fossiliferous rocks, from the earliest Cambrian -to the most recent, is, geologically speaking, short compared with that -which went before it. After quoting Professor Huxley’s enumeration of -the many classes and orders of marine life (identical with those still -existing), whose remains characterize the lowest Cambrian rocks, he -says, “The inference is obvious that in this earliest known varied -life we find no evidence of its having lived near the beginning of -the zoological series. In a broad sense, compared with what must have -gone before, both biologically and physically, all the phenomena -connected with this old period seem to my mind to be quite of a recent -description, and the climates of seas and lands were of the very same -kind as those that the world enjoys at the present day.”... “In the -words of Darwin, when discussing the imperfection of the geological -record of this history, ‘we possess the last volume alone relating -only to two or three countries,’ and the reason why we know so little -of pre-Cambrian faunas and the physical characters of the more ancient -formations as originally deposited, is that below the Cambrian strata -we get at once involved in a sort of chaos of metamorphic strata.’” - -It seems to me that Professor Ramsay’s results lead to the same -conclusion regarding the _positive_ length of geological periods as -those derived from physical considerations. It is true that his views -lead us back to an immense lapse of unknown time prior to the Cambrian -period, but this practically tends to shorten geological periods. For -it is evident that the geological history of our globe must be limited -by the age of the sun’s heat, no matter how long or short its age may -be. This being the case, the greater the length of time which must -have elapsed prior to the Cambrian period, the less must be the time -which has elapsed since that period. Whatever is added to the one -period must be so much taken from the other. Consequently, the longer -we suppose the pre-Cambrian periods to have been, the shorter must we -suppose the post-Cambrian to be. - - - - - CHAPTER XXI. - - THE PROBABLE AGE AND ORIGIN OF THE SUN. - - Gravitation Theory.—Amount of Heat emitted by the Sun.—Meteoric - Theory.—Helmholtz’s Condensation Theory.—Confusion of - Ideas.—Gravitation not the chief Source of the Sun’s - Heat.—Original Heat.—Source of Original Heat.—Original Heat - derived from Motion in Space.—Conclusion as to Date of - Glacial Epoch.—False Analogy.—Probable Date of Eocene and - Miocene Periods. - - -_Gravitation Theory of the Origin and Source of the Sun’s Heat._—There -are two forms in which this theory has been presented: the first, the -meteoric theory, propounded by Dr. Meyer, of Heilbronn; and the second, -the contraction theory, advocated by Helmholtz. - -It is found that 83·4 foot-pounds of heat per second are incident upon -a square foot of the earth’s surface exposed to the perpendicular rays -of the sun. The amount radiated from a square foot of the sun’s surface -is to that incident on a square foot of the earth’s surface as the -square of the sun’s distance to the square of his radius, or as 46,400 -to 1. Consequently 3,869,000 foot-pounds of heat are radiated off every -square foot of the sun’s surface per second—an amount equal to about -7,000 horse power. The total amount radiated from the whole surface -of the sun per annum is 8,340 × 10^{30} foot-pounds. To maintain the -present rate of radiation, it would require the combustion of about -1,500 lbs. of coal per hour on every square foot of the sun’s surface; -and were the sun composed of that material, it would be all consumed in -less than 5,000 years. The opinion that the sun’s heat is maintained -by combustion cannot be entertained for a single moment. A pound of -coal falling into the sun from an infinite distance would produce by -its concussion more than 6,000 times the amount of heat that would be -generated by its combustion. - -It is well known that the velocity with which a body falling from an -infinite distance would reach the sun would be equal to that which -would be generated by a constant force equal to the weight of the body -at the sun’s surface operating through a space equal to the sun’s -radius. One pound would at the sun’s surface weigh about 28 pounds. -Taking the sun’s radius at 441,000 miles,[201] the energy of a pound -of matter falling into the sun from infinite space would equal that -of a 28-pound weight descending upon the earth from an elevation of -441,000 miles, supposing the force of gravity to be as great at that -elevation as it is at the earth’s surface. It would amount to upwards -of 65,000,000,000 foot-pounds. A better idea of this enormous amount -of energy exerted by a one-pound weight falling into the sun will be -conveyed by stating that it would be sufficient to raise 1,000 tons to -a height of 5½ miles. It would project the _Warrior_, fully equipped -with guns, stores, and ammunition, over the top of Ben Nevis. - -Gravitation is now generally admitted to be the only conceivable -source of the sun’s heat. But if we attribute the energy of the sun to -gravitation as a source, we assign it to a cause the value of which can -be accurately determined. Prodigious as is the energy of a single pound -of matter falling into the sun, nevertheless a range of mountains, -consisting of 176 cubic miles of solid rock, falling into the sun, -would maintain his heat for only a single second. A mass equal to that -of the earth would maintain the heat for only 93 years, and a mass -equal to that of the sun itself falling into the sun would afford but -33,000,000 years’ sun-heat. - -It is quite possible, however, that a meteor may reach the sun with a -velocity far greater than that which it could acquire by gravitation; -for it might have been moving in a direct line towards the sun with -an original velocity before coming under the sensible influence of -the sun’s attraction. In this case a greater amount of heat would -be generated by the meteor than would have resulted from its merely -falling into the sun under the influence of gravitation. But then -meteors of this sort must be of rare occurrence. The meteoric theory -of the sun’s heat has now been pretty generally abandoned for the -contraction theory advanced by Helmholtz. - -Suppose, with Helmholtz, that the sun originally existed as a nebulous -mass, filling the entire space presently occupied by the solar system -and extending into space indefinitely beyond the outermost planet. The -total amount of work in foot-pounds performed by gravitation in the -condensation of this mass to an orb of the sun’s present size can be -found by means of the following formula given by Helmholtz,[202] - - 3 _r_^{2}M^{2} - Work of condensation = — × ———————————— × _g_ - 5 R_m_ - -M is the mass of the sun, _m_ the mass of the earth, R the sun’s -radius, and _r_ the earth’s radius. Taking M = 4230 × 10^{27} lbs., -_m_ = 11,920 × 10^{21} lbs., R = 2,328,500,000 feet, and _r_ = -20,889,272 feet; we have then for the total amount of work performed by -gravitation in foot-pounds, - - 3 (20,889,272·5)^2 × (4230 × 10^{27})^2 - Work = — × ————————————————————————————————————— - 5 2,328,500,000 × 11,920 × 10^{21} - - = 168,790 × 10^{36} foot-pounds. - - -The amount of heat thus produced by gravitation would suffice for -nearly 20,237,500 years. - -These calculations are based upon the assumption that the density of -the sun is uniform throughout. But it is highly probable that the sun’s -density increases towards the centre, in which case the amount of work -performed by gravitation would be somewhat more than the above. - -Some confusion has arisen in reference to this subject by the -introduction of the question of the amount of the sun’s specific heat. -If we simply consider the sun as an incandescent body in the process -of cooling, the question of the amount of the sun’s specific heat is -of the utmost importance; because the absolute amount of heat which -the sun is capable of giving out depends wholly upon his temperature -and specific heat. In this case three things only are required: (1), -the sun’s mass; (2), temperature of the mass; (3), specific heat of -the mass. But if we are considering what is the absolute amount of -heat which could have been given out by the sun on the hypothesis that -gravitation, either according to the meteoric theory suggested by Meyer -or according to the contraction theory advocated by Helmholtz, is the -only source of his heat, then we have nothing whatever to do with any -inquiries regarding the specific heat of the sun. This is evident -because the absolute amount of work which gravitation can perform in -the pulling of the particles of the sun’s mass together, is wholly -independent of the specific heat of those particles. Consequently, the -amount of energy in the form of heat thus imparted to the particles -by gravity must also be wholly independent of specific heat. That is -to say, the amount of heat imparted to a particle will be the same -whatever may be its specific heat. - -Even supposing we limit the geological history of our globe to 100 -millions of years, it is nevertheless evident that gravitation will not -account for the supply of the sun’s heat during so long a period. There -must be some other source of much more importance than gravitation. -What other source of energy greater than that of gravitation can there -be? It is singular that the opinion should have become so common even -among physicists, that there is no other conceivable source than -gravitation from which a greater amount of heat could have been derived. - -_The Origin and Chief Source of the Sun’s Heat._—According to the -foregoing theories regarding the source of the sun’s heat, it is -assumed that the matter composing the sun, when it existed in space as -a nebulous mass, was not originally possessed of temperature, but that -the temperature was given to it as the mass became condensed under the -force of gravitation. It is supposed that the heat given out was simply -the heat of condensation. But it is quite conceivable that the nebulous -mass might have been possessed of an original store of heat previous to -condensation. - -It is quite possible that the very reason why it existed in such a -rarefied or gaseous condition was its excessive temperature, and that -condensation only began to take place when the mass began to cool down. -It seems far more probable that this should have been the case than -that the mass existed in so rarefied a condition without temperature. -For why should the particles have existed in this separated form when -devoid of the repulsive energy of heat, seeing that in virtue of -gravitation they had such a tendency to approach to one another? But -if the mass was originally in a heated condition, then in condensing -it would have to part not only with the heat generated in condensing, -but also with the heat which it originally possessed, a quantity -which would no doubt much exceed that produced by condensation. To -illustrate this principle, let us suppose a pound of air, for example, -to be placed in a cylinder and heat applied to it. If the piston be so -fixed that it cannot move, 234·5 foot-pounds of heat will raise the -temperature of the air 1° C. But if the piston be allowed to rise as -the heat is applied, then it will require 330·2 foot-pounds of heat to -raise the temperature 1° C. It requires 95·7 foot-pounds more heat in -the latter case than in the former. The same amount of energy, viz., -234·5 foot-pounds, in both cases goes to produce temperature; but in -the latter case, where the piston is allowed to move, 95·7 foot-pounds -of additional heat are consumed in the mechanical work of raising the -piston. Suppose, now, that the air is allowed to cool under the same -conditions: in the one case 234·5 foot-pounds of heat will be given -out while the temperature of the air sinks 1° C.; in the other case, -where the piston is allowed to descend, 330·2 foot-pounds will be given -out while the temperature sinks 1° C. In the former case, the air in -cooling has simply to part with the energy which it possesses in -the form of temperature; but in the latter case it has, in addition -to this, to part with the energy bestowed upon its molecules by the -descending piston. While the temperature of the gas is sinking 1°, -95·7 foot-pounds of energy in the form of heat are being imparted to -it by the descending piston; and these have to be got rid of before -the temperature is lowered by 1°. Consequently 234·5 foot-pounds of -the heat given out previously existed in the air under the form of -temperature, and the remaining 95·7 foot-pounds given out were imparted -to the air by the descending piston while the gas was losing its -temperature. 234·5 foot-pounds represent the energy or heat which the -air previously possessed, and 95·7 the energy or heat of condensation. - -In the case of the cooling of the sun from a nebulous mass, there -would of course be no external force or pressure exerted on the mass -analogous to that of the piston on the air; but there would be, what -is equivalent to the same, the gravitation of the particles to each -other. There would be the pressure of the whole mass towards the centre -of convergence. In the case of air, and all perfect gases cooling -under pressure, about 234 foot-pounds of the original heat possessed -by the gas are given out while 95 foot-pounds are being generated by -condensation. We have, however, no reason whatever to believe that in -the case of the cooling of the sun the same proportions would hold -true. The proportion of original heat possessed by the mass of the sun -to that produced by condensation may have been much greater than 234 to -95, or it may have been much less. In the absence of all knowledge on -this point, we may in the meantime assume that to be the proportion. -The total quantity of heat given out by the sun resulting from the -condensation of his mass, on the supposition that the density of the -sun is uniform throughout, we have seen to be equal to 20,237,500 -years’ sun-heat. Then the quantity of heat given out, which previously -existed in the mass as original temperature, must have been 49,850,000 -years’ heat, making in all 70,087,500 years’ heat as the total amount. - -The above quantity represents, of course, the total amount of heat -given out by the mass since it began to condense. But the geological -history of our globe must date its beginning at a period posterior to -that. For at that time the mass would probably occupy a much greater -amount of space than is presently possessed by the entire solar system; -and consequently, before it had cooled down to within the limits of -the earth’s present orbit, our earth could not have had an existence -as a separate planet. Previously to that time it must have existed as -a portion of the sun’s fiery mass. If we assume that it existed as a -globe previously to that, and came in from space after the condensation -of the sun, then it is difficult to conceive how its orbit should be so -nearly circular as it is at present. - -Let us assume that by the time that the mass of the sun had condensed -to within the space encircled by the orbit of the planet Mercury (that -is, to a sphere having, say, a radius of 18,000,000 miles) the earth’s -crust began to form; and let this be the time when the geological -history of our globe dates its commencement. The total amount of heat -generated by the condensation of the sun’s mass from a sphere of this -size to its present volume would equal 19,740,000 years’ sun-heat. -The amount of original heat given out during that time would equal -48,625,000 years’ sun-heat,—thus giving a total of 68,365,000 years’ -sun-heat enjoyed by our globe since that period. The total quantity may -possibly, of course, be considerably more than that, owing to the fact -that the sun’s density may increase greatly towards his centre. But we -should require to make extravagant assumptions regarding the interior -density of the sun and the proportion of original heat to that produced -by condensation before we could manage to account for anything like the -period that geological phenomena are supposed by some to demand. - -The question now arises, by what conceivable means could the mass of -the sun have become possessed of such a prodigious amount of energy -in the form of heat previous to condensation? What power could have -communicated to the mass 50,000,000 years’ heat before condensation -began to take place? - -_The Sun’s Energy may have originally been derived from Motion in -Space._—There is nothing at all absurd or improbable in the supposition -that such an amount of energy might have been communicated to the -mass. The Dynamical Theory of Heat affords an easy explanation of at -least _how_ such an amount of energy _may_ have been communicated. Two -bodies, each one-half the mass of the sun, moving directly towards -each other with a velocity of 476 miles per second, would by their -concussion generate in a _single moment_ the 50,000,000 years’ heat. -For two bodies of that mass moving with a velocity of 476 miles per -second would possess 4149 × 10^{38} foot-pounds of energy in the form -of _vis viva_; and this, converted into heat by the stoppage of their -motion, would give an amount of heat which would cover the present rate -of the sun’s radiation, for a period of 50,000,000 years. - -Why may not the sun have been composed of two such bodies? And why may -not the original store of heat possessed by him have all been derived -from the concussion of these two bodies? Two such bodies coming into -collision with that velocity would be dissipated into vapour by such -an inconceivable amount of heat as would thus be generated; and when -they condensed on cooling, they would form one spherical mass like the -sun. It is perfectly true that two such bodies could never attain the -required amount of velocity by their mutual gravitation towards each -other. But there is no necessity whatever for supposing that their -velocities were derived from their mutual attraction alone. They might -have been approaching towards each other with the required velocity -wholly independent of gravitation. - -We know nothing whatever regarding the absolute motion of bodies in -space. And beyond the limited sphere of our observation, we know -nothing even of their relative motions. There may be bodies moving -in relation to our system with inconceivable velocity. For anything -that we know to the contrary, were one of these bodies to strike our -earth, the shock might be sufficient to generate an amount of heat that -would dissipate the earth into vapour, though the striking body might -not be heavier than a cannon-ball. There is, however, nothing very -extraordinary in the velocity which we have found would be required -in the two supposed bodies to generate the 50,000,000 years’ heat. A -comet, having an orbit extending to the path of the planet Neptune, -approaching so near the sun as to almost graze his surface in passing, -would have a velocity of about 390 miles per second, which is within 86 -miles of the required velocity. - -But in the original heating and expansion of the sun into a gaseous -mass, an amount of work must have been performed against gravitation -equal to that which has been performed by gravitation during his -cooling and condensation, a quantity which we have found amounts to -about 20,000,000 years’ heat. The total amount of energy originally -communicated by the concussion must have been equal to 70,000,000 -years’ sun-heat. A velocity of 563 miles per second would give this -amount. It must be borne in mind, however, that the 563 miles per -second is the velocity at the moment of collision; about one-half of -this velocity would be derived from the mutual attraction of the two -bodies in their approach to each other. Suppose each body to be equal -in volume to the sun, and of course one-half the density, the amount -of velocity which they would acquire by their mutual attraction would -be 274 miles per second, consequently we have to assume an original or -projected velocity of only 289 miles per second. - -If we admit that gravitation is not sufficient to account for the -amount of heat given out by the sun during the geological history of -our globe, we are compelled to assume that the mass of which the sun is -composed existed prior to condensation in a heated condition; and if -so, we are further obliged to admit that the mass must have received -its heat from some source or other. And as the dissipation of heat into -space must have been going on, in all probability, as rapidly before -as after condensation took place, we are further obliged to conclude -that the heat must have been communicated to the mass immediately -before condensation began, for the moment the mass began to lose its -heat condensation would ensue. If we confine our speculations to causes -and agencies known to exist, the cause which has been assigned appears -to be the only conceivable one that will account for the production of -such an enormous amount of heat. - -The general conclusion to which we are therefore led from physical -considerations regarding the age of the sun’s heat is, that the entire -geological history of our globe must be comprised within less than -100 millions of years, and that consequently the commencement of the -glacial epoch cannot date much farther back than 240,000 years. - -The facts of geology, more especially those in connection with -denudation, seem to geologists to require a period of much longer -duration than 100 millions of years, and it is this which has so -long prevented them accepting the conclusions of physical science in -regard to the age of our globe. But the method of measuring subaërial -denudation already detailed seems to me to show convincingly that the -geological data, when properly interpreted, are in perfect accord with -the deductions of physical science. Perhaps there are now few who -have fairly considered the question who will refuse to admit that 100 -millions of years are amply sufficient to comprise the whole geological -history of our globe. - -_A false Analogy supposed to exist between Astronomy and -Geology._—Perhaps one of the things which has tended to mislead on -this point is a false analogy which is supposed to subsist between -astronomy and geology, viz., that geology deals with unlimited _time_, -as astronomy deals with unlimited _space_. A little consideration, -however, will show that there is not much analogy between the two cases. - -Astronomy deals with the countless worlds which lie spread out in the -boundless infinity of space; but geology deals with only one world. -No doubt reason and analogy both favour the idea that the age of the -material universe, like its magnitude, is immeasurable; we have no -reason, however, to conclude that it is eternal, any more than we -have to infer that it is infinite. But when we compare the age of the -material universe with its magnitude, we must not take the age of one -of its members (say, our globe) and compare it with the size of the -universe. Neither must we compare the age of all the presently existing -systems of worlds with the magnitude of the universe; but we must -compare the past history of the universe as it stretches back into the -immensity of bygone _time_, with the presently existing universe as it -stretches out on all sides into limitless _space_. For worlds precede -worlds in time as worlds lie beyond worlds in space. Each world, -each individual, each atom is evidently working out a final purpose, -according to a plan prearranged and predetermined by the Divine Mind -from all eternity. And each world, like each individual, when it -serves the end for which it was called into existence, disappears to -make room for others. This is the grand conception of the universe -which naturally impresses itself on every thoughtful mind that has not -got into confusion about those things called in science the Laws of -Nature.[203] - -But the geologist does not pass back from world to world as they stand -related to each other in the order of _succession in time_, as the -astronomer passes from world to world as they stand related to each -other in the order of _coexistence in space_. The researches of the -geologist, moreover, are not only confined to one world, but it is only -a portion of the history of that one world that can come under his -observation. The oldest of existing formations, so far as is yet known, -the Laurentian Gneiss, is made up of the waste of previously existing -rocks, and it, again, has probably been derived from the degradation -of rocks belonging to some still older period. Regarding what succeeds -these old Laurentian rocks geology tells us much; but of the formations -that preceded, we know nothing whatever. For anything that geology -shows to the contrary, the time which may have elapsed from the -solidifying of the earth’s crust to the deposition of the Laurentian -strata—an absolute blank—may have been as great as the time that has -since intervened. - -_Probable Date of the Eocene and Miocene Periods._—If we take into -consideration the limit which physical science assigns to the age of -our globe, and the rapid rate at which, as we have seen, denudation -takes place, it becomes evident that the enormous period of 3 millions -of years comprehended in the foregoing tables must stretch far back -into the Tertiary age. Supposing that the mean rate of denudation -during that period was not greater than the present rate of denudation, -still we should have no less than 500 feet of rock worn off the face of -the country and carried into the sea during these 3 millions of years. -This fact shows how totally different the appearance and configuration -of the country in all probability was at the commencement of this -period from what it is at the present day. If it be correct that the -glacial epoch resulted from the causes which we have already discussed, -those tables ought to aid us in our endeavour to ascertain _how_ much -of the Tertiary period may be comprehended within these 3 millions of -years. - -We have already seen (Chapter XVIII.) that there is evidence of a -glacial condition of climate at two different periods during the -Tertiary age, namely, about the middle of the Miocene and Eocene -periods respectively. As has already been shown, the more severe a -glacial epoch is, the more marked ought to be the character of its warm -inter-glacial periods; the greater the extension of the ice during the -cold periods of a glacial epoch the further should that ice disappear -in arctic regions during the corresponding warm periods. Thus the -severity of a glacial epoch may in this case be indirectly inferred -from the character of the warm periods and the extent to which the -ice may have disappeared from arctic regions. Judged by this test, we -have every reason to believe that the Miocene glacial epoch was one of -extreme severity. - -The Eocene conglomerate, devoid of all organic remains, and containing -numerous enormous ice-transported blocks, is, as we have seen, -immediately associated with nummulitic strata charged with fossils -characteristic of a warm climate. Referring to this Sir Charles Lyell -says, “To imagine icebergs carrying such huge fragments of stone in so -southern a latitude, and at a period immediately preceded and followed -by the signs of a warm climate, is one of the most perplexing enigmas -which the geologist has yet been called upon to solve.”[204] - -It is perfectly true that, according to the generally received theories -of the cause of a glacial climate the whole is a perplexing enigma, but -if we adopt the Secular theory of change of climate, every difficulty -disappears. According to this theory the very fact of the conglomerate -being formed at a period immediately preceded and succeeded by warm -conditions of climate, is of itself strong presumptive evidence of the -conglomerate being a glacial formation. But this is not all, the very -highness of the temperature of the preceding and succeeding periods -bears testimony to the severity of the intervening glacial period. -Despite the deficiency of direct evidence regarding the character of -the Miocene and Eocene glacial periods, we are not warranted, for -reasons which have been stated in Chapter XVII., to conclude that these -periods were less severe than the one which happened in Quaternary -times. Judging from indirect evidence, we have some grounds for -concluding that the Miocene glacial epoch at least was even more severe -and protracted than our recent glacial epoch. - -By referring to Table I., or the accompanying diagram, it will be seen -that prior to the period which I have assigned as that of the glacial -epoch, there are two periods when the eccentricity almost attained -its superior limit. The first period occurred 2,500,000 years ago, -when it reached 0·0721, and the second period 850,000 years ago, when -it attained a still higher value, viz., 0·0747, being within 0·0028 -of the superior limit. To the first of these periods I am disposed -to assign the glacial epoch of Eocene times, and to the second that -of the Miocene age. With the view of determining the character of -these periods Tables II. and III. have been computed. They give the -eccentricity and longitude of perihelion at intervals of 10,000 years. -It will be seen from Table II. that the Eocene period extends from -about 2,620,000 to about 2,460,000 years ago; and from Table III. it -will be gathered that the Miocene period lasted from about 980,000 to -about 720,000 years ago. - -In order to find whether the eccentricity attained a higher value about -850,000 years ago than 0·0747, I computed the values for one or two -periods immediately before and after that period, and satisfied myself -that the value stated was indeed the highest, as will be seen from the -subjoined table:— - - 851,000 0·07454 - 850,000 0·074664 - 849,500 0·07466 - 849,000 0·07466 - -How totally different must have been the condition of the earth’s -climate at that period from what it is at present! Taking the mean -distance of the sun to be 91,400,000 miles, his present distance at -midwinter is 89,864,480 miles; but at the period in question, when the -winter solstice was in perihelion, his distance at midwinter would be -no less than 98,224,289 miles. But this is not all; our winters are at -present shorter than our summers by 7·8 days, but at that period they -would be longer than the summers by 34·7 days. - -At present the difference between the perihelion and aphelion distance -of the sun amounts to only 3,069,580 miles, but at the period under -consideration it would amount to no less than 13,648,579 miles! - - - - - CHAPTER XXII. - - A METHOD OF DETERMINING THE MEAN THICKNESS OF THE SEDIMENTARY - ROCKS OF THE GLOBE. - - Prevailing Methods defective.—Maximum Thickness of British - Rocks.—Three Elements in the Question.—Professor Huxley - on the Rate of Deposition.—Thickness of Sedimentary Rocks - enormously over-estimated.—Observed Thickness no Measure of - mean Thickness.—Deposition of Sediment principally along - Sea-margin.—Mistaken Inference regarding the Absence of a - Formation.—Immense Antiquity of existing Oceans. - - -Various attempts have been made to measure the positive length of -geological periods. Some geologists have sought to determine, roughly, -the age of the stratified rocks by calculations based upon their -probable thickness and the rate at which they may have been deposited. -This method, however, is worthless, because the rates which have been -adopted are purely arbitrary. One geologist will take the rate of -deposit at a foot in a hundred years, while another will assume it -to be a foot in a thousand or perhaps ten thousand years; and, for -any reasons that have been assigned, the one rate is just as likely -to be correct as the other: for if we examine what is taking place -in the ocean-bed at the present day, we shall find in some places a -foot of sediment laid down in a year, while in other places a foot -may not be deposited in a thousand years. The stratified rocks were -evidently formed at all possible rates. When we speak of the rate of -their formation, we must of course refer to the _mean rate_; and it is -perfectly true that if we knew the thickness of these rocks and the -mean rate at which they were deposited, we should have a ready means -of determining their positive age. But there appears to be nearly as -great uncertainty regarding the thickness of the sedimentary rocks as -regarding the rate at which they were formed. No doubt we can roughly -estimate their probable maximum thickness; for instance, Professor -Ramsay has found from actual measurement, that the sedimentary -formations of Great Britain have a maximum thickness of upwards of -72,000 feet; but all such measurements give us no idea of their mean -thickness. What is the mean thickness of the sedimentary rocks of -the globe? On this point geology does not afford a definite answer. -Whatever the present mean thickness of the sedimentary rocks of our -globe may be, it must be small in comparison to the mean thickness -of all the sedimentary rocks which have been formed. This is obvious -from the fact that the sedimentary rocks of one age are partly formed -from the destruction of the sedimentary rocks of former ages. From the -Laurentian age down to the present day, the stratified rocks have been -undergoing constant denudation. - -Unless we take into consideration the quantity of rock removed during -past ages by denudation, we cannot—even though we knew the actual mean -thickness of the existing sedimentary rocks of the globe, and the rate -at which they were formed—arrive at an estimate regarding the length of -time represented by these rocks. For if we are to determine the age of -the stratified rocks from the rate at which they were formed, we must -have, not the present quantity of sedimentary rocks, but the present -plus the quantity which has been denuded during past ages. In other -words, we must have the absolute quantity formed. In many places the -missing beds must have been of enormous thickness. The time represented -by beds which have disappeared is, doubtless, as already remarked, -much greater than that represented by the beds which now remain. The -greater mass of the sedimentary rocks has been formed out of previously -existing sedimentary rocks, and these again out of sedimentary rocks -still older. As the materials composing our stratified beds may have -passed through many cycles of destruction and re-formation, the time -required to have deposited at a given rate the present existing mass -of sedimentary rocks may be but a fraction of the time required to -have deposited at the same rate the total mass that has actually been -formed. To measure the age of the sedimentary rocks by the present -existing rocks, assumed to be formed at some given rate, even supposing -the rate to be correct, is a method wholly fallacious. - -“The aggregate of sedimentary strata in the earth’s crust,” says Sir -Charles Lyell, “can never exceed in volume the amount of solid matter -which has been ground down and washed away by rivers, waves, and -currents. How vast, then, must be the spaces which this abstraction -of matter has left vacant! How far exceeding in dimensions all the -valleys, however numerous, and the hollows, however vast, which we can -prove to have been cleared out by aqueous erosion!”[205] - -I presume there are few geologists who would not admit that if all the -rocks which have in past ages been removed by denudation were restored, -the mean thickness of the sedimentary rocks of the globe would be at -least equal to their present maximum thickness, which we may take at -72,000 feet. - -There are three elements in the question; of which if two are known, -the third is known in terms of the other two. If we have the mean -thickness of all the sedimentary rocks which have been formed and the -mean rate of formation, then we have the time which elapsed during the -formation; or having the thickness and the time, we have the rate; or, -having the rate and the time, we have the thickness. - -One of these three, namely, the rate, can, however, be determined with -tolerable accuracy if we are simply allowed to assume—what is very -probable, as has already been shown—that the present rate at which the -sedimentary deposits are being formed may be taken as the mean rate -for past ages. If we know the rate at which the land is being denuded, -then we know with perfect accuracy the rate at which the sedimentary -deposits are being formed in the ocean. This is obvious, because all -the materials denuded from the land are deposited in the sea; and -what is deposited in the sea is just what comes off the land, with the -exception of the small proportion of calcareous matter which may not -have been derived from the land, and which in our rough estimate may be -left out of account. - -Now the mean rate of subaërial denudation, we have seen, is about one -foot in 6,000 years. Taking the proportion of land to that of water -at 576 to 1,390, then one foot taken off the land and spread over the -sea-bottom would form a layer 5 inches thick. Consequently, if one foot -in 6,000 years represents the mean rate at which the land is being -denuded, one foot in 14,400 years represents the mean rate at which the -sedimentary rocks are being formed. - -Assuming, as before, that 72,000 feet would represent the mean -thickness of all the sedimentary rocks which have ever been formed, -this, at the rate of one foot in 14,400 years, gives 1,036,800,000 -years as the age of the stratified rocks. - -Professor Huxley, in his endeavour to show that 100,000,000 years is -a period sufficiently long for all the demands of geologists, takes -the thickness of the stratified rocks at 100,000 feet, and the rate -of deposit at a foot in 1,000 years. One foot of rock per 1,000 years -gives, it is true, 100,000 feet in 100,000,000 years. But what about -the rocks which have disappeared? If it takes a hundred millions of -years to produce a mass of rock equal to that which now exists, how -many hundreds of millions of years will it require to produce a mass -equal to what has actually been produced? - -Professor Huxley adds, “I do not know that any one is prepared to -maintain that the stratified rocks may not have been formed on the -average at the rate of 1/83rd of an inch per annum.” When the rate, -however, is accurately determined, it is found to be, not 1/83rd of -an inch per annum, but only 1/1200th of an inch, so that the 100,000 -feet of rock must have taken 1,440,000,000 years in its formation,—a -conclusion which, according to the results of modern physics, is wholly -inadmissible. - -Either the thickness of the sedimentary rocks has been over-estimated, -or the rate of their formation has been under-estimated, or both. -If it be maintained that a foot in 14,400 years is too slow a rate -of deposit, then it must be maintained that the land must have been -denuded at a greater rate than one foot in 6,000 years. But most -geologists probably felt surprised when the announcement was first -made, that at this rate of denudation the whole existing land of the -globe would be brought under the ocean in 6,000,000 of years. - -The error, no doubt, consists in over-estimating the thickness of the -sedimentary rocks. Assuming, for physical reasons already stated, that -100,000,000 years limits the age of the stratified rocks, and that the -proportion of land to water and the rate of denudation have been on the -average the same as at present, the mean thickness of sedimentary rocks -formed in the 100,000,000 years amounts to only 7,000 feet. - -But be it observed that this is the mean thickness on an area equal -to that of the ocean. Over the area of the globe it amounts to only -5,000 feet; and this, let it be observed also, is the total mean -thickness formed, without taking into account what has been removed -by denudation. If we wish to ascertain what is actually the present -mean thickness, we must deduct from this 5,000 feet an amount of rock -equal to all the sedimentary rocks which have been denuded during -the 100,000,000 years; for the 5,000 feet is not the present mean -thickness, but the total mean thickness formed during the whole of the -100,000,000 years. If we assume, what no doubt most geologists would be -willing to grant, that the quantity of sedimentary rocks now remaining -is not over one-half of what has been actually deposited during the -history of the globe, then the actual mean thickness of the stratified -rocks of the globe is not over 2,500 feet. This startling result would -almost necessitate us to suspect that the rate of subaërial denudation -is probably greater than one foot in 6,000 years. But, be this as it -may, we are apt, in estimating the mean thickness of the stratified -rocks of the globe from their ascertained maximum thickness, to arrive -at erroneous conclusions. There are considerations which show that -the mean thickness of these rocks must be small in proportion to their -maximum thickness. The stratified rocks are formed from the sediment -carried down by rivers and streamlets and deposited in the sea. It is -obvious that the greater quantity of this sediment is deposited near -the mouths of rivers, and along a narrow margin extending to no great -distance from the land. Did the land consist of numerous small islands -equally distributed over the globe, the sediment carried off from these -islands would be spread pretty equally over the sea-bottom. But the -greater part of the land-surface consists of two immense continents. -Consequently, the materials removed by denudation are not spread -over the ocean-bottom, but on a narrow fringe surrounding those two -continents. Were the materials spread over the entire ocean-bed, a foot -removed off the general surface of the land would form a layer of rock -only five inches thick. But in the way in which the materials are at -present deposited, the foot removed from the land would form a layer -of rock many feet in thickness. The greater part of the sediment is -deposited within a few miles of the shore. - -The entire coast-line of the globe is about 116,500 miles. I should -think that the quantity of sediment deposited beyond, say, 100 miles -from this coast-line is not very great. No doubt several of the large -rivers carry sediment to a much greater distance from their mouths than -100 miles, and ocean currents may in some cases carry mud and other -materials also to great distances. But it must be borne in mind that -at many places within the 100 miles of this immense coast-line little -or no sediment is deposited, so that the actual area over which the -sediment carried off the land is deposited is probably not greater than -the area of this belt—116,500 miles long and 100 miles broad. This -area on which the sediment is deposited, on the above supposition, is -therefore equal to about 11,650,000 square miles. The amount of land on -the globe is about 57,600,000 square miles. Consequently, one foot of -rock, denuded from the surface of the land and deposited on this belt, -would make a stratum of rock 5 feet in thickness; but were the sediment -spread over the entire bed of the ocean, it would form, as has already -been stated, a stratum of rock of only 5 inches in thickness. - -Suppose that no subsidence of the land should take place for a period -of, say, 3,000,000 of years. During that period 500 feet would be -removed by denudation, on an average, off the land. This would make a -formation 2,500 feet thick, which some future geologist might call the -Post-tertiary formation. But this, be it observed, would be only the -mean thickness of the formation on this area; its maximum thickness -would evidently be much greater, perhaps twice, thrice, or even four -times that thickness. A geologist in the future, measuring the actual -thickness of the formation, might find it in some places 10,000 feet -in thickness, or perhaps far more. But had the materials been spread -over the entire ocean-bed, the formation would have a mean thickness -of little more than 200 feet; and spread over the entire surface of -the globe, would form a stratum of scarcely 150 feet in thickness. -Therefore, in estimating the mean thickness of the stratified rocks of -the globe, a formation with a maximum thickness of 10,000 feet may not -represent more than 150 feet. A formation with a _mean_ thickness of -10,000 feet represents only 600 feet. - -It may be objected that in taking the present rate at which the -sedimentary deposits are being formed as the mean rate for all ages, -we probably under-estimate the total amount of rock formed, because -during the many glacial periods which must have occurred in past ages -the amount of materials ground off the rocky surface of the land in a -given period would be far greater than at present. But, in reply, it -must be remembered that although the destruction in ice-covered regions -would be greater during these periods than at present, yet the quantity -of materials carried down by rivers into the sea would be less. At -the present day the greater part of the materials carried down by our -rivers is not what is being removed off the rocky face of the country, -but the boulder clay, sand, and other materials which were ground off -during the glacial epoch. It is therefore possible, on this account, -that the rate of deposit may have been less during the glacial epoch -than at present. - -When any particular formation is wanting in a given area, the inference -generally drawn is, that either the formation has been denuded off -the area, or the area was a land-surface during the period when that -formation was being deposited. From the foregoing it will be seen that -this inference is not legitimate; for, supposing that the area had been -under water, the chances that materials should have been deposited on -that area are far less than are the chances that there should not. -There are sixteen chances against one that no formation ever existed in -the area. - -If the great depressions of the Atlantic, Pacific, and Indian Oceans -be, for example, as old as the beginning of the Laurentian period—and -they may be so for anything which geology can show to the contrary—then -under these oceans little or no stratified rocks may exist. The -supposition that the great ocean basins are of immense antiquity, and -that consequently only a small proportion of the sedimentary strata -can possibly occupy the deeper bed of the sea, acquires still more -probability when we consider the great extent and thickness of the -Old Red Sandstone, the Permian, and other deposits, which, according -to Professor Ramsay and others, have been accumulated in vast inland -lakes. - - - - - CHAPTER XXIII. - - THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE LAND - DURING THE GLACIAL EPOCH. - - Displacement of the Earth’s Centre of Gravity by Polar - Ice-cap.—Simple Method of estimating Amount of - Displacement.—Note by Sir W. Thomson on foregoing - Method.—Difference between Continental Ice and - a Glacier.—Probable Thickness of the Antarctic - Ice-cap.—Probable Thickness of Greenland Ice-sheet.—The - Icebergs of the Southern Ocean.—Inadequate Conceptions - regarding the Magnitude of Continental Ice. - - -_Displacement of the Earth’s Centre of Gravity by Polar -Ice-cap._[206]—In order to represent the question in its most simple -elementary form, I shall assume an ice-cap of a given thickness at the -pole and gradually diminishing in thickness towards the equator in the -simple proportion of the sines of the latitudes, where at the equator -its thickness of course is zero. Let us assume, what is actually the -case, that the equatorial diameter of the globe is somewhat greater -than the polar, but that when the ice-cap is placed on one hemisphere -the whole forms a perfect sphere. - -I shall begin with a period of glaciation on the southern hemisphere. -Let W N E S′ (Fig. 5) be the solid part of the earth, and _c_ its -centre of gravity. And let E S W be an ice-cap covering the southern -hemisphere. Let us in the first case assume the earth to be of the same -density as the cap. The earth with its cap forms now a perfect sphere -with its centre of gravity at _o_; for W N E S is a circle, and _o_ -is its centre. Suppose now the whole to be covered with an ocean a few -miles deep, the ocean will assume the spherical form, and will be of -uniform depth. Let the southern winter solstice begin now to move round -from the aphelion. The ice-cap will also commence gradually to diminish -in thickness, and another cap will begin to make its appearance on -the northern hemisphere. As the northern cap may be supposed, for -simplicity of calculation, to increase at the same rate that the -southern will diminish, the spherical form of the earth will always be -maintained. By the time that the northern cap has reached a maximum, -the southern cap will have completely disappeared. The circle W N′ E S′ -will now represent the earth with its cap on the northern hemisphere, -and _o′_ will be its centre of gravity; for _o′_ is the centre of the -circle W N′ E S′. And as the distance between the centres _o_ and -_o′_ is equal to N N′, the thickness of the cap at the pole N N′ will -therefore represent the extent to which the centre of gravity has been -displaced. It will also represent the extent to which the ocean has -risen at the north pole and sunk at the south. This is evident; for as -the sphere W N′ E S′ is the same in all respects as the sphere W N E -S, with the exception only that the cap is on the opposite side, the -surface of the ocean at the poles will now be at the same distance from -the centre _o′_ as it was from the centre _o_ when the cap covered -the southern hemisphere. Hence the distance between _o_ and _o′_ must -be equal to the extent of the submergence at the north pole and the -emergence at the south. Neglect the attraction of the altering water on -the water itself, which later on will come under our consideration. - - [Illustration: Fig. 5.] - -We shall now consider the result when the earth is taken at its actual -density, which is generally believed to be about 5·5. The density -of ice being ·92, the density of the cap to that of the earth will -therefore be as 1 to 6. - - [Illustration: Fig. 6.] - -Let Fig. 6 represent the earth with an ice-cap on the northern -hemisphere, whose thickness is, say, 6,000 feet at the pole. The centre -of gravity of the earth without the cap is at _c_. When the cap is on, -the centre of gravity is shifted to _o_, a point a little more than -500 feet to the north of _c_. Had the cap and the earth been of equal -density, the centre of gravity would have been shifted to _o′_ the -centre of the figure, a point situated, of course, 3,000 feet to the -north of _c_. Now it is very approximately true that the ocean will -tend to adjust itself as a sphere around the centre of gravity, _o_. -Thus it would of course sink at the south pole and rise to the same -extent at the north, in any opening or channel in the ice allowing the -water to enter. - -Let the ice-cap be now transferred over to the southern hemisphere, -and the condition of things on the two hemispheres will in every -particular be reversed. The centre of gravity will then lie to -the south of _c_, or about 1,000 feet from its former position. -Consequently the transference of the cap from the one hemisphere to the -other will produce a total submergence of about 1,000 feet. - -It is, of course, absurd to suppose that an ice-cap could ever actually -reach down to the equator. It is probable that the great ice-cap of the -glacial epoch nowhere reached even halfway to the equator. Our cap must -therefore terminate at a moderately high latitude. Let it terminate -somewhere about the latitude of the north of England, say at latitude -55°. All that we have to do now is simply to imagine our cap, up to -that latitude, becoming converted into the fluid state. This would -reduce the cap to less than one-half its former mass. But it would not -diminish the submergence to anything like that extent. For although the -cap would be reduced to less than one-half its former mass, yet its -influence in displacing the centre of gravity would not be diminished -to that extent. This is evident; for the cap now extending down to -only latitude 55°, has its centre of gravity much farther removed from -the earth’s centre of gravity than it had when it extended down to the -equator. Consequently it now possesses, in proportion to its mass, a -much greater power in displacing the earth’s centre of gravity. - -There is another fact which must be taken into account. The common -centre of gravity of the earth and cap is not exactly the point -around which the ocean tends to adjust itself. It adjusts itself not -in relation to the centre of gravity of the solid mass alone, but in -relation to the common centre of gravity of the entire mass, solid and -liquid. Now the water which is pulled over from the one hemisphere to -the other by the attraction of the cap will also aid in displacing the -centre of gravity. It will co-operate with the cap and carry the true -centre of gravity to a point beyond that of the centre of gravity of -the earth and cap, and thus increase the effect. - -It is of course perfectly true that when the ice-cap does not extend -down to the equator, as in the latter supposition, and is of less -density than the globe, the ocean will not adjust itself uniformly -around the centre of gravity; but the deviation from perfect uniformity -is so trifling, as will be seen from the appended note of Sir William -Thomson, that for all practical purposes it may be entirely left out of -account. - -In the _Reader_ for January 13, 1866, I advanced an objection to the -submergence theory on the grounds that the lowering of the ocean-level -by the evaporation of the water to form the ice-cap, would exceed the -submergence resulting from the displacement of the earth’s centre of -gravity. But, after my letter had gone to press, I found that I had -overlooked some important considerations which seem to prove that the -objection had no real foundation. For during a glacial period, say -on the northern hemisphere, the entire mass of ice which presently -exists on the southern hemisphere would be transferred to the northern, -leaving the quantity of liquid water to a great extent unchanged. - - - _Note on the preceding by Sir William Thomson, F.R.S._ - -“Mr. Croll’s estimate of the influence of a cap of ice on the sea-level -is very remarkable in its relation to Laplace’s celebrated analysis, -as being founded on that law of thickness which leads to expressions -involving only the first term of the series of ‘Laplace’s functions,’ -or ‘spherical harmonics.’ The equation of the level surface, as -altered by any given transference of solid matter, is expressed by -equating the altered potential function to a constant. This function, -when expanded in the series of spherical harmonics, has for its first -term the potential due to the whole mass supposed collected at its -altered centre of gravity. Hence a spherical surface round the altered -centre of gravity is the _first_ approximation in Laplace’s method of -solution for the altered level surface. Mr. Croll has with admirable -tact chosen, of all the arbitrary suppositions that may be made -foundations for rough estimates of the change of sea-level due to -variations in the polar ice-crusts, _the_ one which reduces to zero all -terms after the first in the harmonic series, and renders that first -approximation (which always expresses the _essence_ of the result) the -whole solution, undisturbed by terms irrelevant to the great physical -question. - -“Mr. Croll, in the preceding paper, has alluded with remarkable -clearness to the effect of the change in the distribution of the -water in increasing, by its own attraction, the deviation of the -level surface above that which is due to the _given_ change in the -distribution of solid matter. The remark he makes, that it is round -the centre of gravity of the altered solid and altered liquid that -the altering liquid surface adjusts itself, expresses the essence of -Laplace’s celebrated demonstration of the stability of the ocean, and -suggests the proper elementary solution of the problem to find the -true alteration of sea-level produced by a given alteration of the -solid. As an assumption leading to a simple calculation, let us suppose -the solid earth to rise out of the water in a vast number of small -flat-topped islands, each bounded by a perpendicular cliff, and let the -proportion of water area to the whole be equal in all quarters. Let all -of these islands in one hemisphere be covered with ice, of thickness -according to the law assumed by Mr. Croll—that is, varying in simple -proportion of the sine of the latitude. Let this ice be removed from -the first hemisphere and similarly distributed over the islands of -the second. By working out according to Mr. Croll’s directions, it is -easily found that the change of sea-level which this will produce will -consist in a sinking in the first hemisphere and rising in the second, -through heights varying according to the same law (that is, simple -proportionality to sines of latitudes), and amounting at each pole to - - (1 - ω)it - —————————, - 1 - ωw - -where _t_ denotes the thickness of the ice-crust at the pole; _i_ the -ratio of the density of ice, and _w_ that of sea-water to the earth’s -mean density; and ω the ratio of the area of ocean to the whole surface. - -“Thus, for instance, if we suppose ω = ⅔, and _t_ = 6,000 feet, and -take ⅙ and 1/(5½) as the densities of ice and water respectively, we -find for the rise of sea-level at one pole, and depression at the other, - - ⅓ × ⅙ × 6000 - ————————————, - 2 1 - 1 - — × — - 3 5½ - -or approximately 380 feet. - -“I shall now proceed to consider roughly what is the probable extent -of submergence which, during the glacial epoch, may have resulted from -the displacement of the earth’s centre of gravity by means of the -transferrence of the polar ice from the one hemisphere to the other.” - -_Difference between Continental-ice and a Glacier._—An ordinary -glacier descends in virtue of the slope of its bed, and, as a general -rule, it is on this account thin at its commencement, and thickens -as it descends into the lower valleys, where the slope is less and -the resistance to motion greater. But in the case of continental ice -matters are entirely different. The slope of the ground exercises -little or no influence on the motion of the ice. In a continent of one -or two thousand miles across, the general slope of the ground may be -left out of account; for any slight elevation which the centre of such -a continent may have will not compensate for the resistance offered to -the flow of the ice by mountain ridges, hills, and other irregularities -of its surface. The ice can move off such a surface only in consequence -of pressure acting from the interior. In order to produce such a -pressure, there must be a piling up of the ice in the interior; or, in -other words, the ice-sheet must thicken from the edge inwards to the -centre. We are necessarily led to the same conclusion, though we should -not admit that the ice moves in consequence of pressure from behind, -but should hold, on the contrary, that each particle of ice moves by -gravity in virtue of its own weight; for in order to have such a motion -there must be a slope, and as the slope is not on the ground, it must -be on the ice itself: consequently we must conclude that the upper -surface of the ice slopes upwards from the edge to the interior. What, -then, is the least slope at which the ice will descend? Mr. Hopkins -found that ice barely moves on a slope of one degree. We have therefore -some data for arriving at least at a rough estimate of the probable -thickness of an ice-sheet covering a continent, such, for example, as -Greenland or the Antarctic Continent. - -_Probable Thickness of the Antarctic Ice-cap._—The antarctic continent -is generally believed to extend, on an average, from the South Pole -down to about, at least, lat. 70°. In round numbers, we may take the -diameter of this continent at 2,800 miles. The distance from the -edge of this ice-cap to its centre, the South Pole, will, therefore, -be 1,400 miles. The whole of this continent, like Greenland, is -undoubtedly covered with one continuous sheet of ice gradually -thickening inwards from its edge to its centre. A slope of one degree -continued for 1,400 miles will give twenty-four miles as the thickness -of the ice at the pole. But suppose the slope of the upper surface -of the cap to be only one-half this amount, viz., a half degree,—and -we have no evidence that a slope so small would be sufficient to -discharge the ice,—still we have twelve miles as the thickness of the -cap at the pole. To those who have not been accustomed to reflect on -the physical conditions of the problem, this estimate may doubtless -be regarded as somewhat extravagant; but a slight consideration -will show that it would be even more extravagant to assume that a -slope of less than half a degree would be sufficient to produce the -necessary outflow of the ice. In estimating the thickness of a sheet of -continental ice of one or two thousand miles across, our imagination -is apt to deceive us. We can easily form a pretty accurate sensuous -representation of the thickness of the sheet; but we can form no -adequate representation of its superficial area. We can represent -to the mind with tolerable accuracy a thickness of a few miles, but -we cannot do this in reference to the area of a surface 2,800 miles -across. Consequently, in judging what proportion the thickness of the -sheet should bear to its superficial area, we are apt to fall into the -error of under-estimating the thickness. We have a striking example -of this in regard to the ocean. The thing which impresses us most -forcibly in regard to the ocean is its profound depth. A mean depth -of, say, three miles produces a striking impression; but if we could -represent to the mind the vast area of the ocean as correctly as we can -do its depth, _shallowness_ rather than _depth_ would be the impression -produced. A sheet of water 100 yards in diameter, and only one inch -deep, would not be called a _deep_ but a very _shallow_ pool or thin -layer of water. But such a layer would be a correct representation of -the ocean in miniature. Were we in like manner to represent to the eye -in miniature the antarctic ice-cap, we would call it a _thin crust of -ice_. Taking the mean thickness of the ice at four miles, the antarctic -ice-sheet would be represented by a carpet covering the floor of an -ordinary-sized dining-room. Were those who consider the above estimate -of the thickness of the antarctic ice-cap as extravagantly great called -upon to sketch on paper a section of what they should deem a cap of -moderate thickness, ninety-nine out of every hundred would draw one of -much greater thickness than twelve miles at the centre. - -The diagram on following page (Fig. 7) represents a section across the -cap drawn to a natural scale; the upper surface of the sheet having -a slope of half a degree. No one on looking at the section would -pronounce it to be too thick at the centre, unless he were previously -made aware that it represented a thickness of twelve miles at that -place. It may be here mentioned that had the section been drawn upon -a much larger scale—had it, for instance, been made seven feet long, -instead of seven inches—it would have shown to the eye in a more -striking manner the thinness of the cap. - -But to avoid all objections on the score of over-estimating the -thickness of the cap, I shall assume the angle of the upper surface to -be only a quarter of a degree, and the thickness of the sheet one-half -what it is represented in the section. The thickness at the pole will -then be only six miles instead of twelve, and the mean thickness of the -cap two instead of four miles. - - [Illustration: Fig. 7. - - Section across Antarctic Ice-cap, drawn to a natural scale. - - Length represented by section = 2,800 miles. Thickness at centre - (South Pole) = 12 miles. - - Slope of upper surface = half-degree.] - -Is there any well-grounded reason for concluding the above to be an -over-estimate of the actual thickness of the antarctic ice? It is not -so much in consequence of any _à priori_ reason that can be urged -against the probability of such a thickness of ice, but rather because -it so far transcends our previous experience that we are reluctant to -admit such an estimate. If we never had any experience of ice thicker -than what is found in England, we should feel startled on learning for -the first time that in the valleys of Switzerland the ice lay from 200 -to 300 feet in depth. Again, if we had never heard of glaciers thicker -than those of Switzerland, we could hardly credit the statement that -in Greenland they are actually from 2,000 to 3,000 feet thick. We, in -this country, have long been familiar with Greenland; but till very -lately no one ever entertained the idea that that continent was buried -under one continuous mass of ice, with scarcely a mountain top rising -above the icy mantle. And had it not been that the geological phenomena -of the glacial epoch have for so many years accustomed our minds to -such an extraordinary condition of things, Dr. Rink’s description of -the Greenland ice would probably have been regarded as the extravagant -picture of a wild imagination. - -Let us now consider whether or not the facts of observation and -experience, so far as they go, bear out the conclusions to which -physical considerations lead us in reference to the magnitude of -continental ice; and more especially as regards the ice of the -antarctic regions. - -_First._ In so far as the antarctic ice-sheet is concerned, observation -and experience to a great extent may be said to be a perfect blank. One -or two voyagers have seen the outer edge of the sheet at a few places, -and this is all. In fact, we judge of the present condition of the -interior of the antarctic continent in a great measure from what we -know of Greenland. But again, our experience of Greenland ice is almost -wholly confined to the outskirts. - -Few have penetrated into the interior, and, with the exception of Dr. -Hayes and Professor Nordenskjöld, none, as far as I know, have passed -to any considerable distance over the inland ice. Dr. Robert Brown -in his interesting memoir on “Das Innere von Grönland,”[207] gives -an account of an excursion made in 1747 by a Danish officer of the -name of Dalager, from Fredrikshaab, near the southern extremity of -the continent, into the interior. After a journey of a day or two, he -reached an eminence from which he saw the inland ice stretching in an -unbroken mass as far as the eye could reach, but was unable to proceed -further. Dr. Brown gives an account also of an excursion made in the -beginning of March, 1830, by O. B. Kielsen, a Danish whale-fisher, from -Holsteinborg (lat. 67° N.). After a most fatiguing journey of several -days, he reached a high point from which he could see the ice of the -interior. Next morning he got up early, and towards midday reached -an extensive plain. From this the land sank inwards, and Kielsen now -saw fully in view before him the enormous ice-sheet of the interior. -He drove rapidly over all the little hills, lakes, and streams, till -he reached a pretty large lake at the edge of the ice-sheet. This was -the end of his journey, for after vainly attempting to climb up on the -ice-sheet, he was compelled to retrace his steps, and had a somewhat -difficult return. When he arrived at the fiord, he found the ice broken -up, so that he had to go round by the land way, by which he reached -the depôt on the 9th of March. The distance which he traversed in a -straight line from Holsteinborg into the interior measured eighty -English miles. - -Dr. Hayes’s excursion was made, however, not upon the real inland -ice, but upon a smaller ice-field connected with it; while Professor -Nordenskjöld’s excursion was made at a place too far south to -afford an accurate idea of the actual condition of the interior of -North Greenland, even though he had penetrated much farther than he -actually did. However, the state of things as recorded by Hayes and by -Nordenskjöld affords us a glimpse into the condition of things in the -interior of the continent. They both found by observation, what follows -as a necessary result from physical considerations, that the upper -surface of the ice plain, under which hills and valleys are buried, -gradually _slopes upwards towards the interior of the continent_. -Professor Nordenskjöld states that when at the extreme point at which -he reached, thirty geographical miles from the coast, he had attained -an elevation of 2,200 feet, and that the inland ice _continued -constantly to rise_ towards the interior, so that the horizon towards -the east, north, and south, was terminated by an ice-border almost as -smooth as that of the ocean.”[208] - -Dr. Hayes and his party penetrated inwards to the distance of about -seventy miles. On the first day they reached the foot of the great Mer -de Glace; the second day’s journey carried them to the upper surface -of the ice-sheet. On the third day they travelled 30 miles, and the -ascent, which had been about 6°, diminished gradually to about 2°. They -advanced on the fourth day about 25 miles; the temperature being 30° -below zero (Fah.). “Our station at the camp,” he says, “was sublime as -it was dangerous. We had attained an altitude of 5,000 feet above the -sea-level, and were 70 miles from the coast, in the midst of a vast -frozen Sahara immeasurable to the human eye. There was neither hill, -mountain, nor gorge, anywhere in view. We had completely sunk the -strip of land between the Mer de Glace and the sea, and no object met -the eye but our feeble tent, which bent to the storm. Fitful clouds -swept over the face of the full-orbed moon, which, descending towards -the horizon, glimmered through the drifting snow that scudded over the -icy plain—to the eye in undulating lines of downy softness, to the -flesh in showers of piercing darts.”[209] - -Dr. Rink, referring to the inland ice, says that the elevation or -height above the sea of this icy plain at its junction with the -outskirts of the country, and where it begins to lower itself through -the valleys to the firths, is, in the ramifications of the Bay of -Omenak, found to be 2,000 feet, from which level _it gradually rises -towards the interior_.[210] - -Dr. Robert Brown, who, along with Mr. Whymper in 1867, attempted a -journey to some distance over the inland ice, is of opinion that -Greenland is not traversed by any ranges of mountains or high land, -but that the entire continent, 1,200 miles in length and 400 miles in -breadth, is covered with one continuous unbroken field of ice, the -upper surface of which, he says, _rises by a gentle slope towards the -interior_.[211] - -Suppose now the point reached by Hayes to be within 200 miles of -the centre of dispersion of the ice, and the mean slope from that -point to the centre, as in the case of the antarctic cap, to be only -half a degree; this would give 10,000 feet as the elevation of the -centre above the point reached. But the point reached was 5,000 feet -above sea-level, consequently the surface of the ice at the centre -of dispersion would be 15,000 feet above sea-level, which is about -one-fourth what I have concluded to be the elevation of the surface -of the antarctic ice-cap at its centre. And supposing we assume -the general surface of the ground to have in the central region an -elevation as great as 5,000 feet, which is not at all probable, still -this would give 10,000 feet for the thickness of the ice at the centre -of the Greenland continent. But if we admit this conclusion in -reference to the thickness of the Greenland ice, we must admit that -the antarctic ice is far thicker, because the thickness, other things -being equal, will depend upon the size, or, more properly, upon the -diameter of the continent; for the larger the surface the greater is -the thickness of ice required to produce the pressure requisite to make -the rate of discharge of the ice equal to the rate of increase. Now -the area of the antarctic continent must be at least a dozen of times -greater than that of Greenland. - -_Second._ That the antarctic ice must be far thicker than the arctic -is further evident from the dimensions of the icebergs which have been -met with in the Southern Ocean. No icebergs over three hundred feet in -height have been found in the arctic regions, whereas in the antarctic -regions, as we shall see, icebergs of twice and even thrice that height -have been reported. - -_Third._ We have no reason to believe that the thickness of the ice -at present covering the antarctic continent is less than that which -covered a continent of a similar area in temperate regions during the -glacial epoch. Take, for example, the North American continent, or, -more properly, that portion of it covered by ice during the glacial -epoch. Professor Dana has proved that during that period the thickness -of the ice on the American continent must in many places have been -considerably over a mile. He has shown that over the northern border of -New England the ice had a mean thickness of 6,500 feet, while its mean -thickness over the Canada watershed, between St. Lawrence and Hudson’s -Bay, was not less than 12,000 feet, or upwards of two miles and a -quarter (see _American Journal of Science and Art_ for March, 1873). - -_Fourth._ Some may object to the foregoing estimate of the amount of -ice on the antarctic continent, on the grounds that the quantity of -snowfall in that region cannot be much. But it must be borne in mind -that, no matter however small the annual amount of snowfall may be, if -more falls than is melted, the ice must continue to accumulate year by -year till its thickness in the centre of the continent be sufficiently -great to produce motion. The opinion that the snowfall of the antarctic -regions is not great does not, however, appear to be borne out by the -observation and experience of those who have visited those regions. -Captain Wilkes, of the American Exploring Expedition, estimated it at -30 feet per annum; and Sir James Ross says, that during a whole month -they had only three days free from snow. The fact that perpetual snow -is found at the sea-level at lat. 64° S. proves that the snowfall -must be great. But there is another circumstance which must be taken -into account, viz., that the currents carrying moisture move in from -all directions towards the pole, consequently the area on which they -deposit their snow becomes less and less as the pole is reached, and -this must, to a corresponding extent, increase the quantity of snow -falling on a given area. Let us assume, for example, that the clouds -in passing from lat. 60° to lat. 80° deposit moisture sufficient to -produce, say, 30 feet of snow per annum, and that by the time they -reach lat. 80° they are in possession of only one-tenth part of their -original store of moisture. As the area between lat. 80° and the -pole is but one-eighth of that between lat. 60° and 80°, this would, -notwithstanding, give 24 feet as the annual amount of snowfall between -lat. 80° and the pole.[212] - -_Fifth._ The enormous size and thickness of the icebergs which have -been met with in the Southern Ocean testify to the thickness of the -antarctic ice-cap. - -We know from the size of some of the icebergs which have been met with -in the southern hemisphere that the ice at the edge of the cap where -the bergs break off must in some cases be considerably over a mile in -thickness, for icebergs of more than a mile in thickness have been -found in the southern hemisphere. The following are the dimensions of -a few of these enormous bergs taken from the Twelfth Number of the -Meteorological Papers published by the Board of Trade, and from the -excellent paper of Mr. Towson on the Icebergs of the Southern Ocean, -published also by the Board of Trade.[213] With one or two exceptions, -the heights of the bergs were accurately determined by angular -measurement:— - - - Sept. 10th, 1856.—The _Lightning_, when in lat. 55° 33′ S., - long. 140° W., met with an iceberg 420 feet high. - - Nov., 1839.—In lat. 41° S., long. 87° 30′ E., numerous icebergs - 400 feet high were met with. - - Sept., 1840.—In lat. 37° S., long. 15° E., an iceberg 1,000 - feet long and 400 feet high was met with. - - Feb., 1860.—Captain Clark, of the _Lightning_, when in lat. 55° - 20′ S., long. 122° 45′ W., found an iceberg 500 feet high and 3 - miles long. - - Dec. 1st, 1859.—An iceberg, 580 feet high, and from two and a - half to three miles long, was seen by Captain Smithers, of the - _Edmond_, in lat. 50° 52′ S., long. 43° 58′ W. So strongly did - this iceberg resemble land, that Captain Smithers believed it - to be an island, and reported it as such, but there is little - or no doubt that it was in reality an iceberg. There were - pieces of drift-ice under its lee. - - Nov., 1856.—Three large icebergs, 500 feet high, were found in - lat. 41° 0′ S., long. 42° 0′ E. - - Jan., 1861.—Five icebergs, one 500 feet high, were met with in - lat. 55° 46′ S., long. 155° 56′ W. - - Jan., 1861.—In lat. 56° 10′ S., long. 160° 0′ W., an iceberg - 500 feet high and half a mile long was found. - - Jan., 1867.—The barque _Scout_, from the West Coast of - America, on her way to Liverpool, passed some icebergs 600 feet - in height, and of great length. - - April, 1864.—The _Royal Standard_ came in collision with an - iceberg 600 feet in height. - - Dec., 1856.—Four large icebergs, one of them 700 feet high, and - another 500 feet, were met with in lat. 50° 14′ S., long. 42° - 54′ E. - - Dec. 25th, 1861.—The _Queen of Nations_ fell in with an iceberg - in lat. 53° 45′ S., long. 170° 0′ W., 720 feet high. - - Dec., 1856.—Captain P. Wakem, ship _Ellen Radford_, found, in - lat. 52° 31′ S., long. 43° 43′ W., two large icebergs, one at - least 800 feet high. - - Mr. Towson states that one of our most celebrated and talented - naval surveyors informed him that he had seen icebergs in the - southern regions 800 feet high. - - March 23rd, 1855.—The _Agneta_ passed an iceberg in lat. 53° - 14′ S., long. 14° 41′ E., 960 feet in height. - - Aug. 16th, 1840.—The Dutch ship, _General Baron von Geen_, - passed an iceberg 1,000 feet high in lat. 37° 32′ S., long. 14° - 10′ E. - - May 15th, 1859.—The _Roseworth_ found in lat. 53° 40′ S., long. - 123° 17′ W., an iceberg as large as “Tristan d’Acunha.” - -In the regions where most of these icebergs were met with, the mean -density of the sea is about 1·0256. The density of ice is ·92. The -density of icebergs to that of the sea is therefore as 1 to 1·115; -consequently every foot of ice above water indicates 8·7 feet below -water. It therefore follows that those icebergs 400 feet high had 3,480 -feet under water,—3,880 feet would consequently be the total thickness -of the ice. The icebergs which were 500 feet high would be 4,850 feet -thick, those 600 feet high would have a total thickness of 5,820 feet, -and those 700 feet high would be no less than 6,790 feet thick, which -is more than a mile and a quarter. The iceberg 960 feet high, sighted -by the _Agneta_, would be actually 9,312 feet thick, which is upwards -of a mile and three-quarters. - -Although the mass of an iceberg below water compared to that above -may be taken to be about 8·7 to 1, yet it would not be always safe -to conclude that the thickness of the ice below water bears the same -proportion to its height above. If the berg, for example, be much -broader at its base than at its top, the thickness of the ice below -water would bear a less proportion to the height above water than -as 8·7 to 1. But a berg such as that recorded by Captain Clark, 500 -feet high and three miles long, must have had only 1/8·7 of its total -thickness above water. The same remark applies also to the one seen by -Captain Smithers, which was 580 feet high, and so large that it was -taken for an island. This berg must have been 5,628 feet in thickness. -The enormous berg which came in collision with the _Royal Standard_ -must have been 5,820 feet thick. It is not stated what length the -icebergs 730, 960, and 1,000 feet high respectively were; but supposing -that we make considerable allowance for the possibility that the -proportionate thickness of ice below water to that above may have been -less than as 8·7 to 1, still we can hardly avoid the conclusion that -the icebergs were considerably above a mile in thickness. But if there -are icebergs above a mile in thickness, then there must be land-ice -somewhere on the southern hemisphere of that thickness. In short, the -great antarctic ice-cap must in some places be over a mile in thickness -at its edge. - -_Inadequate Conceptions regarding the Magnitude of Continental -Ice._—Few things have tended more to mislead geologists in the -interpretation of glacial phenomena than inadequate conceptions -regarding the magnitude of continental ice. Without the conception -of continental ice the known facts connected with glaciation would -be perfectly inexplicable. It was only when it was found that the -accumulated facts refused to be explained by any other conception, -that belief in the very existence of such a thing as continental ice -became common. But although most geologists now admit the existence of -continental ice, yet, nevertheless, adequate conceptions of its real -magnitude are by no means so common. Year by year, as the outstanding -facts connected with glaciation accumulate, we are compelled to extend -our conceptions of the magnitude of land-ice. Take the following as -an example. It was found that the transport of the Wastdale Crag -blocks, the direction of the striæ on the islands of the Baltic, on -Caithness and on the Orkney, Shetland, and Faroe, islands, the boulder -clay with broken shells in Caithness, Holderness, and other places, -were inexplicable on the theory of land-ice. But it was so only in -consequence of the inadequacy of our conceptions of the magnitude of -the ice; for a slight extension of our ideas of its thickness has -explained not only these phenomena,[214] but others of an equally -remarkable character, such as the striation of the Long Island and -the submerged rock-basins around our coasts described by Mr. James -Geikie. In like manner, if we admit the theory of the glacial epoch -propounded in former chapters, all that is really necessary to account -for the submergence of the land is a slight extension of our hitherto -preconceived estimate of the thickness of the ice on the antarctic -continent. If we simply admit a conclusion to which all physical -considerations, as we have seen, necessarily lead us, viz., that the -antarctic continent is covered with a mantle of ice at least two miles -in thickness, we have then a complete explanation of the cause of the -submergence of the land during the glacial epoch. - -Although of no great importance to the question under consideration, it -may be remarked that, except during the severest part of the glacial -epoch, we have no reason to believe that the total quantity of ice -on the globe was much greater than at present, only it would then be -all on one hemisphere. Remove two miles of ice from the antarctic -continent, and place it on the northern hemisphere, and this, along -with the ice that now exists on this hemisphere, would equal, in all -probability, the quantity existing on our hemisphere during the glacial -epoch; at least, before it reached its maximum severity. - - - - - CHAPTER XXIV. - - THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE LAND - DURING THE GLACIAL EPOCH.—_Continued._ - - Extent of Submergence from Displacement of Earth’s Centre - of Gravity.—Circumstances which show that the Glacial - Submergence resulted from Displacement of the Earth’s - Centre of Gravity.—Agreement between Theory and observed - Facts.—Sir Charles Lyell on submerged Areas during - Tertiary Period.—Oscillations of Sea-level in Relation to - Distribution.—Extent of Submergence on the Hypothesis that - the Earth is fluid in the Interior. - - -_Extent of Submergence from Displacement of Earth’s Centre of -Gravity._—How much, then, would the transference of the two miles of -ice from the southern to the northern hemisphere raise the level of the -ocean on the latter hemisphere? This mass, be it observed, is equal to -only one-half that represented in our section. A considerable amount -of discussion has arisen in regard to the method of determining this -point. According to the method already detailed, which supposes the -rise at the pole to be equal to the extent of the displacement of the -earth’s centre of gravity, the rise at the North Pole would be about -380 feet, taking into account the effect produced by the displaced -water; and the rise in the latitude of Edinburgh would be 312 feet. The -fall of level on the southern hemisphere would, of course, be equal to -the rise of level on the northern. According to the method advanced -by Mr. D. D. Heath,[215] the rise of level at the North Pole would be -about 650 feet. Archdeacon Pratt’s method[216] makes the rise still -greater; while according to Rev. O. Fisher’s method[217] the rise would -be no less than 2,000 feet. There is, however, another circumstance -which must be taken into account, which will give an additional rise of -upwards of one hundred feet. - -The greatest extent of the displacement of the earth’s centre of -gravity, and consequently the greatest rise of the ocean resulting -from that displacement, would of course occur at the time of maximum -glaciation, when the ice was all on one hemisphere. But owing to the -following circumstance, a still greater rise than that resulting from -the displacement of the earth’s centre of gravity alone might take -place at some considerable time, either before or after the period of -maximum glaciation. - -It is not at all probable that the ice would melt on the warm -hemisphere at exactly the same rate as it would form on the cold -hemisphere. It is probable that the ice would melt more rapidly on the -warm hemisphere than it would form on the cold. Suppose that during -the glacial epoch, at a time when the cold was gradually increasing on -the northern and the warmth on the southern hemisphere, the ice should -melt more rapidly off the antarctic continent than it was being formed -on the arctic and subarctic regions; suppose also that, by the time -a quantity of ice, equal to one-half what exists at present on the -antarctic continent, had accumulated on the northern hemisphere, the -whole of the antarctic ice had been melted away, the sea would then be -fuller than at present by the amount of water resulting from the one -mile of melted ice. The height to which this would raise the general -level of the sea would be as follows:— - -The antarctic ice-cap is equal in area to 1/23·46 of that covered by -the ocean. The density of ice to that of water being taken at ·92 to -1, it follows that 25 feet 6 inches of ice melted off the cap would -raise the general level of the ocean one foot, and the one mile of -ice melted off would raise the level 200 feet. This 200 feet of rise -resulting from the melted ice we must add to the rise resulting from -the displacement of the earth’s centre of gravity. The removal of the -two miles of ice from the antarctic continent would displace the -centre of gravity 190 feet, and the formation of a mass of ice equal -to the one-half of this on the arctic regions would carry the centre -of gravity 95 feet farther; giving in all a total displacement of 285 -feet, thus producing a rise of sea-level at the North Pole of 285 feet, -and in the latitude of Edinburgh of 234 feet. Add to this the rise of -200 feet resulting from the melted ice, and we have then 485 feet of -submergence at the pole, and 434 feet in the latitude of Edinburgh. A -rise to a similar extent might probably take place after the period -of maximum glaciation, when the ice would be melting on the northern -hemisphere more rapidly than it would be forming on the southern. - -If we assume the antarctic ice-cap to be as thick as is represented in -the diagram, the extent of the submergence would of course be double -the above, and we might have in this case a rise of sea-level in the -latitude of Edinburgh to the extent of from 800 to 1,000 feet. But be -this as it may, it is evident that the quantity of ice on the antarctic -continent is perfectly sufficient to account for the submergence of -the glacial epoch, for we have little evidence to conclude that the -_general_ submergence much exceeded 400 or 500 feet.[218] We have -evidence in England and other places of submergence to the extent of -from 1,000 to 2,000 feet, but these may be quite local, resulting -from subsidence of the land in those particular areas. Elevations and -depressions of the land have taken place in all ages, and no doubt -during the glacial epoch also. - -_Circumstances which show that the Glacial Submergence resulted from -Displacement of the Earth’s Centre of Gravity._—In favour of this -view of the cause of the submergence of the glacial epoch, it is a -circumstance of some significance, that in every part of the globe -where glaciation has been found evidence of the submergence of the -land has also been found along with it. The invariable occurrence of -submergence along with glaciation points to some physical connection -between the two. It would seem to imply, either that the two were the -direct effects of a common cause, or that the one was the cause of the -other; that is, the submergence the cause of the glaciation, or the -glaciation the cause of the submergence. There is, I presume, no known -cause to which the two can be directly related as effects. Nor do I -think that there is any one who would suppose that the submergence of -the land could have been the cause of its glaciation, even although he -attributed all glacial effects to floating ice. The submergence of our -country would, of course, have allowed floating ice to pass over it had -there been any to pass over; but submergence would not have produced -the ice, neither would it have brought the ice from the arctic regions -where it already existed. But although submergence could not have been -the cause of the glacial epoch, yet we can, as we have just seen, -easily understand how the ice of the glacial epoch could have been the -cause of the submergence. If the glacial epoch was brought about by an -increase in the eccentricity of the earth’s orbit, then a submergence -of the land as the ice accumulated was a physical necessity. - -There is another circumstance connected with glacial submergence which -it is difficult to reconcile with the idea that it resulted from a -subsidence of the land. It is well known that during the glacial -epoch the land was not once under water only, but several times; and, -besides, there were not merely several periods when the land stood -at a lower level in relation to the sea than at present, but there -were also several periods when it stood at a much higher level than -now. And this holds true, not merely of our own country, but of every -country on the northern hemisphere where glaciation has yet been found. -All this follows as a necessary consequence from the theory that the -oscillations of sea-level resulted from the transference of the ice -from the one hemisphere to the other; but it is wholly inconsistent -with the idea that they resulted from upheavals and subsidence of the -land during a very recent period. - -But this is not all, there is more still to be accounted for. It has -been the prevailing opinion that at the time when the land was covered -with ice, it stood at a much greater elevation than at present. It -is, however, not maintained that the facts of geology establish such -a conclusion. The greater elevation of the land is simply assumed as -an hypothesis to account for the cold.[219] The facts of geology, -however, are fast establishing the opposite conclusion, viz., that -when the country was covered with ice, the land stood in relation to -the sea at a lower level than at present, and that the continental -periods or times when the land stood in relation to the sea at a higher -level than now were the warm inter-glacial periods, when the country -was free of snow and ice, and a mild and equable condition of climate -prevailed. This is the conclusion towards which we are being led by the -more recent revelations of surface geology, and also by certain facts -connected with the geographical distribution of plants and animals -during the glacial epoch. - -The simple occurrence of a rise and fall of the land in relation to -the sea-level in one or in two countries during the glacial epoch, -would not necessarily imply any physical connection. The coincidence -of these movements with the glaciation of the land might have been -purely accidental; but when we find that a succession of such movements -occurred, not merely in one or in two countries, but in every glaciated -country where proper observations have been made, we are forced to the -conclusion that the connection between the two is not accidental, but -the result of some fixed cause. - -If we admit that an increase in the eccentricity of the earth’s orbit -was the cause of the glacial epoch, then we must admit that all those -results followed as necessary consequences. For if the glacial epoch -lasted for upwards of one hundred thousand years or so, there would be -a succession of cold and warm periods, and consequently a succession -of elevations and depressions of sea-level. And the elevations of -the sea-level would take place during the cold periods, and the -depressions during the warm periods. - -But the agreement between theory and observed facts does not terminate -here. It follows from theory that the greatest oscillations of -sea-level would take place during the severest part of the glacial -epoch, when the eccentricity of the earth’s orbit would be at its -highest value, and that the oscillations would gradually diminish -in extent as the eccentricity diminished and the climate gradually -became less severe. Now it is well known that this is actually what -took place; the great submergence, as well as the great elevation or -continental period, occurred during the earlier or more severe part of -the glacial epoch, and as the climate grew less severe these changes -became of less extent, till we find them terminating in our submerged -forests and 25-foot raised beach. - -It follows, therefore, according to the theory advanced, that the mere -fact of an area having been under sea does not imply that there has -been any subsidence or elevation of the land, and that consequently the -inference which has been drawn from these submerged areas as to changes -in physical geography may be in many cases not well founded. - -Sir Charles Lyell, in his “Principles,” publishes a map showing the -extent of surface in Europe which has been covered by the sea since -the earlier part of the Tertiary period. This map is intended to show -the extraordinary amount of subsidence and elevation of the land which -has taken place during that period. It is necessary for Sir Charles’s -theory of the cause of the glacial epoch that changes in the physical -geography of the globe to an enormous extent should have taken place -during a very recent period, in order to account for the great change -of climate which occurred at that epoch. But if the foregoing results -be anything like correct, it does not necessarily follow that there -must have been great changes in the physical geography of Europe, -simply because the sea covered those areas marked in the map, for this -may have been produced by oscillations of sea-level, and not by changes -in the land. In fact, the areas marked in Sir Charles’s map as having -been covered by the sea, are just those which would be covered were the -sea-level raised a few hundred feet. No doubt there were elevations and -subsidences in many of the areas marked in the map during the Tertiary -period, and to this cause a considerable amount of the submergence -might be due; but I have little doubt that by far the greater part -must be attributed to oscillations of sea-level. It is no objection -that the greater part of the shells and other organic remains found -in the marine deposits of those areas are not indicative of a cold -or glacial condition of climate, for, as we have seen, the greatest -submergence would probably have taken place either before the more -severe cold had set in or after it had to a great extent passed away. -That the submergence of those areas probably resulted from elevations -of sea-level rather than depressions of the land, is further evident -from the following considerations. If we suppose that the climate of -the glacial epoch was brought about mainly by changes in the physical -geography of the globe, we must assume that these great changes took -place, geologically speaking, at a very recent date. Then when we ask -what ground is there for assuming that any such change in the relations -of sea and land as is required actually took place, the submergence -of those areas is adduced as the proof. Did it follow as a physical -necessity that all submergence must be the result of subsidence of the -land, and not of elevations of the sea, there would be some force in -the reasons adduced. But such a conclusion by no means follows, and, -_à priori_, it is just as likely that the appearance of the ice was -the cause of the submergence as that the submergence was the cause -of the appearance of the ice. Again, a subsidence of the land to the -extent required would to a great extent have altered the configuration -of the country, and the main river systems of Europe; but there is no -evidence that any such change has taken place. All the main valleys -are well known to have existed prior to the glacial epoch, and our -rivers to have occupied the same channels then as they do now. In the -case of some of the smaller streams, it is true, a slight deviation -has resulted at some points from the filling up of their channels with -drift during the glacial epoch; but as a general rule all the principal -valleys and river systems are older than the glacial epoch. This, of -course, could not be the case if a subsidence of the land sufficiently -great to account for the submergence of the areas in question, or -changes in the physical geography of Europe necessary to produce a -glacial epoch, had actually taken place. The total absence of any -geological evidence for the existence of any change which could explain -either the submergence of the areas in question or the climate of the -glacial epoch, is strong evidence that the submergence of the glacial -epoch, as well as of the areas in question, was the result of a simple -oscillation of sea-level resulting from the displacement of the earth’s -centre of gravity by the transferrence of the ice-cap from the southern -to the northern hemisphere. - -_Oscillations of Sea-level in relation to Distribution._—The -oscillations of sea-level resulting from the displacement of the -earth’s centre of gravity help to throw new light on some obscure -points connected with the subject of the geographical distribution -of plants and animals. At the time when the ice was on the southern -hemisphere during the glacial epoch, and the northern hemisphere was -enjoying a warm and equable climate, the sea-level would be several -hundred feet lower than at present, the North Sea would probably be -dry land, and Great Britain and Ireland joined to the continent, thus -opening up a pathway from the continent to our island. As has been -shown in former chapters, during the inter-glacial periods the climate -would be much warmer and more equable than now, so that animals from -the south, such as the hippopotamus, hyæna, lion, _Elephas antiquus_ -and _Rhinoceros megarhinus_, would migrate into this country, where -at present they could not live in consequence of the cold. We have -therefore an explanation, as was suggested on a former occasion,[220] -of the fact that the bones of these animals are found mingled in the -same grave with those of the musk-ox, mammoth, reindeer, and other -animals which lived in this country during the cold periods of the -glacial epoch; the animals from the north would cross over into this -country upon the frozen sea during the cold periods, while those from -the south would find the English Channel dry land during the warm -periods. - -The same reasoning will hold equally true in reference to the old -and new world. The depth of Behring Straits is under 30 fathoms; -consequently a lowering of the sea-level of less than 200 feet would -connect Asia with America, and thus allow plants and animals, as Mr. -Darwin believes, to pass from the one continent to the other.[221] -During this period, when Behring Straits would be dry land, Greenland -would be comparatively free from ice, and the arctic regions enjoying a -comparatively mild climate. In this case plants and animals belonging -to temperate regions could avail themselves of this passage, and thus -we can explain how plants belonging to temperate regions may have, -during the Miocene period, passed from the old to the new continent, -and _vice versâ_. - -As has already been noticed, during the time of the greatest extension -of the ice, the quantity of ice on the southern hemisphere might be -considerably greater than what exists on the entire globe at present. -In that case there might, in addition to the lowering of the sea-level -resulting from the displacement of the earth’s centre of gravity, be a -considerable lowering resulting from the draining of the ocean to form -the additional ice. This decrease and increase in the total quantity -of ice which we have considered would affect the level of the ocean as -much at the equator as at the poles; consequently during the glacial -epoch there might have been at the equator elevations and depressions -of sea-level to the extent of a few hundred feet. - -_Extent of Submergence on the Hypothesis that the Earth is fluid in -the Interior._—But we have been proceeding upon the supposition that -the earth is solid to its centre. If we assume, however, what is the -general opinion among geologists, that it consists of a fluid interior -surrounded by a thick and rigid crust or shell, then the extent of the -submergence resulting from the displacement of the centre of gravity -for a given thickness of ice must be much greater than I have estimated -it to be. This is evident, because, if the interior of the globe be in -a fluid state, it, in all probability, consists of materials differing -in density. The densest materials will be at the centre, and the least -dense at the outside or surface. Now the transferrence of an ice-cap -from the one pole to the other will not merely displace the ocean—the -fluid mass on the outside of the shell—but it will also displace the -heavier fluid materials in the interior of the shell. In other words, -the heavier materials will be attracted by the ice-cap more forcibly -than the lighter, consequently they will approach towards the cap to a -certain extent, sinking, as it were, into the lighter materials, and -displacing them towards the opposite pole. This displacement will of -course tend to shift the earth’s centre of gravity in the direction -of the ice-cap, because the heavier materials are shifted in this -direction, and the lighter materials in the opposite direction. This -process will perhaps be better understood from the following figures. - - [Illustration: Fig. 8. Fig. 9. - - O. The Ocean. - - S. Solid Crust or Shell. - - F, F^1, F^2, F^3. The various concentric layers of the fluid - interior. The layers increase in density towards the centre. - - I. The Ice-cap. - - C. Centre of gravity. - - C^1. The displaced centre of gravity.] - -In Fig. 8, where there is no ice-cap, the centre of gravity of the -earth coincides with the centre of the concentric layers of the fluid -interior. In Fig. 9, where there is an ice-cap placed on one pole, the -concentric layer F^1 being denser than layer F, is attracted towards -the cap more forcibly than F, and consequently sinks to a certain depth -in F. Again, F^2 being denser than F^1, it also sinks to a certain -extent in F^1. And again F^3, the mass at the centre, being denser than -F^2, it also sinks in F^2. All this being combined with the effects -of the ice-cap, and the displaced ocean outside the shell, the centre -of gravity of the entire globe will no longer be at C, but at C^1, a -considerable distance nearer to the side of the shell on which the -cap rests than C, and also a considerable distance nearer than it -would have been had the interior of the globe been solid. There are -here three causes tending to shift the centre of gravity, (1) the -ice-cap, (2) the displaced ocean, and (3) the displaced materials in -the interior. Two of the three causes mutually react on each other in -such a way as to increase each other’s effect. Thus the more the ocean -is drawn in the direction of the ice-cap, the more effect it has in -drawing the heavier materials in the interior in the same direction; -and in turn the more the heavier materials in the interior are drawn -towards the cap, the greater is the displacement of the earth’s centre -of gravity, and of course, as a consequence, the greater is the -displacement of the ocean. It may be observed also that, other things -being equal, the thinner the solid crust or shell is, and the greater -the difference in the density of the fluid materials in the interior, -the greater will be the extent of the displacement of the ocean, -because the greater will be the displacement of the centre of gravity. - -It follows that if we knew (1) the extent of the general submergence of -the glacial epoch, and (2) the present amount of ice on the southern -hemisphere, we could determine whether or not the earth is fluid in the -interior. - - - - - CHAPTER XXV. - - THE INFLUENCE OF THE OBLIQUITY OF THE ECLIPTIC ON CLIMATE AND ON - THE LEVEL OF THE SEA. - - The direct Effect of Change of Obliquity on Climate.—Mr. - Stockwell on the maximum Change of Obliquity.—How Obliquity - affects the Distribution of Heat over the Globe.—Increase of - Obliquity diminishes the Heat at the Equator and increases - that at the Poles.—Influence of Change of Obliquity on the - Level of the Sea.—When the Obliquity was last at its superior - Limit.—Probable Date of the 25-foot raised Beach.—Probable - Extent of Rise of Sea-level resulting from Increase of - Obliquity.—Lieutenant-Colonel Drayson’s and Mr. Belt’s - Theories.—Sir Charles Lyell on Influence of Obliquity. - - -_The direct Effect of Change in the Obliquity of the Ecliptic on -Climate._—There is still another cause which, I feel convinced, must to -a very considerable extent have affected climate during past geological -ages. I refer to the change in the obliquity of the ecliptic. This -cause has long engaged the attention of geologists and physicists, -and the conclusion generally come to is that no great effect can be -attributed to it. After giving special attention to the matter, I have -been led to the very opposite conclusion. It is quite true, as has -been urged, that the changes in the obliquity of the ecliptic cannot -sensibly affect the climate of temperate regions; but it will produce -a slight change on the climate of tropical latitudes, and a very -considerable effect on that of the polar regions, especially at the -poles themselves. We shall now consider the matter briefly. - -It was found by Laplace that the obliquity of the ecliptic will -oscillate to the extent of 1° 22′ 34″ on each side of 23° 28′, the -obliquity in the year 1801.[222] This point has lately been examined -by Mr. Stockwell, and the results at which he has arrived are almost -identical with those of Laplace. “The mean value of the obliquity,” he -says, “of both the apparent and fixed ecliptics to the equator is 23° -17′ 17″. The limits of the obliquity of the apparent ecliptic to the -equator are 24° 35′ 58″ and 21° 58′ 36″; whence it follows that the -greatest and least declinations of the sun at the solstices can never -differ from each other to any greater extent than 2° 37′ 22″.”[223] - -This change will but slightly affect the climate of the temperate -regions, but it will exercise a very considerable influence on -the climate of the polar regions. According to Mr. Meech,[224] if -365·24 thermal days represent the present total annual quantity of -heat received at the equator from the sun, 151·59 thermal days will -represent the quantity received at the poles. Adopting his method of -calculation, it turns out that when the obliquity of the ecliptic is at -the maximum assigned by Laplace the quantity received at the equator -would be 363·51 thermal days, and at the poles 160·04 thermal days. The -equator would therefore receive 1·73 thermal days less heat, and the -poles 8·45 thermal days more heat than at present. - - ANNUAL AMOUNT OF SUN’S HEAT. - - +-------------------+-------------+-----------+ - | Amount in 1801. | Amount at | | - | Obliquity 23° 28′.| maximum, |Difference.| - | | 24° 50′ 34“.| | - +---------+---------+-------------+-----------+ - |Latitude.| Thermal | Thermal | Thermal | - | | days. | days. | days. | - | 0 | 365·24 | 363·51 | −1·73 | - | 40 | 288·55 | 288·32 | −0·23 | - | 70 | 173·04 | 179·14 | +6·10 | - | 80 | 156·63 | 164·63 | +8·00 | - | 90 | 151·59 | 160·04 | +8·45 | - +---------+---------+-------------+-----------+ - -When the obliquity was at a maximum, the poles would therefore be -receiving 19 rays for every 18 they are receiving at present. The -poles would then be receiving nearly as much heat as latitude 76° is -receiving at present. - -The increase of obliquity would not sensibly affect the polar winter. -It is true that it would slightly increase the breadth of the -frigid zone, but the length of the winter at the poles would remain -unaffected. After the sun disappears below the horizon his rays are -completely cut off, so that a further descent of 1° 22′ 34″ would make -no material difference in the climate. In the temperate regions, the -sun’s altitude at the winter solstice would be 1° 22′ 34″ less than -at present. This would slightly increase the cold of winter in those -regions. But the increase in the amount of heat received by the polar -regions would materially affect the condition of the polar summer. -What, then, is the rise of temperature at the poles which would result -from the increase of 8·45 thermal days in the total amount received -from the sun? - -An increase of 8·45 thermal days, or 1/18th of the total quantity -received from the sun, according to the mode of calculation adopted in -Chap. II. would produce, all other things being equal, a rise in the -mean annual temperature equal to 14° or 15°. - -According to Professor Dove[225] there is a difference of 7°·6 -between the mean annual temperature of latitude 76° and the pole; -the temperature of the former being 9°·8, and that of the latter -2°·2. Since it follows that when the obliquity of the ecliptic is -at a maximum the poles would receive about as much heat per annum -as latitude 76° receives at present, it may be supposed that the -temperature of the poles at that period ought to be no higher than -that of latitude 76° at the present time. A little consideration will, -however, show that this by no means follows. Professor Dove’s Tables -represent correctly the mean annual temperature corresponding to every -tenth degree of latitude from the equator to the pole. But it must be -observed that the rate at which the temperature diminishes from the -equator to the pole is not proportionate to the decrease in the total -quantity of heat received from the sun as we pass from the equator to -the pole. Were the mean annual temperature of the various latitudes -proportionate to the amount of direct heat received, the equator -would be much warmer than it actually is at present, and the poles -much colder. The reason of this, as has been shown in Chapter II., is -perfectly obvious. There is a constant transferrence of _heat_ from -the equator to the poles, and of _cold_ from the poles to the equator. -The warm water of the equator is constantly flowing towards the poles, -and the cold water at the poles is constantly flowing to the equator. -The same is the case in regard to the aërial currents. Consequently -a great portion of the direct heat of the sun goes, not to raise the -temperature of the equator, but to heat the poles. And, on the other -hand, the cold materials at the poles are transferred to the equator, -and thus lower the temperature of that part of the globe to a great -extent. The present difference of temperature between lat. 76° and the -pole, determined according to the rate at which the temperature is -found to diminish between the equator and the pole, amounts to only -about 7° or 8°. But were there no mutual transferrence of warm and -cold materials between the equatorial and polar regions, and were the -temperature of each latitude to depend solely upon the direct rays of -the sun, the difference would far exceed that amount. - -Now, when the obliquity of the ecliptic was at its superior limit, and -the poles receiving about 1/18th more direct heat from the sun than -at present, the increase of temperature due to this increase of heat -would be far more than 7° or 8. It would probably be nearly double that -amount. - -“We may, therefore, conclude that when the obliquity of the ecliptic -was at a maximum, and the poles were receiving 1/18th more heat than -at present, the temperature of the poles ought to have been about 14° -or 15° warmer than at the present day, _provided, of course, that -this extra heat was employed wholly in raising the temperature_. Were -the polar regions free from snow and ice, the greater portion of the -extra heat would go to raise the temperature. But as those regions -are covered with snow and ice, the extra heat would have no effect in -raising the temperature, but would simply melt the snow and ice. The -ice-covered surface upon which the rays fell could never rise above -32°. At the period under consideration, the total annual quantity of -ice melted at the poles would be 1/18th more than at present. - -The general effect which the change in the obliquity of the ecliptic -would have upon the climate of the polar regions when combined with the -effects resulting from the eccentricity of the earth’s orbit, would be -this:—When the eccentricity was at a very high value, the hemisphere -whose winter occurred in the aphelion (for physical reasons, which have -already been discussed)[226] would be under a condition of glaciation, -while the other hemisphere, having its winter in perihelion, would be -enjoying a warm and equable climate. When the obliquity of the ecliptic -was at a maximum, and 1/18th more heat falling at the poles than at -present, the effect would be to modify to a great extent the rigour -of the glaciation in the polar zone of the hemisphere under a glacial -condition, and, on the other hand, to produce a more rapid melting -of the ice on the other hemisphere enjoying the equable climate. The -effects of eccentricity and obliquity thus combined would probably -completely remove the polar ice-cap from off the latter hemisphere, -and forest trees might then grow at the pole. Again, when the obliquity -was at its minimum condition and less heat reaching the poles than at -present, the glaciation of the former hemisphere would be increased and -the warmth of the latter diminished. - -_The Influence of Change in the Obliquity of the Ecliptic on the -Level of the Sea._—One very remarkable effect which seems to result -indirectly from a variation of the obliquity under certain conditions, -is an influence on the level of the sea. As this probably may have had -something to do with those recent changes of sea-level with which the -history of the submarine forests and raised beaches have made us all so -familiar, it may be of interest to enter at some length into this part -of this subject. - -It appears almost certain that at the time when the northern winter -solstice was in the aphelion last, a rise of the sea on the northern -hemisphere to a considerable number of feet must have taken place from -the combined effect of eccentricity and obliquity. About 11,700 years -ago, the northern winter solstice was in the aphelion. The eccentricity -at that time was ·0187, being somewhat greater than it is now; but the -winters occurring in aphelion instead of, as now, in perihelion, they -would on that account be probably 10° or 15° colder than they are at -the present day. It is probable, also, for reasons stated in a previous -chapter, that the Gulf-stream at that time would be considerably less -than now. This would tend to lower the temperature to a still greater -extent. As snow instead of rain must have fallen during winter to a -greater extent than at present, this no doubt must have produced a -slight increase in the quantity of ice on the northern hemisphere had -no other cause come into operation. But the condition of things, we -have every reason to believe, must have been affected by the greater -obliquity of the ecliptic at that period. We have no formula, except, -perhaps, that given by Mr. Stockwell, from which to determine with -perfect accuracy the extent of the obliquity at a period so remote as -the one under consideration. If we adopt the formula given by Struve -and Peters, which agrees pretty nearly with that obtained from Mr. -Stockwell’s formula, we have the obliquity at a maximum about the time -that the solstice-point was in the aphelion. The formula given by -Leverrier places the maximum somewhat later. At all events, we cannot -be far from the truth in assuming that at the time the northern winter -solstice was in the aphelion, the obliquity of the ecliptic would be -about a maximum, and that since then it has been gradually diminishing. -It is evident, then, that the annual amount of heat received by the -arctic regions, and especially about the pole, would be considerably -greater than at present. And as the heat received on those regions is -chiefly employed in melting the ice, it is probable that the extra -amount of ice which would then be melted in the arctic regions would -prevent that slight increase of ice which would otherwise have resulted -in consequence of the winter occurring in the aphelion. The winters at -that period would be colder than they are at present, but the total -quantity of ice on the northern hemisphere would not probably be -greater. - -Let us now turn to the southern hemisphere. As the southern winter -would then occur in the perihelion, this would tend to produce a slight -decrease in the quantity of ice on the southern hemisphere. But on this -hemisphere the effects of eccentricity would not, as on the northern -hemisphere, be compensated by those of obliquity; for both causes would -here tend to produce the same effect; namely, a melting of the ice in -the antarctic regions. - -It is probable that at this time the quantity of warm water flowing -from the equatorial regions into the Southern Ocean would be much -greater than at present. This would tend to raise the temperature of -the air of the antarctic regions, and thus assist in melting the ice. -These causes, combined with the great increase of heat resulting from -the change of obliquity, would tend to diminish to a considerable -extent the quantity of ice on the southern hemisphere. I think we may -assume that the slight increase of eccentricity at that period, the -occurrence of the southern winter in perihelion, and the extra quantity -of warm water flowing from the equatorial to the antarctic regions, -would produce an effect on the south polar ice-cap equal to that -produced by the increase in the obliquity of the ecliptic. It would, -therefore, follow that for every eighteen pounds of ice melted annually -at present at the south pole twenty pounds would then be melted. - -Let us now consider the effect that this condition of things would -have upon the level of the sea. It would evidently tend to produce an -elevation of the sea-level on the northern hemisphere in two ways. 1st. -The addition to the sea occasioned by the melting of the ice from off -the antarctic land would tend to raise the general level of the sea. -2ndly. The removal of the ice would also tend to shift the earth’s -centre of gravity to the north of its present position—and as the sea -must shift along with the centre, a rise of the sea on the northern -hemisphere would necessarily take place. - -The question naturally suggests itself, might not the last rise of the -sea, relative to the land, have resulted from this cause? We know that -during the period of the 25-foot beach, the time when the estuarine -mud, which now forms the rich soil of the Carses of the Forth and -Tay, was deposited, the sea, in relation to the land, stood at least -20 or 30 feet higher than at present. But immediately prior to this -period, we have the age of the submarine forests and peat-beds, when -the sea relative to the land stood lower than it does now. We know -also that these changes of level were not mere local affairs. There -seems every reason to believe that our Carse clay, as Mr. Fisher -states, is the equivalent of the marine mud, with _Scrobicularia_, -which covers the submarine forests of England.[227] And on the other -hand, those submarine forests are not confined to one locality. “They -may be traced,” says Mr. Jamieson, “round the whole of Britain and -Ireland, from Orkney to Cornwall, from Mayo to the shores of Fife, and -even, it would seem, along a great part of the western sea-board of -Europe, as if they bore witness to a period of widespread elevation, -when Ireland and Britain, with all its numerous islands, formed one -mass of dry land, united to the continent, and stretching out into the -Atlantic.”[228] “These submarine forests”“ remarks De la Beche, also, -“are to be found under the same general condition from the shores of -Scandinavia to those of Spain and Portugal, and around the British -islands.”[229] Those buried forests are not confined to Europe, but -are found in the valley of the Mississippi and in Nova Scotia, and -other parts of North America. And again, the strata which underlie -those forests and peat-beds bear witness to the fact of a previous -elevation of the sea-level. In short, we have evidence of a number of -oscillations of sea-level during post-tertiary times.[230] - -Had there been only one rise of the land relative to the sea-level, or -one depression, it might quite reasonably, as already remarked, have -been attributed to an upheaval or a sinking of the ground, occasioned -by some volcanic, chemical, or other agency. But certainly those -repeated oscillations of sea-level, extending as they do over so wide -an area, look more like a rising and sinking of the sea than of the -land. But, be this as it may, since it is now established, I presume, -beyond controversy, that the old notion that the general level of the -sea remains permanent, and that the changes must be all attributed to -the land is wholly incorrect, and that the sea, as well as the land, -is subject to changes of level, it is certainly quite legitimate to -consider whether the last elevation of the sea-level relatively to the -land may not have resulted from the rising of the sea rather than from -the sinking of the land, in short, whether it may not be attributed -to the cause we are now considering. The fact that those raised -beaches and terraces are found at so many different heights, and also -so discontinuously along our coasts, might be urged as an objection -to the opinion that they were due to changes in the level of the sea -itself. Space will not permit me to enter upon the discussion of this -point at present; but it may be stated that this objection is more -apparent than real. It by no means follows that beaches of the same -age must be at the same level. This has been shown very clearly by Mr. -W. Pengelly in a paper on “Raised Beaches,” read before the British -Association at Nottingham, 1866. - -We have, as I think, evidence amounting to almost absolute certainty -that 11,700 years ago the general sea-level on the northern hemisphere -must have been higher than at present. And in order to determine the -question of the 25-foot beach, we have merely to consider whether a -rise to something like this extent probably took place at the period in -question. We have at present no means of determining the exact extent -of the rise which must have taken place at that period, for we cannot -tell what quantity of ice was then melted off the antarctic regions. -But we have the means of making a very rough estimate, which, at least, -may enable us to determine whether a rise of some 20 or 30 feet may not -possibly have taken place. - -If we assume that the southern ice-cap extends on an average down -to lat. 70°, we shall have an area equal to 1/33·163 of the entire -surface of the globe. The proportion of land to that of water, taking -into account the antarctic continent, is as 526 to 1272. The southern -ice-cap will therefore be equal to 1/23·46 of the area covered by -water. The density of ice to that of water being taken at ·92 to 1, -it follows that 25 feet 6 inches of ice melted from off the face of -the antarctic continent would raise the level of the ocean one foot. -If 470 feet were melted off—and this is by no means an extravagant -supposition, when we reflect that for every 18 pounds of ice presently -melted an additional pound or two pounds, or perhaps more, would then -be melted, and that for many ages in succession—the water thus produced -from the melted ice would raise the level of the sea 18 feet 5 inches. -The removal of the 470 feet of solid ice— which must be but a very -small fraction of the total quantity of ice lying upon the antarctic -continent—would shift the earth’s centre of gravity about 7 feet to the -north of its present position. The shifting of the centre of gravity -would cause the sea to sink on the southern hemisphere and rise on the -northern. And the quantity of water thus transferred from the southern -hemisphere to the northern would carry the centre of gravity about one -foot further, and thus give a total displacement of the centre to the -extent of about 8 feet. The sea would therefore rise about 8 feet at -the North Pole, and in the latitude of Edinburgh about 6 feet 7 inches. -This, added to the rise of 18 feet 5 inches, occasioned by the melting -of the ice, would give 25 feet as the total rise in the latitude of -Scotland 11,700 years ago. - -Each square foot of surface at the poles 11,700 years ago would be -receiving 18,223,100 foot-pounds more of heat annually than at present. -If we deduct 22 per cent. as the amount absorbed in passing through the -atmosphere, we have 14,214,000 foot-pounds. This would be sufficient -to melt 2·26 feet of ice. But if 50, instead of 22, per cent. were cut -off, 1·45 cubic feet would be melted. In this case the 470 feet of ice -would be melted, independently of the effects of eccentricity, in about -320 years. And supposing that only one-fourth part of the extra heat -reached the ground, 470 feet of ice would be removed in about 640 years. - -As to the exact time that the obliquity was at a maximum, previous -to that of 11,700 years ago, our uncertainty is still greater. If we -are permitted to assume that the ecliptic passes from its maximum to -its minimum state and back to its maximum again with anything like -uniformity, at the rate assigned by Leverrier and others, the obliquity -would not be far from a maximum about 60,000 years ago. Taking the -rate of precession at 50″·21129, and assuming it to be uniform—which -it probably is not—the winter solstice would be in the aphelion about -61,300 years ago.[231] In short, it seems not at all improbable that -at the time the solstice-point was in the aphelion, the obliquity of -the ecliptic would not be far from its maximum state. But at that time -the value of the eccentricity was 0·023, instead of 0·0187, its value -at the last period. Consequently the rise of the sea would probably -be somewhat greater than it was 11,700 years ago. Might not this be -the period of the 40-foot beach? In this case 11,000 or 12,000 years -would be the age of the 25-foot beach, and 60,000 years the age of the -40-foot beach. - -About 22,000 years ago, the winter solstice was in the perihelion, and -as the eccentricity was then somewhat greater than it is at present, -the winters would be a little warmer and the climate more equable than -it is at the present day. This perhaps might be the period of the -submarine forests and lower peat-beds which underlie the Carse clays, -_Scrobicularia_ mud, and other deposits belonging to the age of the -25-foot beach. At any rate, it is perfectly certain that a condition -of climate at this period prevailed exceedingly favourable to the -growth of peat. It follows also that at this time, owing to a greater -accumulation of ice on the southern hemisphere, the sea-level would be -a few feet lower than at present, and that forests and peat may have -then grown on places which are now under the sea-level. - -For a few thousand years before and after 11,700 years ago, when the -winter solstice was evidently not far from the aphelion, and the sea -standing considerably above its present level, would probably, as we -have already stated, be the time when the Carse clays and other recent -deposits lying above the present level of the river were formed. -And it is also a singular fact that the condition of things at that -period must have been exceedingly favourable to the formation of -such estuarine deposits; for at that time the winter temperature of -our island, as has been already shown, would be considerably lower -than at present, and, consequently, during that season, snow, to a -much larger extent than now, would fall instead of rain. The melting -of the winter’s accumulation of snow on the approach of summer would -necessarily produce great floods, similar to what occur in the northern -parts of Asia and America at the present day from this very same -cause. The loose upper soil would be carried down by those floods and -deposited in the estuaries of our rivers. - -The foregoing is a rough and imperfect sketch of the history of the -climate and the physical conditions of our globe for the past 60,000 -years, in so far as physical and cosmical considerations seem to afford -us information on the subject, and its striking agreement with that -derived from geological sources is an additional evidence in favour -of the opinion that geological and cosmical phenomena are physically -related by a bond of causation. - -_Lieutenant-Colonel Drayson’s Theory of the Cause of the Glacial -Epoch._—In a paper read before the Geological Society by -Lieutenant-Colonel Drayson, R.A., on the 22nd February, 1871,[232] that -author states, that after calculating from the recorded positions of -the pole of the heavens during the last 2,000 years, he finds the pole -of the ecliptic is not the centre of the circle traced by the pole of -the heavens. The pole of the heavens, he considers, describes a circle -round a point 6° distant from the pole of the ecliptic and 29° 25′ 47″ -from the pole of the heavens, and that about 13,700 years b.c. the -angular distance of the two poles was 35° 25′ 47″. This would bring -the Arctic Circle down to latitude 54° 34′ 13″ N. I fear that this is -a conclusion that will not be generally accepted by those familiar with -celestial mechanics. But, be this as it may, my present object is not -to discuss the astronomical part of Colonel Drayson’s theory, but to -consider whether the conclusions which he deduces from his theory in -regard to the cause of the glacial epoch be legitimate or not. Assuming -for argument’s sake that the obliquity of the ecliptic can possibly -reach to 35° or 36°, so as to bring the Arctic Circle down to the -centre of England, would this account for the glacial epoch? Colonel -Drayson concludes that the shifting of the Arctic Circle down to the -latitude of England would induce here a condition of climate similar -to that which obtains in arctic regions. This seems to be the radical -error of the theory. It is perfectly true that were the Arctic Circle -brought down to latitude 54° 35′ part of our island would be in the -arctic regions, but it does not on that account follow that our island -would be subjected to an arctic climate. - -The polar regions owe their cold not to the obliquity of the ecliptic, -but to their distance from the equator. Indeed were it not for -obliquity those regions would be much colder than they really are, -and an increase of obliquity, instead of increasing their cold, would -really make them warmer. The general effect of obliquity, as we -have seen, is to diminish the amount of heat received in equatorial -and tropical regions, and to increase it in the polar and temperate -regions. The greater the obliquity, and, consequently, the farther -the sun recedes from the equator, the smaller is the quantity of heat -received by equatorial regions, and the greater the amount bestowed on -polar and temperate regions. If, for example, we represent the present -amount of heat received from the sun at the equator on a given surface -at 100 parts, 42·47 parts will then represent the amount received at -the poles on the same given surface. But were the obliquity increased -to 35° the amount received at the equator would be reduced to 94·93 -parts, and that at the poles increased to 59·81; being an increase at -the poles of nearly one half. At latitude 60° the present quantity -is equal to 57 parts; but about 63 parts would be received were the -obliquity increased to 35°. It therefore follows that although the -Arctic Circle were brought down to the latitude of London so that the -British islands would become a part of the arctic regions, the mean -temperature of these islands would not be lowered, but the reverse. -The winters would no doubt be colder than they are at present, but the -cold of winter would be far more than compensated for by the heat of -summer. It is not a fair representation of the state of things, merely -to say that an increase of obliquity tends to make the winters colder -and the summers hotter, for it affects the summer heat far more than -it does the winter cold. And the greater the obliquity the more does -the increase of heat during summer exceed the decrease during winter. -This is obvious because the greater the obliquity the greater the total -annual amount of heat received. - -If an increase of obliquity tended to produce an increase of ice in -temperate and polar regions, and thus to lead to a glacial epoch, then -the greater the obliquity the greater would be the tendency to produce -such an effect. Conceive, then, the obliquity to go on increasing until -it ultimately reached its absolute limit, 90°, and the earth’s axis to -coincide with the plane of the ecliptic. The Arctic Circle would then -extend to the equator. Would this produce a glacial epoch? Certainly -not. A square foot of surface at the poles would then be receiving -as much heat per annum as a square foot at the equator at present, -supposing the sun remained on the equator during the entire year. Less -heat, however, would be reaching the equatorial regions than now. At -present, as we have just seen, the annual quantity of heat received at -either pole is to that received at the equator as 42·47 to 100; but at -the period under consideration the poles would be actually obtaining -one-half more heat than the equator. The amount received per square -foot at the poles, to that received per square foot at the equator, -would be in the ratio of half the circumference of a circle to its -diameter, or as 1·5708 to 1. But merely to say that the poles would be -receiving more heat per annum than the equator is at present, does not -convey a correct idea of the excessive heat which the poles would then -have to endure; for it must be borne in mind that the heat reaching -the equator is spread over the whole year, whereas the poles would get -their total amount during the six months of their summer. Consequently, -for six months in the year the poles would be obtaining far more than -double the quantity of heat received at present by the equator during -the same length of time, and more than three times the quantity then -received by the equator. The amount reaching the pole during the six -months to that reaching the equator would be as 3·1416 to 1. - -At the equator twelve hours’ darkness alternates with twelve hours’ -sunshine, and this prevents the temperature from rising excessively -high; but at the poles it would be continuous sunshine for six months -without the ground having an opportunity of cooling for a single -hour. At the summer solstice, when the sun would be in the zenith of -the pole, the amount of heat received there every twenty-four hours -would actually be nearly three-and-a-quarter times greater than that -presently received at the equator. Now what holds true with regard to -the poles would hold equally true, though to a lesser extent, of polar -and temperate regions. We can form but a very inadequate idea of the -condition of things which would result from such an enormous increase -of heat. Nothing living on the face of the globe could exist in polar -regions under so fearful a temperature as would then prevail during -summer months. How absurd would it be to suppose that this condition -of things would tend to produce a glacial epoch! Not only would every -particle of ice in polar regions be dissipated, but the very seas -around the pole would be, for several months in the year, at the -boiling point. - -If it could be shown from _physical principles_—which, to say the -least, is highly improbable—that the obliquity of the ecliptic could -ever have been as great as 35°, it would to a very considerable -extent account for the comparative absence of ice in Greenland and -other regions in high latitudes, such as we know was the case during -the Carboniferous, Miocene, and other periods. But although a great -increase of obliquity might cause a melting of the ice, yet it could -not produce that mild condition of climate which we know prevailed in -high latitudes during those periods; while no increase of obliquity, -however great, could in any way tend to produce a glacial epoch. - -Colonel Drayson, however, seems to admit that this great increase of -obliquity would make our summers much warmer than they are at present. -How, then, according to his theory, is the glacial epoch accounted for? -The following is the author’s explanation as stated in his own words:— - -“At the date 13,700 B.C. the same conditions appear to have prevailed -down to about 54° of latitude during winter as regards the sun being -only a few degrees above the horizon. We are, then, warranted in -concluding that the same climate prevailed down to 54° of latitude as -now exists in winter down to 67° of latitude. - -“Thus in the greater part of England and Wales, and in the whole of -Scotland, icebergs of large size would be _formed each winter_; every -river and stream would be frozen and blocked with ice, the whole -country would be covered with a mantle of snow and ice, and those -creatures which could neither migrate nor endure the cold of an arctic -climate would be exterminated.”—“The Last Glacial Epoch,” p. 146. - -“At the summer solstice the midday altitude of the sun for the latitude -54° would be about 71½°, an altitude equal to that which the sun -now attains in the south of Italy, the south of Spain, and in all -localities having a latitude of about 40°.” - -“There would, however, be this singular difference from present -conditions, that in latitude 54° the sun at the period of the summer -solstice would remain the whole twenty-four hours above the horizon; -a fact which would give extreme heat to those very regions which, six -months previously, had been subjected to an arctic cold. Not only -would this greatly increased heat prevail in the latitude of 54°, but -the sun’s altitude would be 12° greater at midday in midsummer, and -also 12° greater at midnight in high northern latitudes, than it -ever attains now; consequently the heat would be far greater than at -present, and high northern regions, even around the pole itself, would -be subjected to a heat during summer far greater than any which now -ever exists in those localities. The natural consequence would be, that -the icebergs and ice which had during the severe winter accumulated in -high latitudes would be rapidly thawed by this heat” (p. 148). - -“Each winter the whole northern and southern hemispheres would be one -mass of ice; each summer nearly the whole of the ice of each hemisphere -would be melted and dispersed” (p. 150). - -According to this theory, not only is the whole country covered each -winter with a continuous mass of ice, but large icebergs are formed -during that short season, and when the summer heat sets in all is -melted away. Here we have a misapprehension not only as to the actual -condition of things during the glacial epoch, but even as to the way -in which icebergs and land-ice are formed. Icebergs are formed from -land-ice, but land-ice is not formed during a single winter, much -less a mass of sufficient thickness to produce icebergs. Land-ice of -this thickness requires the accumulated snows of centuries for its -production. All that we could really have, according to this theory, -would be a thick covering of snow during winter, which would entirely -disappear during summer, so that there could be no land-ice. - -_Mr. Thomas Belt’s Theory._—The theory that the glacial epoch resulted -from a great increase in the obliquity of the ecliptic has recently -been advocated by Mr. Thomas Belt.[233] His conceptions on the subject, -however, appear to me to be even more irreconcilable with physics than -those we have been considering. Lieutenant-Colonel Drayson admits that -the increase of heat to polar regions resulting from the great increase -of obliquity would dissipate the ice there, but Mr. Belt does not even -admit that an increase of obliquity would bring with it an increase of -heat, far less that it would melt the polar ice. On the contrary, he -maintains that the tendency of obliquity is to increase the rigour of -polar climate, and that this is the reason “that now around the poles -some lands are being glaciated, for excepting for that obliquity snow -and ice would not accumulate, excepting on mountain chains.” “Thus,” -he says, “there exist glacial conditions at present around the poles, -due primarily to the obliquity of the ecliptic.” And he also maintains -that if there were no obliquity and the earth’s axis were perpendicular -to the plane of its orbit, an eternal “spring would reign around the -arctic circle,” and that “under such circumstances the piling up of -snow, or even its production at the sea-level, would be impossible, -excepting perhaps in the immediate neighbourhood of the poles, where -the rays of the sun would have but little heating power from its small -altitude.” - -Mr. Belt has apparently been led to these strange conclusions by the -following singular misapprehension of the effects of obliquity on -the distribution of the sun’s heat over the globe. “The obliquity of -the ecliptic,” he remarks, “_does not affect the mean amount of heat -received at any one point from the sun_, but it causes the heat and the -cold to predominate at different seasons of the year.” - -It is not necessary to dwell further on the absurdity of the -supposition that an increase of obliquity can possibly account for the -glacial epoch, but we may in a few words consider whether a decrease -of obliquity would mitigate the climate and remove the snow from -polar regions. Supposing obliquity to disappear and the earth’s axis -to become perpendicular to the plane of its orbit, it is perfectly -true that day and night would be equal all over the globe, but then -the quantity of heat received by the polar regions would be far less -than at present. It is well known that at present at the equinoxes, -when day and night are equal, snow and not rain prevails in the arctic -regions, and can we suppose it could be otherwise in the case under -consideration? How, we may well ask, could these regions, deprived of -their summer, get rid of their snow and ice? - -But even supposing it could be shown that a change in the obliquity of -the ecliptic to the extent assumed by Mr. Belt and Lieutenant-Colonel -Drayson would produce a glacial epoch, still the assumption of such a -change is one which physical astronomy will not permit. Mr. Belt does -not appear to dispute the accuracy of the methods by which it is proved -that the variations of obliquity are confined within narrow limits; but -he maintains that physical astronomers, in making their calculations -have left out of account some circumstances which materially affect the -problem. These, according to Mr. Belt, are the following:—(1) Upheavals -and subsidences of the land which may have taken place in past ages. -(2) The unequal distribution of sea and land on the globe. (3) The fact -that the equatorial protuberance is not a regular one, “but approaches -in a general outline to an ellipse, of which the greater diameter is -two miles longer than the other.” (4) The heaping up of ice around the -poles during the glacial period. - -We may briefly consider whether any or all of these can sensibly affect -the question at issue. In reference to the last-mentioned element, it -is no doubt true that if an immense quantity of water were removed -from the ocean and placed around the poles in the form of ice it would -affect the obliquity of the ecliptic; but this is an element of change -which is not available to Mr. Belt, because according to his theory -the piling up of the ice is an effect which results from the change of -obliquity. - -In reference to the difference of two miles in the equatorial diameters -of the earth, the fact must be borne in mind that the longer diameter -passes through nearly the centre of the great depression of the Pacific -Ocean,[234] whereas the shorter diameter passes through the opposite -continents of Asia and America. Now, when we take into consideration -the fact that these continents are not only two-and-a-half times denser -than the ocean, but have a mean elevation of about 1,000 feet above -the sea-level, it becomes perfectly obvious that the earth’s mass must -be pretty evenly distributed around its axis of rotation, and that -therefore the difference in the equatorial diameters can exercise no -appreciable effect on the change of obliquity. It follows also that the -present arrangement of sea and land is the best that could be chosen to -prevent disturbance of motion. - -That there ever were upheavals and depressions of the land of so -enormous a magnitude as to lead to a change of obliquity to the extent -assumed by Lieutenant-Colonel Drayson and Mr. Belt is what, I presume, -few geologists would be willing to admit. Suppose the great table-land -of Thibet, with the Himalaya Mountains, were to sink under the sea, -it would hardly produce any sensible effect on the obliquity of the -ecliptic. Nay more; supposing that all the land in the globe were sunk -under the sea-level, or the ocean beds converted into dry land, still -this would not materially affect obliquity. The reason is very obvious. -The equatorial bulge is so immense that those upheavals and depressions -would not to any great extent alter the oblate form of the earth. The -only cause which could produce any sensible effect on obliquity, as has -already been noticed, would be the removal of the water of the ocean -and the piling of it up in the form of ice around the poles; but this -is a cause which is not available to Mr. Belt. - -_Sir Charles Lyell’s Theory._—I am also unable to agree with Sir -Charles Lyell’s conclusions in reference to the influence of the -obliquity of the ecliptic on climate. Sir Charles says, “It may be -remarked that if the obliquity of the ecliptic could ever be diminished -to the extent of four degrees below its present inclination, such a -deviation would be of geological interest, in so far as it would cause -the sun’s light to be disseminated over a broader zone inside of the -arctic and antarctic circles. Indeed, if the date of its occurrence in -past time could be ascertained, this greater spread of the solar rays, -implying a shortening of the polar night, might help in some slight -degree to account for a vegetation such as now characterizes lower -latitudes, having had in the Miocene and Carboniferous periods a much -wider range towards the pole.”[235] - -The effects, as we have seen, would be directly the reverse of what is -here stated, viz., the more the obliquity was diminished the _less_ -would the sun’s rays spread over the arctic and antarctic regions, and -conversely the more the obliquity was increased the _greater_ would -be the amount of heat spread over polar latitudes. The farther the -sun recedes from the equator, the greater becomes the amount of heat -diffused over the polar regions; and if the obliquity could possibly -attain its absolute limit (90°), it is obvious that the poles would -then be receiving more heat than the equator is now. - - - - - CHAPTER XXVI. - - COAL AN INTER-GLACIAL FORMATION. - - Climate of Coal Period Inter-glacial in Character.—Coal Plants - indicate an Equable, not a Tropical Climate.—Conditions - necessary for Preservation of Coal Plants.—Oscillations - of Sea-level necessarily implied.—Why our Coal-fields - contain more than One Coal-seam.—Time required to form a - Bed of Coal.—Why Coal Strata contain so little evidence of - Ice-action.—Land Flat during Coal Period.—Leading Idea of the - Theory.—Carboniferous Limestones. - - -_An Inter-glacial Climate the one best suited for the Growth of the -Coal Plants._—No assertion, perhaps, could appear more improbable, -or is more opposed to all hitherto received theories, than the one -that the plants which form our coal grew during a glacial epoch. But, -nevertheless, if the theory of secular changes of climate, discussed -in the foregoing chapters, be correct, we have in warm inter-glacial -periods (as was pointed out several years ago)[236] the very condition -of climate best suited for the growth of those kinds of trees and -vegetation of which our coal is composed. It is the generally received -opinion among both geologists and botanists that the flora of the Coal -period does not indicate the existence of a tropical, but a moist, -equable, and temperate climate. “It seems to have become,” says Sir -Charles Lyell, “a more and more received opinion that the coal plants -do not on the whole indicate a climate resembling that now enjoyed in -the equatorial zone. Tree-ferns range as far south as the southern -parts of New Zealand, and Araucanian pines occur in Norfolk Island. -A great preponderance of ferns and lycopodiums indicates moisture, -equability of temperature, and freedom from frost, rather than intense -heat.”[237] - -Mr. Robert Brown, the eminent botanist, considers that the rapid and -great growth of many of the coal plants showed that they grew in swamps -and shallow water of equable and genial temperature. - -“Generally speaking,” says Professor Page, “we find them resembling -equisetums, marsh-grasses, reeds, club-mosses, tree-ferns, and -coniferous trees; and these in existing nature attain their maximum -development in warm, temperate, and subtropical, rather than in -equatorial regions. The Wellingtonias of California and the pines of -Norfolk Island are more gigantic than the largest coniferous tree yet -discovered in the coal-measures.”[238] - -The Coal period was not only characterized by a great preponderance -over the present in the quantity of ferns growing, but also in the -number of different species. Our island possesses only about 50 -species, while no fewer than 140 species have been enumerated as having -inhabited those few isolated places in England over which the coal has -been worked. And Humboldt has shown that it is not in the hot, but in -the mountainous, humid, and shady parts of the equatorial regions that -the family of ferns produces the greatest number of species. - -“Dr. Hooker thinks that a climate warmer than ours now is, would -probably be indicated by the presence of an increased number of -flowering plants, which would doubtless have been fossilized with -the ferns; whilst a lower temperature, _equal to the mean of the -seasons now prevailing_, would assimilate our climate to that of such -cooler countries as are characterized by a disproportionate amount of -ferns.”[239] - -“The general opinion of the highest authorities,” says Professor Hull, -“appears to be that the climate did not resemble that of the equatorial -regions, but was one in which the temperature was free from extremes; -the atmosphere being warm and moist, somewhat resembling that of New -Zealand and the surrounding islands, which we endeavour to imitate -artificially in our hothouses.”[240] - -The enormous quantity of the carboniferous vegetation shows also that -the climate under which it grew could not have been of a tropical -character, or it must have been decomposed by the heat. Peat, so -abundant in temperate regions, is not to be found in the tropics. - -The condition most favourable to the preservation of vegetable remains, -at least under the form of peat, is a cool, moist, and equable climate, -such as prevails in the Falkland Islands at the present day. “In these -islands,” says Mr. Darwin, “almost every kind of plant, even the coarse -grass which covers the whole surface of the land, becomes converted -into this substance.”[241] - -From the evidence of geology we may reasonably infer that were -the difference between our summer and winter temperature nearly -annihilated, and were we to enjoy an equable climate equal to, or -perhaps a little above, the present mean annual temperature of our -island, we should then have a climate similar to what prevailed during -the Carboniferous epoch. - -But we have already seen that such must have been the character of our -climate at the time that the eccentricity of the earth’s orbit was at -a maximum, and winter occurred when the earth was in the perihelion of -its orbit. For, as we have already shown, the earth would in such a -case be 14,212,700 miles nearer to the sun in winter than in summer. -This enormous difference, along with other causes which have been -discussed, would almost extinguish the difference between summer and -winter temperature. The almost if not entire absence of ice and snow, -resulting from this condition of things, would, as has already been -shown, tend to raise the mean annual temperature of the climate higher -than it is at present. - -_Conditions necessary for the Preservation of the Coal Plants._—But -in order to the formation of coal, it is not simply necessary to have -a condition of climate suitable for the growth, but also for the -preservation, of a luxuriant vegetation. The very existence of coal is -as much due to the latter circumstance as to the former; nay more, as -we shall yet see, the fact that a greater amount of coal belongs to the -Carboniferous period than to any other, was evidently due not so much -to a more extensive vegetable growth during that age, suited to form -coal, as to the fact that that flora has been better preserved. Now, -as will be presently shown, we have not merely in the warm periods of -a glacial epoch a condition of climate best suited for the growth of -coal plants, but we have also in the cold periods of such an epoch the -condition most favourable for the preservation of those plants. - -One circumstance necessary for the preservation of plants is that they -should have been covered over by a thick deposit of sand, mud, or clay, -and for this end it is necessary that the area upon which the plants -grew should have become submerged. It is evident that unless the area -had become submerged, the plants could not have been covered over with -a thick deposit; and, even supposing they had been covered over, they -could not have escaped destruction from subaërial denudation unless -the whole had been under water. Another condition favourable, if not -essential, to the preservation of the plants, is that they should have -been submerged in a cold and not in a warm sea. Assuming that the -coal plants grew during a warm period of a glacial epoch, we have in -the cold period which succeeded all the above conditions necessarily -secured. - -It is now generally admitted that the coal trees grew near broad -estuaries and on immense flat plains but little elevated above -sea-level. But that the _Lepidodendra_, _Sigillariæ_, and other trees, -of which our coal is almost wholly composed, grew on dry ground, -elevated above sea-level, and not in swamps and shallow water, as -was at one time supposed, has been conclusively established by the -researches of Principal Dawson and others. After the growth of many -generations of trees, the plain is eventually submerged under the sea, -and the whole, through course of time, becomes covered over with thick -deposits of sand, gravel, and other sediments carried down by streams -from the adjoining land. After this the submerged plain becomes again -elevated above the sea-level, and forms the site of a second forest, -which, after continuing to flourish for long centuries, is in turn -destroyed by submergence, and, like the former, becomes covered over -with deposits from the land. This alternate process of submergence -and emergence goes on till we have a succession of buried forests -one above another, with immense stratified deposits between. These -buried forests ultimately become converted into beds of coal. This, -I presume, is a fair representation of the generally admitted way in -which our coal-beds had their origin. It is also worthy of notice that -the stratified beds between the coal-seams are of marine and not of -lacustrine origin. On this point I may quote the opinion of Professor -Hull, a well-known authority on the subject: “Whilst admitting,” he -says, “the occasional presence of lacustrine strata associated with the -coal-measures, I think we may conclude that the whole formation has -been essentially of marine and estuarine origin.”[242] - -_Coal-beds necessarily imply Oscillations of Sea-level._—It may also -be observed that each coal-seam indicates both an elevation and a -depression of the land. If, for example, there are six coal-seams, -one above the other, this proves that the land must have been, at -least, six times below and six times above sea-level. This repeated -oscillation of the land has been regarded as a somewhat puzzling and -singular circumstance. But if we assume coal to be an inter-glacial -formation, this difficulty not only disappears, but all the various -circumstances which we have been detailing are readily explained. -We have to begin with a warm inter-glacial period, with a climate -specially suited for the growth of the coal trees. During this period, -as has been shown in the chapter on Submergence, the sea would be -standing at a lower level than at present, laying bare large tracts -of sea-bottom, on which would flourish the coal vegetation. This -condition of things would continue for a period of 8,000 or 10,000 -years, allowing the growth of many generations of trees. When the warm -period came to a close, and the cold and glacial condition set in, the -climate became unsuited for the growth of the coal plants. The sea -would begin to rise, and the old sea-bottoms on which, during so long -a period, the forests grew, would be submerged and become covered by -sedimentary deposits brought down from the land. These forests becoming -submerged in a cold sea, and buried under an immense mass of sediment, -were then now protected from destruction, and in a position to become -converted into coal. The cold continuing for a period of 10,000 years, -or thereby, would be succeeded by another warm period, during which the -submerged areas became again a land-surface, on which a second forest -flourished for another 10,000 years, which in turn became submerged -and buried under drift on the approach of the second cold period. -This alternate process of submergence and emergence of the land, -corresponding to the rise and fall of sea-level during the cold and -warm periods, would continue so long as the eccentricity of the earth’s -orbit remained at a high value, till we might have, perhaps, five or -six submerged forests, one above the other, and separated by great -thicknesses of stratified deposits, these submerged forests being the -coal-beds of the present day. - - [Illustration: Fig. 10.] - -It is probable that the forests of the Coal period would extend inland -over the country, but only such portions as were slightly elevated -above sea-level would be submerged and covered over by sediment and -thus be preserved, and ultimately become coal-seams. The process will -be better understood from the following diagram. Let A B represent the -surface of the ground prior to a glacial epoch, and to the formation -of the beds of coal and stratified deposits represented in the -diagram. Let S S′ be the normal sea-level. Suppose the eccentricity -of the earth’s orbit begins to increase, and the winter solstice -approaches the perihelion, we have then a moderately warm period. The -sea-level sinks to 1, and forests of sigillariæ and other coal trees -cover the country from the sea-shore at 1, stretching away inland in -the direction of B. In course of time the winter solstice moves round -to aphelion and a cold period follows. The sea begins to rise and -continues rising till it reaches 1′. Denudation and the severity of -the climate destroy every vestige of the forest from 1′ backwards into -the interior; but the portion 1 1′ being submerged and covered over -by sediment brought down from the land is preserved. The eccentricity -continuing to increase in extent, the second inter-glacial period is -more warm and equable than the first, and the sea this time sinks to 2. -A second forest now covers the country down to the sea-shore at 2. This -second warm period is followed by the second cold period, more severe -than the first, and the sea-level rises to 2′. Denudation and severity -of climate now destroy every remnant of the forest, from 2′ inland, -but of course the submerged portion of 2 2′, like the former portion 1 -1′, is preserved. During the third warm period (the eccentricity being -still on the increase) the sea-level sinks to 3, and the country for -the third time is covered by forests, which extend down to 3. This -third warm period is followed by a cold glacial period more severe than -the preceding, and the sea-level rises to 3′, and the submerged portion -of the forest from 3 to 3′ becomes covered with drift,—the rest as -before being destroyed by denudation and the severity of the climate. -We shall assume that the eccentricity has now reached a maximum, and -that during the fourth inter-glacial period the sea-level sinks only to -4, the level to which it sank during the second inter-glacial period. -The country is now covered for the fourth time by forests. The cold -period which succeeds not being so severe as the last, the sea rises -only to 4′, which, of course, marks the limit of the fourth forest. The -eccentricity continuing to diminish, the fifth forest is only submerged -up to 5′, and the sixth and last one up to 6′. The epoch of cold and -warm periods being now at a close, the sea-level remains stationary at -its old normal position S S′. Here we have six buried forests, the one -above the other, which, through course of ages, become transformed into -coal-beds. - -It does not, however, necessarily follow that each separate coal-seam -represents a warm period. It is quite possible that two or more seams -separated from each other by thin partings or a few feet of sedimentary -strata might have been formed during one warm period; for during a warm -period minor oscillations of sea-level sufficient to submerge the land -to some depth might quite readily have taken place from the melting of -polar ice, as was shown in the chapter on Submergence. - -It may be noticed that in order to make the section more distinct, its -thickness has been greatly exaggerated. It will also be observed that -beds 4, 5, and 6 extend considerably to the left of what is represented -in the section. - -But it is not to be supposed that the whole phenomena of the -coal-fields can be explained without supposing a subsidence of the -land. The great depth to which the coal-beds have been sunk, in many -cases, must be attributed to a subsidence of the level. A series of -beds formed during a glacial epoch, may, owing to a subsidence of the -land, be sunk to a great depth, and become covered over with thousands -of feet of sediment; and then on the occurrence of another glacial -epoch, a new series of coal-beds may be formed on the surface. Thus -the upper series may be separated from the lower by thousands of feet -of sedimentary rock. There is another consequence resulting from the -sinking of the land, which must be taken into account. Had there been -no sinking of the land during the Carboniferous age, the quantity of -coal-beds now remaining would be far less than it actually is, for it -is in a great measure owing to their being sunk to a great depth that -they have escaped destruction by the enormous amount of denudation -which has taken place since that remote age. It therefore follows that -only a very small fraction of the submerged forests of the Coal period -do actually now exist in the form of coal. Generally it would only be -those areas which happened to be sunk to a considerable depth, by a -subsidence of the land, that would escape destruction from denudation. -But no doubt the areas which would thus be preserved bear but a small -proportion to those destroyed. - -_Length of Inter-glacial Period, as indicated by the Thickness of a -Bed of Coal._—A fact favourable to the idea that the coal-seams were -formed during inter-glacial periods is, that the length of those -periods agrees pretty closely with the length of time supposed to be -required to form a coal-seam of average thickness. Other things being -equal, the thickness of a coal-seam would depend upon the length -of the inter-glacial period. If the rate of precession and motion -of the perihelion were always uniform the periods would all be of -equal length. But although the rate of precession is not subject to -much variation, such is not the case in regard to the motion of the -perihelion, as will be seen from the tables of the longitude of the -perihelion given in Chapter XIX. Sometimes the motion of the perihelion -is rapid, at other times slow, while in some cases its motion is -retrograde. In consequence of this, an inter-glacial period may not be -more than some six or seven thousand years in length, while in other -cases its length may be as much as fifteen or sixteen thousand years. - -According to Boussingault, luxuriant vegetation at the present day -takes from the atmosphere about a half ton of carbon per acre -annually, or fifty tons per acre in a century. Fifty tons of carbon of -the specific gravity of coal, about 1·5, spread evenly over the surface -of an acre, would make a layer nearly one-third of an inch.[243] -Humboldt makes the estimate a little higher, viz., one half-inch. -Taking the latter estimate, it would require 7,200 years to form a -bed of coal a yard thick. Dr. Heer, of Zurich, thinks that it would -not require more than 1,400 years to form a bed of coal one yard -thick;[244] while Mr. Maclaren thinks that a bed of coal one yard thick -would be formed in 1,000 years.[245] Professor Phillip, calculating -from the amount of carbon taken from the atmosphere, as determined by -Liebig, considers that if it were converted into ordinary coal with -about 75 per cent. of carbon, it would yield one inch in 127·5 years, -or a yard in 4,600 years.[246] - -There is here a considerable amount of difference in regard to the time -required to form a yard of coal. The truth, however, may probably be -somewhere between the two extremes, and we may assume 5,000 years to be -about the time. In a warm period of 15,000 years we should then have -deposited a seam of coal 9 feet thick, while during a warm period of -7,000 years we would have a seam of only 4 feet. - -_Reason why the Coal Strata present so little Evidence of -Ice-action._—There are two objections which will, no doubt, present -themselves to the reader’s mind. (1.) If coal be an inter-glacial -formation, why do the coal strata present so little evidence of -ice-action? If the coal-seams represent warm inter-glacial periods, the -intervening beds must represent cold or glacial periods, and if so, -they ought to contain more abundant evidence of ice-action than they -really do. (2.) In the case of the glacial epoch, almost every vestige -of the vegetation of the warm periods was destroyed by the ice of the -cold periods; why then did not the same thing take place during the -glacial epoch of the Carboniferous period? - -During the glacial epoch the face of the country was in all -probability covered for ages with the most luxuriant vegetation; but -scarcely a vestige of that vegetation now remains, indeed the very soil -upon which it grew is not to be found. All that now remains is the -wreck and desolation produced by the ice-sheet that covered the country -during the cold periods of that epoch, consisting of transported blocks -of stones, polished and grooved rocks, and a confused mass of boulder -clay. Here we have in this epoch nothing tangible presenting itself -but the destructive effects of the ice which swept over the land. Why, -then, in reference to the glacial epochs of the Carboniferous age -should we have such abundant evidence of the vegetation of the warm -periods, and yet so little evidence of the effect of the ice of the -cold periods? The answer to these two objections will go a great way -to explain why we have so much coal belonging to the Carboniferous -age, and so little belonging to any other age; and it will, I think, -be found in the peculiar physical character of the country during -the Carboniferous age. The areas on which the forests of the Coal -period grew escaped the destructive power of glaciers and land-ice on -account of the flat nature of the ground. There are few points on which -geologists are more unanimous than in regard to the flat character of -the country during the Coal period. - -There does not seem to be any very satisfactory evidence that the -interior of the country rose to any very great elevation. Mr. -Godwin-Austen thinks that during the Coal period there must have -been “a vast expanse of continuous horizontal surface at very slight -elevations above the sea-level.”[247] Of the widely spread terrestrial -surface of the Coal-measure period, portions, he believes, attained -a considerable elevation. But in contrast to this he states, “There -is a feature which seems to distinguish this period physically from -all subsequent periods, and which consists in the vast expanse of -continuous horizontal surface which the land area presented, bordering -on, and at very slight elevations above, the sea-level.”[248] Hugh -Miller, describing in his usual graphic way the appearance of the -country during the Coal period, says:—“It seems to have been a land -consisting of immense flats, unvaried, mayhap, by a _single hill_, -in which dreary swamps, inhabited by doleful creatures, spread out -on every hand for hundreds and thousands of miles; and a gigantic -and monstrous vegetation formed, as I have shown, the only prominent -features of the scenery.”[249] - -Now, if this is in any way like a just representation of the general -features of the country during the Coal period, it was physically -impossible, no matter however severe the climate may have been, -that there could have been in this country at that period anything -approaching to continental ice, or perhaps even to glaciers of such -dimensions as would reach down to near the sea-level, where the coal -vegetation now preserved is supposed chiefly to have grown. The -condition of things which would prevail would more probably resemble -that of Siberia than that of Greenland. - -The absence of all traces of ice-action in the strata of the -coal-measures can in this case be easily explained. For as by -supposition there were no glaciers, there could have been no -scratching, grooving, or polishing of the rocks; neither could there -have been any icebergs, for the large masses known as icebergs are -the terminal portions of glaciers which have reached down to the sea. -Again, there being no icebergs, there of course could have been no -grinding or scratching of the rocks forming the floor of the ocean. -True, during summer, when the frozen sea broke up, we should then -have immense masses of floating ice, but these masses would not be of -sufficient thickness to rub against the sea-bottom. But even supposing -that they did occasionally touch the bottom here and there, we could -not possibly find the evidence of this in any of the strata of the -coal-measures. We could not expect to find any scratchings or markings -on the sandstone or shale of those strata indicating the action of -ice, for at that period there were no beds of sandstone or shale, but -simply beds of sand and mud, which in future ages became consolidated -into sandstone and shale. A mass of ice might occasionally rub along -the sea-bottom, and leave its markings on the loose sand or soft mud -forming that bottom, but the next wave that passed over it would -obliterate every mark, and leave the surface as smooth as before. -Neither could we expect to find any large erratics or boulders in the -coal strata, for these must come from the land, and as by supposition -there were no glaciers or land-ice at that period, there was therefore -no means of transporting them. In Greenland the icebergs sometimes -carry large boulders, which are dropped into the sea as the icebergs -melt away; but these blocks have all either been transported on the -backs of glaciers from inland tracts, or have fallen on the field-ice -along the shore from the face of crags and overhanging precipices. -But as there were probably neither glaciers reaching to the sea, nor -perhaps precipitous cliffs along the sea-shore, there could have been -few or no blocks transported by ice and dropped into the sea of the -Carboniferous period, and of course we need not expect to find them in -the sandstone and shale which during that epoch formed the bed of the -ocean. There would no doubt be coast-line ice and ground-ice in rivers, -carrying away large quantities of gravel and stones; but these gravels -and stones would of course be all water-worn, and although found in the -strata of the coal-measures, as no doubt they actually are, they would -not be regarded as indicating the action of ice. The simple absence of -relics of ice-action in the coal-measures proves nothing whatever in -regard to whether there were cold periods during their formation or not. - -This comparative absence of continental ice might be one reason why -the forests of the Carboniferous period have been preserved to a much -greater extent than those of any other age. - -It must be observed, however, that the conclusions at which we have -arrived in reference to the comparative absence of continental ice -applies only to the areas which now constitute our coal-fields. The -accumulation of ice on the antarctic regions, and on some parts of -the arctic regions, might have been as great during that age as it -is at present. Had there been no continental ice there could have -been no such oscillations of sea-level as is assumed in the foregoing -theory. The leading idea of the theory, expressed in a few words, -is, that the glacial epochs of the Carboniferous age were as severe, -and the accumulation of ice as great, as during any other age, only -there were large tracts of flat country, but little elevated above the -sea-level, which were not covered by ice. These plains, during the -warm inter-glacial periods, were covered with forests of sigillariæ -and other coal trees. Portions of those forests were protected by the -submergence which resulted from the rise of the sea-level during the -cold or glacial periods and the subsequent subsidence of the land. -Those portions now constitute our coal-beds. - -But that coal may be an inter-glacial formation is no mere hypothesis, -for we have in the well-known Dürnten beds—described in Chapter XV.—an -actual example of such a formation. - -_Carboniferous Limestones._—As a general rule the limestones of the -Carboniferous period, like the coal, are found in beds separated by -masses of sandstone and other stratified deposits, which proves that -the corals, crinoids, and other creatures, of the remains of which it -is composed, did not live continuously on during the entire Limestone -period. These limestones are a marine formation. If the land was -repeatedly submerged the coal must of necessity have been produced in -seams with stratified deposits between, but there is no reason why the -same should have been the case with the limestones. If the climatic -condition of the sea continued the same we should not have expected -this alternate succession of life and death; but, according to the -theory of alternate cold and warm periods, such a condition follows -as a necessary consequence, for during the warm periods, when the -land was covered with a luxuriant vegetation, the sea-bottom would be -covered with mollusca, crinoids, corals, &c., fitted to live only in a -moderately warm sea; but when the cold came on those creatures would -die, and their remains, during the continuance of the cold period, -would become slowly covered over with deposits of sand and clay. On the -return of the warm period those deposits would soon become covered with -life as before, forming another bed of limestone, and this alternation -of life and death would go on as long as the glacial epoch continued. - -It is true that in Derbyshire, and in the south of Ireland and some -other places, the limestone is found in one mass of several hundred -feet in thickness without any beds of sandstone or shale, but then it -is nowhere found in one continuous mass from top to bottom without any -lines of division. These breaks or divisions may as distinctly mark -a cold period as though they had been occupied by beds of sandstone. -The marine creatures ceased to exist, and when the rough surface left -by their remains became smoothed down by the action of the waves into -a flat plain, another bed would begin to form upon this floor so -soon as life again appeared. Two agencies working together probably -conspired to produce these enormous masses of limestone divided only -by breaks marking different periods of elaboration. Corals grow in -warm seas, and there only in water of a depth ranging from 20 to 30 -fathoms. The cold of a period of glaciation would not only serve to -destroy them, but they would be submerged so much beyond the depth -proper for their existence that even were it possible that with the -submergence a sufficient temperature was left, they would inevitably -perish from the superincumbent mass of water. We are therefore, as -it seems to me, warranted in concluding that the separate masses of -Derbyshire limestone were formed during warm inter-glacial periods, -and that the lines of division represent cold periods of glaciation -during which the animals perished by the combined influence of cold and -pressure of water. The submergence of the coral banks in deep water on -a sea-bottom, which, like the land, was characteristically flat and -even, implies its carrying away far into the bosom of the ocean, and -consequently remote from any continent and the river-borne detritus -thereof. - - - - - CHAPTER XXVII. - - PATH OF THE ICE-SHEET IN NORTH-WESTERN EUROPE AND ITS RELATIONS - TO THE BOULDER CLAY OF CAITHNESS.[250] - - Character of Caithness Boulder Clay.—Theories of the Origin - of the Caithness Clay.—Mr. Jamieson’s Theory.—Mr. C. W. - Peach’s Theory.—The proposed Theory.—Thickness of Scottish - Ice-sheet.—Pentlands striated on their Summits.—Scandinavian - Ice-sheet.—North Sea filled with Land-ice.—Great Baltic - Glacier.—Jutland and Denmark crossed by Ice.—Sir R. - Murchison’s Observations.—Orkney, Shetland, and Faroe Islands - striated across.—Loess accounted for.—Professor Geikie’s - Suggestion.—Professor Geikie and B. N. Peach’s Observations - on East Coast of Caithness.—Evidence from Chalk Flints and - Oolitic Fossils in Boulder Clay. - - -_The Nature of the Caithness Boulder Clay._—A considerable amount of -difficulty has been felt by geologists in accounting for the origin of -the boulder clay of Caithness. It is an unstratified clay, of a deep -grey or slaty colour, resembling much that of the Caithness flags on -which it rests. It is thus described by Mr. Jamieson (Quart. Jour. -Geol. Soc., vol. xxii., p. 261):— - -“The glacial drift of Caithness is particularly interesting as an -example of a boulder clay which in its mode of accumulation and -ice-scratched _débris_ very much resembles that unstratified stony mud -which occurs underneath glaciers—the ‘_moraine profonde_,’ as some call -it. - -“The appearance of the drift along the Haster Burn, and in many other -places in Caithness, is in fact precisely the same as that of the old -boulder clay of the rest of Scotland, except that it is charged with -remains of sea-shells and other marine organisms. - -“If want of stratification, hardness of texture, and abundance of -well-glaciated stones and boulders are to be the tests for what we call -genuine boulder clay, then much of the Caithness drift will stand the -ordeal.” - -So far, therefore, as the mere appearance of the drift is concerned, -it would at once be pronounced to be true Lower Till, the product of -land-ice. But there are two circumstances connected with it which have -been generally regarded as fatal to this conclusion. - -(1) The striæ on the rocks show that the ice which formed the clay -must have come from the sea, and not from the interior of the country; -for their direction is almost at right angles to what it would have -been had the ice come from the interior. Over the whole district, the -direction of the grooves and scratches, not only of the rocks but -even of the stones in the clay, is pretty nearly N.W. and S.E. “When -examining the sections along the Haster Burn,” says Mr. Jamieson, “in -company with Mr. Joseph Anderson, I remarked that the striæ on the -imbedded fragments generally agreed in direction with those of the -rocks beneath. The scratches on the boulders, as usual, run lengthways -along the stones when they are of an elongated form; and the position -of these stones, as they lie imbedded in the drift, is, as a rule, such -that their longer axes point in the same direction as do the scratches -on the solid rock beneath; showing that the same agency that scored the -rocks also ground and pushed along the drift.” - -Mr. C. W. Peach informs me that he seldom or never found a stone with -two sets of striæ on it, a fact indicating, as Mr. Jamieson remarks, -that the drift was produced by one great movement invariably in the -same direction. Let it be borne in mind that the ice, which thus moved -over Caithness in this invariable track, must either have come from the -Atlantic to the N.W., or from the Moray Firth to the S.E. - -(2) The boulder clay of Caithness is full of sea-shells and other -marine remains. The shells are in a broken condition, and are -interspersed like the stones through the entire mass of the clay. -Mr. Jamieson states that he nowhere observed any instance of shells -being found in an undisturbed condition, “nor could I hear,” he says, -“of any such having been found; there seems to be no such thing as a -bed of laminated silt with shells _in situ_.” The shell-fragments are -scratched and ice-worn, the same as the stones found in the clay. Not -only are the shells glaciated, but even the foraminifera, when seen -through the microscope, have a rubbed and worn appearance. The shells -have evidently been broken, striated, and pushed along by the ice at -the time the boulder clay was being formed. - -_Theories regarding the Origin of the Caithness Clay._—Mr. Jamieson, as -we have seen, freely admits that the boulder clay of Caithness has the -appearance of true land-ice till, but from the N.W. and S.E. direction -of the striæ on the rocks, and the presence of sea-shells in the clay, -he has come to the conclusion that the glaciation of Caithness has been -effected by floating ice at a time when the district was submerged. I -have always felt convinced that Mr. Jamieson had not hit upon the true -explanation of the phenomena. - -(1) It is physically impossible that any deposit formed by icebergs -could be wholly unstratified. Suppose a mass of the materials which -would form boulder clay is dropped into the sea from, say an iceberg, -the heavier parts, such as stones, will reach the bottom first. Then -will follow lighter materials, such as sand, then clay, and last of all -the mud will settle down over the whole in fine layers. The different -masses dropped from the various icebergs, will, no doubt, lie in -confusion one over the other, but each separate mass will show signs of -stratification. A good deal of boulder clay evidently has been formed -in the sea, but if the clay be unstratified, it must have been formed -under glaciers moving along the sea-bottom as on dry ground. Whether -_unstratified_ boulder clay may happen to be formed under water or on -dry land, it must in either case be the product of land-ice.[251] Those -who imagine that materials, differing in specific gravity like those -which compose boulder clay, dropped into water, can settle down without -assuming the stratified form, should make the experiment, and they -would soon satisfy themselves that the thing is physically impossible. -The notion that unstratified boulder clay could be formed by deposits -from floating ice, is not only erroneous, but positively pernicious, -for it tends to lead those who entertain it astray in regard to the -whole question of the origin of drift. - -(2) It is also physically impossible that ice-markings, such as those -everywhere found on the rocky face of the district, and on the pebbles -and shells imbedded in the clay, could have been effected by any other -agency than that of land-ice. I need not here enter into any discussion -on this point, as this has been done at considerable length in another -place.[252] In the present case, however, it is unnecessary, because -if it can be shown that all the facts are accounted for in the most -natural manner by the theory of land-ice, no one will contend for the -floating-ice theory; for it is admitted that, with the exception of the -direction of the striæ and the presence of the shells, all the facts -agree better with the land-ice than with the floating-ice theory. - -My first impression on the subject was that the glaciation of Caithness -had been effected by the polar ice-cap, which, during the severer part -of the glacial epoch, must have extended down to at least the latitude -of the north of Scotland. - -On a former occasion (see the _Reader_ for 14th October, 1865) it was -shown that all the northern seas, owing to their shallowness, must, -at that period, have been blocked up with solid ice, which displaced -the water and moved along the sea-bottoms the same as on dry land. In -fact, the northern seas, including the German Ocean, being filled at -the time with glacier-ice, might be regarded as dry land. Ice of this -sort, moving along the bed of the German Ocean or North Sea, and over -Caithness, could not fail to push before it the shells and other animal -remains lying on the sea-bottom, and to mix them up with the clay -which now remains upon the land as evidence of its progress. - -About two years ago I had a conversation with Mr. C. W. Peach on the -subject. This gentleman, as is well known, has long been familiar with -the boulder clay of Caithness. He felt convinced that the clay of that -country is the true Lower Till, and not a more recent deposit, as Mr. -Jamieson supposes. He expressed to me his opinion that the glaciation -of Caithness had been effected by masses of land-ice crossing the -Moray Firth from the mountain ranges to the south-east, and passing -over Caithness in its course. The difficulty which seems to beset -this theory is, that a glacier entering the Firth would not leave it -and ascend over the Caithness coast. It would take the path of least -resistance and move into the North Sea, where it would find a free -passage into deeper water. Mr. Peach’s theory is, however, an important -step in the right direction. It is a part of the truth, but I believe -not the whole truth. The following is submitted as a solution of the -question. - -_The Proposed Theory._—It may now be regarded as an established fact -that, during the severer part of the glacial period, Scotland was -covered with one continuous mantle of ice, so thick as to bury under -it the Ochil, Sidlaw, Pentland, Campsie, and other moderately high -mountain ranges. For example, Mr. J. Geikie and Mr. B. N. Peach found -that the great masses of the ice from the North-west Highlands, came -straight over the Ochils of Perthshire and the Lomonds of Fife. In -fact, these mountain ridges were not sufficiently high to deflect the -icy stream either to the right hand or to the left; and the flattened -and rounded tops of the Campsie, Pentland, and Lammermoor ranges bear -ample testimony to the denuding power of ice. - -Further, to quote from Mr. Jamieson, “the detached mountain of -Schehallion in Perthshire, 3,500 feet high, is marked near the top as -well as on its flanks, and this not by ice flowing down the sides of -the hill itself, but by ice pressing over it from the north. On the top -of another isolated hill, called Morven, about 3,000 feet high, and -situated a few miles to the north of the village of Ballater, in the -county of Aberdeen, I found granite boulders unlike the rock of the -hill, and apparently derived from the mountains to the west. Again, -on the highest watersheds of the Ochils, at altitudes of about 2,000 -feet, I found this summer (1864) pieces of mica schist full of garnets, -which seem to have come from the Grampian Hills to the north-west, -showing that the transporting agent had overflowed even the highest -parts of the Ochil ridge. And on the West Lomonds, in Fifeshire, at -Clattering-well Quarry, 1,450 feet high, I found ice-worn pebbles of -Red Sandstone and porphyry in the _débris_ covering the Carboniferous -Limestone of the top of the Bishop Hill. Facts like these meet us -everywhere. Thus on the Perthshire Hills, between Blair Athol and -Dunkeld, I found ice-worn surfaces of rocks on the tops of hills, at -elevations of 2,200 feet, as if caused by ice pressing over them from -the north-west, and transporting boulders at even greater heights.”[253] - -Facts still more important, however, in their bearing on the question -before us were observed on the Pentland range by Mr. Bennie and myself -during the summer of 1870. On ascending Allermuir, one of the hills -forming the northern termination of the Pentland range, we were not a -little surprised to find its summit ice-worn and striated. The top of -the hill is composed of a compact porphyritic felstone, which is very -much broken up; but wherever any remains of the original surface could -be seen, it was found to be polished and striated in a most decided -manner. These striæ are all in one uniform direction, nearly east and -west; and on minutely examining them with a lens we had no difficulty -whatever in determining that the ice which effected them came from the -west and not from the east, a fact which clearly shows that they must -have been made at the time when, as is well known, the entire Midland -valley was filled with ice, coming from the North-west Highlands. On -the summit of the hill we also found patches of boulder clay in hollow -basins of the rock. At one spot it was upwards of a foot in depth, and -rested on the ice-polished surface. The clay was somewhat loose and -sandy, as might be expected of a layer so thin, exposed to rain, frost, -and snow, during the long course of ages which must have elapsed since -it was deposited there. Of 100 pebbles collected from the clay, just as -they turned up, every one, with the exception of three or four composed -of hard quartz, presented a flattened and ice-worn surface; and -forty-four were distinctly striated: in short, every stone which was -capable of receiving and retaining scratches was striated. A number of -these stones must have come from the Highlands to the north-west.[254] - -The height of Allermuir is 1,617 feet, and, from its position, it is -impossible that the ice could have gone over its summit, unless the -entire Midland valley, at this place, had been filled with ice to the -depth of more than 1,600 feet. The hill is situated about four or -five miles to the south of Edinburgh, and forms, as has already been -stated, the northern termination of the Pentland range. Immediately -to the north lies the broad valley of the Firth of Forth, more than -twelve miles across, offering a most free and unobstructed outlet for -the great mass of ice coming along the Midland valley from the west. -Now, when we reflect how easily ice can accommodate itself to the -inequalities of the channel along which it moves, how it can turn to -the right hand or to the left, so as to find for itself the path of -least resistance, it becomes obvious that the ice never would have gone -over Allermuir, unless not only the Midland valley at this point, but -also the whole surrounding country had been covered with one continuous -mass of ice to a depth of more than 1,600 feet. But it must not be -supposed that the height of Allermuir represents the thickness of the -ice; for on ascending Scald Law, a hill four miles to the south-west -of Allermuir, and the highest of the Pentland range, we found, in -the _débris_ covering its summit, hundreds of transported stones of -all sizes, from one to eighteen inches in diameter. We also dug up a -Greenstone boulder about eighteen inches in diameter, which was finely -polished and striated. As the height of this hill is 1,898 feet, the -mass of ice covering the surrounding country must have been at least -1,900 feet deep. But this is not all. Directly to the north of the -Pentlands, in a line nearly parallel with the east coast, and at right -angles to the path of ice from the interior, there is not, with the -exception of the solitary peak of East Lomond, and a low hill or two of -the Sidlaw range, an eminence worthy of the name of a hill nearer than -the Grampians in the north of Forfarshire, distant upwards of sixty -miles. This broad plain, extending from almost the Southern to the -Northern Highlands, was the great channel through which the ice of the -interior of Scotland found an outlet into the North Sea. If the depth -of the ice in the Firth of Forth, which forms the southern side of this -broad hollow, was at least 1,900 feet, it is not at all probable that -its depth in the northern side, formed by the Valley of Strathmore -and the Firth of Tay, which lay more directly in the path of the ice -from the North Highlands, could have been less. Here we have one vast -glacier, more than sixty miles broad and 1,900 feet thick, coming from -the interior of the country. - -It is, therefore, evident that the great mass of ice entering the North -Sea to the east of Scotland, especially about the Firths of Forth -and Tay, could not have been less, and was probably much more, than -from 1,000 to 2,000 feet in thickness. The grand question now to be -considered is, What became of the huge sheet of ice after it entered -the North Sea? Did it break up and float away as icebergs? This appears -to have been hitherto taken for granted; but the shallowness of the -North Sea shows such a process to have been utterly impossible. The -depth of the sea in the English Channel is only about twenty fathoms, -and although it gradually increases to about forty fathoms at the -Moray Firth, yet we must go to the north and west of the Orkney and -Shetland Islands ere we reach the 100 fathom line. Thus the average -depth of the entire North Sea is not over forty fathoms, which is even -insufficient to float an iceberg 300 feet thick. - -No doubt the North Sea, for two reasons, is now much shallower than -it was during the period in question. (1.) There would, at the time -of the great extension of the ice on the northern hemisphere, be a -considerable submergence, resulting from the displacement of the -earth’s centre of gravity.[255] (2.) The sea-bed is now probably -filled up to a larger extent with drift deposits than it was at the -ice period. But, after making the most extravagant allowance for the -additional depth gained on this account, still there could not possibly -have been water sufficiently deep to float a glacier of 1,000 or 2,000 -feet in thickness. Indeed, the North Sea would have required to be -nearly ten times deeper than it is at present to have floated the -ice of the glacial period. We may, therefore, conclude with the most -perfect certainty that the ice-sheet of Scotland could not possibly -have broken up into icebergs in such a channel, but must have moved -along on the bed of the sea in one unbroken mass, and must have found -its way to the deep trough of the Atlantic, west of the Orkney and -Shetland Islands, ere it broke up and floated away in the iceberg form. - -It is hardly necessary to remark that the waters of the North Sea would -have but little effect in melting the ice. A shallow sea like this, -into which large masses of ice were entering, would be kept constantly -about the freezing-point, and water of this temperature has but little -melting power, for it takes 142 lbs. of water, at 33°, to melt one -pound of ice. In fact, an icy sea tends rather to protect the ice -entering it from being melted than otherwise. And besides, owing to -fresh acquisitions of snow, the ice-sheet would be accumulating more -rapidly upon its upper surface than it would be melting at its lower -surface, supposing there were sea-water under that surface. The ice of -Scotland during the glacial period must, of necessity, have found its -way into warmer water than that of the North Sea before it could have -been melted. But this it could not do without reaching the Atlantic, -and in getting there it would have to pass round by the Orkney Islands, -along the bed of the North Sea, as land-ice. - -This will explain how the Orkney Islands may have been glaciated by -land-ice; but it does not, however, explain how Caithness should have -been glaciated by that means. These islands lay in the very track of -the ice on its way to the Atlantic, and could hardly escape being -overridden; but Caithness lay considerably to the left of the path -which we should expect the ice to have taken. The ice would not leave -its channel, turn to the left, and ascend upon Caithness, unless it -were forced to do so. What, then, compelled the ice to pass over -Caithness? - -_Path of the Scandinavian Ice._—We must consider that the ice from -Scotland and England was but a fraction of that which entered the -North Sea. The greater part of the ice of Scandinavia must have gone -into this sea, and if the ice of our island could not find water -sufficiently deep in which to float, far less would the much thicker -ice of Scandinavia do so. The Scandinavian ice, before it could break -up, would thus, like the Scottish ice, have to cross the bed of the -North Sea and pass into the Atlantic. It could not pass to the north, -or to the north-west, for the ocean in these directions would be -blocked up by the polar ice. It is true that along the southern shore -of Norway there extends a comparatively deep trough of from one to two -hundred fathoms. But this is evidently not deep enough to have floated -the Scandinavian ice-sheet; and even supposing it had been sufficiently -deep, the floating ice must have found its way to the Atlantic, and -this it could not have done without passing along the coast. Now, its -passage would not only be obstructed by the mass of ice continually -protruding into the sea directly at right angles to its course, but it -would be met by the still more enormous masses of ice coming off the -entire Norwegian coast-line. And, besides this, the ice entering the -Arctic Ocean from Lapland and the northern parts of Siberia, except -the very small portion which might find an outlet into the Pacific -through Behring’s Straits, would have to pass along the Scandinavian -coast in its way to the Atlantic. No matter, then, what the depth of -this trough may have been, if the ice from the land, after entering -it, could not make its escape, it would continue to accumulate till -the trough became blocked up; and after this, the great mass from the -land would move forward as though the trough had no existence. Thus, -the only path for the ice would be by the Orkney and Shetland Islands. -Its more direct and natural path would, no doubt, be to the south-west, -in the direction of our shores; and in all probability, had Scotland -been a low flat island, instead of being a high and mountainous one, -the ice would have passed completely over it. But its mountainous -character, and the enormous masses of ice at the time proceeding from -its interior, would effectually prevent this, so that the ice of -Scandinavia would be compelled to move round by the Orkney Islands. -Consequently, these two huge masses of moving ice—the one from Scotland -and the much greater one from Scandinavia—would meet in the North Sea, -probably not far from our shores, and would move, as represented in -the diagram, side by side northwards into the Atlantic as one gigantic -glacier. - -Nor can this be regarded as an anomalous state of things; for in -Greenland and the antarctic continent the ice does not break up into -icebergs on reaching the sea, but moves along the sea-bottom in a -continuous mass until it reaches water sufficiently deep to float -it. It is quite possible that the ice at the present day may nowhere -traverse a distance of three or four hundred miles of sea-bottom, but -this is wholly owing to the fact that it finds water sufficiently deep -to float it before having travelled so far. Were Baffin’s Bay and -Davis’s Straits, for example, as shallow as the North Sea, the ice of -Greenland would not break up into icebergs in these seas, but cross in -one continuous mass to and over the American continent. - -The median line of the Scandinavian and Scottish ice-sheets would be -situated not far from the east coast of Scotland. The Scandinavian ice -would press up as near to our coast as the resistance of the ice from -this side permitted. The enormous mass of ice from Scotland, pressing -out into the North Sea, would compel the Scandinavian ice to move round -by the Orkneys, and would also keep it at some little distance from -Scotland. Where, on the other hand, there was but little resistance -offered by ice from the interior of this country (and this might be the -case along many parts of the English coast), the Scandinavian ice might -reach the shores, and even overrun the country for some distance inland. - -We have hitherto confined our attention to the action of ice proceeding -from Norway; but if we now consider what took place in Sweden and the -Baltic, we shall find more conclusive proof of the downward pressure -of Scandinavian ice on our own shores. The western half of Gothland -is striated in the direction of N.E. and S.W., and that this has been -effected by a huge mass of ice covering the country, and not by local -glaciers, is apparent from the fact observed by Robert Chambers,[256] -and officers of the Swedish Geological Survey, that the general -direction of the groovings and striæ on the rocks bears little or no -relation to the conformation of the surface, showing that the ice was -of sufficient thickness to move straight forward, regardless of the -inequalities of the ground. - -At Gottenburg, on the shores of the Cattegat, and all around Lake -Wener and Lake Wetter, the ice-markings are of the most remarkable -character, indicating, in the most decided manner, that the ice came -from the interior of the country to the north-east in one vast mass. -All this mass of ice must have gone into the shallow Cattegat, a sea -not sufficiently deep to float even an ordinary glacier. The ice coming -off Gothland would therefore cross the Cattegat, and thence pass over -Jutland into the North Sea. After entering the North Sea, it would be -obliged to keep between our shores and the ice coming direct from the -western side of Scandinavia. - -But this is not all. A very large proportion of the Scandinavian ice -would pass into the Gulf of Bothnia, where it could not possibly float. -It would then move south into the Baltic as land-ice. After passing -down the Baltic, a portion of the ice would probably move south into -the flat plains in the north of Germany, but the greater portion -would keep in the bed of the Baltic, and of course turn to the right -round the south end of Gothland, and thence cross over Denmark into -the North Sea. That this must have been the path of the ice is, I -think, obvious from the observations of Murchison, Chambers, Hörbye, -and other geologists. Sir Roderick Murchison found—though he does not -attribute it to land-ice—that the Aland Islands, which lie between the -Gulf of Bothnia and the Baltic, are all striated in a north and south -direction.[257] - -Upsala and Stockholm, a tract of flat country projecting for some -distance into the Baltic, is also grooved and striated, not in the -direction that would be effected by ice coming from the interior of -Scandinavia, but north and south, in a direction parallel to what must -have been the course of the ice moving down the Baltic.[258] This part -of the country must have been striated by a mass of ice coming from -the direction of the Gulf of Bothnia. And that this mass must have -been great is apparent from the fact that Lake Malar, which crosses -the country from east to west, at right angles to the path of the ice, -does not seem to have had any influence in deflecting the icy stream. -That the ice came from the north and not from the south is also evident -from the fact that the northern sides of rocky eminences are polished, -rounded, and ice-worn, while the southern sides are comparatively -rough. The northern banks of Lake Malar, for example, which, of course, -face the south, are rough, while the southern banks, which must have -offered opposition to the advance of the ice, are smoothed and rounded -in a most singular manner. - -Again, that the ice, after passing down the Baltic, turned to the -right along the southern end of Gothland, is shown by the direction -of the striæ and ice-groovings observed on such islands as Gothland, -Öland, and Bornholm. Sir R. Murchison found that the island of -Gothland is grooved and striated in one uniform direction from N.E. -to S.W. “These groovings,” says Sir Roderick, “so perfectly resemble -the flutings and striæ produced in the Alps by the actual movement -of glaciers, that neither M. Agassiz nor any one of his supporters -could detect a difference.” He concludes, however, that the markings -could not have been made by land-ice, because Gothland is not only a -low, flat island in the middle of the Baltic, but is “at least 400 -miles distant from any elevation to which the term of mountain can be -applied.” This, of course, is conclusive against the hypothesis that -Gothland and the other islands of the Baltic could have been glaciated -by ordinary glaciers; but it is quite in harmony with the theory -that the Gulf of Bothnia and the entire Baltic were filled with one -continuous mass of land-ice, derived from the drainage of the greater -part of Sweden, Lapland, and Finland. In fact, the whole glacial -phenomena of Scandinavia are inexplicable on the hypothesis of local -glaciers. - -That the Baltic was completely filled by a mass of ice moving from the -north is further evidenced by the fact that the mainland, not only at -Upsala, but at several places along the coast of Gothland, is grooved -and striated parallel to the shore, and often at right angles to the -markings of the ice from the interior, showing that the present bed of -the Baltic was not large enough to contain the icy stream. For example, -along the shores between Kalmar and Karlskrona, as described by Sir -Roderick Murchison and by M. Hörbye, the striations are parallel to the -shore. Perhaps the slight obstruction offered by the island of Öland, -situated so close to the shore, would deflect the edge of the stream at -this point over on the land. The icy stream, after passing Karlskrona, -bent round to the west along the present entrance to the Baltic, and -again invaded the mainland, and crossed over the low headland of -Christianstadt, and thence passed westward in the direction of Zealand. - - [Illustration: PLATE V. - - W. & A. K. Johnston, Edinb^r. and London. - - CHART SHOWING THE PROBABLE PATH OF THE ICE IN NORTH-WESTERN EUROPE - DURING THE PERIOD OF MAXIMUM GLACIATION. - - _The lines also represent the actual direction of the striae on the - rocks._] - -This immense Baltic glacier would in all probability pass over Denmark, -and enter the North Sea somewhere to the north of the River Elbe, and -would then have to find an outlet to the Atlantic through the English -Channel, or pass in between our eastern shores and the mass from -Gothland and the north-western shores of Europe. The entire probable -path of the ice may be seen by a reference to the accompanying chart -(Plate V.) That the ice crossed over Denmark is evident from the fact -that the surface of that country is strewn with _débris_ derived from -the Scandinavian peninsula. - -Taking all these various considerations into account, the conclusion is -inevitable that the great masses of ice from Scotland would be obliged -to turn abruptly to the north, as represented in the diagram, and pass -round into the Atlantic in the direction of Caithness and the Orkney -Islands. - -If the foregoing be a fair representation of the state of matters, -it is physically impossible that Caithness could have escaped being -overridden by the land-ice of the North Sea. Caithness, as is well -known, is not only a low, flat tract of land, little elevated above the -sea-level, and consequently incapable of supporting large glaciers; -but, in addition, it projects in the form of a headland across the -very path of the ice. Unless Caithness could have protected itself by -pushing into the sea glaciers of one or two thousand feet in thickness, -it could not possibly have escaped the inroads of the ice of the -North Sea. But Caithness itself could not have supported glaciers of -this magnitude, neither could it have derived them from the adjoining -mountainous regions of Sutherland, for the ice of this county found a -more direct outlet than along the flat plains of Caithness. - -The shells which the boulder clay of Caithness contains have thus -evidently been pushed out of the bed of the North Sea by the land-ice, -which formed the clay itself. - -The fact that these shells are not so intensely arctic as those found -in some other quarters of Scotland, is no evidence that the clay was -not formed during the most severe part of the glacial epoch, for the -shells did not live in the North Sea at the time that it was filled -with land-ice. The shells must have belonged to a period prior to the -invasion of the ice, and consequently before the cold had reached its -greatest intensity. Neither is there any necessity for supposing the -shells to be pre-glacial, for these shells may have belonged to an -inter-glacial period. In so far as Scotland is concerned, it would be -hazardous to conclude that a plant or an animal is either pre-glacial -or post-glacial simply because it may happen not to be of an arctic or -of a boreal type. - -The same remarks which apply to Caithness apply to a certain extent -to the headland at Fraserburgh. It, too, lay in the path of the ice, -and from the direction of the striæ on the rocks, and the presence of -shells in the clay, as described by Mr. Jamieson, it bears evidence -also of having been overridden by the land-ice of the North Sea. -In fact, we have, in the invasion of Caithness and the headland at -Fraserburgh by the land-ice of the North Sea, a repetition of what we -have seen took place at Upsala, Kalmar, Christianstadt, and other flat -tracts along the sides of the Baltic. - -The scarcity, or perhaps entire absence of Scandinavian boulders in -the Caithness clay is not in any way unfavourable to the theory, for -it would only be the left edge of the North Sea glacier that could -possibly pass over Caithness; and this edge, as we have seen, was -composed of the land-ice from Scotland. We might expect, however, to -find Scandinavian blocks on the Shetland and Faroe Islands, for, as we -shall presently see, there is pretty good evidence to prove that the -Scandinavian ice passed over these islands. - -_The Shetland and Faroe Islands glaciated by Land-ice._—It is also -worthy of notice that the striæ on the rocks in the Orkney, Shetland, -and Faroe Islands, all point in the direction of Scandinavia, and are -what would be effected by land-ice moving in the paths indicated -in the diagram. And it is a fact of some significance, that when we -proceed north to Iceland, the striæ, according to the observations -of Robert Chambers, seem to point towards North Greenland. Is it -possible that the entire Atlantic, from Scandinavia to Greenland, was -filled with land-ice? Astounding as this may at first appear, there -are several considerations which render such a conclusion probable. -The observations of Chambers, Peach, Hibbert, Allan, and others, show -that the rocky face of the Shetland and Faroe Islands has been ground, -polished, and striated in a most remarkable manner. That this could not -have been done by ice belonging to the islands themselves is obvious, -for these islands are much too small to have supported glaciers of any -size, and the smallest of them is striated as well as the largest. -Besides, the uniform direction of the striæ on the rocks shows that -it must have been effected by ice passing over the islands. That the -striations could not have been effected by floating icebergs at a time -when the islands were submerged is, I think, equally obvious, from the -fact that not only are the tops of the highest eminences ice-worn, -but the entire surface down to the present sea-level is smoothed and -striated; and these striations conform to all the irregularities of the -surface. This last fact Professor Geikie has clearly shown is wholly -irreconcilable with the floating-ice theory.[259] Mr. Peach[260] found -vertical precipices in the Shetlands grooved and striated, and the -same thing was observed by Mr. Thomas Allan on the Faroe Islands.[261] -That the whole of these islands have been glaciated by a continuous -sheet of ice passing over them was the impression left on the mind of -Robert Chambers after visiting them.[262] This is the theory which -alone explains all the facts. The only difficulty which besets it is -the enormous thickness of the ice demanded by the theory. But this -difficulty is very much diminished when we reflect that we have good -evidence, from the thickness of icebergs which have been met with -in the Southern Ocean,[263] that the ice moving off the antarctic -continent must be in some places considerably over a mile in thickness. -It is then not so surprising that the ice of the glacial epoch, coming -off Greenland and Northern Europe, should not have been able to float -in the North Atlantic. - -_Why the Ice of Scotland was of such enormous Thickness._—The enormous -thickness of the ice in Scotland, during the glacial epoch, has been -a matter of no little surprise. It is remarkable how an island, not -more than 100 miles across, should have been covered with a sheet -of ice so thick as to bury mountain ranges more than 1,000 feet in -height, situated almost at the sea-shore. But all our difficulties -disappear when we reflect that the seas around Scotland, owing to their -shallowness, were, during the glacial period, blocked up with solid -ice. Scotland, Scandinavia, and the North Sea, would form one immense -table-land of ice, from 1,000 to 2,000 feet above the sea-level. This -table-land would terminate in the deep waters of the Atlantic by a -perpendicular wall of ice, extending probably from the west of Ireland -away in the direction of Iceland. From this barrier icebergs would be -continually breaking off, rivalling in magnitude those which are now to -be met with in the antarctic seas. - -_The great Extension of the Loess accounted for._—An effect which would -result from the blocking up of the North Sea with land-ice, would be -that the waters of the Rhine, Elbe, and Thames would have to find -an outlet into the Atlantic through the English Channel. Professor -Geikie has suggested to me that if the Straits of Dover were not then -open—quite a possible thing—or were they blocked up with land-ice, say -by the great Baltic glacier crossing over from Denmark, the consequence -would be that the waters of the Rhine and Elbe would be dammed back, -and would inundate all the low-lying tracts of country to the south; -and this might account for the extraordinary extension of the Loess in -the basin of the Rhine, and in Belgium and the north of France.[264] - - [Illustration: PLATE VI. - - CHART SHOWING PATH OF THE ICE - - W. & A. K. Johnston, Edinb^r and London. - - Note. - - _Curved lines shew path of Ice. - Arrows shew direction of striae - as observed by Prof. Geikie & B. N. Peach. - Short thick lines shew direction of - striae by other observers._] - - - _Note on the Glaciation of Caithness._ - -I have very lately received a remarkable confirmation of the path of -the Caithness ice in observations communicated to me by Professor -Geikie and Mr. B. N. Peach. The latter geologist says, “Near the Ord -of Caithness and on to Berriedale the striæ pass off the land and out -to sea; but near Dunbeath, 6 miles north-east of Berriedale, they -begin to creep up out of the sea on to the land and range from about -15° to 10° east of north. _Where the striæ pass out to the sea_ the -boulder clay is made up of the materials from inland and contains no -shells, but _immediately the striæ begin to creep up on to the land_ -then shells begin to make their appearance; and there is a difference, -moreover, in the colour of the clay, for in the former case it is -red and incoherent, and in the latter hard and dark-coloured.” The -accompanying chart (Plate VI.) shows the outline of the Caithness coast -and the direction of the striæ as observed by Professor Geikie and Mr. -Peach, and no demonstration could be more conclusive as to the path of -the ice and the obstacles it met than these observations, supplemented -and confirmed as they are by other recorded facts to which I shall -presently allude. Had the ice-current as it entered the North Sea off -the Sutherland coast met with no obstacle it would have ploughed its -way outwards till it broke off in glaciers and floated away. But it is -clear that the great press of Scandinavian ice and the smaller mass of -land-ice from the Morayshire coast converging in the North Sea filled -up its entire bed, and these, meeting the opposing current from the -Sutherland coast, turned it back upon itself, and forced it over the -north-east part of Caithness. The farther south on the Sutherland -coast that the ice entered the sea the deeper would it be able to -penetrate into the ocean-bed before it met an opposition sufficiently -strong to turn its course, and the wider would be its sweep; but when -we come to the Sutherland coast we reach a point where the land-ice—as, -for example, near Dunbeath—is forced to bend round before it even -reaches the sea-shore, as will be seen from the accompanying diagram. - -We are led to the same conclusions regarding the path of the ice in the -North Sea from the presence of oolitic fossils and chalk flints found -likewise in the boulder clay of Caithness, for these, as we shall see, -evidently must have come from the sea. At the meeting of the British -Association, Edinburgh, 1850, Hugh Miller exhibited a collection of -boreal shells with fragments of oolitic fossils, chalk, and chalk -flints from the boulder clay of Caithness collected by Mr. Dick, of -Thurso. My friend, Mr. C. W. Peach, found that the chalk flints in -the boulder clay of Caithness become more abundant as we proceed -northward, while the island of Stroma in the Pentland Firth he found -to be completely strewn with them. This same observer found, also, in -the Caithness clay stones belonging to the Oolitic and Lias formations, -with their characteristic fossils, while ammonites, belemnites, fossil -wood, &c., &c., were also found loose in the clay.[265] The explanation -evidently is, that these remains were derived from an outcrop of -oolitic and cretaceous beds in the North Sea. It is well known that -the eastern coast of Sutherlandshire is fringed with a narrow strip -of oolite, which passes under the sea, but to what distance is not -yet ascertained. Outside the Oolitic formation the chalk beds in all -probability crop out. It will be seen from a glance at the accompanying -chart (Plate VI.) that the ice which passed over the north-eastern part -of Caithness must have crossed the out-cropping chalk beds. - -As has already been stated in the foregoing chapter, the headland of -Fraserburgh, north-eastern corner of Aberdeenshire, bears evidence, -both from the direction of the striæ and broken shells in the boulder -clay, of having been overridden also by land-ice from the North Sea. -This conclusion is strengthened by the fact that chalk flints and -oolitic fossils have also been abundantly met with in the clay by Dr. -Knight, Mr. James Christie, Mr. W. Ferguson, Mr. T. F. Jamieson, and -others. - - - - - CHAPTER XXVIII. - - NORTH OF ENGLAND ICE-SHEET, AND TRANSPORT OF WASTDALE CRAG - BLOCKS.[266] - - Transport of Blocks; Theories of.—Evidence of Continental - Ice.—Pennine Range probably striated on Summit.—Glacial - Drift in Centre of England.—Mr. Lacy on Drift of Cotteswold - Hills.—England probably crossed by Land-ice.—Mr. Jack’s - Suggestion.—Shedding of Ice North and South.—South of England - Ice-sheet.—Glaciation of West Somerset.—Why Ice-markings are - so rare in South of England.—Form of Contortion produced by - Land-ice. - - -Considerable difficulty has been felt in accounting for the transport -of the Wastdale granite boulders across the Pennine chain to the east. -Professors Harkness,[267] and Phillips,[268] Messrs. Searles Wood, -jun.,[269] Mackintosh,[270] and I presume all who have written on -the subject, agree that these blocks could not have been transported -by land-ice. The agency of floating ice under some form or other is -assumed by all. - -We have in Scotland phenomena of an exactly similar nature. The summits -of the Ochils, the Pentlands, and other mountain ranges in the east -of Scotland, at elevations of from 1,500 to 2,000 feet, are not only -ice-marked, but strewn over with boulders derived from rocks to the -west and north-west. Many of them must have come from the Highlands -distant some 50 or 60 miles. It is impossible that these stones could -have been transported, or the summits of the hills striated, by means -of ordinary glaciers. Neither can the phenomena be attributed to the -agency of icebergs carried along by currents. For we should require to -assume not merely a submergence of the land to the extent of 2,000 -feet or so,—an assumption which might be permitted,—but also that the -currents bearing the icebergs took their rise in the elevated mountains -of the Highlands (a most unlikely place), and that these currents -radiated in all directions from that place as a centre. - -In short, the glacial phenomena of Scotland are wholly inexplicable -upon any other theory than that, during at least a part of the -glacial epoch, the entire island from sea to sea was covered with one -continuous mass of ice of not less than 2,000 feet in thickness. - -In my paper on the Boulder Clay of Caithness (see preceding chapter), -I have shown that if the ice was 2,000 feet or so in thickness, it -must, in its motion seawards, have followed the paths indicated by the -curved lines in the chart accompanying that paper (See Plate V.). In -so far as Scotland is concerned [and Scandinavia also], these lines -represent pretty accurately not only the paths actually taken by the -boulders, but also the general direction of the ice-markings on all the -elevated mountain ridges. But if Scotland was covered to such an extent -with ice, it is not at all probable that Westmoreland and the other -mountainous districts of the North of England could have escaped being -enveloped in a somewhat similar manner. Now if we admit the supposition -of a continuous mass of ice covering the North of England, all our -difficulties regarding the transport of the Wastdale blocks across the -Pennine chain disappear. An inspection of the chart above referred to -will show that these blocks followed the paths which they ought to have -done upon the supposition that they were conveyed by continental ice. - -That Wastdale Crag itself suffered abrasion by ice moving over it, in -the direction indicated by the lines in the diagram, is obvious from -what has been recorded by Dr. Nicholson and Mr. Mackintosh. They both -found the Crag itself beautifully _moutonnée_ up to its summit, and -striated in a W.S.W. and E.N.E. direction. Mr. Mackintosh states that -these scorings run obliquely up the sloping face of the crag. Ice -scratches crossing valleys and running up the sloping faces of hills -and over their summits are the sure marks of continental ice, which -meet the eye everywhere in Scotland. Dr. Nicholson found in the drift -covering the lower part of the crag, pebbles of the Coniston flags and -grits from the west.[271] - -The fact that in Westmoreland the direction of the ice-markings, as a -general rule, corresponds with the direction of the main valleys, is -no evidence whatever that the country was not at one period covered -with a continuous sheet of ice; because, for long ages after the period -of continental ice, the valleys would be occupied by glaciers, and -these, of course, would necessarily leave the marks of their presence -behind. This is just what we have everywhere in Scotland. It is on -the summits of the hills and elevated ridges, where no glacier could -possibly reach, that we find the sure evidence of continental ice. -But that land-ice should have passed over the tops of hills 1,000 or -2,000 feet in height is a thing hitherto regarded by geologists as -so unlikely that few of them ever think of searching in such places -for ice-markings, or for transported stones. Although little has been -recorded on this point, I hardly think it likely that there is in -Scotland a hill under 2,000 feet wholly destitute of evidence that ice -has gone over it. If there were hills in Scotland that should have -escaped being overridden by ice, they were surely the Pentland Hills; -but these, as was shown on a former occasion,[272] were completely -buried under the mass of ice covering the flat surrounding country. -I have no doubt whatever that if the summits of the Pennine range -were carefully examined, say under the turf, evidence of ice-action, -in the form of transported stones or scratches on the rock, would be -found.[273] - -Nor is the fact that the Wastdale boulders are not rounded and -ice-marked, or found in the boulder clay, but lie on the surface, any -evidence that they were not transported by land-ice. For it would not -be the stones _under_ the ice, but those falling on the upper surface -of the sheet, that would stand the best chance of being carried over -mountain ridges. But such blocks would not be crushed and ice-worn; -and it is on the surface of the clay, and not imbedded in it, that we -should expect to find them. - -It is quite possible that the dispersion of the Wastdale boulders took -place at various periods. During the period of local glaciers the -blocks would be carried along the line of the valleys. - -All I wish to maintain is that the transport of the blocks across -the Pennine chain is easily accounted for if we admit, what is very -probable, that the great ice-covering of Scotland overlapped the high -grounds of the North of England. The phenomenon is the same in both -places, and why not attribute it to the same cause? - -There is another curious circumstance connected with the drift of -England which seems to indicate the agency of an ice-covering. - -As far back as 1819, Dr. Buckland, in his Memoir on the Quartz Rock -of Lickey Hill,[274] directed attention to the fact, that on the -Cotteswold Hills there are found pebbles of hard red chalk which must -have come from the Wolds of Yorkshire and Lincolnshire. He pointed -out also that the slaty and porphyritic pebbles probably came from -Charnwood Forest, near Leicester. Professor Hull, of the Geological -Survey, considers that “almost all the Northern Drift of this part -of the country had been derived from the _débris_ of the rocks of -the Midland Counties.”[275] He came also to the conclusion that the -slate fragments may have been derived from Charnwood Forest. In the -Vale of Moreton he found erratic boulders from two feet to three -feet in diameter. The same northern character of the drift of this -district is remarked by Professor Ramsay and Mr. Aveline, in their -Memoir of the Geology of parts of Gloucestershire. In Leicestershire -and Northamptonshire the officers of the Geological Survey found in -abundance drift which must have come from Lincolnshire and Yorkshire to -the north-east. - -Mr. Lucy, who has also lately directed attention to the fact that -the Cotteswold Hills are sprinkled over with boulders from Charnwood -Forest, states also that, on visiting the latter place, he found that -many of the stones contained in it had come from Yorkshire, still -further to the north-east.[276] - -Mr. Searles Wood, jun., in his interesting paper on the Boulder Clay -of the North of England,[277] states that enormous quantities of the -chalk _débris_ from the Yorkshire Wold are found in Leicester, Rutland, -Warwick, Northampton, and other places to the south and south-west. -Mr. Wood justly concludes that this chalk _débris_ could not have been -transported by water. “If we consider,” he says, “the soluble nature -of chalk, it must be evident that none of this débris can have been -detached from the parent mass, either by water-action, or by any other -atmospheric agency than moving ice. The action of the sea, of rivers, -or of the atmosphere, upon chalk, would take the form of dissolution, -the degraded chalk being taken up in minute quantities by the water, -and held in suspension by it, and in that form carried away; so that -it seems obvious that this great volume of rolled chalk can have been -produced in no other way than by the agency of moving ice; and for that -agency to have operated to an extent adequate to produce a quantity -that I estimate as exceeding a layer 200 feet thick over the entire -Wold, nothing less than the complete envelopment of a large part of the -Wold by ice for a long period would suffice.” - -I have already assigned my reasons for disbelieving the opinion that -such masses of drift could have been transported by floating ice; but -if we refer it to land-ice, it is obvious that the ice could not have -been in the form of local glaciers, but must have existed as a sheet -moving in a south and south-west direction, from Yorkshire, across the -central part of England. But how is this to harmonize with the theory -of glaciation, which is advanced to explain the transport of the Shap -boulders? - -The explanation has, I think, been pointed out by a writer in the -_Glasgow Herald_,[278] of the 26th November, 1870, in a review of Mr. -Lucy’s paper. - -In my paper on the Boulder Clay of Caithness, I had represented the ice -entering the North Sea from the east coast of Scotland and England, -as all passing round the north of Scotland. But the reviewer suggests -that the ice entering at places to the south of, say, Flamborough Head, -would be deflected southwards instead of northwards, and thus pass over -England. “It is improbable, however,” says the writer, “that this joint -ice-sheet would, as Mr. Croll supposes, all find its way round the -north of Scotland into the deep sea. The southern uplands of Scotland, -and probably also the mountains of Northumberland, propelled, during -the coldest part of the glacial period, a land ice-sheet in an eastward -direction. This sheet would be met by another streaming outward from -the south-western part of Norway—in a diametrically opposite direction. -In other words, an imaginary line might be drawn representing the -course of some particular boulder in the _moraine profonde_ from -England met by a boulder from Norway, in the same straight line. With -a dense ice-sheet to the north of this line, and an open plain to the -south, it is clear that all the ice travelling east or west from points -to the south of the starting-points of our two boulders would be ‘shed’ -off to the south. There would be a point somewhere along the line, at -which the ice would turn as on a pivot—this point being nearer England -or Scandinavia, as the degree of pressure exercised by the respective -ice-sheets should determine. There is very little doubt that the point -in question would be nearer England. Further, the direction of the -joint ice-sheet could not be _due_ south unless the pressure of the -component ice-sheets should be exactly equal. In the event of that from -Scandinavia pressing with greater force, the direction would be to the -south-west. This is the direction in which the drifts described by Mr. -Lucy have travelled.” - -I can perceive no physical objection to this modification of the -theory. What the ice seeks is the path of least resistance, and along -this path it will move, whether it may lie to the south or to the -north. And it is not at all improbable that an outlet to the ice would -be found along the natural hollow formed by the valleys of the Trent, -Avon, and Severn. Ice moving in this direction would no doubt pass down -the Bristol Channel and thence into the Atlantic. - -Might not the shedding of the north of England ice-sheet to the north -and south, somewhere not far from Stainmoor, account for the remarkable -fact pointed out by Mr. Searles Wood, that the boulder clay, with -Shap boulders, to the north of the Wold is destitute of chalk; while, -on the other hand, the chalky boulder clay to the south of the Wold -is destitute of Shap boulders? The ice which passed over Wastdale -Crag moved to the E.N.E., and did not cross the chalk of the Wold; -while the ice which bent round to the south by the Wold came from the -district lying to the south of Wastdale Crag, and consequently did not -carry with it any of the granite from that Crag. In fact, Mr. Searles -Wood has himself represented on the map accompanying his Memoir this -shedding of the ice north and south. - -These theoretical considerations are, of course, advanced for what -they are worth. Hitherto geologists have been proceeding upon the -supposition of an ice-sheet and an open North Sea; but the latter is -an impossibility. But if we suppose the seas around our island to have -been filled with land-ice during the glacial epoch, the entire glacial -problem is changed, and it does not then appear so surprising that ice -should have passed over England. - - - _Note on the South of England Ice-sheet._ - -If what has already been stated regarding the north of England be -anything like correct, it is evident that the south of England -could not possibly have escaped glaciation. If the North Sea was so -completely blocked up by Scandinavian ice, that the great mass of ice -from the Cumberland mountains entering the sea on the east coast was -compelled to bend round and find a way of escape across the centre -of England in the direction of the Bristol Channel, it is scarcely -possible that the immense mass of ice filling the Baltic Sea and -crossing over Denmark could help passing across at least a portion -of the south of England. The North Sea being blocked up, its natural -outlet into the Atlantic would be through the English Channel; and it -is not likely that it could pass through without impinging to some -extent upon the land. Already geologists are beginning to recognise the -evidence of ice in this region. - -Mr. W. C. Lucy, in the _Geological Magazine_ for June, 1874, records -the finding by himself of evidences of glaciation in West Somerset, -in the form of “rounded rocky knolls,” near Minehead, like those of -glaciated districts; of a bed of gravel and clay 70 feet deep, which -he considered to be boulder clay. He also mentions the occurrence near -Portlock of a large mass of sandstone well striated, only partially -detached from the parent rock. In the same magazine for the following -month Mr. H. B. Woodward records the discovery by Mr. Usher of some -“rum stuff” near Yarcombe, in the Black Down Hills of Devonshire, -which, on investigation, proved to be boulder clay; and further, that -it was not a mere isolated patch, but occurred in several other places -in the same district. Mr. C. W. Peach informs me that on the Cornwall -coast, near Dodman Point, at an elevation of about 60 feet above -sea-level, he found the rock surface well striated and ice-polished. -In a paper on the Drift Deposits of the Bath district, read before the -Bath Natural History and Antiquarian Field Club, March 10th, 1874, -Mr. C. Moore describes the rock surfaces as grooved, with deep and -long-continued furrows similar to those usually found on glaciated -rocks, and concludes that during the glacial period they were subjected -to ice-action. This conclusion is confirmed by the fact of there being -found, immediately overlying these glaciated rocks, beds of gravel -with intercalated clay-beds, having a thickness of 30 feet, in which -mammalian remains of arctic types are abundant. The most characteristic -of which are _Elephas primigenius_, _E. antiquus_, _Rhinoceros -tichorhinus_, _Bubalus moschatus_, and _Cervus tarandus_. - -There is little doubt that when the ground is better examined many -other examples will be found. One reason, probably, why so little -evidence of glaciation in the south of England has been recorded, -is the comparative absence of rock surfaces suitable for retaining -ice-markings. There is, however, one class of evidence which might -determine the question of the glaciation of the south of England as -satisfactorily as markings on the rock. The evidence to which I refer -is that of contorted beds of sand or clay. In England contortions from -the sinking of the beds are, of course, quite common, but a thoughtful -observer, who has had a little experience of ice-formed contortions, -can easily, without much trouble, distinguish the latter from the -former. Contortions resulting from the lateral pressure of the ice -assume a different form from those produced by the sinking of the beds. -In Scotland, for example, there is one well-marked form of contortion, -which not only proves the existence of land-ice, but also the direction -in which it moved. The form of contortion to which I refer is the -bending back of the stratified beds upon themselves, somewhat in -the form of a fishing-hook. This form of contortion will be better -understood from the accompanying figure. - -[Illustration: Fig. 11. - -SECTION OF CONTORTED DRIFT NEAR MUSSELBURGH. - -_a_ Boulder Clay; _b_ Laminated Clay; _c_ Sand, Gravel, and Clay, -contorted. - -Depth of Section, twenty-two feet.—H. SKAE.] - - - - - CHAPTER XXIX. - - EVIDENCE FROM BURIED RIVER CHANNELS OF A CONTINENTAL PERIOD IN - BRITAIN.[279] - - Remarks on the Drift Deposits.—Examination of Drift - by Borings.—Buried River Channel from Kilsyth to - Grangemouth.—Channels not excavated by Sea nor by - Ice.—Section of buried Channel at Grangemouth.—Mr. Milne - Home’s Theory.—German Ocean dry Land.—Buried River Channel - from Kilsyth to the Clyde.—Journal of Borings.—Marine - Origin of the Drift Deposits.—Evidence of Inter-glacial - Periods.—Oscillations of Sea-level.—Other buried River - Channels. - - -_Remarks on the Drift Deposits._—The drift and other surface deposits -of the country have chiefly been studied from sections observed on the -banks of streams, railway cuttings, ditches, foundations of buildings, -and other excavations. The great defect of such sections is that they -do not lay open a sufficient depth of surface. They may, no doubt, -represent pretty accurately the character and order of the more recent -deposits which overlie the boulder clay, but we are hardly warranted -in concluding that the succession of deposits belonging to the earlier -part of the glacial epoch, the period of the true till, is fully -exhibited in such limited sections. - -Suppose, for example, the glacial epoch proper—the time of the lower -boulder clay—to have consisted of a succession of alternate cold and -warm periods, there would, in such a case, be a series of separate -formations of boulder clay; but we could hardly expect to find on the -flat and open face of the country, where the surface deposits are -generally not of great depth, those various formations of till lying -the one superimposed upon the other. For it is obvious that the till -formed during one ice-period would, as a general rule, be either swept -away or re-ground and laid down by the ice of the succeeding period. -If the very hardest rocks could not withstand the abrading power -of the enormous masses of ice which passed over the surface of the -country during the glacial epoch, it is hardly to be expected that the -comparatively soft boulder clay would be able to do so. It is probable -that the boulder clay of one period would be used as grinding materials -by the ice of the succeeding periods. The boulder clay which we find in -one continuous mass may, therefore, in many cases, have been ground off -the rocks underneath at widely different periods. - -If we wish to find the boulder clays belonging to each of the -successive cold periods lying, the one superimposed on the other in -the order of time in which they were formed, we must go and search in -some deep gorge or valley, where the clay has not only accumulated -in enormous masses, but has been partially protected from the -destructive power of the ice. But it is seldom that the geologist has -an opportunity of seeing a complete section down to the rock-head in -such a place. In fact, excepting by bores for minerals, or by shafts -of pits, the surface, to a depth of one or two hundred feet, is never -passed through or laid open. - -_Examination of Drift by Borings._—With the view of ascertaining if -additional light would be cast on the sequence of events, during the -formation of the boulder clay, by an examination of the journals of -bores made through a great depth of surface deposits, a collection -of about 250 bores, put down in all parts of the mining districts -of Scotland, was made. An examination of these bores shows most -conclusively that the opinion that the boulder clay, or lower till, is -one great undivided formation, is wholly erroneous. - -These 250 bores, as already stated,[280] represent a total thickness -of 21,348 feet, giving 86 feet as the mean thickness of the deposits -passed through. Twenty of these bores have one boulder clay, with beds -of stratified sand or gravel beneath the clay; 25 have 2 boulder clays, -with stratified beds of sand and gravel between; 10 have 3 boulder -clays; one has 4 boulder clays; 2 have 5 boulder clays; and one has no -fewer than 6 separate masses of boulder clay, with stratified beds of -sand and gravel between; 16 have two or three separate boulder clays, -differing altogether in colour and hardness, without any stratified -beds between. We have, therefore, out of 250 bores, 75 of them -representing a condition of things wholly different from that exhibited -to the geologist in ordinary sections. - -These bores bear testimony to the conclusion that the glacial epoch -consisted of a succession of cold and warm periods, and not of one -continuous and unbroken period of ice, as was at one time generally -supposed. - -The full details of the character of the deposits passed through by -these bores, and their bearing on the history of the glacial epoch, -have been given by Mr. James Bennie, in an interesting paper read -before the Glasgow Geological Society,[281] to which I would refer -all those interested in the subject of surface geology. But it is not -to the mere contents of the bores that I wish at present to direct -attention, but to a new and important result, to which they have -unexpectedly led. - -_Buried River Channel, Kilsyth to Grangemouth, Firth of Forth._—These -borings reveal the existence of a deep pre-glacial, or perhaps -inter-glacial, trough or hollow, extending from the Clyde above Bowling -across the country by Kilsyth, along the valley of the Forth and Clyde -Canal, to the Firth of Forth at Grangemouth. This trough is filled up -with immense deposits of mud, sand, gravel, and boulder clay. These -deposits not only fill it up, but they cover it over to such an extent -that it is absolutely impossible to find on the surface a single trace -of it; and had it not been for borings, and other mining operations, -its existence would probably never have been known. In places where the -bottom of the trough is perhaps 200 feet below the sea-level, we find -on the surface not a hollow, but often an immense ridge or elliptical -knoll of sand, gravel, or boulder clay, rising sometimes to 150 or 200 -feet above the present sea-level. - -I need not here enter into any minute details regarding the form, -depth, and general outline of this trough, or of the character of -the deposits covering it, these having already been described by Mr. -Bennie, but shall proceed to the consideration of circumstances which -seem to throw light on the physical origin of this curious hollow, -and to the proof which it unexpectedly affords that Scotland, during -probably an early part of the glacial epoch, stood higher in relation -to the sea-level than it does at present; or rather, as I would be -disposed to express it, the sea stood much lower than at present. - -From the fact that all along the line of this trough the surface of the -country is covered with enormous beds of stratified sands and gravels -of marine origin, which proves that the sea must have at a recent -period occupied the valley, my first impression was that this hollow -had been scooped out by the sea. This conclusion appeared at first -sight quite natural, for at the time that the sea filled the valley, -owing to the Gulf-stream impinging on our western shores, a strong -current would probably then pass through from the Atlantic on the west -to the German Ocean on the east. However, considerations soon began to -suggest themselves wholly irreconcilable with this hypothesis. - -The question immediately arose, if the tendency of the sea occupying -the valley is to deepen it, by wearing down its rocky bottom, and -removing the abraded materials, then why is the valley filled up to -such a prodigious extent with marine deposits? Does not the fact of the -whole valley being filled up from sea to sea with marine deposits to a -depth of from 100 to 200 feet, and in some places, to even 400 feet, -show that the tendency of the sea filling this valley is to silt it up -rather than to deepen it? What conceivable change of conditions could -account for operations so diverse? - -That the sea could not have cut out this trough, is, however, -susceptible of direct proof. The height of the surface of the valley -at the watershed or highest part, about a mile to the east of -Kilsyth, where the Kelvin and the Bonny Water, running in opposite -directions,—the one west into the Clyde, and the other east into the -Carron,—take their rise, is 160 feet above the sea-level. Consequently, -before the sea could pass through the valley at present, the sea-level -would require to be raised 160 feet. - -But in discussing the question as to the origin of this pre-glacial -hollow, we must suppose the surface deposits of the valley all removed, -for this hollow was formed before these deposits were laid down. Let -us take the average depth of these deposits at the watershed to be 50 -feet. It follows that, assuming the hollow in question to have been -formed by the sea, the sea-level at the time must have been at least -110 feet higher than at present. - -Were the surface deposits of the country entirely removed, the district -to the west and north-west of Glasgow would be occupied by a sea -which would stretch from the Kilpatrick Hills, north of Duntocher, -to Paisley, a distance of about five miles, and from near Houston to -within a short way of Kirkintilloch, a distance of more than twelve -miles. This basin would contain a few small islands and sunken rocks, -but its mean depth, as determined from a great number of surface bores -obtained over its whole area, would be not much under 70 or 80 feet. -But we shall, however, take the depth at only 50 feet. Now, if we raise -the sea-level so as to allow the water just barely to flow over the -watershed of the valley, the sea in this basin would therefore be 160 -feet deep. Let us now see what would be the condition of things on the -east end of the valley. The valley, for several miles to the east of -Kilsyth, continues very narrow, but on reaching Larbert it suddenly -opens into the broad and flat carse lands through which the Forth and -Carron wind. The average depth at which the sea would stand at present -in this tract of country, were the surface removed, as ascertained from -bores, would be at least 100 feet, or about double that in the western -basin. Consequently, when the sea was sufficiently high to pass over -the watershed, the water would be here 210 feet in depth, and several -miles in breadth. - - [Illustration: PLATE VII. - - W. & A. K. Johnston Edinb^r. and London. - - CHART OF THE MIDLAND VALLEY, SHOWING BURIED RIVER CHANNELS. - - _The blue parts represent the area which would be covered by sea - were the land submerged to the extent of 200 feet. The heavy black - lines A and B represent the buried River Channels._] - -But in order to have a current of some strength passing through the -valley, let us suppose the sea at the time to have stood 150 feet -higher in relation to the land than at present. This would give 40 feet -as the depth of the sea on the watershed, and 200 feet as the depth in -the western basin, and 250 feet as the depth in the eastern. - -An examination of the Ordnance Survey map of the district will show -that the 200 feet contour lines which run along each side of the valley -from Kilsyth to Castlecary come, in several places, to within one-third -of a mile of each other. From an inspection of the ground, I found -that, even though the surface deposits were removed off the valley, it -would not sensibly affect the contours at those places. It is therefore -evident that though the sea may have stood even 200 feet higher than at -present, the breadth of the strait at the watershed and several other -points could not have exceeded one-third of a mile. It is also evident -that at those places the current would be flowing with the greatest -velocity, for here it was not only narrowest, but also shallowest. A -reference to Plate VII. will show the form of the basins. The stippled -portion, coloured blue, represents the area which would be covered by -the sea were the land submerged to the extent of 200 feet. - -Let us take the breadth of the current in the western basin at, say, -three miles. This is two miles less than the breadth of the basin -itself. Suppose the current at the narrow parts between Kilsyth and -Castlecary to have had a velocity of, say, five miles an hour. Now, as -the mean velocity of the current at the various parts of its course -would be inversely proportionate to the sectional areas of those parts, -it therefore follows that the mean velocity of the current in the -western basin would be only 1/45th of what it was in the narrow pass -between Kilsyth and Castlecary. This would give a mile in nine hours -as the velocity of the water in the western basin. In the eastern -basin the mean velocity of the current, assuming its breadth to be the -same as in the western, would be only a mile in eleven hours. In the -central part of the current the velocity at the surface would probably -be considerably above the mean, but at the sides and bottom it would, -no doubt, be under the mean. In fact, in these two basins the current -would be almost insensible. - -The effect of such a current would simply be to widen and deepen the -valley all along that part between Kilsyth and Castlecary where the -current would be flowing with considerable rapidity. But it would -have little or no effect in deepening the basins at each end, but the -reverse. It would tend rather to silt them up. If the current flowed -from west to east, the materials removed from the narrow part between -Kilsyth and Castlecary, where the velocity of the water was great, -would be deposited when the current almost disappeared in the eastern -part of the valley. Sediment carried by a current flowing at the rate -of five miles an hour, would not remain in suspension when the velocity -became reduced to less than five miles a day. - -But even supposing it were shown that the sea under such conditions -could have deepened the valley along the whole distance from the Clyde -to the Forth, still this would not explain the origin of the trough -in question. What we are in search of is not the origin of the valley -itself, but the origin of a deep and narrow hollow running along -the bottom of it. A sea filling the whole valley, and flowing with -considerable velocity, would, under certain conditions, no doubt deepen -and widen it, but it would not cut out along its bottom a deep, narrow -trough, with sides often steep, and in some places perpendicular and -even overhanging. - -This hollow is evidently an old river-bed scooped out of the rocky -valley by a stream, flowing probably during an early part of the -glacial period. - -During the latter part of the summer of 1868, I spent two or three -weeks of my holidays in tracing the course of this buried trough from -Kilsyth to the river Forth at Grangemouth, and I found unmistakable -evidence that the eastern portion of it, stretching from the watershed -to the Forth, had been cut out, not by the sea, but by a stream which -must have followed almost the present course of the Bonny Water. - -I found that this deep hollow enters the Forth a few hundred yards to -the north of Grangemouth Harbour, at the extraordinary depth of 260 -feet below the present sea-level. At the period when the sea occupied -the valley of the Forth and Clyde Canal, the bottom of the trough at -this spot would therefore be upwards of 400 feet below the level of the -sea. - -A short distance to the west of Grangemouth, and also at Carron, -several bores were put down in lines almost at right angles across -the trough, and by this means we have been enabled to form a pretty -accurate estimate of its depth, breadth, and shape at those places. I -shall give the details of one of those sections. - -Between Towncroft Farm and the river Carron, a bore was put down to -the depth of 273 feet before the rock was reached. About 150 yards to -the north of this there is another bore, giving 234 feet as the depth -to the rock; 150 yards still further north the depth of the surface -deposits, as determined by a third bore, is 155 feet. This last bore is -evidently outside of the hollow, for one about 150 yards north of it -gives the same depth of surface, which seems to be about its average -depth for a mile or two around. About half a mile to the south of the -hollow at this place the surface deposits are 150 feet deep. From a -number of bores obtained at various points within a circuit of 1½ -miles, the surface appears to have a pretty uniform depth of 150 feet -or thereby. For the particulars of these “bores” I am indebted to the -kindness of Mr. Mackay, of Grangemouth. - -To the south of the trough (see Fig. 12) there is a fault running -nearly parallel to it, having a down-throw to the north, and cutting -off the coal and accompanying strata to the south. But an inspection of -the section will show that the hollow in question is no way due to the -fault, but has been scooped out of the solid strata. - - [Illustration: Fig. 12. - - SECTION OF BURIED RIVER-BED NEAR TOWNCROFT FARM, GRANGEMOUTH.] - -The main coal wrought extensively here is cut off by the trough, -as will be seen from the section. Mr. Dawson, of Carron Iron Works, -informs me that at Carronshore pit, about a mile and a quarter above -where this section is taken, the coal was found to be completely cut -off by this trough. In one of the workings of this pit, about forty -years ago, the miners cut into the trough at 40 fathoms below the -surface, when the sand rushed in with irresistible pressure, and filled -the working. Again, about a mile below where the section is taken, -or about two miles below Carronshore, and just at the spot where the -trough enters the Firth, it was also cut into in one of the workings of -the Heuck pit at a depth of 40 fathoms from the surface. Fortunately, -however, at this point the trough is filled with boulder clay instead -of sand, and no damage was sustained. Here, for a distance of two -miles, the Main coal and “Upper Coxroad” are cut off by this hollow; or -rather, I should say this hollow has been cut through the coal-seams. -The “Under Coxroad,” lying about 14 fathoms below the position of the -“Main” coal, as will be seen in the descriptive section (Fig. 12), is -not reached by the trough, and passes undisturbed under it. - -This hollow would seem to narrow considerably as it recedes westwards, -for at Carronshore pit-shaft the surface is 138 feet deep; but not much -over 150 yards to the south of this is the spot where the coal was cut -off by the trough at a depth of 40 fathoms or 240 feet. Here it deepens -upwards of 100 feet in little more than 150 yards. That it is narrow at -this place is proved by the fact, that a bore put down near Carronbank, -a little to the south, shows the surface to be only 156 feet deep. - -In the section (Fig. 12) the line described as “150 _feet above -sea-level_” registers the height of the sea-level at the time when -the central valley was occupied by sea 40 feet deep at the watershed. -Now, if this hollow, which extends right along the whole length of -the valley, had been cut out by the sea, the surface of the rock 150 -feet below the present surface of the ground would be the sea-bottom -at the time, and the line marked “150 _feet above sea-level_” would be -the surface of the sea. The sea would therefore be here 300 feet deep -for several miles around. It cannot be supposed that the sea acting on -a broad flat plain of several miles in extent should cut out a deep, -narrow hollow, like the one exhibited in the section, and leave the -rest of the plain a flat sea-bottom. - -And it must be observed, that this is not a hollow cut merely in a -sea-beach, but one extending westward to Kilsyth. Now, if this hollow -was cut out by the sea, it must have been done, not by the waves -beating on the beach, but by a current flowing through the valley. -The strongest current that could possibly pass through the narrow -part between Kilsyth and Castlecary would be wholly insensible when it -reached Grangemouth, where the water was 300 feet deep, and several -miles broad. Consequently, it is impossible that the current could have -scooped out the hollow represented in the section. - -Again, if this hollow had been scooped out by the sea, it ought to -be deepest between Kilsyth and Castlecary, where the current was -narrowest; but the reverse is actually the case. It is shallowest at -the place where the current was narrowest, and deepest at the two -ends where the current was broadest. In the case of a trough cut by -a sea current, we must estimate its depth from the level of the sea. -Its depth is the depth of the water in it while it was being scooped -out. The bottom of the trough in the highest and narrowest part of -the valley east of Kilsyth is 40 feet above the present sea-level. -Consequently, its depth at this point at the period in question, when -the sea-level was 150 feet higher than at present, would be 110 feet. -The bottom of the trough at Grangemouth is 260 feet below the present -sea-level; add to this 150 feet, and we have 410 feet as its depth here -at the time in question. If this hollow was scooped out by the sea, -how then does it thus happen that at the place where the current was -strongest and confined to a narrow channel by hills on each side, it -cut its channel to a depth of only 110 feet, whereas at the place where -it had scarcely any motion it has cut, on a flat and open plain several -miles broad, a channel to a depth of 410 feet? - -But, suppose we estimate the relative amount of work performed by the -sea at Kilsyth and Grangemouth, not by the actual depth of the bottom -of the trough at these two places below the sea-level at the time that -the work was performed, but by the present actual depth of the bottom -of the trough below the rocky surface of the valley, this will still -not help us out of the difficulty. Taking, as before, the height of the -rocky bed of the valley at the watershed at 110 feet above the present -sea-level, and the bottom of the trough at 40 feet, this gives 70 feet -as the depth scooped out of the rock at that place. The depth of the -trough at Grangemouth below the rocky surface is 118 feet. Here we have -only 70 feet cut out at the only place where there was any resistance -to the current, as well as the place where it possessed any strength; -whereas at Grangemouth, where there was no resistance, and no strength -of current, 118 feet has been scooped out. Such a result as this is -diametrically opposed to all that we know of the dynamics of running -water. - -We may, therefore, conclude that it is physically impossible that this -hollow could have been cut out by the sea. - -Owing to the present tendency among geologists to attribute effects -of this kind to ocean-currents, I have been induced to enter thus at -much greater length than would otherwise have been necessary into the -facts and arguments against the possibility of the hollow having been -excavated by the sea. In the present case the discussion is specially -necessary, for here we have positive evidence of the sea having -occupied the valley for ages, along which this channel has been cut. -Consequently, unless it is proved that the sea could not possibly have -scooped out the channel, most geologists would be inclined to attribute -it to the sea-current which is known to have passed through the valley -rather than to any other cause. - -But that it is a hollow of denudation, and has been scooped out by some -agent, is perfectly certain. By what agent, then, has the erosion been -made? The only other cause to which it can possibly be attributed is -either land-ice or river-action. - -The supposition that this hollow was scooped out by ice is not more -tenable than the supposition that the work has been done by the sea. -A glacier filling up the entire valley and descending into the German -Ocean would unquestionably not only deepen the valley, but would grind -down the surface over which it passed all along its course. But such a -glacier would not cut a deep and narrow channel along the bottom of the -valley. A glacier that could do this would be a small and narrow one, -just sufficiently large to fill this narrow trough; for if it were -much broader than the trough, it would grind away its edges, and make a -broad trough instead of a narrow one. But a glacier so small and narrow -as only to fill the trough, descending from the hills at Kilsyth to the -sea at Grangemouth, a distance of fifteen miles, is very improbable -indeed. The resistance to the advance of the ice along such a slope -would cause the ice to accumulate till probably the whole valley would -be filled.[282] - -There is no other way of explaining the origin of this hollow, but -upon the supposition of its being an old river-bed. But there is -certainly nothing surprising in the fact of finding an old watercourse -under the boulder clay and other deposits. Unless the present contour -of the country be very different from what it was at the earlier -part of the glacial epoch, there must have then been watercourses -corresponding to the Bonny Water and the river Carron of the present -day; and that the remains of these should be found under the present -surface deposits is not surprising, seeing that these deposits are of -such enormous thickness. When water began to flow down our valleys, on -the disappearance of the ice at the close of the glacial epoch, the -Carron and the Bonny Water would not be able to regain their old rocky -channels, but would be obliged to cut, as they have done, new courses -for themselves on the surface of the deposits under which their old -ones lay buried. - -Although an old pre-glacial or inter-glacial river-bed is in itself an -object of much interest and curiosity, still, it is not on that account -that I have been induced to enter so minutely into the details of this -buried hollow. There is something of far more importance attached to -this hollow than the mere fact of its being an old watercourse. For the -fact that it enters the Firth of Forth at a depth of 260 feet below the -present sea-level, proves incontestably that at the time this hollow -was occupied by a stream, _the land must have stood at least between -200 and 300 feet higher in relation to the sea-level than at present_. - -We have seen that the old surface of the country in the neighbourhood -of Grangemouth, out of which this ancient stream cut its channel, -is at least 150 feet below the present sea-level. Now, unless this -surface had been above the sea-level at that time, the stream would -not have cut a channel in it. But it has not merely cut a channel, but -cut one to a depth of 120 feet. It is impossible that this channel -could have been occupied by a river of sufficient volume to fill it. -It is not at all likely that the river which scooped it out could have -been much larger than the Carron of the present day, for the area of -drainage, from the very formation of the country, could not have been -much greater above Grangemouth than at present. An elevation of the -land would, no doubt, increase the area of the drainage of the stream -measured from its source to where it might then enter the sea, because -it would increase the length of the stream; but it would neither -increase the area of drainage, nor the length of the stream above -Grangemouth. Kilsyth would be the watershed then as it is now. - -What we have here is not the mere channel which had been occupied by -the ancient Carron, but the valley in which the channel lay. It may, -perhaps, be more properly termed a buried river valley; formed, no -doubt, like other river valleys by the denuding action of rain and -river. - -The river Carron at present is only a few feet deep. Suppose the -ancient Carron, which flowed in this old channel, to have been say 10 -feet deep. This would show that the land in relation to the sea at that -time must have stood at least 250 feet higher than at present. If 10 -feet was the depth of this old river, and Grangemouth the place where -it entered the sea, then 250 feet would be the extent of the elevation. -But it is probable that Grangemouth was not the mouth of the river; it -would likely be merely the place where it joined the river Forth of -that period. We have every reason to believe that the bed of the German -Ocean was then dry land, and that the Forth, Tay, Tyne, and other -British rivers flowing eastward, as Mr. Godwin-Austin supposes, were -tributaries to the Rhine, which at that time was a huge river passing -down the bed of the German Ocean, and entering the Atlantic to the west -of the Orkney Islands. That the German Ocean, as well as the sea-bed of -the Western Hebrides, was dry land at a very recent geological period, -is so well known, that, on this point, I need not enter into details. -We may, therefore, conclude that the river Forth, after passing -Grangemouth, would continue to descend until it reached the Rhine. If, -by means of borings, we could trace the old bed of the Forth and the -Rhine up to the point where the latter entered the Atlantic, in the -same way as we have done the Bonny Water and the Carron, we should no -doubt obtain a pretty accurate estimate as to the height at which the -land stood at that remote period. Nothing whatever, I presume, is known -as to the depth of the deposits covering the bed of the German Ocean -along what was then the course of the Rhine. It must, no doubt, be -something enormous. We are also in ignorance as to the thickness of the -deposits covering the ancient bed of the Forth. A considerable number -of bores have been put down at various parts of the Firth of Forth in -connection with the contemplated railway bridge across the Firth, but -in none of those bores has the rock been reached. Bores to a depth of -175 feet have been made without even passing through the deposits of -silt which probably overlie an enormous thickness of sand and boulder -clay. Even in places where the water is 40 fathoms deep and quite -narrow, the bottom is not rock but silt. - -It is, however, satisfactory to find on the land a confirmation of -what has long been believed from evidence found in the seas around our -island, that at a very recent period the sea-level in relation to the -land must have been some hundreds of feet lower than at the present -day, and that our island must have at that time formed a part of the -great eastern continent. - -A curious fact was related to me by Mr. Stirling, the manager of the -Grangemouth collieries, which seems to imply a great elevation of the -land at a period long posterior to the time when this channel was -scooped out. - -In sinking a pit at Orchardhead, about a mile to the north of -Grangemouth, the workmen came upon the boulder clay after passing -through about 110 feet of sand, clay, and gravel. On the upper surface -of the boulder clay they found cut out what Mr. Stirling believes -to have been an old watercourse. It was 17 feet deep, and not much -broader. The sides of the channel appear to have been smooth and -water-worn, and the whole was filled with a fine sharp sand beautifully -stratified. As this channel lay about 100 feet below the present -sea-level, it shows that if it actually be an old watercourse, it must -have been scooped out at a time when the land in relation to the sea -stood at least 100 feet higher than at present. - -_Buried River Channel from Kilsyth to the Clyde._—In all probability -the western half of this great hollow, extending from the watershed -at Kilsyth to the Clyde, is also an old river channel, probably -the ancient bed of the Kelvin. This point cannot, however, be -satisfactorily settled until a sufficient number of bores have been -made along the direct line of the hollow, so as to determine with -certainty its width and general form and extent. That the western -channel is as narrow as the eastern is very probable. It has been -found that its sides at some places, as, for example, at Garscadden, -are very steep. At one place the north side is actually an overhanging -buried precipice, the bottom of which is about 200 feet below the -sea-level. We know also that the coal and ironstone in that quarter are -cut through by the trough, and the miners there have to exercise great -caution in driving their workings, in case they might cut into it. The -trough along this district is filled with sand, and is known to the -miners of the locality as the “sand-dyke.” To cut into running sand at -a depth of 40 or 50 fathoms is a very dangerous proceeding, as will be -seen from the details given in Mr. Bennie’s paper[283] of a disaster -which occurred about twenty years ago to a pit near Duntocher, where -this trough was cut into at a depth of 51 fathoms from the surface. - -The depth of this hollow, below the present sea-level at Drumry, as -ascertained by a bore put down, is 230 feet. For several miles to the -east the depth is nearly as great. Consequently, if this hollow be an -old river-bed, the ancient river that flowed in it must have entered -the Clyde at a depth of more than 200 feet below the present sea-level; -and if so, then it follows that the rocky bed of the ancient Clyde must -lie buried under more than 200 feet of surface deposits from Bowling -downwards to the sea. Whether this is the case or not we have no means -at present of determining. The manager to the Clyde Trustees informs -me, however, that in none of the borings or excavations which have -been made has the rock ever been reached from Bowling downwards. The -probability is, that this deep hollow passes downwards continuously to -the sea on the western side of the island as on the eastern.[284] - -The following journals of a few of the borings will give the reader an -idea of the character of the deposits filling the channels. The beds -which are believed to be boulder clay are printed in italics:— - - - BORINGS MADE THROUGH THE DEPOSITS FILLING THE WESTERN CHANNEL. - - Bore, Drumry Farm, on Lands of Garscadden. - - ft. ins. - Surface soil 2 6 - Sand and gravel 3 6 - Dry sand 11 0 - Blue mud 8 6 - Light mud and sand beds 13 0 - Sand 31 6 - Sand and mud 8 0 - Sand and gravel 19 6 - Sand 8 6 - Gravel 24 4 - Sand 5 0 - Gravel 9 6 - Sand 71 6 - Sand (coaly) 1 0 - Sand 9 0 - Sand (coaly) 1 0 - Sand 10 3 - Red clay and gravel 4 8 - Sand 1 5 - Gravel 2 0 - Sand 2 8 - Gravel 10 6 - Sand 1 6 - Gravel 8 10 - _Clay stones and gravel_ 33 3 - ———————— - 297 10 - - Bore on Mains of Garscadden, one mile north-east of Drumry. - - ft. ins. - Surface soil 1 0 - Blue clay and stones 60 0 - Red clay and stones 18 0 - Soft clay and sand beds 7 0 - Gravel 6 0 - Large gravel 9 0 - Sand and gravel 7 0 - Hard gravel 1 6 - Sand and gravel 16 6 - Dry sand 30 0 - Black sand 2 0 - Dry sand 33 0 - Wet sand 8 0 - Light mud 5 0 - Sand 3 0 - Gravel 5 6 - Sandstone, black 0 6 - Blue clay and stones 1 4 - Whin block 0 10 - Sandy clay 4 6 - ———————— - 219 8 - - Bore nearly half a mile south-west of Millichen. - - ft. ins. - Sandy clay 5 0 - _Brown clay and stones_ 17 0 - Mud 15 0 - Sandy mud 31 0 - Sand and gravel with water 28 0 - Sandy clay and gravel 17 0 - Sand 5 0 - Mud 6 0 - Sand 14 0 - Gravel 30 0 - _Brown sandy clay and stones_ 30 0 - Hard red gravel 4 6 - Light mud and sand 1 8 - _Light clay and stones_ 6 6 - _Light clay and whin block_ 26 0 - Fine sandy mud 36 0 - _Brown clay and gravel and stones_ 14 4 - _Bark clay and stones_ 68 0 - ———————— - 355 0 - - Bore at West Millichen, about 100 yards east of farm-house. - - ft. ins. - Soil 1 6 - _Muddy sand and stones_ 4 6 - Soft mud 4 0 - Sand and gravel 45 0 - _Sandy mud and stones_ 20 6 - Coarse gravel 11 6 - Clay and gravel 1 4 - Fine mud 7 0 - Sand and gravel 2 0 - Sandy mud 30 6 - _Brown sandy clay and stones_ 25 0 - Sand and gravel 6 0 - _Brown sandy clay and stones_ 12 0 - Sand 2 0 - _Brown sandy clay and stones_ 4 0 - Mud 5 0 - Mud and sand 10 9 - Sand and stones 2 9 - _Blue clay and stones_ 5 0 - ———————— - 200 4 - - BORINGS MADE THROUGH THE DEPOSITS FILLING THE EASTERN CHANNEL. - - No. 1. Between Towncroft Farm and Carron River—200 yards from - river. Height of surface, 12 feet above sea-level. - - Feet. - Surface sand 6 - Blue mud 4 - Sand 4 - Gravel 3 - Sand 33 - Red clay 46 - _Soft blue till_ 17 - _Hard blue till_ 140 - Sand 20 - ——— - 273 - - No. 2. About 150 yards north of No. 1. Height of surface, 12 - feet above sea-level. - - Feet. - Surface sand 6 - Blue mud 3 - Shell bed 1 - Gravel 2 - Blue mud 8 - Gravel 3 - Blue muddy sand 15 - Red clay 49 - _Blue till and stones_ 20 - Sand 20 - _Hard blue till and stones_ 24 - Sand 2 - _Hard blue till and stones_ 40 - Sand 7 - _Hard blue till_ 24 - ——— - 234 - - No. 3. About 150 yards north of No. 2. Height of surface, - 12 feet above sea-level. - - Feet. - Surface sand 6 - Soft mud with shells 11 - Blue mud and sand (hard) 3 - Channel (rough gravel) 3 - Fine sand 8 - Running sand (red and fine) 17 - Red clay 30 - _Soft till_ 36 - Sand (pure) 2 - _Soft till and sand_ 17 - Gravel 8 - _Hard blue till_ 14 - ——— - 155 - - No. 4. About 100 yards from No. 1. - - Feet. - Surface 5 - Blue mud 5 - Black sand 3 - Gravel 3 - _Red clay and stones_ 34 - Red clay 44 - _Soft blue till_ 32 - _Hard blue till and stones_ 104 - Grey sand not passed through 22 - ——— - 252 - Rock-head not reached. - - No. 5. About 50 yards north of No. 4. - - Feet. - Surface 6 - Blue mud 3 - Shell bed 1 - Channel 2 - Blue mud 8 - Channel 3 - Blue mud and sand 15 - Red clay and sand 10 - Red clay 49 - _Blue till and stones_ 20 - Sand 20 - _Hard blue till and stones_ 24 - Sand 2 - _Hard blue till and stones_ 40 - Sand 7 - _Hard blue till_ 24 - ——— - 211 - - No. 6. Between Heuck and Carron River. - - Feet. - Sandy clay 7 - Mud 16 - _Brown sandy clay and stones_ 3 - Mud 36 - Brown clay 39 - _Blue till and stones_ 54 - ——— - 155 - -The question arises as to what is the origin of the stratified sands -and gravels filling up the buried river channels. Are they of marine or -of freshwater origin? Mr. Dugald Bell[285] and Mr. James Geikie[286] -are inclined to believe that as far as regards those filling the -western channel they are of lacustrine origin; that they were formed -in lakes, produced by the damming back of the water resulting from the -melting of the ice. I am, however, for the following reasons, inclined -to agree with Mr. Bennie’s opinion that they are of marine origin. -It will be seen, by a comparison of the journals of the borings made -through the deposits in the eastern channel with those in the western, -that they are of a similar character; so that, if we suppose those in -the western channel to be of freshwater origin, we may from analogy -infer the same in reference to the origin of those in the eastern -channel. But, as we have already seen, the deposits extend to the Firth -of Forth at Grangemouth, where they are met with at a depth of 260 feet -below sea-level. Consequently, if we conclude them to be of freshwater -origin, we are forced to the assumption, not that the water formed by -the melted ice was dammed back, but that the sea itself was dammed -back, and that by a wall extending to a depth of not less than two or -three hundred feet, so as to allow of a lake being formed in which the -deposits might accumulate; assuming, of course, that the absolute level -of the land was the same then as it is now. - -But as regards the stratified deposits of Grangemouth, we have direct -evidence of their marine origin down to the bottom of the Red Clay that -immediately overlies the till and its intercalated beds, which on an -average is no less than 85 feet, and in some cases 100 feet, below the -present surface. From this deposit, Foraminifera, indicating an arctic -condition of sea, were determined by Mr. David Robertson. Marine shells -were also found in this bed, and along with them the remains of a -seal, which was determined by Professor Turner to be of an exceedingly -arctic type, thus proving that these deposits were not only marine but -glacial. - -Direct fossil evidence as to the character of the deposits occupying -the western basin, is, however, not so abundant, but this may be owing -to the fact that during the sinking of pits, no special attention -has been paid to the matter. At Blairdardie, in sinking a pit-shaft -through these deposits, shells were found in a bed of sand between two -immense masses of boulder clay. The position of this bed will be better -understood from the following section of the pit-shaft:— - - Feet. - Surface soil 4½ - Blue clay 9 - Hard stony clay 69 - Sand with, a few _shells_ 3 - Stony clay and boulders 46½ - Mud and running sand 11 - Hard clay, boulders, and broken rock 27 - ——— - 170 - -But as the shells were not preserved, we have, of course, no means of -determining whether they were of marine or of freshwater origin. - -In another pit, at a short distance from the above, _Cyprina Islandica_ -was found in a bed at the depth of 54 feet below the surface.[287] - -In a paper read by Mr. James Smith, of Jordanhill, to the Geological -Society, April 24th, 1850,[288] the discovery is recorded of a -stratified bed containing _Tellina proxima_ intercalated between two -distinct boulder clays. The bed was discovered by Mr. James Russell in -sinking a well at Chapelhall, near Airdrie. Its height above sea-level -was 510 feet. The character of the shell not only proves the marine -origin of the bed, but also the existence of a submergence to that -extent during an inter-glacial period. - -On the other hand, the difficulty besetting the theory of the marine -origin of the deposits is this. The intercalated boulder clays bear -no marks of stratification, and are evidently the true unstratified -till formed when the country was covered by ice. But the fact that -these beds are both underlaid and overlaid by stratified deposits -would, on the marine theory, imply not merely the repeated appearance -and disappearance of the ice, but also the repeated submergence and -emergence of the land. If the opinion be correct that the submergences -and emergences of the glacial epoch were due to depressions and -elevations of the land, and not to oscillations of sea-level, then -the difficulty in question is, indeed, a formidable one. But, on the -other hand, if the theory of submergences propounded in Chapters XXIII. -and XXIV. be the true one, the difficulty entirely disappears. The -explanation is as follows, viz., during a cold period of the glacial -epoch, when the winter solstice was in aphelion, the low grounds would -be covered with ice, under which a mass of till would be formed. -After the cold began to decrease, and the ice to disappear from the -plains, the greatest rise of the ocean, for reasons already stated, -would take place. The till covering the low grounds would be submerged -to a considerable depth and would soon be covered over by mud, sand, -and gravel, carried down by streams from the high ground, which, at -the time, would still be covered with snow and ice. In course of time -the sea would begin to sink and a warm and continental period of, -perhaps, from 6,000 to 10,000 years, would follow, when the sea would -be standing at a much lower level than at present. The warm period -would be succeeded by a second cold period, and the ice would again -cover the land and form a second mass of till, which, in some places, -would rest directly on the former till, while in other places it would -be laid down upon the surface of the sands and gravels overlying the -first mass. Again, on the disappearance of the ice the second mass of -till would be covered over in like manner by mud, sand, and gravel, and -so on, while the eccentricity of the earth’s orbit continued at a high -value. In this way we might have three, four, five, or more masses of -till separated by beds of sand and gravel. - -It will be seen from Table IV. of the eccentricity of the earth’s -orbit, given in Chapter XIX., that the former half of that long -succession of cold and warm periods, known as the glacial epoch, -was much more severe than the latter half. That is to say, in the -former half the accumulation of ice during the cold periods, and its -disappearance in polar regions during the warm periods, would be -greater than in the latter half. It was probable that it was during -the warm periods of the earlier part of the glacial epoch that the two -buried channels of the Midland valley were occupied by rivers, and that -it was during the latter and less severe part of the glacial epoch that -these channels became filled up with that remarkable series of deposits -which we have been considering. - -_Other buried River Channels._—A good many examples of buried river -channels have been found both in Scotland and in England, though none -of them of so remarkable a character as the two occupying the valley -of the Forth and Clyde Canal which have been just described. I may, -however, briefly refer to one or two localities where some of these -occur. - -(1.) An ancient buried river channel, similar to the one extending -from Kilsyth to Grangemouth, exists in the coal-fields of Durham, -and is known to miners in the district as the “Wash.” Its course was -traced by Mr. Nicholas Wood, F.G.S., and Mr. E. F. Boyd, from Durham -to Newcastle, a distance of fourteen miles.[289] It traverses, after -passing the city of Durham, a portion of the valley of the Wear, passes -Chester-le-Street, and then follows the valley of the river Team, and -terminates at the river Tyne. And what is remarkable, it enters the -Tyne at a depth of 140 feet below the present level of the sea. This -curious hollow lies buried, like the Scottish one just alluded to, -under an enormous mass of drift, and it is only through means of boring -and other mining operations that its character has been revealed. The -bottom and sides of this channel everywhere bear evidence of long -exposure to the abrading influence of water in motion; the rocky bottom -being smoothed, furrowed, and water-worn. The river Wear of the present -day flows to the sea over the surface of the drift at an elevation of -more than 100 feet above this buried river-bed. At the time that this -channel was occupied by running water the sea-level must have been at -least 140 feet lower than at present. This old river evidently belongs -to the same continental period as those of Scotland. - -(2.) From extensive borings and excavations, made at the docks of Hull -and Grimsby, it is found that the ancient bed of the Humber is buried -under more than 100 feet of silt, clay, and gravel. At Hull the bottom -of this buried trough was found to be 110 feet below the sea-level. -And what is most interesting at both these places, the remains of a -submerged forest was found at a depth of from thirty to fifty feet -below the sea-level. In some places two forests were found divided by a -bed of leafy clay from five to fifteen feet thick. - -(3.) In the valleys of Norfolk we also find the same conditions -exhibited. The ancient bed of the Yare and other rivers of this -district enter the sea at a depth of more than 100 feet below the -present sea-level. At Yarmouth the surface was found 170 feet thick, -and the deep surface extends along the Yare to beyond Norwich. Buried -forests are also found here similar to those on the Humber. - -It is probable that all our British rivers flow into the sea over their -old buried channels, except in cases where they may have changed their -courses since the beginning of the glacial epoch. - -(4.) In the Sanquhar Coal Basin, at the foot of the Kello Water, an -old buried river course was found by Mr. B. N. Peach. It ran at right -angles to the Kello, and was filled with boulder clay which cut off the -coal; but, on driving the mine through the clay, the coal was found in -position on the other side. - -(5.) An old river course, under the boulder clay, is described by Mr. -Milne Home in his memoir on the Mid-Lothian coal-fields. It has been -traced out from Niddry away in a N.E. direction by New Craighall. At -Niddry, the hollow is about 100 yards wide and between 60 and 70 feet -deep. It seems to deepen and widen as it approaches towards the sea, -for at New Craighall it is about 200 yards wide and 97 feet deep. This -old channel will probably enter the sea about Musselburgh. Like the -channels in the Midland Valley of Scotland already described, it is so -completely filled up by drift that not a trace of it is to be seen on -the surface. And like these, also, it must have belonged to a period -when the sea-level stood much lower than at present. - -(6.) At Hailes’ Quarry, near Edinburgh, there is to be seen a portion -of an ancient watercourse under the boulder drift. A short account -of it was given by Dr. Page in a paper read before the Edinburgh -Geological Society.[290] The superincumbent sandstone, he says, has -been cut to a depth of 60 feet. The width of the channel at the surface -varies from 12 to 14 feet, but gradually narrows to 2 or 3 feet at the -bottom. The sides and bottom are smoothed and polished, and the whole -is now filled with till and boulders. - -(7.) One of the most remarkable buried channels is that along the -Valley of Strathmore, supposed to be the ancient bed of the Tay. It -extends from Dunkeld, the south of Blairgowrie, Ruthven, and Forfar, -and enters the German Ocean at Lunan Bay. Its length is about 34 miles. - -“No great river,” says Sir Charles Lyell, “follows this course, but -it is marked everywhere by lakes or ponds, which afford shell-marl, -swamps, and peat moss, commonly surrounded by ridges of detritus from -50 to 70 feet high, consisting in the lower part of till and boulders, -and in the upper of stratified gravels, sand, loam, and clay, in some -instances curved or contorted.”[291] - -“It evidently marks an ancient line, by which, first, a great glacier -descended from the mountains to the sea, and by which, secondly, at -a later period, the principal water drainage of this country was -effected.”[292] - -(8.) A number of examples of ancient river courses, underneath the -boulder clay, are detailed by Professor Geikie in his glacial drift of -Scotland. Some of the cases described by him have acquired additional -interest from the fact of their bearing decided testimony to the -existence of inter-glacial warm periods. I shall briefly refer to a few -of the cases described by him. - -In driving a trial mine in a pit at Chapelhall, near Airdrie, the -workmen came upon what they believed to be an old river course. At -the end of the trial mine the ironstone, with its accompanying coal -and fire-clay, were cut off at an angle of about 20° by a stiff, -dark-coloured earth, stuck full of angular pieces of white sandstone, -coal, and shale, with rounded pebbles of greenstone, basalt, quartz, -&c. Above this lay a fine series of sand and clay beds. Above these -stratified beds lay a depth of 50 or 60 feet of true boulder clay. The -channel ran in the direction of north-east and south-west. Mr. Russell, -of Chapelhall, informs Professor Geikie that another of the same kind, -a mile farther to the north-west, had been traced in some of the pit -workings. - -“It is clear,” says Professor Geikie, “that whatever may be the true -explanation of these channels and basins, they unquestionably belong to -the period of the boulder clay. The Chapelhall basin lies, indeed, in a -hollow of the carboniferous rocks, but its stratified sands and clays -rest on an irregular floor of true till. The old channel near the banks -of the Calder is likewise scooped out of sandstones and shales; but -it has a coating of boulder clay, on which its finely-laminated sands -and clays repose, _as if the channel itself had once been filled with -boulder clay, which was re-excavated to allow of the deposition of the -stratified deposits. In all cases, a thick mantle of coarse, tumultuous -boulder clay buries the whole._”[293] - -Professor Geikie found between the mouth of the Pease Burn and St. -Abb’s Head, Berwickshire, several ancient buried channels. One at -the Menzie Cleuch, near Redheugh Shore, was filled to the brim with -boulder clay. Another, the Lumsden Dean, half a mile to the east of -Fast Castle, on the bank of the Carmichael Burn, near the parish church -of Carmichael,—an old watercourse of the boulder clay period—is to -be seen. The valley of the Mouse Water he instances as a remarkable -example. - -One or two he found in Ayrshire, and also one on the banks of the Lyne -Water, a tributary of the Tweed. - -(9.) In the valley of the Clyde, above Hamilton, several buried river -channels have been observed. They are thus described by Mr. James -Geikie:—[294] - -“In the Wishaw district, two deep, winding troughs, filled with sand -and fine gravel, have been traced over a considerable area in the coal -workings.[295] These troughs form no feature at the surface, but are -entirely concealed below a thick covering of boulder clay. They appear -to be old stream courses, and are in all probability the pre-glacial -ravines of the Calder Water and the Tillon Burn. The ‘sand-dyke’ that -represents the pre-glacial course of the Calder Water runs for some -distance parallel to the present course of the stream down to Wishaw -House, where it is intersected by the Calder, and the deposits which -choke it up are well seen in the steep wooded banks below the house -and in the cliff on the opposite side. It next strikes to south-east, -and is again well exposed on the road-side leading down from Wishaw -to the Calder Water. From this point it has been traced underground, -more or less continuously, as far as Wishaw Ironworks. Beyond this -place the coal-seams sink to a greater depth, and therefore cease to -be intersected by the ancient ravine, the course of which, however, -may still be inferred from the evidence obtained during the sinking -of shafts and trial borings. In all probability it runs south, and -enters the old course of the Clyde a little below Cambusnethan House. -Only a portion of the old ravine of the Tillon Burn is shown upon the -Map. It is first met with in the coal-workings of Cleland Townhead -(Sheet 31). From this place it winds underground in a southerly -direction until it is intersected by the present Tillon Burn, a little -north of Glencleland (Sheet 31). It now runs to south-west, keeping -parallel to the burn, and crosses the valley of the Calder just -immediately above the mouth of the Tillon. From this point it can be -traced in pit-shafts, open-air sections, borings, and coal-workings, -by Ravenscraig, Nether Johnstone, and Robberhall Belting, on to the -Calder Water below Coursington Bridge (Sheet 31). It would thus appear -that in pre-glacial times the Calder and the Tillon were independent -streams, and that since glacial times the Calder Water, forsaking its -pre-glacial course, has cut its way across the intervening ground, -ploughing out deep ravines in the solid rocks, until eventually it -united with the Tillon. Similar buried stream courses occur at other -places. Thus, at Fairholme, near Larkhall, as already mentioned (par. -94), the pre-glacial course of the Avon has been traced in pit-shafts -and borings for some distance to the north. Another old course, filled -up with boulder clay, is exposed in a burn near Plotcock, a mile -south-west from Millheugh; and a similar pre-glacial ravine was met -with in the cement-stone workings at Calderwood.[296] Indeed, it might -be said with truth that nearly all the rocky ravines through which the -waters flow, especially in the carboniferous areas, are of post-glacial -age—the pre-glacial courses lying concealed under masses of drift. Most -frequently, however, the present courses of the streams are partly -pre-glacial and partly post-glacial. In the pre-glacial portions the -streams flow through boulder clay, in the post-glacial reaches their -course, as just mentioned, is usually in rocky ravines. The Avon and -the Calder, with their tributaries, afford numerous illustrations of -these phenomena.” - -The question naturally arises, When were those channels scooped out? -To what geological period must those ancient rivers be referred? It -will not do to conclude that those channels must be pre-glacial simply -because they contain boulder clay. Had the glacial epoch been one -unbroken period of cold, and the boulder clay one continuous formation, -then the fact of finding boulder clay in those channels would show that -they were pre-glacial. But when we find undoubted geological evidence -of a warm condition of climate of long continuance, during the severest -part of the glacial epoch, when the ice, to a great extent, must have -disappeared, and water began to flow as usual down our valleys, all -that can reasonably be inferred from the fact of finding till in those -channels, is that they must be older than the till they contain. We -cannot infer that they are older than all the till lying on the face -of the country. The probability, however, is, that some of them are -of pre-glacial and others of inter-glacial origin. That many of these -channels have been used as watercourses during the glacial epoch, or -rather during warm periods of that epoch, is certain, from the fact -that they have been filled with boulder clay, then re-excavated, and -finally filled up again with the clay. - - - - - CHAPTER XXX. - - THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THEORIES OF - GLACIER-MOTION. - - Why the Question of Glacier-motion has been found to be so - difficult.—The Regelation Theory.—It accounts for the - Continuity of a Glacier, but not for its Motion.—Gravitation - proved by Canon Moseley insufficient to shear the Ice - of a Glacier.—Mr. Mathew’s Experiment.—No Parallel - between the bending of an Ice Plank and the shearing - of a Glacier.—Mr. Ball’s Objection to Canon Moseley’s - Experiment.—Canon Moseley’s Method of determining the Unit - of Shear.—Defect of Method.—Motion of a Glacier in some - Way dependent on Heat.—Canon Moseley’s Theory.—Objections - to his Theory.—Professor James Thomson’s Theory.—This - Theory fails to explain Glacier-motion.—De Saussure and - Hopkins’s “Sliding” Theories.—M. Charpentier’s “Dilatation” - Theory.—Important Element in the Theory. - - -The cause of the motion of glaciers has proved to be one of the most -difficult and perplexing questions within the whole domain of physics. -The main difficulty lies in reconciling the motion of the glacier with -the physical properties of the ice. A glacier moves down a valley -very much in the same way as a river, the motion being least at the -sides and greatest at the centre, and greater at the surface than at -the bottom. In a cross section scarcely two particles will be moving -with the same velocity. Again, a glacier accommodates itself to the -inequalities of the channel in which it moves exactly as a semifluid -or plastic substance would do. So thoroughly does a glacier behave -in the manner of a viscous or plastic body that Professor Forbes was -induced to believe that viscosity was a property of the ice, and that -in virtue of this property it was enabled to move with a differential -motion and accommodate itself to all the inequalities of its channel -without losing its continuity just as a mass of mud or putty would do. -But experience proves that ice is a hard and brittle substance far -more resembling glass than putty. In fact it is one of the most brittle -and unyielding substances in nature. So unyielding is a glacier that -it will snap in two before it will stretch to any perceptible extent. -This is proved by the fact that crevasses resulting from a strain on -the glacier consist at first of a simple crack scarcely wide enough to -admit the blade of a penknife. - -All the effects which were considered to be due to the viscosity of -the ice have been fully explained and accounted for on the principle -of fracture and regelation discovered by Faraday. The principle of -regelation explains why the ice moving with a differential motion and -accommodating itself to the inequalities of its channel is yet enabled -to retain its continuity, but it does not account for the _cause_ of -glacier motion. In fact it rather involves the question in deeper -mystery than before. For it is far more difficult to conceive how the -particles of a hard and brittle solid like that of ice can move with -a differential motion, than it is to conceive how this may take place -in the case of a soft and yielding substance. The particles of ice -have all to be displaced one over another and alongside each other, -and as those particles are rigidly fixed together this connection must -be broken before the one can slide over the other. _Shearing-force_, -as Canon Moseley shows, comes into play. Were ice a plastic substance -there would not be much difficulty in understanding how the particles -should move the one over the other, but it is totally different when -we conceive ice to be a solid and unyielding substance. The difficulty -in connection with glacier-motion is not to account for the continuity -of the ice, for the principle of regelation fully explains this, but -to show how it is that one particle succeeds in sliding over the over. -The principle of regelation, instead of assisting to remove this -difficulty, increases it tenfold. Regelation does not explain the cause -of glacier-motion, but the reverse. It rather tends to show that a -glacier should not move. What, then, is the cause of glacier-motion? -According to the regelation theory, gravitation is the impelling -cause. But is gravitation sufficient to _shear_ the ice in the manner -in which it is actually done in a glacier? - -I presume that few who have given much thought to the subject of -glacier-motion have not had some slight misgivings in regard to the -commonly received theory. There are some facts which I never could -harmonize with this theory. For example, boulder clay is a far looser -substance than ice; its shearing-force must be very much less than -that of ice; yet immense masses of boulder clay will lie immovable for -ages on the slope of a hill so steep that one can hardly venture to -climb it, while a glacier will come crawling down a valley which by -the eye we could hardly detect to be actually off the level. Again, a -glacier moves faster during the day than during the night, and about -twice as fast during summer as during winter. Professor Forbes, for -example, found that the Glacier des Bois near its lower extremity moved -sometimes in December only 11·5 inches daily, while during the month -of July its rate of motion sometimes reached 52·1 inches per day. Why -such a difference in the rate of motion between day and night, summer -and winter? The glacier is not heavier during the day than it is -during the night, or during the summer than it is during the winter; -neither is the shearing-force of the great mass of the ice of a glacier -sensibly less during day than night, or during summer than winter; -for the temperature of the great mass of the ice does not sensibly -vary with the seasons. If this be the case, then gravitation ought to -be as able to shear the ice during the night as during the day, or -during the winter as during the summer. At any rate, if there should -be any difference it ought to be but trifling. It is true that, owing -to the melting of the ice, the crevices of the glacier are more gorged -with water during summer than winter; and this, as Professor Forbes -maintains,[297] may tend to make the glacier move faster during the -former than the latter season. But the advocates of the regelation -theory cannot conclude, with Professor Forbes, that the water favours -the motion of the glacier by making the ice more soft and plastic. The -melting of the ice, according to the regelation theory, cannot very -materially aid the motion of the glacier. - -The theory which has led to the general belief that the ice of a -glacier is sheared by the force of gravity appears to be the following. -It is supposed that the only forces to which the motion of a glacier -can be referred are _gravitation_ and _heat_; but as the great mass -of a glacier remains constantly at the same uniform temperature it -is concluded to be impossible that the motion of the glacier can be -due to this cause, and therefore of course it must be attributed to -gravitation, there being no other cause. - -That gravitation is insufficient to shear the ice of a glacier has been -clearly demonstrated by Canon Moseley.[298] He determined by experiment -the amount of force required to shear one square inch of ice, and found -it to be about 75 lbs. By a process of calculation which will be found -detailed in the Memoir referred to, he demonstrated that to descend -by its own weight at the rate at which Professor Tyndall observed the -ice of the Mer de Glace to be descending at the Tacul, the unit of -shearing force of the ice could not have been more than 1·31931 lbs. -Consequently it will require a force more than 34 times the weight of -the glacier to shear the ice and cause it to descend in the manner in -which it is found to descend. - -It is now six years since Canon Moseley’s results were laid before the -public, and no one, as far as I am aware, has yet attempted to point -out any serious defect in his mathematical treatment of the question. -Seeing the great amount of interest manifested in the question of -glacier-motion, I think we are warranted to conclude that had the -mathematical part of the memoir been inconclusive its defects would -have been pointed out ere this time. The question, then, hinges on -whether the experimental data on which his calculations are based -be correct. Or, in other words, is the unit of shear of ice as much -as 75 lbs.? This part of Mr. Moseley’s researches has not passed -unquestioned. Mr. Ball and Mr. Mathews, both of whom have had much -experience among glaciers, and have bestowed considerable attention on -the subject of glacier-motion, have objected to the accuracy of Mr. -Moseley’s unit of shear. I have carefully read the interesting memoirs -of Mr. Mathews and Mr. Ball in reply to Canon Moseley, but I am unable -to perceive that anything which they have advanced materially affects -his general conclusions as regards the commonly received theory. Mr. -Mathews objects to Canon Moseley’s experiments on the grounds that -extraneous forces are brought to bear upon the substance submitted -to operation, and that conditions are thus introduced which do not -obtain in the case of an actual glacier. “It would throw,” he says, -“great light upon our inquiry if we were to change this method of -procedure and simply to observe the deportment of masses of ice under -the influence of no external forces but the gravitation of their own -particles.”[299] A plank of ice six inches wide and 2⅜ inches in -thickness was supported at each end by bearers six feet apart. From the -moment the plank was placed in position it began to sink, and continued -to do so until it touched the surface over which it was supported. Mr. -Mathews remarks that with this property of ice, viz., its power to -change its form under strains produced by its own gravitation, combined -with the sliding movement demonstrated by Hopkins, we have an adequate -cause for glacier-motion. Mr. Mathews concludes from this experiment -that the unit of shear in ice, instead of being 75 lbs., is less than -1¾ lbs. - -There is, however, no parallel between the bending of the ice-plank and -the shearing of a glacier. Mr. Mathews’ experiment appears to prove too -much, as will be seen from the following reply of Canon Moseley:— - -“Now I will,” he says, “suggest to Mr. Mathews a parallel experiment -and a parallel explanation. If a bar of wrought iron 1 inch square and -20 feet long were supported at its extremities, it would _bend_ by its -weight alone, and would therefore shear. Now the weight of such a -rod would be about 67 lbs. According to Mr. Mathews’s explanation in -the case of the ice-plank, the unit of shear in wrought-iron should -therefore be 67 lbs. per square inch. It is actually 50,000 lbs.”[300] - -Whatever theory we may adopt as to the cause of the motion of glaciers, -the deflection of the plank in the way described by Mr. Mathews -_follows as a necessary consequence_. Although no weight was placed -upon the plank, it does not necessarily follow that the deflection -was caused by the weight of the ice alone; for, according to Canon -Moseley’s own theory of the motion of glaciers by heat, the plank -ought to be deflected in the middle, just as it was in Mr. Mathews’s -experiment. A solid body, when exposed to variations of temperature, -will expand and contract transversely as well as longitudinally. Ice, -according to Canon Moseley’s theory, expands and contracts by heat. -Then if the plank expands transversely, the upper half of the plank -must rise and the lower half descend. But the side which rises has -to perform work against gravity, whereas the side which descends has -work performed upon it by gravity; consequently more of the plank will -descend than rise, and this will, of course, tend to lower or deflect -the plank in the middle. Again, when the plank contracts, the lower -half will rise and the upper half will descend; but as gravitation, -in this case also, favours the descending part and opposes the rising -part, more of the plank will descend than rise, and consequently -the plank will be lowered in the middle by contraction as well as -by expansion. Thus, as the plank changes its temperature, it must, -according to Mr. Moseley’s theory, descend or be deflected in the -middle, step by step—and this not by gravitation alone, but chiefly -by the motive power of heat. I do not, of course, mean to assert that -the descent of the plank was caused by heat; but I assert that Mr. -Mathews’s experiment does not necessarily prove (and this is all that -is required in the meantime) that gravitation alone was the cause of -the deflection of the plank. Neither does this experiment prove that -the ice was deflected without shearing; for although the weight of the -plank was not sufficient to shear the ice, as Mr. Mathews, I presume, -admits, yet Mr. Moseley would reply that the weight of the ice, -assisted by the motive power of heat, was perfectly sufficient. - -I shall now briefly refer to Mr. Ball’s principal objections to Canon -Moseley’s proof that a glacier cannot shear by its weight alone. One -of his chief objections is that Mr. Moseley has assumed the ice to be -homogeneous in structure, and that pressures and tensions acting within -it, are not modified by the varying constitution of the mass.[301] -Although there is, no doubt, some force in this objection (for we have -probably good reason to believe that ice will shear, for example, more -easily along certain planes than others), still I can hardly think that -Canon Moseley’s main conclusion can ever be materially affected by this -objection. The main question is this, Can the ice of the glacier shear -by its own weight in the way generally supposed? Now the shearing force -of ice, take it in whatever direction we may, so enormously exceeds -that required by Mr. Moseley in order to allow a glacier to descend by -its weight only, that it is a matter of indifference whether ice be -regarded as homogeneous in structure or not. Mr. Ball objects also to -Mr. Moseley’s imaginary glacier lying on an even slope and in a uniform -rectangular channel. He thinks that an irregular channel and a variable -slope would be more favourable to the descent of the ice. But surely -if the work by the weight of the ice be not equal to the work by the -resistance in a glacier of uniform breadth and slope, it must be much -less so in the case of one of irregular shape and slope. - -That a relative displacement of the particles of the ice is involved -in the motion of a glacier, is admitted, of course, by Mr. Ball; but -he states that the amount of this displacement is but small, and that -it is effected with extreme slowness. This may be the case; but if the -weight of the ice be not able to overcome the mutual cohesion of the -particles, then the weight of the ice cannot produce the required -displacement, however small it may be. Mr. Ball then objects to Mr. -Moseley’s method of determining the unit of shear on this ground:—The -shearing of the ice in a glacier is effected with extreme slowness; -but the shearing in Canon Moseley’s experiment was effected with -rapidity; and although it required 75 lbs. to shear one square inch of -surface in his experiment, it does not follow that 75 lbs. would be -required to shear the ice if done in the slow manner in which it is -effected in the glacier. “In short,” says Mr. Ball, “to ascertain the -resistance opposed to very slow changes in the relative positions of -the particles, so slight as to be insensible at short distances, Mr. -Moseley measures the resistance opposed to rapid disruption between -contiguous portions of the same substance.” - -There is force in this objection; and here we arrive at a really weak -point in Canon Moseley’s reasoning. His experiments show that if we -want to shear ice quickly a weight of nearly 120 lbs. is required; but -if the thing is to be done more slowly, 75 lbs. will suffice.[302] In -short, the number of pounds required to shear the ice depends, to a -large extent, on the length of time that the weight is allowed to act; -the longer it is allowed to act, the less will be the weight required -to perform the work. “I am curious to know,” says Mr. Mathews, when -referring to this point, “what weight would have sheared the ice -if a _day_ had been allowed for its operation.” I do not know what -would have been the weight required to shear the ice in Mr. Moseley’s -experiments had a day been allowed; but I feel pretty confident that, -should the ice remain unmelted, and sufficient time be allowed, -shearing would be produced without the application of any weight -whatever. There are no weights placed upon a glacier to make it move, -and yet the ice of the glacier shears. If the shearing is effected by -weight, the only weight applied is the weight of the ice; and if the -weight of the ice makes the ice shear in the glacier, why may it not -do the same thing in the experiment? Whatever may be the cause which -displaces the particles of the ice in a glacier, they, as a matter of -fact, are displaced without any weight being applied beyond that of -the ice itself; and if so, why may not the particles of the ice in -the experiment be also displaced without the application of weights? -Allow the ice of the glacier to take its own time and its own way, and -the particles will move over each other without the aid of external -weights, whatever may be the cause of this; well, then, allow the ice -in the experiment to take its own time and its own way, and it will -probably do the same thing. There is something here unsatisfactory. -If, by the unit of shear, be meant the pressure in pounds that must -be applied to the ice to break the connection of one square inch of -two surfaces frozen together and cause the one to slip over the other, -then the amount of pressure required to do this will depend upon the -time you allow for the thing being done. If the thing is to be done -rapidly, as in some of Mr. Moseley’s experiments, it will take, as he -has shown, a pressure of about 120 lbs.; but if the thing has to be -done more slowly, as in some other of his experiments, 75 lbs. will -suffice. And if sufficient time be allowed, as in the case of glaciers, -the thing may be done without any weight whatever being applied to the -ice, and, of course, Mr. Moseley’s argument, that a glacier cannot -descend by its weight alone, falls to the ground. But if, by the unit -of shear, be meant not the _weight_ or _pressure_ necessary to shear -the ice, but the amount of _work_ required to shear a square inch of -surface _in a given time or at a given rate_, then he might be able -to show that in the case of a glacier (say the Mer de Glace) the work -of all the resistances which are opposed to its descent at the _rate_ -at which it is descending is greater than the work of its weight, and -that consequently there must be some cause, in addition to the weight, -urging the glacier forward. But then he would have no right to affirm -that the glacier would not descend by its weight only; all that he -could affirm would simply be that it could not descend by its weight -alone at the _rate_ at which it is descending. - -Mr. Moseley’s unit of shear, however, is not the amount of work -performed in shearing a square inch of ice in a given time, but the -amount of _weight_ or _pressure_ requiring to be applied to the ice -to shear a square inch. But this amount of pressure depends upon the -length of time that the pressure is applied. Here lies the difficulty -in determining what amount of pressure is to be taken as the real unit. -And here also lies the radical defect in Canon Moseley’s result. Time -as well as pressure enters as an element into the process. The key to -the explanation of this curious circumstance will, I think, be found in -the fact that the rate at which a glacier descends depends in some way -or other upon the amount of heat that the ice is receiving. This fact -shows that heat has something to do in the shearing of the ice of the -glacier. But in the communication of heat to the ice _time_ necessarily -enters as an element. There are two different ways in which heat may be -conceived to aid in shearing the ice: (1.) we may conceive that heat -acts as a force along with gravitation in producing displacement of the -particles of the ice; or (2.) we may conceive that heat does not act as -a force in pushing the particles over each other, but that it assists -the shearing processes by diminishing the cohesion of the particles of -the ice, and thus allowing gravitation to produce displacement. The -former is the function attributed to heat in Canon Moseley’s theory -of glacier-motion; the latter is the function attributed to heat in -the theory of glacier-motion which I ventured to advance some time -ago.[303] It results, therefore, from Canon Moseley’s own theory, that -the longer the time that is allowed for the pressure to shear the -ice, the less will be the pressure required; for, according to his -theory, a very large proportion of the displacement is produced by the -motive power of heat entering the ice; and, as it follows of course, -other things being equal, the longer the time during which the heat -is allowed to act, the greater will be the proportionate amount of -displacement produced by the heat; consequently the less will require -to be done by the weight applied. In the case of the glacier, Mr. -Moseley concludes that at least thirty or forty times as much work is -done by the motive power of heat in the way of shearing the ice as is -done by mere pressure or weight. Then, if sufficient time be allowed, -why may not far more be done by heat in shearing the ice in his -experiment than by the weight applied? In this case how is he to know -how much of the shearing is effected by the heat and how much by the -weight? If the greater part of the shearing of the ice in the case of a -glacier is produced, not by pressure, but by the heat which necessarily -enters the ice, it would be inconceivable that in his experiments the -heat entering the ice should not produce, at least to some extent, a -similar effect. And if a portion of the displacement of the particles -is produced by heat, then the weight which is applied cannot be -regarded as the measure of the force employed in the displacement, any -more than it could be inferred that the weight of the glacier is the -measure of the force employed in the shearing of it. If the weight -is not the entire force employed in shearing, but only a part of the -force, then the weight cannot, as in Mr. Moseley’s experiment, be taken -as the measure of the force. - -How, then, are we to determine what is the amount of force required to -shear ice? in other words, how is the unit of shear to be determined? -If we are to measure the unit of shear by the weight required to -produce displacement of the particles of the ice, we must make sure -that the displacement is wholly effected by the weight. We must be -certain that heat does not enter as an element in the process. But -if time be allowed to elapse during the experiment, we can never -be certain that heat has not been at work. It is impossible to -prevent heat entering the ice. We may keep the ice at a constant -temperature, but this would not prevent heat from entering the ice and -producing molecular work. True that, according to Moseley’s theory -of glacier-motion, if the temperature of the ice be not permitted -to _vary_, then no displacement of the particles can take place -from the influence of heat; but according to the molecular theory of -glacier-motion, which will shortly be considered, heat will aid the -displacement of the particles whether the temperature be kept constant -or not. In short, it is absolutely impossible in our experiments to -be certain that heat is not in some way or other concerned in the -displacement of the particles of the ice. But we can shorten the time, -and thus make sure that the amount of heat entering the ice during the -experiments is too small to affect materially the result. We cannot in -this case say that all the displacement has been effected by the weight -applied to the ice, but we can say that so little has been done by heat -that, practically, we may regard it as all done by the weight. - -This consideration, I trust, shows that the unit of shear adopted by -Canon Moseley in his calculations is not too large. For if in half an -hour, after all the work that may have been done by heat, a pressure of -75 lbs. is still required to displace the particles of one square inch, -it is perfectly evident that if no work had been done by heat during -that time, the force required to produce the displacement could not -have been less than 75 lbs. It might have been more than that; but it -could not have been less. Be this, however, as it may, in determining -the unit of shear we cannot be permitted to prolong the experiment for -any considerable length of time, because the weight under which the -ice might then shear could not be taken as the measure of the force -which is required to shear ice. By prolonging the experiment we might -possibly get a unit smaller than that required by Canon Moseley for -a glacier to descend by its own weight. But it would be just as much -begging the whole question at issue to assume that, because the ice -sheared under such a weight, a glacier might descend by its weight -alone, as it would be to assume that, because a glacier shears without -a weight being placed upon it, the glacier descends by its weight alone. - -But why not determine the unit of shear of ice in the same way as we -would the unit of shear of any other solid substance, such, as iron, -stone, or wood? If the shearing force of ice be determined in this -manner, it will be found to be by far too great to allow of the ice -shearing by its weight alone. We shall be obliged to admit either -that the ice of the glacier does not shear (in the ordinary sense of -the term), or if it does shear, that there must, as Canon Moseley -concludes, be some other force in addition to the weight of the ice -urging the glacier forward. - -The fact that the rate of descent of a glacier depends upon the amount -of heat which it receives, proves that heat must be regarded either as -a cause or as a necessary condition of its motion; what, then, is the -necessary relationship between heat and the motion of the glacier? If -heat is to be regarded as a cause, in what way does the heat produce -motion? I shall now briefly refer to one or two theories which have -been advanced on the subject. Let us consider first that of Canon -Moseley. - -_Canon Moseley’s Theory._—He found, from observations and experiments, -that sheets of lead, placed upon an inclined plane, when subjected to -variations of temperature, tend to descend even when the slope is far -less than that which would enable it to slide down under the influence -of gravitation. The cause of the descent he shows to be this. When the -temperature of the sheet is raised, it expands, and, in expanding, its -upper portion moves up the slope, and its lower portion down the slope; -but as gravitation opposes the upward and favours the downward motion, -more of the sheet moves down than up, and consequently the centre -of gravity of the sheet is slightly lowered. Again, when the sheet -is cooled, it contracts, and in contracting the upper portion moves -downwards and the lower portion upwards, and here again, for the same -reason, more of the sheet moves downwards than upwards. Consequently, -at every change of temperature there is a slight displacement of the -sheet downwards. “Now a theory of the descent of glaciers,” says -Canon Moseley, “which I have ventured to propose myself, is that they -descend, as the lead in this experiment does, by reason of the passage -into them and the withdrawal of the sun’s rays, and that the dilatation -and contraction of the ice so produced is the proximate cause of their -descent, as it is of that of the lead.”[304] - -The fundamental condition in Mr. Moseley’s theory of the descent of -solid bodies on an incline, is, not that heat should maintain these -bodies at a high temperature, but that the temperature should vary. -The rate of descent is proportionate, not simply to the amount of -heat received, but to the extent and frequency of the variations of -temperature. As a proof that glaciers are subjected to great variations -of temperature, he adduces the following:—“All alpine travellers,” he -says, “from De Saussure to Forbes and Tyndall, have borne testimony -to the intensity of the solar radiation on the surfaces of glaciers. -‘I scarcely ever,’ says Forbes, ‘remember to have found the sun more -piercing than at the Jardin.’ This heat passes abruptly into a state -of intense cold when any part of the glacier falls into shadow by an -alteration of the position of the sun, or even by the passing over it -of a cloud.”[305] - -Mr. Moseley is here narrating simply what the traveller feels, and -not what the glacier experiences. The traveller is subjected to great -variations of temperature; but there is no proof from this that the -glacier experiences any changes of temperature. It is rather because -the temperature of the glacier is not affected by the sun’s heat that -the traveller is so much chilled when the sun’s rays are cut off. The -sun shines down with piercing rays and the traveller is scorched; the -glacier melts on the surface, but it still remains “cold as ice.” The -sun passes behind a cloud or disappears behind a neighbouring hill; the -scorching rays are then withdrawn, and the traveller is now subjected -to radiation on every side from surfaces at the freezing-point. - -It is also a necessary condition in Mr. Moseley’s theory that the heat -should pass easily into and out of the glacier; for unless this were -the case sudden changes of temperature could produce little or no -effect on the great mass of the glacier. How, then, is it possible that -during the heat of summer the temperature of the glacier could vary -much? During that season, in the lower valleys at least, everything, -with the exception of the glacier, is above the freezing-point; -consequently when the glacier goes into the shade there is nothing -to lower the ice below the freezing-point; and as the sun’s rays do -not raise the temperature of the ice above the freezing-point, the -temperature of the glacier must therefore remain unaltered during that -season. It therefore follows that, instead of a glacier moving more -rapidly during the middle of summer than during the middle of winter, -it should, according to Moseley’s theory, have no motion whatever -during summer. - -The following, written fifteen years ago by Professor Forbes on this -very point, is most conclusive:—“But how stands the fact? Mr. Moseley -quotes from De Saussure the following _daily ranges_ of the temperature -of the air in the month of July at the Col du Géant and at Chamouni, -between which points the glacier lies: - - ° - At the Col du Géant 4·257 Réaumur. - At Chamouni 10·092 〃 - -And he assumes ‘the same mean daily variation of temperature to obtain -throughout the length’ [and depth?] ‘of the Glacier du Géant which De -Saussure observed in July at the Col du Géant.’ But between what limits -does the temperature of the air oscillate? We find, by referring to -the third volume of De Saussure’s ‘Travels,’ that the mean temperature -of the coldest hour (4 A.M.) during his stay at the Col du Géant was -33°·03 Fahrenheit, and of the warmest (2 P.M.) 42°·61 F. So that even -upon that exposed ridge, between 2,000 and 3,000 feet above where the -glacier can be properly said to commence, the air does not, on an -average of the month of July, reach the freezing-point at any hour -of the night. Consequently the _range of temperature attributed to -the glacier is between limits absolutely incapable of effecting the -expansion of the ice in the smallest degree_.”[306] - -Again, during winter, as Mr. Ball remarks, the glacier is completely -covered with snow and thus protected both from the influence of -cold and of heat, so that there can be nothing either to raise the -temperature of the ice above the freezing-point or to bring it below -that point; and consequently the glacier ought to remain immovable -during that season also. - -“There can be no doubt, therefore,” Mr. Moseley states, “that the -rays of the sun, which in those alpine regions are of such remarkable -intensity, find their way into the depths of the glacier. They are -a _power_, and there is no such thing as the loss of power. The -mechanical work which is their equivalent, and into which they are -converted when received into the substance of a solid body, accumulates -and stores itself up in the ice under the form of what we call -elastic force or tendency to dilate, until it becomes sufficient to -produce actual dilatation of the ice in the direction in which the -resistance is weakest, and by its withdrawal to produce contraction. -From this expansion and contraction follows of necessity the descent -of the glacier.”[307] When the temperature of the ice is below -the freezing-point, the rays which are absorbed will, no doubt, -produce dilatation; but during summer, when the ice is not below the -freezing-point, no dilatation can possibly take place. All physicists, -so far as I am aware, agree that the rays that are then absorbed go to -melt the ice, and not to expand it. But to this Mr. Moseley replied -as follows:—“To this there is the obvious answer that radiant heat -does find its way into ice as a matter of common observation, and -that it does not melt it except at its surface. Blocks of ice may be -seen in the windows of ice-shops with the sun shining full upon them, -and melting nowhere but on their surfaces. And the experiment of the -ice-lens shows that heat may stream through ice in abundance (of which -a portion is necessarily stopped in the passage) without melting it, -except on its surface.” But what evidence is there to conclude that -if there is no melting of the ice in the interior of the lens there -is a portion of the rays “necessarily stopped” in the interior? It -will not do to assume a point so much opposed to all that we know of -the physical properties of ice as this really is. It is absolutely -essential to Mr. Moseley’s theory of the motion of glaciers, during -summer at least, that ice should continue to expand after it reaches -the melting-point; and it has therefore to be shown that such is the -case; or it need not be wondered at that we cannot accept his theory, -because it demands the adoption of a conclusion contrary to all our -previous conceptions. But, as a matter of fact, it is not strictly true -that when rays pass through a piece of ice there is no melting of the -ice in the interior. Experiments made by Professor Tyndall show the -contrary.[308] - -There is, however, one fortunate circumstance connected with Canon -Moseley’s theory. It is this: its truth can be easily tested by direct -experiment. The ice, according to this theory, descends not simply -in virtue of heat, but in virtue of _change of temperature_. Try, -then, Hopkins’s famous experiment, but keep the ice at a _constant -temperature_; then, according to Moseley’s theory, the ice will not -descend. Let it be observed, however, that although the ice under this -condition should descend (as there is little doubt but it would), -it would show that Mr. Moseley’s theory of the descent of glaciers -is incorrect, still it would not in the least degree affect the -conclusions which he lately arrived at in regard to the generally -received theory of glacier-motion. It would not prove that the ice -sheared, in the way generally supposed, by its weight only. It might be -the heat, after all, entering the ice, which accounted for its descent, -although gravitation (the weight of the ice) might be the impelling -cause. - -According to this theory, the glacier, like the sheet of lead, must -expand and contract as one entire mass, and it is difficult to -conceive how this could account for the differential motion of the -particles of the ice. - -_Professor James Thomson’s Theory._—It was discovered by this physicist -that the freezing-point of water is lowered by pressure. The extent -of the lowering is equal to ·0075° centigrade for every atmosphere -of pressure. As glacier ice is generally about the melting-point, -it follows that when enormous pressure is brought to bear upon any -given point of a glacier a melting of the ice at that particular spot -will take place in consequence of the lowering of the melting-point. -The melting of the ice will, of course, tend to favour the descent -of the glacier, but I can hardly think the liquefaction produced by -pressure can account for the motion of glaciers. It will help to -explain the giving way of the ice at particular points subjected to -great pressure, but I am unable to comprehend how it can account for -the general descent of the glacier. Conceive a rectangular glacier of -uniform breadth and thickness, and lying upon an even slope. In such a -glacier the pressure at each particular point would remain constant, -for there would be no reason why it should be greater at one time than -at another. Suppose the glacier to be 500 feet in thickness; the ice -at the lower surface of the glacier, owing to pressure, would have its -melting-point permanently lowered one-tenth of a degree centigrade -below that of the upper surface; but the ice at the lower surface would -not, on this account, be in the fluid state. It would simply be ice at -a slightly lower temperature. True, when pressure is exerted the ice -melts in consequence of the lowering of the melting-point, but in the -case under consideration there would, properly speaking, be no exertion -of pressure, but a constant statical pressure resulting from the weight -of the ice. But this statical condition of pressure would not produce -fluidity any more than a statical condition of pressure would produce -heat, and consequently motion could not take place as a result of -fluidity. In short, motion itself is required to produce the fluidity. - -I need not here wait to consider the sliding theories of De Saussure -and Hopkins, as they are now almost universally admitted to be -inadequate to explain the phenomena of glacier-motion, seeing that they -do not account for the displacement of the particles of the ice over -one another. - -According to the dilatation theory of M. Charpentier, a glacier is -impelled by the force exerted by water freezing in the fissures of the -ice. A glacier he considers is full of fissures into which water is -being constantly infiltrated, and when the temperature of the air sinks -below the freezing-point it converts the water into ice. The water, in -passing into ice, expands, and in expanding tends to impel the glacier -in the direction of least resistance. This theory, although it does not -explain glacier-motion, as has been clearly shown by Professor J. D. -Forbes, nevertheless contains one important element which, as we shall -see, must enter into the true explanation. The element to which I refer -is the expansive force exerted on the glacier by water freezing. - - - - - CHAPTER XXXI. - - THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THE - MOLECULAR THEORY. - - Present State of the Question.—Heat necessary to the Motion of - a Glacier.—Ice does not shear in the Solid State.—Motion - of a Glacier _molecular_.—How Heat is transmitted through - Ice.—Momentary Loss of Shearing Force.—The _Rationale_ - of Regelation.—The Origin of “Crevasses.”—Effects of - Tension.—Modification of Theory.—Fluid Molecules crystallize - in Interstices.—Expansive Force of crystallizing Molecules - a Cause of Motion.—Internal molecular Pressure the chief - Moving Power.—How Ice can excavate a Rock Basin.—How Ice can - ascend a Slope.—How deep River Valleys are striated across.—A - remarkable Example in the Valley of the Tay.—How Boulders can - be carried from a lower to a higher Level. - - -The condition which the perplexing question of the cause of the descent -of glaciers has now reached seems to be something like the following. -The ice of a glacier is not in a soft and plastic state, but is solid, -hard, brittle, and unyielding. It nevertheless behaves in some respects -in a manner very like what a soft and plastic substance would do if -placed in similar circumstances, inasmuch as it accommodates itself -to all the inequalities of the channel in which it moves. The ice of -the glacier, though hard and solid, moves with a differential motion; -the particles of the ice are displaced over each other, or, in other -words, the ice shears as it descends. It had been concluded that the -mere weight of the glacier is sufficient to shear the ice. Canon -Moseley has investigated this point, and shown that it is not. He has -found that for a glacier to shear in the way that it is supposed to -do, it would require a force some thirty or forty times as great as -the weight of the glacier. Consequently, for the glacier to descend, -a force in addition to that of gravitation is required. What, then, -is this force? It is found that the rate at which the glacier descends -depends upon the amount of heat which it is receiving. This shows that -the motion of the glacier is in some way or other dependent upon heat. -Is heat, then, the force we are in search of? The answer to this, of -course, is, since heat is a force necessarily required, we have no -right to assume any other till we see whether or not heat will suffice. -In what way, then, does heat aid gravitation in the descent of the -glacier? In what way does heat assist gravitation in the shearing of -the ice? There are two ways whereby we may conceive the thing to be -done: the heat may assist gravitation to shear, by pressing the ice -forward, or it may assist gravitation by diminishing the cohesion of -the particles, and thus allow gravitation to produce motion which it -otherwise could not produce. Every attempt which has yet been made -to explain how heat can act as a force in pushing the ice forward, -has failed. The fact that heat cannot expand the ice of the glacier -may be regarded as a sufficient proof that it does not act as a force -impelling the glacier forward; and we are thus obliged to turn our -attention to the other conception, viz., that heat assists gravitation -to shear the ice, not by direct pressure, but by diminishing the -cohesive force of the particles, so as to enable gravitation to push -the one past the other. But how is this done? Does heat diminish the -cohesion by acting as an expansive force in separating the particles? -Heat cannot do this, because it cannot expand the ice of a glacier; -and besides, were it to do this, it would destroy the solid and firm -character of the ice, and the ice of the glacier would not then, as -a mass, possess the great amount of shearing-force which observation -and experiment show that it does. In short it is because the particles -are so firmly fixed together at the time the glacier is descending, -that we are obliged to call in the aid of some other force in addition -to the weight of the glacier to shear the ice. Heat does not cause -displacement of the particles by making the ice soft and plastic; for -we know that the ice of the glacier is not soft and plastic, but -hard and brittle. The shearing-force of the ice of the moving glacier -is found to be by at least from thirty to forty times too great to -permit of the ice being sheared by the mere force of gravitation; -how, then, is it that gravitation, without the direct assistance of -any other force, can manage to shear the ice? Or to put the question -under another form: heat does not reduce the shearing-force of the ice -of a glacier to something like 1·3193 lb. per square inch of surface, -the unit required by Mr. Moseley to enable a glacier to shear by -its weight; the shearing-force of the ice, notwithstanding all the -heat received, still remains at about 75 lbs.; how, then, can the -glacier shear without any other force than its own weight pushing it -forward? _This is the fundamental question; and the true answer to it -must reveal the mystery of glacier-motion._ We are compelled in the -present state of the problem to admit that glaciers do descend with -a differential motion without any other force than their own weight -pushing them forward; and yet the shearing-force of the ice is actually -found to be thirty or forty times the maximum that would permit of the -glacier shearing by its weight only. _The explanation of this apparent -paradox will remove all our difficulties in reference to the cause of -the descent of glaciers._ - -There seems to be but one explanation (and it is a very obvious -one), viz. that the motion of the glacier is _molecular_. The ice -descends molecule by molecule. The ice of a glacier is in the hard -crystalline state, but it does not descend in this state. Gravitation -is a constantly acting force; if a particle of the ice lose its -shearing-force, though but for the moment, it will descend by its -weight alone. But a particle of the ice will lose its shearing-force -for a moment if the particle loses its crystalline state for the -moment. The passage of heat through ice, whether by conduction or by -radiation, in all probability is a molecular process; that is, the -form of energy termed heat is transmitted from molecule to molecule -of the ice. A particle takes the energy from its neighbour A on the -one side and hands it over to its neighbour B on the opposite side. -But the particle must be in a different state at the moment it is in -possession of the energy from what it was before it received it from -A, and from what it will be after it has handed it over to B. Before -it became possessed of the energy, it was in the crystalline state—it -was ice; and after it loses possession of the energy it will be ice; -but at the moment that it is in possession of the passing energy is -it in the crystalline or icy state? If we assume that it is not, but -that in becoming possessed of the energy, it loses its crystalline form -and for the moment becomes water, all our difficulties regarding the -cause of the motion of glaciers are removed. We know that the ice of a -glacier in the mass cannot become possessed of energy in the form of -heat without becoming fluid; _if it can be shown that the same thing -holds true of the ice particle, we have the key to the mystery of -glacier-motion_. A moment’s reflection will suffice to convince any one -that if the glacier ice in the mass cannot receive energy in the form -of heat without melting, the same must hold true of the ice particles, -for it is inconceivable that the ice in the mass could melt and yet -the ice particles themselves remain in the solid state. It is the -solidity of the particles which constitutes the solidity of the mass. -If the particles lose their solid form the mass loses its solid form, -for the mass has no other solidity than that which is possessed by the -particles. - -The correctness of the conclusion, that the weight of the ice is -not a sufficient cause, depends upon the truth of a certain element -taken for granted in the reasoning, viz. that the _shearing-force_ of -the molecules of the ice remains _constant_. If this force remains -constant, then Canon Moseley’s conclusion is undoubtedly correct, -but not otherwise; for if a molecule should lose its shearing-force, -though it were but for a moment, if no obstacle stood in front of the -molecule, it would descend in virtue of its weight. - -The fact that the shearing-force of a mass of ice is found to be -constant does not prove that the same is the case in regard to the -individual molecules. If we take a mass of molecules in the aggregate, -the shearing-force of the mass taken thus collectively may remain -absolutely constant, while at the same time each individual molecule -may be suffering repeated momentary losses of shearing-force. This is -so obvious as to require no further elucidation. The whole matter, -therefore, resolves itself into this one question, as to whether or not -the shearing-force of a crystalline molecule of ice remains constant. -In the case of ordinary solid bodies we have no reason to conclude that -the shearing-force of the molecules ever disappears, but in regard to -ice it is very different. - -If we analyze the process by which heat is conducted through ice, we -shall find that we have reason to believe _that while a molecule of -ice is in the act of transmitting the energy received (say from a -fire), it loses for the moment its shearing-force if the temperature of -the ice be not under_ 32° F. If we apply heat to the end of a bar of -iron, the molecules at the surface of the end have their temperatures -raised. Molecule A at the surface, whose temperature has been raised, -instantly commences to transfer to B a portion of the energy received. -The tendency of this process is to lower the temperature of A and raise -that of B. B then, with its temperature raised, begins to transfer -the energy to C. The result here is the same; B tends to fall in -temperature, and C to rise. This process goes on from molecule to -molecule until the opposite end of the bar is reached. Here in this -case the energy or heat applied to the end of the bar is transmitted -from molecule to molecule under the form of _heat or temperature_. -The energy applied to the bar does _not change its character; it -passes right along from molecule to molecule under the form of heat or -temperature_. But the nature of the process must be wholly different if -the transferrence takes place through a bar of ice at the temperature -of 32°. Suppose we apply the heat of the fire to the end of the bar -of ice at 32°, the molecules of the ice cannot possibly have their -temperatures raised in the least degree. How, then, can molecule A -take on, _under the form of heat_, the energy received from the fire -without being heated or having its _temperature_ raised? The thing is -impossible. The energy of the fire must appear in A under a different -form from that of heat. The same process of reasoning is equally -applicable to B. The molecule B cannot accept of the energy from A -under the form of heat; it must receive it under some other form. The -same must hold equally true of all the other molecules till we reach -the opposite end of the bar of ice. And yet, strange to say, the last -molecule transmits in the form of heat its energy to the objects -beyond; for we find that the heat applied to one side of a piece of ice -will affect the thermal pile on the opposite side. - -The question is susceptible of a clear and definite answer. When -heat is applied to a molecule of ice at 32°, the heat applied -does not raise the temperature of the molecule, it is consumed in -work against the cohesive forces binding the atoms or particles -together into the crystalline form. The energy then must exist in -the dissolved crystalline molecule, under the statical form of an -affinity—crystalline affinity, or whatever else we may call it. That is -to say, the energy then exists in the particles as a power or tendency -to rush together again into the crystalline form, and the moment they -are allowed to do so they give out the energy that was expended upon -them in their separation. This energy, when it is thus given out again, -assumes the dynamical form of heat; in other words, the molecule gives -out _heat_ in the act of freezing. The heat thus given out may be -employed to melt the next adjoining molecule. The ice-molecules take -on energy from a heated body by melting. That peculiar form of motion -or energy called heat disappears in forcing the particles of the -crystalline molecule separate, and for the time being exists in the -form of a tendency in the separated particles to come together again -into the crystalline form. - -But it must be observed that although the crystalline molecule, when -it is acting as a conductor, takes on energy under this form from the -heated body, it only exists in the molecule under such a form during -the moment of transmission; that is to say, the molecule is melted, but -only for the moment. When B accepts of the energy from A, the molecule -A instantly assumes the crystalline form. B is now melted; and when C -accepts of the energy from B, then B also in turn assumes the solid -state. This process goes on from molecule to molecule till the energy -is transmitted through to the opposite side and the ice is left in its -original solid state. This, as will be shown in the Appendix, is the -_rationale_ of Faraday’s property of regelation. - -This is no mere theory or hypothesis; it is a necessary consequence -from known facts. We know that ice at 32° cannot take on energy from -a heated body without melting; and we know also equally well that a -slab of ice at 32°, notwithstanding this, still, as a mass, retains its -solid state while the heat is being transmitted through it. This proves -that every molecule resumes its crystalline form the moment after the -energy is transferred to the adjoining molecule. - -This point being established, every difficulty regarding the descent -of the glacier entirely disappears; for a molecule the moment that -it assumes the fluid state is completely freed from shearing-force, -and can descend by virtue of its own weight without any impediment. -All that the molecule requires is simply room or space to advance in. -If the molecule were in absolute contact with the adjoining molecule -below, it would not descend unless it could push that molecule before -it, which it probably would not be able to do. But the molecule -actually has room in which to advance; for in passing from the solid -to the liquid state its volume is diminished by about 1/10, and it -consequently can descend. True, when it again assumes the solid form -it will regain its former volume; but the question is, will it go back -to its old position? If we examine the matter thoroughly we shall find -that it cannot. If there were only this one molecule affected by the -heat, this molecule would certainly not descend; but all the molecules -are similarly affected, although not all at the same moment of time. - -Let us observe what takes place, say, at the lower end of the glacier. -The molecule A at the lower end, say, of the surface, receives heat -from the sun’s rays; it melts, and in melting not only loses its -shearing-force and descends by its own weight, but it contracts also. -B immediately above it is now, so far as A is concerned, at liberty to -descend, and will do so the moment that it assumes the liquid state. A -by this time has become solid, and again fixed by shearing-force; but -it is not fixed in its old position, but a little below where it was -before. If B has not already passed into the fluid state in consequence -of heat derived from the sun, the additional supply which it will -receive from the solidifying of A will melt it. The moment that B -becomes fluid it will descend till it reaches A. B then is solidified -a little below its former position. The same process of reasoning is -in a similar manner applicable to every molecule of the glacier. Each -molecule of the glacier consequently descends step by step as it melts -and solidifies, and hence the glacier, considered as a mass, is in a -state of constant motion downwards. The fact observed by Professor -Tyndall that there are certain planes in the ice along which melting -takes place more readily than others will perhaps favour the descent of -the glacier. - -We have in this theory a satisfactory explanation of the origin of -“crevasses” in glaciers. Take, for example, the transverse crevasses -formed at the point where an increase in the inclination of the glacier -takes place. Suppose a change of inclination from, say, 4° to 8° in -the bed of the glacier. The molecules on the slope of 8° will descend -more rapidly than those above on the slope of 4°. A state of tension -will therefore be induced at the point where the change of inclination -occurs. The ice on the slope of 8° will tend to pull after it the mass -of the glacier moving more slowly on the slope above. The pull being -continued, the glacier will snap asunder the moment that the cohesion -of the ice is overcome. The greater the change of inclination is, the -more readily will the rupture of the ice take place. Every species of -crevasse can be explained upon the same principle.[309] - -This theory explains also why a glacier moves at a greater rate during -summer than during winter; for as the supply of heat to the glacier is -greater during the former season than during the latter, the molecules -will pass oftener into the liquid state. - -As regards the denuding power of glaciers, I may observe that, though -a glacier descends molecule by molecule, it will grind the rocky bed -over which it moves as effectually as it would do did it slide down in -a rigid mass in the way generally supposed; for the grinding-effect -is produced not by the ice of the glacier, but by the stones, sand, -and other materials forced along under it. But if all the resistances -opposing the descent of a glacier, internal and external, are overcome -by the mere weight of the ice alone, it can be proved that in the case -of one descending with a given velocity the amount of work performed -in forcing the grinding materials lying under the ice forward must be -as great, supposing the motion of the ice to be molecular in the way -I have explained, as it would be supposing the ice descended in the -manner generally supposed. - -Of course, a glacier could not descend by means of its weight as -rapidly in the latter case as in the former; for, in fact, as Canon -Moseley has shown, it would not in the latter case descend at all; but -assuming for the sake of argument the rate of descent in both cases to -be the same, the conclusion I have stated would follow. Consequently -whatever denuding effects may have been attributed to the glacier, -according to the ordinary theory, must be equally attributable to it -according to the present explanation. - -This theory, however, explains, what has always hitherto excited -astonishment, viz., why a glacier can descend a slope almost -horizontal, or why the ice can move off the face of a continent -perfectly level. - -This is the form in which my explanation was first stated about -half-a-dozen years ago.[310] There is, however another element -which must be taken into account. It is one which will help to cast -additional light on some obscure points connected with glacial -phenomena. - -Ice is evidently not absolutely solid throughout. It is composed of -crystalline particles, which, though in contact with one another, are, -however, not packed together so as to occupy the least possible space, -and, even though they were, the particles would not fit so closely -together as to exclude interstices. The crystalline particles are, -however, united to one another at special points determined by their -polarity, and on this account they require more space; and this in -all probability is the reason, as Professor Tyndall remarks, why ice, -volume for volume, is less dense than water. - -“They (the molecules) like the magnets,” says Professor Tyndall, “are -acted upon by two distinct forces; for a time, while the liquid is -being cooled, they approach each other, in obedience to their general -attraction for each other. But at a certain point new forces, some -attractive some repulsive, _emanating from special points_ of the -molecules, come into play. The attracted points close up, the repelled -points retreat. Thus the molecules turn and rearrange themselves, -demanding as they do so more space, and overcoming all ordinary -resistance by the energy of their demand. This, in general terms, is an -explanation of the expansion of water in solidifying.”[311] - -It will be obvious, then, that when a crystalline molecule melts, it -will not merely descend in the manner already described, but capillary -attraction will cause it to flow into the interstices between the -adjoining molecules. The moment that it parts with the heat received, -it will of course resolidify, as has been shown, but it will not -solidify so as to fit the cavity which it occupied when in the fluid -state. For the liquid molecule in solidifying assumes the crystalline -form, and of course there will be a definite proportion between the -length, breadth, and thickness of the crystal; consequently it will -always happen that the interstice in which it solidifies will be too -narrow to contain it. The result will be that the fluid molecule -in passing into the crystalline form will press the two adjoining -molecules aside in order to make sufficient room for itself between -them, and this it will do, no matter what amount of space it may -possess in all other directions. The crystal will not form to suit the -cavity, the cavity must be made to contain the crystal. And what holds -true of one molecule, holds true of every molecule which melts and -resolidifies. This process is therefore going on incessantly in every -part of the glacier, and in proportion to the amount of heat which the -glacier is receiving. This internal molecular pressure, resulting from -the solidifying of the fluid molecules in the interstices of the ice, -acts on the mass of the ice as an expansive force, tending to cause the -glacier to widen out laterally in all directions. - -Conceive a mass of ice lying on a flat horizontal surface, and -receiving heat on its upper surface, say from the sun; as the heat -passes downwards through the mass, the molecules, acting as conductors, -melt and resolidify. Each fluid molecule solidifies in an interstice, -which has to be widened in order to contain it. The pressure thus -exerted by the continual resolidifying of the molecules will cause the -mass to widen out laterally, and of course as the mass widens out it -will grow thinner and thinner if it does not receive fresh acquisition -on its surface. In the case of a glacier lying in a valley, motion, -however, will only take place in one direction. The sides of the -valley prevent the glacier from widening; and as gravitation opposes -the motion of the ice up, and favours its motion down the valley, the -path of least resistance to molecular pressure will always be down -the slope, and consequently in this direction molecular displacement -will take place. Molecular pressure will therefore produce motion in -the same direction as that of gravity. In other words, it will tend to -cause the glacier to descend the valley. - -The lateral expansion of the ice from internal molecular pressure -explains in a clear and satisfactory manner how rock-basins may be -excavated by means of land-ice. It also removes the difficulties -which have been felt in accounting for the ascent of ice up a steep -slope. The main difficulty besetting the theory of the excavation of -rock-basins by ice is to explain how the ice after entering the basin -manages to get out again—how the ice at the bottom is made to ascend -the sloping sides of the basin. Pressure acting from behind, it has -been argued by some; but if the basin be deep and its sides steep, this -will simply cause the ice lying above the level of the basin to move -forward over the surface of the mass filling it. This conclusion is, -however, incorrect. The ice filling the basin and the glacier overlying -it are united in one solid mass, so that the latter cannot move over -the former without shearing; and although the resistance to motion -offered by the sloping sides of the basin may be much greater than the -resistance to shear, still the ice will be slowly dragged out of the -basin. However, in order to obviate this objection to which I refer, -the advocates of the glacial origin of lake-basins point out that -the length of those basins in proportion to their depth is so great -that the slope up which the ice has to pass is in reality but small. -This no doubt is true of lake-basins in general, but it does not hold -universally true. But the theory does not demand that an ice-formed -lake-basin cannot have steep sides. We have incontestable evidence that -ice will pass up a steep slope; and, if ice can pass up a steep slope, -it can excavate a basin with a steep slope. That ice will pass up a -steep slope is proved by the fact that comparatively deep and narrow -river valleys are often found striated across, while hills which stood -directly in the path of the ice of the glacial epoch are sometimes -found striated _upwards_ from their base to their summit. Some striking -examples of striæ running up hill are given by Professor Geikie in -his “Glacial Drift of Scotland.” I have myself seen a slope striated -upwards so steep that one could not climb it. - -A very good example of a river valley striated across came under -my observation during the past summer. The Tay, between Cargill -and Stanley (in the centre of the broad plain of Strathmore), has -excavated, through the Old Red Sandstone, a channel between 200 and -300 feet in depth. The channel here runs at right angles to the path -taken during the glacial epoch by the great mass of ice coming from -the North-west Highlands. At a short distance below Cargill, the trap -rising out of the bed of the river is beautifully ice-grooved and -striated, at right angles to the stream. A trap-dyke, several miles in -length, crosses the river about a mile above Stanley, forming a rapid, -known as the Linn of Campsie. This dyke is _moutonnée_ and striated -from near the Linn up the sloping bank to the level of the surrounding -country, showing that the ice must have ascended a gradient of one in -seven to a height of 300 feet. - -From what has been already stated in reference to the resolidifying of -the molecules in the interstices of the ice, the application of the -molecular theory to the explanation of the effects under consideration -will no doubt be apparent. Take the case of the passage of the -ice-sheet across a river valley. As the upper surface of the ice-sheet -is constantly receiving heat from the sun and the air in contact with -it, there is consequently a transferrence of heat from above downwards -to the bottom of the sheet. This transferrence of heat from molecule -to molecule is accompanied by the melting and resolidifying of the -successive molecules in the manner already detailed. As the fluid -molecules tend to flow into adjoining interstices before solidifying -and assuming the crystalline form, the interstices of the ice at the -bottom of the valley are constantly being filled by fluid molecules -from above. These molecules no sooner enter the interstices than they -pass into the crystalline form, and become, of course, separated from -their neighbours by fresh interstices, which new interstices become -filled by fluid molecules, which, in turn, crystallize, forming fresh -interstices, and so on. The ice at the bottom of the valley, so long as -this process continues, is constantly receiving fresh additions from -above. The ice must therefore expand laterally to make room for these -additions, which it must do unless the resistance to lateral expansion -be greater than the force exerted by the molecules in crystallizing. -But a resistance sufficient to do this must be enormous. The ice at the -bottom of the valley cannot expand laterally without passing up the -sloping sides. In expanding it will take the path of least resistance, -but the path of least resistance will always be on the side of the -valley towards which the general mass of the ice above is flowing. - -It has been shown (Chapter XXVII.) that the ice passing over Strathmore -must have been over 2,000 feet in thickness. An ice-sheet 2,000 feet -in thickness exerts on its bed a pressure of upwards of 51 tons per -square foot. When we reflect that ice under so enormous a pressure, -with grinding materials lying underneath, was forced by irresistible -molecular energy up an incline of one in seven, it is not at all -surprising that the hard trap should be ground down and striated. - -We can also understand how the softer portions of the rocky surface -over which the ice moved should have been excavated into hollow basins. -We have also an explanation of the transport of boulders from a lower -to a higher level, for if ice can move from a lower to a higher level, -it of course can carry boulders along with it. - -The bearing which the foregoing considerations of the manner in which -heat is transmitted through ice have on the question of the cause of -regelation will be considered in the Appendix. - - - - - APPENDIX. - - - - - I. - - OPINIONS EXPRESSED PREVIOUS TO 1864 REGARDING - THE INFLUENCE OF THE ECCENTRICITY OF THE EARTH’S - ORBIT ON CLIMATE.[312] - - - M. DE MAIRAN. - -M. de Mairan, in an article in the _Memoirs of the Royal Academy of -France_[313] “On the General Cause of Heat in Summer and Cold in -Winter, in so far as depends on the internal and permanent Heat of the -Earth,” makes the following remarks on the influence of the difference -of distance of the sun in apogee and perigee:— - -“Cet élément est constant pour les deux solstices; tandis que les -autres (height of the sun and obliquity of his rays) y varient à raison -des latitudes locales; et il y a encore cela de particulier, qu’il -tend à diminuer la valeur de notre été, et à augmenter celle de notre -hiver dans l’hémisphère boréal où nous sommes, et tout au contraire -dans l’austral. Remarquons cependant que de ces mêmes distances, qui -constituent ce troisième élément, naît en partie un autre principe -de chaleur tout opposé, et qui semble devoir tempérer les effets du -précédent; sçavoir, la lenteur et la vitesse réciproques du mouvement -annuel apparent, en vertu duquel et du réel qui s’y mêle, le soleil -emploie 8 jours de plus à parcourir les signes septentrionaux. -C’est-à-dire, que le soleil passe 186½ jours dans notre hémisphère, et -seulement 178½ dans l’hémisphère opposé. Ce qui, en général, ne peut -manquer de répandre un pen plus de chaleur sur l’été du premier, et un -peu moins sur son hiver.” - - - MR. RICHARD KIRWAN. - -“Œpinus,[314] reasoning on astronomical principles, attributes the -inferior temperature of the southern hemisphere to the shorter abode of -the sun in the southern tropic, shorter by seven days, which produces -a difference of fourteen days in favour of the northern hemisphere, -during which more heat is accumulated, and hence he infers that the -temperature of the northern hemisphere is to that of the southern, as -189·5 to 175·5, or as 14 to 13.”—_Trans. of the Royal Irish Academy_, -vol. viii., p. 417. 1802. - - - SIR CHARLES LYELL. - -“Before the amount of difference between the temperature of the two -hemispheres was ascertained, it was referred by astronomers to the -acceleration of the earth’s motion in its perihelion; in consequence of -which the spring and summer of the southern hemisphere are shorter by -nearly eight days than those seasons north of the equator. A sensible -effect is probably produced by this source of disturbance, but it is -quite inadequate to explain the whole phenomena. It is, however, of -importance to the geologist to bear in mind that in consequence of the -precession of the equinoxes, the two hemispheres receive alternately, -each for a period of upwards of 10,000 years, a greater share of -solar light and heat. This cause may sometimes tend to counterbalance -inequalities resulting from other circumstances of a far more -influential nature; but, on the other hand, it must sometimes tend to -increase the extreme of deviation, which certain combinations of causes -produce at distant epochs.”—_Principles_, First Edition, 1830, p. 110, -vol. i. - - - SIR JOHN F. HERSCHEL, BART. - -The following, in so far as it relates to the effects of eccentricity, -is a copy of Sir John Herschel’s memoir, “On the Astronomical Causes -which may influence Geological Phenomena,” read before the Geological -Society, Dec. 15th, 1830.—_Trans. Geol. Soc._, vol. iii., p. 293, -Second Series:— - -“... Let us next consider the changes arising in the orbit of the earth -itself about the sun, from the disturbing action of the planets. In so -doing it will be obviously unnecessary to consider the effect produced -on the solar tides, to which the above reasoning applies much more -forcibly than in the case of the lunar. It is, therefore, only the -variations in the supply of light and heat received from the sun that -we have now to consider. - -“Geometers having demonstrated the absolute invariability of the _mean_ -distance of the earth from the sun, it would seem to follow that -the mean annual supply of light and heat derived from that luminary -would be alike invariable; but a closer consideration of the subject -will show that this would not be a legitimate conclusion, but that, -on the contrary, the _mean_ amount of solar radiation is dependent -on the eccentricity of the orbit, and therefore liable to variation. -Without going at present into any geometrical investigations, it will -be sufficient for the purpose here to state it as a theorem, of which -any one may easily satisfy himself by no very abstruse geometrical -reasoning, that ‘_the eccentricity of the orbit varying, the_ total -_quantity of heat received by the earth from the sun in one revolution -is inversely proportional to the_ minor _axis of the orbit_.’ Now since -the major axis is, as above observed, invariable, and therefore, of -course, the absolute length of the year, it will follow that the _mean -annual_ average of heat will also be in the same inverse ratio of the -_minor_ axis; and thus we see that the very circumstance which on a -cursory view we should have regarded as demonstrative of the constancy -of our supply of solar heat, forms an essential link in the chain of -strict reasoning by which its variability is proved. - -“The eccentricity of the earth’s orbits is actually diminishing, and -has been so for ages, beyond the records of history. In consequence, -the ellipse is in a state of approach to a circle, and its minor -axis being, therefore, on the increase, the annual average of solar -radiation is actually on the _decrease_. - -“So far this is in accordance with the testimony of geological -evidence, which indicates a general refrigeration of climate; but when -we come to consider the amount of diminution which the eccentricity -must be supposed to have undergone to render an account of the -variation which has taken place, we have to consider that, in the first -place, a great diminution of the eccentricity is required to produce -any sensible increase of the minor axis. This is a purely geometrical -conclusion, and is best shown by the following table:— - - Eccentricity. Minor Axis. Reciprocal or Ratio of - Heat received. - 0·00 1·000 1·000 - 0·05 0·999 1·002 - 0·10 0·995 1·005 - 0·15 0·989 1·011 - 0·20 0·980 1·021 - 0·25 0·968 1·032 - 0·30 0·954 1·048 - -By this it appears that a variation of the eccentricity of the orbit -from the circular form to that of an ellipse, having an eccentricity -of one-fourth of the major axis, would produce only a variation of 3 -per cent. on the _mean_ annual amount of solar radiation, and this -variation takes in the whole range of the planetary eccentricities, -from that of Pallas and Juno downwards. - -“I am not aware that the limit of increase of the eccentricity of the -earth’s orbit has ever been determined. That it has a limit has been -satisfactorily proved; but the celebrated theorem of Laplace, which -is usually cited as demonstrating that none of the planetary orbits -can ever deviate materially from the circular form, leads to no such -conclusion, except in the case of the great preponderant planets -Jupiter and Saturn, while for anything that theorem proves to the -contrary, the orbit of the earth may become elliptic to any amount. - -“In the absence of calculations which though practicable have, I -believe, never been made,[315] and would be no slight undertaking, we -may assume that eccentricities which exist in the orbits of planets, -both interior and exterior to that of the earth, may _possibly_ -have been attained, and may be attained again by that of the earth -itself. It is clear that such eccentricities _existing_ they cannot -be incompatible with the stability of the system generally, and that, -therefore, the question of the possibility of such an amount in the -particular case of the earth’s orbit will depend on the particular -data belonging to that case, and can only be determined by executing -the calculations alluded to, having regard to the simultaneous effects -of at least the four most influential planets, Venus, Mars, Jupiter, -and Saturn, _not only on the orbit of the earth, but on those of each -other_. The principles of this calculation are detailed in the article -of Laplace’s work cited. But before entering on a work of so much -labour, it is quite necessary to inquire what prospect of advantage -there is to induce any one to undertake it. - -“Now it certainly at first sight seems clear that a variation of 3 -per cent. only in the mean annual amount of solar radiation, and -that arising from an extreme supposition, does _not_ hold out such a -prospect. Yet it might be argued that the effects of the sun’s heat is -to maintain the temperature of the earth’s surface at its actual mean -height, not above the zero of Fahrenheit’s or any other thermometer, -but above the temperature of the celestial spaces, out of the reach of -the sun’s influence, and what that temperature is may be a matter of -much discussion. M. Fourier has considered it as demonstrated that it -is not greatly inferior to that of the polar regions of our own globe, -but the grounds of this decision appear to me open to considerable -objection.[316] If those regions be really void of matter, their -temperature can only arise, according to M. Fourier’s own view of -the subject, from the radiation of the stars. It ought, therefore, -to be as much inferior to that due to solar radiation, as the light -of a starlight night is to that of the brightest noon day, in other -words it should be very nearly a total privation of heat—almost the -_absolute zero_ respecting which so much difference of opinion exists, -some placing it at 1,000°, some at 5,000° of Fahrenheit below the -freezing-point, and some still lower, in which case a single unit per -cent. in the mean annual amount of radiation would suffice to produce a -change of climate fully commensurate to the demands of geologists.[317] - -“Without attempting, however, to enter further into the perplexing -difficulties in which this point is involved, which are far greater -than appear on a cursory view, let us next consider, not the _mean_, -but the _extreme_ effects which a variation in the eccentricity of -the earth’s orbit may be expected to produce in the summer and winter -climates in particular regions of its surface, and under the influence -of circumstances favouring a difference of effect. And here, if I -mistake not, it will appear that an amount of variation, which we need -not hesitate to admit (at least, provisionally) as a possible one, may -be productive of considerable diversity of climate, and may operate -during great periods of time either to mitigate or to exaggerate -the difference of winter and summer temperatures, so as to produce -alternately, in the same latitude of either hemisphere, a perpetual -spring, or the extreme vicissitudes of a burning summer and a rigorous -winter. - -“To show this, let us at once take the extreme case of an orbit as -eccentric as that of Juno or Pallas, in which the greatest and least -distances of the sun are to each other as 5 to 3, and consequently the -radiations at those distances as 25 to 9, or very nearly as 3 to 1. To -conceive what would be the _extreme_ effects of this great variation -of the heat received at different periods of the year, let us first -imagine in our latitude the place of the perigee of the sun to coincide -with the summer solstice. In that case, the difference between the -summer and winter temperature would be exaggerated in the same degree -as if three suns were placed side by side in the heavens in the former -season and only one in the latter, which would produce a climate -perfectly intolerable. On the other hand, were the perigee situated -in the winter solstice our three suns would combine to warm us in the -winter, and would afford such an excess of winter radiation as would -probably more than counteract the effect of short days and oblique -sunshine, and throw the summer season into the winter months. - -“The actual diminution of the eccentricity is so slow, that the -transition from a state of the orbit such as we have assumed to the -present nearly circular figure would occupy upwards of 600,000 years, -supposing it uniformly changeable—this, of course, would not be the -case; when near the maximum, however, it would vary slower still, so -that at that point it is evident a period of 10,000 years would elapse -without any perceptible change in the state of the data of the case we -are considering. - -“Now this adopting the very ingenious idea of Mr. Lyell[318] would -suffice, by reason of the combined effect of the precession of the -equinoxes and the motion of the apsides of the orbit itself, to -transfer the perigee from the summer to the winter solstice, and thus -to produce a transition from the one to the other species of climate in -a period sufficiently great to give room for a material change in the -botanical character of country. - -“The supposition above made is an extreme, but it is not demonstrated -to be an impossible one, and should even an approach to such a state -of things be possible, the same consequences, in a mitigated degree, -would follow. But if, on executing the calculations, it should appear -that the limits of the eccentricity of the earth’s orbit are really -narrow, and if, on a full discussion of the very difficult and delicate -point of the actual effect of solar radiation, it should appear that -the mean, as well as the extreme, temperature of our climates would -_not_ be materially affected,—it will be at least satisfactory to -_know_ that the causes of the phenomena in question are to be sought -elsewhere than in the relations of our planet to the system to which -it belongs, since there does not appear to exist any other conceivable -connections between these relations and the facts of geology than -those we have enumerated, the obliquity of the ecliptic being, as we -know, confined within too narrow limits for its variation to have any -sensible influence.”—_J. F. W. Herschel._ - -The influence which this paper might have had on the question as -to whether eccentricity may be regarded as a cause of changes in -geological climate appears to have been completely neutralized by the -following, which appeared shortly afterwards both in his “Treatise” and -“Outlines of Astronomy,” showing evidently that he had changed his mind -on the subject. - -“It appears, therefore, from what has been shown, the supplies of heat -received from the sun will be equal in the two segments, in whatever -direction the line PTQ be drawn. They will, indeed, be described in -unequal times: that in which the perihelion A lies in a shorter, and -the other in a longer, in proportion to their unequal area; but the -greater proximity of the sun in the smaller segment compensates exactly -for its more rapid description, and thus an equilibrium of heat is, as -it were, maintained. - -“Were it not for this the eccentricity of the orbit would materially -influence the transition of seasons. The fluctuation of distance -amounts to nearly 1/30th of the mean quantity, and, consequently, -the fluctuation of the sun’s direct heating power to double this, or -1/15th of the whole.... Were it not for the compensation we have just -described, the effect would be to exaggerate the difference of summer -and winter in the southern hemisphere, and to moderate it in the -northern; thus producing a more violent alternation of climate in the -one hemisphere and an approach to perpetual spring in the other. _As it -is, however, no such inequality subsists_, but an equal and impartial -distribution of heat and light is accorded to both.”—“_Treatise of -Astronomy_,” _Cabinet Cyclopædia_, § 315; _Outlines of Astronomy_, § -368. - -“The fact of a great change in the general climate of large tracts -of the globe, if not of the whole earth, and of a diminution of -general temperature, having been recognised by geologists, from -their examination of the remains of animals and vegetables of former -ages enclosed in the strata, various causes for such diminution of -temperature have been assigned.... It is evident that the _mean_ -temperature of the whole surface of the globe, in so far as it is -maintained by the action of the sun at a higher degree than it would -have were the sun extinguished, must depend on the mean quantity of -the sun’s rays which it receives, or, which comes to the same thing, -on the _total_ quantity received in a given invariable time; and the -length of the year being unchangeable in all the fluctuations of the -planetary system, it follows that the total _annual_ amount of solar -radiation will determine, _cæteris paribus_, the general climate -of the earth. Now, it is not difficult to show that this amount is -inversely proportional to the minor axis of the ellipse described -by the earth about the sun, regarded as slowly variable; and that, -therefore, the major axis remaining, as we know it to be, constant, -and the orbit being actually in a state of approach to a circle, -and consequently the minor axis being on the _increase_, the mean -annual amount of solar radiation received by the whole earth must -be actually on the _decrease_. We have here, therefore, an evident -real cause of sufficient universality, and acting _in the right -direction_, to account for the phenomenon. Its adequacy is another -consideration.”[319]—_Discourse on the Study of Natural Philosophy_, -pp. 145−147 (1830). - - - SIR CHARLES LYELL, BART. - -“_Astronomical Causes of Fluctuations in Climate._—Sir John Herschel -has lately inquired, whether there are any astronomical causes which -may offer a possible explanation of the difference between the actual -climate of the earth’s surface, and those which formerly appear to -have prevailed. He has entered upon this subject, he says, ‘impressed -with the magnificence of that view of geological revolutions, which -regards them rather as regular and necessary effects of great and -general causes, than as resulting from a series of convulsions -and catastrophes, regulated by no laws, and reducible to no fixed -principles.’ Geometers, he adds, have demonstrated the absolute -invariability of the mean distance of the earth from the sun; whence -it would seem to follow that the mean annual supply of light and heat -derived from that luminary would be alike invariable; but a closer -consideration of the subject will show that this would not be a -legitimate conclusion, but that, on the contrary, the _mean_ amount of -solar radiation is dependent on the eccentricity of the earth’s orbit, -and, therefore, liable to variation. - -“Now, the eccentricity of the orbit, he continues, is actually -diminishing, and has been so for ages beyond the records of history. -In consequence, the ellipse is in a state of approach to a circle, and -the annual average of solar heat radiated to the earth is actually on -the _decrease_. So far, this is in accordance with geological evidence, -which indicates a general refrigeration of climate; but the question -remains, whether the amount of diminution which the eccentricity may -have ever undergone can be supposed sufficient to account for any -sensible refrigeration.[320] The calculations necessary to determine -this point, though practicable, have never yet been made, and would be -extremely laborious; for they must embrace all the perturbations which -the most influential planets, Venus, Mars, Jupiter, and Saturn, would -cause in the earth’s orbit and in each other’s movements round the sun. - -“The problem is also very complicated, inasmuch as it depends not -merely on the ellipticity of the earth’s orbit, but on the assumed -temperature of the celestial spaces beyond the earth’s atmosphere; -a matter still open to discussion, and on which M. Fourier and Sir -J. Herschel have arrived at very different opinions. But if, says -Herschel, we suppose an extreme case, as if the earth’s orbit should -ever become as eccentric as that of the planet Juno or Pallas, a great -change of climate might be conceived to result, the winter and summer -temperatures being sometimes mitigated and at others exaggerated, in -the same latitudes. - -“It is much to be desired that the calculations alluded to were -executed, as even if they should demonstrate, as M. Arago thinks highly -probable, that the mean of solar radiation can never be materially -affected by irregularities in the earth’s motion, it would still be -satisfactory to ascertain the point.”—_Principles of Geology_, Ninth -Edition, 1853, p. 127. - - - M. ARAGO. - -“_Can the variations which certain astronomical elements undergo -sensibly modify terrestrial climates?_ - -“The sun is not always equally distant from the earth. At this time -its least distance is observed in the first days of January, and the -greatest, six months after, or in the first days of July. But, on the -other hand, a time will come when the _minimum_ will occur in July, -and the _maximum_ in January. Here, then, this interesting question -presents itself,—Should a summer such as those we now have, in which -the _maximum_ corresponds to the solar distance, differ sensibly, from -a summer with which the _minimum_ of this distance should coincide? - -“At first sight every one probably would answer in the affirmative; -for, between the _maximum_ and the _minimum_ of the sun’s distance -from the earth there is a remarkable difference, a difference in round -numbers of a thirtieth of the whole. Let, however, the consideration of -the velocities be introduced into the problem, elements which cannot -fairly be neglected, and the result will be on the side opposite to -that we originally imagined. - -“The part of the orbit where the sun is found nearest the earth, is, at -the same time, the point where the luminary moves most rapidly along. -The demi-orbit, or, in other words, the 180° comprehended betwixt the -two equinoxes of spring-time and autumn, will then be traversed in the -least possible time, when, in moving from the one of the extremities -of this arc to the other, the sun shall pass, near the middle of -this course of six months, at the point of the smallest distance. To -resume—the hypothesis we have just adopted would give, on account of -the lesser distance, a spring-time and summer hotter than they are in -our days; but on account of the greater rapidity, the sum of the two -seasons would be shorter by about seven days. Thus, then, all things -considered, the compensation is mathematically exact. After this it is -superfluous to add, that the point of the sun’s orbit corresponding to -the earth’s least distance changes very gradually; and that since the -most distant periods, the luminary has always passed by this point, -either at the end of autumn or beginning of winter. - -“We have thus seen that the changes which take place in the _position_ -of the solar orbit, _have no power in modifying the climate of our -globe_. We may now inquire, if it be the same concerning the variations -which this orbit experiences in its _form_.... - -“Herschel, who has recently been occupying himself with this problem, -in the hope of discovering the explanation of several geological -phenomena, allows that the succession of ages might bring the -eccentricity of the terrestrial orbit to the proportion of that of -the planet Pallas, that is to say, to be the 25/100 of a semi-greater -axis. It is exceedingly improbable that in these periodical changes -the eccentricity of our orbit should ever experience such enormous -variations, and even then these twenty-five hundredth parts (25/100), -would not augment the _mean_ annual solar radiation except by about one -hundredth part (1/100). To repeat, an eccentricity of 25/100 _would not -alter in any appreciated manner the mean thermometrical state of the -globe_.... - -“The changes of the form, and of the position, of the terrestrial -orbit are mathematically inoperative, or, at most, their influence is -so minute that it is not indicated by the most delicate instruments. -For the explanation of the changes of climates, then, there only -remains to us either the local circumstances, or some alteration in -the heating or illuminating power of the sun. But of these two causes, -we may continue to reject the last. And thus, in fact, all the changes -would come to be attributed to agricultural operations, to the clearing -of plains and mountains from wood, the draining of morasses, &c. - -“Thus, at one swoop, to confine, the whole earth, the variations -of climates, past and future, within the limits of the naturally -very narrow influence which the labour of man can effect, would be -a meteorological result of the very last importance.”—pp. 221−224, -_Memoir on the “Thermometrical State of the Terrestrial Globe,” in the -Edinburgh New Philosophical Journal_, vol. xvi., 1834. - - - BARON HUMBOLDT. - -“The question,” he says, “has been raised as to whether the increasing -value of this ellipticity is capable during thousands of years of -modifying to any considerable extent the temperature of the earth, -in reference to the daily and annual quantity and distribution of -heat? Whether a partial solution of the great geological problem of -the imbedding of tropical vegetable and animal remains in the now -cold zones may not be found in these _astronomical_ causes proceeding -regularly in accordance with eternal laws?... It might at the first -glance be supposed that the occurrence of the perihelion at an opposite -time of the year (instead of the winter, as, is now the case, in -summer) must necessarily produce great climatic variations; but, on the -above supposition, the sun will no longer remain seven days longer in -the northern hemisphere; no longer, as is now the case, traverse that -part of the ecliptic from the autumnal equinox to the vernal equinox, -in a space of time which is one week shorter than that in which it -traverses the other half of its orbit from the vernal to the autumnal -equinox. - -“The difference of temperature which is considered as the consequence -to be apprehended from the turning of the major axis, _will on the -whole disappear_, principally from the circumstance that the point of -our planet’s orbit in which it is nearest to the sun is at the same -time always that over which it passes with the greatest velocity.... - -“As the altered position of the major axis is capable of exerting -only a very _slight influence upon the temperature of the earth;_ so -likewise the _limit_ of the probable changes in the elliptical form of -the earth’s orbit are, according to Arago and Poisson, so narrow that -these changes could _only very slightly_ modify the climates of the -individual zones, and that in very long periods.”[321]—_Cosmos_, vol. -iv., pp. 458, 459. Bohn’s Edition. 1852. - - - SIR HENRY T. DE LA BECHE. - -“Mr. Herschel, viewing this subject with the eye of an astronomer, -considers that a diminution of the surface-temperature might arise from -a change in ellipticity of the earth’s orbit, which, though slowly, -gradually becomes more circular. No calculations having yet been made -as to the probable amount of decreased temperature from this cause, -it can at present be only considered as a possible explanation of -those geological phenomena which point to considerable alterations in -climates.”—_Geological Manual_. Third Edition. 1833. p. 8. - - - PROFESSOR PHILLIPS. - -“_Temperature of the Globe._—_Influence of the Sun._—No proposition is -more certain than the fundamental dependence of the temperature of the -surface of the globe on the solar influence. - -“It is, therefore, very important for geologists to inquire whether -this be variable or constant; whether the amount of solar heat -communicated to the earth is and has always been the same in every -annual period, or what latitude the laws of planetary movements permit -in this respect. - -“Sir John Herschel has examined this question in a satisfactory manner, -in a paper read to the Geological Society of London. The total amount -of solar radiation which determines the general climate of the earth, -the year being of invariable length, is inversely proportional to -the minor axis of the ellipse described by the earth about the sun, -regarded as slowly variable; the major axis remaining constant and -the orbit being actually in a state of approach to a circle, and, -consequently, the minor axis being on the increase, it follows that -the mean annual amount of solar radiation received by the whole earth -must be actually on the decrease. The limits of the variation in the -eccentricity of the earth’s orbit are not known. It is, therefore, -impossible to say accurately what may have been in former periods of -time, the amount of solar radiation; it is, however, certain that -if the ellipticity has ever been so great as that of the orbit of -Mercury or Pallas, the temperature of the earth must have been sensibly -higher than it is at present. But the difference of a few degrees of -temperature thus occasioned, is of too small an order to be employed -in explaining the growth of tropical plants and corals in the polar -or temperate zones, and other great phenomena of Geology.”—_From A -Treatise on Geology_, p. 11, _forming the article under that head in -the seventh edition of the Encyclopædia Britannica_. 1837. - - - MR. ROBERT BAKEWELL. - -“A change in the form of the earth’s orbit, if considerable, might -change the temperature of the earth, by bringing it nearer to the -sun in one part of its course. The orbit of the earth is an ellipsis -approaching nearly to a circle; the distance from the centre of the -orbit to either focus of the ellipsis is called by astronomers ‘the -eccentricity of the orbit.’ This eccentricity has been for ages slowly -decreasing, or, in other words, the orbit of the earth has been -approaching nearer to the form of a perfect circle; after a long period -it will again increase, and the possible extent of the variation has -not been yet ascertained. From what is known respecting the orbits of -Jupiter and Saturn, it appears highly probable that the eccentricity of -the earth’s orbit is confined within limits that preclude the belief -of any great change in the mean annual temperature of the globe ever -having been occasioned by this cause.”—_Introduction to Geology_, p. -600. 1838. Fifth Edition. - - - MRS. SOMERVILLE. - -“Sir John Herschel has shown that the elliptical form of the earth’s -orbit has but a trifling share in producing the variation of -temperature corresponding to the difference of the seasons.”—_Physical -Geography_, vol. ii., p. 20. Third Edition. - - - MR. L. W. MEECH, A.M. - -“Let us, then, look back to that primeval epoch when the earth -was in aphelion at midsummer, and the eccentricity at its maximum -value—assigned by Leverrier near to ·0777. Without entering into -elaborate computation, it is easy to see that the extreme values -of diurnal intensity, in Section IV., would be altered as by the -multiplier ((1 ± _e_)/(1 ± _e′_))^2, that is 1 - 0·11 in summer, and 1 -+ 0·11 in winter. This would diminish the midsummer intensity by about -9°, and increase the midwinter intensity by 3° or 4°; the temperature -of spring and autumn being nearly unchanged. But this does not appear -to be of itself adequate to the geological effects in question. - -“It is not our purpose, here, to enter into the inquiry whether the -atmosphere was once more dense than now, whether the earth’s axis -had once a different inclination to the orbit, or the sun a greater -emissive power of heat and light. Neither shall we attempt to speculate -upon the primitive heat of the earth, nor of planetary space, nor of -the supposed connection of terrestrial heat and magnetism; nor inquire -how far the existence of coal-fields in this latitude, of fossils, -and other geological remains, have depended upon existing causes. The -preceding discussion seems to prove simply that, under the present -system of physical astronomy, the sun’s intensity could never have been -materially different from what is manifested upon the earth at the -present day. _The causes of notable geological changes must be other -than the relative position of the sun and earth, under their present -laws of motion._”—_“On the Relative Intensity of the Heat and Light of -the Sun.” Smithsonian Contributions to Knowledge_, vol. ix. - - - M. JEAN REYNAUD. - -“La révolution qui pourrait y causer les plus grands changements -thermométriques, celle qui porte l’orbite à s’élargir et à se rétrécir -alternativement et, par suite, la planète à passer, aux époques de -périhélie, plus ou moins près du soleil, embrasse une période de plus -de cent mille années terrestres et demeure comprise dans de si étroites -limites que les habitants doivent être à peine avertis que la chaleur -décroît, par cette raison, depuis une haute antiquité et décroîtra -encore pendant des siècles en variant en même temps dans sa répartition -selon les diverses époques de l’année.... Enfin, le tournoiement de -l’axe du globe s’empreint également d’une manière particulière sur -l’ètablissement des saisons qui, à tour de rôle, dans chacun des deux -hémisphères, deviennent graduellement, durant une période d’environ -vingt-cinq mille ans, de plus en plus uniformes, ou, à l’inverse, de -plus en plus dissemblables. C’est actuellement dans l’hémisphère boréal -que règne l’uniformité, et quoique les étés et les hivers y tendent, -dès à présent, à se trancher de plus en plus, il ne paraît pas douteux -que la modération des saisons n’y produise, pendant longtemps encore, -des effets appréciables. En résumé, de tous ces changements il n’en est -donc aucun ni qui suive un cours précipité, ni qui s’élève jamais à des -valeurs considérables; ils se règlent tous sur un mode de développement -presque insensible, et il s’ensuit que les années de la terre, malgré -leur complexité virtuelle, se distinguent par le constance de leurs -caractères non-seulement de ce qui peut avoir lieu, en vertu des mêmes -principes, dans les autres systèmes planétaires de l’univers, mais -même de ce qui s’observe dans plusieurs des mondes qui composent le -nôtre.”—_Philosophie Religieuse: Terre et Ciel._ - - - M. ADHÉMAR. - -Adhémar does not consider the effects which ought to result from a -change in the eccentricity of the earth’s orbit; he only concerns -himself with those which, in his opinion, arise from the present amount -of such eccentricity. He admits, of course, that both hemispheres -receive from the sun equal quantities of heat per annum; but, as -the southern hemisphere has a winter longer by 168 hours than the -corresponding season in the northern hemisphere, an accumulation of -heat necessarily takes place in the latter, and an accumulation of -cold in the former. Adhémar also measures the loss of heat sustained -by the southern hemisphere in a year by the number of hours by which -the southern exceeds the northern winter. “The south pole,” he says, -“loses in one year more heat than it receives, because the total -duration of its nights surpasses that of the days by 168 hours; and the -contrary takes place for the north pole. If, for example, we take for -unity the mean quantity of heat which the sun sends off in one hour, -the heat accumulated at the end of the year at the north pole will be -expressed by 168, while the heat lost by the south pole will be equal -to 168 times what the radiation lessens it by in one hour; so that at -the end of the year the difference in the heat of the two hemispheres -will be represented by 336 times what the earth receives from the sun -or loses in an hour by radiation,”[322] and at the end of 100 years the -difference will be 33,600 times, and at the end of 1,000 years 336,000 -times, or equal to what the earth receives from the sun in 38½ years, -and so on during the 10,000 years that the southern winter exceeds in -length the northern. This, in his opinion, is all that is required to -melt the ice off the arctic regions, and cover the antarctic regions -with an enormous ice-cap. He further supposes that in about 10,000 -years, when our northern winter will occur in aphelion and the southern -in perihelion, the climatic conditions of the two hemispheres will be -reversed; that is to say, the ice will melt at the south pole, and the -northern hemisphere will become enveloped in one continuous mass of -ice, leagues in thickness, extending down to temperate regions. - -This theory, as shown in Chapter V., is based upon a misconception -regarding the laws of radiant heat. The loss of heat sustained by the -southern hemisphere from radiation, resulting from the greater length -of the southern winter, is vastly over-estimated by M. Adhémar, and -could not possibly produce the effects which he supposes. But I need -not enter into this subject here, as the reader will find the whole -question discussed at length in the chapter above referred to. By far -the most important part of Adhemar’s theory, however, is his conception -of the submergence of the land by means of a polar ice-cap. He appears -to have been the first to put forth the idea that a mass of ice placed -on the globe, say, for example, at the south pole, will shift the -earth’s centre of gravity a little to the south of its former position, -and thus, as a physical consequence, cause the sea to sink at the -north pole and to rise at the south. According to Adhémar, as the one -hemisphere cools and the other grows warmer, the ice at the pole of the -former will increase in thickness and that at the pole of the latter -diminish. - -The sea, as a consequence, will sink on the warm hemisphere where the -ice is decreasing and rise on the cold hemisphere where the ice is -increasing. And, again, in 10,000 years, when the climatic conditions -of the two hemispheres are reversed, the sea will sink on the -hemisphere where it formerly rose, and rise on the hemisphere where it -formerly sank, and so on in like manner through indefinite ages. - -Adhémar, however, acknowledges to have derived the grand conception -of a submergence of the land from the shifting of the earth’s centre -of gravity from the following wild speculation of one Bertrand, of -Hamburgh:— - -“Bertrand de Hambourg, dans un ouvrage imprimé en 1799 et qui a -pour titre: _Renouvellement périodique des Continents_, avait déjà -émis cette idée, que la masse des eaux pouvait être alternativement -entraînée d’un hémisphère à l’autre par le déplacement du centre de -gravité du globe. Or, pour expliquer ce déplacement, il supposait que -la terre était creuse et qu’il y avait dans son intérieur un gros noyau -d’aimant auquel les comètes par leur attraction communiquaient un -mouvement de va-et-vient analogue à celui du pendule.”—_Révolutions de -la Mer_, p. 41. - -The somewhat extravagant notions which Adhémar has advanced in -connection with his theory of submergence have very much retarded -its acceptance. Amongst other remarkable views he supposes the polar -ice-cap to rest on the bottom of the ocean, and to rise out of the -water to the enormous height of twenty leagues. Again, he holds that -on the winter approaching perihelion and the hemisphere becoming warm -the ice waxes soft and rotten from the accumulated heat, and the sea -now beginning to eat into the base of the cap, this is so undermined -as, at last, to be left standing upon a kind of gigantic pedestal. This -disintegrating process goes on till the fatal moment at length arrives, -when the whole mass tumbles down into the sea in huge fragments which -become floating icebergs. The attraction of the opposite ice-cap, which -has by this time nearly reached its maximum thickness, becomes now -predominant. The earth’s centre of gravity suddenly crosses the plain -of the equator, dragging the ocean along with it, and carrying death -and destruction to everything on the surface of the globe. And these -catastrophes, he asserts, occur alternately on the two hemispheres -every 10,000 years.—_Révolutions de la Mer_, pp. 316−328. - -Adhémar’s theory has been advocated by M. Le Hon, of Brussels, in a -work entitled _Périodicité des Grands Déluges_. Bruxelles et Leipzig, -1858. - - - - - II. - - ON THE NATURE OF HEAT-VIBRATIONS.[323] - - From the _Philosophical Magazine_ for May, 1864. - - -In a most interesting paper on “Radiant Heat,” by Professor Tyndall, -read before the Royal Society in March last, it is shown conclusively -that the _period_ of heat-vibrations is not affected by the state -of aggregation of the molecules of the heated body; that is to say, -whether the substance be in the gaseous, the liquid, or, perhaps, the -solid condition, the tendency of its molecules to vibrate according to -a given period remains unchanged. The force of cohesion binding the -molecules together exercises no effect on the rapidity of vibration. - -I had arrived at the same conclusion from theoretical considerations -several years ago, and had also deduced some further conclusions -regarding the nature of heat-vibrations, which seem to be in a measure -confirmed by the experimental results of Professor Tyndall. One of -these conclusions was, that the heat-vibration does not consist in -a motion of an aggregate mass of molecules, but in a motion of the -individual molecules themselves. Each molecule, or rather we should -say each atom, acts as if there were no other in existence but -itself. Whether the atom stands by itself as in the gaseous state, -or is bound to other atoms as in the liquid or the solid state, it -behaves in exactly the same manner. The deeper question then suggested -itself, viz., what is the nature of that mysterious motion called heat -assumed by the atom? Does it consist in excursions across centres -of equilibrium external to the atom itself? It is the generally -received opinion among physicists that it does. But I think that the -experimental results arrived at by Professor Tyndall, as well as some -others which will presently be noticed, are entirely hostile to such an -opinion. The relation of an atom to its centre of equilibrium depends -entirely on the state of aggregation. Now if heat-vibrations consist in -excursions to and fro across these centres, then the _period_ ought to -be affected by the state of aggregation. The higher the _tension_ of -the atom in regard to the centre, the more rapid ought its movement to -be. This is the case in regard to the vibrations constituting sound. -The harder a body becomes, or, in other words, the more firmly its -molecules are bound together, the higher is the _pitch_. Two harp-cords -struck with equal force will vibrate with equal force, however much -they may differ in the rapidity of their vibrations. The _vis viva_ -of vibration depends upon the force of the stroke; but the rapidity -depends, not on the stroke, but upon the tension of the cord. - -That heat-vibrations do not consist in excursions of the molecules -or atoms across centres of equilibrium, follows also as a necessary -consequence from the fact that the real specific heat of a body remains -unchanged under all conditions. All changes in the specific heat of a -body are due to differences in the amount of heat consumed in molecular -work against cohesion or other forces binding the molecules together. -Or, in other words, to produce in a body no other effect than a given -rise of temperature, requires the same amount of force, whatever may be -the physical condition of the body. Whether the body be in the solid, -the fluid, or the gaseous condition, the same rise of temperature -always indicates the same quantity of force consumed in the simple -production of the rise. Now, if heat-vibrations consist in excursions -of the atom to and fro across a centre of equilibrium _external to -itself_, as is generally supposed, then the _real_ specific heat of a -solid body, for example, _ought to decrease with the hardness of the -body_, because an increase in the strength of the force binding the -molecules together would in such a case tend to favour the rise in the -rapidity of the vibrations. - -These conclusions not only afford us an insight into the hidden nature -of heat-vibrations, but they also appear to cast some light on the -physical constitution of the atom itself. They seem to lead to the -conclusion that the ultimate atom itself is _essentially elastic_.[324] -For if heat-vibrations do not consist in excursions of the atom, then -it must consist in alternate expansions and contractions of the atom -itself. This again is opposed to the ordinary idea that the atom is -essentially solid and impenetrable. But it favours the modern idea, -that matter consists of forces of resistance acting from a centre. - -Professor Tyndall in a memoir read before the Royal Society “On a new -Series of Chemical Reactions produced by Light,” has subsequently -arrived at a similar conclusion in reference to the atomic nature of -heat-vibrations. The following are his views on the subject:— - -“A question of extreme importance in molecular physics here -arises:—What is the real mechanism of this absorption, and where is its -seat? - -“I figure, as others do, a molecule as a group of atoms, held together -by their mutual forces, but still capable of motion among themselves. -The vapour of the nitrite of amyl is to be regarded as an assemblage -of such molecules. The question now before us is this:—In the act -of absorption, is it the _molecules_ that are effective, or is it -their constituent _atoms?_ Is the _vis viva_ of the intercepted waves -transferred to the molecule as a whole, or to its constituent parts? - -“The molecule, as a whole, can only vibrate in virtue of the forces -exerted between it and its neighbour molecules. The intensity of these -forces, and consequently the rate of vibration, would, in this case, -be a function of the distance between the molecules. Now the identical -absorption of the liquid and of the vaporous nitrite of amyl indicates -an identical vibrating period on the part of liquid and vapour, and -this, to my mind, amounts to an experimental demonstration that the -absorption occurs in the main _within_ the molecule. For it can hardly -be supposed, if the absorption were the act of the molecule as a whole, -that it could continue to affect waves of the same period after the -substance had passed from the vaporous to the liquid state.”—_Proc. of -Roy. Soc._, No. 105. 1868. - -Professor W. A. Norton, in his memoir on “Molecular Physics,”[325] has -also arrived at results somewhat similar in reference to the nature of -heat-vibrations. “It will be seen,” he says, “that these (Mr. Croll’s) -ideas are in accordance with the conception of the constitution of a -molecule adopted at the beginning of the present memoir (p. 193), and -with the theory of heat-vibrations or heat-pulses deduced therefrom (p. -196).”[326] - - - - - III. - - ON THE REASON WHY THE DIFFERENCE OF READING BETWEEN A - THERMOMETER EXPOSED TO DIRECT SUNSHINE AND ONE SHADED - DIMINISHES AS WE ASCEND IN THE ATMOSPHERE.[327] - - - From the _Philosophical Magazine_ for March, 1867. - -The remarkable fact was observed by Mr. Glaisher, that the difference -of reading between a black-bulb thermometer exposed to the direct rays -of the sun and one shaded diminishes as we ascend in the atmosphere. -On viewing the matter under the light of Professor Tyndall’s important -discovery regarding the influence of aqueous vapour on radiant heat, -the fact stated by Mr. Glaisher appears to be in perfect harmony with -theory. The following considerations will perhaps make this plain. - -The shaded thermometer marks the temperature of the surrounding -air; but the exposed thermometer marks not the temperature of the -air, but that of the bulb heated by the direct rays of the sun. The -temperature of the bulb depends upon two elements: (1) the rate at -which it receives heat by _direct radiation_ from the sun above, the -earth beneath, and all surrounding objects, and by _contact_ with the -air; (2) the rate at which it loses heat by radiation and by contact -with the air. As regards the heat gained and lost by contact with the -surrounding air, both thermometers are under the same conditions, -or nearly so. We therefore require only to consider the element of -radiation. - -We begin by comparing the two thermometers at the earth’s surface, and -we find that they differ by a very considerable number of degrees. -We now ascend some miles into the air, and on again comparing the -thermometers we find that the difference between them has greatly -diminished. It has been often proved, by direct observation, that the -intensity of the sun’s rays increases as we rise in the atmosphere. -How then does the exposed thermometer sink more rapidly than the -shaded one as we ascend? The reason is obviously this. The temperature -of the thermometers depends as much upon the rate at which they are -losing their heat as upon the rate at which they are gaining it. -The higher temperature of the exposed thermometer is the result of -_direct radiation_ from the sun. Now, although this thermometer -receives by radiation more heat from the sun at the upper position -than at the lower, it does not necessarily follow on this account -that its temperature ought to be higher. Suppose that at the upper -position it should receive one-fourth more heat from the sun than at -the lower, yet if the rate at which it loses its heat by radiation -into space be, say, one-third greater at the upper position than at -the lower, the temperature of the bulb would sink to a considerable -extent, notwithstanding the extra amount of heat received. Let us now -reflect on how matters stand in this respect in regard to the actual -case under our consideration. When the exposed thermometer is at the -higher position, it receives more heat from the sun than at the lower, -but it receives less from the earth; for a considerable part of the -radiation from the earth is cut off by the screen of aqueous vapour -intervening between the thermometer and the earth. But, on the whole, -it is probable that the total quantity of radiant heat reaching the -thermometer is greater in the higher position than in the lower. -Compare now the two positions in regard to the rate at which the -thermometer loses its heat by radiation. When the thermometer is at the -lower position, it has the warm surface of the ground against which to -radiate its heat downwards. The high temperature of the ground thus -tends to diminish the rate of radiation. Above, there is a screen of -aqueous vapour throwing back upon the thermometer a very considerable -part of the heat which the instrument is radiating upwards. This, of -course, tends greatly to diminish the loss from radiation. But at -the upper position this very screen, which prevented the thermometer -from throwing off its heat into the cold space above, now affects -the instrument in an opposite manner; for the thermometer has now to -radiate its heat downwards, not upon the warm surface of the ground -as before, but upon the cold upper surface of the aqueous screen -intervening between the instrument and the earth. This of course tends -to lower the mercury. We are now in a great measure above the aqueous -screen, with nothing to protect the thermometer from the influence of -cold stellar space. It is true that the air above is at a temperature -little below that of the thermometer itself; but then the air is dry, -and, owing to its diathermancy, it does not absorb the heat radiated -from the thermometer, and consequently the instrument radiates its heat -directly into the cold stellar space above, some hundreds of degrees -below zero, almost the same as it would do were the air entirely -removed. The enormous loss of heat which the thermometer now sustains -causes it to fall in temperature to a great extent. The molecules of -the comparatively dry air at this elevation, being very bad radiators, -do not throw off their heat into space so rapidly as the bulb of the -exposed thermometer; consequently their temperature does not (for this -reason) tend to sink so rapidly as that of the bulb. Hence the shaded -thermometer, which indicates the temperature of those molecules, is -not affected to such an extent as the exposed one. Hence also the -difference of reading between the two instruments must diminish as we -rise in the atmosphere. - -This difference between the temperature of the two thermometers -evidently does not go on diminishing to an indefinite extent. Were we -able to continue our ascent in the atmosphere, we should certainly -find that a point would be reached beyond which the difference of -reading would begin to increase, and would continue to do so till the -outer limits of the atmosphere were reached. The difference between -the temperatures of the two thermometers beyond the limits of the -atmosphere would certainly be enormous. The thermometer exposed to -the direct rays of the sun would no doubt be much colder than it had -been when at the earth’s surface; but the shaded thermometer would -now indicate the temperature of space, which, according to Sir John -Herschel and M. Pouillet, is more than 200° Fahrenheit below zero. - -It follows also, from what has been stated, that even under direct -sunshine the removal of the earth’s atmosphere would tend to lower the -temperature of the earth’s surface to a great extent. This conclusion -also follows as an immediate inference from the fact that the earth’s -atmosphere, as it exists at present charged with aqueous vapour, -affects terrestrial radiation more than it does radiation from the sun; -for the removal of the atmosphere would increase the rate at which the -earth throws off its heat into space more than it would increase the -rate at which it receives heat from the sun; therefore its temperature -would necessarily fall until the rate of radiation _from_ the earth’s -surface exactly equalled the rate of radiation _to_ the surface. Let -the atmosphere again envelope the earth, and terrestrial radiation -would instantly be diminished; the temperature of the earth’s surface -would therefore necessarily begin to rise, and would continue to do so -till the rate of radiation from the surface would equal the rate of -radiation received by the surface. Equilibrium being thus restored, the -temperature would remain stationary. It is perfectly obvious that if we -envelope the earth with a substance such as our atmosphere, that offers -more resistance to terrestrial radiation than to solar, the temperature -of the earth’s surface must necessarily rise until the heat which is -being radiated off equals that which is being received from the sun. -Remove the air and thus get quit of the resistance, and the temperature -of the surface would fall, because in this case a lower temperature -would maintain equilibrium. - -It follows, therefore, that the moon, which has no atmosphere, must -be much colder than our earth, even on the side exposed to the sun. -Were our earth with its atmosphere as it exists at present removed to -the orbit of Venus or Mars, for example, it certainly would not be -habitable, owing to the great change of temperature that would result. -But a change in the physical constitution of the atmospheric envelope -is really all that would be necessary to retain the earth’s surface at -its present temperature in either position. - - - - - IV. - - REMARKS ON MR. J. Y. BUCHANAN’S THEORY OF THE VERTICAL - DISTRIBUTION OF TEMPERATURE OF THE OCEAN.[328] - - -Since the foregoing was in type, a paper on the “Vertical Distribution -of Temperature of the Ocean,” by Mr. J. Y. Buchanan, chemist on board -the _Challenger_, has been read before the Royal Society.[329] In that -paper Mr. Buchanan endeavours to account for the great depth of warm -water in the middle of the North Atlantic compared with that at the -equator, without referring it to horizontal circulation of any kind. - -The following is the theory as stated by Mr. Buchanan:— - -“Let us assume the winter temperature of the surface-water to be 60° F. -and the summer temperature to be 70° F. If we start from midwinter, we -find that, as summer approaches, the surface-water must get gradually -warmer, and that the temperature of the layers below the surface must -decrease at a very rapid rate, until the stratum of winter temperature, -or 60° F., is reached; in the language of the isothermal charts, the -isothermal line for degrees between 70° F. (if we suppose that we have -arrived at midsummer) and 60° F. open out or increase their distance -from each other as the depth increases. Let us now consider the -conditions after the summer heat has begun to waver. During the whole -period of heating, the water, from its increasing temperature, has been -always becoming lighter, so that heat communication by convection with -the water below has been entirely suspended during the whole period. -The heating of the surface-water has, however, had another effect, -besides increasing its volume; it has, by evaporation, rendered it -denser than it was before, at the same temperature. Keeping in view -this double effect of the summer heat upon the surface-water, let us -consider the effect of the winter cold upon it. The superficial water -having assumed the atmospheric temperature of, say 60° F., will sink -through the warmer water below it, until it reaches the stratum of -water having the same temperature as itself. Arrived here, however, -although it has the same temperature as the surrounding water, -the two are no longer in equilibrium, for the water which has come -from the surface, has a greater density than that below at the same -temperature. It will therefore not be arrested at the stratum of the -same temperature, as would have been the case with fresh water; but it -will continue to sink, carrying of course its higher temperature with -it, and distributing it among the lower layers of colder water. At -the end of the winter, therefore, and just before the summer heating -recommences, we shall have at the surface a more or less thick stratum -of water having a nearly uniform temperature of 60° F., and below this -the temperature decreasing at a considerable but less rapid rate than -at the termination of the summer heating. If we distinguish between -_surface-water_, the temperature of which rises with the atmospheric -temperature (following thus, in direction at least, the variation of -the seasons), and _subsurface_-water, or the stratum immediately below -it, we have for the latter the, at first sight, paradoxical effect of -summer cooling and winter heating. The effect of this agency is to -diffuse the same heat to a greater depth in the ocean, the greater the -yearly range of atmospheric temperature at the surface. This effect -is well shown in the chart of isothermals, on a vertical section, -between Madeira and a position in lat. 3° 8′ N., long. 14° 49′ W. The -isothermal line for 45° F. rises from a depth of 740 fathoms at Madeira -to 240 fathoms at the above-mentioned position. In equatorial regions -there is hardly any variation in the surface-temperature of the sea; -consequently we find cold water very close to the surface all along the -line. On referring to the temperature section between the position lat. -3° 8′ N., long. 14° 49′ W., and St. Paul’s Rocks, it will be seen that, -with a surface-temperature of from 75° F. to 79° F., water at 55° F. is -reached at distances of less than 100 fathoms from the surface. Midway -between the Azores and Bermuda, with a surface-temperature of 70° F., -it is only at a depth of 400 fathoms that we reach water of 55° F.” - -What Mr. Buchanan states will explain why the mean annual temperature -of the water at the surface extends to a greater depth in the middle -of the North Atlantic than at the equator. It also explains why the -temperature from the surface downwards decreases more rapidly at the -equator than in the middle of the North Atlantic; but, if I rightly -understand the theory, it does not explain (and this is the point at -issue) why at a given depth the temperature of the water in the North -Atlantic should be higher than the temperature at a corresponding depth -at the equator. Were there no horizontal circulation the greatest -thickness of warm water would certainly be found at the equator and -the least at the poles. The isothermals would in such a case gradually -slope downwards from the poles to the equator. The slope might not be -uniform, but still it would be a continuous downward slope. - - - - - V. - - ON THE CAUSE OF THE COOLING EFFECT PRODUCED ON SOLIDS BY - TENSION.[330] - - - From the _Philosophical Magazine_ for May, 1864. - -From a series of experiments made by Dr. Joule with his usual accuracy, -he found that when bodies are subjected to tension, a cooling effect -takes place. “The quantity of cold,” he says, “produced by the -application of tension was sensibly equal to the heat evolved by its -removal; and further, that the thermal effects were proportional to -the weight employed.”[331] He found that when a weight was applied to -compress a body, a certain amount of heat was evolved; but the same -weight, if applied to stretch the body, produced a corresponding amount -of cold. - -This, although it does not appear to have been remarked, is a most -singular result. If we employ a force to compress a body, and then ask -what has become of the force applied, it is quite a satisfactory answer -to be told that the force is converted into heat, and reappears in the -molecules of the body as such; but if the same force be employed to -stretch the body, it will be no answer to be told that the force is -converted into cold. Cold cannot be the force under another form, for -cold is a privation of force. If a body, for example, is compressed by -a weight, the _vis viva_ of the descending weight is transmitted to the -molecules of the body and reappears under that form of force called -heat; but if the same weight is applied so as to stretch or expand the -body, not only does the force of the weight disappear without producing -heat, but the molecules which receive the force lose part of that -which they already possessed. Not only does the force of the weight -disappear, but along with it a portion of the force previously existing -in the molecules under the form of heat. We have therefore to inquire, -not merely into what becomes of the force imparted by the weight, but -also what becomes of the force in the form of heat which disappears -from the molecules of the body itself. That the _vis viva_ of the -descending weight should disappear without increasing the heat of the -molecules is not so surprising, because it may be transformed into some -other form of force different from that of heat. For it is by no means -evident _à priori_ that heat should be the only form under which it -may exist. But it is somewhat strange that it should cause the force -previously existing in the molecules in the form of heat also to change -into some other form. - -When a weight, for example, is employed to stretch a solid body, it -is evident that the force exerted by the weight is consumed in work -against the cohesion of the particles, for the entire force is exerted -so as to pull them separate from each other. But the cooling effect -which takes place shows that more force disappears than simply what -is exerted by the weight; for the cooling effect is caused by the -disappearance of force in the shape of heat from the body itself. The -force exerted by the weight disappears in performing work against the -cohesion of the particles of the body stretched. But what becomes -of the energy in the form of heat which disappears from the body at -the same time? It must be consumed in performing work of some kind -or other. The force exerted by the weight cannot be the cause of the -cooling effect. The transferrence of force from the weight to the body -may be the cause of a heating effect—an increase of force in the body; -but this transferrence of force to the body cannot be the cause of a -decrease of force in the body. If a decrease of force actually follows -the application of tension, the weight can only be the occasion, not -the cause of the decrease. - -In what manner, then, does the stretching of the body by the weight -become the occasion of its losing energy in the shape of heat? Or, in -other words, what is the cause of the cooling effects which result -from tension? The probable explanation of the phenomenon seems to -be this: if the molecules of a body are held together by any force, -of whatever nature it may be, which prevents any further separation -taking place, then the entire heat applied to such a body will appear -as temperature; but if this binding force becomes lessened so as to -allow further expansion, then a portion of the heat applied will be -lost in producing expansion. All solids at any given temperature expand -until the expansive force of their heat exactly balances the cohesive -force of their molecules, after which no further expansion at the -same temperature can possibly take place while the cohesive force of -the molecules remains unchanged. But if, by some means or other, the -cohesive force of the molecules become reduced, then instantly the -body will expand under the heat which it possesses, and of course a -portion of the heat will be consumed in expansion, and a cooling effect -will result. Now tension, although it does not actually lessen the -cohesive force of the molecules of the stretched body, yet produces, by -counteracting this force, the same effect; for it allows the molecules -an opportunity of performing work of expansion, and a cooling effect -is the consequence. If the piston of a steam-engine, for example, be -loaded to such an extent that the steam is unable to move it, the steam -in the interior of the cylinder will not lose any of its heat; but if -the piston be raised by some external force, the molecules of the steam -will assist this force, and consequently will suffer loss of heat in -proportion to the amount of work which they perform. The very same -occurs when tension is applied to a solid. Previous to the application -of tension, the heat existing in the molecules is unable to produce -any expansion against the force of cohesion. But when the influence of -cohesion is partly counteracted by the tension applied, the heat then -becomes enabled to perform work of expansion, and a cooling effect is -the result. - - - - - VI. - - THE CAUSE OF REGELATION.[332] - - -There are two theories which have been advanced to explain Regelation, -the one by Professor Faraday, and the other by Professor James Thomson. - -According to Professor James Thomson, pressure is the cause of -regelation. Pressure applied to ice tends to lower the melting-point, -and thus to produce liquefaction; but the water which results is -colder than the ice, and refreezes the moment it is relieved from -pressure. When two pieces of ice are pressed together, a melting takes -place at the points in contact, resulting from the lowering of the -melting-point; the water formed, re-freezing, joins the two pieces -together. - -The objection which has been urged against this theory is that -regelation will take place under circumstances where it is difficult to -conceive how pressure can be regarded as the cause. Two pieces of ice, -for example, suspended by silken threads in an atmosphere above the -melting-point, if but simply allowed to touch each other, will freeze -together. Professor J. Thomson, however, attributes the freezing to -the pressure resulting from the capillary attraction of the two moist -surfaces in contact. But when we reflect that it requires the pressure -of a mile of ice—135 tons on the square foot—to lower the melting-point -one degree, it must be obvious that the lowering effect resulting -from capillary attraction in the case under consideration must be -infinitesimal indeed. - -The following clear and concise account of Faraday’s theory, I quote -from Professor Tyndall’s “Forms of Water:”— - -“Faraday concluded that _in the interior_ of any body, whether solid -or liquid, where every particle is grasped, so to speak, by the -surrounding particles, and grasps them in turn, the bond of cohesion -is so strong as to require a higher temperature to change the state -of aggregation than is necessary _at the surface_. At the surface of -a piece of ice, for example, the molecules are free on one side from -the control of other molecules; and they therefore yield to heat more -readily than in the interior. The bubble of air or steam in overheated -water also frees the molecules on one side; hence the ebullition -consequent upon its introduction. Practically speaking, then, the -point of liquefaction of the interior ice is higher than that of the -superficial ice.... - -“When the surfaces of two pieces of ice, covered with a film of the -water of liquefaction, are brought together, the covering film is -transferred from the surface to the centre of the ice, where the point -of liquefaction, as before shown, is higher than at the surface. -The special solidifying power of ice upon water is now brought -into play _on both sides of the film_. Under these circumstances, -Faraday held that the film would congeal, and freeze the two surfaces -together.”—_The Forms of Water_, p. 173. - -The following appears to be a more simple explanation of the phenomena -than either of the preceding:— - -The freezing-point of water, and the melting-point of ice, as Professor -Tyndall remarks, touch each other as it were at this temperature. At -a hair’s-breadth lower water freezes; at a hair’s-breadth higher ice -melts. Now if we wish, for example, to freeze water, already just about -the freezing-point, or to melt a piece of ice already just about the -melting-point, we can do this either by a change of temperature or -by a change of the melting-point. But it will be always much easier -to effect this by the former than by the latter means. Take the -case already referred to, of the two pieces of ice suspended in an -atmosphere above the melting-point. The pieces at their surfaces are -in a melting condition, and are surrounded by a thin film of water -just an infinitesimal degree above the freezing-point. The film has on -the one side solid ice at the freezing-point, and on the other a warm -atmosphere considerably above the freezing-point. The tendency of the -ice is to lower the temperature of the film, while that of the air is -to raise its temperature. When the two pieces are brought into contact -the two films unite and form one film separating the two pieces of ice. -This film is not like the former in contact with ice on the one side -and warm air on the other. It is surrounded on both sides by solid ice. -The tendency of the ice, of course, is to lower the film to the same -temperature as the ice itself, and thus to produce solidification. -It is evident that the film must either melt the ice or the ice must -freeze the film, if the two are to assume the same temperature. But the -power of the ice to produce solidification, owing to its greater mass, -is enormously greater than the power of the film to produce fluidity, -consequently regelation is the result. - - - - - VII. - - LIST OF PAPERS WHICH HAVE APPEARED IN DR. A. PETERMANN’S - _GEOGRAPHISCHE MITTHEILUNGEN_ RELATING TO THE GULF-STREAM AND - THERMAL CONDITION OF THE ARCTIC REGIONS. - - -The most important memoir which we have on the Gulf-stream and its -influence on the climate of the arctic regions is the one by Dr. A. -Petermann, entitled “Der Golfstrom und Standpunkt der thermometrischen -Kenntniss des nord-atlantischen Oceans und Landgebiets im Jahre 1870.” -_Geographische Mittheilungen_, Band XVI. 1870. - -Dr. Petermann has, in this memoir, by a different line of argument -from that which I have pursued in this volume, shown in the most clear -and convincing manner that the abnormally high temperature of the -north-western shores of Europe and the seas around Spitzbergen is owing -entirely to the Gulf-stream, and not to any general circulation such as -that advocated by Dr. Carpenter. From a series of no fewer than 100,000 -observations of temperature in the North Atlantic and in the arctic -seas, he has been enabled to trace with accuracy on his charts the very -footsteps of the heat in its passage from the Gulf of Mexico up to the -shores of Spitzbergen. - -The following is a list of the more important papers bearing on the -subject which have recently appeared in Dr. Petermann’s _Geogr. -Mittheilungen_:— - -An English translation of Dr. Petermann’s Memoir, and of a few more in -the subjoined list, has been published in a volume, with supplements, -by the Hydrographic Department of the United States, under the -superintendence of Commodore R. H. Wyman. - -The papers whose titles are in English have appeared in the American -volume. In that volume the principal English papers on the subject, -in as far as they relate to the north-eastern extension of the -Gulf-stream, have also been reprinted. - -The System of Oceanic Currents in the Circumpolar Basin of the Northern -Hemisphere. By Dr. A. Mühry. Vol. XIII., Part II. 1867. - -The Scientific Results of the first German North Polar Expedition. By -Dr. W. von Freeden. Vol. XV., Part VI. 1869. - -The Gulf-stream, and the Knowledge of the Thermal Properties of the -North Atlantic Ocean and its Continental Borders, up to 1870. By Dr. A. -Petermann. _Geographische Mittheilungen_, Vol. XVI., Part VI. 1870. - -The Temperature of the North Atlantic Ocean and the Gulf-stream. By -Rear-Admiral C. Irminger. Vol. XVI., Part VI. 1870. - -Meteorological Observations during a Winter Stay on Bear Island, -1865−1866. By Sievert Tobilson. Vol. XVI., Part VII. 1870. - -Die Temperatur-verhältnisse in den arktischen Regionen. Von Dr. -Petermann. Band XVI., Heft VII. 1870. - -Preliminary Reports of the Second German North Polar Expedition, and of -minor Expeditions, in 1870. Vol. XVII. - -Preliminary Report of the Expedition for the Exploration of the -Nova-Zembla Sea (the sea between Spitzbergen and Nova Zembla), by -Lieutenants Weyprecht and Payer, June to September, 1871. By Dr. A. -Petermann. Vol. XVII. 1871. - -Der Golfstrom ostwärts vom Nordkap. Von A. Middendorff. Band XVII., -Heft I. 1871. - -Kapitän E. H. Johannesen’s Umfahrung von Nowaja Semlä im Sommer 1870, -und norwegischer Finwalfang östlich vom Nordkap. Von Th. v. Heuglin. -Band XVII., Heft I. 1871. - -Die Nordpol-Expeditionen, das sagenhafte Gillis-land und der Golfstrom -im Polarmeere. Von Dr. A. Petermann. 5 Nov. 1870. - -Th. v. Heuglin’s Aufnahmen in Ost-Spitzbergen. Begleitworte zur neuen -Karte dieses Gebiets. Tafel 9. 1870. Band XVII., Heft V. 1871. - -Die zweite deutsche Nordpolar-Expedition, 1869−70. Schlittenreise -an der Küste Grönlands nach Norden, 8 März−27 April, 1870. Von -Ober-Lieutenant Julius Payer. Band XVII., Heft V. 1871. - -Die Entdeckung des Kaiser Franz Josef-Fjordes in Ost-Grönland, August, -1870. Von Ober-Lieutenant Julius Payer. Band XVII., Heft V. 1871. - -Die Erschliessung eines Theiles des nördlichen Eismeeres durch die -Fahrten und Beobachtungen der norwegischen Seefahrer Torkildsen, -Ulve, Mack Qvale, und Nedrevaag im karischen Meere, 1870. Von Dr. A. -Petermann. Band XVII., Heft III. 1871. - -Die zweite deutsche Nordpolar-Expedition, 1869−70. Schlittenreise nach -Ardencaple Inlet, 8−29 Mai, 1870. Von Ober-Lieutenant Julius Payer. -Band XVII., Heft XI. 1871. - -Ein Winter unter dem Polarkreise. Von Ober-Lieutenant Julius Payer. -Band XVII., Heft XI. 1871. - -Die Entdeckung eines offenen Polarmeeres durch Payer und Weyprecht im -September, 1871. Von Dr. A. Petermann. Band XVII., Heft XI. 1871. - -James Lamont’s Nordfahrt, Mai-August, 1871. Die Entdeckungen von -Weyprecht, Payer, Tobiesen, Mack, Carlsen, Ulve, und Smyth im Sommer, -1871. - -Stand der Nordpolarfrage zu Ende des Jahres 1871. Von Dr. A. Petermann. -Band XVII., Heft XII. 1871. - -Das Innere von Grönland. Von Dr. Robert Brown. Band XVII., Heft X. 1871. - -Captain T. Torkildsen’s Cruise from Tromsö to Spitzbergen, July 26 to -September 26, 1871. Vol. XVIII. 1872. - -The Sea north of Spitzbergen, and the most northern Meteorological -Observations. Vol. XVIII. 1872. - -Results of the Observations of the Deep-sea Temperature in the Sea -between Greenland, Northern Europe, and Spitzbergen. By Professor H. -Möhn. Vol. XVIII. 1872. - -The Norwegian Cruises to Nova Zembla and the Kara Sea in 1871. Vol. -XVIII. 1872. - -The Cruises in the Polar Sea in 1872. Vol. XVIII. 1872. - -The Cruise of Smyth and Ulve, June 19 to September 27, 1871. Vol. -XVIII. 1872. - -Die fünfmonatliche Schiffbarkeit des sibirischen Eismeeres um Nowaja -Semlja, erwiesen durch die norwegischen Seefahrer in 1869 und 1870, -ganz besonders aber in 1871. Von Dr. A. Petermann. Band XVIII., Heft X. -1872. - -Die neuen norwegischen Aufnahmen des nordöstlichen Theiles von Nowaja -Semlja durch Mack, Dörma, Carlsen, u. A., 1871. Von Dr. Petermann. Band -XVIII., Heft X. 1872. - -Nachrichten über die sieben zurückgekehrten Expeditionen unter Graf -Wiltschek, Altmann, Johnsen, Nilsen, Smith, Gray, Whymper; die -drei Überwinterungs-Expeditionen; die Amerikanische, Schwedische, -Österreichisch-Ungarische; und die zwei neuen: die norwegische -Winter-Expedition und diejenige unter Kapitän Mack. Von Dr. A. -Petermann. Band XVIII., Heft XII. 1872. - -Konig Karl-Land im Osten von Spitzbergen und seine Erreichung und -Aufnahme durch norwegische Schiffer im Sommer 1872. Von Professor H. -Möhn. Band XIX., Heft IV. 1873. - -Resultate der Beobachtungen angestellt auf der Fahrt des Dampfers -“Albert” nach Spitzbergen im November und Dezember, 1872. Von Professor -Möhn. Band XIX., Heft VII. 1873. - -Die amerikanische Nordpolar-Expedition unter C. F. Hall, 1871−3. Von -Dr. A. Petermann. Band XIX., Heft VIII. 1873. - -Die Trift der Hall’schen Nordpolar-Expedition, 16 August bis 15 -Oktober, 1872, und die Schollenfahrt der 20 bis zum 30 April, 1873. Von -Dr. A. Petermann. Band XIX., Heft X. 1873. - -Das offene Polarmeer bestätigt durch das Treibholz an der Nordwestküste -von Grönland. Von Dr. A. Petermann. Band XX., Heft V. 1874. - -Das arktische Festland und Polarmeer. Von Dr. Joseph Chavanne. Band -XX., Heft VII. 1874. - -Die Umkehr der Hall’schen Polar-Expedition nach den Aussagen der -Offiziere. Von Dr. A. Petermann. Band XX., Heft VII. 1874. - -Die zweite österreichisch-ungarische Nordpolar-Expedition unter -Weyprecht und Payer, 1872−4. Von Dr. A. Petermann. Band XX., Heft X. -1874. - -Beiträge zur Klimatologie und Meteorologie des Ost-polar-Meeres. Von -Professor Möhn. Band XX., Heft V. 1874. - -Kapitän David Gray’s Reise und Beobachtungen im ost-grönländischen -Meere, 1874, und seine Ansichten über den besten Weg zum Nordpol. -Original-Mittheilungen an A. Petermann, d.D., Peterhead, Dezember, -1874. Band XXI., Heft III. 1875. - - - - - VIII. - - LIST OF PAPERS BY THE AUTHOR TO WHICH REFERENCE IS MADE - IN THIS VOLUME. - - -On the Influence of the Tidal Wave on the Earth’s Rotation and on the -Acceleration of the Moon’s Mean Motion.—_Phil. Mag._, April, 1864. - -On the Nature of Heat-vibrations.—_Phil. Mag._, May, 1864. - -On the Cause of the Cooling Effect produced on Solids by -Tension.—_Phil. Mag._, May, 1864. - -On the Physical Cause of the Change of Climate during Geological -Epochs.—_Phil. Mag._, August, 1864. - -On the Physical Cause of the Submergence of the Land during the Glacial -Epoch.—The _Reader_, September 2nd and October 14th, 1865. - -On Glacial Submergence.—The _Reader_, December 2nd and 9th, 1865. - -On the Eccentricity of the Earth’s Orbit.—_Phil. Mag._, January, 1866. - -Glacial Submergence on the Supposition that the Interior of the Globe -is in a Fluid Condition.—The _Reader_, January 13th, 1866. - -On the Physical Cause of the Submergence and Emergence of the Land -during the Glacial Epoch, with a Note by Professor Sir William -Thomson.—_Phil. Mag._, April, 1866. - -On the Influence of the Tidal Wave on the Motion of the Moon.—_Phil. -Mag._, August and November, 1866. - -On the Reason why the Change of Climate in Canada since the Glacial -Epoch has been less complete than in Scotland.—_Trans. Geol. Soc. of -Glasgow_, 1866. - -On the Eccentricity of the Earth’s Orbit, and its Physical Relations to -the Glacial Epoch.—_Phil. Mag._, February, 1867. - -On the Reason why the Difference of Reading between a Thermometer -exposed to direct Sunshine and one shaded diminishes as we ascend in -the Atmosphere.—_Phil. Mag._, March, 1867. - -On the Change in the Obliquity of the Ecliptic; its Influence on the -Climate of the Polar Regions and Level of the Sea.—_Trans. Geol. Soc. -of Glasgow_, vol. ii., p. 177. _Phil. Mag._, June, 1867. - -Remarks on the Change in the Obliquity of the Ecliptic, and its -Influence on Climate.—_Phil. Mag._, August, 1867. - -On certain Hypothetical Elements in the Theory of Gravitation -and generally received Conceptions regarding the Constitution of -Matter.—_Phil. Mag._, December, 1867. - -On Geological Time, and the probable Date of the Glacial and the Upper -Miocene Period.—_Phil. Mag._, May, August, and November, 1868. - -On the Physical Cause of the Motions of Glaciers.—_Phil. Mag._, March, -1869. _Scientific Opinion_, April 14th, 1869. - -On the Influence of the Gulf-stream.—_Geol. Mag._, April, 1869. -_Scientific Opinion_, April 21st and 28th, 1869. - -On Mr. Murphy’s Theory of the Cause of the Glacial Climate.—_Geol. -Mag._, August, 1869. _Scientific Opinion_, September 1st, 1869. - -On the Opinion that the Southern Hemisphere loses by Radiation more -Heat than the Northern, and the supposed Influence that this has on -Climate.—_Phil. Mag._, September, 1869. _Scientific Opinion_, September -29th and October 6th, 1869. - -On Two River Channels buried under Drift belonging to a Period when the -Land stood several hundred feet higher than at present.—_Trans. Geol. -Soc. of Edinburgh_, vol. i., p. 330. - -On Ocean-currents: Ocean-currents in Relation to the Distribution of -Heat over the Globe.—_Phil. Mag._, February, 1870. - -On Ocean-currents: Ocean-currents in Relation to the Physical Theory of -Secular Changes of Climate.—_Phil. Mag._, March, 1870. - -The Boulder Clay of Caithness a Product of Land-ice.—_Geol. Mag._, May -and June, 1870. - -On the Cause of the Motion of Glaciers.—_Phil. Mag._, September, 1870. - -On Ocean-currents: On the Physical Cause of Ocean-currents. Examination -of Lieutenant Maury’s Theory.—_Phil. Mag._, October, 1870. - -On the Transport of the Wastdale Granite Boulders.—_Geol. Mag._, -January, 1871. - -On a Method of determining the Mean Thickness of the Sedimentary Rocks -of the Globe.—_Geol. Mag._, March, 1871. - -Mean Thickness of the Sedimentary Rocks.—_Geol. Mag._, June, 1871. - -On the Age of the Earth as determined from Tidal Retardation.—_Nature_, -August 24th, 1871. - -Ocean-currents: On the Physical Cause of Ocean-currents. Examination of -Dr. Carpenter’s Theory.—_Phil. Mag._, October, 1871. - -Ocean-currents: Further Examination of the Gravitation Theory.—_Phil. -Mag._, February, 1874. - -Ocean-currents: The Wind Theory of Oceanic Circulation.—_Phil. Mag._, -March, 1874. - -Ocean-currents.—_Nature_, May 21st, 1874. - -The Physical Cause of Ocean-currents.—_Phil. Mag._, June, 1874. -_American Journal of Science and Art_, September, 1874. - -On the Physical Cause of the Submergence and Emergence of the Land -during the Glacial Epoch.—_Geol. Mag._, July and August, 1874. - - - - - INDEX. - - - Absolute heating-power of ocean-currents, 23 - 〃 amount of heat received from the sun per day, 26 - - Adhémar, M., theory founded upon a mistake in regard to - radiation, 81, 85 - 〃 on submergence, 368 - 〃 on influence of eccentricity on climate, 542 - - Aërial currents increased in action by formation of snow and ice, 76 - 〃 function of, stated, 51 - 〃 heat conveyed by, 27 - - Africa, South, glacial and inter-glacial periods of, 242 - 〃 boulder clay of Permian age, 300 - - Age and origin of the sun, 346 - - Air, on absorption of rays by, 59 - 〃 when humid, absorbs rays which agree with it in period, 59 - 〃 when perfectly dry incapable of absorbing radiant heat, 59 - - Airy, Professor, earth’s axis of rotation permanent, 7 - - Aitken’s, Mr., experiment on density of polar water, 129 - - Aland islands, striation of, 447 - - Alternate cold and warm periods, 236 - - Allermuir, striations on summit of, 441 - - America, low temperature in January, 72 - 〃 thickness of ice-sheet of North, 381 - - Anderson, Captain Sir James, never observed a stone on an iceberg, 282 - - Antarctic regions, mean summer temperature of, below - freezing-point, 63 - - Antarctic ice-cap, probable thickness of, 375 - 〃 diagram representing thickness of, 377 - 〃 thickness of, estimated from icebergs, 384 - - Antarctic snowfall, estimates of, 382 - - Aphelion, glacial conditions at maximum when winter solstice is at, 77 - - Arago, M., on influence of eccentricity on climate, 536 - - Arctic climate, influence of ocean-currents on, during glacial - period, 260 - - Arctic regions, influence of Gulf-stream on climate of, 45 - 〃 mean summer temperature of, 63 - - Arctic regions, amount of heat received by, per unit surface, 195 - 〃 warm periods best marked in, 258 - 〃 warm inter-glacial periods in, 258−265 - 〃 state of, during glacial period, 260 - 〃 evidence of warm periods in, 261 - 〃 occurrence of recent trees in, 261, 265 - 〃 evidence of warm inter-glacial periods, 293 - 〃 warm climate during Old Red Sandstone period in, 295 - 〃 glacial period during Carboniferous age in, 297 - 〃 warm climate during Permian period in, 301 - 〃 list of papers relating to, 556 - - Arctic Ocean, area of, 195 - 〃 according to gravitation theory ought to be warmer than Atlantic - in torrid zone, 195 - 〃 heat conveyed into, by currents, compared with that received by it - from the sun, 195 - 〃 blocked up with polar ice, 444 - - Armagh, boulder beds of, 299 - - Arran, Island of, glacial conglomerate of Permian age in, 299 - - Astronomical causes of change of climate, 10 - - Astronomy and geology, supposed analogy between, 355 - - Atlantic, atmospheric pressure on middle of, 33 - 〃 inability of, to heat the south-west winds without the - Gulf-stream, 34 - 〃 mean annual temperature of, 36 - 〃 mean temperature of, raised by Gulf-stream, 36, 40 - 〃 isothermal lines of, compared with those of the Pacific, 46 - 〃 area of, from equator to Tropic of Cancer, 194 - 〃 inquiry whether the area of, is sufficient to supply heat - according to Dr. Carpenter’s theory, 194 - - Atlantic, North, heat received by, from torrid zone by currents, 194 - 〃 according to Dr. Carpenter’s theory ought to be warmer in - temperate regions than in the torrid zone, 195 - 〃 great depth of warm water in, 198 - 〃 North, an immense whirlpool, 216 - 〃 above the level of equator, 221 - 〃 probable antiquity of, 367 - 〃 from Scandinavia to Greenland probably filled with ice, 451 - - Atmosphere-pressure in Atlantic a cause of south-west winds, 33 - - Atmosphere, on difference between black-bulbed and shaded thermometer - in upper strata of, 547 - - Australia, evidence of ice-action in conglomerate of, 295 - - Ayrshire, ice-action during Silurian period in, 293 - - - Bakewell, Mr. R., on influence of eccentricity on climate, 540 - - Banks’s Land, discovery of ancient forest in, 261 - 〃 Professor Heer, on fossilized wood of, 309 - - Ball, Mr., objection to Canon Moseley’s results, 501 - - Baltic current, 171 - - Baltic, glaciation of islands in, 448 - - Baltic glacier, passage of, over Denmark, 449 - - Bath, grooved rock surfaces of, 464 - - Bay-ice grinds but does not striate rocks, 277 - - Belcher, Sir E., tree dug up by, in latitude 75° N., 263 - 〃 carboniferous fossils found in arctic regions by, 298 - - Belle-Isle, Strait of, observations on action of icebergs in, 276 - - Bell, Mr. A., on Mediterranean forms in glacial bed at Greenock, 254 - - Belt, Mr. Thomas, theory of the cause of glacial epochs, 415 - - Bennie, Mr. James, on surface geology, 468 - 〃 on deposits filling buried channel, 486 - - Blanford, Mr., on ice-action during Carboniferous age in India, 297 - - Borings, evidence of inter-glacial beds from, 254 - 〃 examination of drift by, 467 - 〃 journals of, 483, 484 - - Boulder clays of former glacial epochs, why so rare, 269 - 〃 a product of land-ice, 284 - 〃 if formed from icebergs must be stratified, 284 - 〃 scarcity of fossils in, 285 - 〃 formed chiefly from rock on which it lies, 285 - 〃 of Caithness a product of land-ice, 435 - 〃 on summit of Allermuir, 441 - - Boulders, how carried from a lower to a higher level, 527 - - Boussingault on absorption of carbon by vegetation, 428 - - Britain, climate of, affected most by south-eastern portion of - Gulf-stream, 33 - - Brown, Dr. R., cited on Greenland ice-sheet, 378, 380 - 〃 on inland ice of Greenland, 284 - 〃 on cretaceous formation of Greenland, 305 - 〃 on Miocene beds of the Disco district, 310 - - Brown, Mr. Robert, on growth of coal plants, 421 - - Brown and Dickeson, on sediment of Mississippi, 330 - - Buchan, Mr., on atmosphere-pressure in the Atlantic, 33 - 〃 on force of the wind, 220 - - Buchanan, Mr. J. Y., on vertical distribution of heat of the - ocean, 550 - - Buckland, Dr., observations by, on occurrence of red chalk on - Cotteswold hills, 459 - - Buff, Professor, on oceanic circulation, 145 - - Buried river channels, 466 - 〃 channel from Kilsyth to Grangemouth, 468 - 〃 section at Grangemouth, 474 - 〃 from Kilsyth to Clyde, 481 - 〃 not excavated by sea nor by ice, 469 - 〃 other examples of, 488−494 - - - Caithness, difficulty of accounting for - the origin of the boulder clay of, 435 - - Caithness, boulder clay of, a product of land-ice, 435 - 〃 boulder clay not formed by icebergs, 437 - 〃 theories regarding the origin of the boulder clay of, 437 - 〃 why the ice was forced over it, 444 - 〃 Professor Geikie and B. N. Peach on path of ice over, 453 - - Cambrian conglomerate of Islay, 292 - - Campbell, Mr., observations of, on icebergs, 276 - 〃 on supposed striation of rocks by large icebergs, 278 - 〃 evidence that river-ice does not striate rocks, 279 - - Canada, change of climate less complete than in Scotland, 71 - - Carboniferous period of arctic regions, 298 - 〃 evidence of glacial epoch during, 296−298 - 〃 temperate climate of, 422 - - Carboniferous limestone, mode of formation, 433 - - Carpenter’s, Dr., objections examined, 141 - 〃 theory, mechanics of, 145 - 〃 idea of a 〃vertical circulation〃 stated, 153 - - Carpenter’s, Dr., radical error in theory of, 155 - 〃 on difference of density between waters of Atlantic and - Mediterranean, 168 - 〃 theory, inadequacy of, 191 - 〃 estimate of thermal work of Gulf-stream, 199 - - Charpentier’s, M., theory of glacier-motion, 513 - - Carse clays, date of, 405 - - Cattegat, ice-markings on shore of, 446 - - Cave and river deposits, 251 - - Chalk, erratic blocks found in, 304 - 〃 _débris_, conclusion of Mr. Searles Wood, 460 - - _Challenger’s_ temperature-soundings at equator, 119 - 〃 crucial test of the wind and gravitation theories, 220 - - Chambers, Dr. Robert, on striated pavements, 255 - 〃 observations on glaciation of Gothland, 446 - - Champlain Lake, inter-glacial bed of, 241 - - Chapelhall, ancient buried channel at, 491 - 〃 inter-glacial sand-bed, 244 - - Chart showing the agreement between system of currents and system - of winds, 212 - - Christianstadt, crossed by Baltic glacier, 450 - - Circulation without difference of level, 176 - - Climate, Secular changes of, intensified by reaction of physical - causes, 75, 76 - 〃 affected most by temperature of the surface of ground, 88 - 〃 ocean-currents in relation to, 226 - 〃 cold conditions of, inferred from absence of fossils, 288 - 〃 cold condition of, difficulty of determining, from fossil - remains, 289 - 〃 warm, of arctic regions during Old Red Sandstone period, 295 - 〃 rough sketch of the history of, during the last 60,000 years, 409 - 〃 of Coal period inter-glacial in character, 420 - 〃 alternate changes of, during Coal period, 426 - - Climates, Mr. J. Geikie on difficulty of detecting evidence of ancient - glacial conditions, 289 - 〃 evidence of, from ancient sea-bottoms, 289 - - Coal an inter-glacial formation, 420 - - Coal beds, alternate submergence and emergence during formation - of, 424 - 〃 preservation of, by submergence, 426 - - Coal period, flatness of the land during, 430 - - Coal plants, conditions necessary for, preservation of, 423 - - Coal seams, thickness of, indicative of length of inter-glacial - periods, 428 - - Coal seams, time occupied in formation of, 429 - - Coal strata, on absence of ice-action in, 429 - - Coal measures, oscillations of sea-level during formation of, 425 - - Cold periods best marked in temperate regions, 258 - - Colding, Dr., oceanic circulation, 95 - - Confusion of ideas in reference to the agency of polar cold, 179 - - Continental ice, inadequate conceptions of, 385 - 〃 absence of, during glacial epochs of Coal period, 432 - - Contorted drift near Musselburgh, 465 - - Cook, Captain, description of Sandwich Land by, 60 - 〃 on South Georgia, 60 - - Cornwall, striated rocks of, 464 - - Cotteswold hills, red chalk from Yorkshire found on, 459 - - Couthony, Mr., on action of icebergs, 275 - - Coutts, Mr. J., on buried channel, 493 - - Craig, Mr. Robert, on inter-glacial beds at Overton Hillhead and - Crofthead, 247 - - Craiglockhart hill, inter-glacial bed of, 245 - - “Crawling” theory considered, 507 - - “Crevasses,” origin of, according to molecular theory, 521 - - Cretaceous period, evidence of ice-action during, 303−305 - - Cretaceous age, evidence of warm periods during, 304 - - Cretaceous formation of Greenland, 305 - - Crofthead, inter-glacial bed at, 248 - - Cromer forest bed, 250 - - Crosskey, Rev. Mr., comparison of Clyde and Canada shell beds, 71 - 〃 on southern shells in Clyde beds, 253 - - Croydon, block of granite found in chalk at, 303 - - Crucial test of the wind and gravitation theories, 220 - - Crystallization, force of, a cause of glacier-motion, 523 - - Currents, effects of their stoppage on temperatures of equator and - poles, 42 - 〃 produced by saltness neutralize those produced by temperature, 106 - - - Dalager, excursion in Greenland by, 378 - - Dana, Professor, on action of icebergs, 275 - 〃 on striations by icebergs, 275 - 〃 on thickness of ice-sheet of North America, 381 - - Darwin, Mr., on alternate cold and warm periods, 231 - 〃 on migration of plants and animals during glacial epoch, 395 - 〃 on peat of Falkland Islands, 422 - - Date of the 40-foot beach, 409 - - Date when conditions were favourable to formations of the Carse - clay, 409 - - Davis’ Straits, current of, 132 - - Dawkins, Mr. Boyd, on the animals of cave and river deposits, 251 - - Dawson, Principal, on esker of Carboniferous age, 296 - - 〃 on habitats of coal plants, 424 - - Deflection of ocean-currents chief cause of change of climate, 68 - - De la Beche, Sir H. T., on influence of eccentricity on climate, 539 - - De Mairan, on influence of eccentricity on climate, 528 - - Denmark, crossed by Baltic glacier, 449−452 - - Denudation, method of measuring rate of, 329 - 〃 as a measure of geological time, 329 - 〃 measured by sediment of Mississippi, 330 - 〃 subaërial rate of, 331 - 〃 law which determines rate of, 333 - 〃 marine, trifling, 337 - - Deposition, rates of, generally adopted, quite arbitrary, 360 - 〃 rate of, determined by rate of denudation, 362 - 〃 range of, restricted to a narrow fringe surrounding the - continents, 364 - 〃 area of, 365 - 〃 during glacial epoch probably less than present, 366 - - Deposits from icebergs cannot be wholly unstratified, 437 - - Despretz, tables by, of temperature of maximum density of - sea-water, 117 - - Desor, M., on tropical fauna of the Eocene formation in - Switzerland, 306 - - Derbyshire, breaks in limestone of, marks of cold periods, 434 - - Derbyshire limestone a product of inter-glacial periods, 434 - - Devonshire, boulder clay discovered in, 463 - - Diagram illustrating descent of water from equator to poles, 155 - 〃 showing variations of eccentricity, 313 - 〃 illustrative of fluidity of interior of the earth, 396 - 〃 showing formation of coal beds, 426 - - Dick, Mr., chalk flints in boulder clay, 454 - - Dick, Mr. R., on buried channel, 491 - - Difference of level essential to gravitation theory, 176 - - Dilatation of sea-water by increase of temperature calculated by Sir - John Herschel, 116 - - Disco district, Dr. R. Brown cited on Miocene beds of, 310 - - Disco Island, Upper Miocene period of, 307−308 - - Distribution, how effected by ocean-currents, 231 - - Dove, Professor, method of constructing normal temperature tables - by, 40 - 〃 on mean annual temperature, 401 - - Dover, mass of coal imbedded in chalk found at, 303 - - Drayson, Lieutenant-Colonel, on obliquity of ecliptic, 410 - - Drayson, Lieutenant-Colonel, theory of the cause of the glacial - epoch, 410 - - Drift, examination by borings, 467 - - Drumry, deep surface deposits at, 482 - - Dubuat’s, M., experiments, 182 - 〃 experiments by, on water flowing down an incline, 120 - - Duncan, Captain, on under current in Davis’ Strait, 134 - - Dürnten lignite beds, 240 - - Dürnten beds an example of inter-glacial coal formation, 433 - - Durham, buried river channel at, 488 - - - Earth’s axis of rotation permanent, 7 - - Earth, mean temperature of, increased by water at equator, 30 - 〃 not habitable without ocean-currents, 54 - 〃 mean temperature of, greatest in aphelion, 77, 78 - 〃 centre of gravity of, effects of ice-cap on, 370, 371 - - Eccentricity of the earth’s orbit, Mr. Stockwell’s researches - regarding, 54 - 〃 primary cause of change of climate, 54 - 〃 primary cause of glacial epochs, 77 - 〃 how it affects the winds, 228 - 〃 tables of, 314−321 - 〃 its influence on temperature, 323 - 〃 explanation of tables of, 324 - 〃 De Marian, on influence of, on climate, 528 - 〃 Sir J. F. Herschel, on influence of, on climate, 529 - 〃 Œpinus, on influence of, on climate, 529 - 〃 R. Kirwan, on influence of, on climate, 529 - 〃 of planetary orbits, superior limits as determined by Lagrange, - Leverrier, and Mr. Stockwell, 531 - 〃 Sir Charles Lyell, on influence of, on climate, 529, 535 - 〃 M. Arago, on influence of, on climate, 536 - 〃 Baron Humboldt, on influence of, on climate, 538 - 〃 Sir H. T. de la Beche, on influence of, on climate, 539 - 〃 Professor Phillips, on influence of, on climate, 539 - 〃 Mrs. Somerville, on influence of, on climate, 540 - 〃 L. W. Meech, on influence of, on climate, 540 - 〃 Mr. R. Bakewell, on influence of, on climate, 540 - 〃 M. Jean Reynaud, on influence of, on climate, 541 - 〃 M. Adhémar, on influence of, on climate, 542 - - Equator, reduction of level by denudation, 336 - - Ecliptic, supposed effect of a change of obliquity of, 8 - 〃 changes of, effects on climate, 398−417 - 〃 obliquity of, Lieutenant-Colonel Drayson on, 410 - - Emergence, physical cause of, 368 - - England, inter-glacial beds of, 249 - 〃 glacial origin of Old Red Sandstone of, 294 - 〃 ice-action during Permian period in, 298 - 〃 North of, ice-sheet of, 456 - 〃 ice-sheet of South of, 463 - - Eocene period, total absence of fossils in flysch, 286 - 〃 glacial epoch of, 305 - - Eocene and Miocene periods, date of, 357 - - Equator, heat received per square mile at, 26 - 〃 temperature of earth increased by water at, 30 - 〃 and poles, effects of stoppage of currents on temperature of, 42 - 〃 surface-currents warmer than the under currents, 92 - 〃 heat transferred by currents from southern hemisphere compared - with that received by land at, 93 - 〃 temperature soundings at, 119 - 〃 temperature of sea at, decreases most rapidly at the surface, 119 - 〃 heat received by the three zones compared with that received by - the, 194 - 〃 migration across, 234 - 〃 glaciation of, 234 - - Equatorial current, displacement of, 229 - - Erratic blocks in stratified rocks, evidence of former land-ice, 269 - 〃 in chalk, 304 - 〃 why not found in coal strata, 432 - - Erratics extend further south in America than in Europe, 72 - - Etheridge, R., jun., on glacial conglomerate in Australia of Old Red - Sandstone age, 295 - - Europe, influence of Gulf-stream on climate of, 31 - 〃 effect of deflection of Gulf-stream on condition of, 68 - 〃 glacial condition of, if Gulf-stream was stopped, 71 - 〃 river systems of, unaltered since glacial period, 393 - - - Faraday, Professor, on cause of regelation, 554 - - Faroe Islands glaciated by land-ice from Scandinavia, 450 - - Ferrel, Mr., on Dr. Carpenter’s theory, 126 - 〃 argument from the tides, 184 - - Findlay, Mr. A. G., objection by, considered, 31, 203 - 〃 estimate of heat conveyed by Gulf-stream, 206 - - Fisher, Rev. O., on the 〃trail〃 of Norwich, 251 - 〃 on glacial submergence, 387 - - Fitzroy, Admiral, on temperature of Atlantic, 36 - - Fluid molecules crystallize in interstices, 523 - - Fluvio-marine beds of Norwich, 250 - - “Flysch” of Eocene period, absence of fossils in, 286 - 〃 of Switzerland of glacial origin, 306 - - Fogs prevent the sun’s heat from melting ice and snow in arctic - regions, 60 - - Forbes, Professor J. D., method adopted by, of ascertaining - temperatures, 48 - 〃 on temperature of equator and poles, 48 - 〃 on the conductivity of different kinds of rock, 86 - 〃 on underground temperature, 86 - 〃 experiments by, on the power of different rocks to store up - heat, 86 - - Forest bed of Cromer, 250 - - Former glacial periods, 266−310 - 〃 why so little known of, 266 - 〃 geological evidence of, 292 - - France, evidence of ice-action during Carboniferous period in, 296 - - Fraserburgh, glaciation of, 450 - 〃 crossed by North Sea ice, 454 - - Fundamental problem of geology, 1 - - - Ganges, amount of sediment conveyed by, 331 - - Gases, radiation of, 38 - - Gastaldi, M., on the Miocene glacial epoch of Italy, 306 - - Geikie, Professor, on geological agencies, 1 - 〃 on inter-glacial beds of Scotland, 243 - 〃 remarks on inter-glacial beds, 245 - 〃 on striated pavements, 256 - 〃 on ice-markings on Scandinavian coast, 281 - 〃 striated stones found in carboniferous conglomerate by, 296 - 〃 on sediment of European rivers, 332 - 〃 on modern denudation, 332 - 〃 suggestion regarding the loess, 452 - 〃 on striation of Caithness, 453 - 〃 on buried channel at Chapelhall, 491 - 〃 and Mr. James, on glacial conglomerate of Lower Carboniferous - age, 296 - - Geikie, Mr. James, on Crofthead inter-glacial bed, 248 - 〃 on the gravels of Switzerland, 268 - 〃 on difficulty of recognising former glacial periods, 289 - 〃 on Cambrian conglomerate of north-west of Scotland, 293 - 〃 on ice-action in Ayrshire during Silurian period, 293 - 〃 on boulder conglomerate of Sutherland, 301 - 〃 on buried channels, 492 - - Geogr. Mittheilungen, list of papers in, relating to arctic - regions, 556 - - Geological agencies climatic, 2 - - Geological principle, nature of, 4 - - Geological climates, theories of, 6 - - Geological time, 311−359 - 〃 measurable from astronomical data, 311 - 〃 why it has been over-estimated, 325 - 〃 method of measuring, 328, 329 - 〃 Professor Ramsay on, 343 - - Geology, fundamental problem of, 1 - 〃 a dynamical science, 5 - 〃 and astronomy, supposed analogy between, 355 - - German Polar Expedition on density of polar water, 151 - 〃 list of papers relating to, 556 - - German Ocean once dry land, 479 - - Germany, Professor Ramsay on Permian breccia of, 300 - - Gibraltar current, Dr. Carpenter’s theory of, 167 - 〃 cause of, 215 - - Glacial conditions increased by reaction of various physical - causes, 75 - 〃 reach maximum when winter solstice arrives at aphelion, 77 - - Glacial epoch, date of, 327 - 〃 circumstances which show recent date of, 341 - 〃 Mr. Belt’s theory of cause of, 415 - - Glacial epochs dependent upon deflection of ocean-currents, 68 - 〃 caused primarily by eccentricity, 77 - 〃 why so little known of, formerly, 266 - 〃 boulder clays of former, why so rare, 269 - 〃 geological evidence of former, 292 - - Glacial period in America more severe than in Western Europe, 73 - 〃 mean temperature of the earth greatest at aphelion during, 78 - 〃 records of, fast disappearing, 270 - 〃 of the Eocene formation, 305 - - Glacial periods, indirect evidence of, in Eocene and Miocene - formations, 287 - 〃 difficulty of determining, from fossil remains, 289 - - Glacial submergence resulting from displacement of the earth’s centre - of gravity, 389 - - Glaciation a cause of submergence, 390 - 〃 remains of, found chiefly on land surfaces, 267 - 〃 of Scandinavia inexplicable by theory of local glaciers, 448 - - Glacier des Bois, 497 - - Glacier-motion, Canon Moseley’s theory of, 507 - 〃 Professor James Thomson’s theory of, 512 - 〃 M. Charpentier’s theory of, 513 - 〃 molecular, 516 - - Glacier-motion, present state of the question, 514 - 〃 molecular theory of, 514−527 - 〃 heat necessary to, 515 - 〃 due to force of crystallization, 523 - 〃 due chiefly to internal molecular pressure, 523 - - Glaciers, pressure exerted by, 274 - 〃 physical cause of the motion of, 495−527 - 〃 difficulties in accounting for motion of, 495 - - Glasgow, actual January temperature of, 28° above normal, 72 - - Godwin-Austen, Mr., on ice-action during the Carboniferous period in - France, 296 - 〃 on evidence of ice-action during Cretaceous period, 303 - 〃 on mass of coal found in chalk at Dover, 304 - 〃 on the flatness of the land during Coal period, 430 - - Gothland, glaciation of, 446 - - Grangemouth, buried river channel at, 468 - 〃 surface-drift of, 484 - - Gravitation, the whole work of, performed by descent of water down the - slope, 154 - 〃 of sun’s mass, 348 - 〃 insufficient to account for sun’s heat, 349, 350 - - Gravitation theory, its relation to the theory of Secular changes of - climate, 97 - 〃 three modes of determining it, 115 - 〃 mechanics of, 145 - 〃 of the Gibraltar current, 167 - 〃 inadequacy of, 191 - 〃 _crucial_ test of, 220 - 〃 of the sun’s heat, 346−355 - - Gravity, force of, impelling water from equator to poles, 119, 120 - 〃 force of, insensible at a short distance below the surface, 120 - 〃 work performed by, 150 - 〃 diagram illustrating the action of, in producing currents, 155 - 〃 amount of work performed by, due solely to _difference_ of - temperature between equatorial and polar waters, 164 - 〃 specific difference in, between water of Atlantic and - Mediterranean insufficient to produce currents, 169 - 〃 centre of, displacement, by polar ice-cap, 368 - - Greenland, summer warm if free from ice, 59 - 〃 receives as much heat in summer as England, 66 - 〃 continental ice free from clay or mud, 284 - 〃 North, warm climate during Oolitic period in, 302 - 〃 Cretaceous formation of, 305 - - Greenland, evidence of warm conditions during Miocene period in, 307 - 〃 Professor Heer cited on Miocene flora of, 308, 309 - 〃 state of, during glacial period, 259 - 〃 effect of removal of ice from, 260 - - Greenland ice-sheet, probable thickness of, 378 - 〃 invaded the American continent, 445 - - Greenland inland ice, 379 - - Gulf-stream, estimate of its volume, 24 - 〃 United States’ coast survey of, 24 - 〃 absolute amount of heat conveyed by, 25, 26 - 〃 heat conveyed by, compared with that carried by aërial - currents, 27 - 〃 heat conveyed by, compared with that received by the frigid zone - from the sun, 27 - 〃 influence on climate of Europe, 31 - 〃 efficiency of, due to the slowness of its motion, 32 - 〃 climate of Britain influenced by south-eastern portion of, 33 - 〃 heat conveyed by, compared with that derived by temperate regions - from the sun, 34 - 〃 heat of, expressed in foot-pounds of energy, 35 - 〃 mean temperature of Atlantic increased one-fourth by, 36 - 〃 the only current that can heat arctic regions, 45 - 〃 influence of, on climate of arctic regions, 45 - 〃 the compensating warm current, 46 - 〃 palæontological objections to influence of, 53 - 〃 agencies which deflect the, in glacial periods, 69 - 〃 result, if stopped, 71 - 〃 large portion of the heat derived from southern hemisphere, 94 - 〃 Lieut. Maury on propulsion of, by specific gravity, 102 - 〃 contradictory nature of, the causes supposed by Lieut. Maury for - the, 110 - 〃 higher temperature of, considered by Lieut. Maury as the real - cause of its motion, 111 - 〃 amount of heat conveyed by, not over-estimated, 197 - 〃 amount of heat conveyed by, 192 - 〃 amount of heat conveyed by, compared with that by general oceanic - circulation, 194 - 〃 heat conveyed by, compared with that received by torrid zone from - the sun, 194 - 〃 heat conveyed by, into Arctic Ocean compared with that received by - it from the sun, 195 - 〃 Capt. Nares’s observations of, 198 - 〃 Dr. Carpenter’s estimate of the thermal work of, 199 - - Gulf-stream, volume and temperature of, according to Mr. A. G. - Findlay, 203, 206 - 〃 erroneous notion regarding depth of, 207 - 〃 list of papers relating to, 556 - - - Haughton, Professor, on recent trees in arctic regions, 263 - 〃 on fragments of granite in carboniferous limestone, 296 - 〃 on coal beds of arctic regions, 298 - 〃 on _Ammonites_ of Oolitic period in arctic regions, 303 - - Hayes, Dr., on Greenland ice-sheet, 379 - - Heat received from the sun per day, 26 - 〃 received by temperate regions from the sun, 34 - 〃 radiant, absorbed by ice remains insensible, 60 - 〃 sun’s, amount of, stored up in ground, 87 - 〃 transferred from southern to northern hemisphere, 93 - 〃 internal, supposed influence of, 176 - 〃 received by the three zones compared with that received by the - equator, 194 - 〃 amount radiated from the sun, 346 - 〃 received by polar regions 11,700 years ago, 403 - 〃 necessary to glacier-motion, 515 - 〃 how transmitted through ice, 517 - - Heat-vibrations, nature of, 544 - - Heath, Mr. D. D., on glacial submergence, 387 - - Heer, Professor, on Dürnten lignite beds, 241 - 〃 on Miocene flora of Greenland, 308−310 - 〃 on Miocene flora of Spitzbergen, 309 - - Hills, ice-markings on summits of, as evidence of continental ice, 458 - - Helmholtz’s gravitation theory of sun’s heat, 348 - - Henderson, Mr. John, on inter-glacial bed at Redhall quarry, 247 - - Herschel, Sir John, on influence of eccentricity, 11 - 〃 estimate of the Gulf-stream by, 25 - 〃 on the amount of the sun’s heat, 26 - 〃 on inadequacy of specific gravity to produce ocean-currents, 116 - 〃 his objections to specific gravity not accepted, 117 - 〃 on influence of eccentricity on climate, 529 - - Home, Mr. Milne, on buried river channels, 478 - - Hooker, Sir W., on tree dug up by Capt. Belcher, 264 - - Hooker, Dr., on preponderance of ferns among coal plants, 421 - - Horne, Mr. J., on conglomerates of Isle of Man, 295 - - Hoxne, inter-glacial bed of, 241 - - Hudson’s Bay, low mean temperature of, in June, 62 - - Hull, Professor, on ice-action during Permian age in Ireland, 299 - 〃 on equable temperature of Coal period, 421 - 〃 on estuarine origin of coal measures, 424 - - Hull, buried channel at, 489 - - Humboldt, Baron, on loss of heat from radiation, 82 - 〃 on rate of growth of coal, 429 - 〃 on influence of eccentricity on climate, 538 - - Humphreys and Abbot on sediment of Mississippi, 330 - - - Ice, latent heat of, 60 - - Ice, effects of removal of, from polar regions, 64 - 〃 heat absorbed by, employed wholly in mechanical work, 60 - 〃 slope necessary for motion of continental, 375 - 〃 does not shear in the solid state, 516 - 〃 how heat is transmitted through, 517 - 〃 how it can ascend a slope, 525 - 〃 how it can excavate a rock basin, 525 - - Icebergs do not striate sea-bottom, 272 - 〃 markings made by, are soon effaced, 273 - 〃 exerting little pressure perform little work, 273 - 〃 behaviour of, when stranded, 274 - 〃 action of, on sea-bottoms, 274 - 〃 rocks ground smooth, but not striated by, 276 - 〃 stones seldom seen on, 281 - 〃 evidence of, in Miocene formation of Italy, 307 - 〃 comparative thickness of arctic and antarctic, 381 - 〃 great thickness of antarctic, 382 - - Ice-cap, effects of, on the earth’s centre of gravity, 369 - 〃 probable thickness of antarctic, 375 - 〃 evidence from icebergs as to thickness of antarctic, 383−385 - - Ice-markings, modern, observed by Sir Charles Lyell, 280 - - Ice-sheet, probable thickness of in Greenland, 380 - 〃 of north of England, 456 - - Ice-worn pebbles found on summit of Allermuir, 441 - - Iceland, lignite of Miocene age in, 308 - 〃 probably glaciated by land-ice from North Greenland, 451 - - India, evidences of glacial action of Carboniferous age in, 297 - - Indian Ocean, low temperature at bottom, 123 - - Internal heat, no influence on climate, 6 - 〃 supposed influence of, 176 - - Inter-tropical regions, greater portion of moisture falls as rain, 29 - - Inter-glacial bed at Slitrig, 243 - 〃 at Chapelhall, 244 - 〃 of Craiglockhart hill, 245 - 〃 at Kilmaurs, 248 - - Inter-glacial beds, Professor Geikie on, 243 - 〃 of Dürnten, 240 - 〃 of Scotland, 243 - 〃 of England, 249 - 〃 at Norwich, 250 - 〃 evidence of, from borings, 254 - - Inter-glacial character of cave and river deposits, 251 - - Inter-glacial climate during Old Red Sandstone period in arctic - regions, 295 - - Inter-glacial periods, 236 - 〃 reason why overlooked, 237 - 〃 of Switzerland, 239 - 〃 evidence of, from shell-beds, 252 - 〃 evidence from striated pavements of, 255 - 〃 reasons why so few vestiges remain of, 257 - 〃 in arctic regions, 258−265 - 〃 of Silurian age in arctic regions, 293 - 〃 of Carboniferous age in arctic regions, 297 - 〃 of Eocene formation in Switzerland, 306 - 〃 formation of coal during, 420 - 〃 length of, indicated by thickness of coal-seams, 428 - - Inglefield, Captain, erect trees found in Greenland by, 309 - - Ireland, on ice-action during Permian age in, 299 - - Isbister, Mr., on carboniferous limestone of arctic regions, 297 - - Islay, Cambrian conglomerate of, 292 - - Italy, glacial epoch of Miocene period in, 306 - - - Jack, Mr. R. L., on deflection of ice across England, 461 - - Jamieson, Mr. T. F., on boulder clay of Caithness, 435 - 〃 opinion that Caithness was glaciated by floating ice, 437 - 〃 on thickness of ice in the north Highlands, 439 - 〃 glaciation of headland of Fraserburgh, 450, 455 - - January temperature of Glasgow and Cumberland, difference between, 72 - - Jeffreys, Mr. Gwyn, on Swedish glacial shell beds, 253 - - Johnston, Dr. A. Keith, on coast-line of the globe, 337 - - Joule’s, Dr., experiments on the thermal effect of tension, 552 - - Judd, Mr., on boulders of Jurassic age in the Highlands, 302 - - Jukes, Mr., on warm climate of North Greenland during Oolitic period, 302 - - July, why hotter than June, 89 - - - Kane, Dr., on mean temperature of Von Rensselaer Harbour, 62 - - Karoo beds, glacial character of, 301 - 〃 evidence of subtropical during deposition of, 301 - - Kelvin, ancient bed of, 481 - - Kielsen, Mr., excursion upon Greenland ice-sheet, by, 378 - - Kilmours, inter-glacial bed at, 248 - - Kirwan, Richard, on influence of eccentricity on climate, 529 - - Kyles of Bute, southern shell bed in, 253 - - - Labrador, mean temperature of, for January, 72 - 〃 Mr. Packard on glacial phenomena of, 282 - - Lagrange, M., on eccentricity of the earth’s orbit, 54 - 〃 table of superior limits of eccentricity, 531 - - Land at equator would retain the heat at equator, 30 - 〃 radiates heat faster than water, 91 - 〃 elevation of, will not explain glacial epoch, 391 - 〃 submergence and emergence during glacial epoch, 368−397 - 〃 successive upheavals and depressions of, 391 - - Land-ice necessarily exerts enormous pressure, 274 - 〃 evidence of former, from erratic blocks on stratified - deposits, 269 - - Land-surfaces, remains of glaciation found chiefly on, 267 - 〃 (ancient) scarcity of, 268 - - Laplace, M., on obliquity of ecliptic, 398 - - Laughton, Mr., on cause of Gibraltar current, 215 - - Leith Walk, inter-glacial bed at, 246 - - Leverrier, M., on superior limit of eccentricity, 54 - 〃 on obliquity of ecliptic, 398 - 〃 table, by, of superior limits of eccentricity, 531 - 〃 formulæ, of, 312 - - Lignite beds of Dürnten, 240 - - Loess, origin of, 452 - - London, temperature of, raised 40° degrees by Gulf-stream, 43 - - Lomonds, ice-worn pebbles found on, 439 - - Lubbock, Sir J., on cave and river deposits, 252 - - Lucy, Mr. W. C., on glaciation of West Somerset, 463 - 〃 on northern derivation of drift on Cotteswold hills, 460 - - Lyell’s, Sir C., theory of the effect of distribution of land and - water, 8 - 〃 on action of river-ice, 280 - 〃 on tropical character of the fauna of the Cretaceous - formation, 305 - 〃 on warm conditions during Miocene period in Greenland, 307 - 〃 on influence of eccentricity, 324 - 〃 on sediment of Mississippi, 331 - 〃 on comparison of existing rocks with those removed, 362 - 〃 on submerged areas during Tertiary period, 392 - 〃 on change of obliquity of ecliptic, 418 - 〃 on climate best adapted for coal plants, 420 - 〃 on influence of eccentricity on climate, 529, 535 - - - Mackintosh, Mr., observations on the glaciation of Wastdale Crag, 457 - - Magellan, Straits of, temperature at midsummer, 61 - - Mahony, Mr. J. A., on Crofthead inter-glacial bed, 248 - - Mälar Lake crossed by ice, 447 - - Man, Isle of, Mr. Cumming on glacial origin of Old Red Sandstone - of, 294 - - Mars, uncertainty as to its climatic condition, 80 - 〃 objection from present condition of, 79 - - Marine denudation trifling, 337 - - Markham, Clements, on density of Gulf-stream water, 129 - 〃 on motion of icebergs in Davis’ Straits, 133 - - Martins’s, Professor Charles, objections, 79 - - Mathews, Mr., on Canon Moseley’s experiment, 499 - - Maury, Lieutenant, his estimate of the Gulf-stream, 25 - 〃 his theory examined, 95 - 〃 on temperature as a cause of difference of specific gravity, 102 - 〃 on difference of saltness as a cause of ocean-currents, 103 - 〃 discussion of his views of the causes of ocean-currents, 104 - 〃 his objection to wind theory of ocean-currents, 211 - - McClure, Captain, discovery of ancient forest in Banks’s Land, 261 - - Mecham, Lieutenant, discovery of recent trees in Prince Patrick’s - Island, 261 - - Mechanics of gravitation theory, 145 - - Mediterranean shells in glacial shell bed of Udevalla, 253 - 〃 shells in glacial beds at Greenock, 254 - - Meech, Mr., on amount of sun’s rays cut off by the atmosphere, 26 - 〃 on influence of eccentricity on climate, 540 - - Melville Island, summer temperature of, 65 - 〃 discovery of recent trees in, 262 - 〃 plants found in coal of, 298 - - Mer de Glace, Professor Tyndall’s observations on, 498 - - Meteoric theory of sun’s heat, 347 - - Method of measuring rate of denudation, 329 - - Miller, Hugh, on absence of hills in the land of the Coal period, 431 - - Migration of plants and animals, how influenced by ocean-currents, 231 - 〃 across equator, 234 - - Millichen, remarkable section of drift at, 483 - - Miocene glacial period, 286 - - Miocene period, glacial epoch of, in Italy, 306 - - Miocene, warm period of, in Greenland, 307 - - Miocene and Eocene periods, date of, 357 - - Mississippi, amount of sediment in, 330 - 〃 volume of, 330 - - Mitchell, Mr., on cause of Gulf-stream, 131 - - Molecular theory of origin of 〃Crevasses,” 521 - 〃 modification of, 523 - - Moore, Mr. J. Carrick, on ice-action of Silurian age in - Wigtownshire, 293 - - Moore, Mr. Charles, on grooved rocks in Bath district, 464 - - Morlot, M., on inter-glacial periods of Switzerland, 240 - - Moseley, Canon, experiment to determine unit of shear, 498 - 〃 on motion of glaciers, 498 - 〃 unit of shear uncertain, 504 - 〃 his theory examined, 507 - - Motion of the sea, how communicated to a great depth, 136 - - Motion in space, origin of sun’s heat, 353 - - Mühry, M., on circumpolar basin, 133, 556 - - Mundsley, freshwater beds of, 250 - - Muncke on the expansion of sea-water, 118 - - Murchison, Sir R., on southern shells at Worcester, 253 - 〃 on trees in arctic regions, 262 - 〃 on striation of islands in the Baltic, 448 - - Murphy’s, Mr., theory, 66 - - Musselburgh, section of contorted drift near, 465 - - - Nares, Captain, on low temperature of antarctic regions, 64 - 〃 discovery of great depth of warm water in North Atlantic, 198 - 〃 estimate of volume and temperature of Gulf-stream, 198 - 〃 temperature soundings by, 119, 222 - 〃 thermal condition of Southern Ocean, 225 - - Natal, boulder clay of, 300 - - Newberry, Professor, on inter-glacial peat-bed of Ohio, 249 - 〃 on boulder of quartzite found in seam of coal, 296 - - Nicholson, Dr., on Wastdale Crag, 457 - - Nicol, Professor, on inter-glacial buried channel, 244 - - Nordenskjöld, Professor, on inland ice of Greenland, 379 - - North Sea rendered shallow by drift deposits, 443 - - Northern seas probably filled with land-ice during glacial period, 438 - - Northern hemisphere, condition of, when deprived of heat from - ocean-current, 68 - - Norway, southern species in glacial shell beds, 253 - - Norwich Crag, its glacial character, 249 - - Norwich fluvio-marine beds, 250 - - Norwich inter-glacial beds, 250 - - - Obliquity of ecliptic, its effects on climate, 398−419 - 〃 change of, influence on sea-level, 403 - 〃 Lieutenant-Colonel Drayson on, 410 - 〃 Mr. Belt on change of, 415 - 〃 Sir Charles Lyell on change of, 418 - - Ocean, imperfect conception of its area, 135 - 〃 condition of, inconsistent with the gravitation theory, 136 - 〃 low temperature at bottom a result of under currents, 142 - 〃 circulation, pressure as a cause of, 187 - 〃 antiquity of, 367 - - Ocean-currents, absolute heating power of, 23 - 〃 influence of, on normal temperatures overlooked, 40 - 〃 maximum effects of, reached at equator and poles, 49 - 〃 compensatory at only one point, 49 - 〃 heating effects of, greatest at the poles, 50 - 〃 cooling effects of, greatest at equator, 50 - 〃 earth not habitable without, 51 - 〃 result of deflection into Southern Ocean, 68 - 〃 palæontological objections against influence of, 53 - 〃 deflection of, the chief cause of changes of climate, 68 - 〃 how deflected by eccentricity, 69 - 〃 deflected by trade-winds, 70 - 〃 temperature of southern hemisphere lowered by transference of - heat to northern hemisphere by, 92 - 〃 take their rise in the Southern Ocean, 92 - 〃 cause of, never specially examined by physicists, 95 - 〃 if due to specific gravity, strongest on cold hemisphere, 97 - 〃 if due to eccentricity, strongest on warm hemisphere, 97 - 〃 if due to specific gravity, act only by descent, 99 - 〃 mode by which specific gravity causes, 100, 101 - 〃 the true method of estimating the amount of heat conveyed by, 207 - 〃 due to system of winds, 212 - 〃 system of, agrees with the system of the winds, 213 - 〃 how they mutually intersect, 219 - 〃 in relation to climate, 226 - 〃 direction of, depends on direction of winds, 227 - 〃 causes which deflect, affect climate, 228 - 〃 in relation to distribution of plants and animals, 231 - 〃 effects of, on Greenland during glacial period, 260 - - Œpinus on influence of eccentricity on climate, 529 - - Ohio inter-glacial beds, 249 - - Old Red Sandstone, evidence of ice-action in conglomerate of, 294, 295 - - Oolite of Sutherlandshire, 454 - - Oolitic period, evidence of ice-action during, 301−303 - 〃 warm climate in North Greenland during, 302 - - Organic remains, absence of, in glacial conglomerate of Upper Miocene - period, 286 - - Organic life, paucity of, a characteristic of glacial periods, 287 - - Orkney Islands, glaciated by land-ice, 444 - - Osborne, Captain, remarks on recent forest trees in arctic - regions, 262, 263 - - Oudemans, Dr., on planet Mars, 80 - - Overton Quarry, inter-glacial bed in, 247 - - - Pacific Ocean, depth of, 147 - - Packard, Mr., on glacial phenomena of Labrador, 282 - - Page, Professor, on temperate climate of Coal period, 422 - 〃 on character of coal plants, 421 - 〃 on old watercourse at Hailes quarry, 490 - - Palæontological objections against influence of ocean-currents, 53 - - Palæontological evidence of last glacial period, 285 - - Parry, Captain, discovery of recent trees in Melville Island by, 262 - - Peach, Mr. C. W., on inter-glacial bed at Leith Walk, 246 - 〃 on boulder clay of Caithness, 436 - 〃 on striated rock surfaces in Cornwall, 464 - - Peach, Mr. B. N., on striation of Caithness, 453 - - Pengelly, Mr. W., on raised beaches, 407 - - Perigee, nearness of sun in, cause of snow and ice, 74 - - Perihelion, warm conditions at maximum when winter solstice is at, 77 - - Permian period, evidence of ice-action in, 298−303 - - Perthshire hills, ice-worn surfaces at elevations of 2,200 feet on - the, 440 - - Petermann, Dr. A., on Dr. Carpenter’s theory, 138 - 〃 on thermal condition of the sea, 138 - 〃 chart of Gulf-stream and Polar current, 219 - 〃 _Geogr. Mittheilungen_ of, list of papers in relation to arctic - regions, 556 - - Phillips, Professor, on influence of eccentricity on climate, 539 - - Poisson’s theory of hot and cold parts of space, 7 - - Polar regions, effect of removal of ice from, 64 - 〃 influence of ice on climate, 64 - 〃 low summer temperature of, 66 - - Polar cold considered by Dr. Carpenter the _primum mobile_ of - ocean-currents, 173 - 〃 confusion of ideas regarding its influence, 180 - 〃 influence of, according to Dr. Carpenter, 180 - - Polar ice-cap, displacement of the earth’s centre of gravity by, 368 - - Port Bowen, mean temperature of, 63 - - Portobello, striated pavements near, 255, 256 - - Post-tertiary formations, hypothetical thickness of, 366 - - Pouillet, M., on the amount of the sun’s heat, 26 - 〃 on amount of sun’s rays cut off by the atmosphere, 26 - - Pratt, Archdeacon, on glacial submergence, 387 - - Prestwich, Professor, on Hoxne inter-glacial bed, 241 - - Pressure as a cause of circulation, 187 - - Principles of geology, nature of, 4 - - Prince Patrick’s Island, discovery of recent tree in, 261 - - - Radiation, rate of, increases with increase of temperature, 37 - 〃 of gases, 38 - 〃 the way by which the earth loses heat, 39 - 〃 how affected by snow covering the ground, 58 - 〃 how affected by humid air, 59 - 〃 accelerated by increased formation of snow and ice, 75 - - Raised beaches, date of, 407 - 〃 Mr. Pengelly on, 407 - - Ramsay, Professor, on glacial origin of Old Red Sandstone of North - of England, 294 - 〃 on Old Red Sandstone, 367 - 〃 on geological time, 343 - 〃 on ice-action during Permian period, 298 - 〃 on boulders of Permian age in Natal, 301 - 〃 on thickness of stratified rocks of Britain, 267, 361 - - Redhall Quarry, inter-glacial bed in, 247 - - Red Sea, why almost rainless, 30 - - Regelation, _rationale_ of, 520, 554 - 〃 Professor James Thomson on cause of, 554 - 〃 Professor Faraday on cause of, 554 - - Regnault, M., on specific heat of sandstone, 86 - - Reynaud, Jean, on influence of eccentricity on climate, 541 - - Rhine, ancient, bed in German Ocean, 480 - - Ridge between Capes Trafalgar and Spartel, influence of, 167 - - Rink, Dr., on inland ice of Greenland, 380 - - River-ice, effect of, 279 - - River-ice does not produce striations, 279 - - River systems, carrying-power measure of denudation, 336 - - River valleys, how striated across, 525 - - Robertson, Mr. David, on Crofthead and Hillhead inter-glacial - beds, 247, 248 - 〃 on foraminifera in red clay, 485 - - Rock-basins, how excavated by ice, 525 - - Rocks removed by denudation, 361 - - Ross, Capt. Sir James, on South Shetland, 61 - 〃 on temperature of antarctic regions in summer, 63 - - - Sandwich Land, description by Capt. Cook, 60 - 〃 cold summers of, not due to latitude, 64 - - Salter, Mr., on carboniferous fossils of arctic regions, 298 - 〃 on warm climate of North Greenland during Oolitic period, 302 - - Saltness of the ocean, difference of, as a cause of motion, 103 - 〃 in direct opposition to temperature in producing - ocean-currents, 104 - - Scandinavian ice, track of, 447 - - Scandinavian ice-sheet in the North Sea, 444 - - Scoresby, Dr., on condition of arctic regions in summer, 58, 62 - 〃 on density of Gulf-stream water, 129 - - Scotland, inter-glacial beds of, 243−249 - 〃 evidence of ice-action in carboniferous conglomerate of, 296 - 〃 buried under ice, 439 - 〃 ice-sheet of, in North Sea, 442 - 〃 why ice-sheet was so thick, 452 - - Sea, height of, at equator above poles, 119 - 〃 rise of, due to combined effect of eccentricity and obliquity, 403 - 〃 bottoms not striated by icebergs, 272 - - Sea and land, present arrangement indispensable to life, 52 - - Sea-level, oscillations of, in relation to distribution, 394 - 〃 oscillations of, during formation of coal measures, 424 - 〃 raised, by melting of antarctic ice-cap, 388 - 〃 influence of obliquity of ecliptic on, 403 - - Section of Mid-Atlantic, 222 - - Section across antarctic ice-cap, 377 - - Sedimentary rocks existing fragmentary, 361 - 〃 of the globe, mean thickness of, hitherto unknown, 361 - 〃 how mean thickness might be determined, 362 - 〃 mean thickness of, over-estimated, 364 - - Shearing-force of ice, 496 - 〃 momentary loss of, 518 - - Shetland islands glaciated by land-ice from Scandinavia, 450 - - Shetland, South, glacial condition of, 61 - - Shell-beds, evidence of warm inter-glacial periods from, 252 - - Shells of the boulder clay of Caithness, 450 - - Shore-ice, striations produced by, in Bay of Fundy, 280 - - Silurian period, ice-action in Ayrshire during, 293 - 〃 evidence in Wigtownshire of ice-action during, 293 - - Slitrig, inter-glacial bed of, 243 - - Slope of surface of maximum density has no power to produce - motion, 120 - 〃 from equator to pole, erroneous view regarding, 120 - - Smith, Mr. Leigh, temperature soundings, 129 - - Smith, Mr., of Jordanhill, on striated pavements, 256 - - Snow, how radiation is affected by, 58 - 〃 common in summer in arctic regions, 62 - 〃 rate of accumulation of, increased by sun’s rays being cut off by - fogs, 75 - 〃 formation increased by radiation, 75 - - Somerset, West, glaciation of, 463 - - Somerville, Mrs., on influence of eccentricity on climate, 540 - - South Africa, glaciation of, 242 - 〃 boulder clay of Permian age in, 300 - - South of England ice-sheet, 463 - - South Shetland, glacial condition of, at mid summer, 61 - - South-west winds, heat conveyed by, not derived from equatorial - regions, 28 - 〃 heat conveyed by, derived from Gulf-stream, 28 - - Southern hemisphere, present extension of ice on, due partly to - eccentricity, 78 - 〃 why colder than northern, 81−92 - 〃 absorbs more heat than the northern, 90 - 〃 lower temperature of, due to ocean-currents, 92 - 〃 surface currents from, warmer than under currents to, 92 - 〃 glacial and inter-glacial periods of, 242 - - Southern Ocean, thermal condition of, 225 - - Specific gravity can act only by causing water to descend a slope, 99 - 〃 mode of action in causing ocean-currents, 100 - 〃 inadequacy of, to produce ocean-currents demonstrated by Sir John - Herschel, 116 - - Spitzbergen, Gulf-stream and under current at, 134 - 〃 Miocene flora of, 309 - - Stellar space, temperature of, 35 - 〃 received temperature of, probably too high, 39 - - Stewart, Professor Balfour, experiment on radiation, 37 - 〃 on cause of glacial cold, 79 - - Stirling, Mr., on old watercourse near Grangemouth, 481 - - St. John’s River, action of ice on banks of, 279 - - St. Lawrence, action of ice on bank of river, 279 - - Stockwell, Mr., on eccentricity of earth’s orbit, 54 - 〃 on obliquity of ecliptic, 399 - 〃 table of superior limits of eccentricity, 531 - - Stone, Mr., on eccentricity of the earth’s orbit, 322 - - Stow, G. W., on glacial beds of South Africa, 242 - 〃 on Karoo beds, 301 - - Striæ, direction of, show the clay of Caithness came from the sea, 436 - - Striations obliterated rather than produced by icebergs, 274 - - Striated pavements why so seldom observed, 256 - 〃 evidence of inter-glacial periods from, 255 - - Striated stones found in conglomerate of Lower Carboniferous age by - Professor Geikie, 296 - 〃 in Permian breccias, 299 - 〃 in the glacial conglomerate of the Superga, Turin, 306 - - Stratified rocks may be formed at all possible rates, 360 - 〃 rate of formation of, as estimated by Professor Huxley, 363 - - Struve, M., formula of obliquity of ecliptic, 404 - - Subaërial denudation, rate of, 331 - - Submarine forests, 409 - 〃 (ancient), coal seams the remains of, 428 - - Submergence, physical causes of, 368 - 〃 coincident with glaciation, 389 - 〃 of land resulting from melting of antarctic ice-cap, 389 - 〃 how affected by fluidity of interior of the earth, 395 - 〃 necessary for preservation of coal plants, 423 - 〃 frequent during formation of coal beds, 426 - - Subsidence insufficient to account for general submergence, 390 - 〃 necessary to accumulation of coal seams, 427 - - Sun supposed by some to be a variable star, 8 - 〃 maximum and minimum distance of, 55 - 〃 rays of, cut off by fogs in ice-covered regions, 60 - 〃 nearness in perigee a cause of snow and ice, 74 - 〃 total amount of heat radiated from, 346 - 〃 age and origin of, 346 - 〃 source of its energy, 347 - 〃 heat of, origin and chief source of, 349 - 〃 originally an incandescent mass, 350 - 〃 energy of, may have originally been derived from motion in - space, 355 - - Surface currents which cross the equator warmer than the compensatory - under currents, 92 - - Surface currents from poles to equator, according to Maury, produced - by saltness, 108 - - Sutherland, Dr., observations by, on stranding of icebergs, 275 - 〃 testimony, that icebergs do not striate rocks, 278 - 〃 on the boulder clay of Natal, 300 - - Sutherland, boulder conglomerate of Oolitic period of, 302 - - Sweden, Southern, shells in glacial shell beds of, 253 - - Switzerland, inter-glacial period of, 239 - 〃 M. Morlat on inter-glacial periods of, 240 - 〃 gravels of, by Mr. James Geikie, 268 - 〃 Eocene glacial epoch in, 305 - - - Table of June temperatures in different latitudes, 65 - 〃 soundings in temperate regions, 222 - - Tables of eccentricity, 314−321 - 〃 of eccentricity, explanation of, 322 - - Tay, valley of, striated across, 526 - 〃 ancient buried channel of, 490 - - Temperate regions, cold periods best marked in, 258 - - Temperature of space, 532 - 〃 reasons why it should be reconsidered, 39 - - Temperature (mean) of equator and poles compared, 41 - 〃 why so low in polar regions during summer, 66 - 〃 how difference of specific gravity is caused by, 102 - 〃 higher, of the waters of Gulf-stream considered by Lieutenant - Maury as the real causes of its motion, 111 - 〃 of sea at equator decreases most rapidly at the surface, 119 - 〃 of Greenland in Miocene period, 310 - 〃 of poles when obliquity was at its superior limit, 402 - - Tension, effect of, on ice, 522 - 〃 the cause of the cooling effect produced by, 552 - - Tertiary period, climate of, error in regard to, 288 - - Thermal condition of Southern Ocean, 225 - - Thibet, table-land of, 418 - - Thomson, Professor James, on cause of regelation, 554 - 〃 theory of glacier-motion, 512 - - Thomson, Mr. James, on glacial conglomerate in Arran, 299 - 〃 on ice-action in Cambrian conglomerate of Islay, 292 - - Thomson, Professor Wyville, on Dr. Carpenter’s theory, 129 - 〃 cited, 130 - 〃 thermal condition of the sea, 138 - - Thomson, Sir W., amount of internal heat passing through earth’s - crust, 142 - 〃 on limit to age of the globe, 343 - 〃 on influence of ice-cap on sea-level, 372 - 〃 climate not affected by internal heat, 6 - 〃 earth’s axis of rotation permanent, 7 - 〃 on volume and mass of the sun, 347 - - Tidal wave, effect of friction, 336 - - Tides, supposed argument from, 184 - - Time, geological, 311−359 - 〃 as represented by geological phenomena, 326 - 〃 represented by existing rocks, 361 - - Torrid zone, annual quantity of heat received by, per unit of - surface, 194 - - Towncroft farm, section of channel at, 474 - - Towson, Mr., on icebergs of Southern Ocean, 383 - - Trade-winds (anti), heat conveyed by, over-estimated, 28 - 〃 (anti) derive their heat from the Gulf-stream, 32 - 〃 of warm hemisphere overborne by those of cold hemisphere, 70 - 〃 causes which determine the strength of, 70 - 〃 strongest on glaciated hemisphere, 70 - 〃 reaction upon trade-winds by formation of snow and ice, 76 - 〃 influence of, in turning ocean-currents on warm hemisphere, 97 - 〃 do not explain the antarctic current, 211 - - Tiddeman on North of England ice-sheet, 458 - 〃 displacement of, 230 - - Transport of boulders and rubbish the proper function of icebergs, 281 - - Trafalgar, effect of ridge between Capes Spartel and on Gibraltar - current, 167 - - Turner, Professor, on arctic seal found at Grangemouth, 485 - - Tylor, Alfred, on denudation of Mississippi basin, 333 - - Tyndall, Professor, on heat in aqueous vapour, 29 - 〃 on sifted rays, 47 - 〃 on diathermancy of air, 59 - 〃 on glacial epoch, 78 - - - Udevalla, Mediterranean shell in glacial shells, bed of, 253 - - Under currents to southern hemisphere colder than surface currents - from, 92 - 〃 produced by saltness, flow from equator to poles, 106 - 〃 account for cold water at equator, 124, 142 - 〃 in Davis’ Strait, 134 - 〃 take path of least resistance, 130 - 〃 why considered improbable, 135 - 〃 difficulty regarding, obviated, 217 - 〃 theory of, 217 - - Underground temperature, Professor J. D. Forbes on, 86 - - Underground temperature exerts no influence on the climate, 88 - 〃 absolute amount of heat derived from, 142 - 〃 supposed influence of, 176 - - Uniformity, modern doctrine of, 325 - - United States’ coast survey of Gulf-stream, 24 - 〃 hydrographic department, papers published by, 556 - - Unstratified boulder clay must be the product of land-ice, 437 - - Upsala and Stockholm striated by Baltic glacier, 447 - - - Vertical circulation, Lieutenant Maury’s theory of, 108 - 〃 according to Dr. Carpenter, 153 - - Vertical descent of polar column caused by extra pressure of water - upon it, 154 - 〃 effects of, and slope, the same, whether performed simultaneously - or alternately, 159 - 〃 of polar column illustrated by diagram, 160 - - Vertical distribution of heat in the ocean, Mr. Buchanan’s theory, 550 - - Vogt, Professor, on Dürnten lignite bed, 241 - - - Warm hemisphere made warmer by increased reaction of physical - causes, 76 - - Warm periods best marked in arctic regions, 258 - 〃 in arctic regions, evidence of, 261 - 〃 better represented by fossils than cold periods, 288 - 〃 evidence of, during Cretaceous age, 304 - - Warm inter-glacial periods in arctic regions, 258−265 - - Water at equator the best means of distributing heat derived from the - sun, 30 - - Water, a worse radiator than land, 91 - - Wastdale granite boulders, difficulty of accounting for transport - of, 456 - - Wastdale Crag glaciated by continental ice, 457 - - Weibye, M., striation observed by, 280 - - Wilkes, Lieutenant, on cold experienced in antarctic regions in - summer, 63 - - Wellington Sound, ancient trees found at, 265 - - Winter-drift of ice on coast of Labrador, 276 - - West winds, moisture of, derived from Gulf-stream, 29 - - Wind, work in impelling currents, 219 - - Winds, ocean-currents produced by, 212 - 〃 system of, agrees with the system of ocean-currents, 213 - - Wind theory of oceanic circulation, 210 - 〃 crucial test of, 220 - - Wigtownshire, ice-action during Silurian age, 293 - - Work performed by descent of polar column, 157 - - Wood, Mr. Nicholas, on buried channel, 488 - - Wood, Jun., Mr. Searles, middle drift, 250 - 〃 on occurrence of chalk _débris_ in south-west of England, 460 - - Woodward, Mr. H. B., on boulder clay in Devonshire, 463 - - Wunsch, Mr. E. A., on glacial conglomerate in Arran, 299 - - - Yare, ancient buried channel of, 489 - - Young, Mr. J., objection considered, 482 - - Yorkshire drift common in south of England, 460 - - - Zenger, Professor, on the moon’s influence on climate, 324 - - - THE END. - - PRINTED BY VIRTUE AND CO., CITY ROAD, LONDON. - - - - - THE GREAT ICE AGE, - AND ITS RELATION TO THE ANTIQUITY OF MAN. - - By JAMES GEIKIE, F.R.S.E., F.G.S., &c., of H.M. Geological Survey. - - With Maps, Charts, and numerous illustrations. Demy 8vo, 24s. - -“There is a great charm in the well-balanced union of cultivated powers -of observation and analytical method, with considerable imagination -and much poetical feeling, which runs through the pages of this -volume.... 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Geol. | -| Soc., vol. viii. | -| | -| [6] See Chap. xxv. | -| | -| [7] See Chap. iv. | -| | -| [8] “Treatise on Astronomy,” § 315; “Outlines,” § 368. | -| | -| [9] _Annuaire_ for 1834, p. 199. Edin. New Phil. Journ., April, | -| 1834, p. 224. | -| | -| [10] “Cosmos,” vol. iv. p. 459 (Bohn’s Edition). “Physical | -| Description of the Heavens,” p. 336. | -| | -| [11] Phil. Mag. for February, 1867, p. 127. | -| | -| [12] The Gulf-stream at the narrowest place examined by the Coast | -| Survey, and where also its velocity was greatest, was found to be | -| over 30 statute miles broad and 1,950 feet deep. But we must not | -| suppose that this represents all the warm water which is received | -| by the Atlantic from the equator; a great mass flows into the | -| Atlantic without passing through the Straits of Florida. | -| | -| [13] It is probable that a large proportion of the water | -| constituting the south-eastern branch of the Gulf-stream is never | -| cooled down to 40°; but, on the other hand, the north-eastern | -| branch, which passes into the arctic regions, will be cooled far | -| below 40°, probably below 30°. Hence I cannot be over-estimating | -| the extent to which the water of the Gulf-stream is cooled down in | -| fixing upon 40° as the average minimum temperature. | -| | -| [14] “Physical Geography of the Sea,” § 24, 6th edition. | -| | -| [15] “Physical Geography,” § 54. | -| | -| [16] Trans. of Roy. Soc. of Edin., vol. xxi., p. 57. Phil. Mag., § | -| 4, vol. ix., p. 36. | -| | -| [17] “Smithsonian Contributions to Knowledge,” vol. ix. | -| | -| [18] “Heat as a Mode of Motion,” art. 240. | -| | -| [19] Trans. Roy. Soc. of Edin., vol. xxv., part 2. | -| | -| [20] See “Smithsonian Contributions to Knowledge,” vol. ix. | -| | -| [21] “Meteorology,” section 36. | -| | -| [22] _Comptes-Rendus_, July 9, 1838. Taylor’s “Scientific Memoirs,” | -| vol. iv., p. 44 (1846). | -| | -| [23] The mean temperature of the Atlantic between the tropics and | -| the arctic circle, according to Admiral Fitzroy’s chart, is about | -| 60°. But he assigns far too high a temperature for latitudes above | -| 50°. It is probable that 56° is not far from the truth. | -| | -| [24] The probable physical cause of this will be considered in the | -| Appendix. | -| | -| [25] The mean temperature of the equator, according to Dove, is | -| 79°·7, and that of the north pole 2°·3. But as there is, of course, | -| some uncertainty regarding the actual mean temperature of the | -| poles, we may take the difference in round numbers at 80°. | -| | -| [26] Trans. of Roy. Soc. Edin., vol. xxii., p. 75. | -| | -| [27] _Connaissance des Temps_ for 1863 (Additions). Lagrange’s | -| determination makes the superior limit 0·07641 (Memoirs of the | -| Berlin Academy for 1782, p. 273). Recently the laborious task of | -| re-investigating the whole subject of the secular variations of the | -| elements of the planetary orbits was undertaken by Mr. Stockwell, | -| of the United States. He has taken into account the disturbing | -| influence of the planet Neptune, the existence of which was not | -| known when Leverrier’s computations were made; and he finds that | -| the eccentricity of the earth’s orbit will always be included | -| within the limits of 0 and 0·0693888. Mr. Stockwell’s elaborate | -| Memoir, extending over no fewer than two hundred pages, will be | -| found in the eighteenth volume of the “Smithsonian Contributions to | -| Knowledge.” | -| | -| [28] When the eccentricity is at its superior limit, the absolute | -| quantity of heat received by the earth during the year is, however, | -| about one three-hundredth part greater than at present. But this | -| does not affect the question at issue. | -| | -| [29] Scoresby’s “Arctic Regions,” vol. ii., p. 379. Daniell’s | -| “Meteorology,” vol. ii., p. 123. | -| | -| [30] Tyndall, “On Heat,” article 364. | -| | -| [31] Tyndall, “On Heat,” article 364. | -| | -| [32] See Phil. Mag., March, 1870, p. | -| | -| [33] Captain Cook’s “Second Voyage,” vol. ii., pp. 232, 235. | -| | -| [34] “Antarctic Regions,” vol. ii., pp. 345−349. | -| | -| [35] Ibid., vol. i., p. 167. | -| | -| [36] Ibid., vol. ii., p. 362. | -| | -| [37] Edinburgh Philosophical Journal, vol. iv., p. 266. | -| | -| [38] Scoresby’s “Arctic Regions,” vol. i., p. 378. | -| | -| [39] Ibid., p. 425. | -| | -| [40] See Meech’s memoir “On the Intensity of the Sun’s Heat and | -| Light,” “Smithsonian Contributions,” vol. ix. | -| | -| [41] “Antarctic Regions,” vol. i., p. 240. | -| | -| [42] _Challenger_ Reports, No. 2, p. 10. | -| | -| [43] See “Smithsonian Contributions,” vol. ix. | -| | -| [44] Quart. Journ. Geol. Soc., vol. xxv., p. 350. | -| | -| [45] Trans. of Glasgow Geol. Soc. for 1866. | -| | -| [46] _Revue des Deux Mondes_ for 1867. | -| | -| [47] Letter to the author, February, 1870. | -| | -| [48] “Révolutions de la Mer,” p. 37 (second edition). | -| | -| [49] Edin. Phil. Journ., vol. iv., p. 262 (1821). | -| | -| [50] Phil. Mag., § 4, vol. xxviii., p. 131. _Reader_, December 2nd, | -| 1865. | -| | -| [51] This point will be found discussed at considerable length in | -| the Phil. Mag. for September, 1869. | -| | -| [52] See Phil. Mag. for October, 1870, p. 259. | -| | -| [53] Proceedings of the Royal Society, No. 138, p. 596, foot-note. | -| | -| [54] The edition from which I quote, unless the contrary is stated, | -| is the one published by Messrs. T. Nelson and Sons, 1870, which is | -| a reprint of the new edition published in 1859 by Messrs. Sampson | -| Low and Co. | -| | -| [55] “Physical Geography,” article 57. | -| | -| [56] Philosophical Magazine, vol. xii. p. 1 (1838). | -| | -| [57] “Mémoires par divers Savans,” tom. i., p. 318, St. | -| Petersburgh, 1831. See also twelfth number of Meteorological | -| Papers, published by the Board of Trade, 1865, p. 16. | -| | -| [58] Dubuat’s “Hydraulique,” tom. i., p. 64 (1816). See also | -| British Association Report for 1834, pp. 422, 451. | -| | -| [59] See Proceedings of the Royal Society for December, 1868, | -| November, 1869. Lecture delivered at the Royal Institute, _Nature_, | -| vol. i., p. 490. Proceedings of the Royal Geographical Society, | -| vol. xv. | -| | -| [60] Trans. of Glasgow Geol. Soc. for April, 1867. Phil. Mag. for | -| February, 1867, and June, 1867 (Supplement). | -| | -| [61] Phil. Mag. for February, 1870. | -| | -| [62] “The Depths of the Sea,” pp. 376 and 377. | -| | -| [63] “The Threshold of the Unknown Region,” p. 95. | -| | -| [64] See “Physical Geography of the Sea,” chap. ix., new edition, | -| and Dr. A. Mühry “On Ocean-currents in the Circumpolar Basin of the | -| North Hemisphere.” | -| | -| [65] “Depths of the Sea,” _Nature_ for July 28, 1870. | -| | -| [66] “Memoir on the Gulf-stream,” _Geographische Mittheilungen_, | -| vol. xvi. (1870). | -| | -| [67] Dr. Carpenter “On the Gulf-stream,” Proceedings of Royal | -| Geographical Society for January 9, 1871, § 29. | -| | -| [68] Dr. Petermann’s _Mittheilungen_ for 1872, p. 315. | -| | -| [69] Proceedings of the Royal Society, vol. xvii., p. 187, xviii., | -| p. 463. | -| | -| [70] The average depth of the Pacific Ocean, as found by the | -| soundings of Captain Belknap, of the U.S. steamer _Tuscarora_, made | -| during January and February, 1874, is about 2,400 fathoms. The | -| depth of the Atlantic is somewhat less. | -| | -| [71] Proceedings of Royal Geographical Society, vol. xv., § 22. | -| | -| [72] It is a well-established fact that in polar regions the | -| temperature of the sea decreases from the surface downwards; | -| and the German Polar Expedition found that the water in very | -| high latitudes is actually less dense at the surface than at | -| considerable depths, thus proving that the surface-water could not | -| sink in consequence of its greater density. | -| | -| [73] Proceedings of the Royal Society, vol. xix., p. 215. | -| | -| [74] _Nature_ for July 6, 1871. | -| | -| [75] Since the above objection to the Gravitation Theory of the | -| Gibraltar Current was advanced three years ago, Dr. Carpenter | -| appears to have abandoned the theory to a great extent. He now | -| admits (Proceedings of Royal Geographical Society, vol. xviii., | -| pp. 319−334, 1874) that the current is almost wholly due not to | -| difference of specific gravity, but to an excess of evaporation in | -| the Mediterranean over the return by rain and rivers. | -| | -| [76] Proceedings of Royal Society, No. 138, § 26. | -| | -| [77] Proceedings of Royal Geographical Society, January 9, 1871. | -| | -| [78] Ibid. | -| | -| [79] See §§ 20, 34; also Brit. Assoc. Report for 1872, p. 49, and | -| other places. | -| | -| [80] See also to the same effect Brit. Assoc. Report, 1872, p. 50. | -| | -| [81] Phil. Mag. for Oct. 1871. | -| | -| [82] The actual slope, however, does not amount to more than 1 in | -| 7,000,000. | -| | -| [83] Proc. of Roy. Geog. Soc., January 9, 1871, § 29. | -| | -| [84] Trans. of Geol. Soc. of Glasgow for April, 1867; Phil. Mag. | -| for June, 1867. | -| | -| [85] _Nature_, vol. i., p. 541. Proc. Roy. Soc., vol. xviii., p. | -| 473. | -| | -| [86] Chapter II. | -| | -| [87] Chapter II. | -| | -| [88] Chapter II. | -| | -| [89] Mr. Findlay considers that the daily discharge does not exceed | -| 333 cubic miles (Brit. Assoc. Rep., 1869, p. 160). My estimate | -| makes it 378 cubic miles. Mr. Laughton’s estimate is 630 cubic | -| miles (Paper “On Ocean-currents,” Journal of Royal United-Service | -| Institution, vol. xv.). | -| | -| [90] Proceedings of the Royal Geographical Society, vol. xviii., p. | -| 393. | -| | -| [91] Phil. Mag. for October, 1871, p. 274. | -| | -| [92] Proceedings of the Royal Geographical Society, vol. xv. | -| | -| [93] Phil. Mag., February, 1870. | -| | -| [94] Brit. Assoc. Report, 1869, Sections, p. 160. | -| | -| [95] Journal of Royal United-Service Institute, vol. xv. | -| | -| [96] Dr. Carpenter (Proc. of Roy. Geog. Soc., vol. xviii., p. | -| 334) misapprehends me in supposing that I attribute the Gibraltar | -| current wholly to the Gulf-stream. In the very page from which he | -| derives or could derive his opinion as to my views on the subject | -| (Phil. Mag. for March, 1874, p. 182), I distinctly state that | -| “the excess of evaporation over that of precipitation within the | -| Mediterranean area would of itself produce a considerable current | -| through the Strait.” That the Gibraltar current is due to two | -| causes, (1) the pressure of the Gulf-stream, and (2) excess of | -| evaporation over precipitation in the Mediterranean, has always | -| appeared to me so perfectly obvious, that I never held nor could | -| have held any other opinion on the subject. | -| | -| [97] Paper read to the Edinburgh Botanical Society on January 8, | -| 1874. | -| | -| [98] Proc. Roy. Geog. Soc., vol. xviii., p. 362. A more | -| advantageous section might have been chosen, but this will suffice. | -| The section referred to is shown in Plate III. The peculiarity of | -| this section, as will be observed, is the thinness of the warm | -| strata at the equator, as compared with that of the heated water in | -| the North Atlantic. | -| | -| [99] The temperature of column C in Dr. Carpenter’s section is | -| somewhat less than that given in the foregoing table; so that, | -| according to that section, the difference of level between column C | -| and columns A and B would be greater than my estimate. | -| | -| [100] Captain Nares’s Report, July 30, 1874. | -| | -| [101] See Chapter IV. | -| | -| [102] Phil. Mag. for August, 1864, February, 1867, March, 1870; see | -| Chap. IV. | -| | -| [103] Quarterly Journal of Science for October, 1874. | -| | -| [104] See a paper by M. Morlot, on “The Post-Tertiary and | -| Quaternary Formations of Switzerland.” Edin. New Phil. Journal, New | -| Series, vol. ii., 1855. | -| | -| [105] Edin. New Phil. Journ., New Series, vol. ii., p. 28. | -| | -| [106] Vogt’s “Lectures on Man,” pp. 318−321. | -| | -| [107] See Mr. Prestwich on Flint Implements, Phil. Trans. for 1860 | -| and 1864. Lyell’s “Antiquity of Man,” Second Edition, p. 168. | -| | -| [108] Edin. New Phil. Journ., New Series, vol. ii., p. 28. | -| Silliman’s Journ., vol. xlvii., p. 259 (1844). | -| | -| [109] Quart. Journ. Geol. Soc., vol. xxvii., p. 534. | -| | -| [110] Ibid., vol. xxviii., p. 17. | -| | -| [111] “Glacial Drift of Scotland,” p. 54. | -| | -| [112] “Glacial Drift of Scotland,” p. 58. | -| | -| [113] Quart. Journ. Geol. Soc., vol. v., p. 22. | -| | -| [114] “Glacial Drift of Scotland,” p. 64. | -| | -| [115] Trans. Edin. Geol. Soc., vol. ii., p. 391. | -| | -| [116] Trans. of Geol. Soc. of Glasgow, vol. iv., p. 146. | -| | -| [117] Geol. Mag., vi., p. 391. | -| | -| [118] See “Memoirs of Geological Survey of Scotland,” Explanation | -| of sheet 22, p. 29. See also Trans. Glasgow Geol. Soc., iv., p. 150. | -| | -| [119] “Great Ice Age,” p. 374. | -| | -| [120] “Great Ice Age,” p. 384. | -| | -| [121] “Geological Survey of Ohio, 1869,” p. 165. See also “Great | -| Ice Age,” chap. xxviii. | -| | -| [122] Quart. Journ. Geol. Soc., xxviii., p. 435. | -| | -| [123] Brit. Assoc. Report, 1863. | -| | -| [124] Trans. Glasgow Nat. Hist. Soc., vol. i., p. 115. | -| | -| [125] Trans. of the Geol. Soc. of Glasgow, vol. iii., p. 133. See | -| also “Great Ice Age,” chaps. xii. and xiii. | -| | -| [126] Chap. XXIX. | -| | -| [127] Edin. New Phil. Journ., vol. liv., p. 272. | -| | -| [128] “Newer Pliocene Geology,” p. 129. John Gray & Co., Glasgow. | -| | -| [129] “Glacial Drift of Scotland,” p. 67. | -| | -| [130] “Glacial Drift of Scotland,” p. 12. | -| | -| [131] See Chapter IV. | -| | -| [132] “Discovery of the North-West Passage,” p. 213. | -| | -| [133] “Voyage of the _Resolute_,” p. 294. | -| | -| [134] Quart. Journ. Geol. Soc., vol. xi., p. 540. | -| | -| [135] “McClure’s North-West Passage,” p. 214. Second Edition. | -| | -| [136] “British Association Report for 1855,” p. 381. “The Last of | -| the Arctic Voyages,” vol. i., p. 381. | -| | -| [137] Mr. James Geikie informs me that the great accumulations of | -| gravel which occur so abundantly in the low grounds of Switzerland, | -| and which are, undoubtedly, merely the re-arranged materials | -| originally brought down from the Alps as till and as moraines | -| by the glaciers during the glacial epoch, rarely or never yield | -| a single scratched or glaciated stone. The action of the rivers | -| escaping from the melting ice has succeeded in obliterating all | -| trace of striæ. It is the same, he says, with the heaps of gravel | -| and sand in the lower grounds of Sweden and Norway, Scotland and | -| Ireland. These deposits are evidently in the first place merely the | -| materials carried down by the swollen rivers that issued from the | -| gradually melting ice-fields and glaciers. The stones of the gravel | -| derived from the demolition of moraines and till, have lost all | -| their striæ and become in most cases well water-worn and rounded. | -| | -| [138] Report on Icebergs, read before the Association of American | -| Geologists, _Silliman’s Journal_, vol. xliii., p. 163 (1842). | -| | -| [139] “Manual of Geology,” p. 677. | -| | -| [140] Quart. Journ. Geol. Soc., vol. ix., p. 306. | -| | -| [141] Dana’s “Manual of Geology,” p. 677. | -| | -| [142] Quart. Journ. Geol. Soc., vol. ix., p. 306. | -| | -| [143] “Journal,” vol. i., p. 38. | -| | -| [144] “Short American Tramp,” pp. 168, 174. | -| | -| [145] “Short American Tramp,” pp. 239−241. | -| | -| [146] “Travels in North America,” vol. ii., p. 137. | -| | -| [147] Ibid., vol. ii., p. 174. | -| | -| [148] Proceedings of the Royal Society of Edinburgh, Session | -| 1865−66, p. 537. | -| | -| [149] “Short American Tramp,” pp. 77, 81, 111. | -| | -| [150] “Second Visit,” vol. ii., p. 367. | -| | -| [151] “Memoirs of Boston Society of Natural History,” vol. i. | -| (1867), p. 228. | -| | -| [152] “Antiquity of Man,” p. 268. Third Edition. | -| | -| [153] “Great Ice Age,” p. 512. | -| | -| [154] Brit. Assoc., 1870, p. 88. | -| | -| [155] Quart. Journ. Geol. Soc., vol. v., p. 10. Phil. Mag. for | -| April, 1865, p. 289. | -| | -| [156] “Great Ice Age,” p. 512. | -| | -| [157] Jukes’ “Manual of Geology,” p. 421. | -| | -| [158] See also Quarterly Journal Geological Society, vol. xi., p. | -| 510. | -| | -| [159] The _Reader_ for August 12, 1865. | -| | -| [160] “History of the Isle of Man,” p. 86. My colleague, Mr. John | -| Horne, in his “Sketch of the Geology of the Isle of Man,” Trans. of | -| Edin. Geol. Soc., vol. ii., part iii., considers this conglomerate | -| to be of Lower Carboniferous age. | -| | -| [161] See Selwyn, “Phys. Geography and Geology of Victoria.” 1866. | -| pp. 15−16; Taylor and Etheridge, _Geol. Survey Vict., Quarter Sheet | -| 13, N.E._ | -| | -| [162] Report on the Geology of the District of Ballan, Victoria. | -| 1866. p. 11. | -| | -| [163] _Atrypa reticularis._ | -| | -| [164] Quart. Journ. Geol. Soc., vol. xii., p. 58. | -| | -| [165] “Great Ice Age,” p. 513. | -| | -| [166] “Great Ice Age,” p. 513. | -| | -| [167] Brit. Assoc. Report for 1873. | -| | -| [168] Quart. Journ. Geol. Soc., vol. xi., p. 519. | -| | -| [169] _Orthis resupinata._ | -| | -| [170] _Prod. semireticulatus_ var. _Martini_. Sow. | -| | -| [171] “Belcher’s Voyage,” vol. ii., p. 377. | -| | -| [172] “Journal of a Boat Voyage through Rupert-Land,” vol. ii., p. | -| 208. | -| | -| [173] Quart. Journ. Geol. Soc., vol. xi., p. 197. | -| | -| [174] Explanation Memoir to Sheet 47, “Geological Survey of | -| Ireland.” | -| | -| [175] Phil. Mag., vol. xxix., p. 290. | -| | -| [176] “Memoirs of the Geological Survey of India,” vol. i., part i. | -| | -| [177] Quart. Journ. Geol. Soc., vol. xxvi., p. 514. | -| | -| [178] Ibid., vol. xxvii., p. 544. | -| | -| [179] Phil. Mag., vol. xxix., p. 290. | -| | -| [180] Journal of the Royal Dublin Society for February, 1857. | -| | -| [181] Quart. Journ. Geol. Soc., vol. xi., p. 519. | -| | -| [182] “The Last of the Arctic Voyages,” by Captain Sir E. Belcher, | -| vol. ii., p. 389. Appendix Brit. Assoc. Report for 1855, p. 79. | -| | -| [183] Ibid., vol. ii., p. 379. Appendix. | -| | -| [184] “Manual of Geology,” pp. 395, 493. | -| | -| [185] Appendix to McClintock’s “Arctic Discoveries.” | -| | -| [186] Quart. Journ. Geol. Soc., vol. xiv., p. 262. Brit. Assoc. | -| Report for 1857, p. 62. | -| | -| [187] Quart. Journ. Geol. Soc., vol. xvi., p. 327. _Geologist_, | -| 1860, p. 38. | -| | -| [188] Phil. Mag., vol. xxix., p. 290. | -| | -| [189] Trans. Geol. Soc. of Glasgow, vol. v., p. 64. | -| | -| [190] “Principles,” vol. i., p. 209. Eleventh Edition. | -| | -| [191] “Memoirs of the Royal Academy of Science of Turin,” Second | -| Series, vol. xx. I am indebted for the above particulars to | -| Professor Ramsay, who visited the spot along with M. Gastaldi. | -| | -| [192] “Antiquity of Man,” Second Edition, p. 237. | -| | -| [193] Dr. Robert Brown, in a recent Memoir on the Miocene Beds of | -| the Disco District (Trans. Geol. Soc. Glasg., vol. v., p. 55), | -| has added considerably to our knowledge of these deposits. He | -| describes the strata in detail, and gives lists of the plant and | -| animal remains discovered by himself and others, and described by | -| Professor Heer. Professor Nordenskjöld has likewise increased the | -| data at our command (Transactions of the Swedish Academy, 1873); | -| and still further evidence in favour of a warm climate having | -| prevailed in Greenland during Miocene times has been obtained by | -| the recent second German polar expedition. | -| | -| [194] The following are M. Leverrier’s formulæ for computing the | -| eccentricity of the earth’s orbit, given in his “Memoir” in the | -| _Connaissance des Temps_ for 1843:— | -| | -| Eccentricity in (_t_) years after January 1, 1800 | -| _____________ | -| = √_h_^2 + _l_^2 where | -| | -| _h_ = 0·000526 Sin (_gt_ + _ß_) + 0·016611 Sin (_g_{1}t_ + _ß_{1}_) | -| + 0·002366 Sin (_g_{2}t_ + _ß_{2}_) | -| + 0·010622 Sin (_g_{3}t_ + _ß_{3}_) | -| - 0·018925 Sin (_g_{4}t_ + _ß_{4}_) | -| + 0·011782 Sin (_g_{5}t_ + _ß_{5}_) | -| - 0·016913 Sin (_g_{6}t_ + _ß_{6}_) | -| and | -| | -| _l_ = 0·000526 Cos (_gt_ + _ß_) + 0·016611 Cos (_g_{1}t_ + _ß_{1}_) | -| + 0·002366 Cos (_g_{2}t_ + _ß_{2}_) | -| + 0·010622 Cos (_g_{3}t_ + _ß_{3}_) | -| - 0·018925 Cos (_g_{4}t_ + _ß_{4}_) | -| + 0·011782 Cos (_g_{5}t_ + _ß_{5}_) | -| - 0·016913 Cos (_g_{6}t_ + _ß_{6}_) | -| | -| _g_ = 2″·25842 _ß_ = 126° 43′ 15″ | -| _g_{1}_ = 3″·71364 _ß_{1}_ = 27 21 26 | -| _g_{2}_ = 22″·4273 _ß_{2}_ = 126 44 8 | -| _g_{3}_ = 5″·2989 _ß_{3}_ = 85 47 45 | -| _g_{4}_ = 7″·5747 _ß_{4}_ = 35 38 43 | -| _g_{5}_ = 17″·1527 _ß_{5}_ = −25 11 33 | -| _g_{6}_ = 17″·8633 _ß_{6}_ = −45 28 59 | -| | -| [195] See Professor C. V. Zenger’s paper “On the Periodic Change cf | -| Climate caused by the Moon,” Phil. Mag. for June, 1868. | -| | -| [196] Phil. Mag. for February, 1867. | -| | -| [197] Phil. Mag. for May, 1868. | -| | -| [198] Student’s “Elements of Geology,” p. 91. Second Edition. | -| | -| [199] In an interesting memoir, published in the Phil. Mag. for | -| 1850, Mr. Alfred Tylor estimated that the basin of the Mississippi | -| is being lowered at the rate of one foot in 10,000 years by the | -| removal of the sediment; and he proceeds further, and reasons that | -| one foot removed off the general surface of the land during that | -| period would raise the sea-level three inches. Had it not been that | -| Mr. Tylor’s attention was directed to the effects produced by the | -| removal of sediment in raising the level of the ocean rather than | -| in lowering the level of the land, he could not have failed to | -| perceive that he was in possession of a key to unfold the mystery | -| of geological time. | -| | -| [200] Proc. Roy. Soc., No. 152, 1874. | -| | -| [201] I have taken for the volume and mass of the sun the values | -| given in Professor Sir William Thomson’s memoir, Phil. Mag., vol. | -| viii. (1854). | -| | -| [202] Phil. Mag., § 4, vol. xi., p. 516 (1856). | -| | -| [203] Phil. Mag. for July, 1872, p. 1. | -| | -| [204] “Principles,” p. 210. Eleventh Edition. | -| | -| [205] “Principles,” vol. i., p. 107. Tenth Edition. | -| | -| [206] The conception of submergence resulting from displacement of | -| the earth’s centre of gravity, caused by a heaping up of ice at | -| one of the poles, was first advanced by M. Adhémar, in his work | -| “_Révolutions de la Mer_,” 1842. When the views stated in this | -| chapter appeared in the _Reader_, I was not aware that M. Adhémar | -| had written on the subject. An account of his mode of viewing the | -| question is given in the Appendix. | -| | -| [207] Petermann’s _Geog. Mittheilungen_, 1871, Heft. x., p. 377. | -| | -| [208] Geol. Mag., 1872, vol. ix., p. 360. | -| | -| [209] “Open Polar Sea,” p. 134. | -| | -| [210] Journal of the Royal Geographical Society, 1853, vol. xxiii. | -| | -| [211] “Physics of Arctic Ice,” Quart. Journ. Geol. Soc. for | -| February, 1871. | -| | -| [212] Some writers have objected to the conclusion that the | -| antarctic ice-cap is thickest at the pole, on the ground that the | -| snowfall there is probably less than at lower latitudes. The fact | -| is, however, overlooked, that the greater thickness of an ice-cap | -| at its centre is a physical necessity not depending on the rate of | -| snowfall. Supposing the snowfall to be greater at, say, lat. 70° | -| than at 80°, and greater at 80° than at the pole; nevertheless, the | -| ice will continue to accumulate till it is thicker at 80° than at | -| 70°, and at the pole than it is at 80°. | -| | -| [213] It is a pity that at present no record is kept, either by | -| the Board of Trade or by the Admiralty, of remarkable icebergs | -| which may from time to time be met with. Such a record might be of | -| little importance to navigation, but it would certainly be of great | -| service to science. | -| | -| [214] See Chapter XXVII., and also Geol. Mag. for May and June, | -| 1870, and January, 1871. | -| | -| [215] Phil. Mag. for April, 1866, p. 323. | -| | -| [216] Ibid., for March, 1866, p. 172. | -| | -| [217] _Reader_, February 10, 1866. | -| | -| [218] In a former paper I considered the effects of another cause, | -| viz., the melting of polar ice resulting from an increase of the | -| Obliquity of the Earth’s Orbit.—Trans. Glasgow Geol. Soc., vol. | -| ii., p. 177. Phil. Mag., June, 1867. See also Chapter XXV. | -| | -| [219] Phil. Mag. for November, 1868, p. 376. | -| | -| [220] Phil. Mag., November, 1868. | -| | -| [221] “Origin of Species,” chap. xi. Fifth Edition. | -| | -| [222] Lieutenant-Colonel Drayson (“Last Glacial Epoch of Geology”) | -| and also Mr. Belt (Quart. Journ. of Science, October, 1874) state | -| that Leverrier has lately investigated the question as to the | -| extent of the variation of the plane of the ecliptic, and has | -| arrived at results differing considerably from those of Laplace; | -| viz., that the variation may amount to 4° 52′, whereas, according | -| to Laplace, it amounts to only 1° 21′. I fear they are comparing | -| things that are totally different; viz., the variation of the | -| plane of the ecliptic in relation to its mean position with its | -| variation in relation to the equator. Laplace estimated that the | -| plane of the ecliptic would oscillate to the extent of 4° 53′ 33″ | -| on each side of its mean position, a result almost identical with | -| that of Leverrier, who makes it 4° 51′ 42″. But neither of these | -| geometricians ever imagined that the ecliptic could change in | -| relation to the equator to even one-third of that amount. | -| | -| Laplace demonstrated that the change in the plane of the ecliptic | -| affected the position of the equator, causing it to vary along with | -| it, so that the equator could never possibly recede further than | -| 1° 22′ 34″ from its mean position in relation to the ecliptic | -| (“_Mécanique Céleste_,” vol. ii., p. 856, Bowditch’s Translation; | -| see also Laplace’s memoir, “Sur les Variations de l’Obliquité de | -| l’Écliptique,” _Connaissance des Temps_ for 1827, p. 234), and I am | -| not aware that Leverrier has arrived at a different conclusion. | -| | -| [223] Memoir on the Secular Variations of the Elements of the | -| Orbits of the Planets, “Smithsonian Contributions to Knowledge,” | -| vol. xvii. | -| | -| [224] “Smithsonian Contributions to Knowledge,” vol. ix. | -| | -| [225] “Distribution of Heat on the Surface of the Globe,” p. 14. | -| | -| [226] Chapter IV. | -| | -| [227] Quart. Journ. Geol. Soc., June, 1866, p. 564. | -| | -| [228] Quart. Journ. Geol. Soc., vol. xxi., p. 186. | -| | -| [229] “Geological Observer,” p. 446. See also Mr. James Geikie’s | -| valuable Memoir, “On the Buried Forests and Peat Mosses of | -| Scotland.” Trans. of the Royal Society of Edinburgh, vol. xxiv., | -| and Chambers’ “Ancient Sea-Margins.” | -| | -| [230] See Lyell’s “Antiquity of Man,” Second Edition, p. 282; | -| “Elements,” Sixth Edition, p. 162. | -| | -| [231] In order to determine the position of the solstice-point | -| in relation to the aphelion, it will not do to assume, as is | -| commonly done, that the point makes a revolution from aphelion to | -| aphelion in any regular given period, such as 21,000 years; for | -| it is perfectly evident that owing to the great irregularity in | -| the motion of the aphelion, no two revolutions will probably be | -| performed in the same length of period. For example, the winter | -| solstice was in the aphelion about the following dates: 11,700, | -| 33,300, and 61,300 years ago. Here are two consecutive revolutions, | -| the one performed in 21,600 years and the other in 28,000 years; | -| the difference in the length of the two periods amounting to no | -| fewer than 6,400 years. | -| | -| [232] Quart. Journ. Geol. Soc., vol. xxvii., p. 232. See also “The | -| Last Glacial Epoch of Geology,” by the same author. | -| | -| [233] Quart. Journ. of Science, October, 1874. | -| | -| [234] The longer diameter passes from long. 14° 23′ E. to long. | -| 165° 37′ W. | -| | -| [235] “Principles,” vol. i., p. 294. Eleventh Edition. | -| | -| [236] Phil. Mag. for August, 1864. | -| | -| [237] “Elementary Geology,” p. 399. | -| | -| [238] “The Past and Present Life of the Globe,” p. 102. | -| | -| [239] “Memoirs of the Geological Survey,” vol. ii., Part 2, p. 404. | -| | -| [240] “Coal Fields of Great Britain,” p. 45. Third Edition. | -| | -| [241] “Journal of Researches,” chap. xiii. | -| | -| [242] “Coal Fields of Great Britain,” p. 67. | -| | -| [243] See “Smithsonian Report for 1857,” p. 138. | -| | -| [244] Quart. Journ. Geol. Soc., May, 1865, p. civ. | -| | -| [245] “Geology of Fife and the Lothians,” p. 116. | -| | -| [246] “Life on the Earth,” p. 133. | -| | -| [247] Quart. Journ. Geol. Soc., vol. xi., p. 535. | -| | -| [248] Ibid., vol. xii., p. 39. | -| | -| [249] Miller’s “Sketch Book of Practical Geology,” p. 192. | -| | -| [250] From Geological Magazine, May and June, 1870; with a few | -| verbal corrections, and a slight re-arrangement of the paragraphs. | -| | -| [251] See Phil. Mag. for November, 1868, p. 374. | -| | -| [252] See Phil. Mag. for November, 1868, pp. 366−374. | -| | -| [253] Journ. Geol. Soc., vol. xxi., p. 165. | -| | -| [254] Specimens of the striated summit and boulder clay stones are | -| to be seen in the Edinburgh Museum of Science and Art. | -| | -| [255] Phil. Mag. for April, 1866. | -| | -| [256] “Tracings of the North of Europe,” 1850, pp. 48−51. | -| | -| [257] Quart. Journ. Geol. Soc., vol. ii., p. 364. | -| | -| [258] “Tracings of the North of Europe,” by Robert Chambers, pp. | -| 259, 285. “Observations sur les Phénomènes d’Erosion en Norvège,” | -| by M. Hörbye, 1857. See also Professor Erdmann’s “Formations | -| Quaternaires de la Suède.” | -| | -| [259] “Glacial Drift of Scotland,” p. 29. | -| | -| [260] Geological Magazine, vol. ii., p. 343. Brit. Assoc. Rep., | -| 1864 (sections), p. 59. | -| | -| [261] Trans. Roy. Soc. Edin., vol. vii., p. 265. | -| | -| [262] “Tracings of Iceland and the Faroe Islands,” p. 49. | -| | -| [263] See Chap. XXIII. | -| | -| [264] Mr. Thomas Belt has subsequently advanced (Quart. Jour. Geol. | -| Soc., vol. xxx., p. 490), a similar explanation of the steppes of | -| Siberia. He supposes that an overflow of ice from the polar basin | -| dammed back all the rivers flowing northward, and formed an immense | -| lake which extended over the lowlands of Siberia, and deposited the | -| great beds of sand and silt with occasional freshwater shells and | -| elephant remains, of which the steppes consist. | -| | -| [265] Proc. Roy. Phys. Soc., Edin., vols. ii. and iii. | -| | -| [266] From Geol. Mag. for January, 1871. | -| | -| [267] Quart. Journ. Geol. Soc., xxvi., p. 517. | -| | -| [268] British Assoc. Report for 1864 (sections), p. 65. | -| | -| [269] Quart. Journ. Geol. Soc., xxvi., p. 90. | -| | -| [270] Geol. Mag., vii., p. 349. | -| | -| [271] Trans. Edin. Geol. Soc., vol. i., p. 136. | -| | -| [272] Geol. Mag. for June, 1870. See Chap. XXVII. | -| | -| [273] This was done by Mr. R. H. Tiddeman of the Geological Survey | -| of England (Quart. Journ. Geol. Soc. for November, 1872), and the | -| result established the correctness of the above opinion as to the | -| existence of a North of England ice-sheet. Additional confirmation | -| has been derived from the important observations of Mr. D. | -| Mackintosh, and also of Mr. Goodchild, of the Geological Survey of | -| England. | -| | -| [274] Trans. Geol. Soc., vol. v., p. 516 (first series). | -| | -| [275] Quart. Journ. Geol. Soc., vol. xi., p. 492. “Memoir of the | -| Country around Cheltenham,” 1857. “Geology of the Country around | -| Woodstock,” 1859. | -| | -| [276] Geol. Mag., vol. vii., p. 497. | -| | -| [277] Quart. Journ. Geol. Soc., vol. xxvi., p. 90. | -| | -| [278] My colleague, Mr. R. L. Jack. | -| | -| [279] The greater portion of this chapter is from the Trans. of | -| Geol. Soc. of Edinburgh, for 1869. | -| | -| [280] Chapter XV., p. 253. | -| | -| [281] Trans. of the Geol. Soc. of Glasgow, vol. iii., part i., page | -| 133. | -| | -| [282] Mr. Milne Home has advanced, in his “Estuary of the Firth of | -| Forth,” p. 91, the theory that this trough had been scooped out | -| during the glacial epoch by icebergs floating through the Midland | -| valley from west to east when it was submerged. The bottom of the | -| trough, be it observed, at the watershed at Kilsyth, is 300 feet | -| above the level of its bottom at Grangemouth; and this Mr. Milne | -| Home freely admits. But he has not explained how an iceberg, which | -| could float across the shallow water at Kilsyth, say, 100 feet | -| deep, could manage to grind the rocky bottom at Grangemouth, where | -| it was not less than 400 feet deep. “The impetus acquired in the | -| Kyle at Kilsyth,” says Mr. Milne Home, “would keep them moving on, | -| and the prevailing westerly winds would also aid, so that when | -| _grating_ on the subjacent carboniferous rocks they would not have | -| much difficulty in scooping out a channel both wider and deeper | -| than at Kilsyth.” But how could they “_grate_ on the subjacent | -| carboniferous rocks” at Grangemouth, if they managed to _float_ | -| at Kilsyth? Surely an iceberg that could “_grate_” at Grangemouth | -| would “_ground_” at Kilsyth. | -| | -| [283] Trans. of the Geol. Soc. of Glasgow, vol. iii., p. 141. | -| | -| [284] Mr. John Young and Mr. Milne Home advanced the objection, | -| that several trap dykes cross the valley of the Clyde near Bowling, | -| and come to so near the present surface of the land, that the | -| Clyde at present flows across them with a depth not exceeding | -| 20 feet. I fear that Mr. Young and Mr. Milne Home have been | -| misinformed in regard to the existence of these dykes. About a mile | -| _above_ Bowling there are one or two dykes which approach to the | -| river-bank, and may probably cross, but these could not possibly | -| cut off a channel entering the Clyde at Bowling. In none of the | -| borings or excavations which have been made by the Clyde Trustees | -| has the rock been reached from Bowling downwards. I may also state | -| that the whole Midland valley, from the Forth of Clyde to the Firth | -| of Forth, has been surveyed by the officers of the Geological | -| Survey, and only a single dyke has been found to cross the buried | -| channels, viz., one (Basalt rock) running eastward from Kilsyth to | -| the canal bridge near Dullatur. But as this is not far from the | -| watershed between the two channels it cannot affect the question at | -| issue. See sheet 31 of Geological Survey Map of Scotland. | -| | -| [285] Trans. Geol. Soc. Glasgow, vol. iv., p. 166. | -| | -| [286] “Great Ice Age,” chap. xiii. | -| | -| [287] See further particulars in Mr. Bennie’s paper on the Surface | -| Geology of the district around Glasgow, Trans. Geol. Soc. of | -| Glasgow, vol. iii. | -| | -| [288] See also Smith’s “Newer Pliocene Geology,” p. 139. | -| | -| [289] British Association Report for 1863, p. 89. _Geologist_ for | -| 1863, p. 384. | -| | -| [290] See Geological Magazine, vol. ii., p. 38. | -| | -| [291] Proc. Geol. Soc., vol. iii., 1840, p. 342. | -| | -| [292] “Antiquity of Man” (Third Edition), p. 249. | -| | -| [293] “Glacial Drift of Scotland,” p. 65. Trans. Geol. Soc. Glas., | -| vol. i., part 2. | -| | -| [294] “Memoir, Geological Survey of Scotland,” Sheet 23, p. 42. | -| | -| [295] Mr. Robert Dick had previously described, in the Trans. Geol. | -| Soc. Edinburgh, vol. i., p. 345, portions of these buried channels. | -| He seems, however, to have thought that they formed part of one and | -| the same channel. | -| | -| [296] A description of this channel was read to the Natural History | -| Society of Glasgow by Mr. James Coutts, the particulars of which | -| will appear in the Transactions of the Society. | -| | -| [297] “Occasional Papers,” pp. 166, 223. | -| | -| [298] Memoir read before the Royal Society, January 7, 1869. | -| | -| [299] “Alpine Journal,” February, 1870. | -| | -| [300] Phil. Mag., January, 1872. | -| | -| [301] Phil. Mag., July, 1870; February, 1871. | -| | -| [302] Philosophical Magazine for January, 1870, p. 8; Proceedings | -| of the Royal Society for January, 1869. | -| | -| [303] Philosophical Magazine for March, 1869. | -| | -| [304] Proceedings of Bristol Naturalists’ Society, p. 37 (1869). | -| | -| [305] Ibid., vol. iv., p. 37 (new series). | -| | -| [306] Phil. Mag., S. 4, vol. x., p. 303. | -| | -| [307] Proceedings of the Bristol Naturalists’ Society, vol. iv., p. | -| 39 (new series). | -| | -| [308] See Philosophical Transactions, December, 1857. | -| | -| [309] There is one circumstance tending slightly to prevent the | -| rupture of the glacier, when under tension, which I do not remember | -| to have seen noticed; that is, the cooling effect which is produced | -| in solids, such as ice, when subjected to tension. Tension would | -| tend to lower the temperature of the ice-molecules, and this | -| lowering of temperature would have the tendency of freezing them | -| more firmly together. The cause of this cooling effect will be | -| explained in the Appendix. | -| | -| [310] Phil. Mag., March, 1869; September, 1870. | -| | -| [311] “Forms of Water,” p. 127. | -| | -| [312] See text, p. 10. | -| | -| [313] Mathematical and Physical Series, vol. xxxvi. (1765). | -| | -| [314] “Memoirs of St. Petersburg Academy,” 1761. | -| | -| [315] The calculations here referred to were made by Lagrange | -| nearly half a century previous to the appearance of this paper, and | -| published in the “Mémoires de l’Académie de Berlin,” for 1782, p. | -| 273. Lagrange’s results differ but slightly from those afterwards | -| obtained by Leverrier, as will be seen from the following table; | -| but as he had assigned erroneous values to the masses of the | -| smaller planets, particularly that of Venus, the mass of which he | -| estimated at one-half more than its true value, full confidence | -| could not be placed in his results. | -| | -| Superior limits of eccentricity as determined by Lagrange, | -| Leverrier, and Mr. Stockwell:— | -| | -| By Lagrange. By Leverrier. By Mr. Stockwell. | -| | -| Mercury 0·22208 0·225646 0·2317185 | -| Venus 0·08271 0·086716 0·0706329 | -| Earth 0·07641 0·077747 0·0693888 | -| Mars 0·14726 0·142243 0·139655 | -| Jupiter 0·06036 0·061548 0·0608274 | -| Saturn 0·08408 0·084919 0·0843289 | -| Uranus — 0·064666 0·0779652 | -| Neptune — — 0·0145066 | -| | -| [J. C.] | -| | -| [316] “Mém. de l’Acad. royale des Sciences.” 1827. Tom. vii., p. | -| 598. | -| | -| [317] Absolute zero is now considered to be only 493° Fah. below | -| the freezing-point, and Herschel himself has lately determined | -| 271° below the freezing-point to be the temperature of space. | -| Consequently, a decrease, or an increase of one per cent. in the | -| mean annual amount of radiation would not produce anything like the | -| effect which is here supposed. But the mean annual amount of heat | -| received cannot vary much more than one-tenth part of one per cent. | -| In short, the effect of eccentricity on the mean annual supply of | -| heat received from the sun, in so far as geological climate is | -| concerned, may be practically disregarded.—[J. C.] | -| | -| [318] “Principles of Geology,” p. 110. “Mr. Lyell, however, in | -| stating the actual excess of eight days in the duration of the | -| sun’s presence in the northern hemisphere over that in the southern | -| as productive of an excess of light and heat annually received by | -| the one over the other hemisphere, appears to have misconceived the | -| effect of elliptic motion in the passage here cited, since it is | -| demonstrable that whatever be the ellipticity of the earth’s orbit | -| the two hemispheres must receive equal absolute quantities of light | -| and heat per annum, the proximity of the sun in perigee exactly | -| compensating the effect of its swifter motion. This follows from a | -| very simple theorem, which may be thus stated: ‘The amount of heat | -| received by the earth from the sun while describing any part of | -| its orbit is proportional to the angle described round the sun’s | -| centre,’ so that if the orbit be divided into two portions by a | -| line drawn _in any direction_ through the sun’s centre, the heats | -| received in describing the two unequal segments of the ellipse so | -| produced will be equal.” | -| | -| [319] When the eccentricity of the earth’s orbit is at its superior | -| limit, the absolute quantity of heat received by the globe during | -| one year will be increased by only 1/300th part; an amount which | -| could produce no sensible influence on climate.—[J. C.] | -| | -| [320] Sir Charles has recently, to a certain extent, adopted the | -| views advocated in the present volume, viz., that the cold of the | -| glacial epoch was brought about not by a _decrease_, but by an | -| _increase_ of eccentricity. (See vol. i. of “Principles,” tenth | -| and eleventh editions.) The decrease in the mean annual quantity | -| of heat received from the sun, resulting from the decrease in | -| the eccentricity of the earth’s orbit—the astronomical cause to | -| which he here refers—could have produced no sensible effect on | -| climate.—[J. C.] | -| | -| [321] It is singular that both Arago and Humboldt should appear to | -| have been unaware of the researches of Lagrange on this subject. | -| | -| [322] “Révolutions de la Mer,” p. 37. Second Edition. | -| | -| [323] See text, p. 37. | -| | -| [324] See _Philosophical Magazine_ for December, 1867, p. 457. | -| | -| [325] _Silliman’s American Journal_ for July, 1864. _Philosophical | -| Magazine_ for September, 1864, pp. 193, 196. | -| | -| [326] _Philosophical Magazine_ for August, 1865, p. 95. | -| | -| [327] See text, p. 80. | -| | -| [328] See text, p. 222. | -| | -| [329] Proc. Roy. Soc., No. 157, 1875. | -| | -| [330] See text, p. 522. | -| | -| [331] Phil. Trans. for 1859, p. 91. | -| | -| [332] See text, p. 527. | -| | -+----------------------------------------------------------------------+ - - -Transcriber’s Notes: - - - Text enclosed by underscores is in italics (_italics_). - - Blank pages have been removed. - - Obvious typographical errors have been silently corrected. - - Some spelling and hyphenation variations have been made consistent. - - - - - -End of the Project Gutenberg EBook of Climate and Time in their Geological -Relations, by James Croll - -*** END OF THIS PROJECT GUTENBERG EBOOK CLIMATE AND TIME *** - -***** This file should be named 62693-0.txt or 62693-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/6/2/6/9/62693/ - -Produced by WebRover, MWS, Robert Tonsing, and the Online -Distributed Proofreading Team at https://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive/American Libraries.) - - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll -have to check the laws of the country where you are located before using -this ebook. - - - -Title: Climate and Time in their Geological Relations - A Theory of Secular Changes of the Earth's Climate - -Author: James Croll - -Release Date: July 18, 2020 [EBook #62693] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK CLIMATE AND TIME *** - - - - -Produced by WebRover, MWS, Robert Tonsing, and the Online -Distributed Proofreading Team at https://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive/American Libraries.) - - - - - - -</pre> - - <div class="figcenter"> - <img id="coverpage" src="images/cover.jpg" alt="" width="500" height="800" /> - </div> - - <hr class="page" /> - <div class="center xlarge mt20 mb20">CLIMATE AND TIME</div> - <hr class="page" /> - - <div class="figcenter illow600" id="Frontispiece" > - <div class="attribt">FRONTISPIECE</div> - <img src="images/i_frontis.jpg" width="600" height="354" alt="" /> - <div class="attribr">W. & A. K. Johnston, Edinb<sup>r</sup>. and London.</div> - </div> - - <div class="titlepage"> - <h1><span class="gespertt">CLIMATE <span class="xlarge">AND</span> TIME</span><br /> - <span class="medium"><i>IN THEIR GEOLOGICAL RELATIONS</i></span></h1> - - <div class="mt5 lh2">A THEORY OF<br /> - SECULAR CHANGES OF THE EARTH’S CLIMATE</div> - - <div class="mt20"><span class="xlarge"><span class="smcap">By</span> JAMES CROLL</span><br /> - <span class="small">OF H.M. GEOLOGICAL SURVEY OF SCOTLAND</span></div> - - <div class="mt20">LONDON<br /> - DALDY, ISBISTER, & CO.<br /> - 56, LUDGATE HILL<br /> - 1875</div> - </div> - - <hr class="page" /> - <div class="center small mt20 mb20">LONDON:<br /> - PRINTED BY VIRTUE AND CO.,<br /> - CITY ROAD. - </div> - - <hr class="page" /> - <div class="chapter" id="PREFACE"> - <span class="pagenum" id="Page_v">v</span> - <h2>PREFACE</h2> - </div> - - <div class="figcenter"> - <img src="images/diamondbar.png" width="100" height="8" alt="" /> - </div> - - <p class="noindent"><span class="smcap">In</span> the following pages I have endeavoured to give a full and concise - statement of the facts and arguments adduced in support of the theory - of Secular Changes of the Earth’s Climate. Considerable portions of - the volume have already appeared in substance as separate papers in - the Philosophical Magazine and other journals during the past ten or - twelve years. The theory, especially in as far as it relates to the - cause of the glacial epoch, appears to be gradually gaining acceptance - with geologists. This, doubtless, is owing to the greatly increased - and constantly increasing knowledge of the drift-phenomena, which has - induced the almost general conviction that a climate such as that of - the glacial epoch could only have resulted from cosmical causes.</p> - - <p>Considerable attention has been devoted to objections, and to the - removal of slight misapprehensions, which have naturally arisen in - regard to a subject comparatively new and, in many respects, complex, - and beset with formidable difficulties.</p> - - <p>I have studiously avoided introducing anything of a hypothetical - character. All the conclusions are based either on known facts or - admitted physical principles. In short, the aim of the work, as will be - shown in the introductory chapter, is to prove that secular changes of - climate follow, as a necessary effect, from admitted physical agencies, - and that these changes,<span class="pagenum" id="Page_vi">vi</span> in as far as the past climatic condition of - the globe is concerned, fully meet the demand of the geologist.</p> - - <p>The volume, though not intended as a popular treatise, will be found, - I trust, to be perfectly plain and intelligible even to readers not - familiar with physical science.</p> - - <p>I avail myself of this opportunity of expressing my obligations to my - colleagues, Mr. James Geikie, Mr. Robert L. Jack, Mr. Robert Etheridge, - jun., and also to Mr. James Paton, of the Edinburgh Museum of Science - and Art, for their valuable assistance rendered while these pages were - passing through the press. To the kindness of Mr. James Bennie I am - indebted for the copious index at the end of the volume, as well as - for many of the facts relating to the glacial deposits of the West of - Scotland.</p> - - <div class="right">JAMES CROLL.</div> - - <p class="small"> - <span class="smcap">Edinburgh</span>, <i>March</i>, 1875. - </p> - - <hr class="page" /> - <div class="chapter" id="CONTENTS"> - <span class="pagenum" id="Page_vii">vii</span> - <h2>CONTENTS</h2> - </div> - - <div class="figcenter"> - <img src="images/diamondbar.png" width="100" height="8" alt="" /> - </div> - - <table summary="Contents"> - <tbody> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_I">CHAPTER I.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>INTRODUCTION.</div></td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdr xsmall"><div>PAGE</div></td> - </tr> - <tr> - <td class="tdl">The Fundamental Problem of Geology.—Geology a Dynamical Science.—The - Nature of a Geological Principle.—Theories of Geological Climate.—Geological - Climate dependent on Astronomical Causes.—An Important - Consideration overlooked.—Abstract of the Line of Argument pursued - in the Volume</td> - <td class="tdr"><div>1</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_II">CHAPTER II.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER - THE GLOBE.</div></td> - </tr> - <tr> - <td class="tdl">The absolute Heating-power of Ocean-currents.—Volume of the Gulf-stream.—Absolute - Amount of Heat conveyed by it.—Greater Portion - of Moisture in Inter-tropical Regions falls as Rain in those Regions.—Land - along the Equator tends to lower the Temperature of the Globe.—Influence - of Gulf-stream on Climate of Europe.—Temperature of - Space.—Radiation of a Particle.—Professor Dove on Normal Temperature.—Temperature - of Equator and Poles in the Absence of - Ocean-currents.—Temperature of London, how much due to Ocean-currents</td> - <td class="tdr"><div>23</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_III">CHAPTER III.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER - THE GLOBE.—(<i>Continued.</i>)</div></td> - </tr> - <tr> - <td class="tdl">Influence of the Gulf-stream on the Climate of the Arctic Regions.—Absolute - Amount of Heat received by the Arctic Regions from the Sun.—Influence - of Ocean-currents shown by another Method.—Temperature - of a Globe all Water or all Land according to Professor J. D. Forbes.—An - important Consideration overlooked.—Without Ocean-currents - the Globe would not be habitable.—Conclusions not affected by Imperfection - of Data</td> - <td class="tdr"><div>45</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><span class="pagenum" id="Page_viii">viii</span><div><a href="#CHAPTER_IV">CHAPTER IV.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>OUTLINE OF THE PHYSICAL AGENCIES WHICH LEAD TO SECULAR - CHANGES OF CLIMATE.</div></td> - </tr> - <tr> - <td class="tdl">Eccentricity of the Earth’s Orbit; its Effect on Climate.—Glacial Epoch - not the direct Result of an Increase of Eccentricity.—An important - Consideration overlooked.—Change of Eccentricity affects Climate only - indirectly.—Agencies which are brought into Operation by an Increase - of Eccentricity.—How an Accumulation of Snow is produced.—The - Effect of Snow on the Summer Temperature.—Reason of the Low - Summer Temperature of Polar Regions.—Deflection of Ocean-currents - the chief Cause of Secular Changes of Climate.—How the foregoing - Causes deflect Ocean-currents.—Nearness of the Sun in Perigee a - Cause of the Accumulation of Ice.—A remarkable Circumstance regarding - the Causes which lead to Secular Changes of Climate.—The - primary Cause an Increase of Eccentricity.—Mean Temperature of - whole Earth should be greater in Aphelion than in Perihelion.—Professor - Tyndall on the Glacial Epoch.—A general Reduction of Temperature - will not produce a Glacial Epoch.—Objection from the present - Condition of the Planet Mars</td> - <td class="tdr"><div>54</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_V">CHAPTER V.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>REASON WHY THE SOUTHERN HEMISPHERE IS COLDER THAN THE - NORTHERN.</div></td> - </tr> - <tr> - <td class="tdl">Adhémar’s Explanation.—Adhémar’s Theory founded upon a physical Mistake - in regard to Radiation.—Professor J. D. Forbes on Underground - Temperature.—Generally accepted Explanation.—Low Temperature - of Southern Hemisphere attributed to Preponderance of Sea.—Heat - transferred from Southern to Northern Hemisphere by Ocean-current - the true Explanation.—A large Portion of the Heat of the Gulf-stream - derived from the Southern Hemisphere</td> - <td class="tdr"><div>81</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_VI">CHAPTER VI.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.—LIEUT. - MAURY’S THEORY.</div></td> - </tr> - <tr> - <td class="tdl">Introduction.—Ocean-currents, according to Maury, due to Difference of - Specific Gravity.—Difference of Specific Gravity resulting from Difference - of Temperature.—Difference of Specific Gravity resulting from - Difference of Saltness.—Maury’s two Causes neutralize each other.—How, - according to him, Difference in Saltness acts as a Cause</td> - <td class="tdr"><div>95</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><span class="pagenum" id="Page_ix">ix</span><a href="#CHAPTER_VII">CHAPTER VII.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.—LIEUT. - MAURY’S THEORY.—(<i>Continued.</i>)</div></td> - </tr> - <tr> - <td class="tdl">Methods of determining the Question.—The Force resulting from Difference - of Specific Gravity.—Sir John Herschel’s Estimate of the Force.—Maximum - Density of Sea-Water.—Rate of Decrease of Temperature - of Ocean at Equator.—The actual Amount of Force resulting from - Difference of Specific Gravity.—M. Dubuat’s Experiments</td> - <td class="tdr"><div>115</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_VIII">CHAPTER VIII.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.—DR. - CARPENTER’S THEORY.</div></td> - </tr> - <tr> - <td class="tdl">Gulf-stream according to Dr. Carpenter not due to Difference of Specific - Gravity.—Facts to be Explained.—The Explanation of the Facts.—The - Explanation hypothetical.—The Cause assigned for the hypothetical - Mode of Circulation.—Under Currents account for all the - Facts better than the Gravitation Hypothesis.—Known Condition of - the Ocean inconsistent with that Hypothesis</td> - <td class="tdr"><div>122</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_IX">CHAPTER IX.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.—THE - MECHANICS OF DR. CARPENTER’S THEORY.</div></td> - </tr> - <tr> - <td class="tdl">Experimental Illustration of the Theory.—The Force exerted by Gravity.—Work - performed by Gravity.—Circulation not by Convection.—Circulation - depends on Difference in Density of the Equatorial and - Polar Columns.—Absolute Amount of Work which can be performed - by Gravity.—How Underflow is produced.—How Vertical Descent at - the Poles and Ascent at the Equator is produced.—The Gibraltar - Current.—Mistake in Mechanics concerning it.—The Baltic Current</td> - <td class="tdr"><div>145</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_X">CHAPTER X.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.—DR. - CARPENTER’S THEORY.—OBJECTIONS CONSIDERED.</div></td> - </tr> - <tr> - <td class="tdl"><i lang="la">Modus Operandi</i> of the Matter.—Polar Cold considered by Dr. Carpenter - the <i lang="la">Primum Mobile</i>.—Supposed Influence of Heat derived from the - Earth’s Crust.—Circulation without Difference of Level.—A Confusion - of Ideas in Reference to the supposed Agency of Polar Cold.—M. - Dubuat’s Experiments.—A Begging of the Question at Issue.—Pressure - as a Cause of Circulation</td> - <td class="tdr"><div>172</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><span class="pagenum" id="Page_x">x</span><a href="#CHAPTER_XI">CHAPTER XI.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>THE INADEQUACY OF THE GRAVITATION THEORY PROVED BY - ANOTHER METHOD.</div></td> - </tr> - <tr> - <td class="tdl">Quantity of Heat which can be conveyed by the General Oceanic Circulation - trifling.—Tendency in the Advocates of the Gravitation Theory to - under-estimate the Volume of the Gulf-stream.—Volume of the Stream - as determined by the <cite>Challenger</cite>.—Immense Volume of Warm Water - discovered by Captain Nares.—Condition of North Atlantic inconsistent - with the Gravitation Theory.—Dr. Carpenter’s Estimate of the - Thermal Work of the Gulf-stream</td> - <td class="tdr"><div>191</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XII">CHAPTER XII.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>MR. A. G. FINDLAY’S OBJECTIONS CONSIDERED.</div></td> - </tr> - <tr> - <td class="tdl">Mr. Findlay’s Estimate of the Volume of the Gulf-stream.—Mean Temperature - of a Cross Section less than Mean Temperature of Stream.—Reason - of such Diversity of Opinion regarding Ocean-currents.—More - rigid Method of Investigation necessary</td> - <td class="tdr"><div>203</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XIII">CHAPTER XIII.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>THE WIND THEORY OF OCEANIC CIRCULATION.</div></td> - </tr> - <tr> - <td class="tdl">Ocean-Currents not due alone to the Trade-winds.—An Objection by - Maury.—Trade-winds do not explain the Great Antarctic Current.—Ocean-currents - due to the System of Winds.—The System of Currents - agrees with the System of the Winds.—Chart showing the Agreement - between the System of Currents and System of Winds.—Cause of the - Gibraltar Current.—North Atlantic an immense Whirlpool.—Theory - of Under Currents.—Difficulty regarding Under Currents obviated.—Work - performed by the Wind in impelling the Water forward.—The - <cite>Challenger’s</cite> crucial Test of the Wind and Gravitation Theories.—North - Atlantic above the Level of Equator.—Thermal Condition of the - Southern Ocean irreconcilable with the Gravitation Theory</td> - <td class="tdr"><div>210</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XIV">CHAPTER XIV.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>THE WIND THEORY OF OCEANIC CIRCULATION IN RELATION TO - CHANGE OF CLIMATE.</div></td> - </tr> - <tr> - <td class="tdl">Direction of Currents depends on Direction of the Winds.—Causes which - affect the Direction of Currents will affect Climate.—How Change of - Eccentricity affects the Mode of Distribution of the Winds.—Mutual - Reaction of Cause and Effect.—Displacement of the Great Equatorial - Current.—Displacement of the Median Line between the Trades, and - its Effect on Currents.—Ocean-currents in Relation to the Distribution - of Plants and Animals.—Alternate Cold and Warm Periods in North - and South.—Mr. Darwin’s Views quoted.—How Glaciers at the - Equator may be accounted for.—Migration across the Equator</td> - <td class="tdr"><div>226</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><span class="pagenum" id="Page_xi">xi</span><a href="#CHAPTER_XV">CHAPTER XV.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>WARM INTER-GLACIAL PERIODS.</div></td> - </tr> - <tr> - <td class="tdl">Alternate Cold and Warm Periods.—Warm Inter-glacial Periods a Test of - Theories.—Reason why their Occurrence has not been hitherto recognised.—Instances - of Warm Inter-glacial Periods.—Dranse, Dürnten, - Hoxne, Chapelhall, Craiglockhart, Leith Walk, Redhall Quarry, Beith, - Crofthead, Kilmaurs, Sweden, Ohio, Cromer, Mundesley, &c., &c.—Cave - and River Deposits.—Occurrence of Arctic and Warm Animals - in some Beds accounted for.—Mr. Boyd Dawkins’s Objections.—Occurrence - of Southern Shells in Glacial Deposits.—Evidence of Warm - Inter-glacial Periods from Mineral Borings.—Striated Pavements.—Reason - why Inter-glacial Land-surfaces are so rare</td> - <td class="tdr"><div>236</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XVI">CHAPTER XVI.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>WARM INTER-GLACIAL PERIODS IN ARCTIC REGIONS.</div></td> - </tr> - <tr> - <td class="tdl">Cold Periods best marked in Temperate, and Warm Periods in Arctic, - Regions.—State of Arctic Regions during Glacial Period.—Effects of - Removal of Ice from Arctic Regions.—Ocean-currents; Influence on - Arctic Climate.—Reason why Remains of Inter-glacial Period are rare - in Arctic Regions.—Remains of Ancient Forests in Banks’s Land, - Prince Patrick’s Island, &c.—Opinions of Sir R. Murchison, Captain - Osborn, and Professor Haughton.—Tree dug up by Sir E. Belcher in - lat. 75° N.</td> - <td class="tdr"><div>258</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XVII">CHAPTER XVII.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>FORMER GLACIAL EPOCHS.—REASON OF THE IMPERFECTION OF - GEOLOGICAL RECORDS IN REFERENCE TO THEM.</div></td> - </tr> - <tr> - <td class="tdl">Two Reasons why so little is known of Glacial Epochs.—Evidence of - Glaciation to be found on Land-surfaces.—Where are all our ancient - Land-surfaces?—The stratified Rocks consist of a Series of old Sea-bottoms.—Transformation - of a Land-surface into a Sea-bottom obliterates - all Traces of Glaciation.—Why so little remains of the Boulder - Clays of former Glacial Epochs.—Records of the Glacial Epoch are fast - disappearing.—Icebergs do not striate the Sea-bottom.—Mr. Campbell’s - Observations on the Coast of Labrador.—Amount of Material transported - by Icebergs much exaggerated.—Mr. Packard on the Glacial - Phenomena of Labrador.—Boulder Clay the Product of Land-ice.—Palæontological - Evidence.—Paucity of Life characteristic of a Glacial - Period.—Warm Periods better represented by Organic Remains than - cold.—Why the Climate of the Tertiary Period was supposed to be - warmer than the present.—Mr. James Geikie on the Defects of - Palæontological Evidence.—Conclusion</td> - <td class="tdr"><div>266</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><span class="pagenum" id="Page_xii">xii</span><div><a href="#CHAPTER_XVIII">CHAPTER XVIII.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>FORMER GLACIAL EPOCHS; GEOLOGICAL EVIDENCE OF.</div></td> - </tr> - <tr> - <td class="tdl">Cambrian Conglomerate of Islay and North-west of Scotland.—Ice-action - in Ayrshire and Wigtownshire during Silurian Period.—Silurian - Limestones in Arctic Legions.—Professor Ramsay on Ice-action during - Old Red Sandstone Period.—Warm Climate in Arctic Regions during - Old Red Sandstone Period.—Professor Geikie and Mr. James Geikie - on a Glacial Conglomerate of Lower Carboniferous Age.—Professor - Haughton and Professor Dawson on Evidence of Ice-action during - Coal Period.—Mr. W. T. Blanford on Glaciation in India during - Carboniferous Period.—Carboniferous Formations of Arctic Regions.—Professor - Ramsay on Permian Glaciers.—Permian Conglomerate in - Arran.—Professor Hull on Boulder Clay of Permian Age.—Permian - Boulder Clay of Natal.—Oolitic Boulder Conglomerate in Sutherlandshire.—-Warm - Climate in North Greenland during Oolitic Period.—Mr. - Godwin-Austen on Ice-action during Cretaceous Period.—Glacial - Conglomerates of Eocene Age in the Alps.—M. Gastaldi on the Ice-transported - Limestone Blocks of the Superga.—Professor Heer on the - Climate of North Greenland during Miocene Period</td> - <td class="tdr"><div>292</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XIX">CHAPTER XIX.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>GEOLOGICAL TIME.—PROBABLE DATE OF THE GLACIAL EPOCH.</div></td> - </tr> - <tr> - <td class="tdl">Geological Time measurable from Astronomical Data.—M. Leverrier’s Formulæ.—Tables - of Eccentricity for 3,000,000 Years in the Past and - 1,000,000 Years in the Future.—How the Tables have been computed.—Why - the Glacial Epoch is more recent than had been supposed.—Figures - convey a very inadequate Conception of immense Duration.—Mode - of representing a Million of Years.—Probable Date of the - Glacial Epoch</td> - <td class="tdr"><div>311</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XX">CHAPTER XX.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>GEOLOGICAL TIME.—METHOD OF MEASURING THE RATE OF SUBAËRIAL DENUDATION.</div></td> - </tr> - <tr> - <td class="tdl">Rate of Subaërial Denudation a Measure of Time.—Rate determined from - Sediment of the Mississippi.—Amount of Sediment carried down by - the Mississippi; by the Ganges.—Professor Geikie on Modern Denudation.—Professor - Geikie on the Amount of Sediment conveyed by - European Rivers.—Rate at which the Surface of the Globe is being - denuded.—Alfred Tylor on the Sediment of the Mississippi.—The - Law which determines the Rate of Denudation.—The Globe becoming - less oblate.—Carrying Power of our River Systems the true Measure - of Denudation.—Marine Denudation, trifling in comparison to Subaërial.—Previous - Methods of measuring Geological Time.—Circumstances - which show the recent Date of the Glacial Epoch.—Professor - Ramsay on Geological Time</td> - <td class="tdr"><div>329</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><span class="pagenum" id="Page_xiii">xiii</span><a href="#CHAPTER_XXI">CHAPTER XXI.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>THE PROBABLE AGE AND ORIGIN OF THE SUN.</div></td> - </tr> - <tr> - <td class="tdl">Gravitation Theory.—Amount of Heat emitted by the Sun.—Meteoric - Theory.—Helmholtz’s Condensation Theory.—Confusion of Ideas.—Gravitation - not the chief Source of the Sun’s Heat.—Original Heat.—Source - of Original Heat.—Original Heat derived from Motion in Space.—Conclusion - as to Date of Glacial Epoch.—False Analogy.—Probable - Date of Eocene and Miocene Periods</td> - <td class="tdr"><div>346</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XXII">CHAPTER XXII.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>A METHOD OF DETERMINING THE MEAN THICKNESS OF THE - SEDIMENTARY ROCKS OF THE GLOBE.</div></td> - </tr> - <tr> - <td class="tdl">Prevailing Methods defective.—Maximum Thickness of British Rocks.—Three - Elements in the Question.—Professor Huxley on the Rate of - Deposition.—Thickness of Sedimentary Rocks enormously over-estimated.—Observed - Thickness no Measure of mean Thickness.—Deposition - of Sediment principally along Sea-margin.—Mistaken Inference - regarding the Absence of a Formation.—Immense Antiquity - of existing Oceans</td> - <td class="tdr"><div>360</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XXIII">CHAPTER XXIII.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE - OF THE LAND DURING THE GLACIAL EPOCH.</div></td> - </tr> - <tr> - <td class="tdl">Displacement of the Earth’s Centre of Gravity by Polar Ice-cap.—Simple - Method of estimating Amount of Displacement.—Note by Sir W. - Thomson on foregoing Method.—Difference between Continental Ice - and a Glacier.—Probable Thickness of the Antarctic Ice-cap.—Probable - Thickness of Greenland Ice-sheet.—The Icebergs of the - Southern Ocean.—Inadequate Conceptions regarding the Magnitude - of Continental Ice</td> - <td class="tdr"><div>368</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XXIV">CHAPTER XXIV.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF - THE LAND DURING THE GLACIAL EPOCH.—(<i>Continued.</i>)</div></td> - </tr> - <tr> - <td class="tdl">Extent of Submergence from Displacement of Earth’s Centre of Gravity.—Circumstances - which show that the Glacial Submergence resulted from - Displacement of the Earth’s Centre of Gravity.—Agreement between - Theory and Observed Facts.—Sir Charles Lyell on submerged Areas - during Tertiary Period.—Oscillations of Sea-Level in Relation to Distribution.—Extent - of Submergence on the Hypothesis that the Earth - is fluid in the Interior</td> - <td class="tdr"><div>387</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><span class="pagenum" id="Page_xiv">xiv</span><a href="#CHAPTER_XXV">CHAPTER XXV.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>THE INFLUENCE OF THE OBLIQUITY OF THE ECLIPTIC ON CLIMATE - AND ON THE LEVEL OF THE SEA.</div></td> - </tr> - <tr> - <td class="tdl">The direct Effect of Change of Obliquity on Climate.—Mr. Stockwell on - the maximum Change of Obliquity.—How Obliquity affects the Distribution - of Heat over the Globe.—Increase of Obliquity diminishes - the Heat at the Equator and increases that at the Poles.—Influence of - Change of Obliquity on the Level of the Sea.—When the Obliquity - was last at its superior Limit.—Probable Date of the 25-foot raised - Beach.—Probable Extent of Rise of Sea-level resulting from Increase - of Obliquity.—Lieutenant-Colonel Drayson’s and Mr. Belt’s Theories.—Sir - Charles Lyell on Influence of Obliquity</td> - <td class="tdr"><div>398</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XXVI">CHAPTER XXVI.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>COAL AN INTER-GLACIAL FORMATION.</div></td> - </tr> - <tr> - <td class="tdl">Climate of Coal Period Inter-glacial in Character.—Coal Plants indicate - an Equable, not a Tropical Climate.—Conditions necessary for Preservation - of Coal Plants.—Oscillations of Sea-level necessarily implied.—Why - our Coal-fields contain more than One Coal-seam.—Time required - to form a Bed of Coal.—Why Coal Strata contain so little - evidence of Ice-action.—Land Flat during Coal Period.—Leading Idea - of the Theory.—Carboniferous Limestones</td> - <td class="tdr"><div>420</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XXVII">CHAPTER XXVII.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>PATH OF THE ICE-SHEET IN NORTH-WESTERN EUROPE AND ITS - RELATIONS TO THE BOULDER CLAY OF CAITHNESS.</div></td> - </tr> - <tr> - <td class="tdl">Character of Caithness Boulder Clay.—Theories of the Origin of the Caithness - Clay.—Mr. Jamieson’s Theory.—Mr. C. W. Peach’s Theory.—The - proposed Theory.—Thickness of Scottish Ice-sheet.—Pentlands - striated on their Summits.—Scandinavian Ice-sheet.—North Sea filled - with Land-ice.—Great Baltic Glacier.—Jutland and Denmark crossed - by Ice.—Sir R. Murchison’s Observations.—Orkney, Shetland, and - Faroe Islands striated across.—Loess accounted for.—Professor Geikie’s - Suggestion.—Professor Geikie and B. N. Peach’s Observations on East - Coast of Caithness.—Evidence from Chalk Flints and Oolitic Fossils in - Boulder Clay</td> - <td class="tdr"><div>435</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XXVIII">CHAPTER XXVIII.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>NORTH OF ENGLAND ICE-SHEET, AND TRANSPORT OF WASTDALE - CRAG BLOCKS.</div></td> - </tr> - <tr> - <td class="tdl">Transport of Blocks; Theories of.—Evidence of Continental Ice.—Pennine - Range probably striated on Summit.—Glacial Drift in Centre of England.—Mr. - Lacy on Drift of Cotteswold Hills.—England probably - crossed by Land-ice.—Mr. Jack’s Suggestion.—Shedding of Ice North - and South.—South of England Ice-sheet.—Glaciation of West Somerset.—Why - Ice-markings are so rare in South of England.—Form of - Contortion produced by Land-ice</td> - <td class="tdr"><div>456</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XXIX">CHAPTER XXIX.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>EVIDENCE FROM BURIED RIVER CHANNELS OF A CONTINENTAL PERIOD - IN BRITAIN.</div></td> - </tr> - <tr> - <td class="tdl">Remarks on the Drift Deposits.—Examination of Drift by Borings.—Buried - River Channel from Kilsyth to Grangemouth.—Channels not - excavated by Sea nor by Ice.—Section of buried Channel at Grangemouth.—Mr. - Milne Home’s Theory.—German Ocean dry Land.—Buried - River Channel from Kilsyth to the Clyde.—Journal of Borings.—Marine - Origin of the Drift Deposits.—Evidence of Inter-glacial - Periods.—Oscillations of Sea-Level.—Other buried River Channels</td> - <td class="tdr"><div>466</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><a href="#CHAPTER_XXX">CHAPTER XXX.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THEORIES - OF GLACIER-MOTION.</div></td> - </tr> - <tr> - <td class="tdl">Why the Question of Glacier-motion has been found to be so difficult.—The - Regelation Theory.—It accounts for the Continuity of a Glacier, - but not for its Motion.—Gravitation proved by Canon Moseley insufficient - to shear the Ice of a Glacier.—Mr. Matthew’s Experiment.—No - Parallel between the bending of an Ice Plank and the shearing of - a Glacier.—Mr. Ball’s Objection to Canon Moseley’s Experiment.—Canon - Moseley’s Method of determining the Unit of Shear.—Defect of - Method.—Motion of a Glacier in some Way dependent on Heat.—Canon - Moseley’s Theory.—Objections to his Theory.—Professor James - Thomson’s Theory.—This Theory fails to explain Glacier-motion.—De - Saussure and Hopkins’s “Sliding” Theories.—M. Charpentier’s “Dilatation” - Theory.—Important Element in the Theory</td> - <td class="tdr"><div>495</div></td> - </tr> - <tr> - <td colspan="2" class="tdc1"><div><span class="pagenum" id="Page_xvi">xvi18</span><a href="#CHAPTER_XXXI">CHAPTER XXXI.</a></div></td> - </tr> - <tr> - <td colspan="2" class="tdc2 small"><div>THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THE - MOLECULAR THEORY.</div></td> - </tr> - <tr> - <td class="tdl">Present State of the Question.—Heat necessary to the Motion of a Glacier.—Ice - does not shear in the Solid State.—Motion of a Glacier <em>molecular</em>.—How - Heat is transmitted through Ice.—Momentary Loss of Shearing - Force.—The <i lang="fr">Rationale</i> of Regelation.—The Origin of “Crevasses.”—Effects - of Tension.—Modification of Theory.—Fluid Molecules - crystallize in Interstices.—Expansive Force of crystallizing - Molecules a Cause of Motion.—Internal molecular Pressure the chief - Moving Power.—How Ice can excavate a Rock Basin.—How Ice can - ascend a Slope.—How deep River Valleys are striated across.—A - remarkable Example in the Valley of the Tay.—How Boulders can he - carried from a lower to a higher Level</td> - <td class="tdr"><div>514</div></td> - </tr> - </tbody> - </table> - - <table class="mt5" summary="Appendix contents"> - <tbody> - <tr> - <td colspan="3" class="tdc2"><div>APPENDIX.</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#APPENDIX_I">I.</a></div></td> - <td class="tdj">Opinions expressed previous to 1864 regarding the Influence of the - Eccentricity of the Earth’s Orbit on Climate</td> - <td class="tdr"><div>528</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#APPENDIX_II">II.</a></div></td> - <td class="tdj">On the Nature of Heat-Vibrations</td> - <td class="tdr"><div>544</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#APPENDIX_III">III.</a></div></td> - <td class="tdj">On the Reason why the Difference of Reading between a Thermometer - exposed to direct Sunshine and One Shaded diminishes as we - ascend in the Atmosphere</td> - <td class="tdr"><div>547</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#APPENDIX_IV">IV.</a></div></td> - <td class="tdj">Remarks on Mr. J. Y. Buchanan’s Theory of the Vertical Distribution - of Temperature of the Ocean</td> - <td class="tdr"><div>550</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#APPENDIX_V">V.</a></div></td> - <td class="tdj">On the Cause of the Cooling Effect produced on Solids by Tension</td> - <td class="tdr"><div>552</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#APPENDIX_VI">VI.</a></div></td> - <td class="tdj">The Cause of Regelation</td> - <td class="tdr"><div>554</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#APPENDIX_VII">VII.</a></div></td> - <td class="tdj">List of Papers which have appeared in Dr. A. Petermann’s <cite>Geographische - Mittheilungen</cite> relating to the Gulf-stream and Thermal Condition of the Arctic Regions</td> - <td class="tdr"><div>556</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#APPENDIX_VIII">VIII.</a></div></td> - <td class="tdj">List of Papers by the Author to which Reference is made in this Volume</td> - <td class="tdr"><div>560</div></td> - </tr> - <tr> - <td colspan="3" class="tdc"><div>————</div></td> - </tr> - <tr> - <td class="chapnum smcap"><div><a href="#INDEX">Index</a></div></td> - <td> </td> - <td class="tdr"><div>563</div></td> - </tr> - </tbody> - </table> - - <hr class="page" /> - <div class="chapter" id="LIST_OF_PLATES"> - <span class="pagenum" id="Page_xvii">xvii</span> - <h2>LIST OF PLATES.</h2> - </div> - - <div class="figcenter"> - <img src="images/diamondbar.png" width="100" height="8" alt="" /> - </div> - - <table summary="List of plates"> - <tbody> - <tr> - <td colspan="2" class="tdl smcap">Earth’s Orbit when Eccentricity is at its Superior Limit</td> - <td class="tdr"><div><a href="#Frontispiece"><i>Frontispiece.</i></a></div></td> - </tr> - <tr> - <td class="tdr xsmall"><div>PLATE</div></td> - <td> </td> - <td class="tdr xxsmall"><div><i>To face page</i></div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#PLATE_I">I.</a></div></td> - <td class="tdl smcap pt2">Showing Agreement between the System of Ocean-Currents and Winds</td> - <td class="tdr"><div>212</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#PLATE_II">II.</a></div></td> - <td class="tdl smcap pt2">Showing how opposing Currents intersect each other</td> - <td class="tdr"><div>219</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#PLATE_III">III.</a></div></td> - <td class="tdl smcap pt2">Section of Mid-Atlantic</td> - <td class="tdr"><div>222</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#PLATE_IV">IV.</a></div></td> - <td class="tdl smcap pt2">Diagram representing the Variations of Eccentricity - of the Earth’s Orbit</td> - <td class="tdr"><div>313</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#PLATE_V">V.</a></div></td> - <td class="tdl smcap pt2">Showing probable Path of the Ice in North-Western Europe</td> - <td class="tdr"><div>449</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#PLATE_VI">VI.</a></div></td> - <td class="tdl smcap pt2">Showing Path of Ice across Caithness</td> - <td class="tdr"><div>453</div></td> - </tr> - <tr> - <td class="chapnum"><div><a href="#PLATE_VII">VII.</a></div></td> - <td class="tdl smcap pt2">Map of the Midland Valley (Scotland), showing buried River Channels</td> - <td class="tdr"><div>471</div></td> - </tr> - </tbody> - </table> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_I"> - <span class="pagenum" id="Page_1">1</span> - <h2> - CHAPTER I.<br /><br /> - <span class="small">INTRODUCTION.</span> - </h2> - </div> - <div class="subhead">The Fundamental Problem of Geology.—Geology a Dynamical - Science.—The Nature of a Geological Principle.—Theories - of Geological Climate.—Geological Climate dependent - on Astronomical Causes.—An Important Consideration - overlooked.—Abstract of the Line of Argument pursued in the - Volume.</div> - - <p><em>The Fundamental Problem of Geology.</em>—The investigation of the - successive changes and modifications which the earth’s crust has - undergone during past ages is the province of geology. It will be - at once admitted that an acquaintance with the agencies by means of - which those successive changes and modifications were effected, is of - paramount importance to the geologist. What, then, are those agencies? - Although volcanic and other subterranean eruptions, earthquakes, - upheavals, and subsidences of the land have taken place in all ages, - yet no truth is now better established than that it is not by these - convulsions and cataclysms of nature that those great changes were - effected. It was rather by the ordinary agencies that we see every day - at work around us, such as rain, rivers, heat and cold, frost and snow. - The valleys were not produced by violent dislocations, nor the hills - by sudden upheavals, but were actually carved out of the solid rock, - silently and gently, by the agencies to which we have referred. “The - tools,” to quote the words of Professor Geikie, “by which this great - work has been done are of the simplest and most every-day order—the - air, rain, frosts, springs, brooks, rivers, glaciers, icebergs, and the - sea. These tools have been at work from the earliest times of which - any geological record has been preserved. Indeed, it is out of<span class="pagenum" id="Page_2">2</span> the - accumulated chips and dust which they have made, afterwards hardened - into solid rock and upheaved, that the very framework of our continents - has been formed.”<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">[1]</a></p> - - <p>It will be observed—and this is the point requiring particular - attention—that the agencies referred to are the ordinary meteorological - or climatic agencies. In fact, it is these agencies which constitute - climate. The various peculiarities or modifications of climate result - from a preponderance of one or more of these agencies over the rest. - When heat, for example, predominates, we have a hot or tropical - climate. When cold and frost predominate, we have a rigorous or arctic - climate. With moisture in excess, we have a damp and rainy climate; - and so on. But this is not all. These climatic agencies are not only - the factors which carved out the rocky face of the globe into hill - and dale, and spread over the whole a mantle of soil; but by them are - determined the character of the <i lang="la">flora</i> and <i lang="la">fauna</i> which exist on - that soil. The flora and fauna of a district are determined mainly by - the character of the climate, and not by the nature of the soil, or - the conformation of the ground. It is from difference of climate that - tropical life differs so much from arctic, and both these from the life - of temperate regions. It is climate, and climate alone, that causes - the orange and the vine to blossom, and the olive to flourish, in the - south, but denies them to the north, of Europe. It is climate, and - climate alone, that enables the forest tree to grow on the plain, but - not on the mountain top; that causes wheat and barley to flourish on - the mainland of Scotland, but not on the steppes of Siberia.</p> - - <p>Again, if we compare flat countries with mountainous, highlands with - lowlands, or islands with continents, we shall find that difference of - climatic conditions is the chief reason why life in the one differs - so much from life in the other. And if we turn to the sea we find - that organic life is there as much under the domain of climate as on - the land, only the conditions are much less complex. For in the case - of the sea, difference in the temperature of the water may be said - to constitute almost the only <span class="pagenum" id="Page_3">3</span>difference of climatic conditions. - If there is one fact more clearly brought out than another by the - recent deep-sea explorations, it is this, that nothing exercises so - much influence on organic life in the ocean as the temperature of the - water. In fact, so much is this the case, that warm zones were found - to be almost equivalent to zones of life. It was found that even the - enormous pressure at the bottom of the ocean does not exercise so much - influence on life as the temperature of the water. There are few, I - presume, who reflect on the subject that will not readily admit that, - whether as regards the great physical changes which are taking place on - the surface of our globe, or as regards the growth and distribution of - plant and animal life, the ordinary climatic agents are the real agents - at work, and that, compared with them, all other agencies sink into - insignificance.</p> - - <p>It will also be admitted that what holds true of the present holds - equally true of the past. Climatic agents are not only now the most - important and influential; they have been so during all past geological - ages. They were so during the Cainozoic as much as during the present; - and there is no reason for supposing they were otherwise during the - remoter Mesozoic and Palæozoic epochs. They have been the principal - factors concerned in that long succession of events and changes which - have taken place since the time of the solidification of the earth’s - crust. The stratified rocks of the globe contain all the records which - now remain of their action, and it is the special duty of the geologist - to investigate and read those records. It will be at once admitted that - in order to a proper understanding of the events embodied in these - records, an acquaintance with the agencies by which they were produced - is of the utmost importance. In fact, it is only by this means that we - can hope to arrive at their rational explanation. A knowledge of the - agents, and of the laws of their operations, is, in all the physical - sciences, the means by which we arrive at a rational comprehension - of the effects produced. If we have before us some complex and - intricate effects which have been<span class="pagenum" id="Page_4">4</span> produced by heat, or by light, or by - electricity, &c., in order to understand them we must make ourselves - acquainted with the agents by which they were produced and the laws of - their action. If the effects to be considered be, for example, those of - heat, then we must make ourselves acquainted with this agent and its - laws. If they be of electricity, then a knowledge of electricity and - its laws becomes requisite.</p> - - <p>This is no mere arbitrary mode of procedure which may be adopted in - one science and rejected in another. It is in reality a necessity of - thought arising out of the very constitution of our intellect; for the - objective law of the agent is the conception by means of which the - effects are subjectively united in a rational unity. We may describe, - arrange, and classify the effects as we may, but without a knowledge of - the laws of the agent we can have no rational unity. We have not got - the higher conception by which they can be <em>comprehended</em>. It is this - relationship between the effects and the laws of the agent, a knowledge - of which really constitutes a science. We might examine, arrange, and - describe for a thousand years the effects produced by heat, and still - we should have no science of heat unless we had a knowledge of the - laws of that agent. The effects would never be seen to be necessarily - connected with anything known to us; we could not connect them with - any rational principle from which they could be deduced <i lang="la">à priori</i>. - The same remarks hold, of course, equally true of all sciences, in - which the things to be considered stand in the relationship of cause - and effect. Geology is no exception. It is not like systematic botany, - a mere science of classification. It has to explain and account for - effects produced; and these effects can no more be explained without - a knowledge of the laws of the agents which produced them, than can - the effects of heat without a knowledge of the laws of heat. The only - distinction between geology and heat, light, electricity, &c., is, - that in geology the effects to be explained have almost all occurred - already, whereas in these other sciences effects actually taking place - have to be explained. But this distinction is of no<span class="pagenum" id="Page_5">5</span> importance to - our present purpose, for effects which have already occurred can no - more be explained without a knowledge of the laws of the agent which - produced them than can effects which are in the act of occurring. It - is, moreover, not strictly true that all the effects to be explained - by the geologist are already past. It falls within the scope of his - science to account for the changes which are at present taking place on - the earth’s crust.</p> - - <p>No amount of description, arrangement, and classification, however - perfect or accurate, of the facts which come under the eye of the - geologist can ever constitute a science of geology any more than a - description and classification of the effects of heat could constitute - a science of heat. This will, no doubt, be admitted by every one who - reflects upon the subject, and it will be maintained that geology, - like every other science, must possess principles applicable to the - facts. But here confusion and misconception will arise unless there be - distinct and definite ideas as to what ought to constitute a geological - principle. It is not every statement or rule that may apply to a great - many facts, which will constitute a geological principle. A geological - principle must bear the same characteristics as the principles of those - sciences to which we have referred. What, then, is the nature of the - principles of light, heat, electricity, &c.? The principles of heat - are the laws of heat. The principles of electricity are the laws of - electricity. And these laws are nothing more nor less than the ways - according to which these agents produce their effects. The principles - of geology are therefore the laws of geology. But the laws of geology - must be simply the laws of the geological agents, or, in other words, - the methods by which they produce their effects. Any other so-called - principle can be nothing more than an empirical rule, adopted for - convenience. Possessing no rationality in itself, it cannot be justly - regarded as a principle. In order to rationality the principle must be - either resolvable into, or logically deducible from, the laws of the - agents. Unless it possess this quality we cannot give the explanation - <i lang="la">à priori</i>.</p> - - <p><span class="pagenum" id="Page_6">6</span></p> - - <p>The reason of all this is perfectly obvious. The things to be explained - are effects; and the relationship between cause and effect affords the - subjective connection between the principle and the explanation. The - explanation follows from the principle simply as the effect results - from the laws of the agent or cause.</p> - - <p><em>Theories of Geological Climate.</em>—We have already seen that the - geological agents are chiefly the ordinary climatic agents. - Consequently, the main principles of geology must be the laws of the - climatic agents, or some logical deductions from them. It therefore - follows that, in order to a purely scientific geology, the grand - problem must be one of geological climate. It is through geological - climate that we can hope to arrive ultimately at principles which will - afford a rational explanation of the multifarious facts which have - been accumulating during the past century. The facts of geology are - as essential to the establishment of the principles, as the facts of - heat, light, and electricity are essential to the establishment of the - principles of these sciences. A theory of geological climate devised - without reference to the facts would be about as worthless as a theory - of heat or of electricity devised without reference to the facts of - these sciences.</p> - - <p>It has all along been an admitted opinion among geologists that the - climatic condition of our globe has not, during past ages, been - uniformly the same as at present. For a long time it was supposed that - during the Cambrian, Silurian, and other early geological periods, the - climate of our globe was much hotter than now, and that ever since - it has been gradually becoming cooler. And this high temperature of - Palæozoic ages was generally referred to the influence of the earth’s - internal heat. It has, however, been proved by Sir William Thomson<a id="FNanchor_2" href="#Footnote_2" class="fnanchor">[2]</a> - that the general climate of our globe could not have been sensibly - affected by internal heat at any time more than ten thousand years - after the commencement of the solidification of the surface. This - physicist has proved that the present influence of internal heat on - the temperature amounts to about only 1/75th of a degree. Not only - is the theory of internal <span class="pagenum" id="Page_7">7</span>heat now generally abandoned, but it is - admitted that we have no good geological evidence that climate was much - hotter during Palæozoic ages than now; and much less, that it has been - becoming <em>uniformly</em> colder.</p> - - <p>The great discovery of the glacial epoch, and more lately that of a - mild and temperate condition of climate extending during the Miocene - and other periods to North Greenland, have introduced a complete - revolution of ideas in reference to geological climate. Those - discoveries showed that our globe has not only undergone changes of - climate, but changes of the most extraordinary character. They showed - that at one time not only an arctic condition of climate prevailed in - our island, but that the greater part of the temperate region down - to comparatively low latitudes was buried under ice, while at other - periods Greenland and the Arctic regions, probably up to the North - Pole, were not only free from ice, but were covered with a rich and - luxuriant vegetation.</p> - - <p>To account for these extraordinary changes of climate has generally - been regarded as the most difficult and perplexing problem which has - fallen to the lot of the geologist. Some have attempted to explain - them by assuming a displacement of the earth’s axis of rotation in - consequence of the uprising of large mountain masses on some part - of the earth’s surface. But it has been shown by Professor Airy,<a id="FNanchor_3" href="#Footnote_3" class="fnanchor">[3]</a> - Sir William Thomson,<a id="FNanchor_4" href="#Footnote_4" class="fnanchor">[4]</a> and others, that the earth’s equatorial - protuberance is such that no geological change on its surface could - ever possibly alter the position of the axis of rotation to an extent - which could at all sensibly affect climate. Others, again, have tried - to explain the change of climate by supposing, with Poisson, that the - earth during its past geological history may have passed through hotter - and colder parts of space. This is not a very satisfactory hypothesis. - There is no doubt a difference in the quantity of force in the form of - heat passing through different parts of space; but space itself is not - a substance <span class="pagenum" id="Page_8">8</span>which can possibly be either cold or hot. If, therefore, - we were to adopt this hypothesis, we must assume that the earth during - the hot periods must have been in the vicinity of some other great - source of heat and light besides the sun. But the proximity of a - mass of such magnitude as would be sufficient to affect to any great - extent the earth’s climate would, by its gravity, seriously disarrange - the mechanism of our solar system. Consequently, if our solar system - had ever, during any former period of its history, really come into - the vicinity of such a mass, the orbits of the planets ought at the - present day to afford some evidence of it. But again, in order to - account for a cold period, such as the glacial epoch, we have to assume - that the earth must have come into the vicinity of a cold body.<a id="FNanchor_5" href="#Footnote_5" class="fnanchor">[5]</a> - But recent discoveries in regard to inter-glacial periods are wholly - irreconcilable with this theory.</p> - - <p>A change in the obliquity of the ecliptic has frequently been, and - still is, appealed to as an explanation of geological climate. This - theory appears, however, to be beset by a twofold objection: (1), it - can be shown from celestial mechanics, that the variations in the - obliquity of the ecliptic must always have been so small that they - could not materially affect the climatic condition of the globe; and - (2), even admitting that the obliquity could change to an indefinite - extent, it can be shown<a id="FNanchor_6" href="#Footnote_6" class="fnanchor">[6]</a> that no increase or decrease, however great, - could possibly account for either the glacial epoch or a warm temperate - condition of climate in polar regions.</p> - - <p>The theory that the sun is a variable star, and that the glacial - epochs of the geologists may correspond to periods of decrease in the - sun’s heat, has lately been advanced. This theory is also open to two - objections: (1), a general diminution of heat<a id="FNanchor_7" href="#Footnote_7" class="fnanchor">[7]</a> never could produce - a glacial epoch; and (2), even if it could, it would not explain - inter-glacial periods.</p> - - <p>The only other theory on the subject worthy of notice is that - <span class="pagenum" id="Page_9">9</span> - of Sir Charles Lyell. Those extraordinary changes of climate are, - according to his theory, attributed to differences in the distribution - of land and water. Sir Charles concludes that, were the land all - collected round the poles, while the equatorial zones were occupied by - the ocean, the general temperature would be lowered to an extent that - would account for the glacial epoch. And, on the other hand, were the - land all collected along the equator, while the polar regions were - covered with sea, this would raise the temperature of the globe to an - enormous extent. It will be shown in subsequent chapters that this - theory does not duly take into account the prodigious influence exerted - on climate by means of the heat conveyed from equatorial to temperate - and polar regions by means of ocean-currents. In Chapters <a href="#CHAPTER_II">II.</a> and <a href="#CHAPTER_III">III.</a> - I have endeavoured to prove (1), that were it not for the heat conveyed - from equatorial to temperate and polar regions by this means, the - thermal condition of the globe would be totally different from what it - is at present; and (2), that the effect of placing all the land along - the equator would be diametrically the opposite of that which Sir - Charles supposes.</p> - - <p>But supposing that difference in the distribution of land and water - would produce the effects attributed to it, nevertheless it would not - account for those extraordinary changes of climate which have occurred - during geological epochs. Take, for example, the glacial epoch. - Geologists almost all agree that little or no change has taken place - in the relative distribution of sea and land since that <em>epoch</em>. All - our main continents and islands not only existed then as they do now, - but every year is adding to the amount of evidence which goes to show - that so recent, geologically considered, is the glacial epoch that the - very contour of the surface was pretty much the same then as it is at - the present day. But this is not all; for even should we assume (1), - that a difference in the distribution of sea and land would produce the - effects referred to, and (2), that we had good geological evidence to - show that at a very recent period a form of distribution existed which - would produce the necessary<span class="pagenum" id="Page_10">10</span> glacial conditions, still the glacial - epoch would not be explained, for the phenomena of warm inter-glacial - periods would completely upset the theory.</p> - - <p><em>Geological Climate depending on Astronomical Causes.</em>—For a good many - years past, an impression has been gradually gaining ground amongst - geologists that the glacial epoch, as well as the extraordinary - condition of climate which prevailed in arctic regions during the - Miocene and other periods, must some way or other have resulted from - a cosmical cause; but all seemed at a loss to conjecture what that - cause could possibly be. It was apparent that the cosmical cause must - be sought for in the relations of our earth to the sun; but a change - in the obliquity of the ecliptic and the eccentricity of the earth’s - orbit are the only changes from which any sensible effect on climate - could possibly be expected to result. It was shown, however, by Laplace - that the change of obliquity was confined within so narrow limits that - it has scarcely ever been appealed to as a cause seriously affecting - climate. The only remaining cause to which appeal could be made was - the change in the eccentricity of the earth’s orbit—precession of the - equinoxes without eccentricity producing, of course, no effect whatever - on climate. Upwards of forty years ago Sir John Herschel and a few - other astronomers directed their attention to the consideration of this - cause, but the result arrived at was adverse to the supposition that - change of eccentricity could greatly affect the climate of our globe.</p> - - <p>As some misapprehension seems to prevail with reference to this, I - would take the liberty of briefly adverting to the history of the - matter,—referring the reader to the Appendix for fuller details.</p> - - <p>About the beginning of the century some writers attributed the lower - temperature of the southern hemisphere to the fact that the sun remains - about seven days less on that hemisphere than on the northern; their - view being that the southern hemisphere on this account receives - seven days less heat than the northern. Sir Charles Lyell, in the - first edition of his “Principles,” <span class="pagenum" id="Page_11">11</span>published in 1830, refers to this - as a cause which might produce some slight effect on climate. Sir - Charles’s remarks seem to have directed Sir John Herschel’s attention - to the subject, for in the latter part of the same year he read a - paper before the Geological Society on the astronomical causes which - may influence geological phenomena, in which, after pointing out the - mistake into which Sir Charles had been led in concluding that the - southern hemisphere receives less heat than the northern, he considers - the question as to whether geological climate could be influenced by - changes in the eccentricity of the earth’s orbit. He did not appear at - the time to have been aware of the conclusions arrived at by Lagrange - regarding the superior limit of the eccentricity of the earth’s orbit; - but he came to the conclusion that possibly the climate of our globe - may have been affected by variations in the eccentricity of its orbit. - “An amount of variation,” he says, “which we need not hesitate to - admit (at least provisionally) as a possible one, may be productive - of considerable diversity of climate, and may operate during great - periods of time either to mitigate or to exaggerate the difference of - winter and summer temperatures, so as to produce alternately in the - same latitude of either hemisphere a perpetual spring, or the extreme - vicissitudes of a burning summer and a rigorous winter.”</p> - - <p>This opinion, however, was unfortunately to a great extent nullified - by the statement which shortly afterwards appeared in his “Treatise - on Astronomy,” and also in the “Outlines of Astronomy,” to the effect - that the elliptic form of the earth’s orbit has but a very trifling - influence in producing variation of temperature corresponding to the - sun’s distance; the reason being that whatever may be the ellipticity - of the orbit, it follows that equal amounts of heat are received - from the sun in passing over equal angles round it, in whatever part - of the ellipse those angles may be situated. Those angles will of - course be described in unequal times, but the greater proximity of - the sun exactly compensates for the more rapid description, and thus - an equilibrium of heat is maintained. The sun, for example, is - <span class="pagenum" id="Page_12">12</span> much - nearer the earth when he is over the southern hemisphere than he is - when over the northern; but the southern hemisphere does not on this - account receive more heat than the northern; for, owing to the greater - velocity of the earth when nearest the sun, the sun does not remain - so long on the southern hemisphere as he does on the northern. These - two effects so exactly counterbalance each other that, whatever be - the extent of the eccentricity, the total amount of heat reaching - both hemispheres is the same. And he considered that this beautiful - compensating principle would protect the climate of our globe from - being seriously affected by an increase in the eccentricity of its - orbit, unless the extent of that increase was very great.</p> - - <p>“Were it not,” he says, “for this, the eccentricity of the orbit - would materially influence the transition of seasons. The fluctuation - of distance amounts to nearly 1/30th of its mean quantity, and - consequently the fluctuation in the sun’s direct heating power to - double this, or 1/15th of the whole. Now the perihelion of the orbit is - situated nearly at the place of the northern winter solstice; so that, - were it not for the compensation we have just described, the effect - would be to exaggerate the difference of summer and winter in the - southern hemisphere, and to moderate it in the northern; thus producing - a more violent alternation of climate in the one hemisphere, and an - approach to perpetual spring in the other. As it is, however, no such - inequality subsists, but an equal and impartial distribution of heat - and light is accorded to both.”<a id="FNanchor_8" href="#Footnote_8" class="fnanchor">[8]</a></p> - - <p>Herschel’s opinion was shortly afterwards adopted and advocated by - Arago<a id="FNanchor_9" href="#Footnote_9" class="fnanchor">[9]</a> and by Humboldt.<a id="FNanchor_10" href="#Footnote_10" class="fnanchor">[10]</a></p> - - <p>Arago, for example, states that so little is the climate of our globe - affected by the eccentricity of its orbit, that even were the orbit to - become as eccentric as that of the planet Pallas (that is, as great as - 0·24), “still this would not alter in <span class="pagenum" id="Page_13">13</span>any appreciable manner the mean - thermometrical state of the globe.”</p> - - <p>This idea, supported by these great authorities, got possession of the - public mind; and ever since it has been almost universally regarded - as settled that the great changes of climate indicated by geological - phenomena could not have resulted from any change in the relation of - the earth to the sun.</p> - - <p>There is, however, one effect that was not regarded as compensated. The - total amount of heat received by the earth is inversely proportional - to the minor axis of its orbit; and it follows, therefore, that the - greater the eccentricity, the greater is the total amount of heat - received by the earth. On this account it was concluded that an - increase of eccentricity would tend to a certain extent to produce a - warmer climate.</p> - - <p>All those conclusions to which I refer, arrived at by astronomers, are - perfectly legitimate so far as the direct effects of eccentricity are - concerned; and it was quite natural, and, in fact, proper to conclude - that there was nothing in the mere increase of eccentricity that could - produce a glacial epoch. How unnatural would it have been to have - concluded that an increase in the quantity of heat received from the - sun should lower the temperature, and cover the country with snow and - ice! Neither would excessively cold winters, followed by excessively - hot summers, produce a glacial epoch. To assert, therefore, that the - purely astronomical causes could produce such an effect would be simply - absurd.</p> - - <p><em>Important Consideration overlooked.</em>—The important fact, however, was - overlooked that, although the glacial epoch could not result <em>directly</em> - from an increase of eccentricity, it might nevertheless do so - <em>indirectly</em>. Although an increase of eccentricity could have no direct - tendency to lower the temperature and cover our country with ice, yet - it might bring into operation physical agents which would produce this - effect.</p> - - <p>If, instead of endeavouring to trace a direct connection between a high - condition of eccentricity and a glacial condition of climate, we turn - our attention to the consideration of what<span class="pagenum" id="Page_14">14</span> are the physical effects - which result from an increase of eccentricity, we shall find that a - host of physical agencies are brought into operation, the combined - effect of which is to lower to a very great extent the temperature of - the hemisphere whose winters occur in aphelion, and to raise to nearly - as great an extent the temperature of the opposite hemisphere, whose - winters of course occur in perihelion. Until attention was directed to - those physical circumstances to which I refer, it was impossible that - the true cause of the glacial epoch could have been discovered; and, - moreover, many of the indirect and physical effects, which in reality - were those that brought about the glacial epoch, could not, in the - nature of things, have been known previously to recent discoveries in - the science of heat.</p> - - <p>The consideration and discussion of those various physical agencies are - the chief aim of the following pages.</p> - - <p><em>Abstract of the Line of Argument pursued in this Volume.</em>—I shall - now proceed to give a brief abstract of the line of argument pursued - in this volume. But as a considerable portion of it is devoted to the - consideration of objections and difficulties bearing either directly - or indirectly on the theory, it will be necessary to point out what - those difficulties are, how they arose, and the methods which have been - adopted to overcome them.</p> - - <p><a href="#CHAPTER_IV">Chapter IV.</a> contains an outline of the physical agencies affecting - climate which are brought into operation by an increase of - eccentricity. By far the most important of all those agencies, and the - one which mainly brought about the glacial epoch, is the <em>Deflection</em> - of Ocean-Currents. The consideration of the indirect physical - connection between a high state of eccentricity and the deflection - of ocean-currents, and also the enormous influence on climate which - results from this deflection constitute not only the most important - part of the subject, but the one beset with the greatest amount of - difficulties.</p> - - <p>The difficulties besetting this part of the theory arise mainly from - the imperfect state of our knowledge, (1st) with reference - <span class="pagenum" id="Page_15">15</span> to the - absolute amount of heat transferred from equatorial to temperate and - polar regions by means of ocean-currents and the influence which the - heat thus transferred has on the distribution of temperature on the - earth’s surface; and (2nd) in connection with the physical cause of - ocean circulation.</p> - - <p>In Chapters <a href="#CHAPTER_II">II.</a> and <a href="#CHAPTER_III">III.</a> I have entered at considerable length into - the consideration of the effects of ocean currents on the distribution - of heat over the globe. The only current of which anything like - an accurate estimate of volume and temperature has been made is - the Gulf-stream. In reference to this stream we have a means of - determining in absolute measure the quantity of heat conveyed by it. - On the necessary computation being made, it is found that the amount - transferred by the Gulf-stream from equatorial regions into the North - Atlantic is enormously greater than was ever anticipated, amounting - to no less than one-fifth part of the entire heat possessed by the - North Atlantic. This striking fact casts a new light on the question - of the distribution of heat over the globe. It will be seen that to - such an extent is the temperature of the equatorial regions lowered, - and that of high temperate, and polar regions raised, by means of ocean - currents, that were they to cease, and each latitude to depend solely - on the heat received directly from the sun, only a very small portion - of the globe would be habitable by the present order of beings. This - being the case, it becomes obvious to what an extent the deflection - of ocean currents must affect temperature. For example, were the - Gulf-stream stopped, and the heat conveyed by it deflected into the - Southern Ocean, how enormously would this tend to lower the temperature - of the northern hemisphere, and raise the temperature south of the - equator.</p> - - <p>Chapters <a href="#CHAPTER_VI">VI.</a>, <a href="#CHAPTER_VII">VII.</a>, <a href="#CHAPTER_VIII">VIII.</a>, <a href="#CHAPTER_IX">IX.</a>, <a href="#CHAPTER_X">X.</a>, and <a href="#CHAPTER_XIII">XIII.</a>, are devoted to the - consideration of the physical cause of oceanic circulation. This has - been found to be the most difficult and perplexing part of the whole - inquiry. The difficulties mainly arise from the great diversity of - opinion and confusion of ideas prevailing<span class="pagenum" id="Page_16">16</span> in regard to the mechanics - of the subject. There are two theories propounded to account for - oceanic circulation; the one which may be called the <em>Wind</em> theory, and - the other the <em>Gravitation</em> theory; and this diversity of opinion and - confusion of ideas prevail in connection with both theories. As the - question of the cause of oceanic circulation has not only a direct and - important bearing on the subject of the present volume, but is further - one of much general interest, I have entered somewhat fully into the - matter.</p> - - <p>The Gravitation theories may be divided into two classes. The first of - these attributes the Gulf-stream and other sensible currents of the - ocean to difference of specific gravity, resulting from difference - of temperature between the sea in equatorial and polar regions. The - leading advocate of this theory was the late Lieutenant Maury, who - brought it so much into prominence in his interesting book on the - “Physical Geography of the Sea.” The other class does not admit that - the sensible currents of the ocean can be produced by difference of - specific gravity; but they maintain that difference of temperature - between the sea in equatorial and polar regions produces a general - movement of the upper portion of the sea from the equator to the - poles, and a counter-movement of the under portion from the poles - to the equator. This form of the gravitation theory has been ably - and zealously advocated by Dr. Carpenter, who may be regarded as - its representative. The Wind theories also divide into two classes. - According to the one ocean currents are caused and maintained by the - impulse of the trade-winds, while according to the other they are - due not to the impulse of the trade-winds alone, but to that of the - prevailing winds of the globe, regarded as a general system. The former - of these is the one generally accepted; the latter is that advocated in - the present volume.</p> - - <p>The relations which these theories bear to the question of secular - change of climate, will be found stated at length in <a href="#CHAPTER_VI">Chapter VI.</a> It - will, however, be better to state here in a few words what those - relations are. When the eccentricity of the<span class="pagenum" id="Page_17">17</span> earth’s orbit attains a - high value, the hemisphere, whose winter solstice occurs in aphelion, - has, for reasons which are explained in <a href="#CHAPTER_IV">Chapter IV.</a>, its temperature - lowered, while that of the opposite hemisphere is raised. Let us - suppose the northern hemisphere to be the cold one, and the southern - the warm one. The difference of temperature between the equator and - the North Pole will then be greater than between the equator and the - South Pole; according, therefore, to theory, the trades of the northern - hemisphere will be stronger than those of the southern, and will - consequently blow across the equator to some distance on the southern - hemisphere. This state of things will tend to deflect equatorial - currents southwards, impelling the warm water of the equatorial regions - more into the southern or warm hemisphere than into the northern or - cold hemisphere. The tendency of all this will be to exaggerate the - difference of temperature already existing between the two hemispheres. - If, on the other hand, the great ocean currents which convey the warm - equatorial waters to temperate and polar regions be not produced by - the impulse of the winds, but by difference of temperature, as Maury - maintains, then in the case above supposed the equatorial waters would - be deflected more into the northern or cold hemisphere than into the - southern or warm hemisphere, because the difference of temperature - between the equator and the poles would be greater on the cold than - on the warm hemisphere. This, of course, would tend to neutralize or - counteract that difference of temperature between the two hemispheres - which had been previously produced by eccentricity. In short, this - theory of circulation would effectually prevent eccentricity from - seriously affecting climate.</p> - - <p>Chapters <a href="#CHAPTER_VI">VI.</a> and <a href="#CHAPTER_VII">VII.</a> have been devoted to an examination of this form - of the gravitation theory.</p> - - <p>The above remarks apply equally to Dr. Carpenter’s form of the theory; - for according to a doctrine of General Oceanic Circulation resulting - from difference of specific gravity between the water at the equator - and at the poles, the equatorial water will<span class="pagenum" id="Page_18">18</span> be carried more to the - cold than to the warm hemisphere. It is perfectly true that a belief - in a general oceanic circulation may be held quite consistently with - the theory of secular changes of climate, provided it be admitted - that not this general circulation but ocean currents are the great - agency employed in distributing heat over the globe. The advocates of - the theory, however, admit no such thing, but regard ocean currents - as of secondary importance. It may be stated that the existence of - this general ocean circulation has never been detected by actual - observation. It is simply assumed in order to account for certain - facts, and it is asserted that such a circulation must take place as - a physical necessity. I freely admit that were it not that the warm - water of equatorial regions is being constantly carried off by means - of ocean currents such as the Gulf-stream, it would accumulate till, - in order to restoration of equilibrium, such a general movement as is - supposed would be generated. But it will be shown that the warm water - in equatorial regions is being drained off so rapidly by ocean currents - that the actual density of an equatorial column differs so little - from that of a polar column that the force of gravity resulting from - that difference is so infinitesimal that it is doubtful whether it is - sufficient to produce sensible motion. I have also shown in <a href="#CHAPTER_VIII">Chapter - VIII.</a> that all the facts which this theory is designed to explain are - not only explained by the wind theory, but are deducible from it as - necessary consequences. In <a href="#CHAPTER_XI">Chapter XI.</a> it is proved, by contrasting - the quantity of heat conveyed by ocean currents from inter-tropical to - temperate and polar regions with such an amount as could possibly be - conveyed by means of a general oceanic circulation, that the latter - sinks into insignificance before the former. In Chapters <a href="#CHAPTER_X">X.</a> and <a href="#CHAPTER_XII">XII.</a> - the various objections which have been advanced by Dr. Carpenter and - Mr. Findlay are discussed at considerable length, and in <a href="#CHAPTER_IX">Chapter IX.</a> - I have entered somewhat minutely into an examination of the mechanics - of the gravitation theory. A statement of the wind theory is given in - <a href="#CHAPTER_XIII">Chapter XIII.</a>; and in <a href="#CHAPTER_XIV">Chapter XIV.</a> is shown the relation of this theory - to the<span class="pagenum" id="Page_19">19</span> theory of Secular changes of climate. This terminates the part - of the inquiry relating to oceanic circulation.</p> - - <p>We now come to the <em>crucial test</em> of the theories respecting the cause - of the glacial epoch, viz., Warm Inter-glacial Periods. In Chapters - <a href="#CHAPTER_XV">XV.</a> and <a href="#CHAPTER_XVI">XVI.</a> I have given a statement of the geological facts which - go to prove that that long epoch known as the Glacial was not one - of continuous cold, but consisted of a succession of cold and warm - periods. This condition of things is utterly inexplicable on every - theory of the cause of the glacial epoch which has hitherto been - advanced; but, according to the physical theory of secular changes of - climate under consideration, it follows as a necessary consequence. - In fact, the amount of geological evidence which has already been - accumulated in reference to inter-glacial periods may now be regarded - as perfectly sufficient to establish the truth of that theory.</p> - - <p>If the glacial epoch resulted from some accidental distribution of sea - and land, then there may or may not have been more than one glacial - epoch, but if it resulted from the cause which we have assigned, then - there must have been during the geological history of the globe a - succession of glacial epochs corresponding to the secular variations - in the eccentricity of the earth’s orbit. A belief in the existence - of recurring glacial epochs has been steadily gaining ground for many - years past. I have, in <a href="#CHAPTER_XVIII">Chapter XVIII.</a>, given at some length the facts - on which this belief rests. It is true that the geological evidence of - glacial epochs in prior ages is meagre in comparison with that of the - glacial epoch of Post-tertiary times; but there is a reason for this in - the nature of geological evidence itself. <a href="#CHAPTER_XVII">Chapter XVII.</a> deals with the - geological records of former glacial epochs, showing that they are not - only imperfect, but that there is good reason why they should be so, - and that the imperfection of the records in reference to them cannot be - advanced as an argument against their existence.</p> - - <p>If the glacial epoch resulted from a high condition of eccentricity, we - have not only a means of determining the positive date of that epoch, - but we have also a means of determining<span class="pagenum" id="Page_20">20</span> geological time in absolute - measure. For if the glacial epochs of prior ages correspond to periods - of high eccentricity, then the intervals between those periods of high - eccentricity become the measure of the intervals between the glacial - epochs. The researches of Lagrange and Leverrier into the secular - variations of the elements of the orbits of the planets enable us - to determine with tolerable accuracy the values of the eccentricity - of the earth’s orbit for, at least, four millions of years past and - future. With the view of determining those values, I several years - ago computed from Leverrier’s formula the eccentricity of the earth’s - orbit and longitude of the perihelion, at intervals of ten thousand and - fifty thousand years during a period of three millions of years in the - past, and one million of years in the future. The tables containing - these values will be found in <a href="#CHAPTER_XIX">Chapter XIX.</a> These tables not only give - us the date of the glacial epoch, but they afford, as will be seen - from <a href="#CHAPTER_XXI">Chapter XXI.</a>, evidence as to the probable date of the Eocene and - Miocene periods.</p> - - <p>Ten years ago, when the theory was first advanced, it was beset by - a very formidable difficulty, arising from the opinions which then - prevailed in reference to geological time. One or two glacial epochs in - the course of a million of years was a conclusion which at that time - scarcely any geologist would admit, and most would have felt inclined - to have placed the last glacial epoch at least one million of years - back. But then if we assume that the glacial epoch was due to a high - state of eccentricity, we should be compelled to admit of at least two - glacial epochs during that lapse of time. It was the modern doctrine - that the great changes undergone by the earth’s crust were produced, - not by convulsions of nature, but by the slow and almost imperceptible - action, of rain, rivers, snow, frost, ice, &c., which impressed so - strongly on the mind of the geologist the vast duration of geological - periods. When it was considered that the rocky face of our globe had - been carved into hills and dales, and ultimately worn down to the - sea-level by means of those apparently trifling agents, not only once - or twice, but many times, during past ages, it was not surprising - <span class="pagenum" id="Page_21">21</span> that - the views entertained by geologists regarding the immense antiquity of - our globe should not have harmonised with the deductions of physical - science on the subject. It had been shown by Sir William Thomson and - others, from physical considerations relating to the age of the sun’s - heat and the secular cooling of our globe, that the geological history - of our earth’s crust must be limited to a period of something like - one hundred millions of years. But these speculations had but little - weight when pitted against the stern and undeniable facts of subaërial - denudation. How, then, were the two to be reconciled? Was it the - physicist who had under-estimated geological time, or the geologist - who had over-estimated it? Few familiar with modern physics, and who - have given special attention to the subject, would admit that the sun - could have been dissipating his heat at the present enormous rate for - a period much beyond one hundred millions of years. The probability - was that the amount of work performed on the earth’s crust by the - denuding agents in a period so immense as a million of years was, for - reasons stated in <a href="#CHAPTER_XX">Chapter XX.</a>, very much under-estimated. But the - difficulty was how to prove this. How was it possible to measure the - rate of operation of agents so numerous and diversified acting with - such extreme slowness and irregularity over so immense areas? In other - words, how was it possible to measure the rate of subaërial denudation? - Pondering over this problem about ten years ago, an extremely simple - and obvious method of solving it suggested itself to my mind. This - method—the details of which will be found in <a href="#CHAPTER_XX">Chapter XX.</a>—showed that - the rate of subaërial denudation is enormously greater than had been - supposed. The method is now pretty generally accepted, and the result - has already been to bring about a complete reconciliation between - physics and geology in reference to time.</p> - - <p><a href="#CHAPTER_XXI">Chapter XXI.</a> contains an account of the gravitation theories of the - origin of the sun’s heat. The energy possessed by the sun is generally - supposed to have been derived from gravitation, combustion being - totally inadequate as a source. But something more than gravitation - is required before we can<span class="pagenum" id="Page_22">22</span> account for even one hundred millions of - years’ heat. Gravitation could not supply even one-half that amount. - There must be some other and greater source than that of gravitation. - There is, however, as is indicated, an obvious source from which far - more energy may have been derived than could have been obtained from - gravitation.</p> - - <p>The method of determining the rate of subaërial denudation enables us - also to arrive at a rough estimate of the actual mean thickness of the - stratified rocks of the globe. It will be seen from <a href="#CHAPTER_XXII">Chapter XXII.</a> that - the mean thickness is far less than is generally supposed.</p> - - <p>The physical cause of the submergence of the land during the glacial - epoch, and the influence of change in the obliquity of the ecliptic on - climate, are next considered. In <a href="#CHAPTER_XXVI">Chapter XXVI.</a> I have given the reasons - which induce me to believe that coal is an inter-glacial formation.</p> - - <p>The next two chapters—the one on the path of the ice in north-western - Europe, the other on the north of England ice-sheet—are reprints of - papers which appeared a few years ago in the <cite>Geological Magazine</cite>. - Recent observations have confirmed the truth of the views advanced - in these two chapters, and they are rapidly gaining acceptance among - geologists.</p> - - <p>I have given, at the conclusion, a statement of the molecular theory of - glacier motion—a theory which I have been led to modify considerably on - one particular point.</p> - - <p>There is one point to which I wish particularly to direct - attention—viz., that I have studiously avoided introducing into the - theories propounded anything of a hypothetical nature. There is not, - so far as I am aware, from beginning to end of this volume, a single - hypothetical element: nowhere have I attempted to give a hypothetical - explanation. The conclusions are in every case derived either from - facts or from what I believe to be admitted principles. In short, I - have aimed to prove that the theory of secular changes of climate - follows, as a necessary consequence, from the admitted principles of - physical science.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_II"> - <span class="pagenum" id="Page_23">23</span> - <h2> - CHAPTER II.<br /><br /> - <span class="small">OCEANS-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER THE GLOBE.</span> - </h2> - </div> - <div class="subhead">The absolute Heating-power of Ocean-currents.—Volume of the - Gulf-stream.—Absolute Amount of Heat conveyed by it.—Greater - Portion of Moisture in inter-tropical Regions falls as Rain - in those Regions.—Land along the Equator tends to lower - the Temperature of the Globe.—Influence of Gulf-stream on - Climate of Europe.—Temperature of Space.—Radiation of a - Particle.—Professor Dove on Normal Temperature.—Temperature of - Equator and Poles in the Absence of Ocean-currents.—Temperature - of London, how much due to Ocean-currents.</div> - - <p><em>The absolute Heating-power of Ocean-currents.</em>—There is perhaps no - physical agent concerned in the distribution of heat over the surface - of the globe the influence of which has been so much underrated as that - of ocean-currents. This is, no doubt, owing to the fact that although - their surface-temperature, direction, and general influence have - obtained considerable attention, yet little or nothing has been done - towards determining the absolute amount of heat or of cold conveyed by - them or the resulting absolute increase or decrease of temperature.</p> - - <p>The modern method of determining the amount of heat-effects in absolute - measure is, doubtless, destined to cast new light on all questions - connected with climate, as it has done, and is still doing, in every - department of physics where energy, under the form of heat, is being - studied. But this method has hardly as yet been attempted in questions - of meteorology; and owing to the complicated nature of the phenomena - with which the meteorologist has generally to deal, its application - will very often prove practically impossible. Nevertheless, it is - particularly suitable to all questions relating to the direct<span class="pagenum" id="Page_24">24</span> thermal - effects of currents, whatever the nature of these currents may happen - to be.</p> - - <p>In the application of the method to an ocean-current, the two most - important elements required as data are the volume of the stream and - its mean temperature. But although we know something of the temperature - of most of the great ocean-currents, yet, with the exception of the - Gulf-stream, little has been ascertained regarding their volume.</p> - - <p>The breadth, depth, and temperature of the Gulf-stream have formed the - subject of extensive and accurate observations by the United States - Coast Survey. In the memoirs and charts of that survey cross-sections - of the stream at various places are given, showing its breadth and - depth, and also the temperature of the water from the surface to the - bottom. We are thus enabled to determine with some precision the - mean temperature of the stream. And knowing its mean velocity at any - given section, we have likewise a means of determining the number of - cubic feet of water passing through that section in a given time. But - although we can obtain with tolerable accuracy the mean temperature, - yet observations regarding the velocity of the water at all depths have - unfortunately not been made at any particular section. Consequently we - have no means of estimating as accurately as we could wish the volume - of the current. Nevertheless, since we know the surface-velocity of the - water at places where some of the sections were taken, we are enabled - to make at least a rough estimate of the volume.</p> - - <p>From an examination of the published sections, I came to the conclusion - some years ago<a id="FNanchor_11" href="#Footnote_11" class="fnanchor">[11]</a> that the total quantity of water conveyed by the - stream is probably equal to that of a stream fifty miles broad and - 1,000 feet deep,<a id="FNanchor_12" href="#Footnote_12" class="fnanchor">[12]</a> flowing at the rate of <span class="pagenum" id="Page_25">25</span>four miles an hour, - and that the mean temperature of the entire mass of moving water is - not under 65° at the moment of leaving the Gulf. But to prevent the - possibility of any objections being raised on the grounds that I may - have over-estimated the volume of the stream, I shall take the velocity - to be <em>two</em> miles instead of four miles an hour. We are warranted, - I think, in concluding that the stream before it returns from its - northern journey is on an average cooled down to at least 40°,<a id="FNanchor_13" href="#Footnote_13" class="fnanchor">[13]</a> - consequently it loses 25° of heat. Each cubic foot of water, therefore, - in this case carries from the tropics for distribution upwards of - 1,158,000 foot-pounds of heat. According to the above estimate of the - size and velocity of the stream, which in <a href="#CHAPTER_XI">Chapter XI.</a> will be shown - to be an under-estimate, 2,787,840,000,000 cubic feet of water are - conveyed from the Gulf per hour, or 66,908,160,000,000 cubic feet - daily. Consequently the total quantity of heat thus transferred per day - amounts to 77,479,650,000,000,000,000 foot-pounds.</p> - - <p>This estimate of the volume of the stream is considerably less by - one-half than that given both by Captain Maury and by Sir John - Herschel. Captain Maury considers the Gulf-stream equal to a stream - thirty-two miles broad and 1,200 feet deep, flowing at the rate of five - knots an hour.<a id="FNanchor_14" href="#Footnote_14" class="fnanchor">[14]</a> This gives 6,165,700,000,000 cubic feet per hour - as the quantity of water conveyed by this stream. Sir John Herschel’s - estimate is still greater. He considers it equal to a stream thirty - miles broad and 2,200 feet deep, flowing at the rate of four miles - an hour.<a id="FNanchor_15" href="#Footnote_15" class="fnanchor">[15]</a> This makes the quantity 7,359,900,000,000 cubic feet - per hour. Dr. Colding, in his elaborate memoir on the Gulf-stream, - estimates the volume at 5,760,000,000,000 cubic feet per hour, while - Mr. Laughton’s estimate is nearly double that of mine.</p> - - <p><span class="pagenum" id="Page_26">26</span></p> - - <p>From observations made by Sir John Herschel and by M. Pouillet on the - direct heat of the sun, it is found that, were no heat absorbed by the - atmosphere, about eighty-three foot-pounds per second would fall upon - a square foot of surface placed at right angles to the sun’s rays.<a id="FNanchor_16" href="#Footnote_16" class="fnanchor">[16]</a> - Mr. Meech estimates that the quantity of heat cut off by the atmosphere - is equal to about twenty-two per cent. of the total amount received - from the sun. M. Pouillet estimates the loss at twenty-four per cent. - Taking the former estimate, 64·74 foot-pounds per second will therefore - be the quantity of heat falling on a square foot of the earth’s surface - when the sun is in the zenith. And were the sun to remain stationary in - the zenith for twelve hours, 2,796,768 foot-pounds would fall upon the - surface.</p> - - <p>It can be shown that the total amount of heat received upon a unit - surface on the equator, during the twelve hours from sunrise till - sunset at the time of the equinoxes, is to the total amount which - would be received upon that surface, were the sun to remain in the - zenith during those twelve hours, as the diameter of a circle to half - its circumference, or as 1 to 1·5708. It follows, therefore, that - a square foot of surface on the equator receives from the sun at - the time of the equinoxes 1,780,474 foot-pounds daily, and a square - mile 49,636,750,000,000 foot-pounds daily. But this amounts to only - 1/1560935th part of the quantity of heat daily conveyed from the - tropics by the Gulf-stream. In other words, the Gulf-stream conveys as - much heat as is received from the sun by 1,560,935 square miles at the - equator. The amount thus conveyed is equal to all the heat which falls - upon the globe within thirty-two miles on each side of the equator. - According to calculations made by Mr. Meech,<a id="FNanchor_17" href="#Footnote_17" class="fnanchor">[17]</a> the annual quantity - of heat received by a unit surface on the frigid zone, taking the - mean of the whole zone, is 5·45/12th of that received at the equator; - consequently the quantity of heat conveyed by the Gulf-stream in one - year is<span class="pagenum" id="Page_27">27</span> - equal to the heat which falls on an average on 3,436,900 square miles - of the arctic regions. The frigid zone or arctic regions contain - 8,130,000 square miles. There is actually, therefore, nearly one-half - as much heat transferred from tropical regions by the Gulf-stream as - is received from the sun by the entire arctic regions, the quantity - conveyed from the tropics by the stream to that received from the sun - by the arctic regions being nearly as two to five.</p> - - <p>But we have been assuming in our calculations that the percentage of - heat absorbed by the atmosphere is no greater in polar regions than - it is at the equator, which is not the case. If we make due allowance - for the extra amount absorbed in polar regions in consequence of the - obliqueness of the sun’s rays, the total quantity of heat conveyed by - the Gulf-stream will probably be nearly equal to one-half the amount - received from the sun by the entire arctic regions.</p> - - <p>If we compare the quantity of heat conveyed by the Gulf-stream with - that conveyed by means of aërial currents, the result is equally - startling. The density of air to that of water is as 1 to 770, and - its specific heat to that of water is as 1 to 4·2; consequently the - same amount of heat that would raise 1 cubic foot of water 1° would - raise 770 cubic feet of air 4°·2, or 3,234 cubic feet 1°. The quantity - of heat conveyed by the Gulf-stream is therefore equal to that which - would be conveyed by a current of air 3,234 times the volume of the - Gulf-stream, at the same temperature and moving with the same velocity. - Taking, as before, the width of the stream at fifty miles, and its - depth at 1,000 feet, and its velocity at two miles an hour, it follows - that, in order to convey an equal amount of heat from the tropics by - means of an aërial current, it would be necessary to have a current - about 1¼ mile deep, and at the temperature of 65°, blowing at the - rate of two miles an hour from every part of the equator over the - northern hemisphere towards the pole. If its velocity were equal to - that of a good sailing-breeze, which Sir John Herschel states to be - about twenty-one miles an hour, the current would require to be<span class="pagenum" id="Page_28">28</span> above - 600 feet deep. A greater quantity of heat is probably conveyed by the - Gulf-stream alone from the tropical to the temperate and arctic regions - than by all the aërial currents which flow from the equator.</p> - - <p>We are apt, on the other hand, to over-estimate the amount of the heat - conveyed from tropical regions to us by means of aërial currents. The - only currents which flow from the equatorial regions are the upper - currents, or anti-trades as they are called. But it is not possible - that much heat can be conveyed directly by them. The upper currents of - the trade-winds, even at the equator, are nowhere below the snow-line; - they must therefore lie in a region of which the temperature is - actually below the freezing-point. In fact, if those currents were - warm, they would elevate the snow-line above themselves. The heated air - rising off the hot burning ground at the equator, after ascending a - few miles, becomes exposed to the intense cold of the upper regions of - the atmosphere; it then very soon loses all its heat, and returns from - the equator much colder than it went thither. It is impossible that - we can receive any heat directly from the equatorial regions by means - of aërial currents. It is perfectly true that the south-west wind, to - which we owe so much of our warmth in this country, is a continuation - of the anti-trade; but the heat which this wind brings to us is not - derived from the equatorial regions. This will appear evident, if we - but reflect that, before the upper current descends to the snow-line - after leaving the equator, it must traverse a space of at least 2,000 - miles; and to perform this long journey several days will be required. - During all this time the air is in a region below the freezing-point; - and it is perfectly obvious that by the time it begins to descend it - must have acquired the temperature of the region in which it has been - travelling.</p> - - <p>If such be the case, it is evident that a wind whose temperature - is below 32° could never warm a country such as ours, where the - temperature does not fall below 38° or 39°. The heat of our south-west - winds is derived, not directly from the<span class="pagenum" id="Page_29">29</span> equator, but from the warm - water of the Atlantic—in fact, from the Gulf-stream. The upper current - acquires its heat after it descends to the earth. There is one way, - however, whereby heat is indirectly conveyed from the equator by the - anti-trades; that is, in the form of aqueous vapour. In the formation - of one pound of water from aqueous vapour, as Professor Tyndall - strikingly remarks, a quantity of heat is given out sufficient to melt - five pounds of cast iron.<a id="FNanchor_18" href="#Footnote_18" class="fnanchor">[18]</a> It must, however, be borne in mind that - the greater part of the moisture of the south-west and west winds is - derived from the ocean in temperate regions. The upper current receives - the greater part of its moisture after it descends to the earth, whilst - the moisture received at the equator is in great part condensed, and - falls as rain in those regions.</p> - - <p>This latter assertion has been so frequently called in question - that I shall give my reasons for making it. According to Dr. Keith - Johnston (“Physical Atlas”) the mean rainfall of the torrid regions - is ninety-six inches per annum, while that of the temperate regions - amounts to only thirty-seven inches. If the greater part of the - moisture of the torrid regions does not fall as rain in those regions, - it must fall as such beyond them. Now the area of the torrid to that - of the two temperate regions is about as 39·3 to 51. Consequently - ninety-six inches of rain spread over the temperate regions would give - seventy-four inches; but this is double the actual rainfall of the - temperate regions. If, again, it were spread over both temperate and - polar regions this would yield sixty-four inches, which, however, is - nearly double the mean rainfall of the temperate and polar regions. If - we add to this the amount of moisture derived from the ocean within - temperate and polar regions, we should have a far greater rainfall for - these latitudes than for the torrid region, and we know, of course, - that it is actually far less. This proves the truth of the assertion - that by far the greater part of the moisture of the torrid regions - falls in those regions as rain. It will hardly do to object that the - above may <span class="pagenum" id="Page_30">30</span>probably be an over-estimate of the amount of rainfall in - the torrid zone, for it is not at all likely that any error will ever - be found which will affect the general conclusion at which we have - arrived.</p> - - <p>Dr. Carpenter, in proof of the small rainfall of the torrid zone, - adduces the case of the Red Sea, where, although evaporation is - excessive, almost no rain falls. But the reason why the vapour raised - from the Red Sea does not fall in that region as rain, is no doubt - owing to the fact that this sea is only a narrow strip of water in a - dry and parched land, the air above which is too greedy of moisture - to admit of the vapour being deposited as rain. Over a wide expanse - of ocean, however, where the air above is kept to a great extent in a - constant state of saturation, the case is totally different.</p> - - <p><em>Land at the Equator tends to Lower the Temperature of the Globe.</em>—The - foregoing considerations, as well as many others which might be stated, - lead to the conclusion that, in order to raise the mean temperature of - the whole earth, <em>water</em> should be placed along the equator, and not - <em>land</em>, as is supposed by Sir Charles Lyell and others. For if land is - placed at the equator, the possibility of conveying the sun’s heat from - the equatorial regions by means of ocean-currents is prevented. The - transference of heat could then be effected only by means of the upper - currents of the trades; for the heat conveyed by <em>conduction</em> along the - solid crust, if any, can have no sensible effect on climate. But these - currents, as we have just seen, are ill-adapted for conveying heat.</p> - - <p>The surface of the ground at the equator becomes intensely heated by - the sun’s rays. This causes it to radiate its heat more rapidly into - space than a surface of water heated under the same conditions. Again, - the air in contact with the hot ground becomes also more rapidly - heated than in contact with water, and consequently the ascending - current of air carries off a greater amount of heat. But were the - heat thus carried away transferred by means of the upper currents to - high latitudes and there employed to warm the earth, then it might to - a considerable <span class="pagenum" id="Page_31">31</span>extent compensate for the absence of ocean-currents, - and in this case land at the equator might be nearly as well adapted - as water for raising the temperature of the whole earth. But such is - not the case; for the heat carried up by the ascending current at the - equator is not employed in warming the earth, but is thrown off into - the cold stellar space above. This ascending current, instead of being - employed in warming the globe, is in reality one of the most effectual - means that the earth has of getting quit of the heat received from the - sun, and of thus maintaining a much lower temperature than it would - otherwise possess. It is in the equatorial regions that the earth loses - as well as gains the greater part of its heat; so that, of all places, - here ought to be placed the substance best adapted for preventing the - dissipation of the earth’s heat into space, in order to raise the - general temperature of the earth. Water, of all substances in nature, - seems to possess this quality to the greatest extent; and, besides, it - is a fluid, and therefore adapted by means of currents to carry the - heat which it receives from the sun to every region of the globe.</p> - - <p>These results show (although they have reference to only one stream) - that the general influence of ocean-currents on the distribution of - heat over the surface of the globe must be very great. If the quantity - of heat transferred from equatorial regions by the Gulf-stream - alone is nearly equal to all the heat received from the sun by the - arctic regions, then how enormous must be the quantity conveyed from - equatorial regions by all the ocean-currents together!</p> - - <p><em>Influence of the Gulf-stream on the Climate of Europe.</em>—In a paper - read before the British Association at Exeter, Mr. A. G. Findlay - objects to the conclusions at which I have arrived in former papers - on the subject, that I have not taken into account the great length - of time that the water requires in order to circulate, and the - interference it has to encounter in its passage.</p> - - <p>The objection is, that a stream so comparatively small as the - Gulf-stream, after spreading out over such a large area of the - Atlantic, and moving so slowly across to the shores of Europe,<span class="pagenum" id="Page_32">32</span> losing - heat all the way, would not be able to produce any very sensible - influence on the climate of Europe.</p> - - <p>I am unable to perceive the force of this objection. Why, the very - efficiency of the stream as a heating agent necessarily depends upon - the slowness of its motion. Did the Gulf-stream move as rapidly along - its whole course as it does in the Straits of Florida, it could produce - no sensible effect on the climate of Europe. It does not require much - consideration to perceive this. (1) If the stream during its course - continued narrow, deep, and rapid, it would have little opportunity of - losing its heat, and the water would carry back to the tropics the heat - which it ought to have given off in the temperate and polar regions. - (2) The Gulf-stream does not heat the shores of Europe by direct - radiation. Our island, for example, is not heated by radiation from a - stream of warm water flowing along its shores. The Gulf-stream heats - our island <em>indirectly</em> by heating the winds which blow over it to our - shores.</p> - - <p>The anti-trades, or upper return-currents, as we have seen, bring no - heat from the tropical regions. After traversing some 2,000 miles - in a region of extreme cold they descend on the Atlantic as a cold - current, and there absorb the heat and moisture which they carry to - north-eastern Europe. Those aërial currents derive their heat from the - Gulf-stream, or if it is preferred, from the warm water poured into the - Atlantic by the Gulf-stream.</p> - - <p>How, then, are these winds heated by the warm water? The air is heated - in two ways, viz., by direct <em>radiation</em> from the water, and by - <em>contact</em> with the water. Now, if the Gulf-stream continued a narrow - and deep current during its entire course similar to what it is at - the Straits of Florida, it could have little or no opportunity of - communicating its heat to the air either by radiation or by contact. If - the stream were only about forty or fifty miles in breadth, the aërial - particles in their passage across it would not be in contact with the - warm water more than an hour or two. Moreover, the number of particles - in contact with the water, owing to the narrowness of the<span class="pagenum" id="Page_33">33</span> stream, - would be small, and there would therefore be little opportunity for - the air becoming heated by contact. The same also holds true in regard - to radiation. The more we widen the stream and increase its area, the - more we increase its radiating surface; and the greater the radiating - surface, the greater is the quantity of heat thrown off. But this is - not all; the number of aërial particles heated by radiation increases - in proportion to the area of the radiating surface; consequently, the - wider the area over which the waters of the Gulf-stream are spread, - the more effectual will the stream be as a heating agent. And, again, - in order that a very wide area of the Atlantic may be covered with the - warm waters of the stream, slowness of motion is essential.</p> - - <p>Mr. Findlay supposes that fully one-half of the Gulf-stream passes into - the south-eastern branch, and that it is only the north-eastern branch - of the current that can be effectual in raising the temperature of - Europe. But it appears to me that it is to this south-eastern portion - of the current, and not to the north-eastern, that we, in this country, - are chiefly indebted for our heat. The south-west winds, to which we - owe our heat, derive their temperature from this south-eastern portion - which flows away in the direction of the Azores. The south-west winds - which blow over the northern portion of the current which flows past - our island up into the arctic seas cannot possibly cross this country, - but will go to heat Norway and northern Europe. The north-eastern - portion of the stream, no doubt, protects us from the ice of Greenland - by warming the north-west winds which come to us from that cold region.</p> - - <p>Mr. Buchan, Secretary of the Scottish Meteorological Society, has - shown<a id="FNanchor_19" href="#Footnote_19" class="fnanchor">[19]</a> that in a large tract of the Atlantic between latitudes 20° - and 40° N., the mean pressure of the atmosphere is greater than in any - other place on the globe. To the west of Madeira, between longitude - 10° and 40° W., the mean annual pressure amounts to 30·2 inches, while - between Iceland and Spitzbergen it is only 29·6, a lower mean pressure - than is found <span class="pagenum" id="Page_34">34</span>in any other place on the northern hemisphere. There - must consequently, he concludes, be a general tendency in the air to - flow from the former to the latter place along the earth’s surface. - Now, the air in moving from the lower to the higher latitudes tends - to take a north-easterly direction, and in this case will pass over - our island in its course. This region of high pressure, however, - is situated in the very path of the south-eastern branch of the - Gulf-stream, and consequently the winds blowing therefrom will carry - directly to Britain the heat of the Gulf-stream.</p> - - <p>As we shall presently see, it is as essential to the heating of our - island as to that of the southern portion of Europe, that a very large - proportion of the waters of the Gulf-stream should spread over the - surface of the Atlantic and never pass up into the arctic regions.</p> - - <p>Even according to Mr. Findlay’s own theory, it is to the south-west - wind, heated by the warm waters of the Atlantic, that we are indebted - for the high temperature of our climate. But he seems to be under the - impression that the Atlantic would be able to supply the necessary - heat independently of the Gulf-stream. This, it seems to me, is the - fundamental error of all those who doubt the efficiency of the stream. - It is a mistake, however, into which one is very apt to fall who does - not adopt the more rigid method of determining heat-results in absolute - measure. When we apply this method, we find that the Atlantic, without - the aid of such a current as the Gulf-stream, would be wholly unable to - supply the necessary amount of heat to the south-west winds.</p> - - <p>The quantity of heat conveyed by the Gulf-stream, as we have seen, - is equal to all the heat received from the sun by 1,560,935 square - miles at the equator. The mean annual quantity of heat received from - the sun by the temperate regions per unit surface is to that received - by the equator as 9·08 to 12.<a id="FNanchor_20" href="#Footnote_20" class="fnanchor">[20]</a> Consequently, the quantity of heat - conveyed by the stream is equal to all the heat received from the sun - by 2,062,960 square <span class="pagenum" id="Page_35">35</span>miles of the temperate regions. The total area of - the Atlantic from the latitude of the Straits of Florida, 200 miles - north of the tropic of Cancer, up to the Arctic Circle, including also - the German Ocean, is about 8,500,000 square miles. In this case the - quantity of heat carried by the Gulf-stream into the Atlantic through - the Straits of Florida, is to that received by this entire area from - the sun as 1 to 4·12, or in round numbers as 1 to 4. It therefore - follows that one-fifth of all the heat possessed by the waters of the - Atlantic over that area, even supposing that they absorb every ray that - falls upon them, is derived from the Gulf-stream. Would those who call - in question the efficiency of the Gulf-stream be willing to admit that - a decrease of one-fourth in the total amount of heat received from the - sun, over the entire area of the Atlantic from within 200 miles of - the tropical zone up to the arctic regions, would not sensibly affect - the climate of northern Europe? If they would not willingly admit - this, why, then, contend that the Gulf-stream does not affect climate? - for the stoppage of the Gulf-stream would deprive the Atlantic of - 77,479,650,000,000,000,000 foot-pounds of energy in the form of heat - per day, a quantity equal to one-fourth of all the heat received from - the sun by that area.</p> - - <p>How much, then, of the temperature of the south-west winds derived from - the water of the Atlantic is due to the Gulf-stream?</p> - - <p>Were the sun extinguished, the temperature over the whole earth - would sink to <em>nearly</em> that of stellar space, which, according to - the investigations of Sir John Herschel<a id="FNanchor_21" href="#Footnote_21" class="fnanchor">[21]</a> and of M. Pouillet,<a id="FNanchor_22" href="#Footnote_22" class="fnanchor">[22]</a> - is not above −239° F. Were the earth possessed of no atmosphere, the - temperature of its surface would sink to exactly that of space, or to - that indicated by a thermometer exposed to no other heat-influence than - that of radiation from the stars. But the presence of the atmospheric - envelope would slightly modify <span class="pagenum" id="Page_36">36</span>the conditions of things; for the - heat from the stars (which of course constitutes what is called the - temperature of space) would, like the sun’s heat, pass more freely - through the atmosphere than the heat radiated back from the earth, and - there would in consequence of this be an accumulation of heat on the - earth’s surface. The temperature would therefore stand a little higher - than that of space; or, in other words, it would stand a little higher - than it would otherwise do were the earth exposed in space to the - direct radiation of the stars without the atmospheric envelope. But, - for reasons which will presently be stated, we may in the meantime, - till further light is cast upon this matter, take −239° F. as probably - not far from what would be the temperature of the earth’s surface were - the sun extinguished.</p> - - <p>Suppose now that we take the mean annual temperature of the Atlantic - at, say, 56°.<a id="FNanchor_23" href="#Footnote_23" class="fnanchor">[23]</a> Then 239° + 56° = 295° represents the number of - degrees of rise due to the heat which it receives. In other words, - it takes all the heat that the Atlantic receives to maintain its - temperature 295° above the temperature of space. Stop the Gulf-stream, - and the Atlantic would be deprived of one-fifth of the heat which - it possesses. Then, if it takes five parts of heat to maintain a - temperature of 295° above that of space, the four parts which would - remain after the stream was stopped would only be able to maintain a - temperature of four-fifths of 295°, or 236° above that of space: the - stoppage of the Gulf-stream would therefore deprive the Atlantic of an - amount of heat which would be sufficient to maintain its temperature - 59° above what it would otherwise be, did it depend alone upon the heat - received directly from the sun. It does not, of course, follow that - the Gulf-stream actually maintains the temperature 59° above what it - would otherwise be were there no ocean-currents, because the actual - heating-effect of the stream is neutralized to a very considerable - extent by cold currents from <span class="pagenum" id="Page_37">37</span>the arctic regions. But 59° of rise - represents its actual power; consequently 59°, minus the lowering - effect of the cold currents, represents the actual rise. What the rise - may amount to at any particular place must be determined by other means.</p> - - <p>This method of calculating how much the temperature of the earth’s - surface would rise or fall from an increase or a decrease in the - absolute amount of heat received is that adopted by Sir John Herschel - in his “Outlines of Astronomy,” § 369<sup>a</sup>.</p> - - <p>About three years ago, in an article in the <cite>Reader</cite>, I endeavoured - to show that this method is not rigidly correct. It has been shown - from the experiments of Dulong and Petit, Dr. Balfour Stewart, - Professor Draper, and others, that the rate at which a body radiates - its heat off into space is not directly proportionate to its absolute - temperature. The rate at which a body loses its heat as its temperature - rises increases more rapidly than the temperature. As a body rises - in temperature the rate at which it radiates off its heat increases; - the <em>rate</em> of this increase, however, is not uniform, but increases - with the temperature. Consequently the temperature is not lowered in - proportion to the decrease of the sun’s heat. But at the comparatively - low temperature with which we have at present to deal, the error - resulting from assuming the decrease of temperature to be proportionate - to the decrease of heat would not be great.</p> - - <p>It may be remarked, however, that the experiments referred to were - made on solids; but, from certain results arrived at by Dr. Balfour - Stewart, it would seem that the radiation of a material particle may - be proportionate to its absolute temperature.<a id="FNanchor_24" href="#Footnote_24" class="fnanchor">[24]</a> This physicist found - that the radiation of a thick plate of glass increases more rapidly - than that of a thin plate as the temperature rises, and that, if we go - on continually diminishing the thickness of the plate whose radiation - at different temperatures we are ascertaining, we find that as it grows - thinner and thinner the rate at which it radiates off its heat as its - temperature rises becomes less and less. In other words, as the <span class="pagenum" id="Page_38">38</span>plate - grows thinner and thinner its rate of radiation becomes more and more - proportionate to its absolute temperature. And we can hardly resist the - conviction that if we could possibly go on diminishing the thickness - of the plate till we reached a film so thin as to embrace but only one - particle in its thickness, its rate of radiation would be proportionate - to its temperature. Dr. Balfour Stewart has very ingeniously suggested - the probable reason why the rate of radiation of thick plates increases - with rise of temperature more rapidly than that of thin. It is this: - all substances are more diathermanous for heat of high temperatures - than for heat of low temperatures. When a body is at a low temperature, - we may suppose that only the exterior rows of particles supply the - radiation, the heat from the interior particles being all stopped by - the exterior ones, the substance being very opaque for heat of low - temperature; while at a high temperature we may imagine that part - of the heat from the interior particles is allowed to pass, thereby - swelling the total radiation. But as the plate becomes thinner and - thinner, the obstructions to interior radiation become less and less, - and as these obstructions are greater for radiation at low temperatures - than for radiation at high temperatures, it necessarily follows that, - by reducing the thickness of the plate, we assist radiation at low - temperatures more than we do at high.</p> - - <p>In a gas, where each particle may be assumed to radiate by itself, and - where the particles stand at a considerable distance from one another, - the obstruction to interior radiation must be far less than in a - solid. In this case the rate at which a gas radiates off its heat as - its temperature rises must increase more slowly than that of a solid - substance. In other words, its rate of radiation must correspond more - nearly to its absolute temperature than that of a solid. If this be the - case, a reduction in the amount of heat received from the sun, owing to - an increase of his distance, should tend to produce a greater lowering - effect on the temperature of the air than it does on the temperature of - the solid ground. But as the temperature of our climate is determined - by the temperature of the air, it must follow that<span class="pagenum" id="Page_39">39</span> the error of - assuming that the decrease of temperature would be proportionate to the - decrease in the intensity of the sun’s heat may not be great.</p> - - <p>It may be observed here, although it does not bear directly on this - point, that although the air in a room, for example, or at the earth’s - surface is principally cooled by convection rather than by radiation, - yet it is by radiation alone that the earth’s atmosphere parts with its - heat to stellar space; and this is the chief matter with which we are - at present concerned. Air, like all other gases, is a bad radiator; - and this tends to protect it from being cooled to such an extent as it - would otherwise be, were it a good radiator like solids. True, it is - also a bad absorber; but as it is cooled by radiation into space, and - heated, not altogether by absorption, but to a very large extent by - convection, it on the whole gains its heat more easily than it loses - it, and consequently must stand at a higher temperature than it would - do were it heated by absorption alone.</p> - - <p>But, to return; the error of regarding the decrease of temperature - as proportionate to the decrease in the amount of heat received, is - probably neutralized by one of an opposite nature, viz., that of taking - space at too high a temperature; for by so doing we make the result too - small.</p> - - <p>We know that absolute zero is at least 493° below the melting-point - of ice. This is 222° below that of space. Consequently, if the heat - derived from the stars is able to maintain a temperature of −239°, - or 222° of absolute temperature, then nearly as much heat is derived - from the stars as from the sun. But if so, why do the stars give so - much heat and so very little light? If the radiation from the stars - could maintain a thermometer 222° above absolute zero, then space must - be far more transparent to heat-rays than to light-rays, or else the - stars give out a great amount of heat, but very little light, neither - of which suppositions is probably true. The probability is, I venture - to presume, that the temperature of space is not very much above - absolute zero. At the time when these investigations into the probable - temperature of space were made, at<span class="pagenum" id="Page_40">40</span> least as regards the labours of - Pouillet, the modern science of heat had no existence, and little or - nothing was then known with certainty regarding absolute zero. In this - case the whole matter would require to be reconsidered. The result of - such an investigation in all probability would be to assign a lower - temperature to stellar space than −239°.</p> - - <p>Taking all these various considerations into account, it is probable - that if we adopt −239° as the temperature of space, we shall not be far - from the truth in assuming that the absolute temperature of a place - above that of space is proportionate to the amount of heat received - from the sun.</p> - - <p>We may, therefore, in this case conclude that 59° of rise is probably - not very far from the truth, as representing the influence of the - Gulf-stream. The Gulf-stream, instead of producing little or no effect, - produces an effect far greater than is generally supposed.</p> - - <p>Our island has a mean annual temperature of about 12° above the normal - due to its latitude. This excess of temperature has been justly - attributed to the influence of the Gulf-stream. But it is singular - how this excess should have been taken as the measure of the <em>rise - resulting from the influence of the stream</em>. These figures only - represent the number of degrees that the mean normal temperature of - our island stands above what is called the normal temperature of the - latitude.</p> - - <p>The mode in which Professor Dove constructed his Tables of normal - temperature was as follows:—He took the temperature of thirty-six - equidistant points on every ten degrees of latitude. The mean - temperature of these thirty-six points he calls in each case the - <em>normal</em> temperature of the parallel. The excess above the normal - merely represents how much the stream raises our temperature above - the mean of all places on the same latitude, but it affords us no - information regarding the absolute rise produced. In the Pacific, as - well as in the Atlantic, there are immense masses of water flowing - from the tropical to the temperate regions. Now, unless we know how - much of the normal temperature of a latitude is due to ocean-currents, - and how<span class="pagenum" id="Page_41">41</span> much to the direct heat of the sun, we could not possibly, - from Professor Dove’s Tables, form the most distant conjecture as - to how much of our temperature is derived from the Gulf-stream. The - overlooking of this fact has led to a general misconception regarding - the positive influence of the Gulf-stream on temperature. The 12° - marked in Tables of normal temperature do not represent the absolute - effect of the stream, but merely show how much the stream raises the - temperature of our country above the mean of all places on the same - latitude. Other places have their temperature raised by ocean-currents - as well as this country; only the Gulf-stream produces a rise of - several degrees over and above that produced by other streams in the - same latitude.</p> - - <p>At present there is a difference merely of 80° between the mean - temperature of the equator and the poles;<a id="FNanchor_25" href="#Footnote_25" class="fnanchor">[25]</a> but were each part of the - globe’s surface to depend only upon the direct heat which it receives - from the sun, there ought, according to theory, to be a difference of - more than 200°. The annual quantity of heat received at the equator is - to that received at the poles (supposing the proportionate quantity - absorbed by the atmosphere to be the same in both cases) as 12 to 4·98, - or, say, as 12 to 5. Consequently, if the temperatures of the equator - and the poles be taken as proportionate to the absolute amount of heat - received from the sun, then the temperature of the equator above that - of space must be to that of the poles above that of space as 12 to 5. - What ought, therefore, to be the temperatures of the equator and the - poles, did each place depend solely upon the heat which it receives - directly from the sun? Were all ocean and aërial currents stopped, - so that there could be no transference of heat from one part of the - earth’s surface to another, what ought to be the temperatures of the - equator and the poles? We can at least arrive at a rough estimate - on this <span class="pagenum" id="Page_42">42</span>point. If we diminish the quantity of warm water conveyed - from the equatorial regions to the temperate and arctic regions, the - temperature of the equator will begin to rise, and that of the poles - to sink. It is probable, however, that this process would affect the - temperature of the poles more than it would that of the equator; for as - the warm water flows from the equator to the poles, the area over which - it is spread becomes less and less. But as the water from the tropics - has to raise the temperature of the temperate regions as well as the - polar, the difference of effect at the equator and poles might not, on - that account, be so very great. Let us take a rough estimate. Say that, - as the temperature of the equator rises one degree, the temperature of - the poles sinks one degree and a half. The mean annual temperature of - the globe is about 58°. The mean temperature of the equator is 80°, and - that of the poles 0°. Let ocean and aërial currents now begin to cease, - the temperature of the equator commences to rise and the temperature - of the poles to sink. For every degree that the temperature of the - equator rises, that of the poles sinks 1½°; and when the currents are - all stopped and each place becomes dependent solely upon the direct - rays of the sun, the mean annual temperature of the equator above that - of space will be to that of the poles, above that of space, as 12 to - 5. When this proportion is reached, the equator will be 374° above - that of space, and the poles 156°; for 374 is to 156 as 12 is to 5. - The temperature of space we have seen to be −239°, consequently the - temperature of the equator will in this case be 135°, reckoned from the - zero of the Fahrenheit thermometer, and the poles 83° below zero. The - equator would therefore be 55° warmer than at present, and the poles - 83° colder. The difference between the temperature of the equator and - the poles will in this case amount to 218°.</p> - - <p>Now, if we take into account the quantity of positive energy in the - form of heat carried by warm currents from the equator to the temperate - and polar regions, and also the quantity of negative energy (cold) - carried by cold currents from the polar regions to the equator, we - shall find that they are sufficient to<span class="pagenum" id="Page_43">43</span> reduce the difference of - temperature between the poles and the equator from 218° to 80°.</p> - - <p>The quantity of heat received in the latitude of London, for example, - is to that received at the equator nearly as 12 to 8. This, according - to theory, should produce a difference of about 125°. The temperature - of the equator above that of space, as we have seen, would be 374°. - Therefore 249° above that of space would represent the temperature - of the latitude of London. This would give 10° as its temperature. - The stoppage of all ocean and aërial currents would thus increase the - difference between the equator and the latitude of London by about - 85°. The stoppage of ocean-currents would not be nearly so much felt, - of course, in the latitude of London as at the equator and the poles, - because, as has been already noticed, in all latitudes midway between - the equator and the poles the two sets of currents to a considerable - extent compensate each other—the warm currents from the equator - raise the temperature, while the cold ones from the poles lower it; - but as the warm currents chiefly keep on the surface and the cold - return-currents are principally under-currents, the heating effect very - greatly exceeds the cooling effect. Now, as we have seen, the stoppage - of all currents would raise the temperature of the equator 55°; that - is to say, the rise at the equator alone would increase the difference - of temperature between the equator and that of London by 55°. But the - actual difference, as we have seen, ought to be 85°; consequently the - temperature of London would be lowered 30° by the stoppage of the - currents. For if we raise the temperature of the equator 55° and lower - the temperature of London 30°, we then increase the difference by - 85°. The normal temperature of the latitude of London being 40°, the - stoppage of all ocean and aërial currents would thus reduce it to 10°. - But the Gulf-stream raises the actual mean temperature of London 10° - above the normal. Consequently 30° + 10° = 40° represents the actual - rise at London due to the influence of the Gulf-stream over and above - all the lowering effects resulting from arctic currents. On some parts<span class="pagenum" id="Page_44">44</span> - of the American shores on the latitude of London, the temperature is - 10° below the normal. The stoppage of all ocean and aërial currents - would therefore lower the temperature there only 20°.</p> - - <p>It is at the equator and the poles that the great system of ocean and - aërial currents produces its maximum effects. The influence becomes - less and less as we recede from those places, and between them there - is a point where the influence of warm currents from the equator and - of cold currents from the poles exactly neutralize each other. At - this point the stoppage of ocean-currents would not sensibly affect - temperature. This point, of course, is not situated on the same - latitude in all meridians, but varies according to the position of the - meridian in relation to land, and ocean-currents, whether cold or hot, - and other circumstances. A line drawn round the globe through these - various points would be very irregular. At one place, such as on the - western side of the Atlantic, where the arctic current predominates, - the neutral line would be deflected towards the equator, while on - the eastern side, where warm currents predominate, the line would be - deflected towards the north. It is a difficult problem to determine the - mean position of this line; it probably lies somewhere not far north of - the tropics.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_III"> - <span class="pagenum" id="Page_45">45</span> - <h2> - CHAPTER III.<br /><br /> - <span class="small">OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER THE - GLOBE.—(<i>Continued.</i>)</span> - </h2> - </div> - <div class="subhead">Influence of the Gulf-stream on the Climate of the Arctic - Regions.—Absolute Amount of Heat received by the Arctic - Regions from the Sun.—Influence of Ocean-currents shown by - another Method.—Temperature of a Globe all Water or all Land - according to Professor J. D. Forbes.—An important Consideration - overlooked.—Without Ocean-currents the Globe would not be - habitable.—Conclusions not affected by Imperfection of Data.</div> - - <p><em>Influence of the Gulf-stream on the Climate of the Arctic - Regions.</em>—Does the Gulf-stream pass into the arctic regions? Are the - seas around Spitzbergen and North Greenland heated by the warm water of - the stream?</p> - - <p>Those who deny this nevertheless admit the existence of an arctic - current. They admit that an immense mass of cold water is continually - flowing south from the polar regions around Greenland into the - Atlantic. If it be admitted, then, that a mass of water flows across - the arctic circle from north to south, it must also be admitted that an - equal mass flows across from south to north. It is also evident that - the water crossing from south to north must be warmer than the water - crossing from north to south; for the temperate regions are warmer than - the arctic, and the ocean in temperate regions warmer than the ocean in - the arctic; consequently the current which flows into the arctic seas, - to compensate for the cold arctic current, must be a warmer current.</p> - - <p>Is the Gulf-stream this warm current? Does this compensating warm - current proceed from the Atlantic or from the Pacific? If it proceeds - from the Atlantic, it is simply the<span class="pagenum" id="Page_46">46</span> warm water of the Gulf-stream. - We may call it the warm water of the Atlantic if we choose; but this - cannot materially affect the question at issue, for the heat which - the waters of the Atlantic possess is derived, as we have seen, to - an enormous extent from the water brought from the tropics by the - Gulf-stream. If we deny that the warm compensating current comes from - the Atlantic, then we must assume that it comes from the Pacific. But - if the cold current flows from the arctic regions into the Atlantic, - and the warm compensating current from the Pacific into the arctic - regions, the highest temperature should be found on the Pacific side of - the arctic regions and not on the Atlantic side; the reverse, however, - is the case. In the Atlantic, for example, the 41° isothermal line - reaches to latitude 65°30′, while in the Pacific it nowhere goes beyond - latitude 57°. The 27° isotherm reaches to latitude 75° in the Atlantic, - but in the Pacific it does not pass beyond 64°. And the 14° isotherm - reaches the north of Spitzbergen in latitude 80°, whereas on the - Pacific side of the arctic regions it does not reach to latitude 72°.</p> - - <p>On no point of the earth’s surface does the mean annual temperature - rise so high above the normal as in the northern Atlantic, just at - the arctic circle, at a spot believed to be in the middle of the - Gulf-stream. This place is no less than 22°·5 above the normal, while - in the northern Pacific the temperature does not anywhere rise more - than 9° above the normal. These facts prove that the warm current - passes up the Atlantic into the arctic regions and not up the Pacific, - or at least that the larger amount of warm water must pass into the - arctic regions through the Atlantic. In other words, the Gulf-stream is - the warm compensating current. Not only must there be a warm stream, - but one of very considerable magnitude, in order to compensate for the - great amount of cold water that is constantly flowing from the arctic - regions, and also to maintain the temperature of those regions so much - above the temperature of space as they actually are.</p> - - <p>No doubt, when the results of the late dredging expedition<span class="pagenum" id="Page_47">47</span> are - published, they will cast much additional light on the direction and - character of the currents forming the north-eastern branch of the - Gulf-stream.</p> - - <p>The average quantity of heat received by the arctic regions as a whole - per unit surface to that received at the equator, as we have already - seen, is as 5·45 to 12, assuming that the percentage of rays cut off by - the atmosphere is the same at both places. In this case the mean annual - temperature of the arctic regions, taken as a whole, would be about - −69°, did those regions depend entirely for their temperature upon the - heat received directly from the sun. But the temperature would not even - reach to this; for the percentage of rays cut off by the atmosphere in - arctic regions is generally believed to be greater than at the equator, - and consequently the actual mean quantity of heat received by the - arctic regions will be less than 5·45−12ths of what is received at the - equator.</p> - - <p>In the article on Climate in the “Encyclopædia Britannica” there is - a Table calculated upon the principle that the quantity of heat cut - off is proportionate to the number of aërial particles which the rays - have to encounter before reaching the surface of the earth—that, as - a general rule, if the tracts of the rays follow an arithmetical - progression, the diminished force with which the rays reach the ground - will form a decreasing geometrical progression. According to this Table - about 75 per cent. of the sun’s rays are cut off by the atmosphere - in arctic regions. If 75 per cent. of the rays were cut off by the - atmosphere in arctic regions, then the direct rays of the sun could - not maintain a mean temperature 100° above that of space. But this is - no doubt much too high a percentage for the quantity of heat cut off; - for recent discoveries in regard to the absorption of radiant heat by - gases and vapours prove that Tables computed on this principle must be - incorrect. The researches of Tyndall and Melloni show that when rays - pass through any substance, the absorption is rapid at first: but the - rays are soon “sifted,” as it is called, and they then pass onwards - with but little further obstruction. Still, however, owing to the dense - fogs<span class="pagenum" id="Page_48">48</span> which prevail in arctic regions, the quantity of heat cut off - must be considerable. If as much as 50 per cent. of the sun’s rays - are cut off by the atmosphere in arctic regions, the amount of heat - received directly from the sun would not be sufficient to maintain a - mean annual temperature of −100°. Consequently the arctic regions must - depend to an enormous extent upon ocean-currents for their temperature.</p> - - <p><em>Influence of Ocean-currents shown by another Method.</em>—That the - temperature of the arctic regions would sink enormously, and the - temperature of the equator rise enormously, were all ocean-currents - stopped, can be shown by another method—viz., by taking the mean annual - temperature from the equator to the pole along a meridian passing - through the ocean, say, the Atlantic, and comparing it with the mean - annual temperature taken along a meridian passing through a great - continent, say, the Asiatic.</p> - - <p>Professor J. D. Forbes, in an interesting memoir,<a id="FNanchor_26" href="#Footnote_26" class="fnanchor">[26]</a> has endeavoured - by this method to determine what would be the temperature of the - equator and the poles were the globe all water or all land. He has - taken the temperature of the two meridians from the tables and charts - of Professor Dove, and ascertained the exact proportion of land and - water on every 10° of latitude from the equator to the poles, with the - view of determining what proportion of the average temperature of the - globe in each parallel is due to the land, and what to the water which - respectively belongs to it. He next endeavours to obtain a formula for - expressing the mean temperature of a given parallel, and thence arrives - at “an approximate answer to the inquiry as to what would have been the - equatorial or polar temperature of the globe, or that of any latitude, - had its surface been entirely composed of land or of water.”</p> - - <p>The result at which he arrived is this: that, were the surface of - the globe all water, 71°·7 would be the temperature of the equator, - and 12°·5 the temperature of the poles; and were the <span class="pagenum" id="Page_49">49</span>surface all - land, 109°·8 would be the temperature of the equator, and −25°·6 the - temperature of the poles.</p> - - <p>But in Professor Forbes’s calculations no account whatever is taken - of the influence of currents, whether of water or of air, and the - difference of temperature is attributed wholly to difference of - latitude and the physical properties of land and water in relation to - their powers in absorbing and detaining the sun’s rays, and to the laws - of conduction and of convection which regulate the internal motion of - heat in the one and in the other. He considers that the effects of - currents are all compensatory.</p> - - <p>“If a current of hot water,” he says, “moderates the cold of a Lapland - winter, the counter-current, which brings the cold of Greenland to the - shores of the United States, in a great measure restores the balance of - temperature, so far as it is disturbed by this particular influence. - The prevalent winds, in like manner, including the trade-winds, though - they render some portions of continents, on the average, hotter or - colder than others, produce just the contrary effect elsewhere. Each - continent, if it has a cold eastern shore, has likewise a warm western - one; and even local winds have for the most part established laws of - compensation. In a given parallel of latitude all these secondary - causes of local climate may be imagined to be mutually compensatory, - and the outstanding gradation of mean or normal temperature will - mainly depend, 1st, upon the effect of latitude simply; 2nd, on - the distribution of land and water considered in their primary or - <em>statical</em> effect.”</p> - - <p>It is singular that a physicist so acute as Professor Forbes should, - in a question such as this, leave out of account the influence of - currents, under the impression that their effects were compensatory.</p> - - <p>If there is a constant transference of hot water from the equatorial - regions to the polar, and of cold water from the polar regions to the - equatorial (a thing which Professor Forbes admitted), then there can - only be one place between the equator and the pole where the two sets - of currents compensate each<span class="pagenum" id="Page_50">50</span> other. At all places on the equatorial - side of this point a cooling effect is the result. Starting from this - neutral point, the preponderance of the cooling effect over the heating - increases as we approach towards the equator, and the preponderance of - the heating effect over the cooling increases as we recede from this - point towards the pole—the cooling effect reaching a maximum at the - equator, and the heating effect a maximum at the pole.</p> - - <p>Had Professor Forbes observed this important fact, he would have - seen at once that the low temperature of the land in high latitudes, - in comparison with that of the sea, was no index whatever as to - how much the temperature of those regions would sink were the sea - entirely removed and the surface to become land; for the present - high temperature of the sea is not due wholly to the mere physical - properties of water, but to a great extent is due to the heat brought - by currents from the equator. Now, unless it is known how much of - the absolute temperature of the ocean in those latitudes is due to - currents, we cannot tell how much the removal of the sea would lower - the absolute temperature of those places. Were the sea removed, - the continents in high latitudes would not simply lose the heating - advantages which they presently derive from the mere fact of their - proximity to so much sea, but the removal would, in addition to this, - deprive them of an enormous amount of heat which they at present - receive from the tropics by means of ocean-currents. And, on the other - hand, at the equator, were the sea removed, the continents there - would not simply lose the cooling influences which result from their - proximity to so much water, but, in addition to this, they would have - to endure the scorching effects which would result from the heat which - is at present carried away from the tropics by ocean-currents.</p> - - <p>We have already seen that Professor Forbes concluded that the - removal of the sea would raise the mean temperature of the equator - 30°, and lower the temperature of the poles 28°; it is therefore - perfectly certain that, had he added to his result the<span class="pagenum" id="Page_51">51</span> effect due to - ocean-currents, and had he been aware that about one-fifth of all the - heat possessed by the Atlantic is actually derived from the equator by - means of the Gulf-stream, he would have assigned a temperature to the - equator and the poles, of a globe all land, differing not very far from - what I have concluded would be the temperature of those places were all - ocean and aërial currents stopped, and each place to depend solely upon - the heat which it received directly from the sun.</p> - - <p><em>Without Ocean-currents the Globe would not be habitable.</em>—All these - foregoing considerations show to what an extent the climatic condition - of our globe is due to the thermal influences of ocean-currents.</p> - - <p>As regards the northern hemisphere, we have two immense oceans, the - Pacific and the Atlantic, extending from the equator to near the north - pole, or perhaps to the pole altogether. Between these two oceans lie - two great continents, the eastern and the western. Owing to the earth’s - spherical form, far too much heat is received at the equator and far - too little at high latitudes to make the earth a suitable habitation - for sentient beings. The function of these two great oceans is to - remove the heat from the equator and carry it to temperate and polar - regions. Aërial currents could not do this. They might remove the heat - from the equator, but they could not, as we have already seen, carry - it to the temperate and polar regions; for the greater portion of the - heat which aërial currents remove from the equator is dissipated into - stellar space: the ocean alone can convey the heat to distant shores. - But aërial currents have a most important function; for of what avail - would it be, though ocean-currents should carry heat to high latitudes, - if there were no means of distributing the heat thus conveyed over the - land? The function of aërial currents is to do this. Upon this twofold - arrangement depends the thermal condition of the globe. Exclude the - waters of the Pacific and the Atlantic from temperate and polar regions - and place them at the equator, and nothing now existing on the globe - could live in high latitudes.</p> - - <p><span class="pagenum" id="Page_52">52</span></p> - - <p>Were these two great oceans placed beside each other on one side of the - globe, and the two great continents placed beside each other on the - other side, the northern hemisphere would not then be suitable for the - present order of things: the land on the central and on the eastern - side of the united continent would be far too cold.</p> - - <p><em>The foregoing Conclusions not affected by the Imperfection of the - Data.</em>—The general results at which we have arrived in reference to the - influence of ocean-currents on the climatic condition of the globe are - not affected by the imperfection of the data employed. It is perfectly - true that considerable uncertainty prevails regarding some of the data; - but, after making the fullest allowance for every possible error, the - influence of currents is so enormous that the general conclusion cannot - be materially affected. I can hardly imagine that any one familiar - with the physics of the subject will be likely to think that, owing to - possible errors in the data, the effects have probably been doubled. - Even admitting, however, that this were proved to be the case, still - that would not materially alter the general conclusion at which we - have arrived. The influence of ocean-currents in the distribution of - heat over the surface of the globe would still be admittedly enormous, - whether we concluded that owing to them the present temperature of the - equator is 55° or 27° colder than it would otherwise be, or the poles - 83° or 41° hotter than they would be did no currents exist.</p> - - <p>Nay, more, suppose we should again halve the result; even in that case - we should have to admit that, owing to ocean-currents, the equator - is about 14° colder and the poles about 21° hotter than they would - otherwise be; in other words, we should have to admit that, were it not - for ocean-currents, the mean temperature of the equator would be about - 100° and the mean temperature of the poles about −21°.</p> - - <p>If the influence of ocean-currents in reducing the difference between - the temperature of the equator and poles amounted to only a few - degrees, it would of course be needless to put much weight on any - results arrived at by the method of calculation<span class="pagenum" id="Page_53">53</span> which I have adopted; - but when it is a matter of two hundred degrees, it is not at all likely - that the general results will be very much affected by any errors which - may ever be found in the data.</p> - - <p>Objections of a palæontological nature have frequently been urged - against the opinion that our island is much indebted for its mild - climate to the influence of the Gulf-stream; but, from what has already - been stated, it must be apparent that all objections of that nature - are of little avail. The palæontologist may detect, from the character - of the flora and fauna brought up from the sea-bottom by dredging and - other means, the presence of a warm or of a cold current; but this can - never enable him to prove that the temperate and polar regions are - not affected to an enormous extent by warm water conveyed from the - equatorial regions. For anything that palæontology can show to the - contrary, were ocean-currents to cease, the mean annual temperature - of our island might sink below the present midwinter temperature of - Siberia. What would be the thermal condition of our globe were there no - ocean-currents is a question for the physicist; not for the naturalist.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_IV"> - <span class="pagenum" id="Page_54">54</span> - <h2> - CHAPTER IV.<br /><br /> - <span class="small">OUTLINE OF THE PHYSICAL AGENCIES WHICH LEAD TO SECULAR CHANGES OF CLIMATE.</span> - </h2> - </div> - <div class="subhead">Eccentricity of the Earth’s Orbit; its Effect on - Climate.—Glacial Epoch not the direct Result of an Increase of - Eccentricity.—An important Consideration overlooked.—Change of - Eccentricity affects Climate only indirectly.—Agencies which - are brought into Operation by an Increase of Eccentricity.—How - an Accumulation of Snow is produced.—The Effect of Snow on - the Summer Temperature.—Reason of the low Summer Temperature - of Polar Regions.—Deflection of Ocean-currents the chief - Cause of secular Changes of Climate.—How the foregoing Causes - deflect Ocean-currents.—Nearness of the Sun in Perigee a - Cause of the Accumulation of Ice.—A remarkable Circumstance - regarding the Causes which lead to secular Changes of - Climate.—The primary Cause an Increase of Eccentricity.—Mean - Temperature of whole Earth should be greater in Aphelion than - in Perihelion.—Professor Tyndall on the Glacial Epoch.—A - general Reduction of Temperature will not produce a Glacial - Epoch.—Objection from the present Condition of the Planet Mars.</div> - - <p><em>Primary cause of Change of Eccentricity of the Earth’s Orbit.</em>—There - are two causes affecting the position of the earth in relation to - the sun, which must, to a very large extent, influence the earth’s - climate; viz., the precession of the equinoxes and the change in the - eccentricity of the earth’s orbit. If we duly examine the combined - influence of these two causes, we shall find that the northern and - southern portions of the globe are subject to an excessively slow - secular change of climate, consisting in a slow periodic change of - alternate warmer and colder cycles.</p> - - <p>According to the calculations of Leverrier, the superior limit of the - earth’s eccentricity is 0·07775.<a id="FNanchor_27" href="#Footnote_27" class="fnanchor">[27]</a> The eccentricity is at <span class="pagenum" id="Page_55">55</span>present - diminishing, and will continue to do so during 23,980 years, from the - year 1800 <span class="smcap">a.d.</span>, when its value will be then ·00314.</p> - - <p>The change in the eccentricity of the earth’s orbit may affect - the climate in two different ways; viz., by either increasing or - diminishing the mean annual amount of heat received from the sun, or - by increasing or diminishing the difference between summer and winter - temperature.</p> - - <p>Let us consider the former case first. The total quantity of heat - received from the sun during one revolution is inversely proportional - to the minor axis.</p> - - <p>The difference of the minor axis of the orbit when at its maximum and - its minimum state of eccentricity is as 997 to 1000. This small amount - of difference cannot therefore sensibly affect the climate. Hence we - must seek for our cause in the second case under consideration.</p> - - <p>There is of course as yet some little uncertainty in regard to the - exact mean distance of the sun. I shall, however, in the present volume - assume it to be 91,400,000 miles. When the eccentricity is at its - superior limit, the distance of the sun from the earth, when the latter - is in the aphelion of its orbit, is no less than 98,506,350 miles; - and when in the perihelion it is only 84,293,650 miles. The earth is - therefore 14,212,700 miles further from the sun in the former position - than in the latter. The direct heat of the sun being inversely as the - square of the distance, it follows that the amount of heat received - by the earth when in these two positions will be as 19 to 26. Taking - the present eccentricity to be ·0168, the earth’s distance during - winter, when nearest to the sun, is 89,864,480 miles. Suppose now that, - according to the precession of the equinoxes, winter in our northern - hemisphere should happen when the earth is in <span class="pagenum" id="Page_56">56</span>the aphelion of its - orbit, at the time when the orbit is at its greatest eccentricity; the - earth would then be 8,641,870 miles further from the sun in winter than - at present. The direct heat of the sun would therefore be one-fifth - less during that season than at present; and in summer one-fifth - greater. This enormous difference would affect the climate to a very - great extent. But if winter under these circumstances should happen - when the earth is in the perihelion of its orbit, the earth would then - be 14,212,700 miles nearer the sun in winter than in summer. In this - case the difference between winter and summer in the latitude of this - country would be almost annihilated. But as the winter in the one - hemisphere corresponds with the summer in the other, it follows that - while the one hemisphere would be enduring the greatest extremes of - summer heat and winter cold, the other would be enjoying a perpetual - summer.</p> - - <p>It is quite true that whatever may be the eccentricity of the earth’s - orbit, the two hemispheres must receive equal quantities of heat per - annum; for proximity to the sun is exactly compensated by the effect of - swifter motion—the total amount of heat received from the sun between - the two equinoxes is the same in both halves of the year, whatever the - eccentricity of the earth’s orbit may be. For example, whatever extra - heat the southern hemisphere may at present receive from the sun during - its summer months owing to greater proximity to the sun, is exactly - compensated by a corresponding loss arising from the shortness of the - season; and, on the other hand, whatever deficiency of heat we in the - northern hemisphere may at present have during our summer half year - in consequence of the earth’s distance from the sun, is also exactly - compensated by a corresponding length of season.</p> - - <p>It has been shown in the introductory chapter that a simple change in - the sun’s distance would not alone produce a glacial epoch, and that - those physicists who confined their attention to purely astronomical - effects were perfectly correct in affirming that no increase of - eccentricity of the earth’s orbit could account for that epoch. But - the important fact was overlooked that<span class="pagenum" id="Page_57">57</span> although the glacial epoch - could not result directly from an increase of eccentricity, it might - nevertheless do so indirectly. The glacial epoch, as I hope to show, - was not due directly to an increase in the eccentricity of the earth’s - orbit, but to a number of physical agents that were brought into - operation as a result of an increase.</p> - - <p>I shall now proceed to give an outline of what these physical agents - were, how they were brought into operation, and the way in which they - led to the glacial epoch.</p> - - <p>When the eccentricity is about its superior limit, the combined - effect of all those causes to which I allude is to lower to a very - great extent the temperature of the hemisphere whose winters occur in - aphelion, and to raise to nearly as great an extent the temperature of - the opposite hemisphere, where winter of course occurs in perihelion.</p> - - <p>With the eccentricity at its superior limit and the winter occurring - in the aphelion, the earth would be 8,641,870 miles further from the - sun during that season than at present. The reduction in the amount - of heat received from the sun owing to this increased distance would, - upon the principle we have stated in <a href="#CHAPTER_II">Chapter II.</a>, lower the midwinter - temperature to an enormous extent. In temperate regions the greater - portion of the moisture of the air is at present precipitated in the - form of rain, and the very small portion which falls as snow disappears - in the course of a few weeks at most. But in the circumstances under - consideration, the mean winter temperature would be lowered so much - below the freezing-point that what now falls as rain during that season - would then fall as snow. This is not all; the winters would then not - only be colder than now, but they would also be much longer. At present - the winters are nearly eight days shorter than the summers; but with - the eccentricity at its superior limit and the winter solstice in - aphelion, the length of the winters would exceed that of the summers by - no fewer than thirty-six days. The lowering of the temperature and the - lengthening of the winter would both tend to the same effect, viz., to - increase the amount of snow<span class="pagenum" id="Page_58">58</span> accumulated during the winter; for, other - things being equal, the larger the snow-accumulating period the greater - the accumulation. I may remark, however, that the absolute quantity - of heat received during winter is not affected by the decrease in the - sun’s heat,<a id="FNanchor_28" href="#Footnote_28" class="fnanchor">[28]</a> for the additional length of the season compensates - for this decrease. As regards the absolute amount of heat received, - increase of the sun’s distance and lengthening of the winter are - compensatory, but not so in regard to the amount of snow accumulated.</p> - - <p>The consequence of this state of things would be that, at the - commencement of the short summer, the ground would be covered with the - winter’s accumulation of snow.</p> - - <p>Again, the presence of so much snow would lower the summer temperature, - and prevent to a great extent the melting of the snow.</p> - - <p>There are three separate ways whereby accumulated masses of snow and - ice tend to lower the summer temperature, viz.:—</p> - - <p><em>First.</em> By means of direct radiation. No matter what the intensity of - the sun’s rays may be, the temperature of snow and ice can never rise - above 32°. Hence the presence of snow and ice tends by direct radiation - to lower the temperature of all surrounding bodies to 32°.</p> - - <p>In Greenland, a country covered with snow and ice, the pitch has been - seen to melt on the side of a ship exposed to the direct rays of the - sun, while at the same time the surrounding air was far below the - freezing-point; a thermometer exposed to the direct radiation of the - sun has been observed to stand above 100°, while the air surrounding - the instrument was actually 12° below the freezing-point.<a id="FNanchor_29" href="#Footnote_29" class="fnanchor">[29]</a> A similar - experience has been recorded by travellers on the snow-fields of the - Alps.<a id="FNanchor_30" href="#Footnote_30" class="fnanchor">[30]</a></p> - - <p>These results, surprising as they no doubt appear, are what <span class="pagenum" id="Page_59">59</span>we ought - to expect under the circumstances. The diathermancy of air has been - well established by the researches of Professor Tyndall on radiant - heat. Perfectly dry air seems to be nearly incapable of absorbing - radiant heat. The entire radiation passes through it almost without any - sensible absorption. Consequently the pitch on the side of the ship may - be melted, or the bulb of the thermometer raised to a high temperature - by the direct rays of the sun, while the surrounding air remains - intensely cold. “A joint of meat,” says Professor Tyndall, “might be - roasted before a fire, the air around the joint being cold as ice.”<a id="FNanchor_31" href="#Footnote_31" class="fnanchor">[31]</a> - The air is cooled by <em>contact</em> with the snow-covered ground, but is not - heated by the radiation from the sun.</p> - - <p>When the air is humid and charged with aqueous vapour, a similar - cooling effect also takes place, but in a slightly different way. Air - charged with aqueous vapour is a good absorber of radiant heat, but - it can only absorb those rays which agree with it in <em>period</em>. It so - happens that rays from snow and ice are, of all others, those which it - absorbs best. The humid air will absorb the total radiation from the - snow and ice, but it will allow the greater part of, if not nearly all, - the sun’s rays to pass unabsorbed. But during the day, when the sun is - shining, the radiation from the snow and ice to the air is negative; - that is, the snow and ice cool the air by radiation. The result is, the - air is cooled by radiation from the snow and ice (or rather, we should - say, <em>to</em> the snow and ice) more rapidly than it is heated by the sun; - and, as a consequence, in a country like Greenland, covered with an - icy mantle, the temperature of the air, even during summer, seldom - rises above the freezing-point. Snow is a good reflector, but as simple - reflection does not change the character of the rays they would not be - absorbed by the air, but would pass into stellar space.</p> - - <p>Were it not for the ice, the summers of North Greenland, owing to the - continuance of the sun above the horizon, would be as warm as those of - England; but, instead of this, the <span class="pagenum" id="Page_60">60</span>Greenland summers are colder than - our winters. Cover India with an ice sheet, and its summers would be - colder than those of England.</p> - - <p><em>Second.</em> Another cause of the cooling effect is that the rays which - fall on snow and ice are to a great extent reflected back into - space.<a id="FNanchor_32" href="#Footnote_32" class="fnanchor">[32]</a> But those that are not reflected, but absorbed, do not raise - the temperature, for they disappear in the mechanical work of melting - the ice. The latent heat of ice is about 142° F.; consequently in the - melting of every pound of ice a quantity of heat sufficient to raise - one pound of water 142° disappears, and is completely lost, so far - as temperature is concerned. This quantity of heat is consumed, not - in raising the temperature of the ice, but in the mechanical work of - tearing the molecules separate against the forces of cohesion binding - them together into the solid form. No matter what the intensity of the - sun’s heat may be, the surface of the ground will remain permanently at - 32° so long as the snow and ice continue unmelted. [**P1:missing page - number]</p> - - <p><em>Third.</em> Snow and ice lower the temperature by chilling the air and - condensing the vapour into thick fogs. The great strength of the sun’s - rays during summer, due to his nearness at that season, would, in the - first place, tend to produce an increased amount of evaporation. But - the presence of snow-clad mountains and an icy sea would chill the - atmosphere and condense the vapour into thick fogs. The thick fogs - and cloudy sky would effectually prevent the sun’s rays from reaching - the earth, and the snow, in consequence, would remain unmelted during - the entire summer. In fact, we have this very condition of things - exemplified in some of the islands of the Southern Ocean at the present - day. Sandwich Land, which is in the same parallel of latitude as the - north of Scotland, is covered with ice and snow the entire summer; - and in the island of South Georgia, which is in the same parallel - as the centre of England, the perpetual snow descends to the very - sea-beach. The following is Captain Cook’s description of this dismal - place:—“We thought it very extraordinary,” he says, <span class="pagenum" id="Page_61">61</span>“that an island - between the latitudes of 54° and 55° should, in the very height of - summer, be almost wholly covered with frozen snow, in some places many - fathoms deep.... The head of the bay was terminated by ice-cliffs of - considerable height; pieces of which were continually breaking off, - which made a noise like a cannon. Nor were the interior parts of the - country less horrible. The savage rocks raised their lofty summits till - lost in the clouds, and valleys were covered with seemingly perpetual - snow. Not a tree nor a shrub of any size were to be seen. The only - signs of vegetation were a strong-bladed grass growing in tufts, wild - burnet, and a plant-like moss seen on the rocks.... We are inclined to - think that the interior parts, on account of their elevation, never - enjoy heat enough to melt the snow in such quantities as to produce - a river, nor did we find even a stream of fresh water on the whole - coast.”<a id="FNanchor_33" href="#Footnote_33" class="fnanchor">[33]</a></p> - - <p>Captain Sir James Ross found the perpetual snow at the sea-level at - Admiralty Inlet, South Shetland, in lat. 64°; and while near this - place the thermometer in the very middle of summer fell at night to - 23° F.; and so rapidly was the young ice forming around the ship that - he began, he says, “to have serious apprehensions of the ships being - frozen in.”<a id="FNanchor_34" href="#Footnote_34" class="fnanchor">[34]</a> At the comparatively low latitude of 59° S., in long. - 171° E. (the corresponding latitude of our Orkney Islands), snow was - falling on the longest day, and the surface of the sea at 32°.<a id="FNanchor_35" href="#Footnote_35" class="fnanchor">[35]</a> And - during the month of February (the month corresponding to August in our - hemisphere) there were only three days in which they were not assailed - by snow-showers.<a id="FNanchor_36" href="#Footnote_36" class="fnanchor">[36]</a></p> - - <p>In the Straits of Magellan, in 53° S. lat., where the direct heat of - the sun ought to be as great as in the centre of England, MM. Churrca - and Galcano have seen snow fall in the middle of summer; and though the - day was eighteen hours long, the thermometer seldom rose above 42° or - 44°, and never above 51°.<a id="FNanchor_37" href="#Footnote_37" class="fnanchor">[37]</a></p> - - <p><span class="pagenum" id="Page_62">62</span></p> - - <p>This rigorous condition of climate chiefly results from the rays - of the sun being intercepted by the dense fogs which envelope those - regions during the entire summer; and the fogs again are due to the - air being chilled by the presence of the snow-clad mountains and the - immense masses of floating ice which come from the antarctic seas. The - reduction of the sun’s heat and lengthening of the winter, which would - take place when the eccentricity is near to its superior limit and the - winter in aphelion, would in this country produce a state of things - perhaps as bad as, if not worse than, that which at present exists in - South Georgia and South Shetland.</p> - - <p>If we turn our attention to the polar regions, we shall find that - the cooling effects of snow and ice are even still more marked. The - coldness of the summers in polar regions is owing almost solely to this - cause. Captain Scoresby states that, in regard to the arctic regions, - the general obscurity of the atmosphere arising from fogs or clouds is - such that the sun is frequently invisible during several successive - days. At such times, when the sun is near the northern tropic, there is - scarcely any sensible quantity of light from noon till midnight.<a id="FNanchor_38" href="#Footnote_38" class="fnanchor">[38]</a> - “And snow,” he says, “is so common in the arctic regions, that it may - be boldly stated that in nine days out of ten during the months of - April, May, and June more or less falls.”<a id="FNanchor_39" href="#Footnote_39" class="fnanchor">[39]</a></p> - - <p>On the north side of Hudson’s Bay, for example, where the quantity of - floating ice during summer is enormous, and dense fogs prevail, the - mean temperature of June does not rise above the freezing-point, being - actually 13°·5 below the normal temperature; while in some parts of - Asia under the same latitude, where there is comparatively little ice, - the mean temperature of June is as high as 60°.</p> - - <p>The mean temperature of Van Rensselaer Harbour, in lat. 78° 37′ N., - long. 70° 53′ W., was accurately determined from hourly observations - made day and night over a period of two years by Dr. Kane. It was found - to be as follows:—</p> - - <p><span class="pagenum" id="Page_63">63</span></p> - - <table summary="Van Rensselaer Harbour temeratures"> - <tbody> - <tr> - <td> </td> - <td class="tdr"><div>° </div></td> - </tr> - <tr> - <td>Winter</td> - <td class="tdr"><div>−28·59</div></td> - </tr> - <tr> - <td>Spring</td> - <td class="tdr"><div>−10·59</div></td> - </tr> - <tr> - <td>Summer</td> - <td class="tdr"><div>+33·38</div></td> - </tr> - <tr> - <td>Autumn</td> - <td class="tdr"><div>- 4·03</div></td> - </tr> - </tbody> - </table> - - <p class="noindent">But although the quantity of heat received from the sun at that - latitude ought to have been greater during the summer than in - England,<a id="FNanchor_40" href="#Footnote_40" class="fnanchor">[40]</a> yet nevertheless the temperature is only 1°·38 above the - freezing-point.</p> - - <p>The temperature of Port Bowen, lat. 73° 14′ N., was found to be as - follows:—</p> - - <table summary="Port Bowen temeratures"> - <tbody> - <tr> - <td> </td> - <td class="tdr"><div>° </div></td> - </tr> - <tr> - <td>Winter</td> - <td class="tdr"><div>−25·09</div></td> - </tr> - <tr> - <td>Spring</td> - <td class="tdr"><div>- 5·77</div></td> - </tr> - <tr> - <td>Summer</td> - <td class="tdr"><div>+34·40</div></td> - </tr> - <tr> - <td>Autumn</td> - <td class="tdr"><div>+10·58</div></td> - </tr> - </tbody> - </table> - - <p class="noindent">Here the summer is only 2°·4 above the freezing-point.</p> - - <p>The condition of things in the antarctic regions is even still worse - than in the arctic. Captain Sir James Ross, when between lat. 66° S. - and 77° 5′ S., during the months of January and February, 1841, found - the mean temperature to be only 26°·5; and there were only two days - when it rose even to the freezing-point. When near the ice-barrier on - the 8th of February, 1841, a season of the year equivalent to August - in England, he had the thermometer at 12° at noon; and so rapidly was - the young ice forming around the ships, that it was with difficulty - that he escaped being frozen in for the winter. “Three days later,” - he says, “the thick falling snow prevented our seeing to any distance - before us; the waves as they broke over the ships froze as they fell - on the decks and rigging, and covered our clothes with a thick coating - of ice.”<a id="FNanchor_41" href="#Footnote_41" class="fnanchor">[41]</a> On visiting the barrier next year about the same season, - he again ran the risk of being frozen in. He states that the surface - of the sea presented one unbroken sheet of young ice as far as the eye - could discover from the masthead.</p> - - <p>Lieutenant Wilkes, of the American Exploring Expedition, <span class="pagenum" id="Page_64">64</span>says that the - temperature they experienced in the antarctic regions surprised him, - for they seldom, if ever, had it above 30°, even at midday. Captain - Nares, when in latitude 64°S., between the 13th and 25th February last - (1874), found the mean temperature of the air to be 31°·5; a lower - temperature than is met with in the arctic regions, in August, ten - degrees nearer the pole.<a id="FNanchor_42" href="#Footnote_42" class="fnanchor">[42]</a></p> - - <p>These extraordinarily low temperatures during summer, which we have - just been detailing, were due solely to the presence of snow and ice. - In South Georgia, Sandwich Land, and some other places which we have - noticed, the summers ought to be about as warm as those of England; yet - to such an extent is the air cooled by means of floating ice coming - from the antarctic regions, and the rays of the sun enfeebled by the - dense fogs which prevail, that there is actually not heat sufficient - even in the very middle of summer to melt the snow lying on the - sea-beach.</p> - - <p>We read with astonishment that a country in the latitude of England - should in the very middle of summer be covered with snow down to the - sea-shore—the thermometer seldom rising much above the freezing-point. - But we do not consider it so surprising that the summer temperature of - the polar regions should be low, for we are accustomed to regard a low - temperature as the normal condition of things there. We are, however, - mistaken if we suppose that the influence of ice on climate is less - marked at the poles than at such places as South Georgia or Sandwich - Land.</p> - - <p>It is true that a low summer temperature is the normal state of - matters in very high latitudes, but it is so only in consequence of - the perpetual presence of snow and ice. When we speak of the normal - temperature of a place we mean, of course, as we have already seen, - the normal temperature under the present condition of things. But - were the ice removed from those regions, our present Tables of normal - summer temperature would be valueless. These Tables give us the normal - June temperature while the ice remains, but they do not afford us <span class="pagenum" id="Page_65">65</span>the - least idea as to what that temperature would be were the ice removed. - The mere removal of the ice, all things else remaining the same, would - raise the summer temperature enormously. The actual June temperature of - Melville Island, for example, is 37°, and Port Franklin, Nova Zembla, - 36°·5; but were the ice removed from the arctic regions, we should - then find that the summer temperature of those places would be about - as high as that of England. This will be evident from the following - considerations:—</p> - - <p>The temperature of a place, other things being equal, is proportionate - to the quantity of heat received from the sun. If Greenland receives - per given surface as much heat from the sun as England, its temperature - ought to be as high as that of England. Now, from May 10 till August - 3, a period of eighty-five days, the quantity of heat received from - the sun in consequence of his remaining above the horizon is actually - greater at the north pole than at the equator.</p> - - <p>Column II. of the following Table, calculated by Mr. Meech,<a id="FNanchor_43" href="#Footnote_43" class="fnanchor">[43]</a> - represents the quantity of heat received from the sun on the 15th of - June at every 10° of latitude. To simplify the Table, I have taken 100 - as the unit quantity received at the equator on that day instead of the - unit adopted by Mr. Meech:—</p> - - <table summary="Quantity of heat received from the sun"> - <tbody> - <tr> - <td class="bt bb bl"> </td> - <td class="tdc bt bb bl"><div>I.<br />Latitude.</div></td> - <td class="tdc bt bb bl"><div>II.<br />Quantity of heat.</div></td> - <td class="tdc bt br bb bl"><div>III.<br />June temperature.</div></td> - </tr> - <tr> - <td class="bl"> </td> - <td class="tdc bl"><div>°</div></td> - <td class="bl"> </td> - <td class="tdc bl br"><div>°</div></td> - </tr> - <tr> - <td class="bl">Equator</td> - <td class="tdc bl"><div> 0</div></td> - <td class="tdc bl"><div>100</div></td> - <td class="tdc bl br"><div>80·0</div></td> - </tr> - <tr> - <td class="bl"> </td> - <td class="tdc bl"><div>10</div></td> - <td class="tdc bl"><div>111</div></td> - <td class="tdc bl br"><div>81·1</div></td> - </tr> - <tr> - <td class="bl"> </td> - <td class="tdc bl"><div>20</div></td> - <td class="tdc bl"><div>118</div></td> - <td class="tdc bl br"><div>81·1</div></td> - </tr> - <tr> - <td class="bl"> </td> - <td class="tdc bl"><div>30</div></td> - <td class="tdc bl"><div>123</div></td> - <td class="tdc bl br"><div>77·3</div></td> - </tr> - <tr> - <td class="bl"> </td> - <td class="tdc bl"><div>40</div></td> - <td class="tdc bl"><div>125</div></td> - <td class="tdc bl br"><div>68·0</div></td> - </tr> - <tr> - <td class="bl"> </td> - <td class="tdc bl"><div>50</div></td> - <td class="tdc bl"><div>125</div></td> - <td class="tdc bl br"><div>58·8</div></td> - </tr> - <tr> - <td class="bl"> </td> - <td class="tdc bl"><div>60</div></td> - <td class="tdc bl"><div>123</div></td> - <td class="tdc bl br"><div>51·4</div></td> - </tr> - <tr> - <td class="bl"> </td> - <td class="tdc bl"><div>70</div></td> - <td class="tdc bl"><div>127</div></td> - <td class="tdc bl br"><div>39·2</div></td> - </tr> - <tr> - <td class="bl"> </td> - <td class="tdc bl"><div>80</div></td> - <td class="tdc bl"><div>133</div></td> - <td class="tdc bl br"><div>30·2</div></td> - </tr> - <tr> - <td class="bb bl">North Pole</td> - <td class="tdc bb bl"><div>90</div></td> - <td class="tdc bb bl"><div>136</div></td> - <td class="tdc bb bl br"><div>27·4</div></td> - </tr> - </tbody> - </table> - - <p>The calculations are, of course, made upon the supposition that the - quantity of rays cut off in passing through the atmosphere <span class="pagenum" id="Page_66">66</span>is the - same at the poles as at the equator, which, as we know, is not exactly - the case. But, notwithstanding the extra loss of solar heat in high - latitudes caused by the greater amount of rays that are cut off, still, - if the temperature of the arctic summers were at all proportionate to - the quantity of heat received from the sun, it ought to be very much - higher than it actually is. Column III. represents the actual mean June - temperature, according to Prof. Dove, at the corresponding latitudes. - A comparison of these two columns will show the very great deficiency - of temperature in high latitudes during summer. At the equator, for - example, the quantity of heat received is represented by 100 and the - temperature 80°; while at the pole the temperature is only 27°·4, - although the amount of heat received is 136. This low temperature - during summer, from what has been already shown, is due chiefly to the - presence of snow and ice. If by some means or other we could remove - the snow and ice from the arctic regions, they would then enjoy a - temperate, if not a hot, summer. In Greenland, as we have already seen, - snow falls even in the very middle of summer, more or less, nine days - out of ten; but remove the snow from the northern hemisphere, and a - snow-shower in Greenland during summer would be as great a rarity as it - would be on the plains of India.</p> - - <p>Other things being equal, the quantity of solar heat received in - Greenland during summer is considerably greater than in England. - Consequently, were it not for snow and ice, it would enjoy as warm a - climate during summer as that of England. Conversely, let the polar - snow and ice extend to the latitude of England, and the summers of that - country would be as cold as those of Greenland. Our summers would then - be as cold as our winters are at present, and snow in the very middle - of summer would perhaps be as common as rain.</p> - - <p><em>Mr. Murphy’s Theory.</em>—In a paper read before the Geological Society - by Mr. Murphy<a id="FNanchor_44" href="#Footnote_44" class="fnanchor">[44]</a> he admits that the glacial climate was due to an - increase of eccentricity, but maintains in opposition to me that the - glaciated hemisphere must be that in which the <span class="pagenum" id="Page_67">67</span><em>summer</em> occurs in - <em>aphelion</em> during the greatest eccentricity of the earth’s orbit.</p> - - <p>I fear that Mr. Murphy must be resting his theory on the mistaken idea - that a summer in aphelion ought to melt less snow and ice than one in - perihelion. It is quite true that the longer summer in aphelion—other - things being equal—is colder than the shorter one in perihelion, but - the quantity of heat received from the sun is the same in both cases. - Consequently the quantity of snow and ice melted ought also to be the - same; for the amount melted is in proportion to the quantity of energy - in the form of heat received.</p> - - <p>It is true that with us at present less snow and ice are melted during - a cold summer than during a warm one. But this is not a case in point, - for during a cold summer we have less heat than during a warm summer, - the length of both being the same. The coldness of the summers in - this case is owing chiefly to a portion of the heat which we ought to - receive from the sun being cut off by some obstructing cause.</p> - - <p>The reason why we have so little snow, and consequently so little ice, - in temperate regions, is not, as Mr. Murphy seems to suppose, that - the heat of summer melts it all, but that there is so little to melt. - And the reason why we have so little to melt is that, owing to the - warmth of our winters, we have generally rain instead of snow. But - if you increase the eccentricity very much, and place the winter in - perihelion, we should probably have no snow whatever, and, as far as - glaciation is concerned, it would then matter very little what sort of - summer we had.</p> - - <p>But it is not correct to say that the perihelion summer of the glacial - epoch must have been hot. There are physical reasons, as we have just - seen, which go to prove that, notwithstanding the nearness of the sun - at that season, the temperature would seldom, if ever, rise much above - the freezing-point.</p> - - <p>Besides, Mr. Murphy overlooks the fact that the nearness of the sun - during summer was nearly as essential to the production of the ice, as - we shall shortly see, as his great distance during winter.</p> - - <p><span class="pagenum" id="Page_68">68</span></p> - - <p>We must now proceed to the consideration of an agency which is brought - into operation by the foregoing condition of things, an agency far - more potent than any which has yet come under our notice, viz., the - <em>Deflection of Ocean-currents</em>.</p> - - <p><em>Deflection of Ocean-currents the chief Cause of secular Changes - of Climate.</em>—The enormous extent to which the thermal condition of - the globe is affected by ocean-currents seems to cast new light on - the mystery of geological climate. What, for example, would be the - condition of Europe were the Gulf-stream stopped, and the Atlantic thus - deprived of one-fifth of the absolute amount of heat which it is now - receiving above what it has in virtue of the temperature of space? If - the results just arrived at be at all justifiable, it follows that the - stoppage of the stream would lower the temperature of northern Europe - to an extent that would induce a condition of climate as severe as that - of North Greenland; and were the warm currents of the North Pacific - also at the same time to be stopped, the northern hemisphere would - assuredly be subjected to a state of general glaciation.</p> - - <p>Suppose also that the warm currents, having been withdrawn from the - northern hemisphere, should flow into the Southern Ocean: what then - would be the condition of the southern hemisphere? Such a transference - of heat would raise the temperature of the latter hemisphere about - as much as it would lower the temperature of the former. It would - consequently raise the mean temperature of the antarctic regions much - above the freezing-point, and the ice under which those regions are - at present buried would, to a great extent at least, disappear. The - northern hemisphere, thus deprived of the heat from the equator, would - be under a condition of things similar to that which prevailed during - the glacial epoch; while the other hemisphere, receiving the heat from - the equator, would be under a condition of climate similar to what we - know prevailed in the northern hemisphere during a part of the Upper - Miocene period, when North Greenland enjoyed a climate as mild as that - of England at the present day.</p> - - <p><span class="pagenum" id="Page_69">69</span></p> - - <p>This is no mere picture of the imagination, no mere hypothesis devised - to meet a difficult case; for if what has already been stated be not - completely erroneous, all this follows as a necessary consequence from - physical principles. If the warm currents of the equatorial regions - be all deflected into one hemisphere, such must be the condition of - things. How then do the agencies which we have been considering deflect - ocean-currents?</p> - - <p><em>How the foregoing Causes deflect Ocean-currents.</em>—A high condition - of eccentricity tends, we have seen, to produce an accumulation of - snow and ice on the hemisphere whose winters occur in aphelion. This - accumulation tends in turn to lower the summer temperature, to cut - off the sun’s rays, and so to retard the melting of the snow. In - short, it tends to produce on that hemisphere a state of glaciation. - Exactly opposite effects take place on the other hemisphere, which - has its winter in perihelion. There the shortness of the winters and - the highness of the temperature, owing to the sun’s nearness, combine - to prevent the accumulation of snow. The general result is that the - one hemisphere is cooled and the other heated. This state of things - now brings into play the agencies which lead to the deflection of the - Gulf-stream and other great ocean-currents.</p> - - <p>Owing to the great difference between the temperature of the equator - and the poles, there is a constant flow of air from the poles to the - equator. It is to this that the trade-winds owe their existence. Now as - the strength of these winds, as a general rule, will depend upon the - difference of temperature that may exist between the equator and higher - latitudes, it follows that the trades on the cold hemisphere will be - stronger than those on the warm. When the polar and temperate regions - of the one hemisphere are covered to a large extent with snow and ice, - the air, as we have just seen, is kept almost at the freezing-point - during both summer and winter. The trades on that hemisphere will, of - necessity, be exceedingly powerful; while on the other hemisphere, - where there is comparatively little snow and ice, and the air is warm, - the trades will, as a consequence, be weak. Suppose now the northern - hemisphere to be<span class="pagenum" id="Page_70">70</span> the cold one. The north-east trade-winds of this - hemisphere will far exceed in strength the south-east trade-winds of - the southern hemisphere. The <em>median-line</em> between the trades will - consequently lie to a very considerable distance to the south of - the equator. We have a good example of this at the present day. The - difference of temperature between the two hemispheres at present is - but trifling to what it would be in the case under consideration; yet - we find that the south-east trades of the Atlantic blow with greater - force than the north-east trades, and the result is that the south-east - trades sometimes extend to 10° or 15° N. lat., whereas the north-east - trades seldom blow south of the equator. The effect of the northern - trades blowing across the equator to a great distance will be to impel - the warm water of the tropics over into the Southern Ocean. But this - is not all; not only would the median-line of the trades be shifted - southwards, but the great equatorial currents of the globe would also - be shifted southwards.</p> - - <p>Let us now consider how this would affect the Gulf-stream. The South - American continent is shaped somewhat in the form of a triangle, with - one of its angular corners, called Cape St. Roque, pointing eastwards. - The equatorial current of the Atlantic impinges against this corner; - but as the greater portion of the current lies a little to the north - of the corner, it flows westward into the Gulf of Mexico and forms the - Gulf-stream. A considerable portion of the water, however, strikes the - land to the south of the Cape and is deflected along the shores of - Brazil into the Southern Ocean, forming what is known as the Brazilian - current.</p> - - <p>Now it is perfectly obvious that the shifting of the equatorial - current of the Atlantic only a few degrees to the south of its present - position—a thing which would certainly take place under the conditions - which we have been detailing—would turn the entire current into the - Brazilian branch, and instead of flowing chiefly into the Gulf of - Mexico as at present, it would all flow into the Southern Ocean, and - the Gulf-stream would consequently be stopped. The stoppage of the - Gulf-stream, <span class="pagenum" id="Page_71">71</span>combined with all those causes which we have just been - considering, would place Europe under glacial conditions; while, at the - same time, the temperature of the Southern Ocean would, in consequence - of the enormous quantity of warm water received, have its temperature - (already high from other causes) raised enormously.</p> - - <p><em>Deflection of the Gulf-stream during the Glacial Epoch indicated by - the Difference between the Clyde and Canadian Shell-beds.</em>—That the - glaciation of north-western Europe resulted to a great extent from - the stoppage of the Gulf-stream may, I think, be inferred from a - circumstance pointed out by the Rev. Mr. Crosskey, several years ago, - in a paper read before the Glasgow Geological Society.<a id="FNanchor_45" href="#Footnote_45" class="fnanchor">[45]</a> He showed - that the difference between the glacial shells of Canada and those - now existing in the Gulf of St. Lawrence is much less marked than the - difference between the glacial shells of the Clyde beds and those now - existing in the Firth. And from this he justly infers that the change - of climate in Canada since the glacial epoch has been far less complete - than in Scotland.</p> - - <p>The return of the Gulf-stream has raised the mean annual temperature of - our island no less than 15° above the normal, while Canada, deprived of - its influence and exposed to a cold stream from polar regions, has been - kept nearly as much below the normal.</p> - - <p>Let us compare the present temperature of the two countries. In making - our comparison we must, of course, compare places on the same latitude. - It will not do, for example, to compare Glasgow with Montreal or - Quebec, places on the latitude of the south of France and north of - Italy. It will be found that the difference of temperature between - the two countries is so enormous as to appear scarcely credible to - those who have not examined the matter. The temperatures have all been - taken from Professor Dove’s work on the “Distribution of Heat over the - Surface of the Globe,” and his Tables published in the Report of the - British Association for 1847.</p> - - <p><span class="pagenum" id="Page_72">72</span></p> - - <p>The mean temperature of Scotland for January is about 38° F., while - in some parts of Labrador, on the same latitude, and all along the - central parts of North America lying to the north of Upper Canada, - it is actually 10°, and in many places 13° below zero. The January - temperature at the Cumberland House, which is situated on the latitude - of the centre of England, is more than 13° below zero. Here is a - difference of no less than 51°. The normal temperature for the month - of January in the latitude of Glasgow, according to Professor Dove, is - 10°. Consequently, owing to the influence of the Gulf-stream, we are - 28° warmer during that month than we would otherwise be, while vast - tracts of country in America are 23° colder than they should be.</p> - - <p>The July temperature of Glasgow is 61°, while on the same latitude - in Labrador and places to the west it is only 49°. Glasgow during - that month is 3° above the normal temperature, while America, owing - to the influence of the cold polar stream, is 9° below it. The mean - annual temperature of Glasgow is nearly 50°, while in America, on the - same latitude, it is only 30°, and in many places as low as 23°. The - mean normal temperature for the whole year is 35°. Our mean annual - temperature is therefore 15° above the normal, and that of America from - 5° to 12° below it. The American winters are excessively cold, owing - to the continental character of the climate, and the absence of any - benefit from the Gulf-stream, while the summers, which would otherwise - be warm, are, in the latitude of Glasgow, cooled down to a great extent - by the cold ice from Greenland; and the consequence is, that the mean - annual temperature is about 20° or 27° below that of ours. The mean - annual temperature of the Gulf of St. Lawrence is as low as that of - Lapland or Iceland. It is no wonder, then, that the shells which - flourished in Canada during the glacial epoch have not left the gulf - and the neighbouring seas.</p> - - <p>We have good reason to believe that the climate of America during the - glacial epoch was even then somewhat more severe than that of Western - Europe, for the erratics of America extend<span class="pagenum" id="Page_73">73</span> as far south as latitude - 40°, while on the old continent they are not found much beyond latitude - 50°. This difference may have resulted from the fact that the western - side of a continent is always warmer than the eastern.</p> - - <p>In order to determine whether the cold was as great in America during - the glacial epoch as in Western Europe, we must not compare the fossils - found in the glacial beds about Montreal, for example, with those found - in the Clyde beds, for Montreal lies much further to the south than the - Clyde. The Clyde beds must be compared with those of Labrador, while - the beds of Montreal must be compared with those of the south of France - and the north of Italy, if any are to be found there.</p> - - <p>On the whole, it may be concluded that had the Gulf-stream not returned - to our shores at the close of the glacial epoch, and had its place - been supplied by a cold stream from the polar regions, similar to that - which washes the shores of North America, it is highly probable that - nearly every species found in our glacial beds would have had their - representatives flourishing in the British seas at the present day.</p> - - <p>It is no doubt true that when we compare the places in which the - Canadian shell-beds referred to by Mr. Crosskey are situated with - places on the same latitude in Europe, the difference of climate - resulting from the influence of the Gulf-stream is not so great as - between Scotland and those places which we have been considering; but - still the difference is sufficiently great to account for why the - change of climate in Canada has been less complete than in Scotland.</p> - - <p>And what holds true in regard to the currents of the Atlantic holds - also true, though perhaps not to the same extent, of the currents of - the Pacific.</p> - - <p><em>Nearness of the Sun in Perigee a Cause of the Accumulation of - Ice.</em>—But there is still another cause which must be noticed:—A strong - under current of air <em>from</em> the north implies an equally strong upper - current <em>to</em> the north. Now if the effect of the under current would - be to impel the warm water at the equator to the south, the effect - of the upper current would be to carry<span class="pagenum" id="Page_74">74</span> the aqueous vapour formed at - the equator to the north; the upper current, on reaching the snow and - ice of temperate regions, would deposit its moisture in the form of - snow; so that, notwithstanding the great cold of the glacial epoch, - it is probable that the quantity of snow falling in the northern - regions would be enormous. This would be particularly the case during - summer, when the earth would be in the perihelion and the heat at the - equator great. The equator would be the furnace where evaporation would - take place, and the snow and ice of temperate regions would act as a - condenser.</p> - - <p>Heat to produce <em>evaporation</em> is just as essential to the accumulation - of snow and ice as cold to produce <em>condensation</em>. Now at Midsummer, - on the supposition of the eccentricity being at its superior limit, - the sun would be 8,641,870 miles nearer than at present during that - season. The effect would be that the intensity of the sun’s rays would - be one-fifth greater than now. That is to say, for every five rays - received by the ocean at present, six rays would be received then, - consequently the evaporation during summer would be excessive. But the - ice-covered land would condense the vapour into snow. It would, no - doubt, be during summer that the greatest snowfall would take place. In - fact, the nearness of the sun during that season was as essential to - the production of the glacial epoch as was his distance during winter.</p> - - <p>The direct effect of eccentricity is to produce on one of the - hemispheres a long and cold winter. This alone would not lead to a - condition of things so severe as that which we know prevailed during - the glacial epoch. But the snow and ice thus produced would bring into - operation, as we have seen, a host of physical agencies whose combined - efforts would be quite sufficient to do this.</p> - - <p><em>A remarkable Circumstance regarding those Causes which lead to Secular - Changes of Climate.</em>—There is one remarkable circumstance connected - with those physical causes which deserves special notice. They not only - all lead to one result, viz., an accumulation of snow and ice, but - they react on one another.<span class="pagenum" id="Page_75">75</span> It is quite a common thing in physics for - the effect to react on the cause. In electricity and magnetism, for - example, cause and effect in almost every case mutually act and react - upon each other. But it is usually, if not universally, the case that - the reaction of the effect tends to weaken the cause. The weakening - influences of this reaction tend to impose a limit on the efficiency - of the cause. But, strange to say, in regard to the physical causes - concerned in the bringing about of the glacial condition of climate, - cause and effect mutually reacted so as to strengthen each other. And - this circumstance had a great deal to do with the extraordinary results - produced.</p> - - <p>We have seen that the accumulation of snow and ice on the ground - resulting from the long and cold winters tended to cool the air - and produce fogs which cut off the sun’s rays. The rays thus cut - off diminished the melting power of the sun, and so increased the - accumulation. As the snow and ice continued to accumulate, more and - more of the rays were cut off; and on the other hand, as the rays - continued to be cut off, the <em>rate</em> of accumulation increased, because - the quantity of snow and ice melted became thus annually less and less.</p> - - <p>Again, during the long and dreary winters of the glacial epoch the - earth would be radiating off its heat into space. Had the heat thus - lost simply gone to lower the temperature, the lowering of the - temperature would have tended to diminish the rate of loss; but the - necessary result of this was the formation of snow and ice rather than - the lowering of temperature.</p> - - <p>And, again, the formation of snow and ice facilitated the rate at which - the earth lost its heat; and on the other hand, the more rapidly the - earth parted with its heat, the more rapidly were the snow and ice - formed.</p> - - <p>Further, as the snow and ice accumulated on the one hemisphere, they - at the same time continued to diminish on the other. This tended to - increase the strength of the trade-winds on the cold hemisphere, and - to weaken those on the warm. The effect of this on ocean currents - would be to impel the warm water of the tropics more to the warm - hemisphere<span class="pagenum" id="Page_76">76</span> than to the cold. Suppose the northern hemisphere to be - the cold one, then as the snow and ice began gradually to accumulate - there, the ocean currents of that hemisphere would begin to decrease in - volume, while those on the southern, or warm, hemisphere, would <i lang="la">pari - passu</i> increase. This withdrawal of heat from the northern hemisphere - would tend, of course, to lower the temperature of that hemisphere - and thus favour the accumulation of snow and ice. As the snow and ice - accumulated the ocean currents would decrease, and, on the other hand, - as the ocean currents diminished the snow and ice would accumulate,—the - two effects mutually strengthening each other.</p> - - <p>The same must have held true in regard to aërial currents. The more - the polar and temperate regions became covered with snow and ice, the - stronger would become the trades and anti-trades of the hemisphere; and - the stronger those winds became, the greater would be the amount of - moisture transferred from the tropical regions by the anti-trades to - the temperate regions; and on the other hand, the more moisture those - winds brought to temperate regions, the greater would be the quantity - of snow produced.</p> - - <p>The same process of mutual action and reaction would take place among - the agencies in operation on the warm hemisphere, only the result - produced would be diametrically opposite of that produced in the cold - hemisphere. On this warm hemisphere action and reaction would tend to - raise the mean temperature and diminish the quantity of snow and ice - existing in temperate and polar regions.</p> - - <p>Had it been possible for each of those various physical agents which we - have been considering to produce its direct effects without influencing - the other agents or being influenced by them, its real efficiency in - bringing about either the glacial condition of climate or the warm - condition of climate would not have been so great.</p> - - <p>The primary cause that set all those various physical agencies in - operation which brought about the glacial epoch, was a high state of - eccentricity of the earth’s orbit. When the eccentricity<span class="pagenum" id="Page_77">77</span> is at a - high value, snow and ice begin to accumulate, owing to the increasing - length and coldness of the winter on that hemisphere whose winter - solstice is approaching toward the aphelion. The accumulating snow - then begins to bring into operation all the various agencies which - we have been describing; and, as we have just seen, these, when once - in full operation, mutually aid one another. As the eccentricity - increases century by century, the temperate regions become more and - more covered with snow and ice, first by reason of the continued - increase in the coldness and length of the winters, and secondly, - and chiefly, owing to the continued increase in the potency of those - physical agents which have been called into operation. This glacial - state of things goes on at an increasing rate, and reaches a maximum - when the solstice-point arrives at the aphelion. After the solstice - passes the aphelion, a contrary process commences. The snow and ice - gradually begin to diminish on the cold hemisphere and to make their - appearance on the other hemisphere. The glaciated hemisphere turns, by - degrees, warmer and the warm hemisphere colder, and this continues to - go on for a period of ten or twelve thousand years, until the winter - solstice reaches the perihelion. By this time the conditions of the two - hemispheres have been reversed; the formerly glaciated hemisphere has - now become the warm one, and the warm hemisphere the glaciated. The - transference of the ice from the one hemisphere to the other continues - as long as the eccentricity remains at a high value. This will, - perhaps, be better understood from an inspection of the frontispiece.</p> - - <p><em>The Mean Temperature of the whole Earth should be greater in Aphelion - than in Perihelion.</em>—When the eccentricity becomes reduced to about - its present value, its influence on climate is but little felt. - It is, however, probable that the present extension of ice on the - southern hemisphere may, to a considerable extent, be the result of - eccentricity. The difference in the climatic conditions of the two - hemispheres is just what should be according to theory:—(1) The mean - temperature of that hemisphere is less than that of the northern. - (2) The winters<span class="pagenum" id="Page_78">78</span> of the southern hemisphere are colder than those of - the northern. (3) The summers, though occurring in perihelion, are - also comparatively cold; this, as we have seen, is what ought to be - according to theory. (4) The mean temperature of the whole earth is - greater in June, when the earth is in aphelion, than in December, when - it is in perihelion. This, I venture to affirm, is also what ought to - follow according to theory, although this very fact has been adduced - as a proof that eccentricity has at present but little effect on the - climatic condition of our globe.</p> - - <p>That the mean temperature of the whole earth would, during the - glacial epoch, be greater when the earth was in aphelion than - when in perihelion will, I think, be apparent from the following - considerations:—When the earth was in the perihelion, the sun would - be over the hemisphere nearly covered with snow and ice. The great - strength of the sun’s rays would in this case have little effect in - raising the temperature; it would be spent in melting the snow and - ice. But when the earth was in the aphelion, the sun would be over the - hemisphere comparatively free, or perhaps wholly free, from snow and - ice. Consequently, though the intensity of the sun’s rays would be less - than when the earth was in perihelion, still it ought to have produced - a higher temperature, because it would be chiefly employed in heating - the ground and not consumed in melting snow and ice.</p> - - <p><em>Professor Tyndall on the Glacial Epoch.</em>—“So natural,” says Professor - Tyndall, “was the association of ice and cold, that even celebrated - men assumed that all that is needed to produce a great extension of - our glaciers is a diminution of the sun’s temperature. Had they gone - through the foregoing reflections and calculations, they would probably - have demanded <em>more</em> heat instead of less for the production of a - glacial epoch. What they really needed were <em>condensers</em> sufficiently - powerful to congeal the vapour generated by the heat of the sun.” (<cite>The - Forms of Water</cite>, p. 154. See also, to the same effect, <cite>Heat Considered - as a Mode of Motion</cite>, chap. vi.)</p> - - <p><span class="pagenum" id="Page_79">79</span></p> - - <p>I do not know to whom Professor Tyndall here refers, but certainly his - remarks have no application to the theory under consideration, for - according to it, as we have just seen, the ice of the glacial epoch was - about as much due to the nearness of the sun in perigee as to his great - distance in apogee.</p> - - <p>There is one theory, however, to which his remarks justly apply, viz., - the theory that the great changes of climate during geological ages - resulted from the passage of our globe through different temperatures - of space. What Professor Tyndall says shows plainly that the glacial - epoch was not brought about by our earth passing through a cold part - of space. A general reduction of temperature over the whole globe - certainly would not produce a glacial epoch. Suppose the sun were - extinguished and our globe exposed to the temperature of stellar space - (−239° F.), this would certainly freeze the ocean solid from its - surface to its bottom, but it would not cover the land with ice.</p> - - <p>Professor Tyndall’s conclusions are, of course, equally conclusive - against Professor Balfour Stewart’s theory, that the glacial epoch may - have resulted from a general diminution in the intensity of the sun’s - heat.</p> - - <p>Nevertheless it would be in direct opposition to the well-established - facts of geology to assume that the ice periods of the glacial epoch - were warm periods. We are as certain from palæontological evidence - that the cold was then much greater than now, as we are from physical - evidence that the accumulation of ice was greater than now. Our glacial - shell-beds and remains of the mammoth, the reindeer, and musk-ox, tell - of cold as truly as the markings on the rocks do of ice.</p> - - <p><em>Objection from the Present Condition of the Planet Mars.</em>—It has been - urged as an objection by Professor Charles Martins<a id="FNanchor_46" href="#Footnote_46" class="fnanchor">[46]</a> and others, - that if a high state of eccentricity could produce a glacial epoch, - the planet Mars ought to be at present under a glacial condition. The - eccentricity of its orbit amounts to 0·09322, and one of its southern - winter solstices is, according to <span class="pagenum" id="Page_80">80</span>Dr. Oudemans, of Batavia,<a id="FNanchor_47" href="#Footnote_47" class="fnanchor">[47]</a> within - 17° 41′ 8″ of aphelion. Consequently, it is supposed that one of the - hemispheres should be in a glacial state and the other free from snow - and ice. But it is believed that the snow accumulates around each pole - during its winter and disappears to a great extent during its summer.</p> - - <p>There would be force in this objection were it maintained that - eccentricity alone can produce a glacial condition of climate, but - such is not the case, and there is no good ground for concluding that - those physical agencies which led to the glacial epoch of our globe - exist in the planet Mars. It is perfectly certain that either water - must be different in constitution in that planet from what it is in our - earth, or else its atmospheric envelope must be totally different from - ours. For it is evident from what has been stated in <a href="#CHAPTER_II">Chapter II.</a>, that - were our globe to be removed to the distance of Mars from the sun, the - lowering of the temperature resulting from the decrease in the sun’s - heat would not only destroy every living thing, but would convert the - ocean into solid ice.</p> - - <p>But it must be observed that the eccentricity of Mars’ orbit is at - present far from its superior limit of 0·14224, and it may so happen in - the economy of nature that when it approaches to that limit a glacial - condition of things may supervene.</p> - - <p>The truth is, however, that very little seems to be known with - certainty regarding the climatic condition of Mars. This is obvious - from the fact that some astronomers believe that the planet possesses - a dense atmosphere which protects it from cold; while others maintain - that its atmosphere is so exceedingly thin that its mean temperature is - below the freezing-point. Some assert that the climatic condition of - Mars resembles very much that of our earth, while others affirm that - its seas are actually frozen solid to the bottom, and the poles covered - with ice thirty or forty miles in thickness. For reasons which will be - explained in the Appendix, Mars, notwithstanding its greater distance - from the sun, may enjoy a climate as warm as that of our earth.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_V"> - <span class="pagenum" id="Page_81">81</span> - <h2> - CHAPTER V.<br /><br /> - <span class="small">REASON WHY THE SOUTHERN HEMISPHERE IS COLDER THAN THE NORTHERN.</span> - </h2> - </div> - <div class="subhead">Adhémar’s Explanation.—Adhémar’s Theory founded upon a physical - Mistake in regard to Radiation.—Professor J. D. Forbes on - Underground Temperature.—Generally accepted Explanation.—Low - Temperature of Southern Hemisphere attributed to Preponderance - of Sea.—Heat transferred from Southern to Northern Hemisphere - by Ocean-current the true Explanation.—A large Portion of the - Heat of the Gulf-stream derived from the Southern Hemisphere.</div> - - <p><em>Adhémar’s Explanation.</em>—It has long been known that on the southern - hemisphere the temperature is lower and the accumulation of ice greater - than on the northern. This difference has usually been attributed to - the great preponderance of sea on the southern hemisphere. M. Adhémar, - on the other hand, attempts to explain this difference by referring it - to the difference in the amount of heat lost by the two hemispheres - in consequence of the difference of seven days in the length of their - respective winters. As the northern winter is shorter than the summer, - he concludes that there is an accumulation of heat on that hemisphere, - while, on the other hand, the southern winter being longer than the - summer, there is therefore a loss of heat on the southern hemisphere. - “The south pole,” he says, “loses in one year more heat than it - receives, because the total duration of its night surpasses that of - its day by 168 hours; and the contrary takes place for the north pole. - If, for example, we take for unity the mean quantity of heat which the - sun sends off in one hour, the heat accumulated at the end of the year - at the north pole will be expressed by 168, while the heat lost by the - south pole will<span class="pagenum" id="Page_82">82</span> be equal to 168 times what the radiation lessens it by - in one hour, so that at the end of the year the difference in the heat - of the two hemispheres will be represented by 336 times what the earth - receives from the sun or loses in an hour by radiation.”<a id="FNanchor_48" href="#Footnote_48" class="fnanchor">[48]</a></p> - - <p>Adhémar supposes that about 10,000 years hence, when our northern - winter will occur in aphelion and the southern in perihelion, the - climatic conditions of the two hemispheres will be reversed; the - ice will melt at the south pole, and the northern hemisphere will - become enveloped in one continuous mass of ice, leagues in thickness, - extending down to temperate regions.</p> - - <p>This theory seems to be based upon an erroneous interpretation of a - principle, first pointed out, so far as I am aware, by Humboldt in - his memoir “On Isothermal Lines and Distribution of Heat over the - Globe.”<a id="FNanchor_49" href="#Footnote_49" class="fnanchor">[49]</a> This principle may be stated as follows:—</p> - - <p>Although the total quantity of heat received by the earth from the - sun in one revolution is inversely proportional to the minor axis of - the orbit, yet this amount, as was proved by D’Alembert, is equally - distributed between the two hemispheres, whatever the eccentricity may - be. Whatever extra heat the southern hemisphere may at present receive - from the sun daily during its summer months owing to greater proximity - to the sun, is exactly compensated by a corresponding loss arising from - the shortness of the season; and, on the other hand, whatever daily - deficiency of heat we in the northern hemisphere may at present have - during our summer half-year, in consequence of the earth’s distance - from the sun, is also exactly compensated by a corresponding length of - season.</p> - - <p>But the surface temperature of our globe depends as much upon the - amount of heat radiated into space as upon the amount derived from the - sun, and it has been thought by some that this compensating principle - holds true only in regard to the <span class="pagenum" id="Page_83">83</span>latter. In the case of the heat - lost by radiation the reverse is supposed to take place. The southern - hemisphere, it is asserted, has not only a colder winter than the - northern in consequence of the sun’s greater distance, but it has also - a longer winter; and the extra loss of heat from radiation during - winter is not compensated by its nearness to the sun during summer, for - it gains no additional heat from this proximity. And in the same way it - is argued that as our winter in the northern hemisphere, owing to the - less distance of the sun, is not only warmer than that of the southern - hemisphere, but is also at the same time shorter, so our hemisphere - is not cooled to such an extent as the southern. And thus the mean - temperature of the winter half-year, as well as the intensity of the - sun’s heat, is affected by a change in the sun’s distance.</p> - - <p>Although I always regarded this cause of Humboldt’s to be utterly - inadequate to produce such effects as those attributed to it by - Adhémar, still, in my earlier papers<a id="FNanchor_50" href="#Footnote_50" class="fnanchor">[50]</a> I stated it to be a <i lang="la">vera - causa</i> which ought to produce some sensible effect on climate. But - shortly afterwards on a more careful consideration of the whole - subject, I was led to suspect that the circumstance in question can, - according to theory, produce little or no effect on the climatic - condition of our globe.</p> - - <p>As there appears to be a considerable amount of misapprehension in - reference to this point, which forms the basis of Adhémar’s theory, I - may here give it a brief consideration.<a id="FNanchor_51" href="#Footnote_51" class="fnanchor">[51]</a></p> - - <p>The rate at which the earth radiates into space the heat received - from the sun depends upon the temperature of its surface; and the - temperature of its surface (other things being equal) depends upon - the rate at which the heat is received. The greater the rate at which - the earth receives heat from the sun, the greater will therefore be - the rate at which it will lose that heat by radiation. Now the total - quantity of heat received during winter by the southern hemisphere is - exactly <span class="pagenum" id="Page_84">84</span>equal to that received during winter by the northern. But as - the southern winter is longer than the northern, the rate at which the - heat is received, and consequently the rate of radiation, during that - season must be less on the southern hemisphere than on the northern. - Thus the southern hemisphere loses heat during a longer period than the - northern, and therefore the less rate of radiation (were it not for a - circumstance presently to be noticed) would wholly compensate for the - longer period, and the total quantity of heat lost during winter would - be the same on both hemispheres. The southern summer is shorter than - the northern, but the heat is more intense, and the surface of the - ground kept at a higher temperature; consequently the rate of radiation - into space is greater.</p> - - <p>When the rate at which a body receives heat is increased, the - temperature of the body rises till the rate of radiation equals the - rate of absorption, after which equilibrium is restored; and when the - rate of absorption is diminished, the temperature falls till the rate - of radiation equals that of absorption.</p> - - <p>But notwithstanding all this, owing to the slow conductivity of the - ground for heat, more heat will pass into it during the longer summer - of aphelion than during the shorter one of perihelion; for the amount - of heat which passes into the ground depends on the length of time - during which the earth is receiving heat, as well as upon the amount - received. In like manner, more heat will pass out of the ground - during the longer winter in aphelion than during the shorter one in - perihelion. Suppose the length of the days on the one hemisphere (say - the northern) to be 23 hours, and the length of the nights, say one - hour; while on the other hemisphere the days are one hour and the - nights 23 hours. Suppose also that the quantity of heat received from - the sun by the southern hemisphere during the day of one hour to be - equal to that received by the northern hemisphere during the day of - 23 hours. It is evident that although the surface of the ground<span class="pagenum" id="Page_85">85</span> on - the southern hemisphere would receive as much heat from the sun during - the short day of one hour as the surface of the northern hemisphere - during the long day of 23 hours, yet, owing to the slow conductivity - of the ground for heat, the amount absorbed would not be nearly so - much on the southern hemisphere as on the northern. The temperature - of the surface during the day, it is true, would be far higher on the - southern hemisphere than on the northern, and consequently the rate - at which the heat would pass into the ground would be greater on that - hemisphere than on the northern; but, notwithstanding the greater rate - of absorption resulting from the high temperature of the surface, it - would not compensate for the shortness of the day. On the other hand, - the surface of the ground on the southern hemisphere would be colder - during the long night of 23 hours than it would be on the northern - during the short night of only one hour; and the low temperature of the - ground would tend to lessen the rate of radiation into space. But the - decrease in the rate of radiation would not compensate fully for the - great length of the night. The general and combined result of all those - causes would be that a slight accumulation of heat would take place on - the northern hemisphere and a slight loss on the southern. But this - loss of heat on the one hemisphere and gain on the other would not go - on accumulating at a uniform rate year by year, as Adhémar supposes.</p> - - <p>Of course we are at present simply considering the earth as an absorber - and radiator of heat, without taking into account the effects of - distribution of sea and land and other modifying causes, and are - assuming that everything is the same in both hemispheres, with the - exception that the winter of the one hemisphere is longer than that of - the other.</p> - - <p>What, then, is the amount of heat stored up by the one hemisphere and - lost by the other? Is it such an amount as to sensibly affect climate?</p> - - <p>The experiments and observations which have been made on underground - temperature afford us a means of making at least<span class="pagenum" id="Page_86">86</span> a rough estimate of - the amount. And from these it will be seen that the influence of an - excess of seven or eight days in the length of the southern winter over - the northern could hardly produce an effect that would be sensible.</p> - - <p>Observations were made at Edinburgh by Professor J. D. Forbes on - three different substances; viz., sandstone, sand, and trap-rock. By - calculation, we find from the data afforded by those observations that - the total quantity of heat accumulated in the ground during the summer - above the mean temperature was as follows:—In the sandstone-rock, a - quantity sufficient to raise the temperature of the rock 1° C. to a - depth of 85 feet 6 inches; in the sand a quantity sufficient to raise - the temperature 1° C. to a depth of 72 feet 6 inches; and in the - trap-rock a quantity only sufficient to raise the temperature 1° C. to - a depth of 61 feet 6 inches.</p> - - <p>Taking the specific heat of the sandstone per unit volume, as - determined by Regnault, at ·4623, and that of sand at ·3006, and - trap at ·5283, and reducing all the results to one standard, viz., - that of water, we find that the quantity of heat stored up in the - sandstone would, if applied to water, raise its temperature 1° C. to - a depth of 39 feet 6 inches; that stored up in the sand would raise - the temperature of the water 1° C. to a depth of 21 feet 8 inches, and - that stored up in the trap would raise the water 1° C. to the depth - of 32 feet 6 inches. We may take the mean of these three results as - representing pretty accurately the quantity stored up in the general - surface of the country. This would be equal to 31 feet 3 inches depth - of water raised 1° C. The quantity of heat lost by radiation during - winter below the mean was found to be about equal to that stored up - during summer.</p> - - <p>The total quantity of heat per square foot of surface received by the - equator from sunrise till sunset at the time of the equinoxes, allowing - 22 per cent. for the amount cut off in passing through the atmosphere, - is 1,780,474 foot-pounds. In the latitude of Edinburgh about 938,460 - foot-pounds per square foot of surface is received, assuming that not - more than 22 per<span class="pagenum" id="Page_87">87</span> cent. is cut off by the atmosphere. At this rate a - quantity of heat would be received from the sun in two days ten hours - (say, three days) sufficient to raise the temperature of the water 1° - C. to the required depth of 31 feet 3 inches. Consequently the total - quantity of heat stored up during summer in the latitude of Edinburgh - is only equal to what we receive from the sun during three days at the - time of the equinoxes. Three days’ sunshine during the middle of March - or September, if applied to raise the temperature of the ground, would - restore all the heat lost during the entire winter; and another three - days’ sunshine would confer on the ground as much heat as is stored - up during the entire summer. But it must be observed that the total - duration of sunshine in winter is to that of summer in the latitude of - Edinburgh only about as 4 to 7. Here is a difference of two months. - But this is not all; the quantity of heat received during winter is - scarcely one-third of that received during summer; yet, notwithstanding - this enormous difference between summer and winter, the ground during - winter loses only about six days’ sun-heat below the maximum amount - possessed by it in summer.</p> - - <p>But if what has already been stated is correct, this loss of heat - sustained by the earth during winter is not chiefly owing to radiation - during the longer absence of the sun, but to the decrease in the - quantity of heat received in consequence of his longer absence combined - with the obliquity of his rays during that season. Now in the case - of the two hemispheres, although the southern winter is longer than - the northern, yet the quantity of heat received by each is the same. - But supposing it held true, which it does not, that the loss of - heat sustained by the earth in winter is as much owing to radiation - resulting from the excess in the length of the winter nights over those - of the summer as to the deficiency of heat received in winter from that - received in summer, three days’ heat would then in this case be the - amount lost by radiation in consequence of this excess in the length of - the winter nights. The total length of the winter nights to those of - the summer is, as we<span class="pagenum" id="Page_88">88</span> have seen, about as 7 to 4. This is a difference - of nearly 1200 hours. But the excess of the south polar winter over the - north amounts to only about 184 hours. Now if 1200 hours give a loss of - three days’ sun-heat, 184 hours will give a loss of scarcely 5½ hours.</p> - - <p>It is no doubt true that the two cases are not exactly analogous; but - it is obvious that any error which can possibly arise from regarding - them as such cannot materially alter the conclusion to which we have - arrived. Supposing the effect were double, or even quadruple, what - we have concluded it to be, still it would not amount to a loss of - two days’ heat, which could certainly have little or no influence on - climate.</p> - - <p>But even assuming all the preceding reasoning to be incorrect, and that - the southern hemisphere, in consequence of its longer winter, loses - heat to the extravagant extent of 168 hours, supposed by Adhémar, still - this could not materially affect climate. The climate is influenced - by the mere <em>temperature</em> of the <em>surface</em> of the ground, and not by - the quantity of heat or cold that may be stored up under the surface. - The climate is determined, so far as the ground is concerned, by - the temperature of the surface, and is wholly independent of the - temperature which may exist under the surface. Underground temperature - can only affect climate through the surface. If the surface could, - for example, be kept covered with perpetual snow, we should have a - cold and sterile climate, although the temperature of the ground under - the snow was actually at the boiling-point. Let the ground to a depth - of, say 40 or 50 feet, be deprived of an amount of heat equal to that - received from the sun in 168 hours. This could produce little or no - sensible effect on climate; for, owing to the slow conductivity of the - ground for heat, this loss would not sensibly affect the temperature - of the surface, as it would take several months for the sun’s heat - to penetrate to that depth and restore the lost heat. The cold, if I - may be allowed to use the expression, would come so slowly out to the - surface that its effect in lowering the temperature of the surface - would scarcely be sensible. And, again, if we suppose the 168 hours’ - heat to be lost by the mere surface of the<span class="pagenum" id="Page_89">89</span> ground, the effect would - certainly be sensible, but it would only be so for a few days. We - might in this case have a week’s frozen soil, but that would be all. - Before the air had time to become very sensibly affected by the low - temperature of the surface the frozen soil would be thawed.</p> - - <p>The storing up of heat or cold in the ground has in reality very little - to do with climate. Some physicists explain, for example, why the month - of July is warmer than June by referring it to the fact that by the - month of July the ground has become possessed of a larger accumulation - of heat than it possessed in June. This explanation is evidently - erroneous. The ground in July certainly possesses a greater store of - heat than it did in June; but this is not the reason why the former - month is hotter than the latter. July is hotter than June because the - <em>air</em> (not the <em>ground</em>) has become possessed of a larger store of - heat than it had in June. Now the air is warmer in July than in June - because, receiving little increase of temperature from the direct rays - of the sun, it is heated chiefly by radiation from the earth and by - contact with its warm surface. Consequently, although the sun’s heat - is greater in June than it is in July, it is near the middle of July - before the air becomes possessed of its maximum store of heat. We - therefore say that July is hotter than June because the air is hotter, - and consequently the temperature in the shade is greater in the former - month than in the latter.</p> - - <p>It is therefore, I presume, quite apparent that Adhémar’s theory fails - to explain why the southern hemisphere is colder than the northern.</p> - - <p><em>The generally accepted Explanation.</em>—The difference in the mean - temperature of the two hemispheres is usually attributed to the - proportion of sea to land in the southern hemisphere and of land to - sea in the northern hemisphere. This, no doubt, will account for the - greater <em>annual range</em> of temperature on the northern hemisphere, - but it seems to me that it will not account for the excess of <em>mean</em> - temperature possessed by that hemisphere over the southern.</p> - - <p>The general influence of land on climate is to exaggerate the<span class="pagenum" id="Page_90">90</span> - variation of temperature due to the seasons. On continents the summers - are hotter and the winters colder than on the ocean. The days are - also hotter and the nights colder on land than on sea. This is a - result which follows from the mere physical properties of land and - water, independently of currents, whether of ocean or of air. But it - nevertheless follows, according to theory (and this is a point which - has been overlooked), that the mean annual temperature of the ocean - ought to be greater than that of the land in equatorial regions as - well as in temperate and polar regions. This will appear obvious for - the following reasons:—(1) The ground stores up heat only by the slow - process of conduction, whereas water, by the mobility of its particles - and its transparency for heat-rays, especially those from the sun, - becomes heated to a considerable depth rapidly. The quantity of heat - stored up in the ground is thus comparatively small, while the quantity - stored up in the ocean is great. (2) The air is probably heated more - rapidly by contact with the ground than with the ocean; but, on the - other hand, it is heated far more rapidly by radiation from the ocean - than from the land. The aqueous vapour of the air is to a great extent - diathermanous to radiation from the ground, while it absorbs the - rays from water and thus becomes heated. (3) The air radiates back a - considerable portion of its heat, and the ocean absorbs this radiation - from the air more readily than the ground does. The ocean will not - reflect the heat from the aqueous vapour of the air, but absorbs it, - while the ground does the opposite. Radiation from the air, therefore, - tends more readily to heat the ocean than it does the land. (4) The - aqueous vapour of the air acts as a screen to prevent the loss by - radiation from water, while it allows radiation from the ground to pass - more freely into space; the atmosphere over the ocean consequently - throws back a greater amount of heat than is thrown back by the - atmosphere over the land. The sea in this case has a much greater - difficulty than the land has in getting quit of the heat received from - the sun; in other words, the land tends to lose its heat more rapidly - than the sea. The<span class="pagenum" id="Page_91">91</span> consequence of all these circumstances is that the - ocean must stand at a higher mean temperature than the land. A state of - equilibrium is never gained until the rate at which a body is receiving - heat is equal to the rate at which it is losing it; but as equal - surfaces of sea and land receive from the sun the same amount of heat, - it therefore follows that, in order that the sea may get quit of its - heat as rapidly as the land, it <em>must stand at a higher temperature</em> - than the land. The temperature of the sea must continue to rise till - the amount of heat thrown off into space equals that received from the - sun; when this point is reached, equilibrium is established and the - temperature remains stationary. But, owing to the greater difficulty - that the sea has in getting rid of its heat, the mean temperature - of equilibrium of the ocean must be higher than that of the land; - consequently the mean temperature of the ocean, and also of the air - immediately over it, in tropical regions should be higher than the mean - temperature of the land and the air over it.</p> - - <p>The greater portion of the southern hemisphere, however, is occupied by - water, and why then, it may be asked, is this water hemisphere colder - than the land hemisphere? Ought it not also to follow that the sea in - inter-tropical regions should be warmer than the land under the same - parallels; yet, as we know, the reverse is actually found to be the - case. How then is all this to be explained, if the foregoing reasoning - be correct? We find when we examine Professor Dove’s charts of mean - annual temperature, that the ocean in inter-tropical regions has a - mean annual temperature below the normal, and the land a mean annual - temperature above the normal. Both in the Pacific and in the Atlantic - the mean temperature sinks to 2°·3 below the normal, while on the - land it rises 4°·6 above the normal. The explanation in this case is - obviously this: the temperature of the ocean in inter-tropical regions, - as we have already seen, is kept much lower than it would otherwise be - by the enormous amount of <em>heat</em> that is being constantly carried away - from those regions into temperate and polar regions, and of <em>cold</em> that - is being constantly carried from temperate and<span class="pagenum" id="Page_92">92</span> polar regions to the - tropical regions by means of ocean-currents. The same principle which - explains why the sea in inter-tropical regions has a lower mean annual - temperature than the land, explains also why the southern hemisphere - has a lower mean annual temperature than the northern. The temperature - of the southern hemisphere is lowered by the transference of heat by - means of ocean-currents.</p> - - <p><em>Heat transferred from the Southern to the Northern Hemisphere by - Ocean-currents the true Explanation.</em>—The great ocean-currents of - the globe take their rise in three immense streams from the Southern - Ocean, which, on reaching the tropical regions, become deflected in - a westerly direction and flow along the southern side of the equator - for thousands of miles. Perhaps more than one half of this mass of - moving water returns into the Southern Ocean without ever crossing the - equator, but the quantity which crosses over to the northern hemisphere - is enormous. This constant flow of water from the southern hemisphere - to the northern in the form of surface currents must be compensated by - <em>under currents</em> of equal magnitude from the northern hemisphere to the - southern. The currents, however, which cross the equator are far higher - in temperature than their compensating under currents; consequently - there is a constant transference of heat from the southern hemisphere - to the northern. Any currents taking their rise in the northern - hemisphere and flowing across into the southern are comparatively - trifling, and the amount of heat transferred by them is also trifling. - There are one or two currents of considerable size, such as the - Brazilian branch of the great equatorial current of the Atlantic, and - a part of the South Equatorial Drift-current of the Pacific, which - cross the equator from north to south; but these cannot be regarded as - northern currents; they are simply southern currents deflected back - after crossing over to the northern hemisphere. The heat which these - currents possess is chiefly obtained on the southern hemisphere before - crossing over to the northern; and although the northern hemisphere may - not gain much<span class="pagenum" id="Page_93">93</span> heat by means of them, it, on the other hand, does not - lose much, for the heat which they give out in their progress along the - southern hemisphere does not belong to the northern hemisphere.</p> - - <p>But, after making the fullest allowance for the amount of heat carried - across the equator from the northern hemisphere to the southern, we - shall find, if we compare the mean temperature of the currents from - south to north with that of the great compensating under currents and - the one or two small surface currents, that the former is very much - higher than the latter. The mean temperature of the water crossing the - equator from south to north is probably not under 65°, that of the - under currents is probably not over 39°. But to the under currents - we must add the surface currents from north to south; and assuming - that this will raise the mean temperature of the entire mass of water - flowing south to, say, 45°, we have still a difference of 20° between - the temperature of the masses flowing north and south. Each cubic - foot of water which crosses the equator will in this case transfer - about 965,000 foot-pounds of heat from the southern hemisphere to the - northern. If we had any means of ascertaining the volume of those great - currents crossing the equator, we should then be able to make a rough - estimate of the total amount of heat transferred from the southern - hemisphere to the northern; but as yet no accurate estimate has been - made on this point. Let us assume, what is probably below the truth, - that the total amount of water crossing the equator is at least double - that of the Gulf-stream as it passes through the Straits of Florida, - which amount we have already found to be equal to 66,908,160,000,000 - cubic feet daily. Taking the quantity of heat conveyed by each cubic - foot of water of the Gulf-stream as 1,158,000 foot-pounds, it is - found, as we have seen, that an amount of heat is conveyed by this - current equal to all the heat that falls within 32 miles on each - side of the equator. Then, if each cubic foot of water crossing the - equator transfers 965,000 foot-pounds, and the quantity of water be - double that<span class="pagenum" id="Page_94">94</span> of the Gulf-stream, it follows that the amount of heat - transferred from the southern hemisphere to the northern is equal to - all the heat falling within 52 miles on each side of the equator, or - equal to all the heat falling on the southern hemisphere within 104 - miles of the equator. This quantity taken from the southern hemisphere - and added to the northern will therefore make a difference in the - amount of heat possessed by the two hemispheres equal to all the heat - which falls on the southern hemisphere within somewhat more than 208 - miles of the equator.</p> - - <p><em>A large Portion of the Heat of the Gulf-stream derived from the - Southern Hemisphere.</em>—It can be proved that a very large portion of the - heat conveyed by the Gulf-stream comes from the southern hemisphere. - The proof is as follows:—</p> - - <p>If all the heat came from the northern hemisphere, it could only come - from that portion of the Atlantic, Caribbean Sea, and Gulf of Mexico - which lies to the north of the equator. The entire area of these seas, - extending to the Tropic of Cancer, is about 7,700,000 square miles. - But this area is not sufficient to supply the current passing through - the “Narrows” with the necessary heat. Were the heat which passes - through the Straits of Florida derived exclusively from this area, the - following table would then represent the relative quantity per unit - surface possessed by the Atlantic in the three zones, assuming that one - half of the heat of the Gulf-stream passes into the arctic regions and - the other half remains to warm the temperate regions<a id="FNanchor_52" href="#Footnote_52" class="fnanchor">[52]</a>:—</p> - - <table summary="Heat per unit surface"> - <tbody> - <tr> - <td>From the equator to the Tropic of Cancer</td> - <td>773</td> - </tr> - <tr> - <td>From the Tropic of Cancer to the Arctic Circle</td> - <td>848</td> - </tr> - <tr> - <td>From the Arctic Circle to the North Pole</td> - <td>610</td> - </tr> - </tbody> - </table> - - <p class="noindent">These figures show that the Atlantic, from the equator to the Tropic - of Cancer, would be as cold as from the Tropic of Cancer to the North - Pole, were it not that a large proportion of the heat possessed by the - Gulf-stream is derived from the southern hemisphere.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_VI"> - <span class="pagenum" id="Page_95">95</span> - <h2> - CHAPTER VI.<br /><br /> - <span class="small">EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.—LIEUT. MAURY’S THEORY.</span> - </h2> - </div> - <div class="subhead">Introduction.—Ocean-currents, according to Maury, due to - Difference of Specific Gravity.—Difference of Specific Gravity - resulting from Difference of Temperature.—Difference of - Specific Gravity resulting from Difference of Saltness.—Maury’s - two Causes neutralize each other.—How, according to him, - Difference in Saltness acts as a Cause.</div> - - <p><em>Introduction.</em>—Few subjects have excited more interest and attention - than the cause of ocean circulation; and yet few are in a more - imperfect and unsatisfactory condition, nor is there any question - regarding which a greater diversity of opinion has prevailed. Our - incomplete acquaintance with the facts relating to the currents of the - ocean and the modes of circulation actually in operation, is no doubt - one reason for this state of things. But doubtless the principal cause - of such diversity of opinion lies in the fact that the question is one - which properly belongs to the domain of physics and mechanics, while - as yet no physicist of note (if we except Dr. Colding, of Copenhagen) - has given, as far as I know, any special attention to the subject. It - is true that in works of meteorology and physical geography reference - is continually made to such eminent physicists as Herschel, Pouillet, - Buff, and others; but when we turn to the writings of these authors we - find merely a few remarks expressive of their opinions on the subject, - and no special discussion or investigation of the matter, nor anything - which could warrant us in concluding that such investigations have ever - been made. At present the question cannot be decided by a reference to - authorities.</p> - - <p><span class="pagenum" id="Page_96">96</span></p> - - <p>The various theories on the subject may be classed under two divisions; - the first of these attributes the motion of the water to the <em>impulse - of the wind</em>, and the second to the <em>force of gravity</em> resulting from - difference of density. But even amongst those who adopt the former - theory, it is generally held that the winds are not the sole cause, - but that, to a certain extent at least, difference of specific gravity - contributes to produce motion of the waters. This is a very natural - conclusion; and in the present state of physical geography on this - subject one can hardly be expected to hold any other view.</p> - - <p>The supporters of the latter theory may be subdivided into two - classes. The first of these (of which Maury may be regarded as the - representative) attributes the Gulf-stream, and other sensible currents - of the ocean, to difference of specific gravity. The other class (at - present the more popular of the two, and of which Dr. Carpenter may be - considered the representative) denies altogether that such currents can - be produced by difference of specific gravity,<a id="FNanchor_53" href="#Footnote_53" class="fnanchor">[53]</a> and affirms that - there is a general movement of the upper portion of the ocean from the - equator to the poles, and a counter-movement of the under portion from - the poles to the equator. This movement is attributed to difference of - specific gravity between equatorial and polar water, resulting from - difference of temperature.</p> - - <p>The widespread popularity of the gravitation theory is no doubt, to a - great extent, owing to the very great prominence given to it by Lieut. - Maury in his interesting and popular work, “The Physical Geography of - the Sea.” Another cause which must have favoured the reception of this - theory is the ease with which it is perceived how, according to it, - circulation of the waters of the ocean is supposed to follow. One has - no difficulty, for example, in perceiving that if the inter-tropical - waters of the ocean are expanded by heat, and the waters around the - poles contracted by cold, the surface of the ocean will stand at a - higher level at the equator than at the poles. Equilibrium being - thus disturbed, the water at the equator <span class="pagenum" id="Page_97">97</span>will tend to flow towards - the poles as a surface current, and the water at the poles towards - the equator as an under current. This, at first sight, looks well, - especially to those who take but a superficial view of the matter.</p> - - <p>We shall examine this theory at some length, for two reasons: 1, - because it lies at the root of a great deal of the confusion and - misconception which have prevailed in regard to the whole subject of - ocean-currents: 2, because, if the theory is correct, it militates - strongly against the physical theory of secular changes of climate - advanced in this volume. We have already seen (<a href="#CHAPTER_IV">Chapter IV.</a>) that - when the eccentricity of the earth’s orbit reaches a high value, a - combination of physical circumstances tends to lower the temperature of - the hemisphere which has its winter solstice in aphelion, and to raise - the temperature of the opposite hemisphere, whose winter solstice will, - of course, be in perihelion. The direct result of this state of things, - as was shown, is to strengthen the force of the trade-winds on the - cold hemisphere, and to weaken their strength on the warm hemisphere: - and this, in turn, we also saw, tends to impel the warm water of - the inter-tropical region on to the warm hemisphere, and to prevent - it, in a very large degree, from passing into the cold hemisphere. - This deflection of the ocean-currents tends to an enormous extent to - increase the difference of temperature previously existing between the - two hemispheres. In other words, the warm and equable condition of the - one hemisphere, and the cold and glacial condition of the other, are, - to a great extent, due to this deflection of ocean-currents. But if - the theory be correct which attributes the motion of ocean-currents to - a difference in density between the sea in inter-tropical and polar - regions, then it follows that these currents (other things being - equal) ought to be stronger on the cold hemisphere than on the warm, - because there is a greater difference of temperature and, consequently, - a greater difference of density, between the polar seas of the cold - hemisphere and the equatorial seas, than between the polar seas of the - warm hemisphere and the equatorial seas. And this being<span class="pagenum" id="Page_98">98</span> the case, - notwithstanding the influence of the trade-winds of the cold hemisphere - blowing over upon the warm, the currents will, in all probability, - be stronger on the cold hemisphere than on the warm. In other words, - the influence of the powerful trade-winds of the cold hemisphere to - transfer the warm water of the equator to the warm hemisphere will - probably be more than counterbalanced by the tendency of the warm - and buoyant waters of the equator to flow towards the dense and cold - waters around the pole of the cold hemisphere. But if ocean-currents - are due not to difference in specific gravity, but to the influence of - the winds, then it is evident that the waters at the equator will be - impelled, not into the cold hemisphere, but into the warm.</p> - - <p>For this reason I have been the more anxious to prove that - inter-tropical heat is conveyed to temperate and polar regions by - ocean-currents, and not by means of any general movement of the ocean - resulting from difference of gravity. I shall therefore on this account - enter more fully into this part of the subject than I otherwise would - have done. Irrespective of all this, however, the important nature of - the whole question, and the very general interest it excites, warrant a - full consideration of the subject.</p> - - <p>I shall consider first that form of the gravitation theory advocated - by Maury in his work on the “Physical Geography of the Sea,” which - attributes the motion of the Gulf-stream and other sensible currents - of the ocean to differences of specific gravity. One reason which has - induced me to select Maury’s work is, that it not only contains a much - fuller discussion on the cause of the motion of ocean-currents than is - to be found anywhere else, but also that it has probably passed through - a greater number of editions than any other book of a scientific - character in the English language in the same length of time.</p> - - <p><em>Examination of Lieut. Maury’s Gravitation Theory.</em>—Although Lieut. - Maury has expounded his views on the cause of ocean-currents at - great length in the various editions of his work, yet it is somewhat - difficult to discover what they really are. This<span class="pagenum" id="Page_99">99</span> arises chiefly - from the generally confused and sometimes contradictory nature of - his hydrodynamical conceptions. After a repeated perusal of several - editions of his book, the following, I trust, will be found to be a - pretty accurate representation of his theory:—</p> - - <p><em>Ocean-currents, according to Maury, due to Difference of Specific - Gravity.</em>—Although Maury alludes to a number of causes which, he - thinks, tend to produce currents, yet he deems their influence so - small that, practically, all currents may be referred to difference of - specific gravity.</p> - - <p>“If we except,” he says, “the tides, and the partial currents of the - sea, such as those that may be created by the wind, we may lay it down - as a rule that all the currents of the ocean owe their origin to the - differences of specific gravity between sea-water at one place and - sea-water at another; for wherever there is such a difference, whether - it be owing to difference of temperature or to difference of saltness, - &c., it is a difference that disturbs equilibrium, and currents are the - consequence” (§ 467)<a id="FNanchor_54" href="#Footnote_54" class="fnanchor">[54]</a>. To the same effect see §§ 896, 37, 512, 520, - and 537.</p> - - <p>Notwithstanding the fact that he is continually referring to difference - of specific gravity as the great cause of currents, it is difficult to - understand in what way he conceives this difference to act as a cause.</p> - - <p>Difference of specific gravity between the waters of the ocean at one - place and another can give rise to currents only through the influence - of the earth’s gravity. All currents resulting from difference of - specific gravity can be ultimately resolved into the general principle - that the molecules that are specifically heavier <em>descend</em> and displace - those that are specifically lighter. If, for example, the ocean at the - equator be expanded by heat or by any other cause, it will be forced by - the denser waters in temperate and polar regions to rise so that its - surface shall stand at a higher level than the surface of the ocean in - <span class="pagenum" id="Page_100">100</span>these regions. The surface of the ocean will become an inclined plane, - sloping from the equator to the poles. Hydro-statically, the ocean, - considered as a mass, will then be in a state of equilibrium; but the - individual molecules will not be in equilibrium. The molecules at the - surface in this case may be regarded as lying on an inclined plane - sloping from the equator down to the poles, and as these molecules - are at liberty to move they will not remain at rest, but will descend - the incline towards the poles. When the waters at the equator are - expanded, or the waters at the poles contracted, gravitation makes, as - it were, a twofold effort to restore equilibrium. It in the first place - sinks the waters at the poles, and raises the waters at the equator, - in order that the two masses may balance each other; but this very - effort of gravitation to restore equilibrium to the mass destroys the - equilibrium of the molecules by disturbing the level of the ocean. It - then, in the second place, endeavours to restore equilibrium to the - molecules by pulling the lighter surface water at the equator down the - incline towards the poles. This tends not only to restore the level - of the ocean, but to bring the lighter water to occupy the surface - and the denser water the bottom of the ocean; and when this is done, - complete equilibrium is restored, both to the mass of the ocean and - to its individual molecules, and all further motion ceases. But if - heat be constantly applied to the waters of the equatorial regions, - and cold to those of the polar regions, and a permanent disturbance of - equilibrium maintained, then the continual effort of gravitation to - restore equilibrium will give rise to a constant current. In this case, - the heat and the cold (the agents which disturb the equilibrium of the - ocean) may be regarded as causes of the current, inasmuch as without - them the current would not exist; but the real efficient cause, that - which impels the water forward, is the force of gravity. But the force - of gravity, as has already been noticed, cannot produce motion (perform - work) unless the thing acted upon <em>descend</em>. Descent is implied in - the very conception of a current produced by difference of specific - gravity.</p> - - <p><span class="pagenum" id="Page_101">101</span></p> - - <p>But Maury speaks as if difference of specific gravity could give rise - to a current without any descent.</p> - - <p>“It is not necessary,” he says, “to associate with oceanic currents - the idea that they must of necessity, as on land, run from a higher to - a lower level. So far from this being the case, some currents of the - sea actually run up hill, while others run on a level. The Gulf-stream - is of the first class” (§ 403). “The top of the Gulf-stream runs on a - level with the ocean; therefore we know it is not a descending current” - (§ 18). And in § 9 he says that between the Straits of Florida and - Cape Hatteras the waters of the Gulf-stream “are actually forced up an - inclined plane, whose submarine ascent is not less than 10 inches to - the mile.” To the same effect see §§ 25, 59.</p> - - <p>It is perfectly true that “it is not necessary to associate with - ocean-currents the idea that they must of necessity, as on land, - run from a higher to a lower level.” But the reason of this is that - ocean-currents do not, like the currents on land, owe their motion to - the force of gravitation. If ocean-currents result from difference of - specific gravity between the waters in tropical and polar regions, - as Maury maintains, then it is necessary to assume that they are - descending currents. Whatever be the cause which may give rise to a - difference of specific gravity, the motion which results from this - difference is due wholly to the force of gravity; but gravity can - produce no motion unless the water <em>descend</em>.</p> - - <p>This fact must be particularly borne in mind while we are considering - Maury’s theory that currents are the result of difference of specific - gravity.</p> - - <p>Ocean-currents, then, according to that writer, owe their existence to - the difference of specific gravity between the waters of inter-tropical - and polar regions. This difference of specific gravity he attributes to - two causes—(1) to difference as to <em>temperature</em>, (2) to difference as - to saltness. There are one or two causes of a minor nature affecting - the specific gravity of the sea, to which he alludes; but these two - determine the general<span class="pagenum" id="Page_102">102</span> result. Let us begin with the consideration of - the first of these two causes, viz.:—</p> - - <p><em>Difference of Specific Gravity resulting from Difference of - Temperature.</em>—Maury explains his views on this point by means of an - illustration. “Let us now suppose,” he says, “that all the water within - the tropics, to the depth of one hundred fathoms, suddenly becomes oil. - The aqueous equilibrium of the planet would thereby be disturbed, and - a general system of currents and counter currents would be immediately - commenced—the oil, in an unbroken sheet on the surface, running toward - the poles, and the water, in an under current, toward the equator. The - oil is supposed, as it reaches the polar basin, to be reconverted into - water, and the water to become oil as it crosses Cancer and Capricorn, - rising to the surface in inter-tropical regions, and returning as - before” (§ 20). “Now,” he says (§ 22), “do not the cold waters of the - north, and the warm waters of the Gulf, made specifically lighter by - tropical heat, and which we see actually preserving such a system of - counter currents, hold, at least in some degree, the relation of the - supposed water and oil?”</p> - - <p>In § 24 he calculates that at the Narrows of Bemini the difference in - weight between the volume of the Gulf-water that crosses a section of - the stream in one second, and an equal volume of water at the ocean - temperature of the latitude, supposing the two volumes to be equally - salt, is fifteen millions of pounds. Consequently the force per second - operating to propel the waters of the Gulf towards the pole would in - this case, he concludes, be the “equilibrating tendency due to fifteen - millions of pounds of water in the latitude of Bemini.” In §§ 511 and - 512 he states that the effect of expanding the waters at the torrid - zone by heat, and of contracting the waters at the frigid zone by cold, - is to produce a set of surface-currents of warm and light water from - the equator towards the poles, and another set of under currents of - cooler and heavy water from the poles towards the equator. (See also to - the same effect §§ 513, 514, 896.)</p> - - <p><span class="pagenum" id="Page_103">103</span></p> - - <p>There can be no doubt that his conclusion is that the waters in - inter-tropical regions are expanded by heat, while those in polar - regions are contracted by cold, and that this tends to produce a - surface current from the equator to the poles, and an under current - from the poles to the equator.</p> - - <p>“We shall now consider his second great cause of ocean currents, viz.:—</p> - - <p><em>Difference of Specific Gravity resulting from Difference in Degree of - Saltness.</em>—Maury maintains, and that correctly, that saltness increases - the density of water—that, other things being equal, the saltest water - is the densest. He suggests “that one of the purposes which, in the - grand design, it was probably intended to accomplish by having the sea - salt and not fresh, was to impart to its waters the forces and powers - necessary to make their circulation complete” (§ 495).</p> - - <p>Now it is perfectly obvious that if difference in saltness is to - co-operate with difference in temperature in the production of - ocean-currents, the saltest waters, and consequently the densest, must - be in the polar regions, and the waters least salt, and consequently - lightest, must be in equatorial and inter-tropical regions. Were the - saltest waters at the equator, and the freshest at the poles, it would - tend to neutralize the effect due to heat, and, instead of producing - a current, would simply tend to prevent the existence of the currents - which otherwise would result from difference of temperature.</p> - - <p>A very considerable portion of his work, however, is devoted to proving - that the waters of equatorial and inter-tropical regions are salter - and heavier than those of the polar regions; and yet, notwithstanding - this, he endeavours to show that this difference in respect to saltness - between the waters of the equatorial and the polar regions is one of - the chief causes, if not the chief cause, of ocean-currents. In fact, - it is for this special end that so much labour is bestowed in proving - that the saltest water is in the equatorial and inter-tropical regions, - and the freshest in the polar.</p> - - <p>“In the present state of our knowledge,” he says, “concerning <span class="pagenum" id="Page_104">104</span>this - wonderful phenomenon (for the Gulf-stream is one of the most marvellous - things in the ocean) we can do little more than conjecture. But we have - two causes in operation which we may safely assume are among those - concerned in producing the Gulf-stream. One of these is the increased - saltness of its water after the trade-winds have been supplied with - vapour from it, be it much or little; and the other is the diminished - quantum of salt which the Baltic and the Northern Seas contain” (§ 37). - “Now here we have, on one side, the Caribbean Sea and Gulf of Mexico, - with their waters of brine; on the other, the great Polar Basin, the - Baltic, and the North Sea, the two latter with waters that are but - little more than brackish. In one set of these sea-basins the water is - heavy, in the other it is light. Between them the ocean intervenes; but - water is bound to seek and to maintain its level; and here, therefore, - we unmask one of the agents concerned in causing the Gulf-stream” (§ - 38). To the same effect see §§ 52, 522, 523, 524, 525, 526, 528, 530, - 554, 556.</p> - - <p>Lieut. Maury’s <em>two causes neutralize each other</em>. Here we have two - theories put forth regarding the cause of ocean-currents, the one - in direct opposition to the other. According to the one theory, - ocean-currents exist because the waters of equatorial regions, in - consequence of their higher temperature, are <em>less dense</em> than the - waters of the polar regions; but according to the other theory, - ocean-currents exist because the waters of equatorial regions, in - consequence of their greater saltness, are <em>more dense</em> than the - waters of the polar regions. If the one cause be assigned as a reason - why ocean-currents exist, then the other can be equally assigned as - a reason why they should not exist. According to both theories it is - the difference of density between the equatorial and polar waters that - gives rise to currents; but while the one theory maintains that the - equatorial waters are <em>lighter</em> than the polar, the other holds that - they are <em>heavier</em>. Either the one theory or the other may be true, - or neither; but it is logically impossible that both of them can. Let - it be observed that it is not two currents, the one contrary<span class="pagenum" id="Page_105">105</span> to the - other, with which we have at present to do; it is not temperature - producing currents in one direction, and saltness producing currents - in the contrary direction. We have two theories regarding the origin - of currents, the one diametrically opposed to the other. The tendency - of the one cause assigned is to prevent the action of the other. If - temperature is allowed to act, it will make the inter-tropical waters - lighter than the polar, and then, according to theory, a current will - result. But if we bring saltness into play (the other cause) it will - do the reverse: it will increase the density of the inter-tropical - waters and diminish the density of the polar; and so far as it acts it - will diminish the currents produced by temperature, because it will - diminish the difference of specific gravity between the inter-tropical - and polar regions which had been previously caused by temperature. And - when the effects of saltness are as powerful as those of temperature, - the difference of specific gravity produced by temperature will be - completely effaced, or, in other words, the waters of the equatorial - and polar seas will be of the same density, and consequently no current - will exist. And so long as the two causes continue in action, no - current can arise, unless the energy of the one cause should happen to - exceed that of the other; and even then a current will only exist to - the extent by which the strength of the one exceeds that of the other.</p> - - <p>The contrary nature of the two theories will be better seen by - considering the way in which it is supposed that difference in saltness - is produced and acts as a cause.</p> - - <p>If there is a constant current resulting from the difference in - saltness between the equatorial and polar waters, then there must be a - cause which maintains this difference. The current is simply the effort - to restore the equilibrium lost by the difference; and the current - would very soon do this, and then all motion would cease, were there - not a constantly operating cause maintaining the disturbance. What, - then, according to Maury, is the cause of this disturbance, or, in - other words, what is it that keeps the equatorial waters salter than - the polar?</p> - - <p><span class="pagenum" id="Page_106">106</span></p> - - <p>The agencies in operation are stated by him to be heat, radiation, - evaporation, precipitation, and secretion of solid matter in the form - of shells, &c. The two most important, however, are evaporation and - precipitation.</p> - - <p>The trade-winds enter the equatorial regions as relatively dry winds - thirsting for vapour; consequently they absorb far more moisture than - they give out; and the result is that in inter-tropical regions, - evaporation is much in excess of precipitation; and as fresh water only - is taken up, the salt being left behind, the process, of course, tends - to increase the saltness of the inter-tropical seas. Again, in polar - and extra-tropical regions the reverse is the case; precipitation is in - excess of evaporation. This tends in turn to diminish the saltness of - the waters of those regions. (See on these points §§ 31, 33, 34, 37, - 179, 517, 526, and 552.)</p> - - <p>In the system of circulation produced by difference of temperature, - as we have already seen, the surface-currents flow from the equator - to the poles, and the under or return currents from the poles to the - equator; but in the system produced by difference of saltness, the - surface currents flow from the poles to the equator, and the return - under currents from the equator to the poles. That the surface currents - produced by difference of saltness flow from the poles to the equator, - Maury thinks is evident for the two following reasons:—</p> - - <p>(1) As evaporation is in excess of precipitation in inter-tropical - regions, more water is taken off the surface of the ocean in those - regions than falls upon it in the form of rain. This excess of water - falls in the form of rain on temperate and polar regions, where, - consequently, precipitation is in excess of evaporation. The lifting - of the water off the equatorial regions and its deposit on the polar - tend to lower the level of the ocean in equatorial regions and to raise - the level in polar; consequently, in order to restore the level of - the ocean, the surface water at the polar regions flows towards the - equatorial regions.</p> - - <p>(2) As the water taken up at the equator is fresh, and the<span class="pagenum" id="Page_107">107</span> salt - is left behind, the ocean, in inter-tropical regions, is thus made - saltier and consequently denser. This dense water, therefore, sinks - and passes away as an under current. This water, evaporated from - inter-tropical regions, falls as fresh and lighter water in temperate - and polar regions; and therefore not only is the level of the ocean - raised, but the waters are made lighter. Hence, in order to restore - equilibrium, the waters in temperate and polar regions will flow as - a surface current towards the equator. Under currents will flow from - the equator to the poles, and surface or upper currents from the poles - to the equator. Difference in temperature and difference in saltness, - therefore, in every respect tend to produce opposite effects.</p> - - <p>That the above is a fair representation of the way in which Maury - supposes difference in saltness to act as a cause in the production of - ocean-currents will appear from the following quotations:—</p> - - <p>“In those regions, as in the trade-wind region, where evaporation is - in excess of precipitation, the general level of this supposed sea - would be altered, and immediately as much water as is carried off by - evaporation would commence to flow in from north and south toward the - trade-wind or evaporation region, to restore the level” (§ 509). “On - the other hand, the winds have taken this vapour, borne it off to the - extra-tropical regions, and precipitated it, we will suppose, where - precipitation is in excess of evaporation. Here is another alteration - of sea-level, by elevation instead of by depression; and hence we - have the motive power for a <em>surface current from each pole towards - the equator</em>, the object of which is only to supply the demand for - evaporation in the trade-wind regions” (§ 510).</p> - - <p>The above result would follow, supposing the ocean to be fresh. He then - proceeds to consider an additional result that follows in consequence - of the saltness of the ocean.</p> - - <p>“Let evaporation now commence in the trade-wind region, as it was - supposed to do in the case of the freshwater seas, and as it actually - goes on in nature—and what takes place? Why a lowering of the sea-level - as before. But as the vapour<span class="pagenum" id="Page_108">108</span> of salt water is fresh, or nearly so, - fresh water only is taken up from the ocean; that which remains behind - is therefore more salt. Thus, while the level is lowered in the salt - sea, the equilibrium is destroyed because of the saltness of the water; - for the water that remains after evaporation takes place is, on account - of the solid matter held in solution, specifically heavier than it was - before any portion of it was converted into vapour” (§ 517).</p> - - <p>“The vapour is taken from the surface-water; the surface-water thereby - becomes more salt, and, under certain conditions, heavier. When it - becomes heavier, it sinks; and hence we have, due to the salts of the - sea, a vertical circulation, namely, a descent of heavier—because - salter and cooler—water from the surface, and an ascent of water that - is lighter—because it is not so salt—from the depths below” (§ 518).</p> - - <p>In section 519 he goes on to show that this vapour removed from the - inter-tropical region is precipitated in the polar regions, where - precipitation is in excess of evaporation. “In the precipitating - regions, therefore, the level is destroyed, as before explained, by - elevation, and in the evaporating regions by depression; which, as - already stated, gives rise to a system of <em>surface</em> currents, moved by - gravity alone, from the <em>poles towards the equator</em>” (§ 520).</p> - - <p>“This fresh water being emptied into the Polar Sea and agitated by the - winds, becomes mixed with the salt; but as the agitation of the sea by - the winds is supposed to extend to no great depth, it is only the upper - layer of salt water, and that to a moderate depth, which becomes mixed - with the fresh. The specific gravity of this upper layer, therefore, is - diminished just as much as the specific gravity of the sea-water in the - evaporating regions was increased. <em>And thus we have a surface current - of saltish water from the poles towards the equator, and an under - current of water salter and heavier from the equator to the poles</em>” (§ - 522).</p> - - <p>“This property of saltness imparts to the waters of the ocean another - peculiarity, by which the sea is still better<span class="pagenum" id="Page_109">109</span> adapted for the - regulation of climates, and it is this: by evaporating fresh water from - the salt in the tropics, the surface water becomes heavier than the - average of sea-water. This heavy water is also warm water; it sinks, - and being a good retainer, but a bad conductor, of heat, this water - is employed in transporting through <em>under currents</em> heat for the - mitigation of climates in far distant regions” (§ 526).</p> - - <p>“For instance, let us suppose the waters in a certain part of the - torrid zone to be 90°, but by reason of the fresh water which has been - taken from them in a state of vapour, and consequently, by reason of - the proportionate increase of salts, these waters are heavier than - waters that may be cooler, but not so salt. This being the case, the - tendency would be for this warm but salt and heavy water to flow off as - an <em>under current towards the polar or some other regions of lighter - water</em>” (§ 554).</p> - - <p>That Maury supposes the warm water at the equator to flow to the polar - regions as an under current is further evident from the fact that he - maintains that the climate of the arctic regions is mitigated by a warm - under current, which comes from the equatorial regions, and passes up - through Davis Straits (see §§ 534−544).</p> - - <p>The question now suggests itself: to which of these two antagonistic - causes does Maury really suppose ocean-currents must be referred? - Whether does he suppose, difference in temperature or difference in - saltness, to be the real cause? I have been unable to find anything - from which we can reasonably conclude that he prefers the one cause - to the other. It would seem that he regards both as real causes, and - that he has failed to perceive that the one is destructive of the - other. But it is difficult to conceive how he could believe that the - sea in equatorial regions, by virtue of its higher temperature, is - lighter than the sea in polar regions, while at the same time it <em>is - not</em> lighter but heavier, in consequence of its greater saltness—how - he could believe that the warm water at the equator flows to the poles - as an upper current, and the cold water at<span class="pagenum" id="Page_110">110</span> the poles to the equator - as an <em>under</em> current, while at the same time the warm water at the - equator does not flow to the poles as a surface current, nor the cold - water at the poles to the equator as an under current, but the reverse. - And yet, unless these absolute impossibilities be possible, how can an - ocean-current be the result of both causes?</p> - - <p>The only explanation of the matter appears to be that Maury has failed - to perceive the contradictory nature of his two theories. This fact is - particularly seen when he comes to apply his two theories to the case - of the Gulf-stream. He maintains, as has already been stated, that - the waters of the Gulf-stream are salter than the waters of the sea - through which they flow (see §§ 3, 28, 29, 30, 34, and several other - places). And he states, as we have already seen (see p. 104), that the - existence of the Gulf-stream is due principally to the difference of - density of the water of the Caribbean Sea and the Gulf of Mexico as - compared with that of the great Polar Basin and the North Sea. There - can be no doubt whatever that it is the <em>density</em> of the waters of the - Gulf-stream at its fountain-head, the Gulf of Mexico, resulting from - its superior saltness, and the deficiency of density of the waters in - polar regions and the North Sea, &c., that is here considered to be - unmasked as one of the agents. If this be a cause of the motion of the - Gulf-stream, how then can the difference of temperature between the - waters of inter-tropical and polar regions assist as a cause? This - difference of temperature will simply tend to undo all that has been - done by difference of saltness: for it will tend to make the waters - of the Gulf of Mexico lighter, and the waters of the polar regions - heavier. But Maury maintains, as we have seen, that this difference of - temperature is also a cause, which shows that he does not perceive the - contradiction.</p> - - <p>This is still further apparent. He holds, as stated, that “the waters - of the Gulf-stream are salter than the waters of the sea through which - they flow,” and that this excess in saltness, by making the water - heavier, is a cause of the motion of the stream. But he maintains that, - notwithstanding the effect<span class="pagenum" id="Page_111">111</span> which greater saltness has in increasing - the density of the waters of the Gulf-stream, yet, owing to their - higher temperature, they are actually lighter than the water through - which they flow; and as a proof that this is the case, he adduces the - fact that the surface of the Gulf-stream is roof-shaped (§§ 39−41), - which it could not be were its waters not actually lighter than the - waters through which the stream flows. So it turns out that, in - contradiction to what he had already stated, it is the lesser density - of the waters of the Gulf-stream that is the real cause of their - motion. The greater saltness of the waters, to which he attributes so - much, can in no way be regarded as a cause of motion. Its effect, so - far as it goes, is to stop the motion of the stream rather than to - assist it.</p> - - <p>But, again, although he asserts that difference of saltness and - difference of temperature are both causes of ocean-currents, yet he - appears actually to admit that temperature and saltness neutralize each - other so as to prevent change in the specific gravity of the ocean, as - will be seen from the following quotation:—</p> - - <p>“It is the trade-winds, then, which prevent the thermal and - specific gravity curves from conforming with each other in - inter-tropical seas. The water they suck up is fresh water; and - the salt it contained, being left behind, is just sufficient to - counterbalance, by its weight, the effect of thermal dilatation upon - the specific gravity of sea-water between the parallels of 34° north - and south. As we go from 34° to the equator, the water grows warmer and - expands. It would become lighter; but the trade-winds, by taking up - vapour without salt, make the water salter, and therefore heavier. The - conclusion is, the proportion of salt in sea-water, its expansibility - between 62° and 82°, and the thirst of the trade-winds for vapour are, - where they blow, so balanced as to produce <em>perfect compensation</em>; and - a more beautiful compensation cannot, it appears to me, be found in the - mechanism of the universe than that which we have here stumbled upon. - It is a triple adjustment; the power of the<span class="pagenum" id="Page_112">112</span> sun to expand, the power - of the winds to evaporate, and the quantity of salts in the sea—these - are so proportioned and adjusted that when both the wind and the sun - have each played with its forces upon the inter-tropical waters of the - ocean, <em>the residuum of heat and of salt should be just such as to - balance each other in their effects; and so the aqueous equilibrium of - the torrid zone is preserved</em>” (§ 436, eleventh edition).</p> - - <p>“Between 35° or 40° and the equator evaporation is in excess of - precipitation; and though, as we approach the equator on either side - from these parallels, the solar ray warms and expands the surface-water - of the sea, the winds, by the vapour they carry off, and the salt they - leave behind, <em>prevent it from making that water lighter</em>” (§ 437, - eleventh edition).</p> - - <p>“Philosophers have admired the relations between the size of the earth, - the force of gravity, and the strength of fibre in the flower-stalks of - plants; but how much more exquisite is the system of counterpoises and - adjustments here presented between the sea and its salts, the winds and - the heat of the sun!” (§ 438, eleventh edition).</p> - - <p>How can this be reconciled with all that precedes regarding - ocean-currents being the result of difference of specific gravity - caused by a difference of temperature and difference of saltness? Here - is a distinct recognition of the fact that difference in saltness, - instead of producing currents, tends rather to prevent the existence of - currents, by counteracting the effects of difference in temperature. - And so effectually does it do this, that for 40°, or nearly 3,000 - miles, on each side of the equator there is absolutely no difference in - the specific gravity of the ocean, and consequently nothing, either as - regards difference of temperature or difference of saltness, that can - possibly give rise to a current.</p> - - <p>But it is evident that, if between the equator and latitude 40° the - two effects completely neutralize each other, it is not at all likely - that between latitude 40° and the poles they will not to a large extent - do the same thing. And if so, how can ocean-currents be due either - to difference in temperature or to<span class="pagenum" id="Page_113">113</span> difference in saltness, far less - to both. If there be any difference of specific gravity of the ocean - between latitude 40° and the poles, it must be only to the extent - by which the one cause has failed to neutralize the other. If, for - example, the waters in latitude 40°, by virtue of higher temperature, - are less dense than the waters in the polar regions, they can be so - only to the extent that difference in saltness has failed to neutralize - the effect of difference in temperature. And if currents result, they - can do so only to the extent that difference in saltness has thus - fallen short of being able to produce complete compensation. Maury, - after stating his views on compensation, seems to become aware of - this; but, strangely, he does not appear to perceive, or, at least, he - does not make any allusion to the fact, that all this is fatal to his - theories about ocean-currents being the combined result of differences - of temperature and of saltness. For, in opposition to all that he - had previously advanced regarding the difficulty of finding a cause - sufficiently powerful to account for such currents as the Gulf-stream, - and the great importance that difference in saltness had in their - production, he now begins to maintain that so great is the influence - of difference in temperature that difference in saltness, and a number - of other compensating causes are actually necessary to prevent the - ocean-currents from becoming too powerful.</p> - - <p>“If all the inter-tropical heat of the sun,” he says, “were to pass - into the seas upon which it falls, simply raising the temperature of - their waters, it would create a thermo-dynamical force in the ocean - capable of transporting water scalding hot from the torrid zone, and - spreading it while still in the tepid state around the poles.... Now, - suppose there were no trade-winds to evaporate and to counteract the - dynamical force of the sun, this hot and light water, by becoming - hotter and lighter, would flow off in currents with almost mill-tail - velocity towards the poles, covering the intervening sea with a mantle - of warmth as a garment. The cool and heavy water of the polar basin, - coming out as under currents, would flow equatorially with equal - velocity.”</p> - - <p><span class="pagenum" id="Page_114">114</span></p> - - <p>“Thus two antagonistic forces are unmasked, and, being unmasked, we - discover in them a most exquisite adjustment—a compensation—by which - the dynamical forces that reside in the sunbeam and the trade-wind - are made to counterbalance each other, by which the climates of - inter-tropical seas are regulated, and by which the set, force, and - volume of oceanic currents are measured” (§§ 437 and 438, eleventh - edition).</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_VII"> - <span class="pagenum" id="Page_115">115</span> - <h2> - CHAPTER VII.<br /><br /> - <span class="small">EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC - CIRCULATION.—LIEUT. MAURY’S THEORY (<i>continued</i>).</span> - </h2> - </div> - <div class="subhead">Methods of determining the Question.—The Force resulting from - Difference of Specific Gravity.—Sir John Herschel’s Estimate of - the Force.—Maximum Density of Sea-Water.—Rate of Decrease of - Temperature of Ocean at Equator.—-The actual Amount of Force - resulting from Difference of Specific Gravity.—M. Dubuat’s - Experiments.</div> - - <p><em>How the Question may be Determined.</em>—Whether the circulation of the - ocean is due to difference in specific gravity or not may be determined - in three ways: viz. (1) by direct experiment; (2) by ascertaining the - absolute amount of <em>force</em> acting on the water to produce motion, in - virtue of difference of specific gravity, and thereafter comparing it - with the force which has been shown by experiment to be necessary to - the production of sensible motion; or (3) by determining the greatest - possible amount of <em>work</em> which gravity can perform on the waters in - virtue of difference of specific gravity, and then ascertaining if the - work of gravity does or does not equal the work of the resistances in - the required motion. But Maury has not adopted either of these methods.</p> - - <p><em>The Force resulting from Difference of Specific Gravity.</em>—I shall - consider first whether the force resulting from difference of specific - gravity be sufficient to account for the motion of ocean-currents.</p> - - <p>The inadequacy of this cause has been so clearly shown by Sir John - Herschel, that one might expect that little else would be required than - simply to quote his words on the subject, which are as follows:—</p> - - <p><span class="pagenum" id="Page_116">116</span></p> - - <p>“First, then, if there were no atmosphere, there would be no - Gulf-stream, or any other considerable ocean-current (as distinguished - from a mere surface-drift) whatever. By the action of the sun’s rays, - the <em>surface</em> of the ocean becomes <em>most</em> heated, and the heated water - will, therefore, neither directly tend to <em>ascend</em> (which it could - not do without leaving the sea) nor to <em>descend</em>, which it cannot do, - being rendered buoyant, nor to move laterally, no lateral impulse being - given, and which it could only do by reason of a general declivity - of surface, the dilated portion occupying a higher level. Let us see - what this declivity would amount to. The equatorial surface-water - has a temperature of 84°. At 7,200 feet deep the temperature is 39°, - the level of which temperature rises to the surface in latitude 56°. - Taking the dilatability of sea-water to be the same as that of fresh, a - uniformly progressive increase of temperature, from 39° to 84° Fahr., - would dilate a column of 7,200 feet by 10 feet, to which height, - therefore, above the spheroid of equilibrium (or above the sea-level in - lat. 56°), the equatorial surface is actually raised by dilatation. An - arc of 56° on the earth’s surface measures 3,360 geographical miles; - so that we have a slope of 1/28th of an inch per geographical mile, or - 1/32nd of an inch per statute mile for the water so raised to run down. - As the accelerating force corresponding to such a slope (of 1/10th of - a second, 0″·1) is less than one two-millionth part of gravity, we - may dismiss this as a cause capable of creating only a very trifling - surface-drift, and not worth considering, even were it in the proper - direction to form, by concentration, a current from east to west, - <em>which it could not be, but the very reverse</em>.”<a id="FNanchor_55" href="#Footnote_55" class="fnanchor">[55]</a></p> - - <p>It is singular how any one, even though he regarded this conclusion as - but a rough approximation to the truth, could entertain the idea that - ocean-currents can be the result of difference in specific gravity. - There are one or two reasons, however, which may be given for the - above not having been generally received as conclusive. Herschel’s - calculations refer to the difference of gravity resulting from - difference of temperature; <span class="pagenum" id="Page_117">117</span>but this is only one of the causes to which - Maury appeals, and even not the one to which he most frequently refers. - He insists so strongly on the effects of difference of saltness, that - many might think that, although Herschel may have shown that difference - in specific gravity arising from difference of temperature could not - account for the motion of ocean-currents, yet nevertheless that this, - combined with the effects resulting from difference in saltness, might - be a sufficient explanation of the phenomena. Such, of course, would - not be the case with those who perceived the contradictory nature of - Maury’s two causes; but probably many read the “Physical Geography of - the Sea” without being aware that the one cause is destructive of the - other. Again, a few plausible objections, which have never received due - consideration, have been strongly urged by Maury and others against the - theory that ocean-currents can be caused by the impulses of the winds; - and probably these objections appear to militate as strongly against - this theory as Herschel’s arguments against Maury’s.</p> - - <p>There is one trifling objection to Herschel’s result: he takes 39° as - the temperature of maximum density. This, however, as we shall see, - does not materially affect his conclusions.</p> - - <p>Observations on the temperature of the maximum density of sea-water - have been made by Erman, Despretz, Rossetti, Neumann, Marcet, Hubbard, - Horner, and others. No two of them have arrived at exactly the same - conclusion. This probably arises from the fact that the temperature - of maximum density depends upon the amount of salt held in solution. - No two seas, unless they are equal as to saltness, have the same - temperature of maximum density. The following Table of Despretz will - show how rapidly the temperature of both the freezing-point and of - maximum density is lowered by additional amounts of salt:—</p> - - <table summary="Table of Despretz"> - <tbody> - <tr> - <th class="bt bb bl">Amount of salt.</th> - <th class="bt bb bl">Temperature of<br />freezing-point.</th> - <th class="bt bb bl br">Temperature of<br />Maximum density.</th> - </tr> - <tr> - <td class="bl"> </td> - <td class="tdc pl5 bl"><div>° </div></td> - <td class="tdc pl5 bl br"><div>° </div></td> - </tr> - <tr> - <td class="tdc pl5 bl"><div>0·000123</div></td> - <td class="tdc pl5 bl"><div>−1·21 C.</div></td> - <td class="tdc pl5 bl br"><div>+ 1·19 C.</div></td> - </tr> - <tr> - <td class="tdc pl5 bl"><div>0·0246 </div></td> - <td class="tdc pl5 bl"><div>−2·24 </div></td> - <td class="tdc pl5 bl br"><div>− 1·69 </div></td> - </tr> - <tr> - <td class="tdc pl5 bl"><div>0·0371 </div></td> - <td class="tdc pl5 bl"><div>−2·77 </div></td> - <td class="tdc pl5 bl br"><div>− 4·75 </div></td> - </tr> - <tr> - <td class="tdc pl5 bl bb"><div>0·0741 </div></td> - <td class="tdc pl5 bl bb"><div>−5·28 </div></td> - <td class="tdc pl5 bl br bb"><div>−16·00 </div></td> - </tr> - </tbody> - </table> - - <p><span class="pagenum" id="Page_118">118</span></p> - - <p>He found the temperature of maximum density of sea-water, whose density - at 20°C. was 1·0273, to be −3°·67C. (25°·4F.), and the temperature of - freezing-point −2°·55C. (27°·4F.).<a id="FNanchor_56" href="#Footnote_56" class="fnanchor">[56]</a> Somewhere between 25° and 26° - F. may therefore be regarded as the temperature of maximum density - of sea-water of average saltness. We have no reason to believe that - the ocean, from the surface to the bottom, even at the poles, is at - 27°·4F., the freezing-point.</p> - - <p>The actual slope resulting from difference of specific gravity, - as we shall presently see, does not amount to 10 feet. Herschel’s - estimate was, however, made on insufficient data, both as to the rate - of expansion of sea-water and that at which the temperature of the - ocean at the equator decreases from the surface downwards. We are - happily now in the possession of data for determining with tolerable - accuracy the amount of slope due to difference of temperature between - the equatorial and polar seas. The rate of expansion of sea-water - from 0°C. to 100°C. has been experimentally determined by Professor - Muncke, of Heidelberg.<a id="FNanchor_57" href="#Footnote_57" class="fnanchor">[57]</a> The valuable reports of Captain Nares, of - H.M.S. <cite>Challenger</cite>, lately published by the Admiralty, give the rate - at which the temperature of the Atlantic at the equator decreases - from the surface downwards. These observations show clearly that the - super-heating effect of the sun’s rays does not extend to any great - depth. They also prove that at the equator the temperature decreases - as the depth increases so rapidly that at 60 fathoms from the surface - the temperature is 62°·4, the same as at Madeira at the same depth; - while at the depth of 150 fathoms it is only 51°, about the same as - that in the Bay of Biscay (Reports, p. 11). Here at the very outset - we have broad and important facts hostile to the theory of a flow of - water resulting from difference of temperature between the ocean in - equatorial and temperate and polar regions.</p> - - <p>Through the kindness of Staff-Captain Evans, Hydrographer <span class="pagenum" id="Page_119">119</span>of the - Admiralty, I have been favoured with a most valuable set of serial - temperature soundings made by Captain Nares of the <cite>Challenger</cite>, close - to the equator, between long. 14° 49′ W. and 32° 16′ W. The following - Table represents the mean of the whole of these observations:—</p> - - <table summary="Temperature soundings"> - <tbody> - <tr> - <th class="bt bb bl">Fathoms.</th> - <th class="bt bb bl br">Temperature.</th> - </tr> - <tr> - <td class="bl"> </td> - <td class="tdc bl br"><div>°</div></td> - </tr> - <tr> - <td class="tdc bl"><div>Surface.</div></td> - <td class="tdc bl br"><div>77·9</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 10</div></td> - <td class="tdc bl br"><div>77·2</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 20</div></td> - <td class="tdc bl br"><div>77·1</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 30</div></td> - <td class="tdc bl br"><div>76·9</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 40</div></td> - <td class="tdc bl br"><div>71·7</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 50</div></td> - <td class="tdc bl br"><div>64·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 60</div></td> - <td class="tdc bl br"><div>60·4</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 70</div></td> - <td class="tdc bl br"><div>59·4</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 80</div></td> - <td class="tdc bl br"><div>58·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 90</div></td> - <td class="tdc bl br"><div>58·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 100</div></td> - <td class="tdc bl br"><div>55·6</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 150</div></td> - <td class="tdc bl br"><div>51·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 200</div></td> - <td class="tdc bl br"><div>46·6</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 300</div></td> - <td class="tdc bl br"><div>42·2</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 400</div></td> - <td class="tdc bl br"><div>40·3</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 500</div></td> - <td class="tdc bl br"><div>38·9</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 600</div></td> - <td class="tdc bl br"><div>39·2</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 700</div></td> - <td class="tdc bl br"><div>39·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 800</div></td> - <td class="tdc bl br"><div>39·1</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 900</div></td> - <td class="tdc bl br"><div>38·2</div></td> - </tr> - <tr> - <td class="tdc bl"><div>1000</div></td> - <td class="tdc bl br"><div>36·9</div></td> - </tr> - <tr> - <td class="tdc bl"><div>1100</div></td> - <td class="tdc bl br"><div>37·6</div></td> - </tr> - <tr> - <td class="tdc bl"><div>1200</div></td> - <td class="tdc bl br"><div>36·7</div></td> - </tr> - <tr> - <td class="tdc bl"><div>1300</div></td> - <td class="tdc bl br"><div>35·8</div></td> - </tr> - <tr> - <td class="tdc bl"><div>1400</div></td> - <td class="tdc bl br"><div>36·4</div></td> - </tr> - <tr> - <td class="tdc bl"><div>1500</div></td> - <td class="tdc bl br"><div>36·1</div></td> - </tr> - <tr> - <td class="tdc bl bb"><div>Bottom.</div></td> - <td class="tdc bl br bb"><div>34·7</div></td> - </tr> - </tbody> - </table> - - <p>We have in this Table data for determining the height at which the - surface of the ocean at the equator ought to stand above that of the - poles. Assuming 32°F. to be the temperature of the ocean at the poles - from the surface to the bottom and the foregoing to be the rate at - which the temperature of the ocean at the equator decreases from the - surface downwards, and then calculating according to Muncke’s Table of - the expansion of sea-water, we have only 4 feet 6 inches as the height - to which the level of the ocean at the equator ought to stand above - that at the poles in order that the ocean may be in static equilibrium. - In other words, the equatorial column requires to be only 4 feet 6 - inches higher than the polar in order that the two may balance each - other.</p> - - <p>Taking the distance from the equator to the poles at 6,200 miles, the - force resulting from the slope of 4½ feet in 6,200 will amount to only - 1/7,340,000th that of gravity, or about 1/1000th of a grain on a pound - of water. But, as we shall shortly see, there can be no permanent - current resulting from difference of temperature while the two columns - remain in equilibrium, for the current is simply an effort to the - retardation of equilibrium. In order to have permanent circulation - there must be a<span class="pagenum" id="Page_120">120</span> permanent disturbance of equilibrium. Or, in other - words, the weight of the polar column must be kept in excess of that - of the equatorial. Suppose, then, that the weight of the polar column - exceeds that of the equatorial by 2 feet of water, the difference of - level between the two columns will, in that case, amount to only 2 - feet 6 inches. This would give a force of only 1/13,200,000th that of - gravity, or not much over 1/1,900th of a grain on a pound of water, - tending to draw the water down the slope from the equator to the poles, - a force which does not much exceed the weight of a grain on a ton of - water. But it must be observed that this force of a grain per ton would - affect only the water at the surface; a very short distance below the - surface the force, small as it is, would be enormously reduced. If - water were a perfect fluid, and offered no resistance to motion, it - would not only flow down an incline, however small it might be, but - would flow down with an accelerated motion. But water is not a perfect - fluid, and its molecules do offer considerable resistance to motion. - Water flowing down an incline, however steep it may be, soon acquires - a uniform motion. There must therefore be a certain inclination below - which no motion can take place. Experiments were made by M. Dubuat - with the view of determining this limit.<a id="FNanchor_58" href="#Footnote_58" class="fnanchor">[58]</a> He found that when the - inclination was 1 in 500,000, the motion of the water was barely - perceptible; and he came to the conclusion that when the inclination - is reduced to 1 in 1,000,000, all motion ceases. But the inclination - afforded by the difference of temperature between the sea in equatorial - and polar regions does not amount to one-seventh of this, and - consequently it can hardly produce even that “trifling surface-drift” - which Sir John Herschel is willing to attribute to it.</p> - - <p>There is an error into which some writers appear to fall to which I - may here refer. Suppose that at the equator we have to descend 10,000 - feet before water equal in density to that at the poles is reached. We - have in this case a plain with a slope <span class="pagenum" id="Page_121">121</span>of 10,000 feet in 6,200 miles, - forming the upper surface of the water of maximum density. Now this - slope exercises no influence in the way of producing a current, as some - seem to think; for it is not a case of disturbed equilibrium, but the - reverse. It is the condition of static equilibrium resulting from a - difference between the temperature of the water at the equator and the - poles. The only slope that has any tendency to produce motion is that - which is formed by the surface of the ocean in the equatorial regions - being higher than the surface at the poles; but this is an inclination - of only 4 feet 6 inches, and is therefore wholly inadequate to produce - such currents as the Gulf-stream.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_VIII"> - <span class="pagenum" id="Page_122">122</span> - <h2> - CHAPTER VIII.<br /><br /> - <span class="small">EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.—DR. - CARPENTER’S THEORY.</span> - </h2> - </div> - <div class="subhead">Gulf-stream according to Dr. Carpenter not due to Difference - of Specific Gravity.—Facts to be Explained.—The Explanation of - the Facts.—The Explanation hypothetical.—The Cause assigned for - the hypothetical Mode of Circulation.—Under currents account - for all the Facts better than the Gravitation Hypothesis.—Known - Condition of the Ocean inconsistent with that Hypothesis.</div> - - <p class="noindent"><span class="smcap">Dr. Carpenter</span> does not suppose, with Lieut. Maury, that the difference - of temperature between the ocean in equatorial and polar regions can - account for the Gulf-stream and other great currents of the ocean. - He maintains, however, that this difference is quite sufficient to - bring about a slow general interchange of water between the polar and - inter-tropical areas—to induce a general movement of the upper portion - of the ocean from the equator to the poles and a counter-movement of - the under portion in a contrary direction. It is this general movement - which, according to that author, is the great agent by which heat is - distributed over the globe.<a id="FNanchor_59" href="#Footnote_59" class="fnanchor">[59]</a></p> - - <p>In attempting to estimate the adequacy of this hypothesis as an - explanation of the phenomena involved, there are obviously two - questions to be considered: namely, (1) is the difference of - temperature between the sea in inter-tropical and polar regions - sufficiently great to produce the required movement? and (2) assuming - that there is such a movement, does it convey the amount of heat which - Dr. Carpenter supposes? I shall begin with the consideration of the - first of these two points.</p> - - <p><span class="pagenum" id="Page_123">123</span></p> - - <p>But before doing so let us see what the facts are which this - gravitation theory is intended to explain.</p> - - <p><em>The Facts to be Explained.</em>—Dr. Carpenter considers that the great - mass of warm water proved during recent dredging expeditions to - occupy the depths of the North Atlantic, must be referred, not to the - Gulf-stream, but to a general movement of water from the equator. “The - inference seems inevitable,” he says, “that the bulk of the water in - the warm area must have come thither from the south-west. The influence - of the Gulf-stream proper (meaning by this the body of super-heated - water which issues through the ‘Narrows’ from the Gulf of Mexico), if - it reaches this locality at all (which is very doubtful), could only - affect the <em>most superficial</em> stratum; and the same may be said of - the surface-drift caused by the prevalence of south-westerly winds, - to which some have attributed the phenomena usually accounted for by - the extension of the Gulf-stream to these regions. And the presence - of the body of water which lies between 100 and 600 fathoms deep, and - the range of whose temperature is from 48° to 42°, can scarcely be - accounted for on any other hypothesis than that of a <em>great general - movement of equatorial water towards the polar area</em>, of which - movement the Gulf-stream constitutes a peculiar case modified by local - conditions. In like manner the Arctic stream which underlies the warm - superficial stratum in our cold area constitutes a peculiar case, - modified by the local conditions to be presently explained, of <em>a great - general movement of polar water towards the equatorial area</em>, which - depresses the temperature of the deepest parts of the great oceanic - basins nearly to the freezing-point.”</p> - - <p>It is well-known that, wherever temperature-observations have been - made in the Atlantic, the bottom of that ocean has been found to be - occupied by water of an ice-cold temperature. And this holds true - not merely of the Atlantic, but also of the ocean in inter-tropical - regions—a fact which has been proved by repeated observations, and more - particularly of late by those of Commander Chimmo in the China Sea and - Indian Ocean,<span class="pagenum" id="Page_124">124</span> where a temperature as low as 32° Fahr. was found at a - depth of 2,656 fathoms. In short, the North Atlantic, and probably the - inter-tropical seas also, may be regarded, Dr. Carpenter considers, as - divided horizontally into two great layers or strata—an upper warm, and - a lower cold stratum. All these facts I, of course, freely admit; nor - am I aware that their truth has been called in question by any one, no - matter what his views may have been as to the mode in which they are to - be explained.</p> - - <p><em>The Explanation of the Facts.</em>—We have next the explanation of the - facts, which is simply this:—The cold water occupying the bottom of - the Atlantic and of inter-tropical seas is to be accounted for by the - supposition that <em>it came from the polar regions</em>. This is obvious, - because the cold possessed by the water could not have been derived - from the crust of the earth beneath: neither could it have come from - the surface; for the temperature of the bottom water is far below the - normal temperature of the latitude in which it is found. Consequently - “the inference seems irresistible that this depression must be produced - and maintained by the convection of cold from the polar towards the - equatorial area.” Of course, if we suppose a flow of water from the - poles towards the equator, we must necessarily infer a counter flow - from the equator towards the poles; and while the water flowing from - equatorial to polar regions will be <em>warm</em>, that flowing from polar to - equatorial regions will be <em>cold</em>. The doctrine of a mutual interchange - of equatorial and polar water is therefore a <em>necessary consequence</em> - from the admission of the foregoing facts. With this <em>explanation - of the facts</em> I need hardly say that I fully agree; nor am I aware - that its correctness has ever been disputed. Dr. Carpenter surely - cannot charge me with overlooking the fact of a mutual interchange of - equatorial and polar water, seeing that my estimate of the thermal - power of the Gulf-stream, from which it is proved that the amount - of heat conveyed from equatorial to temperate and polar regions - is enormously greater than had ever been anticipated, was made a - considerable time<span class="pagenum" id="Page_125">125</span> before he began to write on the subject of oceanic - circulation.<a id="FNanchor_60" href="#Footnote_60" class="fnanchor">[60]</a> And in my paper “On Ocean-currents in relation to the - Distribution of Heat over the Globe”<a id="FNanchor_61" href="#Footnote_61" class="fnanchor">[61]</a> (the substance of which is - reproduced in Chapters <a href="#CHAPTER_II">II.</a> and <a href="#CHAPTER_III">III.</a> of this volume), I have endeavoured - to show that, were it not for the raising of the temperature of polar - and high temperate regions and the lowering of the temperature of - inter-tropical regions by means of this interchange of water, these - portions of the globe would not be habitable by the present existing - orders of beings.</p> - - <p>The explanation goes further:—“It is along the surface and upper - portion of the ocean that the equatorial waters flow towards the - poles, and it is along the bottom and under portion of the ocean that - polar waters flow towards the equator; or, in other words, the warm - water keeps the <em>upper</em> portion of the ocean and the cold water the - <em>under</em> portion.” With this explanation I to a great extent agree. It - is evident that, in reference to the northern hemisphere at least, the - most of the water which flows from inter-tropical to polar regions - (as, for example, the Gulf-stream) keeps to the surface and upper - portion of the ocean; but for reasons which I have already stated, a - very large proportion of this water must return in the form of <em>under</em> - currents; or, which is the same thing, the return compensating current, - whether it consist of the identical water which originally came from - the equator or not, must flow towards the equator as an under current. - That the cold water which is found at the bottom of the Atlantic and - of inter-tropical seas must have come as under currents is perfectly - obvious, because water which should come along the surface of the ocean - from the polar regions would not be cold when it reached inter-tropical - regions.</p> - - <p><em>The Explanation hypothetical.</em>—Here the general agreement between - us in a great measure terminates, for Dr. Carpenter is not satisfied - with the explanation generally adopted by the <span class="pagenum" id="Page_126">126</span>advocates of the - <em>wind theory</em>, viz., that the cold water found in temperate and - inter-tropical areas comes from polar regions as compensating under - currents, but advances a <em>hypothetical</em> form of circulation to account - for the phenomenon. He assumes that there is a <em>general set</em> or flow of - the surface and upper portion of the ocean from the equator to polar - regions, and a <em>general set</em> or flow of the bottom and under portion of - the ocean from polar regions to the equator. Mr. Ferrel (<cite>Nature</cite>, June - 13, 1872) speaks of that “interchanging motion of the water between the - equator and the pole <em>discovered</em> by Dr. Carpenter.” In this, however, - Mr. Ferrel is mistaken; for Dr. Carpenter not only makes no claim to - any discovery of the kind, but distinctly admits that none such has - yet been made. Although in some of his papers he speaks of a “<em>set</em> of - warm surface-water in the southern oceans toward the Antarctic pole” - as being well known to navigators, yet he nowhere affirms, as far as I - know, that the existence of such a general oceanic circulation as he - advocates has ever been directly determined from observations. This - mode of circulation is <em>simply inferred</em> or <em>assumed</em> in order to - account for the facts referred to above. “At present,” Dr. Carpenter - says, “I claim for it no higher character than that of a good working - <em>hypothesis</em> to be used as a guide in further inquiry” (§ 16); and lest - there should be any misapprehension on this point, he closes his memoir - thus:—“At present, as I have already said, I claim for the doctrine of - a general oceanic circulation no higher a character than that of a good - working <em>hypothesis</em> consistent with our present knowledge of facts, - and therefore entitled to be <em>provisionally</em> adopted for the purpose of - stimulating and directing further inquiry.”</p> - - <p>I am unable to agree with him, however, on this latter point. It - seems to me that there is no necessity for adopting any hypothetical - mode of circulation to account for the facts, as they can be quite - well accounted for by means of that mode of circulation which does - <em>actually exist</em>. It has been determined from direct observation that - surface-currents flow from equatorial to polar regions, and their - paths have been actually mapped out.<span class="pagenum" id="Page_127">127</span> But if it is established that - currents flow from equatorial to polar regions, it is equally so that - return currents flow from polar to equatorial regions; for if the one - <em>actually</em> exists, the other of necessity <em>must</em> exist. We know also - on physical grounds, to which I have already referred, and which fall - to be considered more fully in a subsequent chapter, that a very large - portion of the water flowing from polar to equatorial regions must - be in the form of under currents. If there are cold under currents, - therefore, flowing from polar to temperate and equatorial regions, - this is all that we really require to account for the cold water which - is found to occupy the bed of the ocean in those regions. It does not - necessarily follow, because cold water may be found at the bottom of - the ocean all along the equator, that there must be a direct flow - from the polar regions to every point of the equator. Water brought - constantly from the polar regions to various points along the equator - by means of under currents will necessarily accumulate, and in course - of time spread over the bottom of the inter-tropical seas. It must - either do this, or the currents on reaching the equator must bend - upwards and flow to the surface in an unbroken mass. Considerable - portions of some of those currents may no doubt do so and join - surface-currents; but probably the greater portion of the water coming - from polar regions extends itself over the floor of the equatorial - seas. In a letter in <cite>Nature</cite>, January 11, 1872, I endeavoured to show - that the surface-currents of the ocean are not separate and independent - of one another, but form one grand system of circulation, and that - the impelling cause keeping up this system of circulation is not the - <em>trade-winds</em> alone, as is generally supposed, but the <em>prevailing - winds of the entire globe considered also as one grand system</em>. The - evidence for this opinion, however, will be considered more fully in - the sequel.</p> - - <p>Although the under currents are parts of one general system of oceanic - circulation produced by the impulse of the system of prevailing winds, - yet their direction and position are nevertheless, to a large extent, - determined by different laws. The<span class="pagenum" id="Page_128">128</span> water at the surface, being moved - by the force of the wind, will follow the path of <em>greatest pressure - and traction</em>,—the effects resulting from the general contour of the - land, which to a great extent are common to both sets of currents, not - being taken into account; while, on the other hand, the under currents - from polar regions (which to a great extent are simply “indraughts” - compensating for the water drained from equatorial regions by the - Gulf-stream and other surface currents) will follow, as a general rule, - the path of <em>least resistance</em>.</p> - - <p><em>The Cause assigned for the Hypothetical Mode of Circulation.</em>—Dr. - Carpenter assigns a cause for his mode of circulation; and that cause - he finds in the difference of specific gravity between equatorial - and polar waters, resulting from the difference of temperature - between these two regions. “Two separate questions,” he says, “have - to be considered, which have not, perhaps, been kept sufficiently - distinct, either by Mr. Croll or by myself;—<em>first</em>, whether there - is adequate evidence of the existence of a general vertical oceanic - circulation; and <em>second</em>, whether, supposing its existence to be - provisionally admitted, a <i lang="la">vera causa</i> can be found for it in the - difference of temperature between the oceanic waters of the polar and - equatorial areas” (§ 17). It seems to me that the facts adduced by - Dr. Carpenter do not necessarily require the assumption of any such - mode of circulation as that advanced by him. The phenomena can be - satisfactorily accounted for otherwise; and therefore there does not - appear to be any necessity for considering whether his hypothesis be - sufficient to produce the required effect or not.</p> - - <p><em>An important Consideration overlooked.</em>—But there is one important - consideration which seems to have been overlooked—namely, the fact - that the sea is salter in inter-tropical than in polar regions, and - that this circumstance, so far as it goes, must tend to neutralize - the effect of difference of temperature. It is probable, indeed, that - the effect produced by difference of temperature is thus entirely - neutralized, and that no difference of density whatever exists between - the sea in inter-tropical and polar regions, and consequently that - there is no difference of<span class="pagenum" id="Page_129">129</span> level nor anything to produce such a general - motion as Dr. Carpenter supposes. This, I am glad to find, is the - opinion of Professor Wyville Thomson.</p> - - <p>“I am greatly mistaken,” says that author, “if the low specific gravity - of the polar sea, the result of the condensation and precipitation - of vapour evaporated from the inter-tropical area, do not fully - counterbalance the contraction of the superficial film by arctic - cold.... Speaking in the total absence of all reliable data, it is my - general impression that if we were to set aside all other agencies, and - to trust for an oceanic circulation to those conditions only which are - relied upon by Dr. Carpenter, if there were any general circulation at - all, which seems very problematical, the odds are rather in favour of - a warm under current travelling northwards by virtue of its excess of - salt, balanced by a surface return current of fresher though colder - arctic water.”<a id="FNanchor_62" href="#Footnote_62" class="fnanchor">[62]</a></p> - - <p>This is what actually takes place on the west and north-west of - Spitzbergen. There the warm water of the Gulf-stream flows underneath - the cold polar current. And it is the opinion of Dr. Scoresby, Mr. - Clements Markham, and Lieut. Maury that this warm water, in virtue - of its greater saltness, is denser than the polar water. Mr. Leigh - Smith found on the north-west of Spitzbergen the temperature at 500 - fathoms to be 52°, and once even 64°, while the water on the surface - was only a degree or two above freezing.<a id="FNanchor_63" href="#Footnote_63" class="fnanchor">[63]</a> Mr. Aitken, of Darroch, - in a paper lately read before the Royal Scottish Society of Arts, - showed experimentally that the polar water in regions where the ice is - melting is actually less dense than the warm and more salt tropical - waters. Nor will it help the matter in the least to maintain that - difference of specific gravity is not the reason why the warm water of - the Gulf-stream passes under the polar stream—because if difference - of specific gravity be not the cause of the warm water underlying the - cold water in polar regions, then difference of specific gravity may - likewise not <span class="pagenum" id="Page_130">130</span>be the cause of the cold water underlying the warm at - the equator; and if so, then there is no necessity for the gravitation - hypothesis of oceanic circulation.</p> - - <p>There is little doubt that the super-heated stratum at the surface of - the inter-tropical seas, which stratum, according to Dr. Carpenter, - is of no great thickness, is less dense than the polar water: but if - we take a column extending from the surface down to the bottom of the - ocean, this column at the equator will be found to be as heavy as one - of equal length in the polar area. And if this be the case, then there - can be no difference of level between the equator and the poles, and - no disturbance of static equilibrium nor anything else to produce - circulation.</p> - - <p><em>Under Currents account for all the Facts better than Dr. Carpenter’s - Hypothesis.</em>—Assuming, for the present, the system of prevailing winds - to be the true cause of oceanic currents, it necessarily follows (as - will be shown hereafter) that a large quantity of Atlantic water must - be propelled into the Arctic Ocean; and such, as we know, is actually - the case. The Arctic Ocean, however, as Professor Wyville Thomson - remarks, is a well-nigh closed basin, not permitting of a free outflow - into the Pacific Ocean of the water impelled into it.</p> - - <p>But it is evident that the water which is thus being constantly - carried from the inter-tropical to the arctic regions must somehow - or other find its way back to the equator; in other words, there - must be a return current equal in magnitude to the direct current. - Now the question to be determined is, what path must this return - current take? It appears to me that it will take the <em>path of least - resistance</em>, whether that path may happen to be at the surface or under - the surface. But that the path of least resistance will, as a general - rule, lie at a very considerable distance below the surface is, I - think, evident from the following considerations. At the surface the - general direction of the currents is opposite to that of the return - current. The surface motion of the water in the Atlantic is from the - equator to the pole; but the return current must be<span class="pagenum" id="Page_131">131</span> from the pole to - the equator. Consequently the surface currents will oppose the motion - of any return current unless that current lie at a considerable depth - below the surface currents. Again, the winds, as a general rule, blow - in an opposite direction to the course of the return current, because, - according to supposition, the winds blow in the direction of the - surface currents. From all these causes the path of least resistance to - the return current will, as a general rule, not be at the surface, but - at a very considerable depth below it.</p> - - <p>A large portion of the water from the polar regions no doubt leaves - those regions as surface currents; but a surface current of this kind, - on meeting with some resistance to its onward progress along the - surface, will dip down and continue its course as an under current. We - have an example of this in the case of the polar current, which upon - meeting the Gulf-stream on the banks of Newfoundland divides—a portion - of it dipping down and pursuing its course underneath that stream into - the Gulf of Mexico and the Caribbean Sea. And that this under current - is a real and tangible current, in the proper sense of the term, and - not an imperceptible movement of the water, is proved by the fact that - large icebergs deeply immersed in it are often carried southward with - considerable velocity against the united force of the wind and the - Gulf-stream.</p> - - <p>Dr. Carpenter refers at considerable length (§ 134) to Mr. Mitchell’s - opinion as to the origin of the polar current, which is the same as - that advanced by Maury, viz., that the impelling cause is difference - of specific gravity. But although Dr. Carpenter quotes Mr. Mitchell’s - opinion, he nevertheless does not appear to adopt it: for in §§ 90−93 - and various other places he distinctly states that he does not agree - with Lieut. Maury’s view that the Gulf-stream and polar current - are caused by difference of density. In fact, Dr. Carpenter seems - particularly anxious that it should be clearly understood that he - dissents from the theory maintained by Maury. But he does not merely - deny that the Gulf-stream and polar current can be caused by difference - of density; he even goes so far as to<span class="pagenum" id="Page_132">132</span> affirm that no sensible current - whatever can be due to that cause, and adduces the authority of Sir - John Herschel in support of that opinion:—“The doctrine of Lieut. - Maury,” he says, “was powerfully and convincingly opposed by Sir - John Herschel; who showed, beyond all reasonable doubt, first, that - the Gulf-stream really has its origin in the propulsive force of the - trade-winds, and secondly, that the greatest disturbance of equilibrium - which can be supposed to result from the agencies invoked by Lieut. - Maury would be utterly inadequate to generate and maintain either the - Gulf-stream or any other sensible current” (§ 92). This being Dr. - Carpenter’s belief, it is somewhat singular that he should advance the - case of the polar current passing under the Gulf-stream as evidence - in favour of his theory; for in reality he could hardly have selected - a case more hostile to that theory. In short, it is evident that, if - a polar current impelled by a force other than that of gravity can - pass from the banks of Newfoundland to the Gulf of Mexico (a distance - of some thousands of miles) under a current flowing in the opposite - direction and, at the same time, so powerful as the Gulf-stream, it - could pass much more easily under comparatively still water, or water - flowing in the same direction as itself. And if this be so, then all - our difficulties disappear, and we satisfactorily explain the presence - of cold polar water at the bottom of inter-tropical seas without having - recourse to the hypothesis advanced by Dr. Carpenter.</p> - - <p>But we have an example of an under current more inexplicable on the - gravitation hypothesis than even that of the polar current, viz., the - warm under current of Davis Strait.</p> - - <p>There is a strong current flowing north from the Atlantic through Davis - Strait into the Arctic Ocean underneath a surface current passing - southwards in an opposite direction. Large icebergs have been seen to - be carried northwards by this under current at the rate of four knots - an hour against both the wind and the surface current, ripping and - tearing their way with<span class="pagenum" id="Page_133">133</span> terrific force through surface ice of great - thickness.<a id="FNanchor_64" href="#Footnote_64" class="fnanchor">[64]</a> A current so powerful and rapid as this cannot, as Dr. - Carpenter admits, be referred to difference of specific gravity. But - even supposing that it could, still difference of temperature between - the equatorial and polar seas would not account for it; for the current - in question flows in the <em>wrong direction</em>. Nor will it help the matter - the least to adopt Maury’s explanation, viz., that the warm under - current from the south, in consequence of its greater saltness, is - denser than the cold one from the polar regions. For if the water of - the Atlantic, notwithstanding its higher temperature, is in consequence - of its greater saltness so much denser than the polar water on the - west of Greenland as to produce an under current of four knots an hour - in the direction of the pole, then surely the same thing to a certain - extent will hold true in reference to the ocean on the east side of - Greenland. Thus instead of there being, as Dr. Carpenter supposes, - an underflow of polar water south into the Atlantic in virtue of its - <em>greater</em> density, there ought, on the contrary, to be a surface flow - in consequence of its lesser density.</p> - - <p>The true explanation no doubt is, that the warm under current from - the south and the cold upper current from the north are both parts - of one grand system of circulation produced by the winds, difference - of specific gravity having no share whatever either in impelling the - currents, or in determining which shall be the upper and which the - lower.</p> - - <p>The wind in Baffin’s Bay and Davis Strait blows nearly always in one - direction, viz. from the north. The tendency of this is to produce a - surface or upper current from the north down into the Atlantic, and to - prevent or retard any surface current from the south. The warm current - from the Atlantic, taking the path of least resistance, dips under the - polar current and pursues its course as an under current.</p> - - <p>Mr. Clements Markham, in his “Threshold of the Unknown Region,” is - inclined to attribute the motion of the icebergs to <span class="pagenum" id="Page_134">134</span>tidal action or - to counter under currents. That the motion of the icebergs cannot - reasonably be attributed to the tides is, I think, evident from the - descriptions given both by Midshipman Griffin and by Captain Duncan, - who distinctly saw the icebergs moving at the rate of about four knots - an hour against a surface current flowing southwards. And Captain - Duncan states that the bergs continued their course northwards for - several days, till they ultimately disappeared. The probability is that - this northward current is composed partly of Gulf-stream water and - partly of that portion of polar water which is supposed to flow round - Cape Farewell from the east coast of Greenland. This stream, composed - of both warm and cold water, on reaching to about latitude 65°N., where - it encounters the strong northerly winds, dips down under the polar - current and continues its northward course as an under current.</p> - - <p>We have on the west of Spitzbergen, as has already been noticed, a - similar example of a warm current from the south passing under a polar - current. A portion of the Gulf-stream which passes round the west - coast of Spitzbergen flows under an arctic current coming down from - the north; and it does so no doubt because it is here in the region of - prevailing northerly winds, which favour the polar current but oppose - the Gulf-stream. Again, we have a cold and rapid current sweeping - round the east and south of Spitzbergen, a current of which Mr. Lamont - asserts that he is positive he has seen it running at the rate of seven - or eight miles an hour. This current, on meeting the Gulf-stream about - the northern entrance to the German Ocean, dips down under that stream - and pursues its course southwards as an under current.</p> - - <p>Several other cases of under currents might be adduced which cannot - be explained on the gravitation theory, and which must be referred to - a system of oceanic circulation produced by the impulse of the wind; - but these will suffice to show that the assumption that the winds can - produce only a mere surface-drift is directly opposed to facts. And - it will not do to affirm that a current which forms part of a general - system of circulation<span class="pagenum" id="Page_135">135</span> produced by the impulse of the winds cannot - possibly be an under current; for in the case referred to we have - proof that the thing is not only possible but actually exists. This - point, however, will be better understood after we have considered the - evidence in favour of a general system of oceanic currents.</p> - - <p>Much of the difficulty experienced in comprehending how under currents - can be produced by the wind, or how an impulse imparted to the surface - of the ocean can ever be transmitted to the bottom, appears to me to - result, to a considerable extent at least, from a slight deception - of the imagination. The thing which impresses us most forcibly in - regard to the ocean is its profound depth. A mean depth of, say, three - miles produces a striking impression; but if we could represent to - the mind the vast area of the ocean as correctly as we can its depth, - <em>shallowness</em> rather than <em>depth</em> would be the impression produced. If - in crossing a meadow we found a sheet of water one hundred yards in - diameter and only an inch in depth, we should not call that a <em>deep</em>, - but a very <em>shallow</em> pool. The probability is that we should speak of - it as simply a piece of ground covered with a thin layer of water. - Yet such a thin layer of water would be a correct representation in - miniature of the ocean; for the ocean in relation to its superficial - area is as shallow as the pool of our illustration. In reference to - such a pool or thin film of water, we have no difficulty in conceiving - how a disturbance on its surface would be transmitted to its bottom. - In fact our difficulty is in conceiving how any disturbance extending - over its entire surface should not extend to the bottom. Now if we - could form as accurate a sensuous impression of the vast area of the - ocean as we do of such a pool, all our difficulty in understanding how - the impulses of the wind acting on the vast area of the ocean should - communicate motion down to its bottom would disappear. It is certainly - true that sudden commotions caused by storms do not generally extend to - great depths. Neither will winds of short continuance produce a current - extending far below the surface. But prevailing winds which can produce - such immense surface-flow as that of the great<span class="pagenum" id="Page_136">136</span> equatorial currents of - the globe and the Gulf-stream, which follow definite directions, must - communicate their motion to great depths, unless water be frictionless, - a thing which it is not. Suppose the upper layer of the ocean to be - forced on by the direct action of the winds with a constant velocity - of, say, four miles an hour, the layer immediately below will be - dragged along with a constant velocity somewhat less than four miles - an hour. The layer immediately below this second layer will in turn be - also dragged along with a constant velocity somewhat less than the one - above it. The same will take place in regard to each succeeding layer, - the constant velocity of each layer being somewhat less than the one - immediately above it, and greater than the one below it. The question - to be determined is, at what depth will all motion cease? I presume - that at present we have not sufficient data for properly determining - this point. The depth will depend, other things being equal, upon the - amount of molecular resistance offered by the water to motion—in other - words, on the amount of the shearing-force of the one layer over the - other. The fact, however, that motion imparted to the surface will - extend to great depths can be easily shown by direct experiment. If a - constant motion be imparted to the surface of water, say, in a vessel, - motion will ultimately be communicated to the bottom, no matter how - wide or how deep the vessel may be. The same effect will take place - whether the vessel be 5 feet deep or 500 feet deep.</p> - - <p><em>The known Condition of the Ocean inconsistent with Dr. Carpenter’s - Hypothesis.</em>—Dr. Carpenter says that he looks forward with great - satisfaction to the results of the inquiries which are being prosecuted - by the Circumnavigation Expedition, in the hope that the facts brought - to light may establish his theory of a general oceanic circulation; and - he specifies certain of these facts which, if found to be correct, will - establish his theory. It seems to me, however, that the facts to which - he refers are just as explicable on the theory of under currents as on - the theory of a general oceanic circulation. He begins by saying, “If - the<span class="pagenum" id="Page_137">137</span> views I have propounded be correct, it may be expected that near - the border of the great antarctic ice-barrier a temperature below 30° - will be met with (as it has been by Parry, Martens, and Weyprecht near - Spitzbergen) at no great depth beneath the surface, and that instead of - rising at still greater depths, the thermometer will fall to near the - freezing-point of salt water” (§ 39).</p> - - <p>Dr. Carpenter can hardly claim this as evidence in favour of his - theory; for near the borders of the ice-barrier the water, as a matter - of course, could not be expected to have a much higher temperature than - the ice itself. And if the observations be made during summer months, - the temperature of the water at the surface will no doubt be found to - be higher than that of the bottom; but if they be carried on during - winter, the surface-temperature will doubtless be found to be as low as - the bottom-temperature. These are results which do not depend upon any - particular theory of oceanic circulation.</p> - - <p>“The bottom temperature of the North Pacific,” he continues, “will - afford a crucial test of the truth of the doctrine. For since the sole - communication of this vast oceanic area with the arctic basin is a - strait so shallow as only to permit an inflow of warm surface water, - its deep cold stratum must be entirely derived from the antarctic area; - and if its bottom temperature is not actually higher than that of the - South Pacific, the glacial stratum ought to be found at a greater depth - north of the equator than south of it” (§ 39).</p> - - <p>This may probably show that the water came from the antarctic regions, - but cannot possibly prove that it came in the manner which he supposes.</p> - - <p>“In the North Atlantic, again, the comparative limitation of - communication with the arctic area may be expected to prevent its - bottom temperature from being reduced as low as that of the Southern - Atlantic” (§ 39). Supposing the bottom temperature of the South - Atlantic should be found to be lower than the bottom temperature of the - North Atlantic, this fact will be just as consistent with the theory of - under<span class="pagenum" id="Page_138">138</span> currents as with his theory of a general movement of the ocean.</p> - - <p>I am also wholly unable to comprehend how he should imagine, because - the bottom temperature of the South Atlantic happens to be lower, and - the polar water to lie nearer to the surface in this ocean than in the - North Atlantic, that therefore this proves the truth of his theory. - This condition of matters is just as consistent, and even more so, as - will be shown in <a href="#CHAPTER_XIII">Chapter XIII.</a>, with my theory as with his. When we - consider the immense quantity of warm surface water which, as has been - shown (<a href="#CHAPTER_V">Chapter V.</a>), is being constantly transferred from the South into - the North Atlantic, we readily understand how the polar water comes - nearer to the surface in the former ocean than in the latter. Every - pound of water, of course, passing from the southern to the northern - hemisphere must be compensated by an equal amount passing from the - northern to the southern hemisphere. But nevertheless the warm water - drained off the South Atlantic is not replaced directly by water from - the north, but by that cold antarctic current, the existence of which - is, unfortunately, too well known to navigators from the immense masses - of icebergs which it brings along with it. In fact, the whole of the - phenomena are just as easily explained upon the principle of under - currents as upon Dr. Carpenter’s theory. But we shall have to return to - this point in <a href="#CHAPTER_XIII">Chapter XIII.</a>, when we come to discuss a class of facts - which appear to be wholly irreconcilable with the gravitation theory.</p> - - <p>Indeed I fear that even although Dr. Carpenter’s expectations should - eventually be realised in the results of the Circumnavigation - Expedition, yet the advocates of the wind theory will still remain - unconverted. In fact the Director of this Expedition has already, on - the wind theory, offered an explanation of nearly all the phenomena - on which Dr. Carpenter relies;<a id="FNanchor_65" href="#Footnote_65" class="fnanchor">[65]</a> and the same has also been done by - Dr. Petermann,<a id="FNanchor_66" href="#Footnote_66" class="fnanchor">[66]</a> who, as is well known, is equally opposed to <span class="pagenum" id="Page_139">139</span>Dr. - Carpenter’s theory. Dr. Carpenter directs attention to the necessity of - examining the broad and deep channel separating Iceland from Greenland. - The observations which have already been made, however, show that - nearly the entire channel is occupied, on the surface at least, by - water flowing southward from the polar area—a direction the opposite of - what it ought to be according to the gravitation theory. In fact the - surface of one half of the entire area of the ocean, extending from - Greenland to the North Cape, is moving in a direction the opposite of - that which it ought to take according to the theory under review. The - western half of this area is occupied by water which at the surface is - flowing southwards; while the eastern half, which has hitherto been - regarded by almost everybody but Dr. Carpenter himself and Mr. Findlay - as an extension of the Gulf-stream, is moving polewards. The motion of - the western half must be attributed to the winds and not to gravity; - for it is moving in the wrong direction to be accounted for by the - latter cause; but had it been moving in the opposite direction, no - doubt its motion would have been referred to gravitation. To this cause - the motion of the eastern half, which is in the proper direction, is - attributed;<a id="FNanchor_67" href="#Footnote_67" class="fnanchor">[67]</a> but why not assign this motion also to the impulse of - the winds, more especially since the direction of the prevailing winds - blowing over that area coincides with that of the water? If the wind - can produce the motion of the water in the western half, why may not it - do the same in the eastern half?</p> - - <p>If there be such a difference of density between equatorial and polar - waters as to produce a general flow of the upper portion of the ocean - poleward, how does it happen that one half of the water in the above - area is moving in opposition to gravity? How is it that in a wide - open sea gravitation should act so powerfully in the one half of it - and with so little effect in the other half? There is probably little - doubt that the ice-cold water of the western half extends from the - surface down to <span class="pagenum" id="Page_140">140</span>the bottom. And it is also probable that the bottom - water is moving southwards in the same direction as the surface water. - The bottom water in such a case would be moving in harmony with the - gravitation theory; but would Dr. Carpenter on this account attribute - its motion to gravity? Would he attribute the motion of the lower half - to gravity and the upper half to the wind? He could not in consistency - with his theory attribute the motion of the upper half to gravity: for - although the ice-cold water extended to the surface, this could not - explain how gravity should move it southward instead of polewards, as - according to theory it ought to move. He might affirm, if he chose, - that the surface water moves southwards because it is dragged forward - by the bottom water; but if this view be held, he is not entitled to - affirm, as he does, that the winds can only produce a mere surface - drift. If the viscosity and molecular resistance of water be such that, - when the lower strata of the ocean are impelled forward by gravity or - by any other cause, the superincumbent strata extending to the surface - are perforce dragged after them, then, for the same reason, when the - upper strata are impelled forward by the wind or any other cause, the - underlying strata must also be dragged along after them.</p> - - <p>If the condition of the ocean between Greenland and the north-western - shore of Europe is irreconcilable with the gravitation theory, we find - the case even worse for that theory when we direct our attention to - the condition of the ocean on the southern hemisphere; for according - to the researches of Captain Duperrey and others on the currents of - the Southern Ocean, a very large portion of the area of that ocean is - occupied by water moving on the surface more in a northward than a - poleward direction. Referring to the deep trough between the Shetland - and the Faroe Islands, called by him the “Lightning Channel,” Dr. - Carpenter says, “If my view be correct, a current-drag suspended in - the <em>upper</em> stratum ought to have a perceptible movement in the N.E. - direction; whilst another, suspended in the <em>lower</em> stratum, should - move S.W.” (§ 40).</p> - - <p><span class="pagenum" id="Page_141">141</span></p> - - <p>Any one believing in the north-eastern extension of the Gulf-stream - and in the Spitsbergen polar under current, to which I have already - referred, would not feel surprised to learn that the surface strata - have a perceptible north-eastward motion, and the bottom strata a - perceptible south-westward motion. North-east and east of Iceland - there is a general flow of cold polar water in a south-east direction - towards the left edge of the Gulf-stream. This water, as Professor Mohn - concludes, “descends beneath the Gulf-stream and partially finds an - outlet in the lower half of the Faroe-Shetland channel.”<a id="FNanchor_68" href="#Footnote_68" class="fnanchor">[68]</a></p> - - <p><em>An Objection Considered.</em>—In Nature, vol. ix. p. 423, Dr. Carpenter - has advanced the following objection to the foregoing theory of - under-currents:—“According to Mr. Croll’s doctrine, the whole of that - vast mass of water in the North Atlantic, averaging, say, 1,500 fathoms - in thickness and 3,600 miles in breadth, the temperature of which - (from 40° downwards), as ascertained by the <cite>Challenger</cite> soundings, - clearly shows it to be mainly derived from a polar source, is nothing - else than <em>the reflux of the Gulf-stream</em>. Now, even if we suppose - that the whole of this stream, as it passes Sandy Hook, were to go on - into the closed arctic basin, it would only force out an equivalent - body of water. And as, on comparing the sectional areas of the two, - I find that of the Gulf-stream to be about 1/900th that of the North - Atlantic underflow; and as it is admitted that a large part of the - Gulf-stream returns into the Mid-Atlantic circulation, only a branch of - it going on to the north-east, the extreme improbability (may I not say - impossibility?) that so vast a mass of water can be put in motion by - what is by comparison a mere rivulet (the north-east motion of which, - as a distinct current, has not been traced eastward of 30° W. long.) - seems still more obvious.”</p> - - <p>In this objection three things are assumed: (1) that the mass of cold - water 1,500 fathoms deep and 3,600 miles in breadth is in a state of - motion towards the equator; (2) that it cannot be the reflux of the - Gulf-stream, because its sectional <span class="pagenum" id="Page_142">142</span>area is 900 times as great as that - of the Gulf-stream; (3) that the immense mass of water is, according to - my views, set in motion by the Gulf-stream.</p> - - <p>As this objection has an important bearing on the question under - consideration, I shall consider these three assumptions separately - and in their order: (1) That this immense mass of cold water came - originally from the polar regions I, of course, admit, but that the - whole is in a state of motion I certainly do not admit. There is no - warrant whatever for any such assumption. According to Dr. Carpenter - himself, the heating-power of the sun does not extend to any great - depth below the surface; consequently there is nothing whatever to - heat this mass but the heat coming through the earth’s crust. But - the amount of heat derived from this source is so trifling, that an - under current from the arctic regions far less in volume than that - of the Gulf-stream would be quite sufficient to keep the mass at an - ice-cold temperature. Taking the area of the North Atlantic between - the equator and the Tropic of Cancer, including also the Caribbean - Sea and the Gulf of Mexico, to be 7,700,000 square miles, and the - rate at which internal heat passes through the earth’s surface to be - that assigned by Sir William Thomson, we find that the total quantity - of heat derived from the earth’s crust by the above area is equal to - about 88 × 10<sup>15</sup> foot-pounds per day. But this amount is equal to - only 1/894th that conveyed by the Gulf-stream, on the supposition that - each pound of water carries 19,300 foot-pounds of heat. Consequently - an under current from the polar regions of not more than 1/35th the - volume of the Gulf-stream would suffice to keep the entire mass of - water of that area within 1° of what it would be were there no heat - derived from the crust of the earth; that is to say, were the water - conveyed by the under current at 32°, internal heat would not maintain - the mass of the ocean in the above area at more than 33°. The entire - area of the North Atlantic from the equator to the arctic circle is - somewhere about 16,000,000 square miles. An under current of less than - 1/17th that of the Gulf-stream coming from the arctic regions would<span class="pagenum" id="Page_143">143</span> - therefore suffice to keep the entire North Atlantic basin filled with - ice-cold water. In short, whatever theory we adopt regarding oceanic - circulation, it follows equally as a necessary consequence that the - entire mass of the ocean below the stratum heated by the sun’s rays - must consist of cold water. For if cold water be continually coming - from the polar regions either in the form of under currents, or in the - form of a general underflow as Dr. Carpenter supposes, the entire under - portion of the ocean must ultimately become occupied by cold water; for - there is no source from which this influx of water can derive heat, - save from the earth’s crust. But the amount thus derived is so trifling - as to produce no sensible effect. For example, a polar under current - one half the size of the Gulf-stream would be sufficient to keep the - entire water of the globe (below the stratum heated by the sun’s rays) - at an ice-cold temperature. Internal heat would not be sufficient under - such circumstances to maintain the mass 1° Fahr. above the temperature - it possessed when it left the polar regions.</p> - - <p>It follows therefore that the presence of the immense mass of ice-cold - water in the great depths of the ocean is completely accounted for by - under currents, and there is no necessity for supposing it to be all - in a state of motion towards the equator. In fact, this very state of - things, which the general oceanic circulation hypothesis was devised to - explain, results as a necessary consequence of polar under currents. - Unless these were entirely stopped it is physically impossible that the - ocean could be in any other condition.</p> - - <p>But suppose that this immense mass of cold water occupying the great - depths of the ocean were, as Dr. Carpenter assumes it to be, in a - state of constant motion towards the equator, and that its sectional - area were 900 times that of the Gulf-stream, it would not therefore - follow that the quantity of water passing through this large sectional - area must be greater than that flowing through a sectional area of - the Gulf-stream; for the quantity of water flowing through this large - sectional area depends entirely on the rate of motion.</p> - - <p><span class="pagenum" id="Page_144">144</span></p> - - <p>I am wholly unable to understand how it could be supposed that this - underflow, according to my view, is set in motion by the Gulf-stream, - seeing that I have shown that the return under current is as much due - to the impulse of the wind as the Gulf-stream itself.</p> - - <p>Dr. Carpenter lays considerable stress on the important fact - established by the <cite>Challenger</cite> expedition, that the great depths of - the sea in equatorial regions are occupied by ice-cold water, while - the portion heated by the sun’s rays is simply a thin stratum at the - surface. It seems to me that it would be difficult to find a fact more - hostile to his theory than this. Were it not for this upper stratum - of heated water there would be no difference between the equatorial - and polar columns, and consequently nothing to produce motion. But the - thinner this stratum is the less is the difference, and the less there - is to produce motion.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_IX"> - <span class="pagenum" id="Page_145">145</span> - <h2> - CHAPTER IX.<br /><br /> - <span class="small">EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.—THE - MECHANICS OF DR. CARPENTER’S THEORY.</span> - </h2> - </div> - <div class="subhead">Experimental Illustration of the Theory.—The Force exerted - by Gravity.—Work performed by Gravity.—Circulation not by - Convection.—Circulation depends on Difference in Density of the - Equatorial and Polar Columns.—Absolute Amount of Work which - can be performed by Gravity.—How Underflow is produced.—How - Vertical Descent at the Poles and Ascent at the Equator - is produced.—The Gibraltar Current.—Mistake in Mechanics - concerning it.—The Baltic Current.</div> - - <p><em>Experiment to illustrate Theory.</em>—In support of the theory of a - general movement of water between equatorial and polar regions, Dr. - Carpenter adduces the authority of Humboldt and of Prof. Buff.<a id="FNanchor_69" href="#Footnote_69" class="fnanchor">[69]</a> - I have been unable to find anything in the writings of either from - which it can be inferred that they have given this matter special - consideration. Humboldt merely alludes to the theory, and that in the - most casual manner; and that Prof. Buff has not carefully investigated - the subject is apparent from the very illustration quoted by Dr. - Carpenter from the “Physics of the Earth.” “The water of the ocean at - great depths,” says Prof. Buff, “has a temperature, even under the - equator, nearly approaching to the freezing-point. This low temperature - cannot depend on any influence of the sea-bottom.... The fact, however, - is explained by a continual current of cold water flowing from the - polar regions towards the equator. The following well-known experiment - clearly illustrates the manner of this movement. A glass vessel is to - be filled with water with which some powder has been mixed, and is then - to be heated <em>at bottom</em>. It will soon be seen, from <span class="pagenum" id="Page_146">146</span>the motion of the - particles of powder, that currents are set up in opposite directions - through the water. Warm water rises from the bottom up through the - middle of the vessel, and spreads over the surface, while the colder - and therefore heavier liquid falls down at the sides of the glass.”</p> - - <p>This illustration is evidently intended to show not merely the form - and direction of the great system of oceanic circulation, but also the - mode in which the circulation is induced by heat. It is no doubt true - that if we apply heat (say that of a spirit-lamp) to the bottom of a - vessel filled with water, the water at the bottom of the vessel will - become heated and rise to the surface; and if the heat be continued - an ascending current of warm water will be generated; and this, of - course, will give rise to a compensating under current of colder water - from all sides. In like manner it is also true that, if heat were - applied to the bottom of the ocean in equatorial regions, an ascending - current of hot water would be also generated, giving rise to an under - current of cold water from the polar regions. But all this is the - diametrically opposite of what actually takes place in nature. The heat - is not applied to the bottom of the ocean, so as to make the water - there lighter than the water at the surface, and thus to generate an - ascending current; but the heat is applied to the surface of the ocean, - and the effect of this is to prevent an ascending current rather than - to produce one, for it tends to keep the water at the surface lighter - than the water at the bottom. In order to show how the heat of the sun - produces currents in the ocean, Prof. Buff should have applied the - heat, not to the bottom of his vessel, but to the upper surface of the - water. But this is not all, the form of the vessel has something to - do with the matter. The wider we make the vessel in proportion to its - depth, the more difficult it is to produce currents by means of heat. - But in order to represent what takes place in nature, we ought to have - the same proportion between the depth and the superficial area of the - water in our vessel as there is between the depth and the superficial - area of the sea. The mean depth of the sea may be taken roughly to<span class="pagenum" id="Page_147">147</span> be - about three miles.<a id="FNanchor_70" href="#Footnote_70" class="fnanchor">[70]</a> The distance between pole and pole we shall take - in round numbers to be 12,000 miles. The sun may therefore be regarded - as shining upon a circular sea 12,000 miles in diameter and three miles - deep. The depth of the sea to its diameter is therefore as 1 to 4,000. - Suppose, now, that in our experiment we make the depth of our vessel - one inch, we shall require to make its diameter 4,000 inches, or 333 - feet, say, in round numbers, 100 yards in diameter. Let us, then, take - a pool of water 100 yards in diameter, and one inch deep. Suppose the - water to be at 32°. Apply heat to the upper surface of the pool, so - as to raise the temperature of the surface of the water to 80° at the - centre of the pool, the temperature diminishing towards the edge, where - it is at 32°. It is found that at a depth of two miles the temperature - of the water at the equator is about as low as that of the poles. We - must therefore suppose the water at the centre of our pool to diminish - in temperature from the surface downwards, so that at a depth of half - an inch the water is at 32°. We have in this case a thin layer of warm - water half an inch thick at the centre, and gradually thinning off to - nothing at the edge of the pool. The lightest water, be it observed, - is at the surface, so that an ascending or a descending current - is impossible. The only way whereby the heat applied can have any - tendency to produce motion is this:—The heating of the water expands - it, consequently the surface of the pool must stand at a little higher - level at its centre than at its edge, where no expansion takes place; - and therefore, in order to restore the level of the pool, the water at - the centre will tend to flow towards the sides. But what is the amount - of this tendency? Its amount will depend upon the amount of slope, - but the slope in the case under consideration amounts to only 1 in - 7,340,000.</p> - - <p><em>Dr. Carpenter’s Experiment.</em>—In order to obviate the objection to - Professor Buff’s experiment Dr. Carpenter has devised <span class="pagenum" id="Page_148">148</span>another mode. - But I presume his experiment was intended rather to illustrate the way - in which the circulation of the ocean, according to his theory, takes - place, than to prove that it actually does take place. At any rate, all - that can be claimed for the experiment is the proof that water will - circulate in consequence of difference of specific gravity resulting - from difference of temperature. But this does not require proof, for no - physicist denies it. The point which requires to be proved is this. Is - the difference of specific gravity which exists in the ocean sufficient - to produce the supposed circulation? Now his mode of experimenting - will not prove this, unless he makes his experiment agree with the - conditions already stated.</p> - - <p>But I decidedly object to the water being heated in the way in which it - has been done by him in his experiment before the Royal Geographical - Society; for I feel somewhat confident that in this experiment the - circulation resulted not from difference of specific gravity, as was - supposed, but rather from the way in which the heat was applied. In - that experiment the one half of a thick metallic plate was placed in - contact with the upper surface of the water at one end of the trough; - the other half, projecting over the end of the trough, was heated - by means of a spirit-lamp. It is perfectly obvious that though the - temperature of the great mass of the water under the plate might not - be raised over 80° or so, yet the molecules in contact with the metal - would have a very high temperature. These molecules, in consequence of - their expansion, would be unable to sink into the cooler and denser - water underneath, and thus escape the heat which was being constantly - communicated to them from the heated plate. But escape they must, or - their temperature would continue to rise until they would ultimately - burst into vapour. They cannot ascend, neither can they descend: they - therefore must be expelled by the heat from the plate in a horizontal - direction. The next layer of molecules from beneath would take their - place and would be expelled in a similar manner, and this process would - continue so long as the heat was applied to the plate. A circulation - would thus be established by the<span class="pagenum" id="Page_149">149</span> direct expansive force of vapour, and - not in any way due to difference of specific gravity, as Dr. Carpenter - supposes.</p> - - <p>But supposing the heated bar to be replaced by a piece of ice, - circulation would no doubt take place; but this proves nothing more - than that difference of density will produce circulation, which is what - no one calls in question.</p> - - <p>The case referred to by Dr. Carpenter of the heating apparatus in - London University is also unsatisfactory. The water leaves the boiler - at 120° and returns to it at 80°. The difference of specific gravity - between the water leaving the boiler and the water returning to it - is supposed to produce the circulation. It seems to me that this - difference of specific gravity has nothing whatever to do with the - matter. The cause of the circulation must be sought for in the boiler - itself, and not in the pipes. The heat is applied to the bottom of - the boiler, not to the top. What is the temperature of the molecules - in contact with the bottom of the boiler directly over the fire, is - a question which must be considered before we can arrive at a just - determination of the causes which produce circulation in the pipes - of a heating apparatus such as that to which Dr. Carpenter refers. - But, in addition to this, as the heat is applied to the bottom of the - boiler and not to the top, convection comes into play, a cause which, - as we shall find, does not come into play in the theory of oceanic - circulation at present under our consideration.</p> - - <p><em>The Force exerted by Gravity.</em>—Dr. Carpenter speaks of his doctrine of - a general oceanic circulation sustained by difference of temperature - alone, “as one of which physical geographers could not recognise the - importance, so long as they remained under the dominant idea that - the temperature of the deep sea is everywhere 39°.” And he affirms - that “until it is clearly apprehended that sea-water becomes more and - more dense as its temperature is reduced, the immense motive power of - polar cold cannot be understood.” But in chap. vii. and also in the - Phil. Mag. for October, 1870 and 1871, I proved that if we take 39° - as the temperature of maximum density the force<span class="pagenum" id="Page_150">150</span> exerted by gravity - tending to produce circulation is just as great as when we take 32°. - The reason for this is that when we take 32° as the temperature of - maximum density, although we have, it is true, a greater elevation of - the ocean above the place of maximum density, yet this latter occurs at - the poles; while on the other hand, when we take 39°, the difference - of level is less—the place not being at the poles but in about lat. - 56°. Now the shorter slope from the equator to lat. 56° is as steep as - the larger one from the equator to the poles, and consequently gravity - exerts as much force in the production of motion in the one case as in - the other. Sir John Herschel, taking 39° as the temperature of maximum - density, estimated the slope at 1/32nd of an inch per mile, whereas - we, taking 32° as the actual temperature of maximum density of the - polar seas and calculating from modern data, find that the slope is not - one-half that amount, and that the force of gravity tending to produce - circulation is much less than Herschel concluded it to be. The reason, - therefore, why physical geographers did not adopt the theory that - oceanic circulation is the result of difference of temperature could - not possibly be the one assigned by Dr. Carpenter, viz., that they had - under-estimated the force of gravity by taking 39° instead of 32° as - the temperature of maximum density.</p> - - <p><em>The Work performed by Gravity.</em>—But in order clearly to understand - this point, it will be better to treat the matter according to the - third method, and consider not the mere <em>force</em> of gravity impelling - the waters, but the amount of <em>work</em> which gravitation is capable of - performing.</p> - - <p>Let us then assume the correctness of my estimate, that the height of - the surface of the ocean at the equator above that at the poles is 4 - feet 6 inches, for in representing the mode in which difference of - specific gravity produces circulation it is of no importance what we - may fix upon as the amount of the slope. In order, therefore, to avoid - fractions of a foot, I shall take the slope at 4 feet instead of 4½ - feet, which it actually is. A pound of water in flowing down this slope - from the equator to<span class="pagenum" id="Page_151">151</span> either of the poles will perform 4 foot-pounds of - work; or, more properly speaking, gravitation will. Now it is evident - that when this pound of water has reached the pole, it is at the - bottom of the slope, and consequently cannot descend further. Gravity, - therefore, cannot perform any more work upon it; as it can only do so - while the thing acted upon continues to descend—that is, moves under - the force exerted. But the water will not move under the influence - of gravity unless it move downward; it being in this direction only - that gravity acts on the water. “But,” says Dr. Carpenter, “the effect - of surface-cold upon the water of the polar basin will be to reduce - the temperature of its whole mass below the freezing-point of fresh - water, the surface-stratum <em>sinking</em> as it is cooled in virtue of its - diminished bulk and increased density, and being replaced by water not - yet cooled to the same degree.”<a id="FNanchor_71" href="#Footnote_71" class="fnanchor">[71]</a> By the cooling of the whole mass - of polar water by cold and the heating of the water at the equator by - the sun’s rays the polar column of water, as we have seen, is rendered - denser than the equatorial one, and in order that the two may balance - each other, the polar column is necessarily shorter than the equatorial - by 4 feet; and thus it is that the slope of 4 feet is formed. It is - perfectly true that the water which leaves the equator warm and light, - becomes by the time it reaches the pole cold and dense. But unless - it be denser than the underlying polar water it will not sink down - <em>through</em> it.<a id="FNanchor_72" href="#Footnote_72" class="fnanchor">[72]</a> We are not told, however, why it should be colder - than the whole mass underneath, which, according to Dr. Carpenter, - is cooled by polar cold. But that he does suppose it to sink to the - bottom in consequence of its contraction by cold would appear from the - following quotation:—</p> - - <p>“Until it is clearly apprehended that sea-water becomes <span class="pagenum" id="Page_152">152</span>more and more - dense as its temperature is reduced, and that it consequently continues - to sink until it freezes, the immense motor power of polar cold cannot - be apprehended. But when this has been clearly recognised, it is seen - that the application of <em>cold at the surface</em> is precisely equivalent - as a moving power to that application of <em>heat at the bottom</em> by which - the circulation of water is sustained in every heating apparatus that - makes use of it” (§ 25).</p> - - <p>The application of cold at the surface is thus held to be equivalent - as a motor power to the application of heat at the bottom. But heat - applied to the bottom of a vessel produces circulation by <em>convection</em>. - It makes the molecules at the bottom expand, and they, in consequence - of buoyancy, rise <em>through</em> the water in the vessel. Consequently if - the action of cold at the surface in polar regions is equivalent to - that of heat, the cold must contract the molecules at the surface and - make them sink <em>through</em> the mass of polar water beneath. But assuming - this to be the meaning in the passage just quoted, how much colder is - the surface water than the water beneath? Let us suppose the difference - to be one degree. How much work, then, will gravity perform upon this - one pound of water which is one degree colder than the mass beneath - supposed to be at 32°? The force with which the pound of water will - sink will not be proportional to its weight, but to the difference - of weight between it and a similar bulk of the water through which - it sinks. The difference between the weight of a pound of water at - 31° and an equal volume of water at 32° is 1/29,000th of a pound. Now - this pound of water in sinking to a depth of 10,000 feet, which is - about the depth at which a polar temperature is found at the equator, - would perform only one-third of a foot-pound of work. And supposing - it were three degrees colder than the water beneath, it would in - sinking perform only one foot-pound. This would give us only 4 + 1 = 5 - foot-pounds as the total amount that could be performed by gravitation - on the pound of water from the time that it left the equator till - it returned to the point from which<span class="pagenum" id="Page_153">153</span> it started. The amount of work - performed in descending the slope from the equator to the pole and in - sinking to a depth of 10,000 feet or so through the polar water assumed - to be warmer than the surface water, comprehends the total amount of - work that gravitation can possibly perform; so that the amount of force - gained by such a supposition over and above that derived from the slope - is trifling.</p> - - <p>It would appear, however, that this is not what is meant after all. - What Dr. Carpenter apparently means is this: when a quantity of water, - say a layer one foot thick, flows down from the equator to the pole, - the polar column becomes then heavier than the equatorial by the - weight of this additional layer. A layer of water equal in quantity - is therefore pressed away from the bottom of the column and flows off - in the direction of the equator as an under current, the polar column - at the same time sinking down one foot until equilibrium of the polar - and equatorial columns is restored. Another foot of water now flows - down upon the polar column and another foot of water is displaced - from below, causing, of course, the column to descend an additional - foot. The same process being continually repeated, a constant downward - motion of the polar column is the result. Or, perhaps, to express the - matter more accurately, owing to the constant flow of water from the - equatorial regions down the slope, the weight of the polar column is - kept always in excess of that of the equatorial; therefore the polar - column in the effort to restore equilibrium is kept in a constant state - of descent. Hence he terms it a “vertical” circulation. The following - will show Dr. Carpenter’s theory in his own words:—</p> - - <p>“The action of cold on the surface water of each polar area will be - exerted as follows:—</p> - - <p>“(<i>a</i>) In diminishing the height of the polar column as compared with - that of the equatorial, so that a lowering of its <em>level</em> is produced, - which can only be made good by a surface-flow from the latter towards - the former.</p> - - <p>“(<i>b</i>) In producing an excess in the downward <em>pressure</em> of the<span class="pagenum" id="Page_154">154</span> column - when this inflow has restored its level, in virtue of the increase of - specific gravity it has gained by its reduction in volume; whereby a - portion of its heavy bottom-water is displaced laterally, causing a - further reduction of level, which draws in a further supply of the - warmer and lighter water flowing towards its surface.</p> - - <p>“(<i>c</i>) In imparting a downward <em>movement</em> to each new surface-stratum - as its temperature undergoes reduction; so that the <em>entire column</em> may - be said to be in a state of constant descent, like that which exists in - the water of a tall jar when an opening is made at its bottom, and the - water which flows away through it is replaced by an equivalent supply - poured into the top of the jar” (§ 23).</p> - - <p>But if this be his theory, as it evidently is, then the 4 foot-pounds - (the amount of work performed by the descent of the water down the - slope) comprehends all the work that gravitation can perform on a pound - of water in making a complete circuit from the equator to the pole and - from the pole back to the equator.</p> - - <p>This, I trust, will be evident from the following considerations. When - a pound of water has flowed down from the equator to the pole, it has - descended 4 feet, and is then at the foot of the slope. Gravity has - therefore no more power to pull it down to a lower level. It will not - sink through the polar water, for it is not denser than the water - beneath on which it rests. But it may be replied that although it will - not sink through the polar water, it has nevertheless made the polar - column heavier than the equatorial, and this excess of pressure forces - a pound of water out from beneath and allows the column to descend. - Suppose it may be argued that a quantity of water flows down from the - equator, so as to raise the level of the polar water by, say, one foot. - The polar column will now be rendered heavier than the equatorial by - the weight of one foot of water. The pressure of the one foot will - thus force a quantity of water laterally from the bottom and cause the - entire column to descend till the level of equilibrium is restored. <span class="pagenum" id="Page_155">155</span>In - other words, the polar column will sink one foot. Now in the sinking of - this column work is performed by gravity. A certain amount of work is - performed by gravity in causing the water to flow down the slope from - the equator to the pole, and, in addition to this, a certain amount is - performed by gravity in the vertical descent of the column.</p> - - <p>I freely admit this to be sound reasoning, and admit that so much is - due to the slope and so much to the vertical descent of the water. But - here we come to the most important point, viz., is there the full slope - of 4 feet and an additional vertical movement? Dr. Carpenter seems - to conclude that there is, and that this vertical force is something - in addition to the force which I derive from the slope. And here, I - venture to think, is a radical error into which he has fallen in regard - to the whole matter. Let it be observed that, when water circulates - from difference of specific gravity, this vertical movement is just as - real a part of the process as the flow down the slope; but the point - which I maintain is that <em>there is no additional power derived from - this vertical movement over and above what is derived from the full - slope</em>—or, in other words, that this <i lang="la">primum mobile</i>, which he says I - have overlooked, has in reality no existence.</p> - - <p>Perhaps the following diagram will help to make the point still - clearer:—</p> - - <div class="figcenter" id="i_155" > - <div class="caption">Fig. 1.</div> - <img src="images/i_155.jpg" width="600" height="233" alt="" /> - </div> - - <p>Let P (fig. 1) be the surface of the ocean at the pole, and E the - surface at the equator; P O a column of water at the pole, and E Q a - column at the equator. The two columns are of equal weight, and balance - each other; but as the polar water<span class="pagenum" id="Page_156">156</span> is colder, and consequently denser - than the equatorial, the polar column is shorter than the equatorial, - the difference in the length of the two columns being 4 feet. The - surface of the ocean at the equator E is 4 feet higher than the surface - of the ocean at the pole P; there is therefore a slope of 4 feet from E - to P. The molecules of water at E tend to flow down this slope towards - P. The amount of work performed by gravity in the descent of a pound of - water down this slope from E to P is therefore 4 foot-pounds.</p> - - <p>But of course there can be no permanent circulation while the full - slope remains. In order to have circulation the polar column must be - heavier than the equatorial. But any addition to the weight of the - polar column is at the expense of the slope. In proportion as the - weight of the polar column increases the less becomes the slope. This, - however, makes no difference in the amount of work performed by gravity.</p> - - <p>Suppose now that water has flowed down till an addition of one foot - of water is made to the polar column, and the difference of level, - of course, diminished by one foot. The surface of the ocean in this - case will now be represented by the dotted line P′ E, and the slope - reduced from 4 feet to 3 feet. Let us then suppose a pound of water to - leave E and flow down to P′; 3 foot-pounds will be the amount of work - performed. The polar column being now too heavy by the extent of the - mass of water P′ P one foot thick, its extra pressure causes a mass of - water equal to P′ P to flow off laterally from the bottom of the column. - The column therefore sinks down one foot till P′ reaches P. Now the - pound of water in this vertical descent from P′ to P has one foot-pound - of work performed on it by gravity; this added to the 3 foot-pounds - derived from the slope, gives a total of 4 foot-pounds in passing from - E to P′ and then from P′ to P. This is the same amount of work that - would have been performed had it descended directly from E to P. In - like manner it can be proved that 4 foot-pounds is the amount of work - performed in the descent of every pound of water of the mass P′ P. The - first pound which left E flowed<span class="pagenum" id="Page_157">157</span> down the slope directly to P, and - performed 4 foot-pounds of work. The last pound flowed down the slope E - P′, and performed only 3 foot-pounds; but in descending from P′ to P it - performed the other one foot-pound. A pound leaving at a period exactly - intermediate between the two flowed down 3½ feet of slope and descended - vertically half a foot. Whatever path a pound of water might take, by - the time that it reached P, 4 foot-pounds of work would be performed. - But no further work can be performed after it reaches P.</p> - - <p>But some will ask, in regard to the vertical movement, is it only in - the descent of the water from P′ to P that work is performed? Water - cannot descend from P′ to P, it will be urged, unless the entire column - P O underneath descend also. But the column P O descends by means of - gravity. Why, then, it will be asked, is not the descent of the column - a motive power as real as the descent of the mass of water P′ P?</p> - - <p>That neither force nor energy can be derived from the mere descent of - the polar column P O is demonstrable thus:—The reason why the column P - O descends is because, in consequence of the mass of water P′ P resting - on it, its weight is in excess of the equatorial column E Q. But the - force with which the column descends is equal, not to the weight of - the column, but to the weight of the mass P′ P; consequently as much - work would be performed by gravity in the descent of the mass P′ P (the - one foot of water) alone as in the descent of the entire column P′ O, - 10,000 feet in height. Suppose a ton weight is placed in each scale of - a balance: the two scales balance each other. Place a pound weight in - one of the scales along with the ton weight and the scale will descend. - But it descends, not with the pressure of a ton and a pound, but with - the pressure of the pound weight only. In the descent of the scale, - say, one foot, gravity can perform only one foot-pound of work. In like - manner, in the descent of the polar column, the only work available is - the work of the mass P′ P laid on the top of the column. But it must be - observed that in the descent of the column from P′ to P, a distance of - one foot, each pound of<span class="pagenum" id="Page_158">158</span> water of the mass P′ P does not perform one - foot-pound of work; for the moment that a molecule of water reaches P, - it then ceases to perform further work. The molecules at the surface P′ - descend one foot before reaching P; the molecules midway between P′ and - P descend only half a foot before reaching P, and the molecules at the - bottom of the mass are already at P, and therefore cannot perform any - work. The mean distance through which the entire mass performs work is - therefore half a foot. One foot-pound per pound of water represents in - this case the amount of work derived from the vertical movement.</p> - - <p>That such is the case is further evident from the following - considerations. Before the polar column begins to descend, it is - heavier than the equatorial by the weight of one foot of water; but - when the column has descended half a foot, the polar column is heavier - than the equatorial by the weight of only half a foot of water; and, - as the column continues to descend, the force with which it descends - continues to diminish, and when it has sunk to P the force is zero. - Consequently the mean pressure or weight with which the one foot of - water P′ P descended was equal to that of a layer of half a foot of - water; in other words, each pound of water, taking the mass as a whole, - descended with the pressure or weight of half a pound. But a half - pound descending one foot performs half a foot-pound; so that whether - we consider the <em>full pressure acting through the mean distance, or - the mean pressure acting through the full distance, we get the same - result</em>, viz. a half foot-pound as the work of vertical descent.</p> - - <p>Now it will be found, as we shall presently see, that if we calculate - the mean amount of work performed in descending the slope from the - equator to the pole, 3½ foot-pounds per pound of water is the amount. - The water at the bottom of the mass P P′ moved, of course, down the - full slope E P 4 feet. The water at the top of the mass which descended - from E to P′ descended a slope of only 3 feet. The mean descent of the - whole mass is therefore 3½ feet. And this gives 3½ foot-pounds<span class="pagenum" id="Page_159">159</span> as the - mean amount of work per pound of water in descending the slope; this, - added to the half foot-pound derived from vertical descent, gives 4 - foot-pounds as the total amount of work per pound of the mass.</p> - - <p>I have in the above reasoning supposed one foot of water accumulated - on the polar column before any vertical descent takes place. It is - needless to remark that the same conclusion would have been arrived - at, viz., that the total amount of work performed is 4 foot-pounds per - pound of water, supposing we had considered 2 feet, or 3 feet, or even - 4 feet of water to have accumulated on the polar column before vertical - motion took place.</p> - - <p>I have also, in agreement with Dr. Carpenter’s mode of representing - the operation, been considering the two effects, viz., the flowing of - the water down the slope and the vertical descent of the polar column - as taking place alternately. In nature, however, the two effects take - place simultaneously; but it is needless to add that the amount of work - performed would be the same whether the effects took place alternately - or simultaneously.</p> - - <p>I have also represented the level of the ocean at the equator as - remaining permanent while the alterations of level were taking place at - the pole. But in representing the operation as it would actually take - place in nature, we should consider the equatorial column to be lowered - as the polar one is being raised. We should, for example, consider the - one foot of water P′ P put upon the polar column as so much taken off - the equatorial column. But in viewing the problem thus we arrive at - exactly the same results as before.</p> - - <p>Let P (Fig. 2), as in Fig. 1, be the surface of the ocean at the pole, - and E the surface at the equator, there being a slope of 4 feet from E - to P. Suppose now a quantity of water, E E′, say, one foot thick, to - flow from off the equatorial regions down upon the polar. It will thus - lower the level of the equatorial column by one foot, and raise the - level of the polar column by the same amount. I may, however, observe - that the one foot<span class="pagenum" id="Page_160">160</span> of water in passing from E to P would have its - temperature reduced from 80° to 32°, and this would produce a slight - contraction. But as the weight of the mass would not be affected, in - order to simplify our reasoning we may leave this contraction out of - consideration. Any one can easily satisfy himself that the assumption - that E E′ is equal to P′ P does not in any way affect the question at - issue—the only effect of the contraction being to <em>increase</em> by an - infinitesimal amount the work done in descending the slope, and to - <em>diminish</em> by an equally infinitesimal amount the work done in the - vertical descent. If, for example, 3 foot-pounds represent the amount - of work performed in descending the slope, and one foot-pound the - amount performed in the vertical descent, on the supposition that E′ E - does not contract in passing to the pole, then 3·0024 foot-pounds will - represent the work of the slope, and 0·9976 foot-pounds the work of - vertical descent when allowance is made for the contraction. But the - total amount of work performed is the same in both cases. Consequently, - to simplify our reasoning, we may be allowed to assume P′ P to be equal - to E E′.</p> - - <div class="figcenter" id="i_160" > - <div class="caption">Fig. 2.</div> - <img src="images/i_160.jpg" width="600" height="234" alt="" /> - </div> - - <p>The slope E P being 4 feet, the slope E′ P′ is consequently 2 feet; - the mean slope for the entire mass is therefore 3 feet. The mean - amount of work performed by the descent of the mass will of course - be 3 foot-pounds per pound of water. The amount of work performed by - the vertical descent of P′ P ought therefore to be one foot-pound per - pound. That this is the amount will be evident thus:—The transference - of the one<span class="pagenum" id="Page_161">161</span> foot of water from the equatorial column to the polar - disturbs the equilibrium by making the equatorial column too light by - one foot of water and the polar column too heavy by the same amount of - water. The polar column will therefore tend to sink, and the equatorial - to rise till equilibrium is restored. The difference of weight of - the two columns being equal to 2 feet of water, the polar column - will begin to descend with a pressure of 2 feet of water; and the - equatorial column will begin to rise with an equal amount of pressure. - When the polar column has descended half a foot the equatorial column - will have risen half a foot. The pressure of the descending polar - column will now be reduced to one foot of water. And when the polar - column has descended another foot, P′ will have reached P, and E′ - will have reached E; the two columns will then be in equilibrium. It - therefore follows that the mean pressure with which the polar column - descended the one foot was equal to the pressure of one foot of water. - Consequently the mean amount of work performed by the descent of the - mass was equal to one foot-pound per pound of water; this, added to the - 3 foot-pounds derived from the slope, gives a total of 4 foot-pounds.</p> - - <p>In whatever way we view the question, we are led to the conclusion that - if 4 feet represent the amount of slope between the equatorial and - polar columns when the two are in equilibrium, then 4 foot-pounds is - the total amount of work that gravity can perform upon a pound of water - in overcoming the resistance to motion in its passage from the equator - to the pole down the slope, and then in its vertical descent to the - bottom of the ocean.</p> - - <p>But it will be replied, not only does the one foot of water P′ P - descend, but the entire column P O, 10,000 feet in length, descends - also. What, then, it will be asked, becomes of the force which gravity - exerts in the descent of this column? We shall shortly see that this - force is entirely applied in work against gravity in other parts of - the circuit; so that not a single foot-pound of this force goes to - overcome cohesion,<span class="pagenum" id="Page_162">162</span> friction, and other resistances; it is all spent in - counteracting the efforts which gravity exerts to stop the current in - another part of the circuit.</p> - - <p>I shall now consider the next part of the movement, viz., the under - or return current from the bottom of the polar to the bottom of the - equatorial column. What produces this current? It is needless to say - that it cannot be caused directly by gravity. Gravitation cannot - directly draw any body horizontally along the earth’s surface. The - water that forms this current is pressed out laterally by the weight - of the polar column, and flows, or rather is pushed, towards the - equator to supply the vacancy caused by the ascent of the equatorial - column. There is a constant flow of water from the equator to the poles - along the surface, and this draining of the water from the equator is - supplied by the under or return current from the poles. But the only - power which can impel the water from the bottom of the polar column - to the bottom of the equatorial column is the pressure of the polar - column. But whence does the polar column derive its pressure? It can - only press to the extent that its weight exceeds that of the equatorial - column. That which exerts the pressure is therefore the mass of water - which has flowed down the slope from the equator upon the polar column. - It is in this case the vertical movement that causes this under - current. The energy which produces this current must consequently be - derived from the 4 foot-pounds resulting from the slope; for the energy - of the vertical movement, as has already been proved, is derived from - this source; or, in other words, whatever power this vertical movement - may exert is so much deducted from the 4 foot-pounds derived from the - full slope.</p> - - <p>Let us now consider the fourth and last movement, viz., the ascent of - the under current to the surface of the ocean at the equator. When - this cold under current reaches the equatorial regions, it ascends - to the surface to the point whence it originally started on its - circuit. What, then, lifts the water from the bottom of the equatorial - column to its top? This cannot<span class="pagenum" id="Page_163">163</span> be done directly, either by heat or - by gravity. When heat, for example, is applied to the bottom of a - vessel, the heated water at the bottom expands and, becoming lighter - than the water above, rises through it to the surface; but if the - heat be applied to the surface of the water instead of to the bottom, - the heat will not produce an ascending current. It will tend rather - to prevent such a current than to produce one—the reason being that - each successive layer of water will, on account of the heat applied, - become hotter and consequently lighter than the layer below it, and - colder and consequently heavier than the layer above it. It therefore - cannot ascend, because it is too heavy; nor can it descend, because - it is too light. But the sea in equatorial regions is heated from - above, and not from below; consequently the water at the bottom does - not rise to the surface at the equator in virtue of any heat which it - receives. A layer of water can never raise the temperature of a layer - below it to a higher temperature than itself; and since it cannot do - this, it cannot make the layer under it lighter than itself. That which - raises the water at the equator, according to Dr. Carpenter’s theory, - must be the downward pressure of the polar column. When water flows - down the slope from the equator to the pole, the polar column, as we - have seen, becomes too heavy and the equatorial column too light; - the former then sinks and the latter rises. It is the sinking of the - polar column which raises the equatorial one. When the polar column - descends, as much water is pressed in underneath the equatorial column - as is pressed from underneath the polar column. If one foot of water - is pressed from under the polar column, a foot of water is pressed in - under the equatorial column. Thus, when the polar column sinks a foot, - the equatorial column rises to the same extent. The equatorial water - continuing to flow down the slope, the polar column descends: a foot - of water is again pressed from underneath the polar column and a foot - pressed in under the equatorial. As foot after foot is thus removed - from the bottom of the polar column while it sinks, foot after foot is - pushed in under<span class="pagenum" id="Page_164">164</span> the equatorial column while it rises; so by this means - the water at the surface of the ocean in polar regions descends to - the bottom, and the water at the bottom in equatorial regions ascends - to the surface—the effect of solar heat and polar cold continuing, of - course, to maintain the surface of the ocean in equatorial regions at a - higher level than at the poles, and thus keeping up a constant state of - disturbed equilibrium. Or, to state the matter in Dr. Carpenter’s own - words, “The cold and dense polar water, as it flows in at the bottom of - the equatorial column, will not directly take the place of that which - has been drafted off from the surface; but this place will be filled - by the rising of the whole superincumbent column, which, being warmer, - is also lighter than the cold stratum beneath. Every new arrival from - the poles will take its place below that which precedes it, since its - temperature will have been less affected by contact with the warmer - water above it. In this way an ascending movement will be imparted to - the whole equatorial column, and in due course every portion of it will - come under the influence of the surface-heat of the sun.”<a id="FNanchor_73" href="#Footnote_73" class="fnanchor">[73]</a></p> - - <p>But the agency which raises up the water of the under current to the - surface is the pressure of the polar column. The equatorial column - cannot rise directly by means of gravity. Gravity, instead of raising - the column, exerts all its powers to prevent its rising. Gravity - here is a force acting against the current. It is the descent of - the polar column, as has been stated, that raises the equatorial - column. Consequently the entire amount of work performed by gravity - in pulling down the polar column is spent in raising the equatorial - column. Gravity performs exactly as much work in preventing motion - in the equatorial column as it performs in producing motion in the - polar column; so that, so far as the vertical parts of Dr. Carpenter’s - circulation are concerned, gravity may be said neither to produce - motion nor to prevent it. And this remark, be it observed, applies not - only to P O and E Q, but also to the parts P′ P and E E′ of the two - columns. When a mass of <span class="pagenum" id="Page_165">165</span>water E E′, say one foot deep, is removed off - the equatorial column and placed upon the polar column, the latter - column is then heavier than the former by the weight of two feet of - water. Gravity then exerts more force in pulling the polar column down - than it does in preventing the equatorial column from rising; and - the consequence is that the polar column begins to descend and the - equatorial column to rise. But as the polar column continues to descend - and the equatorial to rise, the power of gravity to produce motion in - the polar column diminishes, and the power of gravity to prevent motion - in the equatorial column increases; and when P′ descends to P and E′ - rises to E, the power of gravity to prevent motion in the equatorial - column is exactly equal to the power of gravity to produce motion in - the polar column, and consequently motion ceases. It therefore follows - that the entire amount of work performed by the descent of P′ P is - spent in raising E′ E against gravity.</p> - - <p>It follows also that inequalities in the sea-bottom cannot in any - way aid the circulation; for although the cold under current should - in its progress come to a deep trough filled with water less dense - than itself, it would no doubt sink to the bottom of the hollow; yet - before it could get out again as much work would have to be performed - against gravity as was performed by gravity in sinking it. But whilst - inequalities in the bed of the ocean would not aid the current, they - would nevertheless very considerably retard it by the obstructions - which they would offer to the motion of the water.</p> - - <p>We have been assuming that the weight of P′ P is equal to that of E E′; - but the mass P′ P must be greater than E E′ because P′ P has not only - to raise E E′, but to impel the under current—to push the water along - the sea-bottom from the pole to the equator. So we must have a mass of - water, in addition to P′ P, placed on the polar column to enable it to - produce the under current in addition to the raising of the equatorial - column.</p> - - <p>It follows also that the amount of work which can be performed by - gravity depends entirely on the <em>difference</em> of temperature <span class="pagenum" id="Page_166">166</span>between - the equatorial and the polar waters, and is wholly independent of the - way in which the temperature may decrease from the equator to the - poles. Suppose, in agreement with Dr. Carpenter’s idea,<a id="FNanchor_74" href="#Footnote_74" class="fnanchor">[74]</a> that the - equatorial heat and polar cold should be confined to limited areas, and - that through the intermediate space no great difference of temperature - should prevail. Such an arrangement as this would not increase the - amount of work which gravity could perform; it would simply make the - slope steeper at the two extremes and flatter in the intervening space. - It would no doubt aid the surface-flow of the water near the equator - and the poles, but it would retard in a corresponding degree the flow - of the water in the intermediate regions. In short, it would merely - destroy the uniformity of the slope without aiding in the least degree - the general motion of the water.</p> - - <p>It is therefore demonstrable that <em>the energy derived from the full - slope, whatever that slope may be, comprehends all that can possibly be - obtained from gravity</em>.</p> - - <p>It cannot be urged as an objection to what has been advanced that I - have determined simply the amount of the force acting on the water at - the surface of the ocean and not that on the water at all depths—that I - have estimated the amount of work which gravity can perform on a given - quantity of water at the surface, but not the total amount of work - which gravity can perform on the entire ocean. This objection will not - stand, because it is at the surface of the ocean where the greatest - difference of temperature, and consequently of density, exists between - the equatorial and polar waters, and therefore there that gravity - exerts its greatest force. And if gravity be unable to move the water - at the surface, it is much less able to do so under the surface. So - far as the question at issue is concerned, any calculations as to the - amount of force exerted by gravity at various depths are needless.</p> - - <p>It is maintained also that the winds cannot produce a vertical current - except under some very peculiar conditions. We have <span class="pagenum" id="Page_167">167</span>already seen that, - according to Dr. Carpenter’s theory, the vertical motion is caused - by the water flowing off the equatorial column, down the slope, upon - the polar column, thus destroying the equilibrium between the two by - diminishing the weight of the equatorial column and increasing that of - the polar column. In order that equilibrium may be restored, the polar - column sinks and the equatorial one rises. Now must not the same effect - occur, supposing the water to be transferred from the one column to - the other, by the influence of the winds instead of by the influence - of gravity? The vertical descent and ascent of these columns depend - entirely upon the difference in their weights, and not upon the nature - of the agency which makes this difference. So far as difference of - weight is concerned, 2 feet of water, propelled down the slope from the - equatorial column to the polar by the winds, will produce just the same - effect as though it had been propelled by gravity. If vertical motion - follows as a necessary consequence from a transference of water from - the equator to the poles by gravity, it follows equally as a necessary - consequence from the same transference by the winds; so that one is not - at liberty to advocate a vertical circulation in the one case and to - deny it in the other.</p> - - <p><em>Gravitation Theory of the Gibraltar Current.</em>—If difference of - specific gravity fails to account for the currents of the ocean in - general, it certainly fails in a still more decided manner to account - for the Gibraltar current. The existence of the submarine ridge - between Capes Trafalgar and Spartel, as was shown in the Phil. Mag. - for October, 1871, p. 269, affects currents resulting from difference - of specific gravity in a manner which does not seem to have suggested - itself to Dr. Carpenter. The pressure of water and other fluids is - not like that of a solid—not like that of the weight in the scale of - a balance, simply a downward pressure. Fluids press downwards like - the solids, but they also press laterally. The pressure of water is - hydrostatic. If we fill a basin with water or any other fluid, the - fluid remains in perfect equilibrium, provided the sides of the<span class="pagenum" id="Page_168">168</span> basin - be sufficiently strong to resist the pressure. The Mediterranean and - Atlantic, up to the level of the submarine ridge referred to, may be - regarded as huge basins, the sides of which are sufficiently strong to - resist all pressure. It follows that, however much denser the water - of the Mediterranean may be than that of the Atlantic, it is only the - water above the level of the ridge that can possibly exercise any - influence in the way of disturbing equilibrium, so as to cause the - level of the Mediterranean to stand lower than that of the Atlantic. - The water of the Atlantic below the level of this ridge might be as - light as air, and that of the Mediterranean as heavy as molten lead, - but this could produce no disturbance of equilibrium; and if there be - no difference of density between the Atlantic and the Mediterranean - waters from the surface down to the level of the top of the ridge, then - there can be nothing to produce the circulation which Dr. Carpenter - infers. Suppose both basins empty, and dense water to be poured into - the Mediterranean, and water less dense into the Atlantic, until they - are both filled up to the level of the ridge, it is evident that the - heavier water in the one basin can exercise no influence in raising - the level of the lighter water in the other basin, the entire pressure - being borne by the sides of the basins. But if we continue to pour in - water till the surface is raised, say one foot, above the level of the - ridge, then there is nothing to resist the lateral pressure of this one - foot of water in the Mediterranean but the counter pressure of the one - foot in the Atlantic. But as the Mediterranean water is denser than the - Atlantic, this one foot of water will consequently exert more pressure - than the one foot of water of the Atlantic. We must therefore continue - to pour more water into the Atlantic until its lateral pressure equals - that of the Mediterranean. The two seas will then be in equilibrium, - but the surface of the Atlantic will of course be at a higher level - than the surface of the Mediterranean. The difference of level will be - proportionate to the difference in density of the waters of the two - seas. But here we come to the point of importance. In determining the - difference of level between<span class="pagenum" id="Page_169">169</span> the two seas, or, which is the same thing, - the difference of level between a column of the Atlantic and a column - of the Mediterranean, we must take into consideration <em>only the water - which lies above the level of the ridge</em>. If there be one foot of water - above the ridge, then there is a difference of level proportionate to - the difference of pressure between the one foot of water of the two - seas. If there be 2 feet, 3 feet, or any number of feet of water above - the level of the ridge, the difference of level is proportionate to - the 2 feet, 3 feet, or whatever number of feet there may be of water - above the ridge. If, for example, 13 should represent the density of - the Mediterranean water and 12 the density of the Atlantic water, then - if there were one foot of water in the Mediterranean above the level of - the ridge, there would require to be one foot one inch of water in the - Atlantic above the ridge in order that the two might be in equilibrium. - The difference of level would therefore be one inch. If there were 2 - feet of water, the difference of level would be 2 inches; if 3 feet, - the difference would be 3 inches, and so on. And this would follow, - no matter what the actual depth of the two basins might be; the water - below the level of the ridge exercising no influence whatever on the - level of the surface.</p> - - <p>Taking Dr. Carpenter’s own data as to the density of the Mediterranean - and Atlantic waters, what, then, is the difference of density? The - submarine ridge comes to within 167 fathoms of the surface; say, in - round numbers, to within 1,000 feet. What are the densities of the two - basins down to the depth of 1,000 feet? According to Dr. Carpenter - there is little, if any, difference. His own words on this point are - these:—“A comparison of these results leaves no doubt that there is - an excess of salinity in the water of the Mediterranean above that of - the Atlantic; but that this excess <em>is</em> slight in the surface-water, - whilst somewhat greater in the deeper water” (§ 7). “Again, it was - found by examining samples of water taken from the surface, from 100 - fathoms, from 250 fathoms, and from 400 fathoms respectively, that - whilst the <em>first two</em> had the<span class="pagenum" id="Page_170">170</span> <em>characteristic temperature and density - of Atlantic water</em>, the last two had the characteristics and density of - Mediterranean water” (§ 13). Here, at least to the depth of 100 fathoms - or 600 feet, there is little difference of density between the waters - of the two basins. Consequently down to the depth of 600 feet, there is - nothing to produce any sensible disturbance of equilibrium. If there - be any sensible disturbance of equilibrium, it must be in consequence - of difference of density which may exist between the depths of 600 - feet and the surface of the ridge. We have nothing to do with any - difference which may exist between the water of the Mediterranean and - the Atlantic below the ridge; the water in the Mediterranean basin may - be as heavy as mercury below 1,000 feet: but this can have no effect - in disturbing equilibrium. The water to the depth of 600 feet being of - the same density in both seas, the length of the two columns acting on - each other is therefore reduced to 400 feet—that is, to that stratum of - water lying at a depth of from 600 to the surface of the ridge 1,000 - feet below the surface. But, to give the theory full justice, we shall - take the Mediterranean stratum at the density of the deep water of - the Mediterranean, which he found to be about 1·029, and the density - of the Atlantic stratum at 1·026. The difference of density between - the two columns is therefore ·003. Consequently, if the height of the - Mediterranean column be 400 feet, it will be balanced by the Atlantic - column of 401·2 feet; the difference of level between the Mediterranean - and the Atlantic cannot therefore be more than 1·2 foot. The amount - of work that can be performed by gravity in the case of the Gibraltar - current is little more than one foot-pound per pound of water, an - amount of energy evidently inadequate to produce the current.</p> - - <p>It is true that in his last expedition Dr. Carpenter found the - bottom-water on the ridge somewhat denser than Atlantic water at the - same depth, the former being 1·0292 and the latter 1·0265; but it - also proved to be denser than Mediterranean water at the same depth. - He found, for example, that “the dense Mediterranean water lies about - 100 fathoms nearer the<span class="pagenum" id="Page_171">171</span> surface over a 300-fathoms bottom, than it - does where the bottom sinks to more than 500 fathoms” (§ 51). But any - excess of density which might exist at the ridge could have no tendency - whatever to make the Mediterranean column preponderate over the - Atlantic column, any more than could a weight placed over the fulcrum - of a balance have a tendency to make the one scale weigh down the other.</p> - - <p>If the objection referred to be sound, it shows the mechanical - impossibility of the theory. It proves that whether there be an under - current or not, or whether the dense water lying in the deep trough of - the Mediterranean be carried over the submarine ridge into the Atlantic - or not, the explanation offered by Dr. Carpenter is one which cannot be - admitted. It is incumbent on him to explain either (1) how the almost - infinitesimal difference of density which exists between the Atlantic - and Mediterranean columns down to the level of the ridge can produce - the upper and under currents carrying the deep and dense water of - the Mediterranean over the ridge, or (2) how all this can be done by - means of the difference of density which exists below the level of the - ridge.<a id="FNanchor_75" href="#Footnote_75" class="fnanchor">[75]</a> What the true cause of the Gibraltar current really is will - be considered in Chap. XIII.</p> - - <p><em>The Baltic Current.</em>—The entrance to the Baltic Sea is in some - places not over 50 or 60 feet deep. It follows, therefore, from what - has already been proved in regard to the Gibraltar current, that the - influence of gravity must be even still less in causing a current in - the Baltic strait than in the Gibraltar strait.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_X"> - <span class="pagenum" id="Page_172">172</span> - <h2> - CHAPTER X.<br /><br /> - <span class="small">EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.—DR. - CARPENTER’S THEORY.—OBJECTIONS CONSIDERED.</span> - </h2> - </div> - <div class="subhead"><i lang="la">Modus Operandi</i> of the Matter.—Polar Cold considered by Dr. - Carpenter the <i lang="la">Primum Mobile</i>.—Supposed Influence of Heat - derived from the Earth’s Crust.—Circulation without Difference - of Level.—A Confusion of Ideas in Reference to the supposed - Agency of Polar Cold.—M. Dubuat’s Experiments.—A Begging of the - Question at Issue.—Pressure as a Cause of Circulation.</div> - - <p class="noindent"><span class="smcap">In</span> the foregoing chapter, the substance of which appeared in the - Phil. Mag. for October, 1871, I have represented the manner in which - difference of specific gravity produces circulation. But Dr. Carpenter - appears to think that there are some important points which I have - overlooked. These I shall now proceed to consider in detail.</p> - - <p>“Mr. Croll’s whole manner of treating the subject,” he says, “is so - different from that which it appears to me to require, and he has so - completely misapprehended my own view of the question, that I feel it - requisite to present this in fuller detail in order that physicists and - mathematicians, having both sides fully before them, may judge between - us” (§ 26).<a id="FNanchor_76" href="#Footnote_76" class="fnanchor">[76]</a></p> - - <p>He then refers to a point so obvious as hardly to require - consideration, viz., the effect which results when the surface of - the entire area of a lake or pond of water is cooled. The whole of - the surface-film, being chilled at the same time, sinks through the - subjacent water, and a new film from the warmer layer immediately - beneath the surface rises into its place. This being cooled in its - turn, sinks, and so on. He next considers <span class="pagenum" id="Page_173">173</span>what takes place when only - a portion of the surface of the pond is cooled, and shows that in this - case the surface-film which descends is replaced not from beneath, but - by an inflow from the neighbouring area.</p> - - <p>“That such must be the case,” says Dr. Carpenter, “appears to me so - self-evident that I am surprised that any person conversant with the - principles of physical science should hesitate in admitting it, still - more that he should explicitly deny it. But since others may feel the - same difficulty as Mr. Croll, it may be worth while for me to present - the case in a form of yet more elementary simplicity” (§ 29).</p> - - <p>Then, in order to show the mode in which the general oceanic - circulation takes place, he supposes two cylindrical vessels, W and - C, of equal size, to be filled with sea-water. Cylinder W represents - the equatorial column, and the water contained in it has its - temperature maintained at 60°; whilst the water in the other cylinder - C, representing the polar column, has its temperature maintained at - 30° by means of the constant application of cold at the top. Free - communication is maintained between the two cylinders at top and - bottom; and the water in the cold cylinder being, in virtue of its - low temperature, denser than the water in the warm cylinder, the two - columns are therefore not in static equilibrium. The cold, and hence - heavier column tends to produce an outflow of water from its bottom to - the bottom of the warm column, which outflow is replaced by an inflow - from the top of the warm column to the top of the cold column. In fact, - we have just a simple repetition of what he has given over and over - again in his various memoirs on the subject. But why so repeatedly - enter into the <i lang="la">modus operandi</i> of the matter? Who feels any difficulty - in understanding how the circulation is produced?</p> - - <p><em>Polar Cold considered by Dr. Carpenter the Primum Mobile.</em>—It is - evident that Dr. Carpenter believes that he has found in polar <em>cold</em> - an agency the potency of which, in producing a general oceanic - circulation, has been overlooked by physicists; and it is with the view - of developing his ideas on this subject<span class="pagenum" id="Page_174">174</span> that he has entered so fully - and so frequently into the exposition of his theory. “If I have myself - done anything,” he says, “to strengthen the doctrine, it has been by - showing that polar cold, rather than equatorial heat, is the <i lang="la">primum - mobile</i> of this circulation.”<a id="FNanchor_77" href="#Footnote_77" class="fnanchor">[77]</a></p> - - <p>The influence of the sun in heating the waters of the inter-tropical - seas is, in Dr. Carpenter’s manner of viewing the problem, of no - great importance. The efficient cause of motion he considers resides - in <em>cold</em> rather than in <em>heat</em>. In fact, he even goes the length - of maintaining that, as a power in the production of the general - interchange of equatorial and polar water, the effect of polar cold is - so much superior to that of inter-tropical heat, that the influence of - the latter may be <em>practically disregarded</em>.</p> - - <p>“Suppose two basins of ocean-water,” he says, “connected by a strait to - be placed under such different climatic conditions that the surface of - one is exposed to the heating influence of tropical sunshine, whilst - the surface of the other is subjected to the extreme cold of the - sunless polar winter. The effect of the surface-heat upon the water - of the tropical basin will be for the most part limited (as I shall - presently show) to its uppermost stratum, and may here be <em>practically - disregarded</em>.”<a id="FNanchor_78" href="#Footnote_78" class="fnanchor">[78]</a></p> - - <p>Dr. Carpenter’s idea regarding the efficiency of cold in producing - motion seems to me to be not only opposed to the generally received - views on the subject, but wholly irreconcilable with the ordinary - principles of mechanics. In fact, there are so many points on which - Dr. Carpenter’s theory of a “General <em>Vertical</em> Oceanic Circulation” - differs from the generally received views on the subject of circulation - by means of difference of specific gravity, that I have thought it - advisable to enter somewhat minutely into the consideration of the - mechanics of that theory, the more so as he has so repeatedly asserted - that eminent physicists agree with what he has advanced on the subject.</p> - - <p>According to the generally received theory, the circulation <span class="pagenum" id="Page_175">175</span>is due to - the <em>difference of density</em> between the sea in equatorial and polar - regions. The real efficient cause is gravity; but gravity cannot act - when there is no difference of specific gravity. If the sea were of - equal density from the poles to the equator, gravity could exercise no - influence in the production of circulation; and the influence which it - does possess is in proportion to the difference of density. But the - difference of density between equatorial and polar waters is in turn - due not absolutely either to polar cold or to tropical heat, but to - both—or, in other words, to the <em>difference</em> of temperature between - the polar and equatorial seas. This difference, in the very nature of - things, must be as much the result of equatorial heat as of polar cold. - If the sea in equatorial regions were not being heated by the sun as - rapidly as the sea in polar regions is being cooled, the difference of - temperature between them, and consequently the difference of density, - would be diminishing, and in course of time would disappear altogether. - As has already been shown, it is a necessary consequence that the - water flowing from equatorial to polar regions must be compensated by - an equal amount flowing from polar to equatorial regions. Now, if the - water flowing from polar to equatorial regions were not being heated - as rapidly as the water flowing from equatorial to polar regions is - being cooled, the equatorial seas would gradually become colder and - colder until no sensible difference of temperature existed between - them and the polar oceans. In fact, <em>equality of the two rates</em> is - necessary to the very existence of such a general circulation as that - advocated by Dr. Carpenter. If he admits that the general interchange - of equatorial and polar water advocated by him is caused by the - difference of density between the water at the equator and the poles, - resulting from difference of temperature, then he must admit also that - this difference of density is just as much due to the heating of the - equatorial water by the sun as it is to the cooling of the polar water - by radiation and other means—or, in other words, that it is as much due - to equatorial heat as to polar cold. And if so, it cannot be true that - polar cold rather<span class="pagenum" id="Page_176">176</span> than equatorial heat is the “<i lang="la">primum mobile</i>” of - this circulation; and far less can it be true that the heating of the - equatorial water by the sun is of so little importance that it may be - “practically disregarded.”</p> - - <p><em>Supposed Influence of Heat derived from the Earth’s Crust.</em>—There is, - according to Dr. Carpenter, another agent concerned in the production - of the general oceanic circulation, viz., the heat derived by the - bottom of the ocean from the crust of the earth.<a id="FNanchor_79" href="#Footnote_79" class="fnanchor">[79]</a> We have no reason - to believe that the quantity of internal heat coming through the - earth’s crust is greater in one part of the globe than in another; nor - have we any grounds for concluding that the bottom of inter-tropical - seas receives more heat from the earth’s crust than the bottom of those - in polar regions. But if the polar seas receive as much heat from this - source as the seas within the tropics, then the difference of density - between the two cannot possibly be due to heat received from the - earth’s crust; and this being so, it is mechanically impossible that - internal heat can be a cause in the production of the general oceanic - circulation.</p> - - <p><em>Circulation without Difference of Level.</em>—There is another part of - the theory which appears to me irreconcilable with mechanics. It is - maintained that this general circulation takes place without any - difference of level between the equator and the poles. Referring to the - case of the two cylinders W and C, which represent the equatorial and - polar columns respectively, Dr. Carpenter says:—</p> - - <p>“The force which will thus lift up the entire column of water in W - is that which causes the descent of the entire column in C, namely, - the excess of gravity constantly acting in C,—the levels of the - two columns, and consequently their heights, being maintained at a - <em>constant equality</em> by the free passage of surface-water from W to C.”</p> - - <p>“The whole of Mr. Croll’s discussion of this question, however,” he - continues, “proceeds upon the assumption that the levels of the polar - and equatorial columns are <em>not kept at an</em> <span class="pagenum" id="Page_177">177</span><em>equality</em>, &c.” (§ 30.) - And again, “Now, so far from asserting (as Captain Maury has done) that - the trifling difference of level arising from inequality of temperature - is adequate to the production of ocean-currents, I simply affirm that - as fast as the level is disturbed by change of temperature it will be - restored by gravity.” (§ 23.)<a id="FNanchor_80" href="#Footnote_80" class="fnanchor">[80]</a></p> - - <div class="figcenter" id="i_177" > - <div class="caption">Fig. 3.</div> - <img src="images/i_177.jpg" width="600" height="232" alt="" /> - </div> - - <p>In order to understand more clearly how the circulation under - consideration cannot take place without a difference of level, let W E - (Fig. 3) represent the equatorial column, and C P the polar column. The - equatorial column is warmer than the polar column because it receives - <em>more</em> heat from the sun than the latter; and the polar is colder - than the equatorial column because it receives <em>less</em>. The difference - in the density of the two columns results from their difference of - temperature; and the difference of temperature results in turn from the - difference in the quantity of heat received from the sun by each. Or, - to express the matter in other words, the difference of density (and - consequently the circulation under consideration) is due to the excess - of heat received from the sun by the equatorial over that received by - the polar column; so that to leave out of account the super-heating of - the inter-tropical waters by the sun is to leave out of account the - very thing of all others that is absolutely essential to the existence - of the circulation. The water being assumed to be the same in both - columns and differing only as regards temperature, and the equatorial - column possessing more heat than the polar, and being therefore less - <span class="pagenum" id="Page_178">178</span>dense than the latter, it follows, in order that the two columns may - be in static equilibrium, that the surface of the equatorial column - must stand at a higher level than that of the polar. This produces the - slope W C from the equator to the pole. The extent of the slope will of - course depend upon the extent of the difference of their temperatures. - But, as was shown on a former occasion,<a id="FNanchor_81" href="#Footnote_81" class="fnanchor">[81]</a> it is impossible that - static equilibrium can ever be fully obtained, because the slope - occasioned by the elevation of the equatorial column above the polar - produces what we may be allowed to call a <em>molecular</em> disturbance of - equilibrium. The surface of the ocean, or the molecules of water lying - on the slope, are not in a position of equilibrium, but tend, in virtue - of gravity, to roll down the slope in the direction of the polar column - C. It will be observed that the more we gain of static equilibrium - of the entire ocean the greater is the slope, and consequently the - greater is the disturbance of molecular equilibrium; and, <i lang="la">vice versâ</i>, - the more molecular equilibrium is restored by the reduction of the - slope, the greater is the disturbance of static equilibrium. <em>It is - therefore absolutely impossible that both conditions of equilibrium can - be fulfilled at the same time so long as a difference of temperature - exists between the two columns.</em> And this conclusion holds true even - though we should assume water to be a perfect fluid absolutely devoid - of viscosity. It follows, therefore, that a general oceanic circulation - without a difference of level is a <em>mechanical impossibility</em>.</p> - - <p>In a case of actual circulation due to difference of gravity, there - is always a constant disturbance of both <em>static</em> and molecular - equilibrium. Column C is always higher and column W always lower than - it ought to be were the two in equilibrium; but they never can be at - the same level.</p> - - <p>It is quite conceivable, of course, that the two conditions of - equilibrium may be fulfilled alternately. We can conceive column C - remaining stationary till the water flowing from column W has restored - the level. And after the level is restored <span class="pagenum" id="Page_179">179</span>we can conceive the polar - column C sinking and the equatorial column W rising till the two - perfectly balance each other. Such a mode of circulation, consisting - of an alternate surface-flow and vertical descent and ascent of the - columns, though conceivable, is in reality impossible in nature; for - there are no means by which the polar column C could be supported - from sinking till the level had been restored. But Dr. Carpenter does - not assume that the general oceanic circulation takes place in this - intermitting manner; according to him, the circulation is <em>constant</em>. - He asserts that there is a “<em>continual</em> transference of water from the - bottom of C to the bottom of W, and from the top of W to the top of C, - with a <em>constant</em> descending movement in C and a <em>constant</em> ascending - movement in W” (§ 29). But such a condition of things is irreconcilable - with the idea of “the levels of the two columns, and consequently their - heights, being maintained at a <em>constant</em> equality” (§ 29).</p> - - <p>Although Dr. Carpenter does not admit the existence of a permanent - difference of level between the equator and the pole, he nevertheless - speaks of a depression of level in the polar basin resulting from the - contraction by cooling of the water flowing into it. This reduction of - level induces an inflow of water from the surrounding area; “and since - what is drawn away,” to quote his own words, “is supplied from a yet - greater distance, the continued cooling of the surface-stratum in the - polar basin will cause a ‘set’ of waters towards it, to be propagated - backwards through the whole intervening ocean in communication with - it until it reaches the tropical area.” The slope produced between - the polar basin and the surrounding area, if sufficiently great, will - enable the water in the surrounding area to flow polewards; but unless - this slope extend to the equator, it will not enable the tropical - waters also to flow polewards. One of two things necessarily follows: - either the slope extends from the equator to the pole, or water can - flow from the equator to the pole without a slope. If Dr. Carpenter - maintains the former, he contradicts himself; and if he adopts the - latter, he contradicts an obvious principle of mechanics.</p> - - <p><span class="pagenum" id="Page_180">180</span></p> - - <p><em>A Confusion of Ideas in Reference to the supposed Agency of Polar - Cold.</em>—It seems to me that Dr. Carpenter has been somewhat misled by a - slight confusion of ideas in reference to the supposed agency of polar - cold. This is brought out forcibly in the following passage from his - memoir in the Proceedings of the Royal Geographical Society, vol. xv.</p> - - <p>“Mr. Croll, in arguing against the doctrine of a general oceanic - circulation sustained by difference of temperature, and <em>justly - maintaining</em> that such a circulation cannot be produced by the - application of heat at the surface, has entirely ignored the agency of - cold.”</p> - - <p>It is here supposed that there are two agents at work in the production - of the general oceanic circulation. The one agent is <em>heat</em>, acting - at the equatorial regions; and the other agent is <em>cold</em>, acting at - the polar regions. It is supposed that the agency of cold is far more - powerful than that of heat. In fact so trifling is the agency of - equatorial heat in comparison with that of polar cold that it may be - “practically disregarded”—left out of account altogether,—polar cold - being the <i lang="la">primum mobile</i> of the circulation. It is supposed also that - I have considered the efficiency of one of the agents, viz., heat, and - found it totally inadequate to produce the circulation in question; and - it is admitted also that my conclusions are perfectly correct. But then - I am supposed to have left out of account the other agent, viz., polar - cold, the only agent possessing real potency. Had I taken into account - polar cold, it is supposed that I should have found at once a cause - perfectly adequate to produce the required effect.</p> - - <p>This is a fair statement of Dr. Carpenter’s views on the subject; I am - unable, at least, to attach any other meaning to his words. And I have - no doubt they are also the views which have been adopted by those who - have accepted his theory.</p> - - <p>It must be sufficiently evident from what has already been stated, - that the notion of there being two separate agents at work producing - circulation, namely heat and cold, the one of which is assumed to have - much more potency than the other,<span class="pagenum" id="Page_181">181</span> is not only opposed to the views - entertained by physicists, but is also wholly irreconcilable with the - ordinary principles of mechanics. But more than this, if we analyze the - subject a little so as to remove some of the confusion of ideas which - besets it, we shall find that these views are irreconcilable with even - Dr. Carpenter’s own explanation of the cause of the general oceanic - circulation.</p> - - <p><em>Cold</em> is not a something positive imparted to the polar waters giving - them motion, and of which the tropical waters are deprived. If, dipping - one hand into a basin filled with tropical water at 80° and the other - into one filled with polar water at 32°, we refer to our <em>sensations</em>, - we call the water in the one <em>hot</em> and that in the other <em>cold</em>; but - so far as the water itself is concerned heat and cold simply mean - difference in the amounts of heat possessed. Both the polar and the - tropical water possess a certain amount of energy in the form of heat, - only the polar water does not possess so much of it as the tropical.</p> - - <p>How, then, according to Dr. Carpenter, does polar cold impart motion - to the water? The warm water flowing in upon the polar column becomes - chilled by cold, but it is not cooled below that of the water - underneath; for, according to Dr. Carpenter, the ocean in polar regions - is as cold and as dense underneath as at the surface. The cooled - surface-water does not sink through the water underneath, like the - surface-water of a pond chilled during a frosty night. “The descending - motion in column C will not consist,” he says, “in a successional - descent of surface-films from above downwards, but it will be a - downward movement of the <em>entire mass</em>, as if water in a tall jar - were being drawn off through an orifice at the bottom” (§ 29). There - is a downward motion of the entire column, producing an outflow of - water at the bottom towards the equatorial column W, which outflow is - compensated by an inflow from the top of the equatorial column to the - top of the polar column C. But what causes column C to descend? The - cause of the descent is its excess of weight over that of column W. - Column C descends and column W ascends, for the same reason that in<span class="pagenum" id="Page_182">182</span> - a balance the heavy scale descends and the light scale rises. Column - C descends not simply because it is cold, but because it is <em>colder</em> - than column W. Column C descends not simply because in consequence of - being cold it is dense and therefore heavy, but because in consequence - of being cold it is <em>denser</em> and therefore <em>heavier</em> than column W. - It might be as cold as frozen mercury and as heavy as lead; but it - would not on that account descend unless it were heavier than column - W. The descent of column C and ascent of column W, and consequently - the general oceanic circulation, results, therefore, according to Dr. - Carpenter’s explanation, from the <em>difference</em> in the weights of the - two columns; and the difference in the weights of the two columns - results from their <em>difference</em> of density; and the difference of - density of the two columns in turn results from their <em>difference</em> of - temperature. But it has already been proved that the difference of - temperature between the polar and equatorial columns depends wholly on - the difference in the amount of heat received by each from the sun. The - equatorial column W possesses more heat than the polar column C, solely - because it receives more heat from the sun than column C. Consequently - Dr. Carpenter’s statement that the circulation is produced by polar - cold rather than by equatorial heat, is just as much in contradiction - to his own theory as it is to the principles of mechanics. Again, his - admission that the general oceanic circulation “cannot be produced by - the application of heat to the surface,” is virtually a giving up the - whole point in debate; for according to his gravitation theory, and - every form of that theory, the circulation results from <em>difference</em> of - temperature between equatorial and polar seas; but this difference, as - we have seen, is entirely owing to the difference in the amount of heat - received from the sun at these two places. The heat received, however, - is “surface-heat;” for it is at the surface that the ocean receives all - its heat from the sun; and consequently if surface-heat cannot produce - the effect required, nothing else can.</p> - - <p><em>M. Dubuat’s Experiments.</em>—Referring to the experiments of<span class="pagenum" id="Page_183">183</span> M. Dubuat - adduced by me to show that water would not run down a slope of 1 - in 1,820,000,<a id="FNanchor_82" href="#Footnote_82" class="fnanchor">[82]</a> he says, “Now the experiments of M. Dubuat had - reference, not to the slow restoration of level produced by the motion - of water on itself, but to the sensible movement of water flowing over - solid surfaces and retarded by its friction against them” (§ 22). - Dr. Carpenter’s meaning, I presume, is that if the incline consist - of any solid substance, water will not flow down it; but if it be - made of <em>water</em> itself, <em>water</em> will flow down it. But in M. Dubuat’s - experiments it was only the molecules in actual <em>contact</em> with the - solid incline that could possibly be retarded by friction against it. - The molecules not in contact with the solid incline evidently rested - upon an <em>incline of water</em>, and were at perfect liberty to roll down - that incline if they chose; but they did not do so; and consequently M. - Dubuat’s experiment proved that water will not flow over itself on an - incline of 1 in 1,000,000.</p> - - <p><em>A Begging of the Question at Issue.</em>—“It is to be remembered,” says - Dr. Carpenter, “that, however small the original amount of movement - may be, a <em>momentum</em> tending to its continuance <em>must</em> be generated - from the instant of its commencement; so that if the initiating force - be in constant action, there will be a <em>progressive acceleration</em> of - its rate, until the increase of resistance equalises the tendency to - further acceleration. Now, if it be admitted that the propagation of - the disturbance of equilibrium from one column to another is simply - <em>retarded</em>, <em>not</em> prevented, by the viscosity of the liquid, I cannot - see how the conclusion can be resisted, that the constantly maintained - difference of gravity between the polar and equatorial columns really - acts as a <i lang="la">vis viva</i> in maintaining a circulation between them” (§ 35).</p> - - <p>If it be true, as Dr. Carpenter asserts, that in the case of the - general oceanic circulation advocated by him “viscosity” simply - <em>retards</em> motion, but does not <em>prevent</em> it, I certainly agree with him - “that the constantly maintained difference of gravity between the polar - and equatorial columns really acts as a <i lang="la">vis <span class="pagenum" id="Page_184">184</span>viva</i> in maintaining a - circulation between them.” But to assert that it merely retards, but - does not prevent, motion, is simply <em>begging the question at issue</em>. - It is an established principle that if the <em>force</em> resisting motion be - greater than the force tending to produce it, then no motion can take - place and no work can be performed. The experiments of M. Dubuat prove - that the <em>force</em> of the molecular resistance of water to motion is - <em>greater</em> than the <em>force</em> derived from a slope of 1 in 1,000,000; and - therefore it is simply begging the question at issue to assert that it - is <em>less</em>. The experiments of MM. Barlow, Rainey, and others, to which - he alludes, are scarcely worthy of consideration in relation to the - present question, because we know nothing whatever regarding the actual - amount of force producing motion of the water in these experiments, - further than that it must have been enormously greater than that - derived from a slope of 1 in 1,000,000.</p> - - <p><em>Supposed Argument from the Tides.</em>—Dr. Carpenter advances Mr. - Ferrel’s argument in regard to the tides. The power of the moon to - disturb the earth’s water, he asserts, is, according to Herschel, - only 1/11,400,000th part of gravity, and that of the sun not over - 1/25,736,400th part of gravity; yet the moon’s attractive force, even - when counteracted by the sun, will produce a rise of the ocean. But as - the disturbance of gravity produced by difference of temperature is far - greater than the above, it ought to produce circulation.</p> - - <p>It is here supposed that the force exerted by gravity on the ocean, - resulting from difference of temperature, tending to produce the - general oceanic circulation, is much greater than the force exerted - on the ocean by the moon in the production of the tides. But if we - examine the subject we shall find that the opposite is the case. The - attraction of the moon tending to lift the waters of the ocean acts - directly on every molecule from the surface to the bottom; but the - force of gravity tending to produce the circulation in question acts - directly on only a portion of the ocean. Gravity can exercise no direct - force in impelling the underflow from the polar to the equatorial<span class="pagenum" id="Page_185">185</span> - regions, nor in raising the water to the surface when it reaches the - equatorial regions. Gravity can exercise no direct influence in pulling - the water horizontally along the earth’s surface, nor in raising it - up to the surface. The pull of gravity is always <em>downwards</em>, never - <em>horizontally</em> nor upwards. Gravity will tend to pull the surface-water - from the equator to the poles because here we have <em>descent</em>. Gravity - will tend to sink the polar column because here also we have <em>descent</em>. - But these are the only parts of the circuit where gravity has any - tendency to produce motion. Motion in the other parts of the circuit, - viz., along the bottom of the ocean from the poles to the equator and - in raising the equatorial column, is produced by the <em>pressure</em> of the - polar column; and consequently it is only <em>indirectly</em> that gravity may - be said to produce motion in those parts. It is true that on certain - portions of the ocean the force of gravity tending to produce motion is - greater than the force of the moon’s attraction, tending to produce the - tides; but this portion of the ocean is of inconsiderable extent. The - total force of gravity acting on the entire ocean tending to produce - circulation is in reality prodigiously less than the total force of the - moon tending to produce the tides.</p> - - <p>It is no doubt a somewhat difficult problem to determine accurately - the total amount of force exercised by gravity on the ocean; but for - our present purpose this is not necessary. All that we require at - present is a very rough estimate indeed. And this can be attained by - very simple considerations. Suppose we assume the mean depth of the - sea to be, say, three miles. The mean depth may yet be found to be - somewhat less than this, or it may be found to be somewhat greater; - a slight mistake, however, in regard to the mass of the ocean will - not materially affect our conclusions. Taking the depth at 3 miles, - the force or direct pull of gravity on the entire waters of the ocean - tending to the production of the general circulation will not amount to - more than 1/24,000,000,000th that of gravity, or only about 1/2,100th - that of the attraction of the moon in the production of the tides. Let - it be observed that I am<span class="pagenum" id="Page_186">186</span> referring to the force or pull of gravity, - and not to hydrostatic pressure.</p> - - <p>The moon, by raising the waters of the ocean, will produce a slope of 2 - feet in a quadrant; and because the raised water sinks and the level is - restored, Mr. Ferrel concludes that a similar slope of 2 feet produced - by difference of temperature will therefore be sufficient to produce - motion and restore level. But it is overlooked that the restoration of - level in the case of the tides is as truly the work of the moon as the - disturbance of that level is. For the water raised by the attraction of - the moon at one time is again, six hours afterwards, pulled down by the - moon when the earth has turned round a quadrant.</p> - - <p>No doubt the earth’s gravity alone would in course of time restore - the level; but this does not follow as a logical consequence from Mr. - Ferrel’s premises. If we suppose a slope to be produced in the ocean by - the moon and the moon’s attraction withdrawn so as to allow the water - to sink to its original level, the raised side will be the heaviest and - the depressed side the lightest; consequently the raised side will tend - to sink and the depressed side will tend to rise, in order that the - ocean may regain its static equilibrium. But when a difference of level - is produced by difference of temperature, the raised side is always the - lightest and the depressed side is always the heaviest; consequently - the very effort which the ocean makes to maintain its equilibrium - tends to prevent the level being restored. The moon produces the tides - chiefly by means of a simple yielding of the entire ocean considered as - a mass; whereas in the case of a general oceanic circulation the level - is restored by a <em>flow</em> of water at or near the surface. Consequently - the amount of friction and molecular resistance to be overcome in the - restoration of level in the latter case is much greater than in the - former. The moon, as the researches of Sir William Thomson show, will - produce a tide in a globe composed of a substance where no currents or - general flow of the materials could possibly take place.</p> - - <p><em>Pressure as a Cause of Circulation.</em>—We shall now briefly refer<span class="pagenum" id="Page_187">187</span> to - the influence of pressure (the indirect effects of gravity) in the - production of the circulation under consideration. That which causes - the polar column C to descend and the equatorial column W to ascend, - as has repeatedly been remarked, is the difference in the weight of - the two columns. The efficient cause in the production of the movement - is, properly speaking, gravity; <em>cold</em> at the poles and <em>heat</em> at the - equator, or, what is the same thing, the <em>excess</em> of heat received - by the equator over that received by the poles is what maintains the - difference of temperature between the two columns, and consequently is - that also which maintains the difference of weight between them. In - other words, difference of temperature is the cause which maintains - the <em>state of disturbed equilibrium</em>. But the efficient cause of - the circulation in question is gravity. Gravity, however, could not - act without this state of disturbed equilibrium; and difference of - temperature may therefore be called, in relation to the circulation, - a necessary <em>condition</em>, while gravity may be termed the <em>cause</em>. - Gravity sinks column C <em>directly</em>, but it raises column W <em>indirectly</em> - by means of pressure. The same holds true in regard to the motion of - the bottom-waters from C to W, which is likewise due to pressure. The - pressure of the excess of the weight of column C over that of column W - impels the bottom-water equatorwards and lifts the equatorial column. - But on this point I need not dwell, as I have in the preceding chapter - entered into a full discussion as to how this takes place.</p> - - <p>We come now to the most important part of the inquiry, viz., how is - the surface-water impelled from the equator to the poles? Is pressure - from behind the impelling force here as in the case of the bottom-water - of the ocean? It seems to me that, in attempting to account for the - surface-flow from the equator to the poles, Dr. Carpenter’s theory - signally fails. The force to which he appeals appears to be wholly - inadequate to produce the required effect.</p> - - <p>The experiments of M. Dubuat, as already noticed, prove that, any slope - which can possibly result from the difference of temperature <span class="pagenum" id="Page_188">188</span>between - the equator and the poles is wholly insufficient to enable gravity to - move the waters; but it does not necessarily prove that the <em>pressure</em> - resulting from the raised water at the equator may not be sufficient to - produce motion. This point will be better understood from the following - figure, where, as before, P C represents the polar column and E W the - equatorial column.</p> - - <div class="figcenter" id="i_188" > - <div class="caption">Fig. 4.</div> - <img src="images/i_188.jpg" width="600" height="218" alt="" /> - </div> - - <p>It will be observed that the water in that wedge-shaped portion W C - W′ forming the incline cannot be in a state of static equilibrium. - A molecule of water at O, for example, will be pressed more in the - direction of C than in the direction of W′, and the amount of this - excess of pressure towards C will depend upon the height of W above - the line C W′. It is evident that the pressure tending to move the - molecule at O towards C will be far greater than the direct pull of - gravity tending to draw a molecule at O′ lying on the surface of the - incline towards C. The experiments of M. Dubuat prove that the direct - force of gravity will not move the molecule at O′—that is, cause it to - roll down the incline W C; but they do not prove that it may not yield - to pressure from above, or that the pressure of the column W W′ will - not move the molecule at O. The pressure is caused by gravity, and - cannot, of course, enable gravity to perform more work than what is - derived from the energy of gravity; it will enable gravity, however, - to overcome resistance, which it could not do by direct action. But - whether the pressure resulting from the greater height of the water - at the equator due to its higher temperature be actually sufficient - to<span class="pagenum" id="Page_189">189</span> produce displacement of the water is a question which I am wholly - unable to answer.</p> - - <p>If we suppose 4 feet 6 inches to be the height of the equatorial - surface above the polar required to make the two columns balance - each other, the actual difference of level between the two columns - will certainly not be more than one-half that amount, because, if a - circulation exist, the weight of the polar column must always be in - excess of that of the equatorial. But this excess can only be obtained - at the expense of the surface-slope, as has already been shown at - length. The surface-slope probably will not be more than 2 feet or 2 - feet 6 inches. Suppose the ocean to be of equal density from the poles - to the equator, and that by some means or other the surface of the - ocean at the equator is raised, say, 2 feet above that of the poles, - then there can be little doubt that in such a case the water would - soon regain its level; for the ocean at the equator being heavier than - at the poles by the weight of a layer 2 feet in thickness, it would - sink at the former place and rise at the latter until equilibrium was - restored, producing, of course, a very slight displacement of the - bottom-waters towards the poles. It will be observed, however, that - restoration of level in this case takes place by a simple yielding, as - it were, of the entire mass of the ocean without displacement of the - molecules of the water over each other to any great extent. In the case - of a slope produced by difference of temperature, however, the raised - portion of the ocean is not heavier but lighter than the depressed - portion, and consequently has no tendency to sink. Any movement which - the ocean as a mass makes in order to regain equilibrium tends, as we - have seen, rather to increase the difference of level than to reduce - it. Restoration of level can only be produced by the forces which are - in operation in the wedge-shaped mass W C W′, constituting the slope - itself. But it will be observed by a glance at the Figure that, in - order to the restoration of level, a large portion of the water W W′ at - the equator will require to flow to C, the pole.</p> - - <p>According to the general <em>vertical</em> oceanic circulation theory,<span class="pagenum" id="Page_190">190</span> - pressure from behind is not one of the forces employed in the - production of the flow from the equator to the poles. This is evident; - for there can be no pressure from behind acting on the water if there - be no slope existing between the equator and the poles. Dr. Carpenter - not only denies the actual existence of a slope, but denies the - necessity for its existence. But to deny the existence of a slope is to - deny the existence of pressure, and to deny the necessity for a slope - is to deny the necessity for pressure. That in Dr. Carpenter’s theory - the surface-water is supposed to be <em>drawn</em> from the equator to the - poles, and not <em>pressed</em> forward by a force from behind, is further - evident from the fact that he maintains that the force employed is not - <i lang="la">vis a tergo</i> but <i lang="la">vis a fronte</i>.<a id="FNanchor_83" href="#Footnote_83" class="fnanchor">[83]</a></p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XI"> - <span class="pagenum" id="Page_191">191</span> - <h2> - CHAPTER XI.<br /><br /> - <span class="small">THE INADEQUACY OF THE GRAVITATION THEORY PROVED BY ANOTHER METHOD.</span> - </h2> - </div> - <div class="subhead">Quantity of Heat which can be conveyed by the General - Oceanic Circulation trifling.—Tendency in the Advocates - of the Gravitation Theory to under-estimate the Volume of - the Gulf-stream.—Volume of the Stream as determined by the - <cite>Challenger</cite>.—Immense Volume of Warm Water discovered by - Captain Nares.—Condition of North Atlantic inconsistent with - the Gravitation Theory.—Dr. Carpenter’s Estimate of the Thermal - Work of the Gulf-stream.</div> - - <p class="noindent"><span class="smcap">I shall</span> now proceed by another method to prove the inadequacy of such - a general oceanic circulation as that which Dr. Carpenter advocates. - By contrasting the quantity of heat carried by the Gulf-stream from - inter-tropical to temperate and polar regions with such amount as - can possibly be conveyed in the same direction by means of a general - oceanic circulation, it will become evident that the latter sinks into - utter insignificance before the former.</p> - - <p>In my earlier papers on the amount of heat conveyed by the - Gulf-stream,<a id="FNanchor_84" href="#Footnote_84" class="fnanchor">[84]</a> I estimated the volume of that stream as <em>equal - to that</em> of a current 50 miles broad and 1,000 feet deep, flowing - (from the surface to the bottom) at 4 miles an hour. Of course I did - not mean, as Dr. Carpenter seems to suppose, that the stream at any - particular place is 50 miles broad and 1,000 feet deep, or that it - actually flows at the uniform rate of 4 miles an hour at surface and - bottom. All I meant was, that the Gulf-stream is <em>equal to that</em> of - a current of the above size and velocity. But in my recent papers on - Ocean-currents, the substance of which appears in the present volume, - to obviate any objections <span class="pagenum" id="Page_192">192</span>on the grounds of having over-estimated the - volume, I have taken that at one half this estimate, viz., equal to - a current 50 miles broad and 1,000 feet deep flowing at the rate of - 2 miles an hour. I have estimated the mean temperature of the stream - as it passes the Straits of Florida to be 65°, and have supposed that - the water in its course becomes ultimately cooled down on an average - to 40°. In this case each pound of water conveys 19,300 foot-pounds of - heat from the Gulf of Mexico, to be employed in warming temperate and - polar regions. Assuming these data to be correct, it follows that the - amount of heat transferred from the Gulf of Mexico by this stream per - day amounts to 77,479,650,000,000,000,000 foot-pounds. This enormous - quantity of heat is equal to one-fourth of all that is received from - the sun by the whole of the Atlantic Ocean from the Tropic of Cancer up - to the Arctic Circle.</p> - - <p>This is the amount of heat conveyed from inter-tropical to temperate - and polar regions by the Gulf-stream. What now is the amount conveyed - by means of the General Oceanic Circulation?</p> - - <p>According to this theory there ought to be as much warm water flowing - from inter-tropical regions towards the Antarctic as towards the Arctic - Circle. We may, therefore, in our calculations, consider that the heat - which is received in tropical regions to the south of the equator goes - to warm the southern hemisphere, and that received on the north side - of the equator to warm the northern hemisphere. The warm currents - found in the North Atlantic in temperate regions we may conclude came - from the regions lying to the north of the equator,—or, in other - words, from that part of the Atlantic lying between the equator and - the Tropic of Cancer. At least, according to the gravitation theory, - we have no reason to believe that the quantity of warm water flowing - from tropical to temperate and polar regions in the Atlantic is - greater than the area between the equator and the Tropic of Cancer - can supply—because it is affirmed that a very large proportion of the - cold water found in the North Atlantic comes, not from the arctic, but - from the<span class="pagenum" id="Page_193">193</span> antarctic regions. But if the North Atlantic is cooled by a - cold stream from the southern hemisphere, the southern hemisphere in - turn must be heated by a warm current from the North Atlantic—unless - we assume that the compensating current flowing from the Atlantic into - the southern hemisphere is as cold as the antarctic current, which is - very improbable. But Dr. Carpenter admits that the quantity of warm - water flowing from the Atlantic in equatorial regions towards the - south is even greater than that flowing northwards. “The unrestricted - communication,” he says, “which exists between the antarctic area and - the great Southern Ocean-basins would involve, if the doctrine of a - general oceanic circulation be admitted, a much more considerable - interchange of waters between the antarctic and the equatorial areas - than is possible in the northern hemisphere.”<a id="FNanchor_85" href="#Footnote_85" class="fnanchor">[85]</a></p> - - <p>We have already seen that, were it not for the great mass of warm water - which finds its way to the polar regions, the temperature of these - regions would be enormously lower than they really are. It has been - shown likewise that the comparatively high temperature of north-western - Europe is due to the same cause. But if it be doubtful whether the - Gulf-stream reaches our shores, and if it be true that, even supposing - it did, it “could only affect the <em>most superficial</em> stratum,” and - that the great mass of warm water found by Dr. Carpenter in his - dredging expeditions came directly from the equatorial regions, and not - from the Gulf-stream, then the principal part of the heating-effect - must be attributed, not to the Gulf-stream, but to the general flow - of water from the equatorial regions. It surely would not, then, be - too much to assume that the quantity of heat conveyed from equatorial - regions by this general flow of water into the North Atlantic is at - least equal to that conveyed by the Gulf-stream. If we assume this to - be the amount of heat conveyed by the two agencies into the Atlantic - from inter-tropical regions, it will, of course, be equal to twice that - conveyed by the Gulf-stream alone.</p> - - <p><span class="pagenum" id="Page_194">194</span></p> - - <p>We shall now consider whether the area of the Atlantic to the north of - the equator is sufficient to supply the amount of heat demanded by Dr. - Carpenter’s theory.</p> - - <p>The entire area of the Atlantic, extending from the equator to the - Tropic of Cancer, including the Caribbean Sea and the Gulf of Mexico, - is about 7,700,000 square miles.</p> - - <p>The quantity of heat conveyed by the Gulf-stream through the Straits of - Florida is, as we have already endeavoured to show, equal to all the - heat received from the sun by 1,560,935 square miles at the equator. - The annual quantity of heat received from the sun by the torrid zone - per unit surface, taking the mean of the whole zone, is to that - received by the equator as 39 to 40, consequently the quantity of - heat conveyed by the Gulf-stream is equal to all the heat received by - 1,600,960 square miles of the Atlantic in the torrid zone.</p> - - <p>But if, according to Dr. Carpenter’s views, the quantity of heat - conveyed from the tropical regions is double that conveyed by the - Gulf-stream, the amount of heat in this case conveyed into the Atlantic - in temperate regions will be equal to all the heat received from the - sun by 3,201,920 square miles of the Atlantic between the equator and - the Tropic of Cancer. This is 32/77ths of all the heat received from - the sun by that area.</p> - - <p>Taking the annual quantity received per unit surface at the equator at - 1,000, the quantities received by the three zones would be respectively - as follows:—</p> - - <table summary="Heat quantities"> - <tbody> - <tr> - <td>Equator</td> - <td class="tdr"><div>1000</div></td> - </tr> - <tr> - <td>Torrid zone</td> - <td class="tdr"><div>975</div></td> - </tr> - <tr> - <td>Temperate zone</td> - <td class="tdr"><div>757</div></td> - </tr> - <tr> - <td>Frigid zone</td> - <td class="tdr"><div>454</div></td> - </tr> - </tbody> - </table> - - <p>Now, if we remove from the Atlantic in tropical regions 32/77ths of the - heat received from the sun, we remove 405 parts from every 975 received - from the sun, and consequently only 570 parts per unit surface remain.</p> - - <p>It has been shown<a id="FNanchor_86" href="#Footnote_86" class="fnanchor">[86]</a> that the quantity of heat conveyed by <span class="pagenum" id="Page_195">195</span>the - Gulf-stream from the equatorial regions into the temperate regions - is equal to 100/412ths of all the heat received by the Atlantic in - temperate regions. But according to the theory under consideration the - quantity removed is double this, or equal to 100/206ths of all the heat - received from the sun. But the amount received from the sun is equal - to 757 parts per unit surface; add then to this 100/206ths of 757, or - 367, and we have 1,124 parts of heat per unit surface as the amount - possessed by the Atlantic in temperate regions. The Atlantic should in - this case be much warmer in temperate than in tropical regions; for - in temperate regions it would possess 1,124 parts of heat per unit - surface, whereas in tropical regions it would possess only 570 parts - per unit surface. Of course the heat conveyed from tropical regions - does not all remain in temperate regions; a very considerable portion - of it must pass into the arctic regions. Let us, then, assume that - one half goes to warm the Arctic Ocean, and the other half remains - in the temperate regions. In this case 183·5 parts would remain, and - consequently 757 + 183·5 = 940·5 parts would be the quantity possessed - by the Atlantic in temperate regions, a quantity which still exceeds by - no less than 370·5 parts the heat possessed by the Atlantic in tropical - regions.</p> - - <p>As one half of the amount of heat conveyed from the tropical regions - is assumed to go into the Arctic Ocean, the quantity passing into - that ocean would therefore be equal to that which passes through the - Straits of Florida, an amount which, as we have found, is equal to all - the heat received from the sun by 3,436,900 square miles of the Arctic - Ocean.<a id="FNanchor_87" href="#Footnote_87" class="fnanchor">[87]</a> The entire area covered by sea beyond the Arctic Circle is - under 5,000,000 square miles; but taking the Arctic Ocean in round - numbers at 5,000,000 square miles, the quantity of heat conveyed into - it by currents to that received from the sun would therefore be as - 3,436,900 to 5,000,000.</p> - - <p>The amount received on the unit surface of the arctic regions we have - seen to be 454 parts. The amount received from the <span class="pagenum" id="Page_196">196</span>currents would - therefore be 312 parts. This gives 766 parts of heat per unit surface - as the quantity possessed by the Arctic Ocean. Thus the Arctic Ocean - also would contain more heat than the Atlantic in tropical regions; for - the Atlantic in these regions would, in the case under consideration, - possess only 570 parts, while the Arctic Ocean would possess 766 - parts. It is true that more rays are cut off in arctic regions than in - tropical; but still, after making due allowance for this, the Arctic - Ocean, if the theory we are considering were true, ought to be as warm - as, if not warmer than, the Atlantic in tropical regions. The relative - quantities of heat possessed by the three zones would therefore be as - follows:—</p> - - <table summary="Heat zone quantities 1"> - <tbody> - <tr> - <td>Atlantic, in torrid zone</td> - <td class="tdr"><div>570</div></td> - </tr> - <tr> - <td> 〃 in temperate zone</td> - <td class="tdr"><div>940</div></td> - </tr> - <tr> - <td> 〃 in frigid zone</td> - <td class="tdr"><div>766</div></td> - </tr> - </tbody> - </table> - - <p>It is here assumed, however, that none of the heat possessed by the - Gulf-stream is derived from the southern hemisphere, which, we know, - is not the case. But supposing that as much as one half of the heat - possessed by the stream came from the southern hemisphere, and that the - other half was obtained from the seas lying between the equator and the - Tropic of Cancer, the relative proportions of heat possessed by the - three zones per given area would be as follows:—</p> - - <table summary="Heat zone quantities 2"> - <tbody> - <tr> - <td>Atlantic, in torrid zone</td> - <td class="tdr"><div>671</div></td> - </tr> - <tr> - <td> 〃 in temperate zone</td> - <td class="tdr"><div>940</div></td> - </tr> - <tr> - <td> 〃 in frigid zone</td> - <td class="tdr"><div>766</div></td> - </tr> - </tbody> - </table> - - <p>This proves incontestably that, supposing there is such a general - oceanic circulation as is maintained, the quantity of heat conveyed by - means of it into the North Atlantic and Arctic Oceans must be trifling - in comparison with that conveyed by the Gulf-stream; for if it nearly - equalled that conveyed by the Gulf-stream, then not only the North - Atlantic in temperate regions, but even the Arctic Ocean itself would - be much warmer than the inter-tropical seas. In fact, so far as the - distribution of heat over the globe is concerned, it is a<span class="pagenum" id="Page_197">197</span> matter of - indifference whether there really is or is not such a thing as this - general oceanic circulation. The enormous amount of heat conveyed by - the Gulf-stream alone puts it beyond all doubt that ocean-currents are - the great agents employed in distributing over the globe the excess of - heat received by the sea in inter-tropical regions.</p> - - <p>It is therefore, so far as concerns the theory of a General Oceanic - Circulation, of the utmost importance that the advocates of that - theory should prove that I have over-estimated the thermal power of - the Gulf-stream. This, however, can only be done by detecting some - error either in my computation or in the data on which it is based; - yet neither Dr. Carpenter nor any one else, as far as I know, has - challenged the accuracy of my figures. The question at issue is the - correctness of the data; but the only part of the data which can - possibly admit of being questioned is my estimate of the <em>volume</em> - and <em>temperature</em> of the stream. Dr. Carpenter, however, does not - maintain that I have over-estimated the temperature of the stream; on - the contrary, he affirms that I have really under-estimated it. “If we - assume,” he remarks, “the limit of the stratum above 60° as that of - the real Gulf-stream current, we shall find its average temperature to - be somewhat higher than it has been stated by Mr. Croll, who seems to - have taken 65° as the average of the water flowing through the entire - channel. The average surface temperature of the Florida channel for - the whole year is 80°; and we may fairly set the average of the entire - outgoing stream, down to the plane of 60°, at 70°, instead of 65° as - estimated by Mr. Croll” (§ 141). It follows, then, that every pound of - water of the Gulf-stream actually conveys 5 units of heat more than - I have estimated it to do—the amount conveyed being 30 units instead - of 25 units as estimated by me. Consequently, if the Gulf-stream be - equal to that of a current of merely 41½ miles broad and 1,000 feet - deep, flowing at the rate of 2 miles an hour, it will still convey the - estimated quantity of heat. But this estimate of the volume of the - stream, let it be observed, barely exceeds <em>one-third</em> of that<span class="pagenum" id="Page_198">198</span> given - by Herschel, Maury, and Colding,<a id="FNanchor_88" href="#Footnote_88" class="fnanchor">[88]</a> and is little more than one-half - that assigned to it by Mr. Laughton, while it very little exceeds that - given by Mr. Findlay,<a id="FNanchor_89" href="#Footnote_89" class="fnanchor">[89]</a> an author whom few will consider likely to - overrate either the volume or heating-power of the stream.</p> - - <p>The important results obtained during the <cite>Challenger</cite> expedition have - clearly proved that I have neither over-estimated the temperature nor - the volume of the Gulf-stream. Between Bermuda and Sandy Hook the - stream is 60 miles broad and 600 feet deep, with a maximum velocity of - from 3½ to 4 miles an hour. If the mean velocity of the entire section - amounts to 2¼ miles an hour, which it probably does, the volume of the - stream must equal that given in my estimate. But we have no evidence - that all the water flowing through the Straits of Florida passes - through the section examined by the officers of the <cite>Challenger</cite>. Be - this, however, as it may, the observations made between St. Thomas - and Sandy Hook reveal the existence of an immense flow of warm water, - 2,300 feet deep, entirely distinct from the water included in the above - section of the Gulf-stream proper. As the thickest portion of this - immense body of water joins the warm water of the Gulf-stream, Captain - Nares considers that “it is evidently connected with it, and probably - as an offshoot.” At Sandy Hook, according to him, it extends 1,200 - feet deeper than the Gulf-stream itself, but off Charleston, 600 miles - nearer the source, the same temperature is found at the same depth. - But whether it be an offshoot of the Gulf-stream or not, one thing is - certain, it can only come from the Gulf of Mexico or from the Caribbean - Sea. This mass of water, after flowing northwards for about 1,000 - miles, turns to the right and crosses the Atlantic in the direction of - the Azores, where it appears to thin out.</p> - - <p>If, therefore, we take into account the combined heat conveyed <span class="pagenum" id="Page_199">199</span>by - both streams, my estimate of the heat transferred from inter-tropical - regions into the North Atlantic will be found rather under than above - the truth.</p> - - <p><em>Dr. Carpenter’s Estimate of the Thermal Work of the Gulf-stream.</em>—In - the appendix to an elaborate memoir on Oceanic Circulation lately - read before the Geographical Society, Dr. Carpenter endeavours to - show that I have over-estimated the thermal work of the Gulf-stream. - In that memoir<a id="FNanchor_90" href="#Footnote_90" class="fnanchor">[90]</a> he has also favoured us with his own estimate of - the sectional area, rate of flow, and temperature of the stream. Even - adopting his data, however, I find myself unable to arrive at his - conclusions.</p> - - <p>Let us consider first his estimate of the sectional area of the - stream. He admits that “it is impossible, in the present state of our - knowledge, to arrive at any exact estimate of the sectional area of the - stream; since it is for the most part only from the temperatures of - its different strata that we can judge whether they are, or are not, - in movement, and what is the direction of their movement.” Now it is - perfectly evident that our estimate of the sectional area of the stream - will depend upon what we assume to be its bottom temperature. If, for - example, we assume 70° to be the bottom temperature, we shall have a - small sectional area. Taking the temperature at 60°, the sectional - area will be larger, and if 50° be assumed to be the temperature, the - sectional area will be larger still, and so on. Now the small sectional - area obtained by Dr. Carpenter arises from the fact of his having - assumed the high temperature of 60° to be that of the bottom of the - stream. He concludes that all the water below 60° has an inward flow, - and that it is only that portion from 60° and upwards which constitutes - the Gulf-stream. I have been unable to find any satisfactory evidence - for assuming so high a temperature for the bottom of the stream. It - must be observed that the water underlying the Gulf-stream is not - the ordinary water of the Atlantic, but the cold current from the - arctic regions. In fact, it is the same <span class="pagenum" id="Page_200">200</span>water which reaches the - equator at almost every point with a temperature not much above the - freezing-point. It is therefore highly improbable that the under - surface of the Gulf-stream has a temperature so high as 60°.</p> - - <p>Dr. Carpenter’s method of measuring the mean velocity of the - Gulf-stream is equally objectionable. He takes the mean annual rate at - the surface in the “Narrows” to be two miles an hour and the rate at - the bottom to be zero, and he concludes from this that the average rate - of the whole is one mile an hour—the arithmetical mean between these - two extremes. Now it will be observed that this conclusion only holds - true on the supposition that the breadth of the stream is as great at - the bottom as at the surface, which of course it is not. All admit that - the sides of the Gulf-stream are not perpendicular, but slope somewhat - in the manner of the banks of a river. The stream is broad at the - surface and narrows towards the bottom. It is therefore evident that - the upper half of the section has a much larger area than the lower; - the quantity of water flowing through the upper half with a greater - velocity than one mile an hour must be much larger than the quantity - flowing through the lower half with a less velocity than one mile an - hour.</p> - - <p>His method of estimating the mean temperature of the stream is even - more objectionable. He says, “The average surface temperature of the - Florida Channel for the whole year is 80°, and we may set the average - of the entire outgoing stream down to the plane of 60° at 70°, instead - of 65°, as estimated by Mr. Croll.” If 80° be the surface and 60° be - the bottom temperature, temperature and rate of velocity being assumed - of course to decrease uniformly from the surface downwards, how is it - possible that 70° can be the average temperature? The amount of water - flowing through the upper half of the section, with a temperature above - 70°, is far more than the amount flowing through the under half of the - section, with a temperature below 70°. Supposing the lower half of the - section to be as large as the upper half, which it is not, still the - quantity of<span class="pagenum" id="Page_201">201</span> water flowing through it would only equal one-third of - that flowing through the upper half, because the mean velocity of the - water in the lower half would be only half a mile per hour, whereas - the mean velocity of that in the upper half would be a mile and a half - an hour. But the area of the lower half is much less than that of the - upper half, consequently the amount of water whose temperature is under - 70° must be even much under one-third of that, the temperature of which - is above 70°.</p> - - <p>Had Dr. Carpenter taken the proper method of estimating the mean - temperature, he would have found that 75°, even according to his own - data, was much nearer the truth than 70°. I pointed out, several years - ago,<a id="FNanchor_91" href="#Footnote_91" class="fnanchor">[91]</a> the fallacy of estimating the mean temperature of a stream in - this way.</p> - - <p>So high a mean temperature as 75° for the Gulf-stream, even in the - Florida Channel, is manifestly absurd, but if 60° be the bottom - temperature of the stream, the mean temperature cannot possibly be much - under that amount. It is, of course, by under-estimating the sectional - area of the stream that its mean temperature is over-estimated. We - cannot reduce the mean temperature without increasing the sectional - area. If my estimate of 65° be taken as the mean temperature, which I - have little doubt will yet be found to be not far from the truth, Dr. - Carpenter’s estimate of the sectional area must be abandoned. For if - 65° be the mean temperature of the stream, its bottom temperature must - be far under 60°, and if the bottom temperature be much under 60°, then - the sectional area must be greater than he estimates it to be.</p> - - <p>Be this, however, as it may; even if we suppose that 60° will - eventually be found to be the actual bottom temperature of the - Gulf-stream, nevertheless, if the total quantity of heat conveyed by - the stream from inter-tropical regions be estimated in the proper way, - we shall still find that amount to be so enormous, that there is not - sufficient heat remaining in those <span class="pagenum" id="Page_202">202</span>regions to supply Dr. Carpenter’s - oceanic circulation with a quantity as great for distribution in the - North Atlantic.</p> - - <p>It therefore follows (and so far as regards the theory of Secular - changes of climate, this is all that is worth contending for) that - Ocean-currents and not a General Oceanic Circulation resulting from - gravity, are the great agents employed in the distribution of heat over - the globe.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XII"> - <span class="pagenum" id="Page_203">203</span> - <h2> - CHAPTER XII.<br /><br /> - <span class="small">MR. A. G. FINDLAY’S OBJECTIONS CONSIDERED.</span> - </h2> - </div> - <div class="subhead">Mr. Findlay’s Estimate of the Volume of the Gulf-stream.—Mean - Temperature of a Cross Section less than Mean Temperature - of Stream.—Reason of such Diversity of Opinion regarding - Ocean-currents.—More rigid Method of Investigation necessary.</div> - - <p class="noindent"><span class="smcap">At</span> the conclusion of the reading of Dr. Carpenter’s paper before the - Royal Geographical Society, on January 9th, 1871, Mr. Findlay made the - following remarks:—</p> - - <p>“When, by the direction of the United States Government, ten or eleven - years ago, the narrowest part of the Gulf-stream was examined, figures - were obtained which shut out all idea of its ever reaching our shores - as a heat-bearing current. In the narrowest part, certainly not more - than from 250 to 300 cubic miles of water pass per diem. Six months - afterwards that water reaches the banks of Newfoundland, and nine or - twelve months afterwards the coast of England, by which time it is - popularly supposed to cover an area of 1,500,000 square miles. The - proportion of the water that passes through the Gulf of Florida will - not make a layer of water more than 6 inches thick per diem over such - a space. Every one knows how soon a cup of tea cools; and yet it is - commonly imagined that a film of only a few inches in depth, after the - lapse of so long a time, has an effect upon our climate. There is no - need for calculations; the thing is self-evident.”<a id="FNanchor_92" href="#Footnote_92" class="fnanchor">[92]</a></p> - - <p>About five years ago, Mr. Findlay objected to the conclusions which I - had arrived at regarding the enormous heating-power of the Gulf-stream - on the ground that I had over-estimated the <span class="pagenum" id="Page_204">204</span>volume of the stream. He - stated that its volume was only about the half of what I had estimated - it to be. To obviate this objection, I subsequently reduced the volume - to one-half of my former estimate.<a id="FNanchor_93" href="#Footnote_93" class="fnanchor">[93]</a> But taking the volume at this - low estimate, it was nevertheless found that the quantity of heat - conveyed into the Atlantic through the Straits of Florida by means of - the stream was equal to about <em>one-fourth</em> of all the heat received - from the sun by the Atlantic from the latitude of the Strait of Florida - up to the Arctic Circle.</p> - - <p>Mr. Findlay, in his paper read before the British Association, affirmed - that the volume of the stream is somewhere from 294 to 333 cubic miles - per day; but in his remarks at the close of Dr. Carpenter’s address, he - stated it to be not greater than from 250 to 300 cubic miles per day. I - am unable to reconcile any of those figures with the data from which he - appears to have derived them. In his paper to the British Association, - he remarks that “the Gulf-stream at its outset is not more than 39½ - miles wide, and 1,200 feet deep.” From all attainable data, he computes - the mean annual rate of motion to be 65·4 miles per day; but as the - rate decreases with the depth, the mean velocity of the whole mass does - not exceed 49·4 miles per day. When he speaks of the mean velocity of - the Gulf-stream being so and so, he must refer to the mean velocity at - some particular place. This is evident; for the mean velocity entirely - depends upon the sectional area of the stream. The place where the - mean velocity is 49·4 miles per day must be the place where it is 39½ - miles broad and 1,200 feet deep; for he is here endeavouring to show us - how small the volume of the stream actually is. Now, unless the mean - velocity refers to the place where he gives us the breadth and depth - of the stream, his figures have no bearing on the point in question. - But a stream 39½ miles broad and 1,200 feet deep has a sectional area - of 8·97 square miles, and this, with a mean velocity of 49·4 miles - per day, will give 443 cubic miles of water. The amount, according to - my estimate, is 459 cubic <span class="pagenum" id="Page_205">205</span>miles per day; it therefore exceeds Mr. - Findlay’s estimate by only 16 cubic miles.</p> - - <p>Mr. Findlay does not, as far as I know, consider that I have - over-estimated the mean temperature of the stream. He states<a id="FNanchor_94" href="#Footnote_94" class="fnanchor">[94]</a> that - between Sand Key and Havana the Gulf-stream is about 1,200 feet deep, - and that it does not reach the summit of a submarine ridge, which he - states has a temperature of 60°. It is evident, then, that the bottom - of the stream has a temperature of at least 60°, which is within 5° of - what I regard as the mean temperature of the mass. But the surface of - the stream is at least 17° above this mean. Now, when we consider that - it is at the upper parts of the stream, the place where the temperature - is so much above 65°, that the motion is greatest, it is evident that - the mean temperature of the entire moving mass must, according to Mr. - Findlay, be considerably over 65°. It therefore follows, according - to his own data, that the Gulf-stream conveys into the Atlantic an - amount of heat equal to one-fourth of all the heat which the Atlantic, - from the latitude of the Straits of Florida up to the arctic regions, - derives from the sun.</p> - - <p>But it must be borne in mind that although the mean temperature of the - cross section should be below 65°, it does not therefore follow that - the mean temperature of the <em>water flowing through this cross section</em> - must be below that temperature, for it is perfectly obvious that the - mean temperature of the mass of water flowing through the cross section - in a given time must be much higher than that of the cross section - itself. The reason is very simple. It is in the upper half of the - section where the high temperature exists; but as the velocity of the - stream is much greater in its upper than in its lower half, the greater - portion of the water passing through this cross section is water of - high temperature.</p> - - <p>But even supposing we were to halve Mr. Findlay’s own estimate, and - assume that the volume of the stream is equal to only 222 cubic miles - of water per day instead of 443, still the <span class="pagenum" id="Page_206">206</span>amount of heat conveyed - would be equal to one-eighth part of the heat received from the sun by - the Atlantic. But would not the withdrawal of an amount of heat equal - to one-eighth of that received from the sun greatly affect the climate - of the Atlantic? Supposing we take the mean temperature of the Atlantic - at, say, 56°; this will make its temperature 295° above that of space. - Extinguish the sun and stop the Gulf-stream, and the temperature ought - to sink 295°. How far, then, ought the temperature to sink, supposing - the sun to remain and the Gulf-stream to stop? Would not the withdrawal - of the stream cause the temperature to sink some 30°? Of course, if - the Gulf-stream were withdrawn and everything else were to remain the - same, the temperature of the Atlantic would not actually remain 30° - lower than at present; for heat would flow in from all sides and partly - make up for the loss of the stream. But nevertheless 30° represents the - amount of temperature maintained by means of the heat from the stream. - And this, be it observed, is taking the volume of the stream at a lower - estimate than even Mr. Findlay himself would be willing to admit. Mr. - Findlay says that, by the time the Gulf-stream reaches the shores of - England, it is supposed to cover a space of 1,500,000 square miles. - “The proportion of water that passes through the Straits of Florida - will not make,” according to him, “a layer of water more than 6 inches - thick per diem over such a space.” But a layer of water 6 inches thick - cooling 25° will give out 579,000 foot-pounds of heat per square foot. - If, therefore, the Gulf-stream, as he asserts, supplies 6 inches per - day to that area, then every square foot of the area gives off per - day 579,000 foot-pounds of heat. The amount of heat received from the - sun per square foot in latitude 55°, which is not much above the mean - latitude of Great Britain, is 1,047,730 foot-pounds per day, taking, of - course, the mean of the whole year; <em>consequently this layer of water - gives out an amount of heat equal to more than</em> one-half <em>of all that - is received from the sun</em>. But assuming that the stream should leave - the half of its heat on the<span class="pagenum" id="Page_207">207</span> American shores and carry to the shores of - Britain only 12½° of heat, still we should have 289,500 foot-pounds per - square foot, which notwithstanding <em>is more than equal to</em> one-fourth - <em>of that received from the sun</em>. If an amount of heat so enormous - cannot affect climate, what can?</p> - - <p>I shall just allude to one other erroneous notion which prevails in - regard to the Gulf-stream; but it is an error which I by no means - attribute either to Mr. Findlay or to Dr. Carpenter. The error to which - I refer is that of supposing that when the Gulf-stream widens out to - hundreds of miles, as it does before it reaches our shores, its depth - must on this account be much less than when it issues from the Gulf of - Mexico. Although the stream may be hundreds of miles in breadth, there - is no necessity why it should be only 6 inches, or 6 feet, or 60 feet, - or even 600 feet in depth. It may just as likely be 6,000 feet deep as - 6 inches.</p> - - <p><em>The Reason why such Diversity of Opinion prevails in Regard to - Ocean-currents.</em>—In conclusion I venture to remark that more than - nine-tenths of all the error and uncertainty which prevail, both - in regard to the cause of ocean-currents and to their influence on - climate, is due, not, as is generally supposed, to the intrinsic - difficulties of the subject, but rather to the defective methods - which have hitherto been employed in its investigation—that is, in - not treating the subject according to the rigid methods adopted in - other departments of physics. What I most particularly allude to is - the disregard paid to the modern method of determining the amount of - effects in <em>absolute measure</em>.</p> - - <p>But let me not be misunderstood on this point. I by no means suppose - that the <em>absolute quantity</em> is the thing always required for its - own sake. It is in most cases required simply as a means to an end; - and very often that end is the knowledge of the <em>relative</em> quantity. - Take, for example, the Gulf-stream. Suppose the question is asked, - to what extent does the heat conveyed by that stream influence the - climate of the North Atlantic? In order to the proper answering of this - question,<span class="pagenum" id="Page_208">208</span> the principal thing required is to know what proportion the - amount of heat conveyed by the stream into the Atlantic bears to that - received from the sun by that area. We want the <em>relative proportions</em> - of these two quantities. But how are we to obtain them? We can only - do so by determining first the <em>absolute</em> quantity of each. We must - first measure each before we can know how much the one is greater - than the other, or, in other words, before we can know their relative - proportions. We have the means of determining the absolute amount - of heat received from the sun by a given area at any latitude with - tolerable accuracy; but the same cannot be done with equal accuracy in - regard to the amount of heat conveyed by the Gulf-stream, because the - volume and mean temperature of the stream are not known with certainty. - Nevertheless we have sufficient data to enable us to fix upon such a - maximum and minimum value to these quantities as will induce us to - admit that the truth must lie somewhere between them. In order to give - full justice to those who maintain that the Gulf-stream exercises - but little influence on climate, and to put an end to all further - objections as to the uncertainty of my data, I shall take a minimum - to which none of them surely can reasonably object, viz. that the - volume of the stream is not over 230 cubic miles per day, and the heat - conveyed per pound of water not over 12½ units. Calculating from these - data, we find that the amount of heat carried into the North Atlantic - is equal to one-sixteenth of all the heat received from the sun by that - area. There are, I presume, few who will not admit that the actual - proportion is much higher than this, probably as high as 1 to 3, or 1 - to 4. But, who, without adopting the method I have pursued, could ever - have come to the conclusion that the proportion was even 1 to 16? He - might have guessed it to be 1 to 100 or 1 to 1000, but he never would - have guessed it to be 1 to 16. Hence the reason why the great influence - of the Gulf-stream as a heating agent has been so much under-estimated.</p> - - <p>The same remarks apply to the gravitation theory of the cause of - currents. Viewed simply as a theory it looks very<span class="pagenum" id="Page_209">209</span> reasonable. There - is no one acquainted with physics but will admit that the tendency of - the difference of temperature between the equator and the poles is - to cause a surface current from the equator towards the poles, and - an under current from the poles to the equator. But before we can - prove that this tendency does actually produce such currents, another - question must be settled, viz. is this force sufficiently great to - produce the required motion? Now when we apply the method to which I - refer, and determine the absolute amount of the force resulting from - the difference of specific gravity, we discover that not to be the - powerful agent which the advocates of the gravitation theory suppose, - but a force so infinitesimal as not to be worthy of being taken into - account when considering the causes by which currents are produced.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XIII"> - <span class="pagenum" id="Page_210">210</span> - <h2> - CHAPTER XIII.<br /><br /> - <span class="small">THE WIND THEORY OF OCEANIC CIRCULATION.</span> - </h2> - </div> - <div class="subhead">Ocean-currents not due alone to the Trade-winds.—An Objection - by Maury.—Trade-winds do not explain the Great Antarctic - Current.—Ocean-currents due to the System of Winds.—The System - of Currents agrees with the System of the Winds.—Chart showing - the Agreement between the System of Currents and System of - Winds.—Cause of the Gibraltar Current.—North Atlantic an - immense Whirlpool.—Theory of Under Currents.—Difficulty - regarding Under Currents obviated.—Work performed by the Wind - in impelling the Water forward.—The <cite>Challenger’s</cite> crucial Test - of the Wind and Gravitation Theories.—North Atlantic above - the Level of Equator.—Thermal Condition of the Southern Ocean - irreconcilable with the Gravitation Theory.</div> - - <p><em>Ocean-currents not due alone to the Trade-winds.</em>—The generally - received opinion amongst the advocates of the wind theory of oceanic - circulation is that the Gulf-stream and other currents of the ocean are - due to the impulse of the trade-winds. The tendency of the trade-winds - is to impel the inter-tropical waters along the line of the equator - from east to west; and were those regions not occupied in some places - by land, this equatorial current would flow directly round the - globe. Its westward progress, however, is arrested by the two great - continents, the old and the new. On approaching the land the current - bifurcates, one portion trending northwards and the other southwards. - The northern branch of the equatorial current of the Atlantic passes - into the Caribbean Sea, and after making a circuit of the Gulf of - Mexico, flows northward and continues its course into the Arctic - Ocean. The southern branch, on the other hand, is deflected along the - South-American coast, constituting what is known as the Brazilian - current. In the Pacific a similar deflection occurs against the - Asiatic coast, forming a current somewhat resembling the Gulf-stream, - <span class="pagenum" id="Page_211">211</span>a portion of which (Kamtschatka current) in like manner passes into - the arctic regions. In reference to all these various currents, the - impelling cause is supposed to be the force of the trade-winds.</p> - - <p>It is, however, urged as an objection by Maury and other advocates of - the gravitation theory, that a current like the Gulf-stream, extending - as far as the arctic regions, could not possibly be impelled and - maintained by a force acting at the equatorial regions. But this is - a somewhat weak objection. It seems to be based upon a misconception - of the magnitude of the force in operation. It does not take into - account that this force acts on nearly the whole area of the ocean in - inter-tropical regions. If, in a basin of water, say three feet in - diameter, a force is applied sufficient to produce a surface-flow one - foot broad across the centre of the basin, the water impelled against - the side will be deflected to the extremes of the vessel. And this - result does not in any way depend upon the size of the basin. The - same effect which occurs in a small basin will occur in a large one, - provided the proportion between the breadth of the belt of water put in - motion and the size of the vessel be the same in both cases. It does - not matter, therefore, whether the diameter of the basin be supposed to - be three feet, or three thousand miles, or ten thousand miles.</p> - - <p>There is a more formidable objection, however, to the theory. - The trade-winds will account for the Gulf-stream, Brazil, Japan, - Mozambique, and many other currents; but there are currents, such as - some of the polar currents, which cannot be so accounted for. Take, - for example, the great antarctic current flowing northward into the - Pacific. This current does not bend to the left under the influence - of the earth’s rotation and continue its course in a north-westerly - direction, but actually bends round to the right and flows eastward - against the South-American coast, in direct opposition both to the - influence of rotation and to the trade-winds. The trade-wind theory, - therefore, is insufficient to account for all the facts. But there is - yet another explanation, which satisfactorily solves our difficulties. - <span class="pagenum" id="Page_212">212</span>The currents of the ocean owe their origin, not to the trade-winds - alone, but to the <em>prevailing</em> winds of the globe (including, of - course, the trade-winds).</p> - - <p><em>Ocean-currents due to the System of Winds.</em>—If we leave out of account - a few small inland sheets of water, the globe may be said to have but - one sea, just as it possesses only one atmosphere. We have accustomed - ourselves, however, to speak of parts or geographical divisions of - the one great ocean, such as the Atlantic and the Pacific, as if they - were so many separate oceans. And we have likewise come to regard the - currents of the ocean as separate and independent of one another. This - notion has no doubt to a considerable extent militated against the - acceptance of the theory that the currents are caused by the winds, and - not by difference of specific gravity; for it leads to the conclusion - that currents in a sea must flow in the direction of the prevailing - winds blowing over that particular sea. The proper view of the matter, - as I hope to be able to show, is that which regards the various - currents merely as members of one grand system of circulation produced, - not by the trade-winds alone, nor by the prevailing winds proper alone, - but by the combined action of all the prevailing winds of the globe, - regarded as one system of circulation.</p> - - <p>If the winds be the impelling cause of currents, the <em>direction</em> of the - currents will depend upon two circumstances, viz.:—(1) the direction - of the prevailing winds of the globe, including, of course, under this - term the prevailing winds proper and the trade-winds; and (2) the - conformation of land and sea. It follows, therefore, that as a current - in any given sea is but a member of a general system of circulation, - its direction is determined, not alone by the prevailing winds blowing - over the sea in question, but by the general system of prevailing - winds. It may consequently sometimes happen that the general system - of winds may produce a current directly opposite to the prevailing - wind blowing over the current. The accompanying Chart (<a href="#PLATE_I">Plate I.</a>) shows - how exactly the system of ocean-currents agrees with the system of - the prevailing winds. The fine<span class="pagenum" id="Page_213">213</span> - lines indicate the paths of the - prevailing winds, and the fine arrows the direction in which the wind - blows along those paths. The large arrows show the direction of the - principal ocean-currents.</p> - - <div class="figcenter illow600" id="PLATE_I" > - <div class="attribt">PLATE I</div> - <img src="images/i_212.jpg" width="600" height="411" alt="" /> - <div class="attribr">W. & A. K. Johnston, Edinb<sup>r</sup>. and London.</div> - <div class="caption smcap">CHART SHOWING the GENERAL AGREEMENT BETWEEN the SYSTEM of OCEAN - CURRENTS and WINDS.</div> - </div> - - <p>The directions and paths of the prevailing winds have been taken from - Messrs. Johnston’s small physical Atlas, which, I find, agrees exactly - with the direction of the prevailing winds as deduced from the four - quarterly wind charts lately published by the Hydrographic Department - of the Admiralty. The direction of the ocean-currents has been taken - from the Current-chart published by the Admiralty.</p> - - <p>In every case, without exception, the direction of the main currents of - the globe agrees exactly with the direction of the prevailing winds. - There could not possibly be a more convincing proof that those winds - are the cause of the ocean-currents than this general agreement of the - two systems as indicated by the chart. Take, for example, the North - Atlantic. The Gulf-stream follows exactly the path of the prevailing - winds. The Gulf-stream bifurcates in mid-Atlantic; so does the wind. - The left branch of the stream passes north-eastwards into the arctic - regions, and the right branch south-eastwards by the Azores; so does - the wind. The south-eastern branch of the stream, after passing the - Canaries, re-enters the equatorial current and flows into the Gulf - of Mexico; the same, it will be observed, holds true of the wind. A - like remarkable agreement exists in reference to all the other leading - currents of the ocean. This is particularly seen in the case of the - great antarctic current between long. 140° W. and 160° W. This current, - flowing northwards from the antarctic regions, instead of bending to - the left under the influence of rotation, turns to the right when it - enters the regions of the westerly winds, and flows eastwards towards - the South-American shores. In fact, all the currents in this region of - strong westerly winds flow in an easterly or north-easterly direction.</p> - - <p>Taking into account the effects resulting from the conformation of - sea and land, the system of ocean-currents agrees<span class="pagenum" id="Page_214">214</span> precisely with - the system of the winds. All the principal currents of the globe are - in fact moving in the exact direction in which they ought to move, - assuming the winds to be the sole impelling cause. In short, so perfect - is the agreement between the two systems, that, given the system of - winds and the conformation of sea and land, and the direction of all - the currents of the ocean, or more properly the system of oceanic - circulation, might be determined <i lang="la">à priori</i>. Or given the system of the - ocean-currents together with the conformation of sea and land, and the - direction of the prevailing winds could also be determined <i lang="la">à priori</i>. - Or, thirdly, given the system of winds and the system of currents, - and the conformation of sea and land might be roughly determined. For - example, it can be shown by this means that the antarctic regions - are probably occupied by a continent and not by a number of separate - islands, nor by sea.</p> - - <p>While holding that the currents of the ocean form one system of - circulation, we must not be supposed to mean that the various currents - are connected end to end, having the same water flowing through them - all in succession like that in a heating apparatus. All that is - maintained is simply this, that the currents are so mutually related - that any great change in one would modify the conditions of all the - others. For example, a great increase or decrease in the easterly flow - of antarctic water in the Southern Ocean would decrease or increase, - as the case might be, the strength of the West Australian current; - and this change would modify the equatorial current of the Indian - Ocean, a modification which in like manner would affect the Agulhas - current and the Southern Atlantic current—this last leading in turn - to a modification of the equatorial current of the Atlantic, and - consequently of the Brazilian current and the Gulf-stream. Furthermore, - since a current impelled by the winds, as Mr. Laughton in his excellent - paper on Ocean-currents justly remarks, tends to leave a vacancy - behind, it follows that a decrease or increase in the Gulf-stream would - affect the equatorial current, the<span class="pagenum" id="Page_215">215</span> Agulhas current, and all the other - currents back to the antarctic currents. Again, a large modification - in the great antarctic drift-current would in like manner affect all - the currents of the Pacific. On the other hand, any great change in - the currents of the Pacific would ultimately affect the currents of - the Atlantic and Indian Oceans, through its influence on the Cape Horn - current, the South Australian current, and the current passing through - the Asiatic archipelago; and <i lang="la">vice versâ</i>, any changes in the currents - of the Atlantic or Indian Oceans would modify the currents of the - Pacific.</p> - - <p><em>Cause of Gibraltar Current.</em>—I may now consider the cause of the - Gibraltar current. There can be little doubt that this current owes its - origin (as Mr. Laughton points out) to the Gulf-stream. “I conceive,” - that author remarks, “that the Gibraltar current is distinctly a stream - formed by easterly drift of the North Atlantic, which, although it - forms a southerly current on the coast of Portugal, is still strongly - pressed to the eastward and seeks the first escape it can find. So - great indeed does this pressure seem to be, that more water is forced - through the Straits than the Mediterranean can receive, and a part - of it is ejected in reverse currents, some as lateral currents on - the surface, some, it appears, as an under current at a considerable - depth.”<a id="FNanchor_95" href="#Footnote_95" class="fnanchor">[95]</a> The funnel-shaped nature of the strait through which the - water is impelled helps to explain the existence of the under current. - The water being pressed into the narrow neck of the channel tends to - produce a slight banking up; and as the pressure urging the water - forward is greatest at the surface and diminishes rapidly downwards, - the tendency to the restoration of level will cause an underflow - towards the Atlantic, because below the surface the water will find the - path of least resistance. It is evident indeed that this underflow will - not take place toward the Mediterranean, from the fact that that sea is - already filled to overflowing by the current received from the outside - ocean.</p> - - <p><span class="pagenum" id="Page_216">216</span></p> - - <p>If we examine the Current-chart published by the Hydrographic - Department of the Admiralty, we shall find the Gibraltar current - represented as merely a continuation of the S.E. flow of Gulf-stream - water. Now, if the arrows shown upon this chart indicate correctly the - direction of the flow, we must become convinced that the Gulf-stream - water cannot possibly avoid passing through the Gibraltar Strait. Of - course the excess of evaporation over that of precipitation within - the Mediterranean area would alone suffice to produce a considerable - current through the Strait; but this of itself would not fill that - inland sea to overflowing.<a id="FNanchor_96" href="#Footnote_96" class="fnanchor">[96]</a></p> - - <p>The Atlantic may, in fact, be regarded as an immense whirlpool with the - Saragossa Sea as its vortex; and although it is true, as will be seen - from an inspection of the Chart, that the wind blows round the Atlantic - along the very path taken by the water, impelling the water forward - along every inch of its course, yet nevertheless it must hold equally - true that the water has a tendency to flow off in a straight line at - a tangent to the circular course in which it is moving. But the water - is so hemmed in on all sides that it cannot leave this circular path - except only at two points; and at these two points it actually does - flow outwards. On the east and west sides the land prevents any such - outflow. Similarly, in the south the escape of the water is frustrated - by the pressure of the opposing currents flowing from that quarter; - while in the north it is prevented by the pressure exerted by polar - currents from Davis Strait and the Arctic Ocean. But in the Strait of - Gibraltar and in the north-eastern portion of the Atlantic between - <span class="pagenum" id="Page_217">217</span>Iceland and the north-eastern shores of Europe there is no resistance - offered: and at these two points an outflow does actually take place. - In both cases, however, especially the latter, the outflow is greatly - aided by the impulse of the prevailing winds.</p> - - <p>No one, who will glance at the accompanying chart (<a href="#PLATE_I">Plate I.</a>) showing - how the north-eastern branch of the Gulf-stream bends round and, of - course, necessarily presses against the coast, can fail to understand - how the Atlantic water should be impelled into the Gibraltar Strait, - even although the loss sustained by the Mediterranean from evaporation - did not exceed the gain from rain and rivers.</p> - - <p><em>Theory of Under Currents.</em>—The consideration that ocean-currents are - simply parts of a system of circulation produced by the system of - prevailing winds, and not by the impulse of the trade-winds alone, - helps to remove the difficulty which some have in accounting for the - existence of under currents without referring them to difference of - specific gravity. Take the case of the Gulf-stream, which passes - under the polar stream on the west of Spitzbergen, this latter stream - passing in turn under the Gulf-stream a little beyond Bear Island. The - polar streams have their origin in the region of prevailing northerly - winds, which no doubt extends to the pole. The current flowing past - the western shores of Spitzbergen, throughout its entire course up - to near the point where it disappears under the warm waters of the - Gulf-stream, lies in the region of these same northerly winds. Now why - should this current cease to be a surface current as soon as it passes - out of the region of northerly into that of south-westerly winds? The - explanation seems to be this: when the stream enters the region of - prevailing south-westerly winds, its progress southwards along the - surface of the ocean is retarded both by the wind and by the surface - water moving in opposition to its course; but being continually pressed - forward by the impulse of the northerly winds acting along its whole - course back almost to the pole, perhaps, or as far north at least<span class="pagenum" id="Page_218">218</span> as - the sea is not wholly covered with ice, the polar current cannot stop - when it enters the region of opposing winds and currents; it must move - forward. But the water thus pressed from behind will naturally take - the <em>path of least resistance</em>. Now in the present case this path will - necessarily lie at a considerable distance below the surface. Had the - polar stream simply to contend with the Gulf-stream flowing in the - opposite direction, it would probably keep the surface and continue its - course along the side of that stream; but it is opposed by the winds, - from which it cannot escape except by dipping down under the surface; - and the depth to which it will descend will depend upon the depth of - the surface current flowing in the opposite direction. There is no - necessity for supposing a heaping up of the water in order to produce - by pressure a force sufficient to impel the under current. The pressure - of the water from behind is of itself enough. The same explanation, of - course, applies to the case of the Gulf-stream passing under the polar - stream. And if we reflect that these under currents are but parts of - the general system of circulation, and that in most cases they are - currents compensating for water drained off at some other quarter, we - need not wonder at the distance which they may in some cases flow, as, - for example, from the banks of Newfoundland to the Gulf of Mexico. - The under currents of the Gulf-stream are necessary to compensate for - the water impelled southwards by the northerly winds; and again, the - polar under currents are necessary to compensate for the water impelled - northward by the south and south-westerly winds.</p> - - <p>But it may be asked, how do the opposing currents succeed in crossing - each other? It is evident that the Gulf-stream must plunge through - the whole thickness of the polar stream before it can become an - under current, and so likewise must the cold water of the polar-flow - pass through the genial water of the Gulf-stream in order to get - underneath it and continue on its course towards the south. The - accompanying diagram (Plate II., Fig. 1) will render this sufficiently - intelligible.</p> - - <div class="figcenter illow550" id="PLATE_II" > - <div class="caption"><i>Fig. 3</i></div> - <div class="attribt">PLATE II</div> - <img src="images/i_219a.jpg" width="550" height="674" alt="" /> - <div class="caption"> - <i>Map shewing meeting of the Gulf-stream and Polar Current (from D<sup>r</sup>. - Petermann’s Geographische Mittheilungen.</i>)<br /> - <i>The curved lines are Isotherms; temperatures are in Fahrenheit.</i></div> - </div> - - <div class="figcenter" id="i_219b" > - <div class="caption">N. Winds - <img class="iglyph-a" src="images/r_arrow.jpg" alt=">>>———>" width="150" height="21" /> - <i>Fig. 1</i> - <img class="iglyph-a" src="images/l_arrow.jpg" alt="<———<<<" width="150" height="21" /> - S. Winds</div> - <img src="images/i_219b.jpg" width="550" height="58" alt="" /> - <div class="caption"><i>Diagram to shew how two opposing currents intersect each other</i></div> - </div> - - <div class="figcenter illow550" id="i_219c" > - <div class="caption"><i>Surface Plan to shew how two opposing currents meet each other</i></div> - <img src="images/i_219c.jpg" width="550" height="201" alt="" /> - <div class="attribr">W. & A. K. Johnston, Edinb<sup>r</sup>. and London.</div> - <div class="caption"><i>Fig. 2</i></div> - </div> - - <p><span class="pagenum" id="Page_219">219</span></p> - - <p>Now these two great ocean-currents are so compelled to intersect each - other for the simple reason that they cannot turn aside, the one to the - left and the other to the right. When two broad streams like those in - question are pressed up against each other, they succeed in mutually - intersecting each other’s path by breaking up into bands or belts—the - cold water being invaded and pierced as it were by long tongues of - warm water, while at the same time the latter is similarly intersected - by corresponding protrusions of cold water. The two streams become - in a manner interlocked, and the one passes through the other very - much as we pass the fingers of one hand between the fingers of the - other. The diagram (<a href="#PLATE_II">Plate II.</a>, Fig. 2), representing the surface of - the ocean at the place of meeting of two opposing currents, will show - this better than description. At the surface the bands necessarily - assume the tongue-shaped appearance represented in the diagram, but - when they have succeeded in mutually passing down through the whole - thickness of the opposing currents, they then unite and form two - definite under currents, flowing in opposite directions. The polar - bands, after penetrating the Gulf-stream, unite below to form a - southward-flowing under current, and in the same way the Gulf-stream - bands, uniting underneath the polar current, continue in their - northerly course as a broad under current of warm water. That this is - a correct representation of what actually occurs in nature becomes - evident from an inspection of the current charts. Thus in the chart - of the North Atlantic which accompanies Dr. Petermann’s Memoir on the - Gulf-stream, we observe that south of Spitzbergen the polar current and - the Gulf-stream are mutually interpenetrated—long tongues invading and - dipping down underneath the Gulf-stream, while in like manner the polar - current becomes similarly intersected by well-marked protrusions of - warm water flowing from the south. (See <a href="#PLATE_II">Plate II.</a>, Fig. 3.)</p> - - <p>No accurate observations, as far as I know, have been made regarding - the amount of work performed by the wind in impelling the water - forward; but when we consider the great<span class="pagenum" id="Page_220">220</span> retarding effect of objects - on the earth’s surface, it is quite apparent that the amount of work - performed on the surface of the ocean must be far greater than is - generally supposed. For example, Mr. Buchan, Secretary to the Scottish - Meteorological Society, has shown<a id="FNanchor_97" href="#Footnote_97" class="fnanchor">[97]</a> that a fence made of slabs of - wood three inches in width and three inches apart from each other is a - protection even during high winds to objects on the lee side of it, and - that a wire screen with meshes about an inch apart affords protection - during a gale to flower-pots. The same writer was informed by Mr. Addie - that such a screen put up at Rockville was torn to pieces by a storm of - wind, the wire screen giving way much in the same way as sails during a - hurricane at sea.</p> - - <p><em>The “Challenger’s” Crucial Test of the Wind and Gravitation Theories - of Oceanic Circulation.</em>—It has been shown in former chapters that all - the facts which have been adduced in support of the gravitation theory - are equally well explained by the wind theory. We may now consider a - class of facts which do not appear to harmonize with either theory. The - recent investigations of the <cite>Challenger</cite> Expedition into the thermal - state of the ocean reveal a condition of things which appears to me - utterly irreconcilable with the gravitation theory.</p> - - <p>It is a condition absolutely essential to the gravitation theory that - the surface of the ocean should be highest in equatorial regions and - slope downwards to either pole. Were water absolutely frictionless, an - incline, however small, would be sufficient to produce a surface-flow - from the equator to the poles, but to induce such an effect some slope - there must be, or gravitation could exercise no power in drawing the - surface-water polewards.</p> - - <p>The researches of the <cite>Challenger</cite> Expedition bring to light the - striking and important fact that the general surface of the North - Atlantic in order to produce equilibrium must stand at a higher level - than at the equator. In other words the surface of <span class="pagenum" id="Page_221">221</span>the Atlantic is - lowest at the equator, and rises with a gentle slope to well-nigh the - latitude of England. If this be the case, then it is mechanically - impossible that, as far as the North Atlantic is concerned, there can - be any such general movement as Dr. Carpenter believes. Gravitation can - no more cause the surface-water of the Atlantic to flow towards the - arctic regions than it can compel the waters of the Gulf of Mexico up - the Mississippi into the Missouri. The impossibility is equally great - in both cases.</p> - - <p>In order to prove what has been stated, let us take a section of the - mid-Atlantic, north and south, across the equator; and, to give the - gravitation theory every advantage, let us select that particular - section adopted by Dr. Carpenter as the one of all others most - favourable to his theory, viz., Section marked No. VIII. in his memoir - lately read before the Royal Geographical Society.<a id="FNanchor_98" href="#Footnote_98" class="fnanchor">[98]</a></p> - - <p>The fact that the polar cold water comes so near the surface at the - equator is regarded by Dr. Carpenter as evidence in favour of the - gravitation theory. On first looking at Dr. Carpenter’s section it - forcibly struck me that if it was accurately drawn, the ocean to be - in equilibrium would require to stand at a higher level in the North - Atlantic than at the equator. In order, therefore, to determine - whether this is the case or not I asked the hydrographer of the - Admiralty to favour me with the temperature soundings indicated in the - section, a favour which was most obligingly granted. The following - are the temperature soundings at the three stations A, B, and C. The - temperature of C are the mean of six soundings taken along near the - equator:—</p> - - <p><span class="pagenum" id="Page_222">222</span></p> - - <table summary="Temperature soundings"> - <tbody> - <tr> - <th class="bt bl">Depth in Fathoms.</th> - <th class="bt bl">Lat. 37° 54′ N.<br />Long. 41° 44′ W.</th> - <th class="bt bl">Lat. 23° 10′ N.<br />Long. 38° 42′ W.</th> - <th colspan="2" class="bt bl br bb">Mean of six temperature soundings near equator.</th> - </tr> - <tr> - <th class="bb bl"> </th> - <th class="bb bl">Temperature.</th> - <th class="bb bl">Temperature.</th> - <th class="bb bl">Depth in Fathoms.</th> - <th class="bb bl br">Temperature.</th> - </tr> - <tr> - <td class="bl"> </td> - <td class="tdc bl"><div>°</div></td> - <td class="tdc bl"><div>°</div></td> - <td class="bl"> </td> - <td class="tdc bl br"><div>°</div></td> - </tr> - <tr> - <td class="tdc bl"><div>Surface.</div></td> - <td class="tdc bl"><div>70·0</div></td> - <td class="tdc bl"><div>72·0</div></td> - <td class="tdc bl"><div>Surface.</div></td> - <td class="tdc bl br"><div>77·9</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 100</div></td> - <td class="tdc bl"><div>63·5</div></td> - <td class="tdc bl"><div>67·0</div></td> - <td class="tdc bl"><div> 10</div></td> - <td class="tdc bl br"><div>77·2</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 200</div></td> - <td class="tdc bl"><div>60·6</div></td> - <td class="tdc bl"><div>57·6</div></td> - <td class="tdc bl"><div> 20</div></td> - <td class="tdc bl br"><div>77·1</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 300</div></td> - <td class="tdc bl"><div>60·0</div></td> - <td class="tdc bl"><div>52·5</div></td> - <td class="tdc bl"><div> 30</div></td> - <td class="tdc bl br"><div>76·9</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 400</div></td> - <td class="tdc bl"><div>54·8</div></td> - <td class="tdc bl"><div>47·7</div></td> - <td class="tdc bl"><div> 40</div></td> - <td class="tdc bl br"><div>71·7</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 500</div></td> - <td class="tdc bl"><div>46·7</div></td> - <td class="tdc bl"><div>43·7</div></td> - <td class="tdc bl"><div> 50</div></td> - <td class="tdc bl br"><div>64·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 600</div></td> - <td class="tdc bl"><div>41·6</div></td> - <td class="tdc bl"><div>41·7</div></td> - <td class="tdc bl"><div> 60</div></td> - <td class="tdc bl br"><div>60·4</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 700</div></td> - <td class="tdc bl"><div>40·6</div></td> - <td class="tdc bl"><div>40·6</div></td> - <td class="tdc bl"><div> 70</div></td> - <td class="tdc bl br"><div>59·4</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 800</div></td> - <td class="tdc bl"><div>38·1</div></td> - <td class="tdc bl"><div>39·4</div></td> - <td class="tdc bl"><div> 80</div></td> - <td class="tdc bl br"><div>58·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 900</div></td> - <td class="tdc bl"><div>37·8</div></td> - <td class="tdc bl"><div>39·2</div></td> - <td class="tdc bl"><div> 90</div></td> - <td class="tdc bl br"><div>58·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div>1000</div></td> - <td class="tdc bl"><div>37·9</div></td> - <td class="tdc bl"><div>38·3</div></td> - <td class="tdc bl"><div> 100</div></td> - <td class="tdc bl br"><div>55·6</div></td> - </tr> - <tr> - <td class="tdc bl"><div>1100</div></td> - <td class="tdc bl"><div>37·1</div></td> - <td class="tdc bl"><div>38·0</div></td> - <td class="tdc bl"><div> 150</div></td> - <td class="tdc bl br"><div>51·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div>1200</div></td> - <td class="tdc bl"><div>37·1</div></td> - <td class="tdc bl"><div>37·6</div></td> - <td class="tdc bl"><div> 200</div></td> - <td class="tdc bl br"><div>46·6</div></td> - </tr> - <tr> - <td class="tdc bl"><div>1300</div></td> - <td class="tdc bl"><div>37·2</div></td> - <td class="tdc bl"><div>36·7</div></td> - <td class="tdc bl"><div> 300</div></td> - <td class="tdc bl br"><div>42·2</div></td> - </tr> - <tr> - <td class="tdc bl"><div>1400</div></td> - <td class="tdc bl"><div>37·1</div></td> - <td class="tdc bl"><div>36·9</div></td> - <td class="tdc bl"><div> 400</div></td> - <td class="tdc bl br"><div>40·3</div></td> - </tr> - <tr> - <td class="tdc bl"><div>1500</div></td> - <td class="tdc bl"><div>..</div></td> - <td class="tdc bl"><div>36·7</div></td> - <td class="tdc bl"><div> 500</div></td> - <td class="tdc bl br"><div>38·9</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2700</div></td> - <td class="tdc bl"><div>35·2</div></td> - <td class="tdc bl"><div>..</div></td> - <td class="tdc bl"><div> 600</div></td> - <td class="tdc bl br"><div>39·2</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2720</div></td> - <td class="tdc bl"><div>..</div></td> - <td class="tdc bl"><div>35·4</div></td> - <td class="tdc bl"><div> 700</div></td> - <td class="tdc bl br"><div>39·0</div></td> - </tr> - <tr> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div> 800</div></td> - <td class="tdc bl br"><div>39·1</div></td> - </tr> - <tr> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div> 900</div></td> - <td class="tdc bl br"><div>38·2</div></td> - </tr> - <tr> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div>1000</div></td> - <td class="tdc bl br"><div>36·9</div></td> - </tr> - <tr> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div>1100</div></td> - <td class="tdc bl br"><div>37·6</div></td> - </tr> - <tr> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div>1200</div></td> - <td class="tdc bl br"><div>36·7</div></td> - </tr> - <tr> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div>1300</div></td> - <td class="tdc bl br"><div>35·8</div></td> - </tr> - <tr> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div>1400</div></td> - <td class="tdc bl br"><div>36·4</div></td> - </tr> - <tr> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div>1500</div></td> - <td class="tdc bl br"><div>36·1</div></td> - </tr> - <tr> - <td class="tdc bl bb"> </td> - <td class="tdc bl bb"> </td> - <td class="tdc bl bb"> </td> - <td class="tdc bl bb"><div>Bottom.</div></td> - <td class="tdc bl br bb"><div>34·7</div></td> - </tr> - </tbody> - </table> - - <p>On computing the extent to which the three columns A, B, and C are each - expanded by heat according to Muncke’s table of the expansion of sea - water for every degree Fahrenheit, I found that column B, in order to - be in equilibrium with C (the equatorial column), would require to have - its surface standing fully 2 feet 6 inches above the level of column C, - and column A fully 3 feet 6 inches above that column. In short, it is - evident that there must be a gradual rise from the equator to latitude - 38° N. of 3½ feet. Any one can verify the accuracy of these results by - making the necessary computations for himself.<a id="FNanchor_99" href="#Footnote_99" class="fnanchor">[99]</a></p> - - <div class="figcenter illow600" id="PLATE_III" > - <div class="attribt">PLATE III</div> - <img src="images/i_222.jpg" width="600" height="324" alt="" /> - <div class="attribr">W. & A. K. Johnston, Edinb<sup>r</sup>. and London.</div> - <div class="caption">SECTION OF THE ATLANTIC nearly North and South, between LAT. 38° N. & - LAT. 38° S.</div> - </div> - - <p><span class="pagenum" id="Page_223">223</span></p> - - <p>I may observe that, had column C extended to the same depth as columns - A and B, the difference of level would be considerably greater, for - column C requires to balance only that portion of columns A and B - which lies above the level of its base. Suppose a depth of ocean equal - to that of column C to extend to the north pole, and the polar water - to have a uniform temperature of 32° from the surface to the bottom, - then, in order to produce equilibrium, the surface of the ocean at - the equator would require to be 4 feet 6 inches above that at the - pole. But the surface of the ocean at B would be 7 feet, and at A 8 - feet, above the poles. Gravitation never could have caused the ocean - to assume this form. It is impossible that this immense mass of warm - water, extending to such a depth in the North Atlantic, could have been - brought from equatorial regions by means of gravitation. And, even - if we suppose this accumulation of warm water can be accounted for - by some other means, still its presence precludes the possibility of - any such surface-flow as that advocated by Dr. Carpenter. For so long - as the North Atlantic stands 3½ feet above the level of the equator, - gravitation can never move the equatorial waters polewards.</p> - - <p>There is another feature of this section irreconcilable with the - gravitation theory. It will be observed that the accumulation of warm - water is all in the North Atlantic, and that there is little or none - in the south. But according to the gravitation theory it ought to - have been the reverse. For owing to the unrestricted communication - between the equatorial and antarctic regions, the general flow of - water towards the south pole is, according to that theory, supposed to - be greater than towards the north, and consequently the quantity of - warm equatorial water in the South Atlantic ought also to be greater. - Dr. Carpenter himself seems to be aware of this difficulty besetting - the theory, and meets it by stating that “the upper stratum of the - North Atlantic is not nearly as much cooled down by its limited polar - underflow, as that of the South Atlantic is by the vast movement of - antarctic water which is constantly taking<span class="pagenum" id="Page_224">224</span> place towards the equator.” - But this “vast movement of antarctic water” necessarily implies a vast - counter-movement of warm surface-water. So that if there is more polar - water in the South Atlantic to produce the cooling effect, there should - likewise be more warm water to be cooled.</p> - - <p>According to the wind theory of oceanic circulation the explanation of - the whole phenomena is simple and obvious. It has already been shown - that owing to the fact that the S. E. trades are stronger than the N. - E., and blow constantly over upon the northern hemisphere, the warm - surface-water of the South Atlantic is drifted across the equator. It - is then carried by the equatorial current into the Gulf of Mexico, and - afterwards of course forms a part of the Gulf-stream.</p> - - <p>The North Atlantic, on the other hand, not only does not lose its - surface heat like the equatorial and South Atlantic, but it receives - from the Gulf-stream in the form of warm water an amount of heat, as we - have seen, equal to one-fourth of all the heat which it receives from - the sun. The reason why the warm surface strata are so much thicker - on the North Atlantic than on the equatorial regions is perfectly - obvious. The surface-water at the equator is swept into the Gulf of - Mexico by the trade-winds and the equatorial current, as rapidly as it - is heated by the sun, so that it has not time to gather to any great - depth. But all this warm water is carried by the Gulf-stream into the - North Atlantic, where it accumulates. That this great depth of warm - water in the North Atlantic, represented in the section, is derived - from the Gulf-stream, and not from a direct flow from the equator due - to gravitation, is further evident from the fact that temperature - sounding A in latitude 38° N. is made through that immense body of warm - water, upwards of 300 fathoms thick, extending from Bermuda to near the - Azores, discovered by the <cite>Challenger</cite> Expedition, and justly regarded - by Captain Nares as an offshoot of the Gulf-stream. This, in Captain - Nares’s Report, is No. 8 “temperature sounding,” between Bermuda and - the Azores; sounding <span class="pagenum" id="Page_225">225</span>B is No. 6 “temperature curve,” between Teneriffe - and St. Thomas.</p> - - <p>There is an additional reason to the one already stated why the - surface temperature of the South Atlantic should be so much below - that of the North. It is perfectly true that whatever amount of water - is transferred from the southern hemisphere to the northern must be - compensated by an equal amount from the northern to the southern - hemisphere, nevertheless the warm water which is carried off the South - Atlantic by the winds is not directly compensated by water from the - north, but by that cold antarctic current whose existence is so well - known to mariners from the immense masses of ice which it brings from - the Southern Ocean.</p> - - <p><em>Thermal Condition of Southern Ocean.</em>——The thermal condition of the - Southern Ocean, as ascertained by the <cite>Challenger</cite> Expedition, appears - to me to be also irreconcilable with the gravitation theory. Between - the parallels of latitude 65° 42′ S. and 50° 1′ S., the ocean, with - the exception of a thin stratum at the surface heated by the sun’s - rays, was found, down to the depth of about 200 fathoms, to be several - degrees colder than the water underneath.<a id="FNanchor_100" href="#Footnote_100" class="fnanchor">[100]</a> The cold upper stratum - is evidently an antarctic current, and the warm underlying water an - equatorial under current. But, according to the gravitation theory, the - colder water should be underneath.</p> - - <p>The very fact of a mass of water, 200 fathoms deep and extending over - fifteen degrees of latitude, remaining above water of three or four - degrees higher temperature shows how little influence difference of - temperature has in producing motion. If it had the potency which some - attribute to it, one would suppose that this cold stratum should sink - down and displace the warm water underneath. If difference of density - is sufficient to move the water horizontally, surely it must be more - than sufficient to cause it to sink vertically.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XIV"> - <span class="pagenum" id="Page_226">226</span> - <h2> - CHAPTER XIV.<br /><br /> - <span class="small">THE WIND THEORY OF OCEANIC CIRCULATION IN RELATION TO CHANGE OF CLIMATE.</span> - </h2> - </div> - <div class="subhead">Direction of Currents depends on Direction of the Winds.—Causes - which affect the Direction of Currents will affect Climate.—How - Change of Eccentricity affects the Mode of Distribution of - the Winds.—Mutual Reaction of Cause and Effect.—Displacement - of the Great Equatorial Current.—Displacement of the - Median Line between the Trades, and its Effect on - Currents.—Ocean-currents in Relation to the Distribution of - Plants and Animals.—Alternate Cold and Warm Periods in North - and South.—Mr. Darwin’s Views quoted.—How Glaciers at the - Equator may be accounted for.—Migration across the Equator.</div> - - <p><em>Ocean-currents in Relation to Change of Climate.</em>—In my attempts to - prove that oceanic circulation is produced by the winds and not by - difference of specific gravity, and that ocean-currents are the great - distributors of heat over the globe, my chief aim has been to show - the bearing which these points have on the grand question of secular - changes of climate during geological epochs, more particularly in - reference to that mystery the cause of the glacial epoch.</p> - - <p>In concluding this discussion regarding oceanic circulation, I may - therefore be allowed briefly to recapitulate those points connected - with the subject which seem to shed most light on the question of - changes of climate.</p> - - <p>The complete agreement between the systems of ocean-currents and - winds not only shows that the winds are the impelling cause of the - currents, but it also indicates to what an extent the <em>directions</em> of - the currents are determined by the winds, or, more properly, to what an - extent their directions are determined by the <em>direction</em> of the winds.</p> - - <p>We have seen in <a href="#CHAPTER_II">Chapter II.</a> to what an enormous extent<span class="pagenum" id="Page_227">227</span> the climatic - conditions of the globe are dependent on the distribution of heat - effected by means of ocean-currents. It has been there pointed out - that, if the heat conveyed from inter-tropical to temperate and polar - regions by oceanic circulation were restored to the former, the - equatorial regions would then have a temperature about 55° warmer, - and the high polar regions a climate 83° colder than at present. It - follows, therefore, that any cause which will greatly affect the - currents or greatly change their paths and mode of distribution, will - of necessity seriously affect the climatic condition of the globe. But - as the existence of these currents depends on the winds, and their - direction and form of distribution depend upon the direction and form - of distribution of the winds, any cause which will greatly affect the - winds will also greatly affect the currents, and consequently will - influence the climatic condition of the globe. Again, as the existence - of the winds depends mainly on the difference of temperature between - equatorial and polar regions, any cause which will greatly affect this - difference of temperature will likewise greatly affect the winds; and - these will just as surely react on the currents and climatic conditions - of the globe. A simple increase or decrease in the difference of - temperature between equatorial and polar regions, though it would - certainly produce an increase or a decrease, as the case might be, in - the strength of the winds, and consequently in the strength of the - currents, would not, however, greatly affect the mode of <em>distribution</em> - of the winds, nor, as a consequence, the mode of <em>distribution</em> of - the currents. But although a simple change in the difference of - temperature between the equator and the poles would not produce a - different <em>distribution</em> of aërial, and consequently of ocean-currents, - nevertheless a <em>difference in the difference</em> of temperature between - the equator and the two poles would do so; that is to say, any cause - that should increase the difference of temperature between the equator - and the pole on the one hemisphere, and decrease that difference on - the other, would effect a change in the distribution of the aërial - currents, which change would in turn<span class="pagenum" id="Page_228">228</span> produce a corresponding change in - the distribution of ocean-currents.</p> - - <p>It has been shown<a id="FNanchor_101" href="#Footnote_101" class="fnanchor">[101]</a> that an increase in the eccentricity of the - earth’s orbit tends to lower the temperature of the one hemisphere and - to raise the temperature of the other. It is true that an increase of - eccentricity does not afford more heat to the one hemisphere than to - the other; nevertheless it brings about a condition of things which - tends to lower the temperature of the one hemisphere and to raise the - temperature of the other. Let us imagine the eccentricity to be at its - superior limit, 0·07775, and the winter solstice in the aphelion. The - midwinter temperature, owing to the increased distance of the sun, - would be lowered enormously; and the effect of this would be to cause - all the moisture which now falls as rain during winter in temperate - regions to fall as snow. Nor is this all; the winters would not merely - be colder than now, but they would also be much longer. At present the - summer half-year exceeds the winter half year by nearly eight days; but - at the period in question the winters would be longer than the summers - by upwards of thirty-six days. The heat of the sun during the short - summer, for reasons which have already been explained, would not be - sufficient to melt the snow of winter; so that gradually, year by year, - the snow would continue to accumulate on the ground.</p> - - <p>On the southern hemisphere the opposite condition of things would - obtain. Owing to the nearness of the sun during the winter of that - hemisphere, the moisture of the air would be precipitated as rain in - regions where at present it falls as snow. This and the shortness of - the winter would tend to produce a decrease in the quantity of snow. - The difference of temperature between the equatorial and the temperate - and polar regions would therefore be greater on the northern than on - the southern hemisphere; and, as a consequence, the aërial currents - of the former hemisphere would be stronger than those of the latter. - This would be more especially the case <span class="pagenum" id="Page_229">229</span>with the trade-winds. The - N.E. trades being stronger than the S.E. trades would blow across the - equator, and the median line between them would therefore be at some - distance to the south of the equator. Thus the equatorial waters would - be impelled more to the southern than to the northern hemisphere; and - the warm water carried over in this manner to the southern hemisphere - would tend to increase the difference of temperature between the two - hemispheres. This change, again, would in turn tend to strengthen the - N.E. and to weaken the S.E. trades, and would thus induce a still - greater flow of equatorial waters into the southern hemisphere—a - result which would still more increase the difference of temperature - between the northern and southern hemisphere, and so on—the one cause - so reacting on the other as to increase its effects, as was shown at - length in <a href="#CHAPTER_IV">Chapter IV.</a></p> - - <p>It was this mutual reaction of those physical agents which led, as was - pointed out in <a href="#CHAPTER_IV">Chapter IV.</a>, to that extraordinary condition of climate - which prevailed during the glacial epoch.</p> - - <p>There is another circumstance to be considered which perhaps more - than any thing else would tend to lower the temperature of the one - hemisphere and to raise the temperature of the other; and this is - the <em>displacement of the great equatorial current</em>. During a glacial - period in the northern hemisphere the median line between the trades - would be shifted very considerably south of the equator; and the same - would necessarily be the case with the great equatorial currents, the - only difference being that the equatorial currents, other things being - equal, would be deflected farther south than the median line. For the - water impelled by the strong N.E. trades would be moving with greater - velocity than the waters impelled by the weaker S.E. trades, and, of - course, would cross the median line of the trades before its progress - southwards could be arrested by the counteracting influence of the S.E. - trades. Let us glance briefly at the results which would follow from - such a condition of things. In the first place, as was shown on former<span class="pagenum" id="Page_230">230</span> - occasions,<a id="FNanchor_102" href="#Footnote_102" class="fnanchor">[102]</a> were the equatorial current of the Atlantic (the feeder - of the Gulf-stream) shifted considerably south of its present position, - it would not bifurcate, as it now does, off Cape St. Roque, owing to - the fact that the whole of the waters would strike obliquely against - the Brazilian coast and thus be deflected into the Southern Ocean. The - effect produced on the climate of the North Atlantic and North-Western - Europe by the withdrawal of the water forming the Gulf-stream, may be - conceived from what has already been stated concerning the amount of - heat conveyed by that stream. The heat thus withdrawn from the North - Atlantic would go to raise the temperature of the Southern Ocean and - antarctic regions. A similar result would take place in the Pacific - Ocean. Were the equatorial current of that ocean removed greatly to - the south of its present position, it would not then impinge and be - deflected upon the Asiatic coast, but upon the continent of Australia; - and the greater portion of its waters would then pass southward into - the Southern Ocean, while that portion passing round the north of - Australia (owing to the great strength of the N.E. trades) would rather - flow into the Indian Ocean than turn round, as now, along the east - coast of Asia by the Japan Islands. The stoppage of the Japan current, - combined with the displacement of the equatorial current to the south - of the equator, would greatly lower the temperature of the whole of the - North Pacific and adjoining continents, and raise to a corresponding - degree the temperature of the South Pacific and Southern Ocean. Again, - the waters of the equatorial current of the Indian Ocean (owing to the - opposing N.E. trades), would not, as at present, find their way round - the Cape of Good Hope into the North Atlantic, but would be deflected - southwards into the Antarctic Sea.</p> - - <p>We have in the present state of things a striking example of the extent - to which the median line between the two trades may be shifted, and the - position of the great equatorial currents of the ocean may be affected, - by a slight difference in the <span class="pagenum" id="Page_231">231</span>relative strength of the two aërial - currents. The S.E. trades are at present a little stronger than the - N.E.; and the consequence is that they blow across the equator into the - northern hemisphere to a distance sometimes of 10 or 15°, so that the - mean position of the median line lies at least 6 or 7 degrees north of - the equator.</p> - - <p>And it is doubtless owing to the superior strength of the S.E. trades - that so much warm water crosses the equator from the South to the North - Atlantic, and that the main portion of the equatorial current flows - into the Caribbean Sea rather than along the Brazilian coast. Were the - two trades of equal strength, the transference of heat into the North - Atlantic from the southern hemisphere by means of the Southern Atlantic - and equatorial currents would be much less than at present. The same - would also hold true in regard to the Pacific.</p> - - <p><em>Ocean-currents in Relation to the Distribution of Plants and - Animals.</em>—In the fifth and last editions of the “Origin of Species,” - Mr. Darwin has done me the honour to express his belief that the - foregoing view regarding alternate cold and warm periods in north - and south during the glacial epoch explains a great many facts in - connection with the distribution of plants and animals which have - always been regarded as exceedingly puzzling.</p> - - <p>There are certain species of plants which occur alike in the temperate - regions of the southern and northern hemispheres. At the equator these - same temperate forms are found on elevated mountains, but not on the - lowlands. How, then, did these temperate forms manage to cross the - equator from the northern temperate regions to the southern, and <i lang="la">vice - versâ</i>? Mr. Darwin’s solution of the problem is (in his own words) as - follows:—</p> - - <p>“As the cold became more and more intense, we know that arctic forms - invaded the temperate regions; and from the facts just given, there - can hardly be a doubt that some of the more vigorous, dominant, and - widest-spreading temperate forms invaded the equatorial lowlands. - The inhabitants of these hot<span class="pagenum" id="Page_232">232</span> lowlands would at the same time have - migrated to the tropical and subtropical regions of the south; for the - southern hemisphere was at this period warmer. On the decline of the - glacial period, as both hemispheres gradually recovered their former - temperatures, the northern temperate forms living on the lowlands under - the equator would have been driven to their former homes or have been - destroyed, being replaced by the equatorial forms returning from the - south. Some, however, of the northern temperate forms would almost - certainly have ascended any adjoining high land, where, if sufficiently - lofty, they would have long survived like the arctic forms on the - mountains of Europe.”</p> - - <p>“In the regular course of events the southern hemisphere would in - its turn be subjected to a severe glacial period, with the northern - hemisphere rendered warmer; and then the southern temperate forms - would invade the equatorial lowlands. The northern forms which had - before been left on the mountains would now descend and mingle with the - southern forms. These latter, when the warmth returned, would return - to their former homes, leaving some few species on the mountains, and - carrying southward with them some of the northern temperate forms which - had descended from their mountain fastnesses. Thus we should have some - few species identically the same in the northern and southern temperate - zones and on the mountains of the intermediate tropical regions” (p. - 339, sixth edition).</p> - - <p>Additional light is cast on this subject by the results already stated - in regard to the enormous extent to which the temperature of the - equator is affected by ocean-currents. Were there no transferrence of - heat from equatorial to temperate and polar regions, the temperature - of the equator, as has been remarked, would probably be about 55° - warmer than at present. In such a case no plant existing on the face of - the globe could live at the equator unless on some elevated mountain - region. On the other hand, were the quantity of warm water which is - being transferred from the equator to be very<span class="pagenum" id="Page_233">233</span> much increased, the - temperature of inter-tropical latitudes might be so lowered as easily - to admit of temperate species of plants growing at the equator. A - lowering of the temperature at the equator some 20° or 30° is all that - would be required; and only a moderate increase in the volume of the - currents proceeding from the equator, taken in connection with the - effects flowing from the following considerations, might suffice to - produce that result. During the glacial epoch, when the one hemisphere - was under ice and the other enjoying a warm and equable climate, the - median line between the trades may have been shifted to almost the - tropical line of the warm hemisphere. Under such a condition of things - the warmest part would probably be somewhere about the tropic of the - warm hemisphere, and not, as now, at the equator; for since all, or - nearly all, the surface-water of the equator would then be impelled - over to the warm hemisphere, the tropical regions of that hemisphere - would be receiving nearly double their present amount of warm water.</p> - - <p>Again, as the equatorial current at this time would be shifted towards - the tropic of the warm hemisphere, the surface-water would not, as at - present, be flowing in equatorial regions parallel to the equator, - but obliquely across it from the cold to the warm hemisphere. This of - itself would tend greatly to lower the temperature of the equator.</p> - - <p>It follows, therefore, as a necessary consequence, that during the - glacial epoch, when the one hemisphere was under snow and ice and - the other enjoying a warm and equable climate, the temperature of - the equator would be lower than at present. But when the glaciated - hemisphere (which we may assume to be the northern) began to grow - warmer and the climate of the southern or warm hemisphere to get - colder, the median line of the trades and the equatorial currents - of the ocean also would begin to move back from the southern tropic - towards the equator. This would cause the temperature of the equator - to rise and to continue rising until the equatorial currents reached - their normal position. When the snow began to accumulate<span class="pagenum" id="Page_234">234</span> on the - southern hemisphere and to disappear on the northern, the median line - of the trades and the equatorial currents of the ocean would then - begin to move towards the northern tropic as they had formerly towards - the southern. The temperature of the equator would then again begin - to sink, and continue to do so until the glaciation of the southern - hemisphere reached its maximum. This oscillation of the thermal equator - to and fro across the geographical equator would continue so long as - the alternate glaciation of the two hemispheres continued.</p> - - <p>This lowering of the temperature of the equator during the severest - part of the glacial epoch will help to explain the former existence of - glaciers in inter-tropical regions at no very great elevation above the - sea-level, evidence of which appears recently to have been found by - Professor Agassiz, Mr. Belt, and others.</p> - - <p>The glacial <em>epoch</em> may be considered as contemporaneous in both - hemispheres. But the epoch consisted of a succession of cold and warm - <em>periods</em>, the cold periods of one hemisphere coinciding with the warm - periods of the other, and <i lang="la">vice versâ</i>.</p> - - <p><em>Migration across the Equator.</em>—Mr. Belt<a id="FNanchor_103" href="#Footnote_103" class="fnanchor">[103]</a> and others have felt - some difficulty in understanding how, according to theory, the plants - and animals of temperate regions could manage to migrate from one - hemisphere to the other, seeing that in their passage they would have - to cross the thermal equator. The oscillation to and fro of the thermal - equator across the geographical, removes every difficulty in regard to - how the migration takes place. When, for example, a cold period on the - northern hemisphere and the corresponding warm one on the southern were - at their maximum, the thermal equator would by this time have probably - passed beyond the Tropic of Capricorn. The geographical equator would - then be enjoying a subtropical, if not a temperate condition of - climate, and the plants and animals of the northern hemisphere would - manage then to reach the equator. When the cold began to abate <span class="pagenum" id="Page_235">235</span>on - the northern and to increase on the southern hemisphere, the thermal - equator would commence its retreat towards the geographical. The plants - and animals from the north, in order to escape the increasing heat as - the thermal equator approached them, would begin to ascend the mountain - heights; and when that equator had passed to its northern limit, and - the geographical equator was again enjoying a subtropical condition of - climate, the plants and animals would begin to descend and pursue their - journey southwards as the cold abated on the southern hemisphere.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XV"> - <span class="pagenum" id="Page_236">236</span> - <h2> - CHAPTER XV.<br /><br /> - <span class="small">WARM INTER-GLACIAL PERIODS.</span> - </h2> - </div> - <div class="subhead">Alternate Cold and Warm Periods.—Warm Inter-glacial Periods - a Test of Theories.—Reason why their Occurrence has not - been hitherto recognised.—Instances of Warm Inter-glacial - Periods.—Dranse, Dürnten, Hoxne, Chapelhall, Craiglockhart, - Leith Walk, Redhall Quarry, Beith, Crofthead, Kilmaurs, - Sweden, Ohio, Cromer, Mundesley, &c., &c.—Cave and River - Deposits.—Occurrence of Arctic and Warm Animals in some Beds - accounted for.—Mr. Boyd Dawkins’s Objections.—Occurrence - of Southern Shells in Glacial Deposits.—Evidence of Warm - Inter-glacial Periods from Mineral Borings.—Striated - Pavements.—Reason why Inter-glacial Land-surfaces are so rare.</div> - - <p><em>Alternate Cold and Warm Periods.</em>—If the theory developed in the - foregoing chapters in reference to the cause of secular changes of - climate be correct, it follows that that long age known as the glacial - epoch did not, as has hitherto been generally supposed, consist of one - long unbroken period of cold and ice. Neither did it consist, as some - have concluded, of two long periods of ice with an intervening mild - period, but it must have consisted of a long succession of cold and - warm periods; the warm periods of the one hemisphere corresponding in - time with the cold periods of the other and <i lang="la">vice versâ</i>. It follows - also from theory that as the cold periods became more and more severe, - the warm intervening periods would become more and more warm and - equable. As the ice began to accumulate during the cold periods in - subarctic and temperate regions in places where it previously did not - exist, so in like manner during the corresponding warm periods it would - begin to disappear in arctic regions where it had held enduring sway - throughout the now closing cycle. As the cold periods in the southern - hemisphere became more and more<span class="pagenum" id="Page_237">237</span> severe, the ice would continue to - advance northwards in the temperate regions; but at that very same - time the intervening warm periods in the northern hemisphere would - become warmer and warmer and more equable, and the ice of the arctic - regions would continue to disappear farther and farther to the north, - till by the time that the ice had reached a maximum during the cold - antarctic periods, Greenland and the arctic regions would, during the - warm intervening periods, be probably free of ice and enjoying a mild - and equable climate. Or we may say that as the one hemisphere became - cold the other became warm, and when the cold reached a maximum in the - one hemisphere, the warmth would reach a maximum in the other. The time - when the ice had reached its greatest extension on the one hemisphere - would be the time when it had disappeared from the other.</p> - - <p><em>Inter-glacial Periods a Test of Theories.</em>—Here we have the grand - crucial test of the truth of the foregoing theory of the cause of - the glacial epoch. That the glacial epoch should have consisted of a - succession of cold and warm periods is utterly inconsistent with all - previous theories which have been advanced to account for it. What, - then, is the evidence of geology on this subject? If the glacial epoch - can be proved from geological evidence to have consisted of such a - succession of cold and warm periods, then I have little doubt but the - theory will soon be generally accepted. But at the very outset an - objection meets us, viz., why call an epoch, which consisted as much of - warm periods as of cold, a glacial epoch, or an “Ice Age,” as Mr. James - Geikie tersely expresses it? Why not as well call it a warm epoch as a - cold one, seeing that, according to theory, it was just as much a warm - as a cold epoch? The answer to this objection will be fully discussed - in the chapter on the Reason of the Imperfection of Geological Records. - But in the meantime, I may remark that it will be shown that the epoch - known as the glacial has been justly called the glacial epoch or “Ice - Age,” because the geological evidences of the cold periods remain in - a remarkably<span class="pagenum" id="Page_238">238</span> perfect state, whilst the evidences of the warm periods - have to a great extent disappeared. The reason of this difference - in the two cases will be discussed in the chapter to which I have - referred. Besides, the condition of things during the cold periods was - so extraordinary, so exceptional, so totally different from those now - prevailing, that even supposing the geological records of the warm - periods had been as well preserved as those of the cold, nevertheless - we should have termed the epoch in question a glacial epoch. There - is yet another reason, however, for our limited knowledge of warm - inter-glacial periods. Till very lately, little or no attention was - paid by geologists to this part of the subject in the way of keeping - records of cases of inter-glacial deposits which, from time to time, - have been observed. Few geologists ever dreamt of such a thing as - warm periods during the age of ice, so that when intercalated beds of - sand and gravel, beds of peat, roots, branches, trunks, leaves, and - fruits of trees were found in the boulder clay, no physical importance - was attached to them, and consequently no description or record of - them ever kept. In fact, all such examples were regarded as purely - accidental and exceptional, and were considered not worthy of any - special attention. A case which came under my own observation will - illustrate my meaning. An intelligent geologist, some years ago, read a - paper before one of our local geological societies, giving an account - of a fossiliferous bed of clay found intercalated between two distinct - beds of till. In this intercalated bed were found rootlets and stems of - trees, nuts, and other remains, showing that it had evidently been an - old inter-glacial land surface. In the transactions of the society a - description of the two beds of till was given, but no mention whatever - was made of the intercalated bed containing the organic remains, - although this was the only point of any real importance.</p> - - <p>Since the theory that the glacial epoch resulted from a high state - of eccentricity of the earth’s orbit began to receive some little - acceptance, geologists have paid a good deal of attention to cases of - intercalated beds in the till containing organic<span class="pagenum" id="Page_239">239</span> remains, and the - result is that we have already a great body of evidence of a geological - nature in favour of warm inter-glacial periods, and I have little - doubt that in the course of a few years the former occurrence of warm - inter-glacial periods will be universally admitted.</p> - - <p>I shall now proceed to give a very brief outline of the evidence - bearing on the subject. But the cases to which I shall have to refer - are much too numerous to allow me to enter into details.</p> - - <p><em>Inter-glacial Beds of Switzerland.</em>—The first geologist, so far as I - am aware, who directed attention to evidence of a break in the cold of - the glacial epoch was M. Morlot. It is now twenty years ago since he - announced the existence of a warm period during the glacial epoch from - geological evidence connected with the glacial drift of the Alps.<a id="FNanchor_104" href="#Footnote_104" class="fnanchor">[104]</a></p> - - <p>The rivers of Switzerland, he found, show on their banks three - well-marked terraces of regularly stratified and well-rounded shingle, - identical with the modern deposits of the rivers. They stand at 50, - 100, and 150 feet above the present level of the rivers. These terraces - were evidently formed by the present system of rivers when these flowed - at a higher level, and extend up the Alps to a height of from 3,000 to - 4,000 feet above the level of the sea. There is a terrace bordering the - Rhine at Camischollas, above Disentis, 4,400 feet above the level of - the sea, proving that during the period of its formation the Alps were - free of ice up to the height of 4,400 feet above the sea-level. It is - well known that a glacial period must have succeeded the formation of - these drifts, for they are in many places covered with erratics. At - Geneva, for example, an erratic drift nearly 50 feet thick is seen to - rest on the drift of the middle terrace, which rises 100 feet above the - level of the lake. But it is also evident that a glacial period must - have preceded the formation of the drift beds, for they are found to - lie in many places upon the unstratified boulder <span class="pagenum" id="Page_240">240</span>clay or <em>till</em>. M. - Morlot observed in the neighbourhood of Clareus, from 7 to 9 feet of - drift resting upon a bed of true till 40 feet thick; the latter was - composed of a compact blue clay, containing worn and scratched alpine - boulders and without any trace of stratification. In the gorge of - Dranse, near Thoron, M. Morlot found the whole three formations in a - direct superimposed series. At the bottom was a mass of compact till or - boulder clay, 12 feet thick, containing boulders of alpine limestone. - Over this mass came regularly stratified beds 150 feet thick, made - up of rounded pebbles in horizontal beds. Above this again lay a - second formation of unstratified boulder clay, with erratic blocks and - striated pebbles, which constituted the left lateral moraine of the - great glacier of the Rhone, when it advanced for the second time to the - Lake of Geneva. A condition of things somewhat similar was observed by - M. Ischer in the neighbourhood of Berne.</p> - - <p>These facts, M. Morlot justly considers, prove the existence of two - glacial periods separated by an intermediate one, during which the - ice, which had not only covered Switzerland, but the greater part of - Europe, disappeared even in the principal valleys of the Alps to a - height of more than 4,400 feet above the present level of the sea. This - warm period, after continuing for long ages, was succeeded by a second - glacial period, during which the country was again covered with ice as - before. M. Morlot even suggests the possibility of these alternations - of cold and warm periods depending upon a cosmical cause. “Wild as it - may have appeared,” he says, “when first started, the idea of general - and periodical eras of refrigeration for our planet, connected perhaps - with some cosmic agency, may eventually prove correct.”<a id="FNanchor_105" href="#Footnote_105" class="fnanchor">[105]</a></p> - - <p>Shortly afterwards, evidence of a far more remarkable character was - found in the glacial drift of Switzerland, namely, the famous lignite - beds of Dürnten. In the vicinity of Utznach and Dürnten, on the Lake of - Zurich, and near Mörschwyl, on the Lake of Constance, there are beds of - coal or <span class="pagenum" id="Page_241">241</span>lignite, nearly 12 feet thick, lying directly on the boulder - clay. Overlying these beds is another mass of drift and clay 30 feet - in thickness, with rounded blocks, and on the top of this upper drift - lie long angular erratics, which evidently have been transported on - the back of glaciers.<a id="FNanchor_106" href="#Footnote_106" class="fnanchor">[106]</a> Professor Vogt attributes their transport - to floating ice; but he evidently does so to avoid the hypothesis of a - warm period during the glacial epoch.</p> - - <p>Here we have proof not merely of the disappearance of the ice during - the glacial epoch, but of its absence during a period of sufficient - length to allow of the growth of 10 or 12 feet of coal. Professor Heer - thinks that this coal-bed, when in the condition of peat, must have - been 60 feet thick; and assuming that one foot of peat would be formed - in a century, he concludes that 6,000 years must have been required - for the growth of the coal plants. According to Liebig, 9,600 years - would be required. This, as we have already seen, is about the average - duration of a warm period.</p> - - <p>In these beds have been found the bones of the elephant (<i>E. Merkii</i>), - stag, cave-bear, and other animals. Numerous insects have also been met - with, which further prove the warm, mild condition of climate which - must have prevailed at the time of the formation of the lignite.</p> - - <p>At Hoxne, near Diss, in Suffolk, a black peaty mass several feet thick, - containing fragments of wood of the oak, yew, and fir, was found, - overlying the boulder clay.<a id="FNanchor_107" href="#Footnote_107" class="fnanchor">[107]</a> Professor Vogt believes that this peat - bed is of the same age as the lignite beds of Switzerland.</p> - - <p>In the glacial drift of North America, particularly about Lake - Champlain and the valley of the St. Lawrence, there is similar evidence - of two glacial periods with an intervening non-glacial or warm - period.<a id="FNanchor_108" href="#Footnote_108" class="fnanchor">[108]</a></p> - - <p><span class="pagenum" id="Page_242">242</span></p> - - <p><em>Glacial and Inter-glacial Periods of the Southern Hemisphere</em>—(<em>South - Africa</em>).—Mr. G. W. Stow, in a paper on the “Geology of South - Africa,”<a id="FNanchor_109" href="#Footnote_109" class="fnanchor">[109]</a> describes a recent glaciation extending over a large - portion of Natal, British Kaffraria, the Kaga and Krome mountains, - which he attributes to the action of land-ice. He sums up the phenomena - as follows:—“The rounding off of the hills in the interiors of the - ancient basins; the numerous dome-shaped (<i>roches moutonnée</i>) rocks; - the enormous erratic boulders in positions where water could not have - carried them; the frequency of unstratified clays—clays with imbedded - angular boulders; drift and lofty mounds of boulders; large tracts of - country thickly spread over with unstratified clays and superimposed - fragments of rock; the Oliphant’s-Hoek clay, and the vast piles of Enon - conglomerate.” In addition to these results of ice-action, he records - the discovery by himself of distinct ice-scratches or groovings on the - surface of the rocks at Reit-Poort in the Tarka, and subsequently<a id="FNanchor_110" href="#Footnote_110" class="fnanchor">[110]</a> - the discovery by Mr. G. Gilfillan of a large boulder at Pniel with - <i lang="la">striæ</i> distinctly marked upon it, and also that the same observer - found that almost every boulder in the gravel at “Moonlight Rush” had - unmistakable striæ on one or more sides.</p> - - <p>In South Africa there is evidence not only of a glacial condition - during the Pliocene period, but also of a warmer climate than now - prevails in that region. “The evidence,” says Mr. Stow, “of the - Pliocene shells of the superficial limestone of the Zwartkops heights, - and elsewhere, leads us to believe that the climate of South Africa - must have been of a far more tropical character than at present.</p> - - <p>“Take, for instance, the characteristic <i lang="la">Venericardia</i> of that - limestone. This has migrated along the coast some 29° or 30° and is now - found within a few degrees of the equator, near Zanzibar, gradually - driven, as I presume it must have been, further and further north by a - gradual lowering of the temperature of the more southern parts of this - coast since the limestone was deposited.”</p> - - <p><span class="pagenum" id="Page_243">243</span></p> - - <p>“During the formation of the shell-banks in the Zwartkops estuary, - younger than the Pliocene limestone, the immense number of certain - species of shells, which have as yet been found living only in - latitudes nearer the equator, point to a somewhat similar though a more - modified change of temperature.”</p> - - <p><em>Inter-glacial Beds of Scotland.</em>—Upwards of a dozen years ago, - Professor Geikie arrived, from his own observations of the glacial - drift of Scotland, at a similar conclusion to that of M. Morlot - regarding the intercalation of warm periods during the glacial epoch; - and the facts on which Professor Geikie’s conclusions were based are - briefly as follows. In a cliff of boulder clay on the banks of the - Slitrig Water, near the town of Hawick, he observed a bed of stones - or shingle. Over the lower stratum of stones lay a few inches of - well-stratified sand, silt, and clay, some of the layers being black - and peaty, <em>with enclosed vegetable fibres</em> in a crumbling state.<a id="FNanchor_111" href="#Footnote_111" class="fnanchor">[111]</a> - There were some 30 or 40 feet of boulder clay above these stratified - beds, and 15 or 20 feet under them. The stones in the shingle band - were identical with those of the boulder clay, but they showed no - striations, and were more rounded and water-worn, and resembled in - every respect the stones now lying in the bed of the Slitrig. The - section of the cliff stood as under:—</p> - - <table summary="Inter-glacial Beds of Scotland"> - <tbody> - <tr> - <td colspan="2"> </td> - <td>1. Vegetable soil.</td> - </tr> - <tr> - <td colspan="2"> </td> - <td>2. Boulder clay, thirty to forty feet.</td> - </tr> - <tr> - <td class="vac">Stratified beds</td> - <td class="tdrc"><span class="x400">{</span></td> - <td>3. Yellowish gravelly sand.<br /> - 4. Peaty silt and clay.<br /> - 5. Fine ferruginous sand.<br /> - 6. Coarse shingle, two to three feet. - </td> - </tr> - <tr> - <td colspan="2"> </td> - <td>7. Coarse, stiff boulder clay, fifteen to twenty feet.</td> - </tr> - </tbody> - </table> - - <p>A few more cases of intercalation of stratified materials in the true - till were also found in the same valley.</p> - - <p>In a cliff of stiff brown boulder clay, about 20 feet high, on the - banks of the Carmichael Water, Lanarkshire, Professor <span class="pagenum" id="Page_244">244</span>Geikie observed - a stratified bed of clay about 3 or 4 inches in thickness. About a mile - higher up the stream, he found a series of beds of gravel, sand, and - clay in the true <em>till</em>. “A thin seam of <em>peaty matter</em>,” he says, “was - observed to run for a few inches along the bottom of a bed of clay and - then disappear, while in a band of fine laminated clay with thin sandy - partings occasional <em>fragments of mouldering wood</em> were found.”<a id="FNanchor_112" href="#Footnote_112" class="fnanchor">[112]</a></p> - - <p>At Chapelhall, near Airdrie, a sand-bed has been extensively mined - under about 114 feet of till. This bed of finely stratified sand - is about 20 feet thick. In it were found lenticular beds of fine - pale-coloured clay containing layers of peat and decaying twigs and - branches. Professor Geikie found the vegetable fibres, though much - decayed, still distinct, and the substance when put into the fire - burned with a dull lambent flame. Underlying these stratified beds, and - forming the floor of the mine, is a deposit of <em>the true till</em> about - 24 feet in thickness. In another pit adjoining, the till forming the - floor is 30 feet thick, but it is sometimes absent altogether, so as to - leave the sand beds resting directly on the sandstone and shale of the - coal-measures. At some distance from this sand-pit an old buried river - channel was met with in one of the pit workings. This channel was found - to contain a coating of boulder clay, on which the laminated sands and - clays reposed, showing, as Professor Geikie has pointed out, that this - old channel had been filled with boulder clay, and then re-excavated - to allow of the deposition of the stratified deposits. Over all lay a - thick mantle of boulder clay which buried the whole.</p> - - <p>A case somewhat similar was found by Professor Nicol in a cutting on - the Edinburgh and Leith Railway. In many places the till had been - worn into hollows as if part of it had been removed by the action of - running water.<a id="FNanchor_113" href="#Footnote_113" class="fnanchor">[113]</a> One of these hollows, about 5 or 6 feet wide by 3 - or 4 feet deep, closely resembled the channel of a small stream. It - was also filled <span class="pagenum" id="Page_245">245</span>with gravel and sand, in all respects like that found - in such a stream at the present day. It was seen to exhibit the same - characters on both sides of the cutting, but Professor Nicol was unable - to determine how far it may have extended beyond; but he had no doubt - whatever that it had been formed by a stream of water. Over this old - watercourse was a thick deposit of true till.</p> - - <p>In reference to the foregoing cases, Professor Geikie makes the - following pertinent remarks:—“Here it is evident that the scooping out - of this channel belongs to the era of the boulder clay. It must have - been effected during a pause in the deposition of the clay, when a run - of water could find its way along the inequalities of the surface of - the clay. This pause must have been of sufficient duration to enable - the runnel to excavate a capacious channel for itself, and leave in it - a quantity of sand and shingle. We can scarcely doubt that when this - process was going on the ground must have been a land surface, and - could not have been under the sea. And lastly, we see from the upper - boulder clay that the old conditions returned, the watercourse was - choked up, and another mass of chaotic boulder clay was tumbled down - upon the face of the country. This indicates that the boulder clay is - not the result of one great catastrophe, but of slow and silent, yet - mighty, forces acting sometimes with long pauses throughout a vast - cycle of time.”<a id="FNanchor_114" href="#Footnote_114" class="fnanchor">[114]</a></p> - - <p>At Craiglockhart Hill, about a mile south of Edinburgh, an extensive - bed of fine sand of from one to three feet in thickness was found - between two distinct masses of true boulder clay or till. The sand was - extensively used for building purposes during the erection of the city - poorhouse a few years ago. In this sand-bed I found a great many tree - roots in the position in which they had grown. During the time of the - excavations I visited the place almost daily, and had every opportunity - of satisfying myself that this sand-bed, prior to the time of the - formation of the upper boulder clay, must have <span class="pagenum" id="Page_246">246</span>been a land surface - on which the roots had grown. In no case did I find them penetrating - into the upper boulder clay, and in several places I found stones of - the upper clay resting directly on the broken ends of the roots. These - roots were examined by Professor Balfour, but they were so decayed that - he was unable to determine their character.</p> - - <p>In digging a foundation for a building in Leith Walk, Edinburgh, a few - years ago, two distinct beds of sand were passed through, the upper, - about 10 feet in thickness, rested upon what appeared to be a denuded - surface of the lower bed. In this lower bed, which evidently had been - a land surface, numbers of tree roots were found. I had the pleasure - of examining them along with my friend Mr. C. W. Peach, who first - directed my attention to them. In no instance were the roots found - in the upper bed. That these roots did not belong to trees which had - grown on the present surface and penetrated to that depth, was further - evident from the fact that in one or two cases we found the roots - broken off at the place where they had been joined to the trunk, and - there the upper sand-bed over them was more than 10 feet in thickness. - If we assume that the roots belonged to trees which had grown on the - present surface, then we must also assume, what no one would be willing - to admit, that the trunks of the trees had grown downwards into the - earth to a depth of upwards of ten feet. I have shown these roots to - several botanists, but none of them could determine to what trees they - belonged. The surface of the ground at the spot in question is 45 feet - above sea-level. Mr. Peach and I have found similar roots in the under - sand-bed at several other places in the same neighbourhood. That they - belong to an inter-glacial period appears probable for the following - reasons:—(1.) This upper sand-bed is overlaid by a tough clay, which - in all respects appears to be the same as the Portobello clay, which - we know belongs to the glacial series. In company with Mr. Bennie, - I found the clay in some places to be contorted in a similar manner - to the Portobello clays. (2.) In a sand-pit about one or two hundred - yards to the west of<span class="pagenum" id="Page_247">247</span> where the roots were found, the sand-bed was - found contorted in the most extraordinary manner to a depth of about 15 - feet. In fact, for a space of more than 30 feet, the bedding had been - completely turned up on end without the fine layers being in the least - degree broken or disarranged, showing that they had been upturned by - some enormous powers acting on a large mass of the sand.</p> - - <p>One of the best examples of true till to be met with in the - neighbourhood of Edinburgh is at Redhall Quarry, about three miles to - the south-west of the city. In recently opening up a new quarry near - the old one a bed of peat was found intercalated in the thick mass of - till overlying the rock. The clay overlying and underlying the peat-bed - was carefully examined by Mr. John Henderson,<a id="FNanchor_115" href="#Footnote_115" class="fnanchor">[115]</a> and found to be true - till.</p> - - <p>In a quarry at Overtown, near Beith, Ayrshire, a sedimentary bed of - clay, intercalated between two boulder clays, was some years ago - observed by Mr. Robert Craig, of the Glasgow Geological Society. This - bed filled an elliptical basin about 130 yards long, and about 30 yards - broad. Its thickness averaged from one to two feet. This sedimentary - bed rested on the till on the north-east end of the basin, and was - itself overlaid on the south-west end by the upper bed of till. The - clay bed was found to be full of roots and stems of the common hazel. - That these roots had grown in the position in which they were found - was evident from the fact that they were in many places found to pass - into the “cutters” or fissures of the limestone, and were here found - in a flattened form, having in growing accommodated themselves to the - size and shape of the fissures. Nuts of the hazel were plentifully - found.<a id="FNanchor_116" href="#Footnote_116" class="fnanchor">[116]</a></p> - - <p>At Hillhead, some distance from Overtown, there is a similar - intercalated bed full of hazel remains, and a species of freshwater - <i>Ostracoda</i> was detected by Mr. David Robertson.</p> - - <p>In a railway cutting a short distance from Beith, Mr. Craig pointed out - to my colleague, Mr. Jack, and myself, a thin <span class="pagenum" id="Page_248">248</span>layer of peaty matter, - extending for a considerable distance between an upper and lower mass - of till; and at one place we found a piece of oak about four feet in - length and about seven or eight inches in thickness. This oak boulder - was well polished and striated.</p> - - <p>Not far from this place is the famous Crofthead inter-glacial bed, so - well known from the description given by Mr. James Geikie and others - that I need not here describe it. I had the pleasure of visiting the - section twice while it was well exposed, once, in company with Mr. - James Geikie, and I do not entertain the shadow of a doubt as to its - true inter-glacial character.</p> - - <p>In the silt, evidently the mud of an inter-glacial lake, were found the - upper portion of the skull of the great extinct ox (<i>Bos primigenius</i>), - horns of the Irish elk or deer, and bones of the horse. In the detailed - list of the lesser organic remains found in the intercalated peat-bed - by Mr. J. A. Mahony,<a id="FNanchor_117" href="#Footnote_117" class="fnanchor">[117]</a> are the following, viz., three species of - <i>Desmidaceæ</i>, thirty-one species of <i>Diatomaceæ</i>, eleven species of - mosses, nine species of phanerogamous plants, and several species of - annelids, crustacea, and insects. This list clearly shows that the - inter-glacial period, represented by these remains, was not only mild - and warm, but of considerable duration. Mr. David Robertson found in - the clay under the peat several species of <i>Ostracoda</i>.</p> - - <p>The well-known Kilmaurs bed of peaty matter in which the remains of - the mammoth and reindeer were found, has now by the researches of the - Geological Survey been proved to be of inter-glacial age.<a id="FNanchor_118" href="#Footnote_118" class="fnanchor">[118]</a></p> - - <p>In Ireland, as shown by Professors Hull and Harkness, the inter-glacial - beds, called by them the “manure gravels,” contain numerous fragments - of shells indicating a more genial climate than prevailed when the - boulder clays lying above and below them were formed.<a id="FNanchor_119" href="#Footnote_119" class="fnanchor">[119]</a></p> - - <p><span class="pagenum" id="Page_249">249</span></p> - - <p>In Sweden inter-glacial beds of freshwater origin, containing plants, - have been met with by Herr Nathorst and also by Herr Holmström.<a id="FNanchor_120" href="#Footnote_120" class="fnanchor">[120]</a></p> - - <p>In North America Mr. Whittlesey describes inter-glacial beds of blue - clay enclosing pieces of wood, intercalated with beds of hard pan - (till). Professor Newberry found at Germantown, Ohio, an immense bed of - peat, from 12 to 20 feet in thickness, underlying, in some places 30 - feet, and in other places as much as 80 feet, of till, and overlying - drift beds. The uppermost layers of the peat contain undecomposed - sphagnous mosses, grasses, and sedges, but in the other portions of - the bed abundant fragments of coniferous wood, identified as red cedar - (<i>Juniperus virginiana</i>), have been found. Ash, hickory, sycamore, - together with grape-vines and beech-leaves, were also met with, and - with these the remains of the mastodon and great extinct beaver.<a id="FNanchor_121" href="#Footnote_121" class="fnanchor">[121]</a></p> - - <p><em>Inter-glacial Beds of England.</em>—Scotland has been so much denuded by - the ice sheet with which it was covered during the period of maximum - glaciation that little can be learned in this part of the island - regarding the early history of the glacial epoch. But in England, - and more especially in the south-eastern portion of it, matters are - somewhat different. We have, in the Norwich Crag and Chillesford beds, - a formation pretty well developed, which is now generally regarded as - lying at the base of the Glacial Series. That this formation is of a - glacial character is evident from the fact of its containing shells of - a northern type, such as <i>Leda lanceolata</i>, <i>Cardium Groènlandicum</i>, - <i>Lucina borealis</i>, <i>Cyprina Islandica</i>, <i>Panopæa Norvegica</i>, and - <i>Mya truncata</i>. But the glacial character of the formation is - more strikingly brought out, as Sir Charles Lyell remarks, by the - predominance of such species as <i>Rhynchonella psittacea</i>, <i>Tellina - calcarea</i>, <i>Astarte borealis</i>, <i>Scalaria Groènlandica</i>, and <i>Fusus - carinatus</i>.</p> - - <p><span class="pagenum" id="Page_250">250</span></p> - - <p><em>The “Forest Beds.”</em>—Immediately following this in the order of - time comes the famous “Forest Bed” of Cromer. This buried forest has - been traced for more than forty miles along the coast from Cromer to - near Kessengland, and consists of stumps of trees standing erect, - attached to their roots, penetrating the original soil in which they - grew. Here and in the overlying fluvio-marine beds we have the first - evidence of at least a temperate, if not a warm, inter-glacial period. - This is evident from the character of the flora and fauna belonging - to these beds. Among the trees we have, for example, the Scotch and - spruce fir, the yew, the oak, birch, the alder, and the common sloe. - There have also been found the white and yellow water-lilies, the - pond-weed, and others. Amongst the mammalia have been met with the - <i>Elephas meridionalis</i>, also found in the Lower Pliocene beds of the - Val d’Arno, near Florence; <i>Elephas antiquus</i>, <i>Hippopotamus major</i>, - <i>Rhinoceros Etruscus</i>, the two latter Val d’Arno species, the roebuck, - the horse, the stag, the Irish elk, the <i>Cervus Polignacus</i>, found - also at Mont Perrier, France, <i>C. verticornis</i>, and <i>C. carnutorum</i>, - the latter also found in Pliocene strata of St. Prest, France. In - the fluvio-marine series have been found the <i>Cyclas omnica</i> and the - <i>Paludina marginata</i>, a species of mollusc still found in the South of - France, but no longer inhabiting the British Isles.</p> - - <p>Above the forest bed and fluvio-marine series comes the well-known - unstratified Norwich boulder till, containing immense blocks 6 or 8 - feet in diameter, many of which must have come from Scandinavia, and - above the unstratified till are a series of contorted beds of sand and - gravel. This series may be considered to represent a period of intense - glaciation. Above this again comes the middle drift of Mr. Searles - Wood, junior, yielding shells which indicate, as is now generally - admitted, a comparatively mild condition of climate. Upon this middle - drift lies the upper boulder clay, which is well developed in South - Norfolk and Suffolk, and which is of unmistakable glacial origin. Newer - than all these are the Mundesley freshwater beds, which lie in a hollow - denuded out of the foregoing<span class="pagenum" id="Page_251">251</span> series. In this formation a black peaty - deposit containing seeds of plants, insects, shells, and scales and - bones of fishes, has been found, all indicating a mild and temperate - condition of climate. Among the shells there is, as in the forest bed, - the <i>Paludina marginata</i>. And that an arctic condition of things in - England followed is believed by Mr. Fisher and others, on the evidence - of the “Trail” described by the former observer.</p> - - <p><em>Cave and River Deposits.</em>—Evidence of the existence of warm periods - during the glacial epoch is derived from a class of facts which - have long been regarded by geologists as very puzzling, namely, the - occurrence of mollusca and mammalia of a southern type associated - in England and on the continent with those of an extremely arctic - character. For example, <i>Cyrena fluminalis</i> is a shell which does not - live at present in any European river, but inhabits the Nile and parts - of Asia, especially Cashmere. <i>Unio littoralis</i>, extinct in Britain, - is still abundant in the Loire; <i>Paludina marginata</i> does not exist - in this country. These shells of a southern type have been found in - post-tertiary deposits at Gray’s Thurrock, in Essex; in the valley - of the Ouse, near Bedford; and at Hoxne, in Suffolk, associated with - a <i>Hippopotamus</i> closely allied to that now inhabiting the Nile, and - <i>Elephas antiquus</i>, an animal remarkable for its southern range. - Amongst other forms of a southern type which have been met with in - the cave and river deposits, are the spotted hyæna from Africa, - an animal, says Mr. Dawkins, identical, except in size, with the - cave hyæna, the African elephant (<i>E. Africanus</i>), and the <i>Elephas - meridionalis</i>, the great beaver (<i>Trogontherium</i>), the cave hyæna - (<i>Hyæna spelæa</i>), the cave lion (<i>Felis leo</i>, var. <i>spelæa</i>), the lynx - (<i>Felis lynx</i>), the sabre-toothed tiger (<i>Machairodus latidens</i>), the - rhinoceros (<i>Rhinoceros megarhinus</i> and <i>R. leptorhinus</i>). But the - most extraordinary thing is that along with these, associated in the - same beds, have been found the remains of such animals of an arctic - type as the glutton (<i>Gulo luscus</i>), the ermine (<i>Mustela erminea</i>), - the reindeer (<i>Cervus tarandus</i>), the musk-ox or musk-sheep (<i>Ovibos - moschatus</i>),<span class="pagenum" id="Page_252">252</span> the aurochs (<i>Bison priscus</i>), the woolly rhinoceros - (<i>Rhinoceros tichorhinus</i>), the mammoth (<i>Elephas primigenius</i>), and - others of a like character. According to Mr. Boyd Dawkins, these - southern animals extended as far north as Yorkshire in England, and - the northern animals as far south as the latitude of the Alps and - Pyrenees.<a id="FNanchor_122" href="#Footnote_122" class="fnanchor">[122]</a></p> - - <p><em>The Explanation of the Difficulty.</em>—As an explanation of these - puzzling phenomena, I suggested, in the Philosophical Magazine for - November, 1868, that these southern animals lived in our island during - the warm periods of the glacial epoch, while the northern animals - lived during the cold periods. This view I am happy to find has lately - been supported by Sir John Lubbock; further, Mr. James Geikie, in his - “Great Ice Age,” and also in the Geological Magazine, has entered so - fully into the subject and brought forward such a body of evidence - in support of it, that, in all probability, it will, ere long, be - generally accepted. The only objection which has been advanced, so far - as I am aware, deserving of serious consideration, is that by Mr. Boyd - Dawkins, who holds that if these migrations had been <em>secular</em> instead - of seasonal, as is supposed by Sir Charles Lyell and himself, the - arctic and southern animals would now be found in separate deposits. - It is perfectly true that if there had been only one cold and one warm - period, each of geologically immense duration, the remains might, of - course, be expected to have been found in separate beds; but when - we consider that the glacial epoch consisted of a long <em>succession - of alternate cold and warm periods</em>, of not more than ten or twelve - thousand years each, we can hardly expect that in the river deposits - belonging to this long cycle we should be able to distinguish the - deposits of the cold periods from those of the warm.</p> - - <p><em>Shell Beds.</em>—Evidence of warm inter-glacial periods may be justly - inferred from the presence of shells of a southern type which have been - found in glacial beds, of which some illustrations follow.</p> - - <p>In the southern parts of Norway, from the present sea-level <span class="pagenum" id="Page_253">253</span>up to 500 - feet, are found glacial shell beds, similar to those of Scotland. In - these beds <i>Trochus magus</i>, <i>Tapes decussata</i>, and <i>Pholas candida</i> - have been found, shells which are distributed between the Mediterranean - and the shores of England, but no longer live round the coasts of - Norway.</p> - - <p>At Capellbacken, near Udevalla, in Sweden, there is an extensive bed of - shells 20 to 30 feet in thickness. This formation has been described - by Mr. Gwyn Jeffreys.<a id="FNanchor_123" href="#Footnote_123" class="fnanchor">[123]</a> It consists of several distinct layers, - apparently representing many epochs and conditions. Its shells are of a - highly arctic character, and several of the species have not been found - living south of the arctic circle. But the remarkable circumstance - is that it contains <i>Cypræa lurida</i>, a Mediterranean shell, which - Mr. Jeffreys, after some hesitation, believed to belong to the bed. - Again, at Lilleherstehagen, a short distance from Capellbacken, - another extensive deposit is exposed. “Here the upper layer,” says Mr. - Jeffreys, “gives a singular result. Mixed with the universal <i>Trophon - clathratus</i> (which is a high northern species, and found living only - within the arctic circle) are many shells of a southern type, such are - <i>Ostrea edulis</i>, <i>Tapes pullastra</i>, <i>Corbula gibba</i>, and <i>Aporrhais - pes-pelicani</i>.”</p> - - <p>At Kempsey, near Worcester, a shell bed is described by Sir R. - Murchison in his “Silurian System” (p. 533), in which <i>Bulla ampulla</i> - and a species of <i>Oliva</i>, shells of a southern type, have been found.</p> - - <p>A case somewhat similar to the above is recorded by the Rev. Mr. - Crosskey as having been met with in Scotland at the Kyles of Bute. - “Among the Clyde beds, I have found,” he says, “a layer containing - shells, in which those of a more southern type appear to exist in - greater profusion and perfection than even in our present seas. It is - an open question,” he continues, “whether our climate was not slightly - warmer than it is now between the glacial epoch and the present - day.”<a id="FNanchor_124" href="#Footnote_124" class="fnanchor">[124]</a></p> - - <p><span class="pagenum" id="Page_254">254</span></p> - - <p>In a glacial bed near Greenock, Mr. A. Bell found the fry of living - Mediterranean forms, viz., <i>Conus Mediterraneus</i> and <i>Cardita trapezia</i>.</p> - - <p>Although deposits containing shells of a temperate or of a southern - type in glacial beds have not been often recorded, it by no means - follows that such deposits are actually of rare occurrence. That - glacial beds should contain deposits indicating a temperate or a - warm condition of climate is a thing so contrary to all preconceived - opinions regarding the sequence of events during the glacial epoch, - that most geologists, were they to meet with a shell of a southern - type in one of those beds, would instantly come to the conclusion that - its occurrence there was purely accidental, and would pay no special - attention to the matter.</p> - - <p><em>Evidence derived from “Borings.”</em>—With the view of ascertaining if - additional light would be cast on the sequence of events, during the - formation of the boulder clay, by an examination of the journals of - bores made through a great depth of surface deposits, I collected, - during the summer of 1867, about two hundred and fifty such records, - put down in all parts of the mining districts of Scotland. An - examination of these bores shows most conclusively that the opinion - that the boulder clay, or lower till, is one great undivided formation, - is wholly erroneous.</p> - - <p>These two hundred and fifty bores represent a total thickness of 21,348 - feet, giving 86 feet as the mean thickness of the deposits passed - through. Twenty of these have one boulder clay, with beds of stratified - sand or gravel beneath the clay; twenty-five have <em>two</em> boulder clays, - with stratified beds of sand and gravel between; ten have <em>three</em> - boulder clays; one has <em>four</em> boulder clays; two have <em>five</em> boulder - clays; and no one has fewer than <em>six</em> separate masses of boulder - clay, with stratified beds of sand and gravel between; sixteen have - two or three separate boulder clays, differing altogether in colour - and hardness, without any stratified beds between. We have, therefore, - out of two hundred and fifty bores, seventy-five of them representing - a condition <span class="pagenum" id="Page_255">255</span>of things wholly different from that exhibited to the - geologist in ordinary sections.</p> - - <p>The full details of the character of the deposits passed through by - these bores, and their bearing on the history of the glacial epoch, - have been given by Mr. James Bennie, in an interesting paper read - before the Glasgow Geological Society,<a id="FNanchor_125" href="#Footnote_125" class="fnanchor">[125]</a> to which I would refer all - those interested in the subject of surface geology.</p> - - <p>The evidence afforded by these bores of the existence of warm - inter-glacial periods will, however, fall to be considered in a - subsequent chapter.<a id="FNanchor_126" href="#Footnote_126" class="fnanchor">[126]</a></p> - - <p>Another important and unexpected result obtained from these bores to - which we shall have occasion to refer, was the evidence which they - afforded of a Continental Period.</p> - - <p><em>Striated Pavements.</em>—It has been sometimes observed that in horizontal - sections of the boulder clay, the stones and boulders are all striated - in one uniform direction, and this has been effected over the original - markings on the boulders. It has been inferred from this that a pause - of long duration must have taken place in the formation of the boulder - clay, during which the ice disappeared and the clay became hardened - into a solid mass. After which the old condition of things returned, - glaciers again appeared, passed over the surface of the hardened clay - with its imbedded boulders, and ground it down in the same way as they - had formerly done the solid rocks underneath the clay.</p> - - <p>An instance of striated pavements in the boulder clay was observed by - Mr. Robert Chambers in a cliff between Portobello and Fisherrow. At - several places a narrow train of blocks was observed crossing the line - of the beach, somewhat like a quay or mole, but not more than a foot - above the general level. All the blocks <em>had flat sides uppermost, - and all the flat sides were striated in the same direction</em> as that - of the rocky surface throughout <span class="pagenum" id="Page_256">256</span>the country. A similar instance was - also observed between Leith and Portobello. “There is, in short,” says - Mr. Chambers, “a surface of the boulder clay, deep down in the entire - bed, which, to appearance, has been in precisely the same circumstances - as the fast rock surface below had previously been. It has had in its - turn to sustain the weight and abrading force of the glacial agent, - in whatever form it was applied; and the additional deposits of the - boulder clay left over this surface may be presumed to have been formed - by the agent on that occasion.”<a id="FNanchor_127" href="#Footnote_127" class="fnanchor">[127]</a></p> - - <p>Several cases of a similar character were observed by Mr. James - Smith, of Jordanhill, on the beach at Row, and on the shore of the - Gareloch.<a id="FNanchor_128" href="#Footnote_128" class="fnanchor">[128]</a> Between Dunbar and Cockburnspath, Professor Geikie found - along the beach, for a space of 30 or 40 square yards, numbers of large - blocks of limestone with flattened upper sides, imbedded in a stiff red - clay, and all striated in one direction. On the shores of the Solway he - found another example.<a id="FNanchor_129" href="#Footnote_129" class="fnanchor">[129]</a></p> - - <p>The cases of striated pavements recorded are, however, not very - numerous. But this by no means shows that they are of rare occurrence - in the boulder clay. These pavements, of course, are to be found only - in the interior of the mass, and even there they can only be seen - along a horizontal section. But sections of this kind are rarely to be - met with, for river channels, quarries, railway cuttings, and other - excavations of a similar character which usually lay open the boulder - clay, exhibit vertical sections only. It is therefore only along the - sea-shore, as Professor Geikie remarks, where the surface of the clay - has been worn away by the action of the waves, that opportunities have - hitherto been presented to the geologist for observing them.</p> - - <p>There can be little doubt that during the warm periods of the glacial - epoch our island would be clothed with a luxuriant <span class="pagenum" id="Page_257">257</span>flora. At the end - of a cold period, when the ice had disappeared, the whole face of the - country would be covered over to a considerable depth with a confused - mass of stones and boulder clay. A surface thus wholly destitute of - every seed and germ would probably remain for years without vegetation. - But through course of time life would begin to appear, and during - the thousands of years of perpetual summer which would follow, the - soil, uncongenial as it no doubt must have been, would be forced to - sustain a luxuriant vegetation. But although this was the case, we - need not wonder that now scarcely a single vestige of it remains; for - when the ice sheet again crept over the island everything animate and - inanimate would be ground down to powder. We are certain that prior - to the glacial epoch our island must have been covered with life and - vegetation. But not a single vestige of these are now to be found; - no, not even of the very soil on which the vegetation grew. The solid - rock itself upon which the soil lay has been ground down to mud by the - ice sheet, and, to a large extent, as Professor Geikie remarks, swept - away into the adjoining seas.<a id="FNanchor_130" href="#Footnote_130" class="fnanchor">[130]</a> It is now even more difficult to - find a trace of the ancient soil <em>under</em> the boulder clay than it is - to find remains of the soil of the warm periods <em>in</em> that clay. As - regards Scotland, cases of old land surfaces under the boulder clay are - as seldom recorded as cases of old land surfaces in it. In so far as - geology is concerned, there is as much evidence to show that our island - was clothed with vegetation during the glacial epoch as there is that - it was so clothed prior to that epoch.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XVI"> - <span class="pagenum" id="Page_258">258</span> - <h2> - CHAPTER XVI.<br /><br /> - <span class="small">WARM INTER-GLACIAL PERIODS IN ARCTIC REGIONS.</span> - </h2> - </div> - <div class="subhead">Cold Periods best marked in Temperate, and Warm Periods - in Arctic, Regions.—State of Arctic Regions during - Glacial Period.—Effects of Removal of Ice from Arctic - Regions.—Ocean-Currents; Influence on Arctic Climate.—Reason - why Remains of Inter-glacial Period are rare in Arctic - Regions.—Remains of Ancient Forests in Banks’s Land, Prince - Patrick’s Island, &c.—Opinions of Sir R. Murchison, Captain - Osborn, and Professor Haughton.—Tree dug up by Sir E. Belcher - in lat. 75° N.</div> - - <p class="noindent"><span class="smcap">In</span> the temperate regions the cold periods of the glacial epoch would be - far more marked than the warm inter-glacial periods. The condition of - things which prevailed during the cold periods would differ far more - widely from that which now prevails than would the condition of things - during the warm periods. But as regards the polar regions the reverse - would be the case; there the warm inter-glacial periods would be far - more marked than the cold periods. The condition of things prevailing - in those regions during the warm periods would be in strongest contrast - to what now obtains, but this would not hold true in reference to the - cold periods; for during the latter, matters there would be pretty - much the same as at present, only a good deal more severe. The reason - of this may be seen from what has already been stated in <a href="#CHAPTER_IV">Chapter IV.</a>; - but as it is a point of considerable importance in order to a proper - understanding of the physical state of things prevailing in polar - regions during the glacial epoch, I shall consider this part of the - subject more fully.</p> - - <p>During the cold periods, our island, and nearly all places in the - northern temperate regions down to about the same latitude, would be - covered with snow and ice, and all animal and vegetable life within - the glaciated area would to a great<span class="pagenum" id="Page_259">259</span> extent be destroyed. The presence - of the ice would of itself, for reasons already explained, lower the - mean annual temperature to near the freezing-point. The summers, - notwithstanding the proximity of the sun, would not be warm, on the - contrary their temperature would rise little above the freezing-point. - An excess of evaporation would no doubt take place, owing to the - increase in the intensity of the sun’s rays, but this result would only - tend to increase the snowfall.<a id="FNanchor_131" href="#Footnote_131" class="fnanchor">[131]</a></p> - - <p>During the warm periods our country and the regions under consideration - would experience conditions not differing much from those of the - present, but the climate would probably be somewhat warmer and more - equable. The proximity of the sun during winter would prevent snow - from falling. The summers, owing to the greater distance of the sun, - would probably be somewhat colder than they are now. But the loss of - heat during summer would be to a large extent compensated for by two - causes to which we must here refer. (1.) The much greater amount of - heat conveyed by ocean-currents than at present. (2.) Our summers are - now cooled to a considerable extent by cold aërial currents from the - ice-covered regions of the north. But during the period in question - there would be little or no ice in arctic regions, consequently the - winds would be comparatively warm, whatever direction they came from.</p> - - <p>Let us next direct our attention to the state of things in the arctic - regions during the glacial epoch. At present Greenland and other parts - of the arctic regions occupied by land are almost wholly covered - with ice, and as a consequence nearly destitute of vegetable life. - During the cold periods of the glacial epoch the quantity of snow - falling would doubtless be greater and the ice thicker, but as regards - organic life, matters would not probably be much worse than they are - at present. In fact, so far as Greenland and the antarctic continent - are concerned, they are about as destitute of plant life as they can - be. Although an increase in the thickness of the arctic ice would not - greatly alter the present state of matters in those regions, <span class="pagenum" id="Page_260">260</span>yet what - a transformation would ensue upon the disappearance of the ice! This - would not only raise the summer temperature some twenty degrees or so, - but would afford the necessary conditions for the existence of abundant - animal and plant life. The severity of the climate of Greenland is - due to a very considerable extent, as we have already seen, to the - presence of ice. Get rid of the permanent ice, and the temperature of - the country, <i>cæteris paribus</i>, would instantly rise. That Greenland - should ever have enjoyed a temperate climate, capable of supporting - abundant vegetation, has often been matter of astonishment, but this - wonder diminishes when we reflect that during the warm periods it would - be in the arctic regions that the greatest heating effect would take - place, this being due mainly to the transference of nearly all the warm - inter-tropical waters to one hemisphere.</p> - - <p>It has been shown in <a href="#CHAPTER_II">Chapter II.</a> that the heating effects at present - resulting from the transference of heat by ocean-currents increase as - we approach the poles. As a consequence of this it follows that during - the warm periods, when the quantity of warm water transferred would be - nearly doubled, the <em>increase of heat resulting from this cause would - itself increase</em> as the warm pole was approached. This effect, combined - with the shortness of the winter in perihelion and the nearness of the - sun during that season, would prevent the accumulation of snow. During - summer, the sun, it is true, would be at a much greater distance from - the earth than at present, but it must be borne in mind that for a - period of three months the quantity of heat received from the sun at - the north pole would be greater than that received at the equator. - Consequently, after the winter’s snow was melted, this great amount of - heat would go to raise the temperature, and the arctic summer could - not be otherwise than hot. It is not hot at present, but this, be it - observed, is because of the presence of the ice. When we take all these - facts into consideration we need not be surprised that Greenland once - enjoyed a condition of climate totally different from that which now - obtains in that region.</p> - - <p><span class="pagenum" id="Page_261">261</span></p> - - <p>It is, therefore, in the arctic and antarctic regions where we ought - to find the most marked and decided evidence of warm inter-glacial - periods. And doubtless such evidence would be abundantly forthcoming - had these regions not been subjected to such intense denudation since - the glacial epoch, and were so large a portion of the land not still - buried beneath an icy covering, and therefore beyond the geologist’s - reach. Only on islands and such outlying places as are not shrouded in - snow and ice can we hope to meet with any trace of the warm periods of - the glacial epoch: and we may now proceed to consider what relics of - these warm periods have actually been discovered in arctic regions.</p> - - <p><em>Evidence of Warm Periods in Arctic Regions.</em>—The fact that stumps, - &c., of full-grown trees have been found in places where at present - nothing is to be met with but fields of snow and ice, and where the - mean annual temperature scarcely rises above the zero of the Fahrenheit - thermometer, is good evidence to show that the climate of the arctic - regions was once much warmer than now. The remains of an ancient forest - were discovered by Captain McClure, in Banks’s Land, in latitude 74° - 48′. He found a great accumulation of trees, from the sea-level to an - elevation of upwards of 300 feet. “I entered a ravine,” says Captain - McClure, “some miles inland, and found the north side of it, for a - depth of 40 feet from the surface, composed of one mass of wood similar - to what I had before seen.”<a id="FNanchor_132" href="#Footnote_132" class="fnanchor">[132]</a> In the ravine he observed a tree - protruding about 8 feet, and 3 feet in circumference. And he further - states that, “<em>From the perfect state of the bark</em>, and the position of - the trees so far from the sea, there can be but little doubt that they - grew originally in the country.” A cone of one of these fir-trees was - brought home, and was found to belong apparently to the genus <i>Abies</i>, - resembling <i>A. (Pinus) alba</i>.</p> - - <p>In Prince Patrick’s Island, in latitude 76° 12′ N., longitude 122° - W., near the head of Walker Inlet, and a considerable distance in the - interior in one of the ravines, a tree protruding <span class="pagenum" id="Page_262">262</span>about 10 feet from - a bank was discovered by Lieutenant Mecham. It proved to be 4 feet - in circumference. In its neighbourhood several others were seen, all - of them similar to some he had found at Cape Manning; each of them - measured 4 feet round and 30 feet in length. The carpenter stated that - the trees resembled larch. Lieutenant Mecham, from their appearance and - position, concluded that they must have grown in the country.<a id="FNanchor_133" href="#Footnote_133" class="fnanchor">[133]</a></p> - - <p>Trees under similar conditions were also found by Lieutenant Pim on - Prince Patrick’s Island, and by Captain Parry on Melville Island, all - considerably above the present sea-level and at a distance from the - shore. On the coast of New Siberia, Lieutenant Anjou found a cliff of - clay containing stems of trees still capable of being used for fuel.</p> - - <p>“This remarkable phenomenon,” says Captain Osborn, “opens a vast field - for conjecture, and the imagination becomes bewildered in trying to - realise that period of the world’s history when the absence of ice and - a milder climate allowed forest trees to grow in a region where now the - ground-willow and dwarf-birch have to struggle for existence.”</p> - - <p>Sir Roderick Murchison came to the conclusion that all those trees - were drifted to their present position when the islands of the arctic - archipelago were submerged. But it was the difficulty of accounting - for the growth of trees in such a region which led him to adopt this - hypothesis. His argument is this: “If we imagine,” he says, “that the - timber found in those latitudes grew on the spot we should be driven - to adopt the anomalous hypothesis that, notwithstanding physical - relations of land and water similar to those which now prevail, trees - of large size grew on such <i lang="la">terra firma</i> within a few degrees of the - north pole!—a supposition which I consider to be wholly incompatible - with the data in our possession, and at variance with the laws of the - isothermal lines.”<a id="FNanchor_134" href="#Footnote_134" class="fnanchor">[134]</a> This reasoning of Sir Roderick’s may be quite - correct, on the supposition that <span class="pagenum" id="Page_263">263</span>changes of climate are due to changes - in the distribution of sea and land, as advocated by Sir Charles Lyell. - But these difficulties disappear if we adopt the views advocated in - the foregoing chapters. As Captain Osborn has pointed out, however, - Sir Roderick’s hypothesis leaves the real difficulty untouched. “A - very different climate,” he says, “must then have existed in those - regions to allow driftwood so perfect as to retain its bark to reach - such great distances; and perhaps it may be argued that if that sea was - sufficiently clear of ice to allow such timber to drift unscathed to - Prince Patrick’s Land, that that <em>very absence of a frozen sea would - allow fir-trees to grow in a soil naturally fertile</em>.”<a id="FNanchor_135" href="#Footnote_135" class="fnanchor">[135]</a></p> - - <p>As has been already stated, all who have seen those trees in arctic - regions agree in thinking that they grew <i lang="la">in situ</i>. And Professor - Haughton, in his excellent account of the arctic archipelago appended - to McClintock’s “Narrative of Arctic Discoveries,” after a careful - examination of the entire evidence on the subject, is distinctly of - the same opinion; while the recent researches of Professor Heer put it - beyond doubt that the drift theory must be abandoned.</p> - - <p>Undoubtedly the arctic archipelago was submerged to an extent that - could have admitted of those trees being floated to their present - positions. This, as we shall see, follows from theory; but submergence, - without a warmer condition of climate, would not enable trees to reach - those regions with their bark entire.</p> - - <p>But in reality we are not left to theorise on the subject, for we - have a well-authenticated case of one of those trees being got by - Captain Belcher standing erect in the position in which it grew. It was - found immediately to the northward of the narrow strait opening into - Wellington Sound, in lat. 75° 32′ N. long. 92° W., and about a mile and - a half inland. The tree was dug up out of the frozen ground, and along - with it a portion of the soil which was immediately in contact with the - roots. The whole was packed in canvas and brought to <span class="pagenum" id="Page_264">264</span>England. Near to - the spot several knolls of peat mosses about nine inches in depth were - found, containing the bones of the lemming in great numbers. The tree - in question was examined by Sir William Hooker, who gave the following - report concerning it, which bears out strongly the fact of its having - grown <i lang="la">in situ</i>.</p> - - <p>“The piece of wood brought by Sir Edward Belcher from the shores of - Wellington Channel belongs to a species of pine, probably to the <i>Pinus - (Abies) alba</i>, the most northern conifer. The structure of the wood - of the specimen brought home differs remarkably in its anatomical - character from that of any other conifer with which I am acquainted. - Each concentric ring (or annual growth) consists of two zones of - tissue; one, the outer, that towards the circumference, is broader, of - a pale colour, and consists of ordinary tubes of fibres of wood, marked - with discs common to all coniferæ. These discs are usually opposite - one another when more than one row of them occur in the direction of - the length of the fibre; and, what is very unusual, present radiating - lines from the central depression to the circumference. Secondly, - the inner zone of each annual ring of wood is narrower, of a dark - colour, and formed of more slender woody fibres, with thicker walls in - proportion to their diameter. These tubes have few or no discs upon - them, but are covered with spiral striæ, giving the appearance of each - tube being formed of a twisted band. The above characters prevail in - all parts of the wood, but are slightly modified in different rings. - Thus the outer zone is broader in some than in others, the disc-bearing - fibres of the outer zone are sometimes faintly marked with spiral - striæ, and the spirally marked fibres of the inner zone sometimes bear - discs. These appearances suggest the annual recurrence of some special - cause that shall thus modify the first and last formed fibres of each - year’s deposit, so that that first formed may differ in amount as - well as in kind from that last formed; and the peculiar conditions of - an arctic climate appear to afford an adequate solution. The inner, - or first-formed zone, must be regarded as imperfectly developed,<span class="pagenum" id="Page_265">265</span> - being deposited at a season when the functions of the plant are very - intermittently exercised, and when a few short hours of sunshine are - daily succeeded by many of extreme cold. As the season advances the - sun’s heat and light are continuous during the greater part of the - twenty-four hours, and the newly formed wood fibres are hence more - perfectly developed, they are much longer, present no signs of striæ, - but are studded with discs of a more highly organized structure than - are usual in the natural order to which this tree belongs.”<a id="FNanchor_136" href="#Footnote_136" class="fnanchor">[136]</a></p> - - <p>Another circumstance which shows that the tree had grown where it was - found is the fact that in digging up the roots portions of the leaves - were obtained. It may also be mentioned that near this place was found - an old river channel cut deeply into the rock, which, at some remote - period, when the climate must have been less rigorous than at present, - had been occupied by a river of considerable size.</p> - - <p>Now, it is evident that if a tree could have grown at Wellington Sound, - there is no reason why one might not have grown at Banks’s Land, or - at Prince Patrick’s Island. And, if the climatic condition of the - country would allow one tree to grow, it would equally as well allow - a hundred, a thousand, or a whole forest. If this, then, be the case, - Sir Roderick’s objection to the theory of growth <i lang="la">in situ</i> falls to the - ground.</p> - - <p>Another circumstance which favours the idea that those trees grew - during the glacial epoch is the fact that although they are recent, - geologically speaking, and belong to the drift series, yet they are, - historically speaking, very old. The wood, though not fossilized, is so - hardened and changed by age that it will scarcely burn.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XVII"> - <span class="pagenum" id="Page_266">266</span> - <h2> - CHAPTER XVII.<br /><br /> - <span class="small">FORMER GLACIAL EPOCHS.—REASON OF THE IMPERFECTION OF GEOLOGICAL RECORDS - IN REFERENCE TO THEM.</span> - </h2> - </div> - <div class="subhead">Two Reasons why so little is known of Glacial Epochs.—Evidence - of Glaciation to be found on Land-surfaces.—Where are all our - ancient Land-surfaces?—The stratified Rocks consist of a Series - of old Sea-bottoms.—Transformation of a Land-surface into a - Sea-bottom obliterates all Traces of Glaciation.—Why so little - remains of the Boulder Clays of former Glacial Epochs.—Records - of the Glacial Epoch are fast disappearing.—Icebergs do not - striate the Sea-bottom.—Mr. Campbell’s Observations on the - Coast of Labrador.—Amount of Material transported by Icebergs - much exaggerated.—Mr. Packard on the Glacial Phenomena of - Labrador.—Boulder Clay the Product of Land-ice.—Palæontological - Evidence.—Paucity of Life characteristic of a Glacial - Period.—Warm Periods better represented by Organic Remains than - cold.—Why the Climate of the Tertiary Period was supposed to - be warmer than the present.—Mr. James Geikie on the Defects of - Palæontological Evidence.—Conclusion.</div> - - <p><em>Two Reasons why so little is known of former Glacial Epochs.</em>—If the - glacial epoch resulted from the causes discussed in the foregoing - chapters, then such epochs must have frequently supervened. We may, - therefore, now proceed to consider what evidence there is for the - former occurrence of excessive conditions of climate during previous - geological ages. When we begin our inquiry, however, we soon find - that the facts which have been recorded as evidence in favour of the - action of ice in former geological epochs are very scanty indeed. Two - obvious reasons for this may be given, namely, (1) The imperfection - of the geological records themselves, and (2) the little attention - hitherto paid toward researches of this kind. The notion, once so - prevalent, that the climate of our earth was much warmer in the earlier - geological ages than it is now, and that it has ever since been - gradually becoming cooler, was wholly at variance<span class="pagenum" id="Page_267">267</span> with the idea of - former ice-periods. And this conviction of the <i lang="la">à priori</i> improbability - of cold periods having obtained during Palæozoic and Mesozoic ages - tended to prevent due attention being paid to such facts as seemed to - bear upon the subject. But our limited knowledge of former glacial - epochs must no doubt be attributed chiefly to the actual imperfection - of the geological records. So great is this imperfection that the mere - absence of direct geological evidence cannot reasonably be regarded as - sufficient proof that the conclusions derived from astronomical and - physical considerations regarding former ice-periods are improbable. - Nor is this all. The geological records of ancient glacial conditions - are not only imperfect, but, as I shall endeavour to show, this - imperfection <em>follows as a natural consequence from the principles of - geology itself</em>. There are not merely so many blanks or gaps in the - records, but a reason exists in the very nature of geological evidence - why such breaks in the record might reasonably be expected to occur.</p> - - <p><em>Evidence of Glaciation to be found chiefly on Land-surfaces.</em>—It is on - a land-surface that the principal traces of the action of ice during - a glacial epoch are left, for it is there that the stones are chiefly - striated, the rocks ground down, and the boulder clay formed. But where - are all our ancient land-surfaces? They are not to be found. The total - thickness of the stratified rocks of Great Britain is, according to - Professor Ramsay, nearly fourteen miles. But from the top to the bottom - of this enormous pile of deposits there is hardly a single land-surface - to be detected. True patches of old land-surfaces of a local character - exist, such, for example, as the dirt-beds of Portland; but, with the - exception of coal-seams, every general formation from top to bottom - has been accumulated under water, and none but the under-clays <em>ever - existed as a land</em>-surface. And it is here, in such a formation, - that the geologist has to collect all his information regarding the - existence of former glacial epochs. The entire stratified rocks of the - globe, with the exception of the coal-beds and under-clays (in neither - of which would one expect to find traces of ice-action), consist almost - entirely of a<span class="pagenum" id="Page_268">268</span> <em>series of old sea-bottoms</em>, with here and there an - occasional freshwater deposit. Bearing this in mind, what is the sort - of evidence which we can now hope to find in these old sea-bottoms of - the existence of former ice-periods?</p> - - <p>Every geologist of course admits that the stratified rocks are not - old land-surfaces, but a series of old sea-bottoms formed out of - the accumulated material derived from the degradation of primeval - land-surfaces. And it is true that all land-surfaces once existed - as sea-bottoms; but the stratified rocks consist of a series of old - sea-bottoms which never were land-surfaces. Many of them no doubt - have been repeatedly above the sea-level, and may once have possessed - land-surfaces; but these, with the exception of the under-clays of the - various coal measures, the dirt-beds of Portland, and one or two more - patches, have all been denuded away. The important bearing which this - consideration has on the nature of the evidence which we can now expect - to find of the existence of former glacial epochs has certainly been - very much overlooked.</p> - - <p>If we examine the matter fully we shall be led to conclude that the - <em>transformation of a land-surface into a sea-bottom</em> will probably - completely obliterate every trace of glaciation which that land-surface - may once have presented. We cannot, for example, expect to meet with - polished and striated stones belonging to a former land glaciation; for - such stones are not carried down bodily and unchanged by our rivers - and deposited in the sea. They become broken up by subaërial agencies - into gravel, sand, and clay, and in this condition are transported - seawards. Nor even if we supposed it possible that the stones and - boulders derived from a mass of till could be carried down to sea by - river-action, could we at the same time fail to admit that such stones - would be deprived of all their ice-markings, and become water-worn and - rounded on the way.<a id="FNanchor_137" href="#Footnote_137" class="fnanchor">[137]</a></p> - - <p><span class="pagenum" id="Page_269">269</span></p> - - <p>Nor can we expect to find boulder clay among the stratified rocks, for - boulder clay is not carried down as such and deposited in the sea, but - under the influence of the denuding agents becomes broken up into soft - mud, clay, sand, and gravel, as it is gradually peeled off the land and - swept seawards. Patches of boulder clay may have been now and again - forced into the sea by ice and eventually become covered up; but such - cases are wholly exceptional, and their absence in any formation cannot - fairly be adduced as a proof that that formation does not belong to a - glacial period.</p> - - <p>The only evidence of the existence of land-ice during former periods - which we can reasonably expect to meet with in the stratified rocks, - consists of erratic blocks which may have been transported by icebergs - and dropped into the sea. But unless the glaciers of such epochs - reached the sea, we could not possibly possess even this evidence. - Traces in the stratified rocks of the effects of land-ice during former - epochs must, in the very nature of things, be rare indeed. The only - sort of evidence which, as a general rule, we may expect to detect, - is the presence of large erratic blocks imbedded in strata which from - their constitution have evidently been formed in still water. But this - is quite enough; for it proves the existence of ice at the time the - strata were being deposited as conclusively as though we saw the ice - floating with the blocks upon it. This sort of evidence, when found in - low latitudes, ought to be received as conclusive of the existence of - former glacial epochs; and, no doubt, would have been so received had - it not been for the idea that, if these blocks had been transported - by ice, there ought in addition to have been found striated stones, - boulder clay, and other indications of the agency of land-ice.</p> - - <p>Of course all erratics are not necessarily transported by <span class="pagenum" id="Page_270">270</span>masses of - ice broken from the terminal front of glaciers. The “ice foot,” formed - by the freezing of the sea along the coasts of the higher latitudes of - Greenland, carries seawards immense quantities of blocks and <i lang="fr">débris</i>. - And again stones and boulders are frequently frozen into river-ice, - and when the ice breaks up in spring are swept out to sea, and may be - carried some little distance before they are dropped. But both these - cases can occur only in regions where the winters are excessive; nor - is it at all likely that such ice-rafts will succeed in making a long - voyage. If, therefore, we could assure ourselves that the erratics - occasionally met with in certain old geological formations in low - latitudes were really transported from the land by an ice-foot or a - raft of river-ice, we should be forced to conclude that very severe - climatic conditions must have obtained in such latitudes at the time - the erratics were dispersed.</p> - - <p>The reason why we now have, comparatively speaking, so little direct - evidence of the existence of former glacial periods will be more - forcibly impressed upon the mind, if we reflect on how difficult it - would be in a million or so of years hence to find any trace of what - we now call the glacial epoch. The striated stones would by that time - be all, or nearly all, disintegrated, and the till washed away and - deposited in the bottom of the sea as stratified sands and clays. And - when these became consolidated into rock and were raised into dry land, - the only evidence that we should probably then have that there ever - had been a glacial epoch would be the presence of large blocks of the - older rocks, which would be found imbedded in the upraised formation. - We could only infer that there had been ice at work from the fact that - by no other known agency could we conceive such blocks to have been - transported and dropped in a still sea.</p> - - <p>Probably few geologists believe that during the Middle Eocene and - the Upper Miocene periods our country passed through a condition of - glaciation as severe as it has done during the Post-pliocene period; - yet when we examine the subject carefully, we find that there is - actually no just ground<span class="pagenum" id="Page_271">271</span> to conclude that it has not. For, in all - probability, throughout the strata to be eventually formed out of the - destruction of the now existing land-surfaces, evidence of ice-action - will be as scarce as in Eocene or Miocene strata.</p> - - <p>If the stratified rocks forming the earth’s crust consisted of a series - of old land-surfaces instead (as they actually do) of a series of old - sea-bottoms, then probably traces of many glacial periods might be - detected.</p> - - <p>Nearly all the evidence which we have regarding the glacial epoch - has been derived from what we find on the now existing land-surfaces - of the globe. But probably not a vestige of this will exist in the - stratified beds of future ages, formed out of the destruction of the - present land-surfaces. Even the very arctic shell-beds themselves, - which have afforded to the geologist such clear proofs of a frozen sea - during the glacial epoch, will not be found in those stratified rocks; - for they must suffer destruction along with everything else which now - exists above the sea-level. There is probably not a single relic of - the glacial epoch which has ever been seen by the eye of man that will - be treasured up in the stratified rocks of future ages. Nothing that - does not lie buried in the deeper recesses of the ocean will escape - complete disintegration and appear imbedded in those formations. It - is only those objects which lie in our existing sea-bottoms that will - remain as monuments of the glacial epoch of the Post-tertiary period. - And, moreover, it will only be those portions of the sea-bottoms that - may happen to be upraised into dry land that will be available to the - geologist of future ages. The point to be determined now is this:—<em>Is - it probable that the geologist of the future will find in the rocks - formed out of the now existing sea-bottoms more evidence of a glacial - epoch during Post-tertiary times than we now do of one during, say, the - Miocene, the Eocene, or the Permian period?</em> Unless this can be proved - to be the case, we have no ground whatever to conclude that the cold - periods of the Miocene, Eocene, and Permian periods were not as severe - as that of the glacial epoch. This is evident, for the only relics - which now<span class="pagenum" id="Page_272">272</span> remain of the glacial epochs of those periods are simply - what happened to be protected in the then existing sea-bottoms. Every - vestige that lay on the land would in all probability be destroyed by - subaërial agency and carried into the sea in a sedimentary form. But - before we can determine whether or not there is more evidence of the - glacial epoch in our now existing sea-bottoms than there is of former - glacial epochs in the stratified rocks (which are in reality the - sea-bottoms belonging to ancient epochs), we must first ascertain what - is the nature of those marks of glaciation which are to be found in a - sea-bottom.</p> - - <p><em>Icebergs do not striate the Sea-bottom.</em>—We know that the rocky face - of the country was ground down and striated during the glacial epoch; - and this is now generally believed to have been done by land-ice. But - we have no direct evidence that the floor of the ocean, beyond where it - may have been covered with land-ice, was striated. Beyond the limits - of the land-ice it could be striated only by means of icebergs. But - do icebergs striate the rocky bed of the ocean? Are they adapted for - such work? It seems to be often assumed that they are. But I have been - totally unable to find any rational grounds for such a belief. Clean - ice can have but little or no erosive power, and never could scratch a - rock. To do this it must have grinding materials in the form of sand, - mud, or stones. But the bottoms of icebergs are devoid of all such - materials. Icebergs carry the grinding materials on their backs, not on - their bottoms. No doubt, when the iceberg is launched into the deep, - great masses of sand, mud, and stones will be adhering to its bottom. - But no sooner is the berg immersed, than a melting process commences - at its sides and lower surface in contact with the water; and the - consequence is, the materials adhering to the lower surface soon drop - off and sink to the bottom of the sea. The iceberg, divested of these - materials, can now do very little harm to the rocky sea-bottom over - which it floats. It is true that an iceberg moving with a velocity - of a few miles an hour, if it came in contact with the sea-bottom, - would, by the mere force<span class="pagenum" id="Page_273">273</span> of concussion, tear up loose and disjointed - rocks, and hurl some of the loose materials to a distance; but it would - do but little in the way of grinding down the rock against which it - struck. But even supposing the bottom of the iceberg were properly - shod with the necessary grinding materials, still it would be but a - very inefficient grinding agent; for a <em>floating</em> iceberg would not - be in contact with the sea-bottom. And if it were in contact with the - sea-bottom, it would soon become stranded and, of course, motionless, - and under such conditions could produce no effect.</p> - - <p>It is perfectly true that although the bottom of the berg may be devoid - of grinding materials, yet these may be found lying on the surface - of the submarine rock over which the ice moves. But it must be borne - in mind that the same current which will move the icebergs over the - surface of the rock will move the sand, mud, and other materials - over it also; so that the markings effected by the ice would in all - probability be erased by the current. In the deep recesses of the - ocean the water has been found to have but little or no motion. But - icebergs always follow the path of currents; and it is very evident - that at the comparatively small depth of a thousand feet or so reached - by icebergs the motion of the water will be considerable; and the - continual shifting of the small particles of the mud and sand will in - all probability efface the markings which may be made now and again by - a passing berg.</p> - - <p>Much has been said regarding the superiority of icebergs as grinding - and striating agents in consequence of the great velocity of their - motion in comparison with that of land-ice. But it must be remembered - that it is while the iceberg is floating, and before it touches the - rock, that it possesses high velocity. When the iceberg runs aground, - its motion is suddenly arrested or greatly reduced. But if the iceberg - advancing upon a sloping sea-bottom is raised up so as to exert great - pressure, it will on this account be the more suddenly arrested, - the motion will be slow, and the distance passed over short, before - the berg becomes stranded. If it exerts but little pressure on the - sea-bottom, <span class="pagenum" id="Page_274">274</span>it may retain a considerable amount of motion and advance - to a considerable distance before it is brought to a stand; but, - exerting little pressure, it can perform but little work. Land-ice - moves slowly, but then it exerts enormous pressure. A glacier 1,000 - feet in thickness has a pressure on its rocky bed equal to about 25 - tons on the square foot; but an iceberg a mile in thickness, forced up - on a sloping sea-bottom to an elevation of 20 feet (and this is perhaps - more than any ocean-current could effect), would only exert a pressure - of about half a ton on the square foot, or about 1/50th part of the - pressure of the glacier 1,000 feet in thickness. A great deal has been - said about the erosive and crushing power of icebergs of enormous - thickness, as if their thickness gave them any additional pressure. An - iceberg 100 feet in thickness will exert just as much pressure as one - a mile in thickness. The pressure of an iceberg is not like that of a - glacier, in proportion to its thickness, but to the height to which it - is raised out of the water. An iceberg 100 feet in thickness raised 10 - feet will exert exactly the same pressure as one a mile in thickness - raised to an equal height.</p> - - <p>To be an efficient grinding agent, steadiness of motion, as well as - pressure, is essential. A rolling or rocking motion is ill-adapted - for grinding down and striating a rock. A steady rubbing motion under - pressure is the thing required. But an iceberg is not only deficient in - pressure, but also deficient in steadiness of motion. When an iceberg - moving with considerable velocity comes on an elevated portion of the - sea-bottom, it does not move steadily onwards over the rock, unless - the pressure of the berg on the rock be trifling. The resistance being - entirely at the bottom of the iceberg, its momentum, combined with the - pressure of the current, applied wholly above the point of resistance, - tends to make the berg bend forward, and in some cases upset (when - it is of a cubical form). The momentum of the moving berg, instead - of being applied in forcing it over the rock against which it comes - in contact, is probably all consumed in work against gravitation in - raising the berg upon its<span class="pagenum" id="Page_275">275</span> front edge. After the momentum is consumed, - unless the berg be completely upset, it will fall back under the force - of gravitation to its original position. But the momentum which it - acquires from gravitation in falling backwards carries it beyond its - position of repose in an opposite direction. It will thus continue to - rock backwards and forwards until the friction of the water brings it - to rest. The momentum of the berg, instead of being applied to the work - of grinding and striating the sea-bottom, will chiefly be consumed in - heat in the agitation of the water. But if the berg does advance, it - will do so with a rocking unsteady motion, which, as Mr. Couthouy<a id="FNanchor_138" href="#Footnote_138" class="fnanchor">[138]</a> - and Professor Dana<a id="FNanchor_139" href="#Footnote_139" class="fnanchor">[139]</a> observe, will tend rather to obliterate - striations than produce them.</p> - - <p>A floating berg moves with great steadiness; but a berg that has run - aground cannot advance with a steady motion. If the rock over which the - berg moves offers little resistance, it may do so; but in such a case - the berg could produce but little effect on the rock.</p> - - <p>Dr. Sutherland, who has had good opportunities to witness the effects - of icebergs, makes some most judicious remarks on the subject. “It - will be well” he says, “to bear in mind that when an iceberg <em>touches - the ground, if that ground be hard and resisting, it must come to a - stand</em>, and the propelling power continuing, a slight leaning over in - the water, or yielding motion of the whole mass, may compensate readily - for being so suddenly arrested. If, however, the ground be soft, so - as not to arrest the motion of the iceberg at once, a moraine will be - the result; but the moraine thus raised will tend to bring it to a - stand.”<a id="FNanchor_140" href="#Footnote_140" class="fnanchor">[140]</a></p> - - <p>There is another cause referred to by Professor Dana, which, to a - great extent, must prevent the iceberg from having an opportunity of - striating the sea-bottom, even though it were otherwise well adapted - for so doing. It is this: the bed of the <span class="pagenum" id="Page_276">276</span>ocean in the track of - icebergs must be pretty much covered with stones and rubbish dropped - from the melting bergs. And this mass of rubbish will tend to protect - the rock.<a id="FNanchor_141" href="#Footnote_141" class="fnanchor">[141]</a></p> - - <p>If icebergs cannot be shown <i lang="la">à priori</i>, from mechanical considerations, - to be well adapted for striating the sea-bottom, one would naturally - expect, from the confident way in which it is asserted that they are - so adapted, that the fact has been at least established by actual - observation. But, strange as it may appear, we seem to have little or - no proof that icebergs actually striate the bed of the ocean. This can - be proved from the direct testimony of the advocates of the iceberg - theory themselves.</p> - - <p>We shall take the testimony of Mr. Campbell, the author of two - well-known works in defence of the iceberg theory, viz., “Frost and - Fire,” and “A Short American Tramp.” Mr. Campbell went in the fall of - the year 1864 to the coast of Labrador, the Straits of Belle Isle, and - the Gulf of St. Lawrence, for the express purpose of witnessing the - effects of icebergs, and testing the theory which he had formed, that - the ice-markings of the glacial epoch were caused by floating ice and - not by land-ice, as is now generally believed.</p> - - <p>The following is the result of his observations on the coast of - Labrador.</p> - - <p>Hanly Harbour, Strait of Belle Isle:—“The water is 37° F. in July.... - As fast as one island of ice grounds and bursts, another takes its - place; and in winter the whole strait is blocked up by a mass which - swings bodily up and down, grating along the bottom at all depths.... - Examined the beaches and rocks at the water-line, especially in sounds. - Found the rocks ground smooth, <em>but not striated</em>, in the sounds” - (<cite>Short American Tramp</cite>, pp. 68, 107).</p> - - <p>Cape Charles and Battle Harbour:—“But though these harbours are all - frozen every winter, the <em>rocks at the water-line are not striated</em>” - (p. 68).</p> - - <p>At St. Francis Harbour:—“The water-line is much rubbed, smooth, <em>but - not striated</em>” (p. 72).</p> - - <p><span class="pagenum" id="Page_277">277</span></p> - - <p>Cape Bluff:—“Watched the rocks with a telescope, and <em>failed to make - out striæ anywhere</em>; but the water-line is everywhere rubbed smooth” - (p. 75).</p> - - <p>Seal Islands:—“<em>No striæ are to be seen at the land-wash in these - sounds or on open sea-coasts near the present water-line</em>” (p. 76).</p> - - <p>He only mentions having here found striations in the three following - places along the entire coast of Labrador visited by him; and in regard - to two of these, it seems very doubtful that the markings were made by - modern icebergs.</p> - - <p>Murray’s Harbour:—“This harbour was blocked up with ice on the 20th of - July. The water-line is rubbed, and in <em>some places</em> striated” (p. 69).</p> - - <p>Pack Island:—“The water-line in a narrow sound was polished and - striated in the direction of the sound, about N.N.W. This seems to be - fresh work done by heavy ice drifting from Sandwich Bay; <em>but, on the - other hand, stages with their legs in the sea, and resting on these - very rocks, are not swept away by the ice</em>” (p. 96). If these markings - were modern, why did not the “heavy ice” remove the small fir poles - supporting the fishing-stages?</p> - - <p>Red Bay:—“Landed half-dressed, and found some striæ perfectly fresh at - the water-level, but weathered out a short distance <em>inland</em>” (p. 107). - The striations “inland” could not have been made by modern icebergs; - and it does not follow that because the markings at the water-level - were not weathered they were produced by modern ice.</p> - - <p>These are the evidences which he found that icebergs striate rocks, - on a coast of which he says that, during the year he visited it, “the - winter-drift was one vast solid raft of floes and bergs more than 150 - miles wide, and perhaps 3,000 feet thick at spots, driven by a whole - current bodily over one definite course, year after year, since this - land was found” (p. 85).</p> - - <p>But Mr. Campbell himself freely admits that the floating ice which - comes aground along the shores does not produce striæ. “It is - sufficiently evident,” he says, “<em>that glacial striæ<span class="pagenum" id="Page_278">278</span> are not produced - by thin bay-ice</em>” (p. 76). And in “Frost and Fire,” vol. ii., p. 237, - he states that, “from a careful examination of the water-line at many - spots, it appears that bay-ice grinds rocks, <em>but does not produce - striation</em>.”</p> - - <p>“It is impossible,” he continues, “to get at rocks over which heavy - icebergs now move; but a mass 150 miles wide, perhaps 3,000 feet thick - in some parts, and moving at the rate of a mile an hour, or more, - <em>appears to be an engine amply sufficient</em> to account for striæ on - rising rocks.” And in “American Tramp,” p. 76, he says, “<em>striæ must be - made</em> in deep water by the large masses which seem to pursue the even - tenor of their way in the steady current which flows down the coast.”</p> - - <p>Mr. Campbell, from a careful examination of the sea-bottom along the - coast, finds that the small icebergs do not produce striæ, but the - large ones, which move over rocks impossible to be got at, “must” - produce them. They “appear” to be amply sufficient to do so. If the - smaller bergs cannot striate the sea-bottom, why must the larger ones - do so? There is no reason why the smaller bergs should not move as - swiftly and exert as much pressure on the sea-bottom as the larger - ones. And even supposing that they did not, one would expect that the - light bergs would effect on a smaller scale what the heavy ones would - do on a larger.</p> - - <p>I have no doubt that when Mr. Campbell visited Labrador he expected to - find the sea-coast under the water-line striated by means of icebergs, - and was probably not a little surprised to find that it actually was - not. And I have no doubt that were the sea-bottom in the tracks of the - large icebergs elevated into view, he would find to his surprise that - it was free from striations also.</p> - - <p>So far as observation is concerned, we have no grounds from what Mr. - Campbell witnessed to conclude that icebergs striate the sea-bottom.</p> - - <p>The testimony of Dr. Sutherland, who has had opportunities of seeing - the effects of icebergs in arctic regions, leads us to the same - conclusion. “Except,” he says, “from the evidence<span class="pagenum" id="Page_279">279</span> afforded by plants - and animals at the bottom, we have <em>no means whatever</em> to ascertain - the effect produced by icebergs upon the rocks.<a id="FNanchor_142" href="#Footnote_142" class="fnanchor">[142]</a> In the Malegat - and Waigat I have seen whole clusters of these floating islands, - drawing from 100 to 250 fathoms, moving to and fro with every return - and recession of the tides. I looked very earnestly for grooves and - scratches left by icebergs and glaciers in the rocks, but always failed - to discover any.”<a id="FNanchor_143" href="#Footnote_143" class="fnanchor">[143]</a></p> - - <p>We shall now see whether river-ice actually produces striations or not. - If floating ice under any form can striate rocks, one would expect that - it ought to be done by river-ice, seeing that such ice is obliged to - follow one narrow definite track.</p> - - <p>St. John’s River, New Brunswick:—“This river,” says Mr. Campbell, - “is obstructed by ice during five months of the year. When the ice - goes, there is wild work on the bank. Arrived at St. John, drove - to the suspension-bridge.... At this spot, if <em>anywhere in the - world</em>, river-ice ought to produce striation. The whole drainage of - a wide basin and one of the strongest tides in the world, here work - continually in one rock-groove; and in winter this water-power is armed - with heavy ice. <em>There are no striæ</em> about the water-line.”<a id="FNanchor_144" href="#Footnote_144" class="fnanchor">[144]</a></p> - - <p>River St. Lawrence:—“In winter the power of ice-floats driven by - water-power is tremendous. The river freezes and packs ice till - the flow of water is obstructed. The rock-pass at Quebec is like - the Narrows at St. John’s, Newfoundland. The whole pass, about a - mile wide, was paved with great broken slabs and round boulders of - worn ice as big as small shacks, piled and tossed, and heaped and - scattered upon the level water below and frozen solid.... This kind - of ice does <span class="smcap">not</span> <em>produce striation</em> at the water-margin at - Quebec. At Montreal, when the river ‘goes,’ the ice goes with it - with a vengeance.... The <em>piers are not yet striated</em> by river-ice - at Montreal.... The rocks at the high-water level have <em>no trace</em> of - glacial striæ.... The rock at Ottawa is rubbed by <span class="pagenum" id="Page_280">280</span>river-ice every - spring, and <em>always in one direction, but it is not striated</em>.... - The surfaces are all rubbed smooth, and the edges of broken beds are - rounded where exposed to the ice; <em>but there are no striæ</em>.”<a id="FNanchor_145" href="#Footnote_145" class="fnanchor">[145]</a></p> - - <p>When Sir Charles Lyell visited the St. Lawrence in 1842, at Quebec he - went along with Colonel Codrington “and searched carefully below the - city in the channel of the St. Lawrence, at low water, near the shore, - for the signs of glacial action at the precise point where the chief - pressure and friction of packed ice are exerted every year,” but found - none.</p> - - <p>“At the bridge above the Falls of Montmorenci, over which a large - quantity of ice passes every year, the gneiss is polished, and kept - perfectly free from lichens, but not more so than rocks similarly - situated at waterfalls in Scotland. In none of these places were any - long straight grooves observable.”<a id="FNanchor_146" href="#Footnote_146" class="fnanchor">[146]</a></p> - - <p>The only thing in the shape of modern ice-markings which he seems to - have met with in North America was a few straight furrows half an inch - broad in soft sandstone, at the base of a cliff at Cape Blomidon in the - Bay of Fundy, at a place where during the preceding winter “packed” - ice 15 feet thick had been pushed along when the tide rose over the - sandstone ledges.<a id="FNanchor_147" href="#Footnote_147" class="fnanchor">[147]</a></p> - - <p>The very fact that a geologist so eminent as Sir Charles Lyell, after - having twice visited North America, and searched specially for modern - ice-markings, was able to find only two or three scratches, upon a soft - sandstone rock, which he could reasonably attribute to floating ice, - ought to have aroused the suspicion of the advocates of the iceberg - theory that they had really formed too extravagant notions regarding - the potency of floating ice as a striating agent.</p> - - <p>There is no reason to believe that the grooves and markings noticed - by M. Weibye and others on the Scandinavian coast and other parts of - northern Europe were made by icebergs.</p> - - <p><span class="pagenum" id="Page_281">281</span></p> - - <p>Professor Geikie has clearly shown, from the character and direction - of the markings, that they are the production of land-ice.<a id="FNanchor_148" href="#Footnote_148" class="fnanchor">[148]</a> If - the floating ice of the St. Lawrence and the icebergs of Labrador are - unable to striate and groove the rocks, it is not likely that those of - northern Europe will be able to do so.</p> - - <p>It will not do for the advocates of the iceberg theory to assume, as - they have hitherto done, that, as a matter of course, the sea-bottom is - being striated and grooved by means of icebergs. They must prove that. - They must either show that, as a matter of fact, icebergs are actually - efficient agents in striating the sea-bottom, or prove from mechanical - principles that they must be so. The question must be settled either by - observation or by reason; mere opinion will not do.</p> - - <p><em>The Amount of Material transported by Icebergs much exaggerated.</em>—The - transporting of boulders and rubbish, and not the grinding and - striating of rocks, is evidently the proper function of the iceberg. - But even in this respect I fear too much has been attributed to it.</p> - - <p>In reading the details of voyages in the arctic regions one cannot help - feeling surprised how seldom reference is made to stones and rubbish - being seen on icebergs. Arctic voyagers, like other people, when they - are alluding to the geological effects of icebergs, speak of enormous - quantities of stones being transported by them; but in reading the - details of their voyages, the impression conveyed is that icebergs with - stones and blocks of rock upon them are the exceptions. The greater - portion of the narratives of voyages in arctic regions consists of - interesting and detailed accounts of the voyager’s adventures among the - ice. The general appearance of the icebergs, their shape, their size, - their height, their colour, are all noticed; but rarely is mention - made of stones being seen. That the greater number of icebergs have - no stones or rubbish on them is borne out by the positive evidence of - geologists who have had opportunities of seeing icebergs.</p> - - <p>Mr. Campbell says:—“It is remarkable that up to this <span class="pagenum" id="Page_282">282</span>time we have only - seen a few doubtful stones on bergs which we have passed.... Though - no bergs with stones <em>on them or in them</em> have been approached during - this voyage, many on board the <cite>Ariel</cite> have been close to bergs heavily - laden.... A man who has had some experience of ice has <em>never seen a - stone on a berg</em> in these latitudes. Captain Anderson, of the <cite>Europa</cite>, - who is a geologist, has <em>never seen a stone on a berg</em> in crossing the - Atlantic. <em>No stones were clearly seen on this trip.</em>”<a id="FNanchor_149" href="#Footnote_149" class="fnanchor">[149]</a> Captain Sir - James Anderson (who has long been familiar with geology, has spent a - considerable part of his life on the Atlantic, and has been accustomed - to view the iceberg as a geologist as well as a seaman) has never seen - a stone on an iceberg in the Atlantic. This is rather a significant - fact.</p> - - <p>Sir Charles Lyell states that, when passing icebergs on the Atlantic, - he “was most anxious to ascertain whether there was any mud, stones, - or fragments of rocks on any one of these floating masses; but after - examining about forty of them without perceiving any signs of frozen - matter, I left the deck when it was growing dusk.”<a id="FNanchor_150" href="#Footnote_150" class="fnanchor">[150]</a> After he had - gone below, one was said to be seen with something like stones upon it. - The captain and officers of the ship assured him that they had <em>never - seen a stone upon a berg</em>.</p> - - <p>The following extract from Mr. Packard’s “Memoir on the Glacial - Phenomena of Labrador and Maine,” will show how little is effected by - the great masses of floating ice on the Labrador coast either in the - way of grinding and striating the rocks, or of transporting stones, - clay, and other materials.</p> - - <p>“Upon this coast, which during the summer of 1864 was lined with a - belt of floe-ice and bergs probably two hundred miles broad, and which - extended from the Gulf of St. Lawrence at Belles Amours to the arctic - seas, this immense body of floating ice seemed <em>directly</em> to produce - but little alteration in its physical features. If we were to ascribe - the grooving and polishing of rocks to the action of floating ice-floes - and bergs, <span class="pagenum" id="Page_283">283</span>how is it that the present shores far above (500), and at - least 250 feet below, the water-line are often jagged and angular, - though constantly stopping the course of masses of ice impelled four to - six miles an hour by the joint action of tides, currents, and winds? No - boulders, or gravel, or mud were seen upon any of the bergs or masses - of shore-ice. They had dropped all burdens of this nature nearer their - points of detachment in the high arctic regions.” ...</p> - - <p>“This huge area of floating ice, embracing so many thousands of square - miles, was of greater extent, and remained longer upon the coast, in - 1864, than for forty years previous. It was not only pressed upon the - coast by the normal action of the Labrador and Greenland currents, - which, in consequence of the rotatory motion of the earth, tended to - force the ice in a south-westerly direction, but the presence of the - ice caused the constant passage of cooler currents of air from the - sea over the ice upon the heated land, giving rise during the present - season to a constant succession of north-easterly winds from March - until early in August, which further served to crowd the ice into every - harbour and recess upon the coast. It was the universal complaint - of the inhabitants that the easterly winds were more prevalent, and - the ice ‘held’ later in the harbours this year than for many seasons - previous. Thus the fisheries were nearly a failure, and vegetation - greatly retarded in its development. But so far as polishing and - striating the rocks, depositing drift material, and thus modifying - the contour of the surface of the present coast, this modern mass of - bergs and floating ice effected comparatively little. Single icebergs, - when small enough, entered the harbours, and there stranding, soon - pounded to pieces upon the rocks, melted, and disappeared. From Cape - Harrison, in lat. 55°, to Caribo Island, was an interrupted line of - bergs stranded in 80 to 100 or more fathoms, often miles apart, while - others passed to the seaward down by the eastern coast of Newfoundland, - or through the Straits of Belle Isle.”<a id="FNanchor_151" href="#Footnote_151" class="fnanchor">[151]</a></p> - - <p><span class="pagenum" id="Page_284">284</span></p> - - <p><em>Boulder Clay the Product of Land-ice.</em>—There is still another point - connected with icebergs to which we must allude, viz., the opinion - that great masses of the boulder clay of the glacial epoch were formed - from the droppings of icebergs. If boulder clay is at present being - accumulated in this manner, then traces of the boulder clay deposits of - former epochs might be expected to occur. It is perfectly obvious that - <em>unstratified</em> boulder clay could not have been formed in this way. - Stones, gravel, sand, clay, and mud, the ingredients of boulder clay, - tumbled all together from the back of an iceberg, could not sink to the - bottom of the sea without separating. The stones would reach the bottom - first, then the gravel, then the sand, then the clay, and last of all - the mud, and the whole would settle down in a stratified form. But, - besides, how could the <em>clay</em> be derived from icebergs? Icebergs derive - their materials from the land before they are launched into the deep, - and while they are in the form of land-ice. The materials which are - found on the backs of icebergs are what fell upon the ice from mountain - tops and crags projecting above the ice. Icebergs are chiefly derived - from continental ice, such as that of Greenland, where the whole - country is buried under one continuous mass, with only a lofty mountain - peak here and there rising above the surface. And this is no doubt - the chief reason why so few icebergs have stones upon their backs. - The continental ice of Greenland is not, like the glaciers of the - Alps, covered with loose stones. Dr. Robert Brown informs me that no - moraine matter has ever been seen on the inland ice of Greenland. It is - perfectly plain that clay does not fall upon the ice. What falls upon - the ice is stones, blocks of rocks, and the loose <i lang="fr">débris</i>. Clay and - mud we know, from the accounts given by arctic voyagers, are sometimes - washed down upon the coast-ice; but certainly very little of either can - possibly get upon an iceberg. Arctic voyagers sometimes speak of seeing - clay and mud upon bergs; but it is probable that if they had been near - enough they would have found that what they took for clay and mud were - merely dust and rubbish.</p> - - <p><span class="pagenum" id="Page_285">285</span></p> - - <p>Undoubtedly the boulder clay of many places bears unmistakable evidence - of having been formed under water; but it does not on that account - follow that it was formed from the droppings of icebergs. The fact - that the boulder clay in every case <em>is chiefly composed of materials - derived from the country on which the clay lies</em>, proves that it was - not formed from matter transported by icebergs. The clay, no doubt, - contains stones and boulders belonging to other countries, which in - some cases may have been transported by icebergs; but the clay itself - has not come from another country. But if the clay itself has been - derived from the country on which it lies, then it is absurd to suppose - that it was deposited from icebergs. The clay and materials which are - found on icebergs are derived from the land on which the iceberg is - formed; but to suppose that icebergs, after floating about upon the - ocean, should always return to the country which gave them birth, and - there deposit their loads, is rather an extravagant supposition.</p> - - <p>From the facts and considerations adduced we are, I would venture to - presume, warranted to conclude that, with the exception of what may - have been produced by land-ice, very little in the shape of boulder - clay or striated rocks belonging to the glacial epoch lies buried - under the ocean—and that when the now existing land-surfaces are all - denuded, probably scarcely a trace of the glacial epoch will then be - found, except the huge blocks that were transported by icebergs and - dropped into the sea. It is therefore probable that we have as much - evidence of the existence of a glacial epoch during former periods as - the geologists of future ages will have of the existence of a glacial - epoch during the Post-tertiary period, and that consequently we are not - warranted in concluding that the glacial epoch was something unique in - the geological history of our globe.</p> - - <p><em>Palæontological Evidence.</em>—It might be thought that if glacial epochs - have been numerous, we ought to have abundance of palæontological - evidence of their existence. I do not know if this necessarily follows. - Let us take the glacial epoch itself for example, which is quite a - modern affair. Here we do not<span class="pagenum" id="Page_286">286</span> require to go and search in the bottom - of the sea for the evidence of its existence; for we have the surface - of the land in almost identically the same state in which it was when - the ice left it, with the boulder clay and all the wreck of the ice - lying upon it. But what geologist, with all these materials before him, - would be able to find out from palæontological evidence alone that - there had been such an epoch? He might search the whole, but would not - be able to find fossil evidence from which he could warrantably infer - that the country had ever been covered with ice. We have evidence - in the fossils of the Crag and other deposits of the existence of a - colder condition of climate prior to the true glacial period, and in - the shell-beds of the Clyde and other places of a similar state of - matters after the great ice-sheets had vanished away. But in regard - to the period of the true boulder clay or till, when the country was - enveloped in ice, palæontology has almost nothing whatever to tell - us. “Whatever may be the cause,” says Sir Charles Lyell, “the fact is - certain that over large areas in Scotland, Ireland, and Wales, I might - add throughout the northern hemisphere on both sides of the Atlantic, - the stratified drift of the glacial period is very commonly devoid of - fossils.”<a id="FNanchor_152" href="#Footnote_152" class="fnanchor">[152]</a></p> - - <p>In the “flysch” of the Eocene of the Alps, to which we shall have - occasion to refer in the next chapter, in which the huge blocks are - found which prove the existence of ice-action during that period, few - or no fossils have been found. So devoid of organic remains is that - formation, that it is only from its position, says Sir Charles, that - it is known to belong to the middle or “nummulitic” portion of the - great Eocene series. Again, in the conglomerates at Turin, belonging - to the Upper Miocene period, in which the angular blocks of limestone - are found which prove that during that period Alpine glaciers reached - the sea-level in the latitude of Italy, not a single organic remain has - been found. It would seem that an extreme paucity of organic life is a - characteristic of a glacial period, <span class="pagenum" id="Page_287">287</span>which warrants us in concluding - that the absence of organic remains in any formation otherwise - indicative of a cold climate cannot be regarded as sufficient evidence - that that formation does not belong to a cold period.</p> - - <p>In the last chapter it was shown why so little evidence of the warm - periods of the glacial epoch is now forthcoming. The remains of the - <i lang="la">faunas</i> and <i lang="la">floras</i> of those periods were nearly wholly destroyed and - swept into the adjoining seas by the ice-sheet that covered the land. - It is upon the present land-surface that we find the chief evidence - of the last glacial epoch, but the traces of the warm periods of that - epoch are hardly now to be met with in that position since they have - nearly all been obliterated or carried into the sea.</p> - - <p>In regard to former glacial epochs, however, ice-marked rocks, - scratched stones, moraines, till, &c., no longer exist; the - land-surfaces of those old times have been utterly swept away. The only - evidence, therefore, of such ancient glacial epochs, that we can hope - to detect, must be sought for in the deposits that were laid down upon - the sea-bottom; where also we may expect to find traces of the warm - periods that alternated during such epochs with glacial conditions. It - is plain, moreover, that the palæontological evidence in favour of warm - periods will always be the most abundant and satisfactory.</p> - - <p>Judging from geological evidence alone, we naturally conclude that, as - a general rule, the climate of former periods was somewhat warmer than - it is at the present day. It is from fossil remains that the geologist - principally forms his estimate of the character of the climate during - any period. Now, in regard to fossil remains, the warm periods will - always be far better represented than the cold; for we find that, as - a <em>general rule, those formations which geologists are inclined to - believe indicate a cold condition of climate are remarkably devoid of - fossil remains</em>. If a geologist does not keep this principle in view, - he will be very apt to form a wrong estimate of the general character - of the climate of a period of such enormous length as say the Tertiary.</p> - - <p><span class="pagenum" id="Page_288">288</span></p> - - <p>Suppose that the presently existing sea-bottoms, which have been - forming since the commencement of the glacial epoch, were to become - consolidated into rock and thereafter to be elevated into dry land, we - should then have a formation which might be properly designated the - Post-pliocene. It would represent the time which has elapsed from the - beginning of the glacial epoch to the present day. Suppose one to be - called upon as a geologist to determine from that formation what was - the general character of the climate during the period in question, - what would probably be the conclusion at which he would arrive? He - would probably find here and there patches of boulder clay containing - striated and ice-worn stones. Now and again he would meet with bones - of the mammoth and the reindeer, and shells of an arctic type. He - would likewise stumble upon huge blocks of the older rocks imbedded - in the formation, from which he would infer the existence of icebergs - and glaciers reaching the sea-level. But, on the whole, he would - perceive that the greater portion of the fossil remains met with in - this formation implied a warm and temperate condition of climate. At - the lower part of the formation, corresponding to the time of the true - boulder clay, there would be such a scarcity of organic remains that - he would probably feel at a loss to say whether the climate at that - time was cold or hot. But if the intense cold of the glacial epoch - was not continuous, but broken up by intervening warm periods during - which the ice, to a considerable extent at least, disappeared for a - long period of time (and there are few geologists who have properly - studied the subject who will positively deny that such was the case), - then the country would no doubt during those warm periods possess an - abundance of plant and animal life. It is quite true that we may almost - search in vain on the present land-surface for the organic remains - which belonged to those inter-glacial periods; for they were nearly - all swept away by the ice which followed. But no doubt in the deep - recesses of the ocean, buried under hundreds of feet of sand, mud, - clay, and gravel, lie multitudes of the plants and animals which then - flourished on the land, and were carried down by<span class="pagenum" id="Page_289">289</span> rivers into the sea. - And along with these lie the skeletons, shells, and other exuviæ of - the creatures which flourished in the warm seas of those periods. Now - looking at the great abundance of fossils indicative of warm and genial - conditions which the lower portions of this formation would contain, - the geologist might be in danger of inferring that the earlier part - of the Post-pliocene period was a warmer period, whereas we, at the - present day, looking at the matter from a different standpoint, declare - that part to have been characterized by cold or glacial conditions. No - doubt, if the beds formed during the cold periods of the glacial epoch - could be distinguished from those formed during the warm periods, the - fossil remains of the one would indicate a cold condition of climate, - and those of the other a warm condition; but still, taking the entire - epoch as a whole, the percentage of fossil remains indicative of a - warm condition would probably so much exceed that indicative of a cold - condition, that we should come to the conclusion that the character - of the climate, as a whole, during the epoch in question was warm and - equable.</p> - - <p>As geologists we have, as a rule, no means of arriving at a knowledge - of the character of the climate of any given period but through an - examination of the sea-bottoms belonging to that period; for these - contain all the evidence upon the subject. But unless we exercise - caution, we shall be very apt, in judging of the climate of such - a period, to fall into the same error that we have just now seen - one might naturally fall into were he called upon to determine the - character of the climate during the glacial epoch from the nature of - the organic remains which lie buried in our adjoining seas. On this - point Mr. J. Geikie’s observations are so appropriate, that I cannot - do better than introduce them here. “When we are dealing,” says this - writer, “with formations so far removed from us in time, and in which - the animal and plant remains depart so widely from existing forms of - life, we can hardly expect to derive much aid from the fossils in our - attempts to detect traces of cold climatic conditions. The arctic - shells in our Post-tertiary clays are<span class="pagenum" id="Page_290">290</span> convincing proofs of the former - existence in our latitude of a severe climate; but when we go so far - back as Palæozoic ages, we have no such clear evidence to guide us. - All that palæontologists can say regarding the fossils belonging to - these old times is simply this, that they seem to indicate, generally - speaking, mild, temperate, or genial, and even sometimes tropical, - conditions of climate. Many of the fossils, indeed, if we are to reason - from analogy at all, could not possibly have lived in cold seas. But, - for aught that we know, there may have been alternations of climate - during the deposition of each particular formation; and these changes - may be marked by the presence or absence, or by the greater or less - abundant development, of certain organisms at various horizons in - the strata. Notwithstanding all that has been done, our knowledge of - the natural history of these ancient seas is still very imperfect; - and therefore, in the present state of our information, we are not - entitled to argue, from the general aspect of the fossils in our older - formations, that the temperature of the ancient seas was never other - than mild and genial.”<a id="FNanchor_153" href="#Footnote_153" class="fnanchor">[153]</a></p> - - <p><em>Conclusion.</em>—From what has already been stated it will, I trust, be - apparent that, assuming glacial epochs during past geological ages to - have been as numerous and as severe as the Secular theory demands, - still it would be unreasonable to expect to meet with abundant traces - of them. The imperfection of the geological record is such that we - ought not to be astonished that so few relics of former ice ages have - come down to us. It will also be apparent that the palæontological - evidence of a warm condition of climate having obtained during any - particular age, is no proof that a glacial epoch did not also supervene - during the same cycle of time. Indeed it is quite the reverse; for - the warm conditions of which we have proof may indicate merely the - existence of an inter-glacial period. Furthermore, if the Secular - theory of changes of climate be admitted, then evidence of a warm - condition of climate having <span class="pagenum" id="Page_291">291</span>prevailed in arctic regions during any - past geological age may be regarded as presumptive proof of the - existence of a glacial epoch; that is to say, of an epoch during - which cold and warm conditions of climate alternated. Keeping these - considerations in view, we shall now proceed to examine briefly what - evidence we at present have of the former existence of glacial epochs.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XVIII"> - <span class="pagenum" id="Page_292">292</span> - <h2> - CHAPTER XVIII.<br /><br /> - <span class="small">FORMER GLACIAL EPOCHS; GEOLOGICAL EVIDENCE OF.</span> - </h2> - </div> - <div class="subhead">Cambrian Conglomerate of Islay and North-west of - Scotland.—Ice-action in Ayrshire and Wigtownshire - during Silurian Period.—Silurian Limestones in Arctic - Regions.—Professor Ramsay on Ice-action during Old Red - Sandstone Period.—Warm Climate in Arctic Regions during Old - Red Sandstone Period.—Professor Geikie and Mr. James Geikie on - a Glacial Conglomerate of Lower Carboniferous Age.—Professor - Haughton and Professor Dawson on Evidence of Ice-action - during Coal Period.—Mr. W. T. Blanford on Glaciation in India - during Carboniferous Period.—Carboniferous Formations of - Arctic Regions.—Professor Ramsay on Permian Glaciers.—Permian - Conglomerate in Arran.—Professor Hull on Boulder Clay of - Permian Age.—Permian Boulder Clay of Natal.—Oolitic Boulder - Conglomerate in Sutherlandshire.—Warm Climate in North - Greenland during Oolitic Period.—Mr. Godwin-Austen on - Ice-action during Cretaceous Period.—Glacial Conglomerates of - Eocene Age in the Alps.—M. Gastaldi on the Ice-transported - Limestone Blocks of the Superga.—Professor Heer on the Climate - of North Greenland during Miocene Period.</div> - - <h3>CAMBRIAN PERIOD.</h3> - - <p><em>Island of Islay.</em>—Good evidence of ice-action has been observed by - Mr. James Thomson, F.G.S.,<a id="FNanchor_154" href="#Footnote_154" class="fnanchor">[154]</a> in strata which he believes to be of - Cambrian age. At Port Askaig, Island of Islay, below a precipitous - cliff of quartzite 70 feet in height, there is a mass of arenaceous - talcose schist containing fragments of granite, some angular, but - most of them rounded, and of all sizes, from mere particles to - large boulders. As there is no granite in the island from which - these boulders could have been derived, he justly infers that they - must have been transported by the agency of ice. The probability - of his conclusion is strengthened by the almost total absence of - stratification in the deposit in question.</p> - - <p><span class="pagenum" id="Page_293">293</span></p> - - <p><em>North-west of Scotland.</em>—Mr. J. Geikie tells me that much of the - Cambrian conglomerate in the north-west of Scotland strongly reminds - him of the coarse shingle beds (Alpine diluvium) which so often crowd - the old glacial valleys of Switzerland and Northern Italy. In many - places the stones of the Cambrian conglomerate have a subangular, - blunted shape, like those of the re-arranged moraine débris of Alpine - countries.</p> - - <h3>SILURIAN PERIOD.</h3> - - <p><em>Wigtownshire.</em>—The possibility of glacial action so far back as - the Silurian age has been suggested. In beds of slate and shales in - Wigtownshire of Lower Silurian age Mr. J. Carrick Moore found beds of - conglomerate of a remarkable character. The fragments generally vary - from the size of one inch to a foot in diameter, but in some of the - beds, boulders of 3, 4, and even 5 feet in diameter occur. There are - no rocks in the neighbourhood from which any of these fragments could - have been derived. The matrix of this conglomerate is sometimes a green - trappean-looking sandstone of exceeding toughness, and sometimes an - indurated sandstone indistinguishable from many common varieties of - greywacke.<a id="FNanchor_155" href="#Footnote_155" class="fnanchor">[155]</a></p> - - <p><em>Ayrshire.</em>—Mr. James Geikie states that in Glenapp, and near - Dalmellington, he found embedded in Lower Silurian strata blocks - and boulders from one foot to 5 feet in diameter of gneiss, - syenite, granite, &c., none of which belong to rocks of those - neighbourhoods.<a id="FNanchor_156" href="#Footnote_156" class="fnanchor">[156]</a> Similar cases have been found in Galway, Ireland, - and at Lisbellaw, south of Enniskillen.<a id="FNanchor_157" href="#Footnote_157" class="fnanchor">[157]</a> In America, Professor - Dawson describes Silurian conglomerates with boulders 2 feet in - diameter.</p> - - <p><em>Arctic Regions.</em>—The existence of warm inter-glacial periods - during that age may be inferred from the fact that in the arctic - regions we find widespread masses of Silurian limestones containing - encrinites, corals, and mollusca, and other fossil <span class="pagenum" id="Page_294">294</span>remains, for an - account of which see Professor Haughton’s geological account of the - Arctic Archipelago appended to McClintock’s “Narrative of Arctic - Discoveries.”<a id="FNanchor_158" href="#Footnote_158" class="fnanchor">[158]</a></p> - - <h3>OLD RED SANDSTONE.</h3> - - <p><em>North of England.</em>—According to Professor Ramsay and some other - geologists the brecciated, subangular conglomerates and boulder beds - of the Old Red Sandstone of Scotland and the North of England are of - glacial origin. When these conglomerates and the recent boulder clay - come together it is difficult to draw the line of demarcation between - them.</p> - - <p>Professor Ramsay observed some very remarkable facts in connection - with the Old Red Sandstone conglomerates of Kirkby Lonsdale, and - Sedburgh, in Westmoreland and Yorkshire. I shall give the results of - his observations in his own words.</p> - - <p>“The result is, that we have found many stones and blocks distinctly - scratched, and on others the ghosts of scratches nearly obliterated - by age and chemical action, probably aided by pressure at a time when - these rocks were buried under thousands of feet of carboniferous - strata. In some cases, however, the markings were probably produced - within the body of the rock itself by pressure, accompanied by - disturbance of the strata; but in others the longitudinal and cross - striations convey the idea of glacial action. The shapes of the stones - of these conglomerates, many of which are from 2 to 3 feet long, their - flattened sides and subangular edges, together with the confused manner - in which they are often arranged (like stones in the drift), have - long been enough to convince me of their ice-borne character; and the - scratched specimens, when properly investigated, may possibly convince - others.”<a id="FNanchor_159" href="#Footnote_159" class="fnanchor">[159]</a></p> - - <p><em>Isle of Man.</em>—The conglomerate of the Old Red Sandstone in the Isle of - Man has been compared by Mr. Cumming to “a consolidated ancient boulder - clay.” And he remarks, “Was it so that those strange trilobitic-looking - fishes of that era had to <span class="pagenum" id="Page_295">295</span>endure the buffeting of ice-waves, and to - struggle amidst the wreck of ice-floes and the crush of bergs?”<a id="FNanchor_160" href="#Footnote_160" class="fnanchor">[160]</a></p> - - <p><em>Australia.</em>—A conglomerate similar to that of Scotland has been found - in Victoria, Australia, by Mr. Selwyn, at several localities. Along - the Wild Duck Creek, near Heathcote, and also near the Mia-Mia, Spring - Plains, Redesdale, localities in the Colony of Victoria, where it was - examined by Messrs. Taylor and Etheridge, Junior, this conglomerate - consists of a mixture of granite pebbles and boulders of various - colours and textures, porphyries, indurated sandstone, quartz, and - a peculiar flint-coloured rock in a matrix of bluish-grey very hard - mud-cement.<a id="FNanchor_161" href="#Footnote_161" class="fnanchor">[161]</a> Rocks similar to the pebbles and blocks composing the - conglomerate do not occur in the immediate neighbourhood; and from the - curious mixture of large and small angular and water-worn fragments - it was conjectured that it might possibly be of glacial origin. - Scratched stones were not observed, although a careful examination was - made. From similar mud-pebble beds on the Lerderderg River, Victoria, - Mr. P. Daintree obtained a few pebbles grooved after the manner of - ice-scratched blocks.<a id="FNanchor_162" href="#Footnote_162" class="fnanchor">[162]</a></p> - - <p>And the existence of a warm condition of climate during the Old Red - Sandstone period is evidenced by the fossiliferous limestones of - England, Russia, and America. On the banks of the Athabasca River, - Rupert-Land, Sir John Richardson found beds of limestone containing - <i>Producti</i>, <i>Spiriferi</i>, an <i>Orthis</i> resembling <i>O. resupinata</i>, - <i>Terebratula reticularis</i>,<a id="FNanchor_163" href="#Footnote_163" class="fnanchor">[163]</a> and a <i>Pleurotomaria</i>, which, in the - opinion of the late Dr. Woodward, who examined the specimens, are - characteristic of Devonian rocks of Devonshire.</p> - - <p><span class="pagenum" id="Page_296">296</span></p> - - <h3>CARBONIFEROUS PERIOD.</h3> - - <p><em>France.</em>—It is now a good many years since Mr. Godwin-Austen directed - attention to what he considered evidence of ice-action during the coal - period. This geologist found in the carboniferous strata of France - large angular blocks which he could not account for without inferring - the former action of ice. “Whether from local elevation,” he says, - “or from climatic conditions, there are certain appearances over the - whole which imply that at one time the temperature must have been very - low, as glacier-action can alone account for the presence of the large - angular blocks which occur in the lowest detrital beds of many of the - southern coal-basins.”<a id="FNanchor_164" href="#Footnote_164" class="fnanchor">[164]</a></p> - - <p><em>Scotland.</em>—In Scotland great beds of conglomerate are met with in - various parts, which are now considered by Professor Geikie, Mr. - James Geikie, and other officers of the Geological Survey who have - had opportunities of examining them, to be of glacial origin. “They - are,” says Mr. James Geikie, “quite unstratified, and the stones often - show that peculiar blunted form which is so characteristic of glacial - work.”<a id="FNanchor_165" href="#Footnote_165" class="fnanchor">[165]</a> Many of the stones found by Professor Geikie, several of - which I have had an opportunity of seeing, are well striated.</p> - - <p>In 1851 Professor Haughton brought forward at the Geological Society - of Dublin, a case of angular fragments of granite occurring in the - carboniferous limestone of the county of Dublin; and he explained the - phenomena by the supposition of the transporting power of ice.</p> - - <p><em>North America.</em>—In one of the North American coal-fields Professor - Newberry found a boulder of quartzite 17 inches by 12 inches, imbedded - in a seam of coal. Similar facts have also been recorded both in the - United States, and in Nova Scotia. Professor Dawson describes what he - calls a gigantic esker of Carboniferous age, on the outside of which - large <span class="pagenum" id="Page_297">297</span>travelled boulders were deposited, probably by drift-ice; while - in the swamps within, the coal flora flourished.<a id="FNanchor_166" href="#Footnote_166" class="fnanchor">[166]</a></p> - - <p><em>India.</em>—Mr. W. T. Blanford, of the Geological Survey of India, states - that in beds considered to be of Carboniferous age are found large - boulders, some of them as much as 15 feet in diameter. The bed in - which these occur is a fine silt, and he refers the deposition of the - boulders to ice-action. Within the last three years his views have - received singular confirmation in another part of India, where beds - of limestone were found striated below certain overlying strata. The - probability that these appearances are due, as Mr. Blanford says, to - the action of ice, is strengthened by the consideration that about five - degrees farther to the north of the district in question rises the - cold and high table-land of Thibet, which during a glacial epoch would - undoubtedly be covered with ice that might well descend over the plains - of India.<a id="FNanchor_167" href="#Footnote_167" class="fnanchor">[167]</a></p> - - <p><em>Arctic Regions.</em>—A glacial epoch during the Carboniferous age may be - indirectly inferred from the probable existence of warm inter-glacial - periods, as indicated by the limestones with fossil remains found in - arctic regions.</p> - - <p>That an equable condition of climate extended to near the north pole - is proved by the fact that in the arctic regions vast masses of - carboniferous limestone, having all the characters of the mountain - limestone of England, have been found. “These limestones,” says - Mr. Isbister, “are most extensively developed in the north-east - extremity of the continent, where they occupy the greater part of - the coast-line, from the north side of the Kotzebue Sound to within - a few miles of Point Barrow, and form the chief constituent of the - lofty and conspicuous headlands of Cape Thomson, Cape Lisburn, and - Cape Sabine.”<a id="FNanchor_168" href="#Footnote_168" class="fnanchor">[168]</a> Limestone of the same age occurs extensively - along the Mackenzie River. The following fossils have been found - in these limestones:—<i>Terebratula resupinata</i>,<a id="FNanchor_169" href="#Footnote_169" class="fnanchor">[169]</a> <i>Lithostrotion - basaltiforme</i>, <i>Cyathophyllum dianthum</i>, <i>C. flexuosum</i>, <i>Turbinolia - mitrata</i>, <i>Productus<span class="pagenum" id="Page_298">298</span> Martini</i>,<a id="FNanchor_170" href="#Footnote_170" class="fnanchor">[170]</a> <i>Dentalium Sarcinula</i>, - <i>Spiriferi</i>, <i>Orthidæ</i>, and encrinital fragments in the greatest - abundance.</p> - - <p>Among the fossils brought home from Depôt Point, Albert Land, by - Sir E. Belcher, Mr. Salter found the following, belonging to the - Carboniferous period:—<i>Fusulina hyperborea</i>, <i>Stylastrea inconferta</i>, - <i>Zaphrentis ovibos</i>, <i>Clisiophyllum tumulus</i>, <i>Syringopora (Aulopora)</i>, - <i>Fenestella Arctica</i>, <i>Spirifera Keilhavii</i>, <i>Productus cora</i>, <i>P. - semireticulatus</i>.<a id="FNanchor_171" href="#Footnote_171" class="fnanchor">[171]</a></p> - - <p>Coal-beds of Carboniferous age are extensively developed in arctic - regions. The fuel is of a highly bituminous character, resembling, says - Professor Haughton, the gas coals of Scotland. The occurrence of coal - in such high latitudes indicates beyond doubt that a mild and temperate - condition of climate must, during some part of the Carboniferous age, - have prevailed up to the very pole.</p> - - <p>“In the coal of Jameson’s Land, on the east side of Greenland, lying - in latitude 71°, and in that of Melville Island, in latitude 75° N., - Professor Jameson found plants resembling fossils of the coal-fields of - Britain.”<a id="FNanchor_172" href="#Footnote_172" class="fnanchor">[172]</a></p> - - <h3>PERMIAN PERIOD.</h3> - - <p><em>England.</em>—From the researches of Professor Ramsay in the Permian - breccias, we have every reason to believe that during a part of the - Permian age our country was probably covered with glaciers reaching - to the sea. These brecciated stones, he states, are mostly angular - or subangular, with flattened sides and but very slightly rounded at - the edges, and are imbedded in a deep red marly paste. At Abberley - Hill some of the masses are from 2 to 3 feet in diameter, and in one - of the quarries, near the base of Woodbury Hill, Professor Ramsay - saw one 2 feet in diameter. Another was observed at Woodbury Rock, 4 - feet long, 3 feet broad, and 1½ feet thick. The boulders were found - in South Staffordshire, Enville, in <span class="pagenum" id="Page_299">299</span>Abberley and Malvern Hills, - and other places. “They seem,” he says, “to have been derived from - the conglomerate and green, grey, and purple Cambrian grits of the - Longmynd, and from the Silurian quartz-rocks, slates, felstones, - felspathic ashes, greenstones, and Upper Caradoc rocks of the country - between the Longmynd and Chirbury. But then,” he continues, “the south - end of the Malvern Hills is from forty to fifty miles, the Abberleys - from twenty-five to thirty-five miles, Enville from twenty to thirty - miles, and South Staffordshire from thirty-five to forty miles distant - from that country.”<a id="FNanchor_173" href="#Footnote_173" class="fnanchor">[173]</a></p> - - <p>It is physically impossible, Professor Ramsay remarks, that these - blocks could have been transported to such distances by any other - agency than that of ice. Had they been transported by water, supposing - such a thing possible, they would have been rounded and water-worn, - whereas many of these stones are flat slabs, and most of them have - their edges but little rounded. And besides many of them are highly - polished, and others grooved and finely striated, exactly like those of - the ancient glaciers of Scotland and Wales. Some of these specimens are - to be seen in the Museum of Practical Geology, Jermyn Street.</p> - - <p><em>Scotland.</em>—In the Island of Arran, Mr. E. A. Wunsch and Mr. James - Thomson found a bed of conglomerate which they considered of Permian - age, and probably of glacial origin. This conglomerate enclosed angular - fragments of various schistose, volcanic, and limestone rocks, and - contained carboniferous fossils.</p> - - <p><em>Ireland.</em>—At Armagh, Ireland, Professor Hull found boulder beds of - Permian age, containing pebbles and boulders, sometimes 2 feet in - diameter. Some of the boulders must have been transported from a - region lying about 30 miles to the north-west of the locality in which - they now occur. It is difficult to conceive, says Professor Hull, - how rock fragments of such a size could have been carried to their - present position by any other agency than that of floating ice. This - boulder-bed is <span class="pagenum" id="Page_300">300</span>overlaid by a recent bed of boulder clay. Professor - Ramsay, who also examined the section, agrees with Professor Hull that - the bed is of Permian age, and unquestionably of ice-formation.<a id="FNanchor_174" href="#Footnote_174" class="fnanchor">[174]</a></p> - - <p>Professor Ramsay feels convinced that the same conclusions which he has - drawn in regard to the Permian breccia of England will probably yet be - found to hold good in regard to much of that of North Germany.<a id="FNanchor_175" href="#Footnote_175" class="fnanchor">[175]</a> And - there appears to be some ground for concluding that the cold of that - period even reached to India.<a id="FNanchor_176" href="#Footnote_176" class="fnanchor">[176]</a></p> - - <p><em>South Africa.</em>—An ancient boulder clay, supposed to be either - of Permian or Jurassic age, has been extensively found in Natal, - South Africa. This deposit, discovered by Dr. Sutherland, the - Surveyor-General of the colony, is thus described by Dr. Mann:—</p> - - <p>“The deposit itself consists of a greyish-blue argillaceous matrix, - containing fragments of granite, gneiss, graphite, quartzite, - greenstone, and clay-slate. These imbedded fragments are of various - size, from the minute dimensions of sand-grains up to vast blocks - measuring 6 feet across, and weighing from 5 to 10 tons. They are - smoothed, as if they had been subject to a certain amount of attrition - in a muddy sediment; but they are not rounded like boulders that - have been subjected to sea-breakers. The fracture of the rock is not - conchoidal, and there is manifest, in its substance, a rude disposition - towards wavy stratification.”</p> - - <p>“Dr. Sutherland inclines to think that the transport of vast massive - blocks of several tons’ weight, the scoring of the subjacent surfaces - of sandstone, and the simultaneous deposition of minute sand-grains - and large boulders in the same matrix, all point to one agency as the - only one which can be rationally admitted to account satisfactorily - for the presence of this remarkable formation in the situations in - which it is found. He believes that the boulder-bearing clay of Natal - is of <span class="pagenum" id="Page_301">301</span>analogous nature to the great Scandinavian drift, to which it - is certainly intimately allied in intrinsic mineralogical character; - that it is virtually a vast moraine of olden time; and that ice, in - some form or other, has had to do with its formation, at least so far - as the deposition of the imbedded fragments in the amorphous matrix are - concerned.”<a id="FNanchor_177" href="#Footnote_177" class="fnanchor">[177]</a></p> - - <p>In the discussion which followed the reading of Dr. Sutherland’s paper, - Professor Ramsay pointed out that in the Natal beds enormous blocks of - rock occurred, which were 60 or 80 miles from their original home, and - still remained angular; and there was a difficulty in accounting for - the phenomena on any other hypothesis than that suggested.</p> - - <p>Mr. Stow, in his paper on the Karoo beds, has expressed a similar - opinion regarding the glacial character of the formation.<a id="FNanchor_178" href="#Footnote_178" class="fnanchor">[178]</a></p> - - <p>But we have in the Karoo beds evidence not only of glaciation, but of a - much warmer condition of things than presently exists in that latitude. - This is shown from the fact that the shells of the <i>Trigona</i>-beds - indicate a tropical or subtropical condition of climate.</p> - - <p><em>Arctic Regions.</em>—The evidence which we have of the existence of a - warm climate during the Permian period is equally conclusive. The - close resemblance of the <i lang="la">flora</i> of the Permian period to that of - Carboniferous times evidently points to the former prevalence of a warm - and equable climate. And the existence of the magnesian limestone in - high latitudes seems to indicate that during at least a part of the - Permian period, just as during the accumulation of the carboniferous - limestone, a warm sea must have obtained in those latitudes.</p> - - <h3>OOLITIC PERIOD.</h3> - - <p><em>North of Scotland.</em>—There is not wanting evidence of something like - the action of ice during the Oolitic period.<a id="FNanchor_179" href="#Footnote_179" class="fnanchor">[179]</a></p> - - <p>In the North of Scotland Mr. James Geikie says there is a <span class="pagenum" id="Page_302">302</span>coarse - boulder conglomerate associated with the Jurassic strata in the east - of Sutherland, the possibly glacial origin of which long ago suggested - itself to Professor Ramsay and other observers. Mr. Judd believes the - boulders to have been floated down by ice from the Highland mountains - at the time the Jurassic strata were being accumulated.</p> - - <p><em>North Greenland.</em>—During the Oolitic period a warm condition of - climate extended to North Greenland. For example, in Prince Patrick’s - Island, at Wilkie Point, in lat. 76° 20′ N., and long. 117° 20′ - W., Oolitic rocks containing an ammonite (<i>Ammonites McClintocki</i>, - Haughton), like <i>A. concavus</i> and other shells of Oolitic species, - were found by Captain McClintock.<a id="FNanchor_180" href="#Footnote_180" class="fnanchor">[180]</a> In Katmai Bay, near Behring’s - Straits, the following Oolitic fossils were discovered—<i>Ammonites - Wasnessenskii</i>, <i>A. biplex</i>, <i>Belemnites paxillosus</i>, and <i>Unio - liassinus</i>.<a id="FNanchor_181" href="#Footnote_181" class="fnanchor">[181]</a> Captain McClintock found at Point Wilkie, in Prince - Patrick’s Island, lat. 76° 20′, a bone of <i>Ichthyosaurus</i>, and Sir E. - Belcher found in Exmouth Island, lat. 76° 16′ N., and long. 96° W., at - an elevation of 570 feet above the level of the sea, bones which were - examined by Professor Owen, and pronounced to be those of the same - animal.<a id="FNanchor_182" href="#Footnote_182" class="fnanchor">[182]</a> Mr. Salter remarks that at the time that these fossils - were deposited, “a condition of climate something like that of our own - shores was prevailing in latitudes not far short of 80° N.”<a id="FNanchor_183" href="#Footnote_183" class="fnanchor">[183]</a> And - Mr. Jukes says that during the Oolitic period, “in latitudes where - now sea and land are bound in ice and snow throughout the year, there - formerly flourished animals and plants similar to those living in our - own province at that time. The questions thus raised,” continues Mr. - Jukes, “as to the climate of the globe when cephalopods and reptiles - such as we should expect to find only in warm or temperate seas, - could live in such high latitudes, are not easy to answer.”<a id="FNanchor_184" href="#Footnote_184" class="fnanchor">[184]</a> And - <span class="pagenum" id="Page_303">303</span>Professor Haughton remarks, that he thinks it highly improbable that - any change in the position of land and water could ever have produced a - temperature in the sea at 76° north latitude which would allow of the - existence of ammonites, especially species so like those that lived - at the same time in the tropical warm seas of the South of England - and France at the close of the Liassic, and commencement of the Lower - Oolitic period.<a id="FNanchor_185" href="#Footnote_185" class="fnanchor">[185]</a></p> - - <p>The great abundance of the limestone and coal of the Oolitic system - shows also the warm and equable condition of the climate which must - have then prevailed.</p> - - <h3>CRETACEOUS PERIOD.</h3> - - <p><em>Croydon.</em>—A large block of crystalline rock resembling granite was - found imbedded in a pit, on the side of the old London and Brighton - road near Purley, about two miles south of Croydon. Mr. Godwin-Austen - has shown conclusively that it must have been transported there by - means of floating ice. This boulder was associated with loose sea-sand, - coarse shingle, and a smaller boulder weighing twenty or twenty-five - pounds, and all water-worn. These had all sunk together without - separating. Hence they must have been firmly held together, both during - the time that they were being floated away, and also whilst sinking to - the bottom of the cretaceous sea. Mr. Godwin-Austen supposes the whole - to have been carried away frozen to the bottom of a mass of ground-ice. - When the ice from melting became unable to float the mass attached to - it, the whole would then sink to the bottom together.<a id="FNanchor_186" href="#Footnote_186" class="fnanchor">[186]</a></p> - - <p><em>Dover.</em>—While the workmen were employed in cutting the tunnel on - the London, Chatham, and Dover Railway, between Lydden Hill and - Shepherdswell, a few miles from Dover, they came upon a mass of coal - imbedded in chalk, at a depth of 180 feet. It was about 4 feet square, - and from 4 to 10 inches thick. <span class="pagenum" id="Page_304">304</span>The coal was friable and highly - bituminous. It resembled some of the Wealden or Jurassic coal, and - was unlike the true coal of the coal-measures. The specific gravity - of the coal precluded the supposition that it could have floated away - of itself into the cretaceous sea. “Considering its friability,” says - Mr. Godwin-Austen, “I do not think that the agency of a floating tree - could have been engaged in its transport; but, looking at its flat, - angular form, it seems to me that its history may agree with what I - have already suggested with reference to the boulder in the chalk - at Croydon. We may suppose that during the Cretaceous period some - bituminous beds of the preceding Oolitic period lay so as to be covered - with water near the sea-margin, or along some river-bank, and from - which portions could be carried off by ice, and so drifted away, until - the ice was no longer able to support its load.”<a id="FNanchor_187" href="#Footnote_187" class="fnanchor">[187]</a></p> - - <p>Mr. Godwin-Austen then mentions a number of other cases of blocks - being found in the chalk. In regard to those cases he appropriately - remarks that, as the cases where the occurrence of such blocks has - been observed are likely to be far less numerous than those which have - escaped observation, or failed to have been recorded, and as the chalk - exposed in pits and quarries bears only a most trifling proportion to - the whole horizontal extent of the formation, we have no grounds to - conclude that the above are exceptional cases.</p> - - <p>Boulders have also been found in the cretaceous strata of the Alps by - Escher von der Linth.<a id="FNanchor_188" href="#Footnote_188" class="fnanchor">[188]</a></p> - - <p>The existence of warm periods during the Cretaceous age is plainly - shown by the character of the flora and fauna of that age. The fact - that chalk is of organic origin implies that the climate must have - been warm and genial, and otherwise favourable to animal life. This is - further manifested by such plants as <i>Cycas</i> and <i>Zamia</i>, which betoken - a warm climate, and by the corals and huge sauroid reptiles which then - inhabited our waters.</p> - - <p><span class="pagenum" id="Page_305">305</span></p> - - <p>It is, in fact, the tropical character of the fauna of that period - which induced Sir Charles Lyell to reject Mr. Godwin-Austen’s idea that - the boulders found in the chalk had been transported by floating ice. - Such a supposition, implying a cold climate, “is,” Sir Charles says, - “inconsistent with the luxuriant growth of large chambered univalves, - numerous corals, and many fish, and other fossils of tropical forms.”</p> - - <p>The recent discovery of the Cretaceous formation in Greenland shows - that during that period a mild and temperate condition of climate - must have prevailed in that continent up to high latitudes. “This - formation in Greenland,” says Dr. Robert Brown, “has only been recently - separated from the Miocene formation, with which it is associated and - was supposed to be a part of. It is, as far as we yet know, only found - in the vicinity of Kome or Koke, near the shores of Omenak Fjord, in - about 70° north latitude, though traces have been found elsewhere - on Disco, &c. The fossils hitherto brought to Europe have been very - few, and consist of plants which are now preserved in the Stockholm - and Copenhagen Museums. From these there seems little doubt that the - age assigned to this limited deposit (so far as we yet know) by the - celebrated palæontologist, Professor Oswald Heer, of Zurich, is the - correct one.”<a id="FNanchor_189" href="#Footnote_189" class="fnanchor">[189]</a> Dr. Brown gives a list of the Cretaceous flora found - in Greenland.</p> - - <h3>EOCENE PERIOD.</h3> - - <p><em>Switzerland.</em>—In a coarse conglomerate belonging to the “<i lang="de">flysch</i>” - of Switzerland, an Eocene formation, there are found certain immense - blocks, some of which consist of a variety of granite which is not - known to occur <i lang="la">in situ</i> in any part of the Alps. Some of the blocks - are 10 feet and upwards in length, and one at Halekeren, at the Lake of - Thun, is 105 feet in length, 90 feet in breadth, and 45 feet in height. - Similar blocks are found in the Apennines. These unmistakably <span class="pagenum" id="Page_306">306</span>indicate - the presence of glaciers or floating ice. This conclusion is further - borne out by the fact that the “<i lang="de">flysch</i>” is destitute of organic - remains. But the hypothesis that these huge masses were transported - to their present sites by glaciers or floating ice has been always - objected to, says Sir Charles Lyell, “on the ground that the Eocene - strata of Nummulitic age in Switzerland, as well as in other parts of - Europe, contain genera of fossil plants and animals characteristic of a - warm climate. And it has been particularly remarked,” he continues, “by - M. Desor that the strata most nearly associated with the ‘<i lang="de">flysch</i>’ in - the Alps are rich in echinoderms of the <i>Spatangus</i> family which have a - decided tropical aspect.”<a id="FNanchor_190" href="#Footnote_190" class="fnanchor">[190]</a></p> - - <p>But according to the theory of Secular Changes of Climate, the very - fact that the “<i lang="de">flysch</i>” is immediately associated with beds indicating - a warm or even tropical condition of climate, is one of the strongest - proofs which could be adduced in favour of its glacial character, for - the more severe a cold period of a glacial epoch is, the warmer will be - the periods which immediately precede and succeed. These crocodiles, - tortoises, and tropical flora probably belong to a warm Eocene - inter-glacial period.</p> - - <h3>MIOCENE PERIOD.</h3> - - <p><em>Italy.</em>—We have strong evidence in favour of the opinion that a - glacial epoch existed during the Miocene period. It has been shown - by M. Gastaldi, that during that age Alpine glaciers extended to the - sea-level.</p> - - <p>Near Turin there is a series of hills, rising about 500 or 600 feet - above the valleys, composed of beds of Miocene sandstone, marl, and - gravel, and loose conglomerate. These beds have been carefully examined - and described by M. Gastaldi.<a id="FNanchor_191" href="#Footnote_191" class="fnanchor">[191]</a> The hill of the Luperga has been - particularly noticed by him. Many of the stones in these beds are - striated in a manner similar to those found in the true till or boulder - clay of this <span class="pagenum" id="Page_307">307</span>country. But what is most remarkable is the fact that - large erratic blocks of limestone, many of them from 10 to 15 feet in - diameter, are found in abundance in these beds. It has been shown by - Gastaldi that these blocks have all been derived from the outer ridge - of the Alps on the Italian side, namely, from the range extending from - Ivrea to the Lago Maggiore, and consequently they must have travelled - from twenty to eighty miles. So abundant are these large blocks, that - extensive quarries have been opened in the hills for the sake of - procuring them. These facts prove not only the existence of glaciers - on the Alps during the Miocene period, but of glaciers extending to - the sea and breaking up into icebergs; the stratification of the beds - amongst which the blocks occur sufficiently indicating aqueous action - and the former presence of the sea.</p> - - <p>That the glaciers of the Southern Alps actually reached to the sea, - and sent their icebergs adrift over what are now the sunny plains of - Northern Italy, is sufficient proof that during the cold period of - Miocene times the climate must have been very severe. Indeed, it may - well have been as severe as, if not even more excessive than, the - intensest severity of climate experienced during the last great glacial - epoch.</p> - - <p><em>Greenland.</em>—Of the existence of warm conditions during Miocene times, - geology affords us abundant evidence. I shall quote the opinion of Sir - Charles Lyell on this point:—</p> - - <p>“We know,” says Sir Charles, “that Greenland was not always covered - with snow and ice; for when we examine the tertiary strata of Disco - Island (of the Upper Miocene period), we discover there a multitude - of fossil plants which demonstrate that, like many other parts of the - arctic regions, it formerly enjoyed a mild and genial climate. Among - the fossils brought from that island, lat. 70° N., Professor Heer has - recognised <i>Sequoia Landsdorfii</i>, a coniferous species which flourished - throughout a great part of Europe in the Miocene period. The same - plant has been found fossil by Sir John Richardson within the Arctic - Circle, far to the west on the Mackenzie River, near the entrance of - Bear River; also by some Danish naturalists in<span class="pagenum" id="Page_308">308</span> Iceland, to the east. - The Icelandic surturband or lignite, of this age, has also yielded a - rich harvest of plants, more than thirty-one of them, according to - Steenstrup and Heer, in a good state of preservation, and no less than - fifteen specifically identical with Miocene plants of Europe. Thirteen - of the number are arborescent; and amongst others is a tulip-tree - (<i>Liriodendron</i>), with its fruit and characteristic leaves, a plane - (<i>Platanus</i>), a walnut, and a vine, affording unmistakable evidence - of a climate in the parallel of the Arctic Circle which precludes the - supposition of glaciers then existing in the neighbourhood, still less - any general crust of continental ice like that of Greenland.”<a id="FNanchor_192" href="#Footnote_192" class="fnanchor">[192]</a></p> - - <p>At a meeting of the British Association, held at Nottingham in August - 1866, Professor Heer read a valuable paper on the “Miocene Flora of - North Greenland.” In this paper some remarkable conclusions as to the - probable temperature of Greenland during the Miocene period were given.</p> - - <p>Upwards of sixty different species brought from Atanekerdluk, a place - on the Waigat opposite Disco, in lat. 70° N., have been examined by him.</p> - - <p>A steep hill rises on the coast to a height of 1,080 feet, and at - this level the fossil plants are found. Large quantities of wood in - a fossilized or carbonized condition lie about. Captain Inglefield - observed one trunk thicker than a man’s body standing upright. The - leaves, however, are the most important portion of the deposit. The - rock in which they are found is a sparry iron ore, which turns reddish - brown on exposure to the weather. In this rock the leaves are found, in - places packed closely together, and many of them are in a very perfect - condition. They give us a most valuable insight into the nature of the - vegetation which formed this primeval forest.</p> - - <p>He arrives at the following conclusions:—</p> - - <p>1. <em>The fossilized plants of Atanekerdluk cannot have been drifted from - any great distance. They must have grown on the spot where they were - found.</em></p> - - <p><span class="pagenum" id="Page_309">309</span></p> - - <p>This is shown—</p> - - <p>(<i>a</i>) By the fact that Captain Inglefield and Dr. Ruik observed trunks - of trees standing upright.</p> - - <p>(<i>b</i>) By the great abundance of the leaves, and the perfect state of - preservation in which they are found.</p> - - <p>(<i>c</i>) By the fact that we find in the stone both fruits and seeds of - the trees whose leaves are also found there.</p> - - <p>(<i>d</i>) By the occurrence of insect remains along with the leaves.</p> - - <p>2. <em>The flora of Atanekerdluk is Miocene.</em></p> - - <p>3. <em>The flora is rich in species.</em></p> - - <p>4. <em>The flora proves without a doubt that North Greenland, in the - Miocene epoch, had a climate much warmer than its present one. The - difference must be at least</em> 29° F.</p> - - <p>Professor Heer discusses at considerable length this proposition. He - says that the evidence from Greenland gives a final answer to those - who objected to the conclusions as to the Miocene climate of Europe - drawn by him on a former occasion. It is quite impossible that the - trees found at Atanekerdluk could ever have flourished there if - the temperature were not far higher than it is at present. This is - clear from many of the species, of which we find the nearest living - representative 10° or even 20° of latitude to the south of the locality - in question.</p> - - <p>The trees of Atanekerdluk were not, he says, all at the extreme - northern limit of their range, for in the Miocene flora of Spitzbergen, - lat. 78° N., we find the beech, plane, hazelnut, and some other species - identical with those from Greenland, and we may conclude, he thinks, - that the firs and poplars which we meet at Atanekerdluk and Bell Sound, - Spitzbergen, must have reached up to the North Pole if land existed - there in the tertiary period.</p> - - <p>“The hills of fossilized wood,” he adds, “found by McClure and his - companions in Banks’s Land (lat. 74° 27′ N.), are therefore discoveries - which should not astonish us, they only confirm the evidence as to the - original vegetation of the polar regions which we have derived from - other sources.”</p> - - <p><span class="pagenum" id="Page_310">310</span></p> - - <p>The <i>Sequoia landsdorfii</i> is the most abundant of the trees of - Atanekerdluk. The <i>Sequoia sempervirens</i> is its present representative. - This tree has its extreme northern limit about lat. 53° N. For its - existence it requires a summer temperature of 59° or 61° F. Its fruit - requires a temperature of 64° for ripening. The winter temperature must - not fall below 34°, and that of the whole year must be at least 49°. - The temperature of Atanekerdluk during the time that the Miocene flora - grew could not have been under the above.<a id="FNanchor_193" href="#Footnote_193" class="fnanchor">[193]</a></p> - - <p>Professor Heer concludes his paper as follows:—</p> - - <p>“I think these facts are convincing, and the more so that they are not - insulated, but confirmed by the evidence derivable from the Miocene - flora of Iceland, Spitzbergen, and Northern Canada. These conclusions, - too, are only links in the grand chain of evidence obtained from the - examination of the Miocene flora of the whole of Europe. They prove to - us that we could not by any re-arrangement of the relative positions - of land and water produce for the northern hemisphere a climate which - would explain the phenomena in a satisfactory manner. We must only - admit that we are face to face with a problem, whose solution in all - probability must be attempted, and, we doubt not, completed by the - astronomer.”</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XIX"> - <span class="pagenum" id="Page_311">311</span> - <h2> - CHAPTER XIX.<br /><br /> - <span class="small">GEOLOGICAL TIME.—PROBABLE DATE OF THE GLACIAL EPOCH.</span> - </h2> - </div> - <div class="subhead">Geological Time measurable from Astronomical Data.—M. - Leverrier’s Formulæ.—Tables of Eccentricity for 3,000,000 Years - in the Past and 1,000,000 Years in the Future.—How the Tables - have been computed.—Why the Glacial Epoch is more recent than - had been supposed.—Figures convey a very inadequate Conception - of immense Duration.—Mode of representing a Million of - Years.—Probable Date of the Glacial Epoch.</div> - - <p class="noindent"><span class="smcap">If</span> those great Secular variations of climate which we have been - considering be indirectly the result of changes in the eccentricity - of the earth’s orbit, then we have a means of determining, at least - so far as regards recent epochs, when these variations took place. - If the glacial epoch be due to the causes assigned, we have a means - of ascertaining, with tolerable accuracy, not merely the date of its - commencement, but the length of its duration. M. Leverrier has not - only determined the superior limit of the eccentricity of the earth’s - orbit, but has also given formulæ by means of which the extent of the - eccentricity for any period, past or future, may be computed.</p> - - <p>A well-known astronomer and mathematician, who has specially - investigated the subject, is of opinion that these formulæ give results - which may be depended upon as approximately correct for <em>four millions - of years</em> past and future. An eminent physicist has, however, expressed - to me his doubts as to whether the results can be depended on for a - period so enormous. M. Leverrier in his Memoir has given a table of the - eccentricity for 100,000 years before and after 1800 <span class="smcap">a.d.</span>, - computed for intervals of 10,000 years. This table, no doubt, embraces - a period sufficiently great for ordinary astronomical purposes, but it - is by far too limited to afford information in regard to geological - epochs.</p> - - <p><span class="pagenum" id="Page_312">312</span></p> - - <p>With the view of ascertaining the probable date of the glacial epoch, - as well as the character of the climate for a long course of ages, - <a href="#TABLE_I">Table I.</a> was computed from M. Leverrier’s formulæ.<a id="FNanchor_194" href="#Footnote_194" class="fnanchor">[194]</a> It shows the - eccentricity of the earth’s orbit and longitude of the perihelion for - 3,000,000 of years back, and 1,000,000 of years to come, at periods - 50,000 years apart.</p> - - <p>On looking over the table it will be seen that there are three - principal periods when the eccentricity rose to a very high value, - with a few subordinate maxima between. It will be perceived also that - during each of those periods the eccentricity does not remain at the - same uniform value, but rises and falls, in one case twice, and in the - other two cases three times. About 2,650,000 years back we have the - eccentricity almost at its inferior limit. It then begins to increase, - and fifty thousand years afterwards, namely at 2,600,000 years ago, it - reaches ·0660; fifty thousand years after this period it has diminished - to ·0167, which is about its present value. It then begins to increase, - and in another fifty thousand years, namely at 2,500,000 years ago, it - approaches to almost the superior limit, its value being then ·0721. It - then begins to diminish, and at 2,450,000 years ago it has diminished - to ·0252. These two maxima, separated by a minimum and extending over a - period<span class="pagenum" id="Page_313">313</span> - of 200,000 years, constitute the first great period of high - eccentricity. We then pass onwards for upwards of a million and a half - years, and we come to the second great period. It consists of three - maxima separated by two minima. The first maximum occurred at 950,000 - years ago, the second or middle one at 850,000 years ago, and the - third and last at 750,000 years ago—the whole extending over a period - of nearly 300,000 years. Passing onwards for another million and half - years, or to about 800,000 years in the future, we come to the third - great period. It also consists of three maxima one hundred thousand - years apart. Those occur at the periods 800,000, 900,000, and 1,000,000 - years to come, respectively, separated also by two minima. Those three - great periods, two of them in the past and one of them in the future, - included in the Table, are therefore separated from each other by an - interval of upwards of 1,700,000 years.</p> - - <div class="figcenter illow600" id="PLATE_IV" > - <div class="attribt">PLATE IV</div> - <img src="images/i_313.jpg" width="600" height="282" alt="" /> - <div class="attribr">W. & A. K. Johnston, Edinb<sup>r</sup>. and London.</div> - <div class="caption">DIAGRAM REPRESENTING THE VARIATIONS IN THE ECCENTRICITY OF THE EARTH’S - ORBIT FOR THREE MILLION OF YEARS BEFORE 1800 A.D. ONE MILLION OF YEARS - AFTER IT.<br /> - <i>The Ordinates are joined by straight lines where the values, at - intervals of 10,000 years, between them have not been determined.</i></div> - </div> - - <p>In this Table there are seven periods when the earth’s orbit becomes - nearly circular, four in the past and three in the future.</p> - - <p>The Table shows also four or five subordinate periods of high - eccentricity, the principal one occurring 200,000 years ago.</p> - - <p>The variations of eccentricity during the four millions of years, are - represented to the eye diagrammatically in <a href="#PLATE_IV">Plate IV.</a></p> - - <p>In order to determine with more accuracy the condition of the earth’s - orbit during the three periods of great eccentricity included in Table - I., I computed the values for periods of ten thousand years apart, and - the results are embodied in Tables II., III., and IV.</p> - - <p>There are still eminent astronomers and physicists who are of opinion - that the climate of the globe never could have been seriously affected - by changes in the eccentricity of its orbit. This opinion results, no - doubt, from viewing the question as a purely astronomical one. Viewed - from an astronomical standpoint, as has been already remarked, there - is actually nothing from which any one could reasonably conclude with - certainty whether a change of eccentricity would seriously affect - climate or not. By means of astronomy we ascertain the extent of the<span class="pagenum" id="Page_314">314</span> - eccentricity at any given period, how much the winter may exceed - the summer in length (or the reverse), how much the sun’s heat is - increased or decreased by a decrease or an increase of distance, - and so forth; but we obtain no information whatever regarding how - these will actually affect climate. This, as we have already seen, - must be determined wholly from physical considerations, and it is - an exceedingly complicated problem. An astronomer, unless he has - given special attention to the physics of the question, is just as - apt to come to a wrong conclusion as any one else. The question - involves certain astronomical elements; but when these are determined - everything then connected with the matter is purely physical. Nearly - all the astronomical elements of the question are comprehended in the - accompanying Tables.</p> - - <div class="center mt5 mb2" id="TABLE_I">TABLE I.</div> - - <div class="center small smcap mb2">The Eccentricity and Longitude of the Perihelion of the - Earth’s Orbit for 3,000,000 Years in the Past and 1,000,000 - Years in the Future, computed for Intervals of 50,000 Years. - </div> - - <table summary="Eccentricity and Longitude of the Perihelion of the Earth’s Orbit"> - <tbody> - <tr> - <th colspan="3" class="bt bl br">PAST TIME.</th> - </tr> - <tr> - <th class="bt bl bb">Number of years<br />before epoch 1800.</th> - <th class="bt bl bb">Eccentricity.</th> - <th class="bt bl br bb">Longitude of<br />perihelion.</th> - </tr> - <tr> - <td class="bt bl"> </td> - <td class="bt bl"> </td> - <td class="tdc bt bl br"><div> ° ′</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−3,000,000</div></td> - <td class="tdc bl"><div>0·0365</div></td> - <td class="tdc bl br"><div> 39 30</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,950,000</div></td> - <td class="tdc bl"><div>0·0170</div></td> - <td class="tdc bl br"><div>210 39</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,900,000</div></td> - <td class="tdc bl"><div>0·0442</div></td> - <td class="tdc bl br"><div>200 52</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,850,000</div></td> - <td class="tdc bl"><div>0·0416</div></td> - <td class="tdc bl br"><div> 0 18</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,800,000</div></td> - <td class="tdc bl"><div>0·0352</div></td> - <td class="tdc bl br"><div>339 14</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,750,000</div></td> - <td class="tdc bl"><div>0·0326</div></td> - <td class="tdc bl br"><div>161 22</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,700,000</div></td> - <td class="tdc bl"><div>0·0330</div></td> - <td class="tdc bl br"><div> 65 37</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,650,000</div></td> - <td class="tdc bl"><div>0·0053</div></td> - <td class="tdc bl br"><div>318 40</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,600,000</div></td> - <td class="tdc bl"><div>0·0660</div></td> - <td class="tdc bl br"><div>190 4</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,550,000</div></td> - <td class="tdc bl"><div>0·0167</div></td> - <td class="tdc bl br"><div>298 34</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,500,000</div></td> - <td class="tdc bl"><div>0·0721</div></td> - <td class="tdc bl br"><div>338 36</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,450,000</div></td> - <td class="tdc bl"><div>0·0252</div></td> - <td class="tdc bl br"><div>109 33</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,400,000</div></td> - <td class="tdc bl"><div>0·0415</div></td> - <td class="tdc bl br"><div>116 40</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,350,000</div></td> - <td class="tdc bl"><div>0·0281</div></td> - <td class="tdc bl br"><div>308 23</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,300,000</div></td> - <td class="tdc bl"><div>0·0238</div></td> - <td class="tdc bl br"><div>195 25</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,250,000</div></td> - <td class="tdc bl"><div>0·0328</div></td> - <td class="tdc bl br"><div>141 18</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,200,000</div></td> - <td class="tdc bl"><div>0·0352</div></td> - <td class="tdc bl br"><div>307 6</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,150,000</div></td> - <td class="tdc bl"><div>0·0183</div></td> - <td class="tdc bl br"><div>307 5</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,100,000</div></td> - <td class="tdc bl"><div>0·0304</div></td> - <td class="tdc bl br"><div> 98 40</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,050,000</div></td> - <td class="tdc bl"><div>0·0170</div></td> - <td class="tdc bl br"><div>334 46</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−2,000,000</div></td> - <td class="tdc bl"><div>0·0138</div></td> - <td class="tdc bl br"><div>324 4</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,950,000</div></td> - <td class="tdc bl"><div>0·0427</div></td> - <td class="tdc bl br"><div>120 32</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,900,000</div></td> - <td class="tdc bl"><div>0·0336</div></td> - <td class="tdc bl br"><div>188 31</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,850,000</div></td> - <td class="tdc bl"><div>0·0503</div></td> - <td class="tdc bl br"><div>272 14</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,800,000</div></td> - <td class="tdc bl"><div>0·0334</div></td> - <td class="tdc bl br"><div>354 52</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,750,000</div></td> - <td class="tdc bl"><div>0·0350</div></td> - <td class="tdc bl br"><div> 65 25</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,700,000</div></td> - <td class="tdc bl"><div>0·0085</div></td> - <td class="tdc bl br"><div> 95 13</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,650,000</div></td> - <td class="tdc bl"><div>0·0035</div></td> - <td class="tdc bl br"><div>168 23</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,600,000</div></td> - <td class="tdc bl"><div>0·0305</div></td> - <td class="tdc bl br"><div>158 42</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,550,000</div></td> - <td class="tdc bl"><div>0·0239</div></td> - <td class="tdc bl br"><div>225 57</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,500,000</div></td> - <td class="tdc bl"><div>0·0430</div></td> - <td class="tdc bl br"><div>303 29</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,450,000</div></td> - <td class="tdc bl"><div>0·0195</div></td> - <td class="tdc bl br"><div> 57 11</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,400,000</div></td> - <td class="tdc bl"><div>0·0315</div></td> - <td class="tdc bl br"><div> 97 35</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,350,000</div></td> - <td class="tdc bl"><div>0·0322</div></td> - <td class="tdc bl br"><div>293 38</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,300,000</div></td> - <td class="tdc bl"><div>0·0022</div></td> - <td class="tdc bl br"><div> 0 48</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,250,000</div></td> - <td class="tdc bl"><div>0·0475</div></td> - <td class="tdc bl br"><div>105 50</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,200,000</div></td> - <td class="tdc bl"><div>0·0289</div></td> - <td class="tdc bl br"><div>239 34</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,150,000</div></td> - <td class="tdc bl"><div>0·0473</div></td> - <td class="tdc bl br"><div>250 27</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,100,000</div></td> - <td class="tdc bl"><div>0·0311</div></td> - <td class="tdc bl br"><div> 55 24</div></td> - </tr> - <tr> - <td class="tdc bl"><div>−1,050,000</div></td> - <td class="tdc bl"><div>0·0326</div></td> - <td class="tdc bl br"><div> 4 8</div></td> - </tr> - <tr> - <td class="tdc bl"><span class="pagenum" id="Page_315">315</span><div>−1,000,000</div></td> - <td class="tdc bl"><div>0·0151</div></td> - <td class="tdc bl br"><div>248 22</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 950,000</div></td> - <td class="tdc bl"><div>0·0517</div></td> - <td class="tdc bl br"><div> 97 51</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 900,000</div></td> - <td class="tdc bl"><div>0·0102</div></td> - <td class="tdc bl br"><div>135 2</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 850,000</div></td> - <td class="tdc bl"><div>0·0747</div></td> - <td class="tdc bl br"><div>239 28</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 800,000</div></td> - <td class="tdc bl"><div>0·0132</div></td> - <td class="tdc bl br"><div>343 49</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 750,000</div></td> - <td class="tdc bl"><div>0·0575</div></td> - <td class="tdc bl br"><div> 27 18</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 700,000</div></td> - <td class="tdc bl"><div>0·0220</div></td> - <td class="tdc bl br"><div>208 13</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 650,000</div></td> - <td class="tdc bl"><div>0·0226</div></td> - <td class="tdc bl br"><div>141 29</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 600,000</div></td> - <td class="tdc bl"><div>0·0417</div></td> - <td class="tdc bl br"><div> 32 34</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 550,000</div></td> - <td class="tdc bl"><div>0·0166</div></td> - <td class="tdc bl br"><div>251 50</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 500,000</div></td> - <td class="tdc bl"><div>0·0388</div></td> - <td class="tdc bl br"><div>193 56</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 450,000</div></td> - <td class="tdc bl"><div>0·0308</div></td> - <td class="tdc bl br"><div>356 52</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 400,000</div></td> - <td class="tdc bl"><div>0·0170</div></td> - <td class="tdc bl br"><div>290 7</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 350,000</div></td> - <td class="tdc bl"><div>0·0195</div></td> - <td class="tdc bl br"><div>182 50</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 300,000</div></td> - <td class="tdc bl"><div>0·0424</div></td> - <td class="tdc bl br"><div> 23 29</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 250,000</div></td> - <td class="tdc bl"><div>0·0258</div></td> - <td class="tdc bl br"><div> 59 39</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 200,000</div></td> - <td class="tdc bl"><div>0·0569</div></td> - <td class="tdc bl br"><div>168 18</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 150,000</div></td> - <td class="tdc bl"><div>0·0332</div></td> - <td class="tdc bl br"><div>242 56</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 100,000</div></td> - <td class="tdc bl"><div>0·0473</div></td> - <td class="tdc bl br"><div>316 18</div></td> - </tr> - <tr> - <td class="tdc bl"><div>− 50,000</div></td> - <td class="tdc bl"><div>0·0131</div></td> - <td class="tdc bl br"><div> 50 14</div></td> - </tr> - <tr> - <th colspan="3" class="bt bl br">FUTURE TIME.</th> - </tr> - <tr> - <th class="bt bl bb">Number of years<br />after epoch 1800.</th> - <th class="bt bl bb">Eccentricity.</th> - <th class="bt bl br bb">Longitude of<br />perihelion.</th> - </tr> - <tr> - <td class="bt bl"> </td> - <td class="bt bl"> </td> - <td class="tdc bt bl br"><div> ° ′</div></td> - </tr> - <tr> - <td class="tdc bl"><div><span class="smcap">a.d</span> 1800</div></td> - <td class="tdc bl"><div>0·0168</div></td> - <td class="tdc bl br"><div> 99 30</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 50,000</div></td> - <td class="tdc bl"><div>0·0173</div></td> - <td class="tdc bl br"><div> 38 12</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 100,000</div></td> - <td class="tdc bl"><div>0·0191</div></td> - <td class="tdc bl br"><div>114 50</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 150,000</div></td> - <td class="tdc bl"><div>0·0353</div></td> - <td class="tdc bl br"><div>201 57</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 200,000</div></td> - <td class="tdc bl"><div>0·0246</div></td> - <td class="tdc bl br"><div>279 41</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 250,000</div></td> - <td class="tdc bl"><div>0·0286</div></td> - <td class="tdc bl br"><div>350 54</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 300,000</div></td> - <td class="tdc bl"><div>0·0158</div></td> - <td class="tdc bl br"><div>172 29</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 350,000</div></td> - <td class="tdc bl"><div>0·0098</div></td> - <td class="tdc bl br"><div>201 40</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 400,000</div></td> - <td class="tdc bl"><div>0·0429</div></td> - <td class="tdc bl br"><div> 6 9</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 450,000</div></td> - <td class="tdc bl"><div>0·0231</div></td> - <td class="tdc bl br"><div> 98 37</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 500,000</div></td> - <td class="tdc bl"><div>0·0534</div></td> - <td class="tdc bl br"><div>157 26</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 550,000</div></td> - <td class="tdc bl"><div>0·0259</div></td> - <td class="tdc bl br"><div>287 31</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 600,000</div></td> - <td class="tdc bl"><div>0·0395</div></td> - <td class="tdc bl br"><div>285 43</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 650,000</div></td> - <td class="tdc bl"><div>0·0169</div></td> - <td class="tdc bl br"><div>144 3</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 700,000</div></td> - <td class="tdc bl"><div>0·0357</div></td> - <td class="tdc bl br"><div> 17 12</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 750,000</div></td> - <td class="tdc bl"><div>0·0195</div></td> - <td class="tdc bl br"><div> 0 53</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 800,000</div></td> - <td class="tdc bl"><div>0·0639</div></td> - <td class="tdc bl br"><div>140 38</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 850,000</div></td> - <td class="tdc bl"><div>0·0144</div></td> - <td class="tdc bl br"><div>176 41</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 900,000</div></td> - <td class="tdc bl"><div>0·0659</div></td> - <td class="tdc bl br"><div>291 16</div></td> - </tr> - <tr> - <td class="tdc bl"><div>+ 950,000</div></td> - <td class="tdc bl"><div>0·0086</div></td> - <td class="tdc bl br"><div>115 13</div></td> - </tr> - <tr> - <td class="tdc bl bb"><div>+1,000,000</div></td> - <td class="tdc bl bb"><div>0·0528</div></td> - <td class="tdc bl br bb"><div> 57 31</div></td> - </tr> - </tbody> - </table> - - <p><span class="pagenum" id="Page_316">316</span></p> - - <div class="center mt5 mb2" id="TABLE_II">TABLE II.</div> - - <div class="center small smcap">Eccentricity, Longitude of the Perihelion, &c., &c., for Intervals - of 10,000 Years, from 2,650,000 to 2,450,000 Years ago.</div> - - <div class="center small mt1 mb2"><span class="smcap">the glacial epoch of the</span> <i>Eocene period</i> <span class="smcap">is probably - comprehended within this table</span>.</div> - - <table summary="Eccentricity, Longitude of the Perihelion"> - <tbody> - <tr> - <th class="bt bl">I.</th> - <th class="bt bl">II.</th> - <th class="bt bl">III.</th> - <th class="bt bl">IV.</th> - <th colspan="4" class="bt bl br bb">Winter occurring in aphelion.</th> - </tr> - <tr> - <th class="bl bb">Number of years before <span class="smcap">a.d.</span> 1800.</th> - <th class="bl bb">Eccentricity of orbit.</th> - <th class="bl bb">Longitude of perihelion.</th> - <th class="bl bb">Number of degrees passed over by the perihelion. Motion retrograde at periods marked R.</th> - <th class="bl bb">V.<br />Excess of winter over summer, in days.</th> - <th class="bl bb">VI.<br />Midwinter intensity of the sun’s heat. Present intensity = 1000.</th> - <th class="bl bb">VII.<br />Number of degrees by which the midwinter temperature is lowered.</th> - <th class="bl br bb">VIII.<br />Midwinter temperature of Great Britain.</th> - </tr> - <tr> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div> ° ′</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div>2,650,000</div></td> - <td class="tdc bl"><div>0·0053</div></td> - <td class="tdc bl"><div>318 40</div></td> - <td class="tdc bl"><div> ° ′</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div>F.</div></td> - <td class="tdc bl br"><div>F.</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,640,000</div></td> - <td class="tdc bl"><div>0·0173</div></td> - <td class="tdc bl"><div> 54 25</div></td> - <td class="tdc bl"><div> 95 45</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div>°</div></td> - <td class="tdc bl br"><div>°</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,630,000</div></td> - <td class="tdc bl"><div>0·0331</div></td> - <td class="tdc bl"><div> 93 37</div></td> - <td class="tdc bl"><div> 39 12</div></td> - <td class="tdc bl"><div>15·4</div></td> - <td class="tdc bl"><div>906</div></td> - <td class="tdc bl"><div>26·2</div></td> - <td class="tdc bl br"><div>12·8</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,620,000</div></td> - <td class="tdc bl"><div>0·0479</div></td> - <td class="tdc bl"><div>127 12</div></td> - <td class="tdc bl"><div> 33 35</div></td> - <td class="tdc bl"><div>22·2</div></td> - <td class="tdc bl"><div>884</div></td> - <td class="tdc bl"><div>33·3</div></td> - <td class="tdc bl br"><div> 5·7</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,610,000</div></td> - <td class="tdc bl"><div>0·0591</div></td> - <td class="tdc bl"><div>158 36</div></td> - <td class="tdc bl"><div> 31 24</div></td> - <td class="tdc bl"><div>27·4</div></td> - <td class="tdc bl"><div>862</div></td> - <td class="tdc bl"><div>38·3</div></td> - <td class="tdc bl br"><div> 0·7</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,600,000</div></td> - <td class="tdc bl"><div>0·0660</div></td> - <td class="tdc bl"><div>190 4</div></td> - <td class="tdc bl"><div> 31 28</div></td> - <td class="tdc bl"><div>30·6</div></td> - <td class="tdc bl"><div>851</div></td> - <td class="tdc bl"><div>41·5</div></td> - <td class="tdc bl br"><div> −2·5</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,590,000</div></td> - <td class="tdc bl"><div>0·0666</div></td> - <td class="tdc bl"><div>220 28</div></td> - <td class="tdc bl"><div> 30 24</div></td> - <td class="tdc bl"><div>30·9</div></td> - <td class="tdc bl"><div>850</div></td> - <td class="tdc bl"><div>41·8</div></td> - <td class="tdc bl br"><div>−2·8</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,580,000</div></td> - <td class="tdc bl"><div>0·0609</div></td> - <td class="tdc bl"><div>249 56</div></td> - <td class="tdc bl"><div> 29 28</div></td> - <td class="tdc bl"><div>28·3</div></td> - <td class="tdc bl"><div>859</div></td> - <td class="tdc bl"><div>39·2</div></td> - <td class="tdc bl br"><div>−0·2</div></td> - </tr> - <tr> - <td class="tdc bl"><span class="pagenum" id="Page_317">317</span><div>2,570,000</div></td> - <td class="tdc bl"><div>0·0492</div></td> - <td class="tdc bl"><div>277 24</div></td> - <td class="tdc bl"><div> 27 28</div></td> - <td class="tdc bl"><div>22·9</div></td> - <td class="tdc bl"><div>878</div></td> - <td class="tdc bl"><div>33·9</div></td> - <td class="tdc bl br"><div> 5·1</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,560,000</div></td> - <td class="tdc bl"><div>0·0350</div></td> - <td class="tdc bl"><div>305 2</div></td> - <td class="tdc bl"><div> 27 38</div></td> - <td class="tdc bl"><div>16·2</div></td> - <td class="tdc bl"><div>902</div></td> - <td class="tdc bl"><div>27·1</div></td> - <td class="tdc bl br"><div>11·9</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,550,000</div></td> - <td class="tdc bl"><div>0·0167</div></td> - <td class="tdc bl"><div>298 34</div></td> - <td class="tdc bl"><div>R 6 28</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div>2,540,000</div></td> - <td class="tdc bl"><div>0·0192</div></td> - <td class="tdc bl"><div>253 58</div></td> - <td class="tdc bl"><div>R 44 36</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div>2,530,000</div></td> - <td class="tdc bl"><div>0·0369</div></td> - <td class="tdc bl"><div>259 19</div></td> - <td class="tdc bl"><div> 5 21</div></td> - <td class="tdc bl"><div>17·1</div></td> - <td class="tdc bl"><div>899</div></td> - <td class="tdc bl"><div>28·0</div></td> - <td class="tdc bl br"><div>11·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,520,000</div></td> - <td class="tdc bl"><div>0·0537</div></td> - <td class="tdc bl"><div>283 7</div></td> - <td class="tdc bl"><div> 23 48</div></td> - <td class="tdc bl"><div>25·0</div></td> - <td class="tdc bl"><div>871</div></td> - <td class="tdc bl"><div>35·9</div></td> - <td class="tdc bl br"><div> 3·1</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,510,000</div></td> - <td class="tdc bl"><div>0·0660</div></td> - <td class="tdc bl"><div>310 4</div></td> - <td class="tdc bl"><div> 26 57</div></td> - <td class="tdc bl"><div>30·6</div></td> - <td class="tdc bl"><div>851</div></td> - <td class="tdc bl"><div>41·5</div></td> - <td class="tdc bl br"><div>−2·5</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,500,000</div></td> - <td class="tdc bl"><div>0·0721</div></td> - <td class="tdc bl"><div>338 36</div></td> - <td class="tdc bl"><div> 28 32</div></td> - <td class="tdc bl"><div>33·5</div></td> - <td class="tdc bl"><div>841</div></td> - <td class="tdc bl"><div>44·2</div></td> - <td class="tdc bl br"><div>−5·2</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,490,000</div></td> - <td class="tdc bl"><div>0·0722</div></td> - <td class="tdc bl"><div> 7 36</div></td> - <td class="tdc bl"><div> 29 0</div></td> - <td class="tdc bl"><div>33·6</div></td> - <td class="tdc bl"><div>841</div></td> - <td class="tdc bl"><div>44·3</div></td> - <td class="tdc bl br"><div>−5·3</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,480,000</div></td> - <td class="tdc bl"><div>0·0662</div></td> - <td class="tdc bl"><div> 35 46</div></td> - <td class="tdc bl"><div> 28 10</div></td> - <td class="tdc bl"><div>30·8</div></td> - <td class="tdc bl"><div>850</div></td> - <td class="tdc bl"><div>41·7</div></td> - <td class="tdc bl br"><div>−2·7</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,470,000</div></td> - <td class="tdc bl"><div>0·0553</div></td> - <td class="tdc bl"><div> 63 26</div></td> - <td class="tdc bl"><div> 27 40</div></td> - <td class="tdc bl"><div>25·7</div></td> - <td class="tdc bl"><div>868</div></td> - <td class="tdc bl"><div>36·6</div></td> - <td class="tdc bl br"><div> 2·4</div></td> - </tr> - <tr> - <td class="tdc bl"><div>2,460,000</div></td> - <td class="tdc bl"><div>0·0410</div></td> - <td class="tdc bl"><div> 89 13</div></td> - <td class="tdc bl"><div> 25 47</div></td> - <td class="tdc bl"><div>19·1</div></td> - <td class="tdc bl"><div>892</div></td> - <td class="tdc bl"><div>30·0</div></td> - <td class="tdc bl br"><div> 9·0</div></td> - </tr> - <tr> - <td class="tdc bl bb"><div>2,450,000</div></td> - <td class="tdc bl bb"><div>0·0252</div></td> - <td class="tdc bl bb"><div>109 33</div></td> - <td class="tdc bl bb"><div> 20 20</div></td> - <td class="tdc bl bb"><div>11·7</div></td> - <td class="tdc bl bb"> </td> - <td class="tdc bl bb"> </td> - <td class="tdc bl br bb"> </td> - </tr> - </tbody> - </table> - - <p><span class="pagenum" id="Page_318">318</span></p> - - <div class="center mt5 mb2" id="TABLE_III">TABLE III.</div> - - <div class="center small smcap">Eccentricity, Longitude of the Perihelion, &c., &c., for Intervals - of 10,000 Years, from 1,000,000 to 750,000 Years ago.</div> - - <div class="center small mt1 mb2"><span class="smcap">the glacial epoch of the</span> <i>Eocene period</i> <span class="smcap">is probably - comprehended within this table</span>.</div> - - <table summary="Eccentricity, Longitude of the Perihelion"> - <tbody> - <tr> - <th class="bt bl">I.</th> - <th class="bt bl">II.</th> - <th class="bt bl">III.</th> - <th class="bt bl">IV.</th> - <th colspan="4" class="bt bl br bb">Winter occurring in aphelion.</th> - </tr> - <tr> - <th class="bl bb">Number of years before <span class="smcap">a.d.</span> 1800.</th> - <th class="bl bb">Eccentricity of orbit.</th> - <th class="bl bb">Longitude of perihelion.</th> - <th class="bl bb">Number of degrees passed over by the perihelion. Motion retrograde at periods marked R.</th> - <th class="bl bb">V.<br />Excess of winter over summer, in days.</th> - <th class="bl bb">VI.<br />Midwinter intensity of the sun’s heat. Present intensity = 1000.</th> - <th class="bl bb">VII.<br />Number of degrees by which the midwinter temperature is lowered.</th> - <th class="bl br bb">VIII.<br />Midwinter temperature of Great Britain.</th> - </tr> - <tr> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div> ° ′</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div>1,000,000</div></td> - <td class="tdc bl"><div>0·0151</div></td> - <td class="tdc bl"><div>248 22</div></td> - <td class="tdc bl"><div> ° ′</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div>F.</div></td> - <td class="tdc bl br"><div>F.</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 990,000</div></td> - <td class="tdc bl"><div>0·0224</div></td> - <td class="tdc bl"><div>313 50</div></td> - <td class="tdc bl"><div> 65 28</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div>°</div></td> - <td class="tdc bl br"><div>°</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 980,000</div></td> - <td class="tdc bl"><div>0·0329</div></td> - <td class="tdc bl"><div>358 2</div></td> - <td class="tdc bl"><div> 44 12</div></td> - <td class="tdc bl"><div>15·3</div></td> - <td class="tdc bl"><div>906</div></td> - <td class="tdc bl"><div>26·1</div></td> - <td class="tdc bl br"><div>12·9</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 970,000</div></td> - <td class="tdc bl"><div>0·0441</div></td> - <td class="tdc bl"><div> 32 40</div></td> - <td class="tdc bl"><div> 34 38</div></td> - <td class="tdc bl"><div>20·5</div></td> - <td class="tdc bl"><div>887</div></td> - <td class="tdc bl"><div>31·5</div></td> - <td class="tdc bl br"><div> 7·5</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 960,000</div></td> - <td class="tdc bl"><div>0·0491</div></td> - <td class="tdc bl"><div> 66 49</div></td> - <td class="tdc bl"><div> 34 9</div></td> - <td class="tdc bl"><div>22·8</div></td> - <td class="tdc bl"><div>878</div></td> - <td class="tdc bl"><div>33·8</div></td> - <td class="tdc bl br"><div> 5·2</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 950,000</div></td> - <td class="tdc bl"><div>0·0517</div></td> - <td class="tdc bl"><div> 97 51</div></td> - <td class="tdc bl"><div> 31 2</div></td> - <td class="tdc bl"><div>24·0</div></td> - <td class="tdc bl"><div>874</div></td> - <td class="tdc bl"><div>35·0</div></td> - <td class="tdc bl br"><div> 4·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 940,000</div></td> - <td class="tdc bl"><div>0·0495</div></td> - <td class="tdc bl"><div>127 42</div></td> - <td class="tdc bl"><div> 29 51</div></td> - <td class="tdc bl"><div>23·0</div></td> - <td class="tdc bl"><div>878</div></td> - <td class="tdc bl"><div>34·0</div></td> - <td class="tdc bl br"><div> 5·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 930,000</div></td> - <td class="tdc bl"><div>0·0423</div></td> - <td class="tdc bl"><div>156 11</div></td> - <td class="tdc bl"><div> 28 29</div></td> - <td class="tdc bl"><div>19·7</div></td> - <td class="tdc bl"><div>890</div></td> - <td class="tdc bl"><div>30·6</div></td> - <td class="tdc bl br"><div> 8·4</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 920,000</div></td> - <td class="tdc bl"><div>0·0305</div></td> - <td class="tdc bl"><div>181 40</div></td> - <td class="tdc bl"><div> 25 29</div></td> - <td class="tdc bl"><div>14·2</div></td> - <td class="tdc bl"><div>910</div></td> - <td class="tdc bl"><div>25·0</div></td> - <td class="tdc bl br"><div>14·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 910,000</div></td> - <td class="tdc bl"><div>0·0156</div></td> - <td class="tdc bl"><div>194 15</div></td> - <td class="tdc bl"><div> 12 35</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div> 900,000</div></td> - <td class="tdc bl"><div>0·0102</div></td> - <td class="tdc bl"><div>135 2</div></td> - <td class="tdc bl"><div>R 59 13</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><span class="pagenum" id="Page_319">319</span><div> 890,000</div></td> - <td class="tdc bl"><div>0·0285</div></td> - <td class="tdc bl"><div>127 1</div></td> - <td class="tdc bl"><div>R 8 1</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div> 880,000</div></td> - <td class="tdc bl"><div>0·0456</div></td> - <td class="tdc bl"><div>152 33</div></td> - <td class="tdc bl"><div> 25 32</div></td> - <td class="tdc bl"><div>21·2</div></td> - <td class="tdc bl"><div>884</div></td> - <td class="tdc bl"><div>32·2</div></td> - <td class="tdc bl br"><div> 6·8</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 870,000</div></td> - <td class="tdc bl"><div>0·0607</div></td> - <td class="tdc bl"><div>180 23</div></td> - <td class="tdc bl"><div> 27 50</div></td> - <td class="tdc bl"><div>28·2</div></td> - <td class="tdc bl"><div>859</div></td> - <td class="tdc bl"><div>39·0</div></td> - <td class="tdc bl br"><div> 0·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 860,000</div></td> - <td class="tdc bl"><div>0·0708</div></td> - <td class="tdc bl"><div>209 41</div></td> - <td class="tdc bl"><div> 29 18</div></td> - <td class="tdc bl"><div>32·9</div></td> - <td class="tdc bl"><div>843</div></td> - <td class="tdc bl"><div>43·6</div></td> - <td class="tdc bl br"><div>−4·6</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 850,000</div></td> - <td class="tdc bl"><div>0·0747</div></td> - <td class="tdc bl"><div>239 28</div></td> - <td class="tdc bl"><div> 29 47</div></td> - <td class="tdc bl"><div>34·7</div></td> - <td class="tdc bl"><div>837</div></td> - <td class="tdc bl"><div>45·3</div></td> - <td class="tdc bl br"><div>−6·3</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 840,000</div></td> - <td class="tdc bl"><div>0·0698</div></td> - <td class="tdc bl"><div>269 14</div></td> - <td class="tdc bl"><div> 29 46</div></td> - <td class="tdc bl"><div>32·4</div></td> - <td class="tdc bl"><div>845</div></td> - <td class="tdc bl"><div>43·2</div></td> - <td class="tdc bl br"><div>−4·2</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 830,000</div></td> - <td class="tdc bl"><div>0·0623</div></td> - <td class="tdc bl"><div>298 28</div></td> - <td class="tdc bl"><div> 29 14</div></td> - <td class="tdc bl"><div>29·0</div></td> - <td class="tdc bl"><div>857</div></td> - <td class="tdc bl"><div>40·0</div></td> - <td class="tdc bl br"><div>−1·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 820,000</div></td> - <td class="tdc bl"><div>0·0476</div></td> - <td class="tdc bl"><div>326 4</div></td> - <td class="tdc bl"><div> 27 36</div></td> - <td class="tdc bl"><div>22·1</div></td> - <td class="tdc bl"><div>881</div></td> - <td class="tdc bl"><div>33·1</div></td> - <td class="tdc bl br"><div> 5·9</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 810,000</div></td> - <td class="tdc bl"><div>0·0296</div></td> - <td class="tdc bl"><div>348 30</div></td> - <td class="tdc bl"><div> 22 26</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div> 800,000</div></td> - <td class="tdc bl"><div>0·0132</div></td> - <td class="tdc bl"><div>343 49</div></td> - <td class="tdc bl"><div>R 4 41</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div> 790,000</div></td> - <td class="tdc bl"><div>0·0171</div></td> - <td class="tdc bl"><div>293 19</div></td> - <td class="tdc bl"><div>R 50 30</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div> 780,000</div></td> - <td class="tdc bl"><div>0·0325</div></td> - <td class="tdc bl"><div>303 37</div></td> - <td class="tdc bl"><div> 10 18</div></td> - <td class="tdc bl"><div>15·2</div></td> - <td class="tdc bl"><div>907</div></td> - <td class="tdc bl"><div>26·0</div></td> - <td class="tdc bl br"><div>13·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 770,000</div></td> - <td class="tdc bl"><div>0·0455</div></td> - <td class="tdc bl"><div>328 38</div></td> - <td class="tdc bl"><div> 25 1</div></td> - <td class="tdc bl"><div>21·2</div></td> - <td class="tdc bl"><div>884</div></td> - <td class="tdc bl"><div>32·2</div></td> - <td class="tdc bl br"><div> 6·8</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 760,000</div></td> - <td class="tdc bl"><div>0·0540</div></td> - <td class="tdc bl"><div>357 12</div></td> - <td class="tdc bl"><div> 28 34</div></td> - <td class="tdc bl"><div>25·1</div></td> - <td class="tdc bl"><div>870</div></td> - <td class="tdc bl"><div>36·0</div></td> - <td class="tdc bl br"><div> 3·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 750,000</div></td> - <td class="tdc bl"><div>0·0575</div></td> - <td class="tdc bl"><div> 27 18</div></td> - <td class="tdc bl"><div> 30 6</div></td> - <td class="tdc bl"><div>26·7</div></td> - <td class="tdc bl"><div>864</div></td> - <td class="tdc bl"><div>37·7</div></td> - <td class="tdc bl br"><div> 1·3</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 740,000</div></td> - <td class="tdc bl"><div>0·0561</div></td> - <td class="tdc bl"><div> 58 30</div></td> - <td class="tdc bl"><div> 31 12</div></td> - <td class="tdc bl"><div>26·1</div></td> - <td class="tdc bl"><div>867</div></td> - <td class="tdc bl"><div>37·0</div></td> - <td class="tdc bl br"><div> 2·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 730,000</div></td> - <td class="tdc bl"><div>0·0507</div></td> - <td class="tdc bl"><div> 90 55</div></td> - <td class="tdc bl"><div> 32 25</div></td> - <td class="tdc bl"><div>23·6</div></td> - <td class="tdc bl"><div>876</div></td> - <td class="tdc bl"><div>34·6</div></td> - <td class="tdc bl br"><div> 4·4</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 720,000</div></td> - <td class="tdc bl"><div>0·0422</div></td> - <td class="tdc bl"><div>125 14</div></td> - <td class="tdc bl"><div> 34 19</div></td> - <td class="tdc bl"><div>19·6</div></td> - <td class="tdc bl"><div>890</div></td> - <td class="tdc bl"><div>30·6</div></td> - <td class="tdc bl br"><div> 8·4</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 710,000</div></td> - <td class="tdc bl"><div>0·0307</div></td> - <td class="tdc bl"><div>177 26</div></td> - <td class="tdc bl"><div> 52 12</div></td> - <td class="tdc bl"><div>14·3</div></td> - <td class="tdc bl"><div>910</div></td> - <td class="tdc bl"><div>25·0</div></td> - <td class="tdc bl br"><div>14·0</div></td> - </tr> - <tr> - <td class="tdc bl bb"><div> 700,000</div></td> - <td class="tdc bl bb"><div>0·0220</div></td> - <td class="tdc bl bb"><div>208 13</div></td> - <td class="tdc bl bb"><div> 30 47</div></td> - <td class="tdc bl bb"> </td> - <td class="tdc bl bb"> </td> - <td class="tdc bl bb"> </td> - <td class="tdc bl br bb"> </td> - </tr> - </tbody> - </table> - - <p><span class="pagenum" id="Page_320">320</span></p> - - <div class="center mt5 mb2" id="TABLE_IV">TABLE IV.</div> - - <div class="center small smcap">Eccentricity, Longitude of the Perihelion, &c., &c., for Intervals - of 10,000 Years, from 250,000 Years ago to the present Date.</div> - - <div class="center small mt1 mb2"><span class="smcap">the</span> <i>Glacial epoch</i> <span class="smcap">is probably - comprehended within this table</span>.</div> - - <table summary="Eccentricity, Longitude of the Perihelion"> - <tbody> - <tr> - <th class="bt bl">I.</th> - <th class="bt bl">II.</th> - <th class="bt bl">III.</th> - <th class="bt bl">IV.</th> - <th colspan="4" class="bt bl br bb">Winter occurring in aphelion.</th> - </tr> - <tr> - <th class="bl bb">Number of years before <span class="smcap">a.d.</span> 1800.</th> - <th class="bl bb">Eccentricity of orbit.</th> - <th class="bl bb">Longitude of perihelion.</th> - <th class="bl bb">Number of degrees passed over by the perihelion. Motion retrograde at periods marked R.</th> - <th class="bl bb">V.<br />Excess of winter over summer, in days.</th> - <th class="bl bb">VI.<br />Midwinter intensity of the sun’s heat. Present intensity = 1000.</th> - <th class="bl bb">VII.<br />Number of degrees by which the midwinter temperature is lowered.</th> - <th class="bl br bb">VIII.<br />Midwinter temperature of Great Britain.</th> - </tr> - <tr> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div> ° ′</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div>F.</div></td> - <td class="tdc bl br"><div>F.</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 250,000</div></td> - <td class="tdc bl"><div>0·0258</div></td> - <td class="tdc bl"><div> 59 39</div></td> - <td class="tdc bl"><div> ° ′</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"><div>°</div></td> - <td class="tdc bl br"><div>°</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 240,000</div></td> - <td class="tdc bl"><div>0·0374</div></td> - <td class="tdc bl"><div> 74 58</div></td> - <td class="tdc bl"><div> 15 19</div></td> - <td class="tdc bl"><div>17·4</div></td> - <td class="tdc bl"><div>898</div></td> - <td class="tdc bl"><div>28·3</div></td> - <td class="tdc bl br"><div>10·7</div></td> - </tr> - <tr> - <td class="tdc bl"><div>S 230,000</div></td> - <td class="tdc bl"><div>0·0477</div></td> - <td class="tdc bl"><div>102 49</div></td> - <td class="tdc bl"><div> 27 51</div></td> - <td class="tdc bl"><div>22·2</div></td> - <td class="tdc bl"><div>885</div></td> - <td class="tdc bl"><div>33·2</div></td> - <td class="tdc bl br"><div> 5·8</div></td> - </tr> - <tr> - <td class="tdc bl"><div>S 220,000</div></td> - <td class="tdc bl"><div>0·0497</div></td> - <td class="tdc bl"><div>124 33</div></td> - <td class="tdc bl"><div> 21 44</div></td> - <td class="tdc bl"><div>23·2</div></td> - <td class="tdc bl"><div>877</div></td> - <td class="tdc bl"><div>34·1</div></td> - <td class="tdc bl br"><div> 4·9</div></td> - </tr> - <tr> - <td class="tdc bl"><div>S 210,000</div></td> - <td class="tdc bl"><div>0·0575</div></td> - <td class="tdc bl"><div>144 55</div></td> - <td class="tdc bl"><div> 20 22</div></td> - <td class="tdc bl"><div>26·7</div></td> - <td class="tdc bl"><div>864</div></td> - <td class="tdc bl"><div>37·7</div></td> - <td class="tdc bl br"><div> 1·3</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 200,000</div></td> - <td class="tdc bl"><div>0·0569</div></td> - <td class="tdc bl"><div>168 18</div></td> - <td class="tdc bl"><div> 23 23</div></td> - <td class="tdc bl"><div>26·5</div></td> - <td class="tdc bl"><div>865</div></td> - <td class="tdc bl"><div>37·4</div></td> - <td class="tdc bl br"><div> 1·6</div></td> - </tr> - <tr> - <td class="tdc bl"><div>S 190,000</div></td> - <td class="tdc bl"><div>0·0532</div></td> - <td class="tdc bl"><div>190 4</div></td> - <td class="tdc bl"><div> 21 46</div></td> - <td class="tdc bl"><div>24·7</div></td> - <td class="tdc bl"><div>871</div></td> - <td class="tdc bl"><div>35·7</div></td> - <td class="tdc bl br"><div> 3·3</div></td> - </tr> - <tr> - <td class="tdc bl"><div>S 180,000</div></td> - <td class="tdc bl"><div>0·0476</div></td> - <td class="tdc bl"><div>209 22</div></td> - <td class="tdc bl"><div> 19 18</div></td> - <td class="tdc bl"><div>22·1</div></td> - <td class="tdc bl"><div>881</div></td> - <td class="tdc bl"><div>33·1</div></td> - <td class="tdc bl br"><div> 5·9</div></td> - </tr> - <tr> - <td class="tdc bl"><div>S 170,000</div></td> - <td class="tdc bl"><div>0·0437</div></td> - <td class="tdc bl"><div>228 7</div></td> - <td class="tdc bl"><div> 18 45</div></td> - <td class="tdc bl"><div>20·3</div></td> - <td class="tdc bl"><div>887</div></td> - <td class="tdc bl"><div>31·3</div></td> - <td class="tdc bl br"><div> 7·7</div></td> - </tr> - <tr> - <td class="tdc bl"><span class="pagenum" id="Page_321">321</span><div> 160,000</div></td> - <td class="tdc bl"><div>0·0364</div></td> - <td class="tdc bl"><div>236 38</div></td> - <td class="tdc bl"><div> 8 31</div></td> - <td class="tdc bl"><div>16·9</div></td> - <td class="tdc bl"><div>900</div></td> - <td class="tdc bl"><div>27·8</div></td> - <td class="tdc bl br"><div>11·2</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 150,000</div></td> - <td class="tdc bl"><div>0·0332</div></td> - <td class="tdc bl"><div>242 56</div></td> - <td class="tdc bl"><div> 6 18</div></td> - <td class="tdc bl"><div>15·4</div></td> - <td class="tdc bl"><div>905</div></td> - <td class="tdc bl"><div>26·2</div></td> - <td class="tdc bl br"><div>12·8</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 140,000</div></td> - <td class="tdc bl"><div>0·0346</div></td> - <td class="tdc bl"><div>246 29</div></td> - <td class="tdc bl"><div> 3 33</div></td> - <td class="tdc bl"><div>16·1</div></td> - <td class="tdc bl"><div>903</div></td> - <td class="tdc bl"><div>26·9</div></td> - <td class="tdc bl br"><div>12·1</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 130,000</div></td> - <td class="tdc bl"><div>0·0384</div></td> - <td class="tdc bl"><div>259 34</div></td> - <td class="tdc bl"><div> 13 5</div></td> - <td class="tdc bl"><div>17·8</div></td> - <td class="tdc bl"><div>896</div></td> - <td class="tdc bl"><div>28·8</div></td> - <td class="tdc bl br"><div>10·2</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 120,000</div></td> - <td class="tdc bl"><div>0·0431</div></td> - <td class="tdc bl"><div>274 47</div></td> - <td class="tdc bl"><div> 15 13</div></td> - <td class="tdc bl"><div>20·1</div></td> - <td class="tdc bl"><div>888</div></td> - <td class="tdc bl"><div>31·0</div></td> - <td class="tdc bl br"><div> 8·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 110,000</div></td> - <td class="tdc bl"><div>0·0460</div></td> - <td class="tdc bl"><div>293 48</div></td> - <td class="tdc bl"><div> 19 1</div></td> - <td class="tdc bl"><div>21·4</div></td> - <td class="tdc bl"><div>883</div></td> - <td class="tdc bl"><div>32·4</div></td> - <td class="tdc bl br"><div> 6·6</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 100,000</div></td> - <td class="tdc bl"><div>0·0473</div></td> - <td class="tdc bl"><div>316 18</div></td> - <td class="tdc bl"><div> 22 30</div></td> - <td class="tdc bl"><div>22·0</div></td> - <td class="tdc bl"><div>881</div></td> - <td class="tdc bl"><div>33·0</div></td> - <td class="tdc bl br"><div> 6·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div>L 90,000</div></td> - <td class="tdc bl"><div>0·0452</div></td> - <td class="tdc bl"><div>340 2</div></td> - <td class="tdc bl"><div> 23 44</div></td> - <td class="tdc bl"><div>21·0</div></td> - <td class="tdc bl"><div>885</div></td> - <td class="tdc bl"><div>32·0</div></td> - <td class="tdc bl br"><div> 7·0</div></td> - </tr> - <tr> - <td class="tdc bl"><div>L 80,000</div></td> - <td class="tdc bl"><div>0·0398</div></td> - <td class="tdc bl"><div> 4 13</div></td> - <td class="tdc bl"><div> 24 11</div></td> - <td class="tdc bl"><div>18·5</div></td> - <td class="tdc bl"><div>894</div></td> - <td class="tdc bl"><div>29·4</div></td> - <td class="tdc bl br"><div> 9·6</div></td> - </tr> - <tr> - <td class="tdc bl"><div>L 70,000</div></td> - <td class="tdc bl"><div>0·0316</div></td> - <td class="tdc bl"><div>27 22</div></td> - <td class="tdc bl"><div> 23 9</div></td> - <td class="tdc bl"><div>14·7</div></td> - <td class="tdc bl"><div>908</div></td> - <td class="tdc bl"><div>25·5</div></td> - <td class="tdc bl br"><div>13·5</div></td> - </tr> - <tr> - <td class="tdc bl"><div>L 60,000</div></td> - <td class="tdc bl"><div>0·0218</div></td> - <td class="tdc bl"><div>46 8</div></td> - <td class="tdc bl"><div> 18 46</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div> 50,000</div></td> - <td class="tdc bl"><div>0·0131</div></td> - <td class="tdc bl"><div>50 14</div></td> - <td class="tdc bl"><div> 4 6</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div>L 40,000</div></td> - <td class="tdc bl"><div>0·0109</div></td> - <td class="tdc bl"><div>28 36</div></td> - <td class="tdc bl"><div>R 21 38</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div>L 30,000</div></td> - <td class="tdc bl"><div>0·0151</div></td> - <td class="tdc bl"><div> 5 50</div></td> - <td class="tdc bl"><div>R 22 46</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div>L 20,000</div></td> - <td class="tdc bl"><div>0·0188</div></td> - <td class="tdc bl"><div>44 0</div></td> - <td class="tdc bl"><div> 38 10</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl"><div>L 10,000</div></td> - <td class="tdc bl"><div>0·0187</div></td> - <td class="tdc bl"><div>78 28</div></td> - <td class="tdc bl"><div> 34 28</div></td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl"> </td> - <td class="tdc bl br"> </td> - </tr> - <tr> - <td class="tdc bl bb"><div><span class="smcap">a.d.</span> 1800</div></td> - <td class="tdc bl bb"><div>0·0168</div></td> - <td class="tdc bl bb"><div>99 30</div></td> - <td class="tdc bl bb"><div> 21 2</div></td> - <td class="tdc bl bb"> </td> - <td class="tdc bl bb"> </td> - <td class="tdc bl bb"> </td> - <td class="tdc bl br bb"> </td> - </tr> - </tbody> - </table> - - <p><span class="pagenum" id="Page_322">322</span></p> - - <p>In Tables II., III., and IV., column I. represents the dates of the - periods, column II. the eccentricity, column III. the longitude of - the perihelion. In Table IV. the eccentricity and the longitude of - the perihelion of the six periods marked with an S are copied from a - letter of Mr. Stone to Sir Charles Lyell, published in the Supplement - of the Phil. Mag. for June, 1865; the eight periods marked L are copied - from M. Leverrier’s Table, to which reference has been made. For the - correctness of everything else, both in this Table and in the other - three, I alone am responsible.</p> - - <p>Column IV. gives the number of degrees passed over by the perihelion - during each 10,000 years. From this column it will be seen how - irregular is the motion of the perihelion. At four different periods - it had a retrograde motion for 20,000 years. Column V. shows the - number of days by which the winter exceeds the summer when the winter - occurs in aphelion. Column VI. shows the intensity of the sun’s heat - during midwinter, when the winter occurs in aphelion, the present - midwinter intensity being taken at 1,000. These six columns comprehend - all the astronomical part of the Tables. Regarding the correctness of - the principles upon which these columns are constructed, there is no - diversity of opinion. But these columns<span class="pagenum" id="Page_323">323</span> afford no direct information - as to the character of the climate, or how much the temperature is - increased or diminished. To find this we pass on to columns VII. and - VIII., calculated on physical principles. Now, unless the physical - principles upon which these three columns are calculated be wholly - erroneous, change of eccentricity must undoubtedly very seriously - affect climate. Column VII. shows how many degrees Fahrenheit the - temperature is lowered by a decrease in the intensity of the sun’s heat - corresponding to column VI. For example, 850,000 years ago, if the - winters occurred then in aphelion, the direct heat of the sun during - midwinter would be only 837/1000 of what it is at present at the same - season of the year, and column VII. shows that this decrease in the - intensity of the sun’s heat would lower the temperature 45°·3 F.</p> - - <p>The principle upon which this result is arrived at is this:—The - temperature of space, as determined by Sir John Herschel, is −239° - F. M. Pouillet, by a different method, arrived at almost the same - result. If we take the midwinter temperature of Great Britain at - 39°, then 239° + 39° = 278° will represent the number of degrees of - rise due to the sun’s heat at midwinter; in other words, it takes a - quantity of sun-heat which we have represented by 1000 to maintain the - temperature of the earth’s surface in Great Britain 278° above the - temperature of space. Were the sun extinguished, the temperature of - our island would sink 278° below its present midwinter temperature, - or to the temperature of space. But 850,000 years ago, as will be - seen from <a href="#TABLE_III">Table III.</a>, if the winters occurred in aphelion, the heat - of the sun at midwinter would only equal 837 instead of 1000 as at - present. Consequently, if it takes 1,000 parts of heat to maintain the - temperature 278° above the temperature of space, 837 parts of heat will - only be able to maintain the temperature 232°·7 above the temperature - of space; for 232°·7 is to 278 as 837 is to 1,000. Therefore, if the - temperature was then only 232°·7 above that of space, it would be - 45°·3 below what it is at present. This is what the temperature would - be on the supposition, of course, that it depended wholly on the<span class="pagenum" id="Page_324">324</span> - sun’s intensity and was not modified by other causes. This method has - already been discussed at some length in <a href="#CHAPTER_II">Chapter II.</a> But whether these - values be too high or too low, one thing is certain, that a very slight - increase or a very slight decrease in the quantity of heat received - from the sun must affect temperature to a considerable extent. The - direct heat of the moon, for example, cannot be detected by the finest - instruments which we possess; yet from 238,000 observations made at - Prague during 1840−66, it would seem that the temperature is sensibly - affected by the mere change in the lunar perigee and inclination of the - moon’s orbit.<a id="FNanchor_195" href="#Footnote_195" class="fnanchor">[195]</a></p> - - <p>Column VIII. gives the midwinter temperature. It is found by - subtracting the numbers in column VII. from 39°, the present midwinter - temperature.</p> - - <p>I have not given a Table showing the temperature of the summers at - the corresponding periods. This could not well be done; for there is - no relation at the periods in question between the intensity of the - sun’s heat and the temperature of the summers. One is apt to suppose, - without due consideration, that the summers ought to be then as much - warmer than they are at present, as the winters were then colder than - now. Sir Charles Lyell, in his “Principles,” has given a column of - summer temperatures calculated from my table upon this principle. - Astronomically the principle is correct, but physically, as was shown - in <a href="#CHAPTER_IV">Chapter IV.</a>, it is totally erroneous, and calculated to convey a - wrong impression regarding the whole subject of geological climate. - The summers at those periods, instead of being much warmer than they - are at present, would in reality be much colder, notwithstanding the - great increase in the intensity of the sun’s heat resulting from the - diminished distance of the sun.</p> - - <p>What, then, is the date of the glacial epoch? It is perfectly obvious - that if the glacial epoch resulted from a high state of eccentricity, - it must be referred either to the period included <span class="pagenum" id="Page_325">325</span>in <a href="#TABLE_III">Table III.</a> or - to the one in <a href="#TABLE_IV">Table IV.</a> In <a href="#TABLE_III">Table III.</a> we have a period extending from - about 980,000 to about 720,000 years ago, and in <a href="#TABLE_IV">Table IV.</a> we have a - period beginning about 240,000 years ago, and extending down to about - 80,000 years ago. As the former period was of greater duration than - the latter, and the eccentricity also attained to a higher value, I at - first felt disposed to refer the glacial epoch proper (the time of the - till and boulder clay) to the former period; and the latter period, I - was inclined to believe, must have corresponded to the time of local - glaciers towards the close of the glacial epoch, the evidence for which - (moraines) is to be found in almost every one of our Highland glens. - On this point I consulted several eminent geologists, and they all - agreed in referring the glacial epoch to the former period; the reason - assigned being that they considered the latter period to be much too - recent and of too short duration to represent that epoch.</p> - - <p>Pondering over the subject during the early part of 1866, reasons soon - suggested themselves which convinced me that the glacial epoch must - be referred to the latter and not to the former period. Those reasons - I shall now proceed to state at some length, since they have a direct - bearing, as will be seen, on the whole question of geological time.</p> - - <p>It is the modern and philosophic doctrine of uniformity that has - chiefly led geologists to over-estimate the length of geological - periods. This philosophic school teaches, and that truly, that the - great changes undergone by the earth’s crust must have been produced, - not by convulsions and cataclysms of nature, but by those ordinary - agencies that we see at work every day around us, such as rain, snow, - frost, ice, and chemical action, &c. It teaches that the valleys - were not produced by violent dislocations, nor the hills by sudden - upheavals, but that they were actually carved out of the solid rock - by the silent and gentle agency of chemical action, frost, rain, ice, - and running water. It teaches, in short, that the rocky face of our - globe has been carved into hill and dale, and ultimately worn<span class="pagenum" id="Page_326">326</span> down - to the sea-level, by means of these apparently trifling agents, not - only once or twice, but probably dozens of times over during past - ages. Now, when we reflect that with such extreme slowness do these - agents perform their work, that we might watch their operations from - year to year, and from century to century, if we could, without being - able to perceive that they make any very sensible advance, we are - necessitated to conclude that geological periods must be enormous. And - the conclusion at which we thus arrive is undoubtedly correct. It is, - in fact, impossible to form an adequate conception of the length of - geological time. It is something too vast to be fully grasped by our - minds. But here we come to the point where the fundamental mistake - arises; Geologists do not err in forming too great a conception of the - extent of geological periods, <em>but in the mode in which they represent - the length of these periods in numbers</em>. When we speak of units, tens, - hundreds, thousands, we can form some notion of what these quantities - represent; but when we come to millions, tens of millions, hundreds - of millions, thousands of millions, the mind is then totally unable - to follow, and we can only use these numbers as representations of - quantities that turn up in calculation. We know, from the way in which - they do turn up in our process of calculation, whether they are correct - representations of things in actual nature or not; but we could not, - from a mere comparison of these quantities with the thing represented - by them, say whether they were actually too small or too great.</p> - - <p>At present, geological estimates of time are little else than mere - conjectures. Geological science has hitherto afforded no trustworthy - means of estimating the positive length of geological epochs. - Geological phenomena tell us most emphatically that these periods - must be long; but how long they have hitherto failed to inform us. - Geological phenomena represent time to the mind under a most striking - and imposing form. They present to the eye, as it were, a sensuous - representation of time; the mind thus becomes deeply impressed with - a sense of immense duration; and when one under these feelings is<span class="pagenum" id="Page_327">327</span> - called upon to put down in figures what he believes will represent that - duration, he is very apt to be deceived. If, for example, a million of - years as represented by geological phenomena and a million of years as - represented by figures were placed before our eyes, we should certainly - feel startled. We should probably feel that a unit with six ciphers - after it was really something far more formidable than we had hitherto - supposed it to be. Could we stand upon the edge of a gorge a mile and - a half in depth that had been cut out of the solid rock by a tiny - stream, scarcely visible at the bottom of this fearful abyss, and were - we informed that this little streamlet was able to wear off annually - only 1/10 of an inch from its rocky bed, what would our conceptions be - of the prodigious length of time that this stream must have taken to - excavate the gorge? We should certainly feel startled when, on making - the necessary calculations, we found that the stream had performed this - enormous amount of work in something less than a million of years.</p> - - <p>If, for example, we could possibly form some adequate conception of a - period so prodigious as one hundred millions of years, we should not - then feel so dissatisfied with Sir W. Thomson’s estimate that the age - of the earth’s crust is not greater than that.</p> - - <p>Here is one way of conveying to the mind some idea of what a million - of years really is. Take a narrow strip of paper an inch broad, or - more, and 83 feet 4 inches in length, and stretch it along the wall of - a large hall, or round the walls of an apartment somewhat over 20 feet - square. Recall to memory the days of your boyhood, so as to get some - adequate conception of what a period of a hundred years is. Then mark - off from one of the ends of the strip 1/10 of an inch. The 1/10 of the - inch will then represent one hundred years, and the entire length of - the strip a million of years. It is well worth making the experiment, - just in order to feel the striking impression that it produces on the - mind.</p> - - <p>The latter period, which we have concluded to be that of the<span class="pagenum" id="Page_328">328</span> glacial - epoch, extended, as we have seen, over a period of 160,000 years. But - as the glaciation was only on one hemisphere at a time, 80,000 years - or so would represent the united length of the cold periods. In order - to satisfy ourselves that this period is sufficiently long to account - for all the amount of denudation effected during the glacial epoch, - let us make some rough estimate of the probable rate at which the - surface of the country would be ground down by the ice. Suppose the - ice to grind off only one-tenth of an inch annually this would give - upwards of 650 feet as the quantity of rock removed during the time. - But it is probable that it did not amount to one-fourth part of that - quantity. Whether one-tenth of an inch per annum be an over-estimate or - an under-estimate of the rate of denudation by the ice, it is perfectly - evident that the period in question is sufficiently long, so far as - denudation is concerned, to account for the phenomena of the glacial - epoch.</p> - - <p>But admitting that the period under consideration is sufficiently - <em>long</em> to account for all the denudation which took place <em>during</em> - the glacial epoch, we have yet to satisfy ourselves that it is also - sufficiently <em>remote</em> to account for all the denudation which has taken - place <em>since</em> the glacial epoch. Are the facts of geology consistent - with the idea that the close of the glacial epoch does not date back - beyond 80,000 years?</p> - - <p>This question could be answered if we knew the present rate of - subaërial denudation, for the present rate evidently does not differ - greatly from that which has obtained since the close of the glacial - epoch.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XX"> - <span class="pagenum" id="Page_329">329</span> - <h2> - CHAPTER XX.<br /><br /> - <span class="small">GEOLOGICAL TIME.—METHOD OF MEASURING THE RATE OF SUBAËRIAL DENUDATION.</span> - </h2> - </div> - <div class="subhead">Rate of Subaërial Denudation a Measure of Time.—Rate determined - from Sediment of the Mississippi.—Amount of Sediment carried - down by the Mississippi; by the Ganges.—Professor Geikie on - Modern Denudation.—Professor Geikie on the Amount of Sediment - conveyed by European Rivers.—Rate at which the Surface of - the Globe is being denuded.—Alfred Tylor on the Sediment - of the Mississippi.—The Law which determines the Rate of - Denudation.—The Globe becoming less oblate.—Carrying Power - of our River Systems the true Measure of Denudation.—Marine - Denudation trifling in comparison to Subaërial.—Previous - Methods of measuring Geological Time.—Circumstances which show - the recent Date of the Glacial Epoch.—Professor Ramsay on - Geological Time.</div> - - <p class="noindent"><span class="smcap">It</span> is almost self-evident that the rate of subaërial denudation must - be equal to the rate at which the materials are carried off the land - into the sea, but the rate at which the materials are carried off the - land is measured by the rate at which sediment is carried down by our - river systems. <em>Consequently, in order to determine the present rate - of subaërial denudation, we have only to ascertain the quantity of - sediment annually carried down by the river systems.</em></p> - - <p>Knowing the quantity of sediment transported by a river, say annually, - and the area of its drainage, we have the means of determining the - rate at which the surface of this area is being lowered by subaërial - denudation. And if we know this in reference to a few of the great - continental rivers draining immense areas in various latitudes, we - could then ascertain with tolerable correctness the rate at which the - surface of the globe is being lowered by subaërial denudation, and - also the length of time which our present continents can remain above - the sea-level. Explaining this to Professor Ramsay during the winter<span class="pagenum" id="Page_330">330</span> - of 1865, I learned from him that accurate measurements had been made - of the amount of sediment annually carried down by the Mississippi - River, full particulars of which investigations were to be found - in the Proceedings of the American Association for the Advancement - of Science for 1848. These proceedings contain a report by Messrs. - Brown and Dickeson, which unfortunately over-estimated the amount of - sediment transported by the Mississippi by nearly four times what - was afterwards found by Messrs. Humphreys and Abbot to be the actual - amount. From this estimate, I was led to the conclusion that if the - Mississippi is a fair representative of rivers in general, our existing - continents would not remain longer than one million and a half years - above the sea-level.<a id="FNanchor_196" href="#Footnote_196" class="fnanchor">[196]</a> This was a conclusion so startling as to - excite suspicion that there must have been some mistake in reference - to Messrs. Brown and Dickeson’s data. It showed beyond doubt, however, - that the rate of subaërial denudation, when accurately determined by - this method, would be found to be enormously greater than had been - supposed. Shortly afterwards, on estimating the rate from the data - furnished by Humphreys and Abbot, I found the rate of denudation to - be about one foot in 6,000 years. Taking the mean elevation of all - the land as ascertained by Humboldt to be 1,000 feet, the whole would - therefore be carried down into the ocean by our river systems in about - 6,000,000 of years if no elevation of the land took place.<a id="FNanchor_197" href="#Footnote_197" class="fnanchor">[197]</a> The - following are the data and mode of computation by which this conclusion - was arrived at. It was found by Messrs. Humphreys and Abbot that the - average amount of sediment held in suspension in the waters of the - Mississippi is about 1/1500 of the weight of the water, or 1/2900 - by bulk. The annual discharge of the river is 19,500,000,000,000 - cubic feet of water. The quantity of sediment carried down into the - Gulf of Mexico amounts to 6,724,000,000 cubic feet. But besides - that which is held in suspension, the river pushes down into the - sea about 750,000,000 cubic feet of earthy matter, making in all a - total of 7,474,000,000 cubic feet <span class="pagenum" id="Page_331">331</span>transferred from the land to the - sea annually. Where does this enormous mass of material come from? - Unquestionably it comes from the ground drained by the Mississippi. The - area drained by the river is 1,244,000 square miles. Now 7,474,000,000 - cubic feet removed off 1,224,000 square miles of surface is equal to - 1/4566 of a foot off that surface per annum, or one foot in 4,566 - years. The specific gravity of the sediment is taken at 1·9, that of - rock is about 2·5; consequently the amount removed is equal to one foot - of rock in about 6,000 years. The average height of the North American - continent above the sea-level, according to Humboldt, is 748 feet; - consequently, at the present rate of denudation, the whole area of - drainage will be brought down to the sea-level in less than 4,500,000 - years, if no elevation of the land takes place.</p> - - <p>Referring to the above, Sir Charles Lyell makes the following - appropriate remarks:—“There seems no danger of our overrating the - mean rate of waste by selecting the Mississippi as our example, for - that river drains a country equal to more than half the continent of - Europe, extends through twenty degrees of latitude, and therefore - through regions enjoying a great variety of climate, and some of its - tributaries descend from mountains of great height. The Mississippi - is also more likely to afford us a fair test of ordinary denudation, - because, unlike the St. Lawrence and its tributaries, there are no - great lakes in which the fluviatile sediment is thrown down and - arrested on its way to the sea.”<a id="FNanchor_198" href="#Footnote_198" class="fnanchor">[198]</a></p> - - <p>The rate of denudation of the area drained by the river Ganges is much - greater than that of the Mississippi. The annual discharge of that - river is 6,523,000,000,000 cubic feet of water. The sediment held in - suspension is equal to 1/510 by weight; area of drainage 432,480 square - miles. This gives one foot of rock in 2,358 years as the amount removed.</p> - - <p>Rough estimates have been made of the amount of sediment carried down - by some eight or ten European rivers; and although those estimates - cannot be depended upon as being <span class="pagenum" id="Page_332">332</span>anything like perfectly accurate, - still they show (what there is very little reason to doubt) that it is - extremely probable that the European continent is being denuded about - as rapidly as the American.</p> - - <p>For a full account of all that is known on this subject I must - refer to Professor Geikie’s valuable memoir on Modern Denudation - (Transactions of Geological Society of Glasgow, vol. iii.; also Jukes - and Geikie’s “Manual of Geology,” chap. xxv.) It is mainly through the - instrumentality of this luminous and exhaustive memoir that the method - under consideration has gained such wide acceptance amongst geologists.</p> - - <p>Professor Geikie finds that at the present rate of erosion the - following is the number of years required by the undermentioned rivers - to remove one foot of rock from the general surface of their basins. - Professor Geikie thus shows that the rate of denudation, as determined - from the amount of sediment carried down the Mississippi, is certainly - not too high.</p> - - <table summary="Rate of erosion"> - <tbody> - <tr> - <td>Danube</td> - <td>6,846 years.</td> - </tr> - <tr> - <td>Mississippi</td> - <td>6,000 〃</td> - </tr> - <tr> - <td>Nith</td> - <td>4,723 〃</td> - </tr> - <tr> - <td>Ganges</td> - <td>2,358 〃</td> - </tr> - <tr> - <td>Rhone</td> - <td>1,528 〃</td> - </tr> - <tr> - <td>Hoang Ho</td> - <td>1,464 〃</td> - </tr> - <tr> - <td>Po</td> - <td> 729 〃</td> - </tr> - </tbody> - </table> - - <p>By means of subaërial agencies continents are being cut up into - islands, the islands into smaller islands, and so on till the whole - ultimately disappears.</p> - - <p>No proper estimate has been made of the quantity of sediment carried - down into the sea by our British rivers. But, from the principles just - stated, we may infer that it must be as great in proportion to the area - of drainage as that carried down by the Mississippi. For example, the - river Tay, which drains a great portion of the central Highlands of - Scotland, carries to the sea three times as much water in proportion - to its area of drainage as is carried by the Mississippi. And any one - who has seen this rapidly running river during a flood, red and turbid - with sediment, will easily be convinced that the<span class="pagenum" id="Page_333">333</span> quantity of solid - material carried down by it into the German Ocean must be very great. - Mr. John Dougall has found that the waters of the Clyde during a flood - hold in suspension 1/800 by bulk of sediment. The observations were - made about a mile above the city of Glasgow. But even supposing the - amount of sediment held in suspension by the waters of the Tay to be - only one-third (which is certainly an under-estimate) of that of the - Mississippi, viz. 1/4500 by weight, still this would give the rate of - denudation of the central Highlands at one foot in 6,000 years, or - 1,000 feet in 6 millions of years.</p> - - <p>It is remarkable that although so many measurements have been made of - the amount of fluviatile sediment being transported seawards, yet that - the bearing which this has on the broad questions of geological time - and the rate of subaërial denudation should have been overlooked. One - reason for this, no doubt, is that the measurements were made, not - with a view to determine the rate at which the river basins are being - lowered, but mainly to ascertain the age of the river deltas and the - rate at which these are being formed.<a id="FNanchor_199" href="#Footnote_199" class="fnanchor">[199]</a></p> - - <p><em>The Law which determines the Rate at which any Country is being - denuded.</em>—By means of subaërial agencies continents are being cut up - into islands, the islands into smaller islands, and so on till the - whole ultimately disappears.</p> - - <p>So long as the present order of things remains, the rate of denudation - will continue while land remains above the sea-level; and we have no - warrant for supposing that the rate was during past ages less than it - is at the present day. It will not do to object that, as a considerable - amount of the sediment carried down by rivers is boulder clay and - other materials belonging to <span class="pagenum" id="Page_334">334</span>the Ice age, the total amount removed - by the rivers is on that account greater than it would otherwise be. - Were this objection true, it would follow that, prior to the glacial - period, when it is assumed that there was no boulder clay, the face of - the country must have consisted of bare rock; for in this case no soil - could have accumulated from the disintegration and decomposition of the - rocks, <em>since, unless the rocks of a country disintegrate more rapidly - than the river systems are able to carry the disintegrated materials - to the sea, no surface soil can form on that country</em>. The rate at - which rivers carry down sediment is evidently not determined by the - rate at which the rocks are disintegrated and decomposed, but by the - quantity of rain falling, and the velocity with which it moves off the - face of the country. Every river system possesses a definite amount of - carrying-power, depending upon the slope of the ground, the quantity of - rain falling per annum, the manner in which the rain falls, whether it - falls gradually or in torrents, and a few other circumstances. When it - so happens, as it generally does, that the amount of rock disintegrated - on the face of the country is greater than the carrying-power of the - river systems can remove, then a soil necessarily forms. But when the - reverse is the case no soil can form on that country, and it will - present nothing but barren rock. This is no doubt the reason why in - places like the Island of Skye, for example, where the rocks are - exceedingly hard and difficult to decompose and separate, the ground - steep, and the quantity of rain falling very great, there is so much - bare rock to be seen. If, prior to the glacial epoch, the rocks of - the area drained by the Mississippi did not produce annually more - material from their destruction under atmospheric agency than was being - carried down by that river, then it follows that the country must have - presented nothing but bare rock, if the amount of rain falling then was - as great as at present.</p> - - <p>But, after all, one foot removed off the general level of the country - since the creation of man, according to Mosaic chronology, is certainly - not a very great quantity. No person but<span class="pagenum" id="Page_335">335</span> one who had some preconceived - opinions to maintain, would ever think of concluding that one foot of - soil during 6,000 years was an extravagant quantity to be washed off - the face of the country by rain and floods during that long period. - Those who reside in the country and are eye-witnesses of the actual - effects of heavy rains upon the soil, our soft country roads, ditches, - brooks, and rivers, will have considerable difficulty in actually - believing that only one foot has been washed away during the past 6,000 - years.</p> - - <p>Some may probably admit that a foot of soil may be washed off during - a period so long as 6,000 years, and may tell us that what they deny - is not that a foot of loose and soft soil, but a foot of solid rock - can be washed away during that period. But a moment’s reflection must - convince them that, unless the rocks of the country were disintegrating - and decomposing as rapidly into soil as the rain is carrying the soil - away, the surface of the country would ultimately become bare rock. It - is true that the surface of our country in many places is protected by - a thick covering of boulder clay; but when this has once been removed, - the rocks will then disintegrate far more rapidly than they are doing - at present.</p> - - <p>But slow as is the rate at which the country is being denuded, yet - when we take into consideration a period so enormous as 6 millions of - years, we find that the results of denudation are really startling. - One thousand feet of solid rock during that period would be removed - from off the face of the country. But if the mean level of the country - would be lowered 1,000 feet in 6 millions of years, how much would our - valleys and glens be deepened during that period? This is a problem - well worthy of the consideration of those who treat with ridicule the - idea that the general features of our country have been carved out by - subaërial agency.</p> - - <p>In consequence of the retardation of the earth’s rotation, occasioned - by the friction of the tidal wave, the sea-level must be slowly sinking - at the equator and rising at the poles. But it is probable that the - land at the equator is being lowered by<span class="pagenum" id="Page_336">336</span> denudation as rapidly as - the sea-level is sinking. <em>Nearly one mile must have been worn off - the equator during the past 12 millions of years</em>, if the rate of - denudation all along the equator be equal to that of the basin of the - Ganges. It therefore follows that we cannot infer from the present - shape of our globe what was its form, or the rate at which it was - rotating, at the time when its crust became solidified. Although it - had been as oblate as the planet Jupiter, denudation must in time have - given it its present form.</p> - - <p>There is another effect which would result from the denudation of the - equator and the sinking of the ocean at the equator and its rise at - the poles. This, namely, that it would tend to increase the rate of - rotation; or, more properly, it would tend to <em>lessen</em> the rate of - tidal retardation.</p> - - <p>But if the rate of denudation be at present so great, what must it - have been during the glacial epoch? It must have been something - enormous. At present, denudation is greatly retarded by the limited - power of our river systems to remove the loose materials resulting - from the destruction of the rocks. These materials accumulate and form - a thick soil over the surface of the rocks, which protects them, to a - great extent, from the weathering effects of atmospheric agents. So - long as the amount of rock disintegrated exceeds that which is being - removed by the river systems, the soil will continue to accumulate - till the amount of rock destroyed per annum is brought to equal that - which is being removed. It therefore follows from this principle that - the <span class="smcap">carrying-power of our river systems is the true measure of - denudation</span>. But during the glacial epoch the thickness of the soil - would have but little effect in diminishing the waste of the rocks; for - at that period the rocks were not decomposed by atmospheric agency, - but were ground down by the mechanical friction of the ice. But the - presence of a thick soil at this period, instead of retarding the rate - of denudation, would tend to increase it tenfold, for the soil would - then be used as grinding-material for the ice-sheet. In places where - the ice was, say, 2,000 feet in thickness, the<span class="pagenum" id="Page_337">337</span> soil would be forced - along over the rocky face of the country, exerting a pressure on the - rocks equal to 50 tons on the square foot.</p> - - <p>It is true that the rate at which many kinds of rocks decompose and - disintegrate is far less than what has been concluded to be the mean - rate of denudation of the whole country. This is evident from the fact - which has been adduced by some writers, that inscriptions on stones - which have been exposed to atmospheric agency for a period of 2,000 - years or so, have not been obliterated. But in most cases epitaphs on - monuments and tombstones, and inscriptions on the walls of buildings, - 200 years old, can hardly be read. And this is not all: the stone on - which the letters were cut has during that time rotted in probably to - the depth of several inches; and during the course of a few centuries - more the whole mass will crumble into dust.</p> - - <p>The facts which we have been considering show also how trifling is the - amount of denudation effected by the sea in comparison with that by - subaërial agents. The entire sea-coast of the globe, according to Dr. - A. Keith Johnston, is 116,531 miles. Suppose we take the average height - of the coast-line at 25 feet, and take also the rate at which the sea - is advancing on the land at one foot in 100 years, then this gives - 15,382,500,000 cubic feet of rock as the total amount removed in 100 - years by the action of the sea. The total amount of land is 57,600,000 - square miles, or 1,605,750,000,000,000 square feet; and if one foot is - removed off the surface in 6,000 years, then 26,763,000,000,000 cubic - feet is removed by subaërial agency in 100 years, or about 1,740 times - as much as that removed by the sea. Before the sea could denude the - globe as rapidly as the subaërial agents, it would have to advance on - the land at the rate of upwards of 17 feet annually.</p> - - <p>It will not do, however, to measure marine denudation by the rate at - which the sea is advancing on the land. There is no relation whatever - between the rate at which the sea is <em>advancing</em> on the land and the - rate at which the sea is <em>denuding</em> the land. For it is evident that as - the subaërial agents bring<span class="pagenum" id="Page_338">338</span> the coast down to the sea-level, all that - the sea has got to do is simply to advance, or at most to remove the - loose materials which may lie in its path. The amount of denudation - which has been effected by the sea during past geological ages, - compared with what has been effected by subaërial agency, is evidently - but trifling. Denudation is not the proper function of the sea. The - great denuding agents are land-ice, frost, rain, running-water, - chemical agency, &c. The proper work which belongs to the sea is the - transporting of the loose materials carried down by the rivers, and the - spreading of these out so as to form the stratified beds of future ages.</p> - - <p><em>Previous Methods of measuring Geological Time unreliable.</em>—The method - which has just been detailed of estimating the rate of subaërial - denudation seems to afford the only reliable means of a geological - character of determining geological time in absolute measure. The - methods which have hitherto been adopted not only fail to give the - positive length of geological periods, but some of them are actually - calculated to mislead.</p> - - <p>The common method of calculating the length of a period from the - thickness of the stratified rocks belonging to that period is one of - that class. Nothing whatever can be inferred from the thickness of a - deposit as to the length of time which was required to form it. The - thickness of a deposit will depend upon a great many circumstances, - such as whether the deposition took place near to land or far away in - the deep recesses of the ocean, whether it occurred at the mouth of a - great river or along the sea-shore, or at a time when the sea-bottom - was rising, subsiding, or remaining stationary. Stratified formations - 10,000 feet in thickness, for example, may, under some conditions, have - been formed in as many years, while under other conditions it may have - required as many centuries. Nothing whatever can be safely inferred as - to the absolute length of a period from the thickness of the stratified - formations belonging to that period. Neither will this method give us a - trustworthy estimate of the <em>relative</em> lengths of geological periods. - Suppose we find the average thickness of the Cambrian rocks<span class="pagenum" id="Page_339">339</span> to be, - say, 26,000 feet, the Silurian to be 28,000 feet, the Devonian to be - 6,000 feet, and the Tertiary to be 10,000 feet, it would not be safe - to assume, as is sometimes done, that the relative duration of those - periods must have corresponded to these numbers. Were we sure that we - had got the correct average thickness of all the rocks belonging to - each of those formations, we might probably be able to arrive at the - relative lengths of those periods; but we can never be sure of this. - Those formations all, at one time, formed sea-bottoms; and we can only - measure such deposits as are now raised above the sea-level. But is - not it probable that the relative positions of sea and land during the - Cambrian, Silurian, Old Red Sandstone, Carboniferous, and other early - periods of the earth’s history, differed more from the present than the - distribution of sea and land during the Tertiary period differed from - that which obtains now? May not the greater portion of the Tertiary - deposits be still under the sea-bottom? And if this be the case, it may - yet be found at some day in the distant future, when these deposits - are elevated into dry land, that they are much thicker than we now - conclude them to be. Of course, it is by no means asserted that this - is so, but only that they <em>may</em> be thicker for anything we know to the - contrary; and the possibility that they may, destroys our confidence - in the accuracy of this method of determining the relative lengths of - geological periods.</p> - - <p>Neither does palæontology afford any better mode of measuring - geological time. In fact, the palæontological method of estimating - geological time, either absolute or relative, from the rate at which - species change, appears to be even still more unsatisfactory. If we - could ascertain by some means or other the time that has elapsed from - some given epoch (say, for example, the glacial) till the present - day, and were we sure at the same time that species have changed at a - uniform rate during all past ages, then, by ascertaining the percentage - of change that has taken place since the glacial epoch, we should - have a means of making something like a rough estimate of the length - of the<span class="pagenum" id="Page_340">340</span> various periods. But without some such period to start with, - the palæontological method is useless. It will not do to take the - historic period as a base-line. It is far too short to be used with - safety in determining the distance of periods so remote as those which - concern the geologist. But even supposing the palæontologist had a - period of sufficient length measured off correctly to begin with, his - results would still be unsatisfactory; for it is perfectly obvious, - that unless the climatic conditions of the globe during the various - periods were nearly the same, the rate at which the species change - would certainly not be uniform; but such has not been the case, as an - examination of the Tables of eccentricity will show. Take, for example, - that long epoch of 260,000 years, beginning about 980,000 years ago - and terminating about 720,000 years ago. During that long period the - changes from cold to warm conditions of climate every 10,000 or 12,000 - years must have been of the most extreme character. Compare that - period with the period beginning, say, 80,000 years ago, and extending - to nearly 150,000 years into the future, during which there will be - no extreme variations of climate, and how great is the contrast! How - extensive the changes in species must have been during the first period - as compared with those which are likely to take place during the latter!</p> - - <p>Besides, it must also be taken into consideration that organization was - of a far more simple type in the earlier Palæozoic ages than during the - Tertiary period, and would probably on this account change much more - slowly in the former than in the latter.</p> - - <p>The foregoing considerations render it highly probable, if not - certain, that the rate at which the general surface of the globe is - being lowered by subaërial denudation cannot be much under one foot - in 6,000 years. How, if we assign the glacial epoch to that period of - high eccentricity beginning 980,000 years ago, and terminating 720,000 - years ago, then we must conclude that as much as 120 feet must have - been denuded off the face of the country since the close of the glacial - epoch.<span class="pagenum" id="Page_341">341</span> But if as much as this had been carried down by our rivers into - the sea, hardly a patch of boulder clay, or any trace of the glacial - epoch, should be now remaining on the land. It is therefore evident - that the glacial epoch cannot be assigned to that remote period, but - ought to be referred to the period terminating about 80,000 years ago. - We have, in this latter case, 13 feet, equal to about 18 feet of drift, - as the amount removed from the general surface of the country since - the glacial epoch. This amount harmonizes very well with the direct - evidence of geology on this point. Had the amount of denudation since - the close of the glacial epoch been much greater than this, the drift - deposits would not only have been far less complete, but the general - appearance and outline of the surface of all glaciated countries would - have been very different from what they really are.</p> - - <p><em>Circumstances which show the Recent Date of the Glacial Epoch.</em>—One - of the circumstances to which I refer is this. When we examine the - surface of any glaciated country, such as Scotland, we can easily - satisfy ourselves that the upper surface of the ground differs very - much from what it would have been had its external features been due - to the action of rain and rivers and the ordinary agencies which have - been at work since the close of the Ice period. Go where one will in - the Lowlands of Scotland, and he shall hardly find a single acre whose - upper surface bears the marks of being formed by the denuding agents - which are presently in operation. He will observe everywhere mounds - and hollows, the existence of which cannot be accounted for by the - present agencies at work. In fact these agencies are slowly denuding - pre-existing heights and silting up pre-existing hollows. Everywhere - one comes upon patches of alluvium which upon examination prove to be - simply old glacially formed hollows silted up. True, the main rivers, - streams, and even brooks, occupy channels which have been formed by - running water, either since or prior to the glacial epoch, but, in - regard to the general surface of the country, the present agencies may - be said to be just beginning to carve a new line of features out of - the old glacially formed surface.<span class="pagenum" id="Page_342">342</span> But so little progress has yet been - made, that the kames, gravel mounds, knolls of boulder clay, &c., still - retain in most cases their original form. Now, when we reflect that - more than a foot of drift is being removed from the general surface of - the country every 5,000 years or so, it becomes perfectly obvious that - the close of the glacial epoch must be of comparatively recent date.</p> - - <p>There is another circumstance which shows that the glacial epoch must - be referred to the latest period of great eccentricity. If we refer the - glacial epoch to the penultimate period of extreme eccentricity, and - place its commencement one million of years back, then we must also - lengthen out to a corresponding extent the entire geological history - of the globe. Sir Charles Lyell, who is inclined to assign the glacial - epoch to this penultimate period, considers that when we go back as far - as the Lower Miocene formations, we arrive at a period when the marine - shells differed as a whole from those now existing. But only 5 per - cent. of the shells existing at the commencement of the glacial epoch - have since died out. Hence, assuming the rate at which the species - change to be uniform, it follows that the Lower Miocene period must - be twenty times as remote as the commencement of the glacial epoch. - Consequently, if it be one million of years since the commencement - of the glacial epoch, 20 millions of years, Sir Charles concludes, - must have elapsed since the time of the Lower Miocene period, and - 60 millions of years since the beginning of the Eocene period, and - about 160 millions of years since the Carboniferous period, and about - 240 millions of years must be the time which has elapsed since the - beginning of the Cambrian period. But, on the other hand, if we refer - the glacial epoch to the latest period of great eccentricity, and take - 250,000 years ago as the beginning of that period, then, according - to the same mode of calculation, we have 15 millions of years since - the beginning of the Eocene period, and 40 millions of years since - the Carboniferous period, and 60 millions of years in all since the - beginning of the Cambrian period.</p> - - <p><span class="pagenum" id="Page_343">343</span></p> - - <p>If the beginning of the glacial epoch be carried back a million years, - then it is probable, as Sir Charles Lyell concludes, that the beginning - of the Cambrian period will require to be placed 240 millions of years - back. But it is very probable that the length of time embraced by the - pre-Cambrian ages of geological history may be as great as that which - has elapsed since the close of the Cambrian period, and, if this be - so, then we shall be compelled to admit that nearly 500 millions of - years have passed away since the beginning of the earth’s geological - history. But we have evidence of a physical nature which proves that it - is absolutely impossible that the existing order of things, as regards - our globe, can date so far back as anything like 500 millions of years. - The arguments to which I refer are those which have been advanced by - Professor Sir William Thomson at various times. These arguments are - well known, and to all who have really given due attention to them must - be felt to be conclusive. It would be superfluous to state them here; I - shall, however, for reasons which will presently appear, refer briefly - to one of them, and that one which seems to be the most conclusive of - all, viz., the argument derived from the limit to the age of the sun’s - heat.</p> - - <p><em>Professor Ramsay on Geological Time.</em>—In an interesting suggestive - memoir, “On Geological Ages as items of Geological Time,”<a id="FNanchor_200" href="#Footnote_200" class="fnanchor">[200]</a> - Professor Ramsay discusses the comparative values of certain groups of - formations as representative of geological time, and arrives at the - following general conclusion, viz., “That the local continental era - which began with the Old Red Sandstone and closed with the New Red Marl - is comparable, in point of geological time, to that occupied in the - deposition of the whole of the Mesozoic, or Secondary series, later - than the New Red Marl and all the Cainozoic or Tertiary formations, - and indeed of all the time that has elapsed since the beginning of - the deposition of the Lias down to the present day.” This conclusion - is derived partly from a comparison of the physical character of - the formations constituting each <span class="pagenum" id="Page_344">344</span>group, but principally from the - zoological changes which took place during the time represented by them.</p> - - <p>The earlier period represented by the Cambrian and Silurian rocks he - also, from the same considerations, considers to have been very long, - but he does not attempt to fix its relative length. Of the absolute - length of any or all of these great eras of geological time no - estimate or guess is given. He believes, however, that the whole time - represented by all the fossiliferous rocks, from the earliest Cambrian - to the most recent, is, geologically speaking, short compared with that - which went before it. After quoting Professor Huxley’s enumeration of - the many classes and orders of marine life (identical with those still - existing), whose remains characterize the lowest Cambrian rocks, he - says, “The inference is obvious that in this earliest known varied - life we find no evidence of its having lived near the beginning of - the zoological series. In a broad sense, compared with what must have - gone before, both biologically and physically, all the phenomena - connected with this old period seem to my mind to be quite of a recent - description, and the climates of seas and lands were of the very same - kind as those that the world enjoys at the present day.”... “In the - words of Darwin, when discussing the imperfection of the geological - record of this history, ‘we possess the last volume alone relating - only to two or three countries,’ and the reason why we know so little - of pre-Cambrian faunas and the physical characters of the more ancient - formations as originally deposited, is that below the Cambrian strata - we get at once involved in a sort of chaos of metamorphic strata.’”</p> - - <p>It seems to me that Professor Ramsay’s results lead to the same - conclusion regarding the <em>positive</em> length of geological periods as - those derived from physical considerations. It is true that his views - lead us back to an immense lapse of unknown time prior to the Cambrian - period, but this practically tends to shorten geological periods. For - it is evident that the geological history of our globe must be limited - by the age of the sun’s heat, no matter how long or short its age may - be. This<span class="pagenum" id="Page_345">345</span> being the case, the greater the length of time which must - have elapsed prior to the Cambrian period, the less must be the time - which has elapsed since that period. Whatever is added to the one - period must be so much taken from the other. Consequently, the longer - we suppose the pre-Cambrian periods to have been, the shorter must we - suppose the post-Cambrian to be.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XXI"> - <span class="pagenum" id="Page_346">346</span> - <h2> - CHAPTER XXI.<br /><br /> - <span class="small">THE PROBABLE AGE AND ORIGIN OF THE SUN.</span> - </h2> - </div> - <div class="subhead">Gravitation Theory.—Amount of Heat emitted by the Sun.—Meteoric - Theory.—Helmholtz’s Condensation Theory.—Confusion of - Ideas.—Gravitation not the chief Source of the Sun’s - Heat.—Original Heat.—Source of Original Heat.—Original Heat - derived from Motion in Space.—Conclusion as to Date of Glacial - Epoch.—False Analogy.—Probable Date of Eocene and Miocene Periods.</div> - - <p><em>Gravitation Theory of the Origin and Source of the Sun’s Heat.</em>—There - are two forms in which this theory has been presented: the first, the - meteoric theory, propounded by Dr. Meyer, of Heilbronn; and the second, - the contraction theory, advocated by Helmholtz.</p> - - <p>It is found that 83·4 foot-pounds of heat per second are incident upon - a square foot of the earth’s surface exposed to the perpendicular rays - of the sun. The amount radiated from a square foot of the sun’s surface - is to that incident on a square foot of the earth’s surface as the - square of the sun’s distance to the square of his radius, or as 46,400 - to 1. Consequently 3,869,000 foot-pounds of heat are radiated off every - square foot of the sun’s surface per second—an amount equal to about - 7,000 horse power. The total amount radiated from the whole surface - of the sun per annum is 8,340 × 10<sup>30</sup> foot-pounds. To maintain the - present rate of radiation, it would require the combustion of about - 1,500 lbs. of coal per hour on every square foot of the sun’s surface; - and were the sun composed of that material, it would be all consumed in - less than 5,000 years. The opinion that the sun’s heat is maintained - by combustion cannot be entertained for a single moment. A pound of - coal falling into the sun from an infinite distance would produce by<span class="pagenum" id="Page_347">347</span> - its concussion more than 6,000 times the amount of heat that would be - generated by its combustion.</p> - - <p>It is well known that the velocity with which a body falling from an - infinite distance would reach the sun would be equal to that which - would be generated by a constant force equal to the weight of the body - at the sun’s surface operating through a space equal to the sun’s - radius. One pound would at the sun’s surface weigh about 28 pounds. - Taking the sun’s radius at 441,000 miles,<a id="FNanchor_201" href="#Footnote_201" class="fnanchor">[201]</a> the energy of a pound - of matter falling into the sun from infinite space would equal that - of a 28-pound weight descending upon the earth from an elevation of - 441,000 miles, supposing the force of gravity to be as great at that - elevation as it is at the earth’s surface. It would amount to upwards - of 65,000,000,000 foot-pounds. A better idea of this enormous amount - of energy exerted by a one-pound weight falling into the sun will be - conveyed by stating that it would be sufficient to raise 1,000 tons to - a height of 5½ miles. It would project the <em>Warrior</em>, fully equipped - with guns, stores, and ammunition, over the top of Ben Nevis.</p> - - <p>Gravitation is now generally admitted to be the only conceivable - source of the sun’s heat. But if we attribute the energy of the sun to - gravitation as a source, we assign it to a cause the value of which can - be accurately determined. Prodigious as is the energy of a single pound - of matter falling into the sun, nevertheless a range of mountains, - consisting of 176 cubic miles of solid rock, falling into the sun, - would maintain his heat for only a single second. A mass equal to that - of the earth would maintain the heat for only 93 years, and a mass - equal to that of the sun itself falling into the sun would afford but - 33,000,000 years’ sun-heat.</p> - - <p>It is quite possible, however, that a meteor may reach the sun with a - velocity far greater than that which it could acquire by gravitation; - for it might have been moving in a direct line towards the sun with - an original velocity before coming under <span class="pagenum" id="Page_348">348</span>the sensible influence of - the sun’s attraction. In this case a greater amount of heat would - be generated by the meteor than would have resulted from its merely - falling into the sun under the influence of gravitation. But then - meteors of this sort must be of rare occurrence. The meteoric theory - of the sun’s heat has now been pretty generally abandoned for the - contraction theory advanced by Helmholtz.</p> - - <p>Suppose, with Helmholtz, that the sun originally existed as a nebulous - mass, filling the entire space presently occupied by the solar system - and extending into space indefinitely beyond the outermost planet. The - total amount of work in foot-pounds performed by gravitation in the - condensation of this mass to an orb of the sun’s present size can be - found by means of the following formula given by Helmholtz,<a id="FNanchor_202" href="#Footnote_202" class="fnanchor">[202]</a></p> - - <div class="center">Work of condensation = <span class="frac"><sup>3</sup><span>/</span><sub>5</sub></span> - × <span class="frac"><sup><i>r</i><sup>2</sup>M<sup>2</sup></sup><span>/</span><sub>R<i>m</i></sub></span> × <i>g</i> - </div> - - <p class="noindent">M is the mass of the sun, <i>m</i> the mass of the earth, R the sun’s - radius, and <i>r</i> the earth’s radius. Taking M = 4230 × 10<sup>27</sup> lbs., - <i>m</i> = 11,920 × 10<sup>21</sup> lbs., R = 2,328,500,000 feet, and <i>r</i> = - 20,889,272 feet; we have then for the total amount of work performed by - gravitation in foot-pounds,</p> - - <div class="center"> - Work = <span class="frac"><sup>3</sup><span>/</span><sub>5</sub></span> × - <span class="frac"><sup>(20,889,272·5)<sup>2</sup> × (4230 × 10<sup>27</sup>)<sup>2</sup></sup><span>/</span><sub>2,328,500,000 × 11,920 × 10<sup>21</sup></sub></span> - </div> - - <div class="center">= 168,790 × 10<sup>36</sup> foot-pounds.</div> - - <p class="noindent">The amount of heat thus produced by gravitation would suffice for - nearly 20,237,500 years.</p> - - <p>These calculations are based upon the assumption that the density of - the sun is uniform throughout. But it is highly probable that the sun’s - density increases towards the centre, in which case the amount of work - performed by gravitation would be somewhat more than the above.</p> - - <p>Some confusion has arisen in reference to this subject by the - introduction of the question of the amount of the sun’s specific heat. - If we simply consider the sun as an incandescent body <span class="pagenum" id="Page_349">349</span>in the process - of cooling, the question of the amount of the sun’s specific heat is - of the utmost importance; because the absolute amount of heat which - the sun is capable of giving out depends wholly upon his temperature - and specific heat. In this case three things only are required: (1), - the sun’s mass; (2), temperature of the mass; (3), specific heat of - the mass. But if we are considering what is the absolute amount of - heat which could have been given out by the sun on the hypothesis that - gravitation, either according to the meteoric theory suggested by Meyer - or according to the contraction theory advocated by Helmholtz, is the - only source of his heat, then we have nothing whatever to do with any - inquiries regarding the specific heat of the sun. This is evident - because the absolute amount of work which gravitation can perform in - the pulling of the particles of the sun’s mass together, is wholly - independent of the specific heat of those particles. Consequently, the - amount of energy in the form of heat thus imparted to the particles - by gravity must also be wholly independent of specific heat. That is - to say, the amount of heat imparted to a particle will be the same - whatever may be its specific heat.</p> - - <p>Even supposing we limit the geological history of our globe to 100 - millions of years, it is nevertheless evident that gravitation will not - account for the supply of the sun’s heat during so long a period. There - must be some other source of much more importance than gravitation. - What other source of energy greater than that of gravitation can there - be? It is singular that the opinion should have become so common even - among physicists, that there is no other conceivable source than - gravitation from which a greater amount of heat could have been derived.</p> - - <p><em>The Origin and Chief Source of the Sun’s Heat.</em>—According to the - foregoing theories regarding the source of the sun’s heat, it is - assumed that the matter composing the sun, when it existed in space as - a nebulous mass, was not originally possessed of temperature, but that - the temperature was given to it<span class="pagenum" id="Page_350">350</span> as the mass became condensed under the - force of gravitation. It is supposed that the heat given out was simply - the heat of condensation. But it is quite conceivable that the nebulous - mass might have been possessed of an original store of heat previous to - condensation.</p> - - <p>It is quite possible that the very reason why it existed in such a - rarefied or gaseous condition was its excessive temperature, and that - condensation only began to take place when the mass began to cool down. - It seems far more probable that this should have been the case than - that the mass existed in so rarefied a condition without temperature. - For why should the particles have existed in this separated form when - devoid of the repulsive energy of heat, seeing that in virtue of - gravitation they had such a tendency to approach to one another? But - if the mass was originally in a heated condition, then in condensing - it would have to part not only with the heat generated in condensing, - but also with the heat which it originally possessed, a quantity - which would no doubt much exceed that produced by condensation. To - illustrate this principle, let us suppose a pound of air, for example, - to be placed in a cylinder and heat applied to it. If the piston be so - fixed that it cannot move, 234·5 foot-pounds of heat will raise the - temperature of the air 1° C. But if the piston be allowed to rise as - the heat is applied, then it will require 330·2 foot-pounds of heat to - raise the temperature 1° C. It requires 95·7 foot-pounds more heat in - the latter case than in the former. The same amount of energy, viz., - 234·5 foot-pounds, in both cases goes to produce temperature; but in - the latter case, where the piston is allowed to move, 95·7 foot-pounds - of additional heat are consumed in the mechanical work of raising the - piston. Suppose, now, that the air is allowed to cool under the same - conditions: in the one case 234·5 foot-pounds of heat will be given - out while the temperature of the air sinks 1° C.; in the other case, - where the piston is allowed to descend, 330·2 foot-pounds will be given - out while the temperature sinks 1° C. In the former case, the air in - cooling has simply to part with the energy which it possesses <span class="pagenum" id="Page_351">351</span>in - the form of temperature; but in the latter case it has, in addition - to this, to part with the energy bestowed upon its molecules by the - descending piston. While the temperature of the gas is sinking 1°, - 95·7 foot-pounds of energy in the form of heat are being imparted to - it by the descending piston; and these have to be got rid of before - the temperature is lowered by 1°. Consequently 234·5 foot-pounds of - the heat given out previously existed in the air under the form of - temperature, and the remaining 95·7 foot-pounds given out were imparted - to the air by the descending piston while the gas was losing its - temperature. 234·5 foot-pounds represent the energy or heat which the - air previously possessed, and 95·7 the energy or heat of condensation.</p> - - <p>In the case of the cooling of the sun from a nebulous mass, there - would of course be no external force or pressure exerted on the mass - analogous to that of the piston on the air; but there would be, what - is equivalent to the same, the gravitation of the particles to each - other. There would be the pressure of the whole mass towards the centre - of convergence. In the case of air, and all perfect gases cooling - under pressure, about 234 foot-pounds of the original heat possessed - by the gas are given out while 95 foot-pounds are being generated by - condensation. We have, however, no reason whatever to believe that in - the case of the cooling of the sun the same proportions would hold - true. The proportion of original heat possessed by the mass of the sun - to that produced by condensation may have been much greater than 234 to - 95, or it may have been much less. In the absence of all knowledge on - this point, we may in the meantime assume that to be the proportion. - The total quantity of heat given out by the sun resulting from the - condensation of his mass, on the supposition that the density of the - sun is uniform throughout, we have seen to be equal to 20,237,500 - years’ sun-heat. Then the quantity of heat given out, which previously - existed in the mass as original temperature, must have been 49,850,000 - years’ heat, making in all 70,087,500 years’ heat as the total amount.</p> - - <p><span class="pagenum" id="Page_352">352</span></p> - - <p>The above quantity represents, of course, the total amount of heat - given out by the mass since it began to condense. But the geological - history of our globe must date its beginning at a period posterior to - that. For at that time the mass would probably occupy a much greater - amount of space than is presently possessed by the entire solar system; - and consequently, before it had cooled down to within the limits of - the earth’s present orbit, our earth could not have had an existence - as a separate planet. Previously to that time it must have existed as - a portion of the sun’s fiery mass. If we assume that it existed as a - globe previously to that, and came in from space after the condensation - of the sun, then it is difficult to conceive how its orbit should be so - nearly circular as it is at present.</p> - - <p>Let us assume that by the time that the mass of the sun had condensed - to within the space encircled by the orbit of the planet Mercury (that - is, to a sphere having, say, a radius of 18,000,000 miles) the earth’s - crust began to form; and let this be the time when the geological - history of our globe dates its commencement. The total amount of heat - generated by the condensation of the sun’s mass from a sphere of this - size to its present volume would equal 19,740,000 years’ sun-heat. - The amount of original heat given out during that time would equal - 48,625,000 years’ sun-heat,—thus giving a total of 68,365,000 years’ - sun-heat enjoyed by our globe since that period. The total quantity may - possibly, of course, be considerably more than that, owing to the fact - that the sun’s density may increase greatly towards his centre. But we - should require to make extravagant assumptions regarding the interior - density of the sun and the proportion of original heat to that produced - by condensation before we could manage to account for anything like the - period that geological phenomena are supposed by some to demand.</p> - - <p>The question now arises, by what conceivable means could the mass of - the sun have become possessed of such a prodigious amount of energy - in the form of heat previous to condensation?<span class="pagenum" id="Page_353">353</span> What power could have - communicated to the mass 50,000,000 years’ heat before condensation - began to take place?</p> - - <p><em>The Sun’s Energy may have originally been derived from Motion in - Space.</em>—There is nothing at all absurd or improbable in the supposition - that such an amount of energy might have been communicated to the - mass. The Dynamical Theory of Heat affords an easy explanation of at - least <em>how</em> such an amount of energy <em>may</em> have been communicated. Two - bodies, each one-half the mass of the sun, moving directly towards - each other with a velocity of 476 miles per second, would by their - concussion generate in a <em>single moment</em> the 50,000,000 years’ heat. - For two bodies of that mass moving with a velocity of 476 miles per - second would possess 4149 × 10<sup>38</sup> foot-pounds of energy in the form - of <i lang="la">vis viva</i>; and this, converted into heat by the stoppage of their - motion, would give an amount of heat which would cover the present rate - of the sun’s radiation, for a period of 50,000,000 years.</p> - - <p>Why may not the sun have been composed of two such bodies? And why may - not the original store of heat possessed by him have all been derived - from the concussion of these two bodies? Two such bodies coming into - collision with that velocity would be dissipated into vapour by such - an inconceivable amount of heat as would thus be generated; and when - they condensed on cooling, they would form one spherical mass like the - sun. It is perfectly true that two such bodies could never attain the - required amount of velocity by their mutual gravitation towards each - other. But there is no necessity whatever for supposing that their - velocities were derived from their mutual attraction alone. They might - have been approaching towards each other with the required velocity - wholly independent of gravitation.</p> - - <p>We know nothing whatever regarding the absolute motion of bodies in - space. And beyond the limited sphere of our observation, we know - nothing even of their relative motions. There may be bodies moving - in relation to our system with inconceivable velocity. For anything - that we know to the<span class="pagenum" id="Page_354">354</span> contrary, were one of these bodies to strike our - earth, the shock might be sufficient to generate an amount of heat that - would dissipate the earth into vapour, though the striking body might - not be heavier than a cannon-ball. There is, however, nothing very - extraordinary in the velocity which we have found would be required - in the two supposed bodies to generate the 50,000,000 years’ heat. A - comet, having an orbit extending to the path of the planet Neptune, - approaching so near the sun as to almost graze his surface in passing, - would have a velocity of about 390 miles per second, which is within 86 - miles of the required velocity.</p> - - <p>But in the original heating and expansion of the sun into a gaseous - mass, an amount of work must have been performed against gravitation - equal to that which has been performed by gravitation during his - cooling and condensation, a quantity which we have found amounts to - about 20,000,000 years’ heat. The total amount of energy originally - communicated by the concussion must have been equal to 70,000,000 - years’ sun-heat. A velocity of 563 miles per second would give this - amount. It must be borne in mind, however, that the 563 miles per - second is the velocity at the moment of collision; about one-half of - this velocity would be derived from the mutual attraction of the two - bodies in their approach to each other. Suppose each body to be equal - in volume to the sun, and of course one-half the density, the amount - of velocity which they would acquire by their mutual attraction would - be 274 miles per second, consequently we have to assume an original or - projected velocity of only 289 miles per second.</p> - - <p>If we admit that gravitation is not sufficient to account for the - amount of heat given out by the sun during the geological history of - our globe, we are compelled to assume that the mass of which the sun is - composed existed prior to condensation in a heated condition; and if - so, we are further obliged to admit that the mass must have received - its heat from some source or other. And as the dissipation of heat into - space must have been going on, in all probability, as rapidly before - as after condensation<span class="pagenum" id="Page_355">355</span> took place, we are further obliged to conclude - that the heat must have been communicated to the mass immediately - before condensation began, for the moment the mass began to lose its - heat condensation would ensue. If we confine our speculations to causes - and agencies known to exist, the cause which has been assigned appears - to be the only conceivable one that will account for the production of - such an enormous amount of heat.</p> - - <p>The general conclusion to which we are therefore led from physical - considerations regarding the age of the sun’s heat is, that the entire - geological history of our globe must be comprised within less than - 100 millions of years, and that consequently the commencement of the - glacial epoch cannot date much farther back than 240,000 years.</p> - - <p>The facts of geology, more especially those in connection with - denudation, seem to geologists to require a period of much longer - duration than 100 millions of years, and it is this which has so - long prevented them accepting the conclusions of physical science in - regard to the age of our globe. But the method of measuring subaërial - denudation already detailed seems to me to show convincingly that the - geological data, when properly interpreted, are in perfect accord with - the deductions of physical science. Perhaps there are now few who - have fairly considered the question who will refuse to admit that 100 - millions of years are amply sufficient to comprise the whole geological - history of our globe.</p> - - <p><em>A false Analogy supposed to exist between Astronomy and - Geology.</em>—Perhaps one of the things which has tended to mislead on - this point is a false analogy which is supposed to subsist between - astronomy and geology, viz., that geology deals with unlimited <em>time</em>, - as astronomy deals with unlimited <em>space</em>. A little consideration, - however, will show that there is not much analogy between the two cases.</p> - - <p>Astronomy deals with the countless worlds which lie spread out in the - boundless infinity of space; but geology deals with only one world. - No doubt reason and analogy both favour the idea that the age of the - material universe, like its magnitude,<span class="pagenum" id="Page_356">356</span> is immeasurable; we have no - reason, however, to conclude that it is eternal, any more than we - have to infer that it is infinite. But when we compare the age of the - material universe with its magnitude, we must not take the age of one - of its members (say, our globe) and compare it with the size of the - universe. Neither must we compare the age of all the presently existing - systems of worlds with the magnitude of the universe; but we must - compare the past history of the universe as it stretches back into the - immensity of bygone <em>time</em>, with the presently existing universe as it - stretches out on all sides into limitless <em>space</em>. For worlds precede - worlds in time as worlds lie beyond worlds in space. Each world, - each individual, each atom is evidently working out a final purpose, - according to a plan prearranged and predetermined by the Divine Mind - from all eternity. And each world, like each individual, when it - serves the end for which it was called into existence, disappears to - make room for others. This is the grand conception of the universe - which naturally impresses itself on every thoughtful mind that has not - got into confusion about those things called in science the Laws of - Nature.<a id="FNanchor_203" href="#Footnote_203" class="fnanchor">[203]</a></p> - - <p>But the geologist does not pass back from world to world as they stand - related to each other in the order of <em>succession in time</em>, as the - astronomer passes from world to world as they stand related to each - other in the order of <em>coexistence in space</em>. The researches of the - geologist, moreover, are not only confined to one world, but it is only - a portion of the history of that one world that can come under his - observation. The oldest of existing formations, so far as is yet known, - the Laurentian Gneiss, is made up of the waste of previously existing - rocks, and it, again, has probably been derived from the degradation - of rocks belonging to some still older period. Regarding what succeeds - these old Laurentian rocks geology tells us much; but of the formations - that preceded, we know nothing whatever. For anything that geology - shows to the contrary, the time which may have elapsed from the - solidifying of the earth’s <span class="pagenum" id="Page_357">357</span>crust to the deposition of the Laurentian - strata—an absolute blank—may have been as great as the time that has - since intervened.</p> - - <p><em>Probable Date of the Eocene and Miocene Periods.</em>—If we take into - consideration the limit which physical science assigns to the age of - our globe, and the rapid rate at which, as we have seen, denudation - takes place, it becomes evident that the enormous period of 3 millions - of years comprehended in the foregoing tables must stretch far back - into the Tertiary age. Supposing that the mean rate of denudation - during that period was not greater than the present rate of denudation, - still we should have no less than 500 feet of rock worn off the face of - the country and carried into the sea during these 3 millions of years. - This fact shows how totally different the appearance and configuration - of the country in all probability was at the commencement of this - period from what it is at the present day. If it be correct that the - glacial epoch resulted from the causes which we have already discussed, - those tables ought to aid us in our endeavour to ascertain <em>how</em> much - of the Tertiary period may be comprehended within these 3 millions of - years.</p> - - <p>We have already seen (<a href="#CHAPTER_XVIII">Chapter XVIII.</a>) that there is evidence of a - glacial condition of climate at two different periods during the - Tertiary age, namely, about the middle of the Miocene and Eocene - periods respectively. As has already been shown, the more severe a - glacial epoch is, the more marked ought to be the character of its warm - inter-glacial periods; the greater the extension of the ice during the - cold periods of a glacial epoch the further should that ice disappear - in arctic regions during the corresponding warm periods. Thus the - severity of a glacial epoch may in this case be indirectly inferred - from the character of the warm periods and the extent to which the - ice may have disappeared from arctic regions. Judged by this test, we - have every reason to believe that the Miocene glacial epoch was one of - extreme severity.</p> - - <p>The Eocene conglomerate, devoid of all organic remains, and containing - numerous enormous ice-transported blocks, is, as we<span class="pagenum" id="Page_358">358</span> have seen, - immediately associated with nummulitic strata charged with fossils - characteristic of a warm climate. Referring to this Sir Charles Lyell - says, “To imagine icebergs carrying such huge fragments of stone in so - southern a latitude, and at a period immediately preceded and followed - by the signs of a warm climate, is one of the most perplexing enigmas - which the geologist has yet been called upon to solve.”<a id="FNanchor_204" href="#Footnote_204" class="fnanchor">[204]</a></p> - - <p>It is perfectly true that, according to the generally received theories - of the cause of a glacial climate the whole is a perplexing enigma, but - if we adopt the Secular theory of change of climate, every difficulty - disappears. According to this theory the very fact of the conglomerate - being formed at a period immediately preceded and succeeded by warm - conditions of climate, is of itself strong presumptive evidence of the - conglomerate being a glacial formation. But this is not all, the very - highness of the temperature of the preceding and succeeding periods - bears testimony to the severity of the intervening glacial period. - Despite the deficiency of direct evidence regarding the character of - the Miocene and Eocene glacial periods, we are not warranted, for - reasons which have been stated in <a href="#CHAPTER_XVII">Chapter XVII.</a>, to conclude that these - periods were less severe than the one which happened in Quaternary - times. Judging from indirect evidence, we have some grounds for - concluding that the Miocene glacial epoch at least was even more severe - and protracted than our recent glacial epoch.</p> - - <p>By referring to <a href="#TABLE_III">Table III.</a>, or the accompanying diagram, it will be seen - that prior to the period which I have assigned as that of the glacial - epoch, there are two periods when the eccentricity almost attained - its superior limit. The first period occurred 2,500,000 years ago, - when it reached 0·0721, and the second period 850,000 years ago, when - it attained a still higher value, viz., 0·0747, being within 0·0028 - of the superior limit. To the first of these periods I am disposed - to assign the glacial epoch of Eocene times, and to the second that - of the Miocene age. With the view of determining the character of - these <span class="pagenum" id="Page_359">359</span>periods <a href="#TABLE_II">Tables II.</a> and <a href="#TABLE_III">III.</a> have been computed. They give the - eccentricity and longitude of perihelion at intervals of 10,000 years. - It will be seen from <a href="#TABLE_II">Table II.</a> that the Eocene period extends from - about 2,620,000 to about 2,460,000 years ago; and from <a href="#TABLE_III">Table III.</a> it - will be gathered that the Miocene period lasted from about 980,000 to - about 720,000 years ago.</p> - - <p>In order to find whether the eccentricity attained a higher value about - 850,000 years ago than 0·0747, I computed the values for one or two - periods immediately before and after that period, and satisfied myself - that the value stated was indeed the highest, as will be seen from the - subjoined table:—</p> - - <table summary="Eccentricity "> - <tbody> - <tr> - <td>851,000</td> - <td>0·07454</td> - </tr> - <tr> - <td>850,000</td> - <td>0·074664</td> - </tr> - <tr> - <td>849,500</td> - <td>0·07466</td> - </tr> - <tr> - <td>849,000</td> - <td>0·07466</td> - </tr> - </tbody> - </table> - - <p>How totally different must have been the condition of the earth’s - climate at that period from what it is at present! Taking the mean - distance of the sun to be 91,400,000 miles, his present distance at - midwinter is 89,864,480 miles; but at the period in question, when the - winter solstice was in perihelion, his distance at midwinter would be - no less than 98,224,289 miles. But this is not all; our winters are at - present shorter than our summers by 7·8 days, but at that period they - would be longer than the summers by 34·7 days.</p> - - <p>At present the difference between the perihelion and aphelion distance - of the sun amounts to only 3,069,580 miles, but at the period under - consideration it would amount to no less than 13,648,579 miles!</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XXII"> - <span class="pagenum" id="Page_360">360</span> - <h2> - CHAPTER XXII.<br /><br /> - <span class="small">A METHOD OF DETERMINING THE MEAN THICKNESS OF THE SEDIMENTARY ROCKS OF - THE GLOBE.</span> - </h2> - </div> - <div class="subhead">Prevailing Methods defective.—Maximum Thickness of British - Rocks.—Three Elements in the Question.—Professor Huxley - on the Rate of Deposition.—Thickness of Sedimentary Rocks - enormously over-estimated.—Observed Thickness no Measure of - mean Thickness.—Deposition of Sediment principally along - Sea-margin.—Mistaken Inference regarding the Absence of a - Formation.—Immense Antiquity of existing Oceans.</div> - - <p class="noindent"><span class="smcap">Various</span> attempts have been made to measure the positive length of - geological periods. Some geologists have sought to determine, roughly, - the age of the stratified rocks by calculations based upon their - probable thickness and the rate at which they may have been deposited. - This method, however, is worthless, because the rates which have been - adopted are purely arbitrary. One geologist will take the rate of - deposit at a foot in a hundred years, while another will assume it - to be a foot in a thousand or perhaps ten thousand years; and, for - any reasons that have been assigned, the one rate is just as likely - to be correct as the other: for if we examine what is taking place - in the ocean-bed at the present day, we shall find in some places a - foot of sediment laid down in a year, while in other places a foot - may not be deposited in a thousand years. The stratified rocks were - evidently formed at all possible rates. When we speak of the rate of - their formation, we must of course refer to the <em>mean rate</em>; and it is - perfectly true that if we knew the thickness of these rocks and the - mean rate at which they were deposited, we should have a ready means - of determining their positive age. But there appears to be nearly as - great uncertainty regarding the thickness of the sedimentary rocks as<span class="pagenum" id="Page_361">361</span> - regarding the rate at which they were formed. No doubt we can roughly - estimate their probable maximum thickness; for instance, Professor - Ramsay has found from actual measurement, that the sedimentary - formations of Great Britain have a maximum thickness of upwards of - 72,000 feet; but all such measurements give us no idea of their mean - thickness. What is the mean thickness of the sedimentary rocks of - the globe? On this point geology does not afford a definite answer. - Whatever the present mean thickness of the sedimentary rocks of our - globe may be, it must be small in comparison to the mean thickness - of all the sedimentary rocks which have been formed. This is obvious - from the fact that the sedimentary rocks of one age are partly formed - from the destruction of the sedimentary rocks of former ages. From the - Laurentian age down to the present day, the stratified rocks have been - undergoing constant denudation.</p> - - <p>Unless we take into consideration the quantity of rock removed during - past ages by denudation, we cannot—even though we knew the actual mean - thickness of the existing sedimentary rocks of the globe, and the rate - at which they were formed—arrive at an estimate regarding the length of - time represented by these rocks. For if we are to determine the age of - the stratified rocks from the rate at which they were formed, we must - have, not the present quantity of sedimentary rocks, but the present - plus the quantity which has been denuded during past ages. In other - words, we must have the absolute quantity formed. In many places the - missing beds must have been of enormous thickness. The time represented - by beds which have disappeared is, doubtless, as already remarked, - much greater than that represented by the beds which now remain. The - greater mass of the sedimentary rocks has been formed out of previously - existing sedimentary rocks, and these again out of sedimentary rocks - still older. As the materials composing our stratified beds may have - passed through many cycles of destruction and re-formation, the time - required to have deposited at a given rate the present existing mass - of<span class="pagenum" id="Page_362">362</span> sedimentary rocks may be but a fraction of the time required to - have deposited at the same rate the total mass that has actually been - formed. To measure the age of the sedimentary rocks by the present - existing rocks, assumed to be formed at some given rate, even supposing - the rate to be correct, is a method wholly fallacious.</p> - - <p>“The aggregate of sedimentary strata in the earth’s crust,” says Sir - Charles Lyell, “can never exceed in volume the amount of solid matter - which has been ground down and washed away by rivers, waves, and - currents. How vast, then, must be the spaces which this abstraction - of matter has left vacant! How far exceeding in dimensions all the - valleys, however numerous, and the hollows, however vast, which we can - prove to have been cleared out by aqueous erosion!”<a id="FNanchor_205" href="#Footnote_205" class="fnanchor">[205]</a></p> - - <p>I presume there are few geologists who would not admit that if all the - rocks which have in past ages been removed by denudation were restored, - the mean thickness of the sedimentary rocks of the globe would be at - least equal to their present maximum thickness, which we may take at - 72,000 feet.</p> - - <p>There are three elements in the question; of which if two are known, - the third is known in terms of the other two. If we have the mean - thickness of all the sedimentary rocks which have been formed and the - mean rate of formation, then we have the time which elapsed during the - formation; or having the thickness and the time, we have the rate; or, - having the rate and the time, we have the thickness.</p> - - <p>One of these three, namely, the rate, can, however, be determined with - tolerable accuracy if we are simply allowed to assume—what is very - probable, as has already been shown—that the present rate at which the - sedimentary deposits are being formed may be taken as the mean rate - for past ages. If we know the rate at which the land is being denuded, - then we know with perfect accuracy the rate at which the sedimentary - deposits are being formed in the ocean. This is obvious, because all - the materials denuded from the land are deposited in <span class="pagenum" id="Page_363">363</span>the sea; and - what is deposited in the sea is just what comes off the land, with the - exception of the small proportion of calcareous matter which may not - have been derived from the land, and which in our rough estimate may be - left out of account.</p> - - <p>Now the mean rate of subaërial denudation, we have seen, is about one - foot in 6,000 years. Taking the proportion of land to that of water - at 576 to 1,390, then one foot taken off the land and spread over the - sea-bottom would form a layer 5 inches thick. Consequently, if one foot - in 6,000 years represents the mean rate at which the land is being - denuded, one foot in 14,400 years represents the mean rate at which the - sedimentary rocks are being formed.</p> - - <p>Assuming, as before, that 72,000 feet would represent the mean - thickness of all the sedimentary rocks which have ever been formed, - this, at the rate of one foot in 14,400 years, gives 1,036,800,000 - years as the age of the stratified rocks.</p> - - <p>Professor Huxley, in his endeavour to show that 100,000,000 years is - a period sufficiently long for all the demands of geologists, takes - the thickness of the stratified rocks at 100,000 feet, and the rate - of deposit at a foot in 1,000 years. One foot of rock per 1,000 years - gives, it is true, 100,000 feet in 100,000,000 years. But what about - the rocks which have disappeared? If it takes a hundred millions of - years to produce a mass of rock equal to that which now exists, how - many hundreds of millions of years will it require to produce a mass - equal to what has actually been produced?</p> - - <p>Professor Huxley adds, “I do not know that any one is prepared to - maintain that the stratified rocks may not have been formed on the - average at the rate of 1/83rd of an inch per annum.” When the rate, - however, is accurately determined, it is found to be, not 1/83rd of - an inch per annum, but only 1/1200th of an inch, so that the 100,000 - feet of rock must have taken 1,440,000,000 years in its formation,—a - conclusion which, according to the results of modern physics, is wholly - inadmissible.</p> - - <p>Either the thickness of the sedimentary rocks has been over-estimated, - <span class="pagenum" id="Page_364">364</span>or the rate of their formation has been under-estimated, or both. - If it be maintained that a foot in 14,400 years is too slow a rate - of deposit, then it must be maintained that the land must have been - denuded at a greater rate than one foot in 6,000 years. But most - geologists probably felt surprised when the announcement was first - made, that at this rate of denudation the whole existing land of the - globe would be brought under the ocean in 6,000,000 of years.</p> - - <p>The error, no doubt, consists in over-estimating the thickness of the - sedimentary rocks. Assuming, for physical reasons already stated, that - 100,000,000 years limits the age of the stratified rocks, and that the - proportion of land to water and the rate of denudation have been on the - average the same as at present, the mean thickness of sedimentary rocks - formed in the 100,000,000 years amounts to only 7,000 feet.</p> - - <p>But be it observed that this is the mean thickness on an area equal - to that of the ocean. Over the area of the globe it amounts to only - 5,000 feet; and this, let it be observed also, is the total mean - thickness formed, without taking into account what has been removed - by denudation. If we wish to ascertain what is actually the present - mean thickness, we must deduct from this 5,000 feet an amount of rock - equal to all the sedimentary rocks which have been denuded during - the 100,000,000 years; for the 5,000 feet is not the present mean - thickness, but the total mean thickness formed during the whole of the - 100,000,000 years. If we assume, what no doubt most geologists would be - willing to grant, that the quantity of sedimentary rocks now remaining - is not over one-half of what has been actually deposited during the - history of the globe, then the actual mean thickness of the stratified - rocks of the globe is not over 2,500 feet. This startling result would - almost necessitate us to suspect that the rate of subaërial denudation - is probably greater than one foot in 6,000 years. But, be this as it - may, we are apt, in estimating the mean thickness of the stratified - rocks of the globe from their ascertained maximum thickness, to arrive - at erroneous conclusions. There are considerations<span class="pagenum" id="Page_365">365</span> which show that - the mean thickness of these rocks must be small in proportion to their - maximum thickness. The stratified rocks are formed from the sediment - carried down by rivers and streamlets and deposited in the sea. It is - obvious that the greater quantity of this sediment is deposited near - the mouths of rivers, and along a narrow margin extending to no great - distance from the land. Did the land consist of numerous small islands - equally distributed over the globe, the sediment carried off from these - islands would be spread pretty equally over the sea-bottom. But the - greater part of the land-surface consists of two immense continents. - Consequently, the materials removed by denudation are not spread - over the ocean-bottom, but on a narrow fringe surrounding those two - continents. Were the materials spread over the entire ocean-bed, a foot - removed off the general surface of the land would form a layer of rock - only five inches thick. But in the way in which the materials are at - present deposited, the foot removed from the land would form a layer - of rock many feet in thickness. The greater part of the sediment is - deposited within a few miles of the shore.</p> - - <p>The entire coast-line of the globe is about 116,500 miles. I should - think that the quantity of sediment deposited beyond, say, 100 miles - from this coast-line is not very great. No doubt several of the large - rivers carry sediment to a much greater distance from their mouths than - 100 miles, and ocean currents may in some cases carry mud and other - materials also to great distances. But it must be borne in mind that - at many places within the 100 miles of this immense coast-line little - or no sediment is deposited, so that the actual area over which the - sediment carried off the land is deposited is probably not greater than - the area of this belt—116,500 miles long and 100 miles broad. This - area on which the sediment is deposited, on the above supposition, is - therefore equal to about 11,650,000 square miles. The amount of land on - the globe is about 57,600,000 square miles. Consequently, one foot of - rock, denuded from the surface of the land and deposited on this<span class="pagenum" id="Page_366">366</span> belt, - would make a stratum of rock 5 feet in thickness; but were the sediment - spread over the entire bed of the ocean, it would form, as has already - been stated, a stratum of rock of only 5 inches in thickness.</p> - - <p>Suppose that no subsidence of the land should take place for a period - of, say, 3,000,000 of years. During that period 500 feet would be - removed by denudation, on an average, off the land. This would make a - formation 2,500 feet thick, which some future geologist might call the - Post-tertiary formation. But this, be it observed, would be only the - mean thickness of the formation on this area; its maximum thickness - would evidently be much greater, perhaps twice, thrice, or even four - times that thickness. A geologist in the future, measuring the actual - thickness of the formation, might find it in some places 10,000 feet - in thickness, or perhaps far more. But had the materials been spread - over the entire ocean-bed, the formation would have a mean thickness - of little more than 200 feet; and spread over the entire surface of - the globe, would form a stratum of scarcely 150 feet in thickness. - Therefore, in estimating the mean thickness of the stratified rocks of - the globe, a formation with a maximum thickness of 10,000 feet may not - represent more than 150 feet. A formation with a <em>mean</em> thickness of - 10,000 feet represents only 600 feet.</p> - - <p>It may be objected that in taking the present rate at which the - sedimentary deposits are being formed as the mean rate for all ages, - we probably under-estimate the total amount of rock formed, because - during the many glacial periods which must have occurred in past ages - the amount of materials ground off the rocky surface of the land in a - given period would be far greater than at present. But, in reply, it - must be remembered that although the destruction in ice-covered regions - would be greater during these periods than at present, yet the quantity - of materials carried down by rivers into the sea would be less. At - the present day the greater part of the materials carried down by our - rivers is not what is being removed off the rocky face of the country, - but the boulder clay, sand, and other<span class="pagenum" id="Page_367">367</span> materials which were ground off - during the glacial epoch. It is therefore possible, on this account, - that the rate of deposit may have been less during the glacial epoch - than at present.</p> - - <p>When any particular formation is wanting in a given area, the inference - generally drawn is, that either the formation has been denuded off - the area, or the area was a land-surface during the period when that - formation was being deposited. From the foregoing it will be seen that - this inference is not legitimate; for, supposing that the area had been - under water, the chances that materials should have been deposited on - that area are far less than are the chances that there should not. - There are sixteen chances against one that no formation ever existed in - the area.</p> - - <p>If the great depressions of the Atlantic, Pacific, and Indian Oceans - be, for example, as old as the beginning of the Laurentian period—and - they may be so for anything which geology can show to the contrary—then - under these oceans little or no stratified rocks may exist. The - supposition that the great ocean basins are of immense antiquity, and - that consequently only a small proportion of the sedimentary strata - can possibly occupy the deeper bed of the sea, acquires still more - probability when we consider the great extent and thickness of the - Old Red Sandstone, the Permian, and other deposits, which, according - to Professor Ramsay and others, have been accumulated in vast inland - lakes.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XXIII"> - <span class="pagenum" id="Page_368">368</span> - <h2> - CHAPTER XXIII.<br /><br /> - <span class="small">THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE LAND DURING - THE GLACIAL EPOCH.</span> - </h2> - </div> - <div class="subhead">Displacement of the Earth’s Centre of Gravity by - Polar Ice-cap.—Simple Method of estimating Amount of - Displacement.—Note by Sir W. Thomson on foregoing - Method.—Difference between Continental Ice and a - Glacier.—Probable Thickness of the Antarctic Ice-cap.—Probable - Thickness of Greenland Ice-sheet.—The Icebergs of the Southern - Ocean.—Inadequate Conceptions regarding the Magnitude of - Continental Ice.</div> - - <p><em>Displacement of the Earth’s Centre of Gravity by Polar - Ice-cap.</em><a id="FNanchor_206" href="#Footnote_206" class="fnanchor">[206]</a>—In order to represent the question in its most simple - elementary form, I shall assume an ice-cap of a given thickness at the - pole and gradually diminishing in thickness towards the equator in the - simple proportion of the sines of the latitudes, where at the equator - its thickness of course is zero. Let us assume, what is actually the - case, that the equatorial diameter of the globe is somewhat greater - than the polar, but that when the ice-cap is placed on one hemisphere - the whole forms a perfect sphere.</p> - - <p>I shall begin with a period of glaciation on the southern hemisphere. - Let W N E S′ (Fig. 5) be the solid part of the earth, and <i>c</i> its - centre of gravity. And let E S W be an ice-cap covering the southern - hemisphere. Let us in the first case assume the earth to be of the same - density as the cap. The earth with its cap forms now a perfect sphere - with its <span class="pagenum" id="Page_369">369</span>centre of gravity at <i>o</i>; for W N E S is a circle, and <i>o</i> - is its centre. Suppose now the whole to be covered with an ocean a few - miles deep, the ocean will assume the spherical form, and will be of - uniform depth. Let the southern winter solstice begin now to move round - from the aphelion. The ice-cap will also commence gradually to diminish - in thickness, and another cap will begin to make its appearance on - the northern hemisphere. As the northern cap may be supposed, for - simplicity of calculation, to increase at the same rate that the - southern will diminish, the spherical form of the earth will always be - maintained. By the time that the northern cap has reached a maximum, - the southern cap will have completely disappeared. The circle W N′ E S′ - will now represent the earth with its cap on the northern hemisphere, - and <i>o′</i> will be its centre of gravity; for <i>o′</i> is the centre of the - circle W N′ E S′. And as the distance between the centres <i>o</i> and - <i>o′</i> is equal to N N′, the thickness of the cap at the pole N N′ will - therefore represent the extent to which the centre of gravity has been - displaced. It will also represent the extent to which the ocean has - risen at the north pole and sunk at the south. This is evident; for as - the sphere W N′ E S′ is the same in all respects as the sphere W N E - S, with the exception only that the cap is on the opposite side, the - surface of the ocean at the poles will now be at the same distance from - the centre <i>o′</i> as it<span class="pagenum" id="Page_370">370</span> was from the centre <i>o</i> when the cap covered - the southern hemisphere. Hence the distance between <i>o</i> and <i>o′</i> must - be equal to the extent of the submergence at the north pole and the - emergence at the south. Neglect the attraction of the altering water on - the water itself, which later on will come under our consideration.</p> - - <div class="figcenter" id="i_369" > - <div class="caption mb2">Fig. 5.</div> - <img src="images/i_369.jpg" width="350" height="315" alt="" /> - </div> - - <p>We shall now consider the result when the earth is taken at its actual - density, which is generally believed to be about 5·5. The density - of ice being ·92, the density of the cap to that of the earth will - therefore be as 1 to 6.</p> - - <div class="figcenter" id="i_370" > - <div class="caption mb2">Fig. 6.</div> - <img src="images/i_370.jpg" width="350" height="342" alt="" /> - </div> - - <p>Let Fig. 6 represent the earth with an ice-cap on the northern - hemisphere, whose thickness is, say, 6,000 feet at the pole. The centre - of gravity of the earth without the cap is at <i>c</i>. When the cap is on, - the centre of gravity is shifted to <i>o</i>, a point a little more than - 500 feet to the north of <i>c</i>. Had the cap and the earth been of equal - density, the centre of gravity would have been shifted to <i>o′</i> the - centre of the figure, a point situated, of course, 3,000 feet to the - north of <i>c</i>. Now it is very approximately true that the ocean will - tend to adjust itself as a sphere around the centre of gravity, <i>o</i>. - Thus it would of course sink at the south pole and rise to the same - extent at the north, in any opening or channel in the ice allowing the - water to enter.</p> - - <p>Let the ice-cap be now transferred over to the southern<span class="pagenum" id="Page_371">371</span> hemisphere, - and the condition of things on the two hemispheres will in every - particular be reversed. The centre of gravity will then lie to - the south of <i>c</i>, or about 1,000 feet from its former position. - Consequently the transference of the cap from the one hemisphere to the - other will produce a total submergence of about 1,000 feet.</p> - - <p>It is, of course, absurd to suppose that an ice-cap could ever actually - reach down to the equator. It is probable that the great ice-cap of the - glacial epoch nowhere reached even halfway to the equator. Our cap must - therefore terminate at a moderately high latitude. Let it terminate - somewhere about the latitude of the north of England, say at latitude - 55°. All that we have to do now is simply to imagine our cap, up to - that latitude, becoming converted into the fluid state. This would - reduce the cap to less than one-half its former mass. But it would not - diminish the submergence to anything like that extent. For although the - cap would be reduced to less than one-half its former mass, yet its - influence in displacing the centre of gravity would not be diminished - to that extent. This is evident; for the cap now extending down to - only latitude 55°, has its centre of gravity much farther removed from - the earth’s centre of gravity than it had when it extended down to the - equator. Consequently it now possesses, in proportion to its mass, a - much greater power in displacing the earth’s centre of gravity.</p> - - <p>There is another fact which must be taken into account. The common - centre of gravity of the earth and cap is not exactly the point - around which the ocean tends to adjust itself. It adjusts itself not - in relation to the centre of gravity of the solid mass alone, but in - relation to the common centre of gravity of the entire mass, solid and - liquid. Now the water which is pulled over from the one hemisphere to - the other by the attraction of the cap will also aid in displacing the - centre of gravity. It will co-operate with the cap and carry the true - centre of gravity to a point beyond that of the centre of gravity of - the earth and cap, and thus increase the effect.</p> - - <p><span class="pagenum" id="Page_372">372</span></p> - - <p>It is of course perfectly true that when the ice-cap does not extend - down to the equator, as in the latter supposition, and is of less - density than the globe, the ocean will not adjust itself uniformly - around the centre of gravity; but the deviation from perfect uniformity - is so trifling, as will be seen from the appended note of Sir William - Thomson, that for all practical purposes it may be entirely left out of - account.</p> - - <p>In the <cite>Reader</cite> for January 13, 1866, I advanced an objection to the - submergence theory on the grounds that the lowering of the ocean-level - by the evaporation of the water to form the ice-cap, would exceed the - submergence resulting from the displacement of the earth’s centre of - gravity. But, after my letter had gone to press, I found that I had - overlooked some important considerations which seem to prove that the - objection had no real foundation. For during a glacial period, say - on the northern hemisphere, the entire mass of ice which presently - exists on the southern hemisphere would be transferred to the northern, - leaving the quantity of liquid water to a great extent unchanged.</p> - - <div class="center"><i>Note on the preceding by Sir William Thomson, F.R.S.</i></div> - - <p>“Mr. Croll’s estimate of the influence of a cap of ice on the sea-level - is very remarkable in its relation to Laplace’s celebrated analysis, - as being founded on that law of thickness which leads to expressions - involving only the first term of the series of ‘Laplace’s functions,’ - or ‘spherical harmonics.’ The equation of the level surface, as - altered by any given transference of solid matter, is expressed by - equating the altered potential function to a constant. This function, - when expanded in the series of spherical harmonics, has for its first - term the potential due to the whole mass supposed collected at its - altered centre of gravity. Hence a spherical surface round the altered - centre of gravity is the <em>first</em> approximation in Laplace’s method of - solution for the altered level surface. Mr. Croll has with admirable - tact chosen, of all the arbitrary suppositions that may be made - foundations for rough estimates<span class="pagenum" id="Page_373">373</span> of the change of sea-level due to - variations in the polar ice-crusts, <em>the</em> one which reduces to zero all - terms after the first in the harmonic series, and renders that first - approximation (which always expresses the <em>essence</em> of the result) the - whole solution, undisturbed by terms irrelevant to the great physical - question.</p> - - <p>“Mr. Croll, in the preceding paper, has alluded with remarkable - clearness to the effect of the change in the distribution of the - water in increasing, by its own attraction, the deviation of the - level surface above that which is due to the <em>given</em> change in the - distribution of solid matter. The remark he makes, that it is round - the centre of gravity of the altered solid and altered liquid that - the altering liquid surface adjusts itself, expresses the essence of - Laplace’s celebrated demonstration of the stability of the ocean, and - suggests the proper elementary solution of the problem to find the - true alteration of sea-level produced by a given alteration of the - solid. As an assumption leading to a simple calculation, let us suppose - the solid earth to rise out of the water in a vast number of small - flat-topped islands, each bounded by a perpendicular cliff, and let the - proportion of water area to the whole be equal in all quarters. Let all - of these islands in one hemisphere be covered with ice, of thickness - according to the law assumed by Mr. Croll—that is, varying in simple - proportion of the sine of the latitude. Let this ice be removed from - the first hemisphere and similarly distributed over the islands of - the second. By working out according to Mr. Croll’s directions, it is - easily found that the change of sea-level which this will produce will - consist in a sinking in the first hemisphere and rising in the second, - through heights varying according to the same law (that is, simple - proportionality to sines of latitudes), and amounting at each pole to</p> - - <div class="center"> - <span class="frac"><sup>(1 - ω)it</sup><span>/</span><sub>1 - ωw</sub></span>, - </div> - - <p class="noindent">where <i>t</i> denotes the thickness of the ice-crust at the pole; <i>i</i> the - ratio of the density of ice, and <i>w</i> that of sea-water to the earth’s<span class="pagenum" id="Page_374">374</span> - mean density; and ω the ratio of the area of ocean to the whole surface.</p> - - <p>“Thus, for instance, if we suppose ω = ⅔, and <i>t</i> = 6,000 feet, and - take ⅙ and 1/(5½) as the densities of ice and water respectively, we - find for the rise of sea-level at one pole, and depression at the other,</p> - - <div class="center"> - <span class="frac"> - <sup>⅓ × ⅙ × 6000</sup><span>/</span><sub>1 − <span class="frac"><sup>2</sup><span>/</span><sub>3</sub></span> - × <span class="frac"><sup>1</sup><span>/</span><sub>5½</sub></span></sub> - </span>, - </div> - - <p class="noindent">or approximately 380 feet.</p> - - <p>“I shall now proceed to consider roughly what is the probable - extent of submergence which, during the glacial epoch, may have - resulted from the displacement of the earth’s centre of gravity - by means of the transferrence of the polar ice from the one - hemisphere to the other.”</p> - - <p><em>Difference between Continental-ice and a Glacier.</em>—An ordinary - glacier descends in virtue of the slope of its bed, and, as a general - rule, it is on this account thin at its commencement, and thickens - as it descends into the lower valleys, where the slope is less and - the resistance to motion greater. But in the case of continental ice - matters are entirely different. The slope of the ground exercises - little or no influence on the motion of the ice. In a continent of one - or two thousand miles across, the general slope of the ground may be - left out of account; for any slight elevation which the centre of such - a continent may have will not compensate for the resistance offered to - the flow of the ice by mountain ridges, hills, and other irregularities - of its surface. The ice can move off such a surface only in consequence - of pressure acting from the interior. In order to produce such a - pressure, there must be a piling up of the ice in the interior; or, in - other words, the ice-sheet must thicken from the edge inwards to the - centre. We are necessarily led to the same conclusion, though we should - not admit that the ice moves in consequence of pressure from behind, - but should hold, on the contrary, that each particle of<span class="pagenum" id="Page_375">375</span> ice moves by - gravity in virtue of its own weight; for in order to have such a motion - there must be a slope, and as the slope is not on the ground, it must - be on the ice itself: consequently we must conclude that the upper - surface of the ice slopes upwards from the edge to the interior. What, - then, is the least slope at which the ice will descend? Mr. Hopkins - found that ice barely moves on a slope of one degree. We have therefore - some data for arriving at least at a rough estimate of the probable - thickness of an ice-sheet covering a continent, such, for example, as - Greenland or the Antarctic Continent.</p> - - <p><em>Probable Thickness of the Antarctic Ice-cap.</em>—The antarctic continent - is generally believed to extend, on an average, from the South Pole - down to about, at least, lat. 70°. In round numbers, we may take the - diameter of this continent at 2,800 miles. The distance from the - edge of this ice-cap to its centre, the South Pole, will, therefore, - be 1,400 miles. The whole of this continent, like Greenland, is - undoubtedly covered with one continuous sheet of ice gradually - thickening inwards from its edge to its centre. A slope of one degree - continued for 1,400 miles will give twenty-four miles as the thickness - of the ice at the pole. But suppose the slope of the upper surface - of the cap to be only one-half this amount, viz., a half degree,—and - we have no evidence that a slope so small would be sufficient to - discharge the ice,—still we have twelve miles as the thickness of the - cap at the pole. To those who have not been accustomed to reflect on - the physical conditions of the problem, this estimate may doubtless - be regarded as somewhat extravagant; but a slight consideration - will show that it would be even more extravagant to assume that a - slope of less than half a degree would be sufficient to produce the - necessary outflow of the ice. In estimating the thickness of a sheet of - continental ice of one or two thousand miles across, our imagination - is apt to deceive us. We can easily form a pretty accurate sensuous - representation of the thickness of the sheet; but we can form no - adequate representation of its superficial area. We can represent - to the mind with tolerable accuracy a thickness of a few miles, but - we<span class="pagenum" id="Page_376">376</span> cannot do this in reference to the area of a surface 2,800 miles - across. Consequently, in judging what proportion the thickness of the - sheet should bear to its superficial area, we are apt to fall into the - error of under-estimating the thickness. We have a striking example - of this in regard to the ocean. The thing which impresses us most - forcibly in regard to the ocean is its profound depth. A mean depth - of, say, three miles produces a striking impression; but if we could - represent to the mind the vast area of the ocean as correctly as we can - do its depth, <em>shallowness</em> rather than <em>depth</em> would be the impression - produced. A sheet of water 100 yards in diameter, and only one inch - deep, would not be called a <em>deep</em> but a very <em>shallow</em> pool or thin - layer of water. But such a layer would be a correct representation of - the ocean in miniature. Were we in like manner to represent to the eye - in miniature the antarctic ice-cap, we would call it a <em>thin crust of - ice</em>. Taking the mean thickness of the ice at four miles, the antarctic - ice-sheet would be represented by a carpet covering the floor of an - ordinary-sized dining-room. Were those who consider the above estimate - of the thickness of the antarctic ice-cap as extravagantly great called - upon to sketch on paper a section of what they should deem a cap of - moderate thickness, ninety-nine out of every hundred would draw one of - much greater thickness than twelve miles at the centre.</p> - - <p>The diagram on following page (Fig. 7) represents a section across the - cap drawn to a natural scale; the upper surface of the sheet having - a slope of half a degree. No one on looking at the section would - pronounce it to be too thick at the centre, unless he were previously - made aware that it represented a thickness of twelve miles at that - place. It may be here mentioned that had the section been drawn upon - a much larger scale—had it, for instance, been made seven feet long, - instead of seven inches—it would have shown to the eye in a more - striking manner the thinness of the cap.</p> - - <p>But to avoid all objections on the score of over-estimating the - thickness of the cap, I shall assume the angle of the upper - <span class="pagenum" id="Page_377">377</span> surface to - be only a quarter of a degree, and the thickness of the sheet one-half - what it is represented in the section. The thickness at the pole will - then be only six miles instead of twelve, and the mean thickness of the - cap two instead of four miles.</p> - - <div class="figcenter" id="i_377" > - <div class="caption">Fig. 7.<br /> - <span class="smcap">S. Pole.</span></div> - <img src="images/i_377.jpg" width="600" height="11" alt="" /> - <div class="caption"> - Section across Antarctic Ice-cap, drawn to a natural scale.<br /> - Length represented by section = 2,800 miles. Thickness at centre (South Pole) = 12 miles.<br /> - Slope of upper surface = half-degree.</div> - </div> - - <p>Is there any well-grounded reason for concluding the above to be an - over-estimate of the actual thickness of the antarctic ice? It is not - so much in consequence of any <i lang="la">à priori</i> reason that can be urged - against the probability of such a thickness of ice, but rather because - it so far transcends our previous experience that we are reluctant to - admit such an estimate. If we never had any experience of ice thicker - than what is found in England, we should feel startled on learning for - the first time that in the valleys of Switzerland the ice lay from 200 - to 300 feet in depth. Again, if we had never heard of glaciers thicker - than those of Switzerland, we could hardly credit the statement that - in Greenland they are actually from 2,000 to 3,000 feet thick. We, in - this country, have long been familiar with Greenland; but till very - lately no one ever entertained the idea that that continent was buried - under one continuous mass of ice, with scarcely a mountain top rising - above the icy mantle. And had it not been that the geological phenomena - of the glacial epoch have for so many years accustomed our minds to - such an extraordinary condition of things, Dr. Rink’s description of - the Greenland ice would probably have been regarded as the extravagant - picture of a wild imagination.</p> - - <p>Let us now consider whether or not the facts of observation and - experience, so far as they go,<span class="pagenum" id="Page_378">378</span> bear out the conclusions to which - physical considerations lead us in reference to the magnitude of - continental ice; and more especially as regards the ice of the - antarctic regions.</p> - - <p><em>First.</em> In so far as the antarctic ice-sheet is concerned, observation - and experience to a great extent may be said to be a perfect blank. One - or two voyagers have seen the outer edge of the sheet at a few places, - and this is all. In fact, we judge of the present condition of the - interior of the antarctic continent in a great measure from what we - know of Greenland. But again, our experience of Greenland ice is almost - wholly confined to the outskirts.</p> - - <p>Few have penetrated into the interior, and, with the exception of Dr. - Hayes and Professor Nordenskjöld, none, as far as I know, have passed - to any considerable distance over the inland ice. Dr. Robert Brown - in his interesting memoir on “Das Innere von Grönland,”<a id="FNanchor_207" href="#Footnote_207" class="fnanchor">[207]</a> gives - an account of an excursion made in 1747 by a Danish officer of the - name of Dalager, from Fredrikshaab, near the southern extremity of - the continent, into the interior. After a journey of a day or two, he - reached an eminence from which he saw the inland ice stretching in an - unbroken mass as far as the eye could reach, but was unable to proceed - further. Dr. Brown gives an account also of an excursion made in the - beginning of March, 1830, by O. B. Kielsen, a Danish whale-fisher, from - Holsteinborg (lat. 67° N.). After a most fatiguing journey of several - days, he reached a high point from which he could see the ice of the - interior. Next morning he got up early, and towards midday reached - an extensive plain. From this the land sank inwards, and Kielsen now - saw fully in view before him the enormous ice-sheet of the interior. - He drove rapidly over all the little hills, lakes, and streams, till - he reached a pretty large lake at the edge of the ice-sheet. This was - the end of his journey, for after vainly attempting to climb up on the - ice-sheet, he was compelled to retrace his steps, and had a somewhat - difficult return. When he arrived at the fiord, he found the ice broken - up, so that he had to go round <span class="pagenum" id="Page_379">379</span>by the land way, by which he reached - the depôt on the 9th of March. The distance which he traversed in a - straight line from Holsteinborg into the interior measured eighty - English miles.</p> - - <p>Dr. Hayes’s excursion was made, however, not upon the real inland - ice, but upon a smaller ice-field connected with it; while Professor - Nordenskjöld’s excursion was made at a place too far south to - afford an accurate idea of the actual condition of the interior of - North Greenland, even though he had penetrated much farther than he - actually did. However, the state of things as recorded by Hayes and by - Nordenskjöld affords us a glimpse into the condition of things in the - interior of the continent. They both found by observation, what follows - as a necessary result from physical considerations, that the upper - surface of the ice plain, under which hills and valleys are buried, - gradually <em>slopes upwards towards the interior of the continent</em>. - Professor Nordenskjöld states that when at the extreme point at which - he reached, thirty geographical miles from the coast, he had attained - an elevation of 2,200 feet, and that the inland ice <em>continued - constantly to rise</em> towards the interior, so that the horizon towards - the east, north, and south, was terminated by an ice-border almost as - smooth as that of the ocean.”<a id="FNanchor_208" href="#Footnote_208" class="fnanchor">[208]</a></p> - - <p>Dr. Hayes and his party penetrated inwards to the distance of about - seventy miles. On the first day they reached the foot of the great Mer - de Glace; the second day’s journey carried them to the upper surface - of the ice-sheet. On the third day they travelled 30 miles, and the - ascent, which had been about 6°, diminished gradually to about 2°. They - advanced on the fourth day about 25 miles; the temperature being 30° - below zero (Fah.). “Our station at the camp,” he says, “was sublime as - it was dangerous. We had attained an altitude of 5,000 feet above the - sea-level, and were 70 miles from the coast, in the midst of a vast - frozen Sahara immeasurable to the human eye. There was neither hill, - mountain, nor gorge, <span class="pagenum" id="Page_380">380</span>anywhere in view. We had completely sunk the - strip of land between the Mer de Glace and the sea, and no object met - the eye but our feeble tent, which bent to the storm. Fitful clouds - swept over the face of the full-orbed moon, which, descending towards - the horizon, glimmered through the drifting snow that scudded over the - icy plain—to the eye in undulating lines of downy softness, to the - flesh in showers of piercing darts.”<a id="FNanchor_209" href="#Footnote_209" class="fnanchor">[209]</a></p> - - <p>Dr. Rink, referring to the inland ice, says that the elevation or - height above the sea of this icy plain at its junction with the - outskirts of the country, and where it begins to lower itself through - the valleys to the firths, is, in the ramifications of the Bay of - Omenak, found to be 2,000 feet, from which level <em>it gradually rises - towards the interior</em>.<a id="FNanchor_210" href="#Footnote_210" class="fnanchor">[210]</a></p> - - <p>Dr. Robert Brown, who, along with Mr. Whymper in 1867, attempted a - journey to some distance over the inland ice, is of opinion that - Greenland is not traversed by any ranges of mountains or high land, - but that the entire continent, 1,200 miles in length and 400 miles in - breadth, is covered with one continuous unbroken field of ice, the - upper surface of which, he says, <em>rises by a gentle slope towards the - interior</em>.<a id="FNanchor_211" href="#Footnote_211" class="fnanchor">[211]</a></p> - - <p>Suppose now the point reached by Hayes to be within 200 miles of - the centre of dispersion of the ice, and the mean slope from that - point to the centre, as in the case of the antarctic cap, to be only - half a degree; this would give 10,000 feet as the elevation of the - centre above the point reached. But the point reached was 5,000 feet - above sea-level, consequently the surface of the ice at the centre - of dispersion would be 15,000 feet above sea-level, which is about - one-fourth what I have concluded to be the elevation of the surface - of the antarctic ice-cap at its centre. And supposing we assume - the general surface of the ground to have in the central region an - elevation as great as 5,000 feet, which is not at all probable, still - this would give 10,000 feet for the thickness of the ice at the centre - <span class="pagenum" id="Page_381">381</span>of the Greenland continent. But if we admit this conclusion in - reference to the thickness of the Greenland ice, we must admit that - the antarctic ice is far thicker, because the thickness, other things - being equal, will depend upon the size, or, more properly, upon the - diameter of the continent; for the larger the surface the greater is - the thickness of ice required to produce the pressure requisite to make - the rate of discharge of the ice equal to the rate of increase. Now - the area of the antarctic continent must be at least a dozen of times - greater than that of Greenland.</p> - - <p><em>Second.</em> That the antarctic ice must be far thicker than the arctic - is further evident from the dimensions of the icebergs which have been - met with in the Southern Ocean. No icebergs over three hundred feet in - height have been found in the arctic regions, whereas in the antarctic - regions, as we shall see, icebergs of twice and even thrice that height - have been reported.</p> - - <p><em>Third.</em> We have no reason to believe that the thickness of the ice - at present covering the antarctic continent is less than that which - covered a continent of a similar area in temperate regions during the - glacial epoch. Take, for example, the North American continent, or, - more properly, that portion of it covered by ice during the glacial - epoch. Professor Dana has proved that during that period the thickness - of the ice on the American continent must in many places have been - considerably over a mile. He has shown that over the northern border of - New England the ice had a mean thickness of 6,500 feet, while its mean - thickness over the Canada watershed, between St. Lawrence and Hudson’s - Bay, was not less than 12,000 feet, or upwards of two miles and a - quarter (see <cite>American Journal of Science and Art</cite> for March, 1873).</p> - - <p><em>Fourth.</em> Some may object to the foregoing estimate of the amount of - ice on the antarctic continent, on the grounds that the quantity of - snowfall in that region cannot be much. But it must be borne in mind - that, no matter however small the annual amount of snowfall may be, if - more falls than is melted,<span class="pagenum" id="Page_382">382</span> the ice must continue to accumulate year by - year till its thickness in the centre of the continent be sufficiently - great to produce motion. The opinion that the snowfall of the antarctic - regions is not great does not, however, appear to be borne out by the - observation and experience of those who have visited those regions. - Captain Wilkes, of the American Exploring Expedition, estimated it at - 30 feet per annum; and Sir James Ross says, that during a whole month - they had only three days free from snow. The fact that perpetual snow - is found at the sea-level at lat. 64° S. proves that the snowfall - must be great. But there is another circumstance which must be taken - into account, viz., that the currents carrying moisture move in from - all directions towards the pole, consequently the area on which they - deposit their snow becomes less and less as the pole is reached, and - this must, to a corresponding extent, increase the quantity of snow - falling on a given area. Let us assume, for example, that the clouds - in passing from lat. 60° to lat. 80° deposit moisture sufficient to - produce, say, 30 feet of snow per annum, and that by the time they - reach lat. 80° they are in possession of only one-tenth part of their - original store of moisture. As the area between lat. 80° and the - pole is but one-eighth of that between lat. 60° and 80°, this would, - notwithstanding, give 24 feet as the annual amount of snowfall between - lat. 80° and the pole.<a id="FNanchor_212" href="#Footnote_212" class="fnanchor">[212]</a></p> - - <p><em>Fifth.</em> The enormous size and thickness of the icebergs which have - been met with in the Southern Ocean testify to the thickness of the - antarctic ice-cap.</p> - - <p>We know from the size of some of the icebergs which have been met with - in the southern hemisphere that the ice at the edge of the cap where - the bergs break off must in some cases be considerably over a mile in - thickness, for icebergs of more <span class="pagenum" id="Page_383">383</span>than a mile in thickness have been - found in the southern hemisphere. The following are the dimensions of - a few of these enormous bergs taken from the Twelfth Number of the - Meteorological Papers published by the Board of Trade, and from the - excellent paper of Mr. Towson on the Icebergs of the Southern Ocean, - published also by the Board of Trade.<a id="FNanchor_213" href="#Footnote_213" class="fnanchor">[213]</a> With one or two exceptions, - the heights of the bergs were accurately determined by angular - measurement:—</p> - - <p class="hang">Sept. 10th, 1856.—The <em>Lightning</em>, when in lat. 55° 33′ S., - long. 140° W., met with an iceberg 420 feet high.</p> - - <p class="hang">Nov., 1839.—In lat. 41° S., long. 87° 30′ E., numerous icebergs - 400 feet high were met with.</p> - - <p class="hang">Sept., 1840.—In lat. 37° S., long. 15° E., an iceberg 1,000 - feet long and 400 feet high was met with.</p> - - <p class="hang">Feb., 1860.—Captain Clark, of the <em>Lightning</em>, when in lat. 55° - 20′ S., long. 122° 45′ W., found an iceberg 500 feet high and 3 - miles long.</p> - - <p class="hang">Dec. 1st, 1859.—An iceberg, 580 feet high, and from two and a - half to three miles long, was seen by Captain Smithers, of the - <cite>Edmond</cite>, in lat. 50° 52′ S., long. 43° 58′ W. So strongly did - this iceberg resemble land, that Captain Smithers believed it - to be an island, and reported it as such, but there is little - or no doubt that it was in reality an iceberg. There were - pieces of drift-ice under its lee.</p> - - <p class="hang">Nov., 1856.—Three large icebergs, 500 feet high, were found in - lat. 41° 0′ S., long. 42° 0′ E.</p> - - <p class="hang">Jan., 1861.—Five icebergs, one 500 feet high, were met with in - lat. 55° 46′ S., long. 155° 56′ W.</p> - - <p class="hang">Jan., 1861.—In lat. 56° 10′ S., long. 160° 0′ W., an iceberg - 500 feet high and half a mile long was found.</p> - - <p class="hang">Jan., 1867.—The barque <cite>Scout</cite>, from the West Coast of<span class="pagenum" id="Page_384">384</span> - America, on her way to Liverpool, passed some icebergs 600 feet - in height, and of great length.</p> - - <p class="hang">April, 1864.—The <cite>Royal Standard</cite> came in collision with an - iceberg 600 feet in height.</p> - - <p class="hang">Dec., 1856.—Four large icebergs, one of them 700 feet high, and - another 500 feet, were met with in lat. 50° 14′ S., long. 42° - 54′ E.</p> - - <p class="hang">Dec. 25th, 1861.—The <cite>Queen of Nations</cite> fell in with an iceberg - in lat. 53° 45′ S., long. 170° 0′ W., 720 feet high.</p> - - <p class="hang">Dec., 1856.—Captain P. Wakem, ship <cite>Ellen Radford</cite>, found, in - lat. 52° 31′ S., long. 43° 43′ W., two large icebergs, one at - least 800 feet high. - </p> - - <p class="p2">Mr. Towson states that one of our most celebrated and talented - naval surveyors informed him that he had seen icebergs in the - southern regions 800 feet high.</p> - - <p class="hang">March 23rd, 1855.—The <cite>Agneta</cite> passed an iceberg in lat. 53° 14′ S., - long. 14° 41′ E., 960 feet in height.</p> - - <p class="hang">Aug. 16th, 1840.—The Dutch ship, <cite>General Baron von Geen</cite>, passed an - iceberg 1,000 feet high in lat. 37° 32′ S., long. 14° 10′ E.</p> - - <p class="hang">May 15th, 1859.—The <cite>Roseworth</cite> found in lat. 53° 40′ S., long. 123° - 17′ W., an iceberg as large as “Tristan d’Acunha.”</p> - - <p>In the regions where most of these icebergs were met with, the mean - density of the sea is about 1·0256. The density of ice is ·92. The - density of icebergs to that of the sea is therefore as 1 to 1·115; - consequently every foot of ice above water indicates 8·7 feet below - water. It therefore follows that those icebergs 400 feet high had 3,480 - feet under water,—3,880 feet would consequently be the total thickness - of the ice. The icebergs which were 500 feet high would be 4,850 feet - thick, those 600 feet high would have a total thickness of 5,820 feet, - and those 700 feet high would be no less than 6,790 feet thick, which - is more than a mile and a quarter. The iceberg 960 feet high, sighted - by the <cite>Agneta</cite>, would be actually 9,312 feet thick, which is upwards - of a mile and three-quarters.</p> - - <p>Although the mass of an iceberg below water compared to<span class="pagenum" id="Page_385">385</span> that above - may be taken to be about 8·7 to 1, yet it would not be always safe - to conclude that the thickness of the ice below water bears the same - proportion to its height above. If the berg, for example, be much - broader at its base than at its top, the thickness of the ice below - water would bear a less proportion to the height above water than - as 8·7 to 1. But a berg such as that recorded by Captain Clark, 500 - feet high and three miles long, must have had only 1/8·7 of its total - thickness above water. The same remark applies also to the one seen by - Captain Smithers, which was 580 feet high, and so large that it was - taken for an island. This berg must have been 5,628 feet in thickness. - The enormous berg which came in collision with the <cite>Royal Standard</cite> - must have been 5,820 feet thick. It is not stated what length the - icebergs 730, 960, and 1,000 feet high respectively were; but supposing - that we make considerable allowance for the possibility that the - proportionate thickness of ice below water to that above may have been - less than as 8·7 to 1, still we can hardly avoid the conclusion that - the icebergs were considerably above a mile in thickness. But if there - are icebergs above a mile in thickness, then there must be land-ice - somewhere on the southern hemisphere of that thickness. In short, the - great antarctic ice-cap must in some places be over a mile in thickness - at its edge.</p> - - <p><em>Inadequate Conceptions regarding the Magnitude of Continental - Ice.</em>—Few things have tended more to mislead geologists in the - interpretation of glacial phenomena than inadequate conceptions - regarding the magnitude of continental ice. Without the conception - of continental ice the known facts connected with glaciation would - be perfectly inexplicable. It was only when it was found that the - accumulated facts refused to be explained by any other conception, - that belief in the very existence of such a thing as continental ice - became common. But although most geologists now admit the existence of - continental ice, yet, nevertheless, adequate conceptions of its real - magnitude are by no means so common. Year by year, as the outstanding - facts connected with glaciation accumulate, we are compelled to<span class="pagenum" id="Page_386">386</span> extend - our conceptions of the magnitude of land-ice. Take the following as - an example. It was found that the transport of the Wastdale Crag - blocks, the direction of the striæ on the islands of the Baltic, on - Caithness and on the Orkney, Shetland, and Faroe, islands, the boulder - clay with broken shells in Caithness, Holderness, and other places, - were inexplicable on the theory of land-ice. But it was so only in - consequence of the inadequacy of our conceptions of the magnitude of - the ice; for a slight extension of our ideas of its thickness has - explained not only these phenomena,<a id="FNanchor_214" href="#Footnote_214" class="fnanchor">[214]</a> but others of an equally - remarkable character, such as the striation of the Long Island and - the submerged rock-basins around our coasts described by Mr. James - Geikie. In like manner, if we admit the theory of the glacial epoch - propounded in former chapters, all that is really necessary to account - for the submergence of the land is a slight extension of our hitherto - preconceived estimate of the thickness of the ice on the antarctic - continent. If we simply admit a conclusion to which all physical - considerations, as we have seen, necessarily lead us, viz., that the - antarctic continent is covered with a mantle of ice at least two miles - in thickness, we have then a complete explanation of the cause of the - submergence of the land during the glacial epoch.</p> - - <p>Although of no great importance to the question under consideration, it - may be remarked that, except during the severest part of the glacial - epoch, we have no reason to believe that the total quantity of ice - on the globe was much greater than at present, only it would then be - all on one hemisphere. Remove two miles of ice from the antarctic - continent, and place it on the northern hemisphere, and this, along - with the ice that now exists on this hemisphere, would equal, in all - probability, the quantity existing on our hemisphere during the glacial - epoch; at least, before it reached its maximum severity.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XXIV"> - <span class="pagenum" id="Page_387">387</span> - <h2> - CHAPTER XXIV.<br /><br /> - <span class="small">THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE LAND - DURING THE GLACIAL EPOCH.—<i>Continued.</i></span> - </h2> - </div> - <div class="subhead">Extent of Submergence from Displacement of Earth’s Centre - of Gravity.—Circumstances which show that the Glacial - Submergence resulted from Displacement of the Earth’s - Centre of Gravity.—Agreement between Theory and observed - Facts.—Sir Charles Lyell on submerged Areas during - Tertiary Period.—Oscillations of Sea-level in Relation to - Distribution.—Extent of Submergence on the Hypothesis that the - Earth is fluid in the Interior.</div> - - <p><em>Extent of Submergence from Displacement of Earth’s Centre of - Gravity.</em>—How much, then, would the transference of the two miles of - ice from the southern to the northern hemisphere raise the level of the - ocean on the latter hemisphere? This mass, be it observed, is equal to - only one-half that represented in our section. A considerable amount - of discussion has arisen in regard to the method of determining this - point. According to the method already detailed, which supposes the - rise at the pole to be equal to the extent of the displacement of the - earth’s centre of gravity, the rise at the North Pole would be about - 380 feet, taking into account the effect produced by the displaced - water; and the rise in the latitude of Edinburgh would be 312 feet. The - fall of level on the southern hemisphere would, of course, be equal to - the rise of level on the northern. According to the method advanced - by Mr. D. D. Heath,<a id="FNanchor_215" href="#Footnote_215" class="fnanchor">[215]</a> the rise of level at the North Pole would be - about 650 feet. Archdeacon Pratt’s method<a id="FNanchor_216" href="#Footnote_216" class="fnanchor">[216]</a> makes the rise still - greater; while according to Rev. O. Fisher’s method<a id="FNanchor_217" href="#Footnote_217" class="fnanchor">[217]</a> the rise would - be no <span class="pagenum" id="Page_388">388</span>less than 2,000 feet. There is, however, another circumstance - which must be taken into account, which will give an additional rise of - upwards of one hundred feet.</p> - - <p>The greatest extent of the displacement of the earth’s centre of - gravity, and consequently the greatest rise of the ocean resulting - from that displacement, would of course occur at the time of maximum - glaciation, when the ice was all on one hemisphere. But owing to the - following circumstance, a still greater rise than that resulting from - the displacement of the earth’s centre of gravity alone might take - place at some considerable time, either before or after the period of - maximum glaciation.</p> - - <p>It is not at all probable that the ice would melt on the warm - hemisphere at exactly the same rate as it would form on the cold - hemisphere. It is probable that the ice would melt more rapidly on the - warm hemisphere than it would form on the cold. Suppose that during - the glacial epoch, at a time when the cold was gradually increasing on - the northern and the warmth on the southern hemisphere, the ice should - melt more rapidly off the antarctic continent than it was being formed - on the arctic and subarctic regions; suppose also that, by the time - a quantity of ice, equal to one-half what exists at present on the - antarctic continent, had accumulated on the northern hemisphere, the - whole of the antarctic ice had been melted away, the sea would then be - fuller than at present by the amount of water resulting from the one - mile of melted ice. The height to which this would raise the general - level of the sea would be as follows:—</p> - - <p>The antarctic ice-cap is equal in area to 1/23·46 of that covered by - the ocean. The density of ice to that of water being taken at ·92 to - 1, it follows that 25 feet 6 inches of ice melted off the cap would - raise the general level of the ocean one foot, and the one mile of - ice melted off would raise the level 200 feet. This 200 feet of rise - resulting from the melted ice we must add to the rise resulting from - the displacement of the earth’s centre of gravity. The removal of the - two miles of ice from the antarctic<span class="pagenum" id="Page_389">389</span> continent would displace the - centre of gravity 190 feet, and the formation of a mass of ice equal - to the one-half of this on the arctic regions would carry the centre - of gravity 95 feet farther; giving in all a total displacement of 285 - feet, thus producing a rise of sea-level at the North Pole of 285 feet, - and in the latitude of Edinburgh of 234 feet. Add to this the rise of - 200 feet resulting from the melted ice, and we have then 485 feet of - submergence at the pole, and 434 feet in the latitude of Edinburgh. A - rise to a similar extent might probably take place after the period - of maximum glaciation, when the ice would be melting on the northern - hemisphere more rapidly than it would be forming on the southern.</p> - - <p>If we assume the antarctic ice-cap to be as thick as is represented in - the diagram, the extent of the submergence would of course be double - the above, and we might have in this case a rise of sea-level in the - latitude of Edinburgh to the extent of from 800 to 1,000 feet. But be - this as it may, it is evident that the quantity of ice on the antarctic - continent is perfectly sufficient to account for the submergence of - the glacial epoch, for we have little evidence to conclude that the - <em>general</em> submergence much exceeded 400 or 500 feet.<a id="FNanchor_218" href="#Footnote_218" class="fnanchor">[218]</a> We have - evidence in England and other places of submergence to the extent of - from 1,000 to 2,000 feet, but these may be quite local, resulting - from subsidence of the land in those particular areas. Elevations and - depressions of the land have taken place in all ages, and no doubt - during the glacial epoch also.</p> - - <p><em>Circumstances which show that the Glacial Submergence resulted from - Displacement of the Earth’s Centre of Gravity.</em>—In favour of this - view of the cause of the submergence of the glacial epoch, it is a - circumstance of some significance, that in every part of the globe - where glaciation has been found evidence of the submergence of the - land has also been found along with it. The invariable occurrence of - submergence along with glaciation <span class="pagenum" id="Page_390">390</span>points to some physical connection - between the two. It would seem to imply, either that the two were the - direct effects of a common cause, or that the one was the cause of the - other; that is, the submergence the cause of the glaciation, or the - glaciation the cause of the submergence. There is, I presume, no known - cause to which the two can be directly related as effects. Nor do I - think that there is any one who would suppose that the submergence of - the land could have been the cause of its glaciation, even although he - attributed all glacial effects to floating ice. The submergence of our - country would, of course, have allowed floating ice to pass over it had - there been any to pass over; but submergence would not have produced - the ice, neither would it have brought the ice from the arctic regions - where it already existed. But although submergence could not have been - the cause of the glacial epoch, yet we can, as we have just seen, - easily understand how the ice of the glacial epoch could have been the - cause of the submergence. If the glacial epoch was brought about by an - increase in the eccentricity of the earth’s orbit, then a submergence - of the land as the ice accumulated was a physical necessity.</p> - - <p>There is another circumstance connected with glacial submergence which - it is difficult to reconcile with the idea that it resulted from a - subsidence of the land. It is well known that during the glacial - epoch the land was not once under water only, but several times; and, - besides, there were not merely several periods when the land stood - at a lower level in relation to the sea than at present, but there - were also several periods when it stood at a much higher level than - now. And this holds true, not merely of our own country, but of every - country on the northern hemisphere where glaciation has yet been found. - All this follows as a necessary consequence from the theory that the - oscillations of sea-level resulted from the transference of the ice - from the one hemisphere to the other; but it is wholly inconsistent - with the idea that they resulted from upheavals and subsidence of the - land during a very recent period.</p> - - <p><span class="pagenum" id="Page_391">391</span></p> - - <p>But this is not all, there is more still to be accounted for. It has - been the prevailing opinion that at the time when the land was covered - with ice, it stood at a much greater elevation than at present. It - is, however, not maintained that the facts of geology establish such - a conclusion. The greater elevation of the land is simply assumed as - an hypothesis to account for the cold.<a id="FNanchor_219" href="#Footnote_219" class="fnanchor">[219]</a> The facts of geology, - however, are fast establishing the opposite conclusion, viz., that - when the country was covered with ice, the land stood in relation to - the sea at a lower level than at present, and that the continental - periods or times when the land stood in relation to the sea at a higher - level than now were the warm inter-glacial periods, when the country - was free of snow and ice, and a mild and equable condition of climate - prevailed. This is the conclusion towards which we are being led by the - more recent revelations of surface geology, and also by certain facts - connected with the geographical distribution of plants and animals - during the glacial epoch.</p> - - <p>The simple occurrence of a rise and fall of the land in relation to - the sea-level in one or in two countries during the glacial epoch, - would not necessarily imply any physical connection. The coincidence - of these movements with the glaciation of the land might have been - purely accidental; but when we find that a succession of such movements - occurred, not merely in one or in two countries, but in every glaciated - country where proper observations have been made, we are forced to the - conclusion that the connection between the two is not accidental, but - the result of some fixed cause.</p> - - <p>If we admit that an increase in the eccentricity of the earth’s orbit - was the cause of the glacial epoch, then we must admit that all those - results followed as necessary consequences. For if the glacial epoch - lasted for upwards of one hundred thousand years or so, there would be - a succession of cold and warm periods, and consequently a succession - of elevations and depressions of sea-level. And the elevations of - the sea-level would <span class="pagenum" id="Page_392">392</span>take place during the cold periods, and the - depressions during the warm periods.</p> - - <p>But the agreement between theory and observed facts does not terminate - here. It follows from theory that the greatest oscillations of - sea-level would take place during the severest part of the glacial - epoch, when the eccentricity of the earth’s orbit would be at its - highest value, and that the oscillations would gradually diminish - in extent as the eccentricity diminished and the climate gradually - became less severe. Now it is well known that this is actually what - took place; the great submergence, as well as the great elevation or - continental period, occurred during the earlier or more severe part of - the glacial epoch, and as the climate grew less severe these changes - became of less extent, till we find them terminating in our submerged - forests and 25-foot raised beach.</p> - - <p>It follows, therefore, according to the theory advanced, that the mere - fact of an area having been under sea does not imply that there has - been any subsidence or elevation of the land, and that consequently the - inference which has been drawn from these submerged areas as to changes - in physical geography may be in many cases not well founded.</p> - - <p>Sir Charles Lyell, in his “Principles,” publishes a map showing the - extent of surface in Europe which has been covered by the sea since - the earlier part of the Tertiary period. This map is intended to show - the extraordinary amount of subsidence and elevation of the land which - has taken place during that period. It is necessary for Sir Charles’s - theory of the cause of the glacial epoch that changes in the physical - geography of the globe to an enormous extent should have taken place - during a very recent period, in order to account for the great change - of climate which occurred at that epoch. But if the foregoing results - be anything like correct, it does not necessarily follow that there - must have been great changes in the physical geography of Europe, - simply because the sea covered those areas marked in the map, for this - may have been produced by oscillations of sea-level, and not by changes - in the land. In fact,<span class="pagenum" id="Page_393">393</span> the areas marked in Sir Charles’s map as having - been covered by the sea, are just those which would be covered were the - sea-level raised a few hundred feet. No doubt there were elevations and - subsidences in many of the areas marked in the map during the Tertiary - period, and to this cause a considerable amount of the submergence - might be due; but I have little doubt that by far the greater part - must be attributed to oscillations of sea-level. It is no objection - that the greater part of the shells and other organic remains found - in the marine deposits of those areas are not indicative of a cold - or glacial condition of climate, for, as we have seen, the greatest - submergence would probably have taken place either before the more - severe cold had set in or after it had to a great extent passed away. - That the submergence of those areas probably resulted from elevations - of sea-level rather than depressions of the land, is further evident - from the following considerations. If we suppose that the climate of - the glacial epoch was brought about mainly by changes in the physical - geography of the globe, we must assume that these great changes took - place, geologically speaking, at a very recent date. Then when we ask - what ground is there for assuming that any such change in the relations - of sea and land as is required actually took place, the submergence - of those areas is adduced as the proof. Did it follow as a physical - necessity that all submergence must be the result of subsidence of the - land, and not of elevations of the sea, there would be some force in - the reasons adduced. But such a conclusion by no means follows, and, - <i lang="la">à priori</i>, it is just as likely that the appearance of the ice was - the cause of the submergence as that the submergence was the cause - of the appearance of the ice. Again, a subsidence of the land to the - extent required would to a great extent have altered the configuration - of the country, and the main river systems of Europe; but there is no - evidence that any such change has taken place. All the main valleys - are well known to have existed prior to the glacial epoch, and our - rivers to have occupied the same channels then as they do now. In the - case of some of the smaller streams, it is true, a<span class="pagenum" id="Page_394">394</span> slight deviation - has resulted at some points from the filling up of their channels with - drift during the glacial epoch; but as a general rule all the principal - valleys and river systems are older than the glacial epoch. This, of - course, could not be the case if a subsidence of the land sufficiently - great to account for the submergence of the areas in question, or - changes in the physical geography of Europe necessary to produce a - glacial epoch, had actually taken place. The total absence of any - geological evidence for the existence of any change which could explain - either the submergence of the areas in question or the climate of the - glacial epoch, is strong evidence that the submergence of the glacial - epoch, as well as of the areas in question, was the result of a simple - oscillation of sea-level resulting from the displacement of the earth’s - centre of gravity by the transferrence of the ice-cap from the southern - to the northern hemisphere.</p> - - <p><em>Oscillations of Sea-level in relation to Distribution.</em>—The - oscillations of sea-level resulting from the displacement of the - earth’s centre of gravity help to throw new light on some obscure - points connected with the subject of the geographical distribution - of plants and animals. At the time when the ice was on the southern - hemisphere during the glacial epoch, and the northern hemisphere was - enjoying a warm and equable climate, the sea-level would be several - hundred feet lower than at present, the North Sea would probably be - dry land, and Great Britain and Ireland joined to the continent, thus - opening up a pathway from the continent to our island. As has been - shown in former chapters, during the inter-glacial periods the climate - would be much warmer and more equable than now, so that animals from - the south, such as the hippopotamus, hyæna, lion, <i>Elephas antiquus</i> - and <i>Rhinoceros megarhinus</i>, would migrate into this country, where - at present they could not live in consequence of the cold. We have - therefore an explanation, as was suggested on a former occasion,<a id="FNanchor_220" href="#Footnote_220" class="fnanchor">[220]</a> - of the fact that the bones of these animals are found mingled in the - same grave with those of the musk-ox, mammoth, reindeer, and other - animals which lived in this <span class="pagenum" id="Page_395">395</span>country during the cold periods of the - glacial epoch; the animals from the north would cross over into this - country upon the frozen sea during the cold periods, while those from - the south would find the English Channel dry land during the warm - periods.</p> - - <p>The same reasoning will hold equally true in reference to the old - and new world. The depth of Behring Straits is under 30 fathoms; - consequently a lowering of the sea-level of less than 200 feet would - connect Asia with America, and thus allow plants and animals, as Mr. - Darwin believes, to pass from the one continent to the other.<a id="FNanchor_221" href="#Footnote_221" class="fnanchor">[221]</a> - During this period, when Behring Straits would be dry land, Greenland - would be comparatively free from ice, and the arctic regions enjoying a - comparatively mild climate. In this case plants and animals belonging - to temperate regions could avail themselves of this passage, and thus - we can explain how plants belonging to temperate regions may have, - during the Miocene period, passed from the old to the new continent, - and <i lang="la">vice versâ</i>.</p> - - <p>As has already been noticed, during the time of the greatest extension - of the ice, the quantity of ice on the southern hemisphere might be - considerably greater than what exists on the entire globe at present. - In that case there might, in addition to the lowering of the sea-level - resulting from the displacement of the earth’s centre of gravity, be a - considerable lowering resulting from the draining of the ocean to form - the additional ice. This decrease and increase in the total quantity - of ice which we have considered would affect the level of the ocean as - much at the equator as at the poles; consequently during the glacial - epoch there might have been at the equator elevations and depressions - of sea-level to the extent of a few hundred feet.</p> - - <p><em>Extent of Submergence on the Hypothesis that the Earth is fluid in - the Interior.</em>—But we have been proceeding upon the supposition that - the earth is solid to its centre. If we assume, however, what is the - general opinion among geologists, that it <span class="pagenum" id="Page_396">396</span>consists of a fluid interior - surrounded by a thick and rigid crust or shell, then the extent of the - submergence resulting from the displacement of the centre of gravity - for a given thickness of ice must be much greater than I have estimated - it to be. This is evident, because, if the interior of the globe be in - a fluid state, it, in all probability, consists of materials differing - in density. The densest materials will be at the centre, and the least - dense at the outside or surface. Now the transferrence of an ice-cap - from the one pole to the other will not merely displace the ocean—the - fluid mass on the outside of the shell—but it will also displace the - heavier fluid materials in the interior of the shell. In other words, - the heavier materials will be attracted by the ice-cap more forcibly - than the lighter, consequently they will approach towards the cap to a - certain extent, sinking, as it were, into the lighter materials, and - displacing them towards the opposite pole. This displacement will of - course tend to shift the earth’s centre of gravity in the direction - of the ice-cap, because the heavier materials are shifted in this - direction, and the lighter materials in the opposite direction. This - process will perhaps be better understood from the following figures.</p> - - <div class="figcenter" id="i_396" > - <div class="caption">Fig. 8. - - Fig. 9.</div> - <img src="images/i_396.jpg" width="600" height="323" alt="" /> - <div class="caption">O. The Ocean. S. Solid Crust or Shell.<br /> - F, F<sup>1</sup>, F<sup>2</sup>, F<sup>3</sup>. The various concentric layers of the fluid interior. - The layers increase in density towards the centre.<br /> - I. The Ice-cap. C. Centre of gravity.<br /> - C<sup>1</sup>. The displaced centre of gravity.</div> - </div> - - <p><span class="pagenum" id="Page_397">397</span></p> - - <p>In Fig. 8, where there is no ice-cap, the centre of gravity of the - earth coincides with the centre of the concentric layers of the fluid - interior. In Fig. 9, where there is an ice-cap placed on one pole, the - concentric layer F<sup>1</sup> being denser than layer F, is attracted towards - the cap more forcibly than F, and consequently sinks to a certain depth - in F. Again, F<sup>2</sup> being denser than F<sup>1</sup>, it also sinks to a certain - extent in F<sup>1</sup>. And again F<sup>3</sup>, the mass at the centre, being denser than - F<sup>2</sup>, it also sinks in F<sup>2</sup>. All this being combined with the effects - of the ice-cap, and the displaced ocean outside the shell, the centre - of gravity of the entire globe will no longer be at C, but at C<sup>1</sup>, a - considerable distance nearer to the side of the shell on which the - cap rests than C, and also a considerable distance nearer than it - would have been had the interior of the globe been solid. There are - here three causes tending to shift the centre of gravity, (1) the - ice-cap, (2) the displaced ocean, and (3) the displaced materials in - the interior. Two of the three causes mutually react on each other in - such a way as to increase each other’s effect. Thus the more the ocean - is drawn in the direction of the ice-cap, the more effect it has in - drawing the heavier materials in the interior in the same direction; - and in turn the more the heavier materials in the interior are drawn - towards the cap, the greater is the displacement of the earth’s centre - of gravity, and of course, as a consequence, the greater is the - displacement of the ocean. It may be observed also that, other things - being equal, the thinner the solid crust or shell is, and the greater - the difference in the density of the fluid materials in the interior, - the greater will be the extent of the displacement of the ocean, - because the greater will be the displacement of the centre of gravity.</p> - - <p>It follows that if we knew (1) the extent of the general submergence of - the glacial epoch, and (2) the present amount of ice on the southern - hemisphere, we could determine whether or not the earth is fluid in the - interior.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XXV"> - <span class="pagenum" id="Page_398">398</span> - <h2> - CHAPTER XXV.<br /><br /> - <span class="small">THE INFLUENCE OF THE OBLIQUITY OF THE ECLIPTIC ON CLIMATE AND ON THE - LEVEL OF THE SEA.</span> - </h2> - </div> - <div class="subhead">The direct Effect of Change of Obliquity on Climate.—Mr. - Stockwell on the maximum Change of Obliquity.—How Obliquity - affects the Distribution of Heat over the Globe.—Increase of - Obliquity diminishes the Heat at the Equator and increases - that at the Poles.—Influence of Change of Obliquity on the - Level of the Sea.—When the Obliquity was last at its superior - Limit.—Probable Date of the 25-foot raised Beach.—Probable - Extent of Rise of Sea-level resulting from Increase of - Obliquity.—Lieutenant-Colonel Drayson’s and Mr. Belt’s - Theories.—Sir Charles Lyell on Influence of Obliquity.</div> - - <p><em>The direct Effect of Change in the Obliquity of the Ecliptic on - Climate.</em>—There is still another cause which, I feel convinced, must to - a very considerable extent have affected climate during past geological - ages. I refer to the change in the obliquity of the ecliptic. This - cause has long engaged the attention of geologists and physicists, - and the conclusion generally come to is that no great effect can be - attributed to it. After giving special attention to the matter, I have - been led to the very opposite conclusion. It is quite true, as has - been urged, that the changes in the obliquity of the ecliptic cannot - sensibly affect the climate of temperate regions; but it will produce - a slight change on the climate of tropical latitudes, and a very - considerable effect on that of the polar regions, especially at the - poles themselves. We shall now consider the matter briefly.</p> - - <p>It was found by Laplace that the obliquity of the ecliptic will - oscillate to the extent of 1° 22′ 34″ on each side of 23° 28′, the - obliquity in the year 1801.<a id="FNanchor_222" href="#Footnote_222" class="fnanchor">[222]</a> This point has lately been <span class="pagenum" id="Page_399">399</span>examined - by Mr. Stockwell, and the results at which he has arrived are almost - identical with those of Laplace. “The mean value of the obliquity,” he - says, “of both the apparent and fixed ecliptics to the equator is 23° - 17′ 17″. The limits of the obliquity of the apparent ecliptic to the - equator are 24° 35′ 58″ and 21° 58′ 36″; whence it follows that the - greatest and least declinations of the sun at the solstices can never - differ from each other to any greater extent than 2° 37′ 22″.”<a id="FNanchor_223" href="#Footnote_223" class="fnanchor">[223]</a></p> - - <p>This change will but slightly affect the climate of the temperate - regions, but it will exercise a very considerable influence on - the climate of the polar regions. According to Mr. Meech,<a id="FNanchor_224" href="#Footnote_224" class="fnanchor">[224]</a> if - 365·24 thermal days represent the present total annual quantity of - heat received at the equator from the sun, 151·59 thermal days will - represent the quantity received at the poles. Adopting his method of - calculation, it turns out that when the obliquity of the ecliptic is at - the maximum assigned by Laplace the quantity received at the equator - would be 363·51 thermal days, and at the poles 160·04 thermal days. The - equator would therefore receive 1·73 thermal days less heat, and the - poles 8·45 thermal days more heat than at present.</p> - - <p><span class="pagenum" id="Page_400">400</span></p> - - <div class="center small mb2">ANNUAL AMOUNT OF SUN’S HEAT.</div> - - <table summary="Annual amount of sun’s heat"> - <tbody> - <tr> - <th colspan="2" class="bt bl bb">Amount in 1801.<br />Obliquity 23° 28′.</th> - <th class="bt bl bb">Amount at maximum,<br />24° 50′ 34″.</th> - <th class="bt bl br bb">Difference.</th> - </tr> - <tr> - <td class="tdc bl"><div>Latitude.</div></td> - <td class="tdc bl"><div>Thermal days.</div></td> - <td class="tdc bl"><div>Thermal days.</div></td> - <td class="tdc bl br"><div>Thermal days.</div></td> - </tr> - <tr> - <td class="tdc bl"><div> 0</div></td> - <td class="tdc bl"><div>365·24</div></td> - <td class="tdc bl"><div>363·51</div></td> - <td class="tdc bl br"><div>−1·73</div></td> - </tr> - <tr> - <td class="tdc bl"><div>40</div></td> - <td class="tdc bl"><div>288·55</div></td> - <td class="tdc bl"><div>288·32</div></td> - <td class="tdc bl br"><div>−0·23</div></td> - </tr> - <tr> - <td class="tdc bl"><div>70</div></td> - <td class="tdc bl"><div>173·04</div></td> - <td class="tdc bl"><div>179·14</div></td> - <td class="tdc bl br"><div>+6·10</div></td> - </tr> - <tr> - <td class="tdc bl"><div>80</div></td> - <td class="tdc bl"><div>156·63</div></td> - <td class="tdc bl"><div>164·63</div></td> - <td class="tdc bl br"><div>+8·00</div></td> - </tr> - <tr> - <td class="tdc bl bb"><div>90</div></td> - <td class="tdc bl bb"><div>151·59</div></td> - <td class="tdc bl bb"><div>160·04</div></td> - <td class="tdc bl br bb"><div>+8·45</div></td> - </tr> - </tbody> - </table> - - <p>When the obliquity was at a maximum, the poles would therefore be - receiving 19 rays for every 18 they are receiving at present. The - poles would then be receiving nearly as much heat as latitude 76° is - receiving at present.</p> - - <p>The increase of obliquity would not sensibly affect the polar winter. - It is true that it would slightly increase the breadth of the - frigid zone, but the length of the winter at the poles would remain - unaffected. After the sun disappears below the horizon his rays are - completely cut off, so that a further descent of 1° 22′ 34″ would make - no material difference in the climate. In the temperate regions, the - sun’s altitude at the winter solstice would be 1° 22′ 34″ less than - at present. This would slightly increase the cold of winter in those - regions. But the increase in the amount of heat received by the polar - regions would materially affect the condition of the polar summer. - What, then, is the rise of temperature at the poles which would result - from the increase of 8·45 thermal days in the total amount received - from the sun?</p> - - <p>An increase of 8·45 thermal days, or 1/18th of the total quantity - received from the sun, according to the mode of calculation adopted in - Chap. II. would produce, all other things being equal, a rise in the - mean annual temperature equal to 14° or 15°.</p> - - <p>According to Professor Dove<a id="FNanchor_225" href="#Footnote_225" class="fnanchor">[225]</a> there is a difference of 7°·6 - between the mean annual temperature of latitude 76° and the <span class="pagenum" id="Page_401">401</span>pole; - the temperature of the former being 9°·8, and that of the latter - 2°·2. Since it follows that when the obliquity of the ecliptic is - at a maximum the poles would receive about as much heat per annum - as latitude 76° receives at present, it may be supposed that the - temperature of the poles at that period ought to be no higher than - that of latitude 76° at the present time. A little consideration will, - however, show that this by no means follows. Professor Dove’s Tables - represent correctly the mean annual temperature corresponding to every - tenth degree of latitude from the equator to the pole. But it must be - observed that the rate at which the temperature diminishes from the - equator to the pole is not proportionate to the decrease in the total - quantity of heat received from the sun as we pass from the equator to - the pole. Were the mean annual temperature of the various latitudes - proportionate to the amount of direct heat received, the equator - would be much warmer than it actually is at present, and the poles - much colder. The reason of this, as has been shown in <a href="#CHAPTER_II">Chapter II.</a>, is - perfectly obvious. There is a constant transferrence of <em>heat</em> from - the equator to the poles, and of <em>cold</em> from the poles to the equator. - The warm water of the equator is constantly flowing towards the poles, - and the cold water at the poles is constantly flowing to the equator. - The same is the case in regard to the aërial currents. Consequently - a great portion of the direct heat of the sun goes, not to raise the - temperature of the equator, but to heat the poles. And, on the other - hand, the cold materials at the poles are transferred to the equator, - and thus lower the temperature of that part of the globe to a great - extent. The present difference of temperature between lat. 76° and the - pole, determined according to the rate at which the temperature is - found to diminish between the equator and the pole, amounts to only - about 7° or 8°. But were there no mutual transferrence of warm and - cold materials between the equatorial and polar regions, and were the - temperature of each latitude to depend solely upon the direct rays of - the sun, the difference would far exceed that amount.</p> - - <p><span class="pagenum" id="Page_402">402</span></p> - - <p>Now, when the obliquity of the ecliptic was at its superior limit, and - the poles receiving about 1/18th more direct heat from the sun than - at present, the increase of temperature due to this increase of heat - would be far more than 7° or 8. It would probably be nearly double that - amount.</p> - - <p>“We may, therefore, conclude that when the obliquity of the ecliptic - was at a maximum, and the poles were receiving 1/18th more heat than - at present, the temperature of the poles ought to have been about 14° - or 15° warmer than at the present day, <em>provided, of course, that - this extra heat was employed wholly in raising the temperature</em>. Were - the polar regions free from snow and ice, the greater portion of the - extra heat would go to raise the temperature. But as those regions - are covered with snow and ice, the extra heat would have no effect in - raising the temperature, but would simply melt the snow and ice. The - ice-covered surface upon which the rays fell could never rise above - 32°. At the period under consideration, the total annual quantity of - ice melted at the poles would be 1/18th more than at present.</p> - - <p>The general effect which the change in the obliquity of the ecliptic - would have upon the climate of the polar regions when combined with the - effects resulting from the eccentricity of the earth’s orbit, would be - this:—When the eccentricity was at a very high value, the hemisphere - whose winter occurred in the aphelion (for physical reasons, which have - already been discussed)<a id="FNanchor_226" href="#Footnote_226" class="fnanchor">[226]</a> would be under a condition of glaciation, - while the other hemisphere, having its winter in perihelion, would be - enjoying a warm and equable climate. When the obliquity of the ecliptic - was at a maximum, and 1/18th more heat falling at the poles than at - present, the effect would be to modify to a great extent the rigour - of the glaciation in the polar zone of the hemisphere under a glacial - condition, and, on the other hand, to produce a more rapid melting - of the ice on the other hemisphere enjoying the equable climate. The - effects of eccentricity and obliquity thus combined would probably - completely <span class="pagenum" id="Page_403">403</span>remove the polar ice-cap from off the latter hemisphere, - and forest trees might then grow at the pole. Again, when the obliquity - was at its minimum condition and less heat reaching the poles than at - present, the glaciation of the former hemisphere would be increased and - the warmth of the latter diminished.</p> - - <p><em>The Influence of Change in the Obliquity of the Ecliptic on the - Level of the Sea.</em>—One very remarkable effect which seems to result - indirectly from a variation of the obliquity under certain conditions, - is an influence on the level of the sea. As this probably may have had - something to do with those recent changes of sea-level with which the - history of the submarine forests and raised beaches have made us all so - familiar, it may be of interest to enter at some length into this part - of this subject.</p> - - <p>It appears almost certain that at the time when the northern winter - solstice was in the aphelion last, a rise of the sea on the northern - hemisphere to a considerable number of feet must have taken place from - the combined effect of eccentricity and obliquity. About 11,700 years - ago, the northern winter solstice was in the aphelion. The eccentricity - at that time was ·0187, being somewhat greater than it is now; but the - winters occurring in aphelion instead of, as now, in perihelion, they - would on that account be probably 10° or 15° colder than they are at - the present day. It is probable, also, for reasons stated in a previous - chapter, that the Gulf-stream at that time would be considerably less - than now. This would tend to lower the temperature to a still greater - extent. As snow instead of rain must have fallen during winter to a - greater extent than at present, this no doubt must have produced a - slight increase in the quantity of ice on the northern hemisphere had - no other cause come into operation. But the condition of things, we - have every reason to believe, must have been affected by the greater - obliquity of the ecliptic at that period. We have no formula, except, - perhaps, that given by Mr. Stockwell, from which to determine with - perfect accuracy the extent of the obliquity at a period so remote as - the one under consideration. If we adopt<span class="pagenum" id="Page_404">404</span> the formula given by Struve - and Peters, which agrees pretty nearly with that obtained from Mr. - Stockwell’s formula, we have the obliquity at a maximum about the time - that the solstice-point was in the aphelion. The formula given by - Leverrier places the maximum somewhat later. At all events, we cannot - be far from the truth in assuming that at the time the northern winter - solstice was in the aphelion, the obliquity of the ecliptic would be - about a maximum, and that since then it has been gradually diminishing. - It is evident, then, that the annual amount of heat received by the - arctic regions, and especially about the pole, would be considerably - greater than at present. And as the heat received on those regions is - chiefly employed in melting the ice, it is probable that the extra - amount of ice which would then be melted in the arctic regions would - prevent that slight increase of ice which would otherwise have resulted - in consequence of the winter occurring in the aphelion. The winters at - that period would be colder than they are at present, but the total - quantity of ice on the northern hemisphere would not probably be - greater.</p> - - <p>Let us now turn to the southern hemisphere. As the southern winter - would then occur in the perihelion, this would tend to produce a slight - decrease in the quantity of ice on the southern hemisphere. But on this - hemisphere the effects of eccentricity would not, as on the northern - hemisphere, be compensated by those of obliquity; for both causes would - here tend to produce the same effect; namely, a melting of the ice in - the antarctic regions.</p> - - <p>It is probable that at this time the quantity of warm water flowing - from the equatorial regions into the Southern Ocean would be much - greater than at present. This would tend to raise the temperature of - the air of the antarctic regions, and thus assist in melting the ice. - These causes, combined with the great increase of heat resulting from - the change of obliquity, would tend to diminish to a considerable - extent the quantity of ice on the southern hemisphere. I think we may - assume that the slight increase of eccentricity at that period,<span class="pagenum" id="Page_405">405</span> the - occurrence of the southern winter in perihelion, and the extra quantity - of warm water flowing from the equatorial to the antarctic regions, - would produce an effect on the south polar ice-cap equal to that - produced by the increase in the obliquity of the ecliptic. It would, - therefore, follow that for every eighteen pounds of ice melted annually - at present at the south pole twenty pounds would then be melted.</p> - - <p>Let us now consider the effect that this condition of things would - have upon the level of the sea. It would evidently tend to produce an - elevation of the sea-level on the northern hemisphere in two ways. 1st. - The addition to the sea occasioned by the melting of the ice from off - the antarctic land would tend to raise the general level of the sea. - 2ndly. The removal of the ice would also tend to shift the earth’s - centre of gravity to the north of its present position—and as the sea - must shift along with the centre, a rise of the sea on the northern - hemisphere would necessarily take place.</p> - - <p>The question naturally suggests itself, might not the last rise of the - sea, relative to the land, have resulted from this cause? We know that - during the period of the 25-foot beach, the time when the estuarine - mud, which now forms the rich soil of the Carses of the Forth and - Tay, was deposited, the sea, in relation to the land, stood at least - 20 or 30 feet higher than at present. But immediately prior to this - period, we have the age of the submarine forests and peat-beds, when - the sea relative to the land stood lower than it does now. We know - also that these changes of level were not mere local affairs. There - seems every reason to believe that our Carse clay, as Mr. Fisher - states, is the equivalent of the marine mud, with <i>Scrobicularia</i>, - which covers the submarine forests of England.<a id="FNanchor_227" href="#Footnote_227" class="fnanchor">[227]</a> And on the other - hand, those submarine forests are not confined to one locality. “They - may be traced,” says Mr. Jamieson, “round the whole of Britain and - Ireland, from Orkney to Cornwall, from Mayo to the shores of Fife, and - even, it would seem, along a great part of the western sea-board of - Europe, <span class="pagenum" id="Page_406">406</span>as if they bore witness to a period of widespread elevation, - when Ireland and Britain, with all its numerous islands, formed one - mass of dry land, united to the continent, and stretching out into the - Atlantic.”<a id="FNanchor_228" href="#Footnote_228" class="fnanchor">[228]</a> “These submarine forests”“ remarks De la Beche, also, - “are to be found under the same general condition from the shores of - Scandinavia to those of Spain and Portugal, and around the British - islands.”<a id="FNanchor_229" href="#Footnote_229" class="fnanchor">[229]</a> Those buried forests are not confined to Europe, but - are found in the valley of the Mississippi and in Nova Scotia, and - other parts of North America. And again, the strata which underlie - those forests and peat-beds bear witness to the fact of a previous - elevation of the sea-level. In short, we have evidence of a number of - oscillations of sea-level during post-tertiary times.<a id="FNanchor_230" href="#Footnote_230" class="fnanchor">[230]</a></p> - - <p>Had there been only one rise of the land relative to the sea-level, or - one depression, it might quite reasonably, as already remarked, have - been attributed to an upheaval or a sinking of the ground, occasioned - by some volcanic, chemical, or other agency. But certainly those - repeated oscillations of sea-level, extending as they do over so wide - an area, look more like a rising and sinking of the sea than of the - land. But, be this as it may, since it is now established, I presume, - beyond controversy, that the old notion that the general level of the - sea remains permanent, and that the changes must be all attributed to - the land is wholly incorrect, and that the sea, as well as the land, - is subject to changes of level, it is certainly quite legitimate to - consider whether the last elevation of the sea-level relatively to the - land may not have resulted from the rising of the sea rather than from - the sinking of the land, in short, whether it may not be attributed - to the cause we are now considering. The fact that those raised - beaches and terraces are found at so many different heights, and also - so discontinuously <span class="pagenum" id="Page_407">407</span>along our coasts, might be urged as an objection - to the opinion that they were due to changes in the level of the sea - itself. Space will not permit me to enter upon the discussion of this - point at present; but it may be stated that this objection is more - apparent than real. It by no means follows that beaches of the same - age must be at the same level. This has been shown very clearly by Mr. - W. Pengelly in a paper on “Raised Beaches,” read before the British - Association at Nottingham, 1866.</p> - - <p>We have, as I think, evidence amounting to almost absolute certainty - that 11,700 years ago the general sea-level on the northern hemisphere - must have been higher than at present. And in order to determine the - question of the 25-foot beach, we have merely to consider whether a - rise to something like this extent probably took place at the period in - question. We have at present no means of determining the exact extent - of the rise which must have taken place at that period, for we cannot - tell what quantity of ice was then melted off the antarctic regions. - But we have the means of making a very rough estimate, which, at least, - may enable us to determine whether a rise of some 20 or 30 feet may not - possibly have taken place.</p> - - <p>If we assume that the southern ice-cap extends on an average down - to lat. 70°, we shall have an area equal to 1/33·163 of the entire - surface of the globe. The proportion of land to that of water, taking - into account the antarctic continent, is as 526 to 1272. The southern - ice-cap will therefore be equal to 1/23·46 of the area covered by - water. The density of ice to that of water being taken at ·92 to 1, - it follows that 25 feet 6 inches of ice melted from off the face of - the antarctic continent would raise the level of the ocean one foot. - If 470 feet were melted off—and this is by no means an extravagant - supposition, when we reflect that for every 18 pounds of ice presently - melted an additional pound or two pounds, or perhaps more, would then - be melted, and that for many ages in succession—the water thus produced - from the melted ice would raise the level of the sea 18 feet 5 inches. - The removal of the 470 feet of solid ice<span class="pagenum" id="Page_408">408</span>— which must be but a very - small fraction of the total quantity of ice lying upon the antarctic - continent—would shift the earth’s centre of gravity about 7 feet to the - north of its present position. The shifting of the centre of gravity - would cause the sea to sink on the southern hemisphere and rise on the - northern. And the quantity of water thus transferred from the southern - hemisphere to the northern would carry the centre of gravity about one - foot further, and thus give a total displacement of the centre to the - extent of about 8 feet. The sea would therefore rise about 8 feet at - the North Pole, and in the latitude of Edinburgh about 6 feet 7 inches. - This, added to the rise of 18 feet 5 inches, occasioned by the melting - of the ice, would give 25 feet as the total rise in the latitude of - Scotland 11,700 years ago.</p> - - <p>Each square foot of surface at the poles 11,700 years ago would be - receiving 18,223,100 foot-pounds more of heat annually than at present. - If we deduct 22 per cent. as the amount absorbed in passing through the - atmosphere, we have 14,214,000 foot-pounds. This would be sufficient - to melt 2·26 feet of ice. But if 50, instead of 22, per cent. were cut - off, 1·45 cubic feet would be melted. In this case the 470 feet of ice - would be melted, independently of the effects of eccentricity, in about - 320 years. And supposing that only one-fourth part of the extra heat - reached the ground, 470 feet of ice would be removed in about 640 years.</p> - - <p>As to the exact time that the obliquity was at a maximum, previous - to that of 11,700 years ago, our uncertainty is still greater. If we - are permitted to assume that the ecliptic passes from its maximum to - its minimum state and back to its maximum again with anything like - uniformity, at the rate assigned by Leverrier and others, the obliquity - would not be far from a maximum about 60,000 years ago. Taking the - rate of precession at 50″·21129, and assuming it to be uniform—which - it probably is not—the winter solstice would be in the aphelion about - 61,300 years ago.<a id="FNanchor_231" href="#Footnote_231" class="fnanchor">[231]</a> In short, it seems not at all improbable that - <span class="pagenum" id="Page_409">409</span>at the time the solstice-point was in the aphelion, the obliquity of - the ecliptic would not be far from its maximum state. But at that time - the value of the eccentricity was 0·023, instead of 0·0187, its value - at the last period. Consequently the rise of the sea would probably - be somewhat greater than it was 11,700 years ago. Might not this be - the period of the 40-foot beach? In this case 11,000 or 12,000 years - would be the age of the 25-foot beach, and 60,000 years the age of the - 40-foot beach.</p> - - <p>About 22,000 years ago, the winter solstice was in the perihelion, and - as the eccentricity was then somewhat greater than it is at present, - the winters would be a little warmer and the climate more equable than - it is at the present day. This perhaps might be the period of the - submarine forests and lower peat-beds which underlie the Carse clays, - <i>Scrobicularia</i> mud, and other deposits belonging to the age of the - 25-foot beach. At any rate, it is perfectly certain that a condition - of climate at this period prevailed exceedingly favourable to the - growth of peat. It follows also that at this time, owing to a greater - accumulation of ice on the southern hemisphere, the sea-level would be - a few feet lower than at present, and that forests and peat may have - then grown on places which are now under the sea-level.</p> - - <p>For a few thousand years before and after 11,700 years ago, when the - winter solstice was evidently not far from the aphelion, and the sea - standing considerably above its present level, would probably, as we - have already stated, be the time when the Carse clays and other recent - deposits lying above the present level of the river were formed. - And it is also a singular fact that the condition of things at that - period must <span class="pagenum" id="Page_410">410</span>have been exceedingly favourable to the formation of - such estuarine deposits; for at that time the winter temperature of - our island, as has been already shown, would be considerably lower - than at present, and, consequently, during that season, snow, to a - much larger extent than now, would fall instead of rain. The melting - of the winter’s accumulation of snow on the approach of summer would - necessarily produce great floods, similar to what occur in the northern - parts of Asia and America at the present day from this very same - cause. The loose upper soil would be carried down by those floods and - deposited in the estuaries of our rivers.</p> - - <p>The foregoing is a rough and imperfect sketch of the history of the - climate and the physical conditions of our globe for the past 60,000 - years, in so far as physical and cosmical considerations seem to afford - us information on the subject, and its striking agreement with that - derived from geological sources is an additional evidence in favour - of the opinion that geological and cosmical phenomena are physically - related by a bond of causation.</p> - - <p><em>Lieutenant-Colonel Drayson’s Theory of the Cause of the Glacial - Epoch.</em>—In a paper read before the Geological Society by - Lieutenant-Colonel Drayson, R.A., on the 22nd February, 1871,<a id="FNanchor_232" href="#Footnote_232" class="fnanchor">[232]</a> that - author states, that after calculating from the recorded positions of - the pole of the heavens during the last 2,000 years, he finds the pole - of the ecliptic is not the centre of the circle traced by the pole of - the heavens. The pole of the heavens, he considers, describes a circle - round a point 6° distant from the pole of the ecliptic and 29° 25′ 47″ - from the pole of the heavens, and that about 13,700 years b.c. the - angular distance of the two poles was 35° 25′ 47″. This would bring - the Arctic Circle down to latitude 54° 34′ 13″ N. I fear that this is - a conclusion that will not be generally accepted by those familiar with - celestial mechanics. But, be this as it may, my present object is not - to discuss the astronomical part of Colonel Drayson’s theory, but <span class="pagenum" id="Page_411">411</span>to - consider whether the conclusions which he deduces from his theory in - regard to the cause of the glacial epoch be legitimate or not. Assuming - for argument’s sake that the obliquity of the ecliptic can possibly - reach to 35° or 36°, so as to bring the Arctic Circle down to the - centre of England, would this account for the glacial epoch? Colonel - Drayson concludes that the shifting of the Arctic Circle down to the - latitude of England would induce here a condition of climate similar - to that which obtains in arctic regions. This seems to be the radical - error of the theory. It is perfectly true that were the Arctic Circle - brought down to latitude 54° 35′ part of our island would be in the - arctic regions, but it does not on that account follow that our island - would be subjected to an arctic climate.</p> - - <p>The polar regions owe their cold not to the obliquity of the ecliptic, - but to their distance from the equator. Indeed were it not for - obliquity those regions would be much colder than they really are, - and an increase of obliquity, instead of increasing their cold, would - really make them warmer. The general effect of obliquity, as we - have seen, is to diminish the amount of heat received in equatorial - and tropical regions, and to increase it in the polar and temperate - regions. The greater the obliquity, and, consequently, the farther - the sun recedes from the equator, the smaller is the quantity of heat - received by equatorial regions, and the greater the amount bestowed on - polar and temperate regions. If, for example, we represent the present - amount of heat received from the sun at the equator on a given surface - at 100 parts, 42·47 parts will then represent the amount received at - the poles on the same given surface. But were the obliquity increased - to 35° the amount received at the equator would be reduced to 94·93 - parts, and that at the poles increased to 59·81; being an increase at - the poles of nearly one half. At latitude 60° the present quantity - is equal to 57 parts; but about 63 parts would be received were the - obliquity increased to 35°. It therefore follows that although the - Arctic Circle were brought down to the latitude of London so<span class="pagenum" id="Page_412">412</span> that the - British islands would become a part of the arctic regions, the mean - temperature of these islands would not be lowered, but the reverse. - The winters would no doubt be colder than they are at present, but the - cold of winter would be far more than compensated for by the heat of - summer. It is not a fair representation of the state of things, merely - to say that an increase of obliquity tends to make the winters colder - and the summers hotter, for it affects the summer heat far more than - it does the winter cold. And the greater the obliquity the more does - the increase of heat during summer exceed the decrease during winter. - This is obvious because the greater the obliquity the greater the total - annual amount of heat received.</p> - - <p>If an increase of obliquity tended to produce an increase of ice in - temperate and polar regions, and thus to lead to a glacial epoch, then - the greater the obliquity the greater would be the tendency to produce - such an effect. Conceive, then, the obliquity to go on increasing until - it ultimately reached its absolute limit, 90°, and the earth’s axis to - coincide with the plane of the ecliptic. The Arctic Circle would then - extend to the equator. Would this produce a glacial epoch? Certainly - not. A square foot of surface at the poles would then be receiving - as much heat per annum as a square foot at the equator at present, - supposing the sun remained on the equator during the entire year. Less - heat, however, would be reaching the equatorial regions than now. At - present, as we have just seen, the annual quantity of heat received at - either pole is to that received at the equator as 42·47 to 100; but at - the period under consideration the poles would be actually obtaining - one-half more heat than the equator. The amount received per square - foot at the poles, to that received per square foot at the equator, - would be in the ratio of half the circumference of a circle to its - diameter, or as 1·5708 to 1. But merely to say that the poles would be - receiving more heat per annum than the equator is at present, does not - convey a correct idea of the excessive heat which the poles would then - have to endure; for<span class="pagenum" id="Page_413">413</span> it must be borne in mind that the heat reaching - the equator is spread over the whole year, whereas the poles would get - their total amount during the six months of their summer. Consequently, - for six months in the year the poles would be obtaining far more than - double the quantity of heat received at present by the equator during - the same length of time, and more than three times the quantity then - received by the equator. The amount reaching the pole during the six - months to that reaching the equator would be as 3·1416 to 1.</p> - - <p>At the equator twelve hours’ darkness alternates with twelve hours’ - sunshine, and this prevents the temperature from rising excessively - high; but at the poles it would be continuous sunshine for six months - without the ground having an opportunity of cooling for a single - hour. At the summer solstice, when the sun would be in the zenith of - the pole, the amount of heat received there every twenty-four hours - would actually be nearly three-and-a-quarter times greater than that - presently received at the equator. Now what holds true with regard to - the poles would hold equally true, though to a lesser extent, of polar - and temperate regions. We can form but a very inadequate idea of the - condition of things which would result from such an enormous increase - of heat. Nothing living on the face of the globe could exist in polar - regions under so fearful a temperature as would then prevail during - summer months. How absurd would it be to suppose that this condition - of things would tend to produce a glacial epoch! Not only would every - particle of ice in polar regions be dissipated, but the very seas - around the pole would be, for several months in the year, at the - boiling point.</p> - - <p>If it could be shown from <em>physical principles</em>—which, to say the - least, is highly improbable—that the obliquity of the ecliptic could - ever have been as great as 35°, it would to a very considerable - extent account for the comparative absence of ice in Greenland and - other regions in high latitudes, such as we know was the case during - the Carboniferous, Miocene, and other periods. But although a great - increase of obliquity<span class="pagenum" id="Page_414">414</span> might cause a melting of the ice, yet it could - not produce that mild condition of climate which we know prevailed in - high latitudes during those periods; while no increase of obliquity, - however great, could in any way tend to produce a glacial epoch.</p> - - <p>Colonel Drayson, however, seems to admit that this great increase of - obliquity would make our summers much warmer than they are at present. - How, then, according to his theory, is the glacial epoch accounted for? - The following is the author’s explanation as stated in his own words:—</p> - - <p>“At the date 13,700 <span class="smcap">b.c.</span> the same conditions appear to have - prevailed down to about 54° of latitude during winter as regards the - sun being only a few degrees above the horizon. We are, then, warranted - in concluding that the same climate prevailed down to 54° of latitude - as now exists in winter down to 67° of latitude.</p> - - <p>“Thus in the greater part of England and Wales, and in the whole of - Scotland, icebergs of large size would be <em>formed each winter</em>; every - river and stream would be frozen and blocked with ice, the whole - country would be covered with a mantle of snow and ice, and those - creatures which could neither migrate nor endure the cold of an arctic - climate would be exterminated.”—“The Last Glacial Epoch,” p. 146.</p> - - <p>“At the summer solstice the midday altitude of the sun for the latitude - 54° would be about 71½°, an altitude equal to that which the sun - now attains in the south of Italy, the south of Spain, and in all - localities having a latitude of about 40°.”</p> - - <p>“There would, however, be this singular difference from present - conditions, that in latitude 54° the sun at the period of the summer - solstice would remain the whole twenty-four hours above the horizon; - a fact which would give extreme heat to those very regions which, six - months previously, had been subjected to an arctic cold. Not only - would this greatly increased heat prevail in the latitude of 54°, but - the sun’s altitude would be 12° greater at midday in midsummer, and - also 12° greater at midnight in high northern latitudes, than<span class="pagenum" id="Page_415">415</span> it - ever attains now; consequently the heat would be far greater than at - present, and high northern regions, even around the pole itself, would - be subjected to a heat during summer far greater than any which now - ever exists in those localities. The natural consequence would be, that - the icebergs and ice which had during the severe winter accumulated in - high latitudes would be rapidly thawed by this heat” (p. 148).</p> - - <p>“Each winter the whole northern and southern hemispheres would be one - mass of ice; each summer nearly the whole of the ice of each hemisphere - would be melted and dispersed” (p. 150).</p> - - <p>According to this theory, not only is the whole country covered each - winter with a continuous mass of ice, but large icebergs are formed - during that short season, and when the summer heat sets in all is - melted away. Here we have a misapprehension not only as to the actual - condition of things during the glacial epoch, but even as to the way - in which icebergs and land-ice are formed. Icebergs are formed from - land-ice, but land-ice is not formed during a single winter, much - less a mass of sufficient thickness to produce icebergs. Land-ice of - this thickness requires the accumulated snows of centuries for its - production. All that we could really have, according to this theory, - would be a thick covering of snow during winter, which would entirely - disappear during summer, so that there could be no land-ice.</p> - - <p><em>Mr. Thomas Belt’s Theory.</em>—The theory that the glacial epoch resulted - from a great increase in the obliquity of the ecliptic has recently - been advocated by Mr. Thomas Belt.<a id="FNanchor_233" href="#Footnote_233" class="fnanchor">[233]</a> His conceptions on the subject, - however, appear to me to be even more irreconcilable with physics than - those we have been considering. Lieutenant-Colonel Drayson admits that - the increase of heat to polar regions resulting from the great increase - of obliquity would dissipate the ice there, but Mr. Belt does not even - admit that an increase of obliquity would bring with it an increase of - heat, far less that it would melt the polar ice. <span class="pagenum" id="Page_416">416</span>On the contrary, he - maintains that the tendency of obliquity is to increase the rigour of - polar climate, and that this is the reason “that now around the poles - some lands are being glaciated, for excepting for that obliquity snow - and ice would not accumulate, excepting on mountain chains.” “Thus,” - he says, “there exist glacial conditions at present around the poles, - due primarily to the obliquity of the ecliptic.” And he also maintains - that if there were no obliquity and the earth’s axis were perpendicular - to the plane of its orbit, an eternal “spring would reign around the - arctic circle,” and that “under such circumstances the piling up of - snow, or even its production at the sea-level, would be impossible, - excepting perhaps in the immediate neighbourhood of the poles, where - the rays of the sun would have but little heating power from its small - altitude.”</p> - - <p>Mr. Belt has apparently been led to these strange conclusions by the - following singular misapprehension of the effects of obliquity on - the distribution of the sun’s heat over the globe. “The obliquity of - the ecliptic,” he remarks, “<em>does not affect the mean amount of heat - received at any one point from the sun</em>, but it causes the heat and the - cold to predominate at different seasons of the year.”</p> - - <p>It is not necessary to dwell further on the absurdity of the - supposition that an increase of obliquity can possibly account for the - glacial epoch, but we may in a few words consider whether a decrease - of obliquity would mitigate the climate and remove the snow from - polar regions. Supposing obliquity to disappear and the earth’s axis - to become perpendicular to the plane of its orbit, it is perfectly - true that day and night would be equal all over the globe, but then - the quantity of heat received by the polar regions would be far less - than at present. It is well known that at present at the equinoxes, - when day and night are equal, snow and not rain prevails in the arctic - regions, and can we suppose it could be otherwise in the case under - consideration? How, we may well ask, could these regions, deprived of - their summer, get rid of their snow and ice?</p> - - <p><span class="pagenum" id="Page_417">417</span></p> - - <p>But even supposing it could be shown that a change in the obliquity of - the ecliptic to the extent assumed by Mr. Belt and Lieutenant-Colonel - Drayson would produce a glacial epoch, still the assumption of such a - change is one which physical astronomy will not permit. Mr. Belt does - not appear to dispute the accuracy of the methods by which it is proved - that the variations of obliquity are confined within narrow limits; but - he maintains that physical astronomers, in making their calculations - have left out of account some circumstances which materially affect the - problem. These, according to Mr. Belt, are the following:—(1) Upheavals - and subsidences of the land which may have taken place in past ages. - (2) The unequal distribution of sea and land on the globe. (3) The fact - that the equatorial protuberance is not a regular one, “but approaches - in a general outline to an ellipse, of which the greater diameter is - two miles longer than the other.” (4) The heaping up of ice around the - poles during the glacial period.</p> - - <p>We may briefly consider whether any or all of these can sensibly affect - the question at issue. In reference to the last-mentioned element, it - is no doubt true that if an immense quantity of water were removed - from the ocean and placed around the poles in the form of ice it would - affect the obliquity of the ecliptic; but this is an element of change - which is not available to Mr. Belt, because according to his theory - the piling up of the ice is an effect which results from the change of - obliquity.</p> - - <p>In reference to the difference of two miles in the equatorial diameters - of the earth, the fact must be borne in mind that the longer diameter - passes through nearly the centre of the great depression of the Pacific - Ocean,<a id="FNanchor_234" href="#Footnote_234" class="fnanchor">[234]</a> whereas the shorter diameter passes through the opposite - continents of Asia and America. Now, when we take into consideration - the fact that these continents are not only two-and-a-half times denser - than the ocean, but have a mean elevation of about 1,000 feet above - the sea-level, it becomes perfectly obvious that the earth’s mass must - <span class="pagenum" id="Page_418">418</span>be pretty evenly distributed around its axis of rotation, and that - therefore the difference in the equatorial diameters can exercise no - appreciable effect on the change of obliquity. It follows also that the - present arrangement of sea and land is the best that could be chosen to - prevent disturbance of motion.</p> - - <p>That there ever were upheavals and depressions of the land of so - enormous a magnitude as to lead to a change of obliquity to the extent - assumed by Lieutenant-Colonel Drayson and Mr. Belt is what, I presume, - few geologists would be willing to admit. Suppose the great table-land - of Thibet, with the Himalaya Mountains, were to sink under the sea, - it would hardly produce any sensible effect on the obliquity of the - ecliptic. Nay more; supposing that all the land in the globe were sunk - under the sea-level, or the ocean beds converted into dry land, still - this would not materially affect obliquity. The reason is very obvious. - The equatorial bulge is so immense that those upheavals and depressions - would not to any great extent alter the oblate form of the earth. The - only cause which could produce any sensible effect on obliquity, as has - already been noticed, would be the removal of the water of the ocean - and the piling of it up in the form of ice around the poles; but this - is a cause which is not available to Mr. Belt.</p> - - <p><em>Sir Charles Lyell’s Theory.</em>—I am also unable to agree with Sir - Charles Lyell’s conclusions in reference to the influence of the - obliquity of the ecliptic on climate. Sir Charles says, “It may be - remarked that if the obliquity of the ecliptic could ever be diminished - to the extent of four degrees below its present inclination, such a - deviation would be of geological interest, in so far as it would cause - the sun’s light to be disseminated over a broader zone inside of the - arctic and antarctic circles. Indeed, if the date of its occurrence in - past time could be ascertained, this greater spread of the solar rays, - implying a shortening of the polar night, might help in some slight - degree to account for a vegetation such as now characterizes lower - latitudes, having had in the Miocene and Carboniferous periods a much - wider range towards the pole.”<a id="FNanchor_235" href="#Footnote_235" class="fnanchor">[235]</a></p> - - <p><span class="pagenum" id="Page_419">419</span></p> - - <p>The effects, as we have seen, would be directly the reverse of what is - here stated, viz., the more the obliquity was diminished the <em>less</em> - would the sun’s rays spread over the arctic and antarctic regions, and - conversely the more the obliquity was increased the <em>greater</em> would - be the amount of heat spread over polar latitudes. The farther the - sun recedes from the equator, the greater becomes the amount of heat - diffused over the polar regions; and if the obliquity could possibly - attain its absolute limit (90°), it is obvious that the poles would - then be receiving more heat than the equator is now.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XXVI"> - <span class="pagenum" id="Page_420">420</span> - <h2> - CHAPTER XXVI.<br /><br /> - <span class="small">COAL AN INTER-GLACIAL FORMATION.</span> - </h2> - </div> - <div class="subhead">Climate of Coal Period Inter-glacial in Character.—Coal Plants - indicate an Equable, not a Tropical Climate.—Conditions - necessary for Preservation of Coal Plants.—Oscillations of - Sea-level necessarily implied.—Why our Coal-fields contain more - than One Coal-seam.—Time required to form a Bed of Coal.—Why - Coal Strata contain so little evidence of Ice-action.—Land Flat - during Coal Period.—Leading Idea of the Theory.—Carboniferous Limestones.</div> - - <p><em>An Inter-glacial Climate the one best suited for the Growth of the - Coal Plants.</em>—No assertion, perhaps, could appear more improbable, - or is more opposed to all hitherto received theories, than the one - that the plants which form our coal grew during a glacial epoch. But, - nevertheless, if the theory of secular changes of climate, discussed - in the foregoing chapters, be correct, we have in warm inter-glacial - periods (as was pointed out several years ago)<a id="FNanchor_236" href="#Footnote_236" class="fnanchor">[236]</a> the very condition - of climate best suited for the growth of those kinds of trees and - vegetation of which our coal is composed. It is the generally received - opinion among both geologists and botanists that the flora of the Coal - period does not indicate the existence of a tropical, but a moist, - equable, and temperate climate. “It seems to have become,” says Sir - Charles Lyell, “a more and more received opinion that the coal plants - do not on the whole indicate a climate resembling that now enjoyed in - the equatorial zone. Tree-ferns range as far south as the southern - parts of New Zealand, and Araucanian pines occur in Norfolk Island. - A great preponderance of ferns and lycopodiums <span class="pagenum" id="Page_421">421</span>indicates moisture, - equability of temperature, and freedom from frost, rather than intense - heat.”<a id="FNanchor_237" href="#Footnote_237" class="fnanchor">[237]</a></p> - - <p>Mr. Robert Brown, the eminent botanist, considers that the rapid and - great growth of many of the coal plants showed that they grew in swamps - and shallow water of equable and genial temperature.</p> - - <p>“Generally speaking,” says Professor Page, “we find them resembling - equisetums, marsh-grasses, reeds, club-mosses, tree-ferns, and - coniferous trees; and these in existing nature attain their maximum - development in warm, temperate, and subtropical, rather than in - equatorial regions. The Wellingtonias of California and the pines of - Norfolk Island are more gigantic than the largest coniferous tree yet - discovered in the coal-measures.”<a id="FNanchor_238" href="#Footnote_238" class="fnanchor">[238]</a></p> - - <p>The Coal period was not only characterized by a great preponderance - over the present in the quantity of ferns growing, but also in the - number of different species. Our island possesses only about 50 - species, while no fewer than 140 species have been enumerated as having - inhabited those few isolated places in England over which the coal has - been worked. And Humboldt has shown that it is not in the hot, but in - the mountainous, humid, and shady parts of the equatorial regions that - the family of ferns produces the greatest number of species.</p> - - <p>“Dr. Hooker thinks that a climate warmer than ours now is, would - probably be indicated by the presence of an increased number of - flowering plants, which would doubtless have been fossilized with - the ferns; whilst a lower temperature, <em>equal to the mean of the - seasons now prevailing</em>, would assimilate our climate to that of such - cooler countries as are characterized by a disproportionate amount of - ferns.”<a id="FNanchor_239" href="#Footnote_239" class="fnanchor">[239]</a></p> - - <p>“The general opinion of the highest authorities,” says Professor Hull, - “appears to be that the climate did not resemble that of the equatorial - regions, but was one in which <span class="pagenum" id="Page_422">422</span>the temperature was free from extremes; - the atmosphere being warm and moist, somewhat resembling that of New - Zealand and the surrounding islands, which we endeavour to imitate - artificially in our hothouses.”<a id="FNanchor_240" href="#Footnote_240" class="fnanchor">[240]</a></p> - - <p>The enormous quantity of the carboniferous vegetation shows also that - the climate under which it grew could not have been of a tropical - character, or it must have been decomposed by the heat. Peat, so - abundant in temperate regions, is not to be found in the tropics.</p> - - <p>The condition most favourable to the preservation of vegetable remains, - at least under the form of peat, is a cool, moist, and equable climate, - such as prevails in the Falkland Islands at the present day. “In these - islands,” says Mr. Darwin, “almost every kind of plant, even the coarse - grass which covers the whole surface of the land, becomes converted - into this substance.”<a id="FNanchor_241" href="#Footnote_241" class="fnanchor">[241]</a></p> - - <p>From the evidence of geology we may reasonably infer that were - the difference between our summer and winter temperature nearly - annihilated, and were we to enjoy an equable climate equal to, or - perhaps a little above, the present mean annual temperature of our - island, we should then have a climate similar to what prevailed during - the Carboniferous epoch.</p> - - <p>But we have already seen that such must have been the character of our - climate at the time that the eccentricity of the earth’s orbit was at - a maximum, and winter occurred when the earth was in the perihelion of - its orbit. For, as we have already shown, the earth would in such a - case be 14,212,700 miles nearer to the sun in winter than in summer. - This enormous difference, along with other causes which have been - discussed, would almost extinguish the difference between summer and - winter temperature. The almost if not entire absence of ice and snow, - resulting from this condition of things, would, as has already been - shown, tend to raise the <span class="pagenum" id="Page_423">423</span>mean annual temperature of the climate higher - than it is at present.</p> - - <p><em>Conditions necessary for the Preservation of the Coal Plants.</em>—But - in order to the formation of coal, it is not simply necessary to have - a condition of climate suitable for the growth, but also for the - preservation, of a luxuriant vegetation. The very existence of coal is - as much due to the latter circumstance as to the former; nay more, as - we shall yet see, the fact that a greater amount of coal belongs to the - Carboniferous period than to any other, was evidently due not so much - to a more extensive vegetable growth during that age, suited to form - coal, as to the fact that that flora has been better preserved. Now, - as will be presently shown, we have not merely in the warm periods of - a glacial epoch a condition of climate best suited for the growth of - coal plants, but we have also in the cold periods of such an epoch the - condition most favourable for the preservation of those plants.</p> - - <p>One circumstance necessary for the preservation of plants is that they - should have been covered over by a thick deposit of sand, mud, or clay, - and for this end it is necessary that the area upon which the plants - grew should have become submerged. It is evident that unless the area - had become submerged, the plants could not have been covered over with - a thick deposit; and, even supposing they had been covered over, they - could not have escaped destruction from subaërial denudation unless - the whole had been under water. Another condition favourable, if not - essential, to the preservation of the plants, is that they should have - been submerged in a cold and not in a warm sea. Assuming that the - coal plants grew during a warm period of a glacial epoch, we have in - the cold period which succeeded all the above conditions necessarily - secured.</p> - - <p>It is now generally admitted that the coal trees grew near broad - estuaries and on immense flat plains but little elevated above - sea-level. But that the <i>Lepidodendra</i>, <i>Sigillariæ</i>, and other trees, - of which our coal is almost wholly composed, grew on dry ground, - elevated above sea-level, and not in swamps and<span class="pagenum" id="Page_424">424</span> shallow water, as - was at one time supposed, has been conclusively established by the - researches of Principal Dawson and others. After the growth of many - generations of trees, the plain is eventually submerged under the sea, - and the whole, through course of time, becomes covered over with thick - deposits of sand, gravel, and other sediments carried down by streams - from the adjoining land. After this the submerged plain becomes again - elevated above the sea-level, and forms the site of a second forest, - which, after continuing to flourish for long centuries, is in turn - destroyed by submergence, and, like the former, becomes covered over - with deposits from the land. This alternate process of submergence - and emergence goes on till we have a succession of buried forests - one above another, with immense stratified deposits between. These - buried forests ultimately become converted into beds of coal. This, - I presume, is a fair representation of the generally admitted way in - which our coal-beds had their origin. It is also worthy of notice that - the stratified beds between the coal-seams are of marine and not of - lacustrine origin. On this point I may quote the opinion of Professor - Hull, a well-known authority on the subject: “Whilst admitting,” he - says, “the occasional presence of lacustrine strata associated with the - coal-measures, I think we may conclude that the whole formation has - been essentially of marine and estuarine origin.”<a id="FNanchor_242" href="#Footnote_242" class="fnanchor">[242]</a></p> - - <p><em>Coal-beds necessarily imply Oscillations of Sea-level.</em>—It may also - be observed that each coal-seam indicates both an elevation and a - depression of the land. If, for example, there are six coal-seams, - one above the other, this proves that the land must have been, at - least, six times below and six times above sea-level. This repeated - oscillation of the land has been regarded as a somewhat puzzling and - singular circumstance. But if we assume coal to be an inter-glacial - formation, this difficulty not only disappears, but all the various - circumstances which we have been detailing are readily explained. - We have to begin with a warm inter-glacial period, with a climate - <span class="pagenum" id="Page_425">425</span>specially suited for the growth of the coal trees. During this period, - as has been shown in the chapter on Submergence, the sea would be - standing at a lower level than at present, laying bare large tracts - of sea-bottom, on which would flourish the coal vegetation. This - condition of things would continue for a period of 8,000 or 10,000 - years, allowing the growth of many generations of trees. When the warm - period came to a close, and the cold and glacial condition set in, the - climate became unsuited for the growth of the coal plants. The sea - would begin to rise, and the old sea-bottoms on which, during so long - a period, the forests grew, would be submerged and become covered by - sedimentary deposits brought down from the land. These forests becoming - submerged in a cold sea, and buried under an immense mass of sediment, - were then now protected from destruction, and in a position to become - converted into coal. The cold continuing for a period of 10,000 years, - or thereby, would be succeeded by another warm period, during which the - submerged areas became again a land-surface, on which a second forest - flourished for another 10,000 years, which in turn became submerged - and buried under drift on the approach of the second cold period. - This alternate process of submergence and emergence of the land, - corresponding to the rise and fall of sea-level during the cold and - warm periods, would continue so long as the eccentricity of the earth’s - orbit remained at a high value, till we might have, perhaps, five or - six submerged forests, one above the other, and separated by great - thicknesses of stratified deposits, these submerged forests being the - coal-beds of the present day.</p> - - <div class="figcenter" id="i_426" > - <div class="caption mb2">Fig. 10.</div> - <img src="images/i_426.jpg" width="600" height="57" alt="" /> - </div> - - <p>It is probable that the forests of the Coal period would extend inland - over the country, but only such portions as were slightly elevated - above sea-level would be submerged and covered over by sediment and - thus be preserved, and ultimately become coal-seams. The process will - be better understood from the following diagram. Let A B represent the - surface of the ground prior to a glacial epoch, and to the formation - of the beds of coal and stratified deposits represented in<span class="pagenum" id="Page_426">426</span> the - diagram. Let S S′ be the normal sea-level. Suppose the eccentricity - of the earth’s orbit begins to increase, and the winter solstice - approaches the perihelion, we have then a moderately warm period. The - sea-level sinks to 1, and forests of sigillariæ and other coal trees - cover the country from the sea-shore at 1, stretching away inland in - the direction of B. In course of time the winter solstice moves round - to aphelion and a cold period follows. The sea begins to rise and - continues rising till it reaches 1′. Denudation and the severity of - the climate destroy every vestige of the forest from 1′ backwards into - the interior; but the portion 1 1′ being submerged and covered over - by sediment brought down from the land is preserved. The eccentricity - continuing to increase in extent, the second inter-glacial period is - more warm and equable than the first, and the sea this time sinks to 2. - A second forest now covers the country down to the sea-shore at 2. This - second warm period is followed by the second cold period, more severe - than the first, and the sea-level rises to 2′. Denudation and severity - of climate now destroy every remnant of the forest, from 2′ inland, - but of course the submerged portion of 2 2′, like the former portion 1 - 1′, is preserved. During the third warm period (the eccentricity being - still on the increase) the sea-level sinks to 3, and the country for - the third time is covered by forests, which extend down to 3. This - third warm period is followed by a cold glacial period more severe than - the preceding, and the sea-level rises to 3′, and the submerged portion - of the forest<span class="pagenum" id="Page_427">427</span> from 3 to 3′ becomes covered with drift,—the rest as - before being destroyed by denudation and the severity of the climate. - We shall assume that the eccentricity has now reached a maximum, and - that during the fourth inter-glacial period the sea-level sinks only to - 4, the level to which it sank during the second inter-glacial period. - The country is now covered for the fourth time by forests. The cold - period which succeeds not being so severe as the last, the sea rises - only to 4′, which, of course, marks the limit of the fourth forest. The - eccentricity continuing to diminish, the fifth forest is only submerged - up to 5′, and the sixth and last one up to 6′. The epoch of cold and - warm periods being now at a close, the sea-level remains stationary at - its old normal position S S′. Here we have six buried forests, the one - above the other, which, through course of ages, become transformed into - coal-beds.</p> - - <p>It does not, however, necessarily follow that each separate coal-seam - represents a warm period. It is quite possible that two or more seams - separated from each other by thin partings or a few feet of sedimentary - strata might have been formed during one warm period; for during a warm - period minor oscillations of sea-level sufficient to submerge the land - to some depth might quite readily have taken place from the melting of - polar ice, as was shown in the chapter on Submergence.</p> - - <p>It may be noticed that in order to make the section more distinct, its - thickness has been greatly exaggerated. It will also be observed that - beds 4, 5, and 6 extend considerably to the left of what is represented - in the section.</p> - - <p>But it is not to be supposed that the whole phenomena of the - coal-fields can be explained without supposing a subsidence of the - land. The great depth to which the coal-beds have been sunk, in many - cases, must be attributed to a subsidence of the level. A series of - beds formed during a glacial epoch, may, owing to a subsidence of the - land, be sunk to a great depth, and become covered over with thousands - of feet of sediment; and then on the occurrence of another glacial - epoch, a new series of coal-beds may be formed on the surface. Thus - the<span class="pagenum" id="Page_428">428</span> upper series may be separated from the lower by thousands of feet - of sedimentary rock. There is another consequence resulting from the - sinking of the land, which must be taken into account. Had there been - no sinking of the land during the Carboniferous age, the quantity of - coal-beds now remaining would be far less than it actually is, for it - is in a great measure owing to their being sunk to a great depth that - they have escaped destruction by the enormous amount of denudation - which has taken place since that remote age. It therefore follows that - only a very small fraction of the submerged forests of the Coal period - do actually now exist in the form of coal. Generally it would only be - those areas which happened to be sunk to a considerable depth, by a - subsidence of the land, that would escape destruction from denudation. - But no doubt the areas which would thus be preserved bear but a small - proportion to those destroyed.</p> - - <p><em>Length of Inter-glacial Period, as indicated by the Thickness of a - Bed of Coal.</em>—A fact favourable to the idea that the coal-seams were - formed during inter-glacial periods is, that the length of those - periods agrees pretty closely with the length of time supposed to be - required to form a coal-seam of average thickness. Other things being - equal, the thickness of a coal-seam would depend upon the length - of the inter-glacial period. If the rate of precession and motion - of the perihelion were always uniform the periods would all be of - equal length. But although the rate of precession is not subject to - much variation, such is not the case in regard to the motion of the - perihelion, as will be seen from the tables of the longitude of the - perihelion given in <a href="#CHAPTER_XIX">Chapter XIX.</a> Sometimes the motion of the perihelion - is rapid, at other times slow, while in some cases its motion is - retrograde. In consequence of this, an inter-glacial period may not be - more than some six or seven thousand years in length, while in other - cases its length may be as much as fifteen or sixteen thousand years.</p> - - <p>According to Boussingault, luxuriant vegetation at the present day - takes from the atmosphere about a half ton of carbon<span class="pagenum" id="Page_429">429</span> per acre - annually, or fifty tons per acre in a century. Fifty tons of carbon of - the specific gravity of coal, about 1·5, spread evenly over the surface - of an acre, would make a layer nearly one-third of an inch.<a id="FNanchor_243" href="#Footnote_243" class="fnanchor">[243]</a> - Humboldt makes the estimate a little higher, viz., one half-inch. - Taking the latter estimate, it would require 7,200 years to form a - bed of coal a yard thick. Dr. Heer, of Zurich, thinks that it would - not require more than 1,400 years to form a bed of coal one yard - thick;<a id="FNanchor_244" href="#Footnote_244" class="fnanchor">[244]</a> while Mr. Maclaren thinks that a bed of coal one yard thick - would be formed in 1,000 years.<a id="FNanchor_245" href="#Footnote_245" class="fnanchor">[245]</a> Professor Phillip, calculating - from the amount of carbon taken from the atmosphere, as determined by - Liebig, considers that if it were converted into ordinary coal with - about 75 per cent. of carbon, it would yield one inch in 127·5 years, - or a yard in 4,600 years.<a id="FNanchor_246" href="#Footnote_246" class="fnanchor">[246]</a></p> - - <p>There is here a considerable amount of difference in regard to the time - required to form a yard of coal. The truth, however, may probably be - somewhere between the two extremes, and we may assume 5,000 years to be - about the time. In a warm period of 15,000 years we should then have - deposited a seam of coal 9 feet thick, while during a warm period of - 7,000 years we would have a seam of only 4 feet.</p> - - <p><em>Reason why the Coal Strata present so little Evidence of - Ice-action.</em>—There are two objections which will, no doubt, present - themselves to the reader’s mind. (1.) If coal be an inter-glacial - formation, why do the coal strata present so little evidence of - ice-action? If the coal-seams represent warm inter-glacial periods, the - intervening beds must represent cold or glacial periods, and if so, - they ought to contain more abundant evidence of ice-action than they - really do. (2.) In the case of the glacial epoch, almost every vestige - of the vegetation of the warm periods was destroyed by the ice of the - cold periods; why then did not the same thing take place during the - glacial epoch of the Carboniferous period?</p> - - <p><span class="pagenum" id="Page_430">430</span></p> - - <p>During the glacial epoch the face of the country was in all - probability covered for ages with the most luxuriant vegetation; but - scarcely a vestige of that vegetation now remains, indeed the very soil - upon which it grew is not to be found. All that now remains is the - wreck and desolation produced by the ice-sheet that covered the country - during the cold periods of that epoch, consisting of transported blocks - of stones, polished and grooved rocks, and a confused mass of boulder - clay. Here we have in this epoch nothing tangible presenting itself - but the destructive effects of the ice which swept over the land. Why, - then, in reference to the glacial epochs of the Carboniferous age - should we have such abundant evidence of the vegetation of the warm - periods, and yet so little evidence of the effect of the ice of the - cold periods? The answer to these two objections will go a great way - to explain why we have so much coal belonging to the Carboniferous - age, and so little belonging to any other age; and it will, I think, - be found in the peculiar physical character of the country during - the Carboniferous age. The areas on which the forests of the Coal - period grew escaped the destructive power of glaciers and land-ice on - account of the flat nature of the ground. There are few points on which - geologists are more unanimous than in regard to the flat character of - the country during the Coal period.</p> - - <p>There does not seem to be any very satisfactory evidence that the - interior of the country rose to any very great elevation. Mr. - Godwin-Austen thinks that during the Coal period there must have - been “a vast expanse of continuous horizontal surface at very slight - elevations above the sea-level.”<a id="FNanchor_247" href="#Footnote_247" class="fnanchor">[247]</a> Of the widely spread terrestrial - surface of the Coal-measure period, portions, he believes, attained - a considerable elevation. But in contrast to this he states, “There - is a feature which seems to distinguish this period physically from - all subsequent periods, and which consists in the vast expanse of - continuous horizontal surface which the land area presented, bordering - on, and at very slight elevations above, the sea-level.”<a id="FNanchor_248" href="#Footnote_248" class="fnanchor">[248]</a> Hugh - <span class="pagenum" id="Page_431">431</span>Miller, describing in his usual graphic way the appearance of the - country during the Coal period, says:—“It seems to have been a land - consisting of immense flats, unvaried, mayhap, by a <em>single hill</em>, - in which dreary swamps, inhabited by doleful creatures, spread out - on every hand for hundreds and thousands of miles; and a gigantic - and monstrous vegetation formed, as I have shown, the only prominent - features of the scenery.”<a id="FNanchor_249" href="#Footnote_249" class="fnanchor">[249]</a></p> - - <p>Now, if this is in any way like a just representation of the general - features of the country during the Coal period, it was physically - impossible, no matter however severe the climate may have been, - that there could have been in this country at that period anything - approaching to continental ice, or perhaps even to glaciers of such - dimensions as would reach down to near the sea-level, where the coal - vegetation now preserved is supposed chiefly to have grown. The - condition of things which would prevail would more probably resemble - that of Siberia than that of Greenland.</p> - - <p>The absence of all traces of ice-action in the strata of the - coal-measures can in this case be easily explained. For as by - supposition there were no glaciers, there could have been no - scratching, grooving, or polishing of the rocks; neither could there - have been any icebergs, for the large masses known as icebergs are - the terminal portions of glaciers which have reached down to the sea. - Again, there being no icebergs, there of course could have been no - grinding or scratching of the rocks forming the floor of the ocean. - True, during summer, when the frozen sea broke up, we should then - have immense masses of floating ice, but these masses would not be of - sufficient thickness to rub against the sea-bottom. But even supposing - that they did occasionally touch the bottom here and there, we could - not possibly find the evidence of this in any of the strata of the - coal-measures. We could not expect to find any scratchings or markings - on the sandstone or shale of those strata indicating the action of - ice, for at that period there were no beds of sandstone or shale, but - simply beds of sand and mud, which in <span class="pagenum" id="Page_432">432</span>future ages became consolidated - into sandstone and shale. A mass of ice might occasionally rub along - the sea-bottom, and leave its markings on the loose sand or soft mud - forming that bottom, but the next wave that passed over it would - obliterate every mark, and leave the surface as smooth as before. - Neither could we expect to find any large erratics or boulders in the - coal strata, for these must come from the land, and as by supposition - there were no glaciers or land-ice at that period, there was therefore - no means of transporting them. In Greenland the icebergs sometimes - carry large boulders, which are dropped into the sea as the icebergs - melt away; but these blocks have all either been transported on the - backs of glaciers from inland tracts, or have fallen on the field-ice - along the shore from the face of crags and overhanging precipices. - But as there were probably neither glaciers reaching to the sea, nor - perhaps precipitous cliffs along the sea-shore, there could have been - few or no blocks transported by ice and dropped into the sea of the - Carboniferous period, and of course we need not expect to find them in - the sandstone and shale which during that epoch formed the bed of the - ocean. There would no doubt be coast-line ice and ground-ice in rivers, - carrying away large quantities of gravel and stones; but these gravels - and stones would of course be all water-worn, and although found in the - strata of the coal-measures, as no doubt they actually are, they would - not be regarded as indicating the action of ice. The simple absence of - relics of ice-action in the coal-measures proves nothing whatever in - regard to whether there were cold periods during their formation or not.</p> - - <p>This comparative absence of continental ice might be one reason why - the forests of the Carboniferous period have been preserved to a much - greater extent than those of any other age.</p> - - <p>It must be observed, however, that the conclusions at which we have - arrived in reference to the comparative absence of continental ice - applies only to the areas which now constitute our coal-fields. The - accumulation of ice on the antarctic regions, and on some parts of - the arctic regions, might have been as<span class="pagenum" id="Page_433">433</span> great during that age as it - is at present. Had there been no continental ice there could have - been no such oscillations of sea-level as is assumed in the foregoing - theory. The leading idea of the theory, expressed in a few words, - is, that the glacial epochs of the Carboniferous age were as severe, - and the accumulation of ice as great, as during any other age, only - there were large tracts of flat country, but little elevated above the - sea-level, which were not covered by ice. These plains, during the - warm inter-glacial periods, were covered with forests of sigillariæ - and other coal trees. Portions of those forests were protected by the - submergence which resulted from the rise of the sea-level during the - cold or glacial periods and the subsequent subsidence of the land. - Those portions now constitute our coal-beds.</p> - - <p>But that coal may be an inter-glacial formation is no mere hypothesis, - for we have in the well-known Dürnten beds—described in <a href="#CHAPTER_XV">Chapter XV.</a>—an - actual example of such a formation.</p> - - <p><em>Carboniferous Limestones.</em>—As a general rule the limestones of the - Carboniferous period, like the coal, are found in beds separated by - masses of sandstone and other stratified deposits, which proves that - the corals, crinoids, and other creatures, of the remains of which it - is composed, did not live continuously on during the entire Limestone - period. These limestones are a marine formation. If the land was - repeatedly submerged the coal must of necessity have been produced in - seams with stratified deposits between, but there is no reason why the - same should have been the case with the limestones. If the climatic - condition of the sea continued the same we should not have expected - this alternate succession of life and death; but, according to the - theory of alternate cold and warm periods, such a condition follows - as a necessary consequence, for during the warm periods, when the - land was covered with a luxuriant vegetation, the sea-bottom would be - covered with mollusca, crinoids, corals, &c., fitted to live only in a - moderately warm sea; but when the cold came on those creatures would - die, and their remains,<span class="pagenum" id="Page_434">434</span> during the continuance of the cold period, - would become slowly covered over with deposits of sand and clay. On the - return of the warm period those deposits would soon become covered with - life as before, forming another bed of limestone, and this alternation - of life and death would go on as long as the glacial epoch continued.</p> - - <p>It is true that in Derbyshire, and in the south of Ireland and some - other places, the limestone is found in one mass of several hundred - feet in thickness without any beds of sandstone or shale, but then it - is nowhere found in one continuous mass from top to bottom without any - lines of division. These breaks or divisions may as distinctly mark - a cold period as though they had been occupied by beds of sandstone. - The marine creatures ceased to exist, and when the rough surface left - by their remains became smoothed down by the action of the waves into - a flat plain, another bed would begin to form upon this floor so - soon as life again appeared. Two agencies working together probably - conspired to produce these enormous masses of limestone divided only - by breaks marking different periods of elaboration. Corals grow in - warm seas, and there only in water of a depth ranging from 20 to 30 - fathoms. The cold of a period of glaciation would not only serve to - destroy them, but they would be submerged so much beyond the depth - proper for their existence that even were it possible that with the - submergence a sufficient temperature was left, they would inevitably - perish from the superincumbent mass of water. We are therefore, as - it seems to me, warranted in concluding that the separate masses of - Derbyshire limestone were formed during warm inter-glacial periods, - and that the lines of division represent cold periods of glaciation - during which the animals perished by the combined influence of cold and - pressure of water. The submergence of the coral banks in deep water on - a sea-bottom, which, like the land, was characteristically flat and - even, implies its carrying away far into the bosom of the ocean, and - consequently remote from any continent and the river-borne detritus - thereof.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XXVII"> - <span class="pagenum" id="Page_435">435</span> - <h2> - CHAPTER XXVII.<br /><br /> - <span class="small">PATH OF THE ICE-SHEET IN NORTH-WESTERN EUROPE AND ITS RELATIONS TO THE - BOULDER CLAY OF CAITHNESS.<a id="FNanchor_250" href="#Footnote_250" class="fnanchor">[250]</a></span> - </h2> - </div> - <div class="subhead">Character of Caithness Boulder Clay.—Theories of the Origin - of the Caithness Clay.—Mr. Jamieson’s Theory.—Mr. C. W. - Peach’s Theory.—The proposed Theory.—Thickness of Scottish - Ice-sheet.—Pentlands striated on their Summits.—Scandinavian - Ice-sheet.—North Sea filled with Land-ice.—Great Baltic - Glacier.—Jutland and Denmark crossed by Ice.—Sir R. - Murchison’s Observations.—Orkney, Shetland, and Faroe Islands - striated across.—Loess accounted for.—Professor Geikie’s - Suggestion.—Professor Geikie and B. N. Peach’s Observations on - East Coast of Caithness.—Evidence from Chalk Flints and Oolitic - Fossils in Boulder Clay.</div> - - <p><em>The Nature of the Caithness Boulder Clay.</em>—A considerable amount of - difficulty has been felt by geologists in accounting for the origin of - the boulder clay of Caithness. It is an unstratified clay, of a deep - grey or slaty colour, resembling much that of the Caithness flags on - which it rests. It is thus described by Mr. Jamieson (Quart. Jour. - Geol. Soc., vol. xxii., p. 261):—</p> - - <p>“The glacial drift of Caithness is particularly interesting as an - example of a boulder clay which in its mode of accumulation and - ice-scratched <i lang="fr">débris</i> very much resembles that unstratified stony mud - which occurs underneath glaciers—the ‘<i>moraine profonde</i>,’ as some call it.</p> - - <p>“The appearance of the drift along the Haster Burn, and in many other - places in Caithness, is in fact precisely the same as that of the old - boulder clay of the rest of Scotland, except that it is charged with - remains of sea-shells and other marine organisms.</p> - - <p><span class="pagenum" id="Page_436">436</span></p> - - <p>“If want of stratification, hardness of texture, and abundance of - well-glaciated stones and boulders are to be the tests for what we call - genuine boulder clay, then much of the Caithness drift will stand the - ordeal.”</p> - - <p>So far, therefore, as the mere appearance of the drift is concerned, - it would at once be pronounced to be true Lower Till, the product of - land-ice. But there are two circumstances connected with it which have - been generally regarded as fatal to this conclusion.</p> - - <p>(1) The striæ on the rocks show that the ice which formed the clay - must have come from the sea, and not from the interior of the country; - for their direction is almost at right angles to what it would have - been had the ice come from the interior. Over the whole district, the - direction of the grooves and scratches, not only of the rocks but - even of the stones in the clay, is pretty nearly N.W. and S.E. “When - examining the sections along the Haster Burn,” says Mr. Jamieson, “in - company with Mr. Joseph Anderson, I remarked that the striæ on the - imbedded fragments generally agreed in direction with those of the - rocks beneath. The scratches on the boulders, as usual, run lengthways - along the stones when they are of an elongated form; and the position - of these stones, as they lie imbedded in the drift, is, as a rule, such - that their longer axes point in the same direction as do the scratches - on the solid rock beneath; showing that the same agency that scored the - rocks also ground and pushed along the drift.”</p> - - <p>Mr. C. W. Peach informs me that he seldom or never found a stone with - two sets of striæ on it, a fact indicating, as Mr. Jamieson remarks, - that the drift was produced by one great movement invariably in the - same direction. Let it be borne in mind that the ice, which thus moved - over Caithness in this invariable track, must either have come from the - Atlantic to the N.W., or from the Moray Firth to the S.E.</p> - - <p>(2) The boulder clay of Caithness is full of sea-shells and other - marine remains. The shells are in a broken condition, and are - interspersed like the stones through the entire mass of<span class="pagenum" id="Page_437">437</span> the clay. - Mr. Jamieson states that he nowhere observed any instance of shells - being found in an undisturbed condition, “nor could I hear,” he says, - “of any such having been found; there seems to be no such thing as a - bed of laminated silt with shells <i lang="la">in situ</i>.” The shell-fragments are - scratched and ice-worn, the same as the stones found in the clay. Not - only are the shells glaciated, but even the foraminifera, when seen - through the microscope, have a rubbed and worn appearance. The shells - have evidently been broken, striated, and pushed along by the ice at - the time the boulder clay was being formed.</p> - - <p><em>Theories regarding the Origin of the Caithness Clay.</em>—Mr. Jamieson, as - we have seen, freely admits that the boulder clay of Caithness has the - appearance of true land-ice till, but from the N.W. and S.E. direction - of the striæ on the rocks, and the presence of sea-shells in the clay, - he has come to the conclusion that the glaciation of Caithness has been - effected by floating ice at a time when the district was submerged. I - have always felt convinced that Mr. Jamieson had not hit upon the true - explanation of the phenomena.</p> - - <p>(1) It is physically impossible that any deposit formed by icebergs - could be wholly unstratified. Suppose a mass of the materials which - would form boulder clay is dropped into the sea from, say an iceberg, - the heavier parts, such as stones, will reach the bottom first. Then - will follow lighter materials, such as sand, then clay, and last of all - the mud will settle down over the whole in fine layers. The different - masses dropped from the various icebergs, will, no doubt, lie in - confusion one over the other, but each separate mass will show signs of - stratification. A good deal of boulder clay evidently has been formed - in the sea, but if the clay be unstratified, it must have been formed - under glaciers moving along the sea-bottom as on dry ground. Whether - <em>unstratified</em> boulder clay may happen to be formed under water or on - dry land, it must in either case be the product of land-ice.<a id="FNanchor_251" href="#Footnote_251" class="fnanchor">[251]</a> Those - who imagine that materials, <span class="pagenum" id="Page_438">438</span>differing in specific gravity like those - which compose boulder clay, dropped into water, can settle down without - assuming the stratified form, should make the experiment, and they - would soon satisfy themselves that the thing is physically impossible. - The notion that unstratified boulder clay could be formed by deposits - from floating ice, is not only erroneous, but positively pernicious, - for it tends to lead those who entertain it astray in regard to the - whole question of the origin of drift.</p> - - <p>(2) It is also physically impossible that ice-markings, such as those - everywhere found on the rocky face of the district, and on the pebbles - and shells imbedded in the clay, could have been effected by any other - agency than that of land-ice. I need not here enter into any discussion - on this point, as this has been done at considerable length in another - place.<a id="FNanchor_252" href="#Footnote_252" class="fnanchor">[252]</a> In the present case, however, it is unnecessary, because - if it can be shown that all the facts are accounted for in the most - natural manner by the theory of land-ice, no one will contend for the - floating-ice theory; for it is admitted that, with the exception of the - direction of the striæ and the presence of the shells, all the facts - agree better with the land-ice than with the floating-ice theory.</p> - - <p>My first impression on the subject was that the glaciation of Caithness - had been effected by the polar ice-cap, which, during the severer part - of the glacial epoch, must have extended down to at least the latitude - of the north of Scotland.</p> - - <p>On a former occasion (see the <cite>Reader</cite> for 14th October, 1865) it was - shown that all the northern seas, owing to their shallowness, must, - at that period, have been blocked up with solid ice, which displaced - the water and moved along the sea-bottoms the same as on dry land. In - fact, the northern seas, including the German Ocean, being filled at - the time with glacier-ice, might be regarded as dry land. Ice of this - sort, moving along the bed of the German Ocean or North Sea, and over - Caithness, could not fail to push before it the shells and other animal - remains lying on the sea-bottom, and to mix <span class="pagenum" id="Page_439">439</span>them up with the clay - which now remains upon the land as evidence of its progress.</p> - - <p>About two years ago I had a conversation with Mr. C. W. Peach on the - subject. This gentleman, as is well known, has long been familiar with - the boulder clay of Caithness. He felt convinced that the clay of that - country is the true Lower Till, and not a more recent deposit, as Mr. - Jamieson supposes. He expressed to me his opinion that the glaciation - of Caithness had been effected by masses of land-ice crossing the - Moray Firth from the mountain ranges to the south-east, and passing - over Caithness in its course. The difficulty which seems to beset - this theory is, that a glacier entering the Firth would not leave it - and ascend over the Caithness coast. It would take the path of least - resistance and move into the North Sea, where it would find a free - passage into deeper water. Mr. Peach’s theory is, however, an important - step in the right direction. It is a part of the truth, but I believe - not the whole truth. The following is submitted as a solution of the - question.</p> - - <p><em>The Proposed Theory.</em>—It may now be regarded as an established fact - that, during the severer part of the glacial period, Scotland was - covered with one continuous mantle of ice, so thick as to bury under - it the Ochil, Sidlaw, Pentland, Campsie, and other moderately high - mountain ranges. For example, Mr. J. Geikie and Mr. B. N. Peach found - that the great masses of the ice from the North-west Highlands, came - straight over the Ochils of Perthshire and the Lomonds of Fife. In - fact, these mountain ridges were not sufficiently high to deflect the - icy stream either to the right hand or to the left; and the flattened - and rounded tops of the Campsie, Pentland, and Lammermoor ranges bear - ample testimony to the denuding power of ice.</p> - - <p>Further, to quote from Mr. Jamieson, “the detached mountain of - Schehallion in Perthshire, 3,500 feet high, is marked near the top as - well as on its flanks, and this not by ice flowing down the sides of - the hill itself, but by ice pressing over it from the north. On the top - of another isolated hill, called<span class="pagenum" id="Page_440">440</span> Morven, about 3,000 feet high, and - situated a few miles to the north of the village of Ballater, in the - county of Aberdeen, I found granite boulders unlike the rock of the - hill, and apparently derived from the mountains to the west. Again, - on the highest watersheds of the Ochils, at altitudes of about 2,000 - feet, I found this summer (1864) pieces of mica schist full of garnets, - which seem to have come from the Grampian Hills to the north-west, - showing that the transporting agent had overflowed even the highest - parts of the Ochil ridge. And on the West Lomonds, in Fifeshire, at - Clattering-well Quarry, 1,450 feet high, I found ice-worn pebbles of - Red Sandstone and porphyry in the <i lang="fr">débris</i> covering the Carboniferous - Limestone of the top of the Bishop Hill. Facts like these meet us - everywhere. Thus on the Perthshire Hills, between Blair Athol and - Dunkeld, I found ice-worn surfaces of rocks on the tops of hills, at - elevations of 2,200 feet, as if caused by ice pressing over them from - the north-west, and transporting boulders at even greater heights.”<a id="FNanchor_253" href="#Footnote_253" class="fnanchor">[253]</a></p> - - <p>Facts still more important, however, in their bearing on the question - before us were observed on the Pentland range by Mr. Bennie and myself - during the summer of 1870. On ascending Allermuir, one of the hills - forming the northern termination of the Pentland range, we were not a - little surprised to find its summit ice-worn and striated. The top of - the hill is composed of a compact porphyritic felstone, which is very - much broken up; but wherever any remains of the original surface could - be seen, it was found to be polished and striated in a most decided - manner. These striæ are all in one uniform direction, nearly east and - west; and on minutely examining them with a lens we had no difficulty - whatever in determining that the ice which effected them came from the - west and not from the east, a fact which clearly shows that they must - have been made at the time when, as is well known, the entire Midland - valley was filled with ice, coming from the North-west Highlands. On - the summit of the hill we also found patches of boulder clay in <span class="pagenum" id="Page_441">441</span>hollow - basins of the rock. At one spot it was upwards of a foot in depth, and - rested on the ice-polished surface. The clay was somewhat loose and - sandy, as might be expected of a layer so thin, exposed to rain, frost, - and snow, during the long course of ages which must have elapsed since - it was deposited there. Of 100 pebbles collected from the clay, just as - they turned up, every one, with the exception of three or four composed - of hard quartz, presented a flattened and ice-worn surface; and - forty-four were distinctly striated: in short, every stone which was - capable of receiving and retaining scratches was striated. A number of - these stones must have come from the Highlands to the north-west.<a id="FNanchor_254" href="#Footnote_254" class="fnanchor">[254]</a></p> - - <p>The height of Allermuir is 1,617 feet, and, from its position, it is - impossible that the ice could have gone over its summit, unless the - entire Midland valley, at this place, had been filled with ice to the - depth of more than 1,600 feet. The hill is situated about four or - five miles to the south of Edinburgh, and forms, as has already been - stated, the northern termination of the Pentland range. Immediately - to the north lies the broad valley of the Firth of Forth, more than - twelve miles across, offering a most free and unobstructed outlet for - the great mass of ice coming along the Midland valley from the west. - Now, when we reflect how easily ice can accommodate itself to the - inequalities of the channel along which it moves, how it can turn to - the right hand or to the left, so as to find for itself the path of - least resistance, it becomes obvious that the ice never would have gone - over Allermuir, unless not only the Midland valley at this point, but - also the whole surrounding country had been covered with one continuous - mass of ice to a depth of more than 1,600 feet. But it must not be - supposed that the height of Allermuir represents the thickness of the - ice; for on ascending Scald Law, a hill four miles to the south-west - of Allermuir, and the highest of the Pentland range, we found, in - the <i lang="fr">débris</i> covering its summit, hundreds <span class="pagenum" id="Page_442">442</span>of transported stones of - all sizes, from one to eighteen inches in diameter. We also dug up a - Greenstone boulder about eighteen inches in diameter, which was finely - polished and striated. As the height of this hill is 1,898 feet, the - mass of ice covering the surrounding country must have been at least - 1,900 feet deep. But this is not all. Directly to the north of the - Pentlands, in a line nearly parallel with the east coast, and at right - angles to the path of ice from the interior, there is not, with the - exception of the solitary peak of East Lomond, and a low hill or two of - the Sidlaw range, an eminence worthy of the name of a hill nearer than - the Grampians in the north of Forfarshire, distant upwards of sixty - miles. This broad plain, extending from almost the Southern to the - Northern Highlands, was the great channel through which the ice of the - interior of Scotland found an outlet into the North Sea. If the depth - of the ice in the Firth of Forth, which forms the southern side of this - broad hollow, was at least 1,900 feet, it is not at all probable that - its depth in the northern side, formed by the Valley of Strathmore - and the Firth of Tay, which lay more directly in the path of the ice - from the North Highlands, could have been less. Here we have one vast - glacier, more than sixty miles broad and 1,900 feet thick, coming from - the interior of the country.</p> - - <p>It is, therefore, evident that the great mass of ice entering the North - Sea to the east of Scotland, especially about the Firths of Forth - and Tay, could not have been less, and was probably much more, than - from 1,000 to 2,000 feet in thickness. The grand question now to be - considered is, What became of the huge sheet of ice after it entered - the North Sea? Did it break up and float away as icebergs? This appears - to have been hitherto taken for granted; but the shallowness of the - North Sea shows such a process to have been utterly impossible. The - depth of the sea in the English Channel is only about twenty fathoms, - and although it gradually increases to about forty fathoms at the - Moray Firth, yet we must go to the north and west of the Orkney and - Shetland Islands ere we<span class="pagenum" id="Page_443">443</span> reach the 100 fathom line. Thus the average - depth of the entire North Sea is not over forty fathoms, which is even - insufficient to float an iceberg 300 feet thick.</p> - - <p>No doubt the North Sea, for two reasons, is now much shallower than - it was during the period in question. (1.) There would, at the time - of the great extension of the ice on the northern hemisphere, be a - considerable submergence, resulting from the displacement of the - earth’s centre of gravity.<a id="FNanchor_255" href="#Footnote_255" class="fnanchor">[255]</a> (2.) The sea-bed is now probably - filled up to a larger extent with drift deposits than it was at the - ice period. But, after making the most extravagant allowance for the - additional depth gained on this account, still there could not possibly - have been water sufficiently deep to float a glacier of 1,000 or 2,000 - feet in thickness. Indeed, the North Sea would have required to be - nearly ten times deeper than it is at present to have floated the - ice of the glacial period. We may, therefore, conclude with the most - perfect certainty that the ice-sheet of Scotland could not possibly - have broken up into icebergs in such a channel, but must have moved - along on the bed of the sea in one unbroken mass, and must have found - its way to the deep trough of the Atlantic, west of the Orkney and - Shetland Islands, ere it broke up and floated away in the iceberg form.</p> - - <p>It is hardly necessary to remark that the waters of the North Sea would - have but little effect in melting the ice. A shallow sea like this, - into which large masses of ice were entering, would be kept constantly - about the freezing-point, and water of this temperature has but little - melting power, for it takes 142 lbs. of water, at 33°, to melt one - pound of ice. In fact, an icy sea tends rather to protect the ice - entering it from being melted than otherwise. And besides, owing to - fresh acquisitions of snow, the ice-sheet would be accumulating more - rapidly upon its upper surface than it would be melting at its lower - surface, supposing there were sea-water under that surface. The ice of - Scotland during the glacial period must, of necessity, have found its - way into warmer water than that of <span class="pagenum" id="Page_444">444</span>the North Sea before it could have - been melted. But this it could not do without reaching the Atlantic, - and in getting there it would have to pass round by the Orkney Islands, - along the bed of the North Sea, as land-ice.</p> - - <p>This will explain how the Orkney Islands may have been glaciated by - land-ice; but it does not, however, explain how Caithness should have - been glaciated by that means. These islands lay in the very track of - the ice on its way to the Atlantic, and could hardly escape being - overridden; but Caithness lay considerably to the left of the path - which we should expect the ice to have taken. The ice would not leave - its channel, turn to the left, and ascend upon Caithness, unless it - were forced to do so. What, then, compelled the ice to pass over - Caithness?</p> - - <p><em>Path of the Scandinavian Ice.</em>—We must consider that the ice from - Scotland and England was but a fraction of that which entered the - North Sea. The greater part of the ice of Scandinavia must have gone - into this sea, and if the ice of our island could not find water - sufficiently deep in which to float, far less would the much thicker - ice of Scandinavia do so. The Scandinavian ice, before it could break - up, would thus, like the Scottish ice, have to cross the bed of the - North Sea and pass into the Atlantic. It could not pass to the north, - or to the north-west, for the ocean in these directions would be - blocked up by the polar ice. It is true that along the southern shore - of Norway there extends a comparatively deep trough of from one to two - hundred fathoms. But this is evidently not deep enough to have floated - the Scandinavian ice-sheet; and even supposing it had been sufficiently - deep, the floating ice must have found its way to the Atlantic, and - this it could not have done without passing along the coast. Now, its - passage would not only be obstructed by the mass of ice continually - protruding into the sea directly at right angles to its course, but it - would be met by the still more enormous masses of ice coming off the - entire Norwegian coast-line. And, besides this, the ice entering the - Arctic Ocean from Lapland and the northern<span class="pagenum" id="Page_445">445</span> parts of Siberia, except - the very small portion which might find an outlet into the Pacific - through Behring’s Straits, would have to pass along the Scandinavian - coast in its way to the Atlantic. No matter, then, what the depth of - this trough may have been, if the ice from the land, after entering - it, could not make its escape, it would continue to accumulate till - the trough became blocked up; and after this, the great mass from the - land would move forward as though the trough had no existence. Thus, - the only path for the ice would be by the Orkney and Shetland Islands. - Its more direct and natural path would, no doubt, be to the south-west, - in the direction of our shores; and in all probability, had Scotland - been a low flat island, instead of being a high and mountainous one, - the ice would have passed completely over it. But its mountainous - character, and the enormous masses of ice at the time proceeding from - its interior, would effectually prevent this, so that the ice of - Scandinavia would be compelled to move round by the Orkney Islands. - Consequently, these two huge masses of moving ice—the one from Scotland - and the much greater one from Scandinavia—would meet in the North Sea, - probably not far from our shores, and would move, as represented in - the diagram, side by side northwards into the Atlantic as one gigantic - glacier.</p> - - <p>Nor can this be regarded as an anomalous state of things; for in - Greenland and the antarctic continent the ice does not break up into - icebergs on reaching the sea, but moves along the sea-bottom in a - continuous mass until it reaches water sufficiently deep to float - it. It is quite possible that the ice at the present day may nowhere - traverse a distance of three or four hundred miles of sea-bottom, but - this is wholly owing to the fact that it finds water sufficiently deep - to float it before having travelled so far. Were Baffin’s Bay and - Davis’s Straits, for example, as shallow as the North Sea, the ice of - Greenland would not break up into icebergs in these seas, but cross in - one continuous mass to and over the American continent.</p> - - <p>The median line of the Scandinavian and Scottish ice-sheets<span class="pagenum" id="Page_446">446</span> would be - situated not far from the east coast of Scotland. The Scandinavian ice - would press up as near to our coast as the resistance of the ice from - this side permitted. The enormous mass of ice from Scotland, pressing - out into the North Sea, would compel the Scandinavian ice to move round - by the Orkneys, and would also keep it at some little distance from - Scotland. Where, on the other hand, there was but little resistance - offered by ice from the interior of this country (and this might be the - case along many parts of the English coast), the Scandinavian ice might - reach the shores, and even overrun the country for some distance inland.</p> - - <p>We have hitherto confined our attention to the action of ice proceeding - from Norway; but if we now consider what took place in Sweden and the - Baltic, we shall find more conclusive proof of the downward pressure - of Scandinavian ice on our own shores. The western half of Gothland - is striated in the direction of N.E. and S.W., and that this has been - effected by a huge mass of ice covering the country, and not by local - glaciers, is apparent from the fact observed by Robert Chambers,<a id="FNanchor_256" href="#Footnote_256" class="fnanchor">[256]</a> - and officers of the Swedish Geological Survey, that the general - direction of the groovings and striæ on the rocks bears little or no - relation to the conformation of the surface, showing that the ice was - of sufficient thickness to move straight forward, regardless of the - inequalities of the ground.</p> - - <p>At Gottenburg, on the shores of the Cattegat, and all around Lake - Wener and Lake Wetter, the ice-markings are of the most remarkable - character, indicating, in the most decided manner, that the ice came - from the interior of the country to the north-east in one vast mass. - All this mass of ice must have gone into the shallow Cattegat, a sea - not sufficiently deep to float even an ordinary glacier. The ice coming - off Gothland would therefore cross the Cattegat, and thence pass over - Jutland into the North Sea. After entering the North Sea, it would be - obliged to keep between our shores and the ice coming direct from the - western side of Scandinavia.</p> - - <p><span class="pagenum" id="Page_447">447</span></p> - - <p>But this is not all. A very large proportion of the Scandinavian ice - would pass into the Gulf of Bothnia, where it could not possibly float. - It would then move south into the Baltic as land-ice. After passing - down the Baltic, a portion of the ice would probably move south into - the flat plains in the north of Germany, but the greater portion - would keep in the bed of the Baltic, and of course turn to the right - round the south end of Gothland, and thence cross over Denmark into - the North Sea. That this must have been the path of the ice is, I - think, obvious from the observations of Murchison, Chambers, Hörbye, - and other geologists. Sir Roderick Murchison found—though he does not - attribute it to land-ice—that the Aland Islands, which lie between the - Gulf of Bothnia and the Baltic, are all striated in a north and south - direction.<a id="FNanchor_257" href="#Footnote_257" class="fnanchor">[257]</a></p> - - <p>Upsala and Stockholm, a tract of flat country projecting for some - distance into the Baltic, is also grooved and striated, not in the - direction that would be effected by ice coming from the interior of - Scandinavia, but north and south, in a direction parallel to what must - have been the course of the ice moving down the Baltic.<a id="FNanchor_258" href="#Footnote_258" class="fnanchor">[258]</a> This part - of the country must have been striated by a mass of ice coming from - the direction of the Gulf of Bothnia. And that this mass must have - been great is apparent from the fact that Lake Malar, which crosses - the country from east to west, at right angles to the path of the ice, - does not seem to have had any influence in deflecting the icy stream. - That the ice came from the north and not from the south is also evident - from the fact that the northern sides of rocky eminences are polished, - rounded, and ice-worn, while the southern sides are comparatively - rough. The northern banks of Lake Malar, for example, which, of course, - face the south, are rough, while the southern banks, which must have - offered opposition to the advance of the ice, are smoothed and rounded - in a most singular manner.</p> - - <p><span class="pagenum" id="Page_448">448</span></p> - - <p>Again, that the ice, after passing down the Baltic, turned to the - right along the southern end of Gothland, is shown by the direction - of the striæ and ice-groovings observed on such islands as Gothland, - Öland, and Bornholm. Sir R. Murchison found that the island of - Gothland is grooved and striated in one uniform direction from N.E. - to S.W. “These groovings,” says Sir Roderick, “so perfectly resemble - the flutings and striæ produced in the Alps by the actual movement - of glaciers, that neither M. Agassiz nor any one of his supporters - could detect a difference.” He concludes, however, that the markings - could not have been made by land-ice, because Gothland is not only a - low, flat island in the middle of the Baltic, but is “at least 400 - miles distant from any elevation to which the term of mountain can be - applied.” This, of course, is conclusive against the hypothesis that - Gothland and the other islands of the Baltic could have been glaciated - by ordinary glaciers; but it is quite in harmony with the theory - that the Gulf of Bothnia and the entire Baltic were filled with one - continuous mass of land-ice, derived from the drainage of the greater - part of Sweden, Lapland, and Finland. In fact, the whole glacial - phenomena of Scandinavia are inexplicable on the hypothesis of local - glaciers.</p> - - <p>That the Baltic was completely filled by a mass of ice moving from the - north is further evidenced by the fact that the mainland, not only at - Upsala, but at several places along the coast of Gothland, is grooved - and striated parallel to the shore, and often at right angles to the - markings of the ice from the interior, showing that the present bed of - the Baltic was not large enough to contain the icy stream. For example, - along the shores between Kalmar and Karlskrona, as described by Sir - Roderick Murchison and by M. Hörbye, the striations are parallel to the - shore. Perhaps the slight obstruction offered by the island of Öland, - situated so close to the shore, would deflect the edge of the stream at - this point over on the land. The icy stream, after passing Karlskrona, - bent round to the west along the present entrance to the Baltic, and - again<span class="pagenum" id="Page_449">449</span> - invaded the mainland, and crossed over the low headland of - Christianstadt, and thence passed westward in the direction of Zealand.</p> - - <div class="figcenter illow600" id="PLATE_V" > - <div class="attribt">PLATE V.</div> - <img src="images/i_449.jpg" width="600" height="468" alt="" /> - <div class="attribr">W. & A. K. Johnston, Edinb<sup>r</sup>. and London.</div> - <div class="caption">CHART SHOWING THE PROBABLE PATH OF THE ICE IN NORTH-WESTERN EUROPE - DURING THE PERIOD OF MAXIMUM GLACIATION.<br /> - <i>The lines also represent the actual direction of the striae on the rocks.</i></div> - </div> - - <p>This immense Baltic glacier would in all probability pass over Denmark, - and enter the North Sea somewhere to the north of the River Elbe, and - would then have to find an outlet to the Atlantic through the English - Channel, or pass in between our eastern shores and the mass from - Gothland and the north-western shores of Europe. The entire probable - path of the ice may be seen by a reference to the accompanying chart - (<a href="#PLATE_V">Plate V.</a>) That the ice crossed over Denmark is evident from the fact - that the surface of that country is strewn with <i lang="fr">débris</i> derived from - the Scandinavian peninsula.</p> - - <p>Taking all these various considerations into account, the conclusion is - inevitable that the great masses of ice from Scotland would be obliged - to turn abruptly to the north, as represented in the diagram, and pass - round into the Atlantic in the direction of Caithness and the Orkney - Islands.</p> - - <p>If the foregoing be a fair representation of the state of matters, - it is physically impossible that Caithness could have escaped being - overridden by the land-ice of the North Sea. Caithness, as is well - known, is not only a low, flat tract of land, little elevated above the - sea-level, and consequently incapable of supporting large glaciers; - but, in addition, it projects in the form of a headland across the - very path of the ice. Unless Caithness could have protected itself by - pushing into the sea glaciers of one or two thousand feet in thickness, - it could not possibly have escaped the inroads of the ice of the - North Sea. But Caithness itself could not have supported glaciers of - this magnitude, neither could it have derived them from the adjoining - mountainous regions of Sutherland, for the ice of this county found a - more direct outlet than along the flat plains of Caithness.</p> - - <p>The shells which the boulder clay of Caithness contains have thus - evidently been pushed out of the bed of the North Sea by the land-ice, - which formed the clay itself.</p> - - <p><span class="pagenum" id="Page_450">450</span></p> - - <p>The fact that these shells are not so intensely arctic as those found - in some other quarters of Scotland, is no evidence that the clay was - not formed during the most severe part of the glacial epoch, for the - shells did not live in the North Sea at the time that it was filled - with land-ice. The shells must have belonged to a period prior to the - invasion of the ice, and consequently before the cold had reached its - greatest intensity. Neither is there any necessity for supposing the - shells to be pre-glacial, for these shells may have belonged to an - inter-glacial period. In so far as Scotland is concerned, it would be - hazardous to conclude that a plant or an animal is either pre-glacial - or post-glacial simply because it may happen not to be of an arctic or - of a boreal type.</p> - - <p>The same remarks which apply to Caithness apply to a certain extent - to the headland at Fraserburgh. It, too, lay in the path of the ice, - and from the direction of the striæ on the rocks, and the presence of - shells in the clay, as described by Mr. Jamieson, it bears evidence - also of having been overridden by the land-ice of the North Sea. - In fact, we have, in the invasion of Caithness and the headland at - Fraserburgh by the land-ice of the North Sea, a repetition of what we - have seen took place at Upsala, Kalmar, Christianstadt, and other flat - tracts along the sides of the Baltic.</p> - - <p>The scarcity, or perhaps entire absence of Scandinavian boulders in - the Caithness clay is not in any way unfavourable to the theory, for - it would only be the left edge of the North Sea glacier that could - possibly pass over Caithness; and this edge, as we have seen, was - composed of the land-ice from Scotland. We might expect, however, to - find Scandinavian blocks on the Shetland and Faroe Islands, for, as we - shall presently see, there is pretty good evidence to prove that the - Scandinavian ice passed over these islands.</p> - - <p><em>The Shetland and Faroe Islands glaciated by Land-ice.</em>—It is also - worthy of notice that the striæ on the rocks in the Orkney, Shetland, - and Faroe Islands, all point in the direction of Scandinavia, and are - what would be effected by land-ice moving in<span class="pagenum" id="Page_451">451</span> the paths indicated - in the diagram. And it is a fact of some significance, that when we - proceed north to Iceland, the striæ, according to the observations - of Robert Chambers, seem to point towards North Greenland. Is it - possible that the entire Atlantic, from Scandinavia to Greenland, was - filled with land-ice? Astounding as this may at first appear, there - are several considerations which render such a conclusion probable. - The observations of Chambers, Peach, Hibbert, Allan, and others, show - that the rocky face of the Shetland and Faroe Islands has been ground, - polished, and striated in a most remarkable manner. That this could not - have been done by ice belonging to the islands themselves is obvious, - for these islands are much too small to have supported glaciers of any - size, and the smallest of them is striated as well as the largest. - Besides, the uniform direction of the striæ on the rocks shows that - it must have been effected by ice passing over the islands. That the - striations could not have been effected by floating icebergs at a time - when the islands were submerged is, I think, equally obvious, from the - fact that not only are the tops of the highest eminences ice-worn, - but the entire surface down to the present sea-level is smoothed and - striated; and these striations conform to all the irregularities of the - surface. This last fact Professor Geikie has clearly shown is wholly - irreconcilable with the floating-ice theory.<a id="FNanchor_259" href="#Footnote_259" class="fnanchor">[259]</a> Mr. Peach<a id="FNanchor_260" href="#Footnote_260" class="fnanchor">[260]</a> found - vertical precipices in the Shetlands grooved and striated, and the - same thing was observed by Mr. Thomas Allan on the Faroe Islands.<a id="FNanchor_261" href="#Footnote_261" class="fnanchor">[261]</a> - That the whole of these islands have been glaciated by a continuous - sheet of ice passing over them was the impression left on the mind of - Robert Chambers after visiting them.<a id="FNanchor_262" href="#Footnote_262" class="fnanchor">[262]</a> This is the theory which - alone explains all the facts. The only difficulty which besets it is - the enormous thickness of the ice demanded by the theory. But this - difficulty is very much diminished <span class="pagenum" id="Page_452">452</span>when we reflect that we have good - evidence, from the thickness of icebergs which have been met with - in the Southern Ocean,<a id="FNanchor_263" href="#Footnote_263" class="fnanchor">[263]</a> that the ice moving off the antarctic - continent must be in some places considerably over a mile in thickness. - It is then not so surprising that the ice of the glacial epoch, coming - off Greenland and Northern Europe, should not have been able to float - in the North Atlantic.</p> - - <p><em>Why the Ice of Scotland was of such enormous Thickness.</em>—The enormous - thickness of the ice in Scotland, during the glacial epoch, has been - a matter of no little surprise. It is remarkable how an island, not - more than 100 miles across, should have been covered with a sheet - of ice so thick as to bury mountain ranges more than 1,000 feet in - height, situated almost at the sea-shore. But all our difficulties - disappear when we reflect that the seas around Scotland, owing to their - shallowness, were, during the glacial period, blocked up with solid - ice. Scotland, Scandinavia, and the North Sea, would form one immense - table-land of ice, from 1,000 to 2,000 feet above the sea-level. This - table-land would terminate in the deep waters of the Atlantic by a - perpendicular wall of ice, extending probably from the west of Ireland - away in the direction of Iceland. From this barrier icebergs would be - continually breaking off, rivalling in magnitude those which are now to - be met with in the antarctic seas.</p> - - <p><em>The great Extension of the Loess accounted for.</em>—An effect which would - result from the blocking up of the North Sea with land-ice, would be - that the waters of the Rhine, Elbe, and Thames would have to find - an outlet into the Atlantic through the English Channel. Professor - Geikie has suggested to me that if the Straits of Dover were not then - open—quite a possible thing—or were they blocked up with land-ice, say - by the great Baltic glacier crossing over from Denmark, the consequence - would be that the waters of the Rhine and Elbe would be dammed back, - and would inundate all the low-lying tracts of country to the south; - and this might account for the extraordinary <span class="pagenum" id="Page_453">453</span>extension of the Loess in - the basin of the Rhine, and in Belgium and the north of France.<a id="FNanchor_264" href="#Footnote_264" class="fnanchor">[264]</a></p> - - <div class="figcenter illow418" id="PLATE_VI" > - <div class="attribt">PLATE VI.</div> - <img src="images/i_453.jpg" width="418" height="700" alt="" /> - <div class="attribr">W. & A. K. Johnston, Edinb<sup>r</sup> and London.</div> - <div class="caption">CHART SHOWING PATH OF THE ICE<br /> - Note.<br /> - <span class="left"><i>Curved lines shew path of Ice.<br /> - Arrows shew direction of striae<br /> - as observed by Prof. Geikie & B. N. Peach.<br /> - Short thick lines shew direction of<br /> - striae by other observers.</i></span> - </div> - </div> - - <div class="center"><i>Note on the Glaciation of Caithness.</i></div> - - <p>I have very lately received a remarkable confirmation of the path of - the Caithness ice in observations communicated to me by Professor - Geikie and Mr. B. N. Peach. The latter geologist says, “Near the Ord - of Caithness and on to Berriedale the striæ pass off the land and out - to sea; but near Dunbeath, 6 miles north-east of Berriedale, they - begin to creep up out of the sea on to the land and range from about - 15° to 10° east of north. <em>Where the striæ pass out to the sea</em> the - boulder clay is made up of the materials from inland and contains no - shells, but <em>immediately the striæ begin to creep up on to the land</em> - then shells begin to make their appearance; and there is a difference, - moreover, in the colour of the clay, for in the former case it is - red and incoherent, and in the latter hard and dark-coloured.” The - accompanying chart (<a href="#PLATE_VI">Plate VI.</a>) shows the outline of the Caithness coast - and the direction of the striæ as observed by Professor Geikie and Mr. - Peach, and no demonstration could be more conclusive as to the path of - the ice and the obstacles it met than these observations, supplemented - and confirmed as they are by other recorded facts to which I shall - presently allude. Had the ice-current as it entered the North Sea off - the Sutherland coast met with no obstacle it would have ploughed its - way outwards till it broke off in glaciers and floated away. But it is - clear that the great press of Scandinavian ice and the smaller mass of - land-ice from the Morayshire coast converging in the North Sea filled - up its entire bed, and these, meeting the opposing current from the - Sutherland coast, turned it back upon itself, and forced it over the - north-east <span class="pagenum" id="Page_454">454</span>part of Caithness. The farther south on the Sutherland - coast that the ice entered the sea the deeper would it be able to - penetrate into the ocean-bed before it met an opposition sufficiently - strong to turn its course, and the wider would be its sweep; but when - we come to the Sutherland coast we reach a point where the land-ice—as, - for example, near Dunbeath—is forced to bend round before it even - reaches the sea-shore, as will be seen from the accompanying diagram.</p> - - <p>We are led to the same conclusions regarding the path of the ice in the - North Sea from the presence of oolitic fossils and chalk flints found - likewise in the boulder clay of Caithness, for these, as we shall see, - evidently must have come from the sea. At the meeting of the British - Association, Edinburgh, 1850, Hugh Miller exhibited a collection of - boreal shells with fragments of oolitic fossils, chalk, and chalk - flints from the boulder clay of Caithness collected by Mr. Dick, of - Thurso. My friend, Mr. C. W. Peach, found that the chalk flints in - the boulder clay of Caithness become more abundant as we proceed - northward, while the island of Stroma in the Pentland Firth he found - to be completely strewn with them. This same observer found, also, in - the Caithness clay stones belonging to the Oolitic and Lias formations, - with their characteristic fossils, while ammonites, belemnites, fossil - wood, &c., &c., were also found loose in the clay.<a id="FNanchor_265" href="#Footnote_265" class="fnanchor">[265]</a> The explanation - evidently is, that these remains were derived from an outcrop of - oolitic and cretaceous beds in the North Sea. It is well known that - the eastern coast of Sutherlandshire is fringed with a narrow strip - of oolite, which passes under the sea, but to what distance is not - yet ascertained. Outside the Oolitic formation the chalk beds in all - probability crop out. It will be seen from a glance at the accompanying - chart (<a href="#PLATE_VI">Plate VI.</a>) that the ice which passed over the north-eastern part - of Caithness must have crossed the out-cropping chalk beds.</p> - - <p>As has already been stated in the foregoing chapter, the headland of - Fraserburgh, north-eastern corner of Aberdeenshire, <span class="pagenum" id="Page_455">455</span>bears evidence, - both from the direction of the striæ and broken shells in the boulder - clay, of having been overridden also by land-ice from the North Sea. - This conclusion is strengthened by the fact that chalk flints and - oolitic fossils have also been abundantly met with in the clay by Dr. - Knight, Mr. James Christie, Mr. W. Ferguson, Mr. T. F. Jamieson, and - others.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XXVIII"> - <span class="pagenum" id="Page_456">456</span> - <h2> - CHAPTER XXVIII.<br /><br /> - <span class="small">NORTH OF ENGLAND ICE-SHEET, AND TRANSPORT OF WASTDALE CRAG BLOCKS.<a id="FNanchor_266" href="#Footnote_266" class="fnanchor">[266]</a></span> - </h2> - </div> - <div class="subhead">Transport of Blocks; Theories of.—Evidence of Continental - Ice.—Pennine Range probably striated on Summit.—Glacial - Drift in Centre of England.—Mr. Lacy on Drift of Cotteswold - Hills.—England probably crossed by Land-ice.—Mr. Jack’s - Suggestion.—Shedding of Ice North and South.—South of England - Ice-sheet.—Glaciation of West Somerset.—Why Ice-markings are - so rare in South of England.—Form of Contortion produced by Land-ice.</div> - - <p class="noindent"><span class="smcap">Considerable</span> difficulty has been felt in accounting for the transport - of the Wastdale granite boulders across the Pennine chain to the east. - Professors Harkness,<a id="FNanchor_267" href="#Footnote_267" class="fnanchor">[267]</a> and Phillips,<a id="FNanchor_268" href="#Footnote_268" class="fnanchor">[268]</a> Messrs. Searles Wood, - jun.,<a id="FNanchor_269" href="#Footnote_269" class="fnanchor">[269]</a> Mackintosh,<a id="FNanchor_270" href="#Footnote_270" class="fnanchor">[270]</a> and I presume all who have written on - the subject, agree that these blocks could not have been transported - by land-ice. The agency of floating ice under some form or other is - assumed by all.</p> - - <p>We have in Scotland phenomena of an exactly similar nature. The summits - of the Ochils, the Pentlands, and other mountain ranges in the east - of Scotland, at elevations of from 1,500 to 2,000 feet, are not only - ice-marked, but strewn over with boulders derived from rocks to the - west and north-west. Many of them must have come from the Highlands - distant some 50 or 60 miles. It is impossible that these stones could - have been transported, or the summits of the hills striated, by means - of ordinary glaciers. Neither can the phenomena be attributed to the - agency of icebergs carried along by currents. For we should require to - assume not merely a submergence of the land <span class="pagenum" id="Page_457">457</span>to the extent of 2,000 - feet or so,—an assumption which might be permitted,—but also that the - currents bearing the icebergs took their rise in the elevated mountains - of the Highlands (a most unlikely place), and that these currents - radiated in all directions from that place as a centre.</p> - - <p>In short, the glacial phenomena of Scotland are wholly inexplicable - upon any other theory than that, during at least a part of the - glacial epoch, the entire island from sea to sea was covered with one - continuous mass of ice of not less than 2,000 feet in thickness.</p> - - <p>In my paper on the Boulder Clay of Caithness (see preceding chapter), - I have shown that if the ice was 2,000 feet or so in thickness, it - must, in its motion seawards, have followed the paths indicated by the - curved lines in the chart accompanying that paper (See <a href="#PLATE_I">Plate I.</a>). In - so far as Scotland is concerned [and Scandinavia also], these lines - represent pretty accurately not only the paths actually taken by the - boulders, but also the general direction of the ice-markings on all the - elevated mountain ridges. But if Scotland was covered to such an extent - with ice, it is not at all probable that Westmoreland and the other - mountainous districts of the North of England could have escaped being - enveloped in a somewhat similar manner. Now if we admit the supposition - of a continuous mass of ice covering the North of England, all our - difficulties regarding the transport of the Wastdale blocks across the - Pennine chain disappear. An inspection of the chart above referred to - will show that these blocks followed the paths which they ought to have - done upon the supposition that they were conveyed by continental ice.</p> - - <p>That Wastdale Crag itself suffered abrasion by ice moving over it, in - the direction indicated by the lines in the diagram, is obvious from - what has been recorded by Dr. Nicholson and Mr. Mackintosh. They both - found the Crag itself beautifully <i lang="fr">moutonnée</i> up to its summit, and - striated in a W.S.W. and E.N.E. direction. Mr. Mackintosh states that - these scorings run obliquely up the sloping face of the crag. Ice - scratches<span class="pagenum" id="Page_458">458</span> crossing valleys and running up the sloping faces of hills - and over their summits are the sure marks of continental ice, which - meet the eye everywhere in Scotland. Dr. Nicholson found in the drift - covering the lower part of the crag, pebbles of the Coniston flags and - grits from the west.<a id="FNanchor_271" href="#Footnote_271" class="fnanchor">[271]</a></p> - - <p>The fact that in Westmoreland the direction of the ice-markings, as a - general rule, corresponds with the direction of the main valleys, is - no evidence whatever that the country was not at one period covered - with a continuous sheet of ice; because, for long ages after the period - of continental ice, the valleys would be occupied by glaciers, and - these, of course, would necessarily leave the marks of their presence - behind. This is just what we have everywhere in Scotland. It is on - the summits of the hills and elevated ridges, where no glacier could - possibly reach, that we find the sure evidence of continental ice. - But that land-ice should have passed over the tops of hills 1,000 or - 2,000 feet in height is a thing hitherto regarded by geologists as - so unlikely that few of them ever think of searching in such places - for ice-markings, or for transported stones. Although little has been - recorded on this point, I hardly think it likely that there is in - Scotland a hill under 2,000 feet wholly destitute of evidence that ice - has gone over it. If there were hills in Scotland that should have - escaped being overridden by ice, they were surely the Pentland Hills; - but these, as was shown on a former occasion,<a id="FNanchor_272" href="#Footnote_272" class="fnanchor">[272]</a> were completely - buried under the mass of ice covering the flat surrounding country. - I have no doubt whatever that if the summits of the Pennine range - were carefully examined, say under the turf, evidence of ice-action, - in the form of transported stones or scratches on the rock, would be - found.<a id="FNanchor_273" href="#Footnote_273" class="fnanchor">[273]</a></p> - - <p><span class="pagenum" id="Page_459">459</span></p> - - <p>Nor is the fact that the Wastdale boulders are not rounded and - ice-marked, or found in the boulder clay, but lie on the surface, any - evidence that they were not transported by land-ice. For it would not - be the stones <em>under</em> the ice, but those falling on the upper surface - of the sheet, that would stand the best chance of being carried over - mountain ridges. But such blocks would not be crushed and ice-worn; - and it is on the surface of the clay, and not imbedded in it, that we - should expect to find them.</p> - - <p>It is quite possible that the dispersion of the Wastdale boulders took - place at various periods. During the period of local glaciers the - blocks would be carried along the line of the valleys.</p> - - <p>All I wish to maintain is that the transport of the blocks across - the Pennine chain is easily accounted for if we admit, what is very - probable, that the great ice-covering of Scotland overlapped the high - grounds of the North of England. The phenomenon is the same in both - places, and why not attribute it to the same cause?</p> - - <p>There is another curious circumstance connected with the drift of - England which seems to indicate the agency of an ice-covering.</p> - - <p>As far back as 1819, Dr. Buckland, in his Memoir on the Quartz Rock - of Lickey Hill,<a id="FNanchor_274" href="#Footnote_274" class="fnanchor">[274]</a> directed attention to the fact, that on the - Cotteswold Hills there are found pebbles of hard red chalk which must - have come from the Wolds of Yorkshire and Lincolnshire. He pointed - out also that the slaty and porphyritic pebbles probably came from - Charnwood Forest, near Leicester. Professor Hull, of the Geological - Survey, considers that “almost all the Northern Drift of this part - of the country had been derived from the <i lang="fr">débris</i> of the rocks of - the Midland Counties.”<a id="FNanchor_275" href="#Footnote_275" class="fnanchor">[275]</a> He came also to the conclusion that the - slate fragments may have been derived from Charnwood Forest. In <span class="pagenum" id="Page_460">460</span>the - Vale of Moreton he found erratic boulders from two feet to three - feet in diameter. The same northern character of the drift of this - district is remarked by Professor Ramsay and Mr. Aveline, in their - Memoir of the Geology of parts of Gloucestershire. In Leicestershire - and Northamptonshire the officers of the Geological Survey found in - abundance drift which must have come from Lincolnshire and Yorkshire to - the north-east.</p> - - <p>Mr. Lucy, who has also lately directed attention to the fact that - the Cotteswold Hills are sprinkled over with boulders from Charnwood - Forest, states also that, on visiting the latter place, he found that - many of the stones contained in it had come from Yorkshire, still - further to the north-east.<a id="FNanchor_276" href="#Footnote_276" class="fnanchor">[276]</a></p> - - <p>Mr. Searles Wood, jun., in his interesting paper on the Boulder Clay - of the North of England,<a id="FNanchor_277" href="#Footnote_277" class="fnanchor">[277]</a> states that enormous quantities of the - chalk <i lang="fr">débris</i> from the Yorkshire Wold are found in Leicester, Rutland, - Warwick, Northampton, and other places to the south and south-west. - Mr. Wood justly concludes that this chalk <i lang="fr">débris</i> could not have been - transported by water. “If we consider,” he says, “the soluble nature - of chalk, it must be evident that none of this débris can have been - detached from the parent mass, either by water-action, or by any other - atmospheric agency than moving ice. The action of the sea, of rivers, - or of the atmosphere, upon chalk, would take the form of dissolution, - the degraded chalk being taken up in minute quantities by the water, - and held in suspension by it, and in that form carried away; so that - it seems obvious that this great volume of rolled chalk can have been - produced in no other way than by the agency of moving ice; and for that - agency to have operated to an extent adequate to produce a quantity - that I estimate as exceeding a layer 200 feet thick over the entire - Wold, nothing less than the complete envelopment of a large part of the - Wold by ice for a long period would suffice.”</p> - - <p><span class="pagenum" id="Page_461">461</span></p> - - <p>I have already assigned my reasons for disbelieving the opinion that - such masses of drift could have been transported by floating ice; but - if we refer it to land-ice, it is obvious that the ice could not have - been in the form of local glaciers, but must have existed as a sheet - moving in a south and south-west direction, from Yorkshire, across the - central part of England. But how is this to harmonize with the theory - of glaciation, which is advanced to explain the transport of the Shap - boulders?</p> - - <p>The explanation has, I think, been pointed out by a writer in the - <cite>Glasgow Herald</cite>,<a id="FNanchor_278" href="#Footnote_278" class="fnanchor">[278]</a> of the 26th November, 1870, in a review of Mr. - Lucy’s paper.</p> - - <p>In my paper on the Boulder Clay of Caithness, I had represented the ice - entering the North Sea from the east coast of Scotland and England, - as all passing round the north of Scotland. But the reviewer suggests - that the ice entering at places to the south of, say, Flamborough Head, - would be deflected southwards instead of northwards, and thus pass over - England. “It is improbable, however,” says the writer, “that this joint - ice-sheet would, as Mr. Croll supposes, all find its way round the - north of Scotland into the deep sea. The southern uplands of Scotland, - and probably also the mountains of Northumberland, propelled, during - the coldest part of the glacial period, a land ice-sheet in an eastward - direction. This sheet would be met by another streaming outward from - the south-western part of Norway—in a diametrically opposite direction. - In other words, an imaginary line might be drawn representing the - course of some particular boulder in the <i>moraine profonde</i> from - England met by a boulder from Norway, in the same straight line. With - a dense ice-sheet to the north of this line, and an open plain to the - south, it is clear that all the ice travelling east or west from points - to the south of the starting-points of our two boulders would be ‘shed’ - off to the south. There would be a point somewhere along the line, at - which the <span class="pagenum" id="Page_462">462</span>ice would turn as on a pivot—this point being nearer England - or Scandinavia, as the degree of pressure exercised by the respective - ice-sheets should determine. There is very little doubt that the point - in question would be nearer England. Further, the direction of the - joint ice-sheet could not be <em>due</em> south unless the pressure of the - component ice-sheets should be exactly equal. In the event of that from - Scandinavia pressing with greater force, the direction would be to the - south-west. This is the direction in which the drifts described by Mr. - Lucy have travelled.”</p> - - <p>I can perceive no physical objection to this modification of the - theory. What the ice seeks is the path of least resistance, and along - this path it will move, whether it may lie to the south or to the - north. And it is not at all improbable that an outlet to the ice would - be found along the natural hollow formed by the valleys of the Trent, - Avon, and Severn. Ice moving in this direction would no doubt pass down - the Bristol Channel and thence into the Atlantic.</p> - - <p>Might not the shedding of the north of England ice-sheet to the north - and south, somewhere not far from Stainmoor, account for the remarkable - fact pointed out by Mr. Searles Wood, that the boulder clay, with - Shap boulders, to the north of the Wold is destitute of chalk; while, - on the other hand, the chalky boulder clay to the south of the Wold - is destitute of Shap boulders? The ice which passed over Wastdale - Crag moved to the E.N.E., and did not cross the chalk of the Wold; - while the ice which bent round to the south by the Wold came from the - district lying to the south of Wastdale Crag, and consequently did not - carry with it any of the granite from that Crag. In fact, Mr. Searles - Wood has himself represented on the map accompanying his Memoir this - shedding of the ice north and south.</p> - - <p>These theoretical considerations are, of course, advanced for what - they are worth. Hitherto geologists have been proceeding upon the - supposition of an ice-sheet and an open North Sea;<span class="pagenum" id="Page_463">463</span> but the latter is - an impossibility. But if we suppose the seas around our island to have - been filled with land-ice during the glacial epoch, the entire glacial - problem is changed, and it does not then appear so surprising that ice - should have passed over England.</p> - - <div class="center"><i>Note on the South of England Ice-sheet.</i></div> - - <p>If what has already been stated regarding the north of England be - anything like correct, it is evident that the south of England - could not possibly have escaped glaciation. If the North Sea was so - completely blocked up by Scandinavian ice, that the great mass of ice - from the Cumberland mountains entering the sea on the east coast was - compelled to bend round and find a way of escape across the centre - of England in the direction of the Bristol Channel, it is scarcely - possible that the immense mass of ice filling the Baltic Sea and - crossing over Denmark could help passing across at least a portion - of the south of England. The North Sea being blocked up, its natural - outlet into the Atlantic would be through the English Channel; and it - is not likely that it could pass through without impinging to some - extent upon the land. Already geologists are beginning to recognise the - evidence of ice in this region.</p> - - <p>Mr. W. C. Lucy, in the <cite>Geological Magazine</cite> for June, 1874, records - the finding by himself of evidences of glaciation in West Somerset, - in the form of “rounded rocky knolls,” near Minehead, like those of - glaciated districts; of a bed of gravel and clay 70 feet deep, which - he considered to be boulder clay. He also mentions the occurrence near - Portlock of a large mass of sandstone well striated, only partially - detached from the parent rock. In the same magazine for the following - month Mr. H. B. Woodward records the discovery by Mr. Usher of some - “rum stuff” near Yarcombe, in the Black Down Hills of Devonshire, - which, on investigation, proved to be boulder<span class="pagenum" id="Page_464">464</span> clay; and further, that - it was not a mere isolated patch, but occurred in several other places - in the same district. Mr. C. W. Peach informs me that on the Cornwall - coast, near Dodman Point, at an elevation of about 60 feet above - sea-level, he found the rock surface well striated and ice-polished. - In a paper on the Drift Deposits of the Bath district, read before the - Bath Natural History and Antiquarian Field Club, March 10th, 1874, - Mr. C. Moore describes the rock surfaces as grooved, with deep and - long-continued furrows similar to those usually found on glaciated - rocks, and concludes that during the glacial period they were subjected - to ice-action. This conclusion is confirmed by the fact of there being - found, immediately overlying these glaciated rocks, beds of gravel - with intercalated clay-beds, having a thickness of 30 feet, in which - mammalian remains of arctic types are abundant. The most characteristic - of which are <i>Elephas primigenius</i>, <i>E. antiquus</i>, <i>Rhinoceros - tichorhinus</i>, <i>Bubalus moschatus</i>, and <i>Cervus tarandus</i>.</p> - - <p>There is little doubt that when the ground is better examined many - other examples will be found. One reason, probably, why so little - evidence of glaciation in the south of England has been recorded, - is the comparative absence of rock surfaces suitable for retaining - ice-markings. There is, however, one class of evidence which might - determine the question of the glaciation of the south of England as - satisfactorily as markings on the rock. The evidence to which I refer - is that of contorted beds of sand or clay. In England contortions from - the sinking of the beds are, of course, quite common, but a thoughtful - observer, who has had a little experience of ice-formed contortions, - can easily, without much trouble, distinguish the latter from the - former. Contortions resulting from the lateral pressure of the ice - assume a different form from those produced by the sinking of the beds. - In Scotland, for example, there is one well-marked form of contortion, - which not only proves the existence of land-ice, but also the direction - in which it moved. The form of contortion to which I refer is the - bending back of the stratified beds upon themselves, somewhat in - the form of a<span class="pagenum" id="Page_465">465</span> fishing-hook. This form of contortion will be better - understood from the accompanying figure.</p> - - <div class="figcenter" id="i_465" > - <div class="caption">Fig. 11.</div> - <img src="images/i_465.jpg" width="600" height="386" alt="" /> - <div class="caption"><span class="smcap">Section of Contorted Drift near Musselburgh.</span><br /> - <i>a</i> Boulder Clay; <i>b</i> Laminated Clay; <i>c</i> Sand, Gravel, and Clay, contorted.<br /> - Depth of Section, twenty-two feet.—<span class="smcap">H. Skae.</span></div> - </div> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XXIX"> - <span class="pagenum" id="Page_466">466</span> - <h2> - CHAPTER XXIX.<br /><br /> - <span class="small">EVIDENCE FROM BURIED RIVER CHANNELS OF A CONTINENTAL PERIOD IN - BRITAIN.<a id="FNanchor_279" href="#Footnote_279" class="fnanchor">[279]</a></span> - </h2> - </div> - <div class="subhead">Remarks on the Drift Deposits.—Examination of Drift - by Borings.—Buried River Channel from Kilsyth to - Grangemouth.—Channels not excavated by Sea nor by Ice.—Section - of buried Channel at Grangemouth.—Mr. Milne Home’s - Theory.—German Ocean dry Land.—Buried River Channel from - Kilsyth to the Clyde.—Journal of Borings.—Marine Origin of the - Drift Deposits.—Evidence of Inter-glacial Periods.—Oscillations - of Sea-level.—Other buried River Channels.</div> - - <p><em>Remarks on the Drift Deposits.</em>—The drift and other surface deposits - of the country have chiefly been studied from sections observed on the - banks of streams, railway cuttings, ditches, foundations of buildings, - and other excavations. The great defect of such sections is that they - do not lay open a sufficient depth of surface. They may, no doubt, - represent pretty accurately the character and order of the more recent - deposits which overlie the boulder clay, but we are hardly warranted - in concluding that the succession of deposits belonging to the earlier - part of the glacial epoch, the period of the true till, is fully - exhibited in such limited sections.</p> - - <p>Suppose, for example, the glacial epoch proper—the time of the lower - boulder clay—to have consisted of a succession of alternate cold and - warm periods, there would, in such a case, be a series of separate - formations of boulder clay; but we could hardly expect to find on the - flat and open face of the country, where the surface deposits are - generally not of great depth, those various formations of till lying - the one superimposed upon the other. For it is obvious that the till - formed during one ice-period would, as a general rule, be either swept - away <span class="pagenum" id="Page_467">467</span>or re-ground and laid down by the ice of the succeeding period. - If the very hardest rocks could not withstand the abrading power - of the enormous masses of ice which passed over the surface of the - country during the glacial epoch, it is hardly to be expected that the - comparatively soft boulder clay would be able to do so. It is probable - that the boulder clay of one period would be used as grinding materials - by the ice of the succeeding periods. The boulder clay which we find in - one continuous mass may, therefore, in many cases, have been ground off - the rocks underneath at widely different periods.</p> - - <p>If we wish to find the boulder clays belonging to each of the - successive cold periods lying, the one superimposed on the other in - the order of time in which they were formed, we must go and search in - some deep gorge or valley, where the clay has not only accumulated - in enormous masses, but has been partially protected from the - destructive power of the ice. But it is seldom that the geologist has - an opportunity of seeing a complete section down to the rock-head in - such a place. In fact, excepting by bores for minerals, or by shafts - of pits, the surface, to a depth of one or two hundred feet, is never - passed through or laid open.</p> - - <p><em>Examination of Drift by Borings.</em>—With the view of ascertaining if - additional light would be cast on the sequence of events, during the - formation of the boulder clay, by an examination of the journals of - bores made through a great depth of surface deposits, a collection - of about 250 bores, put down in all parts of the mining districts - of Scotland, was made. An examination of these bores shows most - conclusively that the opinion that the boulder clay, or lower till, is - one great undivided formation, is wholly erroneous.</p> - - <p>These 250 bores, as already stated,<a id="FNanchor_280" href="#Footnote_280" class="fnanchor">[280]</a> represent a total thickness - of 21,348 feet, giving 86 feet as the mean thickness of the deposits - passed through. Twenty of these bores have one boulder clay, with beds - of stratified sand or gravel beneath the clay; 25 have 2 boulder clays, - with stratified beds of sand and <span class="pagenum" id="Page_468">468</span>gravel between; 10 have 3 boulder - clays; one has 4 boulder clays; 2 have 5 boulder clays; and one has no - fewer than 6 separate masses of boulder clay, with stratified beds of - sand and gravel between; 16 have two or three separate boulder clays, - differing altogether in colour and hardness, without any stratified - beds between. We have, therefore, out of 250 bores, 75 of them - representing a condition of things wholly different from that exhibited - to the geologist in ordinary sections.</p> - - <p>These bores bear testimony to the conclusion that the glacial epoch - consisted of a succession of cold and warm periods, and not of one - continuous and unbroken period of ice, as was at one time generally - supposed.</p> - - <p>The full details of the character of the deposits passed through by - these bores, and their bearing on the history of the glacial epoch, - have been given by Mr. James Bennie, in an interesting paper read - before the Glasgow Geological Society,<a id="FNanchor_281" href="#Footnote_281" class="fnanchor">[281]</a> to which I would refer - all those interested in the subject of surface geology. But it is not - to the mere contents of the bores that I wish at present to direct - attention, but to a new and important result, to which they have - unexpectedly led.</p> - - <p><em>Buried River Channel, Kilsyth to Grangemouth, Firth of Forth.</em>—These - borings reveal the existence of a deep pre-glacial, or perhaps - inter-glacial, trough or hollow, extending from the Clyde above Bowling - across the country by Kilsyth, along the valley of the Forth and Clyde - Canal, to the Firth of Forth at Grangemouth. This trough is filled up - with immense deposits of mud, sand, gravel, and boulder clay. These - deposits not only fill it up, but they cover it over to such an extent - that it is absolutely impossible to find on the surface a single trace - of it; and had it not been for borings, and other mining operations, - its existence would probably never have been known. In places where the - bottom of the trough is perhaps 200 feet below the sea-level, we find - on the surface not a hollow, but often an immense ridge or elliptical - knoll of sand, gravel, or boulder clay, rising sometimes to 150 or 200 - feet above the present sea-level.</p> - - <p><span class="pagenum" id="Page_469">469</span></p> - - <p>I need not here enter into any minute details regarding the form, - depth, and general outline of this trough, or of the character of - the deposits covering it, these having already been described by Mr. - Bennie, but shall proceed to the consideration of circumstances which - seem to throw light on the physical origin of this curious hollow, - and to the proof which it unexpectedly affords that Scotland, during - probably an early part of the glacial epoch, stood higher in relation - to the sea-level than it does at present; or rather, as I would be - disposed to express it, the sea stood much lower than at present.</p> - - <p>From the fact that all along the line of this trough the surface of the - country is covered with enormous beds of stratified sands and gravels - of marine origin, which proves that the sea must have at a recent - period occupied the valley, my first impression was that this hollow - had been scooped out by the sea. This conclusion appeared at first - sight quite natural, for at the time that the sea filled the valley, - owing to the Gulf-stream impinging on our western shores, a strong - current would probably then pass through from the Atlantic on the west - to the German Ocean on the east. However, considerations soon began to - suggest themselves wholly irreconcilable with this hypothesis.</p> - - <p>The question immediately arose, if the tendency of the sea occupying - the valley is to deepen it, by wearing down its rocky bottom, and - removing the abraded materials, then why is the valley filled up to - such a prodigious extent with marine deposits? Does not the fact of the - whole valley being filled up from sea to sea with marine deposits to a - depth of from 100 to 200 feet, and in some places, to even 400 feet, - show that the tendency of the sea filling this valley is to silt it up - rather than to deepen it? What conceivable change of conditions could - account for operations so diverse?</p> - - <p>That the sea could not have cut out this trough, is, however, - susceptible of direct proof. The height of the surface of the valley - at the watershed or highest part, about a mile to the east of - Kilsyth, where the Kelvin and the Bonny Water, running <span class="pagenum" id="Page_470">470</span>in opposite - directions,—the one west into the Clyde, and the other east into the - Carron,—take their rise, is 160 feet above the sea-level. Consequently, - before the sea could pass through the valley at present, the sea-level - would require to be raised 160 feet.</p> - - <p>But in discussing the question as to the origin of this pre-glacial - hollow, we must suppose the surface deposits of the valley all removed, - for this hollow was formed before these deposits were laid down. Let - us take the average depth of these deposits at the watershed to be 50 - feet. It follows that, assuming the hollow in question to have been - formed by the sea, the sea-level at the time must have been at least - 110 feet higher than at present.</p> - - <p>Were the surface deposits of the country entirely removed, the district - to the west and north-west of Glasgow would be occupied by a sea - which would stretch from the Kilpatrick Hills, north of Duntocher, - to Paisley, a distance of about five miles, and from near Houston to - within a short way of Kirkintilloch, a distance of more than twelve - miles. This basin would contain a few small islands and sunken rocks, - but its mean depth, as determined from a great number of surface bores - obtained over its whole area, would be not much under 70 or 80 feet. - But we shall, however, take the depth at only 50 feet. Now, if we raise - the sea-level so as to allow the water just barely to flow over the - watershed of the valley, the sea in this basin would therefore be 160 - feet deep. Let us now see what would be the condition of things on the - east end of the valley. The valley, for several miles to the east of - Kilsyth, continues very narrow, but on reaching Larbert it suddenly - opens into the broad and flat carse lands through which the Forth and - Carron wind. The average depth at which the sea would stand at present - in this tract of country, were the surface removed, as ascertained from - bores, would be at least 100 feet, or about double that in the western - basin. Consequently, when the sea was sufficiently high to pass over - the watershed, the water would be here 210 feet in depth, and several - miles in breadth.</p> - - <div class="figcenter illow600" id="PLATE_VII" > - <div class="attribt">PLATE VII.</div> - <img src="images/i_471.jpg" width="600" height="307" alt="" /> - <div class="attribr">W. & A. K. Johnston Edinb<sup>r</sup>. and London.</div> - <div class="caption"><span class="smcap">Chart of the MIDLAND VALLEY, SHOWING BURIED RIVER CHANNELS.</span><br /> - <i>The blue parts represent the area which would be covered by sea were - the land submerged to the extent of 200 feet. The heavy black lines A - and B represent the buried River Channels.</i></div> - </div> - - <p><span class="pagenum" id="Page_471">471</span></p> - - <p>But in order to have a current of some strength passing through the - valley, let us suppose the sea at the time to have stood 150 feet - higher in relation to the land than at present. This would give 40 feet - as the depth of the sea on the watershed, and 200 feet as the depth in - the western basin, and 250 feet as the depth in the eastern.</p> - - <p>An examination of the Ordnance Survey map of the district will show - that the 200 feet contour lines which run along each side of the valley - from Kilsyth to Castlecary come, in several places, to within one-third - of a mile of each other. From an inspection of the ground, I found - that, even though the surface deposits were removed off the valley, it - would not sensibly affect the contours at those places. It is therefore - evident that though the sea may have stood even 200 feet higher than at - present, the breadth of the strait at the watershed and several other - points could not have exceeded one-third of a mile. It is also evident - that at those places the current would be flowing with the greatest - velocity, for here it was not only narrowest, but also shallowest. A - reference to <a href="#PLATE_VII">Plate VII.</a> will show the form of the basins. The stippled - portion, coloured blue, represents the area which would be covered by - the sea were the land submerged to the extent of 200 feet.</p> - - <p>Let us take the breadth of the current in the western basin at, say, - three miles. This is two miles less than the breadth of the basin - itself. Suppose the current at the narrow parts between Kilsyth and - Castlecary to have had a velocity of, say, five miles an hour. Now, as - the mean velocity of the current at the various parts of its course - would be inversely proportionate to the sectional areas of those parts, - it therefore follows that the mean velocity of the current in the - western basin would be only 1/45th of what it was in the narrow pass - between Kilsyth and Castlecary. This would give a mile in nine hours - as the velocity of the water in the western basin. In the eastern - basin the mean velocity of the current, assuming its breadth to be the - same as in the western, would be only a mile in eleven hours. In the - central part of the current the<span class="pagenum" id="Page_472">472</span> velocity at the surface would probably - be considerably above the mean, but at the sides and bottom it would, - no doubt, be under the mean. In fact, in these two basins the current - would be almost insensible.</p> - - <p>The effect of such a current would simply be to widen and deepen the - valley all along that part between Kilsyth and Castlecary where the - current would be flowing with considerable rapidity. But it would - have little or no effect in deepening the basins at each end, but the - reverse. It would tend rather to silt them up. If the current flowed - from west to east, the materials removed from the narrow part between - Kilsyth and Castlecary, where the velocity of the water was great, - would be deposited when the current almost disappeared in the eastern - part of the valley. Sediment carried by a current flowing at the rate - of five miles an hour, would not remain in suspension when the velocity - became reduced to less than five miles a day.</p> - - <p>But even supposing it were shown that the sea under such conditions - could have deepened the valley along the whole distance from the Clyde - to the Forth, still this would not explain the origin of the trough - in question. What we are in search of is not the origin of the valley - itself, but the origin of a deep and narrow hollow running along - the bottom of it. A sea filling the whole valley, and flowing with - considerable velocity, would, under certain conditions, no doubt deepen - and widen it, but it would not cut out along its bottom a deep, narrow - trough, with sides often steep, and in some places perpendicular and - even overhanging.</p> - - <p>This hollow is evidently an old river-bed scooped out of the rocky - valley by a stream, flowing probably during an early part of the - glacial period.</p> - - <p>During the latter part of the summer of 1868, I spent two or three - weeks of my holidays in tracing the course of this buried trough from - Kilsyth to the river Forth at Grangemouth, and I found unmistakable - evidence that the eastern portion of it, stretching from the watershed - to the Forth, had been cut<span class="pagenum" id="Page_473">473</span> out, not by the sea, but by a stream which - must have followed almost the present course of the Bonny Water.</p> - - <p>I found that this deep hollow enters the Forth a few hundred yards to - the north of Grangemouth Harbour, at the extraordinary depth of 260 - feet below the present sea-level. At the period when the sea occupied - the valley of the Forth and Clyde Canal, the bottom of the trough at - this spot would therefore be upwards of 400 feet below the level of the - sea.</p> - - <p>A short distance to the west of Grangemouth, and also at Carron, - several bores were put down in lines almost at right angles across - the trough, and by this means we have been enabled to form a pretty - accurate estimate of its depth, breadth, and shape at those places. I - shall give the details of one of those sections.</p> - - <p>Between Towncroft Farm and the river Carron, a bore was put down to - the depth of 273 feet before the rock was reached. About 150 yards to - the north of this there is another bore, giving 234 feet as the depth - to the rock; 150 yards still further north the depth of the surface - deposits, as determined by a third bore, is 155 feet. This last bore is - evidently outside of the hollow, for one about 150 yards north of it - gives the same depth of surface, which seems to be about its average - depth for a mile or two around. About half a mile to the south of the - hollow at this place the surface deposits are 150 feet deep. From a - number of bores obtained at various points within a circuit of 1½ - miles, the surface appears to have a pretty uniform depth of 150 feet - or thereby. For the particulars of these “bores” I am indebted to the - kindness of Mr. Mackay, of Grangemouth.</p> - - <p>To the south of the trough (see Fig. 12) there is a fault running - nearly parallel to it, having a down-throw to the north, and cutting - off the coal and accompanying strata to the south. But an inspection of - the section will show that the hollow in question is no way due to the - fault, but has been scooped out of the solid strata.</p> - - <div class="figcenter" id="i_474" > - <div class="caption">Fig. 12.</div> - <img src="images/i_474.jpg" width="600" height="190" alt="" /> - <div class="caption"><span class="smcap">Section of buried River-bed near Towncroft Farm, Grangemouth.</span></div> - </div> - - <p>The main coal wrought extensively here is cut off by the<span class="pagenum" id="Page_474">474</span> trough, - as will be seen from the section. Mr. Dawson, of Carron Iron Works, - informs me that at Carronshore pit, about a mile and a quarter above - where this section is taken, the coal was found to be completely cut - off by this trough. In one of the workings of this pit, about forty - years ago, the miners cut into the trough at 40 fathoms below the - surface, when the sand rushed in with irresistible pressure, and filled - the working. Again, about a mile below where the section is taken, - or about two miles below Carronshore, and just at the spot where the - trough enters the Firth, it was also cut into in one of the workings of - the Heuck pit at a depth of 40<span class="pagenum" id="Page_475">475</span> fathoms from the surface. Fortunately, - however, at this point the trough is filled with boulder clay instead - of sand, and no damage was sustained. Here, for a distance of two - miles, the Main coal and “Upper Coxroad” are cut off by this hollow; or - rather, I should say this hollow has been cut through the coal-seams. - The “Under Coxroad,” lying about 14 fathoms below the position of the - “Main” coal, as will be seen in the descriptive section (Fig. 12), is - not reached by the trough, and passes undisturbed under it.</p> - - <p>This hollow would seem to narrow considerably as it recedes westwards, - for at Carronshore pit-shaft the surface is 138 feet deep; but not much - over 150 yards to the south of this is the spot where the coal was cut - off by the trough at a depth of 40 fathoms or 240 feet. Here it deepens - upwards of 100 feet in little more than 150 yards. That it is narrow at - this place is proved by the fact, that a bore put down near Carronbank, - a little to the south, shows the surface to be only 156 feet deep.</p> - - <p>In the section (Fig. 12) the line described as “150 <em>feet above - sea-level</em>” registers the height of the sea-level at the time when - the central valley was occupied by sea 40 feet deep at the watershed. - Now, if this hollow, which extends right along the whole length of - the valley, had been cut out by the sea, the surface of the rock 150 - feet below the present surface of the ground would be the sea-bottom - at the time, and the line marked “150 <em>feet above sea-level</em>” would be - the surface of the sea. The sea would therefore be here 300 feet deep - for several miles around. It cannot be supposed that the sea acting on - a broad flat plain of several miles in extent should cut out a deep, - narrow hollow, like the one exhibited in the section, and leave the - rest of the plain a flat sea-bottom.</p> - - <p>And it must be observed, that this is not a hollow cut merely in a - sea-beach, but one extending westward to Kilsyth. Now, if this hollow - was cut out by the sea, it must have been done, not by the waves - beating on the beach, but by a current flowing through the valley. - The strongest current that could<span class="pagenum" id="Page_476">476</span> possibly pass through the narrow - part between Kilsyth and Castlecary would be wholly insensible when it - reached Grangemouth, where the water was 300 feet deep, and several - miles broad. Consequently, it is impossible that the current could have - scooped out the hollow represented in the section.</p> - - <p>Again, if this hollow had been scooped out by the sea, it ought to - be deepest between Kilsyth and Castlecary, where the current was - narrowest; but the reverse is actually the case. It is shallowest at - the place where the current was narrowest, and deepest at the two - ends where the current was broadest. In the case of a trough cut by - a sea current, we must estimate its depth from the level of the sea. - Its depth is the depth of the water in it while it was being scooped - out. The bottom of the trough in the highest and narrowest part of - the valley east of Kilsyth is 40 feet above the present sea-level. - Consequently, its depth at this point at the period in question, when - the sea-level was 150 feet higher than at present, would be 110 feet. - The bottom of the trough at Grangemouth is 260 feet below the present - sea-level; add to this 150 feet, and we have 410 feet as its depth here - at the time in question. If this hollow was scooped out by the sea, - how then does it thus happen that at the place where the current was - strongest and confined to a narrow channel by hills on each side, it - cut its channel to a depth of only 110 feet, whereas at the place where - it had scarcely any motion it has cut, on a flat and open plain several - miles broad, a channel to a depth of 410 feet?</p> - - <p>But, suppose we estimate the relative amount of work performed by the - sea at Kilsyth and Grangemouth, not by the actual depth of the bottom - of the trough at these two places below the sea-level at the time that - the work was performed, but by the present actual depth of the bottom - of the trough below the rocky surface of the valley, this will still - not help us out of the difficulty. Taking, as before, the height of the - rocky bed of the valley at the watershed at 110 feet above the present - sea-level, and the bottom of the trough at 40 feet, this<span class="pagenum" id="Page_477">477</span> gives 70 feet - as the depth scooped out of the rock at that place. The depth of the - trough at Grangemouth below the rocky surface is 118 feet. Here we have - only 70 feet cut out at the only place where there was any resistance - to the current, as well as the place where it possessed any strength; - whereas at Grangemouth, where there was no resistance, and no strength - of current, 118 feet has been scooped out. Such a result as this is - diametrically opposed to all that we know of the dynamics of running - water.</p> - - <p>We may, therefore, conclude that it is physically impossible that this - hollow could have been cut out by the sea.</p> - - <p>Owing to the present tendency among geologists to attribute effects - of this kind to ocean-currents, I have been induced to enter thus at - much greater length than would otherwise have been necessary into the - facts and arguments against the possibility of the hollow having been - excavated by the sea. In the present case the discussion is specially - necessary, for here we have positive evidence of the sea having - occupied the valley for ages, along which this channel has been cut. - Consequently, unless it is proved that the sea could not possibly have - scooped out the channel, most geologists would be inclined to attribute - it to the sea-current which is known to have passed through the valley - rather than to any other cause.</p> - - <p>But that it is a hollow of denudation, and has been scooped out by some - agent, is perfectly certain. By what agent, then, has the erosion been - made? The only other cause to which it can possibly be attributed is - either land-ice or river-action.</p> - - <p>The supposition that this hollow was scooped out by ice is not more - tenable than the supposition that the work has been done by the sea. - A glacier filling up the entire valley and descending into the German - Ocean would unquestionably not only deepen the valley, but would grind - down the surface over which it passed all along its course. But such a - glacier would not cut a deep and narrow channel along the bottom of the - valley. A glacier that could do this would be a small and narrow one, - just sufficiently large to fill this narrow trough;<span class="pagenum" id="Page_478">478</span> for if it were - much broader than the trough, it would grind away its edges, and make a - broad trough instead of a narrow one. But a glacier so small and narrow - as only to fill the trough, descending from the hills at Kilsyth to the - sea at Grangemouth, a distance of fifteen miles, is very improbable - indeed. The resistance to the advance of the ice along such a slope - would cause the ice to accumulate till probably the whole valley would - be filled.<a id="FNanchor_282" href="#Footnote_282" class="fnanchor">[282]</a></p> - - <p>There is no other way of explaining the origin of this hollow, but - upon the supposition of its being an old river-bed. But there is - certainly nothing surprising in the fact of finding an old watercourse - under the boulder clay and other deposits. Unless the present contour - of the country be very different from what it was at the earlier - part of the glacial epoch, there must have then been watercourses - corresponding to the Bonny Water and the river Carron of the present - day; and that the remains of these should be found under the present - surface deposits is not surprising, seeing that these deposits are of - such enormous thickness. When water began to flow down our valleys, on - the disappearance of the ice at the close of the glacial epoch, the - Carron and the Bonny Water would not be able to regain their old rocky - channels, but would be obliged to cut, as they have done, new courses - for themselves on the surface of the deposits under which their old - ones lay buried.</p> - - <p>Although an old pre-glacial or inter-glacial river-bed is in <span class="pagenum" id="Page_479">479</span>itself an - object of much interest and curiosity, still, it is not on that account - that I have been induced to enter so minutely into the details of this - buried hollow. There is something of far more importance attached to - this hollow than the mere fact of its being an old watercourse. For the - fact that it enters the Firth of Forth at a depth of 260 feet below the - present sea-level, proves incontestably that at the time this hollow - was occupied by a stream, <em>the land must have stood at least between - 200 and 300 feet higher in relation to the sea-level than at present</em>.</p> - - <p>We have seen that the old surface of the country in the neighbourhood - of Grangemouth, out of which this ancient stream cut its channel, - is at least 150 feet below the present sea-level. Now, unless this - surface had been above the sea-level at that time, the stream would - not have cut a channel in it. But it has not merely cut a channel, but - cut one to a depth of 120 feet. It is impossible that this channel - could have been occupied by a river of sufficient volume to fill it. - It is not at all likely that the river which scooped it out could have - been much larger than the Carron of the present day, for the area of - drainage, from the very formation of the country, could not have been - much greater above Grangemouth than at present. An elevation of the - land would, no doubt, increase the area of the drainage of the stream - measured from its source to where it might then enter the sea, because - it would increase the length of the stream; but it would neither - increase the area of drainage, nor the length of the stream above - Grangemouth. Kilsyth would be the watershed then as it is now.</p> - - <p>What we have here is not the mere channel which had been occupied by - the ancient Carron, but the valley in which the channel lay. It may, - perhaps, be more properly termed a buried river valley; formed, no - doubt, like other river valleys by the denuding action of rain and - river.</p> - - <p>The river Carron at present is only a few feet deep. Suppose the - ancient Carron, which flowed in this old channel, to have been say 10 - feet deep. This would show that the land in relation to the sea at that - time must have stood at least 250 feet<span class="pagenum" id="Page_480">480</span> higher than at present. If 10 - feet was the depth of this old river, and Grangemouth the place where - it entered the sea, then 250 feet would be the extent of the elevation. - But it is probable that Grangemouth was not the mouth of the river; it - would likely be merely the place where it joined the river Forth of - that period. We have every reason to believe that the bed of the German - Ocean was then dry land, and that the Forth, Tay, Tyne, and other - British rivers flowing eastward, as Mr. Godwin-Austin supposes, were - tributaries to the Rhine, which at that time was a huge river passing - down the bed of the German Ocean, and entering the Atlantic to the west - of the Orkney Islands. That the German Ocean, as well as the sea-bed of - the Western Hebrides, was dry land at a very recent geological period, - is so well known, that, on this point, I need not enter into details. - We may, therefore, conclude that the river Forth, after passing - Grangemouth, would continue to descend until it reached the Rhine. If, - by means of borings, we could trace the old bed of the Forth and the - Rhine up to the point where the latter entered the Atlantic, in the - same way as we have done the Bonny Water and the Carron, we should no - doubt obtain a pretty accurate estimate as to the height at which the - land stood at that remote period. Nothing whatever, I presume, is known - as to the depth of the deposits covering the bed of the German Ocean - along what was then the course of the Rhine. It must, no doubt, be - something enormous. We are also in ignorance as to the thickness of the - deposits covering the ancient bed of the Forth. A considerable number - of bores have been put down at various parts of the Firth of Forth in - connection with the contemplated railway bridge across the Firth, but - in none of those bores has the rock been reached. Bores to a depth of - 175 feet have been made without even passing through the deposits of - silt which probably overlie an enormous thickness of sand and boulder - clay. Even in places where the water is 40 fathoms deep and quite - narrow, the bottom is not rock but silt.</p> - - <p>It is, however, satisfactory to find on the land a confirmation<span class="pagenum" id="Page_481">481</span> of - what has long been believed from evidence found in the seas around our - island, that at a very recent period the sea-level in relation to the - land must have been some hundreds of feet lower than at the present - day, and that our island must have at that time formed a part of the - great eastern continent.</p> - - <p>A curious fact was related to me by Mr. Stirling, the manager of the - Grangemouth collieries, which seems to imply a great elevation of the - land at a period long posterior to the time when this channel was - scooped out.</p> - - <p>In sinking a pit at Orchardhead, about a mile to the north of - Grangemouth, the workmen came upon the boulder clay after passing - through about 110 feet of sand, clay, and gravel. On the upper surface - of the boulder clay they found cut out what Mr. Stirling believes - to have been an old watercourse. It was 17 feet deep, and not much - broader. The sides of the channel appear to have been smooth and - water-worn, and the whole was filled with a fine sharp sand beautifully - stratified. As this channel lay about 100 feet below the present - sea-level, it shows that if it actually be an old watercourse, it must - have been scooped out at a time when the land in relation to the sea - stood at least 100 feet higher than at present.</p> - - <p><em>Buried River Channel from Kilsyth to the Clyde.</em>—In all probability - the western half of this great hollow, extending from the watershed - at Kilsyth to the Clyde, is also an old river channel, probably - the ancient bed of the Kelvin. This point cannot, however, be - satisfactorily settled until a sufficient number of bores have been - made along the direct line of the hollow, so as to determine with - certainty its width and general form and extent. That the western - channel is as narrow as the eastern is very probable. It has been - found that its sides at some places, as, for example, at Garscadden, - are very steep. At one place the north side is actually an overhanging - buried precipice, the bottom of which is about 200 feet below the - sea-level. We know also that the coal and ironstone in that quarter are - cut through by the trough, and the miners there have to exercise great - caution in driving their workings, in<span class="pagenum" id="Page_482">482</span> case they might cut into it. The - trough along this district is filled with sand, and is known to the - miners of the locality as the “sand-dyke.” To cut into running sand at - a depth of 40 or 50 fathoms is a very dangerous proceeding, as will be - seen from the details given in Mr. Bennie’s paper<a id="FNanchor_283" href="#Footnote_283" class="fnanchor">[283]</a> of a disaster - which occurred about twenty years ago to a pit near Duntocher, where - this trough was cut into at a depth of 51 fathoms from the surface.</p> - - <p>The depth of this hollow, below the present sea-level at Drumry, as - ascertained by a bore put down, is 230 feet. For several miles to the - east the depth is nearly as great. Consequently, if this hollow be an - old river-bed, the ancient river that flowed in it must have entered - the Clyde at a depth of more than 200 feet below the present sea-level; - and if so, then it follows that the rocky bed of the ancient Clyde must - lie buried under more than 200 feet of surface deposits from Bowling - downwards to the sea. Whether this is the case or not we have no means - at present of determining. The manager to the Clyde Trustees informs - me, however, that in none of the borings or excavations which have - been made has the rock ever been reached from Bowling downwards. The - probability is, that this deep hollow passes downwards continuously to - the sea on the western side of the island as on the eastern.<a id="FNanchor_284" href="#Footnote_284" class="fnanchor">[284]</a></p> - - <p>The following journals of a few of the borings will give the reader an - idea of the character of the deposits filling the <span class="pagenum" id="Page_483">483</span>channels. The beds - which are believed to be boulder clay are printed in italics:—</p> - - <div class="center smcap mt5">Borings made through the Deposits filling the Western Channel.</div> - - <div class="center mt1 mb2">Bore, Drumry Farm, on Lands of Garscadden.</div> - - <table summary="Bore, Drumry Farm"> - <tbody> - <tr> - <th> </th> - <th class="tdr"><div>ft.</div></th> - <th class="tdr"><div>ins.</div></th> - </tr> - <tr> - <td>Surface soil</td> - <td class="tdr"><div>2</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Sand and gravel</td> - <td class="tdr"><div>3</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Dry sand</td> - <td class="tdr"><div>11</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Blue mud</td> - <td class="tdr"><div>8</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Light mud and sand beds</td> - <td class="tdr"><div>13</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>31</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Sand and mud</td> - <td class="tdr"><div>8</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand and gravel</td> - <td class="tdr"><div>19</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>8</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>24</div></td> - <td class="tdr"><div>4</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>5</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>9</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>71</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Sand (coaly)</td> - <td class="tdr"><div>1</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>9</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand (coaly)</td> - <td class="tdr"><div>1</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>3</div></td> - </tr> - <tr> - <td>Red clay and gravel</td> - <td class="tdr"><div>4</div></td> - <td class="tdr"><div>8</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>1</div></td> - <td class="tdr"><div>5</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>2</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>2</div></td> - <td class="tdr"><div>8</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>1</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>8</div></td> - <td class="tdr"><div>10</div></td> - </tr> - <tr> - <td><i>Clay stones and gravel</i></td> - <td class="tdr"><div>33</div></td> - <td class="tdr"><div>3</div></td> - </tr> - <tr> - <td> </td> - <td colspan="2" class="tdr"><div><b>———————</b></div></td> - </tr> - <tr> - <td> </td> - <td class="tdr"><div>297</div></td> - <td class="tdr"><div>10</div></td> - </tr> - </tbody> - </table> - - <div class="center mt2 mb2">Bore on Mains of Garscadden, one mile north-east of Drumry.</div> - - <table summary="Bore on Mains of Garscadden"> - <tbody> - <tr> - <th> </th> - <th class="tdr"><div>ft.</div></th> - <th class="tdr"><div>ins.</div></th> - </tr> - <tr> - <td>Surface soil</td> - <td class="tdr"><div>1</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Blue clay and stones</td> - <td class="tdr"><div>60</div></td> - <td class="tdr"><div>1</div></td> - </tr> - <tr> - <td>Red clay and stones</td> - <td class="tdr"><div>18</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Soft clay and sand beds</td> - <td class="tdr"><div>7</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>6</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Large gravel</td> - <td class="tdr"><div>9</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand and gravel</td> - <td class="tdr"><div>7</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Hard gravel</td> - <td class="tdr"><div>1</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Sand and gravel</td> - <td class="tdr"><div>16</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Dry sand</td> - <td class="tdr"><div>30</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Black sand</td> - <td class="tdr"><div>2</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Dry sand</td> - <td class="tdr"><div>33</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Wet sand</td> - <td class="tdr"><div>8</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Light mud</td> - <td class="tdr"><div>5</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>3</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>5</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Sandstone, black</td> - <td class="tdr"><div>0</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Blue clay and stones</td> - <td class="tdr"><div>1</div></td> - <td class="tdr"><div>4</div></td> - </tr> - <tr> - <td>Whin block</td> - <td class="tdr"><div>0</div></td> - <td class="tdr"><div>10</div></td> - </tr> - <tr> - <td>Sandy clay</td> - <td class="tdr"><div>4</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td> </td> - <td colspan="2" class="tdr"><div><b>———————</b></div></td> - </tr> - <tr> - <td> </td> - <td class="tdr"><div>219</div></td> - <td class="tdr"><div>8</div></td> - </tr> - </tbody> - </table> - - <div class="center mt2 mb2">Bore nearly half a mile south-west of Millichen.</div> - - <table summary="Bore Millichen"> - <tbody> - <tr> - <th> </th> - <th class="tdr"><div>ft.</div></th> - <th class="tdr"><div>ins.</div></th> - </tr> - <tr> - <td>Sandy clay</td> - <td class="tdr"><div>5</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td><i>Brown clay and stones</i></td> - <td class="tdr"><div>17</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Mud</td> - <td class="tdr"><div>6</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sandy mud</td> - <td class="tdr"><div>31</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand and gravel with water</td> - <td class="tdr"><div>28</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sandy clay and gravel</td> - <td class="tdr"><div>17</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>5</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Mud</td> - <td class="tdr"><div>6</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>14</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>30</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td><i>Brown sandy clay and stones</i></td> - <td class="tdr"><div>30</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Hard red gravel</td> - <td class="tdr"><div>4</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Light mud and sand</td> - <td class="tdr"><div>1</div></td> - <td class="tdr"><div>8</div></td> - </tr> - <tr> - <td><i>Light clay and stones</i></td> - <td class="tdr"><div>6</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td><i>Light clay and whin block</i></td> - <td class="tdr"><div>26</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Fine sandy mud</td> - <td class="tdr"><div>36</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td><i>Brown clay and gravel and stones</i></td> - <td class="tdr"><div>14</div></td> - <td class="tdr"><div>4</div></td> - </tr> - <tr> - <td><i>Bark clay and stones</i></td> - <td class="tdr"><div>68</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td> </td> - <td colspan="2" class="tdr"><div><b>———————</b></div></td> - </tr> - <tr> - <td> </td> - <td class="tdr"><div>355</div></td> - <td class="tdr"><div>0</div></td> - </tr> - </tbody> - </table> - - <div class="center mt2 mb2">Bore at West Millichen, about 100 yards east of farm-house.</div> - - <table summary="Bore at West Millichen"> - <tbody> - <tr> - <th> </th> - <th class="tdr"><div>ft.</div></th> - <th class="tdr"><div>ins.</div></th> - </tr> - <tr> - <td>Soil</td> - <td class="tdr"><div>1</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td><i>Muddy sand and stones</i></td> - <td class="tdr"><div>4</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Soft mud</td> - <td class="tdr"><div>4</div></td> - <td class="tdr"><div>4</div></td> - </tr> - <tr> - <td>Sand and gravel</td> - <td class="tdr"><div>45</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td><i>Sandy mud and stones</i></td> - <td class="tdr"><div>20</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Coarse gravel</td> - <td class="tdr"><div>11</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Clay and gravel</td> - <td class="tdr"><div>1</div></td> - <td class="tdr"><div>4</div></td> - </tr> - <tr> - <td>Fine mud</td> - <td class="tdr"><div>7</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand and gravel</td> - <td class="tdr"><div>2</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sandy mud</td> - <td class="tdr"><div>30</div></td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td><i>Brown sandy clay and stones</i></td> - <td class="tdr"><div>25</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand and gravel</td> - <td class="tdr"><div>6</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td><i>Brown sandy clay and stones</i></td> - <td class="tdr"><div>12</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>2</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td><i>Brown sandy clay and stones</i></td> - <td class="tdr"><div>4</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Mud</td> - <td class="tdr"><div>5</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td>Mud and sand</td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>9</div></td> - </tr> - <tr> - <td>Sand and stones</td> - <td class="tdr"><div>2</div></td> - <td class="tdr"><div>9</div></td> - </tr> - <tr> - <td><i>Blue clay and stones</i></td> - <td class="tdr"><div>5</div></td> - <td class="tdr"><div>0</div></td> - </tr> - <tr> - <td> </td> - <td colspan="2" class="tdr"><div><b>———————</b></div></td> - </tr> - <tr> - <td> </td> - <td class="tdr"><div>200</div></td> - <td class="tdr"><div>4</div></td> - </tr> - </tbody> - </table> - - <p><span class="pagenum" id="Page_484">484</span></p> - - <div class="center smcap mt5 mb2">Borings made through the Deposits filling the Eastern Channel.</div> - - <div class="center mt1 mb2">No. 1. Between Towncroft Farm and Carron River—200 yards from river. - Height of surface, 12 feet above sea-level.</div> - - <table summary="Bore No. 1"> - <tbody> - <tr> - <th> </th> - <th class="tdr"><div>Feet.</div></th> - </tr> - <tr> - <td>Surface sand</td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Blue mud</td> - <td class="tdr"><div>4</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>4</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>3</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>33</div></td> - </tr> - <tr> - <td>Red clay</td> - <td class="tdr"><div>46</div></td> - </tr> - <tr> - <td><i>Soft blue till</i></td> - <td class="tdr"><div>17</div></td> - </tr> - <tr> - <td><i>Hard blue till</i></td> - <td class="tdr"><div>140</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>20</div></td> - </tr> - <tr> - <td> </td> - <td colspan="2" class="tdr"><div><b>——</b></div></td> - </tr> - <tr> - <td> </td> - <td class="tdr"><div>273</div></td> - </tr> - </tbody> - </table> - - <div class="center mt2 mb2">No. 2. About 150 yards north of No. 1. Height of surface, 12 feet above - sea-level.</div> - - <table summary="Bore No. 2"> - <tbody> - <tr> - <th> </th> - <th class="tdr"><div>Feet.</div></th> - </tr> - <tr> - <td>Surface sand</td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Blue mud</td> - <td class="tdr"><div>3</div></td> - </tr> - <tr> - <td>Shell bed</td> - <td class="tdr"><div>1</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>2</div></td> - </tr> - <tr> - <td>Blue mud</td> - <td class="tdr"><div>8</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>3</div></td> - </tr> - <tr> - <td>Blue muddy sand</td> - <td class="tdr"><div>15</div></td> - </tr> - <tr> - <td>Red clay</td> - <td class="tdr"><div>49</div></td> - </tr> - <tr> - <td><i>Blue till and stones</i></td> - <td class="tdr"><div>20</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>20</div></td> - </tr> - <tr> - <td><i>Hard blue till and stones</i></td> - <td class="tdr"><div>24</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>2</div></td> - </tr> - <tr> - <td><i>Hard blue till and stones</i></td> - <td class="tdr"><div>40</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>7</div></td> - </tr> - <tr> - <td><i>Hard blue till</i></td> - <td class="tdr"><div>24</div></td> - </tr> - <tr> - <td> </td> - <td colspan="2" class="tdr"><div><b>——</b></div></td> - </tr> - <tr> - <td> </td> - <td class="tdr"><div>234</div></td> - </tr> - </tbody> - </table> - - <div class="center mt2 mb2">No. 3. About 150 yards north of No. 2. Height of surface, 12 feet above - sea-level.</div> - - <table summary="Bore No. 3"> - <tbody> - <tr> - <th> </th> - <th class="tdr"><div>Feet.</div></th> - </tr> - <tr> - <td>Surface sand</td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Soft mud with shells</td> - <td class="tdr"><div>11</div></td> - </tr> - <tr> - <td>Blue mud and sand (hard)</td> - <td class="tdr"><div>3</div></td> - </tr> - <tr> - <td>Channel (rough gravel)</td> - <td class="tdr"><div>3</div></td> - </tr> - <tr> - <td>Fine sand</td> - <td class="tdr"><div>8</div></td> - </tr> - <tr> - <td>Running sand (red and fine)</td> - <td class="tdr"><div>17</div></td> - </tr> - <tr> - <td>Red clay</td> - <td class="tdr"><div>30</div></td> - </tr> - <tr> - <td><i>Soft till</i></td> - <td class="tdr"><div>36</div></td> - </tr> - <tr> - <td>Sand (pure)</td> - <td class="tdr"><div>2</div></td> - </tr> - <tr> - <td><i>Soft till and sand</i></td> - <td class="tdr"><div>17</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>8</div></td> - </tr> - <tr> - <td><i>Hard blue till</i></td> - <td class="tdr"><div>14</div></td> - </tr> - <tr> - <td> </td> - <td colspan="2" class="tdr"><div><b>——</b></div></td> - </tr> - <tr> - <td> </td> - <td class="tdr"><div>155</div></td> - </tr> - </tbody> - </table> - - <div class="center mt2 mb2">No. 4. About 100 yards from No. 1.</div> - - <table summary="Bore No. 4"> - <tbody> - <tr> - <th> </th> - <th class="tdr"><div>Feet.</div></th> - </tr> - <tr> - <td>Surface</td> - <td class="tdr"><div>5</div></td> - </tr> - <tr> - <td>Blue mud</td> - <td class="tdr"><div>5</div></td> - </tr> - <tr> - <td>Black sand</td> - <td class="tdr"><div>3</div></td> - </tr> - <tr> - <td>Gravel</td> - <td class="tdr"><div>3</div></td> - </tr> - <tr> - <td><i>Red clay and stones</i></td> - <td class="tdr"><div>34</div></td> - </tr> - <tr> - <td>Red clay</td> - <td class="tdr"><div>44</div></td> - </tr> - <tr> - <td><i>Soft blue till</i></td> - <td class="tdr"><div>32</div></td> - </tr> - <tr> - <td><i>Hard blue till and stones</i></td> - <td class="tdr"><div>104</div></td> - </tr> - <tr> - <td>Grey sand not passed through</td> - <td class="tdr"><div>22</div></td> - </tr> - <tr> - <td> </td> - <td colspan="2" class="tdr"><div><b>——</b></div></td> - </tr> - <tr> - <td> </td> - <td class="tdr"><div>252</div></td> - </tr> - <tr> - <td> Rock-head not reached.</td> - <td> </td> - </tr> - </tbody> - </table> - - <div class="center mt5 mb2">No. 5. About 50 yards north of No. 4.</div> - - <table summary="Bore No. 5"> - <tbody> - <tr> - <th> </th> - <th class="tdr"><div>Feet.</div></th> - </tr> - <tr> - <td>Surface</td> - <td class="tdr"><div>6</div></td> - </tr> - <tr> - <td>Blue mud</td> - <td class="tdr"><div>3</div></td> - </tr> - <tr> - <td>Shell bed</td> - <td class="tdr"><div>1</div></td> - </tr> - <tr> - <td>Channel</td> - <td class="tdr"><div>2</div></td> - </tr> - <tr> - <td>Blue mud</td> - <td class="tdr"><div>8</div></td> - </tr> - <tr> - <td>Channel</td> - <td class="tdr"><div>3</div></td> - </tr> - <tr> - <td>Blue mud and sand</td> - <td class="tdr"><div>15</div></td> - </tr> - <tr> - <td>Red clay and sand</td> - <td class="tdr"><div>10</div></td> - </tr> - <tr> - <td>Red clay</td> - <td class="tdr"><div>49</div></td> - </tr> - <tr> - <td><i>Blue till and stones</i></td> - <td class="tdr"><div>20</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>20</div></td> - </tr> - <tr> - <td><i>Hard blue till and stones</i></td> - <td class="tdr"><div>24</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>2</div></td> - </tr> - <tr> - <td><i>Hard blue till and stones</i></td> - <td class="tdr"><div>40</div></td> - </tr> - <tr> - <td>Sand</td> - <td class="tdr"><div>7</div></td> - </tr> - <tr> - <td><i>Hard blue till</i></td> - <td class="tdr"><div>24</div></td> - </tr> - <tr> - <td> </td> - <td colspan="2" class="tdr"><div><b>——</b></div></td> - </tr> - <tr> - <td> </td> - <td class="tdr"><div>211</div></td> - </tr> - </tbody> - </table> - - <div class="center mt2 mb2">No. 6. Between Heuck and Carron River.</div> - - <table summary="Bore No. 6"> - <tbody> - <tr> - <th> </th> - <th class="tdr"><div>Feet.</div></th> - </tr> - <tr> - <td>Sandy clay</td> - <td class="tdr"><div>7</div></td> - </tr> - <tr> - <td>Mud</td> - <td class="tdr"><div>16</div></td> - </tr> - <tr> - <td><i>Brown sandy clay and stones</i></td> - <td class="tdr"><div>3</div></td> - </tr> - <tr> - <td>Mud</td> - <td class="tdr"><div>36</div></td> - </tr> - <tr> - <td>Brown clay</td> - <td class="tdr"><div>39</div></td> - </tr> - <tr> - <td><i>Blue till and stones</i></td> - <td class="tdr"><div>54</div></td> - </tr> - <tr> - <td> </td> - <td colspan="2" class="tdr"><div><b>——</b></div></td> - </tr> - <tr> - <td></td> - <td class="tdr"><div>155</div></td> - </tr> - </tbody> - </table> - - <p><span class="pagenum" id="Page_485">485</span></p> - - <p>The question arises as to what is the origin of the stratified sands - and gravels filling up the buried river channels. Are they of marine or - of freshwater origin? Mr. Dugald Bell<a id="FNanchor_285" href="#Footnote_285" class="fnanchor">[285]</a> and Mr. James Geikie<a id="FNanchor_286" href="#Footnote_286" class="fnanchor">[286]</a> - are inclined to believe that as far as regards those filling the - western channel they are of lacustrine origin; that they were formed - in lakes, produced by the damming back of the water resulting from the - melting of the ice. I am, however, for the following reasons, inclined - to agree with Mr. Bennie’s opinion that they are of marine origin. - It will be seen, by a comparison of the journals of the borings made - through the deposits in the eastern channel with those in the western, - that they are of a similar character; so that, if we suppose those in - the western channel to be of freshwater origin, we may from analogy - infer the same in reference to the origin of those in the eastern - channel. But, as we have already seen, the deposits extend to the Firth - of Forth at Grangemouth, where they are met with at a depth of 260 feet - below sea-level. Consequently, if we conclude them to be of freshwater - origin, we are forced to the assumption, not that the water formed by - the melted ice was dammed back, but that the sea itself was dammed - back, and that by a wall extending to a depth of not less than two or - three hundred feet, so as to allow of a lake being formed in which the - deposits might accumulate; assuming, of course, that the absolute level - of the land was the same then as it is now.</p> - - <p>But as regards the stratified deposits of Grangemouth, we have direct - evidence of their marine origin down to the bottom of the Red Clay that - immediately overlies the till and its intercalated beds, which on an - average is no less than 85 feet, and in some cases 100 feet, below the - present surface. From this deposit, Foraminifera, indicating an arctic - condition of sea, were determined by Mr. David Robertson. Marine shells - were also found in this bed, and along with them the remains of a - seal, which was determined by Professor Turner to be of an exceedingly - <span class="pagenum" id="Page_486">486</span>arctic type, thus proving that these deposits were not only marine but - glacial.</p> - - <p>Direct fossil evidence as to the character of the deposits occupying - the western basin, is, however, not so abundant, but this may be owing - to the fact that during the sinking of pits, no special attention - has been paid to the matter. At Blairdardie, in sinking a pit-shaft - through these deposits, shells were found in a bed of sand between two - immense masses of boulder clay. The position of this bed will be better - understood from the following section of the pit-shaft:—</p> - - <table summary=""> - <tbody> - <tr> - <th> </th> - <th class="tdr"><div>Feet.</div></th> - </tr> - <tr> - <td>Surface soil</td> - <td class="tdr"><div>4½</div></td> - </tr> - <tr> - <td>Blue clay</td> - <td class="tdr"><div>9 </div></td> - </tr> - <tr> - <td>Hard stony clay</td> - <td class="tdr"><div>69 </div></td> - </tr> - <tr> - <td>Sand with, a few <i>shells</i></td> - <td class="tdr"><div>3 </div></td> - </tr> - <tr> - <td>Stony clay and boulders</td> - <td class="tdr"><div>46½</div></td> - </tr> - <tr> - <td>Mud and running sand</td> - <td class="tdr"><div>11 </div></td> - </tr> - <tr> - <td>Hard clay, boulders, and broken rock</td> - <td class="tdr"><div>27 </div></td> - </tr> - <tr> - <td> </td> - <td colspan="2" class="tdr"><div><b>———</b></div></td> - </tr> - <tr> - <td> </td> - <td class="tdr"><div>170 </div></td> - </tr> - </tbody> - </table> - - <p class="noindent">But as the shells were not preserved, we have, of course, no means of - determining whether they were of marine or of freshwater origin.</p> - - <p>In another pit, at a short distance from the above, <i>Cyprina Islandica</i> - was found in a bed at the depth of 54 feet below the surface.<a id="FNanchor_287" href="#Footnote_287" class="fnanchor">[287]</a></p> - - <p>In a paper read by Mr. James Smith, of Jordanhill, to the Geological - Society, April 24th, 1850,<a id="FNanchor_288" href="#Footnote_288" class="fnanchor">[288]</a> the discovery is recorded of a - stratified bed containing <i>Tellina proxima</i> intercalated between two - distinct boulder clays. The bed was discovered by Mr. James Russell in - sinking a well at Chapelhall, near Airdrie. Its height above sea-level - was 510 feet. The character of the shell not only proves the marine - origin of the bed, but also the existence of a submergence to that - extent during an inter-glacial period.</p> - - <p>On the other hand, the difficulty besetting the theory of the marine - origin of the deposits is this. The intercalated boulder clays bear - no marks of stratification, and are evidently the true unstratified - till formed when the country was covered <span class="pagenum" id="Page_487">487</span>by ice. But the fact that - these beds are both underlaid and overlaid by stratified deposits - would, on the marine theory, imply not merely the repeated appearance - and disappearance of the ice, but also the repeated submergence and - emergence of the land. If the opinion be correct that the submergences - and emergences of the glacial epoch were due to depressions and - elevations of the land, and not to oscillations of sea-level, then - the difficulty in question is, indeed, a formidable one. But, on the - other hand, if the theory of submergences propounded in Chapters <a href="#CHAPTER_XXIII">XXIII.</a> - and <a href="#CHAPTER_XXIV">XXIV.</a> be the true one, the difficulty entirely disappears. The - explanation is as follows, viz., during a cold period of the glacial - epoch, when the winter solstice was in aphelion, the low grounds would - be covered with ice, under which a mass of till would be formed. - After the cold began to decrease, and the ice to disappear from the - plains, the greatest rise of the ocean, for reasons already stated, - would take place. The till covering the low grounds would be submerged - to a considerable depth and would soon be covered over by mud, sand, - and gravel, carried down by streams from the high ground, which, at - the time, would still be covered with snow and ice. In course of time - the sea would begin to sink and a warm and continental period of, - perhaps, from 6,000 to 10,000 years, would follow, when the sea would - be standing at a much lower level than at present. The warm period - would be succeeded by a second cold period, and the ice would again - cover the land and form a second mass of till, which, in some places, - would rest directly on the former till, while in other places it would - be laid down upon the surface of the sands and gravels overlying the - first mass. Again, on the disappearance of the ice the second mass of - till would be covered over in like manner by mud, sand, and gravel, and - so on, while the eccentricity of the earth’s orbit continued at a high - value. In this way we might have three, four, five, or more masses of - till separated by beds of sand and gravel.</p> - - <p>It will be seen from <a href="#TABLE_IV">Table IV.</a> of the eccentricity of the<span class="pagenum" id="Page_488">488</span> earth’s - orbit, given in <a href="#CHAPTER_XIX">Chapter XIX.</a>, that the former half of that long - succession of cold and warm periods, known as the glacial epoch, - was much more severe than the latter half. That is to say, in the - former half the accumulation of ice during the cold periods, and its - disappearance in polar regions during the warm periods, would be - greater than in the latter half. It was probable that it was during - the warm periods of the earlier part of the glacial epoch that the two - buried channels of the Midland valley were occupied by rivers, and that - it was during the latter and less severe part of the glacial epoch that - these channels became filled up with that remarkable series of deposits - which we have been considering.</p> - - <p><em>Other buried River Channels.</em>—A good many examples of buried river - channels have been found both in Scotland and in England, though none - of them of so remarkable a character as the two occupying the valley - of the Forth and Clyde Canal which have been just described. I may, - however, briefly refer to one or two localities where some of these - occur.</p> - - <p>(1.) An ancient buried river channel, similar to the one extending - from Kilsyth to Grangemouth, exists in the coal-fields of Durham, - and is known to miners in the district as the “Wash.” Its course was - traced by Mr. Nicholas Wood, F.G.S., and Mr. E. F. Boyd, from Durham - to Newcastle, a distance of fourteen miles.<a id="FNanchor_289" href="#Footnote_289" class="fnanchor">[289]</a> It traverses, after - passing the city of Durham, a portion of the valley of the Wear, passes - Chester-le-Street, and then follows the valley of the river Team, and - terminates at the river Tyne. And what is remarkable, it enters the - Tyne at a depth of 140 feet below the present level of the sea. This - curious hollow lies buried, like the Scottish one just alluded to, - under an enormous mass of drift, and it is only through means of boring - and other mining operations that its character has been revealed. The - bottom and sides of this channel everywhere bear evidence of long - exposure to the abrading influence of water in motion; the rocky bottom - being smoothed, furrowed, and water-worn. The river Wear of the present - day flows to <span class="pagenum" id="Page_489">489</span>the sea over the surface of the drift at an elevation of - more than 100 feet above this buried river-bed. At the time that this - channel was occupied by running water the sea-level must have been at - least 140 feet lower than at present. This old river evidently belongs - to the same continental period as those of Scotland.</p> - - <p>(2.) From extensive borings and excavations, made at the docks of Hull - and Grimsby, it is found that the ancient bed of the Humber is buried - under more than 100 feet of silt, clay, and gravel. At Hull the bottom - of this buried trough was found to be 110 feet below the sea-level. - And what is most interesting at both these places, the remains of a - submerged forest was found at a depth of from thirty to fifty feet - below the sea-level. In some places two forests were found divided by a - bed of leafy clay from five to fifteen feet thick.</p> - - <p>(3.) In the valleys of Norfolk we also find the same conditions - exhibited. The ancient bed of the Yare and other rivers of this - district enter the sea at a depth of more than 100 feet below the - present sea-level. At Yarmouth the surface was found 170 feet thick, - and the deep surface extends along the Yare to beyond Norwich. Buried - forests are also found here similar to those on the Humber.</p> - - <p>It is probable that all our British rivers flow into the sea over their - old buried channels, except in cases where they may have changed their - courses since the beginning of the glacial epoch.</p> - - <p>(4.) In the Sanquhar Coal Basin, at the foot of the Kello Water, an - old buried river course was found by Mr. B. N. Peach. It ran at right - angles to the Kello, and was filled with boulder clay which cut off the - coal; but, on driving the mine through the clay, the coal was found in - position on the other side.</p> - - <p>(5.) An old river course, under the boulder clay, is described by Mr. - Milne Home in his memoir on the Mid-Lothian coal-fields. It has been - traced out from Niddry away in a N.E. direction by New Craighall. At - Niddry, the hollow is about 100 yards wide and between 60 and 70 feet - deep. It seems to<span class="pagenum" id="Page_490">490</span> deepen and widen as it approaches towards the sea, - for at New Craighall it is about 200 yards wide and 97 feet deep. This - old channel will probably enter the sea about Musselburgh. Like the - channels in the Midland Valley of Scotland already described, it is so - completely filled up by drift that not a trace of it is to be seen on - the surface. And like these, also, it must have belonged to a period - when the sea-level stood much lower than at present.</p> - - <p>(6.) At Hailes’ Quarry, near Edinburgh, there is to be seen a portion - of an ancient watercourse under the boulder drift. A short account - of it was given by Dr. Page in a paper read before the Edinburgh - Geological Society.<a id="FNanchor_290" href="#Footnote_290" class="fnanchor">[290]</a> The superincumbent sandstone, he says, has - been cut to a depth of 60 feet. The width of the channel at the surface - varies from 12 to 14 feet, but gradually narrows to 2 or 3 feet at the - bottom. The sides and bottom are smoothed and polished, and the whole - is now filled with till and boulders.</p> - - <p>(7.) One of the most remarkable buried channels is that along the - Valley of Strathmore, supposed to be the ancient bed of the Tay. It - extends from Dunkeld, the south of Blairgowrie, Ruthven, and Forfar, - and enters the German Ocean at Lunan Bay. Its length is about 34 miles.</p> - - <p>“No great river,” says Sir Charles Lyell, “follows this course, but - it is marked everywhere by lakes or ponds, which afford shell-marl, - swamps, and peat moss, commonly surrounded by ridges of detritus from - 50 to 70 feet high, consisting in the lower part of till and boulders, - and in the upper of stratified gravels, sand, loam, and clay, in some - instances curved or contorted.”<a id="FNanchor_291" href="#Footnote_291" class="fnanchor">[291]</a></p> - - <p>“It evidently marks an ancient line, by which, first, a great glacier - descended from the mountains to the sea, and by which, secondly, at - a later period, the principal water drainage of this country was - effected.”<a id="FNanchor_292" href="#Footnote_292" class="fnanchor">[292]</a></p> - - <p><span class="pagenum" id="Page_491">491</span></p> - - <p>(8.) A number of examples of ancient river courses, underneath the - boulder clay, are detailed by Professor Geikie in his glacial drift of - Scotland. Some of the cases described by him have acquired additional - interest from the fact of their bearing decided testimony to the - existence of inter-glacial warm periods. I shall briefly refer to a few - of the cases described by him.</p> - - <p>In driving a trial mine in a pit at Chapelhall, near Airdrie, the - workmen came upon what they believed to be an old river course. At - the end of the trial mine the ironstone, with its accompanying coal - and fire-clay, were cut off at an angle of about 20° by a stiff, - dark-coloured earth, stuck full of angular pieces of white sandstone, - coal, and shale, with rounded pebbles of greenstone, basalt, quartz, - &c. Above this lay a fine series of sand and clay beds. Above these - stratified beds lay a depth of 50 or 60 feet of true boulder clay. The - channel ran in the direction of north-east and south-west. Mr. Russell, - of Chapelhall, informs Professor Geikie that another of the same kind, - a mile farther to the north-west, had been traced in some of the pit - workings.</p> - - <p>“It is clear,” says Professor Geikie, “that whatever may be the true - explanation of these channels and basins, they unquestionably belong to - the period of the boulder clay. The Chapelhall basin lies, indeed, in a - hollow of the carboniferous rocks, but its stratified sands and clays - rest on an irregular floor of true till. The old channel near the banks - of the Calder is likewise scooped out of sandstones and shales; but - it has a coating of boulder clay, on which its finely-laminated sands - and clays repose, <em>as if the channel itself had once been filled with - boulder clay, which was re-excavated to allow of the deposition of the - stratified deposits. In all cases, a thick mantle of coarse, tumultuous - boulder clay buries the whole.</em>”<a id="FNanchor_293" href="#Footnote_293" class="fnanchor">[293]</a></p> - - <p>Professor Geikie found between the mouth of the Pease Burn and St. - Abb’s Head, Berwickshire, several ancient buried channels. One at - the Menzie Cleuch, near Redheugh Shore, was <span class="pagenum" id="Page_492">492</span>filled to the brim with - boulder clay. Another, the Lumsden Dean, half a mile to the east of - Fast Castle, on the bank of the Carmichael Burn, near the parish church - of Carmichael,—an old watercourse of the boulder clay period—is to - be seen. The valley of the Mouse Water he instances as a remarkable - example.</p> - - <p>One or two he found in Ayrshire, and also one on the banks of the Lyne - Water, a tributary of the Tweed.</p> - - <p>(9.) In the valley of the Clyde, above Hamilton, several buried river - channels have been observed. They are thus described by Mr. James - Geikie:—<a id="FNanchor_294" href="#Footnote_294" class="fnanchor">[294]</a></p> - - <p>“In the Wishaw district, two deep, winding troughs, filled with sand - and fine gravel, have been traced over a considerable area in the coal - workings.<a id="FNanchor_295" href="#Footnote_295" class="fnanchor">[295]</a> These troughs form no feature at the surface, but are - entirely concealed below a thick covering of boulder clay. They appear - to be old stream courses, and are in all probability the pre-glacial - ravines of the Calder Water and the Tillon Burn. The ‘sand-dyke’ that - represents the pre-glacial course of the Calder Water runs for some - distance parallel to the present course of the stream down to Wishaw - House, where it is intersected by the Calder, and the deposits which - choke it up are well seen in the steep wooded banks below the house - and in the cliff on the opposite side. It next strikes to south-east, - and is again well exposed on the road-side leading down from Wishaw - to the Calder Water. From this point it has been traced underground, - more or less continuously, as far as Wishaw Ironworks. Beyond this - place the coal-seams sink to a greater depth, and therefore cease to - be intersected by the ancient ravine, the course of which, however, - may still be inferred from the evidence obtained during the sinking - of shafts and trial borings. In all probability it runs south, and - enters the old course of the Clyde a little below Cambusnethan House. - Only a portion of the old ravine <span class="pagenum" id="Page_493">493</span>of the Tillon Burn is shown upon the - Map. It is first met with in the coal-workings of Cleland Townhead - (Sheet 31). From this place it winds underground in a southerly - direction until it is intersected by the present Tillon Burn, a little - north of Glencleland (Sheet 31). It now runs to south-west, keeping - parallel to the burn, and crosses the valley of the Calder just - immediately above the mouth of the Tillon. From this point it can be - traced in pit-shafts, open-air sections, borings, and coal-workings, - by Ravenscraig, Nether Johnstone, and Robberhall Belting, on to the - Calder Water below Coursington Bridge (Sheet 31). It would thus appear - that in pre-glacial times the Calder and the Tillon were independent - streams, and that since glacial times the Calder Water, forsaking its - pre-glacial course, has cut its way across the intervening ground, - ploughing out deep ravines in the solid rocks, until eventually it - united with the Tillon. Similar buried stream courses occur at other - places. Thus, at Fairholme, near Larkhall, as already mentioned (par. - 94), the pre-glacial course of the Avon has been traced in pit-shafts - and borings for some distance to the north. Another old course, filled - up with boulder clay, is exposed in a burn near Plotcock, a mile - south-west from Millheugh; and a similar pre-glacial ravine was met - with in the cement-stone workings at Calderwood.<a id="FNanchor_296" href="#Footnote_296" class="fnanchor">[296]</a> Indeed, it might - be said with truth that nearly all the rocky ravines through which the - waters flow, especially in the carboniferous areas, are of post-glacial - age—the pre-glacial courses lying concealed under masses of drift. Most - frequently, however, the present courses of the streams are partly - pre-glacial and partly post-glacial. In the pre-glacial portions the - streams flow through boulder clay, in the post-glacial reaches their - course, as just mentioned, is usually in rocky ravines. The Avon and - the Calder, with their tributaries, afford numerous illustrations of - these phenomena.”</p> - - <p><span class="pagenum" id="Page_494">494</span></p> - - <p>The question naturally arises, When were those channels scooped out? - To what geological period must those ancient rivers be referred? It - will not do to conclude that those channels must be pre-glacial simply - because they contain boulder clay. Had the glacial epoch been one - unbroken period of cold, and the boulder clay one continuous formation, - then the fact of finding boulder clay in those channels would show that - they were pre-glacial. But when we find undoubted geological evidence - of a warm condition of climate of long continuance, during the severest - part of the glacial epoch, when the ice, to a great extent, must have - disappeared, and water began to flow as usual down our valleys, all - that can reasonably be inferred from the fact of finding till in those - channels, is that they must be older than the till they contain. We - cannot infer that they are older than all the till lying on the face - of the country. The probability, however, is, that some of them are - of pre-glacial and others of inter-glacial origin. That many of these - channels have been used as watercourses during the glacial epoch, or - rather during warm periods of that epoch, is certain, from the fact - that they have been filled with boulder clay, then re-excavated, and - finally filled up again with the clay.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XXX"> - <span class="pagenum" id="Page_495">495</span> - <h2> - CHAPTER XXX.<br /><br /> - <span class="small">THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THEORIES OF - GLACIER-MOTION.</span> - </h2> - </div> - <div class="subhead">Why the Question of Glacier-motion has been found to be - so difficult.—The Regelation Theory.—It accounts for the - Continuity of a Glacier, but not for its Motion.—Gravitation - proved by Canon Moseley insufficient to shear the Ice of a - Glacier.—Mr. Mathew’s Experiment.—No Parallel between the - bending of an Ice Plank and the shearing of a Glacier.—Mr. - Ball’s Objection to Canon Moseley’s Experiment.—Canon - Moseley’s Method of determining the Unit of Shear.—Defect - of Method.—Motion of a Glacier in some Way dependent - on Heat.—Canon Moseley’s Theory.—Objections to his - Theory.—Professor James Thomson’s Theory.—This Theory fails to - explain Glacier-motion.—De Saussure and Hopkins’s “Sliding” - Theories.—M. Charpentier’s “Dilatation” Theory.—Important - Element in the Theory.</div> - - <p class="noindent"><span class="smcap">The</span> cause of the motion of glaciers has proved to be one of the most - difficult and perplexing questions within the whole domain of physics. - The main difficulty lies in reconciling the motion of the glacier with - the physical properties of the ice. A glacier moves down a valley - very much in the same way as a river, the motion being least at the - sides and greatest at the centre, and greater at the surface than at - the bottom. In a cross section scarcely two particles will be moving - with the same velocity. Again, a glacier accommodates itself to the - inequalities of the channel in which it moves exactly as a semifluid - or plastic substance would do. So thoroughly does a glacier behave - in the manner of a viscous or plastic body that Professor Forbes was - induced to believe that viscosity was a property of the ice, and that - in virtue of this property it was enabled to move with a differential - motion and accommodate itself to all the inequalities of its channel - without losing its continuity just as a mass of mud or putty would do. - But experience proves that ice is a hard and brittle substance far<span class="pagenum" id="Page_496">496</span> - more resembling glass than putty. In fact it is one of the most brittle - and unyielding substances in nature. So unyielding is a glacier that - it will snap in two before it will stretch to any perceptible extent. - This is proved by the fact that crevasses resulting from a strain on - the glacier consist at first of a simple crack scarcely wide enough to - admit the blade of a penknife.</p> - - <p>All the effects which were considered to be due to the viscosity of - the ice have been fully explained and accounted for on the principle - of fracture and regelation discovered by Faraday. The principle of - regelation explains why the ice moving with a differential motion and - accommodating itself to the inequalities of its channel is yet enabled - to retain its continuity, but it does not account for the <em>cause</em> of - glacier motion. In fact it rather involves the question in deeper - mystery than before. For it is far more difficult to conceive how the - particles of a hard and brittle solid like that of ice can move with - a differential motion, than it is to conceive how this may take place - in the case of a soft and yielding substance. The particles of ice - have all to be displaced one over another and alongside each other, - and as those particles are rigidly fixed together this connection must - be broken before the one can slide over the other. <em>Shearing-force</em>, - as Canon Moseley shows, comes into play. Were ice a plastic substance - there would not be much difficulty in understanding how the particles - should move the one over the other, but it is totally different when - we conceive ice to be a solid and unyielding substance. The difficulty - in connection with glacier-motion is not to account for the continuity - of the ice, for the principle of regelation fully explains this, but - to show how it is that one particle succeeds in sliding over the over. - The principle of regelation, instead of assisting to remove this - difficulty, increases it tenfold. Regelation does not explain the cause - of glacier-motion, but the reverse. It rather tends to show that a - glacier should not move. What, then, is the cause of glacier-motion? - According to the regelation theory, gravitation is the impelling - cause.<span class="pagenum" id="Page_497">497</span> But is gravitation sufficient to <em>shear</em> the ice in the manner - in which it is actually done in a glacier?</p> - - <p>I presume that few who have given much thought to the subject of - glacier-motion have not had some slight misgivings in regard to the - commonly received theory. There are some facts which I never could - harmonize with this theory. For example, boulder clay is a far looser - substance than ice; its shearing-force must be very much less than - that of ice; yet immense masses of boulder clay will lie immovable for - ages on the slope of a hill so steep that one can hardly venture to - climb it, while a glacier will come crawling down a valley which by - the eye we could hardly detect to be actually off the level. Again, a - glacier moves faster during the day than during the night, and about - twice as fast during summer as during winter. Professor Forbes, for - example, found that the Glacier des Bois near its lower extremity moved - sometimes in December only 11·5 inches daily, while during the month - of July its rate of motion sometimes reached 52·1 inches per day. Why - such a difference in the rate of motion between day and night, summer - and winter? The glacier is not heavier during the day than it is - during the night, or during the summer than it is during the winter; - neither is the shearing-force of the great mass of the ice of a glacier - sensibly less during day than night, or during summer than winter; - for the temperature of the great mass of the ice does not sensibly - vary with the seasons. If this be the case, then gravitation ought to - be as able to shear the ice during the night as during the day, or - during the winter as during the summer. At any rate, if there should - be any difference it ought to be but trifling. It is true that, owing - to the melting of the ice, the crevices of the glacier are more gorged - with water during summer than winter; and this, as Professor Forbes - maintains,<a id="FNanchor_297" href="#Footnote_297" class="fnanchor">[297]</a> may tend to make the glacier move faster during the - former than the latter season. But the advocates of the regelation - theory cannot conclude, with Professor Forbes, that the water favours - the motion of the glacier <span class="pagenum" id="Page_498">498</span>by making the ice more soft and plastic. The - melting of the ice, according to the regelation theory, cannot very - materially aid the motion of the glacier.</p> - - <p>The theory which has led to the general belief that the ice of a - glacier is sheared by the force of gravity appears to be the following. - It is supposed that the only forces to which the motion of a glacier - can be referred are <em>gravitation</em> and <em>heat</em>; but as the great mass - of a glacier remains constantly at the same uniform temperature it - is concluded to be impossible that the motion of the glacier can be - due to this cause, and therefore of course it must be attributed to - gravitation, there being no other cause.</p> - - <p>That gravitation is insufficient to shear the ice of a glacier has been - clearly demonstrated by Canon Moseley.<a id="FNanchor_298" href="#Footnote_298" class="fnanchor">[298]</a> He determined by experiment - the amount of force required to shear one square inch of ice, and found - it to be about 75 lbs. By a process of calculation which will be found - detailed in the Memoir referred to, he demonstrated that to descend - by its own weight at the rate at which Professor Tyndall observed the - ice of the Mer de Glace to be descending at the Tacul, the unit of - shearing force of the ice could not have been more than 1·31931 lbs. - Consequently it will require a force more than 34 times the weight of - the glacier to shear the ice and cause it to descend in the manner in - which it is found to descend.</p> - - <p>It is now six years since Canon Moseley’s results were laid before the - public, and no one, as far as I am aware, has yet attempted to point - out any serious defect in his mathematical treatment of the question. - Seeing the great amount of interest manifested in the question of - glacier-motion, I think we are warranted to conclude that had the - mathematical part of the memoir been inconclusive its defects would - have been pointed out ere this time. The question, then, hinges on - whether the experimental data on which his calculations are based - be correct. Or, in other words, is the unit of shear of ice as much - as 75 lbs.? This part of Mr. Moseley’s researches has not passed - <span class="pagenum" id="Page_499">499</span>unquestioned. Mr. Ball and Mr. Mathews, both of whom have had much - experience among glaciers, and have bestowed considerable attention on - the subject of glacier-motion, have objected to the accuracy of Mr. - Moseley’s unit of shear. I have carefully read the interesting memoirs - of Mr. Mathews and Mr. Ball in reply to Canon Moseley, but I am unable - to perceive that anything which they have advanced materially affects - his general conclusions as regards the commonly received theory. Mr. - Mathews objects to Canon Moseley’s experiments on the grounds that - extraneous forces are brought to bear upon the substance submitted - to operation, and that conditions are thus introduced which do not - obtain in the case of an actual glacier. “It would throw,” he says, - “great light upon our inquiry if we were to change this method of - procedure and simply to observe the deportment of masses of ice under - the influence of no external forces but the gravitation of their own - particles.”<a id="FNanchor_299" href="#Footnote_299" class="fnanchor">[299]</a> A plank of ice six inches wide and 2⅜ inches in - thickness was supported at each end by bearers six feet apart. From the - moment the plank was placed in position it began to sink, and continued - to do so until it touched the surface over which it was supported. Mr. - Mathews remarks that with this property of ice, viz., its power to - change its form under strains produced by its own gravitation, combined - with the sliding movement demonstrated by Hopkins, we have an adequate - cause for glacier-motion. Mr. Mathews concludes from this experiment - that the unit of shear in ice, instead of being 75 lbs., is less than - 1¾ lbs.</p> - - <p>There is, however, no parallel between the bending of the ice-plank and - the shearing of a glacier. Mr. Mathews’ experiment appears to prove too - much, as will be seen from the following reply of Canon Moseley:—</p> - - <p>“Now I will,” he says, “suggest to Mr. Mathews a parallel experiment - and a parallel explanation. If a bar of wrought iron 1 inch square and - 20 feet long were supported at its extremities, it would <em>bend</em> by its - weight alone, and would therefore <span class="pagenum" id="Page_500">500</span>shear. Now the weight of such a - rod would be about 67 lbs. According to Mr. Mathews’s explanation in - the case of the ice-plank, the unit of shear in wrought-iron should - therefore be 67 lbs. per square inch. It is actually 50,000 lbs.”<a id="FNanchor_300" href="#Footnote_300" class="fnanchor">[300]</a></p> - - <p>Whatever theory we may adopt as to the cause of the motion of glaciers, - the deflection of the plank in the way described by Mr. Mathews - <em>follows as a necessary consequence</em>. Although no weight was placed - upon the plank, it does not necessarily follow that the deflection - was caused by the weight of the ice alone; for, according to Canon - Moseley’s own theory of the motion of glaciers by heat, the plank - ought to be deflected in the middle, just as it was in Mr. Mathews’s - experiment. A solid body, when exposed to variations of temperature, - will expand and contract transversely as well as longitudinally. Ice, - according to Canon Moseley’s theory, expands and contracts by heat. - Then if the plank expands transversely, the upper half of the plank - must rise and the lower half descend. But the side which rises has - to perform work against gravity, whereas the side which descends has - work performed upon it by gravity; consequently more of the plank will - descend than rise, and this will, of course, tend to lower or deflect - the plank in the middle. Again, when the plank contracts, the lower - half will rise and the upper half will descend; but as gravitation, - in this case also, favours the descending part and opposes the rising - part, more of the plank will descend than rise, and consequently - the plank will be lowered in the middle by contraction as well as - by expansion. Thus, as the plank changes its temperature, it must, - according to Mr. Moseley’s theory, descend or be deflected in the - middle, step by step—and this not by gravitation alone, but chiefly - by the motive power of heat. I do not, of course, mean to assert that - the descent of the plank was caused by heat; but I assert that Mr. - Mathews’s experiment does not necessarily prove (and this is all that - is required in the meantime) that gravitation alone was the cause of - the deflection of the plank. Neither does this experiment prove that - the ice was <span class="pagenum" id="Page_501">501</span>deflected without shearing; for although the weight of the - plank was not sufficient to shear the ice, as Mr. Mathews, I presume, - admits, yet Mr. Moseley would reply that the weight of the ice, - assisted by the motive power of heat, was perfectly sufficient.</p> - - <p>I shall now briefly refer to Mr. Ball’s principal objections to Canon - Moseley’s proof that a glacier cannot shear by its weight alone. One - of his chief objections is that Mr. Moseley has assumed the ice to be - homogeneous in structure, and that pressures and tensions acting within - it, are not modified by the varying constitution of the mass.<a id="FNanchor_301" href="#Footnote_301" class="fnanchor">[301]</a> - Although there is, no doubt, some force in this objection (for we have - probably good reason to believe that ice will shear, for example, more - easily along certain planes than others), still I can hardly think that - Canon Moseley’s main conclusion can ever be materially affected by this - objection. The main question is this, Can the ice of the glacier shear - by its own weight in the way generally supposed? Now the shearing force - of ice, take it in whatever direction we may, so enormously exceeds - that required by Mr. Moseley in order to allow a glacier to descend by - its weight only, that it is a matter of indifference whether ice be - regarded as homogeneous in structure or not. Mr. Ball objects also to - Mr. Moseley’s imaginary glacier lying on an even slope and in a uniform - rectangular channel. He thinks that an irregular channel and a variable - slope would be more favourable to the descent of the ice. But surely - if the work by the weight of the ice be not equal to the work by the - resistance in a glacier of uniform breadth and slope, it must be much - less so in the case of one of irregular shape and slope.</p> - - <p>That a relative displacement of the particles of the ice is involved - in the motion of a glacier, is admitted, of course, by Mr. Ball; but - he states that the amount of this displacement is but small, and that - it is effected with extreme slowness. This may be the case; but if the - weight of the ice be not able to overcome the mutual cohesion of the - particles, then the weight <span class="pagenum" id="Page_502">502</span>of the ice cannot produce the required - displacement, however small it may be. Mr. Ball then objects to Mr. - Moseley’s method of determining the unit of shear on this ground:—The - shearing of the ice in a glacier is effected with extreme slowness; - but the shearing in Canon Moseley’s experiment was effected with - rapidity; and although it required 75 lbs. to shear one square inch of - surface in his experiment, it does not follow that 75 lbs. would be - required to shear the ice if done in the slow manner in which it is - effected in the glacier. “In short,” says Mr. Ball, “to ascertain the - resistance opposed to very slow changes in the relative positions of - the particles, so slight as to be insensible at short distances, Mr. - Moseley measures the resistance opposed to rapid disruption between - contiguous portions of the same substance.”</p> - - <p>There is force in this objection; and here we arrive at a really weak - point in Canon Moseley’s reasoning. His experiments show that if we - want to shear ice quickly a weight of nearly 120 lbs. is required; but - if the thing is to be done more slowly, 75 lbs. will suffice.<a id="FNanchor_302" href="#Footnote_302" class="fnanchor">[302]</a> In - short, the number of pounds required to shear the ice depends, to a - large extent, on the length of time that the weight is allowed to act; - the longer it is allowed to act, the less will be the weight required - to perform the work. “I am curious to know,” says Mr. Mathews, when - referring to this point, “what weight would have sheared the ice - if a <em>day</em> had been allowed for its operation.” I do not know what - would have been the weight required to shear the ice in Mr. Moseley’s - experiments had a day been allowed; but I feel pretty confident that, - should the ice remain unmelted, and sufficient time be allowed, - shearing would be produced without the application of any weight - whatever. There are no weights placed upon a glacier to make it move, - and yet the ice of the glacier shears. If the shearing is effected by - weight, the only weight applied is the weight of the ice; and if the - <span class="pagenum" id="Page_503">503</span>weight of the ice makes the ice shear in the glacier, why may it not - do the same thing in the experiment? Whatever may be the cause which - displaces the particles of the ice in a glacier, they, as a matter of - fact, are displaced without any weight being applied beyond that of - the ice itself; and if so, why may not the particles of the ice in - the experiment be also displaced without the application of weights? - Allow the ice of the glacier to take its own time and its own way, and - the particles will move over each other without the aid of external - weights, whatever may be the cause of this; well, then, allow the ice - in the experiment to take its own time and its own way, and it will - probably do the same thing. There is something here unsatisfactory. - If, by the unit of shear, be meant the pressure in pounds that must - be applied to the ice to break the connection of one square inch of - two surfaces frozen together and cause the one to slip over the other, - then the amount of pressure required to do this will depend upon the - time you allow for the thing being done. If the thing is to be done - rapidly, as in some of Mr. Moseley’s experiments, it will take, as he - has shown, a pressure of about 120 lbs.; but if the thing has to be - done more slowly, as in some other of his experiments, 75 lbs. will - suffice. And if sufficient time be allowed, as in the case of glaciers, - the thing may be done without any weight whatever being applied to the - ice, and, of course, Mr. Moseley’s argument, that a glacier cannot - descend by its weight alone, falls to the ground. But if, by the unit - of shear, be meant not the <em>weight</em> or <em>pressure</em> necessary to shear - the ice, but the amount of <em>work</em> required to shear a square inch of - surface <em>in a given time or at a given rate</em>, then he might be able - to show that in the case of a glacier (say the Mer de Glace) the work - of all the resistances which are opposed to its descent at the <em>rate</em> - at which it is descending is greater than the work of its weight, and - that consequently there must be some cause, in addition to the weight, - urging the glacier forward. But then he would have no right to affirm - that the glacier would not descend by its weight only; all that he - could affirm would<span class="pagenum" id="Page_504">504</span> simply be that it could not descend by its weight - alone at the <em>rate</em> at which it is descending.</p> - - <p>Mr. Moseley’s unit of shear, however, is not the amount of work - performed in shearing a square inch of ice in a given time, but the - amount of <em>weight</em> or <em>pressure</em> requiring to be applied to the ice - to shear a square inch. But this amount of pressure depends upon the - length of time that the pressure is applied. Here lies the difficulty - in determining what amount of pressure is to be taken as the real unit. - And here also lies the radical defect in Canon Moseley’s result. Time - as well as pressure enters as an element into the process. The key to - the explanation of this curious circumstance will, I think, be found in - the fact that the rate at which a glacier descends depends in some way - or other upon the amount of heat that the ice is receiving. This fact - shows that heat has something to do in the shearing of the ice of the - glacier. But in the communication of heat to the ice <em>time</em> necessarily - enters as an element. There are two different ways in which heat may be - conceived to aid in shearing the ice: (1.) we may conceive that heat - acts as a force along with gravitation in producing displacement of the - particles of the ice; or (2.) we may conceive that heat does not act as - a force in pushing the particles over each other, but that it assists - the shearing processes by diminishing the cohesion of the particles of - the ice, and thus allowing gravitation to produce displacement. The - former is the function attributed to heat in Canon Moseley’s theory - of glacier-motion; the latter is the function attributed to heat in - the theory of glacier-motion which I ventured to advance some time - ago.<a id="FNanchor_303" href="#Footnote_303" class="fnanchor">[303]</a> It results, therefore, from Canon Moseley’s own theory, that - the longer the time that is allowed for the pressure to shear the - ice, the less will be the pressure required; for, according to his - theory, a very large proportion of the displacement is produced by the - motive power of heat entering the ice; and, as it follows of course, - other things being equal, the longer the time during which the heat - is allowed to act, the greater will be the proportionate <span class="pagenum" id="Page_505">505</span>amount of - displacement produced by the heat; consequently the less will require - to be done by the weight applied. In the case of the glacier, Mr. - Moseley concludes that at least thirty or forty times as much work is - done by the motive power of heat in the way of shearing the ice as is - done by mere pressure or weight. Then, if sufficient time be allowed, - why may not far more be done by heat in shearing the ice in his - experiment than by the weight applied? In this case how is he to know - how much of the shearing is effected by the heat and how much by the - weight? If the greater part of the shearing of the ice in the case of a - glacier is produced, not by pressure, but by the heat which necessarily - enters the ice, it would be inconceivable that in his experiments the - heat entering the ice should not produce, at least to some extent, a - similar effect. And if a portion of the displacement of the particles - is produced by heat, then the weight which is applied cannot be - regarded as the measure of the force employed in the displacement, any - more than it could be inferred that the weight of the glacier is the - measure of the force employed in the shearing of it. If the weight - is not the entire force employed in shearing, but only a part of the - force, then the weight cannot, as in Mr. Moseley’s experiment, be taken - as the measure of the force.</p> - - <p>How, then, are we to determine what is the amount of force required to - shear ice? in other words, how is the unit of shear to be determined? - If we are to measure the unit of shear by the weight required to - produce displacement of the particles of the ice, we must make sure - that the displacement is wholly effected by the weight. We must be - certain that heat does not enter as an element in the process. But - if time be allowed to elapse during the experiment, we can never - be certain that heat has not been at work. It is impossible to - prevent heat entering the ice. We may keep the ice at a constant - temperature, but this would not prevent heat from entering the ice and - producing molecular work. True that, according to Moseley’s theory - of glacier-motion, if the temperature of the ice be not permitted - to <em>vary</em>, then no displacement of the particles can take place<span class="pagenum" id="Page_506">506</span> - from the influence of heat; but according to the molecular theory of - glacier-motion, which will shortly be considered, heat will aid the - displacement of the particles whether the temperature be kept constant - or not. In short, it is absolutely impossible in our experiments to - be certain that heat is not in some way or other concerned in the - displacement of the particles of the ice. But we can shorten the time, - and thus make sure that the amount of heat entering the ice during the - experiments is too small to affect materially the result. We cannot in - this case say that all the displacement has been effected by the weight - applied to the ice, but we can say that so little has been done by heat - that, practically, we may regard it as all done by the weight.</p> - - <p>This consideration, I trust, shows that the unit of shear adopted by - Canon Moseley in his calculations is not too large. For if in half an - hour, after all the work that may have been done by heat, a pressure of - 75 lbs. is still required to displace the particles of one square inch, - it is perfectly evident that if no work had been done by heat during - that time, the force required to produce the displacement could not - have been less than 75 lbs. It might have been more than that; but it - could not have been less. Be this, however, as it may, in determining - the unit of shear we cannot be permitted to prolong the experiment for - any considerable length of time, because the weight under which the - ice might then shear could not be taken as the measure of the force - which is required to shear ice. By prolonging the experiment we might - possibly get a unit smaller than that required by Canon Moseley for - a glacier to descend by its own weight. But it would be just as much - begging the whole question at issue to assume that, because the ice - sheared under such a weight, a glacier might descend by its weight - alone, as it would be to assume that, because a glacier shears without - a weight being placed upon it, the glacier descends by its weight alone.</p> - - <p>But why not determine the unit of shear of ice in the same way as we - would the unit of shear of any other solid substance,<span class="pagenum" id="Page_507">507</span> such, as iron, - stone, or wood? If the shearing force of ice be determined in this - manner, it will be found to be by far too great to allow of the ice - shearing by its weight alone. We shall be obliged to admit either - that the ice of the glacier does not shear (in the ordinary sense of - the term), or if it does shear, that there must, as Canon Moseley - concludes, be some other force in addition to the weight of the ice - urging the glacier forward.</p> - - <p>The fact that the rate of descent of a glacier depends upon the amount - of heat which it receives, proves that heat must be regarded either as - a cause or as a necessary condition of its motion; what, then, is the - necessary relationship between heat and the motion of the glacier? If - heat is to be regarded as a cause, in what way does the heat produce - motion? I shall now briefly refer to one or two theories which have - been advanced on the subject. Let us consider first that of Canon - Moseley.</p> - - <p><em>Canon Moseley’s Theory.</em>—He found, from observations and experiments, - that sheets of lead, placed upon an inclined plane, when subjected to - variations of temperature, tend to descend even when the slope is far - less than that which would enable it to slide down under the influence - of gravitation. The cause of the descent he shows to be this. When the - temperature of the sheet is raised, it expands, and, in expanding, its - upper portion moves up the slope, and its lower portion down the slope; - but as gravitation opposes the upward and favours the downward motion, - more of the sheet moves down than up, and consequently the centre - of gravity of the sheet is slightly lowered. Again, when the sheet - is cooled, it contracts, and in contracting the upper portion moves - downwards and the lower portion upwards, and here again, for the same - reason, more of the sheet moves downwards than upwards. Consequently, - at every change of temperature there is a slight displacement of the - sheet downwards. “Now a theory of the descent of glaciers,” says - Canon Moseley, “which I have ventured to propose myself, is that they - descend, as the lead in this<span class="pagenum" id="Page_508">508</span> experiment does, by reason of the passage - into them and the withdrawal of the sun’s rays, and that the dilatation - and contraction of the ice so produced is the proximate cause of their - descent, as it is of that of the lead.”<a id="FNanchor_304" href="#Footnote_304" class="fnanchor">[304]</a></p> - - <p>The fundamental condition in Mr. Moseley’s theory of the descent of - solid bodies on an incline, is, not that heat should maintain these - bodies at a high temperature, but that the temperature should vary. - The rate of descent is proportionate, not simply to the amount of - heat received, but to the extent and frequency of the variations of - temperature. As a proof that glaciers are subjected to great variations - of temperature, he adduces the following:—“All alpine travellers,” he - says, “from De Saussure to Forbes and Tyndall, have borne testimony - to the intensity of the solar radiation on the surfaces of glaciers. - ‘I scarcely ever,’ says Forbes, ‘remember to have found the sun more - piercing than at the Jardin.’ This heat passes abruptly into a state - of intense cold when any part of the glacier falls into shadow by an - alteration of the position of the sun, or even by the passing over it - of a cloud.”<a id="FNanchor_305" href="#Footnote_305" class="fnanchor">[305]</a></p> - - <p>Mr. Moseley is here narrating simply what the traveller feels, and - not what the glacier experiences. The traveller is subjected to great - variations of temperature; but there is no proof from this that the - glacier experiences any changes of temperature. It is rather because - the temperature of the glacier is not affected by the sun’s heat that - the traveller is so much chilled when the sun’s rays are cut off. The - sun shines down with piercing rays and the traveller is scorched; the - glacier melts on the surface, but it still remains “cold as ice.” The - sun passes behind a cloud or disappears behind a neighbouring hill; the - scorching rays are then withdrawn, and the traveller is now subjected - to radiation on every side from surfaces at the freezing-point.</p> - - <p>It is also a necessary condition in Mr. Moseley’s theory that the heat - should pass easily into and out of the glacier; for <span class="pagenum" id="Page_509">509</span>unless this were - the case sudden changes of temperature could produce little or no - effect on the great mass of the glacier. How, then, is it possible that - during the heat of summer the temperature of the glacier could vary - much? During that season, in the lower valleys at least, everything, - with the exception of the glacier, is above the freezing-point; - consequently when the glacier goes into the shade there is nothing - to lower the ice below the freezing-point; and as the sun’s rays do - not raise the temperature of the ice above the freezing-point, the - temperature of the glacier must therefore remain unaltered during that - season. It therefore follows that, instead of a glacier moving more - rapidly during the middle of summer than during the middle of winter, - it should, according to Moseley’s theory, have no motion whatever - during summer.</p> - - <p>The following, written fifteen years ago by Professor Forbes on this - very point, is most conclusive:—“But how stands the fact? Mr. Moseley - quotes from De Saussure the following <em>daily ranges</em> of the temperature - of the air in the month of July at the Col du Géant and at Chamouni, - between which points the glacier lies:</p> - - <table summary="Daily temperature ranges"> - <tbody> - <tr> - <th> </th> - <th class="tdl"> °</th> - </tr> - <tr> - <td>At the Col du Géant</td> - <td> 4·257 Réaumur.</td> - </tr> - <tr> - <td>At Chamouni</td> - <td>10·092 〃</td> - </tr> - </tbody> - </table> - - <p class="noindent">And he assumes ‘the same mean daily variation of temperature to obtain - throughout the length’ [and depth?] ‘of the Glacier du Géant which De - Saussure observed in July at the Col du Géant.’ But between what limits - does the temperature of the air oscillate? We find, by referring to the - third volume of De Saussure’s ‘Travels,’ that the mean temperature of - the coldest hour (4 <span class="smcap">a.m.</span>) during his stay at the Col du Géant - was 33°·03 Fahrenheit, and of the warmest (2 <span class="smcap">p.m.</span>) 42°·61 F. - So that even upon that exposed ridge, between 2,000 and 3,000 feet - above where the glacier can be properly said to commence, the air does - not, on an average of the month of July, reach the freezing-point - at any hour of the night. Consequently the <em>range of temperature - <span class="pagenum" id="Page_510">510</span>attributed to the glacier is between limits absolutely incapable of - effecting the expansion of the ice in the smallest degree</em>.”<a id="FNanchor_306" href="#Footnote_306" class="fnanchor">[306]</a></p> - - <p>Again, during winter, as Mr. Ball remarks, the glacier is completely - covered with snow and thus protected both from the influence of - cold and of heat, so that there can be nothing either to raise the - temperature of the ice above the freezing-point or to bring it below - that point; and consequently the glacier ought to remain immovable - during that season also.</p> - - <p>“There can be no doubt, therefore,” Mr. Moseley states, “that the - rays of the sun, which in those alpine regions are of such remarkable - intensity, find their way into the depths of the glacier. They are - a <em>power</em>, and there is no such thing as the loss of power. The - mechanical work which is their equivalent, and into which they are - converted when received into the substance of a solid body, accumulates - and stores itself up in the ice under the form of what we call - elastic force or tendency to dilate, until it becomes sufficient to - produce actual dilatation of the ice in the direction in which the - resistance is weakest, and by its withdrawal to produce contraction. - From this expansion and contraction follows of necessity the descent - of the glacier.”<a id="FNanchor_307" href="#Footnote_307" class="fnanchor">[307]</a> When the temperature of the ice is below - the freezing-point, the rays which are absorbed will, no doubt, - produce dilatation; but during summer, when the ice is not below the - freezing-point, no dilatation can possibly take place. All physicists, - so far as I am aware, agree that the rays that are then absorbed go to - melt the ice, and not to expand it. But to this Mr. Moseley replied - as follows:—“To this there is the obvious answer that radiant heat - does find its way into ice as a matter of common observation, and - that it does not melt it except at its surface. Blocks of ice may be - seen in the windows of ice-shops with the sun shining full upon them, - and melting nowhere but on their surfaces. And the experiment of the - ice-lens shows that heat may stream through ice in abundance (of <span class="pagenum" id="Page_511">511</span>which - a portion is necessarily stopped in the passage) without melting it, - except on its surface.” But what evidence is there to conclude that - if there is no melting of the ice in the interior of the lens there - is a portion of the rays “necessarily stopped” in the interior? It - will not do to assume a point so much opposed to all that we know of - the physical properties of ice as this really is. It is absolutely - essential to Mr. Moseley’s theory of the motion of glaciers, during - summer at least, that ice should continue to expand after it reaches - the melting-point; and it has therefore to be shown that such is the - case; or it need not be wondered at that we cannot accept his theory, - because it demands the adoption of a conclusion contrary to all our - previous conceptions. But, as a matter of fact, it is not strictly true - that when rays pass through a piece of ice there is no melting of the - ice in the interior. Experiments made by Professor Tyndall show the - contrary.<a id="FNanchor_308" href="#Footnote_308" class="fnanchor">[308]</a></p> - - <p>There is, however, one fortunate circumstance connected with Canon - Moseley’s theory. It is this: its truth can be easily tested by direct - experiment. The ice, according to this theory, descends not simply - in virtue of heat, but in virtue of <em>change of temperature</em>. Try, - then, Hopkins’s famous experiment, but keep the ice at a <em>constant - temperature</em>; then, according to Moseley’s theory, the ice will not - descend. Let it be observed, however, that although the ice under this - condition should descend (as there is little doubt but it would), - it would show that Mr. Moseley’s theory of the descent of glaciers - is incorrect, still it would not in the least degree affect the - conclusions which he lately arrived at in regard to the generally - received theory of glacier-motion. It would not prove that the ice - sheared, in the way generally supposed, by its weight only. It might be - the heat, after all, entering the ice, which accounted for its descent, - although gravitation (the weight of the ice) might be the impelling - cause.</p> - - <p>According to this theory, the glacier, like the sheet of lead, must - expand and contract as one entire mass, and it is difficult <span class="pagenum" id="Page_512">512</span>to - conceive how this could account for the differential motion of the - particles of the ice.</p> - - <p><em>Professor James Thomson’s Theory.</em>—It was discovered by this physicist - that the freezing-point of water is lowered by pressure. The extent - of the lowering is equal to ·0075° centigrade for every atmosphere - of pressure. As glacier ice is generally about the melting-point, - it follows that when enormous pressure is brought to bear upon any - given point of a glacier a melting of the ice at that particular spot - will take place in consequence of the lowering of the melting-point. - The melting of the ice will, of course, tend to favour the descent - of the glacier, but I can hardly think the liquefaction produced by - pressure can account for the motion of glaciers. It will help to - explain the giving way of the ice at particular points subjected to - great pressure, but I am unable to comprehend how it can account for - the general descent of the glacier. Conceive a rectangular glacier of - uniform breadth and thickness, and lying upon an even slope. In such a - glacier the pressure at each particular point would remain constant, - for there would be no reason why it should be greater at one time than - at another. Suppose the glacier to be 500 feet in thickness; the ice - at the lower surface of the glacier, owing to pressure, would have its - melting-point permanently lowered one-tenth of a degree centigrade - below that of the upper surface; but the ice at the lower surface would - not, on this account, be in the fluid state. It would simply be ice at - a slightly lower temperature. True, when pressure is exerted the ice - melts in consequence of the lowering of the melting-point, but in the - case under consideration there would, properly speaking, be no exertion - of pressure, but a constant statical pressure resulting from the weight - of the ice. But this statical condition of pressure would not produce - fluidity any more than a statical condition of pressure would produce - heat, and consequently motion could not take place as a result of - fluidity. In short, motion itself is required to produce the fluidity.</p> - - <p>I need not here wait to consider the sliding theories of<span class="pagenum" id="Page_513">513</span> De Saussure - and Hopkins, as they are now almost universally admitted to be - inadequate to explain the phenomena of glacier-motion, seeing that they - do not account for the displacement of the particles of the ice over - one another.</p> - - <p>According to the dilatation theory of M. Charpentier, a glacier is - impelled by the force exerted by water freezing in the fissures of the - ice. A glacier he considers is full of fissures into which water is - being constantly infiltrated, and when the temperature of the air sinks - below the freezing-point it converts the water into ice. The water, in - passing into ice, expands, and in expanding tends to impel the glacier - in the direction of least resistance. This theory, although it does not - explain glacier-motion, as has been clearly shown by Professor J. D. - Forbes, nevertheless contains one important element which, as we shall - see, must enter into the true explanation. The element to which I refer - is the expansive force exerted on the glacier by water freezing.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XXXI"> - <span class="pagenum" id="Page_514">514</span> - <h2> - CHAPTER XXXI.<br /><br /> - <span class="small">THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THE MOLECULAR THEORY.</span> - </h2> - </div> - <div class="subhead">Present State of the Question.—Heat necessary to the Motion - of a Glacier.—Ice does not shear in the Solid State.—Motion - of a Glacier <em>molecular</em>.—How Heat is transmitted through - Ice.—Momentary Loss of Shearing Force.—The <i lang="fr">Rationale</i> - of Regelation.—The Origin of “Crevasses.”—Effects of - Tension.—Modification of Theory.—Fluid Molecules crystallize - in Interstices.—Expansive Force of crystallizing Molecules a - Cause of Motion.—Internal molecular Pressure the chief Moving - Power.—How Ice can excavate a Rock Basin.—How Ice can ascend a - Slope.—How deep River Valleys are striated across.—A remarkable - Example in the Valley of the Tay.—How Boulders can be carried - from a lower to a higher Level.</div> - - <p class="noindent"><span class="smcap">The</span> condition which the perplexing question of the cause of the descent - of glaciers has now reached seems to be something like the following. - The ice of a glacier is not in a soft and plastic state, but is solid, - hard, brittle, and unyielding. It nevertheless behaves in some respects - in a manner very like what a soft and plastic substance would do if - placed in similar circumstances, inasmuch as it accommodates itself - to all the inequalities of the channel in which it moves. The ice of - the glacier, though hard and solid, moves with a differential motion; - the particles of the ice are displaced over each other, or, in other - words, the ice shears as it descends. It had been concluded that the - mere weight of the glacier is sufficient to shear the ice. Canon - Moseley has investigated this point, and shown that it is not. He has - found that for a glacier to shear in the way that it is supposed to - do, it would require a force some thirty or forty times as great as - the weight of the glacier. Consequently, for the glacier to descend, - a force in addition to that of gravitation<span class="pagenum" id="Page_515">515</span> is required. What, then, - is this force? It is found that the rate at which the glacier descends - depends upon the amount of heat which it is receiving. This shows that - the motion of the glacier is in some way or other dependent upon heat. - Is heat, then, the force we are in search of? The answer to this, of - course, is, since heat is a force necessarily required, we have no - right to assume any other till we see whether or not heat will suffice. - In what way, then, does heat aid gravitation in the descent of the - glacier? In what way does heat assist gravitation in the shearing of - the ice? There are two ways whereby we may conceive the thing to be - done: the heat may assist gravitation to shear, by pressing the ice - forward, or it may assist gravitation by diminishing the cohesion of - the particles, and thus allow gravitation to produce motion which it - otherwise could not produce. Every attempt which has yet been made - to explain how heat can act as a force in pushing the ice forward, - has failed. The fact that heat cannot expand the ice of the glacier - may be regarded as a sufficient proof that it does not act as a force - impelling the glacier forward; and we are thus obliged to turn our - attention to the other conception, viz., that heat assists gravitation - to shear the ice, not by direct pressure, but by diminishing the - cohesive force of the particles, so as to enable gravitation to push - the one past the other. But how is this done? Does heat diminish the - cohesion by acting as an expansive force in separating the particles? - Heat cannot do this, because it cannot expand the ice of a glacier; - and besides, were it to do this, it would destroy the solid and firm - character of the ice, and the ice of the glacier would not then, as - a mass, possess the great amount of shearing-force which observation - and experiment show that it does. In short it is because the particles - are so firmly fixed together at the time the glacier is descending, - that we are obliged to call in the aid of some other force in addition - to the weight of the glacier to shear the ice. Heat does not cause - displacement of the particles by making the ice soft and plastic; for - we know that the ice of the glacier is not soft and plastic, but<span class="pagenum" id="Page_516">516</span> - hard and brittle. The shearing-force of the ice of the moving glacier - is found to be by at least from thirty to forty times too great to - permit of the ice being sheared by the mere force of gravitation; - how, then, is it that gravitation, without the direct assistance of - any other force, can manage to shear the ice? Or to put the question - under another form: heat does not reduce the shearing-force of the ice - of a glacier to something like 1·3193 lb. per square inch of surface, - the unit required by Mr. Moseley to enable a glacier to shear by - its weight; the shearing-force of the ice, notwithstanding all the - heat received, still remains at about 75 lbs.; how, then, can the - glacier shear without any other force than its own weight pushing it - forward? <em>This is the fundamental question; and the true answer to it - must reveal the mystery of glacier-motion.</em> We are compelled in the - present state of the problem to admit that glaciers do descend with - a differential motion without any other force than their own weight - pushing them forward; and yet the shearing-force of the ice is actually - found to be thirty or forty times the maximum that would permit of the - glacier shearing by its weight only. <em>The explanation of this apparent - paradox will remove all our difficulties in reference to the cause of - the descent of glaciers.</em></p> - - <p>There seems to be but one explanation (and it is a very obvious - one), viz. that the motion of the glacier is <em>molecular</em>. The ice - descends molecule by molecule. The ice of a glacier is in the hard - crystalline state, but it does not descend in this state. Gravitation - is a constantly acting force; if a particle of the ice lose its - shearing-force, though but for the moment, it will descend by its - weight alone. But a particle of the ice will lose its shearing-force - for a moment if the particle loses its crystalline state for the - moment. The passage of heat through ice, whether by conduction or by - radiation, in all probability is a molecular process; that is, the - form of energy termed heat is transmitted from molecule to molecule - of the ice. A particle takes the energy from its neighbour A on the - one side and hands it over to its neighbour<span class="pagenum" id="Page_517">517</span> B on the opposite side. - But the particle must be in a different state at the moment it is in - possession of the energy from what it was before it received it from - A, and from what it will be after it has handed it over to B. Before - it became possessed of the energy, it was in the crystalline state—it - was ice; and after it loses possession of the energy it will be ice; - but at the moment that it is in possession of the passing energy is - it in the crystalline or icy state? If we assume that it is not, but - that in becoming possessed of the energy, it loses its crystalline form - and for the moment becomes water, all our difficulties regarding the - cause of the motion of glaciers are removed. We know that the ice of a - glacier in the mass cannot become possessed of energy in the form of - heat without becoming fluid; <em>if it can be shown that the same thing - holds true of the ice particle, we have the key to the mystery of - glacier-motion</em>. A moment’s reflection will suffice to convince any one - that if the glacier ice in the mass cannot receive energy in the form - of heat without melting, the same must hold true of the ice particles, - for it is inconceivable that the ice in the mass could melt and yet - the ice particles themselves remain in the solid state. It is the - solidity of the particles which constitutes the solidity of the mass. - If the particles lose their solid form the mass loses its solid form, - for the mass has no other solidity than that which is possessed by the - particles.</p> - - <p>The correctness of the conclusion, that the weight of the ice is - not a sufficient cause, depends upon the truth of a certain element - taken for granted in the reasoning, viz. that the <em>shearing-force</em> of - the molecules of the ice remains <em>constant</em>. If this force remains - constant, then Canon Moseley’s conclusion is undoubtedly correct, - but not otherwise; for if a molecule should lose its shearing-force, - though it were but for a moment, if no obstacle stood in front of the - molecule, it would descend in virtue of its weight.</p> - - <p>The fact that the shearing-force of a mass of ice is found to be - constant does not prove that the same is the case in regard to the - individual molecules. If we take a mass of molecules in<span class="pagenum" id="Page_518">518</span> the aggregate, - the shearing-force of the mass taken thus collectively may remain - absolutely constant, while at the same time each individual molecule - may be suffering repeated momentary losses of shearing-force. This is - so obvious as to require no further elucidation. The whole matter, - therefore, resolves itself into this one question, as to whether or not - the shearing-force of a crystalline molecule of ice remains constant. - In the case of ordinary solid bodies we have no reason to conclude that - the shearing-force of the molecules ever disappears, but in regard to - ice it is very different.</p> - - <p>If we analyze the process by which heat is conducted through ice, we - shall find that we have reason to believe <em>that while a molecule of - ice is in the act of transmitting the energy received (say from a - fire), it loses for the moment its shearing-force if the temperature of - the ice be not under</em> 32° F. If we apply heat to the end of a bar of - iron, the molecules at the surface of the end have their temperatures - raised. Molecule A at the surface, whose temperature has been raised, - instantly commences to transfer to B a portion of the energy received. - The tendency of this process is to lower the temperature of A and raise - that of B. B then, with its temperature raised, begins to transfer - the energy to C. The result here is the same; B tends to fall in - temperature, and C to rise. This process goes on from molecule to - molecule until the opposite end of the bar is reached. Here in this - case the energy or heat applied to the end of the bar is transmitted - from molecule to molecule under the form of <em>heat or temperature</em>. - The energy applied to the bar does <em>not change its character; it - passes right along from molecule to molecule under the form of heat or - temperature</em>. But the nature of the process must be wholly different if - the transferrence takes place through a bar of ice at the temperature - of 32°. Suppose we apply the heat of the fire to the end of the bar - of ice at 32°, the molecules of the ice cannot possibly have their - temperatures raised in the least degree. How, then, can molecule A - take on, <em>under the form of heat</em>, the energy received from the fire - without being heated or having its <em>temperature</em><span class="pagenum" id="Page_519">519</span> raised? The thing is - impossible. The energy of the fire must appear in A under a different - form from that of heat. The same process of reasoning is equally - applicable to B. The molecule B cannot accept of the energy from A - under the form of heat; it must receive it under some other form. The - same must hold equally true of all the other molecules till we reach - the opposite end of the bar of ice. And yet, strange to say, the last - molecule transmits in the form of heat its energy to the objects - beyond; for we find that the heat applied to one side of a piece of ice - will affect the thermal pile on the opposite side.</p> - - <p>The question is susceptible of a clear and definite answer. When - heat is applied to a molecule of ice at 32°, the heat applied - does not raise the temperature of the molecule, it is consumed in - work against the cohesive forces binding the atoms or particles - together into the crystalline form. The energy then must exist in - the dissolved crystalline molecule, under the statical form of an - affinity—crystalline affinity, or whatever else we may call it. That is - to say, the energy then exists in the particles as a power or tendency - to rush together again into the crystalline form, and the moment they - are allowed to do so they give out the energy that was expended upon - them in their separation. This energy, when it is thus given out again, - assumes the dynamical form of heat; in other words, the molecule gives - out <em>heat</em> in the act of freezing. The heat thus given out may be - employed to melt the next adjoining molecule. The ice-molecules take - on energy from a heated body by melting. That peculiar form of motion - or energy called heat disappears in forcing the particles of the - crystalline molecule separate, and for the time being exists in the - form of a tendency in the separated particles to come together again - into the crystalline form.</p> - - <p>But it must be observed that although the crystalline molecule, when - it is acting as a conductor, takes on energy under this form from the - heated body, it only exists in the molecule under such a form during - the moment of transmission; that is to say, the molecule is melted, but - only for the moment. When<span class="pagenum" id="Page_520">520</span> B accepts of the energy from A, the molecule - A instantly assumes the crystalline form. B is now melted; and when C - accepts of the energy from B, then B also in turn assumes the solid - state. This process goes on from molecule to molecule till the energy - is transmitted through to the opposite side and the ice is left in its - original solid state. This, as will be shown in the Appendix, is the - <i lang="fr">rationale</i> of Faraday’s property of regelation.</p> - - <p>This is no mere theory or hypothesis; it is a necessary consequence - from known facts. We know that ice at 32° cannot take on energy from - a heated body without melting; and we know also equally well that a - slab of ice at 32°, notwithstanding this, still, as a mass, retains its - solid state while the heat is being transmitted through it. This proves - that every molecule resumes its crystalline form the moment after the - energy is transferred to the adjoining molecule.</p> - - <p>This point being established, every difficulty regarding the descent - of the glacier entirely disappears; for a molecule the moment that - it assumes the fluid state is completely freed from shearing-force, - and can descend by virtue of its own weight without any impediment. - All that the molecule requires is simply room or space to advance in. - If the molecule were in absolute contact with the adjoining molecule - below, it would not descend unless it could push that molecule before - it, which it probably would not be able to do. But the molecule - actually has room in which to advance; for in passing from the solid - to the liquid state its volume is diminished by about 1/10, and it - consequently can descend. True, when it again assumes the solid form - it will regain its former volume; but the question is, will it go back - to its old position? If we examine the matter thoroughly we shall find - that it cannot. If there were only this one molecule affected by the - heat, this molecule would certainly not descend; but all the molecules - are similarly affected, although not all at the same moment of time.</p> - - <p>Let us observe what takes place, say, at the lower end of the glacier. - The molecule A at the lower end, say, of the surface,<span class="pagenum" id="Page_521">521</span> receives heat - from the sun’s rays; it melts, and in melting not only loses its - shearing-force and descends by its own weight, but it contracts also. - B immediately above it is now, so far as A is concerned, at liberty to - descend, and will do so the moment that it assumes the liquid state. A - by this time has become solid, and again fixed by shearing-force; but - it is not fixed in its old position, but a little below where it was - before. If B has not already passed into the fluid state in consequence - of heat derived from the sun, the additional supply which it will - receive from the solidifying of A will melt it. The moment that B - becomes fluid it will descend till it reaches A. B then is solidified - a little below its former position. The same process of reasoning is - in a similar manner applicable to every molecule of the glacier. Each - molecule of the glacier consequently descends step by step as it melts - and solidifies, and hence the glacier, considered as a mass, is in a - state of constant motion downwards. The fact observed by Professor - Tyndall that there are certain planes in the ice along which melting - takes place more readily than others will perhaps favour the descent of - the glacier.</p> - - <p>We have in this theory a satisfactory explanation of the origin of - “crevasses” in glaciers. Take, for example, the transverse crevasses - formed at the point where an increase in the inclination of the glacier - takes place. Suppose a change of inclination from, say, 4° to 8° in - the bed of the glacier. The molecules on the slope of 8° will descend - more rapidly than those above on the slope of 4°. A state of tension - will therefore be induced at the point where the change of inclination - occurs. The ice on the slope of 8° will tend to pull after it the mass - of the glacier moving more slowly on the slope above. The pull being - continued, the glacier will snap asunder the moment that the cohesion - of the ice is overcome. The greater the change of inclination is, the - more readily will the rupture of the ice take place. Every species of - crevasse can be explained upon the same principle.<a id="FNanchor_309" href="#Footnote_309" class="fnanchor">[309]</a></p> - - <p><span class="pagenum" id="Page_522">522</span></p> - - <p>This theory explains also why a glacier moves at a greater rate during - summer than during winter; for as the supply of heat to the glacier is - greater during the former season than during the latter, the molecules - will pass oftener into the liquid state.</p> - - <p>As regards the denuding power of glaciers, I may observe that, though - a glacier descends molecule by molecule, it will grind the rocky bed - over which it moves as effectually as it would do did it slide down in - a rigid mass in the way generally supposed; for the grinding-effect - is produced not by the ice of the glacier, but by the stones, sand, - and other materials forced along under it. But if all the resistances - opposing the descent of a glacier, internal and external, are overcome - by the mere weight of the ice alone, it can be proved that in the case - of one descending with a given velocity the amount of work performed - in forcing the grinding materials lying under the ice forward must be - as great, supposing the motion of the ice to be molecular in the way - I have explained, as it would be supposing the ice descended in the - manner generally supposed.</p> - - <p>Of course, a glacier could not descend by means of its weight as - rapidly in the latter case as in the former; for, in fact, as Canon - Moseley has shown, it would not in the latter case descend at all; but - assuming for the sake of argument the rate of descent in both cases to - be the same, the conclusion I have stated would follow. Consequently - whatever denuding effects may have been attributed to the glacier, - according to the ordinary theory, must be equally attributable to it - according to the present explanation.</p> - - <p>This theory, however, explains, what has always hitherto excited - astonishment, viz., why a glacier can descend a slope almost - horizontal, or why the ice can move off the face of a continent - perfectly level.</p> - - <p><span class="pagenum" id="Page_523">523</span></p> - - <p>This is the form in which my explanation was first stated about - half-a-dozen years ago.<a id="FNanchor_310" href="#Footnote_310" class="fnanchor">[310]</a> There is, however another element - which must be taken into account. It is one which will help to cast - additional light on some obscure points connected with glacial - phenomena.</p> - - <p>Ice is evidently not absolutely solid throughout. It is composed of - crystalline particles, which, though in contact with one another, are, - however, not packed together so as to occupy the least possible space, - and, even though they were, the particles would not fit so closely - together as to exclude interstices. The crystalline particles are, - however, united to one another at special points determined by their - polarity, and on this account they require more space; and this in - all probability is the reason, as Professor Tyndall remarks, why ice, - volume for volume, is less dense than water.</p> - - <p>“They (the molecules) like the magnets,” says Professor Tyndall, “are - acted upon by two distinct forces; for a time, while the liquid is - being cooled, they approach each other, in obedience to their general - attraction for each other. But at a certain point new forces, some - attractive some repulsive, <em>emanating from special points</em> of the - molecules, come into play. The attracted points close up, the repelled - points retreat. Thus the molecules turn and rearrange themselves, - demanding as they do so more space, and overcoming all ordinary - resistance by the energy of their demand. This, in general terms, is an - explanation of the expansion of water in solidifying.”<a id="FNanchor_311" href="#Footnote_311" class="fnanchor">[311]</a></p> - - <p>It will be obvious, then, that when a crystalline molecule melts, it - will not merely descend in the manner already described, but capillary - attraction will cause it to flow into the interstices between the - adjoining molecules. The moment that it parts with the heat received, - it will of course resolidify, as has been shown, but it will not - solidify so as to fit the cavity which it occupied when in the fluid - state. For the liquid molecule in solidifying assumes the crystalline - form, and of <span class="pagenum" id="Page_524">524</span>course there will be a definite proportion between the - length, breadth, and thickness of the crystal; consequently it will - always happen that the interstice in which it solidifies will be too - narrow to contain it. The result will be that the fluid molecule - in passing into the crystalline form will press the two adjoining - molecules aside in order to make sufficient room for itself between - them, and this it will do, no matter what amount of space it may - possess in all other directions. The crystal will not form to suit the - cavity, the cavity must be made to contain the crystal. And what holds - true of one molecule, holds true of every molecule which melts and - resolidifies. This process is therefore going on incessantly in every - part of the glacier, and in proportion to the amount of heat which the - glacier is receiving. This internal molecular pressure, resulting from - the solidifying of the fluid molecules in the interstices of the ice, - acts on the mass of the ice as an expansive force, tending to cause the - glacier to widen out laterally in all directions.</p> - - <p>Conceive a mass of ice lying on a flat horizontal surface, and - receiving heat on its upper surface, say from the sun; as the heat - passes downwards through the mass, the molecules, acting as conductors, - melt and resolidify. Each fluid molecule solidifies in an interstice, - which has to be widened in order to contain it. The pressure thus - exerted by the continual resolidifying of the molecules will cause the - mass to widen out laterally, and of course as the mass widens out it - will grow thinner and thinner if it does not receive fresh acquisition - on its surface. In the case of a glacier lying in a valley, motion, - however, will only take place in one direction. The sides of the - valley prevent the glacier from widening; and as gravitation opposes - the motion of the ice up, and favours its motion down the valley, the - path of least resistance to molecular pressure will always be down - the slope, and consequently in this direction molecular displacement - will take place. Molecular pressure will therefore produce motion in - the same direction as that of gravity. In other words, it will tend to - cause the glacier to descend the valley.</p> - - <p><span class="pagenum" id="Page_525">525</span></p> - - <p>The lateral expansion of the ice from internal molecular pressure - explains in a clear and satisfactory manner how rock-basins may be - excavated by means of land-ice. It also removes the difficulties - which have been felt in accounting for the ascent of ice up a steep - slope. The main difficulty besetting the theory of the excavation of - rock-basins by ice is to explain how the ice after entering the basin - manages to get out again—how the ice at the bottom is made to ascend - the sloping sides of the basin. Pressure acting from behind, it has - been argued by some; but if the basin be deep and its sides steep, this - will simply cause the ice lying above the level of the basin to move - forward over the surface of the mass filling it. This conclusion is, - however, incorrect. The ice filling the basin and the glacier overlying - it are united in one solid mass, so that the latter cannot move over - the former without shearing; and although the resistance to motion - offered by the sloping sides of the basin may be much greater than the - resistance to shear, still the ice will be slowly dragged out of the - basin. However, in order to obviate this objection to which I refer, - the advocates of the glacial origin of lake-basins point out that - the length of those basins in proportion to their depth is so great - that the slope up which the ice has to pass is in reality but small. - This no doubt is true of lake-basins in general, but it does not hold - universally true. But the theory does not demand that an ice-formed - lake-basin cannot have steep sides. We have incontestable evidence that - ice will pass up a steep slope; and, if ice can pass up a steep slope, - it can excavate a basin with a steep slope. That ice will pass up a - steep slope is proved by the fact that comparatively deep and narrow - river valleys are often found striated across, while hills which stood - directly in the path of the ice of the glacial epoch are sometimes - found striated <em>upwards</em> from their base to their summit. Some striking - examples of striæ running up hill are given by Professor Geikie in - his “Glacial Drift of Scotland.” I have myself seen a slope striated - upwards so steep that one could not climb it.</p> - - <p>A very good example of a river valley striated across came<span class="pagenum" id="Page_526">526</span> under - my observation during the past summer. The Tay, between Cargill - and Stanley (in the centre of the broad plain of Strathmore), has - excavated, through the Old Red Sandstone, a channel between 200 and - 300 feet in depth. The channel here runs at right angles to the path - taken during the glacial epoch by the great mass of ice coming from - the North-west Highlands. At a short distance below Cargill, the trap - rising out of the bed of the river is beautifully ice-grooved and - striated, at right angles to the stream. A trap-dyke, several miles in - length, crosses the river about a mile above Stanley, forming a rapid, - known as the Linn of Campsie. This dyke is <i lang="fr">moutonnée</i> and striated - from near the Linn up the sloping bank to the level of the surrounding - country, showing that the ice must have ascended a gradient of one in - seven to a height of 300 feet.</p> - - <p>From what has been already stated in reference to the resolidifying of - the molecules in the interstices of the ice, the application of the - molecular theory to the explanation of the effects under consideration - will no doubt be apparent. Take the case of the passage of the - ice-sheet across a river valley. As the upper surface of the ice-sheet - is constantly receiving heat from the sun and the air in contact with - it, there is consequently a transferrence of heat from above downwards - to the bottom of the sheet. This transferrence of heat from molecule - to molecule is accompanied by the melting and resolidifying of the - successive molecules in the manner already detailed. As the fluid - molecules tend to flow into adjoining interstices before solidifying - and assuming the crystalline form, the interstices of the ice at the - bottom of the valley are constantly being filled by fluid molecules - from above. These molecules no sooner enter the interstices than they - pass into the crystalline form, and become, of course, separated from - their neighbours by fresh interstices, which new interstices become - filled by fluid molecules, which, in turn, crystallize, forming fresh - interstices, and so on. The ice at the bottom of the valley, so long as - this process continues, is constantly receiving fresh additions from<span class="pagenum" id="Page_527">527</span> - above. The ice must therefore expand laterally to make room for these - additions, which it must do unless the resistance to lateral expansion - be greater than the force exerted by the molecules in crystallizing. - But a resistance sufficient to do this must be enormous. The ice at the - bottom of the valley cannot expand laterally without passing up the - sloping sides. In expanding it will take the path of least resistance, - but the path of least resistance will always be on the side of the - valley towards which the general mass of the ice above is flowing.</p> - - <p>It has been shown (<a href="#CHAPTER_XXVII">Chapter XXVII.</a>) that the ice passing over Strathmore - must have been over 2,000 feet in thickness. An ice-sheet 2,000 feet - in thickness exerts on its bed a pressure of upwards of 51 tons per - square foot. When we reflect that ice under so enormous a pressure, - with grinding materials lying underneath, was forced by irresistible - molecular energy up an incline of one in seven, it is not at all - surprising that the hard trap should be ground down and striated.</p> - - <p>We can also understand how the softer portions of the rocky surface - over which the ice moved should have been excavated into hollow basins. - We have also an explanation of the transport of boulders from a lower - to a higher level, for if ice can move from a lower to a higher level, - it of course can carry boulders along with it.</p> - - <p>The bearing which the foregoing considerations of the manner in which - heat is transmitted through ice have on the question of the cause of - regelation will be considered in the Appendix.</p> - - <hr class="page" /> - <div class="chapter" id="APPENDIX"> - <span class="pagenum" id="Page_528">528</span> - <h2>APPENDIX.</h2> - </div> - - <h3 id="APPENDIX_I">I.</h3> - <div class="hang mb3">OPINIONS EXPRESSED PREVIOUS TO 1864 REGARDING THE INFLUENCE OF THE - ECCENTRICITY OF THE EARTH’S ORBIT ON CLIMATE.<a id="FNanchor_312" href="#Footnote_312" class="fnanchor">[312]</a></div> - - <h4>M. DE MAIRAN.</h4> - - <p>M. de Mairan, in an article in the <cite>Memoirs of the Royal Academy of - France</cite><a id="FNanchor_313" href="#Footnote_313" class="fnanchor">[313]</a> “On the General Cause of Heat in Summer and Cold in - Winter, in so far as depends on the internal and permanent Heat of the - Earth,” makes the following remarks on the influence of the difference - of distance of the sun in apogee and perigee:—</p> - - <p lang="fr">“Cet élément est constant pour les deux solstices; tandis que les - autres (height of the sun and obliquity of his rays) y varient à raison - des latitudes locales; et il y a encore cela de particulier, qu’il - tend à diminuer la valeur de notre été, et à augmenter celle de notre - hiver dans l’hémisphère boréal où nous sommes, et tout au contraire - dans l’austral. Remarquons cependant que de ces mêmes distances, qui - constituent ce troisième élément, naît en partie un autre principe - de chaleur tout opposé, et qui semble devoir tempérer les effets du - précédent; sçavoir, la lenteur et la vitesse réciproques du mouvement - annuel apparent, en vertu duquel et du réel qui s’y mêle, le soleil - emploie 8 jours de plus à parcourir les signes septentrionaux. - C’est-à-dire, que le soleil passe 186½ jours dans notre hémisphère, et - seulement 178½ dans l’hémisphère opposé. Ce qui, en général, ne peut - manquer de répandre un pen plus de chaleur sur l’été du premier, et un - peu moins sur son hiver.”</p> - - <p><span class="pagenum" id="Page_529">529</span></p> - - <h4>MR. RICHARD KIRWAN.</h4> - - <p>“Œpinus,<a id="FNanchor_314" href="#Footnote_314" class="fnanchor">[314]</a> reasoning on astronomical principles, attributes the - inferior temperature of the southern hemisphere to the shorter abode of - the sun in the southern tropic, shorter by seven days, which produces - a difference of fourteen days in favour of the northern hemisphere, - during which more heat is accumulated, and hence he infers that the - temperature of the northern hemisphere is to that of the southern, as - 189·5 to 175·5, or as 14 to 13.”—<cite>Trans. of the Royal Irish Academy</cite>, - vol. viii., p. 417. 1802.</p> - - <h4>SIR CHARLES LYELL.</h4> - - <p>“Before the amount of difference between the temperature of the two - hemispheres was ascertained, it was referred by astronomers to the - acceleration of the earth’s motion in its perihelion; in consequence of - which the spring and summer of the southern hemisphere are shorter by - nearly eight days than those seasons north of the equator. A sensible - effect is probably produced by this source of disturbance, but it is - quite inadequate to explain the whole phenomena. It is, however, of - importance to the geologist to bear in mind that in consequence of the - precession of the equinoxes, the two hemispheres receive alternately, - each for a period of upwards of 10,000 years, a greater share of - solar light and heat. This cause may sometimes tend to counterbalance - inequalities resulting from other circumstances of a far more - influential nature; but, on the other hand, it must sometimes tend to - increase the extreme of deviation, which certain combinations of causes - produce at distant epochs.”—<cite>Principles</cite>, First Edition, 1830, p. 110, - vol. i.</p> - - <h4>SIR JOHN F. HERSCHEL, <span class="smcap">Bart.</span></h4> - - <p>The following, in so far as it relates to the effects of eccentricity, - is a copy of Sir John Herschel’s memoir, “On the Astronomical Causes - which may influence Geological Phenomena,” read before the Geological - Society, Dec. 15th, 1830.—<cite>Trans. Geol. Soc.</cite>, vol. iii., p. 293, - Second Series:—</p> - - <p>“... Let us next consider the changes arising in the orbit of the earth - itself about the sun, from the disturbing action of the planets. In so - doing it will be obviously unnecessary to consider the effect produced - on the solar tides, to which the above reasoning applies much more - forcibly than in the case of the lunar. It is, <span class="pagenum" id="Page_530">530</span>therefore, only the - variations in the supply of light and heat received from the sun that - we have now to consider.</p> - - <p>“Geometers having demonstrated the absolute invariability of the <em>mean</em> - distance of the earth from the sun, it would seem to follow that - the mean annual supply of light and heat derived from that luminary - would be alike invariable; but a closer consideration of the subject - will show that this would not be a legitimate conclusion, but that, - on the contrary, the <em>mean</em> amount of solar radiation is dependent - on the eccentricity of the orbit, and therefore liable to variation. - Without going at present into any geometrical investigations, it will - be sufficient for the purpose here to state it as a theorem, of which - any one may easily satisfy himself by no very abstruse geometrical - reasoning, that ‘<em>the eccentricity of the orbit varying, the</em> total - <em>quantity of heat received by the earth from the sun in one revolution - is inversely proportional to the</em> minor <em>axis of the orbit</em>.’ Now since - the major axis is, as above observed, invariable, and therefore, of - course, the absolute length of the year, it will follow that the <em>mean - annual</em> average of heat will also be in the same inverse ratio of the - <em>minor</em> axis; and thus we see that the very circumstance which on a - cursory view we should have regarded as demonstrative of the constancy - of our supply of solar heat, forms an essential link in the chain of - strict reasoning by which its variability is proved.</p> - - <p>“The eccentricity of the earth’s orbits is actually diminishing, and - has been so for ages, beyond the records of history. In consequence, - the ellipse is in a state of approach to a circle, and its minor - axis being, therefore, on the increase, the annual average of solar - radiation is actually on the <em>decrease</em>.</p> - - <p>“So far this is in accordance with the testimony of geological - evidence, which indicates a general refrigeration of climate; but when - we come to consider the amount of diminution which the eccentricity - must be supposed to have undergone to render an account of the - variation which has taken place, we have to consider that, in the first - place, a great diminution of the eccentricity is required to produce - any sensible increase of the minor axis. This is a purely geometrical - conclusion, and is best shown by the following table:—</p> - - <table summary="Ratio of Heat received"> - <tbody> - <tr> - <th>Eccentricity.</th> - <th>Minor Axis.</th> - <th>Reciprocal or Ratio<br />of Heat received.</th> - </tr> - <tr> - <td class="tdc"><div>0·00</div></td> - <td class="tdc"><div>1·000</div></td> - <td class="tdc"><div>1·000</div></td> - </tr> - <tr> - <td class="tdc"><div>0·05</div></td> - <td class="tdc"><div>0·999</div></td> - <td class="tdc"><div>1·002</div></td> - </tr> - <tr> - <td class="tdc"><div>0·10</div></td> - <td class="tdc"><div>0·995</div></td> - <td class="tdc"><div>1·005</div></td> - </tr> - <tr> - <td class="tdc"><div>0·15</div></td> - <td class="tdc"><div>0·989</div></td> - <td class="tdc"><div>1·011</div></td> - </tr> - <tr> - <td class="tdc"><div>0·20</div></td> - <td class="tdc"><div>0·980</div></td> - <td class="tdc"><div>1·021</div></td> - </tr> - <tr> - <td class="tdc"><div>0·25</div></td> - <td class="tdc"><div>0·968</div></td> - <td class="tdc"><div>1·032</div></td> - </tr> - <tr> - <td class="tdc"><div>0·30</div></td> - <td class="tdc"><div>0·954</div></td> - <td class="tdc"><div>1·048</div></td> - </tr> - </tbody> - </table> - - <p class="noindent">By this it appears that a variation of the eccentricity of the orbit - from the circular form to that of an ellipse, having an eccentricity<span class="pagenum" id="Page_531">531</span> - of one-fourth of the major axis, would produce only a variation of 3 - per cent. on the <em>mean</em> annual amount of solar radiation, and this - variation takes in the whole range of the planetary eccentricities, - from that of Pallas and Juno downwards.</p> - - <p>“I am not aware that the limit of increase of the eccentricity of the - earth’s orbit has ever been determined. That it has a limit has been - satisfactorily proved; but the celebrated theorem of Laplace, which - is usually cited as demonstrating that none of the planetary orbits - can ever deviate materially from the circular form, leads to no such - conclusion, except in the case of the great preponderant planets - Jupiter and Saturn, while for anything that theorem proves to the - contrary, the orbit of the earth may become elliptic to any amount.</p> - - <p>“In the absence of calculations which though practicable have, I - believe, never been made,<a id="FNanchor_315" href="#Footnote_315" class="fnanchor">[315]</a> and would be no slight undertaking, we - may assume that eccentricities which exist in the orbits of planets, - both interior and exterior to that of the earth, may <em>possibly</em> - have been attained, and may be attained again by that of the earth - itself. It is clear that such eccentricities <em>existing</em> they cannot - be incompatible with the stability of the system generally, and that, - therefore, the question of the possibility of such an amount in the - particular case of the earth’s orbit will depend on the particular - data belonging to that case, and can only be determined by executing - the calculations alluded to, having regard to the simultaneous effects - of at least the four most influential planets, Venus, Mars, Jupiter, - and Saturn, <em>not only on the orbit of the earth, but on those of each - other</em>. The principles of this calculation are detailed in the article - of Laplace’s work cited. But before entering on a work of so much - labour, it is quite necessary to inquire what prospect of advantage - there is to induce any one to undertake it.</p> - - <p>“Now it certainly at first sight seems clear that a variation of <span class="pagenum" id="Page_532">532</span>3 - per cent. only in the mean annual amount of solar radiation, and - that arising from an extreme supposition, does <em>not</em> hold out such a - prospect. Yet it might be argued that the effects of the sun’s heat is - to maintain the temperature of the earth’s surface at its actual mean - height, not above the zero of Fahrenheit’s or any other thermometer, - but above the temperature of the celestial spaces, out of the reach of - the sun’s influence, and what that temperature is may be a matter of - much discussion. M. Fourier has considered it as demonstrated that it - is not greatly inferior to that of the polar regions of our own globe, - but the grounds of this decision appear to me open to considerable - objection.<a id="FNanchor_316" href="#Footnote_316" class="fnanchor">[316]</a> If those regions be really void of matter, their - temperature can only arise, according to M. Fourier’s own view of - the subject, from the radiation of the stars. It ought, therefore, - to be as much inferior to that due to solar radiation, as the light - of a starlight night is to that of the brightest noon day, in other - words it should be very nearly a total privation of heat—almost the - <em>absolute zero</em> respecting which so much difference of opinion exists, - some placing it at 1,000°, some at 5,000° of Fahrenheit below the - freezing-point, and some still lower, in which case a single unit per - cent. in the mean annual amount of radiation would suffice to produce a - change of climate fully commensurate to the demands of geologists.<a id="FNanchor_317" href="#Footnote_317" class="fnanchor">[317]</a></p> - - <p>“Without attempting, however, to enter further into the perplexing - difficulties in which this point is involved, which are far greater - than appear on a cursory view, let us next consider, not the <em>mean</em>, - but the <em>extreme</em> effects which a variation in the eccentricity of - the earth’s orbit may be expected to produce in the summer and winter - climates in particular regions of its surface, and under the influence - of circumstances favouring a difference of effect. And here, if I - mistake not, it will appear that an amount of variation, which we need - not hesitate to admit (at least, provisionally) as a possible one, may - be productive of considerable diversity of climate, and may operate - during great periods of time either to mitigate or to exaggerate - the difference of winter and summer temperatures, so as to produce - alternately, in the same latitude of either hemisphere, a perpetual - spring, or the extreme vicissitudes of a burning summer and a rigorous - winter.</p> - - <p><span class="pagenum" id="Page_533">533</span></p> - - <p>“To show this, let us at once take the extreme case of an orbit as - eccentric as that of Juno or Pallas, in which the greatest and least - distances of the sun are to each other as 5 to 3, and consequently the - radiations at those distances as 25 to 9, or very nearly as 3 to 1. To - conceive what would be the <em>extreme</em> effects of this great variation - of the heat received at different periods of the year, let us first - imagine in our latitude the place of the perigee of the sun to coincide - with the summer solstice. In that case, the difference between the - summer and winter temperature would be exaggerated in the same degree - as if three suns were placed side by side in the heavens in the former - season and only one in the latter, which would produce a climate - perfectly intolerable. On the other hand, were the perigee situated - in the winter solstice our three suns would combine to warm us in the - winter, and would afford such an excess of winter radiation as would - probably more than counteract the effect of short days and oblique - sunshine, and throw the summer season into the winter months.</p> - - <p>“The actual diminution of the eccentricity is so slow, that the - transition from a state of the orbit such as we have assumed to the - present nearly circular figure would occupy upwards of 600,000 years, - supposing it uniformly changeable—this, of course, would not be the - case; when near the maximum, however, it would vary slower still, so - that at that point it is evident a period of 10,000 years would elapse - without any perceptible change in the state of the data of the case we - are considering.</p> - - <p>“Now this adopting the very ingenious idea of Mr. Lyell<a id="FNanchor_318" href="#Footnote_318" class="fnanchor">[318]</a> would - suffice, by reason of the combined effect of the precession of the - equinoxes and the motion of the apsides of the orbit itself, to - transfer the perigee from the summer to the winter solstice, and thus - to produce a transition from the one to the other species of climate in - a period sufficiently great to give room for a material change in the - botanical character of country.</p> - - <p>“The supposition above made is an extreme, but it is not demonstrated - to be an impossible one, and should even an approach to such a state - of things be possible, the same consequences, <span class="pagenum" id="Page_534">534</span>in a mitigated degree, - would follow. But if, on executing the calculations, it should appear - that the limits of the eccentricity of the earth’s orbit are really - narrow, and if, on a full discussion of the very difficult and delicate - point of the actual effect of solar radiation, it should appear that - the mean, as well as the extreme, temperature of our climates would - <em>not</em> be materially affected,—it will be at least satisfactory to - <em>know</em> that the causes of the phenomena in question are to be sought - elsewhere than in the relations of our planet to the system to which - it belongs, since there does not appear to exist any other conceivable - connections between these relations and the facts of geology than - those we have enumerated, the obliquity of the ecliptic being, as we - know, confined within too narrow limits for its variation to have any - sensible influence.”—<cite>J. F. W. Herschel.</cite></p> - - <p>The influence which this paper might have had on the question as - to whether eccentricity may be regarded as a cause of changes in - geological climate appears to have been completely neutralized by the - following, which appeared shortly afterwards both in his “Treatise” and - “Outlines of Astronomy,” showing evidently that he had changed his mind - on the subject.</p> - - <p>“It appears, therefore, from what has been shown, the supplies of heat - received from the sun will be equal in the two segments, in whatever - direction the line PTQ be drawn. They will, indeed, be described in - unequal times: that in which the perihelion A lies in a shorter, and - the other in a longer, in proportion to their unequal area; but the - greater proximity of the sun in the smaller segment compensates exactly - for its more rapid description, and thus an equilibrium of heat is, as - it were, maintained.</p> - - <p>“Were it not for this the eccentricity of the orbit would materially - influence the transition of seasons. The fluctuation of distance - amounts to nearly 1/30th of the mean quantity, and, consequently, - the fluctuation of the sun’s direct heating power to double this, or - 1/15th of the whole.... Were it not for the compensation we have just - described, the effect would be to exaggerate the difference of summer - and winter in the southern hemisphere, and to moderate it in the - northern; thus producing a more violent alternation of climate in the - one hemisphere and an approach to perpetual spring in the other. <em>As it - is, however, no such inequality subsists</em>, but an equal and impartial - distribution of heat and light is accorded to both.”—“<em>Treatise of - Astronomy</em>,” <cite>Cabinet Cyclopædia</cite>, § 315; <cite>Outlines of Astronomy</cite>, § - 368.</p> - - <p>“The fact of a great change in the general climate of large tracts - of the globe, if not of the whole earth, and of a diminution of - general temperature, having been recognised by geologists, from - their examination of the remains of animals and vegetables of former - ages enclosed in the strata, various causes for such diminution of - temperature have been assigned.... It is evident that<span class="pagenum" id="Page_535">535</span> the <em>mean</em> - temperature of the whole surface of the globe, in so far as it is - maintained by the action of the sun at a higher degree than it would - have were the sun extinguished, must depend on the mean quantity of - the sun’s rays which it receives, or, which comes to the same thing, - on the <em>total</em> quantity received in a given invariable time; and the - length of the year being unchangeable in all the fluctuations of the - planetary system, it follows that the total <em>annual</em> amount of solar - radiation will determine, <i>cæteris paribus</i>, the general climate - of the earth. Now, it is not difficult to show that this amount is - inversely proportional to the minor axis of the ellipse described - by the earth about the sun, regarded as slowly variable; and that, - therefore, the major axis remaining, as we know it to be, constant, - and the orbit being actually in a state of approach to a circle, - and consequently the minor axis being on the <em>increase</em>, the mean - annual amount of solar radiation received by the whole earth must - be actually on the <em>decrease</em>. We have here, therefore, an evident - real cause of sufficient universality, and acting <em>in the right - direction</em>, to account for the phenomenon. Its adequacy is another - consideration.”<a id="FNanchor_319" href="#Footnote_319" class="fnanchor">[319]</a>—<cite>Discourse on the Study of Natural Philosophy</cite>, - pp. 145−147 (1830).</p> - - <h4>SIR CHARLES LYELL, <span class="smcap">Bart.</span></h4> - - <p>“<em>Astronomical Causes of Fluctuations in Climate.</em>—Sir John Herschel - has lately inquired, whether there are any astronomical causes which - may offer a possible explanation of the difference between the actual - climate of the earth’s surface, and those which formerly appear to - have prevailed. He has entered upon this subject, he says, ‘impressed - with the magnificence of that view of geological revolutions, which - regards them rather as regular and necessary effects of great and - general causes, than as resulting from a series of convulsions - and catastrophes, regulated by no laws, and reducible to no fixed - principles.’ Geometers, he adds, have demonstrated the absolute - invariability of the mean distance of the earth from the sun; whence - it would seem to follow that the mean annual supply of light and heat - derived from that luminary would be alike invariable; but a closer - consideration of the subject will show that this would not be a - legitimate conclusion, but that, on the contrary, the <em>mean</em> amount of - solar radiation is dependent on the eccentricity of the earth’s orbit, - and, therefore, liable to variation.</p> - - <p>“Now, the eccentricity of the orbit, he continues, is actually - <span class="pagenum" id="Page_536">536</span>diminishing, and has been so for ages beyond the records of history. - In consequence, the ellipse is in a state of approach to a circle, and - the annual average of solar heat radiated to the earth is actually on - the <em>decrease</em>. So far, this is in accordance with geological evidence, - which indicates a general refrigeration of climate; but the question - remains, whether the amount of diminution which the eccentricity may - have ever undergone can be supposed sufficient to account for any - sensible refrigeration.<a id="FNanchor_320" href="#Footnote_320" class="fnanchor">[320]</a> The calculations necessary to determine - this point, though practicable, have never yet been made, and would be - extremely laborious; for they must embrace all the perturbations which - the most influential planets, Venus, Mars, Jupiter, and Saturn, would - cause in the earth’s orbit and in each other’s movements round the sun.</p> - - <p>“The problem is also very complicated, inasmuch as it depends not - merely on the ellipticity of the earth’s orbit, but on the assumed - temperature of the celestial spaces beyond the earth’s atmosphere; - a matter still open to discussion, and on which M. Fourier and Sir - J. Herschel have arrived at very different opinions. But if, says - Herschel, we suppose an extreme case, as if the earth’s orbit should - ever become as eccentric as that of the planet Juno or Pallas, a great - change of climate might be conceived to result, the winter and summer - temperatures being sometimes mitigated and at others exaggerated, in - the same latitudes.</p> - - <p>“It is much to be desired that the calculations alluded to were - executed, as even if they should demonstrate, as M. Arago thinks highly - probable, that the mean of solar radiation can never be materially - affected by irregularities in the earth’s motion, it would still be - satisfactory to ascertain the point.”—<cite>Principles of Geology</cite>, Ninth - Edition, 1853, p. 127.</p> - - <h4>M. ARAGO.</h4> - - <p>“<i>Can the variations which certain astronomical elements undergo - sensibly modify terrestrial climates?</i></p> - - <p>“The sun is not always equally distant from the earth. At this time - its least distance is observed in the first days of January, and the - greatest, six months after, or in the first days of July. But, on the - other hand, a time will come when the <em>minimum</em> will occur in July, - and the <em>maximum</em> in January. Here, then, this <span class="pagenum" id="Page_537">537</span>interesting question - presents itself,—Should a summer such as those we now have, in which - the <em>maximum</em> corresponds to the solar distance, differ sensibly, from - a summer with which the <em>minimum</em> of this distance should coincide?</p> - - <p>“At first sight every one probably would answer in the affirmative; - for, between the <em>maximum</em> and the <em>minimum</em> of the sun’s distance - from the earth there is a remarkable difference, a difference in round - numbers of a thirtieth of the whole. Let, however, the consideration of - the velocities be introduced into the problem, elements which cannot - fairly be neglected, and the result will be on the side opposite to - that we originally imagined.</p> - - <p>“The part of the orbit where the sun is found nearest the earth, is, at - the same time, the point where the luminary moves most rapidly along. - The demi-orbit, or, in other words, the 180° comprehended betwixt the - two equinoxes of spring-time and autumn, will then be traversed in the - least possible time, when, in moving from the one of the extremities - of this arc to the other, the sun shall pass, near the middle of - this course of six months, at the point of the smallest distance. To - resume—the hypothesis we have just adopted would give, on account of - the lesser distance, a spring-time and summer hotter than they are in - our days; but on account of the greater rapidity, the sum of the two - seasons would be shorter by about seven days. Thus, then, all things - considered, the compensation is mathematically exact. After this it is - superfluous to add, that the point of the sun’s orbit corresponding to - the earth’s least distance changes very gradually; and that since the - most distant periods, the luminary has always passed by this point, - either at the end of autumn or beginning of winter.</p> - - <p>“We have thus seen that the changes which take place in the <em>position</em> - of the solar orbit, <em>have no power in modifying the climate of our - globe</em>. We may now inquire, if it be the same concerning the variations - which this orbit experiences in its <em>form</em>....</p> - - <p>“Herschel, who has recently been occupying himself with this problem, - in the hope of discovering the explanation of several geological - phenomena, allows that the succession of ages might bring the - eccentricity of the terrestrial orbit to the proportion of that of - the planet Pallas, that is to say, to be the 25/100 of a semi-greater - axis. It is exceedingly improbable that in these periodical changes - the eccentricity of our orbit should ever experience such enormous - variations, and even then these twenty-five hundredth parts (25/100), - would not augment the <em>mean</em> annual solar radiation except by about one - hundredth part (1/100). To repeat, an eccentricity of 25/100 <em>would not - alter in any appreciated manner the mean thermometrical state of the - globe</em>....</p> - - <p>“The changes of the form, and of the position, of the terrestrial<span class="pagenum" id="Page_538">538</span> - orbit are mathematically inoperative, or, at most, their influence is - so minute that it is not indicated by the most delicate instruments. - For the explanation of the changes of climates, then, there only - remains to us either the local circumstances, or some alteration in - the heating or illuminating power of the sun. But of these two causes, - we may continue to reject the last. And thus, in fact, all the changes - would come to be attributed to agricultural operations, to the clearing - of plains and mountains from wood, the draining of morasses, &c.</p> - - <p>“Thus, at one swoop, to confine, the whole earth, the variations - of climates, past and future, within the limits of the naturally - very narrow influence which the labour of man can effect, would be - a meteorological result of the very last importance.”—pp. 221−224, - <cite>Memoir on the “Thermometrical State of the Terrestrial Globe,” in the - Edinburgh New Philosophical Journal</cite>, vol. xvi., 1834.</p> - - <h4>BARON HUMBOLDT.</h4> - - <p>“The question,” he says, “has been raised as to whether the increasing - value of this ellipticity is capable during thousands of years of - modifying to any considerable extent the temperature of the earth, - in reference to the daily and annual quantity and distribution of - heat? Whether a partial solution of the great geological problem of - the imbedding of tropical vegetable and animal remains in the now - cold zones may not be found in these <em>astronomical</em> causes proceeding - regularly in accordance with eternal laws?... It might at the first - glance be supposed that the occurrence of the perihelion at an opposite - time of the year (instead of the winter, as, is now the case, in - summer) must necessarily produce great climatic variations; but, on the - above supposition, the sun will no longer remain seven days longer in - the northern hemisphere; no longer, as is now the case, traverse that - part of the ecliptic from the autumnal equinox to the vernal equinox, - in a space of time which is one week shorter than that in which it - traverses the other half of its orbit from the vernal to the autumnal - equinox.</p> - - <p>“The difference of temperature which is considered as the consequence - to be apprehended from the turning of the major axis, <em>will on the - whole disappear</em>, principally from the circumstance that the point of - our planet’s orbit in which it is nearest to the sun is at the same - time always that over which it passes with the greatest velocity....</p> - - <p>“As the altered position of the major axis is capable of exerting - only a very <em>slight influence upon the temperature of the earth;</em> so - likewise the <em>limit</em> of the probable changes in the elliptical form of - the earth’s orbit are, according to Arago and Poisson, so narrow that<span class="pagenum" id="Page_539">539</span> - these changes could <em>only very slightly</em> modify the climates of the - individual zones, and that in very long periods.”<a id="FNanchor_321" href="#Footnote_321" class="fnanchor">[321]</a>—<cite>Cosmos</cite>, vol. - iv., pp. 458, 459. Bohn’s Edition. 1852.</p> - - <h4>SIR HENRY T. DE LA BECHE.</h4> - - <p>“Mr. Herschel, viewing this subject with the eye of an astronomer, - considers that a diminution of the surface-temperature might arise from - a change in ellipticity of the earth’s orbit, which, though slowly, - gradually becomes more circular. No calculations having yet been made - as to the probable amount of decreased temperature from this cause, - it can at present be only considered as a possible explanation of - those geological phenomena which point to considerable alterations in - climates.”—<cite>Geological Manual</cite>. Third Edition. 1833. p. 8.</p> - - <h4>PROFESSOR PHILLIPS.</h4> - - <p>“<em>Temperature of the Globe.</em>—<em>Influence of the Sun.</em>—No proposition is - more certain than the fundamental dependence of the temperature of the - surface of the globe on the solar influence.</p> - - <p>“It is, therefore, very important for geologists to inquire whether - this be variable or constant; whether the amount of solar heat - communicated to the earth is and has always been the same in every - annual period, or what latitude the laws of planetary movements permit - in this respect.</p> - - <p>“Sir John Herschel has examined this question in a satisfactory manner, - in a paper read to the Geological Society of London. The total amount - of solar radiation which determines the general climate of the earth, - the year being of invariable length, is inversely proportional to - the minor axis of the ellipse described by the earth about the sun, - regarded as slowly variable; the major axis remaining constant and - the orbit being actually in a state of approach to a circle, and, - consequently, the minor axis being on the increase, it follows that - the mean annual amount of solar radiation received by the whole earth - must be actually on the decrease. The limits of the variation in the - eccentricity of the earth’s orbit are not known. It is, therefore, - impossible to say accurately what may have been in former periods of - time, the amount of solar radiation; it is, however, certain that - if the ellipticity has ever been so great as that of the orbit of - Mercury or Pallas, the temperature of the earth must have been sensibly - higher than it is at present. But the difference of a few degrees of - temperature thus occasioned, is of too small an order to be <span class="pagenum" id="Page_540">540</span>employed - in explaining the growth of tropical plants and corals in the polar - or temperate zones, and other great phenomena of Geology.”—<em>From A - Treatise on Geology</em>, p. 11, <em>forming the article under that head in - the seventh edition of the Encyclopædia Britannica</em>. 1837.</p> - - <h4>MR. ROBERT BAKEWELL.</h4> - - <p>“A change in the form of the earth’s orbit, if considerable, might - change the temperature of the earth, by bringing it nearer to the - sun in one part of its course. The orbit of the earth is an ellipsis - approaching nearly to a circle; the distance from the centre of the - orbit to either focus of the ellipsis is called by astronomers ‘the - eccentricity of the orbit.’ This eccentricity has been for ages slowly - decreasing, or, in other words, the orbit of the earth has been - approaching nearer to the form of a perfect circle; after a long period - it will again increase, and the possible extent of the variation has - not been yet ascertained. From what is known respecting the orbits of - Jupiter and Saturn, it appears highly probable that the eccentricity of - the earth’s orbit is confined within limits that preclude the belief - of any great change in the mean annual temperature of the globe ever - having been occasioned by this cause.”—<cite>Introduction to Geology</cite>, p. - 600. 1838. Fifth Edition.</p> - - <h4>MRS. SOMERVILLE.</h4> - - <p>“Sir John Herschel has shown that the elliptical form of the earth’s - orbit has but a trifling share in producing the variation of - temperature corresponding to the difference of the seasons.”—<cite>Physical - Geography</cite>, vol. ii., p. 20. Third Edition.</p> - - <h4>MR. L. W. MEECH, A.M.</h4> - - <p>“Let us, then, look back to that primeval epoch when the earth - was in aphelion at midsummer, and the eccentricity at its maximum - value—assigned by Leverrier near to ·0777. Without entering into - elaborate computation, it is easy to see that the extreme values - of diurnal intensity, in Section IV., would be altered as by the - multiplier <span class="xxlarge">(</span><span class="frac"><sup>1 ± <i>e</i></sup><span>/</span><sub>1 ± <i>e′</i></sub></span><span class="xxlarge">)</span><sup>2</sup>, - that is 1 − 0·11 in summer, and 1 - + 0·11 in winter. This would diminish the midsummer intensity by about - 9°, and increase the midwinter intensity by 3° or 4°; the temperature - of spring and autumn being nearly unchanged. But this does not appear - to be of itself adequate to the geological effects in question.</p> - - <p><span class="pagenum" id="Page_541">541</span></p> - - <p>“It is not our purpose, here, to enter into the inquiry whether the - atmosphere was once more dense than now, whether the earth’s axis - had once a different inclination to the orbit, or the sun a greater - emissive power of heat and light. Neither shall we attempt to speculate - upon the primitive heat of the earth, nor of planetary space, nor of - the supposed connection of terrestrial heat and magnetism; nor inquire - how far the existence of coal-fields in this latitude, of fossils, - and other geological remains, have depended upon existing causes. The - preceding discussion seems to prove simply that, under the present - system of physical astronomy, the sun’s intensity could never have been - materially different from what is manifested upon the earth at the - present day. <em>The causes of notable geological changes must be other - than the relative position of the sun and earth, under their present - laws of motion.</em>”—<cite>“On the Relative Intensity of the Heat and Light of - the Sun.” Smithsonian Contributions to Knowledge</cite>, vol. ix.</p> - - <h4>M. JEAN REYNAUD.</h4> - - <p lang="fr">“La révolution qui pourrait y causer les plus grands changements - thermométriques, celle qui porte l’orbite à s’élargir et à se rétrécir - alternativement et, par suite, la planète à passer, aux époques de - périhélie, plus ou moins près du soleil, embrasse une période de plus - de cent mille années terrestres et demeure comprise dans de si étroites - limites que les habitants doivent être à peine avertis que la chaleur - décroît, par cette raison, depuis une haute antiquité et décroîtra - encore pendant des siècles en variant en même temps dans sa répartition - selon les diverses époques de l’année.... Enfin, le tournoiement de - l’axe du globe s’empreint également d’une manière particulière sur - l’ètablissement des saisons qui, à tour de rôle, dans chacun des deux - hémisphères, deviennent graduellement, durant une période d’environ - vingt-cinq mille ans, de plus en plus uniformes, ou, à l’inverse, de - plus en plus dissemblables. C’est actuellement dans l’hémisphère boréal - que règne l’uniformité, et quoique les étés et les hivers y tendent, - dès à présent, à se trancher de plus en plus, il ne paraît pas douteux - que la modération des saisons n’y produise, pendant longtemps encore, - des effets appréciables. En résumé, de tous ces changements il n’en est - donc aucun ni qui suive un cours précipité, ni qui s’élève jamais à des - valeurs considérables; ils se règlent tous sur un mode de développement - presque insensible, et il s’ensuit que les années de la terre, malgré - leur complexité virtuelle, se distinguent par le constance de leurs - caractères non-seulement de ce qui peut avoir lieu, en vertu des mêmes - principes, dans les autres systèmes planétaires de l’univers, mais - même de ce qui s’observe dans plusieurs des mondes qui composent le - nôtre.”—<cite>Philosophie Religieuse: Terre et Ciel.</cite></p> - - <p><span class="pagenum" id="Page_542">542</span></p> - - <h4>M. ADHÉMAR.</h4> - - <p>Adhémar does not consider the effects which ought to result from a - change in the eccentricity of the earth’s orbit; he only concerns - himself with those which, in his opinion, arise from the present amount - of such eccentricity. He admits, of course, that both hemispheres - receive from the sun equal quantities of heat per annum; but, as - the southern hemisphere has a winter longer by 168 hours than the - corresponding season in the northern hemisphere, an accumulation of - heat necessarily takes place in the latter, and an accumulation of - cold in the former. Adhémar also measures the loss of heat sustained - by the southern hemisphere in a year by the number of hours by which - the southern exceeds the northern winter. “The south pole,” he says, - “loses in one year more heat than it receives, because the total - duration of its nights surpasses that of the days by 168 hours; and the - contrary takes place for the north pole. If, for example, we take for - unity the mean quantity of heat which the sun sends off in one hour, - the heat accumulated at the end of the year at the north pole will be - expressed by 168, while the heat lost by the south pole will be equal - to 168 times what the radiation lessens it by in one hour; so that at - the end of the year the difference in the heat of the two hemispheres - will be represented by 336 times what the earth receives from the sun - or loses in an hour by radiation,”<a id="FNanchor_322" href="#Footnote_322" class="fnanchor">[322]</a> and at the end of 100 years the - difference will be 33,600 times, and at the end of 1,000 years 336,000 - times, or equal to what the earth receives from the sun in 38½ years, - and so on during the 10,000 years that the southern winter exceeds in - length the northern. This, in his opinion, is all that is required to - melt the ice off the arctic regions, and cover the antarctic regions - with an enormous ice-cap. He further supposes that in about 10,000 - years, when our northern winter will occur in aphelion and the southern - in perihelion, the climatic conditions of the two hemispheres will be - reversed; that is to say, the ice will melt at the south pole, and the - northern hemisphere will become enveloped in one continuous mass of - ice, leagues in thickness, extending down to temperate regions.</p> - - <p>This theory, as shown in <a href="#CHAPTER_V">Chapter V.</a>, is based upon a misconception - regarding the laws of radiant heat. The loss of heat sustained by the - southern hemisphere from radiation, resulting from the greater length - of the southern winter, is vastly over-estimated by M. Adhémar, and - could not possibly produce the effects which he supposes. But I need - not enter into this subject here, as the reader will find the whole - question discussed at length in the chapter above referred to. By far - the most important part of Adhemar’s theory, however, is his conception - of the submergence <span class="pagenum" id="Page_543">543</span>of the land by means of a polar ice-cap. He appears - to have been the first to put forth the idea that a mass of ice placed - on the globe, say, for example, at the south pole, will shift the - earth’s centre of gravity a little to the south of its former position, - and thus, as a physical consequence, cause the sea to sink at the - north pole and to rise at the south. According to Adhémar, as the one - hemisphere cools and the other grows warmer, the ice at the pole of the - former will increase in thickness and that at the pole of the latter - diminish.</p> - - <p>The sea, as a consequence, will sink on the warm hemisphere where the - ice is decreasing and rise on the cold hemisphere where the ice is - increasing. And, again, in 10,000 years, when the climatic conditions - of the two hemispheres are reversed, the sea will sink on the - hemisphere where it formerly rose, and rise on the hemisphere where it - formerly sank, and so on in like manner through indefinite ages.</p> - - <p>Adhémar, however, acknowledges to have derived the grand conception - of a submergence of the land from the shifting of the earth’s centre - of gravity from the following wild speculation of one Bertrand, of - Hamburgh:—</p> - - <p lang="fr">“Bertrand de Hambourg, dans un ouvrage imprimé en 1799 et qui a - pour titre: <i lang="fr">Renouvellement périodique des Continents</i>, avait déjà - émis cette idée, que la masse des eaux pouvait être alternativement - entraînée d’un hémisphère à l’autre par le déplacement du centre de - gravité du globe. Or, pour expliquer ce déplacement, il supposait que - la terre était creuse et qu’il y avait dans son intérieur un gros noyau - d’aimant auquel les comètes par leur attraction communiquaient un - mouvement de va-et-vient analogue à celui du pendule.”—<cite>Révolutions de - la Mer</cite>, p. 41.</p> - - <p>The somewhat extravagant notions which Adhémar has advanced in - connection with his theory of submergence have very much retarded - its acceptance. Amongst other remarkable views he supposes the polar - ice-cap to rest on the bottom of the ocean, and to rise out of the - water to the enormous height of twenty leagues. Again, he holds that - on the winter approaching perihelion and the hemisphere becoming warm - the ice waxes soft and rotten from the accumulated heat, and the sea - now beginning to eat into the base of the cap, this is so undermined - as, at last, to be left standing upon a kind of gigantic pedestal. This - disintegrating process goes on till the fatal moment at length arrives, - when the whole mass tumbles down into the sea in huge fragments which - become floating icebergs. The attraction of the opposite ice-cap, which - has by this time nearly reached its maximum thickness, becomes now - predominant. The earth’s centre of gravity suddenly crosses the plain - of the equator, dragging the ocean along with it, and carrying death - and destruction to everything on the surface of the globe. And these - catastrophes, he asserts, occur alternately on<span class="pagenum" id="Page_544">544</span> the two hemispheres - every 10,000 years.—<cite>Révolutions de la Mer</cite>, pp. 316−328.</p> - - <p>Adhémar’s theory has been advocated by M. Le Hon, of Brussels, in a - work entitled <cite>Périodicité des Grands Déluges</cite>. Bruxelles et Leipzig, - 1858.</p> - - <hr class="short mt5" /> - <h3 id="APPENDIX_II">II.</h3> - <div class="center mb3">ON THE NATURE OF HEAT-VIBRATIONS.<a id="FNanchor_323" href="#Footnote_323" class="fnanchor">[323]</a></div> - - <div class="center">From the <cite>Philosophical Magazine</cite> for May, 1864.</div> - - <p>In a most interesting paper on “Radiant Heat,” by Professor Tyndall, - read before the Royal Society in March last, it is shown conclusively - that the <em>period</em> of heat-vibrations is not affected by the state - of aggregation of the molecules of the heated body; that is to say, - whether the substance be in the gaseous, the liquid, or, perhaps, the - solid condition, the tendency of its molecules to vibrate according to - a given period remains unchanged. The force of cohesion binding the - molecules together exercises no effect on the rapidity of vibration.</p> - - <p>I had arrived at the same conclusion from theoretical considerations - several years ago, and had also deduced some further conclusions - regarding the nature of heat-vibrations, which seem to be in a measure - confirmed by the experimental results of Professor Tyndall. One of - these conclusions was, that the heat-vibration does not consist in - a motion of an aggregate mass of molecules, but in a motion of the - individual molecules themselves. Each molecule, or rather we should - say each atom, acts as if there were no other in existence but - itself. Whether the atom stands by itself as in the gaseous state, - or is bound to other atoms as in the liquid or the solid state, it - behaves in exactly the same manner. The deeper question then suggested - itself, viz., what is the nature of that mysterious motion called heat - assumed by the atom? Does it consist in excursions across centres - of equilibrium external to the atom itself? It is the generally - received opinion among physicists that it does. But I think that the - experimental results arrived at by Professor Tyndall, as well as some - others which will presently be noticed, are entirely hostile to such an - opinion. The relation of an atom to its centre of equilibrium depends - entirely on the state of aggregation. Now if heat-vibrations consist in - <span class="pagenum" id="Page_545">545</span>excursions to and fro across these centres, then the <em>period</em> ought to - be affected by the state of aggregation. The higher the <em>tension</em> of - the atom in regard to the centre, the more rapid ought its movement to - be. This is the case in regard to the vibrations constituting sound. - The harder a body becomes, or, in other words, the more firmly its - molecules are bound together, the higher is the <em>pitch</em>. Two harp-cords - struck with equal force will vibrate with equal force, however much - they may differ in the rapidity of their vibrations. The <i lang="la">vis viva</i> - of vibration depends upon the force of the stroke; but the rapidity - depends, not on the stroke, but upon the tension of the cord.</p> - - <p>That heat-vibrations do not consist in excursions of the molecules - or atoms across centres of equilibrium, follows also as a necessary - consequence from the fact that the real specific heat of a body remains - unchanged under all conditions. All changes in the specific heat of a - body are due to differences in the amount of heat consumed in molecular - work against cohesion or other forces binding the molecules together. - Or, in other words, to produce in a body no other effect than a given - rise of temperature, requires the same amount of force, whatever may be - the physical condition of the body. Whether the body be in the solid, - the fluid, or the gaseous condition, the same rise of temperature - always indicates the same quantity of force consumed in the simple - production of the rise. Now, if heat-vibrations consist in excursions - of the atom to and fro across a centre of equilibrium <em>external to - itself</em>, as is generally supposed, then the <em>real</em> specific heat of a - solid body, for example, <em>ought to decrease with the hardness of the - body</em>, because an increase in the strength of the force binding the - molecules together would in such a case tend to favour the rise in the - rapidity of the vibrations.</p> - - <p>These conclusions not only afford us an insight into the hidden nature - of heat-vibrations, but they also appear to cast some light on the - physical constitution of the atom itself. They seem to lead to the - conclusion that the ultimate atom itself is <em>essentially elastic</em>.<a id="FNanchor_324" href="#Footnote_324" class="fnanchor">[324]</a> - For if heat-vibrations do not consist in excursions of the atom, then - it must consist in alternate expansions and contractions of the atom - itself. This again is opposed to the ordinary idea that the atom is - essentially solid and impenetrable. But it favours the modern idea, - that matter consists of forces of resistance acting from a centre.</p> - - <p>Professor Tyndall in a memoir read before the Royal Society “On a new - Series of Chemical Reactions produced by Light,” has subsequently - arrived at a similar conclusion in reference to the atomic nature of - heat-vibrations. The following are his views on the subject:—</p> - - <p><span class="pagenum" id="Page_546">546</span></p> - - <p>“A question of extreme importance in molecular physics here - arises:—What is the real mechanism of this absorption, and where is its - seat?</p> - - <p>“I figure, as others do, a molecule as a group of atoms, held together - by their mutual forces, but still capable of motion among themselves. - The vapour of the nitrite of amyl is to be regarded as an assemblage - of such molecules. The question now before us is this:—In the act - of absorption, is it the <em>molecules</em> that are effective, or is it - their constituent <em>atoms?</em> Is the <i lang="la">vis viva</i> of the intercepted waves - transferred to the molecule as a whole, or to its constituent parts?</p> - - <p>“The molecule, as a whole, can only vibrate in virtue of the forces - exerted between it and its neighbour molecules. The intensity of these - forces, and consequently the rate of vibration, would, in this case, - be a function of the distance between the molecules. Now the identical - absorption of the liquid and of the vaporous nitrite of amyl indicates - an identical vibrating period on the part of liquid and vapour, and - this, to my mind, amounts to an experimental demonstration that the - absorption occurs in the main <em>within</em> the molecule. For it can hardly - be supposed, if the absorption were the act of the molecule as a whole, - that it could continue to affect waves of the same period after the - substance had passed from the vaporous to the liquid state.”—<cite>Proc. of - Roy. Soc.</cite>, No. 105. 1868.</p> - - <p>Professor W. A. Norton, in his memoir on “Molecular Physics,”<a id="FNanchor_325" href="#Footnote_325" class="fnanchor">[325]</a> has - also arrived at results somewhat similar in reference to the nature of - heat-vibrations. “It will be seen,” he says, “that these (Mr. Croll’s) - ideas are in accordance with the conception of the constitution of a - molecule adopted at the beginning of the present memoir (p. 193), and - with the theory of heat-vibrations or heat-pulses deduced therefrom (p. - 196).”<a id="FNanchor_326" href="#Footnote_326" class="fnanchor">[326]</a></p> - - <p><span class="pagenum" id="Page_547">547</span></p> - - <hr class="short mt5" /> - <h3 id="APPENDIX_III">III.</h3> - <div class="hang mb3">ON THE REASON WHY THE DIFFERENCE OF READING BETWEEN A - THERMOMETER EXPOSED TO DIRECT SUNSHINE AND ONE SHADED - DIMINISHES AS WE ASCEND IN THE ATMOSPHERE.<a id="FNanchor_327" href="#Footnote_327" class="fnanchor">[327]</a></div> - - <div class="center">From the <cite>Philosophical Magazine</cite> for March, 1867.</div> - - <p>The remarkable fact was observed by Mr. Glaisher, that the difference - of reading between a black-bulb thermometer exposed to the direct rays - of the sun and one shaded diminishes as we ascend in the atmosphere. - On viewing the matter under the light of Professor Tyndall’s important - discovery regarding the influence of aqueous vapour on radiant heat, - the fact stated by Mr. Glaisher appears to be in perfect harmony with - theory. The following considerations will perhaps make this plain.</p> - - <p>The shaded thermometer marks the temperature of the surrounding - air; but the exposed thermometer marks not the temperature of the - air, but that of the bulb heated by the direct rays of the sun. The - temperature of the bulb depends upon two elements: (1) the rate at - which it receives heat by <em>direct radiation</em> from the sun above, the - earth beneath, and all surrounding objects, and by <em>contact</em> with the - air; (2) the rate at which it loses heat by radiation and by contact - with the air. As regards the heat gained and lost by contact with the - surrounding air, both thermometers are under the same conditions, - or nearly so. We therefore require only to consider the element of - radiation.</p> - - <p>We begin by comparing the two thermometers at the earth’s surface, and - we find that they differ by a very considerable number of degrees. - We now ascend some miles into the air, and on again comparing the - thermometers we find that the difference between them has greatly - diminished. It has been often proved, by direct observation, that the - intensity of the sun’s rays increases as we rise in the atmosphere. - How then does the exposed thermometer sink more rapidly than the - shaded one as we ascend? The reason is obviously this. The temperature - of the thermometers depends as much upon the rate at which they are - losing their heat as upon the rate at which they are gaining it. - The higher temperature of the exposed thermometer is the result of - <em>direct radiation</em> from the sun. Now, although this thermometer - receives by radiation more heat <span class="pagenum" id="Page_548">548</span>from the sun at the upper position - than at the lower, it does not necessarily follow on this account - that its temperature ought to be higher. Suppose that at the upper - position it should receive one-fourth more heat from the sun than at - the lower, yet if the rate at which it loses its heat by radiation - into space be, say, one-third greater at the upper position than at - the lower, the temperature of the bulb would sink to a considerable - extent, notwithstanding the extra amount of heat received. Let us now - reflect on how matters stand in this respect in regard to the actual - case under our consideration. When the exposed thermometer is at the - higher position, it receives more heat from the sun than at the lower, - but it receives less from the earth; for a considerable part of the - radiation from the earth is cut off by the screen of aqueous vapour - intervening between the thermometer and the earth. But, on the whole, - it is probable that the total quantity of radiant heat reaching the - thermometer is greater in the higher position than in the lower. - Compare now the two positions in regard to the rate at which the - thermometer loses its heat by radiation. When the thermometer is at the - lower position, it has the warm surface of the ground against which to - radiate its heat downwards. The high temperature of the ground thus - tends to diminish the rate of radiation. Above, there is a screen of - aqueous vapour throwing back upon the thermometer a very considerable - part of the heat which the instrument is radiating upwards. This, of - course, tends greatly to diminish the loss from radiation. But at - the upper position this very screen, which prevented the thermometer - from throwing off its heat into the cold space above, now affects - the instrument in an opposite manner; for the thermometer has now to - radiate its heat downwards, not upon the warm surface of the ground - as before, but upon the cold upper surface of the aqueous screen - intervening between the instrument and the earth. This of course tends - to lower the mercury. We are now in a great measure above the aqueous - screen, with nothing to protect the thermometer from the influence of - cold stellar space. It is true that the air above is at a temperature - little below that of the thermometer itself; but then the air is dry, - and, owing to its diathermancy, it does not absorb the heat radiated - from the thermometer, and consequently the instrument radiates its heat - directly into the cold stellar space above, some hundreds of degrees - below zero, almost the same as it would do were the air entirely - removed. The enormous loss of heat which the thermometer now sustains - causes it to fall in temperature to a great extent. The molecules of - the comparatively dry air at this elevation, being very bad radiators, - do not throw off their heat into space so rapidly as the bulb of the - exposed thermometer; consequently their temperature does not (for this - reason) tend to sink so rapidly as that of the bulb. Hence the shaded - thermometer, which indicates the<span class="pagenum" id="Page_549">549</span> temperature of those molecules, is - not affected to such an extent as the exposed one. Hence also the - difference of reading between the two instruments must diminish as we - rise in the atmosphere.</p> - - <p>This difference between the temperature of the two thermometers - evidently does not go on diminishing to an indefinite extent. Were we - able to continue our ascent in the atmosphere, we should certainly - find that a point would be reached beyond which the difference of - reading would begin to increase, and would continue to do so till the - outer limits of the atmosphere were reached. The difference between - the temperatures of the two thermometers beyond the limits of the - atmosphere would certainly be enormous. The thermometer exposed to - the direct rays of the sun would no doubt be much colder than it had - been when at the earth’s surface; but the shaded thermometer would - now indicate the temperature of space, which, according to Sir John - Herschel and M. Pouillet, is more than 200° Fahrenheit below zero.</p> - - <p>It follows also, from what has been stated, that even under direct - sunshine the removal of the earth’s atmosphere would tend to lower the - temperature of the earth’s surface to a great extent. This conclusion - also follows as an immediate inference from the fact that the earth’s - atmosphere, as it exists at present charged with aqueous vapour, - affects terrestrial radiation more than it does radiation from the sun; - for the removal of the atmosphere would increase the rate at which the - earth throws off its heat into space more than it would increase the - rate at which it receives heat from the sun; therefore its temperature - would necessarily fall until the rate of radiation <em>from</em> the earth’s - surface exactly equalled the rate of radiation <em>to</em> the surface. Let - the atmosphere again envelope the earth, and terrestrial radiation - would instantly be diminished; the temperature of the earth’s surface - would therefore necessarily begin to rise, and would continue to do so - till the rate of radiation from the surface would equal the rate of - radiation received by the surface. Equilibrium being thus restored, the - temperature would remain stationary. It is perfectly obvious that if we - envelope the earth with a substance such as our atmosphere, that offers - more resistance to terrestrial radiation than to solar, the temperature - of the earth’s surface must necessarily rise until the heat which is - being radiated off equals that which is being received from the sun. - Remove the air and thus get quit of the resistance, and the temperature - of the surface would fall, because in this case a lower temperature - would maintain equilibrium.</p> - - <p>It follows, therefore, that the moon, which has no atmosphere, must - be much colder than our earth, even on the side exposed to the sun. - Were our earth with its atmosphere as it exists at present removed to - the orbit of Venus or Mars, for example, it certainly would not be - habitable, owing to the great change of temperature that would result. - But a change in the physical constitution <span class="pagenum" id="Page_550">550</span>of the atmospheric envelope - is really all that would be necessary to retain the earth’s surface at - its present temperature in either position.</p> - - <hr class="short mt5" /> - <h3 id="APPENDIX_IV">IV.</h3> - <div class="hang mb3">REMARKS ON MR. J. Y. BUCHANAN’S THEORY OF THE VERTICAL - DISTRIBUTION OF TEMPERATURE OF THE OCEAN.<a id="FNanchor_328" href="#Footnote_328" class="fnanchor">[328]</a></div> - - <p>Since the foregoing was in type, a paper on the “Vertical Distribution - of Temperature of the Ocean,” by Mr. J. Y. Buchanan, chemist on board - the <cite>Challenger</cite>, has been read before the Royal Society.<a id="FNanchor_329" href="#Footnote_329" class="fnanchor">[329]</a> In that - paper Mr. Buchanan endeavours to account for the great depth of warm - water in the middle of the North Atlantic compared with that at the - equator, without referring it to horizontal circulation of any kind.</p> - - <p>The following is the theory as stated by Mr. Buchanan:—</p> - - <p>“Let us assume the winter temperature of the surface-water to be 60° F. - and the summer temperature to be 70° F. If we start from midwinter, we - find that, as summer approaches, the surface-water must get gradually - warmer, and that the temperature of the layers below the surface must - decrease at a very rapid rate, until the stratum of winter temperature, - or 60° F., is reached; in the language of the isothermal charts, the - isothermal line for degrees between 70° F. (if we suppose that we have - arrived at midsummer) and 60° F. open out or increase their distance - from each other as the depth increases. Let us now consider the - conditions after the summer heat has begun to waver. During the whole - period of heating, the water, from its increasing temperature, has been - always becoming lighter, so that heat communication by convection with - the water below has been entirely suspended during the whole period. - The heating of the surface-water has, however, had another effect, - besides increasing its volume; it has, by evaporation, rendered it - denser than it was before, at the same temperature. Keeping in view - this double effect of the summer heat upon the surface-water, let us - consider the effect of the winter cold upon it. The superficial water - having assumed the atmospheric temperature of, say 60° F., will sink - through the warmer water below it, until it reaches the stratum of - water having the same temperature as itself. Arrived here, however, - although it has the same temperature <span class="pagenum" id="Page_551">551</span>as the surrounding water, - the two are no longer in equilibrium, for the water which has come - from the surface, has a greater density than that below at the same - temperature. It will therefore not be arrested at the stratum of the - same temperature, as would have been the case with fresh water; but it - will continue to sink, carrying of course its higher temperature with - it, and distributing it among the lower layers of colder water. At - the end of the winter, therefore, and just before the summer heating - recommences, we shall have at the surface a more or less thick stratum - of water having a nearly uniform temperature of 60° F., and below this - the temperature decreasing at a considerable but less rapid rate than - at the termination of the summer heating. If we distinguish between - <em>surface-water</em>, the temperature of which rises with the atmospheric - temperature (following thus, in direction at least, the variation of - the seasons), and <em>subsurface</em>-water, or the stratum immediately below - it, we have for the latter the, at first sight, paradoxical effect of - summer cooling and winter heating. The effect of this agency is to - diffuse the same heat to a greater depth in the ocean, the greater the - yearly range of atmospheric temperature at the surface. This effect - is well shown in the chart of isothermals, on a vertical section, - between Madeira and a position in lat. 3° 8′ N., long. 14° 49′ W. The - isothermal line for 45° F. rises from a depth of 740 fathoms at Madeira - to 240 fathoms at the above-mentioned position. In equatorial regions - there is hardly any variation in the surface-temperature of the sea; - consequently we find cold water very close to the surface all along the - line. On referring to the temperature section between the position lat. - 3° 8′ N., long. 14° 49′ W., and St. Paul’s Rocks, it will be seen that, - with a surface-temperature of from 75° F. to 79° F., water at 55° F. is - reached at distances of less than 100 fathoms from the surface. Midway - between the Azores and Bermuda, with a surface-temperature of 70° F., - it is only at a depth of 400 fathoms that we reach water of 55° F.”</p> - - <p>What Mr. Buchanan states will explain why the mean annual temperature - of the water at the surface extends to a greater depth in the middle - of the North Atlantic than at the equator. It also explains why the - temperature from the surface downwards decreases more rapidly at the - equator than in the middle of the North Atlantic; but, if I rightly - understand the theory, it does not explain (and this is the point at - issue) why at a given depth the temperature of the water in the North - Atlantic should be higher than the temperature at a corresponding depth - at the equator. Were there no horizontal circulation the greatest - thickness of warm water would certainly be found at the equator and - the least at the poles. The isothermals would in such a case gradually - slope downwards from the poles to the equator. The slope might not be - uniform, but still it would be a continuous downward slope.</p> - - <p><span class="pagenum" id="Page_552">552</span></p> - - <hr class="short mt5" /> - <h3 id="APPENDIX_V">V.</h3> - <div class="center mb3">ON THE CAUSE OF THE COOLING EFFECT PRODUCED ON SOLIDS BY - TENSION.<a id="FNanchor_330" href="#Footnote_330" class="fnanchor">[330]</a></div> - - <div class="center">From the <cite>Philosophical Magazine</cite> for May, 1864.</div> - - <p>From a series of experiments made by Dr. Joule with his usual accuracy, - he found that when bodies are subjected to tension, a cooling effect - takes place. “The quantity of cold,” he says, “produced by the - application of tension was sensibly equal to the heat evolved by its - removal; and further, that the thermal effects were proportional to - the weight employed.”<a id="FNanchor_331" href="#Footnote_331" class="fnanchor">[331]</a> He found that when a weight was applied to - compress a body, a certain amount of heat was evolved; but the same - weight, if applied to stretch the body, produced a corresponding amount - of cold.</p> - - <p>This, although it does not appear to have been remarked, is a most - singular result. If we employ a force to compress a body, and then ask - what has become of the force applied, it is quite a satisfactory answer - to be told that the force is converted into heat, and reappears in the - molecules of the body as such; but if the same force be employed to - stretch the body, it will be no answer to be told that the force is - converted into cold. Cold cannot be the force under another form, for - cold is a privation of force. If a body, for example, is compressed by - a weight, the <i lang="la">vis viva</i> of the descending weight is transmitted to the - molecules of the body and reappears under that form of force called - heat; but if the same weight is applied so as to stretch or expand the - body, not only does the force of the weight disappear without producing - heat, but the molecules which receive the force lose part of that - which they already possessed. Not only does the force of the weight - disappear, but along with it a portion of the force previously existing - in the molecules under the form of heat. We have therefore to inquire, - not merely into what becomes of the force imparted by the weight, but - also what becomes of the force in the form of heat which disappears - from the molecules of the body itself. That the <i lang="la">vis viva</i> of the - descending weight should disappear without increasing the heat of the - molecules is not so surprising, because it may be transformed into some - other form of force different from that of heat. For it is by no means - evident <i lang="la">à priori</i> that heat should be the only form under which it - may exist. But it is somewhat strange that it should cause the force - previously existing in the molecules in the form of heat also to change - into some other form.</p> - - <p>When a weight, for example, is employed to stretch a solid body, <span class="pagenum" id="Page_553">553</span>it - is evident that the force exerted by the weight is consumed in work - against the cohesion of the particles, for the entire force is exerted - so as to pull them separate from each other. But the cooling effect - which takes place shows that more force disappears than simply what - is exerted by the weight; for the cooling effect is caused by the - disappearance of force in the shape of heat from the body itself. The - force exerted by the weight disappears in performing work against the - cohesion of the particles of the body stretched. But what becomes - of the energy in the form of heat which disappears from the body at - the same time? It must be consumed in performing work of some kind - or other. The force exerted by the weight cannot be the cause of the - cooling effect. The transferrence of force from the weight to the body - may be the cause of a heating effect—an increase of force in the body; - but this transferrence of force to the body cannot be the cause of a - decrease of force in the body. If a decrease of force actually follows - the application of tension, the weight can only be the occasion, not - the cause of the decrease.</p> - - <p>In what manner, then, does the stretching of the body by the weight - become the occasion of its losing energy in the shape of heat? Or, in - other words, what is the cause of the cooling effects which result - from tension? The probable explanation of the phenomenon seems to - be this: if the molecules of a body are held together by any force, - of whatever nature it may be, which prevents any further separation - taking place, then the entire heat applied to such a body will appear - as temperature; but if this binding force becomes lessened so as to - allow further expansion, then a portion of the heat applied will be - lost in producing expansion. All solids at any given temperature expand - until the expansive force of their heat exactly balances the cohesive - force of their molecules, after which no further expansion at the - same temperature can possibly take place while the cohesive force of - the molecules remains unchanged. But if, by some means or other, the - cohesive force of the molecules become reduced, then instantly the - body will expand under the heat which it possesses, and of course a - portion of the heat will be consumed in expansion, and a cooling effect - will result. Now tension, although it does not actually lessen the - cohesive force of the molecules of the stretched body, yet produces, by - counteracting this force, the same effect; for it allows the molecules - an opportunity of performing work of expansion, and a cooling effect - is the consequence. If the piston of a steam-engine, for example, be - loaded to such an extent that the steam is unable to move it, the steam - in the interior of the cylinder will not lose any of its heat; but if - the piston be raised by some external force, the molecules of the steam - will assist this force, and consequently will suffer loss of heat in - proportion to the amount of work which they perform. The very same - occurs when<span class="pagenum" id="Page_554">554</span> tension is applied to a solid. Previous to the application - of tension, the heat existing in the molecules is unable to produce - any expansion against the force of cohesion. But when the influence of - cohesion is partly counteracted by the tension applied, the heat then - becomes enabled to perform work of expansion, and a cooling effect is - the result.</p> - - <hr class="short mt5" /> - <h3 id="APPENDIX_VI">VI.</h3> - <div class="center mb3">THE CAUSE OF REGELATION.<a id="FNanchor_332" href="#Footnote_332" class="fnanchor">[332]</a></div> - - <p>There are two theories which have been advanced to explain Regelation, - the one by Professor Faraday, and the other by Professor James Thomson.</p> - - <p>According to Professor James Thomson, pressure is the cause of - regelation. Pressure applied to ice tends to lower the melting-point, - and thus to produce liquefaction; but the water which results is - colder than the ice, and refreezes the moment it is relieved from - pressure. When two pieces of ice are pressed together, a melting takes - place at the points in contact, resulting from the lowering of the - melting-point; the water formed, re-freezing, joins the two pieces - together.</p> - - <p>The objection which has been urged against this theory is that - regelation will take place under circumstances where it is difficult to - conceive how pressure can be regarded as the cause. Two pieces of ice, - for example, suspended by silken threads in an atmosphere above the - melting-point, if but simply allowed to touch each other, will freeze - together. Professor J. Thomson, however, attributes the freezing to - the pressure resulting from the capillary attraction of the two moist - surfaces in contact. But when we reflect that it requires the pressure - of a mile of ice—135 tons on the square foot—to lower the melting-point - one degree, it must be obvious that the lowering effect resulting - from capillary attraction in the case under consideration must be - infinitesimal indeed.</p> - - <p>The following clear and concise account of Faraday’s theory, I quote - from Professor Tyndall’s “Forms of Water:”—</p> - - <p>“Faraday concluded that <em>in the interior</em> of any body, whether solid - or liquid, where every particle is grasped, so to speak, by the - surrounding particles, and grasps them in turn, the bond of cohesion - is so strong as to require a higher temperature to change the state - of aggregation than is necessary <em>at the surface</em>. At the surface of - a piece of ice, for example, the molecules are free on one side from - <span class="pagenum" id="Page_555">555</span>the control of other molecules; and they therefore yield to heat more - readily than in the interior. The bubble of air or steam in overheated - water also frees the molecules on one side; hence the ebullition - consequent upon its introduction. Practically speaking, then, the - point of liquefaction of the interior ice is higher than that of the - superficial ice....</p> - - <p>“When the surfaces of two pieces of ice, covered with a film of the - water of liquefaction, are brought together, the covering film is - transferred from the surface to the centre of the ice, where the point - of liquefaction, as before shown, is higher than at the surface. - The special solidifying power of ice upon water is now brought - into play <em>on both sides of the film</em>. Under these circumstances, - Faraday held that the film would congeal, and freeze the two surfaces - together.”—<cite>The Forms of Water</cite>, p. 173.</p> - - <p>The following appears to be a more simple explanation of the phenomena - than either of the preceding:—</p> - - <p>The freezing-point of water, and the melting-point of ice, as Professor - Tyndall remarks, touch each other as it were at this temperature. At - a hair’s-breadth lower water freezes; at a hair’s-breadth higher ice - melts. Now if we wish, for example, to freeze water, already just about - the freezing-point, or to melt a piece of ice already just about the - melting-point, we can do this either by a change of temperature or - by a change of the melting-point. But it will be always much easier - to effect this by the former than by the latter means. Take the - case already referred to, of the two pieces of ice suspended in an - atmosphere above the melting-point. The pieces at their surfaces are - in a melting condition, and are surrounded by a thin film of water - just an infinitesimal degree above the freezing-point. The film has on - the one side solid ice at the freezing-point, and on the other a warm - atmosphere considerably above the freezing-point. The tendency of the - ice is to lower the temperature of the film, while that of the air is - to raise its temperature. When the two pieces are brought into contact - the two films unite and form one film separating the two pieces of ice. - This film is not like the former in contact with ice on the one side - and warm air on the other. It is surrounded on both sides by solid ice. - The tendency of the ice, of course, is to lower the film to the same - temperature as the ice itself, and thus to produce solidification. - It is evident that the film must either melt the ice or the ice must - freeze the film, if the two are to assume the same temperature. But the - power of the ice to produce solidification, owing to its greater mass, - is enormously greater than the power of the film to produce fluidity, - consequently regelation is the result.</p> - - <p><span class="pagenum" id="Page_556">556</span></p> - <hr class="short mt5" /> - <h3 id="APPENDIX_VII">VII.</h3> - <div class="hang mb3">LIST OF PAPERS WHICH HAVE APPEARED IN DR. A. PETERMANN’S - <cite>GEOGRAPHISCHE MITTHEILUNGEN</cite> RELATING TO THE GULF-STREAM AND - THERMAL CONDITION OF THE ARCTIC REGIONS.</div> - - <p>The most important memoir which we have on the Gulf-stream and its - influence on the climate of the arctic regions is the one by Dr. A. - Petermann, entitled “Der Golfstrom und Standpunkt der thermometrischen - Kenntniss des nord-atlantischen Oceans und Landgebiets im Jahre 1870.” - <cite>Geographische Mittheilungen</cite>, Band XVI. 1870.</p> - - <p>Dr. Petermann has, in this memoir, by a different line of argument - from that which I have pursued in this volume, shown in the most clear - and convincing manner that the abnormally high temperature of the - north-western shores of Europe and the seas around Spitzbergen is owing - entirely to the Gulf-stream, and not to any general circulation such as - that advocated by Dr. Carpenter. From a series of no fewer than 100,000 - observations of temperature in the North Atlantic and in the arctic - seas, he has been enabled to trace with accuracy on his charts the very - footsteps of the heat in its passage from the Gulf of Mexico up to the - shores of Spitzbergen.</p> - - <p>The following is a list of the more important papers bearing on the - subject which have recently appeared in Dr. Petermann’s <cite>Geogr. - Mittheilungen</cite>:—</p> - - <p>An English translation of Dr. Petermann’s Memoir, and of a few more in - the subjoined list, has been published in a volume, with supplements, - by the Hydrographic Department of the United States, under the - superintendence of Commodore R. H. Wyman.</p> - - <p>The papers whose titles are in English have appeared in the American - volume. In that volume the principal English papers on the subject, - in as far as they relate to the north-eastern extension of the - Gulf-stream, have also been reprinted.</p> - - <p>The System of Oceanic Currents in the Circumpolar Basin of the Northern - Hemisphere. By Dr. A. Mühry. Vol. XIII., Part II. 1867.</p> - - <p>The Scientific Results of the first German North Polar Expedition. By - Dr. W. von Freeden. Vol. XV., Part VI. 1869.</p> - - <p><span class="pagenum" id="Page_557">557</span></p> - - <p>The Gulf-stream, and the Knowledge of the Thermal Properties of the - North Atlantic Ocean and its Continental Borders, up to 1870. By Dr. A. - Petermann. <cite>Geographische Mittheilungen</cite>, Vol. XVI., Part VI. 1870.</p> - - <p>The Temperature of the North Atlantic Ocean and the Gulf-stream. By - Rear-Admiral C. Irminger. Vol. XVI., Part VI. 1870.</p> - - <p>Meteorological Observations during a Winter Stay on Bear Island, - 1865−1866. By Sievert Tobilson. Vol. XVI., Part VII. 1870.</p> - - <p>Die Temperatur-verhältnisse in den arktischen Regionen. Von Dr. - Petermann. Band XVI., Heft VII. 1870.</p> - - <p>Preliminary Reports of the Second German North Polar Expedition, and of - minor Expeditions, in 1870. Vol. XVII.</p> - - <p>Preliminary Report of the Expedition for the Exploration of the - Nova-Zembla Sea (the sea between Spitzbergen and Nova Zembla), by - Lieutenants Weyprecht and Payer, June to September, 1871. By Dr. A. - Petermann. Vol. XVII. 1871.</p> - - <p>Der Golfstrom ostwärts vom Nordkap. Von A. Middendorff. Band XVII., - Heft I. 1871.</p> - - <p>Kapitän E. H. Johannesen’s Umfahrung von Nowaja Semlä im Sommer 1870, - und norwegischer Finwalfang östlich vom Nordkap. Von Th. v. Heuglin. - Band XVII., Heft I. 1871.</p> - - <p>Die Nordpol-Expeditionen, das sagenhafte Gillis-land und der Golfstrom - im Polarmeere. Von Dr. A. Petermann. 5 Nov. 1870.</p> - - <p>Th. v. Heuglin’s Aufnahmen in Ost-Spitzbergen. Begleitworte zur neuen - Karte dieses Gebiets. Tafel 9. 1870. Band XVII., Heft V. 1871.</p> - - <p>Die zweite deutsche Nordpolar-Expedition, 1869−70. Schlittenreise - an der Küste Grönlands nach Norden, 8 März−27 April, 1870. Von - Ober-Lieutenant Julius Payer. Band XVII., Heft V. 1871.</p> - - <p>Die Entdeckung des Kaiser Franz Josef-Fjordes in Ost-Grönland, August, - 1870. Von Ober-Lieutenant Julius Payer. Band XVII., Heft V. 1871.</p> - - <p>Die Erschliessung eines Theiles des nördlichen Eismeeres durch die - Fahrten und Beobachtungen der norwegischen Seefahrer Torkildsen, - Ulve, Mack Qvale, und Nedrevaag im karischen Meere, 1870. Von Dr. A. - Petermann. Band XVII., Heft III. 1871.</p> - - <p><span class="pagenum" id="Page_558">558</span></p> - - <p>Die zweite deutsche Nordpolar-Expedition, 1869−70. Schlittenreise nach - Ardencaple Inlet, 8−29 Mai, 1870. Von Ober-Lieutenant Julius Payer. - Band XVII., Heft XI. 1871.</p> - - <p>Ein Winter unter dem Polarkreise. Von Ober-Lieutenant Julius Payer. - Band XVII., Heft XI. 1871.</p> - - <p>Die Entdeckung eines offenen Polarmeeres durch Payer und Weyprecht im - September, 1871. Von Dr. A. Petermann. Band XVII., Heft XI. 1871.</p> - - <p>James Lamont’s Nordfahrt, Mai-August, 1871. Die Entdeckungen von - Weyprecht, Payer, Tobiesen, Mack, Carlsen, Ulve, und Smyth im Sommer, - 1871.</p> - - <p>Stand der Nordpolarfrage zu Ende des Jahres 1871. Von Dr. A. Petermann. - Band XVII., Heft XII. 1871.</p> - - <p>Das Innere von Grönland. Von Dr. Robert Brown. Band XVII., Heft X. 1871.</p> - - <p>Captain T. Torkildsen’s Cruise from Tromsö to Spitzbergen, July 26 to - September 26, 1871. Vol. XVIII. 1872.</p> - - <p>The Sea north of Spitzbergen, and the most northern Meteorological - Observations. Vol. XVIII. 1872.</p> - - <p>Results of the Observations of the Deep-sea Temperature in the Sea - between Greenland, Northern Europe, and Spitzbergen. By Professor H. - Möhn. Vol. XVIII. 1872.</p> - - <p>The Norwegian Cruises to Nova Zembla and the Kara Sea in 1871. Vol. - XVIII. 1872.</p> - - <p>The Cruises in the Polar Sea in 1872. Vol. XVIII. 1872.</p> - - <p>The Cruise of Smyth and Ulve, June 19 to September 27, 1871. Vol. - XVIII. 1872.</p> - - <p>Die fünfmonatliche Schiffbarkeit des sibirischen Eismeeres um Nowaja - Semlja, erwiesen durch die norwegischen Seefahrer in 1869 und 1870, - ganz besonders aber in 1871. Von Dr. A. Petermann. Band XVIII., Heft X. - 1872.</p> - - <p>Die neuen norwegischen Aufnahmen des nordöstlichen Theiles von Nowaja - Semlja durch Mack, Dörma, Carlsen, u. A., 1871. Von Dr. Petermann. Band - XVIII., Heft X. 1872.</p> - - <p><span class="pagenum" id="Page_559">559</span></p> - - <p>Nachrichten über die sieben zurückgekehrten Expeditionen unter Graf - Wiltschek, Altmann, Johnsen, Nilsen, Smith, Gray, Whymper; die - drei Überwinterungs-Expeditionen; die Amerikanische, Schwedische, - Österreichisch-Ungarische; und die zwei neuen: die norwegische - Winter-Expedition und diejenige unter Kapitän Mack. Von Dr. A. - Petermann. Band XVIII., Heft XII. 1872.</p> - - <p>Konig Karl-Land im Osten von Spitzbergen und seine Erreichung und - Aufnahme durch norwegische Schiffer im Sommer 1872. Von Professor H. - Möhn. Band XIX., Heft IV. 1873.</p> - - <p>Resultate der Beobachtungen angestellt auf der Fahrt des Dampfers - “Albert” nach Spitzbergen im November und Dezember, 1872. Von Professor - Möhn. Band XIX., Heft VII. 1873.</p> - - <p>Die amerikanische Nordpolar-Expedition unter C. F. Hall, 1871−3. Von - Dr. A. Petermann. Band XIX., Heft VIII. 1873.</p> - - <p>Die Trift der Hall’schen Nordpolar-Expedition, 16 August bis 15 - Oktober, 1872, und die Schollenfahrt der 20 bis zum 30 April, 1873. Von - Dr. A. Petermann. Band XIX., Heft X. 1873.</p> - - <p>Das offene Polarmeer bestätigt durch das Treibholz an der Nordwestküste - von Grönland. Von Dr. A. Petermann. Band XX., Heft V. 1874.</p> - - <p>Das arktische Festland und Polarmeer. Von Dr. Joseph Chavanne. Band - XX., Heft VII. 1874.</p> - - <p>Die Umkehr der Hall’schen Polar-Expedition nach den Aussagen der - Offiziere. Von Dr. A. Petermann. Band XX., Heft VII. 1874.</p> - - <p>Die zweite österreichisch-ungarische Nordpolar-Expedition unter - Weyprecht und Payer, 1872−4. Von Dr. A. Petermann. Band XX., Heft X. - 1874.</p> - - <p>Beiträge zur Klimatologie und Meteorologie des Ost-polar-Meeres. Von - Professor Möhn. Band XX., Heft V. 1874.</p> - - <p>Kapitän David Gray’s Reise und Beobachtungen im ost-grönländischen - Meere, 1874, und seine Ansichten über den besten Weg zum Nordpol. - Original-Mittheilungen an A. Petermann, d.D., Peterhead, Dezember, - 1874. Band XXI., Heft III. 1875.</p> - - <p><span class="pagenum" id="Page_560">560</span></p> - <hr class="short mt5" /> - <h3 id="APPENDIX_VIII">VIII.</h3> - <div class="hang mb3">LIST OF PAPERS BY THE AUTHOR TO WHICH REFERENCE IS MADE IN THIS VOLUME.</div> - - <p>On the Influence of the Tidal Wave on the Earth’s Rotation and on the - Acceleration of the Moon’s Mean Motion.—<cite>Phil. Mag.</cite>, April, 1864.</p> - - <p>On the Nature of Heat-vibrations.—<cite>Phil. Mag.</cite>, May, 1864.</p> - - <p>On the Cause of the Cooling Effect produced on Solids by - Tension.—<cite>Phil. Mag.</cite>, May, 1864.</p> - - <p>On the Physical Cause of the Change of Climate during Geological - Epochs.—<cite>Phil. Mag.</cite>, August, 1864.</p> - - <p>On the Physical Cause of the Submergence of the Land during the Glacial - Epoch.—The <cite>Reader</cite>, September 2nd and October 14th, 1865.</p> - - <p>On Glacial Submergence.—The <cite>Reader</cite>, December 2nd and 9th, 1865.</p> - - <p>On the Eccentricity of the Earth’s Orbit.—<cite>Phil. Mag.</cite>, January, 1866.</p> - - <p>Glacial Submergence on the Supposition that the Interior of the Globe - is in a Fluid Condition.—The <cite>Reader</cite>, January 13th, 1866.</p> - - <p>On the Physical Cause of the Submergence and Emergence of the Land - during the Glacial Epoch, with a Note by Professor Sir William - Thomson.—<cite>Phil. Mag.</cite>, April, 1866.</p> - - <p>On the Influence of the Tidal Wave on the Motion of the Moon.—<cite>Phil. - Mag.</cite>, August and November, 1866.</p> - - <p>On the Reason why the Change of Climate in Canada since the Glacial - Epoch has been less complete than in Scotland.—<cite>Trans. Geol. Soc. of - Glasgow</cite>, 1866.</p> - - <p>On the Eccentricity of the Earth’s Orbit, and its Physical Relations to - the Glacial Epoch.—<cite>Phil. Mag.</cite>, February, 1867.</p> - - <p>On the Reason why the Difference of Reading between a Thermometer - exposed to direct Sunshine and one shaded diminishes as we ascend in - the Atmosphere.—<cite>Phil. Mag.</cite>, March, 1867.</p> - - <p><span class="pagenum" id="Page_561">561</span></p> - - <p>On the Change in the Obliquity of the Ecliptic; its Influence on the - Climate of the Polar Regions and Level of the Sea.—<cite>Trans. Geol. Soc. - of Glasgow</cite>, vol. ii., p. 177. <cite>Phil. Mag.</cite>, June, 1867.</p> - - <p>Remarks on the Change in the Obliquity of the Ecliptic, and its - Influence on Climate.—<cite>Phil. Mag.</cite>, August, 1867.</p> - - <p>On certain Hypothetical Elements in the Theory of Gravitation - and generally received Conceptions regarding the Constitution of - Matter.—<cite>Phil. Mag.</cite>, December, 1867.</p> - - <p>On Geological Time, and the probable Date of the Glacial and the Upper - Miocene Period.—<cite>Phil. Mag.</cite>, May, August, and November, 1868.</p> - - <p>On the Physical Cause of the Motions of Glaciers.—<cite>Phil. Mag.</cite>, March, - 1869. <cite>Scientific Opinion</cite>, April 14th, 1869.</p> - - <p>On the Influence of the Gulf-stream.—<cite>Geol. Mag.</cite>, April, 1869. - <cite>Scientific Opinion</cite>, April 21st and 28th, 1869.</p> - - <p>On Mr. Murphy’s Theory of the Cause of the Glacial Climate.—<cite>Geol. - Mag.</cite>, August, 1869. <cite>Scientific Opinion</cite>, September 1st, 1869.</p> - - <p>On the Opinion that the Southern Hemisphere loses by Radiation more - Heat than the Northern, and the supposed Influence that this has on - Climate.—<cite>Phil. Mag.</cite>, September, 1869. <cite>Scientific Opinion</cite>, September - 29th and October 6th, 1869.</p> - - <p>On Two River Channels buried under Drift belonging to a Period when the - Land stood several hundred feet higher than at present.—<cite>Trans. Geol. - Soc. of Edinburgh</cite>, vol. i., p. 330.</p> - - <p>On Ocean-currents: Ocean-currents in Relation to the Distribution of - Heat over the Globe.—<cite>Phil. Mag.</cite>, February, 1870.</p> - - <p>On Ocean-currents: Ocean-currents in Relation to the Physical Theory of - Secular Changes of Climate.—<cite>Phil. Mag.</cite>, March, 1870.</p> - - <p>The Boulder Clay of Caithness a Product of Land-ice.—<cite>Geol. Mag.</cite>, May - and June, 1870.</p> - - <p>On the Cause of the Motion of Glaciers.—<cite>Phil. Mag.</cite>, September, 1870.</p> - - <p>On Ocean-currents: On the Physical Cause of Ocean-currents. Examination - of Lieutenant Maury’s Theory.—<cite>Phil. Mag.</cite>, October, 1870.</p> - - <p>On the Transport of the Wastdale Granite Boulders.—<cite>Geol. Mag.</cite>, - January, 1871.</p> - - <p><span class="pagenum" id="Page_562">562</span></p> - - <p>On a Method of determining the Mean Thickness of the Sedimentary Rocks - of the Globe.—<cite>Geol. Mag.</cite>, March, 1871.</p> - - <p>Mean Thickness of the Sedimentary Rocks.—<cite>Geol. Mag.</cite>, June, 1871.</p> - - <p>On the Age of the Earth as determined from Tidal Retardation.—<cite>Nature</cite>, - August 24th, 1871.</p> - - <p>Ocean-currents: On the Physical Cause of Ocean-currents. Examination of - Dr. Carpenter’s Theory.—<cite>Phil. Mag.</cite>, October, 1871.</p> - - <p>Ocean-currents: Further Examination of the Gravitation Theory.—<cite>Phil. - Mag.</cite>, February, 1874.</p> - - <p>Ocean-currents: The Wind Theory of Oceanic Circulation.—<cite>Phil. Mag.</cite>, - March, 1874.</p> - - <p>Ocean-currents.—<cite>Nature</cite>, May 21st, 1874.</p> - - <p>The Physical Cause of Ocean-currents.—<cite>Phil. Mag.</cite>, June, 1874. - <cite>American Journal of Science and Art</cite>, September, 1874.</p> - - <p>On the Physical Cause of the Submergence and Emergence of the Land - during the Glacial Epoch.—<cite>Geol. Mag.</cite>, July and August, 1874.</p> - - <div class="chapter" id="INDEX"> - <span class="pagenum" id="Page_563">563</span> - <h2>INDEX.</h2> - </div> - - <div class="figcenter"> - <img src="images/diamondbar.png" width="100" height="8" alt="" /> - </div> - - <ul class="index"> - <li class="ifrst">Absolute heating-power of ocean-currents, <a href="#Page_23">23</a></li> - <li class="isub2">〃 amount of heat received from the sun per day, <a href="#Page_26">26</a></li> - - <li class="indx">Adhémar, M., theory founded upon a mistake in regard to radiation, <a href="#Page_81">81</a>, <a href="#Page_85">85</a></li> - <li class="isub2">〃 on submergence, <a href="#Page_368">368</a></li> - <li class="isub2">〃 on influence of eccentricity on climate, <a href="#Page_542">542</a></li> - - <li class="indx">Aërial currents increased in action by formation of snow and ice, <a href="#Page_76">76</a></li> - <li class="isub2">〃 function of, stated, <a href="#Page_51">51</a></li> - <li class="isub2">〃 heat conveyed by, <a href="#Page_27">27</a></li> - - <li class="indx">Africa, South, glacial and inter-glacial periods of, <a href="#Page_242">242</a></li> - <li class="isub2">〃 boulder clay of Permian age, <a href="#Page_300">300</a></li> - - <li class="indx">Age and origin of the sun, <a href="#Page_346">346</a></li> - - <li class="indx">Air, on absorption of rays by, <a href="#Page_59">59</a></li> - <li class="isub2">〃 when humid, absorbs rays which agree with it in period, <a href="#Page_59">59</a></li> - <li class="isub2">〃 when perfectly dry incapable of absorbing radiant heat, <a href="#Page_59">59</a></li> - - <li class="indx">Airy, Professor, earth’s axis of rotation permanent, <a href="#Page_7">7</a></li> - - <li class="indx">Aitken’s, Mr., experiment on density of polar water, <a href="#Page_129">129</a></li> - - <li class="indx">Aland islands, striation of, <a href="#Page_447">447</a></li> - - <li class="indx">Alternate cold and warm periods, <a href="#Page_236">236</a></li> - - <li class="indx">Allermuir, striations on summit of, <a href="#Page_441">441</a></li> - - <li class="indx">America, low temperature in January, <a href="#Page_72">72</a></li> - <li class="isub2">〃 thickness of ice-sheet of North, <a href="#Page_381">381</a></li> - - <li class="indx">Anderson, Captain Sir James, never observed a stone on an iceberg, <a href="#Page_282">282</a></li> - - <li class="indx">Antarctic ice-cap, probable thickness of, <a href="#Page_375">375</a></li> - <li class="isub2">〃 diagram representing thickness of, <a href="#Page_377">377</a></li> - <li class="isub2">〃 thickness of, estimated from icebergs, <a href="#Page_384">384</a></li> - - <li class="indx">Antarctic regions, mean summer temperature of, below freezing-point, <a href="#Page_63">63</a></li> - - <li class="indx">Antarctic snowfall, estimates of, <a href="#Page_382">382</a></li> - - <li class="indx">Aphelion, glacial conditions at maximum when winter solstice is at, <a href="#Page_77">77</a></li> - - <li class="indx">Arago, M., on influence of eccentricity on climate, <a href="#Page_536">536</a></li> - - <li class="indx">Arctic climate, influence of ocean-currents on, during glacial period, <a href="#Page_260">260</a></li> - - <li class="indx">Arctic regions, influence of Gulf-stream on climate of, <a href="#Page_45">45</a></li> - <li class="isub2">〃 mean summer temperature of, <a href="#Page_63">63</a></li> - - <li class="indx">Arctic regions, amount of heat received by, per unit surface, <a href="#Page_195">195</a></li> - <li class="isub2">〃 warm periods best marked in, <a href="#Page_258">258</a></li> - <li class="isub2">〃 warm inter-glacial periods in, <a href="#Page_258">258−265</a></li> - <li class="isub2">〃 state of, during glacial period, <a href="#Page_260">260</a></li> - <li class="isub2">〃 evidence of warm periods in, <a href="#Page_261">261</a></li> - <li class="isub2">〃 occurrence of recent trees in, <a href="#Page_261">261</a>, <a href="#Page_265">265</a></li> - <li class="isub2">〃 evidence of warm inter-glacial periods, <a href="#Page_293">293</a></li> - <li class="isub2">〃 warm climate during Old Red Sandstone period in, <a href="#Page_295">295</a></li> - <li class="isub2">〃 glacial period during Carboniferous age in, <a href="#Page_297">297</a></li> - <li class="isub2">〃 warm climate during Permian period in, <a href="#Page_301">301</a></li> - <li class="isub2">〃 list of papers relating to, <a href="#Page_556">556</a></li> - - <li class="indx">Arctic Ocean, area of, <a href="#Page_195">195</a></li> - <li class="isub2">〃 according to gravitation theory ought to be warmer than Atlantic in torrid zone, <a href="#Page_195">195</a></li> - <li class="isub2">〃 heat conveyed into, by currents, compared with that received by it from the sun, <a href="#Page_195">195</a></li> - <li class="isub2">〃 blocked up with polar ice, <a href="#Page_444">444</a></li> - - <li class="indx">Armagh, boulder beds of, <a href="#Page_299">299</a></li> - - <li class="indx">Arran, Island of, glacial conglomerate of Permian age in, <a href="#Page_299">299</a></li> - - <li class="indx">Astronomical causes of change of climate, <a href="#Page_10">10</a></li> - - <li class="indx">Astronomy and geology, supposed analogy between, <a href="#Page_355">355</a></li> - - <li class="indx">Atlantic, atmospheric pressure on middle of, <a href="#Page_33">33</a></li> - <li class="isub2">〃 inability of, to heat the south-west winds without the Gulf-stream, <a href="#Page_34">34</a></li> - <li class="isub2">〃 mean annual temperature of, <a href="#Page_36">36</a></li> - <li class="isub2">〃 mean temperature of, raised by Gulf-stream, <a href="#Page_36">36</a>, <a href="#Page_40">40</a></li> - <li class="isub2">〃 isothermal lines of, compared with those of the Pacific, <a href="#Page_46">46</a></li> - <li class="isub2">〃 area of, from equator to Tropic of Cancer, <a href="#Page_194">194</a></li> - <li class="isub2">〃 inquiry whether the area of, is sufficient to supply heat according to Dr. Carpenter’s theory, <a href="#Page_194">194</a></li> - - <li class="indx"><span class="pagenum">564</span>Atlantic, North, heat received by, from torrid zone by currents, <a href="#Page_194">194</a></li> - <li class="isub2">〃 according to Dr. Carpenter’s theory ought to be warmer in temperate regions than in the torrid zone, <a href="#Page_195">195</a></li> - <li class="isub2">〃 great depth of warm water in, <a href="#Page_198">198</a></li> - <li class="isub2">〃 North, an immense whirlpool, <a href="#Page_216">216</a></li> - <li class="isub2">〃 above the level of equator, <a href="#Page_221">221</a></li> - <li class="isub2">〃 probable antiquity of, <a href="#Page_367">367</a></li> - <li class="isub2">〃 from Scandinavia to Greenland probably filled with ice, <a href="#Page_451">451</a></li> - - <li class="indx">Atmosphere-pressure in Atlantic a cause of south-west winds, <a href="#Page_33">33</a></li> - - <li class="indx">Atmosphere, on difference between black-bulbed and shaded thermometer in upper strata of, <a href="#Page_547">547</a></li> - - <li class="indx">Australia, evidence of ice-action in conglomerate of, <a href="#Page_295">295</a></li> - - <li class="indx">Ayrshire, ice-action during Silurian period in, <a href="#Page_293">293</a></li> - - <li class="ifrst">Bakewell, Mr. R., on influence of eccentricity on climate, <a href="#Page_540">540</a></li> - - <li class="indx">Banks’s Land, discovery of ancient forest in, <a href="#Page_261">261</a></li> - <li class="isub2">〃 Professor Heer, on fossilized wood of, <a href="#Page_309">309</a></li> - - <li class="indx">Ball, Mr., objection to Canon Moseley’s results, <a href="#Page_501">501</a></li> - - <li class="indx">Baltic current, <a href="#Page_171">171</a></li> - - <li class="indx">Baltic, glaciation of islands in, <a href="#Page_448">448</a></li> - - <li class="indx">Baltic glacier, passage of, over Denmark, <a href="#Page_449">449</a></li> - - <li class="indx">Bath, grooved rock surfaces of, <a href="#Page_464">464</a></li> - - <li class="indx">Bay-ice grinds but does not striate rocks, <a href="#Page_277">277</a></li> - - <li class="indx">Belcher, Sir E., tree dug up by, in latitude 75° N., <a href="#Page_263">263</a></li> - <li class="isub2">〃 carboniferous fossils found in arctic regions by, <a href="#Page_298">298</a></li> - - <li class="indx">Belle-Isle, Strait of, observations on action of icebergs in, <a href="#Page_276">276</a></li> - - <li class="indx">Bell, Mr. A., on Mediterranean forms in glacial bed at Greenock, <a href="#Page_254">254</a></li> - - <li class="indx">Belt, Mr. Thomas, theory of the cause of glacial epochs, <a href="#Page_415">415</a></li> - - <li class="indx">Bennie, Mr. James, on surface geology, <a href="#Page_468">468</a></li> - <li class="isub2">〃 on deposits filling buried channel, <a href="#Page_486">486</a></li> - - <li class="indx">Blanford, Mr., on ice-action during Carboniferous age in India, <a href="#Page_297">297</a></li> - - <li class="indx">Borings, evidence of inter-glacial beds from, <a href="#Page_254">254</a></li> - <li class="isub2">〃 examination of drift by, <a href="#Page_467">467</a></li> - <li class="isub2">〃 journals of, <a href="#Page_483">483</a>, <a href="#Page_484">484</a></li> - - <li class="indx">Boulder clays of former glacial epochs, why so rare, <a href="#Page_269">269</a></li> - <li class="isub2">〃 a product of land-ice, <a href="#Page_284">284</a></li> - <li class="isub2">〃 if formed from icebergs must be stratified, <a href="#Page_284">284</a></li> - <li class="isub2">〃 scarcity of fossils in, <a href="#Page_285">285</a></li> - <li class="isub2">〃 formed chiefly from rock on which it lies, <a href="#Page_285">285</a></li> - <li class="isub2">〃 of Caithness a product of land-ice, <a href="#Page_435">435</a></li> - <li class="isub2">〃 on summit of Allermuir, <a href="#Page_441">441</a></li> - - <li class="indx">Boulders, how carried from a lower to a higher level, <a href="#Page_527">527</a></li> - - <li class="indx">Boussingault on absorption of carbon by vegetation, <a href="#Page_428">428</a></li> - - <li class="indx">Britain, climate of, affected most by south-eastern portion of Gulf-stream, <a href="#Page_33">33</a></li> - - <li class="indx">Brown, Dr. R., cited on Greenland ice-sheet, <a href="#Page_378">378</a>, <a href="#Page_380">380</a></li> - <li class="isub2">〃 on inland ice of Greenland, <a href="#Page_284">284</a></li> - <li class="isub2">〃 on cretaceous formation of Greenland, <a href="#Page_305">305</a></li> - <li class="isub2">〃 on Miocene beds of the Disco district, <a href="#Page_310">310</a></li> - - <li class="indx">Brown, Mr. Robert, on growth of coal plants, <a href="#Page_421">421</a></li> - - <li class="indx">Brown and Dickeson, on sediment of Mississippi, <a href="#Page_330">330</a></li> - - <li class="indx">Buchan, Mr., on atmosphere-pressure in the Atlantic, <a href="#Page_33">33</a></li> - <li class="isub2">〃 on force of the wind, <a href="#Page_220">220</a></li> - - <li class="indx">Buchanan, Mr. J. Y., on vertical distribution of heat of the ocean, <a href="#Page_550">550</a></li> - - <li class="indx">Buckland, Dr., observations by, on occurrence of red chalk on Cotteswold hills, <a href="#Page_459">459</a></li> - - <li class="indx">Buff, Professor, on oceanic circulation, <a href="#Page_145">145</a></li> - - <li class="indx">Buried river channels, <a href="#Page_466">466</a></li> - <li class="isub2">〃 channel from Kilsyth to Grangemouth, <a href="#Page_468">468</a></li> - <li class="isub2">〃 section at Grangemouth, <a href="#Page_474">474</a></li> - <li class="isub2">〃 from Kilsyth to Clyde, <a href="#Page_481">481</a></li> - <li class="isub2">〃 not excavated by sea nor by ice, <a href="#Page_469">469</a></li> - <li class="isub2">〃 other examples of, <a href="#Page_488">488−494</a></li> - - - <li class="ifrst">Caithness, difficulty of accounting for the origin of the boulder clay of, <a href="#Page_435">435</a></li> - - <li class="indx">Caithness, boulder clay of, a product of land-ice, <a href="#Page_435">435</a></li> - <li class="isub2">〃 boulder clay not formed by icebergs, <a href="#Page_437">437</a></li> - <li class="isub2">〃 theories regarding the origin of the boulder clay of, <a href="#Page_437">437</a></li> - <li class="isub2">〃 why the ice was forced over it, <a href="#Page_444">444</a></li> - <li class="isub2">〃 Professor Geikie and B. N. Peach on path of ice over, <a href="#Page_453">453</a></li> - - <li class="indx">Cambrian conglomerate of Islay, <a href="#Page_292">292</a></li> - - <li class="indx">Campbell, Mr., observations of, on icebergs, <a href="#Page_276">276</a></li> - <li class="isub2">〃 on supposed striation of rocks by large icebergs, <a href="#Page_278">278</a></li> - <li class="isub2">〃 evidence that river-ice does not striate rocks, <a href="#Page_279">279</a></li> - - <li class="indx">Canada, change of climate less complete than in Scotland, <a href="#Page_71">71</a></li> - - <li class="indx">Carboniferous period of arctic regions, <a href="#Page_298">298</a></li> - <li class="isub2">〃 evidence of glacial epoch during, <a href="#Page_296">296−298</a></li> - <li class="isub2">〃 temperate climate of, <a href="#Page_422">422</a></li> - - <li class="indx">Carboniferous limestone, mode of formation, <a href="#Page_433">433</a></li> - - <li class="indx">Carpenter’s, Dr., objections examined, <a href="#Page_141">141</a></li> - <li class="isub2">〃 theory, mechanics of, <a href="#Page_145">145</a></li> - <li class="isub2">〃 idea of a 〃vertical circulation〃 stated, <a href="#Page_153">153</a></li> - - <li class="indx"><span class="pagenum">565</span>Carpenter’s, Dr., radical error in theory of, <a href="#Page_155">155</a></li> - <li class="isub2">〃 on difference of density between waters of Atlantic and Mediterranean, <a href="#Page_168">168</a></li> - <li class="isub2">〃 theory, inadequacy of, <a href="#Page_191">191</a></li> - <li class="isub2">〃 estimate of thermal work of Gulf-stream, <a href="#Page_199">199</a></li> - - <li class="indx">Charpentier’s, M., theory of glacier-motion, <a href="#Page_513">513</a></li> - - <li class="indx">Carse clays, date of, <a href="#Page_405">405</a></li> - - <li class="indx">Cattegat, ice-markings on shore of, <a href="#Page_446">446</a></li> - - <li class="indx">Cave and river deposits, <a href="#Page_251">251</a></li> - - <li class="indx">Chalk, erratic blocks found in, <a href="#Page_304">304</a></li> - <li class="isub2">〃 <i>débris</i>, conclusion of Mr. Searles Wood, <a href="#Page_460">460</a></li> - - <li class="indx"><i>Challenger’s</i> temperature-soundings at equator, <a href="#Page_119">119</a></li> - <li class="isub2">〃 crucial test of the wind and gravitation theories, <a href="#Page_220">220</a></li> - - <li class="indx">Chambers, Dr. Robert, on striated pavements, <a href="#Page_255">255</a></li> - <li class="isub2">〃 observations on glaciation of Gothland, <a href="#Page_446">446</a></li> - - <li class="indx">Champlain Lake, inter-glacial bed of, <a href="#Page_241">241</a></li> - - <li class="indx">Chapelhall, ancient buried channel at, <a href="#Page_491">491</a></li> - <li class="isub2">〃 inter-glacial sand-bed, <a href="#Page_244">244</a></li> - - <li class="indx">Chart showing the agreement between system of currents and system of winds, <a href="#Page_212">212</a></li> - - <li class="indx">Christianstadt, crossed by Baltic glacier, <a href="#Page_450">450</a></li> - - <li class="indx">Circulation without difference of level, <a href="#Page_176">176</a></li> - - <li class="indx">Climate, Secular changes of, intensified by reaction of physical causes, <a href="#Page_75">75</a>, <a href="#Page_76">76</a></li> - <li class="isub2">〃 affected most by temperature of the surface of ground, <a href="#Page_88">88</a></li> - <li class="isub2">〃 ocean-currents in relation to, <a href="#Page_226">226</a></li> - <li class="isub2">〃 cold conditions of, inferred from absence of fossils, <a href="#Page_288">288</a></li> - <li class="isub2">〃 cold condition of, difficulty of determining, from fossil remains, <a href="#Page_289">289</a></li> - <li class="isub2">〃 warm, of arctic regions during Old Red Sandstone period, <a href="#Page_295">295</a></li> - <li class="isub2">〃 rough sketch of the history of, during the last <a href="#Page_60">60</a>,000 years, <a href="#Page_409">409</a></li> - <li class="isub2">〃 of Coal period inter-glacial in character, <a href="#Page_420">420</a></li> - <li class="isub2">〃 alternate changes of, during Coal period, <a href="#Page_426">426</a></li> - - <li class="indx">Climates, Mr. J. Geikie on difficulty of detecting evidence of ancient glacial conditions, <a href="#Page_289">289</a></li> - <li class="isub2">〃 evidence of, from ancient sea-bottoms, <a href="#Page_289">289</a></li> - - <li class="indx">Coal an inter-glacial formation, <a href="#Page_420">420</a></li> - - <li class="indx">Coal beds, alternate submergence and emergence during formation of, <a href="#Page_424">424</a></li> - <li class="isub2">〃 preservation of, by submergence, <a href="#Page_426">426</a></li> - - <li class="indx">Coal period, flatness of the land during, <a href="#Page_430">430</a></li> - - <li class="indx">Coal plants, conditions necessary for, preservation of, <a href="#Page_423">423</a></li> - - <li class="indx">Coal seams, thickness of, indicative of length of inter-glacial periods, <a href="#Page_428">428</a></li> - - <li class="indx">Coal seams, time occupied in formation of, <a href="#Page_429">429</a></li> - - <li class="indx">Coal strata, on absence of ice-action in, <a href="#Page_429">429</a></li> - - <li class="indx">Coal measures, oscillations of sea-level during formation of, <a href="#Page_425">425</a></li> - - <li class="indx">Cold periods best marked in temperate regions, <a href="#Page_258">258</a></li> - - <li class="indx">Colding, Dr., oceanic circulation, <a href="#Page_95">95</a></li> - - <li class="indx">Confusion of ideas in reference to the agency of polar cold, <a href="#Page_179">179</a></li> - - <li class="indx">Continental ice, inadequate conceptions of, <a href="#Page_385">385</a></li> - <li class="isub2">〃 absence of, during glacial epochs of Coal period, <a href="#Page_432">432</a></li> - - <li class="indx">Contorted drift near Musselburgh, <a href="#Page_465">465</a></li> - - <li class="indx">Cook, Captain, description of Sandwich Land by, <a href="#Page_60">60</a></li> - <li class="isub2">〃 on South Georgia, <a href="#Page_60">60</a></li> - - <li class="indx">Cornwall, striated rocks of, <a href="#Page_464">464</a></li> - - <li class="indx">Cotteswold hills, red chalk from Yorkshire found on, <a href="#Page_459">459</a></li> - - <li class="indx">Couthony, Mr., on action of icebergs, <a href="#Page_275">275</a></li> - - <li class="indx">Coutts, Mr. J., on buried channel, <a href="#Page_493">493</a></li> - - <li class="indx">Craig, Mr. Robert, on inter-glacial beds at Overton Hillhead and Crofthead, <a href="#Page_247">247</a></li> - - <li class="indx">Craiglockhart hill, inter-glacial bed of, <a href="#Page_245">245</a></li> - - <li class="indx">“Crawling” theory considered, <a href="#Page_507">507</a></li> - - <li class="indx">“Crevasses,” origin of, according to molecular theory, <a href="#Page_521">521</a></li> - - <li class="indx">Cretaceous period, evidence of ice-action during, <a href="#Page_303">303−305</a></li> - - <li class="indx">Cretaceous age, evidence of warm periods during, <a href="#Page_304">304</a></li> - - <li class="indx">Cretaceous formation of Greenland, <a href="#Page_305">305</a></li> - - <li class="indx">Crofthead, inter-glacial bed at, <a href="#Page_248">248</a></li> - - <li class="indx">Cromer forest bed, <a href="#Page_250">250</a></li> - - <li class="indx">Crosskey, Rev. Mr., comparison of Clyde and Canada shell beds, <a href="#Page_71">71</a></li> - <li class="isub2">〃 on southern shells in Clyde beds, <a href="#Page_253">253</a></li> - - <li class="indx">Croydon, block of granite found in chalk at, <a href="#Page_303">303</a></li> - - <li class="indx">Crucial test of the wind and gravitation theories, <a href="#Page_220">220</a></li> - - <li class="indx">Crystallization, force of, a cause of glacier-motion, <a href="#Page_523">523</a></li> - - <li class="indx">Currents, effects of their stoppage on temperatures of equator and poles, <a href="#Page_42">42</a></li> - <li class="isub2">〃 produced by saltness neutralize those produced by temperature, <a href="#Page_106">106</a></li> - - <li class="ifrst">Dalager, excursion in Greenland by, <a href="#Page_378">378</a></li> - - <li class="indx">Dana, Professor, on action of icebergs, <a href="#Page_275">275</a></li> - <li class="isub2">〃 on striations by icebergs, <a href="#Page_275">275</a></li> - <li class="isub2">〃 on thickness of ice-sheet of North America, <a href="#Page_381">381</a></li> - - <li class="indx">Darwin, Mr., on alternate cold and warm periods, <a href="#Page_231">231</a></li> - <li class="isub2">〃 on migration of plants and animals during glacial epoch, <a href="#Page_395">395</a></li> - <li class="isub2">〃 on peat of Falkland Islands, <a href="#Page_422">422</a></li> - - <li class="indx">Date of the 40-foot beach, <a href="#Page_409">409</a></li> - - <li class="indx">Date when conditions were favourable to formations of the Carse clay, <a href="#Page_409">409</a></li> - - <li class="indx">Davis’ Straits, current of, <a href="#Page_132">132</a></li> - - <li class="indx"><span class="pagenum">566</span>Dawkins, Mr. Boyd, on the animals of cave and river deposits, <a href="#Page_251">251</a></li> - - <li class="indx">Dawson, Principal, on esker of Carboniferous age, <a href="#Page_296">296</a></li> - - <li class="indx">〃 on habitats of coal plants, <a href="#Page_424">424</a></li> - - <li class="indx">Deflection of ocean-currents chief cause of change of climate, <a href="#Page_68">68</a></li> - - <li class="indx">De la Beche, Sir H. T., on influence of eccentricity on climate, <a href="#Page_539">539</a></li> - - <li class="indx">De Mairan, on influence of eccentricity on climate, <a href="#Page_528">528</a></li> - - <li class="indx">Denmark, crossed by Baltic glacier, <a href="#Page_449">449−452</a></li> - - <li class="indx">Denudation, method of measuring rate of, <a href="#Page_329">329</a></li> - <li class="isub2">〃 as a measure of geological time, <a href="#Page_329">329</a></li> - <li class="isub2">〃 measured by sediment of Mississippi, <a href="#Page_330">330</a></li> - <li class="isub2">〃 subaërial rate of, <a href="#Page_331">331</a></li> - <li class="isub2">〃 law which determines rate of, <a href="#Page_333">333</a></li> - <li class="isub2">〃 marine, trifling, <a href="#Page_337">337</a></li> - - <li class="indx">Deposition, rates of, generally adopted, quite arbitrary, <a href="#Page_360">360</a></li> - <li class="isub2">〃 rate of, determined by rate of denudation, <a href="#Page_362">362</a></li> - <li class="isub2">〃 range of, restricted to a narrow fringe surrounding the continents, <a href="#Page_364">364</a></li> - <li class="isub2">〃 area of, <a href="#Page_365">365</a></li> - <li class="isub2">〃 during glacial epoch probably less than present, <a href="#Page_366">366</a></li> - - <li class="indx">Deposits from icebergs cannot be wholly unstratified, <a href="#Page_437">437</a></li> - - <li class="indx">Despretz, tables by, of temperature of maximum density of sea-water, <a href="#Page_117">117</a></li> - - <li class="indx">Desor, M., on tropical fauna of the Eocene formation in Switzerland, <a href="#Page_306">306</a></li> - - <li class="indx">Derbyshire, breaks in limestone of, marks of cold periods, <a href="#Page_434">434</a></li> - - <li class="indx">Derbyshire limestone a product of inter-glacial periods, <a href="#Page_434">434</a></li> - - <li class="indx">Devonshire, boulder clay discovered in, <a href="#Page_463">463</a></li> - - <li class="indx">Diagram illustrating descent of water from equator to poles, <a href="#Page_155">155</a></li> - <li class="isub2">〃 showing variations of eccentricity, <a href="#Page_313">313</a></li> - <li class="isub2">〃 illustrative of fluidity of interior of the earth, <a href="#Page_396">396</a></li> - <li class="isub2">〃 showing formation of coal beds, <a href="#Page_426">426</a></li> - - <li class="indx">Dick, Mr., chalk flints in boulder clay, <a href="#Page_454">454</a></li> - - <li class="indx">Dick, Mr. R., on buried channel, <a href="#Page_491">491</a></li> - - <li class="indx">Difference of level essential to gravitation theory, <a href="#Page_176">176</a></li> - - <li class="indx">Dilatation of sea-water by increase of temperature calculated by Sir John Herschel, <a href="#Page_116">116</a></li> - - <li class="indx">Disco district, Dr. R. Brown cited on Miocene beds of, <a href="#Page_310">310</a></li> - - <li class="indx">Disco Island, Upper Miocene period of, <a href="#Page_307">307−308</a></li> - - <li class="indx">Distribution, how effected by ocean-currents, <a href="#Page_231">231</a></li> - - <li class="indx">Dove, Professor, method of constructing normal temperature tables by, <a href="#Page_40">40</a></li> - <li class="isub2">〃 on mean annual temperature, <a href="#Page_401">401</a></li> - - <li class="indx">Dover, mass of coal imbedded in chalk found at, <a href="#Page_303">303</a></li> - - <li class="indx">Drayson, Lieutenant-Colonel, on obliquity of ecliptic, <a href="#Page_410">410</a></li> - - <li class="indx">Drayson, Lieutenant-Colonel, theory of the cause of the glacial epoch, <a href="#Page_410">410</a></li> - - <li class="indx">Drift, examination by borings, <a href="#Page_467">467</a></li> - - <li class="indx">Drumry, deep surface deposits at, <a href="#Page_482">482</a></li> - - <li class="indx">Dubuat’s, M., experiments, <a href="#Page_182">182</a></li> - <li class="isub2">〃 experiments by, on water flowing down an incline, <a href="#Page_120">120</a></li> - - <li class="indx">Duncan, Captain, on under current in Davis’ Strait, <a href="#Page_134">134</a></li> - - <li class="indx">Dürnten lignite beds, <a href="#Page_240">240</a></li> - - <li class="indx">Dürnten beds an example of inter-glacial coal formation, <a href="#Page_433">433</a></li> - - <li class="indx">Durham, buried river channel at, <a href="#Page_488">488</a></li> - - <li class="ifrst">Earth’s axis of rotation permanent, <a href="#Page_7">7</a></li> - - <li class="indx">Earth, mean temperature of, increased by water at equator, <a href="#Page_30">30</a></li> - <li class="isub2">〃 not habitable without ocean-currents, <a href="#Page_54">54</a></li> - <li class="isub2">〃 mean temperature of, greatest in aphelion, <a href="#Page_77">77</a>, <a href="#Page_78">78</a></li> - <li class="isub2">〃 centre of gravity of, effects of ice-cap on, <a href="#Page_370">370</a>, <a href="#Page_371">371</a></li> - - <li class="indx">Eccentricity of the earth’s orbit, Mr. Stockwell’s researches regarding, <a href="#Page_54">54</a></li> - <li class="isub2">〃 primary cause of change of climate, <a href="#Page_54">54</a></li> - <li class="isub2">〃 primary cause of glacial epochs, <a href="#Page_77">77</a></li> - <li class="isub2">〃 how it affects the winds, <a href="#Page_228">228</a></li> - <li class="isub2">〃 tables of, <a href="#Page_314">314−321</a></li> - <li class="isub2">〃 its influence on temperature, <a href="#Page_323">323</a></li> - <li class="isub2">〃 explanation of tables of, <a href="#Page_324">324</a></li> - <li class="isub2">〃 De Marian, on influence of, on climate, <a href="#Page_528">528</a></li> - <li class="isub2">〃 Sir J. F. Herschel, on influence of, on climate, <a href="#Page_529">529</a></li> - <li class="isub2">〃 Œpinus, on influence of, on climate, <a href="#Page_529">529</a></li> - <li class="isub2">〃 R. Kirwan, on influence of, on climate, <a href="#Page_529">529</a></li> - <li class="isub2">〃 of planetary orbits, superior limits as determined by Lagrange, Leverrier, and Mr. Stockwell, <a href="#Page_531">531</a></li> - <li class="isub2">〃 Sir Charles Lyell, on influence of, on climate, <a href="#Page_529">529</a>, <a href="#Page_535">535</a></li> - <li class="isub2">〃 M. Arago, on influence of, on climate, <a href="#Page_536">536</a></li> - <li class="isub2">〃 Baron Humboldt, on influence of, on climate, <a href="#Page_538">538</a></li> - <li class="isub2">〃 Sir H. T. de la Beche, on influence of, on climate, <a href="#Page_539">539</a></li> - <li class="isub2">〃 Professor Phillips, on influence of, on climate, <a href="#Page_539">539</a></li> - <li class="isub2">〃 Mrs. Somerville, on influence of, on climate, <a href="#Page_540">540</a></li> - <li class="isub2">〃 L. W. Meech, on influence of, on climate, <a href="#Page_540">540</a></li> - <li class="isub2">〃 Mr. R. Bakewell, on influence of, on climate, <a href="#Page_540">540</a></li> - <li class="isub2">〃 M. Jean Reynaud, on influence of, on climate, <a href="#Page_541">541</a></li> - <li class="isub2">〃 M. Adhémar, on influence of, on climate, <a href="#Page_542">542</a></li> - - <li class="indx">Equator, reduction of level by denudation, <a href="#Page_336">336</a></li> - - <li class="indx"><span class="pagenum">567</span>Ecliptic, supposed effect of a change of obliquity of, <a href="#Page_8">8</a></li> - <li class="isub2">〃 changes of, effects on climate, <a href="#Page_398">398−417</a></li> - <li class="isub2">〃 obliquity of, Lieutenant-Colonel Drayson on, <a href="#Page_410">410</a></li> - - <li class="indx">Emergence, physical cause of, <a href="#Page_368">368</a></li> - - <li class="indx">England, inter-glacial beds of, <a href="#Page_249">249</a></li> - <li class="isub2">〃 glacial origin of Old Red Sandstone of, <a href="#Page_294">294</a></li> - <li class="isub2">〃 ice-action during Permian period in, <a href="#Page_298">298</a></li> - <li class="isub2">〃 North of, ice-sheet of, <a href="#Page_456">456</a></li> - <li class="isub2">〃 ice-sheet of South of, <a href="#Page_463">463</a></li> - - <li class="indx">Eocene period, total absence of fossils in flysch, <a href="#Page_286">286</a></li> - <li class="isub2">〃 glacial epoch of, <a href="#Page_305">305</a></li> - - <li class="indx">Eocene and Miocene periods, date of, <a href="#Page_357">357</a></li> - - <li class="indx">Equator, heat received per square mile at, <a href="#Page_26">26</a></li> - <li class="isub2">〃 temperature of earth increased by water at, <a href="#Page_30">30</a></li> - <li class="isub2">〃 and poles, effects of stoppage of currents on temperature of, <a href="#Page_42">42</a></li> - <li class="isub2">〃 surface-currents warmer than the under currents, <a href="#Page_92">92</a></li> - <li class="isub2">〃 heat transferred by currents from southern hemisphere compared with that received by land at, <a href="#Page_93">93</a></li> - <li class="isub2">〃 temperature soundings at, <a href="#Page_119">119</a></li> - <li class="isub2">〃 temperature of sea at, decreases most rapidly at the surface, <a href="#Page_119">119</a></li> - <li class="isub2">〃 heat received by the three zones compared with that received by the, <a href="#Page_194">194</a></li> - <li class="isub2">〃 migration across, <a href="#Page_234">234</a></li> - <li class="isub2">〃 glaciation of, <a href="#Page_234">234</a></li> - - <li class="indx">Equatorial current, displacement of, <a href="#Page_229">229</a></li> - - <li class="indx">Erratic blocks in stratified rocks, evidence of former land-ice, <a href="#Page_269">269</a></li> - <li class="isub2">〃 in chalk, <a href="#Page_304">304</a></li> - <li class="isub2">〃 why not found in coal strata, <a href="#Page_432">432</a></li> - - <li class="indx">Erratics extend further south in America than in Europe, <a href="#Page_72">72</a></li> - - <li class="indx">Etheridge, R., jun., on glacial conglomerate in Australia of Old Red Sandstone age, <a href="#Page_295">295</a></li> - - <li class="indx">Europe, influence of Gulf-stream on climate of, <a href="#Page_31">31</a></li> - <li class="isub2">〃 effect of deflection of Gulf-stream on condition of, <a href="#Page_68">68</a></li> - <li class="isub2">〃 glacial condition of, if Gulf-stream was stopped, <a href="#Page_71">71</a></li> - <li class="isub2">〃 river systems of, unaltered since glacial period, <a href="#Page_393">393</a></li> - - <li class="ifrst">Faraday, Professor, on cause of regelation, <a href="#Page_554">554</a></li> - - <li class="indx">Faroe Islands glaciated by land-ice from Scandinavia, <a href="#Page_450">450</a></li> - - <li class="indx">Ferrel, Mr., on Dr. Carpenter’s theory, <a href="#Page_126">126</a></li> - <li class="isub2">〃 argument from the tides, <a href="#Page_184">184</a></li> - - <li class="indx">Findlay, Mr. A. G., objection by, considered, <a href="#Page_31">31</a>, <a href="#Page_203">203</a></li> - <li class="isub2">〃 estimate of heat conveyed by Gulf-stream, <a href="#Page_206">206</a></li> - - <li class="indx">Fisher, Rev. O., on the 〃trail〃 of Norwich, <a href="#Page_251">251</a></li> - <li class="isub2">〃 on glacial submergence, <a href="#Page_387">387</a></li> - - <li class="indx">Fitzroy, Admiral, on temperature of Atlantic, <a href="#Page_36">36</a></li> - - <li class="indx">Fluid molecules crystallize in interstices, <a href="#Page_523">523</a></li> - - <li class="indx">Fluvio-marine beds of Norwich, <a href="#Page_250">250</a></li> - - <li class="indx">“Flysch” of Eocene period, absence of fossils in, <a href="#Page_286">286</a></li> - <li class="isub2">〃 of Switzerland of glacial origin, <a href="#Page_306">306</a></li> - - <li class="indx">Fogs prevent the sun’s heat from melting ice and snow in arctic regions, <a href="#Page_60">60</a></li> - - <li class="indx">Forbes, Professor J. D., method adopted by, of ascertaining temperatures, <a href="#Page_48">48</a></li> - <li class="isub2">〃 on temperature of equator and poles, <a href="#Page_48">48</a></li> - <li class="isub2">〃 on the conductivity of different kinds of rock, <a href="#Page_86">86</a></li> - <li class="isub2">〃 on underground temperature, <a href="#Page_86">86</a></li> - <li class="isub2">〃 experiments by, on the power of different rocks to store up heat, <a href="#Page_86">86</a></li> - - <li class="indx">Forest bed of Cromer, <a href="#Page_250">250</a></li> - - <li class="indx">Former glacial periods, <a href="#Page_266">266−310</a></li> - <li class="isub2">〃 why so little known of, <a href="#Page_266">266</a></li> - <li class="isub2">〃 geological evidence of, <a href="#Page_292">292</a></li> - - <li class="indx">France, evidence of ice-action during Carboniferous period in, <a href="#Page_296">296</a></li> - - <li class="indx">Fraserburgh, glaciation of, <a href="#Page_450">450</a></li> - <li class="isub2">〃 crossed by North Sea ice, <a href="#Page_454">454</a></li> - - <li class="indx">Fundamental problem of geology, <a href="#Page_1">1</a></li> - - <li class="ifrst">Ganges, amount of sediment conveyed by, <a href="#Page_331">331</a></li> - - <li class="indx">Gases, radiation of, <a href="#Page_38">38</a></li> - - <li class="indx">Gastaldi, M., on the Miocene glacial epoch of Italy, <a href="#Page_306">306</a></li> - - <li class="indx">Geikie, Professor, on geological agencies, <a href="#Page_1">1</a></li> - <li class="isub2">〃 on inter-glacial beds of Scotland, <a href="#Page_243">243</a></li> - <li class="isub2">〃 remarks on inter-glacial beds, <a href="#Page_245">245</a></li> - <li class="isub2">〃 on striated pavements, <a href="#Page_256">256</a></li> - <li class="isub2">〃 on ice-markings on Scandinavian coast, <a href="#Page_281">281</a></li> - <li class="isub2">〃 striated stones found in carboniferous conglomerate by, <a href="#Page_296">296</a></li> - <li class="isub2">〃 on sediment of European rivers, <a href="#Page_332">332</a></li> - <li class="isub2">〃 on modern denudation, <a href="#Page_332">332</a></li> - <li class="isub2">〃 suggestion regarding the loess, <a href="#Page_452">452</a></li> - <li class="isub2">〃 on striation of Caithness, <a href="#Page_453">453</a></li> - <li class="isub2">〃 on buried channel at Chapelhall, <a href="#Page_491">491</a></li> - <li class="isub2">〃 and Mr. James, on glacial conglomerate of Lower Carboniferous age, <a href="#Page_296">296</a></li> - - <li class="indx">Geikie, Mr. James, on Crofthead inter-glacial bed, <a href="#Page_248">248</a></li> - <li class="isub2">〃 on the gravels of Switzerland, <a href="#Page_268">268</a></li> - <li class="isub2">〃 on difficulty of recognising former glacial periods, <a href="#Page_289">289</a></li> - <li class="isub2">〃 on Cambrian conglomerate of north-west of Scotland, <a href="#Page_293">293</a></li> - <li class="isub2">〃 on ice-action in Ayrshire during Silurian period, <a href="#Page_293">293</a></li> - <li class="isub2">〃 on boulder conglomerate of Sutherland, <a href="#Page_301">301</a></li> - <li class="isub2"><span class="pagenum">568</span>〃 on buried channels, <a href="#Page_492">492</a></li> - - <li class="indx">Geogr. Mittheilungen, list of papers in, relating to arctic regions, <a href="#Page_556">556</a></li> - - <li class="indx">Geological agencies climatic, <a href="#Page_2">2</a></li> - - <li class="indx">Geological principle, nature of, <a href="#Page_4">4</a></li> - - <li class="indx">Geological climates, theories of, <a href="#Page_6">6</a></li> - - <li class="indx">Geological time, <a href="#Page_311">311−359</a></li> - <li class="isub2">〃 measurable from astronomical data, <a href="#Page_311">311</a></li> - <li class="isub2">〃 why it has been over-estimated, <a href="#Page_325">325</a></li> - <li class="isub2">〃 method of measuring, <a href="#Page_328">328</a>, <a href="#Page_329">329</a></li> - <li class="isub2">〃 Professor Ramsay on, <a href="#Page_343">343</a></li> - - <li class="indx">Geology, fundamental problem of, <a href="#Page_1">1</a></li> - <li class="isub2">〃 a dynamical science, <a href="#Page_5">5</a></li> - <li class="isub2">〃 and astronomy, supposed analogy between, <a href="#Page_355">355</a></li> - - <li class="indx">German Polar Expedition on density of polar water, <a href="#Page_151">151</a></li> - <li class="isub2">〃 list of papers relating to, <a href="#Page_556">556</a></li> - - <li class="indx">German Ocean once dry land, <a href="#Page_479">479</a></li> - - <li class="indx">Germany, Professor Ramsay on Permian breccia of, <a href="#Page_300">300</a></li> - - <li class="indx">Gibraltar current, Dr. Carpenter’s theory of, <a href="#Page_167">167</a></li> - <li class="isub2">〃 cause of, <a href="#Page_215">215</a></li> - - <li class="indx">Glacial conditions increased by reaction of various physical causes, <a href="#Page_75">75</a></li> - <li class="isub2">〃 reach maximum when winter solstice arrives at aphelion, <a href="#Page_77">77</a></li> - - <li class="indx">Glacial epoch, date of, <a href="#Page_327">327</a></li> - <li class="isub2">〃 circumstances which show recent date of, <a href="#Page_341">341</a></li> - <li class="isub2">〃 Mr. Belt’s theory of cause of, <a href="#Page_415">415</a></li> - - <li class="indx">Glacial epochs dependent upon deflection of ocean-currents, <a href="#Page_68">68</a></li> - <li class="isub2">〃 caused primarily by eccentricity, <a href="#Page_77">77</a></li> - <li class="isub2">〃 why so little known of, formerly, <a href="#Page_266">266</a></li> - <li class="isub2">〃 boulder clays of former, why so rare, <a href="#Page_269">269</a></li> - <li class="isub2">〃 geological evidence of former, <a href="#Page_292">292</a></li> - - <li class="indx">Glacial period in America more severe than in Western Europe, <a href="#Page_73">73</a></li> - <li class="isub2">〃 mean temperature of the earth greatest at aphelion during, <a href="#Page_78">78</a></li> - <li class="isub2">〃 records of, fast disappearing, <a href="#Page_270">270</a></li> - <li class="isub2">〃 of the Eocene formation, <a href="#Page_305">305</a></li> - - <li class="indx">Glacial periods, indirect evidence of, in Eocene and Miocene formations, <a href="#Page_287">287</a></li> - <li class="isub2">〃 difficulty of determining, from fossil remains, <a href="#Page_289">289</a></li> - - <li class="indx">Glacial submergence resulting from displacement of the earth’s centre of gravity, <a href="#Page_389">389</a></li> - - <li class="indx">Glaciation a cause of submergence, <a href="#Page_390">390</a></li> - <li class="isub2">〃 remains of, found chiefly on land surfaces, <a href="#Page_267">267</a></li> - <li class="isub2">〃 of Scandinavia inexplicable by theory of local glaciers, <a href="#Page_448">448</a></li> - - <li class="indx">Glacier des Bois, <a href="#Page_497">497</a></li> - - <li class="indx">Glacier-motion, Canon Moseley’s theory of, <a href="#Page_507">507</a></li> - <li class="isub2">〃 Professor James Thomson’s theory of, <a href="#Page_512">512</a></li> - <li class="isub2">〃 M. Charpentier’s theory of, <a href="#Page_513">513</a></li> - <li class="isub2">〃 molecular, <a href="#Page_516">516</a></li> - - <li class="indx">Glacier-motion, present state of the question, <a href="#Page_514">514</a></li> - <li class="isub2">〃 molecular theory of, <a href="#Page_514">514−527</a></li> - <li class="isub2">〃 heat necessary to, <a href="#Page_515">515</a></li> - <li class="isub2">〃 due to force of crystallization, <a href="#Page_523">523</a></li> - <li class="isub2">〃 due chiefly to internal molecular pressure, <a href="#Page_523">523</a></li> - - <li class="indx">Glaciers, pressure exerted by, <a href="#Page_274">274</a></li> - <li class="isub2">〃 physical cause of the motion of, <a href="#Page_495">495−527</a></li> - <li class="isub2">〃 difficulties in accounting for motion of, <a href="#Page_495">495</a></li> - - <li class="indx">Glasgow, actual January temperature of, 28° above normal, <a href="#Page_72">72</a></li> - - <li class="indx">Godwin-Austen, Mr., on ice-action during the Carboniferous period in France, <a href="#Page_296">296</a></li> - <li class="isub2">〃 on evidence of ice-action during Cretaceous period, <a href="#Page_303">303</a></li> - <li class="isub2">〃 on mass of coal found in chalk at Dover, <a href="#Page_304">304</a></li> - <li class="isub2">〃 on the flatness of the land during Coal period, <a href="#Page_430">430</a></li> - - <li class="indx">Gothland, glaciation of, <a href="#Page_446">446</a></li> - - <li class="indx">Grangemouth, buried river channel at, <a href="#Page_468">468</a></li> - <li class="isub2">〃 surface-drift of, <a href="#Page_484">484</a></li> - - <li class="indx">Gravitation, the whole work of, performed by descent of water down the slope, <a href="#Page_154">154</a></li> - <li class="isub2">〃 of sun’s mass, <a href="#Page_348">348</a></li> - <li class="isub2">〃 insufficient to account for sun’s heat, <a href="#Page_349">349</a>, <a href="#Page_350">350</a></li> - - <li class="indx">Gravitation theory, its relation to the theory of Secular changes of climate, <a href="#Page_97">97</a></li> - <li class="isub2">〃 three modes of determining it, <a href="#Page_115">115</a></li> - <li class="isub2">〃 mechanics of, <a href="#Page_145">145</a></li> - <li class="isub2">〃 of the Gibraltar current, <a href="#Page_167">167</a></li> - <li class="isub2">〃 inadequacy of, <a href="#Page_191">191</a></li> - <li class="isub2">〃 <i>crucial</i> test of, <a href="#Page_220">220</a></li> - <li class="isub2">〃 of the sun’s heat, <a href="#Page_346">346−355</a></li> - - <li class="indx">Gravity, force of, impelling water from equator to poles, <a href="#Page_119">119</a>, <a href="#Page_120">120</a></li> - <li class="isub2">〃 force of, insensible at a short distance below the surface, <a href="#Page_120">120</a></li> - <li class="isub2">〃 work performed by, <a href="#Page_150">150</a></li> - <li class="isub2">〃 diagram illustrating the action of, in producing currents, <a href="#Page_155">155</a></li> - <li class="isub2">〃 amount of work performed by, due solely to <i>difference</i> of temperature between equatorial and polar waters, <a href="#Page_164">164</a></li> - <li class="isub2">〃 specific difference in, between water of Atlantic and Mediterranean insufficient to produce currents, <a href="#Page_169">169</a></li> - <li class="isub2">〃 centre of, displacement, by polar ice-cap, <a href="#Page_368">368</a></li> - - <li class="indx">Greenland, summer warm if free from ice, <a href="#Page_59">59</a></li> - <li class="isub2">〃 receives as much heat in summer as England, <a href="#Page_66">66</a></li> - <li class="isub2">〃 continental ice free from clay or mud, <a href="#Page_284">284</a></li> - <li class="isub2">〃 North, warm climate during Oolitic period in, <a href="#Page_302">302</a></li> - <li class="isub2">〃 Cretaceous formation of, <a href="#Page_305">305</a></li> - - <li class="indx"><span class="pagenum">569</span>Greenland, evidence of warm conditions during Miocene period in, <a href="#Page_307">307</a></li> - <li class="isub2">〃 Professor Heer cited on Miocene flora of, <a href="#Page_308">308</a>, <a href="#Page_309">309</a></li> - <li class="isub2">〃 state of, during glacial period, <a href="#Page_259">259</a></li> - <li class="isub2">〃 effect of removal of ice from, <a href="#Page_260">260</a></li> - - <li class="indx">Greenland ice-sheet, probable thickness of, <a href="#Page_378">378</a></li> - <li class="isub2">〃 invaded the American continent, <a href="#Page_445">445</a></li> - - <li class="indx">Greenland inland ice, <a href="#Page_379">379</a></li> - - <li class="indx">Gulf-stream, estimate of its volume, <a href="#Page_24">24</a></li> - <li class="isub2">〃 United States’ coast survey of, <a href="#Page_24">24</a></li> - <li class="isub2">〃 absolute amount of heat conveyed by, <a href="#Page_25">25</a>, <a href="#Page_26">26</a></li> - <li class="isub2">〃 heat conveyed by, compared with that carried by aërial currents, <a href="#Page_27">27</a></li> - <li class="isub2">〃 heat conveyed by, compared with that received by the frigid zone from the sun, <a href="#Page_27">27</a></li> - <li class="isub2">〃 influence on climate of Europe, <a href="#Page_31">31</a></li> - <li class="isub2">〃 efficiency of, due to the slowness of its motion, <a href="#Page_32">32</a></li> - <li class="isub2">〃 climate of Britain influenced by south-eastern portion of, <a href="#Page_33">33</a></li> - <li class="isub2">〃 heat conveyed by, compared with that derived by temperate regions from the sun, <a href="#Page_34">34</a></li> - <li class="isub2">〃 heat of, expressed in foot-pounds of energy, <a href="#Page_35">35</a></li> - <li class="isub2">〃 mean temperature of Atlantic increased one-fourth by, <a href="#Page_36">36</a></li> - <li class="isub2">〃 the only current that can heat arctic regions, <a href="#Page_45">45</a></li> - <li class="isub2">〃 influence of, on climate of arctic regions, <a href="#Page_45">45</a></li> - <li class="isub2">〃 the compensating warm current, <a href="#Page_46">46</a></li> - <li class="isub2">〃 palæontological objections to influence of, <a href="#Page_53">53</a></li> - <li class="isub2">〃 agencies which deflect the, in glacial periods, <a href="#Page_69">69</a></li> - <li class="isub2">〃 result, if stopped, <a href="#Page_71">71</a></li> - <li class="isub2">〃 large portion of the heat derived from southern hemisphere, <a href="#Page_94">94</a></li> - <li class="isub2">〃 Lieut. Maury on propulsion of, by specific gravity, <a href="#Page_102">102</a></li> - <li class="isub2">〃 contradictory nature of, the causes supposed by Lieut. Maury for the, <a href="#Page_110">110</a></li> - <li class="isub2">〃 higher temperature of, considered by Lieut. Maury as the real cause of its motion, <a href="#Page_111">111</a></li> - <li class="isub2">〃 amount of heat conveyed by, not over-estimated, <a href="#Page_197">197</a></li> - <li class="isub2">〃 amount of heat conveyed by, <a href="#Page_192">192</a></li> - <li class="isub2">〃 amount of heat conveyed by, compared with that by general oceanic circulation, <a href="#Page_194">194</a></li> - <li class="isub2">〃 heat conveyed by, compared with that received by torrid zone from the sun, <a href="#Page_194">194</a></li> - <li class="isub2">〃 heat conveyed by, into Arctic Ocean compared with that received by it from the sun, <a href="#Page_195">195</a></li> - <li class="isub2">〃 Capt. Nares’s observations of, <a href="#Page_198">198</a></li> - <li class="isub2">〃 Dr. Carpenter’s estimate of the thermal work of, <a href="#Page_199">199</a></li> - - <li class="indx">Gulf-stream, volume and temperature of, according to Mr. A. G. Findlay, <a href="#Page_203">203</a>, <a href="#Page_206">206</a></li> - <li class="isub2">〃 erroneous notion regarding depth of, <a href="#Page_207">207</a></li> - <li class="isub2">〃 list of papers relating to, <a href="#Page_556">556</a></li> - - <li class="ifrst">Haughton, Professor, on recent trees in arctic regions, <a href="#Page_263">263</a></li> - <li class="isub2">〃 on fragments of granite in carboniferous limestone, <a href="#Page_296">296</a></li> - <li class="isub2">〃 on coal beds of arctic regions, <a href="#Page_298">298</a></li> - <li class="isub2">〃 on <i>Ammonites</i> of Oolitic period in arctic regions, <a href="#Page_303">303</a></li> - - <li class="indx">Hayes, Dr., on Greenland ice-sheet, <a href="#Page_379">379</a></li> - - <li class="indx">Heat received from the sun per day, <a href="#Page_26">26</a></li> - <li class="isub2">〃 received by temperate regions from the sun, <a href="#Page_34">34</a></li> - <li class="isub2">〃 radiant, absorbed by ice remains insensible, <a href="#Page_60">60</a></li> - <li class="isub2">〃 sun’s, amount of, stored up in ground, <a href="#Page_87">87</a></li> - <li class="isub2">〃 transferred from southern to northern hemisphere, <a href="#Page_93">93</a></li> - <li class="isub2">〃 internal, supposed influence of, <a href="#Page_176">176</a></li> - <li class="isub2">〃 received by the three zones compared with that received by the equator, <a href="#Page_194">194</a></li> - <li class="isub2">〃 amount radiated from the sun, <a href="#Page_346">346</a></li> - <li class="isub2">〃 received by polar regions <a href="#Page_11">11</a>,700 years ago, <a href="#Page_403">403</a></li> - <li class="isub2">〃 necessary to glacier-motion, <a href="#Page_515">515</a></li> - <li class="isub2">〃 how transmitted through ice, <a href="#Page_517">517</a></li> - - <li class="indx">Heat-vibrations, nature of, <a href="#Page_544">544</a></li> - - <li class="indx">Heath, Mr. D. D., on glacial submergence, <a href="#Page_387">387</a></li> - - <li class="indx">Heer, Professor, on Dürnten lignite beds, <a href="#Page_241">241</a></li> - <li class="isub2">〃 on Miocene flora of Greenland, <a href="#Page_308">308−310</a></li> - <li class="isub2">〃 on Miocene flora of Spitzbergen, <a href="#Page_309">309</a></li> - - <li class="indx">Hills, ice-markings on summits of, as evidence of continental ice, <a href="#Page_458">458</a></li> - - <li class="indx">Helmholtz’s gravitation theory of sun’s heat, <a href="#Page_348">348</a></li> - - <li class="indx">Henderson, Mr. John, on inter-glacial bed at Redhall quarry, <a href="#Page_247">247</a></li> - - <li class="indx">Herschel, Sir John, on influence of eccentricity, <a href="#Page_11">11</a></li> - <li class="isub2">〃 estimate of the Gulf-stream by, <a href="#Page_25">25</a></li> - <li class="isub2">〃 on the amount of the sun’s heat, <a href="#Page_26">26</a></li> - <li class="isub2">〃 on inadequacy of specific gravity to produce ocean-currents, <a href="#Page_116">116</a></li> - <li class="isub2">〃 his objections to specific gravity not accepted, <a href="#Page_117">117</a></li> - <li class="isub2">〃 on influence of eccentricity on climate, <a href="#Page_529">529</a></li> - - <li class="indx">Home, Mr. Milne, on buried river channels, <a href="#Page_478">478</a></li> - - <li class="indx">Hooker, Sir W., on tree dug up by Capt. Belcher, <a href="#Page_264">264</a></li> - - <li class="indx">Hooker, Dr., on preponderance of ferns among coal plants, <a href="#Page_421">421</a></li> - - <li class="indx">Horne, Mr. J., on conglomerates of Isle of Man, <a href="#Page_295">295</a></li> - - <li class="indx"><span class="pagenum">570</span>Hoxne, inter-glacial bed of, <a href="#Page_241">241</a></li> - - <li class="indx">Hudson’s Bay, low mean temperature of, in June, <a href="#Page_62">62</a></li> - - <li class="indx">Hull, Professor, on ice-action during Permian age in Ireland, <a href="#Page_299">299</a></li> - <li class="isub2">〃 on equable temperature of Coal period, <a href="#Page_421">421</a></li> - <li class="isub2">〃 on estuarine origin of coal measures, <a href="#Page_424">424</a></li> - - <li class="indx">Hull, buried channel at, <a href="#Page_489">489</a></li> - - <li class="indx">Humboldt, Baron, on loss of heat from radiation, <a href="#Page_82">82</a></li> - <li class="isub2">〃 on rate of growth of coal, <a href="#Page_429">429</a></li> - <li class="isub2">〃 on influence of eccentricity on climate, <a href="#Page_538">538</a></li> - - <li class="indx">Humphreys and Abbot on sediment of Mississippi, <a href="#Page_330">330</a></li> - - - <li class="ifrst">Ice, latent heat of, <a href="#Page_60">60</a></li> - - <li class="indx">Ice, effects of removal of, from polar regions, <a href="#Page_64">64</a></li> - <li class="isub2">〃 heat absorbed by, employed wholly in mechanical work, <a href="#Page_60">60</a></li> - <li class="isub2">〃 slope necessary for motion of continental, <a href="#Page_375">375</a></li> - <li class="isub2">〃 does not shear in the solid state, <a href="#Page_516">516</a></li> - <li class="isub2">〃 how heat is transmitted through, <a href="#Page_517">517</a></li> - <li class="isub2">〃 how it can ascend a slope, <a href="#Page_525">525</a></li> - <li class="isub2">〃 how it can excavate a rock basin, <a href="#Page_525">525</a></li> - - <li class="indx">Icebergs do not striate sea-bottom, <a href="#Page_272">272</a></li> - <li class="isub2">〃 markings made by, are soon effaced, <a href="#Page_273">273</a></li> - <li class="isub2">〃 exerting little pressure perform little work, <a href="#Page_273">273</a></li> - <li class="isub2">〃 behaviour of, when stranded, <a href="#Page_274">274</a></li> - <li class="isub2">〃 action of, on sea-bottoms, <a href="#Page_274">274</a></li> - <li class="isub2">〃 rocks ground smooth, but not striated by, <a href="#Page_276">276</a></li> - <li class="isub2">〃 stones seldom seen on, <a href="#Page_281">281</a></li> - <li class="isub2">〃 evidence of, in Miocene formation of Italy, <a href="#Page_307">307</a></li> - <li class="isub2">〃 comparative thickness of arctic and antarctic, <a href="#Page_381">381</a></li> - <li class="isub2">〃 great thickness of antarctic, <a href="#Page_382">382</a></li> - - <li class="indx">Ice-cap, effects of, on the earth’s centre of gravity, <a href="#Page_369">369</a></li> - <li class="isub2">〃 probable thickness of antarctic, <a href="#Page_375">375</a></li> - <li class="isub2">〃 evidence from icebergs as to thickness of antarctic, <a href="#Page_383">383−385</a></li> - - <li class="indx">Ice-markings, modern, observed by Sir Charles Lyell, <a href="#Page_280">280</a></li> - - <li class="indx">Ice-sheet, probable thickness of in Greenland, <a href="#Page_380">380</a></li> - <li class="isub2">〃 of north of England, <a href="#Page_456">456</a></li> - - <li class="indx">Ice-worn pebbles found on summit of Allermuir, <a href="#Page_441">441</a></li> - - <li class="indx">Iceland, lignite of Miocene age in, <a href="#Page_308">308</a></li> - <li class="isub2">〃 probably glaciated by land-ice from North Greenland, <a href="#Page_451">451</a></li> - - <li class="indx">India, evidences of glacial action of Carboniferous age in, <a href="#Page_297">297</a></li> - - <li class="indx">Indian Ocean, low temperature at bottom, <a href="#Page_123">123</a></li> - - <li class="indx">Internal heat, no influence on climate, <a href="#Page_6">6</a></li> - <li class="isub2">〃 supposed influence of, <a href="#Page_176">176</a></li> - - <li class="indx">Inter-tropical regions, greater portion of moisture falls as rain, <a href="#Page_29">29</a></li> - - <li class="indx">Inter-glacial bed at Slitrig, <a href="#Page_243">243</a></li> - <li class="isub2">〃 at Chapelhall, <a href="#Page_244">244</a></li> - <li class="isub2">〃 of Craiglockhart hill, <a href="#Page_245">245</a></li> - <li class="isub2">〃 at Kilmaurs, <a href="#Page_248">248</a></li> - - <li class="indx">Inter-glacial beds, Professor Geikie on, <a href="#Page_243">243</a></li> - <li class="isub2">〃 of Dürnten, <a href="#Page_240">240</a></li> - <li class="isub2">〃 of Scotland, <a href="#Page_243">243</a></li> - <li class="isub2">〃 of England, <a href="#Page_249">249</a></li> - <li class="isub2">〃 at Norwich, <a href="#Page_250">250</a></li> - <li class="isub2">〃 evidence of, from borings, <a href="#Page_254">254</a></li> - - <li class="indx">Inter-glacial character of cave and river deposits, <a href="#Page_251">251</a></li> - - <li class="indx">Inter-glacial climate during Old Red Sandstone period in arctic regions, <a href="#Page_295">295</a></li> - - <li class="indx">Inter-glacial periods, <a href="#Page_236">236</a></li> - <li class="isub2">〃 reason why overlooked, <a href="#Page_237">237</a></li> - <li class="isub2">〃 of Switzerland, <a href="#Page_239">239</a></li> - <li class="isub2">〃 evidence of, from shell-beds, <a href="#Page_252">252</a></li> - <li class="isub2">〃 evidence from striated pavements of, <a href="#Page_255">255</a></li> - <li class="isub2">〃 reasons why so few vestiges remain of, <a href="#Page_257">257</a></li> - <li class="isub2">〃 in arctic regions, <a href="#Page_258">258−265</a></li> - <li class="isub2">〃 of Silurian age in arctic regions, <a href="#Page_293">293</a></li> - <li class="isub2">〃 of Carboniferous age in arctic regions, <a href="#Page_297">297</a></li> - <li class="isub2">〃 of Eocene formation in Switzerland, <a href="#Page_306">306</a></li> - <li class="isub2">〃 formation of coal during, <a href="#Page_420">420</a></li> - <li class="isub2">〃 length of, indicated by thickness of coal-seams, <a href="#Page_428">428</a></li> - - <li class="indx">Inglefield, Captain, erect trees found in Greenland by, <a href="#Page_309">309</a></li> - - <li class="indx">Ireland, on ice-action during Permian age in, <a href="#Page_299">299</a></li> - - <li class="indx">Isbister, Mr., on carboniferous limestone of arctic regions, <a href="#Page_297">297</a></li> - - <li class="indx">Islay, Cambrian conglomerate of, <a href="#Page_292">292</a></li> - - <li class="indx">Italy, glacial epoch of Miocene period in, <a href="#Page_306">306</a></li> - - - <li class="ifrst">Jack, Mr. R. L., on deflection of ice across England, <a href="#Page_461">461</a></li> - - <li class="indx">Jamieson, Mr. T. F., on boulder clay of Caithness, <a href="#Page_435">435</a></li> - <li class="isub2">〃 opinion that Caithness was glaciated by floating ice, <a href="#Page_437">437</a></li> - <li class="isub2">〃 on thickness of ice in the north Highlands, <a href="#Page_439">439</a></li> - <li class="isub2">〃 glaciation of headland of Fraserburgh, <a href="#Page_450">450</a>, <a href="#Page_455">455</a></li> - - <li class="indx">January temperature of Glasgow and Cumberland, difference between, <a href="#Page_72">72</a></li> - - <li class="indx">Jeffreys, Mr. Gwyn, on Swedish glacial shell beds, <a href="#Page_253">253</a></li> - - <li class="indx">Johnston, Dr. A. Keith, on coast-line of the globe, <a href="#Page_337">337</a></li> - - <li class="indx">Joule’s, Dr., experiments on the thermal effect of tension, <a href="#Page_552">552</a></li> - - <li class="indx">Judd, Mr., on boulders of Jurassic age in the Highlands, <a href="#Page_302">302</a></li> - - <li class="indx">Jukes, Mr., on warm climate of North Greenland during Oolitic period, <a href="#Page_302">302</a></li> - - <li class="indx">July, why hotter than June, <a href="#Page_89">89</a></li> - - - <li class="ifrst">Kane, Dr., on mean temperature of Von Rensselaer Harbour, <a href="#Page_62">62</a></li> - - <li class="indx"><span class="pagenum">571</span>Karoo beds, glacial character of, <a href="#Page_301">301</a></li> - <li class="isub2">〃 evidence of subtropical during deposition of, <a href="#Page_301">301</a></li> - - <li class="indx">Kelvin, ancient bed of, <a href="#Page_481">481</a></li> - - <li class="indx">Kielsen, Mr., excursion upon Greenland ice-sheet, by, <a href="#Page_378">378</a></li> - - <li class="indx">Kilmours, inter-glacial bed at, <a href="#Page_248">248</a></li> - - <li class="indx">Kirwan, Richard, on influence of eccentricity on climate, <a href="#Page_529">529</a></li> - - <li class="indx">Kyles of Bute, southern shell bed in, <a href="#Page_253">253</a></li> - - - <li class="ifrst">Labrador, mean temperature of, for January, <a href="#Page_72">72</a></li> - <li class="isub2">〃 Mr. Packard on glacial phenomena of, <a href="#Page_282">282</a></li> - - <li class="indx">Lagrange, M., on eccentricity of the earth’s orbit, <a href="#Page_54">54</a></li> - <li class="isub2">〃 table of superior limits of eccentricity, <a href="#Page_531">531</a></li> - - <li class="indx">Land at equator would retain the heat at equator, <a href="#Page_30">30</a></li> - <li class="isub2">〃 radiates heat faster than water, <a href="#Page_91">91</a></li> - <li class="isub2">〃 elevation of, will not explain glacial epoch, <a href="#Page_391">391</a></li> - <li class="isub2">〃 submergence and emergence during glacial epoch, <a href="#Page_368">368−397</a></li> - <li class="isub2">〃 successive upheavals and depressions of, <a href="#Page_391">391</a></li> - - <li class="indx">Land-ice necessarily exerts enormous pressure, <a href="#Page_274">274</a></li> - <li class="isub2">〃 evidence of former, from erratic blocks on stratified deposits, <a href="#Page_269">269</a></li> - - <li class="indx">Land-surfaces, remains of glaciation found chiefly on, <a href="#Page_267">267</a></li> - <li class="isub2">〃 (ancient) scarcity of, <a href="#Page_268">268</a></li> - - <li class="indx">Laplace, M., on obliquity of ecliptic, <a href="#Page_398">398</a></li> - - <li class="indx">Laughton, Mr., on cause of Gibraltar current, <a href="#Page_215">215</a></li> - - <li class="indx">Leith Walk, inter-glacial bed at, <a href="#Page_246">246</a></li> - - <li class="indx">Leverrier, M., on superior limit of eccentricity, <a href="#Page_54">54</a></li> - <li class="isub2">〃 on obliquity of ecliptic, <a href="#Page_398">398</a></li> - <li class="isub2">〃 table, by, of superior limits of eccentricity, <a href="#Page_531">531</a></li> - <li class="isub2">〃 formulæ, of, <a href="#Page_312">312</a></li> - - <li class="indx">Lignite beds of Dürnten, <a href="#Page_240">240</a></li> - - <li class="indx">Loess, origin of, <a href="#Page_452">452</a></li> - - <li class="indx">London, temperature of, raised 40° degrees by Gulf-stream, <a href="#Page_43">43</a></li> - - <li class="indx">Lomonds, ice-worn pebbles found on, <a href="#Page_439">439</a></li> - - <li class="indx">Lubbock, Sir J., on cave and river deposits, <a href="#Page_252">252</a></li> - - <li class="indx">Lucy, Mr. W. C., on glaciation of West Somerset, <a href="#Page_463">463</a></li> - <li class="isub2">〃 on northern derivation of drift on Cotteswold hills, <a href="#Page_460">460</a></li> - - <li class="indx">Lyell’s, Sir C., theory of the effect of distribution of land and water, <a href="#Page_8">8</a></li> - <li class="isub2">〃 on action of river-ice, <a href="#Page_280">280</a></li> - <li class="isub2">〃 on tropical character of the fauna of the Cretaceous formation, <a href="#Page_305">305</a></li> - <li class="isub2">〃 on warm conditions during Miocene period in Greenland, <a href="#Page_307">307</a></li> - <li class="isub2">〃 on influence of eccentricity, <a href="#Page_324">324</a></li> - <li class="isub2">〃 on sediment of Mississippi, <a href="#Page_331">331</a></li> - <li class="isub2">〃 on comparison of existing rocks with those removed, <a href="#Page_362">362</a></li> - <li class="isub2">〃 on submerged areas during Tertiary period, <a href="#Page_392">392</a></li> - <li class="isub2">〃 on change of obliquity of ecliptic, <a href="#Page_418">418</a></li> - <li class="isub2">〃 on climate best adapted for coal plants, <a href="#Page_420">420</a></li> - <li class="isub2">〃 on influence of eccentricity on climate, <a href="#Page_529">529</a>, <a href="#Page_535">535</a></li> - - <li class="ifrst">Mackintosh, Mr., observations on the glaciation of Wastdale Crag, <a href="#Page_457">457</a></li> - - <li class="indx">Magellan, Straits of, temperature at midsummer, <a href="#Page_61">61</a></li> - - <li class="indx">Mahony, Mr. J. A., on Crofthead inter-glacial bed, <a href="#Page_248">248</a></li> - - <li class="indx">Mälar Lake crossed by ice, <a href="#Page_447">447</a></li> - - <li class="indx">Man, Isle of, Mr. Cumming on glacial origin of Old Red Sandstone of, <a href="#Page_294">294</a></li> - - <li class="indx">Mars, uncertainty as to its climatic condition, <a href="#Page_80">80</a></li> - <li class="isub2">〃 objection from present condition of, <a href="#Page_79">79</a></li> - - <li class="indx">Marine denudation trifling, <a href="#Page_337">337</a></li> - - <li class="indx">Markham, Clements, on density of Gulf-stream water, <a href="#Page_129">129</a></li> - <li class="isub2">〃 on motion of icebergs in Davis’ Straits, <a href="#Page_133">133</a></li> - - <li class="indx">Martins’s, Professor Charles, objections, <a href="#Page_79">79</a></li> - - <li class="indx">Mathews, Mr., on Canon Moseley’s experiment, <a href="#Page_499">499</a></li> - - <li class="indx">Maury, Lieutenant, his estimate of the Gulf-stream, <a href="#Page_25">25</a></li> - <li class="isub2">〃 his theory examined, <a href="#Page_95">95</a></li> - <li class="isub2">〃 on temperature as a cause of difference of specific gravity, <a href="#Page_102">102</a></li> - <li class="isub2">〃 on difference of saltness as a cause of ocean-currents, <a href="#Page_103">103</a></li> - <li class="isub2">〃 discussion of his views of the causes of ocean-currents, <a href="#Page_104">104</a></li> - <li class="isub2">〃 his objection to wind theory of ocean-currents, <a href="#Page_211">211</a></li> - - <li class="indx">McClure, Captain, discovery of ancient forest in Banks’s Land, <a href="#Page_261">261</a></li> - - <li class="indx">Mecham, Lieutenant, discovery of recent trees in Prince Patrick’s Island, <a href="#Page_261">261</a></li> - - <li class="indx">Mechanics of gravitation theory, <a href="#Page_145">145</a></li> - - <li class="indx">Mediterranean shells in glacial shell bed of Udevalla, <a href="#Page_253">253</a></li> - <li class="isub2">〃 shells in glacial beds at Greenock, <a href="#Page_254">254</a></li> - - <li class="indx">Meech, Mr., on amount of sun’s rays cut off by the atmosphere, <a href="#Page_26">26</a></li> - <li class="isub2">〃 on influence of eccentricity on climate, <a href="#Page_540">540</a></li> - - <li class="indx">Melville Island, summer temperature of, <a href="#Page_65">65</a></li> - <li class="isub2">〃 discovery of recent trees in, <a href="#Page_262">262</a></li> - <li class="isub2">〃 plants found in coal of, <a href="#Page_298">298</a></li> - - <li class="indx">Mer de Glace, Professor Tyndall’s observations on, <a href="#Page_498">498</a></li> - - <li class="indx">Meteoric theory of sun’s heat, <a href="#Page_347">347</a></li> - - <li class="indx">Method of measuring rate of denudation, <a href="#Page_329">329</a></li> - - <li class="indx">Miller, Hugh, on absence of hills in the land of the Coal period, <a href="#Page_431">431</a></li> - - <li class="indx"><span class="pagenum">572</span>Migration of plants and animals, how influenced by ocean-currents, <a href="#Page_231">231</a></li> - <li class="isub2">〃 across equator, <a href="#Page_234">234</a></li> - - <li class="indx">Millichen, remarkable section of drift at, <a href="#Page_483">483</a></li> - - <li class="indx">Miocene glacial period, <a href="#Page_286">286</a></li> - - <li class="indx">Miocene period, glacial epoch of, in Italy, <a href="#Page_306">306</a></li> - - <li class="indx">Miocene, warm period of, in Greenland, <a href="#Page_307">307</a></li> - - <li class="indx">Miocene and Eocene periods, date of, <a href="#Page_357">357</a></li> - - <li class="indx">Mississippi, amount of sediment in, <a href="#Page_330">330</a></li> - <li class="isub2">〃 volume of, <a href="#Page_330">330</a></li> - - <li class="indx">Mitchell, Mr., on cause of Gulf-stream, <a href="#Page_131">131</a></li> - - <li class="indx">Molecular theory of origin of 〃Crevasses,” <a href="#Page_521">521</a></li> - <li class="isub2">〃 modification of, <a href="#Page_523">523</a></li> - - <li class="indx">Moore, Mr. J. Carrick, on ice-action of Silurian age in Wigtownshire, <a href="#Page_293">293</a></li> - - <li class="indx">Moore, Mr. Charles, on grooved rocks in Bath district, <a href="#Page_464">464</a></li> - - <li class="indx">Morlot, M., on inter-glacial periods of Switzerland, <a href="#Page_240">240</a></li> - - <li class="indx">Moseley, Canon, experiment to determine unit of shear, <a href="#Page_498">498</a></li> - <li class="isub2">〃 on motion of glaciers, <a href="#Page_498">498</a></li> - <li class="isub2">〃 unit of shear uncertain, <a href="#Page_504">504</a></li> - <li class="isub2">〃 his theory examined, <a href="#Page_507">507</a></li> - - <li class="indx">Motion of the sea, how communicated to a great depth, <a href="#Page_136">136</a></li> - - <li class="indx">Motion in space, origin of sun’s heat, <a href="#Page_353">353</a></li> - - <li class="indx">Mühry, M., on circumpolar basin, <a href="#Page_133">133</a>, <a href="#Page_556">556</a></li> - - <li class="indx">Mundsley, freshwater beds of, <a href="#Page_250">250</a></li> - - <li class="indx">Muncke on the expansion of sea-water, <a href="#Page_118">118</a></li> - - <li class="indx">Murchison, Sir R., on southern shells at Worcester, <a href="#Page_253">253</a></li> - <li class="isub2">〃 on trees in arctic regions, <a href="#Page_262">262</a></li> - <li class="isub2">〃 on striation of islands in the Baltic, <a href="#Page_448">448</a></li> - - <li class="indx">Murphy’s, Mr., theory, <a href="#Page_66">66</a></li> - - <li class="indx">Musselburgh, section of contorted drift near, <a href="#Page_465">465</a></li> - - - <li class="ifrst">Nares, Captain, on low temperature of antarctic regions, <a href="#Page_64">64</a></li> - <li class="isub2">〃 discovery of great depth of warm water in North Atlantic, <a href="#Page_198">198</a></li> - <li class="isub2">〃 estimate of volume and temperature of Gulf-stream, <a href="#Page_198">198</a></li> - <li class="isub2">〃 temperature soundings by, <a href="#Page_119">119</a>, <a href="#Page_222">222</a></li> - <li class="isub2">〃 thermal condition of Southern Ocean, <a href="#Page_225">225</a></li> - - <li class="indx">Natal, boulder clay of, <a href="#Page_300">300</a></li> - - <li class="indx">Newberry, Professor, on inter-glacial peat-bed of Ohio, <a href="#Page_249">249</a></li> - <li class="isub2">〃 on boulder of quartzite found in seam of coal, <a href="#Page_296">296</a></li> - - <li class="indx">Nicholson, Dr., on Wastdale Crag, <a href="#Page_457">457</a></li> - - <li class="indx">Nicol, Professor, on inter-glacial buried channel, <a href="#Page_244">244</a></li> - - <li class="indx">Nordenskjöld, Professor, on inland ice of Greenland, <a href="#Page_379">379</a></li> - - <li class="indx">North Sea rendered shallow by drift deposits, <a href="#Page_443">443</a></li> - - <li class="indx">Northern seas probably filled with land-ice during glacial period, <a href="#Page_438">438</a></li> - - <li class="indx">Northern hemisphere, condition of, when deprived of heat from ocean-current, <a href="#Page_68">68</a></li> - - <li class="indx">Norway, southern species in glacial shell beds, <a href="#Page_253">253</a></li> - - <li class="indx">Norwich Crag, its glacial character, <a href="#Page_249">249</a></li> - - <li class="indx">Norwich fluvio-marine beds, <a href="#Page_250">250</a></li> - - <li class="indx">Norwich inter-glacial beds, <a href="#Page_250">250</a></li> - - <li class="ifrst">Obliquity of ecliptic, its effects on climate, <a href="#Page_398">398−419</a></li> - <li class="isub2">〃 change of, influence on sea-level, <a href="#Page_403">403</a></li> - <li class="isub2">〃 Lieutenant-Colonel Drayson on, <a href="#Page_410">410</a></li> - <li class="isub2">〃 Mr. Belt on change of, <a href="#Page_415">415</a></li> - <li class="isub2">〃 Sir Charles Lyell on change of, <a href="#Page_418">418</a></li> - - <li class="indx">Ocean, imperfect conception of its area, <a href="#Page_135">135</a></li> - <li class="isub2">〃 condition of, inconsistent with the gravitation theory, <a href="#Page_136">136</a></li> - <li class="isub2">〃 low temperature at bottom a result of under currents, <a href="#Page_142">142</a></li> - <li class="isub2">〃 circulation, pressure as a cause of, <a href="#Page_187">187</a></li> - <li class="isub2">〃 antiquity of, <a href="#Page_367">367</a></li> - - <li class="indx">Ocean-currents, absolute heating power of, <a href="#Page_23">23</a></li> - <li class="isub2">〃 influence of, on normal temperatures overlooked, <a href="#Page_40">40</a></li> - <li class="isub2">〃 maximum effects of, reached at equator and poles, <a href="#Page_49">49</a></li> - <li class="isub2">〃 compensatory at only one point, <a href="#Page_49">49</a></li> - <li class="isub2">〃 heating effects of, greatest at the poles, <a href="#Page_50">50</a></li> - <li class="isub2">〃 cooling effects of, greatest at equator, <a href="#Page_50">50</a></li> - <li class="isub2">〃 earth not habitable without, <a href="#Page_51">51</a></li> - <li class="isub2">〃 result of deflection into Southern Ocean, <a href="#Page_68">68</a></li> - <li class="isub2">〃 palæontological objections against influence of, <a href="#Page_53">53</a></li> - <li class="isub2">〃 deflection of, the chief cause of changes of climate, <a href="#Page_68">68</a></li> - <li class="isub2">〃 how deflected by eccentricity, <a href="#Page_69">69</a></li> - <li class="isub2">〃 deflected by trade-winds, <a href="#Page_70">70</a></li> - <li class="isub2">〃 temperature of southern hemisphere lowered by transference of heat to northern hemisphere by, <a href="#Page_92">92</a></li> - <li class="isub2">〃 take their rise in the Southern Ocean, <a href="#Page_92">92</a></li> - <li class="isub2">〃 cause of, never specially examined by physicists, <a href="#Page_95">95</a></li> - <li class="isub2">〃 if due to specific gravity, strongest on cold hemisphere, <a href="#Page_97">97</a></li> - <li class="isub2">〃 if due to eccentricity, strongest on warm hemisphere, <a href="#Page_97">97</a></li> - <li class="isub2">〃 if due to specific gravity, act only by descent, <a href="#Page_99">99</a></li> - <li class="isub2">〃 mode by which specific gravity causes, <a href="#Page_100">100</a>, <a href="#Page_101">101</a></li> - <li class="isub2">〃 the true method of estimating the amount of heat conveyed by, <a href="#Page_207">207</a></li> - <li class="isub2">〃 due to system of winds, <a href="#Page_212">212</a></li> - <li class="isub2">〃 system of, agrees with the system of the winds, <a href="#Page_213">213</a></li> - <li class="isub2">〃 how they mutually intersect, <a href="#Page_219">219</a></li> - <li class="isub2">〃 in relation to climate, <a href="#Page_226">226</a></li> - <li class="isub2">〃 direction of, depends on direction of winds, <a href="#Page_227">227</a></li> - <li class="isub2">〃 causes which deflect, affect climate, <a href="#Page_228">228</a></li> - <li class="isub2">〃 in relation to distribution of plants and animals, <a href="#Page_231">231</a></li> - <li class="isub2">〃 effects of, on Greenland during glacial period, <a href="#Page_260">260</a></li> - - <li class="indx"><span class="pagenum">573</span>Œpinus on influence of eccentricity on climate, <a href="#Page_529">529</a></li> - - <li class="indx">Ohio inter-glacial beds, <a href="#Page_249">249</a></li> - - <li class="indx">Old Red Sandstone, evidence of ice-action in conglomerate of, <a href="#Page_294">294</a>, <a href="#Page_295">295</a></li> - - <li class="indx">Oolite of Sutherlandshire, <a href="#Page_454">454</a></li> - - <li class="indx">Oolitic period, evidence of ice-action during, <a href="#Page_301">301−303</a></li> - <li class="isub2">〃 warm climate in North Greenland during, <a href="#Page_302">302</a></li> - - <li class="indx">Organic remains, absence of, in glacial conglomerate of Upper Miocene period, <a href="#Page_286">286</a></li> - - <li class="indx">Organic life, paucity of, a characteristic of glacial periods, <a href="#Page_287">287</a></li> - - <li class="indx">Orkney Islands, glaciated by land-ice, <a href="#Page_444">444</a></li> - - <li class="indx">Osborne, Captain, remarks on recent forest trees in arctic regions, <a href="#Page_262">262</a>, <a href="#Page_263">263</a></li> - - <li class="indx">Oudemans, Dr., on planet Mars, <a href="#Page_80">80</a></li> - - <li class="indx">Overton Quarry, inter-glacial bed in, <a href="#Page_247">247</a></li> - - <li class="ifrst">Pacific Ocean, depth of, <a href="#Page_147">147</a></li> - - <li class="indx">Packard, Mr., on glacial phenomena of Labrador, <a href="#Page_282">282</a></li> - - <li class="indx">Page, Professor, on temperate climate of Coal period, <a href="#Page_422">422</a></li> - <li class="isub2">〃 on character of coal plants, <a href="#Page_421">421</a></li> - <li class="isub2">〃 on old watercourse at Hailes quarry, <a href="#Page_490">490</a></li> - - <li class="indx">Palæontological objections against influence of ocean-currents, <a href="#Page_53">53</a></li> - - <li class="indx">Palæontological evidence of last glacial period, <a href="#Page_285">285</a></li> - - <li class="indx">Parry, Captain, discovery of recent trees in Melville Island by, <a href="#Page_262">262</a></li> - - <li class="indx">Peach, Mr. C. W., on inter-glacial bed at Leith Walk, <a href="#Page_246">246</a></li> - <li class="isub2">〃 on boulder clay of Caithness, <a href="#Page_436">436</a></li> - <li class="isub2">〃 on striated rock surfaces in Cornwall, <a href="#Page_464">464</a></li> - - <li class="indx">Peach, Mr. B. N., on striation of Caithness, <a href="#Page_453">453</a></li> - - <li class="indx">Pengelly, Mr. W., on raised beaches, <a href="#Page_407">407</a></li> - - <li class="indx">Perigee, nearness of sun in, cause of snow and ice, <a href="#Page_74">74</a></li> - - <li class="indx">Perihelion, warm conditions at maximum when winter solstice is at, <a href="#Page_77">77</a></li> - - <li class="indx">Permian period, evidence of ice-action in, <a href="#Page_298">298−303</a></li> - - <li class="indx">Perthshire hills, ice-worn surfaces at elevations of <a href="#Page_2">2</a>,200 feet on the, <a href="#Page_440">440</a></li> - - <li class="indx">Petermann, Dr. A., on Dr. Carpenter’s theory, <a href="#Page_138">138</a></li> - <li class="isub2">〃 on thermal condition of the sea, <a href="#Page_138">138</a></li> - <li class="isub2">〃 chart of Gulf-stream and Polar current, <a href="#Page_219">219</a></li> - <li class="isub2">〃 <i>Geogr. Mittheilungen</i> of, list of papers in relation to arctic regions, <a href="#Page_556">556</a></li> - - <li class="indx">Phillips, Professor, on influence of eccentricity on climate, <a href="#Page_539">539</a></li> - - <li class="indx">Poisson’s theory of hot and cold parts of space, <a href="#Page_7">7</a></li> - - <li class="indx">Polar regions, effect of removal of ice from, <a href="#Page_64">64</a></li> - <li class="isub2">〃 influence of ice on climate, <a href="#Page_64">64</a></li> - <li class="isub2">〃 low summer temperature of, <a href="#Page_66">66</a></li> - - <li class="indx">Polar cold considered by Dr. Carpenter the <i>primum mobile</i> of ocean-currents, <a href="#Page_173">173</a></li> - <li class="isub2">〃 confusion of ideas regarding its influence, <a href="#Page_180">180</a></li> - <li class="isub2">〃 influence of, according to Dr. Carpenter, <a href="#Page_180">180</a></li> - - <li class="indx">Polar ice-cap, displacement of the earth’s centre of gravity by, <a href="#Page_368">368</a></li> - - <li class="indx">Port Bowen, mean temperature of, <a href="#Page_63">63</a></li> - - <li class="indx">Portobello, striated pavements near, <a href="#Page_255">255</a>, <a href="#Page_256">256</a></li> - - <li class="indx">Post-tertiary formations, hypothetical thickness of, <a href="#Page_366">366</a></li> - - <li class="indx">Pouillet, M., on the amount of the sun’s heat, <a href="#Page_26">26</a></li> - <li class="isub2">〃 on amount of sun’s rays cut off by the atmosphere, <a href="#Page_26">26</a></li> - - <li class="indx">Pratt, Archdeacon, on glacial submergence, <a href="#Page_387">387</a></li> - - <li class="indx">Prestwich, Professor, on Hoxne inter-glacial bed, <a href="#Page_241">241</a></li> - - <li class="indx">Pressure as a cause of circulation, <a href="#Page_187">187</a></li> - - <li class="indx">Principles of geology, nature of, <a href="#Page_4">4</a></li> - - <li class="indx">Prince Patrick’s Island, discovery of recent tree in, <a href="#Page_261">261</a></li> - - <li class="ifrst">Radiation, rate of, increases with increase of temperature, <a href="#Page_37">37</a></li> - <li class="isub2">〃 of gases, <a href="#Page_38">38</a></li> - <li class="isub2">〃 the way by which the earth loses heat, <a href="#Page_39">39</a></li> - <li class="isub2">〃 how affected by snow covering the ground, <a href="#Page_58">58</a></li> - <li class="isub2">〃 how affected by humid air, <a href="#Page_59">59</a></li> - <li class="isub2">〃 accelerated by increased formation of snow and ice, <a href="#Page_75">75</a></li> - - <li class="indx">Raised beaches, date of, <a href="#Page_407">407</a></li> - <li class="isub2">〃 Mr. Pengelly on, <a href="#Page_407">407</a></li> - - <li class="indx">Ramsay, Professor, on glacial origin of Old Red Sandstone of North of England, <a href="#Page_294">294</a></li> - <li class="isub2">〃 on Old Red Sandstone, <a href="#Page_367">367</a></li> - <li class="isub2">〃 on geological time, <a href="#Page_343">343</a></li> - <li class="isub2">〃 on ice-action during Permian period, <a href="#Page_298">298</a></li> - <li class="isub2">〃 on boulders of Permian age in Natal, <a href="#Page_301">301</a></li> - <li class="isub2">〃 on thickness of stratified rocks of Britain, <a href="#Page_267">267</a>, <a href="#Page_361">361</a></li> - - <li class="indx">Redhall Quarry, inter-glacial bed in, <a href="#Page_247">247</a></li> - - <li class="indx">Red Sea, why almost rainless, <a href="#Page_30">30</a></li> - - <li class="indx">Regelation, <i>rationale</i> of, <a href="#Page_520">520</a>, <a href="#Page_554">554</a></li> - <li class="isub2">〃 Professor James Thomson on cause of, <a href="#Page_554">554</a></li> - <li class="isub2">〃 Professor Faraday on cause of, <a href="#Page_554">554</a></li> - - <li class="indx">Regnault, M., on specific heat of sandstone, <a href="#Page_86">86</a></li> - - <li class="indx">Reynaud, Jean, on influence of eccentricity on climate, <a href="#Page_541">541</a></li> - - <li class="indx">Rhine, ancient, bed in German Ocean, <a href="#Page_480">480</a></li> - - <li class="indx">Ridge between Capes Trafalgar and Spartel, influence of, <a href="#Page_167">167</a></li> - - <li class="indx">Rink, Dr., on inland ice of Greenland, <a href="#Page_380">380</a></li> - - <li class="indx">River-ice, effect of, <a href="#Page_279">279</a></li> - - <li class="indx"><span class="pagenum">574</span>River-ice does not produce striations, <a href="#Page_279">279</a></li> - - <li class="indx">River systems, carrying-power measure of denudation, <a href="#Page_336">336</a></li> - - <li class="indx">River valleys, how striated across, <a href="#Page_525">525</a></li> - - <li class="indx">Robertson, Mr. David, on Crofthead and Hillhead inter-glacial beds, <a href="#Page_247">247</a>, <a href="#Page_248">248</a></li> - <li class="isub2">〃 on foraminifera in red clay, <a href="#Page_485">485</a></li> - - <li class="indx">Rock-basins, how excavated by ice, <a href="#Page_525">525</a></li> - - <li class="indx">Rocks removed by denudation, <a href="#Page_361">361</a></li> - - <li class="indx">Ross, Capt. Sir James, on South Shetland, <a href="#Page_61">61</a></li> - <li class="isub2">〃 on temperature of antarctic regions in summer, <a href="#Page_63">63</a></li> - - - <li class="ifrst">Sandwich Land, description by Capt. Cook, <a href="#Page_60">60</a></li> - <li class="isub2">〃 cold summers of, not due to latitude, <a href="#Page_64">64</a></li> - - <li class="indx">Salter, Mr., on carboniferous fossils of arctic regions, <a href="#Page_298">298</a></li> - <li class="isub2">〃 on warm climate of North Greenland during Oolitic period, <a href="#Page_302">302</a></li> - - <li class="indx">Saltness of the ocean, difference of, as a cause of motion, <a href="#Page_103">103</a></li> - <li class="isub2">〃 in direct opposition to temperature in producing ocean-currents, <a href="#Page_104">104</a></li> - - <li class="indx">Scandinavian ice, track of, <a href="#Page_447">447</a></li> - - <li class="indx">Scandinavian ice-sheet in the North Sea, <a href="#Page_444">444</a></li> - - <li class="indx">Scoresby, Dr., on condition of arctic regions in summer, <a href="#Page_58">58</a>, <a href="#Page_62">62</a></li> - <li class="isub2">〃 on density of Gulf-stream water, <a href="#Page_129">129</a></li> - - <li class="indx">Scotland, inter-glacial beds of, <a href="#Page_243">243−249</a></li> - <li class="isub2">〃 evidence of ice-action in carboniferous conglomerate of, <a href="#Page_296">296</a></li> - <li class="isub2">〃 buried under ice, <a href="#Page_439">439</a></li> - <li class="isub2">〃 ice-sheet of, in North Sea, <a href="#Page_442">442</a></li> - <li class="isub2">〃 why ice-sheet was so thick, <a href="#Page_452">452</a></li> - - <li class="indx">Sea, height of, at equator above poles, <a href="#Page_119">119</a></li> - <li class="isub2">〃 rise of, due to combined effect of eccentricity and obliquity, <a href="#Page_403">403</a></li> - <li class="isub2">〃 bottoms not striated by icebergs, <a href="#Page_272">272</a></li> - - <li class="indx">Sea and land, present arrangement indispensable to life, <a href="#Page_52">52</a></li> - - <li class="indx">Sea-level, oscillations of, in relation to distribution, <a href="#Page_394">394</a></li> - <li class="isub2">〃 oscillations of, during formation of coal measures, <a href="#Page_424">424</a></li> - <li class="isub2">〃 raised, by melting of antarctic ice-cap, <a href="#Page_388">388</a></li> - <li class="isub2">〃 influence of obliquity of ecliptic on, <a href="#Page_403">403</a></li> - - <li class="indx">Section of Mid-Atlantic, <a href="#Page_222">222</a></li> - - <li class="indx">Section across antarctic ice-cap, <a href="#Page_377">377</a></li> - - <li class="indx">Sedimentary rocks existing fragmentary, <a href="#Page_361">361</a></li> - <li class="isub2">〃 of the globe, mean thickness of, hitherto unknown, <a href="#Page_361">361</a></li> - <li class="isub2">〃 how mean thickness might be determined, <a href="#Page_362">362</a></li> - <li class="isub2">〃 mean thickness of, over-estimated, <a href="#Page_364">364</a></li> - - <li class="indx">Shearing-force of ice, <a href="#Page_496">496</a></li> - <li class="isub2">〃 momentary loss of, <a href="#Page_518">518</a></li> - - <li class="indx">Shetland islands glaciated by land-ice from Scandinavia, <a href="#Page_450">450</a></li> - - <li class="indx">Shetland, South, glacial condition of, <a href="#Page_61">61</a></li> - - <li class="indx">Shell-beds, evidence of warm inter-glacial periods from, <a href="#Page_252">252</a></li> - - <li class="indx">Shells of the boulder clay of Caithness, <a href="#Page_450">450</a></li> - - <li class="indx">Shore-ice, striations produced by, in Bay of Fundy, <a href="#Page_280">280</a></li> - - <li class="indx">Silurian period, ice-action in Ayrshire during, <a href="#Page_293">293</a></li> - <li class="isub2">〃 evidence in Wigtownshire of ice-action during, <a href="#Page_293">293</a></li> - - <li class="indx">Slitrig, inter-glacial bed of, <a href="#Page_243">243</a></li> - - <li class="indx">Slope of surface of maximum density has no power to produce motion, <a href="#Page_120">120</a></li> - <li class="isub2">〃 from equator to pole, erroneous view regarding, <a href="#Page_120">120</a></li> - - <li class="indx">Smith, Mr. Leigh, temperature soundings, <a href="#Page_129">129</a></li> - - <li class="indx">Smith, Mr., of Jordanhill, on striated pavements, <a href="#Page_256">256</a></li> - - <li class="indx">Snow, how radiation is affected by, <a href="#Page_58">58</a></li> - <li class="isub2">〃 common in summer in arctic regions, <a href="#Page_62">62</a></li> - <li class="isub2">〃 rate of accumulation of, increased by sun’s rays being cut off by fogs, <a href="#Page_75">75</a></li> - <li class="isub2">〃 formation increased by radiation, <a href="#Page_75">75</a></li> - - <li class="indx">Somerset, West, glaciation of, <a href="#Page_463">463</a></li> - - <li class="indx">Somerville, Mrs., on influence of eccentricity on climate, <a href="#Page_540">540</a></li> - - <li class="indx">South Africa, glaciation of, <a href="#Page_242">242</a></li> - <li class="isub2">〃 boulder clay of Permian age in, <a href="#Page_300">300</a></li> - - <li class="indx">South of England ice-sheet, <a href="#Page_463">463</a></li> - - <li class="indx">South Shetland, glacial condition of, at mid summer, <a href="#Page_61">61</a></li> - - <li class="indx">South-west winds, heat conveyed by, not derived from equatorial regions, <a href="#Page_28">28</a></li> - <li class="isub2">〃 heat conveyed by, derived from Gulf-stream, <a href="#Page_28">28</a></li> - - <li class="indx">Southern hemisphere, present extension of ice on, due partly to eccentricity, <a href="#Page_78">78</a></li> - <li class="isub2">〃 why colder than northern, <a href="#Page_81">81−92</a></li> - <li class="isub2">〃 absorbs more heat than the northern, <a href="#Page_90">90</a></li> - <li class="isub2">〃 lower temperature of, due to ocean-currents, <a href="#Page_92">92</a></li> - <li class="isub2">〃 surface currents from, warmer than under currents to, <a href="#Page_92">92</a></li> - <li class="isub2">〃 glacial and inter-glacial periods of, <a href="#Page_242">242</a></li> - - <li class="indx">Southern Ocean, thermal condition of, <a href="#Page_225">225</a></li> - - <li class="indx">Specific gravity can act only by causing water to descend a slope, <a href="#Page_99">99</a></li> - <li class="isub2">〃 mode of action in causing ocean-currents, <a href="#Page_100">100</a></li> - <li class="isub2">〃 inadequacy of, to produce ocean-currents demonstrated by Sir John Herschel, <a href="#Page_116">116</a></li> - - <li class="indx">Spitzbergen, Gulf-stream and under current at, <a href="#Page_134">134</a></li> - <li class="isub2">〃 Miocene flora of, <a href="#Page_309">309</a></li> - - <li class="indx">Stellar space, temperature of, <a href="#Page_35">35</a></li> - <li class="isub2">〃 received temperature of, probably too high, <a href="#Page_39">39</a></li> - - <li class="indx">Stewart, Professor Balfour, experiment on radiation, <a href="#Page_37">37</a></li> - <li class="isub2">〃 on cause of glacial cold, <a href="#Page_79">79</a></li> - - <li class="indx">Stirling, Mr., on old watercourse near Grangemouth, <a href="#Page_481">481</a></li> - - <li class="indx">St. John’s River, action of ice on banks of, <a href="#Page_279">279</a></li> - - <li class="indx">St. Lawrence, action of ice on bank of river, <a href="#Page_279">279</a></li> - - <li class="indx"><span class="pagenum">575</span>Stockwell, Mr., on eccentricity of earth’s orbit, <a href="#Page_54">54</a></li> - <li class="isub2">〃 on obliquity of ecliptic, <a href="#Page_399">399</a></li> - <li class="isub2">〃 table of superior limits of eccentricity, <a href="#Page_531">531</a></li> - - <li class="indx">Stone, Mr., on eccentricity of the earth’s orbit, <a href="#Page_322">322</a></li> - - <li class="indx">Stow, G. W., on glacial beds of South Africa, <a href="#Page_242">242</a></li> - <li class="isub2">〃 on Karoo beds, <a href="#Page_301">301</a></li> - - <li class="indx">Striæ, direction of, show the clay of Caithness came from the sea, <a href="#Page_436">436</a></li> - - <li class="indx">Striations obliterated rather than produced by icebergs, <a href="#Page_274">274</a></li> - - <li class="indx">Striated pavements why so seldom observed, <a href="#Page_256">256</a></li> - <li class="isub2">〃 evidence of inter-glacial periods from, <a href="#Page_255">255</a></li> - - <li class="indx">Striated stones found in conglomerate of Lower Carboniferous age by Professor Geikie, <a href="#Page_296">296</a></li> - <li class="isub2">〃 in Permian breccias, <a href="#Page_299">299</a></li> - <li class="isub2">〃 in the glacial conglomerate of the Superga, Turin, <a href="#Page_306">306</a></li> - - <li class="indx">Stratified rocks may be formed at all possible rates, <a href="#Page_360">360</a></li> - <li class="isub2">〃 rate of formation of, as estimated by Professor Huxley, <a href="#Page_363">363</a></li> - - <li class="indx">Struve, M., formula of obliquity of ecliptic, <a href="#Page_404">404</a></li> - - <li class="indx">Subaërial denudation, rate of, <a href="#Page_331">331</a></li> - - <li class="indx">Submarine forests, <a href="#Page_409">409</a></li> - <li class="isub2">〃 (ancient), coal seams the remains of, <a href="#Page_428">428</a></li> - - <li class="indx">Submergence, physical causes of, <a href="#Page_368">368</a></li> - <li class="isub2">〃 coincident with glaciation, <a href="#Page_389">389</a></li> - <li class="isub2">〃 of land resulting from melting of antarctic ice-cap, <a href="#Page_389">389</a></li> - <li class="isub2">〃 how affected by fluidity of interior of the earth, <a href="#Page_395">395</a></li> - <li class="isub2">〃 necessary for preservation of coal plants, <a href="#Page_423">423</a></li> - <li class="isub2">〃 frequent during formation of coal beds, <a href="#Page_426">426</a></li> - - <li class="indx">Subsidence insufficient to account for general submergence, <a href="#Page_390">390</a></li> - <li class="isub2">〃 necessary to accumulation of coal seams, <a href="#Page_427">427</a></li> - - <li class="indx">Sun supposed by some to be a variable star, <a href="#Page_8">8</a></li> - <li class="isub2">〃 maximum and minimum distance of, <a href="#Page_55">55</a></li> - <li class="isub2">〃 rays of, cut off by fogs in ice-covered regions, <a href="#Page_60">60</a></li> - <li class="isub2">〃 nearness in perigee a cause of snow and ice, <a href="#Page_74">74</a></li> - <li class="isub2">〃 total amount of heat radiated from, <a href="#Page_346">346</a></li> - <li class="isub2">〃 age and origin of, <a href="#Page_346">346</a></li> - <li class="isub2">〃 source of its energy, <a href="#Page_347">347</a></li> - <li class="isub2">〃 heat of, origin and chief source of, <a href="#Page_349">349</a></li> - <li class="isub2">〃 originally an incandescent mass, <a href="#Page_350">350</a></li> - <li class="isub2">〃 energy of, may have originally been derived from motion in space, <a href="#Page_355">355</a></li> - - <li class="indx">Surface currents which cross the equator warmer than the compensatory under currents, <a href="#Page_92">92</a></li> - - <li class="indx">Surface currents from poles to equator, according to Maury, produced by saltness, <a href="#Page_108">108</a></li> - - <li class="indx">Sutherland, Dr., observations by, on stranding of icebergs, <a href="#Page_275">275</a></li> - <li class="isub2">〃 testimony, that icebergs do not striate rocks, <a href="#Page_278">278</a></li> - <li class="isub2">〃 on the boulder clay of Natal, <a href="#Page_300">300</a></li> - - <li class="indx">Sutherland, boulder conglomerate of Oolitic period of, <a href="#Page_302">302</a></li> - - <li class="indx">Sweden, Southern, shells in glacial shell beds of, <a href="#Page_253">253</a></li> - - <li class="indx">Switzerland, inter-glacial period of, <a href="#Page_239">239</a></li> - <li class="isub2">〃 M. Morlat on inter-glacial periods of, <a href="#Page_240">240</a></li> - <li class="isub2">〃 gravels of, by Mr. James Geikie, <a href="#Page_268">268</a></li> - <li class="isub2">〃 Eocene glacial epoch in, <a href="#Page_305">305</a></li> - - - <li class="ifrst">Table of June temperatures in different latitudes, <a href="#Page_65">65</a></li> - <li class="isub2">〃 soundings in temperate regions, <a href="#Page_222">222</a></li> - - <li class="indx">Tables of eccentricity, <a href="#Page_314">314−321</a></li> - <li class="isub2">〃 of eccentricity, explanation of, <a href="#Page_322">322</a></li> - - <li class="indx">Tay, valley of, striated across, <a href="#Page_526">526</a></li> - <li class="isub2">〃 ancient buried channel of, <a href="#Page_490">490</a></li> - - <li class="indx">Temperate regions, cold periods best marked in, <a href="#Page_258">258</a></li> - - <li class="indx">Temperature of space, <a href="#Page_532">532</a></li> - <li class="isub2">〃 reasons why it should be reconsidered, <a href="#Page_39">39</a></li> - - <li class="indx">Temperature (mean) of equator and poles compared, <a href="#Page_41">41</a></li> - <li class="isub2">〃 why so low in polar regions during summer, <a href="#Page_66">66</a></li> - <li class="isub2">〃 how difference of specific gravity is caused by, <a href="#Page_102">102</a></li> - <li class="isub2">〃 higher, of the waters of Gulf-stream considered by Lieutenant Maury as the real causes of its motion, <a href="#Page_111">111</a></li> - <li class="isub2">〃 of sea at equator decreases most rapidly at the surface, <a href="#Page_119">119</a></li> - <li class="isub2">〃 of Greenland in Miocene period, <a href="#Page_310">310</a></li> - <li class="isub2">〃 of poles when obliquity was at its superior limit, <a href="#Page_402">402</a></li> - - <li class="indx">Tension, effect of, on ice, <a href="#Page_522">522</a></li> - <li class="isub2">〃 the cause of the cooling effect produced by, <a href="#Page_552">552</a></li> - - <li class="indx">Tertiary period, climate of, error in regard to, <a href="#Page_288">288</a></li> - - <li class="indx">Thermal condition of Southern Ocean, <a href="#Page_225">225</a></li> - - <li class="indx">Thibet, table-land of, <a href="#Page_418">418</a></li> - - <li class="indx">Thomson, Professor James, on cause of regelation, <a href="#Page_554">554</a></li> - <li class="isub2">〃 theory of glacier-motion, <a href="#Page_512">512</a></li> - - <li class="indx">Thomson, Mr. James, on glacial conglomerate in Arran, <a href="#Page_299">299</a></li> - <li class="isub2">〃 on ice-action in Cambrian conglomerate of Islay, <a href="#Page_292">292</a></li> - - <li class="indx">Thomson, Professor Wyville, on Dr. Carpenter’s theory, <a href="#Page_129">129</a></li> - <li class="isub2">〃 cited, <a href="#Page_130">130</a></li> - <li class="isub2">〃 thermal condition of the sea, <a href="#Page_138">138</a></li> - - <li class="indx"><span class="pagenum">576</span>Thomson, Sir W., amount of internal heat passing through earth’s crust, <a href="#Page_142">142</a></li> - <li class="isub2">〃 on limit to age of the globe, <a href="#Page_343">343</a></li> - <li class="isub2">〃 on influence of ice-cap on sea-level, <a href="#Page_372">372</a></li> - <li class="isub2">〃 climate not affected by internal heat, <a href="#Page_6">6</a></li> - <li class="isub2">〃 earth’s axis of rotation permanent, <a href="#Page_7">7</a></li> - <li class="isub2">〃 on volume and mass of the sun, <a href="#Page_347">347</a></li> - - <li class="indx">Tidal wave, effect of friction, <a href="#Page_336">336</a></li> - - <li class="indx">Tides, supposed argument from, <a href="#Page_184">184</a></li> - - <li class="indx">Time, geological, <a href="#Page_311">311−359</a></li> - <li class="isub2">〃 as represented by geological phenomena, <a href="#Page_326">326</a></li> - <li class="isub2">〃 represented by existing rocks, <a href="#Page_361">361</a></li> - - <li class="indx">Torrid zone, annual quantity of heat received by, per unit of surface, <a href="#Page_194">194</a></li> - - <li class="indx">Towncroft farm, section of channel at, <a href="#Page_474">474</a></li> - - <li class="indx">Towson, Mr., on icebergs of Southern Ocean, <a href="#Page_383">383</a></li> - - <li class="indx">Trade-winds (anti), heat conveyed by, over-estimated, <a href="#Page_28">28</a></li> - <li class="isub2">〃 (anti) derive their heat from the Gulf-stream, <a href="#Page_32">32</a></li> - <li class="isub2">〃 of warm hemisphere overborne by those of cold hemisphere, <a href="#Page_70">70</a></li> - <li class="isub2">〃 causes which determine the strength of, <a href="#Page_70">70</a></li> - <li class="isub2">〃 strongest on glaciated hemisphere, <a href="#Page_70">70</a></li> - <li class="isub2">〃 reaction upon trade-winds by formation of snow and ice, <a href="#Page_76">76</a></li> - <li class="isub2">〃 influence of, in turning ocean-currents on warm hemisphere, <a href="#Page_97">97</a></li> - <li class="isub2">〃 do not explain the antarctic current, <a href="#Page_211">211</a></li> - - <li class="indx">Tiddeman on North of England ice-sheet, <a href="#Page_458">458</a></li> - <li class="isub2">〃 displacement of, <a href="#Page_230">230</a></li> - - <li class="indx">Transport of boulders and rubbish the proper function of icebergs, <a href="#Page_281">281</a></li> - - <li class="indx">Trafalgar, effect of ridge between Capes Spartel and on Gibraltar current, <a href="#Page_167">167</a></li> - - <li class="indx">Turner, Professor, on arctic seal found at Grangemouth, <a href="#Page_485">485</a></li> - - <li class="indx">Tylor, Alfred, on denudation of Mississippi basin, <a href="#Page_333">333</a></li> - - <li class="indx">Tyndall, Professor, on heat in aqueous vapour, <a href="#Page_29">29</a></li> - <li class="isub2">〃 on sifted rays, <a href="#Page_47">47</a></li> - <li class="isub2">〃 on diathermancy of air, <a href="#Page_59">59</a></li> - <li class="isub2">〃 on glacial epoch, <a href="#Page_78">78</a></li> - - - <li class="ifrst">Udevalla, Mediterranean shell in glacial shells, bed of, <a href="#Page_253">253</a></li> - - <li class="indx">Under currents to southern hemisphere colder than surface currents from, <a href="#Page_92">92</a></li> - <li class="isub2">〃 produced by saltness, flow from equator to poles, <a href="#Page_106">106</a></li> - <li class="isub2">〃 account for cold water at equator, <a href="#Page_124">124</a>, <a href="#Page_142">142</a></li> - <li class="isub2">〃 in Davis’ Strait, <a href="#Page_134">134</a></li> - <li class="isub2">〃 take path of least resistance, <a href="#Page_130">130</a></li> - <li class="isub2">〃 why considered improbable, <a href="#Page_135">135</a></li> - <li class="isub2">〃 difficulty regarding, obviated, <a href="#Page_217">217</a></li> - <li class="isub2">〃 theory of, <a href="#Page_217">217</a></li> - - <li class="indx">Underground temperature, Professor J. D. Forbes on, <a href="#Page_86">86</a></li> - - <li class="indx">Underground temperature exerts no influence on the climate, <a href="#Page_88">88</a></li> - <li class="isub2">〃 absolute amount of heat derived from, <a href="#Page_142">142</a></li> - <li class="isub2">〃 supposed influence of, <a href="#Page_176">176</a></li> - - <li class="indx">Uniformity, modern doctrine of, <a href="#Page_325">325</a></li> - - <li class="indx">United States’ coast survey of Gulf-stream, <a href="#Page_24">24</a></li> - <li class="isub2">〃 hydrographic department, papers published by, <a href="#Page_556">556</a></li> - - <li class="indx">Unstratified boulder clay must be the product of land-ice, <a href="#Page_437">437</a></li> - - <li class="indx">Upsala and Stockholm striated by Baltic glacier, <a href="#Page_447">447</a></li> - - <li class="ifrst">Vertical circulation, Lieutenant Maury’s theory of, <a href="#Page_108">108</a></li> - <li class="isub2">〃 according to Dr. Carpenter, <a href="#Page_153">153</a></li> - - <li class="indx">Vertical descent of polar column caused by extra pressure of water upon it, <a href="#Page_154">154</a></li> - <li class="isub2">〃 effects of, and slope, the same, whether performed simultaneously or alternately, <a href="#Page_159">159</a></li> - <li class="isub2">〃 of polar column illustrated by diagram, <a href="#Page_160">160</a></li> - - <li class="indx">Vertical distribution of heat in the ocean, Mr. Buchanan’s theory, <a href="#Page_550">550</a></li> - - <li class="indx">Vogt, Professor, on Dürnten lignite bed, <a href="#Page_241">241</a></li> - - <li class="ifrst">Warm hemisphere made warmer by increased reaction of physical causes, <a href="#Page_76">76</a></li> - - <li class="indx">Warm periods best marked in arctic regions, <a href="#Page_258">258</a></li> - <li class="isub2">〃 in arctic regions, evidence of, <a href="#Page_261">261</a></li> - <li class="isub2">〃 better represented by fossils than cold periods, <a href="#Page_288">288</a></li> - <li class="isub2">〃 evidence of, during Cretaceous age, <a href="#Page_304">304</a></li> - - <li class="indx">Warm inter-glacial periods in arctic regions, <a href="#Page_258">258−265</a></li> - - <li class="indx">Water at equator the best means of distributing heat derived from the sun, <a href="#Page_30">30</a></li> - - <li class="indx">Water, a worse radiator than land, <a href="#Page_91">91</a></li> - - <li class="indx">Wastdale granite boulders, difficulty of accounting for transport of, <a href="#Page_456">456</a></li> - - <li class="indx">Wastdale Crag glaciated by continental ice, <a href="#Page_457">457</a></li> - - <li class="indx">Weibye, M., striation observed by, <a href="#Page_280">280</a></li> - - <li class="indx">Wilkes, Lieutenant, on cold experienced in antarctic regions in summer, <a href="#Page_63">63</a></li> - - <li class="indx">Wellington Sound, ancient trees found at, <a href="#Page_265">265</a></li> - - <li class="indx">Winter-drift of ice on coast of Labrador, <a href="#Page_276">276</a></li> - - <li class="indx">West winds, moisture of, derived from Gulf-stream, <a href="#Page_29">29</a></li> - - <li class="indx">Wind, work in impelling currents, <a href="#Page_219">219</a></li> - - <li class="indx">Winds, ocean-currents produced by, <a href="#Page_212">212</a></li> - <li class="isub2">〃 system of, agrees with the system of ocean-currents, <a href="#Page_213">213</a></li> - - <li class="indx">Wind theory of oceanic circulation, <a href="#Page_210">210</a></li> - <li class="isub2">〃 crucial test of, <a href="#Page_220">220</a></li> - - <li class="indx">Wigtownshire, ice-action during Silurian age, <a href="#Page_293">293</a></li> - - <li class="indx">Work performed by descent of polar column, <a href="#Page_157">157</a></li> - - <li class="indx">Wood, Mr. Nicholas, on buried channel, <a href="#Page_488">488</a></li> - - <li class="indx">Wood, Jun., Mr. Searles, middle drift, <a href="#Page_250">250</a></li> - <li class="isub2">〃 on occurrence of chalk <i lang="fr">débris</i> in south-west of England, <a href="#Page_460">460</a></li> - - <li class="indx">Woodward, Mr. H. B., on boulder clay in Devonshire, <a href="#Page_463">463</a></li> - - <li class="indx">Wunsch, Mr. E. A., on glacial conglomerate in Arran, <a href="#Page_299">299</a></li> - - - <li class="ifrst">Yare, ancient buried channel of, <a href="#Page_489">489</a></li> - - <li class="indx">Young, Mr. J., objection considered, <a href="#Page_482">482</a></li> - - <li class="indx">Yorkshire drift common in south of England, <a href="#Page_460">460</a></li> - - - <li class="ifrst">Zenger, Professor, on the moon’s influence on climate, <a href="#Page_324">324</a></li> - </ul> - - <div class="center mt10 mb10">THE END.</div> - - <div class="center small">PRINTED BY VIRTUE AND CO., CITY ROAD, LONDON.</div> - - <hr class="page" /> - - <div class="ph2">THE GREAT ICE AGE,</div> - - <div class="center large">AND ITS RELATION TO THE ANTIQUITY OF MAN.</div> - - <div class="center mt2">By JAMES GEIKIE, F.R.S.E., F.G.S., &c., of H.M. 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He writes in a light, agreeable, - and graphic style, and has the gift of the pencil as well as - the pen.”—<cite>World.</cite></p> - </blockquote> - - <div><b>Our Inheritance in the Great Pyramid.</b></div> - <blockquote class="mt0"> - <p class="noindent">By Professor <span class="smcap">C. Piazzi Smyth</span>, Astronomer Royal for Scotland. - New and Enlarged Edition, including all the most important Discoveries - up to the Present Time. With 17 Explanatory Plates. Post 8vo, 18s.</p> - </blockquote> - - <div><b>Man and Beast, Here and Hereafter.</b></div> - <blockquote class="mt0"> - <p class="noindent">With Illustrative Anecdotes. By the Rev. <span class="smcap">J. G. Wood</span>, M.A., - Author of “Homes without Hands,” &c. 2 vols. post 8vo, 21s.</p> - - <p class="small">“The book is delightful.”—<cite>British Quarterly Review.</cite></p> - - <p class="small">“It is filled with anecdotes which are very - entertaining.”—<cite>Saturday Review.</cite></p> - - <p class="small">“Extremely readable and interesting.... If the talk runs on - dogs, cats, canaries, horses, elephants, or even pigs or ducks, - he who has ‘Man and Beast’ at his fingers’ end may be sure - of a story good enough to cap the best that is likely to be - told.”—<cite>Pall Mall Gazette.</cite></p> - </blockquote> - </div> - - <div class="footnotes"> - <div class="footheader"><b>FOOTNOTES:</b></div> - - <div class="footnote"><a id="Footnote_1" href="#FNanchor_1" class="label">[1]</a> Trans. of Edin. Geol. Soc., vol. ii. p. 252.</div> - - <div class="footnote"><a id="Footnote_2" href="#FNanchor_2" class="label">[2]</a> Phil. Mag., January, 1863.</div> - - <div class="footnote"><a id="Footnote_3" href="#FNanchor_3" class="label">[3]</a> <cite>Athenæum</cite>, September 22, 1860.</div> - - <div class="footnote"><a id="Footnote_4" href="#FNanchor_4" class="label">[4]</a> Trans. Glasgow Geol. Soc., vol. iv., p. 313.</div> - - <div class="footnote"> - <a id="Footnote_5" href="#FNanchor_5" class="label">[5]</a> See Mr. Hopkin’s remarks on this theory, Quart. Journ. - Geol. Soc., vol. viii. - </div> - - <div class="footnote"><a id="Footnote_6" href="#FNanchor_6" class="label">[6]</a> See Chap. xxv.</div> - - <div class="footnote"><a id="Footnote_7" href="#FNanchor_7" class="label">[7]</a> See Chap. iv.</div> - - <div class="footnote"><a id="Footnote_8" href="#FNanchor_8" class="label">[8]</a> “Treatise on Astronomy,” § 315; “Outlines,” § 368.</div> - - <div class="footnote"> - <a id="Footnote_9" href="#FNanchor_9" class="label">[9]</a> <cite>Annuaire</cite> for 1834, p. 199. Edin. New Phil. Journ., - April, 1834, p. 224. - </div> - - <div class="footnote"> - <a id="Footnote_10" href="#FNanchor_10" class="label">[10]</a> “Cosmos,” vol. iv. p. 459 (Bohn’s Edition). “Physical - Description of the Heavens,” p. 336. - </div> - - <div class="footnote"><a id="Footnote_11" href="#FNanchor_11" class="label">[11]</a> Phil. Mag. for February, 1867, p. 127.</div> - - <div class="footnote"> - <a id="Footnote_12" href="#FNanchor_12" class="label">[12]</a> The Gulf-stream at the narrowest place examined by the - Coast Survey, and where also its velocity was greatest, was found to - be over 30 statute miles broad and 1,950 feet deep. But we must not - suppose that this represents all the warm water which is received by - the Atlantic from the equator; a great mass flows into the Atlantic - without passing through the Straits of Florida. - </div> - - <div class="footnote"> - <a id="Footnote_13" href="#FNanchor_13" class="label">[13]</a> It is probable that a large proportion of the water - constituting the south-eastern branch of the Gulf-stream is never - cooled down to 40°; but, on the other hand, the north-eastern branch, - which passes into the arctic regions, will be cooled far below 40°, - probably below 30°. Hence I cannot be over-estimating the extent to - which the water of the Gulf-stream is cooled down in fixing upon 40° as - the average minimum temperature. - </div> - - <div class="footnote"><a id="Footnote_14" href="#FNanchor_14" class="label">[14]</a> “Physical Geography of the Sea,” § 24, 6th edition.</div> - - <div class="footnote"><a id="Footnote_15" href="#FNanchor_15" class="label">[15]</a> “Physical Geography,” § 54.</div> - - <div class="footnote"> - <a id="Footnote_16" href="#FNanchor_16" class="label">[16]</a> Trans. of Roy. Soc. of Edin., vol. xxi., p. 57. Phil. - Mag., § 4, vol. ix., p. 36. - </div> - - <div class="footnote"><a id="Footnote_17" href="#FNanchor_17" class="label">[17]</a> “Smithsonian Contributions to Knowledge,” vol. ix.</div> - - <div class="footnote"><a id="Footnote_18" href="#FNanchor_18" class="label">[18]</a> “Heat as a Mode of Motion,” art. 240.</div> - - <div class="footnote"><a id="Footnote_19" href="#FNanchor_19" class="label">[19]</a> Trans. Roy. Soc. of Edin., vol. xxv., part 2.</div> - - <div class="footnote"><a id="Footnote_20" href="#FNanchor_20" class="label">[20]</a> See “Smithsonian Contributions to Knowledge,” vol. ix.</div> - - <div class="footnote"><a id="Footnote_21" href="#FNanchor_21" class="label">[21]</a> “Meteorology,” section 36.</div> - - <div class="footnote"> - <a id="Footnote_22" href="#FNanchor_22" class="label">[22]</a> <cite>Comptes-Rendus</cite>, July 9, 1838. Taylor’s “Scientific - Memoirs,” vol. iv., p. 44 (1846). - </div> - - <div class="footnote"> - <a id="Footnote_23" href="#FNanchor_23" class="label">[23]</a> The mean temperature of the Atlantic between the tropics - and the arctic circle, according to Admiral Fitzroy’s chart, is about 60°. But he assigns far too high a temperature for latitudes above 50°. - It is probable that 56° is not far from the truth. - </div> - - <div class="footnote"> - <a id="Footnote_24" href="#FNanchor_24" class="label">[24]</a> The probable physical cause of this will be considered in - the Appendix. - </div> - - <div class="footnote"> - <a id="Footnote_25" href="#FNanchor_25" class="label">[25]</a> The mean temperature of the equator, according to Dove, - is 79°·7, and that of the north pole 2°·3. But as there is, of course, - some uncertainty regarding the actual mean temperature of the poles, we - may take the difference in round numbers at 80°. - </div> - - <div class="footnote"><a id="Footnote_26" href="#FNanchor_26" class="label">[26]</a> Trans. of Roy. Soc. Edin., vol. xxii., p. 75.</div> - - <div class="footnote"> - <a id="Footnote_27" href="#FNanchor_27" class="label">[27]</a> <cite>Connaissance des Temps</cite> for 1863 (Additions). Lagrange’s - determination makes the superior limit 0·07641 (Memoirs of the - Berlin Academy for 1782, p. 273). Recently the laborious task of - re-investigating the whole subject of the secular variations of the - elements of the planetary orbits was undertaken by Mr. Stockwell, of - the United States. He has taken into account the disturbing influence - of the planet Neptune, the existence of which was not known when - Leverrier’s computations were made; and he finds that the eccentricity - of the earth’s orbit will always be included within the limits of 0 and - 0·0693888. Mr. Stockwell’s elaborate Memoir, extending over no fewer - than two hundred pages, will be found in the eighteenth volume of the - “Smithsonian Contributions to Knowledge.” - </div> - - <div class="footnote"> - <a id="Footnote_28" href="#FNanchor_28" class="label">[28]</a> When the eccentricity is at its superior limit, the - absolute quantity of heat received by the earth during the year is, - however, about one three-hundredth part greater than at present. But - this does not affect the question at issue. - </div> - - <div class="footnote"> - <a id="Footnote_29" href="#FNanchor_29" class="label">[29]</a> Scoresby’s “Arctic Regions,” vol. ii., p. 379. Daniell’s - “Meteorology,” vol. ii., p. 123. - </div> - - <div class="footnote"><a id="Footnote_30" href="#FNanchor_30" class="label">[30]</a> Tyndall, “On Heat,” article 364.</div> - - <div class="footnote"><a id="Footnote_31" href="#FNanchor_31" class="label">[31]</a> Tyndall, “On Heat,” article 364.</div> - - <div class="footnote"><a id="Footnote_32" href="#FNanchor_32" class="label">[32]</a> See Phil. Mag., March, 1870, p.</div> - - <div class="footnote"><a id="Footnote_33" href="#FNanchor_33" class="label">[33]</a> Captain Cook’s “Second Voyage,” vol. ii., pp. 232, 235.</div> - - <div class="footnote"><a id="Footnote_34" href="#FNanchor_34" class="label">[34]</a> “Antarctic Regions,” vol. ii., pp. 345−349.</div> - - <div class="footnote"><a id="Footnote_35" href="#FNanchor_35" class="label">[35]</a> Ibid., vol. i., p. 167.</div> - - <div class="footnote"><a id="Footnote_36" href="#FNanchor_36" class="label">[36]</a> Ibid., vol. ii., p. 362.</div> - - <div class="footnote"><a id="Footnote_37" href="#FNanchor_37" class="label">[37]</a> Edinburgh Philosophical Journal, vol. iv., p. 266.</div> - - <div class="footnote"><a id="Footnote_38" href="#FNanchor_38" class="label">[38]</a> Scoresby’s “Arctic Regions,” vol. i., p. 378.</div> - - <div class="footnote"><a id="Footnote_39" href="#FNanchor_39" class="label">[39]</a> Ibid., p. 425.</div> - - <div class="footnote"> - <a id="Footnote_40" href="#FNanchor_40" class="label">[40]</a> See Meech’s memoir “On the Intensity of the Sun’s Heat - and Light,” “Smithsonian Contributions,” vol. ix. - </div> - - <div class="footnote"><a id="Footnote_41" href="#FNanchor_41" class="label">[41]</a> “Antarctic Regions,” vol. i., p. 240.</div> - - <div class="footnote"><a id="Footnote_42" href="#FNanchor_42" class="label">[42]</a> <i>Challenger</i> Reports, No. 2, p. 10.</div> - - <div class="footnote"><a id="Footnote_43" href="#FNanchor_43" class="label">[43]</a> See “Smithsonian Contributions,” vol. ix.</div> - - <div class="footnote"><a id="Footnote_44" href="#FNanchor_44" class="label">[44]</a> Quart. Journ. Geol. Soc., vol. xxv., p. 350.</div> - - <div class="footnote"><a id="Footnote_45" href="#FNanchor_45" class="label">[45]</a> Trans. of Glasgow Geol. Soc. for 1866.</div> - - <div class="footnote"><a id="Footnote_46" href="#FNanchor_46" class="label">[46]</a> <cite>Revue des Deux Mondes</cite> for 1867.</div> - - <div class="footnote"><a id="Footnote_47" href="#FNanchor_47" class="label">[47]</a> Letter to the author, February, 1870.</div> - - <div class="footnote"><a id="Footnote_48" href="#FNanchor_48" class="label">[48]</a> “Révolutions de la Mer,” p. 37 (second edition).</div> - - <div class="footnote"><a id="Footnote_49" href="#FNanchor_49" class="label">[49]</a> Edin. Phil. Journ., vol. iv., p. 262 (1821).</div> - - <div class="footnote"> - <a id="Footnote_50" href="#FNanchor_50" class="label">[50]</a> Phil. Mag., § 4, vol. xxviii., p. 131. <cite>Reader</cite>, December - 2nd, 1865. - </div> - - <div class="footnote"> - <a id="Footnote_51" href="#FNanchor_51" class="label">[51]</a> This point will be found discussed at considerable length - in the Phil. Mag. for September, 1869. - </div> - - <div class="footnote"><a id="Footnote_52" href="#FNanchor_52" class="label">[52]</a> See Phil. Mag. for October, 1870, p. 259.</div> - - <div class="footnote"> - <a id="Footnote_53" href="#FNanchor_53" class="label">[53]</a> Proceedings of the Royal Society, No. 138, p. 596, foot-note. - </div> - - <div class="footnote"> - <a id="Footnote_54" href="#FNanchor_54" class="label">[54]</a> The edition from which I quote, unless the contrary is - stated, is the one published by Messrs. T. Nelson and Sons, 1870, which - is a reprint of the new edition published in 1859 by Messrs. Sampson - Low and Co. - </div> - - <div class="footnote"><a id="Footnote_55" href="#FNanchor_55" class="label">[55]</a> “Physical Geography,” article 57.</div> - - <div class="footnote"><a id="Footnote_56" href="#FNanchor_56" class="label">[56]</a> Philosophical Magazine, vol. xii. p. 1 (1838).</div> - - <div class="footnote"> - <a id="Footnote_57" href="#FNanchor_57" class="label">[57]</a> “Mémoires par divers Savans,” tom. i., p. 318, St. - Petersburgh, 1831. See also twelfth number of Meteorological Papers, - published by the Board of Trade, 1865, p. 16. - </div> - - <div class="footnote"> - <a id="Footnote_58" href="#FNanchor_58" class="label">[58]</a> Dubuat’s “Hydraulique,” tom. i., p. 64 (1816). See also - British Association Report for 1834, pp. 422, 451. - </div> - - <div class="footnote"> - <a id="Footnote_59" href="#FNanchor_59" class="label">[59]</a> See Proceedings of the Royal Society for December, 1868, - November, 1869. Lecture delivered at the Royal Institute, <cite>Nature</cite>, - vol. i., p. 490. Proceedings of the Royal Geographical Society, vol. - xv. - </div> - - <div class="footnote"> - <a id="Footnote_60" href="#FNanchor_60" class="label">[60]</a> Trans. of Glasgow Geol. Soc. for April, 1867. Phil. Mag. - for February, 1867, and June, 1867 (Supplement). - </div> - - <div class="footnote"><a id="Footnote_61" href="#FNanchor_61" class="label">[61]</a> Phil. Mag. for February, 1870.</div> - - <div class="footnote"><a id="Footnote_62" href="#FNanchor_62" class="label">[62]</a> “The Depths of the Sea,” pp. 376 and 377.</div> - - <div class="footnote"><a id="Footnote_63" href="#FNanchor_63" class="label">[63]</a> “The Threshold of the Unknown Region,” p. 95.</div> - - <div class="footnote"> - <a id="Footnote_64" href="#FNanchor_64" class="label">[64]</a> See “Physical Geography of the Sea,” chap. ix., new - edition, and Dr. A. Mühry “On Ocean-currents in the Circumpolar Basin of the North Hemisphere.” - </div> - - <div class="footnote"><a id="Footnote_65" href="#FNanchor_65" class="label">[65]</a> “Depths of the Sea,” <cite>Nature</cite> for July 28, 1870.</div> - - <div class="footnote"> - <a id="Footnote_66" href="#FNanchor_66" class="label">[66]</a> “Memoir on the Gulf-stream,” <cite>Geographische - Mittheilungen</cite>, vol. xvi. (1870). - </div> - - <div class="footnote"> - <a id="Footnote_67" href="#FNanchor_67" class="label">[67]</a> Dr. Carpenter “On the Gulf-stream,” Proceedings of Royal - Geographical Society for January 9, 1871, § 29. - </div> - - <div class="footnote"><a id="Footnote_68" href="#FNanchor_68" class="label">[68]</a> Dr. Petermann’s <cite>Mittheilungen</cite> for 1872, p. 315.</div> - - <div class="footnote"> - <a id="Footnote_69" href="#FNanchor_69" class="label">[69]</a> Proceedings of the Royal Society, vol. xvii., p. 187, - xviii., p. 463. - </div> - - <div class="footnote"> - <a id="Footnote_70" href="#FNanchor_70" class="label">[70]</a> The average depth of the Pacific Ocean, as found by the - soundings of Captain Belknap, of the U.S. steamer <cite>Tuscarora</cite>, made - during January and February, 1874, is about 2,400 fathoms. The depth of - the Atlantic is somewhat less. - </div> - - <div class="footnote"> - <a id="Footnote_71" href="#FNanchor_71" class="label">[71]</a> Proceedings of Royal Geographical Society, vol. xv., § 22. - </div> - - <div class="footnote"> - <a id="Footnote_72" href="#FNanchor_72" class="label">[72]</a> It is a well-established fact that in polar regions the - temperature of the sea decreases from the surface downwards; and the - German Polar Expedition found that the water in very high latitudes is - actually less dense at the surface than at considerable depths, thus - proving that the surface-water could not sink in consequence of its greater density. - </div> - - <div class="footnote"><a id="Footnote_73" href="#FNanchor_73" class="label">[73]</a> Proceedings of the Royal Society, vol. xix., p. 215.</div> - - <div class="footnote"><a id="Footnote_74" href="#FNanchor_74" class="label">[74]</a> <cite>Nature</cite> for July 6, 1871.</div> - - <div class="footnote"> - <a id="Footnote_75" href="#FNanchor_75" class="label">[75]</a> Since the above objection to the Gravitation Theory of - the Gibraltar Current was advanced three years ago, Dr. Carpenter - appears to have abandoned the theory to a great extent. He now admits - (Proceedings of Royal Geographical Society, vol. xviii., pp. 319−334, - 1874) that the current is almost wholly due not to difference of - specific gravity, but to an excess of evaporation in the Mediterranean - over the return by rain and rivers. - </div> - - <div class="footnote"><a id="Footnote_76" href="#FNanchor_76" class="label">[76]</a> Proceedings of Royal Society, No. 138, § 26.</div> - - <div class="footnote"> - <a id="Footnote_77" href="#FNanchor_77" class="label">[77]</a> Proceedings of Royal Geographical Society, January 9, 1871. - </div> - - <div class="footnote"><a id="Footnote_78" href="#FNanchor_78" class="label">[78]</a> Ibid.</div> - - <div class="footnote"> - <a id="Footnote_79" href="#FNanchor_79" class="label">[79]</a> See §§ 20, 34; also Brit. Assoc. Report for 1872, p. 49, - and other places. - </div> - - <div class="footnote"> - <a id="Footnote_80" href="#FNanchor_80" class="label">[80]</a> See also to the same effect Brit. Assoc. Report, 1872, p. 50. - </div> - - <div class="footnote"><a id="Footnote_81" href="#FNanchor_81" class="label">[81]</a> Phil. Mag. for Oct. 1871.</div> - - <div class="footnote"> - <a id="Footnote_82" href="#FNanchor_82" class="label">[82]</a> The actual slope, however, does not amount to more than 1 - in 7,000,000. - </div> - - <div class="footnote"><a id="Footnote_83" href="#FNanchor_83" class="label">[83]</a> Proc. of Roy. Geog. Soc., January 9, 1871, § 29.</div> - - <div class="footnote"> - <a id="Footnote_84" href="#FNanchor_84" class="label">[84]</a> Trans. of Geol. Soc. of Glasgow for April, 1867; Phil. - Mag. for June, 1867. - </div> - - <div class="footnote"> - <a id="Footnote_85" href="#FNanchor_85" class="label">[85]</a> <cite>Nature</cite>, vol. i., p. 541. Proc. Roy. Soc., vol. xviii., p. 473. - </div> - - <div class="footnote"><a id="Footnote_86" href="#FNanchor_86" class="label">[86]</a> <a href="#CHAPTER_II">Chapter II.</a></div> - - <div class="footnote"><a id="Footnote_87" href="#FNanchor_87" class="label">[87]</a> <a href="#CHAPTER_II">Chapter II.</a></div> - - <div class="footnote"><a id="Footnote_88" href="#FNanchor_88" class="label">[88]</a> <a href="#CHAPTER_II">Chapter II.</a></div> - - <div class="footnote"> - <a id="Footnote_89" href="#FNanchor_89" class="label">[89]</a> Mr. Findlay considers that the daily discharge does - not exceed 333 cubic miles (Brit. Assoc. Rep., 1869, p. 160). My - estimate makes it 378 cubic miles. Mr. Laughton’s estimate is 630 cubic - miles (Paper “On Ocean-currents,” Journal of Royal United-Service Institution, vol. xv.). - </div> - - <div class="footnote"> - <a id="Footnote_90" href="#FNanchor_90" class="label">[90]</a> Proceedings of the Royal Geographical Society, vol. - xviii., p. 393. - </div> - - <div class="footnote"><a id="Footnote_91" href="#FNanchor_91" class="label">[91]</a> Phil. Mag. for October, 1871, p. 274.</div> - - <div class="footnote"><a id="Footnote_92" href="#FNanchor_92" class="label">[92]</a> Proceedings of the Royal Geographical Society, vol. xv.</div> - - <div class="footnote"><a id="Footnote_93" href="#FNanchor_93" class="label">[93]</a> Phil. Mag., February, 1870.</div> - - <div class="footnote"><a id="Footnote_94" href="#FNanchor_94" class="label">[94]</a> Brit. Assoc. Report, 1869, Sections, p. 160.</div> - - <div class="footnote"><a id="Footnote_95" href="#FNanchor_95" class="label">[95]</a> Journal of Royal United-Service Institute, vol. xv.</div> - - <div class="footnote"> - <a id="Footnote_96" href="#FNanchor_96" class="label">[96]</a> Dr. Carpenter (Proc. of Roy. Geog. Soc., vol. xviii., - p. 334) misapprehends me in supposing that I attribute the Gibraltar - current wholly to the Gulf-stream. In the very page from which he - derives or could derive his opinion as to my views on the subject - (Phil. Mag. for March, 1874, p. 182), I distinctly state that - “the excess of evaporation over that of precipitation within the - Mediterranean area would of itself produce a considerable current - through the Strait.” That the Gibraltar current is due to two causes, - (1) the pressure of the Gulf-stream, and (2) excess of evaporation - over precipitation in the Mediterranean, has always appeared to me so - perfectly obvious, that I never held nor could have held any other opinion on the subject. - </div> - - <div class="footnote"> - <a id="Footnote_97" href="#FNanchor_97" class="label">[97]</a> Paper read to the Edinburgh Botanical Society on January 8, 1874. - </div> - - <div class="footnote"> - <a id="Footnote_98" href="#FNanchor_98" class="label">[98]</a> Proc. Roy. Geog. Soc., vol. xviii., p. 362. A more - advantageous section might have been chosen, but this will suffice. - The section referred to is shown in <a href="#PLATE_III">Plate III.</a> The peculiarity of this - section, as will be observed, is the thinness of the warm strata at - the equator, as compared with that of the heated water in the North Atlantic. - </div> - - <div class="footnote"> - <a id="Footnote_99" href="#FNanchor_99" class="label">[99]</a> The temperature of column C in Dr. Carpenter’s section - is somewhat less than that given in the foregoing table; so that, - according to that section, the difference of level between column C and - columns A and B would be greater than my estimate. - </div> - - <div class="footnote"> - <a id="Footnote_100" href="#FNanchor_100" class="label">[100]</a> Captain Nares’s Report, July 30, 1874. - </div> - - <div class="footnote"><a id="Footnote_101" href="#FNanchor_101" class="label">[101]</a> See <a href="#CHAPTER_IV">Chapter IV.</a></div> - - <div class="footnote"> - <a id="Footnote_102" href="#FNanchor_102" class="label">[102]</a> Phil. Mag. for August, 1864, February, 1867, March, - 1870; see Chap. IV. - </div> - - <div class="footnote"><a id="Footnote_103" href="#FNanchor_103" class="label">[103]</a> Quarterly Journal of Science for October, 1874.</div> - - <div class="footnote"> - <a id="Footnote_104" href="#FNanchor_104" class="label">[104]</a> See a paper by M. Morlot, on “The Post-Tertiary and - Quaternary Formations of Switzerland.” Edin. New Phil. Journal, New Series, vol. ii., 1855. - </div> - - <div class="footnote"><a id="Footnote_105" href="#FNanchor_105" class="label">[105]</a> Edin. New Phil. Journ., New Series, vol. ii., p. 28.</div> - - <div class="footnote"><a id="Footnote_106" href="#FNanchor_106" class="label">[106]</a> Vogt’s “Lectures on Man,” pp. 318−321.</div> - - <div class="footnote"> - <a id="Footnote_107" href="#FNanchor_107" class="label">[107]</a> See Mr. Prestwich on Flint Implements, Phil. Trans. for - 1860 and 1864. Lyell’s “Antiquity of Man,” Second Edition, p. 168. - </div> - - <div class="footnote"> - <a id="Footnote_108" href="#FNanchor_108" class="label">[108]</a> Edin. New Phil. Journ., New Series, vol. ii., p. 28. - Silliman’s Journ., vol. xlvii., p. 259 (1844). - </div> - - <div class="footnote"><a id="Footnote_109" href="#FNanchor_109" class="label">[109]</a> Quart. Journ. Geol. Soc., vol. xxvii., p. 534.</div> - - <div class="footnote"><a id="Footnote_110" href="#FNanchor_110" class="label">[110]</a> Ibid., vol. xxviii., p. 17.</div> - - <div class="footnote"><a id="Footnote_111" href="#FNanchor_111" class="label">[111]</a> “Glacial Drift of Scotland,” p. 54.</div> - - <div class="footnote"><a id="Footnote_112" href="#FNanchor_112" class="label">[112]</a> “Glacial Drift of Scotland,” p. 58.</div> - - <div class="footnote"><a id="Footnote_113" href="#FNanchor_113" class="label">[113]</a> Quart. Journ. Geol. Soc., vol. v., p. 22.</div> - - <div class="footnote"><a id="Footnote_114" href="#FNanchor_114" class="label">[114]</a> “Glacial Drift of Scotland,” p. 64.</div> - - <div class="footnote"><a id="Footnote_115" href="#FNanchor_115" class="label">[115]</a> Trans. Edin. Geol. Soc., vol. ii., p. 391.</div> - - <div class="footnote"><a id="Footnote_116" href="#FNanchor_116" class="label">[116]</a> Trans. of Geol. Soc. of Glasgow, vol. iv., p. 146.</div> - - <div class="footnote"><a id="Footnote_117" href="#FNanchor_117" class="label">[117]</a> Geol. Mag., vi., p. 391.</div> - - <div class="footnote"> - <a id="Footnote_118" href="#FNanchor_118" class="label">[118]</a> See “Memoirs of Geological Survey of Scotland,” - Explanation of sheet 22, p. 29. See also Trans. Glasgow Geol. Soc., iv., p. 150. - </div> - - <div class="footnote"><a id="Footnote_119" href="#FNanchor_119" class="label">[119]</a> “Great Ice Age,” p. 374.</div> - - <div class="footnote"><a id="Footnote_120" href="#FNanchor_120" class="label">[120]</a> “Great Ice Age,” p. 384.</div> - - <div class="footnote"> - <a id="Footnote_121" href="#FNanchor_121" class="label">[121]</a> “Geological Survey of Ohio, 1869,” p. 165. See also - “Great Ice Age,” chap. xxviii. - </div> - - <div class="footnote"><a id="Footnote_122" href="#FNanchor_122" class="label">[122]</a> Quart. Journ. Geol. Soc., xxviii., p. 435.</div> - - <div class="footnote"><a id="Footnote_123" href="#FNanchor_123" class="label">[123]</a> Brit. Assoc. Report, 1863.</div> - - <div class="footnote"><a id="Footnote_124" href="#FNanchor_124" class="label">[124]</a> Trans. Glasgow Nat. Hist. Soc., vol. i., p. 115.</div> - - <div class="footnote"> - <a id="Footnote_125" href="#FNanchor_125" class="label">[125]</a> Trans. of the Geol. Soc. of Glasgow, vol. iii., p. 133. - See also “Great Ice Age,” chaps. xii. and xiii. - </div> - - <div class="footnote"><a id="Footnote_126" href="#FNanchor_126" class="label">[126]</a> Chap. XXIX.</div> - - <div class="footnote"><a id="Footnote_127" href="#FNanchor_127" class="label">[127]</a> Edin. New Phil. Journ., vol. liv., p. 272.</div> - - <div class="footnote"> - <a id="Footnote_128" href="#FNanchor_128" class="label">[128]</a> “Newer Pliocene Geology,” p. 129. John Gray & Co., Glasgow. - </div> - - <div class="footnote"><a id="Footnote_129" href="#FNanchor_129" class="label">[129]</a> “Glacial Drift of Scotland,” p. 67.</div> - - <div class="footnote"><a id="Footnote_130" href="#FNanchor_130" class="label">[130]</a> “Glacial Drift of Scotland,” p. 12.</div> - - <div class="footnote"><a id="Footnote_131" href="#FNanchor_131" class="label">[131]</a> See <a href="#CHAPTER_IV">Chapter IV.</a></div> - - <div class="footnote"><a id="Footnote_132" href="#FNanchor_132" class="label">[132]</a> “Discovery of the North-West Passage,” p. 213.</div> - - <div class="footnote"><a id="Footnote_133" href="#FNanchor_133" class="label">[133]</a> “Voyage of the <cite>Resolute</cite>,” p. 294.</div> - - <div class="footnote"><a id="Footnote_134" href="#FNanchor_134" class="label">[134]</a> Quart. Journ. Geol. Soc., vol. xi., p. 540.</div> - - <div class="footnote"><a id="Footnote_135" href="#FNanchor_135" class="label">[135]</a> “McClure’s North-West Passage,” p. 214. Second Edition.</div> - - <div class="footnote"> - <a id="Footnote_136" href="#FNanchor_136" class="label">[136]</a> “British Association Report for 1855,” p. 381. “The Last - of the Arctic Voyages,” vol. i., p. 381. - </div> - - <div class="footnote"> - <a id="Footnote_137" href="#FNanchor_137" class="label">[137]</a> Mr. James Geikie informs me that the great accumulations - of gravel which occur so abundantly in the low grounds of Switzerland, - and which are, undoubtedly, merely the re-arranged materials originally - brought down from the Alps as till and as moraines by the glaciers - during the glacial epoch, rarely or never yield a single scratched or - glaciated stone. The action of the rivers escaping from the melting ice - has succeeded in obliterating all trace of striæ. It is the same, he - says, with the heaps of gravel and sand in the lower grounds of Sweden - and Norway, Scotland and Ireland. These deposits are evidently in the - first place merely the materials carried down by the swollen rivers - that issued from the gradually melting ice-fields and glaciers. The - stones of the gravel derived from the demolition of moraines and till, - have lost all their striæ and become in most cases well water-worn and - rounded. - </div> - - <div class="footnote"> - <a id="Footnote_138" href="#FNanchor_138" class="label">[138]</a> Report on Icebergs, read before the Association of - American Geologists, <cite>Silliman’s Journal</cite>, vol. xliii., p. 163 (1842). - </div> - - <div class="footnote"><a id="Footnote_139" href="#FNanchor_139" class="label">[139]</a> “Manual of Geology,” p. 677.</div> - - <div class="footnote"><a id="Footnote_140" href="#FNanchor_140" class="label">[140]</a> Quart. Journ. Geol. Soc., vol. ix., p. 306.</div> - - <div class="footnote"><a id="Footnote_141" href="#FNanchor_141" class="label">[141]</a> Dana’s “Manual of Geology,” p. 677.</div> - - <div class="footnote"><a id="Footnote_142" href="#FNanchor_142" class="label">[142]</a> Quart. Journ. Geol. Soc., vol. ix., p. 306.</div> - - <div class="footnote"><a id="Footnote_143" href="#FNanchor_143" class="label">[143]</a> “Journal,” vol. i., p. 38.</div> - - <div class="footnote"><a id="Footnote_144" href="#FNanchor_144" class="label">[144]</a> “Short American Tramp,” pp. 168, 174.</div> - - <div class="footnote"><a id="Footnote_145" href="#FNanchor_145" class="label">[145]</a> “Short American Tramp,” pp. 239−241.</div> - - <div class="footnote"><a id="Footnote_146" href="#FNanchor_146" class="label">[146]</a> “Travels in North America,” vol. ii., p. 137.</div> - - <div class="footnote"><a id="Footnote_147" href="#FNanchor_147" class="label">[147]</a> Ibid., vol. ii., p. 174.</div> - - <div class="footnote"> - <a id="Footnote_148" href="#FNanchor_148" class="label">[148]</a> Proceedings of the Royal Society of Edinburgh, Session - 1865−66, p. 537. - </div> - - <div class="footnote"><a id="Footnote_149" href="#FNanchor_149" class="label">[149]</a> “Short American Tramp,” pp. 77, 81, 111.</div> - - <div class="footnote"><a id="Footnote_150" href="#FNanchor_150" class="label">[150]</a> “Second Visit,” vol. ii., p. 367.</div> - - <div class="footnote"> - <a id="Footnote_151" href="#FNanchor_151" class="label">[151]</a> “Memoirs of Boston Society of Natural History,” vol. i. - (1867), p. 228. - </div> - - <div class="footnote"><a id="Footnote_152" href="#FNanchor_152" class="label">[152]</a> “Antiquity of Man,” p. 268. Third Edition.</div> - - <div class="footnote"><a id="Footnote_153" href="#FNanchor_153" class="label">[153]</a> “Great Ice Age,” p. 512.</div> - - <div class="footnote"><a id="Footnote_154" href="#FNanchor_154" class="label">[154]</a> Brit. Assoc., 1870, p. 88.</div> - - <div class="footnote"> - <a id="Footnote_155" href="#FNanchor_155" class="label">[155]</a> Quart. Journ. Geol. Soc., vol. v., p. 10. Phil. Mag. for - April, 1865, p. 289. - </div> - - <div class="footnote"><a id="Footnote_156" href="#FNanchor_156" class="label">[156]</a> “Great Ice Age,” p. 512.</div> - - <div class="footnote"><a id="Footnote_157" href="#FNanchor_157" class="label">[157]</a> Jukes’ “Manual of Geology,” p. 421.</div> - - <div class="footnote"> - <a id="Footnote_158" href="#FNanchor_158" class="label">[158]</a> See also Quarterly Journal Geological Society, vol. xi., p. 510. - </div> - - <div class="footnote"><a id="Footnote_159" href="#FNanchor_159" class="label">[159]</a> The <cite>Reader</cite> for August 12, 1865.</div> - - <div class="footnote"> - <a id="Footnote_160" href="#FNanchor_160" class="label">[160]</a> “History of the Isle of Man,” p. 86. My colleague, Mr. - John Horne, in his “Sketch of the Geology of the Isle of Man,” Trans. - of Edin. Geol. Soc., vol. ii., part iii., considers this conglomerate to be of Lower Carboniferous age. - </div> - - <div class="footnote"> - <a id="Footnote_161" href="#FNanchor_161" class="label">[161]</a> See Selwyn, “Phys. Geography and Geology of Victoria.” - 1866. pp. 15−16; Taylor and Etheridge, <i>Geol. Survey Vict., Quarter Sheet 13, N.E.</i> - </div> - - <div class="footnote"> - <a id="Footnote_162" href="#FNanchor_162" class="label">[162]</a> Report on the Geology of the District of Ballan, - Victoria. 1866. p. 11. - </div> - - <div class="footnote"><a id="Footnote_163" href="#FNanchor_163" class="label">[163]</a> <i>Atrypa reticularis.</i></div> - - <div class="footnote"><a id="Footnote_164" href="#FNanchor_164" class="label">[164]</a> Quart. Journ. Geol. Soc., vol. xii., p. 58.</div> - - <div class="footnote"><a id="Footnote_165" href="#FNanchor_165" class="label">[165]</a> “Great Ice Age,” p. 513.</div> - - <div class="footnote"><a id="Footnote_166" href="#FNanchor_166" class="label">[166]</a> “Great Ice Age,” p. 513.</div> - - <div class="footnote"><a id="Footnote_167" href="#FNanchor_167" class="label">[167]</a> Brit. Assoc. Report for 1873.</div> - - <div class="footnote"><a id="Footnote_168" href="#FNanchor_168" class="label">[168]</a> Quart. Journ. Geol. Soc., vol. xi., p. 519.</div> - - <div class="footnote"><a id="Footnote_169" href="#FNanchor_169" class="label">[169]</a> <i>Orthis resupinata.</i></div> - - <div class="footnote"><a id="Footnote_170" href="#FNanchor_170" class="label">[170]</a> <i>Prod. semireticulatus</i> var. <i>Martini</i>. Sow.</div> - - <div class="footnote"><a id="Footnote_171" href="#FNanchor_171" class="label">[171]</a> “Belcher’s Voyage,” vol. ii., p. 377.</div> - - <div class="footnote"> - <a id="Footnote_172" href="#FNanchor_172" class="label">[172]</a> “Journal of a Boat Voyage through Rupert-Land,” vol. - ii., p. 208. - </div> - - <div class="footnote"><a id="Footnote_173" href="#FNanchor_173" class="label">[173]</a> Quart. Journ. Geol. Soc., vol. xi., p. 197.</div> - - <div class="footnote"> - <a id="Footnote_174" href="#FNanchor_174" class="label">[174]</a> Explanation Memoir to Sheet 47, “Geological Survey of Ireland.” - </div> - - <div class="footnote"><a id="Footnote_175" href="#FNanchor_175" class="label">[175]</a> Phil. Mag., vol. xxix., p. 290.</div> - - <div class="footnote"> - <a id="Footnote_176" href="#FNanchor_176" class="label">[176]</a> “Memoirs of the Geological Survey of India,” vol. i., - part i. - </div> - - <div class="footnote"><a id="Footnote_177" href="#FNanchor_177" class="label">[177]</a> Quart. Journ. Geol. Soc., vol. xxvi., p. 514.</div> - - <div class="footnote"><a id="Footnote_178" href="#FNanchor_178" class="label">[178]</a> Ibid., vol. xxvii., p. 544.</div> - - <div class="footnote"><a id="Footnote_179" href="#FNanchor_179" class="label">[179]</a> Phil. Mag., vol. xxix., p. 290.</div> - - <div class="footnote"><a id="Footnote_180" href="#FNanchor_180" class="label">[180]</a> Journal of the Royal Dublin Society for February, 1857.</div> - - <div class="footnote"><a id="Footnote_181" href="#FNanchor_181" class="label">[181]</a> Quart. Journ. Geol. Soc., vol. xi., p. 519.</div> - - <div class="footnote"> - <a id="Footnote_182" href="#FNanchor_182" class="label">[182]</a> “The Last of the Arctic Voyages,” by Captain Sir E. - Belcher, vol. ii., p. 389. Appendix Brit. Assoc. Report for 1855, p. - 79. - </div> - - <div class="footnote"><a id="Footnote_183" href="#FNanchor_183" class="label">[183]</a> Ibid., vol. ii., p. 379. Appendix.</div> - - <div class="footnote"><a id="Footnote_184" href="#FNanchor_184" class="label">[184]</a> “Manual of Geology,” pp. 395, 493.</div> - - <div class="footnote"><a id="Footnote_185" href="#FNanchor_185" class="label">[185]</a> Appendix to McClintock’s “Arctic Discoveries.”</div> - - <div class="footnote"> - <a id="Footnote_186" href="#FNanchor_186" class="label">[186]</a> Quart. Journ. Geol. Soc., vol. xiv., p. 262. Brit. - Assoc. Report for 1857, p. 62. - </div> - - <div class="footnote"> - <a id="Footnote_187" href="#FNanchor_187" class="label">[187]</a> Quart. Journ. Geol. Soc., vol. xvi., p. 327. - <cite>Geologist</cite>, 1860, p. 38. - </div> - - <div class="footnote"><a id="Footnote_188" href="#FNanchor_188" class="label">[188]</a> Phil. Mag., vol. xxix., p. 290.</div> - - <div class="footnote"><a id="Footnote_189" href="#FNanchor_189" class="label">[189]</a> Trans. Geol. Soc. of Glasgow, vol. v., p. 64.</div> - - <div class="footnote"><a id="Footnote_190" href="#FNanchor_190" class="label">[190]</a> “Principles,” vol. i., p. 209. Eleventh Edition.</div> - - <div class="footnote"> - <a id="Footnote_191" href="#FNanchor_191" class="label">[191]</a> “Memoirs of the Royal Academy of Science of Turin,” - Second Series, vol. xx. I am indebted for the above particulars to - Professor Ramsay, who visited the spot along with M. Gastaldi. - </div> - - <div class="footnote"><a id="Footnote_192" href="#FNanchor_192" class="label">[192]</a> “Antiquity of Man,” Second Edition, p. 237.</div> - - <div class="footnote"> - <a id="Footnote_193" href="#FNanchor_193" class="label">[193]</a> Dr. Robert Brown, in a recent Memoir on the Miocene Beds - of the Disco District (Trans. Geol. Soc. Glasg., vol. v., p. 55), has - added considerably to our knowledge of these deposits. He describes - the strata in detail, and gives lists of the plant and animal remains - discovered by himself and others, and described by Professor Heer. - Professor Nordenskjöld has likewise increased the data at our command - (Transactions of the Swedish Academy, 1873); and still further evidence - in favour of a warm climate having prevailed in Greenland during - Miocene times has been obtained by the recent second German polar expedition. - </div> - - <div class="footnote"> - <a id="Footnote_194" href="#FNanchor_194" class="label">[194]</a> The following are M. Leverrier’s formulæ for computing - the eccentricity of the earth’s orbit, given in his “Memoir” in the <cite>Connaissance des Temps</cite> for 1843:— - - <p>Eccentricity in (<i>t</i>) years after January 1, 1800 = √<span class="sqrt"><i>h</i><sup>2</sup> + <i>l</i><sup>2</sup></span> where</p> - <p class="noindent"> - <i>h</i> = 0·000526 Sin (<i>gt</i> + ß) + 0·016611 Sin (<i>g<sub>1</sub>t</i> + ß<sub>1</sub>) + 0·002366 Sin (<i>g<sub>2</sub>t</i> + ß<sub>2</sub>)<br /> - <span class="ml20">+ 0·010622 Sin (<i>g</i><sub>3</sub><i>t</i> + ß<sub>3</sub>) − 0·018925 Sin (<i>g</i><sub>4</sub><i>t</i> + ß<sub>4</sub>)</span><br /> - <span class="ml20">+ 0·011782 Sin (<i>g</i><sub>5</sub><i>t</i> + ß<sub>5</sub>) − 0·016913 Sin (<i>g</i><sub>6</sub><i>t</i> + ß<sub>6</sub>)</span> - </p> - <p class="noindent">and</p> - <p class="noindent"> - <i>l</i> = 0·000526 Cos (<i>gt</i> + ß) + 0·016611 Cos (<i>g<sub>1</sub>t</i> + ß<sub>1</sub>) + 0·002366 Cos (<i>g</i><sub>2</sub><i>t</i> + ß<sub>2</sub>)<br /> - <span class="ml20">+ 0·010622 Cos (<i>g</i><sub>3</sub><i>t</i> + ß<sub>3</sub>) − 0·018925 Cos (<i>g</i><sub>4</sub><i>t</i> + ß<sub>4</sub>)</span><br /> - <span class="ml20">+ 0·011782 Cos (<i>g</i><sub>5</sub><i>t</i> + ß<sub>5</sub>) − 0·016913 Cos (<i>g</i><sub>6</sub><i>t</i> + ß<sub>6</sub>)</span> - </p> - <table summary="Formulae"> - <tbody> - <tr> - <td class="tdc"><i>g</i> = 2″·25842</td> - <td class="tdc"> ß = 126° 43′ 15″</td> - </tr> - <tr> - <td class="tdc"><i>g</i><sub>1</sub> = 3″·71364</td> - <td class="tdc">ß<sub>1</sub> = 27 21 26 </td> - </tr> - <tr> - <td class="tdc"><i>g</i><sub>2</sub> = 22″·4273 </td> - <td class="tdc">ß<sub>2</sub> = 126 44 8 </td> - </tr> - <tr> - <td class="tdc"><i>g</i><sub>3</sub> = 5″·2989 </td> - <td class="tdc">ß<sub>3</sub> = 85 47 45 </td> - </tr> - <tr> - <td class="tdc"><i>g</i><sub>4</sub> = 7″·5747 </td> - <td class="tdc">ß<sub>4</sub> = 35 38 43 </td> - </tr> - <tr> - <td class="tdc"><i>g</i><sub>5</sub> = 17″·1527 </td> - <td class="tdc">ß<sub>5</sub> = −25 11 33 </td> - </tr> - <tr> - <td class="tdc"><i>g</i><sub>6</sub> = 17″·8633 </td> - <td class="tdc">ß<sub>6</sub> = −45 28 59 </td> - </tr> - </tbody> - </table> - </div> - - <div class="footnote"> - <a id="Footnote_195" href="#FNanchor_195" class="label">[195]</a> See Professor C. V. Zenger’s paper “On the Periodic - Change cf Climate caused by the Moon,” Phil. Mag. for June, 1868. - </div> - - <div class="footnote"><a id="Footnote_196" href="#FNanchor_196" class="label">[196]</a> Phil. Mag. for February, 1867.</div> - - <div class="footnote"><a id="Footnote_197" href="#FNanchor_197" class="label">[197]</a> Phil. Mag. for May, 1868.</div> - - <div class="footnote"><a id="Footnote_198" href="#FNanchor_198" class="label">[198]</a> Student’s “Elements of Geology,” p. 91. Second Edition.</div> - - <div class="footnote"> - <a id="Footnote_199" href="#FNanchor_199" class="label">[199]</a> In an interesting memoir, published in the Phil. Mag. - for 1850, Mr. Alfred Tylor estimated that the basin of the Mississippi - is being lowered at the rate of one foot in 10,000 years by the removal - of the sediment; and he proceeds further, and reasons that one foot - removed off the general surface of the land during that period would - raise the sea-level three inches. Had it not been that Mr. Tylor’s - attention was directed to the effects produced by the removal of - sediment in raising the level of the ocean rather than in lowering the - level of the land, he could not have failed to perceive that he was in - possession of a key to unfold the mystery of geological time. - </div> - - <div class="footnote"><a id="Footnote_200" href="#FNanchor_200" class="label">[200]</a> Proc. Roy. Soc., No. 152, 1874.</div> - - <div class="footnote"> - <a id="Footnote_201" href="#FNanchor_201" class="label">[201]</a> I have taken for the volume and mass of the sun the - values given in Professor Sir William Thomson’s memoir, Phil. Mag., vol. viii. (1854). - </div> - - <div class="footnote"><a id="Footnote_202" href="#FNanchor_202" class="label">[202]</a> Phil. Mag., § 4, vol. xi., p. 516 (1856).</div> - - <div class="footnote"><a id="Footnote_203" href="#FNanchor_203" class="label">[203]</a> Phil. Mag. for July, 1872, p. 1.</div> - - <div class="footnote"><a id="Footnote_204" href="#FNanchor_204" class="label">[204]</a> “Principles,” p. 210. Eleventh Edition.</div> - - <div class="footnote"><a id="Footnote_205" href="#FNanchor_205" class="label">[205]</a> “Principles,” vol. i., p. 107. Tenth Edition.</div> - - <div class="footnote"> - <a id="Footnote_206" href="#FNanchor_206" class="label">[206]</a> The conception of submergence resulting from - displacement of the earth’s centre of gravity, caused by a heaping up - of ice at one of the poles, was first advanced by M. Adhémar, in his - work “<cite>Révolutions de la Mer</cite>,” 1842. When the views stated in this - chapter appeared in the <cite>Reader</cite>, I was not aware that M. Adhémar had - written on the subject. An account of his mode of viewing the question - is given in the Appendix. - </div> - - <div class="footnote"> - <a id="Footnote_207" href="#FNanchor_207" class="label">[207]</a> Petermann’s <cite>Geog. Mittheilungen</cite>, 1871, Heft. x., p. 377. - </div> - - <div class="footnote"><a id="Footnote_208" href="#FNanchor_208" class="label">[208]</a> Geol. Mag., 1872, vol. ix., p. 360.</div> - - <div class="footnote"><a id="Footnote_209" href="#FNanchor_209" class="label">[209]</a> “Open Polar Sea,” p. 134.</div> - - <div class="footnote"> - <a id="Footnote_210" href="#FNanchor_210" class="label">[210]</a> Journal of the Royal Geographical Society, 1853, vol. xxiii. - </div> - - <div class="footnote"> - <a id="Footnote_211" href="#FNanchor_211" class="label">[211]</a> “Physics of Arctic Ice,” Quart. Journ. Geol. Soc. for - February, 1871. - </div> - - <div class="footnote"> - <a id="Footnote_212" href="#FNanchor_212" class="label">[212]</a> Some writers have objected to the conclusion that the - antarctic ice-cap is thickest at the pole, on the ground that the - snowfall there is probably less than at lower latitudes. The fact is, - however, overlooked, that the greater thickness of an ice-cap at its - centre is a physical necessity not depending on the rate of snowfall. - Supposing the snowfall to be greater at, say, lat. 70° than at 80°, and - greater at 80° than at the pole; nevertheless, the ice will continue to - accumulate till it is thicker at 80° than at 70°, and at the pole than - it is at 80°. - </div> - - <div class="footnote"> - <a id="Footnote_213" href="#FNanchor_213" class="label">[213]</a> It is a pity that at present no record is kept, either - by the Board of Trade or by the Admiralty, of remarkable icebergs which - may from time to time be met with. Such a record might be of little - importance to navigation, but it would certainly be of great service to - science. - </div> - - <div class="footnote"> - <a id="Footnote_214" href="#FNanchor_214" class="label">[214]</a> See <a href="#CHAPTER_XXVII">Chapter XXVII.</a>, and also Geol. Mag. for May and - June, 1870, and January, 1871. - </div> - - <div class="footnote"><a id="Footnote_215" href="#FNanchor_215" class="label">[215]</a> Phil. Mag. for April, 1866, p. 323.</div> - - <div class="footnote"><a id="Footnote_216" href="#FNanchor_216" class="label">[216]</a> Ibid., for March, 1866, p. 172.</div> - - <div class="footnote"><a id="Footnote_217" href="#FNanchor_217" class="label">[217]</a> <cite>Reader</cite>, February 10, 1866.</div> - - <div class="footnote"> - <a id="Footnote_218" href="#FNanchor_218" class="label">[218]</a> In a former paper I considered the effects of another - cause, viz., the melting of polar ice resulting from an increase of the - Obliquity of the Earth’s Orbit.—Trans. Glasgow Geol. Soc., vol. ii., p. - 177. Phil. Mag., June, 1867. See also <a href="#CHAPTER_XXV">Chapter XXV.</a> - </div> - - <div class="footnote"><a id="Footnote_219" href="#FNanchor_219" class="label">[219]</a> Phil. Mag. for November, 1868, p. 376.</div> - - <div class="footnote"><a id="Footnote_220" href="#FNanchor_220" class="label">[220]</a> Phil. Mag., November, 1868.</div> - - <div class="footnote"><a id="Footnote_221" href="#FNanchor_221" class="label">[221]</a> “Origin of Species,” chap. xi. Fifth Edition.</div> - - <div class="footnote"> - <a id="Footnote_222" href="#FNanchor_222" class="label">[222]</a> Lieutenant-Colonel Drayson (“Last Glacial Epoch of - Geology”) and also Mr. Belt (Quart. Journ. of Science, October, 1874) - state that Leverrier has lately investigated the question as to the - extent of the variation of the plane of the ecliptic, and has arrived - at results differing considerably from those of Laplace; viz., that - the variation may amount to 4° 52′, whereas, according to Laplace, - it amounts to only 1° 21′. I fear they are comparing things that are - totally different; viz., the variation of the plane of the ecliptic - in relation to its mean position with its variation in relation to - the equator. Laplace estimated that the plane of the ecliptic would - oscillate to the extent of 4° 53′ 33″ on each side of its mean - position, a result almost identical with that of Leverrier, who makes - it 4° 51′ 42″. But neither of these geometricians ever imagined that - the ecliptic could change in relation to the equator to even one-third of that amount. - - <p> - Laplace demonstrated that the change in the plane of the ecliptic - affected the position of the equator, causing it to vary along with it, - so that the equator could never possibly recede further than 1° 22′ - 34″ from its mean position in relation to the ecliptic (“<cite>Mécanique - Céleste</cite>,” vol. ii., p. 856, Bowditch’s Translation; see also - Laplace’s memoir, “Sur les Variations de l’Obliquité de l’Écliptique,” - <cite>Connaissance des Temps</cite> for 1827, p. 234), and I am not aware that - Leverrier has arrived at a different conclusion.</p> - </div> - - <div class="footnote"> - <a id="Footnote_223" href="#FNanchor_223" class="label">[223]</a> Memoir on the Secular Variations of the Elements of the - Orbits of the Planets, “Smithsonian Contributions to Knowledge,” vol. xvii. - </div> - - <div class="footnote"><a id="Footnote_224" href="#FNanchor_224" class="label">[224]</a> “Smithsonian Contributions to Knowledge,” vol. ix.</div> - - <div class="footnote"> - <a id="Footnote_225" href="#FNanchor_225" class="label">[225]</a> “Distribution of Heat on the Surface of the Globe,” p. 14. - </div> - - <div class="footnote"><a id="Footnote_226" href="#FNanchor_226" class="label">[226]</a> <a href="#CHAPTER_IV">Chapter IV.</a></div> - - <div class="footnote"><a id="Footnote_227" href="#FNanchor_227" class="label">[227]</a> Quart. Journ. Geol. Soc., June, 1866, p. 564.</div> - - <div class="footnote"><a id="Footnote_228" href="#FNanchor_228" class="label">[228]</a> Quart. Journ. Geol. Soc., vol. xxi., p. 186.</div> - - <div class="footnote"> - <a id="Footnote_229" href="#FNanchor_229" class="label">[229]</a> “Geological Observer,” p. 446. See also Mr. James - Geikie’s valuable Memoir, “On the Buried Forests and Peat Mosses of - Scotland.” Trans. of the Royal Society of Edinburgh, vol. xxiv., and - Chambers’ “Ancient Sea-Margins.” - </div> - - <div class="footnote"> - <a id="Footnote_230" href="#FNanchor_230" class="label">[230]</a> See Lyell’s “Antiquity of Man,” Second Edition, p. 282; - “Elements,” Sixth Edition, p. 162. - </div> - - <div class="footnote"> - <a id="Footnote_231" href="#FNanchor_231" class="label">[231]</a> In order to determine the position of the solstice-point - in relation to the aphelion, it will not do to assume, as is commonly - done, that the point makes a revolution from aphelion to aphelion in - any regular given period, such as 21,000 years; for it is perfectly - evident that owing to the great irregularity in the motion of the - aphelion, no two revolutions will probably be performed in the same - length of period. For example, the winter solstice was in the aphelion - about the following dates: 11,700, 33,300, and 61,300 years ago. Here - are two consecutive revolutions, the one performed in 21,600 years and - the other in 28,000 years; the difference in the length of the two - periods amounting to no fewer than 6,400 years. - </div> - - <div class="footnote"> - <a id="Footnote_232" href="#FNanchor_232" class="label">[232]</a> Quart. Journ. Geol. Soc., vol. xxvii., p. 232. See also - “The Last Glacial Epoch of Geology,” by the same author. - </div> - - <div class="footnote"><a id="Footnote_233" href="#FNanchor_233" class="label">[233]</a> Quart. Journ. of Science, October, 1874.</div> - - <div class="footnote"> - <a id="Footnote_234" href="#FNanchor_234" class="label">[234]</a> The longer diameter passes from long. 14° 23′ E. to - long. 165° 37′ W. - </div> - - <div class="footnote"><a id="Footnote_235" href="#FNanchor_235" class="label">[235]</a> “Principles,” vol. i., p. 294. Eleventh Edition.</div> - - <div class="footnote"><a id="Footnote_236" href="#FNanchor_236" class="label">[236]</a> Phil. Mag. for August, 1864.</div> - - <div class="footnote"><a id="Footnote_237" href="#FNanchor_237" class="label">[237]</a> “Elementary Geology,” p. 399.</div> - - <div class="footnote"><a id="Footnote_238" href="#FNanchor_238" class="label">[238]</a> “The Past and Present Life of the Globe,” p. 102.</div> - - <div class="footnote"> - <a id="Footnote_239" href="#FNanchor_239" class="label">[239]</a> “Memoirs of the Geological Survey,” vol. ii., Part 2, p. 404. - </div> - - <div class="footnote"><a id="Footnote_240" href="#FNanchor_240" class="label">[240]</a> “Coal Fields of Great Britain,” p. 45. Third Edition.</div> - - <div class="footnote"><a id="Footnote_241" href="#FNanchor_241" class="label">[241]</a> “Journal of Researches,” chap. xiii.</div> - - <div class="footnote"><a id="Footnote_242" href="#FNanchor_242" class="label">[242]</a> “Coal Fields of Great Britain,” p. 67.</div> - - <div class="footnote"><a id="Footnote_243" href="#FNanchor_243" class="label">[243]</a> See “Smithsonian Report for 1857,” p. 138.</div> - - <div class="footnote"><a id="Footnote_244" href="#FNanchor_244" class="label">[244]</a> Quart. Journ. Geol. Soc., May, 1865, p. civ.</div> - - <div class="footnote"><a id="Footnote_245" href="#FNanchor_245" class="label">[245]</a> “Geology of Fife and the Lothians,” p. 116.</div> - - <div class="footnote"><a id="Footnote_246" href="#FNanchor_246" class="label">[246]</a> “Life on the Earth,” p. 133.</div> - - <div class="footnote"><a id="Footnote_247" href="#FNanchor_247" class="label">[247]</a> Quart. Journ. Geol. Soc., vol. xi., p. 535.</div> - - <div class="footnote"><a id="Footnote_248" href="#FNanchor_248" class="label">[248]</a> Ibid., vol. xii., p. 39.</div> - - <div class="footnote"><a id="Footnote_249" href="#FNanchor_249" class="label">[249]</a> Miller’s “Sketch Book of Practical Geology,” p. 192.</div> - - <div class="footnote"> - <a id="Footnote_250" href="#FNanchor_250" class="label">[250]</a> From Geological Magazine, May and June, 1870; with a few - verbal corrections, and a slight re-arrangement of the paragraphs. - </div> - - <div class="footnote"><a id="Footnote_251" href="#FNanchor_251" class="label">[251]</a> See Phil. Mag. for November, 1868, p. 374.</div> - - <div class="footnote"><a id="Footnote_252" href="#FNanchor_252" class="label">[252]</a> See Phil. Mag. for November, 1868, pp. 366−374.</div> - - <div class="footnote"><a id="Footnote_253" href="#FNanchor_253" class="label">[253]</a> Journ. Geol. Soc., vol. xxi., p. 165.</div> - - <div class="footnote"> - <a id="Footnote_254" href="#FNanchor_254" class="label">[254]</a> Specimens of the striated summit and boulder clay stones - are to be seen in the Edinburgh Museum of Science and Art. - </div> - - <div class="footnote"><a id="Footnote_255" href="#FNanchor_255" class="label">[255]</a> Phil. Mag. for April, 1866.</div> - - <div class="footnote"><a id="Footnote_256" href="#FNanchor_256" class="label">[256]</a> “Tracings of the North of Europe,” 1850, pp. 48−51.</div> - - <div class="footnote"><a id="Footnote_257" href="#FNanchor_257" class="label">[257]</a> Quart. Journ. Geol. Soc., vol. ii., p. 364.</div> - - <div class="footnote"> - <a id="Footnote_258" href="#FNanchor_258" class="label">[258]</a> “Tracings of the North of Europe,” by Robert Chambers, - pp. 259, 285. “Observations sur les Phénomènes d’Erosion en Norvège,” - by M. Hörbye, 1857. See also Professor Erdmann’s “Formations Quaternaires de la Suède.” - </div> - - <div class="footnote"><a id="Footnote_259" href="#FNanchor_259" class="label">[259]</a> “Glacial Drift of Scotland,” p. 29.</div> - - <div class="footnote"> - <a id="Footnote_260" href="#FNanchor_260" class="label">[260]</a> Geological Magazine, vol. ii., p. 343. Brit. Assoc. - Rep., 1864 (sections), p. 59. - </div> - - <div class="footnote"><a id="Footnote_261" href="#FNanchor_261" class="label">[261]</a> Trans. Roy. Soc. Edin., vol. vii., p. 265.</div> - - <div class="footnote"><a id="Footnote_262" href="#FNanchor_262" class="label">[262]</a> “Tracings of Iceland and the Faroe Islands,” p. 49.</div> - - <div class="footnote"><a id="Footnote_263" href="#FNanchor_263" class="label">[263]</a> See Chap. XXIII.</div> - - <div class="footnote"> - <a id="Footnote_264" href="#FNanchor_264" class="label">[264]</a> Mr. Thomas Belt has subsequently advanced (Quart. Jour. - Geol. Soc., vol. xxx., p. 490), a similar explanation of the steppes - of Siberia. He supposes that an overflow of ice from the polar basin - dammed back all the rivers flowing northward, and formed an immense - lake which extended over the lowlands of Siberia, and deposited the - great beds of sand and silt with occasional freshwater shells and - elephant remains, of which the steppes consist. - </div> - - <div class="footnote"><a id="Footnote_265" href="#FNanchor_265" class="label">[265]</a> Proc. Roy. Phys. Soc., Edin., vols. ii. and iii.</div> - - <div class="footnote"><a id="Footnote_266" href="#FNanchor_266" class="label">[266]</a> From Geol. Mag. for January, 1871.</div> - - <div class="footnote"><a id="Footnote_267" href="#FNanchor_267" class="label">[267]</a> Quart. Journ. Geol. Soc., xxvi., p. 517.</div> - - <div class="footnote"><a id="Footnote_268" href="#FNanchor_268" class="label">[268]</a> British Assoc. Report for 1864 (sections), p. 65.</div> - - <div class="footnote"><a id="Footnote_269" href="#FNanchor_269" class="label">[269]</a> Quart. Journ. Geol. Soc., xxvi., p. 90.</div> - - <div class="footnote"><a id="Footnote_270" href="#FNanchor_270" class="label">[270]</a> Geol. Mag., vii., p. 349.</div> - - <div class="footnote"><a id="Footnote_271" href="#FNanchor_271" class="label">[271]</a> Trans. Edin. Geol. Soc., vol. i., p. 136.</div> - - <div class="footnote"><a id="Footnote_272" href="#FNanchor_272" class="label">[272]</a> Geol. Mag. for June, 1870. See Chap. XXVII.</div> - - <div class="footnote"> - <a id="Footnote_273" href="#FNanchor_273" class="label">[273]</a> This was done by Mr. R. H. Tiddeman of the Geological - Survey of England (Quart. Journ. Geol. Soc. for November, 1872), and - the result established the correctness of the above opinion as to the - existence of a North of England ice-sheet. Additional confirmation has - been derived from the important observations of Mr. D. Mackintosh, and - also of Mr. Goodchild, of the Geological Survey of England. - </div> - - <div class="footnote"><a id="Footnote_274" href="#FNanchor_274" class="label">[274]</a> Trans. Geol. Soc., vol. v., p. 516 (first series).</div> - - <div class="footnote"> - <a id="Footnote_275" href="#FNanchor_275" class="label">[275]</a> Quart. Journ. Geol. Soc., vol. xi., p. 492. “Memoir of - the Country around Cheltenham,” 1857. “Geology of the Country around Woodstock,” 1859. - </div> - - <div class="footnote"><a id="Footnote_276" href="#FNanchor_276" class="label">[276]</a> Geol. Mag., vol. vii., p. 497.</div> - - <div class="footnote"><a id="Footnote_277" href="#FNanchor_277" class="label">[277]</a> Quart. Journ. Geol. Soc., vol. xxvi., p. 90.</div> - - <div class="footnote"><a id="Footnote_278" href="#FNanchor_278" class="label">[278]</a> My colleague, Mr. R. L. Jack.</div> - - <div class="footnote"> - <a id="Footnote_279" href="#FNanchor_279" class="label">[279]</a> The greater portion of this chapter is from the Trans. - of Geol. Soc. of Edinburgh, for 1869. - </div> - - <div class="footnote"><a id="Footnote_280" href="#FNanchor_280" class="label">[280]</a> <a href="#CHAPTER_XV">Chapter XV.</a>, p. 253.</div> - - <div class="footnote"> - <a id="Footnote_281" href="#FNanchor_281" class="label">[281]</a> Trans. of the Geol. Soc. of Glasgow, vol. iii., part i., - page 133. - </div> - - <div class="footnote"> - <a id="Footnote_282" href="#FNanchor_282" class="label">[282]</a> Mr. Milne Home has advanced, in his “Estuary of the - Firth of Forth,” p. 91, the theory that this trough had been scooped - out during the glacial epoch by icebergs floating through the Midland - valley from west to east when it was submerged. The bottom of the - trough, be it observed, at the watershed at Kilsyth, is 300 feet - above the level of its bottom at Grangemouth; and this Mr. Milne Home - freely admits. But he has not explained how an iceberg, which could - float across the shallow water at Kilsyth, say, 100 feet deep, could - manage to grind the rocky bottom at Grangemouth, where it was not less - than 400 feet deep. “The impetus acquired in the Kyle at Kilsyth,” - says Mr. Milne Home, “would keep them moving on, and the prevailing - westerly winds would also aid, so that when <i>grating</i> on the subjacent - carboniferous rocks they would not have much difficulty in scooping out - a channel both wider and deeper than at Kilsyth.” But how could they - “<i>grate</i> on the subjacent carboniferous rocks” at Grangemouth, if they - managed to <i>float</i> at Kilsyth? Surely an iceberg that could “<i>grate</i>” - at Grangemouth would “<i>ground</i>” at Kilsyth. - </div> - - <div class="footnote"><a id="Footnote_283" href="#FNanchor_283" class="label">[283]</a> Trans. of the Geol. Soc. of Glasgow, vol. iii., p. 141.</div> - - <div class="footnote"> - <a id="Footnote_284" href="#FNanchor_284" class="label">[284]</a> Mr. John Young and Mr. Milne Home advanced the - objection, that several trap dykes cross the valley of the Clyde near - Bowling, and come to so near the present surface of the land, that - the Clyde at present flows across them with a depth not exceeding 20 - feet. I fear that Mr. Young and Mr. Milne Home have been misinformed in - regard to the existence of these dykes. About a mile <i>above</i> Bowling - there are one or two dykes which approach to the river-bank, and may - probably cross, but these could not possibly cut off a channel entering - the Clyde at Bowling. In none of the borings or excavations which have - been made by the Clyde Trustees has the rock been reached from Bowling - downwards. I may also state that the whole Midland valley, from the - Forth of Clyde to the Firth of Forth, has been surveyed by the officers - of the Geological Survey, and only a single dyke has been found to - cross the buried channels, viz., one (Basalt rock) running eastward - from Kilsyth to the canal bridge near Dullatur. But as this is not - far from the watershed between the two channels it cannot affect the - question at issue. See sheet 31 of Geological Survey Map of Scotland. - </div> - - <div class="footnote"><a id="Footnote_285" href="#FNanchor_285" class="label">[285]</a> Trans. Geol. Soc. Glasgow, vol. iv., p. 166.</div> - - <div class="footnote"><a id="Footnote_286" href="#FNanchor_286" class="label">[286]</a> “Great Ice Age,” chap. xiii.</div> - - <div class="footnote"> - <a id="Footnote_287" href="#FNanchor_287" class="label">[287]</a> See further particulars in Mr. Bennie’s paper on the - Surface Geology of the district around Glasgow, Trans. Geol. Soc. of Glasgow, vol. iii. - </div> - - <div class="footnote"><a id="Footnote_288" href="#FNanchor_288" class="label">[288]</a> See also Smith’s “Newer Pliocene Geology,” p. 139.</div> - - <div class="footnote"> - <a id="Footnote_289" href="#FNanchor_289" class="label">[289]</a> British Association Report for 1863, p. 89. <cite>Geologist</cite> - for 1863, p. 384. - </div> - - <div class="footnote"><a id="Footnote_290" href="#FNanchor_290" class="label">[290]</a> See Geological Magazine, vol. ii., p. 38.</div> - - <div class="footnote"><a id="Footnote_291" href="#FNanchor_291" class="label">[291]</a> Proc. Geol. Soc., vol. iii., 1840, p. 342.</div> - - <div class="footnote"><a id="Footnote_292" href="#FNanchor_292" class="label">[292]</a> “Antiquity of Man” (Third Edition), p. 249.</div> - - <div class="footnote"> - <a id="Footnote_293" href="#FNanchor_293" class="label">[293]</a> “Glacial Drift of Scotland,” p. 65. Trans. Geol. Soc. - Glas., vol. i., part 2. - </div> - - <div class="footnote"> - <a id="Footnote_294" href="#FNanchor_294" class="label">[294]</a> “Memoir, Geological Survey of Scotland,” Sheet 23, p. 42. - </div> - - <div class="footnote"> - <a id="Footnote_295" href="#FNanchor_295" class="label">[295]</a> Mr. Robert Dick had previously described, in the Trans. - Geol. Soc. Edinburgh, vol. i., p. 345, portions of these buried - channels. He seems, however, to have thought that they formed part of one and the same channel. - </div> - - <div class="footnote"> - <a id="Footnote_296" href="#FNanchor_296" class="label">[296]</a> A description of this channel was read to the Natural - History Society of Glasgow by Mr. James Coutts, the particulars of - which will appear in the Transactions of the Society. - </div> - - <div class="footnote"><a id="Footnote_297" href="#FNanchor_297" class="label">[297]</a> “Occasional Papers,” pp. 166, 223.</div> - - <div class="footnote"><a id="Footnote_298" href="#FNanchor_298" class="label">[298]</a> Memoir read before the Royal Society, January 7, 1869.</div> - - <div class="footnote"><a id="Footnote_299" href="#FNanchor_299" class="label">[299]</a> “Alpine Journal,” February, 1870.</div> - - <div class="footnote"><a id="Footnote_300" href="#FNanchor_300" class="label">[300]</a> Phil. Mag., January, 1872.</div> - - <div class="footnote"><a id="Footnote_301" href="#FNanchor_301" class="label">[301]</a> Phil. Mag., July, 1870; February, 1871.</div> - - <div class="footnote"> - <a id="Footnote_302" href="#FNanchor_302" class="label">[302]</a> Philosophical Magazine for January, 1870, p. 8; - Proceedings of the Royal Society for January, 1869. - </div> - - <div class="footnote"><a id="Footnote_303" href="#FNanchor_303" class="label">[303]</a> Philosophical Magazine for March, 1869.</div> - - <div class="footnote"> - <a id="Footnote_304" href="#FNanchor_304" class="label">[304]</a> Proceedings of Bristol Naturalists’ Society, p. 37 (1869). - </div> - - <div class="footnote"><a id="Footnote_305" href="#FNanchor_305" class="label">[305]</a> Ibid., vol. iv., p. 37 (new series).</div> - - <div class="footnote"><a id="Footnote_306" href="#FNanchor_306" class="label">[306]</a> Phil. Mag., S. 4, vol. x., p. 303.</div> - - <div class="footnote"> - <a id="Footnote_307" href="#FNanchor_307" class="label">[307]</a> Proceedings of the Bristol Naturalists’ Society, vol. - iv., p. 39 (new series). - </div> - - <div class="footnote"><a id="Footnote_308" href="#FNanchor_308" class="label">[308]</a> See Philosophical Transactions, December, 1857.</div> - - <div class="footnote"> - <a id="Footnote_309" href="#FNanchor_309" class="label">[309]</a> There is one circumstance tending slightly to prevent - the rupture of the glacier, when under tension, which I do not remember - to have seen noticed; that is, the cooling effect which is produced - in solids, such as ice, when subjected to tension. Tension would tend - to lower the temperature of the ice-molecules, and this lowering of - temperature would have the tendency of freezing them more firmly - together. The cause of this cooling effect will be explained in the - Appendix. - </div> - - <div class="footnote"><a id="Footnote_310" href="#FNanchor_310" class="label">[310]</a> Phil. Mag., March, 1869; September, 1870.</div> - - <div class="footnote"><a id="Footnote_311" href="#FNanchor_311" class="label">[311]</a> “Forms of Water,” p. 127.</div> - - <div class="footnote"><a id="Footnote_312" href="#FNanchor_312" class="label">[312]</a> See text, p. 10.</div> - - <div class="footnote"><a id="Footnote_313" href="#FNanchor_313" class="label">[313]</a> Mathematical and Physical Series, vol. xxxvi. (1765).</div> - - <div class="footnote"><a id="Footnote_314" href="#FNanchor_314" class="label">[314]</a> “Memoirs of St. Petersburg Academy,” 1761.</div> - - <div class="footnote"> - <a id="Footnote_315" href="#FNanchor_315" class="label">[315]</a> The calculations here referred to were made by Lagrange - nearly half a century previous to the appearance of this paper, and - published in the “Mémoires de l’Académie de Berlin,” for 1782, p. 273. - Lagrange’s results differ but slightly from those afterwards obtained - by Leverrier, as will be seen from the following table; but as he - had assigned erroneous values to the masses of the smaller planets, - particularly that of Venus, the mass of which he estimated at one-half - more than its true value, full confidence could not be placed in his - results. - - <p>Superior limits of eccentricity as determined by Lagrange, Leverrier,and Mr. Stockwell:—</p> - - <table summary="Superior limits of eccentricity"> - <tbody> - <tr> - <th> </th> - <th class="tdc"><div>By Lagrange.</div></th> - <th class="tdc"><div>By Leverrier.</div></th> - <th class="tdc"><div>By Mr. Stockwell.</div></th> - </tr> - <tr> - <td>Mercury</td> - <td class="tdc"><div>0·22208</div></td> - <td class="tdc"><div>0·225646</div></td> - <td class="tdc"><div>0·2317185</div></td> - </tr> - <tr> - <td>Venus</td> - <td class="tdc"><div>0·08271</div></td> - <td class="tdc"><div>0·086716</div></td> - <td class="tdc"><div>0·0706329</div></td> - </tr> - <tr> - <td>Earth</td> - <td class="tdc"><div>0·07641</div></td> - <td class="tdc"><div>0·077747</div></td> - <td class="tdc"><div>0·0693888</div></td> - </tr> - <tr> - <td>Mars</td> - <td class="tdc"><div>0·14726</div></td> - <td class="tdc"><div>0·142243</div></td> - <td class="tdc"><div>0·139655</div></td> - </tr> - <tr> - <td>Jupiter</td> - <td class="tdc"><div>0·06036</div></td> - <td class="tdc"><div>0·061548</div></td> - <td class="tdc"><div>0·0608274</div></td> - </tr> - <tr> - <td>Saturn</td> - <td class="tdc"><div>0·08408</div></td> - <td class="tdc"><div>0·084919</div></td> - <td class="tdc"><div>0·0843289</div></td> - </tr> - <tr> - <td>Uranus</td> - <td class="tdc"><div>—</div></td> - <td class="tdc"><div>0·064666</div></td> - <td class="tdc"><div>0·0779652</div></td> - </tr> - <tr> - <td>Neptune</td> - <td class="tdc"><div>—</div></td> - <td class="tdc"><div>—</div></td> - <td class="tdc"><div>0·0145066</div></td> - </tr> - </tbody> - </table> - - <div class="right">[J. C.]</div> - </div> - - <div class="footnote"> - <a id="Footnote_316" href="#FNanchor_316" class="label">[316]</a> “Mém. de l’Acad. royale des Sciences.” 1827. Tom. vii., - p. 598. - </div> - - <div class="footnote"> - <a id="Footnote_317" href="#FNanchor_317" class="label">[317]</a> Absolute zero is now considered to be only 493° Fah. - below the freezing-point, and Herschel himself has lately determined - 271° below the freezing-point to be the temperature of space. - Consequently, a decrease, or an increase of one per cent. in the mean - annual amount of radiation would not produce anything like the effect - which is here supposed. But the mean annual amount of heat received - cannot vary much more than one-tenth part of one per cent. In short, - the effect of eccentricity on the mean annual supply of heat received - from the sun, in so far as geological climate is concerned, may be - practically disregarded.—[J. C.] - </div> - - <div class="footnote"> - <a id="Footnote_318" href="#FNanchor_318" class="label">[318]</a> “Principles of Geology,” p. 110. “Mr. Lyell, however, - in stating the actual excess of eight days in the duration of the - sun’s presence in the northern hemisphere over that in the southern as - productive of an excess of light and heat annually received by the one - over the other hemisphere, appears to have misconceived the effect of - elliptic motion in the passage here cited, since it is demonstrable - that whatever be the ellipticity of the earth’s orbit the two - hemispheres must receive equal absolute quantities of light and heat - per annum, the proximity of the sun in perigee exactly compensating the - effect of its swifter motion. This follows from a very simple theorem, - which may be thus stated: ‘The amount of heat received by the earth - from the sun while describing any part of its orbit is proportional to - the angle described round the sun’s centre,’ so that if the orbit be - divided into two portions by a line drawn <i>in any direction</i> through - the sun’s centre, the heats received in describing the two unequal - segments of the ellipse so produced will be equal.” - </div> - - <div class="footnote"> - <a id="Footnote_319" href="#FNanchor_319" class="label">[319]</a> When the eccentricity of the earth’s orbit is at its - superior limit, the absolute quantity of heat received by the globe - during one year will be increased by only 1/300th part; an amount which - could produce no sensible influence on climate.—[J. C.] - </div> - - <div class="footnote"> - <a id="Footnote_320" href="#FNanchor_320" class="label">[320]</a> Sir Charles has recently, to a certain extent, adopted - the views advocated in the present volume, viz., that the cold of - the glacial epoch was brought about not by a <i>decrease</i>, but by an - <i>increase</i> of eccentricity. (See vol. i. of “Principles,” tenth and - eleventh editions.) The decrease in the mean annual quantity of heat - received from the sun, resulting from the decrease in the eccentricity - of the earth’s orbit—the astronomical cause to which he here - refers—could have produced no sensible effect on climate.—[J. C.] - </div> - - <div class="footnote"> - <a id="Footnote_321" href="#FNanchor_321" class="label">[321]</a> It is singular that both Arago and Humboldt should - appear to have been unaware of the researches of Lagrange on this subject. - </div> - - <div class="footnote"><a id="Footnote_322" href="#FNanchor_322" class="label">[322]</a> “Révolutions de la Mer,” p. 37. Second Edition.</div> - - <div class="footnote"><a id="Footnote_323" href="#FNanchor_323" class="label">[323]</a> See text, p. 37.</div> - - <div class="footnote"> - <a id="Footnote_324" href="#FNanchor_324" class="label">[324]</a> See <cite>Philosophical Magazine</cite> for December, 1867, p. 457. - </div> - - <div class="footnote"> - <a id="Footnote_325" href="#FNanchor_325" class="label">[325]</a> <cite>Silliman’s American Journal</cite> for July, 1864. - <cite>Philosophical Magazine</cite> for September, 1864, pp. 193, 196. - </div> - - <div class="footnote"><a id="Footnote_326" href="#FNanchor_326" class="label">[326]</a> <cite>Philosophical Magazine</cite> for August, 1865, p. 95.</div> - - <div class="footnote"><a id="Footnote_327" href="#FNanchor_327" class="label">[327]</a> See text, p. 80.</div> - - <div class="footnote"><a id="Footnote_328" href="#FNanchor_328" class="label">[328]</a> See text, p. 222.</div> - - <div class="footnote"><a id="Footnote_329" href="#FNanchor_329" class="label">[329]</a> Proc. Roy. Soc., No. 157, 1875.</div> - - <div class="footnote"><a id="Footnote_330" href="#FNanchor_330" class="label">[330]</a> See text, p. 522.</div> - - <div class="footnote"><a id="Footnote_331" href="#FNanchor_331" class="label">[331]</a> Phil. Trans. for 1859, p. 91.</div> - - <div class="footnote"><a id="Footnote_332" href="#FNanchor_332" class="label">[332]</a> See text, p. 527.</div> - </div> - - <div class="transnote mt10"> - <div class="large center mb2"><b>Transcriber’s Notes:</b></div> - <ul class="spaced"> - <li>Blank pages have been removed.</li> - <li>Obvious typographical errors have been silently corrected.</li> - <li>Advertisements have been moved to the back.</li> - </ul> - </div> - - - - - - - - -<pre> - - - - - -End of the Project Gutenberg EBook of Climate and Time in their Geological -Relations, by James Croll - -*** END OF THIS PROJECT GUTENBERG EBOOK CLIMATE AND TIME *** - -***** This file should be named 62693-h.htm or 62693-h.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/6/2/6/9/62693/ - -Produced by WebRover, MWS, Robert Tonsing, and the Online -Distributed Proofreading Team at https://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive/American Libraries.) - - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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