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+Project Gutenberg (https://www.gutenberg.org) public repository for
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-The Project Gutenberg EBook of The Evolution of Worlds, by Percival Lowell
-
-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: The Evolution of Worlds
-
-Author: Percival Lowell
-
-Release Date: August 21, 2020 [EBook #62992]
-
-Language: English
-
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-*** START OF THIS PROJECT GUTENBERG EBOOK THE EVOLUTION OF WORLDS ***
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-Transcriber’s Notes:
-
- Underscores “_” before and after a word or phrase indicate _italics_
-    in the original text.
- A single underscore after a symbol indicates a
-    subscript.
- Carat symbol “^” designates a superscript.
- Small capitals have been converted to SOLID capitals.
- Illustrations have been moved so they do not break up paragraphs.
- Typographical errors have been silently corrected.
- The numerical references to the NOTES, e. g. [1] have been changed to
-    “(see NOTE x)” in order to avoid confusion with the true footnotes.
-
-
-
-
-THE EVOLUTION OF WORLDS
-
-
-
-
-                            [Illustration]
-
-                         THE MACMILLAN COMPANY
-                      NEW YORK · BOSTON · CHICAGO
-                        ATLANTA · SAN FRANCISCO
-
-                       MACMILLAN & CO., LIMITED
-                      LONDON · BOMBAY · CALCUTTA
-                               MELBOURNE
-
-                   THE MACMILLAN CO. OF CANADA, LTD.
-                                TORONTO
-
-     [Illustration: SATURN—PHOTOGRAPHED AT THE LOWELL OBSERVATORY
-                BY MR. E. C. SLIPHER. SEPTEMBER, 1909.]
-
-
-
-
-                            THE EVOLUTION OF WORLDS
-
-                                      BY
-                         PERCIVAL LOWELL, A.B., LL.D.
-
-                 AUTHOR OF “MARS AND ITS CANALS,” “MARS AS THE
-                             ABODE OF LIFE,” ETC.
-
-        DIRECTOR OF THE OBSERVATORY AT FLAGSTAFF, ARIZONA; NON-RESIDENT
-           PROFESSOR OF ASTRONOMY AT THE MASSACHUSETTS INSTITUTE OF
-       TECHNOLOGY; FELLOW OF THE AMERICAN ACADEMY OF ARTS AND SCIENCES;
-          MEMBRE DE LA SOCIÉTÉ ASTRONOMIQUE DE FRANCE; MEMBER OF THE
-          ASTRONOMICAL AND ASTROPHYSICAL SOCIETY OF AMERICA; MITGLIED
-             DER ASTRONOMISCHE GESELLSCHAFT; MEMBRE DE LA SOCIÉTÉ
-              BELGE D’ASTRONOMIE; HONORARY MEMBER OF THE SOCIEDAD
-                ASTRONOMICA DE MEXICO; JANSSEN MEDALLIST OF THE
-                   SOCIÉTÉ ASTRONOMIQUE DE FRANCE, 1904, FOR
-                     RESEARCHES ON MARS; MEDALLIST OF THE
-                      SOCIEDAD ASTRONOMICA DE MEXICO FOR
-                             STUDIES ON MARS, 1908
-
-                              _ILLUSTRATED_
-
-                                   New York
-                             THE MACMILLAN COMPANY
-                                     1909
-
-                          _All rights reserved_
-
-                           COPYRIGHT, 1909,
-                      BY THE MACMILLAN COMPANY.
-
-              Set up and electrotyped. Published December, 1909.
-
-                                 Norwood Press
-                    J. S. Cushing Co.—Berwick & Smith Co.
-                            Norwood, Mass., U.S.A.
-
-                                      TO
-                             THE PRESIDENT OF THE
-                     MASSACHUSETTS INSTITUTE OF TECHNOLOGY
-                            TO MY COLLEAGUES THERE
-                            AND TO ITS STUDENT BODY
-                     TO WHOSE INTEREST AND ATTENTION THESE
-                             LECTURES ARE INDEBTED
-                       THEY ARE APPRECIATIVELY INSCRIBED
-
-    “Si je n’étais pas devenu général en chef et l’instrument du sort
-    d’un grand people, j’aurais couru les bureaux et les salons pour me
-    mettre dans la dépendance de qui que ce fût, en qualité de ministre
-    ou d’ambassadeur? Non, non! je me serais jeté dans l’étude des
-    sciences exactes. J’aurais fait mon chemin dans la route des
-    Galilée, des Newton. Et puisque j’ai réussi constamment dans mes
-    grandes entreprises, eh bien, je me serais hautement distingué aussi
-    par des travaux scientifiques. J’aurais laissé le souvenir de belles
-    découvertes. Aucune autre gloire n’aurait pu tenter mon ambition.”
-
-                                     —NAPOLEON Iᴱᴿ, QUOTED BY ARAGO.
-
-The substance of the following pages was written and presented in
-a university course of lectures before the Massachusetts Institute
-of Technology—in February and March of this year. The kind interest
-with which the lectures were received, not only by the students and
-professional bodies, but by the public, was followed by an immediate
-request from The Macmillan Company to issue them in book form, and as
-such they now appear.
-
-                                               PERCIVAL LOWELL.
-    BOSTON, MASS., May 29, 1909.
-
-
-
-
-CONTENTS
-
-
-    CHAPTER                                                      PAGE
-        I. BIRTH OF A SOLAR SYSTEM                                 1
-       II. EVIDENCE OF THE INITIAL CATASTROPHE IN OUR OWN CASE    31
-      III. THE INNER PLANETS                                      58
-       IV. THE OUTER PLANETS                                      94
-        V. FORMATION OF PLANETS                                  127
-       VI. A PLANET’S HISTORY—SELF-SUSTAINED STAGE               155
-      VII. A PLANET’S HISTORY—SUN-SUSTAINED STAGE                182
-     VIII. DEATH OF A WORLD                                      213
-
-                                  NOTES
-     1. METEOR ORBITS                                            241
-     2. DENSITIES OF THE PLANETS                                 243
-     3. VARIATION IN SPECTROSCOPIC SHIFT                         243
-     4. ON THE PLANETS’ ORBITAL TILTS                            244
-     5. PLANETS AND THEIR SATELLITE SYSTEMS                      245
-     6. ON THE INDUCED CIRCULARITY OF ORBITS THROUGH COLLISION   250
-     7. CAPTURE OF SATELLITES                                    251
-
-     INDEX                                                       253
-
-
-
-
-LIST OF ILLUSTRATIONS
-
-
-                                    PLATES
-
-       I. Saturn                                       _Frontispiece_
-                                                       OPPOSITE PAGE
-      II. The Moving Nebula surrounding Nova Persei, 1901-1902    14
-     III. Representative Stellar Spectra                          24
-      IV. Spectra of the Major Planets                            52
-       V. Venus, 1896-1897                                        82
-      VI. Asteroids: Major Axes of Orbits                         98
-     VII. Saturn—A Drawing showing Agglomerations                108
-    VIII. Spectrogram of Jupiter, Moon Comparison                152
-      IX. Spectrogram showing Water-vapor in Atmosphere of Mars  160
-       X. Tree Fern                                              176
-      XI. Ten Views of Mercury, showing Effect of Libration      222
-     XII. Spectrogram of Saturn                                  232
-
-                            CUTS APPEARING IN TEXT
-                                                                PAGE
-    Algol and its Dark Companion                                   4
-    Nova Persei                                                   11
-    Spectrum of Nova Persei                                       12
-    The Moving Nebula surrounding Nova Persei, 1901               13
-    Great Nebula in Orion                                         17
-    Great Nebula in Andromeda                                     18
-    Nebula M. 100 Comæ                                            19
-    Nebula ♅ I. 226 Ursæ Majoris                                 20
-    Nebula ♅ V. 24 Comæ. Showing Globular Structure              21
-    Nebula M. 101 Ursæ Majoris                                    23
-    The Radiant of a Meteoric Shower                              37
-    Diagram explaining Proportionate Visibility of Meteors        38
-    The Mart Iron                                                 41
-    Section of Meteorite showing Widmannstättian Lines            42
-    Meteorite, Toluca                                             43
-    Nebula ♅ V. 14 Cygni                                         45
-    Nebula N.G.C. 1499 Persei                                     46
-    Nebula N.G.C. 6960 in Cygnus                                  47
-    Nebula M. 51 Canum Venaticorum                                48
-    Orbits of the Inner Planets                                   59
-    Sulla Rotazione di Mercurio.—Di G. V. Schiaparelli            64
-    Map of Mercury. Lowell                                        69
-    Venus. October, 1896-March, 1897                              78
-    Venus. April 12, 1909.                                        79
-    Diagram: Convection Currents in Atmosphere of Venus           81
-    Diagram: Shift in Central Barometric Depression               81
-    Spectrogram of Venus, showing its Long Day                    87
-    Spectrogram of Jupiter, giving the Length of its Day by the
-         Tilt of its Spectral Lines                               89
-    Orbits of the Outer Planets                                   95
-    Drawing of Jupiter showing its Ellipticity                   103
-    Two Drawings of Jupiter and its Wisps                        105
-    Photograph of Jupiter, 1909                                  107
-    Diagram of Saturn’s Rings                                    113
-    The Tores of Saturn                                          114
-    Chart showing increasing Tilts of the Major Planets          131
-    Orbital Tilts and Eccentricities of Satellites               133
-    Masses of Planets and Satellites                             136
-    Two Drawings of Jupiter and its “Great Red Spot”             164
-    Sun Spots                                                    165
-    Photograph of a Sun Spot                                     166
-    The Volcano Colima, Mexico, March 24, 1903                   169
-    Jukes Butte, a Denuded Laccolith, as seen from the Northwest 170
-    Ideal Section of a Laccolith                                 170
-    Earth as seen from above.—Photographed at an Altitude of
-          5500 Feet                                              183
-    Tracks of Sauropus Primævus                                  188
-    Adventures of a Heat Ray                                     193
-    Polar Caps of Mars at their Maxima and Minima                198
-    Glacial Map of Eurasia                                       200
-    Map showing the Glaciated Area of North America              201
-    Photograph of the Moon                                       205
-    Petrified Bridge, Third Petrified Forest, near Adamana,
-          Arizona                                                210
-    Three Views of Venus, showing Agreement at Different
-          Distances                                              220
-    Diagram of Libration in Longitude due to Rotation            222
-    Moon,—Full and Half                                          225
-    Diagram illustrating Molecular Motion in a Gas               227
-    Distribution of Molecular Velocities in a Gas                229
-
-
-
-
-THE EVOLUTION OF WORLDS
-
-
-
-
-CHAPTER I
-
-BIRTH OF A SOLAR SYSTEM
-
-
-Astronomy is usually thought of as the study of the bodies visible in
-the sky. And such it largely is when the present state of the universe
-alone is considered. But when we attempt to peer into its past and
-to foresee its future, we find ourselves facing a new side of the
-heavens—the contemplation of the invisible there. For in the evolution
-of worlds not simply must the processes be followed by the mind’s
-eye, so short the span of human life, but they begin and end in what
-we cannot see. What the solar system sprang from, and what it will
-eventually become, is alike matter devoid of light. Out of darkness
-into darkness again: such are the bourns of cosmic action.
-
-The stars are suns; past, present, or potential. Each of those diamond
-points we mark studding the heavens on a winter’s night are globes
-comparable with, and in many cases greatly excelling, our own ruler of
-the day. The telescope discloses myriads more. Yet these self-confessed
-denizens of space form but a fraction of its occupants. Quite as
-near, and perhaps much nearer, are orbs of which most of us have no
-suspicion. Unimpressing our senses and therefore ignored by our minds,
-bodies people it which, except for rare occurrences, remain forever
-invisible. For dark stars in countless numbers course hither and
-thither throughout the universe at speeds as stupendous as the lucent
-ones themselves.
-
-Had we no other knowledge of them, reasoning would suffice to
-demonstrate their existence. It is the logic of unlimited subtraction.
-Every self-shining star is continually giving out light and heat.
-Now such an expenditure cannot go on forever, as the source of its
-replenishing by contraction, accretion, or disintegration is finite.
-Long to our measures of time as the process may last, it must
-eventually have an end and the star finally become a cold dark body,
-pursuing as before its course, but in itself inert and dead; an orb
-grown _orbéd_, in the old French sense. So it must remain unless some
-cosmic catastrophe rekindle it to life. The chance of such occurrence
-in a given time compared with the duration of the star’s light-emitting
-career will determine the number of dark stars relative to the lucent
-ones. The chance is undoubtedly small, and the number of dark bodies
-in space proportionally large. Reasoning, then, informs us first that
-such bodies must exist all about us, and second that their multitude
-must be great.
-
-Valid as this reasoning is, however, we are not left to inference
-for our knowledge of them. There is a certain star amid the polar
-constellations known as Algol,—el Ghoul, the Arabs called it, or The
-Dæmon. The name shows they noticed how it winked its eye and recognized
-something sarcastically sinister in its intent. For once in two days
-and twenty hours its light fades to one-third of its usual amount,
-remains thus for about twenty minutes, and then slowly regains its
-brightness. Seemingly unmoved itself, its steady blinking from the time
-man first observed it took on an uncanniness he felt. To untelescoped
-man it certainly seemed demoniacal, this punctual recurrent wink.
-Spectroscoped man has learnt its cause.
-
-Goodricke in 1795 divined it, and research since has confirmed his keen
-intuition. Its loss of light is occasioned by the passing in front of
-it of a dark companion almost of its own size revolving about it in
-a close elliptic orbit. That this is the explanation of its strange
-behavior, the shift of its spectral lines makes certain, by showing
-that the bright star is receding from us at twenty-seven miles a second
-seventeen hours before the eclipse and coming towards us at about the
-same rate seventeen hours after it; its dark companion, therefore,
-doing the reverse.
-
-Algol is no solitary specimen of a mind-seen invisible star. Many
-eclipsing binaries of the same class are now known; and considering
-that the phenomenon could not be disclosed unless the orbital plane
-of the pair traversed the observer’s eye, an unlikely chance in a
-fortuitous distribution, we perceive how many such in truth there must
-be which escape recognition for their tilt.
-
-[Illustration: ALGOL AND ITS DARK COMPANION,
-
-AS SEEN FROM THE EARTH,]
-
-[Illustration: AS SEEN FROM ABOVE ORBIT.]
-
-But if dark stars exist in connection with lucent ones, there must be
-many more that travel alone. Our own Sun is an instance in embryo. If
-he live long enough, he will become such a solitary shrouded tramp in
-his old age. For he has no companion to betray him. The only way in
-which we could become cognizant of these wanderers would be by their
-chance collision with some other star, dark or lucent as the case might
-be. The impact of the catastrophe would generate so much light and heat
-that the previously dark body would be converted into a blazing sun and
-a new star make its advent in the sky.
-
-Star births of the sort have actually been noted. Every now and then a
-new star suddenly appears in the firmament—a nova as it is technically
-called. These apparitions date from the dawn of astronomic history.
-The earliest chronicled is found in the Chinese Annals of 134 B.C. It
-shone out in Scorpio and was probably the new star which Pliny tells us
-incited Hipparchus, “The Father of Astronomy,” to make his celebrated
-catalogue of stars. From this time down we have recorded instances of
-like character.
-
-One of the most famous was the “Pilgrim Star” of Tycho Brahe. That
-astronomer has left us a full account of it. “While I was living,” he
-tells us, “with my uncle in the monastery of Hearitzwadt, on quitting
-my chemical laboratory one evening, I raised my eyes to the well-known
-vault of heaven and observed, with indescribable astonishment, near the
-zenith, in Cassiopeia, a radiant fixed star of a magnitude never before
-seen. In my amazement I doubted the evidence of my senses. However,
-to convince myself that it was no illusion, and to have the testimony
-of others, I summoned my assistants from the laboratory and inquired
-of them, and of all the country people that passed by, if they also
-observed the star that had thus suddenly burst forth. I subsequently
-heard that in Germany wagoners and other common people first called the
-attention of astronomers to this great phenomenon in the heavens,—a
-circumstance which, as in the case of non-predicted comets, furnished
-fresh occasion for the usual raillery at the expense of the learned.”
-
-The new star, he informs us, was just like all other fixed stars, but
-as bright as Venus at her brightest. Those gifted with keen sight could
-discern it in the daytime and even at noon. It soon began to wane.
-In December, 1572, it resembled Jupiter, and a year and three months
-later had sunk beyond recognition to the naked eye. It changed color
-as it did so, passing from white through yellow to red. In May, 1573,
-it returned to yellow (“the hue of Saturn,” he expressly states), and
-so remained till it disappeared from sight, scintillating strongly in
-proportion to its faintness.
-
-Thirty-two years later another stranger appeared and was seen by
-Kepler, who wrote a paper about it entitled “The New Star in the Foot
-of the Serpent.” It shone out in the same sudden manner and faded in
-the same leisurely way.
-
-Since 1860 there have been several such apparitions, and since 1876
-it has been possible to study them with the spectroscope, which has
-immensely increased our knowledge of their constitution. Indeed, this
-instrument of research has really opened our eyes to what they are.
-Nova Cygni, in 1876, Nova Aurigæ, in 1892, and Nova Persei, in 1901,
-besides several others found by Mrs. Fleming on the Arequipa plates,
-were excellent examples, and all agreed in their main features,
-showing that novæ constitute a type of stars by themselves, whose
-appearing in the first place and whose behavior afterwards prove them
-to have started from like cause and to have pursued parallel lines of
-development.
-
-As a typical case we may review the history of Nova Aurigæ. On February
-1, 1892, an anonymous post-card was received by Dr. Copeland of the
-Royal Observatory, Edinburgh, that read as follows: “Nova in Aurigæ.
-In Milky Way, about 2° south of χ Aurigæ, preceding 26 Aurigæ. Fifth
-magnitude slightly brighter than χ.” The observatory staff at once
-looked for the nova and easily found it with an opera glass. They then
-examined it through a prism placed before their 24-inch reflector and
-found its spectrum. It proved to be that of a “blaze star.”
-
-Dr. Thomas D. Anderson turned out to be the writer of the anonymous
-post-card—his name modestly self-obliterated by the nova’s light.
-He had detected the star on January 24, but had only verified it as
-a new one on the 31st. Harvard College Observatory then looked up
-its archived plates. The plates showed that it had appeared sometime
-between December 1 and 10. Its maximum had been attained on December
-20, after which it declined, to record apparently another maximum on
-February 3 of the 3.5 magnitude. From this time its light steadily
-waned till on April 1 it was only of the 16th magnitude or ¹/₁₀₀₀₀₀ of
-what it had been. In August it brightened again and then waned once
-more.
-
-Meanwhile its spectrum underwent equally strange fluctuations. At
-first it exhibited the bright lines characteristic of the flaming red
-solar prominences, the calcium, hydrogen, and helium lines flanked by
-their dark correlatives upon a continuous background, showing that
-both glowing and cooler gases were here concerned. The sodium lines,
-too, appeared, like those that come out in comets as they approach the
-furnace of the Sun. An outburst such as occurs in miniature in the
-solar chromosphere or outermost gaseous layer of the Sun was here going
-on upon a gigantic scale. A veritable spectral chaos next supervened,
-staying until the star had practically faded away. Then, on its
-reappearance, in August, Holden, Schaeberle, and Campbell discovered
-to their surprise not what had been at all, but something utterly new:
-the soberly bright lines only of a nebula. Finally, ten years later,
-January, 1902, Campbell found its spectrum had become continuous, the
-body having reverted to the condition of a star.
-
-Now how are we to interpret these grandiose vicissitudes, visually and
-spectrally revealed? That we witnessed some great catastrophe is clear.
-The sudden increase of light of many thousand fold from invisibility to
-prominence shows that a tremendous cataclysm occurred. The bright lines
-in the spectrum confirm it and imply that vast upheavals like those
-that shake the Sun were there in progress, but on so stupendous a scale
-that, if for no other reason, we must dismiss the idea that explosions
-alone can possibly be concerned. The dark correlatives of the bright
-lines have been interpreted as indicating that two bodies were
-concerned, each travelling at velocities of hundreds of miles a second.
-But in Nova Aurigæ shiftings of the spectral lines implying six bodies
-at least were recorded, if such be attributed to motion in the line of
-sight, and Vogel was minded to throw in a few planets as well—as Miss
-Clerke pithily puts it. There is not room for so many on the stage of
-the cosmic drama. Other causes, as we now know, may also displace the
-spectral lines. Great pressure has been shown to do it, thanks to the
-labors of Humphreys and Mohler at Baltimore. “Anomalous refraction” may
-do it, as Professor Julius of Utrecht has found out. Finally, changes
-of density may produce it, as Michelson has discovered. To these
-causes we may confidently ascribe most of the shiftings in the stellar
-spectrum, for just such forces must be there at work.
-
-Mr. Monck suggested the idea that new stars are the result of old dark
-stars rushing through gaseous fields in space and rendered luminous
-by the encounter. Seeliger revived and developed this idea, which in
-certain cases is undoubtedly the truth. Probably this occurred to the
-new star of 1885 which suddenly blazed out almost in the centre of the
-great nebula in Andromeda. It behaved like a typical nova and in due
-course faded to indistinguishability. Something like it happened, too,
-in the nova of 1860, which suddenly flared up in the star cluster 80
-Messier, outdoing in lustre the cluster itself, and then, too, faded
-away.
-
-But just as psychology teaches us that not only do we cry because we
-are sorrowful, but that we are sorrowful because we cry, so while a
-nova may be made by a nebula, no less may a nebula be made by a star.
-
-Let us see how this might be brought about and what sign manuals it
-would present. Suppose that the two bodies actually grazed. Then the
-disruption would affect the star’s cuticle, first raising the outer
-parts, consisting rather of carbon than of the metals, since that
-substance is the lighter, to intense heat and the gases about it at
-the same time. The glowing carbon would be intensely bright, and at
-first its light would overpower that from the gases, and not till its
-great glow had partially subsided would theirs be seen. Then the gases,
-hydrogen, helium, and so forth, would make themselves evident. Finally
-only the most tenuous ones, those peculiar to a nebula, would remain
-visible. After which the more solid particles due to the disruption
-would fall together and light up again by their individual collisions.
-Much the same would result if without striking the stars passed close.
-
-[Illustration: 1901 February 20th 1901 February 28th
-
-Before appearance of Nova The Nova
-
-NOVA PERSEI. Photographs by A. STANLEY WILLIAMS, Hove, Sussex.]
-
-[Illustration: SPECTRUM OF NOVA PERSEI. (F. Ellerman, 40 in. Yerkes.)]
-
-Now to put this theory to the proof. In the early morning of the 22d of
-February, 1901, Dr. Anderson, the discoverer of Nova Aurigæ, perceived
-that Algol had a neighbor, a star as bright as itself, which had never
-been there before. Within twenty-four hours of its detection the
-newcomer rivalled Capella, and shortly after took rank as the premier
-star of the northern hemisphere. Its spectrum on the 22d was found at
-Harvard College Observatory to be like that of Rigel, a continuous one
-crossed by some thirty faint dark lines. On the 24th, however, _so soon
-as it began to wane_, the bright lines of hydrogen were conspicuous
-with their dark correlatives, just as they had been with Nova Aurigæ
-and other novæ. At the same time each particular spectral line proved a
-law unto itself, some shifted more than others, thus negativing motion
-as their only cause and indicating change of pressure or density as
-concerned concomitants of the affair. Blue emissions like those of
-Wolf-Rayet stars next made their appearance; then a band, found by
-Wright at the Lick to characterize nebulæ, shone out, and finally in
-July the change to a nebular spectrum stood complete.
-
-[Illustration: THE MOVING NEBULA SURROUNDING NOVA PERSEI.
-
-1901, September 20th. 1901, November 13th.
-
-Drawn by G. W. RITCHEY, from Photographs taken with the 24-in.
-Reflector, YERKES OBSERVATORY.]
-
-Then came what is the most suggestive feature in the whole event. On
-August 22 and 23 Dr. Wolf at Königstahl took with his then new Bruce
-objective some long exposure plates of the nova, and on them found, to
-his surprise, wisps of nebulous matter to the southeast of the star. On
-September 20 Ritchey, with a two-foot mirror of his own constructing
-exposed for four hours, brought the whole formation to light. It turned
-out to be a spiral nebula encircling and apparently emanating from
-the star. Its connection with the nova was patent. But there was more
-to come. Later plates taken at the Lick on November 7 disclosed the
-startling fact that the nebula was visibly expanding, uncoiling outward
-from the star. A plate by Ritchey on November 13 confirmed this, and
-still later plates by him in December, January, and February showed the
-motion to be progressive. At the same time the star showed no parallax,
-and the speed of the motion seemed thus to be indicated as enormous.
-Kapteyn suggested to account for it that appearance, not reality, was
-here concerned; that the nebula had always existed, and was only shown
-up by the light from the conflagration travelling outward from the nova
-at the rate of one hundred and eighty-six thousand miles a second. This
-would make the catastrophe to have occurred as far back as the time of
-James I, of which the news more truthful but less timely than that of
-the morning papers had only just reached us.
-
-[Illustration: December 14, 1901.]
-
-[Illustration: January 7 and 9, 1902.]
-
-[Illustration: 1902. February 8, 1902.
-
-THE MOVING NEBULA SURROUNDING NOVA PERSEI—AFTER RITCHEY.]
-
-But a little of that simple reasoning by which Zadig recovered the
-lost horses of the Sultan, and which from its unaccustomedness in the
-affairs of men got him suspected of having stolen them and very nearly
-caused his death, will show the untenableness of this idea and help us
-to a solution. In the first place we note that the star holds the very
-centre of the nebular stage, a remarkable prominence if the star has
-no creative right to the position. Then the same knots and patches of
-the nebulous configuration are visible in all the photographs, in the
-same relative positions, turned through corresponding angles as one
-will see for himself, all having moved symmetrically from one date to
-another. At the truly marvellous mimicry implied if different objects
-were concerned common sense instinctively shies, and very properly,
-as the chances against it are millions to one. Clearly it was not a
-mere matter of ethereal motion, but a very material motion of matter,
-which was here concerned. Something corpuscular emanating from the nova
-spread outward into space.
-
-Clinching this conclusion is the result of a search by Perrine for
-traces of the nebula on earlier plates. For on one taken by him on
-March 29 (1901) he found the process already started in two close
-coils, its conception thus clearly dating from the time of the star’s
-outburst. In Nova Persei, then, we actually witnessed a spiral nebula
-evolved from a disrupted star.
-
-What was this ejectum and what drove it forth? Professor Very regarded
-it as composed of corpuscles such as give rise to cathode rays
-discharged from the star under the stress of light pressure or electric
-repulsion. But I think we may see in it something simpler still; to
-wit, gaseous molecules driven off by light pressure alone—the smoke,
-as one may say, of the catastrophe—akin exactly to the constituents of
-comet’s tails. The mere light of the conflagration pushed the hydrogen
-molecules away. This would explain their presence and their exceeding
-hurry at the same time. They were started on their travels by domestic
-jars and kept going by the vivid after-effects of that infelicity.
-
-The fairly steady rate of regression from the nova observed may be
-explained by the observed decrease in the light of the repellent
-source. Such combined with the retarding effect of gravity might make
-the regression equable. This is the more explanatory as the speed was
-certainly much less than that of light, though greatly exceeding any
-possible from the direct disruption. At the same time both the bright
-and the dark lines of hydrogen seen in the spectrum stand accounted
-for; the colliding molecules, at their starting on their travels
-from the star, shining through their sparser fellows farther out. An
-interesting biograph of the levity of light!
-
-Nova Persei thus introduces us at its birth to one of a class of most
-interesting objects comparatively recently discovered and of most
-pregnant import,—the spiral nebulæ.
-
-[Illustration: GREAT NEBULA IN ORION—AFTER RITCHEY.]
-
-[Illustration: GREAT NEBULA IN ANDROMEDA—AFTER RITCHEY.]
-
-[Illustration: NEBULA M. 100 COMÆ—AFTER ROBERTS.]
-
-In 1843 when Lord Rosse’s giant speculum, six feet across, was turned
-upon the sky, a nebula was brought to light which was unlike any ever
-before seen. It was neither irregular like the great nebula in Orion
-nor round like the so called planetary nebulæ,—the two great classes
-at that time known,—but exhibited a striking spiral structure. It
-proved the forerunner of a remarkable revelation. For the specimen
-thus disclosed has turned out to typify not only the most interesting
-form of those heavenly wreaths of light, but by far the commonest as
-well. As telescopic and especially photographic means improved, the
-number of such objects detected steadily increased until about thirteen
-years ago Keeler by his systematic discoveries of them came to the
-conclusion that a spiral structure pervaded the great majority of all
-the nebulæ visible. Their relative universality was outdone only by
-the invariability of their form. For they all represent spirals of one
-type: two coiled arms radiating diametrically from a central nucleus
-and dilating outward. Even nebulæ not originally supposed spiral have
-disclosed on better revelation the dominant form. Thus the great nebula
-in Andromeda formerly thought lens-shaped proves to be a huge spiral
-coiled in a plane not many degrees inclined to the plane of sight.
-
-[Illustration: NEBULA ♅ I. 226 URSÆ MAJORIS—AFTER ROBERTS.]
-
-As should happen if the spirals are unrelated, left-handed and
-right-handed ones are about equally common. In Dr. Roberts’ great
-collection of those in which the structure is distinctly discernible,
-nine are right-handed, ten left-handed, showing that they partake of
-the ambidextrous impartiality of space.
-
-[Illustration: NEBULA ♅ V. 24 COMÆ—AFTER ROBERTS.
-
-Showing globular structure.]
-
-Lastly the spirals are evidently thicker near the centre, thinning out
-at the edge, and when the central nucleus is pronounced, it seems to
-have a certain globularity not shared by the arms, and more or less
-detached from them. This appears in those cases where they are shown us
-edgewise, and it has been thought perceptible in the great nebula of
-Andromeda. The difficulty in establishing the phenomenon comes from the
-impossibility of both features showing at their best together. For the
-globularity to come out well, the spiral must be presented to us nearly
-in the plane of sight; for the spirality, in a plane at right angles to
-it.
-
-Much may be learnt by pondering on these peculiarities. The widespread
-character of the phenomenon points to some universal law. We are here
-clearly confronted by the embodiment of a great cosmic principle,
-causing the helices it is for us to uncoil. It is a problem in
-mechanics.
-
-In the first place, a spiral structure denotes action on the face
-of it. It implies a rotation combined with motion out or in. We are
-familiar with the fact in the sparks of pin-wheel pyrotechnics. Any
-rotating fluid urged by an outward or an inward impulse must take the
-spiral form. A common example occurs in the water let out of a basin
-through a hole in the centre when we draw out the plug. Here the
-force is inward, and because the bowl and orifice are not perfectly
-symmetric, a rotation is set up in the water trying to escape, and the
-two combine to give us a beautiful conchoidal swirl. In this case the
-particles seek the centre, but the same general shape is assumed when
-they seek to leave it.
-
-Another point to be noticed is that a spiral nebula could not develop
-of itself and subsist. To continue it must have outside help. For if
-it were due to internal explosive action in the pristine body, each
-ejectum must return to the point it started from, or else depart
-forever into space, for the orbit it would describe must either be
-closed or unclosed. If the former, it would revisit its starting-point;
-if the latter, it would never return. Explosion, therefore, of itself
-could not have produced the forms we see, unless they be ephemeral
-apparitions, a supposition their presence throughout the heavens seems
-effectually to exclude.
-
-[Illustration: NEBULA M. 101 URSÆ MAJORIS—AFTER RITCHEY.]
-
-The form of the spiral nebulæ proclaims their motion, but one of its
-particular features discloses more. For it implies the past cause which
-set this motion going. A distinctive detail of these spirals, which so
-far as we know is shared by all of them, are the two arms which leave
-the centre from diametrically opposite sides. This indicates that the
-outward driving force acted only in two places, the one the antipodes
-of the other. Now what kind of force is capable of this peculiar
-effect? If we think of the matter, we shall realize that tidal action
-would produce just this result. We see it daily in the case of the
-Moon; when it is high tide in the open ocean hereabouts, it is high
-tide also at the opposite end of the Earth. The reason is that the
-tideraising body pulls the fluid nearest it more strongly than it pulls
-the Earth as a whole, and pulls the Earth as a whole more than it pulls
-the fluid at the opposite extremity.
-
-Suppose, now, a stranger to approach a body in space near enough; it
-will inevitably raise tides in the other’s mass, and if the approach be
-very close, the tides will be so great as to tear the body in pieces
-along the line due to their action; that is, parts of the body will
-be separated from the main mass in two antipodal directions. This is
-precisely what we see in the spiral nebula. Nor is there any other
-action that we know of which would thus handle the body. If it were
-to disintegrate under increased speed of rotation due to contraction
-upon itself, parts of its periphery should be shed continually and
-a pin-wheel of matter, not a two-armed spiral, be thrown off. If
-explosion were the disintegrating cause, disruption would occur
-unsymmetrically in one or more directions, not symmetrically as here.
-
-[Illustration: REPRESENTATIVE STELLAR SPECTRA
-
-_Photographed, in 1907 and 1908, by_ V. M. SLIPHER, _at_ LOWELL
-OBSERVATORY _Flagstaff, Arizona, with prism spectrograph._]
-
-As the stranger passed on, his effect would diminish until his
-attraction no longer overbalanced that of the body for its disrupted
-portions. These might then be controlled and forced to move in elliptic
-orbits about the mass of which they had originally made part. Thence
-would come into being a solar system, the knots in the nebula going to
-form the planets that were to be.
-
-Before proceeding to what proof we have that it actually did occur in
-this way we may pause to consider some consequences of what we have
-already learned. Thus what brought about the beginning of the system
-may also compass its end. If one random encounter took place in the
-past, a second is as likely to occur in the future. Another celestial
-body may any day run into the Sun, and it is to a dark body that we
-must look for such destruction, because they are so much more numerous
-in space.
-
-That any of the lucent stars, the stars commonly so called, could
-collide with the Sun, or come near enough to amount to the same thing,
-is demonstrably impossible for æons of years. But this is far from the
-case for a dark star. Such a body might well be within a hundredth of
-the distance of the nearest of our known neighbors, Alpha Centauri, at
-the present moment without our being aware of it at all. Our senses
-could only be cognizant of its proximity by the borrowed light it
-reflected from our own Sun. Dark in itself, our own head-lights alone
-would show it up when close upon us. It would loom out of the void thus
-suddenly before the crash.
-
-We can calculate how much warning we should have of the coming
-catastrophe. The Sun with its retinue is speeding through space at
-the rate of eleven miles a second toward a point near the bright star
-Vega. Since the tramp would probably also be in motion with a speed
-comparable with our own, it might hit us coming from any point in
-space, the likelihood depending upon the direction and amount of its
-own speed. So that at the present moment such a body may be in any
-part of the sky. But the chances are greatest if it be coming from the
-direction toward which the sun is travelling, since it would then be
-approaching us head on. If it were travelling itself as fast as the
-Sun, its relative speed of approach would be twenty-two miles a second.
-
-The previousness of the warning would depend upon the stranger’s size.
-The warning would be long according as the stranger was large. Let
-us assume it the mass of the Sun, a most probable supposition. Being
-dark, it must have cooled to a solid, and its density therefore be much
-greater than the Sun’s, probably something like eight times as great,
-giving it a diameter about half his or four hundred and thirty thousand
-miles. Its apparent brightness would depend both upon its distance and
-upon its intrinsic brightness or albedo, and this last would itself
-vary according to its distance from the Sun. While it was still in the
-depths of space and its atmosphere lay inert, owing to the cold there,
-its intrinsic brightness might be that of the Moon or Mercury. As its
-own rotation would greatly affect the speed with which its sunward side
-was warmed, we can form no exact idea of the law of its increase in
-light. That the augmentation would be great we see from the behavior of
-comets as they approach the great hearth of our solar system. But we
-are not called upon to evaluate the question to that nicety. We shall
-assume, therefore, that its brilliancy would be only that of the Moon,
-remembering that the last stages of its fateful journey would be much
-more resplendently set off.
-
-With these data we can find how long it would be visible before
-the collision occurred. As a very small telescopic star it would
-undoubtedly escape detection. It is not likely that the stranger
-would be noticed simply from its appearance until it had attained
-the eleventh magnitude. It would then be one hundred and forty-nine
-astronomical units from the Sun or at five times the distance of
-Neptune. But its detection would come about not through the eye of
-the body, but through the eye of the mind. Long before it could have
-attracted man’s attention to itself directly its effects would have
-betrayed it. Previous, indeed, to its possible showing in any telescope
-the behavior of the outer planets of the system would have revealed
-its presence. The far plummet of man’s analysis would have sounded
-the cause of their disturbance and pointed out the point from which
-that disturbance came. Celestial mechanics would have foretold, as
-once the discovery of another planet, so now the end of the world.
-Unexplained perturbations in the motions of the planets, the far
-tremors of its coming, would have spoken to astronomers as the first
-heralding of the stranger and of the destruction it was about to bring.
-Neptune and Uranus would begin to deviate from their prescribed paths
-in a manner not to be accounted for except by the action of some new
-force. Their perturbations would resemble those caused by an unknown
-exterior planet, but with this difference that the period of the
-disturbance would be exactly that of the disturbed planet’s own period
-of revolution round the Sun.
-
-Our exterior sentinels might fail thus to give us warning of the
-foreign body because of being at the time in the opposite parts of
-their orbits. We should then be first apprised of its coming by Saturn,
-which would give us less prefatory notice.
-
-It would be some twenty-seven years from the time it entered the range
-of vision of our present telescopes before it rose to that of the
-unarmed eye. It would then have reached forty-nine astronomical units’
-distance, or two-thirds as far again as Neptune. From here, however,
-its approach would be more rapid. Humanity by this time would have been
-made acquainted with its sinister intent from astronomic calculation,
-and would watch its slow gaining in conspicuousness with ever growing
-alarm. During the next three years it would have ominously increased
-to a first magnitude star, and two years and three months more have
-reached the distance of Jupiter and surpassed by far in lustre Venus at
-her brightest.
-
-Meanwhile the disturbance occasioned not simply in the outer planets
-but in our own Earth would have become very alarming indeed. The
-seasons would have been already greatly changed, and the year itself
-lengthened, and all these changes fraught with danger to everything
-upon the Earth’s face would momentarily grow worse. In one hundred and
-forty-five days from the time it passed the distance of Jupiter it
-would reach the distance of the Earth. Coming from Vega, it would not
-hit the Earth or any of the outer planets, as the Sun’s way is inclined
-to the planetary planes by some sixty degrees, but the effects would be
-none the less marked for that. Day and night alone of our astronomic
-relations would remain. It would be like going mad and yet remaining
-conscious of the fact. Instead of following the Sun we should now in
-whole or part, according to the direction of its approach, obey the
-stranger. For nineteen more days this frightful chaos would continue;
-as like some comet glorified a thousand fold the tramp dropped silently
-upon the Sun. Toward the close of the nineteenth day the catastrophe
-would occur, and almost in merciful deliverance from the already
-chaotic cataclysm and the yet greater horror of its contemplation, we
-should know no more.
-
-Unless the universe is otherwise articulated than we have reason to
-suppose, such a catastrophe sometime seems certain. But we may bear
-ourselves with equanimity in its prospect for two mitigating details.
-One is that there is no sign whatever at the moment that any such
-stranger is near. The unaccounted-for errors in the planetary theories
-are not such as point to the advent of any tramp. Another is, that
-judged by any scale of time we know, the chance of such occurrence
-is immeasurably remote. Not only may each of us rest content in the
-thought that he will die from causes of his own choosing or neglect,
-but the Earth herself will cease to be a possible abode of life, and
-even the Sun will have become cold and dark and dead so long before
-that day arrives that when the final shock shall come, it will be quite
-ready for another resurrection.
-
-
-
-
-CHAPTER II
-
-EVIDENCE OF THE INITIAL CATASTROPHE IN OUR OWN CASE
-
-
-By quite another class of dark bodies than those we contemplated in
-the last chapter is the immediate space about us tenanted. For that,
-too, is anything but the void our senses give us to understand.
-Could we rise a hundred miles above the Earth’s surface we should be
-highly sorry we came, for we should incontinently be killed by flying
-brickbats. Instead of masses of a sunlike size we should have to do
-with bits of matter on the average smaller than ourselves but hardly on
-that account innocuous, as they would strike us with fifteen hundred
-times the speed of an express train. Only in one respect are the two
-classes of erratics alike, both remain invisible till they are upon
-us. Even so, the cause of their visibility is different. The one is
-announced by the light it reflects, the other by the glow it gives out
-on its destruction. These last are the meteorites or shooting-stars.
-They are as well known to every one for their commonness as,
-fortunately, the first are rare. On any starlight night one need not
-tarry long before one of these visitants darts across the sky, a
-brilliant thread of fire gone almost ere it be descried.
-
-Usually this is all of which one is made aware. Silent, ghostlike, the
-apparition comes and goes, and nothing more of it is either seen or
-heard. But sometimes there is a good deal more. Occasionally a large
-ball of flame shoots through the air, a detonation like distant thunder
-startles the ear, and a luminous train, persisting for several seconds,
-floats slowly away. Finally if one be fortunate to be near,—but not too
-near,—one or more masses of stone are seen to fall swiftly and bury
-themselves in the ground. These are meteorites: far wanderers come at
-last to rest in graves they have dug themselves.
-
-A great revolution has taken place lately in our ideas concerning
-meteorites. Indeed, it was not so very long ago, since modern man
-admitted their astronomic character at all. He looked as askance at
-them as he did at fossils. It was the fall at Aigle, in Switzerland,
-April 26, 1803, that first opened men’s eyes to the fact that such
-falls actually occurred. It is more than a nine days’ wonder at times
-how long men, as well as puppies, can remain blind. To admit that
-stones fell from heaven, however, was not to see whence they came.
-Their paternity was imputed to nearly every body in the sky. They were
-at first supposed to have been ejected from earthly volcanic vents,
-then from volcanoes in the Moon. That they are of domestic manufacture
-is, however, negatived by the paths they severally pursue. Nor can they
-for like reason have been ejected from the Sun.
-
-The Earth was not their birthplace. It is alien ground in which they
-lie at last and from which we transfer them to glass cases in our
-museums. This fact about their parentage they tell by the speed with
-which they enter our air. They become visible 100 miles up and explode
-at from 20 to 10, and their speed has been found to be from 10 to 40
-miles a second, which is that of cosmic bodies moving in large elliptic
-orbits about the Sun,—a speed greater than the Earth could ever have
-imparted.
-
-Four classes of such small celestial bodies tenant space where the
-planets move: sporadic shooting-stars, meteorites, meteor-streams, and
-comets. The discovery of the relation of each of these to the solar
-system and then to each other forms one of the latest chapters of
-astronomic history. For they turn out to be generically one.
-
-It was long, however, before this was perceived. The first step was
-taken simultaneously by Professor Olmstead of Yale and Twining in 1833
-from reasoning on the superb November meteor-shower of that year. All
-the shooting-stars, “thick as snowflakes in a storm,” had a common
-radiant from which they seemed to come. Thus they argued that the
-meteors must all be travelling in parallel lines along an orbit which
-the previous shower, of 1799, showed to be periodic. This was the first
-recognition of a meteor-swarm.
-
-The next advance was when Schiaparelli, in 1862, pointed out the
-remarkable connection between meteor-swarms and comets. On calculation
-the August meteor-stream and the comet of 1862 proved to be pursuing
-exactly the same path. Soon other instances of like association were
-discovered, and we now know mathematically that meteor-streams can
-be, deductively that they must be, and observationally that they are,
-disintegrated comets. More than one comet has even been seen to split.
-
-Then came the recognition that comets are not visitors from space, as
-Sir Isaac Newton and Laplace supposed, but part and parcel of our own
-solar system. Without going into the history of the subject, which
-includes Gauss, Schiaparelli, and finally Fabry’s great Memoir, much
-too little known, the proof can, I think, be made comprehensible
-without too much technique, thanks to the fact that the Sun is speeding
-through space at the rate of eleven miles a second.
-
-Orbits described by bodies under the action of a central force are
-always conic sections, as Sir Isaac Newton proved. There are two
-classes of such curves: those which return into themselves, such as
-the circle and ellipse, and those which do not, the hyperbolæ. If a
-body travel in the first or closed class about the Sun, it is clearly
-a member of his family; if in the second, it is a visitor who bows to
-him only in passing and never returns. Which orbit it shall pursue
-depends at a given distance solely upon the speed of the body; if that
-speed be one the Sun can control, the body will move in an ellipse;
-if greater, in an hyperbola. Obviously the Sun can control just the
-speed he can impart. Now a comet entering the system from without
-would already possess a motion of its own which, when compounded with
-the solar-acquired speed, would make one greater than the Sun could
-master. Comets, therefore, if visitors from space, should all move
-in hyperbolæ. None for certain do; and only six out of four hundred
-even hint at it. Comets, then, are all members of the solar family,
-excentric ones, but not to be denied recognition of kinship for such
-behavior.
-
-Still, admittance to the solar family circle was denied to meteorites
-and shooting-stars. Thus Professor Kirkwood, in 1861, had considered
-“that the motions of some luminous meteors (or cometoids, as perhaps
-they might be called) have been decidedly indicative of an origin
-beyond the limits of the solar system.” Here cometoid was an apt
-coinage, but when comets were later shown not to be of extra-solar
-origin, the reasoning carried luminous meteors in its train.[1] Finally
-Schiaparelli, in 1871, concluded an able Memoir on the subject with the
-decision that “a stellar origin for meteorites was the most likely and
-that meteorites were identifiable with shooting-stars.”[2] A pregnant
-remark this, though not exactly as the author thought, for instead of
-proving both interstellar, as he intended, both have proved to be solar
-bound.
-
-[1] “Mem. del Reale Inst. Lombardo,” Vol. XII. III della serie III.
-
-[2] Quoted in “Luminous Meteors,” Committee’s Report for 1870-1871, p.
-48.
-
-It was Professor Newton, in 1889, who first showed that meteorites
-were pursuing, as a rule, small elliptic orbits about the Sun, and
-that their motion was direct. He, too, was the first to surmise that
-meteorites are but bigger shooting-stars.
-
-Now, as to their connection. Of direct evidence we have little. A
-few meteors have been observed to come from the known radiants of
-shooting-stars. Two instances we have of the fall of meteorites during
-star showers. One in 1095, when the Saxon Chronicle tells us stars
-fell “so thickly that no man could count them, one of which struck the
-ground and when a bystander cast water upon it steam was raised with a
-great noise of boiling.” The second case was the fall of a siderite,
-eight pounds’ worth of nickel-iron, at Mazapil during the Andromede
-shower of 1885, which was by many supposed to be a part of the lost
-Biela comet. It contained graphite enough to pencil its own history,
-but unfortunately could not write. The direction from which it came was
-not recorded, and so the connection between it and the comet not made
-out.
-
-[Illustration: THE RADIANT OF A METEORIC SHOWER, SHOWING ALSO THE PATHS
-OF THREE METEORS WHICH DO NOT BELONG TO THIS SHOWER—AFTER DENNING.]
-
-If our direct knowledge is thus scanty, reasoning affords surer ground
-for belief. For at this point there steps in a bit of news about the
-family relations of shooting-stars from a source hardly to have been
-anticipated. Indeed, it arose from the thought to examine a qualitative
-statement in Young’s “Astronomy” quantitatively. Mathematics is simply
-precise reasoning, applied usually to the discovery that a pet theory
-will not work. But sometimes it presents one with an unexpected find.
-This is what it did here.
-
-It is an interesting fact of observation that more meteors are visible
-at six o’clock in the morning than at six o’clock at night in the
-proportion of 3 to 1. This seeming preference for early rising is due
-to no matutinality on the part of the meteors, but to the matin aspect
-then presented by the Earth combined with its orbital motion round the
-Sun. For at six in the morning the observer stands on the advancing
-side of the Earth, at the bow of the airship; at six at night he is at
-the stern. He, therefore, runs into the meteors at sunrise and slips
-away from them at sunset. He is pelted in the morning in consequence.
-Just as a pedestrian facing a storm gets wetter in front than behind.
-
-[Illustration: METEORS
-
-Diagram explaining their proportionate visibility.
-
-    ——————        _denotes true paths._
-    —— - ——           ”    _apparent paths._
-    —————— - - -      ”    _Earth’s path._]
-
-So far the books. Now let us examine this quantitatively according to
-the direction in which the meteors themselves may be moving before
-the encounter. Suppose, in the first place, that they were travelling
-in every possible direction, with the average velocity of the most
-erratic members of the family, the great comets. On this supposition
-calculation shows that we ought to meet 5.8 times as many at six in the
-morning as at six at night. If their orbits were smaller than this,
-say, something like those of the asteroids, we should find 7.6 to 1 for
-the ratio.
-
-Suppose, however, that they were all travelling in the same sense as
-the Earth, direct as it is called in contradistinction to retrograde,
-and let us calculate what proportion in that case we should meet at the
-two hours respectively. It turns out to be 2.4 to 1 for the parabolic
-ones, 3.3 to 1 for the smaller orbited, or almost precisely what
-observation shows to be the case [see NOTE 1]. Here, then, a bit of
-abstract reasoning has apprized us of a most interesting family fact;
-to wit, that the great majority of shooting-stars are travelling in the
-same orderly sense as ourselves. Furthermore, as some must be moving
-in smaller orbits than the mean, others must be journeying in greater;
-or, in other words, shooting-stars are scattered throughout the system.
-In short, these little bodies are tiny planets themselves, as truly
-planets as the asteroids,—asteroids of a general instead of a localized
-habit.
-
-Thus meteorites and shooting-stars are kin, and from the fact that
-they are pursuing orbits not very unlike our own we get our initial
-hint of a community of origin. Indeed, they are the little bricks out
-of which the whole structure of our solar system was built up. What
-we encounter to-day are the left-over fragments of what once was, the
-fraction that has not as yet been swept up by the larger bodies. And
-this is why these latter-day survivors move, as a rule, direct. To run
-counter to the consensus of trend is to be subjected to greater chance
-of extermination. Those that did so have already been weeded out.
-
-[Illustration: THE MART IRON.
-
-(_Proc. Wash. Acad. of Sci._ vol. II. Plate VI.)]
-
-From the behavior of meteorites we proceed to scan their appearance.
-And here we notice some further telltale facts about them. Their
-conduct informed us of their relationship, their character bespeaks
-their parentage.
-
-Most meteorites are stones, but one or two per cent are nearly pure
-iron mixed with nickel. When picked up, they are usually covered with
-a glossy thin black crust. This overcoat they have put on in coming
-through our air. Air-begotten, too, are the holes with which many of
-them are pitted. For entering our atmosphere with their speed in space
-is equivalent to immersing them suddenly in a blowpipe flame of several
-thousand degrees Fahrenheit. Thus their surface is burnt and fused to
-a cinder. Yet in spite of being warm to the touch their hearts are
-still cosmically cold. The Dhurmsala meteorite falling into moist earth
-was found an hour afterwards coated with frost. Agassiz likened it to
-the Chinese culinary _chef d’œuvre_ “fried ice.” It is the cold of
-space, 200° or more Centigrade below zero, that they bear within, proof
-of their cosmic habitat.
-
-That they are bits of a once larger mass is evident on their face.
-Their shape shows that they are not wholes but parts, while their
-constitution bespeaks them anything but elementary. Diagnosis of it
-yields perhaps their most interesting bit of news. For it shows their
-origin. Their autopsy proves them to contain thirty known elements,
-and not one that is new. The list includes all the substances most
-common on the Earth’s surface, which is suggestive; but, what is still
-more instructive, these are combined into minerals which largely
-differ from those with which we are superficially familiar. Professor
-Newton, whose specialty they were, has said: “In general they show
-no resemblance in their mechanical or mineralogical structure to the
-granitic and surface rocks of the Earth. One condition was certainly
-necessary in their formation, viz. the absence of free oxygen and of
-enough water to oxidize the iron.” Thus they are not of the Earth
-earthy; nor yet, poor little waifs, of the upper crust of any other
-body.
-
-[Illustration: SECTION OF METEORITE SHOWING WIDMANNSTÄTTIAN LINES.
-
-(Field Columbian Museum, Chicago.)]
-
-[Illustration: METEORITE, TOLUCA.
-
-(Field Columbian Museum, Chicago.)]
-
-In them prove to be occluded gases, which can be got out by heating in
-the laboratory, and which must have got in when the meteorites were
-still subjected to great heat and pressure. For only thus could these
-gases have been absorbed. Both such heat and such pressure accuse some
-great solid body as origin of this flotsam of the sky. Fragments now,
-they owe to its disruption their present separate state. This parent
-mass must have been much larger and more massive than the Earth, as the
-grate amount of occluded hydrogen, sometimes one-third the volume at
-500° C., of the meteorite seems to testify.
-
-The two classes of meteorites, the stone and the iron, show this
-further by the very differences they exhibit between themselves. For
-both the amount and the proportions of the occluded gases in the two
-prove to be quite distinct. In the stones the quantity of gas is
-greater and the composition is diverse. In the stones carbonic acid gas
-is common, carbon monoxide rare; in the irons the ratio is just the
-other way. Thus Wright found in nine specimens of the iron meteorites:—
-
-     CO₂     CO      H      CH₄
-    11.5%   32.4%   54.1%   00% of the total;
-
-in ten of stone:—
-
-     CO₂      CO      H      CH₄
-    60.1%    3.4%   32.0%   2.1%
-
-The stones are much lighter than the iron, their specific gravities
-being as 3 to 7 or 8 for the metallic. The stones, therefore, came from
-a more superficial layer of the body torn apart than the iron, and the
-composition of their occluded gases bears this out. Those in the stones
-are such as we may conceive absorbed nearer the surface, those in the
-iron from regions deeper down.
-
-Here, then, the meteorites tell us of another, an earlier, stage of
-our solar system’s history, one that mounts back to before even the
-nebula arose to which we owe our birth. For the large body to whose
-dismemberment the meteorites were due can have been no other than the
-one whose cataclysmic shattering produced that very nebula which was
-for us the origin of things. The meteorites, by continuing unchanged,
-link the present to that far-off past. And they tell us, too, that
-this body must have been dark. For solid, they inform us, it was, and
-solidity in a heavenly body means deficiency of light.
-
-That such corroborative testimony to a cataclysmic origin is
-forthcoming in the sky we shall see by turning again to the spiral
-nebulæ.
-
-Of the two classes of nebulæ which we contemplated in the last chapter,
-the amorphous and the structural, there is more to be said than we
-touched on then.
-
-[Illustration: NEBULA ♅ V. 14 CYGNI—AFTER ROBERTS.]
-
-Not only in look are the two quite unlike, but the spectroscope shows
-that the difference in appearance is associated with dissimilarity of
-character. For the spectrum of the amorphous proves to consist of a
-few bright lines, due to hydrogen and nebulium chiefly, in the green,
-whence the name green nebulæ. That of the spirals, on the other hand,
-is continuous, and therefore white. The great nebula in Andromeda was
-one of the first in which this was recognized; and the perception was
-pregnant, for no nebula defies resolution more determinedly than it.
-We may, therefore, infer that it is not made up of stars, certainly
-big enough for us to see. On the other hand, from the fact that its
-spectrum is continuous it must be solid or liquid. Young pointed out
-that this did not follow, because a gas under great pressure also
-gives a continuous spectrum. But he forgot that here no such pressure
-could exist. A nebula of compressed gas could not have an irregular
-form and would have, in the case of the Andromeda nebula, a mass so
-enormous as to preclude supposition. Continuity of spectrum here means
-discontinuity of mass. The spectral solidity of the nebula speaks of a
-_status quo ante_, not of a condition of condensation now going on.
-
-[Illustration: NEBULA N. G. C. 1499 PERSEI—AFTER ROBERTS.]
-
-[Illustration: NEBULA N. G. C. 6960 IN CYGNUS—AFTER RITCHEY.]
-
-Advanced spectroscopic means reveals that the spectra of these
-“white” nebulæ are not simply continuous. Thus that of the Andromeda
-nebula shows very faint dark lines crossing it, apparently accordant
-with those of the solar spectrum and faint bright ones falling near
-and probably coincident with those of the Wolf-Rayet stars, due
-to hydrogen, helium, and so forth. These later observations make
-practically certain what earlier ones permitted us just now only to
-infer: that it is not composed of stars, but of something subtler
-still; to wit, of meteorites. The reasoning is interesting, as showing
-that if one have hold of a true idea, the stars in their courses fight
-for him.
-
-[Illustration: NEBULA M. 51 CANUM VENATICORUM—AFTER RITCHEY.]
-
-Although Lockyer has long been of opinion that the nebulæ are composed
-of meteorites, the present argument differs from his. The way in which
-their spectra establish their constitution may be outlined as follows:
-the white nebulæ are from their structure evidently in process of
-evolution, and if they are in stable motion, as we suppose them to be,
-their parts are moving round their common centre of gravity. As the
-white nebulæ resist resolution as obstinately as the green, these parts
-must be not only solid but comminuted (composed of small particles).
-Now this would be the case were they flocks of meteorites such as we
-have seen composed our own system once upon a time. Though all are
-travelling round the centre of gravity of the flock, each is pursuing
-its own orbit slightly different from, and intersecting those of, its
-neighbors. Collisions between the meteors must therefore constantly
-occur, and the question is, are these shocks sufficient to cause light.
-Let us take our own system and consider two meteorites at our distance
-from the Sun, travelling in the same sense, the one in an ellipse,
-the other in a circle, with a major axis five per cent greater and
-meeting the other at aphelion. This would be no improper jostle for
-such heavenly bodies. If we calculate the speeds of both and deduct
-the elliptic from the circular, we shall have the relative speed of
-collision. It proves to be a half a mile a second or 30 times the
-speed of an express train. As such a train brought up suddenly against
-a stone wall would certainly elicit sparks, we see that a speed 30
-times as great, whose energy is 900 times greater, is quite competent
-to a shock sufficient to make us see stars _en masse_. But, indeed,
-there must be collisions much more violent than this; both because the
-central mass is often much greater and because the orbits differ much
-more, and the effect would increase as the square of the speed. The
-heat thus generated would cause the meteorites to glow, and at the same
-time raise the temperature of the gases in and about them. Furthermore,
-the light would come to us through other non-affected portions of gas
-between us and the scene of the collision. Thus all three peculiarities
-of the spectra stand explained: we have a continuous background of
-light due to heated solid meteorites, the bright lines of glowing
-gases, and dark lines due to other gases not ignited, lying in our line
-of sight.
-
-In addition we should perceive another result. Collisions would be both
-more numerous and more pronounced toward the centre of the nebula, for
-it must speedily grow denser toward its core owing to the falling in of
-meteorites, in consequence of shock. Being denser in the centre, the
-particles would there be thicker and be travelling at greater speed.
-The nebulæ, therefore, should be brightest at their centres, which is
-accordant with observation.
-
-Thus from having offered themselves exemplars of the way in which our
-own system came into being, the white nebulæ assert their present
-constitution to be that from which we know our system sprang.
-
-Another suggestive fact about the present members of our solar system
-which has something to say about a past collision is the densities of
-the different planets. The average density of the four inner planets,
-Mars, the Earth, Venus, and Mercury is nearly four times that of
-the four outer ones Neptune, Uranus, Saturn, and Jupiter [see NOTE
-2]. The discrepancy is striking and cannot be explained by size, as
-the smallest are the most massive, and if all were primally of like
-constitution, should be the least compressed. Nor can it be explained
-simply by greater heat tending to expand them, for Neptune and Uranus
-show no signs of being very hot. The minor differences between members
-of each group are probably explicable in part by these two factors,
-mass and heat, but the great gulf between the two groups cannot so
-be spanned. We are then driven to the supposition that the materials
-composing the outer ones were originally lighter. Now this is precisely
-what should happen had all eight been formed by disruption of a
-previous body. For its cuticle would be its least dense portion, and on
-disruption would travel farthest away, not because of being lighter,
-but because of being on the outside. Parts coming from deeper down
-would remain near, and be denser intrinsically.
-
-What the present densities of the planets enable us to infer of the
-cataclysm from which they came, a remarkable set of spectrograms taken
-not long ago by Dr. V. M. Slipher, at Flagstaff, seems to confirm.
-
-The spectrograms in question were made possible by his production of a
-new kind of plate. His object was to obtain one which should combine
-sufficient speed with great photographic extension of the spectrum
-into the red. For it is in the red end that the absorption lines due
-to the planets’ atmospheres chiefly lie. With the plates heretofore
-used it was impossible to go much beyond the yellow, the C line marking
-the _Ultima Thule_ of attent. Not only was it advisable to get more
-particularity in the parts previously explored, but it was imperative
-to go beyond into parts as yet unknown. After several attempts he
-succeeded, the plates when exposed showing the spectra beyond even the
-A band. Of their wealth of depiction it is only necessary to say that
-in the spectrum of Neptune 130 lines and bands can easily be counted
-between the wave-lengths 4600 µµ, 7600 µµ. Of these, 31 belong to the
-planet, which compares with 6 found by Huggins, 10 by Vogel, and 9 by
-Keeler in the part of its spectrum they were able to obtain.
-
-[Illustration: THE SPECTRA OF THE MAJOR PLANETS.
-
-_Photographed, in 1907, by V. M. Slipher, at THE LOWELL OBSERVATORY
-Flagstaff, Arizona._]
-
-The result was a revelation. The plates exposed a host of lines never
-previously seen; lines that do not appear in the spectrum of the Sun,
-nor yet in the added spectrum of the atmosphere of the Earth, but are
-due to the planets’ own envelopes. But this was only the starting-point
-of their disclosures. When in this manner he had taken the color
-signatures of Jupiter, Saturn, Uranus, and Neptune, an orderly sequence
-in their respective absorption bands stood strikingly confessed. In
-other words, their atmospheres proved not only peculiar to themselves
-and unlike what we have on Earth, but progressively so according to a
-definite law. That law was distance from the Sun. When the spectra were
-arranged vertically in ordered orbital relation outward from the Sun,
-with that of the lunar for comparison on top, a surprising progression
-showed down the column in the strange bands, an increase in number and
-a progressive deepening in tint. The lunar, of course, gives us the Sun
-and our own air. All else must therefore be of the individual planet’s
-own. Beginning, then, with Jupiter, we note, besides the reënforcement
-of what we know to be the great water-vapor bands ‘_a_,’ several new
-ones, which show still darker in the spectrum of Saturn. The strongest
-of these is apparently not identifiable with a band in the spectra
-of Mira Ceti in spite of falling near it. Passing on to Uranus, we
-perceive these bands still more accentuated, and with them others, some
-strangers, some solar lines enhanced. Thus the hydrogen lines stand out
-as in the Sirian stars. All deepen in Neptune, while further newcomers
-appear.
-
-Thus we are sure that free hydrogen exists in large quantities in
-the atmospheres of the two outermost planets and most so in the one
-farthest off. Helium, too, apparently is there, and other gases which
-in part may be those of long-period stars, decadent suns, in part
-substances we do not know.
-
-From the fact that these bands are not present in the Sun and
-apparently in no type of stars, we may perhaps infer that the
-substances occasioning them are not elements but compounds to us
-unknown. And from the fact that free hydrogen exists there alongside of
-them, and apparently helium, too, we may further conclude that they are
-of a lighter order than can be retained by the Earth.
-
-But now, we may ask, why should these lighter gases be found where they
-are? It cannot be in consequence simply of the kinetic theory of gases
-from which a corollary shows that the heaviest bodies would retain
-their gases longest, because the strange gases are not apportioned
-according to the sizes of their hosts. Jupiter, by all odds the biggest
-in mass, has the least, and Saturn, the next weightiest, the next
-in amount. Nor can title to such gaseous ownership be lodged in the
-planet’s present state. For though Jupiter is the hottest and Saturn
-the next so, the increased mass more than makes up in restraint what
-increased temperature adds in molecular volatility—as we perceive in
-the cases of the Sun and Earth.
-
-No; their envelopes are increasingly strange because their internal
-constituents are different, and as hydrogen is most abundant in
-Neptune, the lightest of all the gases, it is inferable that this
-planet’s material is lighter. As distance from the Sun determines
-their atmospheric clothing, so distance decides upon their bodies,
-too. It was all a case of primogeniture. The light strange matter
-that constitutes them was so because it came from the outer part of
-the dismembered parent orb. Neptune the outermost, Uranus the next,
-then Saturn and Jupiter came in that order from the several successive
-layers of the pristine body, while the inner planets came from parts of
-it deeper down. The major planets were of the skin of the dismembered
-body, we of its lower flesh.
-
-Very interesting the study of these curious spectral lines from the
-outer planets for themselves alone; even more so for what one would
-hardly have imagined: that they should actually tell us something of
-the genesis of our whole solar system. They corroborate in so far what
-the meteorites have to say.
-
-That the meteorites are solid and, except for their experiences in
-coming through our air, bear no marks of external heat, is a fact
-which is itself significant. It seems to hint not at a crash as their
-occasioning but at disruptive tidal strains. The parent body appears
-to have been torn apart without much development of heat. Perhaps,
-then, we had no gloriously pyrotechnic birth, but a more modest coming
-into existence. But about this we must ourselves modestly be content to
-remain for the present in the dark.
-
-Not the least important feature of the theory I have thus outlined is
-that it finishes out the round of evolution. It becomes a conception
-_sapiens in se ipso totus, teres atque rotundus_. To frame a theory
-that carries one back into the past, to leave one there hung up in
-heaven, is for inconclusiveness as bad as the ancient fabulous support
-of the world, which Atlas carried standing on an elephant upheld by a
-tortoise. What supported the tortoise we were not told. So here, if
-meteorites were our occasioning, we must account for the meteorites,
-starting from our present state. This the present presentation does.
-
-Thus do the stones that fall from the sky inform us of two historic
-events in our solar system’s career. They tell us first and directly
-of a nebula made up of them, out of which the several planets were by
-agglomeration formed and of which material they are the last ungathered
-remains. And then they speak to us more remotely but with no less
-certainty of a time antedating that nebula itself, a time when the
-nebula’s constituents still lay enfolded in the womb of a former Sun.
-
-Man’s interest in them hitherto has been, as with other things,
-chiefly proprietary. Greed of them has grown so keen that legal
-questions have been raised of the ownership of their finding, and our
-courts have solemnly declared them not “wild game” but “real estate,”
-and as such belonging to the owner of the land on which they fall.
-
-But to the scientific eye their estate is something more than “real,”
-for theirs is the oldest real estate in the solar system. They were
-what they are now when the Earth we pride ourselves in owning was but a
-molten mass.
-
-So that when in future you see these strange stones in rows upon a
-museum’s shelves, regard them not as rarities, in which each museum
-strives to outdo its neighbors by the quantity it can possess, but as
-rosetta stones telling us of an epoch in cosmic history long since
-passed away—of which they alone hold the key. Look at them as the
-literary do their books, for that which they contain, not as the
-bibliophile to whom a misprint copy outvalues a corrected one and by
-whom “uncuts” are the most prized of all.
-
-
-
-
-CHAPTER III
-
-THE INNER PLANETS
-
-
-When we recall that the Ptolemaic system of the universe was once
-taught side by side with the Copernican at Harvard and at Yale, we
-are impressed, not so much with the age of our universities, as with
-the youth of modern astronomy and with the extraordinary vitality of
-old ideas. That the Ptolemaic system in its fundamental principle
-was antiquated at the start, the older Greeks having had juster
-conceptions, does not lessen our wonder at its tenacity. But the fact
-helps us to understand why so much fossil error holds its ground in
-many astronomic text-books to-day. That stale intellectual bread is
-deemed better for the digestion of the young, is one reason why it
-often seems to them so dry.
-
-[Illustration: ORBITS OF THE INNER PLANETS.]
-
-Before entering upon the problem of the genesis and career of a world,
-it is essential to have acquaintance with the data upon which our
-deductions are to rest. To set forth, therefore, what is known of the
-several planets of our solar system, is a necessary preliminary to any
-understanding of how they came to be or whither they are tending; and
-as our knowledge has been vitally affected by modern discoveries about
-them, it is imperative that this exposition of the facts should be as
-near as possible abreast of the research itself. I shall, therefore,
-give the reader in this chapter a bird’s-eye view of the present state
-of planetary astronomy, which he will find almost a different part of
-speech from what it was thirty years ago. It is not so much in our
-knowledge of their paths as of their persons that our acquaintance
-with the planets has been improved. And this knowledge it is which has
-made possible our study of their evolution as worlds.
-
-Could we get a cosmic view of the solar system by leaving the world we
-live on for some suitable vantage-point in space, two attributes of it
-would impose themselves upon us—the general symmetry of the whole, and
-the impressively graded proportions of its particular parts.
-
-Round a great central globular mass, the Sun, far exceeding in size
-any of his attendants, circle a series of bodies at distances from
-him quite vast, compared with their dimensions. These, his principal
-planets, are in their turn centres to satellite systems of like
-character, but on a correspondingly reduced scale. All of them travel
-substantially in one plane, a fact giving the system thus seen in its
-entirety a remarkably level appearance, as of an ideal surface passing
-through the centre of the Sun. Departing somewhat from this general
-uniformity in their directions of motion, and also deviating more
-from circularity in their paths, some much smaller bodies, a certain
-distance out, dart now up now down across it at different angles and
-from all the points of the compass, agreeing with the others only in
-having the centre of the Sun their seemingly never attained goal of
-endeavor. These bodies are the asteroids. Surrounding the whole, and
-even penetrating within its orderly precincts, a third class would
-be visible which might be described for size as cosmic dust, and for
-display as heavenly pyrotechnics. Coming from all parts of space
-indifferently they would seem to seek the Sun in almost straight lines,
-bow to him in circuit, and then depart whence they came. For in such
-long ellipses do they journey that these seem to be parabolas. These
-visitants are the comets and their associates the meteor-streams.
-
-Although for purposes of discrimination we have labelled the several
-classes apart, an essential fact about the whole company is to be
-noted: that no hard and fast line can be drawn separating the several
-constituents from one another. In size the members of the one class
-merge insensibly into the other. Some of the planets are hardly larger
-than some of the satellites; some of the satellites than some of the
-asteroids; some of the asteroids than comets and shooting-stars. In
-path, too, we find every gradation from almost perfect circularity
-like the orbits of Io and Europa to the very threshold of where one
-step more would cease to leave the body a member of the Sun’s family
-by turning its ellipse into an hyperbola. Finally, in inclination we
-have every angle of departure from orthodox platitude to unconforming
-uprightness. This point, that heavenly bodies, like terrestrial ones,
-show all possible grades of indistinction, is kin to that specific
-generalization by which Darwin revolutionized zoölogy a generation ago.
-It is as fundamental to planets as to plants. For it shows that the
-whole solar system is evolutionarily one.
-
-A second point to be noticed in passing is that undue inclination and
-excessive eccentricity go together. The bodies that have their paths
-least circular have them, as a rule, the most atilt. And with these two
-qualities goes lack of size. It is the smallest bodies that deviate
-most from the general consensus of the system. With so much by way of
-generic preface, the pregnancy of which will become apparent as we
-proceed, we come now to particular consideration of its members in turn.
-
-Nearest to the Sun of all the planets comes Mercury. So close is he to
-that luminary, and so far within the orbit of the earth, that he is
-not a very common object to the unaided eye. Copernicus is said never
-to have seen him, owing, doubtless, to the mists of the Vistula. By
-knowing when to look, however, he may be seen for a few days early in
-the spring in the west after sunset, or before sunrise in the east in
-autumn. He is then conspicuous, being about as bright as Capella, for
-which star or Arcturus he is easily mistaken by one not familiar with
-the constellations.
-
-His mean distance from the Sun is thirty-six million miles, but so
-eccentric is his orbit, the most so of any of the principal planets,
-that he is at times half as far off again as at others. Even his
-orbital behavior is the least understood of any in the solar system.
-His orbit swings round at a rate which so far has defied analysis. It
-may be a case of reflected perturbation, one, that is, of which the
-indirect effect from another body becomes more perceptible than would
-be the direct effect on the body itself. As yet it baffles geometers.
-
-As to his person, our ignorance until lately was profound. It is only
-recently that such fundamental facts about him as his size, his mass,
-and his density have been reached with any approach to precision. This
-was because he so closely hugs the Sun that observations upon his
-full, or nearly full, disk had never been attempted. When I say that
-his volume was not known to within a third of its amount, his mass not
-closer than one-half, while his received density was nearly double
-what we now have reason to suppose the fact, some idea of the depth
-of our nescience may be imagined. This, of course, did not prevent
-text-books from confidently misinstructing youth, or Nautical Almanacs
-from misguiding computers with figures that thus almost achieved
-immortality, so long had they passed current in spite of lacking that
-perfection which is usually assigned as its warrant.
-
-[Illustration: SULLA ROTAZIONE DI MERCURIO—DI G. V. SCHIAPARELLI.]
-
-Schiaparelli first put astronomy on the right track. By attempting
-daylight observations of the planet, not toward night, but actually
-at midday, he made some remarkable discoveries, and though he did
-not detect the hitherto erroneous values of the volume, the mass,
-or the density, his method of observation paved the way for their
-ascertainment. What he sought, and found, was evidence of markings
-upon the disk by which the planet’s time of rotation might be
-determined. Up to then, Schroeter’s value of about twenty-four hours
-had been accepted, on very slender evidence indeed, and passed into
-all the books. But when the planet came to be observed by noon, very
-definite markings stood out on its face, which showed its rotation to
-take place, not in twenty-four hours, but in eighty-eight days. By a
-persistence equal to his able choice of observing time, he established
-this beyond dispute. He proved the revolutionizing fact that Mercury’s
-periods of rotation and of revolution were the same.
-
-He detected, too, the evidence in the position of the markings of the
-planet’s great libratory swing due to the eccentricity of its orbit, a
-result as remarkable as a feat of observation as it was conclusive as a
-proof.
-
-If Schiaparelli had never done any other astronomical work, this study
-of Mercury would have placed him as the first observer of his day. For
-the observations are so difficult that the planet not only baffled all
-his predecessors, but has foiled many since who are credited with being
-observers of eminence.
-
-In 1896 the study of Mercury was taken up at the Lowell Observatory
-in Arizona along the same lines that had proved so successful with
-Schiaparelli, but without using his observations as guide. Indeed,
-his papers had not then been read there. The two conclusions were,
-therefore, independent of one another. The outcome was a complete
-corroboration and an extension of Schiaparelli’s work. We shall begin
-with the consideration of the most fundamental point. In the clear and
-steady air of Flagstaff, permitting of measurement of his disk up to
-within a few degrees of the Sun, Mercury was found to be much larger
-than previously thought.
-
-Instead of a diameter of three thousand miles he proved to have one
-of thirty-four hundred, making his volume nearly half as large again
-as had been credited him. These measures bore intrinsic evidence
-of their trustworthiness in an interesting manner, and at the same
-time produced internal testimony that accounted for the smallness of
-previous determinations. Measures heretofore had been made, usually if
-not invariably, either when the planet transited the Sun or when it
-exhibited a pronounced phase. Now in both these cases the planet looks
-smaller than it is. In the first case this is due to irradiation, the
-surrounding disk of the Sun encroaching both to the eye and to the
-camera upon the silhouette of Mercury. And this inevitable effect had
-not been allowed for in the measures. In the second case the horns of
-the planet never seem to extend quite to their true position. This
-was rendered evident by the Flagstaff series of measures, which began
-when the planet was a half-moon and continued till it was almost
-full. As it did so, the values for the diameter steadily increased,
-even after irradiation was allowed for, although this against the
-brilliant background of the noonday sky must have been exceeding
-small, and tended in part to be diminished as the planet attained the
-full, because of its consequent nearing of the Sun. The measures thus
-explained themselves and vouched for their own accuracy.[3]
-
-Then came a curious bit of unexpected proof to corroborate them. In his
-“Astronomical Constants,”[4] published but a short time before, Newcomb
-had detected a systematic error in the right ascensions of Mercury
-which he was not able to explain. By diligent mousing that eminent
-computer had discovered that Mercury was registered by observers too
-far from the Sun on whichever side of him it happened to be, and
-in proportion roughly not to its distance off but to the phase the
-planet exhibited. When the disk was a crescent the discrepancy between
-observation and theory was large, and thence decreased as the planet
-passed to the full. He suspected the cause, and would have found it
-had he not considered the diametral measures of the planet too well
-assured to permit of doubt. As it was, he neglected a factor which
-has vitiated almost all the observations made on the planets up to
-within a few years, the correction for irradiation. This was the case
-here. The received measures, beginning with Bradley and ending with
-Todd, had almost without exception been made in transit, and, as no
-regard had been paid to the contracting effect of irradiation, had been
-invalidated in consequence. The new method supplied almost exactly the
-amount needed to explain the right ascensions, a second of arc, and in
-precise accordance with the place which the discrepancy demanded.
-
-[3] New Observations of the Planet Mercury, _Memoirs Amer. Acad._ 1897.
-Vol. XII, No. 4.
-
-[4] “Astronomical Constants,” 1895, pp. 67, 68.
-
-About the mass there has been, and still is, great uncertainty. This
-is because it can only be found from the perturbing effect it has on
-Venus, the Earth, or Encke’s comet. Modern determinations, however, are
-smaller than the older ones; thus Backlund in 1894 got from the effect
-on Encke’s comet only one-half the mass that Encke had, fifty-three
-years before. Probably the most reliable information comes from Venus,
-which Tisserand found to give for Mercury ¹/₇₁₀₀₀₀₀ of the mass of the
-Sun, or ¹/₂₁ of the mass of the Earth. If we take ¹/₇₀₀₀₀₀₀ as the
-nearest round number, we find the planet’s density to be 0.66 that of
-the Earth.
-
-[Illustration: MAP OF MERCURY
-
-LOWELL OBSERVATORY 1896-97]
-
-The same observations that disclosed at Flagstaff the planet’s size
-revealed a set of markings on his face so definite as to make the
-rotation period unmistakable. It takes place, as Schiaparelli found,
-in eighty-eight days, or the time of the planet’s revolution round
-the Sun. The markings disclosed the fact, as Schiaparelli had also
-discovered, in a most interesting manner, for the ellipticity of
-the planet’s orbit stood reflected in the swing of the markings
-across the face of the disk, a definiteness in the proof of a really
-surprising kind. What this means we shall see in a subsequent chapter
-when we take up the mechanical problem of the tides. Another result
-that issued from the positions of the markings was the determination
-of the planet’s pole. Except for the libration above noticed, the
-markings kept an invariable longitudinal position upon the illuminated
-disk, showing that the planet turned always the same face to the Sun;
-but latitudinally a difference was noticeable between their place
-in October-November, 1896, and in February-March, 1897, the latter
-being 4° farther north. Now this is just what the orbital position
-should have caused, if the pole stood vertically to it. Thus a
-difference of 4° from perpendicularity should have been discernible,
-had it existed,—a very small amount in such a determination. We may,
-therefore, conclude that the axis stands plumb to the orbit, and this
-is what theory demands.
-
-The state of things this introduces to us upon that other world is
-to our ideas exceeding strange. It is not so much the slowness of
-the diurnal spin, eighty-eight times as long as our own, which is
-surprising, as the fact that this makes its day infinite in length.
-Two antipodal hemispheres divide the planet, the one of which frizzles
-under eternal sun, the other freezes amid everlasting night. The Sun
-does not, indeed, stand stock-still in the sky, but nods like some
-huge pendulum to and fro along a parallel of latitude. In consequence
-of libration the two great domains of day and night are sundered by
-a strip of debatable ground 23½° in breadth on either side, upon
-which the Sun alternately rises and sets. Here there is a true day,
-eighty-eight of our days in length from one sunrise to the next. But
-its day and night are not apportioned alike. The eastern strip has its
-daylight briefer than its starlight hours; the western has them longer.
-Nor are different portions of the strips similarly circumstanced in
-their sunward regard. Only the edge next perpetual day has anything
-approaching an equal distribution of sunlight and shade. The farther
-one just peeps at the Sun for a moment every eighty-eight days, and
-then sinks back again into obscurity.
-
-The transition from day to night is equally instantaneous and profound.
-For little or no twilight here prolongs the light; since the air, if
-there be any at all, is too thin to bend it to service round the edge
-to illuminate the night. When the libratory Sun sets, darkness like
-a mantle falls swiftly over the face of the ground. No evidence of
-atmosphere has ever been perceived, and theory informs that it should
-be nearly, if not wholly, absent.
-
-In consequence of the rigid uprightness of the planet’s axis,
-seasons do not exist. Their nearest simulacrum comes from the
-seeming dilatation of the Sun during half the year, and its apparent
-contraction during the other half. It expands so much between its
-January and its July as to receive more heat in the ratio of nine to
-four. A seasonless, dayless, and almost yearless planet, it is better
-to look at than to look from; but its study opens our eyes to the great
-diversity which even one of our nearest neighbors exhibits from what we
-take as matters of course on Earth.
-
-That what we take offhand to be purely astronomic phenomena should turn
-out to be so essentially of the particular world, worldly, clarifies
-vision of what these really are, and how dependent on and interwoven
-with everyday life astronomy is. Or, we may consider it turned about
-and realize how purely astronomic relations, such abstract mechanical
-matters as rotations and revolutions, result in completely changing the
-very face and character of the globe concerned. Mercury to-day stares
-forever at the Sun. The markings we see have stereotyped this stare
-to its inevitable result. For they seem to mark a globe sun-cracked.
-At such a condition the curious crisscross of dark, irregular lines
-certainly hints, accentuated and perfected as it is by a bounding curve
-where the mean sunward side terminates to the enclosing them as by the
-carapace of a tortoise. Though they cannot probably be actual cracks,
-however much they may resemble such, yet they may well owe their
-existence to that fundamental cause.
-
-In color the planet is ghastly white; of that wan hue that suggests a
-body from which all life has fled. Far whiter than Venus in point of
-fact, the rosy tint with which it sparkles in the sunset glow is all
-borrowed of the dying day and vanishes when the planet is looked at in
-the uncompromising light of noon. Seen close together once at Flagstaff
-it was possible directly to compare the two; when Mercury, although lit
-by the Sun two and a half times as brilliantly as Venus, was, surface
-for surface, more than twice as faint. Müller has found its intrinsic
-brightness about that of our Moon, which in some respects it resembles,
-though it apparently departs widely from any similarity in others. The
-bleached bones of a world; that is what Mercury seems to be.
-
-Venus comes next in order outward from the Sun. To us her incomparable
-beauty is partly the result of propinquity: nearness to ourselves and
-nearness to the Sun. Relatively so close is she to both that she does
-not need the Sun’s withdrawal to appear, but may nearly always be seen
-in the daytime in clear air if one knows where to look for her. Situate
-about seven-tenths of our own distance from our common giver of light
-and heat, she gets about double the amount that falls to our lot, so
-that her surface is proportionately brilliantly illuminated. Being also
-relatively near us, she displays a correspondingly large surface.
-
-But though part of her lustre is due to her position, a part is her
-own. Direct visual observation, as we remarked above, shows her
-intrinsic brightness to be more than five times that of Mercury,
-square mile to square mile of surface for the two. Now this has been
-determined very carefully photometrically by Müller at Potsdam. The
-result of his inquiry was to indicate that Mercury shines with 0.17 of
-absolute reflection, Venus with 0.92. So high a value has seemed to
-many astronomers impossible, because so far surpassing that which has
-tacitly been taken as the _ne plus ultra_ of planetary brightness, that
-of cloud, 0.72.
-
-Now, one of the direct outcomes of the study of Venus at the Lowell
-Observatory was an explanation of this seemingly incredible phenomenon.
-When the planet came to be critically examined there under conditions
-of seeing which permitted discovery, markings very faint, but
-nevertheless assurable, stood presented on the planet’s face. These
-markings, of which we shall have more to say in a moment, had this of
-pertinency to our present point, that they kept an invariable position
-to one another. They thus betrayed themselves to be surface features.
-Furthermore, their dimness was as invariable an attribute of them as
-their place. They were not obscured on some occasions and revealed at
-others, but stayed, so far as one might judge, permanently the same.
-They were thus neither clouds themselves nor subject to the caprice of
-cloud. The old idea that Venus was a cloud-wrapped planet and owed her
-splendor to this envelope, vanished literally into thin air.
-
-It is precisely because she is not cloud-covered that her lustre is so
-great. She “clothes herself with light as with a garment” by a physical
-process of some interest. As becomes the Mother of the Loves, this is
-gauze of the most attenuated character, and yet a wonderful heightener
-of effect. For it consists solely of the atmosphere that compasses her
-about. It is well known that a substance when comminuted reflects much
-more light than when condensed into a solid state. Now an atmosphere is
-itself such a comminuted affair, and, furthermore, holds in suspension
-a variety of dust. This would particularly be the case with the
-atmosphere of Venus, as we shall have reason to see when we consider
-the conditions upon that planet made evident by study of its surface
-markings. To her atmosphere, then, she owes four-fifths or more of her
-brilliancy. And this stands corroborated by the low albedo of both
-Mercury and the Moon, which have no atmosphere, and by the intermediate
-lustre of Mars, which has some, but little.[5]
-
-[5] _Astr. Nach._ No. 3406. Monthly Notices R. A. S., March, 1897.
-
-The rotation time of Venus, the determination, that is, of the planet’s
-day, is one of the fundamental astronomical acquisitions of recent
-years. For upon it turns our whole knowledge of the planet’s physical
-condition. More than this, it adds something which must be reckoned
-with in the framing of any cosmogony. It is not a question of academic
-accuracy merely, of a little more or a little less in actual duration,
-but one which carries in its train a completely new outlook on Venus
-and sheds a valuable side-light upon the history of our whole planetary
-system.
-
-Unconsciously influenced, one is inclined to think, by terrestrial
-analogies, astronomers for more than a couple of centuries, ever
-since the time of the first Cassini in 1666, deemed the day of Venus
-to be just under twenty-four hours in length. So well attested was
-its determination, and so precisely figured to the minute, that it
-imposed itself upon text-books which stated it as an acquired fact
-down to the last second. Nevertheless, Schiaparelli was not so sure,
-and proceeded to look into the matter. He first looked for himself,
-and then looked up all the old observations. His chief observational
-departure was observing by day as near to noon as possible; because
-then the planet was highest, to say nothing of the taking off from its
-glare by the more brilliant sky. From certain dark markings around
-two bright spots near the southern cusp, of one of which spots the
-detection dates from the time of Schroeter, and from a long, dark
-streak stretching thence well down the disk, he convinced himself that
-no such period as twenty-four hours could possibly be correct, inasmuch
-as whenever he looked, the markings were always there. His notes read,
-“Same appearance as yesterday,” day after day, until he would really
-have saved ink and penmanship had he had the phrase cut into a die and
-stamped. He concluded that the rotation was at least six months long,
-and was probably synchronous with the planet’s time of revolution. This
-was in 1889. In 1895 he became still more sure, and showed how the
-older observations were really compatible with what he had found.
-
-In 1896 the subject was taken up at Flagstaff. Very soon it became
-evident there that markings existed on the disk, most noticeable
-as fingerlike streaks pointing in from the terminator, faint but
-unmistakable from the identity of their successive presentation.
-Schroeter’s projection near the south cusp was also clearly discernible
-as well as two others, one in mid-terminator, one near the northern
-cusp. Schiaparelli’s dark markings also came out, developing into a
-sort of collar round the southern pole. Other spots and streaks also
-were discernible, and all proved permanent in place. By watching them
-assiduously it was possible to note that no change in position occurred
-in them, first through an interval of five hours, then through one
-of days, then of weeks. Care was taken to guard against illusion. It
-thus became evident that they bore always the same relation to the
-illuminated portion of the disk. This illuminated part, then, never
-changed. In other words, the planet turned always the same face to
-the Sun. The fact lay beyond a doubt, though of course not beyond a
-doubter.[6]
-
-[6] Monthly notices R. A. S., March, 1897.
-
-[Illustration: VENUS. OCTOBER, 1896—MARCH, 1897—DRAWINGS BY DR.
-LOWELL.]
-
-[Illustration: VENUS. APRIL 12, 1909, 3H 26M—4H 22M—BY DR. LOWELL.]
-
-The years that have passed since these observations were made have
-brought corroboration of them. Several observers at Flagstaff have
-seen and drawn them and added discoveries of their own, among whom are
-especially to be mentioned, of the observatory staff: Miss Leonard, Dr.
-Slipher, and Mr. E. C. Slipher.[7]
-
-[7] Lowell Observatory Bulletin 6.
-
-In character these markings were peculiar and distinctive. In addition
-to some of more ordinary character were a set of spokelike streaks
-which started from the planet’s periphery and ran inwards to a point
-not very distant from the centre. The spokes started well-defined and
-broad at the edge, dwindling and growing fainter as they proceeded,
-requiring the best of definition for their following to their central
-hub.
-
-The peculiar symmetry thus displayed, a symmetry associated with the
-planet’s sunrise and sunset line, or, strictly speaking, what would be
-such did the Sun for Venus ever rise or set, would seem inexplicable,
-except for that very association. When we reflect, however, upon what
-this means, a very potent cause for them becomes apparent, so potent
-that surprise is turned into appreciation that nothing else could well
-exist. That Venus turns on her axis in the same time that she revolves
-about the Sun, in consequence of which she turns always the same face
-to him, must cause a state of things of which we can form but faint
-conception, from any earthly analogy. One face baked for countless
-æons, and still baking, backed by one chilled by everlasting night,
-while both are still surrounded by air, must produce indraughts from
-the cold to the hot side of tremendous power. A funnel-like rise must
-take place in the centre of the illuminated hemisphere, and the partial
-vacuum thus formed would be filled by air drawn from its periphery,
-which, in its turn, would draw from the regions of the night side.
-Such winds would sweep the surface as they entered, becoming less
-superficial as they advanced, and the marks of their inrush might well
-be discernible even at the distance we are off. Deltas of such inroad
-would thus seam the bounding circle of light and shade.
-
-[Illustration: I
-
-Showing convection currents in the planet’s atmosphere.]
-
-
-[Illustration: II
-
-Showing shift in central barometric depression due to rotation of the
-planet affecting the winds.
-
-VENUS.]
-
-Another result of the aërial circulation would be the removal of all
-moisture from the sunward face, and its depositing in the form of
-ice upon the night one. For the heated air would be able to carry
-much water in suspension, which, on cooling, after it had reached the
-dark hemisphere would unload it there. In the low temperature there
-prevailing, this moisture would all be frozen, and so largely estopped
-from return. This process continuing for ages would finally deplete one
-side of all its water to heap it up in the form of ice upon the other.
-
-Now it is not a little odd that a phenomenon has been observed upon
-Venus which seems to display just this state of things. Many observers
-have noted an ashen light on the dark side of her disk. Some have
-tried to account for it as Earth shine, the same earth-reflected light
-that makes dimly visible the old moon in the new moon’s arms. But the
-Earth is too far away from Venus to permit of any such effect; nor is
-there any other body that could thus relieve its night. But if the
-night hemisphere of Venus be one vast polar sheet, we have there a
-substance able to mirror the stars to a ghostlike gleam which might be
-discernible even from our distant post.
-
-[Illustration: _Venus Rotation 225 days._]
-
-Thus when we reason upon them we see that the peculiar markings of the
-planet lose their oddity, becoming the very pattern and prototype of
-what we should expect to view. Interpreted, they present us the picture
-of a plight more pitiable even than that of Mercury. For the nearly
-perfect circularity of Venus’ orbit prevents even that slight change
-from everlasting sameness which the libration of Mercury’s affords. To
-Venus the Sun stands substantially stock-still in the sky,—a fact which
-must prove highly reassuring to Ptolemaic astronomers there, if there
-be any still surviving from her past. No day, no seasons, practically
-no year, diversifies existence or records the flight of time. Monotony
-eternalized,—such is Venus’ lot.
-
-What visual observations have thus discovered of the rotation time of
-Venus, with all that follows from it, the spectroscope at Flagstaff has
-confirmed. At Dr. Slipher’s hands, spectrograms of the planet have told
-the same tale as the markings. It was with special reference to this
-point that the spectrograph there was constructed, and the first object
-to which it was directed was Venus.[8]
-
-[8] Lowell Observatory Bulletin No. 3.
-
-The planet’s rotation time was to be investigated by means of the
-motion it brought about in the line of sight. Visual observation,
-telescopically, reveals motion thwart-wise by the displacement it
-produces in the field of view; spectroscopic observation discloses
-motion to or from the observer by the shift it causes in the spectral
-lines due to a stretching or shortening of their wave-lengths.
-
-The spectroscope is an instrument for analyzing light. Ordinary light
-consists of light of various wave-lengths. By means of a prism or
-grating these are dispersed into a colored ribbon or band, the longer
-waves lying at the red end of the spectrum, as the ribbon is called,
-the shorter at the violet. Now the spectroscope is primarily such a
-prism or grating placed between the image and the observer, by means of
-which a series of colored images of the object are produced. In order
-that these may not overlap and so confuse one another, the light is
-allowed to enter the prism only through a narrow slit placed across the
-telescopic image of the object to be examined. Thus successive images
-of what is contained by the slit are presented arranged according to
-their wave-lengths. In practice the rays of light from the slit enter a
-small telescope called the collimator, and are there rendered parallel,
-in which condition they fall upon the prism. This spreads them out into
-the spectrum and another small telescope focusses them, each according
-to its kind, into a spectral image band which may then be viewed by the
-eye or caught upon a photographic plate.
-
-Now, if an object be coming toward the observer, emitting or reflecting
-light as it does so, each wave-length of its spectrum will be shortened
-in proportion to the relative speed of its approach as compared with
-the speed of light, because each new wave is given out by so much
-nearer the observer and in reflection the body may also meet it.
-Reversely it will be lengthened if the object be receding from the
-observer or he from it. This would change the color of the object were
-it not that while each hue moves into the place of the next, like the
-guests at Alice’s tea-party in Wonderland, some red rays pass off the
-visible spectrum, but new violet rays come up from the infraviolet
-and the spectrum is as complete as before. Fortunately, however, in
-all spectra are gaps where individual wave-lengths are absorbed or
-omitted, and these, the lines in the spectrum, tell the tale of shift.
-Now if a body be rotating, one side of it will be approaching the
-observer, while the opposite side is receding from him, and if the slit
-be placed perpendicular to the axis about which the spin takes place,
-each spectral line will appear not straight across the spectrum of the
-object, but skewed, the approaching side being tilted to the violet
-end, the receding side to the red.
-
-This was to be the procedure adopted for the rotation of Venus. By
-placing the slit parallel to the ecliptic, or, more properly, to the
-orbit of Venus, which is practically the same thing, it found itself
-along what we have reason to suppose the equator of the planet. Even a
-considerable error on this point would make little difference in the
-rotational result. In order that there might be no question of illusion
-or personal bias, photographs instead of eye observations of the
-spectrum were made. For reference and check side by side with that of
-Venus were taken on either hand the spectra of iron, made by sparking a
-tube containing the vapor of that metal. The vapor, of course, had no
-motion with regard to the observer, and therefore its spectral lines
-could have no tilt, but must represent motional verticality.
-
-Dr. Slipher chose his time astutely. He selected the occasion when
-Venus was passing through superior conjunction, or the point in her
-orbit as regards us directly beyond the sun. At first sight this might
-seem to be the worst as well as the most impracticable of epochs,
-inasmuch as the planet is then not only at her farthest from the Earth,
-but in a line with the Sun, and so drowned in his glare. But in point
-of fact any tilt of the spectral lines is then, owing to phase, twice
-what it is at elongation, and exceeds still more what it is when Venus
-has her greatest lustre [see NOTE 3]. In his purpose he was abetted
-by the Flagstaff air, which enabled the planet to be spectrographed
-much nearer the sun than would otherwise have been the case. He thus
-selected the best possible opportunity. To guard against any subsequent
-bias on the part of the examiner of the plates, after the spectroscope
-had taken a plate it was then reversed, and the process repeated on
-another one, the iron being sparked as before. What had been the right
-side of Venus with regard to the red end of the spectrum thus became
-the left one, and _vice versa_. In this manner, when the plates came
-to be measured for tilt, the measurer would have no indication from
-the spectrum itself which way the lines might be expected to tilt; he
-could, therefore, not be influenced either consciously or unconsciously
-in his decision.
-
-[Illustration: SPECTROGRAM OF VENUS, SHOWING ITS LONG DAY—V. M.
-SLIPHER, LOWELL OBSERVATORY, 1903.]
-
-Eight plates with their comparison ferric spectra were thus secured;
-four with the spectroscope direct, four with it reversed. They
-were then shuffled, their numbers hidden, and given to Dr. Slipher
-to measure. The spectral lines told their own story, and without
-prompting. All the plates agreed within the margin of error accordant
-with their possible precision, a precision some thirty times that of
-Belopolski’s experiment on the same lines,—a result not derogatory of
-that investigator, but merely illustrative of superior equipment. They
-showed conclusively that a rotation of anything like twenty-four hours
-was out of the question. They yielded, indeed, testimony to a negative
-rotation of three months, which, interpreted, means that so slow a spin
-as this was beyond their power to precise.
-
-For Dr. Slipher was at no less care to determine just what precision
-was possible in the case, although a speed corresponding to a spin
-of twenty-four hours on a globe the size of Venus is well known to
-be spectroscopically measurable. It would mean a motion toward us
-of one thousand miles an hour, or about a third of a mile a second.
-The tilt occasioned by this speed is well within the spectroscope’s
-ability to disclose. Not content with this, however, by two special
-investigations, he proved the spectroscope’s actual limits of
-performance to be far within the quantity concerned. One of them was
-the determination by the same means and in like manner of the rotation
-time of Mars, the length of that planet’s day, which in other ways we
-know to the hundredth of a second, and which is 24ʰ 37ᵐ 23.66ˢ Now Mars
-offers a test nearly twice as difficult as Venus, even supposing the
-apparent disks of the two the same, because his diameter being less in
-the proportion roughly of one-half, the actual speed of a particle at
-his edge is less for the same time of rotation in the like proportion,
-and it is only with the speed in miles, not in angular amount, that the
-spectroscope is concerned. Nevertheless, when a like number of plates
-were tried on him, they indicated on measurement a rotation time within
-an hour of the true. This corresponds to half an hour on Venus. We see,
-therefore, that had Venus’ day been anywhere in the neighborhood of
-twenty-four hours, Dr. Slipher’s investigation would have disclosed it
-to within thirty-one minutes.
-
-[Illustration: SPECTROGRAM OF JUPITER, GIVING THE LENGTH OF ITS DAY BY
-THE TILT OF ITS SPECTRAL LINES—V. M. SLIPHER, LOWELL OBSERVATORY.]
-
-This result was further borne out by a similar test made by him
-of Jupiter. Inasmuch as the diameter of Jupiter is twelve times
-that of Venus, while the rotation time is 9ʰ 50.4ᵐ at the equator,
-the precision attained on Venus should here have been about a
-minute. And this is what resulted. Slipher found the rotation time
-spectrographically 9ʰ 50ᵐ, or in accordance with the known facts, while
-previous determinations with the spectroscope had somehow fallen short
-of it.
-
-The care at Flagstaff with which the possibility of error was sought
-to be excluded in this investigation of the length of Venus’ day and
-the concordant precision in the results are worthy of notice. For
-it is by thus being particular and systematic that the accuracy of
-the determinations made there, in other lines besides this, has been
-secured.
-
-In size, Venus of all the planets most nearly approaches the Earth.
-She is 7630 miles in diameter to the Earth’s 7918. Her density, too,
-is but just inferior to ours. And she stands next us in place, closest
-in condition and constitution in the primal nebula. Yet in her present
-state she could hardly be more diverse. This shows us how dangerous it
-is to dogmatize upon what can or cannot be, and how enlightening beyond
-expectation often is prolonged and systematic study of the facts.
-
-The next planet outward is our own abode. It is one of which most of
-us think we know considerable from experience and yet about which we
-often reason cosmically so ill. If we knew more, we should not deem
-ourselves nearly so unique. For we really differ from other members of
-our system not more than they do from one another. Much that appears to
-us fundamental is not so in fact. Thus many things which seem matters
-of course are merely accidents of size and position. Our very day and
-night upon which turn the habits of all animals and, even in a measure,
-those of plants, are, as we have seen, not the possession of our
-nearest of cosmic kin. Our seasons which both vegetally and vitally
-mean so much are absent next door. And so the list of our globe’s
-peculiar attributes might be run through to the finding of diversity
-to our familiar ways at every turn. But, as we shall see later, these
-differences from one planet to the next are not only not incompatible
-with a certain oneness of the whole, but actually help to make the
-family relationship discoverable. Analogy alone is a dangerous guide,
-but analogy crossed with diversity is of all clews the most pregnant of
-understanding. The very fact that we can tell them apart when we see
-them together, as the Irishman remarked of two brothers he was in the
-habit of confusing, points to their brotherly relation.
-
-Proceeding still further, we come to Mars at a mean distance of one
-hundred and forty-one million miles. Smaller than ourselves, his
-diameter is but a little over half the Earth’s, or forty-two hundred
-miles, his mass one-ninth of ours, and his density about seven-tenths
-as much. Here, again, but in a different way, we find a planet unlike
-ourselves, and we know more about him than of any body outside the
-Earth and Moon. So much about him has been set forth elsewhere that
-it is enough to mention here that no oceans diversify his surface,
-no mountains relieve it, and but a thin air wraps it about,—an air
-containing water-vapor, but so clear that the surface itself is almost
-never veiled from view.
-
-About the satellites Mars possesses, Deimos and Phobos, we may perhaps
-say a word, as recent knowledge concerning them exemplifies the care
-now taken to such ascertainment and the importance of considering
-factors often overlooked. Soon after they were discovered in 1877, they
-were measured photometrically, with the result of giving a diameter
-of six miles to Deimos and one of seven miles to Phobos, and these
-values unchallenged entered the text-books. When the satellites came
-to be critically considered at Flagstaff, it was found that these
-determinations were markedly in error, Phobos being very much the
-larger of the two, the actual values reaching nearer ten miles for
-Deimos and thirty-six for Phobos.
-
-In getting the Flagstaff values, the size to the eye of the satellite
-was corrected for the background upon which it shone; for the
-background is all-important to the brilliancy of a star. In the case
-of a small star near a planet, the swamping glare of the planet is
-something like the inverse cube of its distance away. Furthermore, the
-Flagstaff observations indicated how the previous error had crept in.
-For before correction for the differing brilliancies of the field of
-view, the apparent size of the satellites judged by conspicuousness
-was about six to seven. The photometric values must have been taken
-just as they came out, no correction apparently having been made for
-the background. Now the background is a fundamental factor in all
-photometric determinations, a factor somewhat too important in this
-case to neglect, since it affected the result 2500 per cent.
-
-
-
-
-CHAPTER IV
-
-THE OUTER PLANETS
-
-
-Beyond Mars lies the domain of the asteroids, a domain vast in extent,
-that, untenanted by any large planet, stretches out to Jupiter.
-Occupied solely by a host of little bodies agreeing only in lack of
-size, even this space seems too small to contain them, for recent
-research has shown some transgressing its bounds. One, Eros, discovered
-by De Witt, more than trenches on Mars’ territory, having an orbit
-smaller than that of the god of war, and may be considered perhaps
-the forerunner of more yet to be found between Mars and the Earth.
-On the other side, three recently detected by Max Wolf at Heidelberg
-have periods equal to that of Jupiter, and in their motions appear
-to exemplify an interesting case of celestial mechanics pointed out
-theoretically by Lagrange long before its corroboration in fact was
-so much as dreamt. Achilles, Patroclus, and Hector, as the triad are
-called, so move as always to keep their angular distance from Jupiter
-unaltered in their similar circuits of the Sun.
-
-[Illustration: ORBITS OF THE OUTER PLANETS.]
-
-Before considering these bodies individually, we may well look upon
-them _en bloc_, inasmuch as one attribute of the asteroids concerns
-them generically rather than specifically, and is of great interest
-both from a mechanical and an historical point of view. For, in fact,
-it is what led to their discovery. Titius of Wittenburg, about the
-middle of the eighteenth century, noticed a curious relation between
-the distances from the Sun of the then known planets. It consisted in
-a sort of regular progression, but with one significant gap. Bode was
-so struck by the gap that he peopled it with a supposed planet, and
-so brought the relation into general regard in 1772. In consequence,
-it usually bears his name. It is this: if we take the geometrical
-series, 3, 6, 12, 24, 48, 96 and add 4 to each term, we shall represent
-to a fair degree of precision the distances of the several planets,
-beginning with Mercury at 4 and ending with Saturn at 100, which
-was the outermost planet then known. All the terms were represented
-except 24 + 4, or 28—a gap lying between Mars and Jupiter. When Uranus
-was discovered by Sir William Herschel in 1781 and was found to be
-travelling at what corresponded to the next outer term 192 + 4, or 196,
-the opinion became quite general that the series represented a real law
-and that 28 must be occupied by a planet. Von Zach actually calculated
-what he called its analogical elements, and finally got up in 1800 a
-company to look for it which he jocularly described as his celestial
-police. Considering that Bode’s law is not a law at all, but a curious
-coincidence, as Gauss early showed in its lack of precision and in its
-failure to mark the place of Mercury with any approach to accuracy, and
-as the discovery of Neptune amply bore out, it was perhaps just in fate
-that the honor of filling the gap did not fall to any of the “celestial
-police,” but to an Italian astronomer, Piazzi, at the time engaged on
-a new star chart. An illness of Piazzi caused it to be lost almost as
-soon as found. In this plight an appeal was made to the remarkable
-Gauss, just starting on his career. Gauss undertook the problem and
-devised formulæ by which its place was predicted and the planet itself
-recovered. It proved to fit admirably the gap. But it had hardly
-been recovered before another planet turned up equally filling the
-conditions. Ceres, the first, lay at 26.67 astronomical units from the
-Sun; Pallas, the second, at 27.72. Two claimants were one too many. But
-the inventive genius of Olbers came to the rescue. By a bold hypothesis
-he suggested that since two had appeared where only one was wanted,
-both must originally have formed parts of a single exploded planet. He
-predicted that others would be detected by watching the place where the
-explosion had occurred, to wit: where the orbits of Ceres and Pallas
-nearly intersected in the signs of the Virgin and the Whale.
-
-For in the case of an explosion the various parts, unless perturbed,
-must all return in time to the scene of the catastrophe. By following
-his precept, two more were in fact detected in the next two years, Juno
-and Vesta. His hypothesis seemed to be confirmed. No new planets were
-discovered, and the old fulfilled fairly what was required of them.
-Lagrange on calculation gave it his mathematical assent.
-
-Nevertheless, it was incorrect, as events eventually showed, though
-for forty years it slept in peace, no new asteroids being found. We
-now know that this was because the rest were all much smaller, and for
-such nobody looked. It was not till 1845 that Hencke, an ex-postmaster
-of Driessen in Prussia, after fifteen years of search detected another,
-Astræa, of the 11th magnitude. After this discoveries of them came on
-apace, until now more than six hundred are known, and their real number
-seems to be legion. But those discovered are smaller each year on the
-average, showing that the larger have already been found. Their orbits
-are such that they cannot possibly ever have all formed part of a
-pristine whole. The idea, not the body, was exploded. For they are now
-recognized as having always been much as they are to-day.
-
-[Illustration: ASTEROIDS.
-
-_MAJOR AXES OF ORBITS._]
-
-They prove to be thickest at nearly the point where Bode’s law
-required, the spot where Ceres and Pallas were found. The mean of
-their distances is less, being 2.65 instead of 2.8 astronomical
-units, probably simply because the nearer ones are easier discovered.
-The fact that they are clustered most thickly just inside 2.8
-astronomical units implies that there of all points within the space
-between Mars and Jupiter a planet would have formed if it could. A
-definite reason exists for its failure to do so—Jupiter’s disturbing
-presence. Throughout this whole region Jupiter’s influence is great;
-so great that his scattering effect upon the particles exceeds their
-own tendency to come together. We see this in the arrangement of the
-orbits. If we plot the orbits of the asteroids, we shall be struck by
-the emergence of certain blanks in the ribbon representing sections
-of their path. It is the woof of a plaid of Jupiter’s weaving. The
-gaps are where asteroids revolving about the Sun would have periods
-commensurate with his, ²/₅, ¹/₂, ³/₅, ⁴/₇, and the like. Such bodies
-would return after a few revolutions, five of theirs, for instance,
-to Jupiter’s two, into the same configurations with him at the
-same points of their orbits. Thus the same perturbation would be
-repeated over and over again until the asteroid’s path was so changed
-that commensurability ceased to exist. And it would be long before
-perturbation brought it back again. Thus the orbits are constantly
-swinging out and in, all of them within certain limits, but those
-are most disturbed which synchronize with his. In this manner he has
-fashioned their arrangement and even prevented any large planet from
-forming in the gap.
-
-Such restrictive action is not only at work to-day in the distribution
-of the asteroids and in the partitions of Saturn’s ring, but it must
-have operated still more in the past while the system was forming. To
-Professor Milham of Williamstown is due the brilliant suggestion that
-this was the force that fashioned the planetary orbits. For a planet
-once given off from a central mass would exercise a prohibitive action
-upon any planet trying to form within. In certain places it would not
-allow it to collect at all. The evolution of the solar family would
-resemble that of some human ones in which each child brings up the next
-in turn. So that the planetary system made itself, as regards position,
-a steadily accumulative set of prohibitions combining to leave only
-certain places tenantable.
-
-In this manner we may perhaps be brought back to Bode’s law as
-representing within a certain degree of approximation a true mechanical
-result, although no such exact relation as the law demands exists.
-That a relation seemingly close to it is necessitated by the several
-successive inhibitions of each planet upon the next to form, is quite
-possible.
-
-One other general trait about their orbits is worth animadversion. In
-spite of being eccentric and inclined, they are all traversed in the
-same sense. Every one of the asteroids travels direct like the larger
-planets. In this they differ from cometary paths, which are as often
-retrograde as direct. Thus in more ways than one they hold a mid-course
-in regularity between the steady, even character of the planets proper
-and what was for long deemed the erratic behavior of the cometary class
-of cosmic bodies. Very telling this fact will be found with regard to
-the genesis of the solar family, as we shall see later.
-
-With regard now to their more individual characteristics, the asteroids
-may be said to agree in one point—their diversity, not only to all
-the larger members of the solar family, but to one another. For they
-travel in orbits ranging in ellipticity all the way from such as nearly
-approach circles to ellipses of cometary eccentricity. They voyage,
-too, without regard to the dynamical plane of the system, or, what is
-close to it, the ecliptic; departing from the general level often 30°
-and, in one instance, that of the little planet dubbed W. D., by as
-much as 48°. This eccentricity and inclination put them in a class by
-themselves. It is associated and unquestionably connected mechanically
-with another trait which likewise distinguishes them from the planets
-more particularly called—their diminutive size. Only four—Vesta, Ceres,
-Pallas, and Juno—out of the six hundred odd now known exceed a hundred
-miles in diameter, and the greater number are hardly over ten or twenty
-miles across. Very tiny worlds indeed they would seem, could we get
-near enough to them to discern their forms and features. Curiously
-enough, reasoning on certain light changes they exhibit has enabled
-us to divine something of their shapes, and even character. Thus it
-was soon perceived that Eros fluctuated in the light he sent us, being
-at times much brighter than at others. In February and March, 1901,
-the changes were such that their maximum exceeded three times their
-minimum two hours and a half later. Then in May the variation vanished.
-More than one explanation has been put forward, but the best so far,
-because the most simple, is that the body is not a sphere but a jagged
-mass, a mountain alone in space, and that as it turns upon its axis
-first one corner and then another is presented to our view or throws
-a shade upon its neighbor. When the pole directly faces us, no great
-change occurs, especially if it also nearly faces the Sun. Yet even
-this fails to explain all its vagaries.
-
-Eros is not alone in thus exhibiting variation. Sirona, Hertha, and
-Tercidina have also shown periodic variability, and it is suspected in
-others. Indeed, it would be surprising did they not show change. For
-they are too small to have drawn their contents into symmetry, and so
-remain as they were when launched in space. Mammoth meteorites they
-undoubtedly are.
-
-With the asteroids we leave the inner half of the Sun’s retinue and
-pass to the outer. Indeed, the asteroids not only mark in place the
-transition bound between the two, but stamp it such mechanically. In
-their own persons they witness that no large body was here allowed to
-form. The culmination of coalition was reached in Jupiter, and that
-very acme of accretion prevented through a long distance any other.
-
-[Illustration: DRAWING OF JUPITER BY DR. LOWELL. APRIL 12, 1907.]
-
-In bulk, the major planets compared with the inner or terrestrial ones
-form a class apart; and among the major Jupiter is by all odds first.
-His mass is 318 times the Earth’s and his volume nearly 1400 times
-hers. From this it appears that his density is very much less. Indeed,
-his substance is only fractionally denser than water. This and its
-tremendous spin, carrying a point at its equator two hundred and eighty
-thousand miles round in less than ten hours, flatten it to a very
-marked oval with an ellipticity of 1/15.5. Not the least beautiful of
-the revelations of astronomy are the geometrical shapes of the heavenly
-bodies, proceeding from nearly perfect spheres like the Sun or Moon to
-marked spheroids like Jupiter or Saturn. So enormous are the masses and
-the forces concerned that the forms assumed under them are mechanically
-regular. They are the visible expression of gravitation, and so delight
-the brain while they satisfy the eye.
-
-It is to appreciation of the detail visible on Jupiter’s disk that
-modern advance in the study of the planet is indebted. Examination has
-shown its features to be of great interest. To Mr. Stanley Williams of
-Brighton, England, much of our knowledge is due, and Mr. Scriven Bolton
-has also made some interesting contributions. The big print of the
-subject, read long ago, is that the planet’s disk is noticeably banded
-by dark belts. Two characteristics of these belts are important. One
-is that they exhibit a regular secular progression with the lapse of
-years, the south tropical belt being broader and more salient for many
-years in succession, and then gradually fading out while the northern
-one increases in prominence. It has been suspected that the rhythm of
-their change is connected with that of sun spots. The second is that
-the belts do not preserve in their several features the same relation
-in longitude toward one another. They all rotate, but at different
-speeds. There could be no better proof that Jupiter is no solid, but
-a seething mass of heavy vapors boiling like a caldron. Tempered by
-distance we can form but a faint idea of the turmoil there going
-on. Further indication of it is furnished by its glow. For all the
-dark belts are a beautiful cherry red, a tint extending even to the
-darkish hoods over the planet’s caps. This hue comes out well in good
-seeing, and best, as with all planetary markings, in twilight, not at
-night, because the excessive brightness of the disk is then taken off,
-preventing the colors from being swamped.
-
-This brings us to the planet’s albedo, which Müller at Potsdam
-has found to be 75 per cent. Now the interest attaching to this
-determination is twofold, that it bespeaks cloud and that it seems to
-imply something else. The albedo of cloud is 72 per cent of absolute
-whiteness. What looks like cloud, then, is such, on that distant
-disk. But Jupiter surpasses cloud in lustre, since his albedo exceeds
-72 per cent. Yet a large part of his surface is strikingly darker
-than that. The inference from this is that he shines by intrinsic
-light, in part at least. The fact may not be stated dogmatically,
-as there is no astronomic determination so uncertain as this one of
-determining albedoes, and therefore Herr Müller’s results must be
-accepted with every reserve, but they suggest that Jupiter is still a
-semi-sun, to be recognized as such by light as well as heat, though his
-self-luminosity, if it exist at all, can hardly exceed a dull red glow.
-
-[Illustration: I.
-
-JUPITER AND ITS WISPS.—A DRAWING BY DR. LOWELL, APRIL 11, 1907.]
-
-[Illustration: II.
-
-JUPITER AND ITS WISPS.—A DRAWING BY DR. LOWELL, APRIL 11, 1907.]
-
-[Illustration: S.
-
-N.
-
-PHOTOGRAPH OF JUPITER, 1909. P. L.]
-
-A modern detection on Jupiter’s disk has been that of wisps or lacings
-across the bright equatorial belt, a detail of importance due to
-Mr. Scriven Bolton. Requested to look for them, the observatory at
-Flagstaff was not long in corroborating this interesting phenomenon.
-The peculiarity about them pointed out by Mr. Bolton is that they
-traverse the belt at an angle of about 45° to the vertical, proceeding
-from caret-shaped dark spots projecting into the bright belt from the
-dark ones on either side. They exist all round the equator and are
-found indifferently dextrous or sinister—sometimes vertical. For there
-are others that go straight across. Nor are they confined to the bright
-equatorial belt, but are to be seen traversing all of the bright belts
-both north or south up to the polar hoods. From its sombreness it seems
-that we are here regarding a phenomenon in the negative; remarking it
-by what it has left behind, not by what it has accomplished. For the
-wisps are not wisps of cloud, since they are dark, not light, but gaps
-strung out in the clouds themselves.
-
-Recently photographs of Jupiter have been secured at Flagstaff, by
-the new methods there of planetary photography, showing a surprising
-amount of detail. The wisps come out with certainty, and the white
-spots, which are such a curious feature of the disk, have also left
-their impress on the plate. Not the least of the services thus rendered
-by the camera is the accurate positioning of the belts made possible
-by it. Micrometric measures are all very well when nothing better
-is attainable, but any one who has made such upon a planet’s disk
-swinging like a lantern in the field of view under a variety of causes
-instrumental and optical, knows how encumbered they inevitably are
-with error. To have the disk caught and fixed on a plate where it
-may be measured at leisure and as often as one likes, is a distinct
-advance toward fundamental accuracy. Measures thus effected upon the
-Jupiter images of 1909 proved the bright equatorial belt to lie exactly
-upon the planet’s equator when allowance was made for the tilt of the
-planet’s axis toward the Earth. This showed that the aspect of the
-planet toward the Sun had no effect upon the position of the belt.
-Jupiter’s cloud formation, therefore, is not dependent, as all ours
-are, upon the solar heat.
-
-A like indifference to solar action is exhibited in the utter
-obliviousness of the belts to day or night. To them darkness and light
-are nugatory alike. They reappear round the sunrise edge of the disk
-just as they left it when they sank from sight round the sunset one,
-and they march across its sunlit face without so much as a flicker on
-their features.
-
-Yet this seeming immobility from moment to moment takes place in what
-is really a seething furnace, the fiery glow of which we catch below
-the vast ebullition of cloud in the cherry hue of its darker portions.
-Distance has merged the turmoil into the semblance of quiescence and
-left only its larger secular changes to show. Even so the Colorado
-River from the brink of the Grand Cañon is seen apparently at rest, the
-billows of its rapids so stereotyped to stability one takes the rippled
-sand bank for the river and the billows of the river for the ripple
-marks of its banks.
-
-At twice the distance of Jupiter we cross the orbit of Saturn. Here the
-ringed planet, with an annual sweep of twenty-nine and a half of our
-years, pursues his majestic circuit of the Sun. Diademed with three or
-more circlets of light and diamonded by ten satellites, he rivals in
-his cortège that of his own lord. In some ways his surpasses the Sun’s.
-For certainly his retinue is the more spectacular of the two; the more
-so that it is much of it fairly comprised within a single glance. Very
-impressive Saturn is as, attended thus, he sails into the field of
-view.
-
-[Illustration: SATURN—A DRAWING BY DR. LOWELL, SHOWING AGGLOMERATIONS.]
-
-In our survey we may best begin with his globe. If Jupiter’s
-compression is striking, Saturn’s is positively startling when well
-displayed. This happens but at rare intervals. As the plane of his
-equator is almost exactly that of the rings, the flattening is
-conspicuous only on those occasions when the rings disappear because
-their plane passes through the line of sight. Seen at such times the
-effect of the discrowned orb is so strange as to suggest delusion.
-This occurred two years ago in 1907, and when the planet was picked up
-by its position and entered the field unheralded by its distinctive
-appendage, it was almost impossible to believe there had not been
-some mistake and a caricatured Jupiter had taken its place. For the
-flattening outdoes that of Jupiter as 3 to 2, being ⅒ of the equatorial
-diameter. Such a bulging almost suggests disruption and is due to
-the extreme lightness of the planet’s substance, which is actually
-only 0.72 of that of water. Like Jupiter, the disk exhibits belts,
-though very much fainter, and, like his, these are of a cherry red. As
-the planet’s albedo is even greater, 0.78 of absolute whiteness, as
-deduced from H. Struve’s measures of the diameter, the same suspicion
-of shining, at least in part, from inherent light, applies equally
-to him. But it is practically certain that in neither case does this
-light equal that of the planet’s clouds, or add anything to them. Both
-planets are red-hot, not white-hot. The determination of the albedo
-depends upon that of the diameter, and an increase in the latter would
-lower the albedo to that of cloud.
-
-His most unique possession are his rings. Broad, yet tenuous, they
-weigh next to nothing, being, as Struve has dubbed them, “Immaterial
-light.” Nevertheless, it is not their lightness but their make-up that
-prevents from lying uneasy the head that wears this crown.
-
-The mechanical marvel was not appreciated by early astronomers, who
-took it for granted that they were what they seemed, solid, flat rings,
-all of a piece. Even Laplace considered it sufficient to divide them up
-concentrically to insure stability. To Edouard Roche of Montpellier,
-as retiringly modest as he was penetratingly profound, is due the
-mathematical detection that to subsist they must be composed of
-discrete particles,—brickbats, Clerk Maxwell called them, when, later,
-unaware of Roche’s work, he proved independently the same thing in his
-essay on Saturn’s rings. Peirce, too, in ignorance of Roche, had half
-taken the same step a little before, showing that they must at least
-be fluid. Then in 1895 Keeler ingeniously photographed the spectrum of
-both ball and rings to the revealing of velocities in the line of sight
-of the different portions of the spectrum exactly agreeing with the
-values mechanics demanded.
-
-The rings have usually been considered to be flat. At the time of
-their disappearance, however, knots have been seen upon them. It is
-as if their filament had suddenly been strung with beads. At the last
-occurrence of the sort in 1907, these beads were particularly well seen
-at several observatories, and were critically studied at Flagstaff. In
-connection with a new phenomenon detected there, that of a dark core
-in the shadow the rings threw across the planet’s face, an explanation
-suggested itself to account for both them and it: to wit, that the
-rings were not really flat, but tores; rings, that is, like an anchor
-ring, any cross-section of which would be of the nature of an oval
-flattened on its inner side. The cogency of the explanation consisted
-in its solution not only of the appearances but of the cause competent
-to bring those appearances about.
-
-For measurement showed that the knots were permanent in position,
-which, since the ring revolved, indicated that they extended all round
-it in spite of their not seeming to do so, and that their distances
-from Saturn were just what this cause should produce.
-
-The action observed was a corollary from the important principle
-of commensurability of orbital period. As we saw in the case of
-the asteroids, if two bodies be travelling round a third and their
-respective periods of revolution be commensurate, they will constantly
-meet one another in such a manner that great perturbation will ensue
-and the bodies be thrown out of commensurability of period.
-
-What has happened to the asteroids has likewise occurred in Saturn’s
-rings. The disturber in this case has been, not Jupiter, as with them,
-but one or other of Saturn’s own satellites. For when we calculate
-the problem, we find that Mimas, Enceladus, and Tethys have periods
-exactly commensurate with the divisions of the rings; in other words,
-these three inner satellites, whose action because of proximity is the
-greatest, have fashioned the rings into the three parts we know, called
-A, the outermost; B, the middle one; and C, the crêpe ring, nearest to
-the body of the planet. Mimas has been the chief actor, though helped
-by the two others, while Enceladus has further subdivided ring A by
-what is known as Encke’s division.
-
-Such has been the chief action of the satellites on the rings: it has
-made them into the system we see. But if we consider the matter, we
-shall realize that a secondary result must have ensued—when we remember
-that the particles composing the rings must be very crowded for the
-rings to show as bright as they do, and also that, though relatively
-thin, the rings are nevertheless some eighty miles through.
-
-Now it is evident that any disturbance in so closely packed a system
-of small bodies as that constituting Saturn’s rings must result in
-collisions between the bodies concerned. Particles pulled out or in
-must come in contact with others pursuing their own paths, and as at
-each collision some energy is lost by the blow, a general falling in
-toward the planet results. At the same time, as the blow will not
-usually be exactly in the plane in which either particle was previously
-moving, both will be thrown more or less out of the general plane of
-their fellows, and the ring at that point, even if originally flat,
-will not remain so. For the ring, though very narrow relatively, has a
-real thickness, quite sufficient for slantwise collision, if the bodies
-impinge.
-
-[Illustration: _Saturn’s Rings._
-
-_November 1907._]
-
-Now the knots or beads on the rings appeared exactly inside the points
-where the satellites’ disturbing action is greatest, or, in other
-words, in precisely their theoretic place. We can hardly doubt that
-such, then, was their origin.[9]
-
-[9] Paper by the writer in the _Phil. Mag._, April, 1908.
-
-The result must be gradually to force the particles as a rule nearer
-the planet, until they fall upon its surface, while a few are forced
-out to where they may coalesce into a satellite,—a result foreseen long
-ago by Maxwell. It is this process which in the knots we are actually
-witnessing take place, and which, like the corona about the eclipsed
-Sun, only comes out to view when the obliterating brightness of the
-main body of the rings is withdrawn by their edgewise presentation.
-
-The reason the out-of-plane particles are most numerous just inside the
-point of disturbance is not only that there the action throwing them
-out is most violent, but that all the time a levelling action quite
-apart from disturbance is all the time tending to reduce them again to
-one plane, as we shall see further on when we come to the mechanical
-forces at work. Thus the tore is most pronounced on its outer edge, and
-falls to a uniform level at its inner boundary. The effect is somewhat
-as represented in the adjoining cut, in which the vertical scale is
-greatly magnified:—
-
-[Illustration: THE TORES OF SATURN. Not drawn to scale.]
-
-With Saturn ended the bounds of the solar system as known to the
-civilized world until 1781. On March 13 of that year Sir William
-Herschel in one of his telescopic voyages through space came upon
-a strange object which he at once saw was not a star, because of
-its very perceptible round disk, and which he therefore took for a
-peculiar kind of comet. Nearly a year rolled by before Lexell showed
-by calculation of its motion that it was no comet, but undoubtedly a
-new planet beyond Saturn travelling at almost twice that body’s mean
-distance from the Sun.
-
-By reckoning backward, it was found to have been seen and mapped
-several times as a star,—no less than twelve times by Lemonnier
-alone,—and yet its planetary character had slipped through his fingers.
-It can even be seen with the naked eye as a star of the 6th magnitude,
-and its course is said to have been watched by savage tribes in
-Polynesia long before Sir William Herschel discovered it.
-
-Its greenish blue disk indicates that it is about thirty-two thousand
-miles in diameter, and its mass that its density is about 0.22 of
-the Earth’s or, like Jupiter’s, somewhat greater than water. Of its
-surface we probably see nothing. Indeed, it is very doubtful if it
-have any surface properly so called, being but a ball of vapors. Its
-flattening, ¹/₁₁ according to Schiaparelli, which is probably the best
-determination, agrees with the density given above, indicating its
-substance to be very light. Belts have faintly been descried traversing
-its disk after the analogy of Jupiter and Saturn. These would be much
-better known than they are but for the great tilt of the planet’s axis
-to the ecliptic, so that during a part of its immense annual sweep its
-poles are pointed nearly at the Earth, and its tropical features, the
-places where the belts lie, are wholly hidden or greatly foreshortened
-from our point of view. As the planet’s year is eighty-four of our
-years long, it is only at intervals of forty odd years that the disk is
-well enough displayed to bring the belts into observable position.
-
-The planet is attended by four satellites,—Ariel, Umbriel, Titania,
-and Oberon,—a midsummer night’s dream to a watcher of the skies. They
-travel in a plane inclined 98° to the ecliptic, so that their motion is
-nearly up and down to that plane and even a little backward. Whether
-their plane is also the equatorial plane of the planet, we do not know
-for certain. The observations as yet are not conclusive one way or the
-other. If the two planes should turn out not to coincide, it will open
-up some new fields in celestial mechanics. The belts have been thought
-to indicate divergence, but the most recent observations by Perrotin on
-them minimize this. They suggest, too, a rotation period of about ten
-hours, which is what we should expect.
-
-Its albedo, or intrinsic brightness, is, according to Müller, 0.73,
-or almost exactly that of cloud. This tallies with the lack of
-pronouncement of the belts and is another argument against the reality
-of the recent diametral measurements, as all Müller’s values are got by
-dividing the amount of light received by the amount of surface sending
-it. If the diameter were much less than thirty-two thousand miles, the
-resulting albedo would become impossibly high.
-
-If we know but little about the actual surface of Uranus, we know now a
-good deal about its atmosphere. And this partly because atmosphere is
-almost all that it is. The satellites are the only solid thing in the
-system. If we needed a telltale that the solar system had evolved, the
-gaseous constitution of its primaries and the condensed state of their
-attendants would sufficiently inform us. Probably all the major planets
-are nothing but gas. It has been debated whether Jupiter be almost all
-vapor with a solid kernel beneath, or vapor entirely. That he grows
-denser toward the core is doubtless the case, but that he is anywhere
-other than a gaseous fluid is very unlikely. For if he had really
-begun to condense, he must have contracted to far within his present
-dimensions. The same is true of Uranus.
-
-The surprising thing about Uranus is the enormous extent of his
-atmosphere. The earliest spectroscopists perceived this, but the more
-spectroscopy advances, the greater and more interesting it proves to
-be. By pushing inquiry into the red end of the spectrum, hitherto a
-terra incognita, Dr. Slipher has uncovered a mass of as yet unexplained
-revelation. Of these remarkable spectrograms we shall speak later.
-Here it is sufficient to say that so great is the absorption in the
-red that only the blue and green in anything like their entirety get
-through; which accounts for the well-known sea-green look of the
-planet. Furthermore, the spectroscope shows that this atmosphere, or
-the great bulk of it, must lie above what we see as the contour of the
-disk. For the spectroscope is as incapable of seeing through opacity as
-the eye, though it distances the eye in seeing the invisible. It is not
-what is condensed into cloud, but what is not, of which it reveals the
-presence. We are thus made aware of a great shell of air enveloping the
-planet.
-
-In Uranus, then, we see a body in an early amorphous state, before the
-solid, the liquid, and the gaseous conditions of matter have become
-differentiate and settled each into distinctive place. Without even an
-embryo core its substance passes from viscosity to cloud.
-
-Neptune has proved a planet of surprises. Though its orbital revolution
-is performed direct, its rotation apparently takes place backward, in a
-plane tilted about 35° to its orbital course. Its satellite certainly
-travels in this retrograde manner. Then its appearance is unexpectedly
-bright, while its spectrum shows bands which as yet, for the most part,
-defy explanation, though they state positively the vast amount of its
-atmosphere and its very peculiar constitution. But first and not least
-of its surprises was its discovery,—a set of surprises, in fact. For
-after owing recognition to one of the most brilliant mathematical
-triumphs, it turned out not to be the planet expected.
-
-“Neptune is much nearer the Sun than it ought to be,” is the
-authoritative way in which a popular historian puts the intruding
-planet in its place. For the planet failed to justify theory by not
-fulfilling Bode’s law, which Leverrier and Adams, in pointing out the
-disturber of Uranus, assumed “as they could do no otherwise.” Though
-not strictly correct, as not only did both geometers do otherwise, but
-neither did otherwise enough, the quotation may serve to bring Bode’s
-law into court, as it was at the bottom of one of the strangest and
-most generally misunderstood chapters in celestial mechanics.
-
-Very soon after Uranus was recognized as a planet, approximate
-ephemerides of its motion resulted in showing that it had several times
-previously been recorded as a fixed star. Bode himself discovered the
-first of these records, one by Mayer in 1756, and Bode and others
-found another made by Flamstead in 1690. These observations enabled an
-elliptic orbit to be calculated which satisfied them all. Subsequently
-others were detected. Lemonnier discovered that he had himself not
-discovered it several times, cataloguing it as a fixed star. Flamstead
-was spared a like mortification by being dead. For both these
-observers had recorded it two or more nights running, from which it
-would seem almost incredible not to have suspected its character from
-its change of place.
-
-Sixteen of these pre-discovery observations were found (there are now
-nineteen known), which with those made upon it since gave a series
-running back a hundred and thirty years, when Alexis Bouvard prepared
-his tables of the planet, the best up to that time, published in 1821.
-In doing so, however, he stated that he had been unable to find any
-orbit which would satisfy both the new and the old observations. He
-therefore rejected the old as untrustworthy, forgetting that they had
-been satisfied thirty years before, and based his tables solely on
-the new, leaving it to posterity, he said, to decide whether the old
-observations were faulty or whether some unknown influence had acted
-on the planet. He had hardly made this invidious distinction against
-the accuracy of the ancient observers when his own tables began to be
-out and grew seriously more so, so that within eleven years they quite
-failed to represent the planet.
-
-The discrepancies between theory and observation attracted the
-attention of the astronomic world, and the idea of another planet
-began to be in the air. The great Bessel was the first to state
-definitely his conviction in a popular lecture at Königsberg in 1840,
-and thereupon encouraged his talented assistant Flemming to begin
-reductions looking to its locating. Unfortunately, in the midst of his
-labors Flemming died, and shortly after Bessel himself, who had taken
-up the matter after Flemming’s death.
-
-Somewhat later Arago, then head of the Paris observatory, who had also
-been impressed with the existence of such a planet, requested one of
-his assistants, a remarkable young mathematician named Leverrier, to
-undertake its investigation. Leverrier, who had already evidenced
-his marked ability in celestial mechanics, proceeded to grapple with
-the problem in the most thorough manner. He began by looking into
-the perturbations of Uranus by Jupiter and Saturn. He started with
-Bouvard’s work, with the result of finding it very much the reverse
-of good. The farther he went, the more errors he found, until he was
-obliged to cast it aside entirely and recompute these perturbations
-himself. The catalogue of Bouvard’s errors he gave must have been an
-eye-opener generally, and it speaks for the ability and precision
-with which Leverrier conducted his investigation that neither Airy,
-Bessel, nor Adams had detected these errors, with the exception of
-one term noticed by Bessel and subsequently by Adams.[10] The result
-of this recalculation of his was to show the more clearly that the
-irregularities in the motion of Uranus could not be explained except by
-the existence of another planet exterior to him. He next set himself
-to locate this body. Influenced by Bode’s law, he began by assuming
-it to lie at twice Uranus’ distance from the Sun, and, expressing
-the observed discrepancies in longitude in equations, comprising the
-perturbations and possible errors in the elements of Uranus, proceeded
-to solve them. He could get no rational solution. He then gave the
-distance and the extreme observations a certain elasticity, and by this
-means was able to find a position for the disturber which sufficiently
-satisfied the conditions of the problem. Leverrier’s first memoir
-on the subject was presented to the French Academy on November 10,
-1845, that giving the place of the disturbing planet on June 1, 1846.
-There is no evidence that the slightest search in consequence was
-made by anybody, with the possible exception of the Naval Observatory
-at Washington. On August 31 he presented his third paper, giving an
-orbit, mass, and more precise place for the unknown. Still no search
-followed. Taking advantage of the acknowledging of a memoir, Leverrier,
-in September, wrote to Dr. Galle in Berlin asking him to look for
-the planet. The letter reached Galle on the 23d, and that very night
-he found a planet showing a disk just as Leverrier had foretold, and
-within 55′ of its predicted place.
-
-[10] Adams, “Explanation of the Motion of Uranus,” 1846.
-
-The planet had scarcely been found when, on October 1, a letter from
-Sir John Herschel appeared in the _London Athenæum_ announcing that
-a young Cambridge graduate, Mr. J. C. Adams, had been engaged on
-the same investigation as Leverrier, and with similar results. This
-was the first public announcement of Mr. Adams’ labors. It then
-appeared that he had started as early as 1843, and had communicated
-his results to Airy in October, 1845, a year before. Into the sad set
-of circumstances which prevented the brilliant young mathematician
-from reaping the fruit of what might have been his discovery, we need
-not go. It reflected no credit on any one concerned except Adams, who
-throughout his life maintained a dignified silence. Suffice it to say
-that Adams had found a place for the unknown within a few degrees of
-Leverrier’s; that he had communicated these results to Airy; that Airy
-had not considered them significant until Leverrier had published an
-almost identical place; that then Challis, the head of the Cambridge
-Observatory, had set to work to search for the planet but so routinely
-that he had actually mapped it several times without finding that he
-had done so, when word arrived of its discovery by Galle.
-
-But now came an even more interesting chapter in this whole
-strange story. Mr. Walker at Washington and Dr. Petersen of Altona
-independently came to the conclusion from a provisional circular orbit
-for the newcomer that Lalande had catalogued in the vicinity of its
-path. They therefore set to work to find out if any Lalande stars were
-missing. Dr. Petersen compared a chart directly with the heavens to
-the finding a star absent, which his calculations showed was about
-where Neptune should have been at the time. Walker found that Lalande
-could only have swept in the neighborhood of Neptune on the 8th and
-10th of May, 1795. By assuming different eccentricities for Neptune’s
-orbit under two hypotheses for the place of its perihelion, he found
-a star catalogued on the latter date which sufficiently satisfied his
-computations. He predicted that on searching the sky this star would be
-found missing. On the next fine evening Professor Hubbard looked for
-it, and the star was gone. It had been Neptune.[11]
-
-[11] Proc. Amer. Acad., Vol. I, p. 64.
-
-This discovery enabled elliptic elements to be computed for it, when
-the surprising fact appeared that it was not moving in anything
-approaching the orbit either Leverrier or Adams had assigned. Instead
-of a mean distance of 36 astronomical units or more, the stranger was
-only at 30. The result so disconcerted Leverrier that he declared that
-“the small eccentricity which appeared to result from Mr. Walker’s
-computations would be incompatible with the nature of the perturbations
-of the planet Herschel,” as he called Uranus. In other words, he
-expressly denied that Neptune was his planet. For the newcomer
-proceeded to follow the path Walker had computed. This was strikingly
-confirmed by Mauvais’ discovering that Lalande had observed the star on
-the 8th of May as well as on the 10th, but because the two places did
-not agree, he had rejected the first observation, and marked the second
-as doubtful, thus carefully avoiding a discovery that actually knocked
-at his door.
-
-Meanwhile Peirce had made a remarkable contribution to the whole
-subject. In a series of profound papers presented to the American
-Academy, he went into the matter more generally than either of the
-discoverers, to the startling conclusion “that the planet Neptune
-is not the planet to which geometrical analysis had directed the
-telescope, and that its discovery by Galle must be regarded as a happy
-accident.”[12] He proved this first by showing that Leverrier’s two
-fundamental propositions,—
-
-    1. That the disturber’s mean distance must be
-       between 35 and 37.9 astronomical units;
-
-    2. That its mean longitude for January 1, 1800,
-       must have been between 243° and 252°,—
-
-were incompatible with Neptune. Either alone might be reconciled with
-the observations, but not both.
-
-[12] Proc. Amer. Acad., Vol. I, p. 65 _et seq._
-
-In justification of his assertion that the discovery was a happy
-accident, he showed that three solutions of the problem Leverrier
-had set himself were possible, all equally complete and decidedly
-different from each other, the positions of the supposed planet being
-120° apart. Had Leverrier and Adams fallen upon either of the other
-two, Neptune would not have been discovered.[13]
-
-He next showed that at 35.3 astronomical units, an important change
-takes place in the character of the perturbations because of the
-commensurability of period of a planet revolving there with that of
-Uranus. In consequence of which, a planet inside of this limit might
-equally account for the observed perturbations with the one outside
-of it supposed by Leverrier. This Neptune actually did. From not
-considering wide enough limits, Leverrier had found one solution,
-Neptune fulfilled the other.[14] And Bode’s law was responsible for
-this. Had Bode’s law not been taken originally as basis for the
-disturber’s distance, those two great geometers, Leverrier and Adams,
-might have looked inside.
-
-[13] Proc. Amer. Acad., Vol. I, p. 144.
-
-[14] Proc. Amer. Acad., Vol. I, p. 332.
-
-This more general solution, as Peirce was careful to state, does not
-detract from the honor due either to Leverrier or to Adams. Their
-masterly calculations, the difficulty of which no one who has not
-had some experience of the subject can appreciate, remain as an
-imperishable monument to both, as does also Peirce’s to him.
-
-
-
-
-CHAPTER V
-
-FORMATION OF PLANETS
-
-
-In our first two chapters we saw what sign-posts in the sky there are
-pointing to the course evolution of a solar system probably follows,
-and secondly, what evidence there is that our system took this road.
-We now come to a question not so easy to precise,—the actual details
-of the journey. It is always difficult to descend from a glittering
-panoramic survey to particular path-finding. The obstacles loom so much
-larger on a near approach.
-
-Most men shy at decisions and shun self-committal to any positive
-course, but when it comes to constructing a cosmogony, few at all
-qualified hesitate to frame one if the old does not suit. The safety
-in so doing lies in the fact that nothing in particular happens if
-it refuses to work. Its absurdity is promptly shown up, it is true,
-by some one else. For there is almost as good a trade in exposing
-cosmogonies as in constructing them. But no special opprobrium attaches
-to failure, because everybody has failed, from Laplace down, or up,
-as you are pleased to consider it. Besides it is really not so easy
-to do, as one is tempted to believe before his book is published.
-Then only does the difficulty dawn, with a speed and clarity inversely
-proportional to the previous relation of the critic to the author. For
-the author himself is apt to be blind. With the fatal fondness of a
-parent for his offspring it is rare for the defects to be so glaringly
-apparent to their perpetrator. At the worst he considers them venial
-faults which can be glossed away.
-
-Attacking the subject in this judicial spirit, the reader can hardly
-expect me to satisfy him with a cosmogony entirely home-made, but
-at best to pursue a happy middle course between creator and critic,
-advocating only such portions as happen to be my own, while sternly
-exposing the mistakes of others.
-
-In undertaking the hazardous climb toward the origin of things two
-qualities are necessary in the explorer: a quick eye for possibilities
-and a steady head in testing them. Without the discernment to perceive
-relations no ascent to first principles is possible; and without the
-support of quantitative criterion, one is in danger of becoming giddy
-from one’s own imagination. Congruities must first hint at a path;
-physical laws then determine its feasibility.
-
-An eye for congruities is the first essential. For congruity alone
-accuses an underlying law. It is the analogic that with logic leads to
-great generalizations. Certain concords of the sort in the motions of
-the planets were what suggested to Laplace his system of the world.
-With the uncommon sense of a mathematician he perceived that such
-accordances were not necessitated by the law of gravitation, and on the
-other hand, could not be due to chance. The laws of probability showed
-millions to one against it. One of these happy harmonies was that all
-the large planets revolved about the Sun in substantially the same
-plane; another that they all travelled in the same sense (direction).
-Had they been unrelated bodies at the start, such agreement in motion
-was mathematically impossible. Their present consensus implied a common
-origin for all. In other words, the solar system must have grown to be
-what it is, not started so.
-
-This basic fact we may consider certain. But from it we would fain
-go on to find out how it evolved. To do so the same process must be
-followed. Considering, then, our solar system from this point of view,
-one cannot but be struck by some further congruities it presents. These
-are not quite those that inspired Laplace, because of discoveries
-since, and demand in consequence a theory different from his.
-
-The out about constructing a theory is that fresh facts will come
-along and knock for admission after the door is shut. They prove
-irreconcilables because they were not consulted in advance. The
-consequence is that since Laplace’s time new relations have come to
-light, and some supposed concords have had to be given up; so that were
-he alive to-day he would himself have formulated some other scheme.
-Two, however, are still as true: that the planets all revolve in the
-same plane and in the same sense, and that sense that of the Sun’s
-rotation. But so general a congruity as this points only to an original
-common moment of momentum and is equally explicable however that motion
-was brought about. It seems quite compatible with an original shock. To
-say that it was caused by a disruption is simply to go one step farther
-back than Laplace. If, then, such a catastrophe did occur as the
-meteorites aver, we may perhaps draw some interesting inferences about
-it from the present state of the system. In a very close approach such
-as we must suppose for the disruption, one within Roches’ limit of 2.5
-diameters, the stranger, supposing him of equal size, would sweep from
-one side of the former Sun to the other in about two hours, and the
-brunt of the disrupting pull occur within that time. That the former
-Sun was rotating slowly seems established by the time, twenty-eight
-days, it now takes to go round. In which case the orbits of the
-masses which were to form the planets would all lie in about the same
-plane,—the plane of the tramp’s approach. If there were exceptions,
-they should be found in the innermost. For such should partake most
-largely of the Sun’s own original rotation and travel therefore most
-nearly in its plane. And as a fact Mercury, the Benjamin, does differ
-from the others by revolving in a plane inclined some 7° to their mean,
-agreeing in this with the Sun’s own rotation, with whose plane it was
-probably originally coincident (digression from it now being due to
-secular retrogression of the planets’ nodes) [see NOTE 4].
-
-From the relations which advance has left unchanged we pass to those
-phenomena which seemed to present congruities in Laplace’s day,
-but which have since proved void owing to subsequent detection of
-exceptions. Time prevents my making the catalogue complete, but the
-reader shall be shown enough to satisfy him of the problem’s complexity
-and to whet his desire for further research—on the part, preferably, of
-others.
-
-[Illustration: CHART SHOWING INCREASING TILTS OF THE MAJOR PLANETS.]
-
-First comes, then, the rotations of the planets upon their axes, which
-Laplace supposed to be all in the same direction, counter to the hands
-of a clock; for the heavens mark time oppositely from us. All those
-within and including Saturn, the only ones he knew, turn, indeed, in
-the same sense that they travel round the Sun. But Uranus departs from
-that direction by a right angle, wallowing rather than spinning in his
-orbit; while Neptune goes still farther in idiosyncratic departure
-and actually turns in the opposite direction. Here, then, Laplace’s
-congruity breaks down, but in its place a little attention will show
-that a new one has arisen. For Saturn’s tilt is 27° and Jupiter’s 3°,
-so that with the major planets there is revealed a systematic righting
-of the planetary axes from inversion through perpendicularity to
-directness as one proceeds inward toward the Sun.
-
-Another congruity supposed to exist a century ago was the exemplary
-agreement of all the satellites to follow in their planetary circuits
-the pattern set them by their primaries round the Sun. But as man has
-penetrated farther into space and photographic plates have come to be
-employed, satellites have been revealed which depart from this orderly
-arrangement. This is the case with the ninth, the outermost, satellite
-of Saturn and with the eighth, the outermost, of Jupiter. But, as
-before, the breaking down of one congruity seems but the establishing
-of another. It appears that only the most distant satellites are
-permitted such unconformity of demeanor. For departure from the
-supposed orthodoxy occurs in both instances where the distance is most,
-and does not occur in the case of all the other satellites found since
-Laplace’s day, eleven in number, nearer their planets.
-
-A third congruity formerly believed in has suffered a like fate; to
-wit, that satellites always moved in or near the equatorial plane of
-their primary. All those first discovered did; the four large ones of
-Jupiter, the main ones of Saturn, and probably those of Uranus and
-Neptune. Even the satellites of Mars conformed. Iapetus alone seemed to
-make exception, and that by a glossable amount. But this orderliness,
-too, has been disposed of, only, like the others, to experience a
-resurrection in a different form.
-
-[Illustration]
-
-On examining more precisely the inclinations of these orbits some years
-ago, an interesting relation between them and the distances of the
-satellites from their primaries forced itself on my notice. The tilt
-increased as the distance grew. The only exceptions were very tiny
-bodies occupying a sort of asteroidal relation to the rest.
-
-A diagram will make this clear. The kernel of it dates from the
-lectures then delivered before the Massachusetts Institute of
-Technology in 1901. The interesting thing now about it is that the
-congruity there pointed out has been conformed to by every satellite
-discovered since,—the sixth, seventh, and eighth of Jupiter and the
-ninth and tenth of Saturn. It is evident that we already know enough of
-the geniture of our system to prophesy something about it and have the
-prophecy come true.
-
-Closely connected with the previous relation is a fourth concordance
-clearly of mechanical origin, the relation of the orbital
-eccentricities of the satellites to their distances from their
-respective planets. The satellites pursue more and more eccentric
-orbits according as they stand removed from planetary proximity.
-
-A fifth congruity is no less striking. All the satellites of all the
-planets that we can observe well enough to judge of turn the same face
-always to their lords. That the Moon does so to the Earth is a fact of
-everyday knowledge, and the telescope hints that the same respectful
-regard is paid by Jupiter’s and Saturn’s retinues to them. What is
-still more remarkable, Mercury and Venus turn out to observe the like
-vassal etiquette with reference to the Sun. And it will be noticed that
-they stand to him the nearest of his court. Here, then, is a law of
-proximity which points conclusively to some well-established force.
-
-Last is a remarkable congruity which study disclosed to me likewise
-some years ago, and which has received corroboration in discoveries
-since. This congruity is the peculiar arrangement of the masses in the
-solar system.
-
-Consider first the way in which the several planets, as respects size,
-stand ordered in distance from the sun. Nearest to him is Mercury, the
-smallest of all the principal ones. Venus and the Earth follow, each
-larger than the last; then comes Mars, of distinctly less bulk, and
-so to the asteroids, of almost none. After this the mass rises again
-to its maximum in Jupiter, and then subsequently falls through Saturn
-to Uranus and Neptune. Here we mark a more or less regular gradation
-between mass and position, a curve in which there are two ups and
-downs, the outer swell being much the larger, though the inner, too, is
-sufficiently pronounced.
-
-Now turn to Saturn and his family, which is the most numerous of the
-secondary systems and that having the greatest span. Under Saturn’s
-wing, as it were, is the ring, itself a congeries of tiny satellites.
-Then comes Mimas, the smallest of the principal ones; then Enceladus,
-a little larger; then Tethys, the biggest of the three. Next stands
-Dione, smaller than Tethys. Then the mass increases with Rhea, reaching
-its culmination in Titan, after which it declines once more. Strangely
-reproductive this of the curve we marked in the arrangement of the
-planets themselves, even to the little inner rise and fall.
-
-[Illustration: MASSES OF PLANETS AND SATELLITES.]
-
-Striking as such analogous ordering is, it is not all. For, scanning
-the Jovian system, we find the main curve here again; Ganymede, the
-Jupiter or Titan of the system, standing in the same medial position
-as they. Lastly, taking up Uranus and his family of satellites, the
-same order is observable there. Titania, the largest, is posted in the
-centre.
-
-Thus the order in which the little and the big are placed with
-reference to their controlling orb is the same in the solar system
-and in that of every one of its satellite families. Method here is
-unmistakable. Nor is it easy to explain unless the cause in all was
-like. That the rule in the placing of the planets should be faithfully
-observed by them in the ordering of their own domestic retinues, is
-not the least strange feature of the arrangement. It argues a common
-principle for both. Not less significant is the secondary hump in
-their distribution, denoting recrudescence farther in of the primary
-procedure shown without.
-
-One point to be particularly noticed in these latter-day congruities
-is that they are not simply general concords like the older ones—the
-fact that the planets move in one plane or in the same sense in that
-plane—but detailed placings, ordered according to the distances of
-the planets from the Sun or of the satellites from the planets. They
-are thus not simply of the combinative but of the permutative order
-of probabilities, a much higher one; in other words, the chance that
-they can be due to chance is multiplicately small. Thus just as these
-analogies are by so much more remarkable, so are they by so much more
-cogent. They tell us not only of an evolution, but they speak of the
-very manner of its work. They do not simply generalize, they specify
-the mode of action. The difficulty is to understand their language. It
-is a case of celestial hieroglyphics to which we lack the key.
-
-In attempting now to discover how all this came about we notice first
-that the system could not have originated in the beautifully simple way
-suggested by Laplace, because of several impossibilities in the path.
-If rings were shed, as he supposed, from a symmetric contracting mass,
-they should have resulted in something even more symmetric than we
-observe to-day. In the next place they could not, it would appear, even
-if formed, have collected into planets.
-
-Nor could there have been an original “fire-mist” with which as a
-stock in trade Laplace thriftily endowed his nebula to start with—the
-necessity for which has been likened to our supposed descent from
-monkeys; but which in truth is as misty a conception of the facts in
-the one case as it is a monkeying with them in the other. Darwin’s
-theory distinctly avers that we were _not_ descended from monkeys;
-and Laplace’s fire-mist under modern examination evaporates away.
-It is an interesting outcome of modern analysis that the very fact
-which suggested the annular genesis of planets to Laplace, the rings
-of Saturn, should now probably be deemed a striking instance of the
-reverse. Far from its being an exemplar in the heavens of the pristine
-state of the solar system, we may now see in it a shining pattern of
-how the devolution of bodies comes about. For instead of typifying an
-unfortunate set of particles which untoward circumstance has prevented
-from coalescing into a single orb, it almost certainly represents the
-distraught state to which a once more compact congeries of them has
-been brought by planetary interference. For to just such fate must
-the stresses in it caused by Saturn have eventually led. Disruption
-inevitable to such a group the observation of comets demonstrates is
-daily taking place. When a comet passes round the Sun or near a planet,
-the partitive pulls of the body tend to dismember it, and the same is
-_a fortiori_ true of matter circulating round a planet as relatively
-near as the meteoric particles that constitute Saturn’s rings. Starting
-as a congeries, it was pulled out more and more into a ring until it
-became practically even throughout. And the very action that produced
-it tends to keep it as surprisingly regular as we note to-day.
-
-No, the planets probably were otherwise generated and may have looked
-in their earlier stages as the knots in the spiral nebulæ do to-day.
-But this does not mean that we can detail the process [see NOTE 5].
-
-Taking now the congruities for guide, we proceed to see what they
-affirm or negative. Laplace, when he ventured on his exposition of
-the system of the world, did so “with the mistrust which everything
-which is not the direct outcome of observation or calculation must
-inspire.” To all who know how even figures can lie this caution will
-seem well timed. The best we can do to keep our heads steady is to
-lay firm hold at each step on the great underlying principles of
-physics. One of these is the conservation of the moment of momentum.
-This expression embodies one of the grandest generalizations of cosmic
-mechanics. The very phrase is fittingly sonorous, with something of
-that religious sublimity which the dear old lady said she found such
-a consolation in the biblical word Mesopotamia. Indeed the idea is
-grand for its very simplicity. Momentum means the quantity of motion
-in a body. It is the speed into the number of particles or the mass.
-Moment of momentum denotes the rotatory power of it round an axis. Now
-the curious and interesting thing about this quantity is that it can
-neither be diminished nor increased. It is an abstraction from which
-nothing can be abstracted—but results. It is the one unalterable thing
-in a universe of change. What it was in the beginning in a system,
-that it forever remains. Because of this unchangeableness we can use
-it very effectively for purposes of deduction. One of these is in
-connection with that other great principle of physics, the conservation
-of energy. By the mutual action of particles on one another, by
-contraction, by tidal pulls, and so on, some energy of motion is
-constantly being changed into heat and thus dissipated away. Energy of
-motion, therefore, is slowly being lost to the system, and the only
-stable state for the bodies composing it is when their energy of motion
-has decreased to the minimum consistent with the initial moment of
-momentum. This principle we shall find very fecund in its application.
-It means that our whole system is evolving in a way to lessen its
-energy of motion while keeping its quantity of motion unchanged. The
-universe always does a thing with the least possible expenditure of
-force and gets rid of its superfluous energy by parting with it to
-space. Philosophers may wrangle over its being the best possible of
-worlds, but it is incontrovertibly mechanically the laziest, which a
-pessimistic friend of mine says proves it the best.
-
-Now this generalization finds immediate use in explaining certain
-features of the solar system. In looking over the congruities it will
-be seen that deviation from the principal plane of the system or
-departure from a circular orbit is always associated with smallness
-in size. The insignificant bodies are the erratic ones. Now it has
-been shown mathematically in several different ways that when small
-particles collect into a larger mass, the collisions tend to make
-the resultant orbit of the combination both more circular and more
-conformant to the general plane than its constituents. But we may see
-this more forthrightly by means of the general principle enunciated
-above. For in fact both results are direct outcomes of the conservation
-of moment of momentum. Given a certain moment of momentum for the
-system, the total energy of the bodies is least when they all move in
-one plane. This is evident at once because the components of motion
-at right angles to the principal plane add nothing to the moment
-of momentum of the system. It is also least when the bodies all
-revolve in circles about the centre of gravity. The circle has some
-interesting properties which almost justify the regard paid to it by
-the ancients as the only perfect figure. It encloses the maximum area
-for a given periphery, so that according to the old legends, if one
-were given as much land as he could enclose with a certain bull’s
-hide, he should, after cutting the hide into strips, arrange these
-along the circumference of a circle. Now this property of the circle
-is intimately connected with the fact that a body revolving in a
-circle has the greatest moment of momentum for the least expenditure
-of energy. For under the same central force all ellipses of the same
-longest diameters—major axes these are technically called—are described
-in the same time, and with the same energy, and of all such, the circle
-encloses the greatest area, which area measures the moment of momentum
-[see NOTE 6].
-
-Given a certain moment of momentum, then the energy is least when the
-bodies all move in one plane and all travel in circles in that plane.
-As energy is constantly being dissipated while any alteration among
-the bodies is going on, to coplanarity and circularity of path all
-the bodies must tend, if by collision they be aggregated into larger
-masses. As in the present state of our system the small bodies travel
-out of the general plane in eccentric ellipses while the big ones
-travel in it in approximate circles, the facts indicate that the origin
-of the larger masses was due to development by aggregation out of
-smaller particles.
-
-The next principle is of a different character. Half a century ago
-celestial mechanics dealt with bodies chiefly as points. The Earth was
-treated as a weighted point, and so was the Sun. This was possible
-because a sphere acts upon outside bodies as if all its mass were
-collected at its centre, and the Sun and many of the planets are
-practically spheres. But when it came to nicer questions of their
-present behavior and especially of their past career, it grew necessary
-to take their shape into account in their mutual effects. One of the
-results was the discovery of the great rôle played in evolution by
-tidal action. Inasmuch as the planets are not perfectly rigid bodies,
-each is subject to tidal deformation by the other, the outside being
-pulled more than the centre on one side and less on the other. Bodily
-tides are thus raised in it analogous to the surface tides we see in
-the ocean, only vastly greater, and these in turn act as a brake on its
-rotation.
-
-Now the retrograde motions occurring in the outermost parts of all
-the systems, principal and subsidiary, only and always there: the
-retrograde rotations of Neptune and Uranus, the retrograde revolutions
-of the ninth satellite of Saturn and of the eighth of Jupiter, point to
-something fundamental. For when we consider that it is precisely in its
-outer portions that any forces shaping the development of the system
-have had less time to produce their effect, we perceive that apparent
-abnormality now is really survival of the original normal state, only
-to be found at present in what has not been sufficiently forced to
-change. It suggests that the pristine motion of the constituents of the
-scattered agglomerations which went to form the planets was retrograde,
-and that their present direct rotations and the direct revolutions of
-most of their satellites have been imposed by some force acting since.
-Let us inquire if there be a force competent to this end, and what its
-mode of action.
-
-Let us see how tidal action would work. Tidal force would raise bulges,
-and these, not being carried round with the planet’s rotation except to
-a certain distance, due to viscosity, must necessarily act as brakes
-upon the planet’s spin. In consequence of the friction they would thus
-exert, energy of motion must be lost. So long, then, as tidal forces
-can come into play, the energy of the system is capable of decrease.
-According to the last principle we considered, the system cannot be
-in stable equilibrium until this superfluous energy is lost or until
-tidal forces become inoperative, which cannot be till all the bodies
-in the system turn the same face to their respective centres of
-attraction.
-
-To see this more clearly, take the case of a retrograde spin of a
-planet as compared with a direct one. The energy of the planet’s spin
-is the same in both cases, because energy depends on the square of a
-quantity; to wit, that of the velocity, and is therefore independent
-of sign. Not so the moment of momentum. For this depends on the first
-power of the speed, and if positive in the one case, must be negative
-in the other. The moment of momentum of the whole system, then, is less
-in the former case, since the moment of momentum of the retrograde
-rotation must be subtracted from, that of the direct rotation be added
-to, that of the rest of the system. For a given initial moment of
-momentum with which the system was endowed at the start, there is,
-then, superfluous energy in the first state which can be got rid of
-through reduction to the second. Nature, according to her principles
-of least exertion, avails herself of the chance of dispensing with it,
-and a direct rotation results. Sir Robert Ball first suggested this
-argument.
-
-Tidal action accomplishes the end. In checking up a body rotating
-contrary to the general consensus of spin, its first effect is to
-start to turn the axis over. For the body is in dynamical unstable
-equilibrium with regard to the rest of the system. The righting would
-continue, practically to the exclusion of any diminution at first of
-the spin, until the body had turned over in its plane so that the spin
-became direct. As the force increases greatly with nearness to the
-Sun, the effect would be most marked on the nearer, and most so on the
-biggest, bodies. This would account for the otherwise strange gradation
-from retrograde to direct in the tilts of the axes of the outer
-planets, and also for the present tilts of all the inner ones.
-
-Related to the initial retrograde rotations of the planets, and in a
-sense survivals from an earlier state of things, are two of the latest
-discoveries of motions in the solar system, the retrograde orbital
-movements of the ninth satellite of Saturn and the eighth of Jupiter.
-Considered so anomalous as scarcely at first to be believed, it has
-been stated that they directly contradict the theory of Laplace.
-This is true; in the same sense and no more in which they directly
-contradict the contradictor, one of the latest theories. For neither
-theory has anything to explain them as the result of law. That they
-cannot be the sport of indifferent chance seems evidenced by their
-occupying similar external positions in their respective systems. As
-the product of a law we must regard them, and to find that law we now
-turn. Suppose the planet originally to have been rotating backward, or
-in the direction of the hands of a clock. At this time the satellite,
-which may never have formed a part of its mass, was travelling backward
-too, according to what we have said. Then under the friction of
-the tides raised on the planet by the Sun, the planet proceeded to
-turn over. It continued to do so until it spun direct. During this
-process there was no passage through zero of its moment of momentum
-_considered with regard to itself_, and therefore no difficulty on that
-score of supposing that it successively generated satellites at all
-degrees of inclination. That its children are of the nature of adopted
-waifs, Babinet’s criterion (1861) would seem to imply. But it must
-be remembered that the Sun has been slowing up the planet’s rotation
-now for æons. As it turned over, its tidal bulges tended to carry
-over with it such satellites as it already had. This effect was much
-greater on the nearer ones, both because they were nearer and because
-they were much larger than the outer. So that the nearer kept with the
-planet, the others lagged proportionately behind. This suggests itself
-to account for the facts, but the subject involves so much that is
-uncertain that I submit the hypothesis with the distrust which Laplace
-has so eminently bespoken. I advance in its favor only the three
-striking facts: that a steady progression in their tilts of rotation
-is observable from Neptune to Jupiter and a substantially accordant
-one from Mars to Mercury; secondly, that the satellites turn their
-faces to their primaries, as likewise do Mercury and Venus to the Sun;
-and, thirdly, that the orbits of the satellites of all the planets are
-themselves tilted in accordance with what it would require [see NOTE 7].
-
-After the axial spins have been made over to the same sense, the second
-consequence of tidal action in the case of two bodies revolving about
-their common centre of gravity is to slow down both spins until first
-the smaller and then the larger turn the same face to each other and
-remain thus constant ever after. Now such is precisely the pass to
-which we observe the satellites of the planets have come. All that we
-can be sure of now turn the same face always to their primary. The Moon
-was the first to betray her attitude, because the one we can best note.
-On scrutiny, however, Jupiter’s satellites, so far as we can make out,
-do the like; and Saturn’s, too. And a very proper attitude it is, this
-regard paid to compelling attraction. Thus one of the congruities we
-noticed stands accounted for. The satellites could hardly have been at
-first so observant; time has brought about this unfailing recognition
-of their lords.
-
-Of the peculiar massing of the bodies in the family of the Sun, and
-the still stranger copying of it in their own domestic circles, little
-can as yet be said in interpretation. That the planetary families and
-their ancestral group should agree is not the least strange part of
-the affair. It shows that none of them was fortuitous, but that at
-the formation of all some common principle presided, apportioning the
-aggregations to their proper place. But it is such fine print of the
-system’s history as at present to preclude discernment.
-
-So much for the details we may deduce of the method of our birth. We
-perceive unmistakably that our solar system grew to be what it is,
-and that it developed by agglomeration of its previously shattered
-fragments into the planets we behold to-day, but exactly how the
-process progressed we are as yet unable to precise. We are, however,
-as what I have mentioned and tabled show, every day accumulating data
-which will enable an eventual determination probably to be reached.
-
-From the fact of agglomeration, the essence of the affair, we turn to
-the traces it has left upon its several offspring.
-
-Just as the continued existence to-day of meteorites _in statu quo_
-informs us of a previous body from which our nebula sprang; so a
-physical characteristic of our own earth at the present time shows it
-to have evolved from that nebula—even though we cannot make out all the
-steps. Of its having done so, we are far more sure than of how it did.
-
-That primitive man perceived that somewhere below him was a fiery
-region which was not an agreeable abode, is plain from his consigning
-to such Tophet those whose religious tenets did not square with
-his own. That his conception of it was not strictly scientific is
-evidenced by his not realizing that to bury his enemies was the way
-to make them take the first step of the journey thither. Indeed, the
-vindictive venting of his notions clearly indicates their source as
-volcanic, rather than bred of a general disapproval of a downward
-descent either in silicates or sin.
-
-It was not till man began to bore into the Earth for metallic or
-potable purposes that he brought to light the generic fact that it
-was everywhere hotter as one went down. And this not only in a very
-regular, but in a most speedy, manner. The temperature increased in a
-really surprising way 1° F. for every sixty-five feet of descent. As
-the rise continued unabated to the limit of his borings, becoming very
-unpleasant at its end, it was clear that at a depth of thirty-five
-miles even so refractory a substance as platinum must melt, and
-practically all the Earth except a thin crust be molten or even gaseous.
-
-Now heat, like money, is easy to dissipate but hard to acquire, as
-primitive man was the first to realize. It does not come without
-cause. Being a mode of motion, other motion must have preceded it from
-which it sprang. So much the doctrine of the conservation of energy
-teaches us, a doctrine considered now to have been the great scientific
-heirloom of the nineteenth century to the twentieth, yet which in its
-day caused the death of its first discoverer, Mayer, of a broken heart
-from non-recognition; its second, Helmholtz, was refused publication by
-the leading Berlin physical magazine of the time. So quick is man to
-delay his own advance.
-
-The only conceivable motion for thus heating the Earth as a whole was
-the falling together of its parts. The present heat of the Earth,
-then, accuses the concourse of particles in the past to its formation,
-or in other words proves that the Earth was evolved out of material
-originally more sparcely strewn. It does so not only in a generic but
-in a most particular manner, for the heat is distributed just where it
-would be by such a process. It is greater to-day within, increasingly,
-because when the globe began to cool, the surface necessarily cooled
-first and established a regular gradient of heat from core to cuticle.
-
-It is possible to test this qualitative inference quantitatively and
-see if the falling together of the meteorites was equal to the task.
-Knowing the mechanical equivalent of heat, what we do is to calculate
-the quantity of motion involved and then evaluate it in heat. As we
-are unaware of the exact law of density of the Earth, and are ignorant
-of how much was radiated away in the process, the problem is a little
-like estimating the fortune of a man when we do not know the stocks in
-which he has invested, and ignore how much he has spent the while. We
-only know what he would have been worth had he followed our advice in
-the matter of investments and lived as frugally as we recommended. For
-here, too, we are obliged to make certain assumptions. Nevertheless
-the figure obtained in the case of the planets’ stores of heat is so
-enormous as to leave a most ample margin for dissipation. Had the Earth
-contracted from a fairly generous expansion to its present state under
-the probable law of density suggested by Laplace in another connection,
-the heat developed would have been enough to raise the whole globe to
-160,000° F. if of iron, 90,000° F. if of stone. As 10,000° F. would
-have sufficed for the Earth to have kept up its past, to say nothing of
-its present, state, we are justified of our deduction.
-
-Nor is the Earth the only body in the system which thus argues itself
-evolved by the falling together of its present constituents. In the
-larger planets Jupiter and Saturn we seem to see the heat, far as we
-are away. For the cherry hue they disclose between their brighter belts
-proves to come from greater absorption there of the green and blue rays
-of the spectrum, indicating a greater depth of atmosphere traversed.
-Thus these parts lie at a lower level, and their ruddy hue is just what
-they should show were they still glowing with a dull red heat.
-
-[Illustration: SPECTROGRAM OF JUPITER, MOON COMPARISON.
-
-LOWELL OBSERVATORY. V. M. SLIPHER.]
-
-Heat is not only the end of the beginning, it is the beginning of the
-end as well. It is both the result of the evolving of definite bodies
-out of the agglomeration of matter-strewn space, and the cause of the
-higher evolution of those globes themselves. For the acquisition of
-heat is the necessary preface to all that follows. Heat is a body’s
-evolutionary capital whose wise expenditure through cooling down makes
-all further advance to higher products possible. A body too small
-to have acquired it must remain forever lifeless, as dead as the
-meteorites themselves that enter our air as mere inert bits of stone or
-iron.
-
-Curiously enough, heat both must have been and then must have been
-lost. Like the loss of fortune or of friends sometimes in the ennobling
-of character, it is through its passing away that its effects are
-realized. For in cooling down from a once heated condition, that train
-of events occurs which we most commonly particularize as evolution.
-So far in our survey the march of advance has been through masses of
-matter, a molar evolution; from this point on it passes into its minute
-constituents and becomes a molecular one. The one is the necessary
-prelude to the other. Up to this great turning-point in the history of
-each member of a solar system we have been busied with the acquisition
-of heat, though we may not have been aware of it the while. All the
-motions we have studied tended to that end. During these three
-chapters, I, II, V, we have been gradually rising in our point of view
-until we stand at the temperature pinnacle of the whole process. In the
-next three we are to descend upon the other side. The slope we have
-come up was of necessity barren; the one we are to go down brings us to
-verdure and the haunts of men. Coming from the causes above, we reach
-at each step effects more and more related to ourselves which those
-causes will help us to explain.
-
-
-
-
-CHAPTER VI
-
-A PLANET’S HISTORY
-
-_Self-sustained Stage_
-
-
-Up to this point in our retrospective survey the long course of
-evolution has taken one line, that of dynamical separation of the
-system’s parts with subsequent reunitement of them according to the
-laws of celestial mechanics. Of this action I have submitted the reader
-my brief: departing in it from common-law practice, in which the cause
-of action is short and the brief long. And I have, I trust, guarded
-against his appealing on exceptions.
-
-From this point on we have two kinds of development to follow: the
-one intrinsic, the chemical; the other incidental, the physical. Not
-that, in a way, the one is divorcible from the other. For the physical
-makes possible the chemical by furnishing it the conditions to act.
-But in another sense, and that which is most thrust upon our notice,
-the two are independent. Thus oceans and land, hills and valleys,
-clouds and blue sky, as we know them,—everything, pretty much, which
-we associate with a world,—are not universal, inevitable, results
-of planetary evolution, but resultant, individual, characteristics
-of our particular abode. They are as much our own as the peculiar
-arithmetic of waiters is theirs, or as used to be the sobriety of the
-country doctor’s horse—his and no other’s. Our whole geologic career is
-essentially earthly. Not that its fundamental laws are not of universal
-application, but the kaleidoscopic patterns they produce depend on the
-little idiosyncrasies of the constituents and the mode in which these
-fall together. Our everyday experiences we should find quite changed,
-could we alight on Venus or on Mars.
-
-On the other hand, the chemical changes which follow a body’s
-acquisition of heat, setting in the moment that heat has reached its
-acme and starts to decline, are as universal as the universe itself.
-They are conditioned, it is true, by the body’s size and by the
-position that body occupied in the primal nebula, but they depend
-directly upon the degree of heat the body had attained. The larger
-the planet, the higher the temperature it reached and the fuller its
-possibilities. Even the planets are born to their estate. Thus the
-little meteorites live their whole waking life during the few seconds
-they spend rushing through our air. For then only does change affect
-their otherwise eternally inert careers. That the time is too short for
-any important experience is evident on their faces.
-
-Heat is most intimately associated with the very constitution of
-matter. It is, in fact, merely the motion of its ultimate particles,
-and plays an essential part in their chemical relations. Just as a
-certain discreet fervor and sufficient exposure for attraction to take,
-make for matrimony, so with the little molecules, a suitable degree of
-warmth and a propitious opportunity similarly conduce to conjunction;
-too fiery a temperament resulting in a vagabondage preventative of
-settled partnership and too cold a one in permanent celibacy. You may
-think the simile a touch too anthropomorphic, but it is a most sober
-statement of fact. Indeed, it is more than probable that in some dull
-sense they feel the impulse, though not the need of expressing it
-in verse. That metals can remember their past states seems to have
-been demonstrated by Bose, and is certainly in keeping with general
-principles as we know them to-day. For memory is the partial retention
-of past changes, rendering those changes more facile of repetition.
-
-A high degree of heat, then, makes chemical union impossible, because
-the great speeds at which the molecules are rushing past each other
-prevents any of them being caught. Lack of speed is equally deterrent.
-Nor is it wholly or even principally, perhaps, a movement of the whole
-which is here concerned, but a partitive throbbing of the molecule
-itself. Certain it is that great cold is as prohibitive of chemic
-combination as great heat. Phosphorus, which evinces such avidity for
-oxygen at ordinary temperatures as to have got its name from the way it
-publishes the fact, at very low ones shows a coolness for its affinity
-amounting to absolute unconcern. Thus only within a certain range of
-temperature does chemical combination occur. To remain above or below
-this is to stay forever immortally dead. To get hot enough in the first
-place, and then subsequently to cool, are therefore essential processes
-to a body which is to know evolutionary advance.
-
-To pen the history of the solar system and leave out of it all mention
-of its most transcendentally wonderful result, the chemical evolution
-attendant upon cooling, would be to play “Hamlet” with Hamlet left out.
-For the thing which makes the second half of the great cosmic drama so
-inconceivably grand is the building up of the infinitely little into
-something far finer than the infinitely great. The mechanical action
-that first tore a sun apart, and then whirled the fragments into the
-beautifully symmetric system we behold to-day, is of a grandeur which
-is at least conceivable; the molecular one that, beginning where the
-other left off, built up first the diamond and then humanity is one
-that passes our power to imagine. That out of the aggregation of
-meteorites should come man, a being able to look back over his own
-genesis, to be cognizant of it, as it were, from its first beginnings,
-is almost to prove him immanent in it from the start. Fortunate it is
-that his powers should seem more limited than his perceptions, and the
-more so as he goes farther, else he had been but the embodiment of
-conceit.
-
-We must sketch, therefore, the steps in this marvellous synthesis;
-hastily, for I have already spoken of it elsewhere in print and
-repetitions dull appreciation,—in the appreciative,—though we have the
-best of precedents for believing that, even in science, to be dull
-and iterative insures success; the dulness passing for wisdom and the
-iteration tiring opposition out.
-
-In the Sun all substances are in their elemental state. Though its
-materials are the same as the Earth’s, we should certainly not feel at
-home there, even if we waived the question of comfort, for we should
-recognize nothing we know. We talk glibly of elements as if we had
-personal acquaintance with them, man’s innate snobbery cropping out.
-For to the chemist alone are they observable entities. No one but he
-has ever beheld calcium or silicon, or magnesium, or manganese, and
-most of us would certainly not know these everyday elements if we met
-them on the street. Of all the substances composing the Earth’s crust,
-or the air above, or the water beneath, practically the only elements
-with which we are personally familiar are iron, copper, and carbon,
-and these only in minute quantities and in that order of acquisition;
-which accounts for the stone, iron, and bronze ages of man, ending we
-may add with the graphite or lead-pencil age of early education.
-
-Yet that elementary substances once existed here we have evidence. We
-find such in volcanic vents. That the Earth was once as hot on its
-surface as it now is underneath, we know from the condition of the
-plutonic rocks where sedimentary strata have not covered them up.
-Volcanoes and geysers are our only avenues now to that earlier state of
-things. From these pathways to the past, and only from them, do we find
-elementary substances produced to-day,—hydrogen, sulphur, chlorine,
-oxygen, and carbon.[15] We are thus made aware that once the Earth was
-simple, too, on the surface as well as deeper down. A side-light, this,
-to what we knew must have been the case.
-
-[15] Geikie, “Geology,” pages 85, 86, and 131-136.
-
-From its primordial state, the least complex compounds were evolved
-first. As the heat lessened, higher and higher combinations became
-possible. And this is why the more complex molecules are so unstable,
-the organic ones the most. Since they are not possible at all under
-much stir of their atomic constituents, it shows that the bond between
-them must be feeble—and, therefore, easily broken by other causes
-besides heat. To the instability of the organic molecule is due its
-power; and to cooling, the possibility of its expression.
-
-[Illustration: LOWELL OBSERVATORY SPECTROGRAM SHOWING WATER-VAPOR IN
-THE ATMOSPHERE OF MARS, JANUARY 1908.—V. M. SLIPHER.]
-
-For the steps in the chemical process from Sun to habitable Earth
-we must look to the spectroscope; not in its older field, the blue
-end of the spectrum, but in that which is unfolding to our view in
-Dr. Slipher’s ingenious hands, the extension of the observable part
-of it into the red. For at that end lie the bands due to planetary
-absorption. Here we have already secured surprising results as to the
-atmospheres of the various planets. We have not only found positive
-evidence of water-vapor in the atmosphere of Mars, but we have detected
-strange envelopes in the major planets which show a constitution
-different from that of the Sun on the one hand, and of the Earth on
-the other. That size and position are for much in these peculiarities,
-I have already shown you; but something, too, is to be laid at the
-door of age. The major planets are not so advanced in their planetary
-history as is our Earth; and Dr. Slipher’s spectrograms of them
-disclose what is now going on in that prefatory, childish stage.
-
-These spectrograms are full of possibilities, and it is not too much to
-say that chemistry may yet be greatly indebted to the stars. Compounds,
-the strange unknown substances there revealed by their spectral
-lines, may be cryptic as yet to us. Some of the elements missing in
-Mendeléeff’s table may be there, too. Helium was first found in the
-Sun; coronium still awaits detection elsewhere. So with these spectral
-lines of the outer planets. It looks as if chemistry had been a thought
-too previous in making free for others with what should have been their
-names, Zenon and Uranium. For we may yet have to speak of Dion and
-Varunium.
-
-From the chemical aspect of evolution we pass to its physical side;
-from the indirectly to the directly visible results. Here again, to
-learn what happened after the sunlike stage, we must turn to the major
-planets. For the cooling which induced both physical and chemical
-change has there progressed less far, inasmuch as a large globe takes
-longer to cool than a small one. To the largest planets, then, we
-should look for types of the early planetary stages to-day.
-
-Almost as soon as the telescope was directed to Jupiter, among the
-details it disclosed were the Jovian belts (in the year 1630), dark
-streaks ruling the planet’s disk parallel to its equator. They are of
-the first objects advertised as visible in small glasses to-day, vying
-with the craters in the Moon as purchasable wonders of the sky. As the
-belts were better and better seen, features came out in them which
-proved more and more interesting. Cassini, in 1692, noticed that the
-markings travelled round Jupiter and those nearest his equator the
-quickest. Sir William Herschel thought them due to Jovian trade-winds,
-the planet’s swift rotation making up for deficiency of sun; why, does
-not appear.
-
-Modern study of the planet shows that the bright longitudinal layers
-between the dark belts are unquestionably belts of cloud. Their
-behavior indicates this, and their intrinsic brightness bears it out.
-For they are of almost exactly that albedo. Whether they are the kind
-of cloud with which we are familiar, clouds of water-vapor, we are not
-yet sure. But whatever their constitution, their conduct is quite other
-than is exhibited by our own.
-
-In the first place, they are of singular permanence for clouds. The
-fleeting forms we know as such assume in the Jovian air a stability
-worthy of Jove himself. In their general outlines, they remain the same
-for years at a time. “Constant as cloud” would be the proper poetic
-simile there. But while remaining true to themselves, they prove to be
-in slow, unequal shift with one another. Thus Jupiter’s official day
-differs according to the watch of the particular belt that times it.
-Spots in different latitudes drift round lazily in appearance, swiftly
-in fact, those near the equator as a rule the fastest. Nor is there any
-hard and fast latitudinal law; it is a go-as-they-please race in which
-one belt passes its neighbor at a rate sometimes of four hundred miles
-an hour. The mean day is 9ʰ 55ᵐ long.
-
-[Illustration: JUPITER AND ITS “GREAT RED SPOT”—A DRAWING BY DR.
-LOWELL, APRIL 12, 7ʰ 0ᵐ-5ᵐ, 1907.]
-
-[Illustration: JUPITER AND ITS “GREAT RED SPOT”—A DRAWING BY DR.
-LOWELL, APRIL 12, 7ʰ 28ᵐ-42ᵐ, 1907.]
-
-A side-light is cast upon the Jovian state of things by the “great red
-spot,” which has been more or less visible for thirty years, and which
-takes five minutes longer than the equatorial band to travel round. Its
-tint bespoke interest in what might be its atmospheric horizon. Yet
-it betrayed no sign of being either depressed or exalted with regard
-to the rest of the surface. “In 1891,” as Miss Clerke puts it, “an
-opportunity was offered of determining its altitude relative to a small
-dark spot on the same parallel, by which, after months of pursuit,
-it was finally overtaken. An occultation appeared to be the only
-alternative from a transit; yet neither occurred. The dark spot chose a
-third. It coasted round the obstacle in its way, and got damaged beyond
-recognition in the process.” It thus astutely refused to testify.
-
-[Illustration: SUN SPOTS—AFTER BOND.]
-
-Now, this exclusiveness on the part of the “great red spot” really
-offers us an insight to its character. Clearly it was no void, but
-occupied space with more than ordinary persistency. As it was neither
-above nor below the dark spot and shattered that spot on approach,
-which its former surroundings had not done, its force must have been
-due to motion. This can be explained by its being formed of a vast
-uprush of heated vapor from the interior. In short, it was a sort of
-baby elephant of a volcano, or geyser, occurring as befits its youth
-in fluid, not solid, conditions, but fairly permanent, nevertheless—a
-bit of kindergarten Jovian geology. This estimate of it is concurred in
-by Dr. Slipher’s spectrogram of the dark and light belts respectively.
-For in the spectrum of the dark one we see the distinctive Jovian bands
-intensified as if the light had traversed a greater depth of Jovian
-air. Its color, a cherry red, abets the conclusion—that in such places
-we look down into the fiery, chaotic turmoil so incessantly going on.
-
-[Illustration: PHOTOGRAPH OF A SUN SPOT—AFTER THE LATE M. JANSSEN.]
-
-It is of interest to note that we have prototypes of this sort of
-extraterrestrial cyclone in the Sun. His spots are probably local
-upsettings of atmospheric equilibrium, using the word atmospheric in
-the widest possible sense. Just as our storms are the mildest examples
-of the like expostulation at the impossibility of keeping up a too
-long continued decorum. Only that with us the Earth is not so much
-to blame as the Sun; while both Jupiter and the Sun are themselves
-responsible for their condition.
-
-Thus we have, in the very depth of their negation, warrant from the
-dark belts of Jupiter that the bright ones are cloud. But also that
-they are not clouds ordered as ours. The Jovian clouds pay no sort
-of regard to the Sun. In orbital matters Jupiter obeys the ruler of
-the system; but he suffers no interference from him in his domestic
-affairs. His cloud-belts behave as if the Sun did not exist. Day and
-night cause no difference in them; nor does the Jovian year. They come
-when they will; last for months, years, decades; and disappear in like
-manner. They are _sui Jovis_, caused by vertical currents from the
-heated core and strung out in longitudinal procession by Jupiter’s
-spin. They are self-raised, not sun-raised, condensations of what is
-vaporized below. Jove is indeed the cloud-compeller his name implies.
-
-Yet Jupiter emits no light, unless the cherry red of his darker belts
-be considered its last lingering glow. He is thus on the road from Sun
-to world, and his present appearance informs us that this incubation
-takes place under cloud.
-
-The like is true of Saturn, in fainter replica, even to the cherry
-hue. In one way Saturn visibly asserts his independence beyond that
-possible by Jupiter. For Jupiter’s equator lies almost in the plane
-of his orbit, and on a hasty view the Sun might be credited with the
-ordering of the belts, as was indeed long the case. But Saturn’s
-inclination to his orbital plane is 27°; yet his belts fit his figure
-as neatly as his rings, and never get displaced, no matter how his body
-be turned.
-
-Uranus and Neptune are in the same self-centred attitude at present as
-the faint traces of belts on their disk, otherwise of the same albedo
-as cloud, lead us to conclude. Yet both their densities and their
-situation give us to believe them further advanced than the giant
-planets, and still they lie wrapped in cloud.
-
-These planets, then, are quite unbeholden to the Sun for all their
-present internal economies. What goes on under that veil of clouds with
-which they discreetly hide their doings from the too curious astronomic
-eye—we can only conjecture. But we discern enough to know that it is
-no placid uneventfulness. That it will continue, too, we are assured.
-For whether these clouds are largely water-vapor now, or not, to
-watery ones they must come as the last of all the wrappers they will
-eventually put off.
-
-The major planets are the only ones at the present moment in this
-self-centred and self-sustained stage. Their great size has kept them
-young. In the smaller terrestrial planets we could not expect to
-witness any such condition to-day. If they experienced an ebullient
-youth, they have long since outgrown it. Only by rummaging their past
-could we find evidence on the point, and this, distance both in time
-and space bars us from doing. There is but one body into whose foretime
-career we could hope to peer with the slightest prospect of success—our
-own Earth.
-
-[Illustration: THE VOLCANO COLIMA, MEXICO, MARCH 24, 1903—JOSÉ MARIA
-ARREOLA, PER FREDERICK STARR.]
-
-[Illustration: JUKES BUTTE, A DENUDED LACCOLITH, AS SEEN FROM THE
-NORTHWEST—GILBERT.]
-
-[Illustration: IDEAL SECTION OF A LACCOLITH—GILBERT.]
-
-Whether our Earth was ever hot enough at the surface to vaporize
-those substances which now form the Jovian or Saturnian clouds, we do
-not know; but that it was once hot enough to vaporize water we are
-perfectly certain. And this from proof both of what did exist and
-of what did not. That the surface temperature was at onetime in the
-thousands of degrees Fahrenheit, the Plutonic magma underlying all the
-sedimentary rocks of the Earth amply shows. Reversely, the absence
-of any effect of water until we reach these sedimentary deposits,
-testifies that during all the earlier stages of the Earth’s career
-water as such was absent, and as water subsequently appeared, it is
-clear that the conditions did not at first allow it to form. We are
-sure, therefore, that there was a time when water existed only as
-steam, and very possibly a period still anterior to that when it did
-not exist at all, its constituent hydrogen and oxygen not having yet
-combined. There was certainly an era, then, in the morning of the
-ages, when the Earth wore her cloud-wrapper much as Jupiter his now.
-
-That the seas were not once and yet are to-day, affords proof positive
-that at some intermediate period they began to be. Avery long
-intermediate one it must have been, too,—all the time it took the Earth
-to cool from about 2000° C. to 100° C. Not till after the temperature
-had fallen to the latter figure in the outer regions of the atmosphere
-could clouds form, and not till it had done so at the solid surface
-could the steam be deposited as water. Reasoning thus presents us with
-a picture of our Earth as a vast seething caldron from which steam
-condensing into cloud was precipitated upon a heated layer of rock,
-to rise in clouds of steam again. The solid surface had by this time
-formed, thickening slowly and more or less irregularly, and into its
-larger dimples the water settled as it grew, deepening them into the
-great ocean basins of to-day. We see the process with as much certainty
-and considerably more comfort than if, in the French sense, we had
-assisted at it. Presence of mind now thus amply makes up for absence of
-body then.
-
-Passing on evolutionarily we reach more and more tolerable conditions
-and solid ground in fact, as well as theory. Thus the crust hardened
-and cooled, while the oceans still remained uncomfortably hot. For
-water requires much more heat to warm it to a given temperature than
-rock, about four and a half times as much. It has therefore by so much
-the more to lose, and is proportionally long in the losing. These
-hot seas must have produced a small universe of cloud, and as the
-conditions were the same all over the Earth, we can see easily with the
-mind’s eye that we could not have seen at all with the bodily one, had
-we occupied the land in those very early days. To be quite shut out
-from curious sight without, was hardly made up for by not being able
-to see more than dimly within. Any one who has stood on the edge of a
-not-extinct crater when the wind was blowing his way, will have as good
-a realization of the then state of things as he probably cares for.
-
-Now this astronomic drawing of the then Earth, which by its lack of
-detail allows of no doubt whatever, permits us to offer help in the
-elucidation of some of their phenomena to our geologic colleagues.
-We are the more emboldened to do so in that they have themselves
-appealed to astronomy for diagnosis, and accepted nostrums devised by
-themselves. It is always better in such cases to call in a regular
-practitioner. Not that he is necessarily more astute, but that he knows
-what will not work. It was in the matter of the paleologic climate that
-they were led to consult astronomy. The singular thing about paleologic
-times was the combination of much warmth with little light; and the
-not less singular fact that these conditions were roughly uniform over
-the whole Earth. From this universality it was clear, as De Lapparent,
-their chief spokesman, puts it, that nothing local could explain the
-fact. It was something which demanded a cause common to the globe.
-
-It thus fell properly within the province of astronomy. For if we are
-to draw any line between the spheres of influence of the two sciences,
-it would seem to lie where totality ends and provincialism begins. I
-use this not as a pejorative, but simply to part local color from one
-universal drab. In the Earth’s general attributes,—its size, shape, and
-weight,—we must have recourse to astronomy to learn the facts. Not less
-so for those principal causes which have shaped its general career; we
-surrender it only at the point where everyday interest begins, when
-those causes that led it through its uninviting youth give way to
-effects which in the least concern humanity at large.
-
-Between the mere aggregation of matter into planetary bodies, of which
-nebular hypotheses treat, and the specific transformation of plants and
-animals upon their surfaces with which organic evolution is concerned,
-lies a long history of development, which, beginning at the time
-the body starts to cool, continues till it become, for one cause or
-another, again an inert mass. In this period is contained its career as
-a world. Planetology I have ventured to call the brand of astronomy
-which deals with this evolution of worlds. It treats of what is general
-and cosmic in that evolution, as geology treats of what is terrestrial
-and specific in the history of one member of the class, our own Earth.
-The two do not interfere, as the one faces questions in time and space
-to which the other remains perforce a stranger. If the picture by the
-one be fuller of detail, the canvas of the other permits of the wider
-perspective. Certain events in the history of our Earth can only be
-explained by astronomy, as geologists have long since recognized. It is
-these that fall into our present province.
-
-Geologists, however, have applied astronomy according to their own
-ideas. Either they called in aurists, so to speak, when what they
-needed was an oculist, or they went to books for their drugs, which
-they then administered themselves—a somewhat dangerous practice. Thus
-they began by displacing the Earth’s axis in hope of effecting a
-result; not realizing that this would only shift the trouble, not cure
-it; in fact, make it rather worse. They next tried what De Lapparent,
-one of the most brilliant geologists of the age, calls “a variation
-in the eccentricity of the ecliptic[16] joined to precession of the
-equinoxes,”—a startling condition unknown to astronomy which does not
-deal in eccentric planes, whatever such geometric anomalies may be,
-but by which its coiner evidently means a change in the eccentricity
-of the orbit, as the context shows. Its effect on the Earth, as he
-wisely points out, would be to reduce its extremities to extremes. To
-get out of his quandary he then embraced a brilliant suggestion of
-a brother geologist, M. Blandet. M. Blandet conceived the idea, and
-brought it forth unaided, that all that was necessary was a sun big
-enough to look down on both poles of the Earth at once. To get this
-he travelled back to the time when, in Laplace’s cosmogony, the Sun
-filled the whole orbit of Mercury. This conception, which, De Lapparent
-remarks, “might, at the time of its apparition, have disconcerted
-spirits accustomed to consider our system as stable,”—an apparition
-which we may add would certainly continue to disconcert them,—he
-says seems to him quite in harmony with that system’s genesis. That
-it labors under two physical impossibilities, one on the score of
-the Sun, the other on that of the Earth, and that in this case two
-negatives do not make an affirmative, need not be repeated here, as
-the reader will find it set forth at length elsewhere,[17] together
-with what I conceive to be the only explanation of paleothermal times
-which will work astronomically—presently to be mentioned. But before
-I do so, it is pertinent to record two things that have come to my
-notice since. One is that in rereading Faye’s “Origine du Monde,” I
-came upon a passage in which it appears that M. Blandet had actually
-consulted Faye about his hypothesis, and that Faye had shown him its
-impossibility on much the same grounds as those above referred to;
-which, however, did not deter M. Blandet from giving it to the world
-nor De Lapparent from god-fathering the conception.
-
-[16] “Abrégé de Geologie,” De Lapparent.
-
-[17] “Mars as the Abode of Life,” Macmillan, 1908.
-
-Faye, meanwhile, developed his theory of the origin of the world, and
-by it explained the greater heat and lesser light of paleologic times
-compared with our own, thus: The Earth evolved before the Sun. In
-paleologic times the Sun was still of great extent,—an ungathered-up
-residue of nebula that had not yet fallen together enough to
-concentrate, not a contracting mass from which the planets had been
-detached,—and was in consequence but feebly luminous and of little
-heating effect; so that there were no seasons on Earth and no climatic
-zones. The Earth itself supplied the heat felt uniformly over its whole
-surface.
-
-This differs from my conception, as the reader will see presently, in
-one vital point—as to why the Earth was not heated by the Sun. In the
-first place Faye’s sun has no _raison d’être_; and in the second no
-visible means of existence. If its matter were not already within the
-orbit of the Earth at the time, there seems no reason why it should
-ever get there; and if there, why it should have been so loath to
-condense. We cannot admit, I think, any such juvenility in the Sun at
-the time the Earth was already so far advanced as geology shows it to
-have been in paleologic times. For the Earth had already cooled below
-the boiling-point of water.
-
-[Illustration: TREE FERN.]
-
-To understand the problem from the Earth’s point of view, let us
-review the facts with which geology presents us. The flora of
-paleologic times, as we see both at their advent in the Devonian and
-from their superb development in the Carboniferous era, consisted
-wholly of forms whose descendants now seek the shade.[18] Tree ferns,
-sigillaria, equisetæ, and other gloom-seeking plants composed it.
-That some tree fern survivals to-day can bear the light does not
-invalidate the racial tendency. We have plenty of instances in nature
-of such adaptability to changed conditions. In fact, the dying out
-and deterioration of most of the order shows that the conditions have
-changed. And these plants, grown to the dimensions of trees, inhabited
-equally the tropic, the temperate, and the frigid zones as we know them
-now. Lastly, no annual rings of growth are to be found on them.[19] In
-other words, they grew right on, day in, day out. The climate, then,
-was as continuous as it was widespread.
-
-[18] De Lapparent, Dana, Geikie, _passim_.
-
-[19] De Lapparent.
-
-On the other hand, astronomy and geology both assert that the seas
-were warm.[20] From this it follows that a vastly greater evaporation
-must have gone on then than now, and that a welkin of cloud must thus
-inevitably have been formed.
-
-[20] De Lapparent, Dana, Geikie, _passim_.
-
-Now put the two facts together, and you have the solution. The climate
-was warm and equable over the whole globe because a thick cloud
-envelope shut off the Sun’s heat, the heat being wholly supplied from
-the steamy seas. At the same time, by the same means the light was
-necessarily so tempered as to produce exactly that half-light the ferns
-so dearly love. One and the same cause thus answers the double riddle
-of greater warmth and less light in those old days than is now the case.
-
-And here comes in the second find I spoke of above, in the person of
-some old trilobites who stepped in unexpectedly in corroboration. It
-has long been known—though its full significance seems to have escaped
-notice—that in 1872 M. Barrande made the discovery that many species
-of trilobites of the Cambrian and lower Silurian, the two lowest, and
-therefore the oldest, strata of paleozoic times, and distant relative
-of our horseshoe crabs, were blind. What is yet more significant,
-the most antediluvian were the least provided with eyes. Thus in the
-primordial strata, one-fourth of the whole number of species were
-eyeless, in the next above one-fifth, and in the latest of all one
-two-hundredth only.[21] Furthermore, they testify to the difficulty of
-seeing, in two distinct ways, some by having no eyes and some colossal
-ones, strenuous individuals increasing their equipment and the lazy
-letting it lapse. It seems more than questionable to attribute this
-blindness to a deep-sea habitat, as Suess does in describing them, for
-they lived in what geologists agree were shallow seas on the site of
-Bohemia to-day. Besides, trilobites never had abyssal proclivities; for
-they are found preserved in littoral deposits, not in deep-sea silt.
-Muddy water may have had some hand in this, but muddy water itself
-testifies to great commotion above and torrential rains. So the light
-in those seas was not what it became later, or would be now. Thus
-these trilobites were antelucan members of their brotherhood, and this
-accuses a lack of light in those earlier eras even greater than in
-Carboniferous times, which is just where it ought to be found if the
-theory is true.
-
-[21] Suess, “The Face of the Earth,” p. 213.
-
-I trust this conception may prove acceptable to geologists, for it
-seems imperative from the astronomic side that something of the sort
-must have occurred. And it is just as well, if not better, to view it
-thus in the light of the dawn of geologic history as to remain in the
-dark about it altogether. Nescience is not science—whether hyphenized
-or apart; for the whole object of science is to synthesize and explain.
-Its body of learning is but the letter, coördination the spirit, of
-its law. Nevertheless, the unpardonable impropriety of a new idea,
-I am aware, is as reprehensible as the atrocious crime of being a
-young man. Yet the world could not get on without both. Time is a sure
-reformer and will render the most hardened case of youth senile in
-the end. So even a new idea may grow respectable at last. And it is
-really as well to make its acquaintance while it still has vigor in it
-as to wait till it is old and may be embraced with impunity. Boasted
-conservatism is troglodytic, and usually proves a self-conferred
-euphuism for dull. For conservatism proceeds from slowness of
-apprehension. It may be necessary for certain minds to be in the rear
-of the procession, but it is of doubtful glory to find distinction in
-the fact.
-
-Thus the youth of a world, like the babyhood of an individual, is
-passed screened from immediate contact from without. That this is
-the only way that life can originate on a planet we cannot say, but
-that it is away in which it does occur, our own Earth attests, and
-that, moreover, it is the way with all planets of sufficient size,
-the present aspect of the major planets shows. It may well be that
-with celestial bodies as with earthly species, some swaddle their
-young, others cast them forth to take their chance, and that those
-that most protect them rear the higher progeny in the end. What
-glories in evolution thus await the giant planets when they shall
-have sufficiently cooled down, we can only dimly imagine. But we can
-foresee enough to realize that we are not the sum of our solar system’s
-possibilities, and by studying the skies read there a future more
-wonderful than anything we know.
-
-
-
-
-CHAPTER VII
-
-A PLANET’S HISTORY
-
-_Sun-sustained Stage_
-
-
-Two stages have characterized the surface history of the Earth,—stages
-which may be likened to the career of the chick within and without the
-egg. In the first of them the Earth lay screened from outside influence
-under a thick shell of cloud, indifferently exclusive of the cold of
-space or of the heating beams of the Sun. Motherless, the warmth of
-its own body brooded over it, keeping its heat from dissipating too
-speedily into space, and so fostering the life that was quickening upon
-its surface.
-
-The second stage began when the egg-shell broke and the chick lay
-exposed to the universe about it, to get its living no longer from its
-little world within, but from the greater one without. One and the same
-event ended the old life to make possible the new. So soon as the cloud
-envelope was pierced, both the Earth’s own heat escaped and the Sun’s
-rays were permitted to come in.
-
-It is not surprising that under such changed conditions development
-itself should have changed, too. In fact, the transformation was
-marked. That its epochal character has failed to impress itself
-generally on geologists, is perhaps because they look too closely,
-missing the march of events in the events themselves, and because, too,
-of the gradual nature of its processional change. We can recall only De
-Lapparent as having particularly signalled it; although not only in its
-cause, but for its effects, it should have delimited two great geologic
-divisions of time.
-
-[Illustration: EARTH AS SEEN FROM ABOVE—PHOTOGRAPHED BY DR. LOWELL AT
-AN ALTITUDE OF 5500 FEET.]
-
-Astronomy and geology are each but part of one universal history. The
-tale each has to tell must prove in keeping with that of the other.
-If they seem at variance, it behooves us very carefully to scan their
-respective stories to find the flaw where the apparent incongruity
-slipped in. Each, too, fittingly supplements the other, and especially
-must geology look to astronomy for its initial data, since astronomy
-deals with the beginning of our own Earth.
-
-That study of our Earth in its entirety falls properly within the
-province of astronomy, is not only deducible from its relationship to
-the other planets, but demonstrable from the cosmic causes that have
-been at work upon it, and the inadequacy of anything but cosmic laws
-to explain them. The ablest geologists to-day are becoming aware of
-this,—we have one of them at the head of the geology department of the
-Institute,—while from the curious astronomy at second hand which gets
-printed in geologic text-books, by eminent men at that, dating from
-some time before the flood,—of modern ideas,—it seems high time that
-the connection should be made clear.
-
-For, after all, our Earth too is a heavenly body, in spite of man’s
-doing his best to make it the reverse. It has some right to astronomic
-regard, even if it is our own mother. At the same time it is quite
-puerile to consider the universe as bounded by our terrestrial
-backyard. If man took himself a thought less importantly, he might
-perceive the humor of so circumscribed a view. Like children we play at
-being alone in the universe, and then go them one better by believing
-it too.
-
-I shall, of course, not touch on any matters purely geologic, for
-fear of committing the very excesses I deplore; mentioning only such
-points as astronomy has information on, and which, by the sidelights
-it throws, may help to illuminate the subject.
-
-Thus it certainly is interesting and may to many be a new point of
-view, that the changes introduced when paleologic times passed into
-neologic ones were in their fundamental aspects essentially astronomic;
-which shows how truly astronomic causes are woven into the whole fabric
-of the Earth. For it was then only, terrestrially speaking, that the
-year began. The orbital period had existed, of course, from the time
-the Earth first made the circuit of the Sun. But the year was more a
-_succès d’estime_ on the Sun’s part than one of popular appreciation.
-As the Sun could not be seen and worked no striking effects upon the
-Earth, the annual round had no recognizable parts, and one revolution
-lapsed into the next without demarcation. Only with the clearing of the
-sky did the seasons come in: to register time by stamping its record
-on the trees. Before that, summer and winter, spring and autumn, were
-unknown.
-
-Climate, too, made then its first appearance; climate, named after
-the sunward obliquity of the Earth, and seeming at times to live down
-to that characterization. Weather there had been before; pejoratively
-speaking, nothing but weather. For the downpours in paleologic times
-must have been exceeded in numbers only by their force. One dull
-perpetual round of rain was the programme for the day, with absolutely
-no hope of a happy clearance to-morrow. It was the golden age only for
-weather prophets whose prognostications could hardly go wrong. With
-climate, however, it was a very different matter. With polyp corals
-building reefs almost to the pole (81° 50′),[22] as far north nearly
-as man has yet by his utmost efforts succeeded in getting, while their
-fellows were busy at the like industry in the tropics, it is clear that
-latitude was laughed at and climate even lacked a name.
-
-[22] Dana. “Geology.”
-
-Another astronomic feature, then for the first time disclosed, was the
-full significance of the day and the revelation of its cause. While
-the Earth brooded under perpetual cloud, there could have been but
-imperfect recognition of day and night. Or perhaps we may put it better
-by saying that the standard of both was greatly depressed, dull days
-alternating with nights black as pitch. But the moment the Sun was
-let in, all this changed, though not in a twinkling. The change came
-on most gradually. We can see in our mind’s eye the first openings in
-the great welkin permitting the Earth its initial peeps of the world
-beyond, and how quickly and tantalously they shut in again like a
-mid-storm morning which dreams of clearing only to find how drowsy it
-still is. But eventually the clouds parted afresh and farther, and the
-Earth began to open its eyes to the universe without.
-
-The cause of the clearing, of course, was the falling temperature of
-the seas. Evaporation went on much less fast as the heat of the water
-lessened. The whole round of aquatic travel from ocean to air, and back
-to ocean again, proceeded at an ever slackening pace. And here, if it
-so please geologists, may be found a reconciling of their demands for
-time to the relative pittance astronomy has been willing to dole them
-out, a paltry 50 or 100 millions of years, which like all framers of
-budgets they have declared utterly insufficient. For in early times the
-forces at work were greater, and by magnifying the means you quicken
-the process and contract the Earth’s earlier eras to reasonable limits.
-
-Upon these various astronomic novelties, the Earth on thus awakening
-looked for the first time. Such regard altered for good its own
-internal relations. The wider outlook made impossible the life of the
-narrower that preceded it. A totally changed set of animals and plants
-arose, to whom the cosmos bore a different aspect. The Earth ceased
-to be the self-centred spot it seemed before. As long ago as this had
-the idea that our globe was the centre of the universe been cosmically
-exploded. The Earth knew it if man did not.
-
-[Illustration: TRACKS OF SAUROPUS PRIMÆVUS (× ½). I. LEA.—DANA, “MANUAL
-OF GEOLOGY.”]
-
-Its denizens responded. The organisms that already inhabited it
-proceeded to change their character and crawl out upon the land. For
-in Devonian times the Earth was the home of fishes. The land was not
-considered a fit abode by anything but insects, and not over-good
-by them. But it looked different when the Sun shone. Some maritime
-dwellers felt tempted to explore, and proceeded in the shape of
-amphibians to spy out the land. They have left very readable accounts
-of their travels in footnotes by the way. As one should always inspect
-the original documents, I will reproduce the footnotes of one early
-explorer. It is one of the few copies we have, as the type is worn out.
-But it tells a pretty full story as it stands. The ripple-marks show
-that a sea beach it was which the discoverer trod in his bold journey
-of a few feet from home and friends, and the pits in the sandstone that
-it was raining at the time of his excursion. No Columbus or Hakluyt
-could have left a record more precise or more eminently trustworthy.
-The pilgrims found it so good that their eventual collaterals, the
-great reptiles, actually took possession of the land and held it for
-many centuries by right of eminent domain. Yet throughout the time of
-these bold adventurers, their skies were only clearing, as the pitting
-of the sandstone eloquently states.
-
-It was not till the chalk cliffs of Dover were being laid down that we
-have evidence that seasons had fully developed, in the shape of the
-first deciduous trees.[23] Cryptogams, cycads, and, finally, conifers
-had in turn represented the highest attainments of vegetation, and
-the last of these had already recognized the seasons by a sort of
-half-hearted hibernation or annual moulting; deeming it wise not to
-be off with the old leaves before they were on with the new. But
-finally the most advanced among them decided unreservedly to accept the
-winter and go to sleep till spring. The larches and ginkgo trees are
-descendants of the leaders of this coniferous progressive party.
-
-At the same time color came in. We are not accustomed to realize that
-nature drew the Earth in grays and greens, and touched it up with color
-afterward. Only the tempered tints of the rocks and the leaden blue of
-the sea, subdued by the disheartening welkin overhead to a dull drab,
-enlivened their abode for the oldest inhabitants. But with Tertiary
-times entered the brilliantly petalled flowers. Beginning with yellow,
-these rose through a chromatic scale of beauty from white through red
-to blue.[24] They decked themselves thus gaudily because the Sun was
-there to see by, as well as eyes to see. For without the Sun those
-unconscious horticulturists, the insects, could not have exercised
-their pictorial profession.
-
-[23] Dana, Geikie, De Lapparent.
-
-[24] Cf. Grant Allen.
-
-To the entering of the Sun upon the scene this wondrous revolution was
-due; and once entered, it became the dominant factor in the Earth’s
-organic life. We are in the habit of apostrophizing the Sun as the
-source of all terrestrial existence. It is true enough to-day, and has
-been so since man entered on the scene. But it was not always thus.
-There was a time when the Sun played no part in the world’s affairs.
-
-As its heat is now all-important, it becomes an interesting matter to
-determine the laws governing its amount. That summer is hotter than
-winter we all know from experience, pleasurable or painful as the case
-may be. This is due to the fact that the Sun is above the horizon for
-a greater number of hours in summer and passes more directly overhead.
-But not so many people are aware that on midsummer day, so far as the
-Sun is concerned, the north pole should be the hottest place on earth.
-That Arctic explorers, who have got within speaking acquaintance of it,
-assure us it is not so, shows that something besides the direct rays
-of the Sun is involved. Indeed, we learn as much from the extensively
-advertised thermometers of winter resorts which, judiciously placed,
-beguile the stranger to sojourn where it is just too cold for comfort.
-The factor in question is the blanketing character of our air. Now
-a blanket may keep heat out as well as keep it in. Our air acts in
-both capacities. It is by no means simply a storer of heat, as many
-people seem to suppose; it is a heat-stopper as well. What it really
-is is a temporizer, a buffer to ease the shocks of sudden change
-like those comfortable, phlegmatic souls who reduce all emotion to a
-level. For the heating power of the Sun, even at the Earth’s distance
-away, is much greater than appears. Knowledge of this we owe most to
-Langley, and then to Very, who continued his results to yet a finer
-determination, the best we have to-day. In consequence we have learnt
-that the amount of heat we should receive from the Sun, could we get
-above our air,—the solar constant, as it is called,—would be over three
-times what it is on the average in our latitude at the surface, and
-is rising still, so to speak. For as man has gone higher he has found
-his inferences rising too, and the limit would seem to be not yet. We
-see then that the air to which we thought ourselves so much indebted,
-actually begins its kindly offices by shutting off two-thirds of what
-was coming to us. As it plays, however, something of the same trick to
-what tries to escape, we are really somewhat beholden to it after all.
-
-But not so much as has been thought. We used to be told that the
-Moon’s temperature even at midday hardly rose above freezing, but Very
-has found it about 350° F., which even the most chilly of souls might
-find warm. By the late afternoon, however, he would need his overcoat,
-and no end of blankets subsequently, for during the long lunar night of
-fourteen days the temperature must fall appallingly low, to -300° F. or
-less.
-
-As the determination of temperature is a vital one, not only to any
-organic existence, but even to inorganic conditions upon a planet,
-it behooves us to look carefully into the question of the effective
-heat received from the Sun. Until recently the only criterion in the
-case was assumed to be distance from the illuminating source, about
-as efficient a mode of computation as estimating a Russian army by
-its official roll. For as we saw in our own case, not all that ought
-to ever gets to the front, to say nothing of what is lost there. Yet
-on this worse than guesswork astronomic text-books still assert as
-a fact that the temperature of other bodies—the Moon and Mars, for
-example—must be excessively low.
-
-Let us now examine into this most interesting problem. It is intricate,
-of course, but I think you will find it more comprehensible than
-you imagine. Indeed, I shall be to blame if you do not. For if one
-knows his subject, he can always explain it, in untechnical language,
-technical terms being merely a sort of shorthand for the profession.
-The physical processes involved can be made clear without difficulty,
-although their quantitative evaluation is less forthrightly
-demonstrable. Let me, then, give you an epitome of my investigation of
-the subject.
-
-[Illustration: ADVENTURES OF A HEAT RAY.]
-
-Consider a ray of light falling on a surface from the Sun. A part of
-it is reflected; that is, is instantly thrown off again. By this part
-the body shines and makes its show in the world, but gets no good
-itself. Another part is absorbed; this alone goes to heat the body. Now
-if the visible rays were all that emanated from the Sun, it would be
-strictly true, and a pretty paradox for believers in the efficacy of
-distance, that what heated the planet was precisely what seemed not to
-do so. Unfortunately there are also invisible rays, and these, too, are
-in part reflected and in part absorbed, and their ratio is different
-from that of the visible ones. To appreciate them, Langley invented
-the bolometer, in which heat falling on a strip of metal produces a
-current of electricity registered by a galvanometer. By thus recording
-the heat received at different parts of the spectrum and at different
-heights in our atmosphere, he was able to find how much the air cut
-off. Very has since determined this still more accurately. By thus
-determining the depletion in the invisible part of the spectrum joined
-to what astronomy tells us of the loss in the visible part, we have a
-value for the whole amount. By knowing, then, the immediate brightness
-of a planet and approximately the amount of atmosphere it owns, we are
-enabled to judge how much heat it actually receives. This proves to be,
-in the case of Mars, more than twice as much as distance alone would
-lead us to infer.
-
-The second question is how much of this it retains. The temperature of
-a body at any moment is the balance struck between what it receives and
-what it radiates. If it gets rid of a great deal of its income, it will
-clearly be less hot than if it is miserly retentive. To find how much
-it radiates we may take the difference in temperature between sunset
-and sunrise, since during this interval the Earth receives no heat from
-the Sun. In the same way the efficacy of different atmospheric blankets
-may be judged. Thus the Earth parts with nine centigrade degrees’ worth
-of its store on clear nights, and only four degrees’ worth on cloudy
-ones, before morning. This is at sea-level. By going up a high mountain
-we get another set of depletions, and from this a relative scale for
-different atmospheric blankets. This is the principle, and we only have
-to fill out the skeleton of theory with appropriate numbers to find how
-warm the body is.
-
-In doing so, we light on some interesting facts. Thus clouds reflect 72
-per cent of the visible rays, letting through only 28 per cent of them.
-We feel chilly when a cloud passes over the Sun. On the other hand,
-slate reflects only 18 per cent of the visible rays, absorbing all the
-rest. This is why slate gets so much hotter in the Sun than chalk, and
-why men wear white in the tropics. White, indeed, is the best color to
-clothe one’s self in the year around, except for the cold effect it has
-on the imagination, for it keeps one’s own heat in as well as keeping
-the Sun’s out. The modest, self-obliterating, white winter habit of the
-polar hares not only enables them to keep still and escape notice, but
-keeps them warm while they wait.
-
-Astronomically, the effect is equally striking. Mars, for example,
-owing to being cloudless and of a duller hue, turns out to have a
-computed mean temperature nearly equal to the Earth’s,—a theoretic
-deduction which the aspect of the planet most obligingly corroborates.
-It thus enjoys a comparatively genial old age.
-
-For what is specially instructive in planetary economy is that, on the
-whole, clear skies add more by what they let in than they subtract
-by what they let out. If the Earth had no clouds at all, its mean
-temperature would be higher than it is to-day. Thus as a planet ages
-a beneficent compensation is brought about, the Sun’s heat increasing
-as its own gives out. Not that the foreign importation, however slight
-the duty levied on it by the air, ever fully makes up for the loss of
-the domestic article, but it tempers the refrigeration which inevitably
-occurs.
-
-The subject of refrigeration leads us to one of the most puzzling and
-vexed problems of geology: how to account for the great Ice Age of
-which the manifest sign manuals both in Europe and in America have so
-intrigued man since he began to read the riddle of the rocks. Upon
-this, also, planetology throws some light.
-
-If I needed an apology to the geologists for seeming again to
-trespass on their particular domain, I might refer to the astrocomico
-expositions put forward to account for the great Ice Age.
-
-We can all remember Croll’s “Climate and Time,” a book which can
-hardly be overpraised for its title and which had things worth reading
-inside, too. It had in consequence no inconsiderable vogue at one time.
-It undertook to account for glacial epochs on astronomic principles.
-It called in such grand cosmic conditions and dealt in such imposing
-periods of time that it fired fancy and almost compelled capitulation
-by the mere marshalling of its figurative array. Secular change in the
-eccentricity of the Earth’s orbit, combined with progression in the
-orbital place of the winter’s solstice, was supposed to have induced
-physical changes of climate which accentuated the snowfall in the
-northern hemisphere and so caused extensive and permanent glaciation
-there. In other words, long, cold winters followed by short, hot
-summers in one hemisphere were credited with accumulating a perpetual
-snow sheet, such as short, warm winters and long, cold summers could
-not effect.
-
-[Illustration: MARS.
-
-         NORTH POLAR CAP.                 SOUTH POLAR CAP.
-    At maximum   full extent of white    At maximum   white
-    At minimum   inner circle            At minimum   nothing]
-
-Now it so happens that these astronomic conditions affecting the Earth
-several thousand years ago, are in process of action on one of our
-nearest planetary neighbors at the present time. The orbit of Mars
-is such that its present eccentricity is greater than what the Earth
-ever can have had, and the winter solstice of the planet’s southern
-hemisphere falls within 23° of its aphelion point. We have then the
-conditions for glaciation if these are the astronomic ones supposed,
-and we should expect a southern polar cap, larger at its maximum
-and still more so, relatively, at its minimum, than in the opposite
-hemisphere. Let us now look at the facts, for we have now a knowledge
-of the Martian polar caps exceeding in some respects what we know of
-our own. The accompanying diagrams exhibit the state of things at
-a glance, the maximum and minimum of each cap being represented in
-a single picture and the two being placed side by side. It will be
-observed that the southern cap outdoes its antipodal counterpart at its
-maximum, showing that the longer, colder winter has its effect in snow
-or hoar-frost deposition. But, on the other hand, instead of excelling
-it at its minimum, which it should do to produce permanent glaciation,
-it so far falls short of its fellow that during the last opposition at
-which it could be well observed, it disappeared entirely. The short,
-hot summer, then, far exceeded in melting capacity that of the longer
-but colder one.
-
-Let us now suppose the precipitation to be increased, the winters and
-summers remaining both in length and temperature what they were before.
-The amount of snow which a summer of given length and warmth can
-dispose of is, roughly speaking, a definite quantity. For it depends
-to a great extent only on its amount of heat. The summer precipitation
-may be taken as offsetting itself in the two hemispheres alike. If,
-then, the snowfall in the winter be for any reason increased daily in
-both, a time will come when the deposition due the longer winter of
-the one will exceed what its summer can melt relatively to the other,
-and a permanent glaciation result in the hemisphere so circumstanced.
-Increased precipitation, then, not eccentricity of orbit, is the real
-cause of an Ice Age. And this astronomic deduction we owe not to
-theoretic conclusions, for which we lack the necessary quantitative
-data, but wholly to study of our neighbor in space. Had any one
-informed our geologic colleagues that they must look to the sky for
-definite information about the cause of an Ice Age, they would probably
-have been surprised.
-
-With this Martian information, received some years ago, it is pleasing
-now to see that Earthly knowledge is gradually catching up. For that
-increased precipitation could account for it, the evidence of pluvial
-eras in the equatorial regions, contemporaneous with glacial periods,
-indicates. But another and probably the chief factor involved was not
-a generally increased precipitation, potent as that would be, but an
-increased snow deposit due to temporary elevation of the ground.
-
-[Illustration: GLACIAL MAP OF EURASIA—AFTER JAMES GEIKIE.]
-
-[Illustration: MAP SHOWING THE GLACIATED AREA OF NORTH AMERICA—THE
-ARROWS INDICATING THE DIRECTION OF ICE MOVEMENT—CHAMBERLIN AND
-SALISBURY.]
-
-For it now appears that there was no glacial _epoch_. Our early
-ideas inculcated by text-books at school received a rude shock when
-it appeared that the glacial _epoch_ was not, as we had been led
-to believe, a polar phenomenon at all, but a local affair which on
-the face of it had nothing to do with the pole. For investigation
-has disclosed that instead of emanating from the pole southward, it
-proceeded from certain centres, descending thence in all directions,
-north as much as south. Thus there was a centre in Norway in 65° N.
-lat. and another in Scotland in 56° N. In North America there were
-three—the Labradorian in latitude 54° N., the Kerwatin to the northwest
-of Hudson’s Bay in latitude 62° N., and the Cordilleran along the
-Pacific coast in latitude 58° N. On the other hand, northern Siberia,
-the coldest region in the world, was not glaciated. That the ice flowed
-off these centres proves them to have been higher than the sides. But
-we have further evidence of their then great height from the fact that
-dead littoral shells have been dredged from 1333 fathoms in the North
-Atlantic, and the prolongation under water of the fiords of Norway and
-of land valleys in North America witness to the same subsidence since.
-
-But evidence refuses to stop here. The Alps were then more glaciated
-than they are now. So was Kilimanjaro and Ruwenzori on the equator;
-and finally at the same time more ice and snow existed round about the
-south pole than is the case to-day. Now this is really going too far
-even for the most ardent believers in the force of eccentricity. For
-if the astronomic causes postulated were true, they must have produced
-just the opposite action at the antipodes, to say nothing of the crux
-of being equally effective at the equator. The theory lies down like
-the ass between two burdens. Whichever load it chooses to saddle, it
-must perforce abandon the other.
-
-So it turns out that the Ice Age was not an Ice Age at all but an
-untoward elevation of certain spots, and is to be relegated to the
-same limbo of exaggeration of a local incident into a world-wide
-cataclysm as the deluge. That some geologists will still cling to
-their former belief I doubt not; for as the philosophic old lady
-remarked: “There always have been two factions on every subject. Just
-as there are the suffragists and anti-suffragists now, so there were
-slaveholders and the anti-slavery people in my time; and even in the
-days of the deluge, there were the diluvians who were in favor of a
-flood and the antediluvians who were opposed to it.” A tale which has a
-peculiarly scientific moral, as in science _anti_ and _ante_ seem often
-interchangeable terms.
-
-When I began the course of lectures that resulted in this volume, I
-labored under the apprehension that an account of cosmic physics might
-prove dull. It soon threatened to prove too startling. I therefore
-hasten to reassure the timid by saying that we are outgrowing ice ages
-and probably deluges. Elevations of the Earth’s crust are likely to be
-less and less pronounced in the future, and meanwhile such as exist are
-being slowly worn down. Secondly, the Sun is sure to continue of much
-the same efficiency for many æons to come. And lastly, the essential
-ingredient of both prodigies, water, is daily becoming more scarce. To
-this latter point we now turn, and perhaps when it is explained to him
-the reader may think that he has been rescued from one fate only to
-fall into the hands of another.
-
-Geology is necessarily limited in its scope to what has happened;
-planetology is not so circumscribed in its domain. It may indulge in
-prognostication of the future, and find countenance for its conclusions
-in the physiognomy of other worlds. Thus one of the things which it
-foresees is the relative drying up of our abode. To those whose studies
-have never led them off this earth, the fact that the oceans are slowly
-evaporating into space may seem as incredible as would, to one marooned
-on a desert island, the march of mankind in the meantime. We live on
-an island in space, but can see something of the islands about us, and
-our conception of what is coming to our limited habitat can be judged
-most surely by what we note has happened to others more advanced than
-ourselves. Just as we look at Jupiter to perceive some likeness of
-what we once were, the real image of which has travelled by this time
-far into the depths of space beyond possibility of recall, so must we
-look to the Moon or Mars if we desire to see some faint adumbration
-of the pass to which we are likely to come. For from their lack of
-size they should have preceded us on the road we are bound to travel.
-Now, both these worlds to-day are water-lacking, in whole or part; the
-Moon practically absolutely so, Mars so far as any oceans or seas are
-concerned. We should do wisely then to take note. But we have more
-definite information than simply their present presentments. For both
-bear upon their faces marks of having held seas once upon a time. They
-were once, then, more as we are now. We cannot of course be sure, as
-we are unable to get near enough to scan their surfaces for signs of
-erosive action. But so far as we can make out, past seas best explain
-their appearance.
-
-[Illustration: THE MOON—PHOTOGRAPHED AT THE LOWELL OBSERVATORY.]
-
-So sealike, indeed, was their look that the first astronomers to note
-them took them unhesitatingly for water expanses. Thus the moment the
-telescope brought the Moon near enough for map making of it we find the
-dark patches at once designated as seas. The Sea of Serenity, the Sea
-of Showers, the Bay of Rainbows, speak still of what once was supposed
-to be the nature of the dark, smooth, lunar surfaces they name.
-Suggestively, indeed, in an opera glass do they seem to lap the land.
-The Lake of Dreams fore-shadowed what was eventually to be thought of
-them. With increasing optical approach the substance evaporated, but
-the form remained. It was speedily evident that there was no water
-there; yet the semblance of its repository still lurked in those
-shadows and suggests itself to one scanning their surfaces to-day.
-If they be not old sea bottoms, they singularly mimic the reality in
-their smooth, sloping floors and their long, curving lines of beach.
-Their strange uniformity shows that something protected them from
-volcanic fury while the rest of the lunar face was being corrugated.
-This preservative points to some superincumbent pressure which can have
-been no other than water. Lava-flows on such a scale seem inadmissible.
-What these surfaces show and what they do not show alike hint them sea
-bottoms once upon a time. In the strange chalk-like hue of the lunar
-landscape they look like plaster of Paris death-masks of the former
-seas.
-
-A like history fell to the lot of the surface features of Mars. There
-too, as soon as the telescope revealed them and their permanency of
-place, the dark patches upon the planet’s face were forthrightly taken
-for seas, and were so called: the Sea of the Sirens and the Great Red
-Sea. Such they long continued to be deemed. The seas of Mars held water
-in theory centuries after the idea of the lunar had vanished into
-air. At last, ruthless science pricked the pretty bubble analogy had
-pictured. Being so much farther off than the Moon, it was much later
-that their true character came out. Come out it has, though, within the
-last few years. Lines—some of the so called canals—have been detected
-crossing the seas, lines persistent in place. This has effectually
-disposed of any water in them. But here again something of semblance
-is left behind. They are still the darkest portions of the planet, and
-their tint changes in places with the progress of the planet’s year.
-That their color is that of vegetation, and that its change obeys the
-seasons, stamp it for vegetation in fact. Thus these regions must be
-more humid than the rest of Mars. They must, therefore, be lower. That
-they are thus lower and possess a modicum of water to-day marks them
-out for the spots where seas would be, were there any seas to be. As we
-know of a _vera causa_ which has for ages been tending to deplete them,
-extrapolation from what is now going on returns them the water they
-have lost and rehabilitates their ancient aquatic character. To the
-far-sight of inference, seas they again become in the morning of the
-ages long ago when Mars itself was young.
-
-Nor is this the end of the evidence. When we compare quantitatively the
-areas occupied by the quondam seas on Mars and on the Moon, we find
-reason to increase our confidence in our deduction. For the smaller
-body, the Moon, should have had less water relatively, at the time when
-the seas there were laid down, than the larger, Mars. Because from the
-moment its mass began to collect, it was in process of parting with its
-gases, water-vapor among the rest, and, as we shall see more in detail
-in the next chapter, it had from the start less hold on them than Mars.
-Its oceans, therefore, should have been less extensive than the Martian
-ones. This is what the present lunar Mare seem to attest. They are less
-extended than the dark areas of Mars. A fact which becomes the more
-evident when we remember that the Moon has long turned the same face
-to the Earth. Her shape, therefore, has been that of an egg, with the
-apex pointing toward our world. Here the water would chiefly collect.
-The greater part of the seas she ever had should be on our side of her
-surface, the one she presents in perpetuity to our gaze.
-
-It is to the heavens that we must look for our surest information on
-such a cosmic point, because of the long perspective other bodies give
-us of our own career. Less conclusive, because dependent upon less
-time, is any evidence our globe can offer. Yet even from it we may
-learn something; if nothing else, that it does not contradict the story
-of the sky. To it, therefore, we return, quickened in apprehension by
-the sights we have elsewhere seen.
-
-The first thing our sharpened sense causes us to note is the spread
-of deserts even within historic times. Just as deserts show by their
-latitudinal girdling of the Earth their direct dependence upon the
-great system of planetary winds, as meteorologists recognizingly
-call them, so a study of the fringes of these belts discloses their
-encroachment upon formerly less arid lands. The southern borders of
-the Mediterranean reveal this all the way from Carthage to Palestine.
-The disappearance of their former peoples, leaving these lands but
-scantily inhabited now, points to this; because other regions, as
-India, which still retain a waterful climate, are as populous as ever.
-Much of this is doubtless due to the overthrow of dynasties and the
-ensuing lapse of irrigation, but query: Is it all? For we have still
-more definite information in the drying up of the streams which have
-left the aqueducts of Carthage without continuation, as much to water
-on the one hand as to its drinkers on the other. Men may leave because
-of lack of water, but water does not leave because of dearth of men to
-drink.
-
-Recent search around the Caspian by Huntington has disclosed the
-like degeneration due to encroaching desertism there. Indeed, it
-is no chance coincidence that just where all the great nations
-thrived in the morning of the historic times should be precisely
-where populous peoples no longer exist. For neither increasing cold
-nor increasing heat is responsible for this, seeing that no general
-change has occurred in either. Nor were they particularly exposed to
-extermination by northern hordes of barbarians. Egypt as a world power
-died a natural death, and Babylonia too; but the common people died of
-thirst, indirect and unconscious and not wholly of their own choosing.
-Prehistoric records make this conclusion doubly sure, by lengthening
-the limit of our observation. Both extinct flora and extinct fauna tell
-the same tale. In the neighborhood of Cairo petrified forests attest
-that Egypt was not always a wiped slate, while the unearthed animals of
-the Fayum bear witness to water where no water is to-day.
-
-[Illustration: PETRIFIED BRIDGE, THIRD PETRIFIED FOREST, NEAR ADAMANA,
-ARIZONA—PHOTOGRAPH BY HARVEY.]
-
-Anywhere we wander along these girdling belts we find the same story
-written for us to read. The great deserts of New Mexico and Arizona
-show castellated structures far beyond the means of its present Indian
-population to inhabit. Yet this retrenchment occurred long before the
-white man came with his exterminating blight on everything he touched.
-Nor have we reason to suppose that it arose in consequence of invasion
-by other alien hordes. Individual communities may thus indeed have
-perished as the preservation of their domiciles intact leads us to
-infer, but all did not thus vanish from off the Earth. Here again
-humanity died or moved away because nature dried the sources of its
-supply. And here, as elsewhere, we find prehistoric record in the rocks
-of a once more smiling state of things, strengthening the testimony we
-deduce from man. The forests, crowning now only the greater heights,
-are but the shrinking residues of what once clothed the land. The
-well-named Arid Zone is becoming more so every day.
-
-If from the land evidence of drying up we turn to the marine, we see
-the same shrinkage at work. It has even been discovered in a lowering
-of the ocean bed, but as this may so easily be disputed, we turn to
-one aspect of the situation which cannot so easily be gainsaid,—the
-bodies of water that have been cut off. That the Dead Sea, the Caspian,
-the Great Salt Lake, are slowly but surely giving way to land, is
-patent. If the climate at least were not more arid than before this
-could not occur; but more than this, if the ocean were not on the whole
-shrinking, there would be no tendency to leave such arms of itself
-behind to shrivel up. That the ocean basins are deepening is possible,
-but we know of one depletion which is not replaced—evaporation into
-space; and of another bound to come—withdrawal into fissures when the
-earth shall cease to be too hot.
-
-This gradual withdrawal of the water may seem an unpleasant one to
-contemplate, but like most things it has its silver lining in the hope
-it holds out that sometime there shall be no more sea. Those of us who
-detest the constant going down to the sea in ships hardly more than the
-occasional going down with them, can take a crumb of comfort in the
-thought. Unfortunately it partakes of a somewhat far-off realization
-in our distant descendants, coming a little too late to be of material
-advantage to ourselves.
-
-But let me not leave the reader wholly disconsolate. For another
-thought we can take with us in closing our sketch of so much of the
-Earth’s life as brings it well down to to-day,—the thought that it has
-grown for us a steadily better place to contemplate from the earliest
-eras to the present time. Indeed, with innate prescience we forbore to
-appear till the prospect did prove pleasing. Finally, we may palliate
-prognostication by considering that if its future seem a thought less
-attractive, we, at least, shall not be there to see.
-
-
-
-
-CHAPTER VIII
-
-DEATH OF A WORLD
-
-
-Everything around us on this Earth we see is subject to one inevitable
-cycle of birth, growth, decay. Nothing that begins but comes at last
-to end. Not less is this true of the Earth as a whole and of each of
-its sister planets. Though our own lives are too brief even to mark
-the slow nearing to that eventual goal, the past history of the Earth
-written in its rocks and the present aspects of the several planets
-that circle similarly round the Sun alike assure us of the course of
-aging as certainly as if time, with all it brings about, passed in one
-long procession before our very eyes.
-
-Death is a distressing thing to contemplate under any circumstances,
-and not less so to a philosopher when that of a whole world is
-concerned. To think that this fair globe with all it has brought forth
-must lapse in time to nothingness; that the generations of men shall
-cease to be, their very records obliterated, is something to strike a
-chill into the heart of the most callous and numb endeavor at its core.
-That æons must roll away before that final day is to the mind of the
-far-seeing no consolation for the end. Not only that we shall pass,
-but that everything to show we ever were shall perish too, seems an
-extinction too overpowering for words.
-
-But vain regret avails not to change the universe’s course. What is
-concerns us and what will be too. From facing it we cannot turn away.
-We may alleviate its poignancy by the thought that our interest is
-after all remote, affecting chiefly descendants we shall never know,
-and commend to ourselves the altruistic example so nobly set us by
-doctors of medicine who, on the demise of others at which—and possibly
-to which—they have themselves assisted, show a fortitude not easily
-surpassed, a fortitude extending even to their bills. If they can act
-thus unshaken at sight of their contemporaries, we should not fall
-behind them in heroism toward posterity.
-
-Having in our last chapter run the gantlet of the geologists, we are in
-some sort fortified to face death—in a world—in this. The more so that
-we have some millenniums of respite before the execution of the decree.
-By the death of a planet we may designate that stage when all change
-on its surface, save disintegration, ceases. For then all we know as
-life in its manifold manifestations is at an end. To this it may come
-by many paths. For a planet, like a man, is exposed to death from a
-variety of untoward events.
-
-Of these the one least likely to occur is death by accident. This,
-celestially speaking, is anything which may happen to the solar system
-from without, and is of the nature of an unforeseen catastrophe. Our
-Sun might, as we remarked, be run into. For so far as we know at
-present the stars are moving among themselves without any too careful
-regard for one another. The swarm may be circling a central Sun as
-André states, but the individual stars behave more like the random
-particles of a gas with licensed freedom to collide; whereas we may
-liken the members of the solar system to molecules in the solid state
-held to a centre from which they can never greatly depart. Their
-motions thus afford a sense of security lacking in the universe at
-large.
-
-Such an accident, a collision actual or virtual with another sun,
-would probably occur with some dark star; of which we sketched the
-ultimate results in our first chapter. The immediate ones would be of
-a most disastrous kind. For prefatory to the new birth would be the
-dissolution to make such resurrection possible. Destruction might come
-direct, or indirectly through the Sun. For though the Sun would be the
-tramp’s objective point, we might inadvertently find ourselves in the
-way. The choice would be purely academic; between being powdered, or
-deorbited and burnt up.
-
-So remote is this contingency that it need cause us no immediate
-alarm, as I carefully pointed out. But so strong is the instinct
-of self-preservation and so pleasurable the sensation of spreading
-appalling news, that the press of America, and incidentally Europe,
-took fire, with the result, so I have been written, that by the time
-the pictured catastrophe reached the Pacific “it had assumed the
-dimensions of a first magnitude fact.”
-
-This is the first way in which our world may come by its death. It is
-possible, but unlikely. For our Earth, long before that, is morally
-certain to perish otherwise.
-
-The second mode is one, incident to the very constitution of our solar
-system. It follows as a direct outcome of that system’s mechanical
-evolution, and may be properly designated, therefore, as due to natural
-causes. It might be diagnosed as death by paralysis. For such it
-resembles in human beings, palsy of individual movement afflicting a
-planet instead of a man.
-
-Tidal friction is the slow undermining cause; a force which is
-constantly at work in the action of every body in the universe upon
-every other. As we previously explained, the pull of one mass upon
-another is inevitably differential. Not only is the second drawn in
-its entirety toward the first, falling literally as it circles round,
-but the nearer parts are drawn more than the centre and the centre
-more than those farthest away. We may liken the result to a stretched
-rotating rubber ball, with, however, one important difference,—that
-each layer is more or less free to shear over the others. The bulge,
-solicited by the rotation to keep up, by the disturber to lag behind,
-is torn two ways, and the friction acts as a break upon the body’s
-rotation, tending first to turn it over if it be rotating backward
-and then to slow it down till the body presents the same face in
-perpetuity to its primary. The tides are the bulge, not simply those
-superficial ones which we observe in our oceans, and know to be so
-strong, but substantial ones of the whole body which we must conceive
-thus as egg-shaped through the action that goes on—the long diameter
-of the egg pointing somewhat ahead of the line joining its centre
-to the distorting mass. All the bodies in the solar system are thus
-really egg-shaped, though the deformation is so slight as to escape
-detection observationally. The knowledge is an instance of how much
-more perceptive the brain is than the eye. For we are certain of the
-fact, and yet to see it with our present means is impossible, and may
-long remain so.
-
-Two concomitant symptoms follow the friction of the tidal ansæ: a shift
-of the plane in which the rotation takes place, and a loss of speed in
-the spin itself. The first tends to bring the plane of rotation down
-to the orbital plane, with rotation and revolution in the same sense.
-This effect takes place quicker than the other, and in consequence
-different stages may be noted in the creeping paralysis by which the
-body is finally overcome. Loss of seasons characterizes the first. For
-the coincidence of the two planes means invariability in the Sun’s
-declination throughout the year for a given latitude. This reduces
-all its days to one dead level in which summer and winter, spring and
-autumn, are always and everywhere the same. There is thus a return at
-the end of the planet’s career to an uneventful condition reminiscent
-of its start; a senility in planets comparable to second childhood in
-man.
-
-In large planets this outgrowing of seasons occurs before they have
-any, while the planet is yet cloud-wrapped. Such planets know nothing
-of some attributes of youth, like those unfortunate men who never
-were boys; just as reversely the meteorites are boys that never grew
-up. For if the planet be large, the action of the tidal forces is
-proportionately more powerful; while on the other hand the self-aging
-of the planet is greatly prolonged, and thus it may come about that
-the former process outstrips the latter to the missing of seasons
-entirely. This is sure to be the case with Jupiter, as the equator
-has already got down to within 3° of the orbit, and threatens to be
-the case with Saturn. These bodies, then, when they shall have put
-off their swaddling clothes of cloud, will wake to climates without
-seasons; globes where conditions are always the same on the same belts
-of latitude, and on which these alter progressively from equator to
-pole. Variety other than diurnal is thus excluded from their surfaces
-and from their skies. For the Sun and stars will rise always the same,
-in punctual obedience only to the slowly shifting year.
-
-The next stage of deprivation is the parting with the day. Although the
-day disappears, the result is too much day or too little, depending on
-where you choose to consider yourself upon the afflicted orb. For tidal
-friction proceeds to lengthen the twenty-four or other hours first to
-weeks, then months, then years, and at last to infinity; thus bringing
-the sun to a stock-still on the meridian, to flood one side of the
-world with perpetual day and plunge the other in eternal night.
-
-Which of these two hemispheres would be the worse abode, is matter
-of personal predilection; dust or glacier, deserts both. Everlasting
-unshielded noon would cause a wind circulation from all points of the
-enlightened periphery to the centre, whence a funnel-shaped current
-would rise to overflow back into the antipodes, thence to return by the
-horizon again. As the night side would be several hundred degrees at
-least colder than the noon one, all the moisture would be evaporated
-on the sunlit hemisphere, to be carried round and deposited as ice on
-the other, there to stay. Life would be either toasted or _frappé_. A
-Sahara backed by polar regions would be the obverse and the reverse of
-the shield.
-
-[Illustration: October 15, 1896.]
-
-[Illustration: February 12, 1897.]
-
-[Illustration: March 26, 1897.
-
-VENUS—DRAWINGS BY DR. LOWELL SHOWING AGREEMENT AT DIFFERENT DISTANCES.]
-
-The reader may deem the picture a fancy sketch which possibly may not
-appeal to him. Nevertheless, it not only is possible, but one which
-has overtaken our nearest of neighbors. To this pass the Mater Amorum,
-Venus herself, has already been brought. She betrays it by the wrinkles
-which modern observation has revealed upon her face. Innocent critics,
-with a gallantry one would hardly have credited them,—which shows how
-one may wrong even the humblest of creatures,—have denied the existence
-of these marks of age, on the chivalrous _a priori_ assumption that it
-could not possibly be true because never seen before. Their negation,
-in naïve ignorance of the facts, partakes the logic of the gallant
-captain, who, when asked by a lady to guess her age, replied: “’Pon
-my word, I haven’t the slightest idea,” hastily adding, “But you don’t
-look it!” Less commendable than this conventional nescience, but
-unfortunately more to the point, is the evidence of prying scientific
-curiosity. Shrewdly divined as much as detected by Schiaparelli,
-made more certain by the crow’s-feet disclosed at Flagstaff, and
-corroborated by the testimony of the spectroscope there, her
-isochronism of rotation and revolution lies beyond a doubt. Attraction
-to her lord has conquered at last her who was the cynosure of all.
-Venus, in her old age, stares forever at the Sun, and we all know how
-ill an aging beauty can support a garish light.
-
-Mercury has been brought to a like pass. This was evident even before
-the facts came out about Venus, for Venus, true to her instincts,
-shields herself with a veil of air which largely baffles man’s too
-curious gaze. Mercury, on the other hand, offers no objection to
-observation. When looked for at the proper time, his markings are
-quite distinct, dark, broken lines suggesting cracks. Schiaparelli,
-again, was the first to perceive the true state of the case, and his
-observations were independently confirmed and extended at Flagstaff in
-1896. In so doing the latter disclosed a very interesting fact. It was
-evident that the markings held in general a definite fixed position
-upon the illuminated part of the disk, showing that the planet kept
-the same face always to the Sun. But systematic observation, continued
-day after day for weeks, disclosed a curious shift, which, though
-slight, was unmistakable. Upon thought the cause suggested itself, and
-on being subjected to calculation proved equal to such accounting. In
-this singular systematic sway stood revealed the libration in longitude
-caused by the eccentricity of the planet’s orbit.
-
-[Illustration: DIAGRAM OF LIBRATION IN LONGITUDE DUE TO ROTATION.]
-
-[Illustration: _Mercury._
-
-_Effect of Libration
-
-Rotation 88 days._]
-
-Mercury revolves about the Sun in an ellipse more eccentric than that
-of any other principal planet. At times he is half as far off again
-from him as he is at others. When near, he travels faster than when
-far. For both reasons, nearness and speed, his angular revolution about
-the Sun varies greatly from point to point according to where he finds
-himself in his orbit. His rotation, however, is necessarily uniform.
-For even the Sun has no power at once to change the enormous moment
-of momentum of his axial spin. In consequence, at times his angular
-velocity of revolution gains on his rotation, at other times loses,
-both coming out together at the end of a complete Mercurial year. The
-result is a superb rhythmic oscillation, a true mercurial pendulum
-compensated by celestial laws to perfect isochronism of swing.
-
-The outward sign of this shows in the movement of the markings. To
-observers in space like ourselves, the planet seems to sway his head as
-he travels along his orbit. For weeks he turns his face, as shown by
-the markings on it, more and more over to the left; then turns it back
-again as far over to the right. It is as if he were looking furtively
-around as he hastens over his planetary path.
-
-Venus, of course, is equally subject to this law of distraction, but
-owing to the almost perfect circularity of her orbit she is less
-visibly affected. In fact, it is not possible to detect her lapse from
-a fixed regard to the Sun. At most it is no more than a glance out of
-the corner of her eyes—her slight deviation from perfect rectitude of
-demeanor. Knowledge of the laws governing such action alone permits us
-to recognize its occurrence.
-
-Mercury and Venus are the only planets as yet that turn a constant
-face to their overruling lord. The reason for this appears when one
-goes into the matter analytically. The tidal force is not the direct
-pull of the Sun on a particle of the body, but the difference in the
-pulls upon a particle at the centre and one at the circumference. Being
-differential, it depends directly upon the radius of the distorted body
-and inversely upon the third power of its distance away. As the space
-through which the force acts is proportional to the force itself, the
-effect is as the squares of the quantities mentioned, or, inversely, as
-the sixth power of the distance and as the square of the body’s radius.
-The result thus proves greatest on the planets nearest to the Sun, and
-diminishes rapidly as we pass outward from him. If, then, the solar
-force had had time enough to produce its effects, it would be first in
-Mercury and then in Venus that it should be seen. And this is precisely
-where we observe it.
-
-The Moon presents us a well-known case of such filial regard, resulting
-in permanent incompetency of action on its own account. It turns always
-the same face to us, following us about with the mute attention of a
-dog to its master. Here again the libration may be detected, for no
-dog but makes excursions on the road. This case differs from those of
-Mercury and Venus in that the body to which the regard is paid is not
-also the dispenser of light and warmth. In consequence, though the side
-of the Moon with which we are presented remains always the same, we do
-not always see it; the light creeping over it with the progress of the
-lunation, from new to full. On this account the worst that happens to
-our Moon in its old age is that its day becomes its month.
-
-[Illustration: MOON—FULL AND HALF, PHOTOGRAPHED AT THE LOWELL
-OBSERVATORY.]
-
-Our Moon is not peculiar in having its day and its month the same. On
-the contrary, it is now the rule with satellites thus to protract their
-days. So far as we can observe, all the large satellites of Jupiter
-turn the same face to him; those of Saturn pay him a like regard; while
-about those of Uranus and Neptune we are too far off to tell. Their
-direct respect for their primary, with only secondary recognition of
-the Sun, keeps them from the full consequences of their fatal yielding
-to attraction. It is bad enough to have the day half a month long, but
-worse to have one that never ends, or, still worse, perpetual night.
-
-In our diagnosis of the cause of death in planets, we now pass from
-paralysis to heart failure. For so we may speak of the next affection
-which ends in their taking off, since it is due to want of circulation
-and lack of breath. It comes of a planet’s losing first its oceans and
-then its air.
-
-To understand how this distressing condition comes about, we must
-consider one of the interesting scientific legacies of the nineteenth
-century to the twentieth: the kinetic theory of gases.
-
-[Illustration: ILLUSTRATING MOLECULAR MOTION IN A GAS (BLACK MOLECULES
-HERE CONSIDERED AT REST).]
-
-The kinetic theory of gases supposes them to be made up of minute
-particles all alike, which are perfectly elastic and are travelling
-hither and thither at great speeds in practically straight lines. In
-consequence, these are forever colliding among themselves, giving and
-taking velocities with bewildering rapidity, resulting in a state of
-confusion calculated to drive a computer mad. Somebody has likened a
-quiet bit of air to a boiler full of furious bees madly bent on getting
-out. The simile flatters the bees. To follow the vicissitudes of any
-one molecule in this hurly-burly would be out of the question; still
-more, it would seem, that of all of them at once. Yet no less Herculean
-a task confronts us. To find out about their motions, we are therefore
-driven to what is called the statistical method of inquiry,—which is
-simply a branch of the doctrine of probabilities. It is the method
-by which we learn how many people are going to catch cold in Boston
-next week when we know nothing about the people, or about colds, or
-about catching them. At first sight it might seem as if we could never
-discover anything in this hopelessly ignorant way, and as if we had
-almost better call in a doctor. But in the multitude of colds—not of
-counsellors—lies wisdom. So in other things not hygienic. As you cannot
-possibly divine, for instance, what each boy in town is going to do
-during the year, nor what is his make of mind, how can you say whether
-he will accidentally discharge a firearm and shoot his playmate or not!
-And yet if you take all the boys of Boston, you can predict to a nicety
-how many will thus let off a gun and “not know that it was loaded.”
-
-In this only genuine method of prophecy, complete ignorance of all the
-actual facts, we are able without knowing anything whatever about each
-of the molecules to predicate a good deal about them all. To begin
-with, the pressure a gas exerts upon the sides of a vessel containing
-it must be the bombardment the sides receive from the little molecules;
-and the heating due this rain of blows, or the temperature to which the
-vessel is raised, must measure their energy of translation. On this
-supposition it is found that the laws of Avogadro and of Boyle are
-perfectly accounted for, besides many more properties of gases which
-the theory explains, and as nothing yet has been encountered seriously
-contradicting it, we may consider it as almost as surely correct as the
-theory of gravitation. To three great geniuses of the last century we
-owe this remarkable discovery—Clausius, Clerk Maxwell, and Boltzmann.
-
-By determining the density of a gas at a given temperature and under
-a given pressure, we can find by the statistical method the average
-speed of its molecules. It depends on the most probable distribution
-of their energy. For hydrogen at the temperature of melting ice, and
-under atmospheric pressure, this speed proves to be a little over a
-mile a second—a speed, curiously enough, which is to that of light
-almost exactly as centimetres to miles. But some of the molecules are
-going at speeds much above the mean; fewer and fewer as the speed
-gets higher. Just how many there are for any assigned speed, we can
-calculate by the same ingenious application of unknown quantities.
-
-[Illustration: DISTRIBUTION OF MOLECULAR VELOCITIES IN A GAS.]
-
-These speeds have been found for a temperature of freezing, and as
-the speed varies as the square root of the absolute temperature, we
-might suppose that when an adventurous or lucky molecule arrived at
-practically the limit of the atmosphere, where the cold is intense, it
-would become numbly sluggish. But let us consider this. When we enclose
-a gas in a cooler vessel, the molecules bombard the sides more than
-they are bombarded back. In consequence, they lose energy; as we say,
-are cooled. But in free air if a molecule be fortunate enough to elude
-its neighbors, there is nothing to take away its motion but the ether
-through radiation, and this is a very slow process. Thus the escaping
-fugitive must arrive at the confines of the air with the speed it had
-at its last encounter. We reach, then, this result: In space there is
-no such thing as temperature; temperature being simply the aggregate
-effect of molecular temperament. The reason we should consider it
-uncommonly cold up there is that fewer molecules would strike us.
-Quantity, therefore, in our estimation replaces quality,—a possible
-substitution which also accounts for some reputations, literary or
-otherwise. The only forces which could affect this lonely molecule
-would be the heating by the Sun, the repellent force of light, and
-gravity.
-
-Now the speed which gravity on the Earth can control is 6.9 miles a
-second. It can impart this to a body falling freely to it from infinite
-space, and can therefore annul it on the way up, and no more. If, then,
-any of the molecules reach the outer boundary of the air going at more
-than this speed, they will pass beyond the Earth’s power to restrain.
-They will become little rovers in space on their own account, and dart
-off on interstellar travels of their own. This extension of the kinetic
-theory and of the consequent voyages of the molecules is due to Dr.
-Johnstone Stoney, who has since, humorously enough, tried to stop the
-very balls he set rolling. First thoughts are usually the best, after
-all.
-
-As among the molecules some are already travelling at speeds in excess
-of this critical velocity, molecules must constantly be attaining
-to this emancipation, and thus be leaving the Earth for good. In
-consequence there is a steady drain upon its gaseous covering.
-Furthermore, as we know from comets’ tails, the repellent power of the
-light-waves, what we may call the levity of light, much exceeds upon
-such volatile vagrants the heat excitement or even the gravity of the
-Sun, so that we arrive at this interesting conclusion—their escape is
-best effected under cover of the night.
-
-Again, the heavier the gas, the less its molecular speed at a given
-temperature, because its kinetic energy which measures that temperature
-is one-half the molecule’s mass into the square of its speed. Thus
-their ponderosity prevents as many of them from following their more
-agile cousins of a different constitution. So that the lighter gases
-are sooner gone. Water-vapor leaves before oxygen. Nor is there any
-escape from this escape of the gases. It may take excessively long,
-but go they must until a solitary individual who happens to have had
-the wrong end of the last collision is alone left hopelessly behind.
-
-Another factor also is concerned. The smaller the planet, the lower the
-utmost velocity it can control, and the quicker, therefore, it must
-lose its atmosphere. For a greater number of molecules must at every
-instant reach the releasing speed. Thus those bodies that are little
-shall, perforce, have less to cover themselves withal.
-
-Now this inevitable depletion of their atmospheric envelopes, the
-aspects of the various planets strikingly attest. They do so in most
-exemplary fashion, according to law. The larger, the major planets, as
-we have already remarked, have a perfect plethora of atmosphere, more
-than we at least know what to do with in the way of cataloguing yet.
-The medium-sized, like our own Earth, have a very comfortable amount;
-Mars, an uncomfortable one, as we consider, and the smallest none at
-all. All the smaller bodies of our system are thus painfully deprived
-so far as we can discover. We are certain of it in the case of our
-Moon and Mercury, the only ones we can see well enough to be sure.
-In further evidence it has been shown at the Yerkes and at Flagstaff
-that no perceptible effect of air betrays itself in the spectroscopic
-imprint of the rings of Saturn, those tiny satellites of his, and very
-recently a spectrogram of Ganymede, Jupiter’s third moon, made at
-Flagstaff for the purpose by Mr. E. C. Slipher has proved equally void
-of atmospheric hint.
-
-[Illustration: SPECTROGRAM OF SATURN—PHOTOGRAPHED BY DR. V. M.
-SLIPHER, LOWELL OBSERVATORY, OCTOBER 11, 1904. EXPOSURE 4ʰ ON “27” GILT
-EDGE PLATE. LONG CAMERA PLACED BENEATH THE SLIT. TITANIUM COMPARISON
-SPECTRUM. ENLARGEMENT BY MR. C. O. LAMPLAND.]
-
-With the loss of water and of air, all possibility of development
-departs. Not only must every organism die, but even the inorganic
-can no longer change its state. In the extinction thus not only of
-inhabitants but of the habitat that made them possible, occurs a
-curious inversion of the order we are familiar with in the life history
-of organisms. In planets it is the grandchildren that die first, then
-the children, and lastly their surviving parent. And this is not
-accidental, but inevitably consequent upon their respective origins.
-For the offspring, as we may spell it with a hyphen, of any cosmic mass
-is of necessity smaller than that from which it issued. Being smaller,
-it must age quicker. In the natural order of events, then, its end must
-be reached first.
-
-Such has been the course taken, or still taking, by the bodies of our
-solar family. The latest generation has already succumbed to this
-ebbing of vitality with time. Every one of the satellites of the
-planets—those of Neptune, Uranus, Saturn, Jupiter, and our own Moon—is
-practically dead; born so the smaller which never were alive. Our own
-Moon carries its decrepitude on its face. To all intents and purposes
-its life is past; and that it had at one time a very fiery existence,
-the great lunar craters amply testify. It is now, for all its flooding
-with radiance our winter nights, the lifeless statue of its former self.
-
-The same inevitable end, in default of others, is now overtaking the
-planetary group. Its approach is stamped on the face of Mars. There
-we see a world dying of exhaustion. The signs of it are legible in
-the markings we descry. How long before its work is done, we ignore.
-But that it is a matter of time only, our study of the laws of the
-inexorable lead us to conclude. Mars has been spared the fate of
-Mercury and Venus to perish by this other form of planetary death.
-
-Last in our enumeration of the causes by which the end of a world may
-be brought about, because the last to occur in order of time, is the
-extinction of the Sun itself. Certain to come and conclude the solar
-system’s history as the abode of life, if all the others should by any
-chance fail to precede it, it fittingly forms the climax, grand in its
-very quietude, of all that went before.
-
-By the same physical laws that caused our Earth once to be hot, the
-Sun shines to-day. Only its greater size has given it a life and
-a brilliancy denied to smaller orbs. The falling together of the
-scattered particles of which it is composed, caused, and still is
-causing, the dazzling splendor it emits. And so long as it remains
-gaseous, its temperature must increase, in spite of its lavish
-expenditure of heat, as Homer Lane discovered forty years ago.
-
-But the Sun’s store of heat, immense as it is to-day, and continued as
-it is bound to be for untold æons by means of contraction of its globe
-upon itself, and possibly by other causes, must some day give out. From
-its present gaseous condition it must gradually but eventually contract
-to a solid one, and this in turn radiate all its heat into space.
-Slowly its lustre must dim as it becomes incapable of replenishing
-its supply of motive power by further shrinkage in size. Fitfully,
-probably, like Mira Ceti to-day, it will show temporary bursts of
-splendor as if striving to regain the brightness it had lost, only to
-sink after each effort into more and more impotent senility. At last
-some day must come, if we may talk of days at all when the great event
-occurs when all days shall be blotted out, that the last flicker shall
-grow extinct in the orb that for so long has made the hearth of the
-whole system. For, presciently enough, the Latin word _focus_ means
-hearth, and the body which includes within it the focus about which all
-the planets revolve also constitutes the hearth from which they all are
-lighted and warmed.
-
-When this ultimate moment arrives and the last spark of solar energy
-goes out, the Sun will have reverted once more to what it was when the
-cataclysm of the foretime stranger awoke it into activity. It will
-again be the dark body it was when our peering into the past first
-descries it down the far vista of unrecorded time. Ghostlike it will
-travel through space, unknown, unheralded, till another collision shall
-cause it to take a place again among the bright company of heaven.
-Thus, in our account of the career of a solar system, we began by
-seeing with the mind’s eye a dark body travelling incognito in space,
-and a dark body we find ourselves again contemplating at the end.
-
-In this kaleidoscopic biograph of the solar system’s life, each
-picture dissolves into its successor by the falling together of its
-parts to fresh adjustments of stability, as in that instrument of
-pleasure which so witched our childish wonder in early youth. Just as
-when a combination had proved so pretty, once gone, to our sorrow no
-turning of the handle could ever bring it back, so in the march of
-worlds no retrace is possible of steps that once are past. Inexorable
-permutations lead from one state to the next, till the last of all be
-reached.
-
-Yet, unlike our childhood’s toy, reasoning can conjure up beside the
-present picture far vistas of what preceded it and of what is yet
-to come. Hidden from thought only by the distraction of the day, as
-the universe to sight lies hid by the day’s overpowering glare, both
-come out on its withdrawal till we wonder we never gazed before. Our
-own surroundings shut out the glories that lie beyond. Our veil of
-atmosphere cloaks them from our view. But wait, as an astronomer, till
-the Sun sinks behind the hills and his gorgeous gold of parting fades
-to amber amid the tender tapestry of trees. The very air takes on a
-meaning which the flood of day had swamped. Seen itself, no longer
-imperfectly seen through, it wakes to semi-sentient existence, a spirit
-come to life aloft to shield us from the too immediate vacancy of
-space. The perfumes of the soil, the trees, the flowers, steal out to
-it, as the twilight glow itself exhales to heaven. In the hushed quiet
-of the gloaming Earth holds her breath, prescient of a revelation to
-come.
-
-Then as the half-light deepens, the universe appears. One by one the
-company of heaven stand forth to human sight. Venus first in all
-her glory brightens amid the dying splendor of the west, growing in
-lustre as her setting fades. From mid-heaven the Moon lets fall a
-sheen of silvery light, the ghostly mantle of her ghostlike self, over
-the silent Earth. Eastward Jupiter, like some great lantern of the
-system’s central sweep, swings upward from the twilight bow to take
-possession of the night. Beyond lies Saturn, or Uranus perchance dim
-with distance, measuring still greater span. All in order in their
-several place the noble cortège of the Sun is exposed to view, seen now
-by the courtesy of his withdrawal, backgrounded against the immensity
-of space. Great worlds, these separate attendants, and yet as nothings
-in the void where stare the silent stars, huge suns themselves with
-retinues unseen, so vast the distances ’twixt us and them.
-
-No less a revelation awaits the opening of the shutters of the mind.
-If night discloses glimpses of the great beyond, knowledge invests it
-with a meaning unfolding and extending as acquaintance grows. Sight
-is human; insight seems divine. To know those points of light for
-other worlds themselves, worlds the telescope approaches as the years
-advance, while study reconstructs their past and visions forth their
-future, is to be made free of the heritage of heaven. Time opens to
-us as space expands. We stand upon the Earth, but in the sky, a vital
-portion not only of our globe, but of all of which it, too, forms part.
-To feel it is to enter upon another life; and if to realization of
-its beauty, its grandeur, and its sublimity of thought these chapters
-of its history have proved in any wise the portal, they have not been
-penned in vain.
-
-
-
-
-NOTES
-
-
-1 METEOR ORBITS
-
-If the space of the solar system be equally filled with meteors
-throughout, or if they diminish as one goes out from the Sun according
-to any rational law, their average speed of encounter with the Earth
-would be nearly parabolic.
-
-If they were travelling in orbits like those of the short-period
-comets, that is with their aphelia at Jupiter’s orbit and their
-perihelia at or within the Earth’s, their major axes would lie between
-6.2 and 5.2. If we suppose their perihelion distances to be equally
-distributed according to distance, we have for the mean a major axis of
-5.7. Their velocity, then, at the point where they cross the Earth’s
-track would be given by
-
-                       2     1
-             _v_² = µ(——— - ——— ),
-                       1   2.85
-
-    in which       µ = 18.5² in miles per second
-                     = 342.25,
-    whence      _v_  = 23.76 in miles per second.
-
-Suppose them to be approaching the Earth indifferently from all
-directions.
-
-At sunset the zenith faces the Earth’s quit; at sunrise the Earth’s
-goal. Let θ be the real angle of the meteor’s approach reckoned
-from the Earth’s quit; θ₁ the apparent angle due to compounding the
-meteor’s velocity-direction with that of the Earth. Then those
-approaching it at any angle 0 less than that which makes θ₁ = 90° will
-be visible at sunset; those at a greater angle, at sunrise. The angle
-01 is given by the relation,
-
-                _a_
-    cos θ₁ = +  ——— ,
-                _x_
-
-in which _a_ is the Earth’s velocity, _x_ the meteor’s, and θ₁ is
-reckoned from the Earth’s quit.
-
-The portion of the celestial dome covered at sunset is, therefore,
-
-    ⌠θ₁ ⌠360°
-    │   │    sin θ·_d_θ·_d_φ,
-    ⌡0  ⌡0
-
-where φ is the azimuth,
-
-                      ⌠180°  ⌠360°
-    that at sunrise,  │      │    sin θ·_d_θ·_d_φ.
-                      ⌡θ₁    ⌡0
-
-If the meteors have direct motion only, θ can never exceed 90°, and the
-limits become,
-
-                      ⌠θ₁ ⌠360°
-    for sunset,       │   │    sin θ·_d_θ·_d_φ,
-                      ⌡0  ⌡0
-
-                      ⌠90°  ⌠360°
-    and for sunrise,  │     │    sin θ·_d_θ·_d_φ.
-                      ⌡θ₁   ⌡0
-
-
-The mean inclination at sunset is
-
-                      ⌠θ₁ ⌠360°
-                      │   │     θ₁·sin θ·_d_θ·_d_φ,
-                      ⌡0  ⌡0
-                      ⸻⸻⸻⸻⸻⸻⸻⸻⸻ ,
-                        ⌠θ₁ ⌠360°
-                        │   │    sin θ·_d_θ·_d_φ,
-                        ⌡0  ⌡0
-
-in which θ₁ must be expressed in terms of θ, etc.
-
-From this it appears that the relative number of bodies, travelling in
-all directions and at parabolic speed, which the Earth would encounter
-at sunrise and sunset respectively would be:—
-
-        sunrise   5.8
-        sunset    1.0
-
-and with the speed of the short-period comets,
-
-        sunrise   8.0
-        sunset    1.0
-
-If, however, the bodies were all moving in the same sense as the Earth,
-_i.e._ direct, the ratios would be:—
-
-    ========+=========+==============+============================
-            |PARABOLIC|SPEED OF SHORT|SPEED OF ACTUAL SHORT-PERIOD
-            |  SPEED  |PERIOD COMETS |    COMETS ABOUT JUPITER
-    --------+---------+--------------+----------------------------
-    Sunrise |   2.4   |     3.5      |            3.3
-    Sunset  |   1.0   |     1.0      |            1.0
-    ========+=========+==============+============================
-
-As the actual number encountered is between 2 and 3 to 1, we see that
-the greater part must be travelling in the same sense as the Earth,
-since they come indifferently at all altitudes from the plane of her
-orbit.
-
-
-2 DENSITIES OF THE PLANETS
-
-The densities of the principal planets, so far as we can determine them
-at present, the density of water being unity, are:—
-
-    Mercury   3.65
-    Venus     5.36
-    Earth     5.53
-    Moon      3.32
-    Mars      3.93
-              ———— mean 4.36
-    Jupiter   1.33
-    Saturn    0.72
-    Uranus    1.22
-    Neptune   1.11
-              ———— mean 1.09
-    Sun       1.38
-
-The second decimal place is not to be considered as anything but an
-indication.
-
-
-3 VARIATION IN SPECTROSCOPIC SHIFT
-
-In the case of a body reflecting light, the shift differs from that
-for a body emitting it. If the planet be on the further side of the
-Sun, the approaching rim advances both toward the Sun and toward the
-Earth, thus doubling the shift. The receding rim recedes in like
-manner. At elongation the rims approach or recede with regard to the
-Earth, but not the Sun, and the shift is single as for emission.
-At inferior conjunction rotational approach to the Earth implies
-rotational recession from the Sun, and the two effects cancel.
-
-
-4 ON THE PLANETS’ ORBITAL TILTS
-
-The tilts of the plane of rotation of the Sun and of the orbits of the
-several planets to the dynamical plane of the system tabulated are:—
-
-    Sun        7°
-    Mercury    6° 14′
-    Venus      2°  4′
-    Earth      1° 41′
-    Mars       1° 38′
-    Asteroids  various
-    Jupiter       20′
-    Saturn        56′
-    Uranus     1°  2′
-    Neptune       43′
-
-where, in the determination of that plane, the latest values of the
-masses of the planets and the rotations of the Sun, Jupiter, and Saturn
-have been taken into account.
-
-These tilts suggest something, doubtless, but it is by no means clear
-what it is they suggest. They are just as compatible with a giving off
-from a slowly condensing nebula as with an origin by shock. The greater
-inclinations of Mercury and Venus may be due to their late birth from
-the central mass without the necessity of a cataclysm, the rotation
-of that central mass out of the general plane being caused by the
-consensus of the motions of the particles from which it was formed. The
-accordance of the larger planetary masses with the dynamical plane of
-the system would necessarily result from their great aggregations. So
-that this, too, is quite possible without shock.
-
-
-5 PLANETS AND THEIR SATELLITE SYSTEMS
-
-If we compute the speeds of satellites about their primaries in the
-solar system and compare them with the velocities in their orbits of
-the planets themselves, a striking parallelism stands displayed between
-the several systems. This is shown in the following table of them:
-
-    ==============+============================+===========+============
-                  |                            | PARABOLIC |
-                  |         MEAN SPEED,        | SPEED AT  | RATIO SPEED
-                  |       MILES A SECOND       |   ORBIT   |  SAT. ABOUT
-                  +------------+---------------+-----------+  PRIMARY TO
-                  | of Primary | of Satellite  |  Miles a  |   PLANET’S
-                  |  in Orbit  | about Primary |  second   |     SPEED
-                  |     _V_    |     _v_       |           |   IN ORBIT
-    --------------+------------+---------------+-----------+------------
-    Jupiter       |     8.1    |               |   11.5    |
-           Sat. 1 |            |     10.7      |           |   1.32
-                2 |            |      8.5      |           |   1.05
-                3 |            |      6.7      |           |   0.83
-                4 |            |      5.1      |           |   0.63
-    Saturn        |     6.0    |               |    8.5    |
-                1 |            |      9.0      |           |   1.50
-                2 |            |      7.9      |           |   1.31
-                3 |            |      8.2      |           |   1.36
-                4 |            |      6.3      |           |   1.05
-                5 |            |      5.3      |           |   0.89
-                6 |            |      3.5      |           |   0.59
-                8 |            |      2.0      |           |   0.34
-    Uranus        |     4.2    |               |    5.9    |
-                1 |            |      3.5      |           |   0.82
-                2 |            |      2.9      |           |   0.70
-                3 |            |      2.3      |           |   0.54
-                4 |            |      2.0      |           |   0.47
-    Neptune       |     3.4    |               |    4.8    |
-                1 |            |      2.7      |           |   0.81
-    ==============+============+===============+===========+============
-
-The relations here disclosed are too systematic to be the result of
-chance.
-
-The orbits of all these satellites have no perceptible eccentricity
-independent of perturbation except Iapetus, of which the eccentricity
-is about .03.
-
-In view of the various cosmogonies which have been advanced for the
-genesis of the solar system it is interesting to note what these
-speeds imply as to the effect upon the satellites of the impact of
-particles circulating in the interplanetary spaces at the time the
-system evolved. To simplify the question we shall suppose—which is
-sufficiently near the truth—that the planets move in circles, the
-interplanetary particles in orbits of any eccentricity.
-
-Taking the Sun’s mass as unity, the distance _R_ of any given planet
-from the Sun also as unity, let the planet’s mass be represented by _M_
-and the radius of its satellite’s orbit, supposed circular, as _r_. We
-have for the space velocity of the satellite on the sunward side of the
-planet, calling that of the planet in its orbit _V_ and that of the
-satellite in its orbit round the planet _v_,
-
-                 _______    _______
-    _V_ - _v_ = √(1/_R_) - √_M_/_r_.
-
-For a particle, the semi-major axis of whose orbit is _a₁_ and which
-shall encounter the satellite, the velocity is
-
-    _v₁_ = (2/(_R_-_r_) - 1/_a₁_)^{½}.
-
-That no effect shall be produced by the impact of these two bodies,
-their velocities must be equal, or
-
-     _____    _______    ____________________
-    √1/_R_ - √_M_/_r_ = √2/(_R_-_r_) - 1/_a₁_
-
-As _R_-_r_ = _a₁_(1 + _e_) for the point of impact if the particle be
-wholly within the orbit of the planet and _e_ the eccentricity of its
-orbit, we find
-
-             _________________
-    _e_ = 2 √_MR_/_r_ - _RM_/_r_ approx.
-
-for the case of no action, the other terms being insensible for the
-satellites in the table, since in all _r_ < _R_/400.
-
-Supposing, now, the particles within the orbit of the planet to be
-equally distributed according to their major axes, then as the velocity
-of any one of them, taking _R_-_r_ = _R_ approx. as unity, is
-
-    _v₁_ = (2/1 - 1/_a₁_)^{½},
-
-the mean velocity of all of those which may encounter the satellite is,
-at the point of collision,
-
-    ⌠¹
-    │  ((2_a₁_ - 1)^{½} / _a₁_^{½})_da₁_
-    ⌡_{½}
-    ————————————————————————————————————
-                      ⌠¹
-                      │  _da₁_
-                      ⌡_{½}
-
-       ┌¹                            __                         _____  ┐
-    = 2│    (2_a₁_² - _a₁_)^{½} - 1/√(2) log{(2_a₁_ - 1)^{½} + √2_a₁_ }│
-       └_{½}                                                           ┘
-
-    = 0.754;
-
-that is, just over three-quarters of the planet’s speed in its orbit.
-
-If we suppose the particles to be equally distributed in space, we
-shall have more with a given major axis in proportion to that axis, and
-our integral will become
-
-    ⌠¹
-    │  (2_a₁_ - 1)^{½}_a₁_^{½} _da₁_
-    ⌡_{½}
-    ————————————————————————————————
-                      ⌠¹
-                      │  _a₁ da₁_
-                      ⌡_{½}
-
-            ₁
-     = 8/3 [_{½} (4_a₁_-1)/8 (2_a₁_² - _a₁_)^{½}
-               __                              __                __
-       - (1/16√ 2 ) log[(2_a₁_² - _a₁_)^{½} + √ 2  · _a₁_ - 1/(2√ 2 )]]
-
-     = 0.792 of the planet’s orbital speed.
-
-The speed _v_, then, at which a satellite must be moving round the
-planet to have the same velocity as the average particle within the
-planet’s orbit, is
-
-    _V_ - _v₁_ = _v_.
-
-This velocity is, for the several planets:—
-
-    ========+====================+================
-            |  DISTRIBUTION OF   | DISTRIBUTION OF
-            | PARTICLES AS THEIR | PARTICLES EQUAL
-            |     MAJOR AXES     |    IN SPACE
-            +--------------------+----------------
-            |   Miles a second   |  Miles a second
-    --------+--------------------+----------------
-    Jupiter |        2.0         |      1.6
-    Saturn  |        1.5         |      1.2
-    Uranus  |        1.0         |      0.9
-    Neptune |        0.8         |      0.7
-    ========+====================+================
-
-If the satellite be moving in its orbit less fast than this, its
-space-speed will exceed that of the average particle; it will strike
-the particle at its own rear and be accelerated by the collision. If
-faster, the particle will strike it in front and retard it in its
-motion round its primary.
-
-From the table it appears that all the large satellites of all the
-planets have an orbital speed round their primaries exceeding those
-in either column. In consequence, all of them must have been retarded
-during their formation by the impact of interplanetary particles and
-forced nearer their primaries than would otherwise have been the case;
-and this whether the particles were distributed more densely toward the
-Sun, as 1/_a₁_, or were equally strewn throughout.
-
-For interplanetary particles whose orbits lie without the particular
-planet’s path the mean speed is the parabolic at the planet’s distance,
-given in the third column of the table. This is the case on either
-supposition of distribution. The orbital speed of the satellite which
-shall not be affected by collisions with them is, for the several
-planets:—
-
-    ========+==============
-            |MILES A SECOND
-    --------+--------------
-    Jupiter |     3.4
-    Saturn  |     2.5
-    Uranus  |     1.7
-    Neptune |     1.4
-    ========+==============
-
-All the satellites but Iapetus have orbital speeds exceeding this, and
-consequently are retarded also by these particles.
-
-For particles crossing the orbit (2) the mean velocity would be
-practically parabolic, 1.4, even if the distribution were as 1/_r_′,
-_r_′ being the distance from the Sun. The effect would depend upon
-the angle of approach and in the mean give a greater velocity for the
-particle than for the satellite within the orbit, a less one without;
-retarding the satellite in both cases. Thus the total effect of all the
-particles encountering the large satellites is to retard them and to
-tend to make them hug their primary.
-
-For retrograde satellites the velocities of impact with inside and
-outside particles moving direct are respectively:
-
-    =========+===========+==========
-             |  INSIDE   |  OUTSIDE
-    ---------+-----------+---------
-    Jupiter  | 2.0 + _v_ | _v_ + 3.4
-    Saturn   | 1.5 + _v_ | _v_ + 2.5
-    Uranus   | 1.0 + _v_ | _v_ + 1.7
-    Neptune  | 0.8 + _v_ | _v_ + 1.4
-    =========+===========+=========
-
-In both cases the impact tends to check the satellite.
-
-Comparing with these the velocities of impact for direct satellites in
-a direct plenum:—
-
-    =========+===========+===========
-             |  INSIDE   |  OUTSIDE
-    ---------+-----------+-----------
-    Jupiter  | 2.0 - _v_ | 3.4 - _v_
-    Saturn   | 1.5 - _v_ | 2.5 - _v_
-    Uranus   | 1.0 - _v_ | 1.7 - _v_
-    Neptune  | 0.8 - _v_ | 1.4 - _v_
-    =========+===========+===========
-
-the signs being taken positive when the motion is direct, we see that
-retrograde satellites would be more arrested than direct ones with the
-same orbital speed round the primary.
-
-In a plenum of direct moving particles, then, the force tending to
-stop the satellite and bring it down upon the planet is greater for
-retrograde satellites than for direct ones.
-
-If, therefore, the positions of the satellites have been controlled
-by the impact of interplanetary particles, the retrograde satellites
-should be found nearer their planets than the direct ones.
-
-
-6 ON THE INDUCED CIRCULARITY OF ORBITS THROUGH COLLISION
-
-Since the moment of momentum is the velocity into the perpendicular
-upon its direction, in the time _dt_ it is:—
-
-    _vp dt_ = _h dt_ = _r_²_d_Θ.
-
-The whole moment of momentum from perihelion to perihelion is
-therefore:—
-
-
-    ⌠360°
-    │    _r_²_d_Θ = _a_²·(1-_e_²)²/1-_e_²
-    ⌡₀
-
-      ┌360°
-      │   (-_e_ sin Θ)/(1+_e_ cos Θ)
-      └₀
-                                         _____________           ┐
-                + 2/(1-_e_²)^{½} tan⁻¹ (√1-_e_)/(1+_e_·tan (Θ/2))│
-                                                                 ┘
-
-    = 2π_a_² · (1 - _e_²)^{½},
-
-which is twice the area of the ellipse.
-
-The energy in the ellipse during an interval _dt_ is
-
-    (½)_mv_²_dt_ = (½)_m_µ(2/_r_ - 1/_a_)_dt_,
-
-from the well-known equation for the velocity in a focal conic. The
-integral of this for the whole ellipse is
-
-    ⌠ᵀ                ⌠360°
-    │ (½)_mv_² _dt_ = │ (½)(_m_µ/_h_)(2_r_ - _r_²/_a_)_d_Θ
-    ⌡₀                ⌡₀
-
-                    = _m_µ^{½}π_a_^{½}.
-
-Since
-
-    ⌠         ⌠
-    │ _rd_Θ = │ (_a_ · 1 - _e_²)/(1 + _e_ cos Θ)_d_Θ
-    ⌡         ⌡
-                                             ________________
-      = (2_a_· 1 -_e_²)/(1 -_e_²)^{½} tan⁻¹(√(1 -_e_)/(1 +_e_)tan (Θ/2))
-
-and ∫_r_² _d_Θ is given above.
-
-By collision a part of this energy is lost, being converted into heat.
-The major axis, _a_, is, therefore, shortened. But from the expression
-2π_a_² · (1-_e_²)^{½} for the moment of momentum we see that this is
-greatest when _e_ is least. If, therefore, _a_ is diminished, _e_ must
-also be diminished, or the moment of momentum would be lessened, which
-is impossible.
-
-
-7 CAPTURE OF SATELLITES
-
-See has recently shown (_Astr. Nach._ No. 4341-42) that a particle
-moving through a resisting medium under the attraction of two bodies
-revolving round one another in circles may eventually be captured
-by one of them though originally under the domination of both. The
-argument consists in introducing the effect of a resisting medium upon
-the motion in the space permitted by Jacobi’s integral, following
-Darwin’s examination of this space. In the actual case of nature the
-effect is much more complicated, and at present is not capable of exact
-solution for masses other than indefinitely small, even supposing
-circular orbits for the chief bodies. It may, however, explain the
-curious relation shown in the arrangement of the direct and retrograde
-movement of satellites.
-
-
-
-
-INDEX
-
-
-                    A
-    Abnormality, the survival of original state, 144, 146.
-    Absorption in spectrum,
-      planetary, 52, 161.
-      of Uranus, 118.
-      of Jupiter, 152.
-      of Saturn, 152.
-    Achilles, 94.
-    Adams, 119, 121.
-    Adams, Mr. J. C., 123-126.
-    Agassiz, 41.
-    Airy, 121, 123.
-    Albedo,
-      of dark star, 27.
-      of Mercury, 62, 73-75.
-      of Venus, 73-75.
-      of Moon, 75.
-      of Jupiter, 104, 105.
-      of Saturn, 109.
-      of Uranus, 116.
-      of Neptune, 168.
-      of clouds, 195.
-    Algol, 3.
-    American Academy, 125.
-    Amphibians, first record of, 188.
-    Anderson, Dr. Thomas D., 8, 12.
-    André, 215.
-    Andromeda, great nebula in, 10, 20, 21.
-      constitution disclosed by spectroscope, 45, 48.
-    Apex of Sun’s way, 26.
-    Arago, 121.
-    Asteroids, 39, 60, 61, 94-102.
-      domain of, 94.
-      diminutive size, 94, 101.
-      number, 94, 101.
-      peculiar discovery of, 95-98.
-      never formed part of a pristine whole, 98.
-      where thickest, 98.
-      formation of large planet from, prevented, 98, 99.
-      mid-course between planets and comets, 100.
-      shape of, 101, 102.
-      mammoth meteorites, 102.
-      mark transition between inner and outer planets, 102.
-    Atmosphere,
-      spectrographic study of, 53, 54, 161.
-      Mercury deprived of, 71, 75, 232.
-      reflecting power, 75.
-      of Venus, 75.
-      Moon deprived of, 75, 232.
-      thin on Mars, 75, 91, 232.
-      of Uranus, enormous, 117, 118, 232.
-      of Neptune, vast, 118, 232.
-      of Jupiter, 166, 232.
-      depletion of, 231-233.
-      none on Ganymede, 232, 233.
-      of Saturn, 232.
-      lacking in Saturn’s rings, 232.
-    Avogadro, 228.
-    Axes of planets,
-      systematic righting of, 132.
-      tilts accounted for, 146.
-
-                    B
-    Babinet, 147.
-    Backland, 68.
-    Ball, Sir Robert, 145.
-    Barrande, M., 178.
-    Belopolski, 87.
-    Bessel, 120, 121.
-    Blandet, M., 175, 176.
-    Bode, 95, 119.
-    Bode’s law, 96, 100, 119, 122, 126.
-    Bolometer, 194.
-    Bolton, Mr. Scriven, 103, 105, 106.
-    Boltzmann, 228.
-    Bose, 157.
-    Bouvard, Alexis, 120, 121.
-    Boyle, 228.
-    Bradley, 68.
-
-                    C
-    Cambrian era, 178.
-    Cambridge Observatory, 123.
-    Campbell, 9.
-    Carboniferous period, 179.
-    Cassini, 76, 162.
-    Celestial mechanics, 28, 94, 155.
-    Ceres, 101.
-    Challis, 123.
-    Chemistry, indebted to the stars, 160.
-    Clausius, 228.
-    Clerke, Miss, 9, 164.
-    Climate, advent of, 185.
-    Clouds,
-      none on Venus, 75.
-      of Jupiter not ordered as ours, 107, 163, 167.
-      Uranus wrapped in, 168.
-      Neptune wrapped in, 168.
-      Earth once wrapped in, 170, 171, 178.
-    Collision of dark star with Sun, 25, 215.
-      warning of, 26-29.
-      disturbances previous to, 29, 30.
-      rarity of event, 30.
-    Collisions between meteorites of a flock, 11, 49.
-      causing light, 49, 50.
-    Columbus, 188.
-    Comets, 33, 61.
-      members of solar system, 34, 35.
-      orbits of, 61, 100.
-    Commensurability of orbital period, 99, 111.
-    Congruities of solar system, 128-137.
-      deviations from, 62, 100, 101, 130, 131, 141.
-      specify mode of evolution, 137.
-    Convection currents, 219.
-      in atmosphere of Venus, 80.
-    Copeland, Dr. 7.
-    Copernican system, 58.
-    Copernicus, 62.
-    Cosmic action, 1, 22, 184.
-    Croll, 196.
-    Cuticle of star, effect of impact on, 11.
-
-                    D
-    Dana, 177, 186, 189.
-    Dark stars,
-      origin, 2.
-      number, 2, 25.
-      evidence of, 3-5.
-      collision of, 10, 11.
-      rendered visible, 26.
-    Darwin, 62, 138, Notes 252.
-    Day,
-      lengthened to infinity, 70, 219.
-      none on Venus, 83.
-      Jovian, 163.
-      first appreciation of, 186.
-      coincides with month, on satellites, 225.
-    Death of a planet,
-      defined, 214.
-      catastrophic cause, 215, 216.
-      due to tidal retardation of rotation, 216-219.
-      outcome of loss of oceans and air, 226, 233.
-      caused by extinction of Sun itself, 234.
-    Density,
-      of dark star, 27.
-      of planets, 51, Notes 243.
-      of Mercury, 63, 64.
-      of Venus, 90.
-      of Jupiter, 103, 117.
-      of Uranus, 115.
-    Deserts, increase of, on Earth, 208-211.
-    Devonian era, 187.
-    Dhurmsala meteorite, 41.
-    Diameter,
-      of Mercury, 63, 64, 66, 67.
-      of Venus, 90.
-      of Earth, 90.
-      of Mars, 91.
-      of satellites of Mars, 92.
-      of Jupiter, 103.
-      of Uranus, 115-117.
-    Dust, in atmosphere of Venus, 75.
-
-                    E
-    Earth,
-      characteristics, not universal, 90, 91, 155.
-      evolved from a nebula, 149.
-      internal heat, 150.
-      early surface temperature, 160, 169, 170.
-      once cloud-wrapped, 170, 171, 178.
-      solid surface formed, 171.
-      hot seas of, 171, 172.
-      self-sustained, 182.
-      study of, within province of astronomy, 184.
-      ceased to be self-centred, 187.
-      Sun becomes dominant factor in organic life of, 190.
-    Earth shine, 82.
-    Eccentricity, orbital,
-      of Mercury, 63, 65, 69, 222.
-      of asteroids, erratic, 100, 101.
-      of satellites, increases with distance from primary, 134.
-    Eclipsing binaries, 3, 4.
-    Ejectum from nova, 5, 16.
-      rate of regression, 16.
-    Elemental substances, 159.
-      in Sun, 159.
-      once in Earth, 160.
-      discovery of, in stars, 161, 162.
-    Ellipticity,
-      of Jupiter, 103.
-      of Saturn, 109.
-      of Uranus, 115.
-    Encke, 68.
-    Energy,
-      conservation of, 140, 150, 151.
-      dissipation, 140-142.
-      conditions for a minimum, 142.
-    Eros, fluctuation of light of, gives evidence of form, 101, 102.
-    Evolution, 153.
-      white nebulæ in process of, 49.
-      rounded out, 56.
-      of solar family, 100.
-      evidence of, in solar system, 117.
-      manner of, lessens energy, 141.
-    Evolution, chemical, 155, 173.
-      universal, 156.
-      temperature conducive to, 157, 158.
-      attendant upon cooling, 158, 162.
-      steps in, shown by spectroscope, 161.
-    Evolution, physical, 155, 162.
-      induced by cooling, 162.
-
-                    F
-    Fabry, 34.
-    Fauna, 178, 179, 187.
-    Faye, 175, 176.
-    Flagstaff, Arizona, 52, 66, 68, 79, 83, 89, 92, 106, 110, 221, 232.
-      clear and steady air of, 66, 86.
-    Flamstead, 119.
-    Fleming, Mrs., 7.
-    Flemming, 120, 121.
-    Flora, of paleologic times, 177.
-    French Academy, 122.
-
-                    G
-    Galle, Dr., 122, 123, 125.
-    Gases,
-      peculiar to nebulæ, 11, 16.
-      occluded in meteorites, 42, 43.
-      in atmospheres of planets, 53-55.
-    Gauss, 34, 96, 97.
-    Geikie, 160, 177, 189.
-    Geology,
-      relation to astronomy, 173, 174, 183, 184.
-      scope of, 174, 203.
-    Geysers, avenues to earlier state, 160.
-    Goodricke, 3.
-
-                    H
-    Hakluyt, 188.
-    Harvard College Observatory, 8, 12.
-    Heat,
-      molecular motion, 150, 157, 230.
-      the result of evolving, 153.
-      the preface to higher evolution, 153, 156.
-      laws governing amount of, 190.
-      atmosphere keeps out, as well as stores, 191.
-      effective, received from Sun, 192-194.
-      invisible rays, 194.
-      retained, 194-196.
-      radiated, 194-196.
-    Heat of condensation of Earth,
-      accuses concourse of particles, 151.
-      evaluated, 151, 152.
-      sufficient for geologic phenomena, 152.
-    Hector, 94.
-    Helmholtz, 151.
-    Hencke, 98.
-    Herschel, Sir John, 122.
-    Herschel, Sir William, 96, 114, 162.
-    Hertha, periodic variability, 102.
-    Hipparchus, 5.
-    Holden, 9.
-    Hubbard, Professor, 124.
-    Huggins, 52.
-    Humphreys, 10.
-    Huntington, 209.
-
-                    I
-    Ice Age, 196.
-      not of orbital occasioning, 197-199.
-      increased precipitation, the cause, 199, 200.
-      a local affair, 200-202.
-    Irradiation, affecting diameter of Mercury, 66, 68.
-
-                    J
-    Jacobi, Notes 252.
-    Julius, Professor, 10.
-    Juno, 101.
-    Jupiter, 103-108.
-      not solid, 104, 107.
-      a semi-sun, 105, 108, 152, 166, 167.
-      white spots of, 106.
-    Jupiter, “great red spot” of, 164.
-      time of rotation, 164.
-      a vast uprush of heated vapor, 165, 166.
-    Jupiter’s belts,
-      secular progression, 104.
-      rotate at different speeds, 104, 162, 163.
-      color, 104.
-      wisps across, 105, 106.
-      bright ones, cloud, 163, 167.
-      spectrographic study of, 166.
-
-                    K
-    Kapteyn, 14.
-    Keeler, 19, 52, 110.
-    Kepler, 6.
-    Kinetic theory of gases, 226, 228.
-      corollary of, 54.
-      extension of, 230, 231.
-    Kirkwood, Professor, 35.
-
-                    L
-    Lagrange, 94, 97.
-    Lalande, 123, 124.
-    Lane, Homer, 234.
-    Langley, 191, 194.
-    Laplace, 34, 110, 127, 129, 131, 132, 138, 139, 147, 152, 175.
-    Laplacian cosmos, 129, 130.
-      false congruities of, 131-133.
-      annular genesis, disproved, 138, 139.
-      original “fire-mist” of, impossible, 138.
-    Lapparent, de, 173-176, 183, 189.
-    Lemonnier, 115, 119.
-    Leonard, Miss, 79.
-    Leverrier, 119, 121-126.
-    Lexell, 115.
-    Libration in longitude,
-      of Mercury, 65, 69, 70, 222, 223.
-      causes true day, 70, 71.
-      of Venus, inappreciable, 83, 223.
-      of Moon, 224.
-    Lick Observatory, 13, 14.
-    Lockyer, 48.
-    Lowell Observatory, 65, 74.
-
-                    M
-    Major planets,
-      gaseous, 117.
-      constitution of, differs from Sun or Earth, 161.
-      types of early planetary stages, 162.
-      self-centred and self-sustained, 168.
-    Man, immanent, 159.
-    Mars,
-      polar caps, 198.
-      canals in dark regions, 206, 207.
-      dying of exhaustion, 234.
-    Mass,
-      of Mercury, 63, 64, 68.
-      of Mars, 91.
-      of Jupiter, 103.
-      arrangement of, in solar system, 135-137, 148.
-    Massachusetts Institute of Technology, 134, 184.
-    Mauvais, 125.
-    Maxwell, Clerk, 110, 113, 228.
-    Mayer, 119, 151.
-    Mendeléeff, 161.
-    Mercury, 62-73.
-      time of rotation and revolution the same, 65, 69.
-      axis stands plumb to orbit, 70.
-      turns same face to the Sun, 70, 72, 134, 221.
-      surface markings, 72, 221.
-      color, 72.
-    Meteorites, 31, 35, 36.
-      cosmic bodies, 32, 33.
-      relation to shooting-stars, 36.
-      members of solar system, 36.
-      composition, 40-44, 55.
-      fused by friction with atmosphere, 40.
-      temperature, 41, 55.
-      fragments of a dark body, 44.
-      link past to present, 44, 56, 57, 130.
-    Meteors,
-      orbits of, 36, 39, Notes 241-243.
-      visibility of, 38.
-    Meteor-streams, 33, 61.
-      first recognition of, 34.
-      disintegrated comets, 34.
-    Michelson, 10.
-    Milham, Professor, 99.
-    Mira Ceti, 235.
-    Mohler, 10.
-    Molecular speeds, gaseous, 228-231.
-      critical velocity, 230, 231.
-    Molecule, organic, power in its instability, 160.
-    Moment of momentum, 140, Notes 250.
-      cause of original, 130.
-    Moment of momentum, conservation of, 140.
-      applied to solar system, 141-143.
-    Momentum, 140.
-    Monck, Mr., 10.
-    Moon,
-      turns same face to Earth, 134, 208, 224, 225.
-      once fiery, now dead, 233, 234.
-    Mountains, none on Mars, 91.
-    Müller, 73, 74, 104, 105, 116.
-
-                    N
-    Naval Observatory at Washington, 122.
-    Nebulæ,
-      origin of, 10, 11.
-      amorphous, 18, 44.
-      planetary, 18.
-      spectrum of amorphous, 45.
-    Nebulæ, spiral, 17-25, 44.
-      evolved from disrupted stars, 10-15.
-      relation to novæ, 14-16.
-      corpuscular character of, 15, 16.
-      knots and patches of, 15.
-      most common, 19, 20.
-      two-armed, 20, 25.
-      central nucleus, globular, 21.
-      not due to explosive action, 22, 23, 25.
-      not caused by disintegration, 24, 25.
-      cause of development, 24, 25.
-      spectrum of, 45-48.
-      composed of flocks of meteorites, 48, 49.
-      constitution established by spectroscope, 49, 50.
-    Nebular hypotheses, 173.
-    Neologic times, clearing of sky in, 185.
-    Neptune, 118.
-      rotates backward, 118.
-      owes discovery to mathematical triumph, 119-126.
-      faint belts on, 168.
-      further advanced than giant planets, 168.
-    Newcomb, 67.
-    Newton, Professor, 36, 42.
-    Newton, Sir Isaac, 34.
-    Nova Aurigæ, 7, 8, 12.
-      history chronicled by its spectrum, 8, 9.
-    Nova Cygni, 7.
-    Novæ, 6, 7.
-      origin 5, 10.
-      first chronicled, 5.
-      spectroscopic study of, 7.
-    Nova Persei, 7.
-      history of, 12-15.
-
-                    O
-    Oceans,
-      none on Mars, 91.
-      evaporation of, 204.
-      basins of, on Moon, 204-208.
-      basins of, on Mars, 206, 207.
-    Olbers, 97.
-    Olmstead, Professor, 33.
-    Orbital distance,
-      of Mercury, 62.
-      of Venus, 73.
-      of Mars, 91.
-      of Eros, 94.
-      of Saturn, 108.
-    Orbital tilts,
-      of asteroids, erratic, 100, 101.
-      of satellites of Uranus, 116.
-      of planets, substantially the same, 129-131, Notes 244.
-      deviation from rule, by Mercury, 131.
-      of satellites, increase with distance from primary, 133, 134.
-    Orbits,
-      determining factors, 35.
-      rendered more circular by collisions, 141-143, Notes 250, 251.
-      made more conformant to general plane by collisions, 141-143.
-    Orion, great nebula in, 18.
-
-                    P
-    Paleologic times,
-      much warmth and little light in, 172.
-      fallacies in geologists’ expositions of, 174-176.
-      climate continuous, 177, 186.
-      seas warm, 177, 178.
-      explained by cloud envelope, 178.
-      corroboration of explanation, 187, 179.
-      excessive rain in, 185, 186.
-      passage into Neologic, essentially astronomic, 185.
-    Pallas, 101.
-    Parabolic speed at orbit, Notes 245.
-    Patroclus, 94.
-    Peirce, 110, 125, 126.
-    Perrine, 15.
-    Perrotin, 116.
-    Perturbations,
-      in motion of planets, heralding a catastrophe, 28, 30.
-      reflected, 63.
-      mass of planet determined by, 68.
-      of asteroids by Jupiter, 98, 99.
-      restrictive action of, 99.
-      the fashioning force of planetary orbits, 99, 100.
-      of rings of Saturn by satellites, 111, 112.
-      of Uranus lead to discovery of Neptune, 121-126.
-    Petersen, Dr., 123.
-    Photometric determinations, 92, 93.
-      background, the fundamental factor in, 92, 93.
-    Piazzi, 96.
-    Pilgrim Star, 5, 6.
-    Planetary astronomy, advance in, 59, 60.
-    Planetology, 203.
-      defined, 173, 174.
-    Planets, 61.
-      knots in spiral nebulæ, 25, 139.
-      developed by agglomeration, 143, 149, 151, 152.
-    Pliny, 5.
-    Plutonic rocks, 160.
-    Pluvial eras, contemporaneous with glacial, 200.
-    Polyp corals, in paleologic times, 186.
-    Pristine motion of planetary particles,
-      retrograde, 144.
-      superfluous energy in, 145.
-      unstable, 145.
-    Ptolemaic system, 58.
-
-                    R
-    Refrigeration, tempered by loss of cloud, 196.
-    Revolutions,
-      of shooting-stars, 39.
-      of asteroids, direct like planets, 100.
-      planetary, in same sense, 129, 130.
-      outermost satellites, retrograde, 132.
-      of satellites explained, 146, 147, Notes 252.
-    Ritchey, 14.
-    Roberts, Dr., 20.
-    Roche, Edouard, 110.
-    Rosse, Lord, 17.
-    Rotation of planets, 131, 132.
-      systematic righting of axes, 132.
-      initially, retrograde, 146.
-    Rotation period,
-      of Venus, spectrographically determined, 83, 85-90.
-      of Mars, spectrographically determined, 88, 89.
-      of Jupiter, spectrographically determined, 89.
-      of Uranus, 116.
-    Royal Observatory, Edinburgh, 7.
-
-                    S
-    Satellites, 61.
-      of Mars, 92.
-      of Saturn, 108, 112.
-      of Uranus, 116.
-      solid, 117.
-      of Neptune, 118.
-      turn same face to primaries, 134, 147, 148, 225.
-      latest discoveries in regard to motions of, 146.
-      origin of, 147.
-      death of, before planet, 233.
-      impact of interplanetary particles on, Notes 246-250.
-      capture of, Notes 251, 252.
-    Saturn, 108-114.
-      belts of, 109, 168.
-      inherent light, 109, 152.
-    Saturn’s rings, 109-114.
-      mechanical marvel of, not early appreciated, 110.
-      discrete particles, 110, 135.
-      knots upon, 110-113.
-      not flat, but tores, 111-114.
-      show devolution—not pristine state of solar system, 138, 139.
-      once a congeries, 139.
-    Schaeberle, 9.
-    Schiaparelli, 34, 36, 64-66, 69, 76, 77, 221.
-    Schroeter, 65, 77.
-    Seasons,
-      loss of, 71, 83, 217, 218.
-      begin with clearing of sky, 185.
-      fully developed, 189.
-    See, Notes 251.
-    Seeliger, 10.
-    Shooting-stars, 33, 35.
-      radiant of, 33, 36.
-      members of solar system, 36-40.
-      tiny planets, 39.
-    Siderite, 36.
-    Silurian era, 178.
-    Sirona, periodic variability of, 102.
-    Sky, cause of clearing, 187.
-    Slipher, Dr. V. M., 52, 79, 83, 86, 88, 89, 117, 161, 166.
-    Slipher, Mr. E. C., 79, 233.
-    Solar constant, 191.
-    Solar system,
-      evolved from a dark star, 44.
-      evidence of origin, 51, 130.
-      characteristics of, 60-62.
-      evolutionarily one, 62.
-      gap in progression of orbital distances, 95-100.
-      bodies of, egg-shaped, 217.
-    Specific gravity, of stone and iron, 44.
-    Spectroscope, 7, 84.
-    Spectroscopic shift, 84.
-      determining velocity, 3.
-      in Nova Aurigæ, 9.
-      produced by great pressure, 10, 13.
-      produced by anomalous refraction, 10.
-      produced by change of density, 10, 13.
-      explained, 85.
-      variation in, Notes 243, 244.
-    Spectrum,
-      of Nova Persei, 12, 13.
-      nebular, 13, 16, 45-48.
-      peculiarities of nebular, explained, 50.
-      photographic extension of, 52, 117, 161.
-      of major planets, 52, 53, 161.
-      of belts of Jupiter, 166.
-    Spiral structure,
-      implies rotation combined with motion out or in, 22.
-    Stability of a system, condition for, 140, 141.
-    Stoney, Dr. Johnstone, 231.
-    Struve, 109.
-    Suess, 179.
-    Sun,
-      original slow rotation of the, 130.
-      heat of, 234, 235.
-      reversion to a dark star, 235, 236.
-    Sun spots, 104, 166.
-
-                    T
-    Temperature,
-      of Moon, 191, 192.
-      of Mars, 192, 194, 196.
-      defined, 230.
-      no such thing as, in space, 230.
-    Tercidina, periodic variability of, 102.
-    Tertiary times, entrance of color with, 189, 190.
-    Tidal action, 143-147, 216-218.
-      causes loss of energy, 144.
-      inoperative, 144, 145, 147.
-      changes retrograde rotation of planet to direct, 145-147, 217.
-      on satellites, 147.
-      slows down spin, 148, 217.
-      brings plane of rotation down to orbital plane, 217.
-      lengthens day to infinity, 219.
-      analytically expressed, 224.
-      greatest on planets near Sun, 135, 224.
-    Tidal action, disruptive, 130.
-      exemplified by spiral nebulæ, 24, 25.
-      hinted at, by meteorites, 55.
-      theory corroborated by densities of planets, 51.
-      theory corroborated by atmospheres of planets, 52-55.
-      on comets, 139.
-      cause of Saturn’s rings, 139.
-    Tisserand, 68.
-    Titius, 95.
-    Todd, 68.
-    Trees, deciduous, first appearance of, 189.
-    Trilobites, blindness of, 178, 179.
-    Twining, 33.
-    Tycho Brahe, 5.
-
-                    U
-    Uranus, 114-118.
-      history of discovery, 114, 115, 119.
-      a ball of vapor, 115, 117.
-      belts of, 115, 116, 168.
-      tilt of axis to ecliptic, great, 115.
-      spectroscopic revelations of, 117, 118.
-      in an early amorphous state, 118.
-      further advanced than the giant planets, 168.
-
-                    V
-    Velocity,
-      of Mercury in orbit, 63.
-      of satellites about primary, Notes 245.
-      of major planets, in orbit, Notes 245.
-    Venus, 73-90.
-      surface markings, 74, 77, 79, 80, 83, 220, 221.
-      brilliancy due to cloudless atmosphere, 75.
-      importance of rotation period, 75, 76.
-      turns same face to the Sun, 77-80, 134, 220, 221.
-      ice on the night side, causes ashen light, 82.
-    Very, Professor, 16, 191, 192, 194.
-    Vesta, 101.
-    Vogel, 52.
-    Volcanoes, avenues to earlier state, 160.
-    Von Zach, 96.
-
-                    W
-    Walker, Mr., 123, 124.
-    Water,
-      becoming more scarce, 203, 204, 211.
-      lacking on Moon, 204.
-    Water-vapor,
-      in atmosphere of Jupiter, 53.
-      in atmosphere of Mars, 91, 161.
-      smaller planet has less hold on, 207.
-    Williams, Mr. Stanley, 103.
-    Witt, de, 94.
-    Wolf, Dr., 13.
-    Wolf, Max, 94.
-    Wolf-Rayet stars, 13, 48.
-    Wright, 13, 43.
-
-                   Y
-    Year, of Uranus, 116.
-    Yerkes Observatory, 232.
-    Young, 46.
-
-
-
-
-PERCIVAL LOWELL’S
-
-Mars and Its Canals
-
-    _Illustrated, 8vo, $2.50 net_
-
-
-“The book makes fascinating reading and is intended for the average
-man of intelligence and scientific curiosity. It represents mature
-reflection, patient investigation and observation, and eleven years’
-additional work and verification.... It is the work of a scientist who
-has found inspiration and joy in his work; it is full of enthusiasm,
-but the enthusiasm is not allowed to influence unduly a single
-conclusion.”—_Chicago Evening Post._
-
-“It seems impossible that Mr. Lowell can raise another girder more
-grandly impressive and expressive of the whole fabric or take another
-step in his scientific syllogism that will hold us any tighter in his
-logic. He has practically reached already his ‘Q. E. D.’ The thing is
-done, apparently, except for filling in the detail. But with his racy,
-epigrammatic brilliancy of style, his delicate, quiet humor, his daring
-scientific imagination—all held in check by instructive modesty of good
-breeding, gayly throwing to the winds all professional airs and mere
-rhetorical bounce—his course will be no doubt as charming to the end
-as it has been steadily illuminating even for the illuminati.”—_Boston
-Transcript._
-
-“Whether or not we choose to follow the author of this book to his
-ultimate inferences, he at least opens up a field of fascinating
-conjecture. The work is written in a style as popular as the precise
-enumeration of the ascertained facts permits, and if the narrative
-is not in all its details as entrancing as a novel, it nevertheless
-transports us into a region of superlatively romantic interest.”—_New
-York Tribune._
-
-“No doubt the highest living authority on Mars and things Martian
-is Prof. Percival Lowell, director of the observatory at Flagstaff,
-Arizona, an astronomical investigator and writer known over the entire
-world. Professor Lowell’s book, ‘Mars and Its Canals,’ is the final
-word, up to the present, on the planet and what we know of it.”—_Review
-of Reviews._
-
-            PUBLISHED BY
-        THE MACMILLAN COMPANY
-    64-66 Fifth Avenue, New York
-
-
-
-
-PERCIVAL LOWELL’S
-
-Mars as the Abode of Life
-
-    _Illustrated, 8vo, $2.50 net_
-
-
-The book is based on a course of lectures delivered at the Lowell
-Institute in 1906, supplemented by the results of later observations.
-It is, in the large, the presentation of the results of the author’s
-research into the genesis and development of what we call a world; not
-the mere aggregating of matter, but the process by which that matter
-comes to be individual as we find it. He bridges with the new science
-of planetology the evolutionary gap between the nebular hypothesis and
-the Darwinian theory.
-
-“It is not only as an astronomer but as a writer that Professor Lowell
-charms the reader in this work. The beguilement of the theme is well
-matched by the grace and literary finish of the style in which it is
-presented. The subject is one to beget enthusiasm in its advocates, and
-the author certainly is not devoid of it. The warmth and earnestness of
-the true lover of his theme shine through the entire work so that in
-its whole style and illustrations it is a charming production.”—_St.
-Louis Globe Democrat._
-
-“Mr. Lowell approaches the subject by outlining the now generally
-accepted theory of the formation of planets and the solar system. He
-describes the stages in the life history of a planet three of which are
-illustrated in the present state of the earth, Mars, and the moon. He
-tells what conditions we would expect to find on a planet in what we
-may call the Martian age, and proceeds to show how the facts revealed
-by observation square with the theories. The book is fascinatingly
-readable.”—_The Outlook._
-
-“So attractive are the style and the illustrations that the work will
-doubtless draw the attention of many new readers to its fascinating
-subject. Professor Lowell has fairly preëmpted that portion of the
-field of astronomy which interests the widest readers, for there
-is no doubt that speculation regarding the possibility of life on
-other planets than our own has a peculiar attraction for the average
-human mind.... For the convenience of the non-technical reader,
-the body of the book has been made as simple and understandable as
-possible.”—_Philadelphia Press._
-
-            PUBLISHED BY
-        THE MACMILLAN COMPANY
-    64-66 Fifth Avenue, New York
-
-
-
-
-
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-The Project Gutenberg EBook of The Evolution of Worlds, by Percival Lowell
-
-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: The Evolution of Worlds
-
-Author: Percival Lowell
-
-Release Date: August 21, 2020 [EBook #62992]
-
-Language: English
-
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-*** START OF THIS PROJECT GUTENBERG EBOOK THE EVOLUTION OF WORLDS ***
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-Produced by Paul Marshall, Tim Lindell and the Online
-Distributed Proofreading Team at https://www.pgdp.net (This
-file was produced from images generously made available
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-</pre>
-
-
-<hr class="chap" />
-
-<div class="transnote bbox epubonly">
-<p class="f120 space-above1">Transcriber’s Notes:</p>
-<hr class="r5" />
-<p class="indent">The cover image was created by the transcriber and is placed in the public domain.</p>
-</div>
-
-<h1 class="space-below3">THE EVOLUTION OF WORLDS</h1>
-
-<div class="figcenter">
-    <img src="images/logo.jpg" alt="" width="200" height="53" />
-</div>
-
-<p class="center">THE MACMILLAN COMPANY<br />NEW YORK · BOSTON · CHICAGO<br />
-ATLANTA · SAN FRANCISCO</p>
-<hr class="r5" />
-<p class="center">MACMILLAN &amp; CO., <span class="smcap">Limited</span><br />
-LONDON · BOMBAY · CALCUTTA MELBOURNE</p>
-<hr class="r5" />
-<p class="center">THE MACMILLAN CO. OF CANADA, <span class="smcap">Ltd.</span><br />
-TORONTO</p>
-<hr class="chap" />
-
-<div class="figcenter">
-    <a name="FRONTIS" id="FRONTIS"> </a>
-    <img src="images/frontispiece.jpg" alt="" width="400" height="530" />
-    <p class="center"><span class="smcap">Saturn—photographed at the
-          Lowell Observatory<br /> by Mr. E. C. Slipher. September, 1909.</span></p>
-</div>
-<hr class="chap" />
-
-<p class="f200"><b>THE EVOLUTION OF WORLDS</b></p>
-
-<p class="center">BY</p>
-<p class="f150">PERCIVAL LOWELL, A.B., LL.D.</p>
-<p class="f80 space-below2">AUTHOR OF “MARS AND ITS CANALS,” “MARS AS THE ABODE OF LIFE,” ETC.</p>
-
-<p class="center">DIRECTOR OF THE OBSERVATORY AT FLAGSTAFF, ARIZONA; NON-RESIDENT<br />
-PROFESSOR OF ASTRONOMY AT THE MASSACHUSETTS INSTITUTE OF<br />
-TECHNOLOGY; FELLOW OF THE AMERICAN ACADEMY OF ARTS AND SCIENCES;<br />
-MEMBRE DE LA SOCIÉTÉ ASTRONOMIQUE DE FRANCE; MEMBER OF THE<br />
-ASTRONOMICAL AND ASTROPHYSICAL SOCIETY OF AMERICA; MITGLIED<br />
-DER ASTRONOMISCHE GESELLSCHAFT; MEMBRE DE LA SOCIÉTÉ<br />
-BELGE D’ASTRONOMIE; HONORARY MEMBER OF THE SOCIEDAD<br />
-ASTRONOMICA DE MEXICO; JANSSEN MEDALLIST OF THE<br />
-SOCIÉTÉ ASTRONOMIQUE DE FRANCE, 1904, FOR<br />
-RESEARCHES ON MARS; MEDALLIST OF THE<br />
-SOCIEDAD ASTRONOMICA DE MEXICO FOR<br />
-STUDIES ON MARS, 1908</p>
-
-<p class="f120 space-above2 space-below2"><i>ILLUSTRATED</i></p>
-
-<p class="f120">New York<br />THE MACMILLAN COMPANY<br />1909</p>
-
-<p class="center"><i>All rights reserved</i></p>
-<hr class="r5" />
-<p class="center"><span class="smcap">Copyright</span>, 1909,</p>
-<p class="center"><span class="smcap">By</span> THE MACMILLAN COMPANY.</p>
-<p class="center">Set up and electrotyped. Published December, 1909.</p>
-<hr class="r5" />
-<p class="center">Norwood Press<br />J. S. Cushing Co.—Berwick &amp; Smith Co.<br />
-Norwood, Mass., U.S.A.</p>
-<hr class="chap" />
-
-<p class="center"><big><b>TO</b></big><br />THE PRESIDENT OF THE<br />
-MASSACHUSETTS INSTITUTE OF TECHNOLOGY<br />TO MY COLLEAGUES THERE<br />
-AND TO ITS STUDENT BODY<br />TO WHOSE INTEREST AND ATTENTION THESE<br />
-LECTURES ARE INDEBTED<br />THEY ARE APPRECIATIVELY INSCRIBED
-<span class="pagenum"><a name="Page_vi" id="Page_vi">[Pg vi]</a></span></p>
-<hr class="chap" />
-
-<div class="blockquot">
-<p>“Si je n’étais pas devenu général en chef et l’instrument du sort
-d’un grand people, j’aurais couru les bureaux et les salons pour me
-mettre dans la dépendance de qui que ce fût, en qualité de ministre
-ou d’ambassadeur? Non, non! je me serais jeté dans l’étude des
-sciences exactes. J’aurais fait mon chemin dans la route des Galilée,
-des Newton. Et puisque j’ai réussi constamment dans mes grandes
-entreprises, eh bien, je me serais hautement distingué aussi par
-des travaux scientifiques. J’aurais laissé le souvenir de belles
-découvertes. Aucune autre gloire n’aurait pu tenter mon ambition.”</p>
-
-<p class="author">—<span class="smcap">Napoleon Iᴱᴿ, quoted by Arago.</span></p>
-
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_vii" id="Page_vii">[Pg vii]</a></span>
-The substance of the following pages was written and presented in a
-university course of lectures before the Massachusetts Institute of
-Technology—in February and March of this year. The kind interest
-with which the lectures were received, not only by the students and
-professional bodies, but by the public, was followed by an immediate
-request from The Macmillan Company to issue them in book form, and as
-such they now appear.</p>
-
-<p class="author">PERCIVAL LOWELL.</p>
-<p><span class="smcap">Boston, Mass.</span>, May 29, 1909.</p>
-</div>
-
-<p><span class="pagenum"><a name="Page_viii" id="Page_viii">[Pg viii]</a></span></p>
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_ix" id="Page_ix">[Pg ix]</a></span></p>
-
-<p class="f150"><b>CONTENTS</b></p>
-
-<table border="0" cellspacing="0" summary="TOC" cellpadding="0" >
-  <tbody><tr>
-      <td class="tdr"><small>CHAPTER</small></td>
-      <td class="tdc"> </td>
-      <td class="tdr"><small>PAGE</small></td>
-   </tr><tr>
-      <td class="tdr">I.</td>
-      <td class="tdl_ws1"><span class="smcap">Birth of a Solar System</span></td>
-      <td class="tdr"><a href="#Page_1"> 1</a></td>
-   </tr><tr>
-      <td class="tdr">II.</td>
-      <td class="tdl_ws1"><span class="smcap">Evidence of the Initial Catastrophe in Our Own Case</span></td>
-      <td class="tdr"><a href="#Page_31">31</a></td>
-   </tr><tr>
-      <td class="tdr">III.</td>
-      <td class="tdl_ws1"><span class="smcap">The Inner Planets</span></td>
-      <td class="tdr"><a href="#Page_58">58</a></td>
-   </tr><tr>
-      <td class="tdr">IV.</td>
-      <td class="tdl_ws1"><span class="smcap">The Outer Planets</span></td>
-      <td class="tdr"><a href="#Page_94">94</a></td>
-   </tr><tr>
-      <td class="tdr">V.</td>
-      <td class="tdl_ws1"><span class="smcap">Formation of Planets</span></td>
-      <td class="tdr"><a href="#Page_127">127</a></td>
-   </tr><tr>
-      <td class="tdr">VI.</td>
-      <td class="tdl_ws1"><span class="smcap">A Planet’s History—Self-sustained Stage</span></td>
-      <td class="tdr"><a href="#Page_155">155</a></td>
-   </tr><tr>
-      <td class="tdr">VII.</td>
-      <td class="tdl_ws1"><span class="smcap">A Planet’s History—Sun-sustained Stage</span></td>
-      <td class="tdr"><a href="#Page_182">182</a></td>
-   </tr><tr>
-      <td class="tdr">VIII.</td>
-      <td class="tdl_ws1"><span class="smcap">Death of a World</span></td>
-      <td class="tdr"><a href="#Page_213">213</a></td>
-   </tr><tr>
-      <td class="tdc" colspan="3"><br />NOTES</td>
-   </tr><tr>
-      <td class="tdr">1.</td>
-      <td class="tdl_ws1"><span class="smcap">Meteor Orbits</span></td>
-      <td class="tdr"><a href="#NOTE_1">241</a></td>
-   </tr><tr>
-      <td class="tdr">2.</td>
-      <td class="tdl_ws1"><span class="smcap">Densities of the Planets</span></td>
-      <td class="tdr"><a href="#NOTE_2">243</a></td>
-   </tr><tr>
-      <td class="tdr">3.</td>
-      <td class="tdl_ws1"><span class="smcap">Variation in Spectroscopic Shift</span></td>
-      <td class="tdr"><a href="#NOTE_3">243</a></td>
-   </tr><tr>
-      <td class="tdr">4.</td>
-      <td class="tdl_ws1"><span class="smcap">On the Planets’ Orbital Tilts</span></td>
-      <td class="tdr"><a href="#NOTE_4">244</a></td>
-   </tr><tr>
-      <td class="tdr">5.</td>
-      <td class="tdl_ws1"><span class="smcap">Planets and their Satellite Systems</span></td>
-      <td class="tdr"><a href="#NOTE_5">245</a></td>
-   </tr><tr>
-      <td class="tdr">6.</td>
-      <td class="tdl_ws1"><span class="smcap">On the Induced Circularity of Orbits through Collision  </span></td>
-      <td class="tdr"><a href="#NOTE_6">250</a></td>
-   </tr><tr>
-      <td class="tdr">7.</td>
-      <td class="tdl_ws1"><span class="smcap">Capture of Satellites</span></td>
-      <td class="tdr"><a href="#NOTE_7">251</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1"><span class="smcap">Index</span></td>
-      <td class="tdr"><a href="#Page_253">253</a></td>
-   </tr>
- </tbody>
-</table>
-
-<p><span class="pagenum"><a name="Page_x" id="Page_x">[Pg x]</a></span></p>
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_xi" id="Page_xi">[Pg xi]</a></span></p>
-
-<p class="f150"><b>LIST OF ILLUSTRATIONS</b></p>
-
-<table border="0" cellspacing="0" summary="LOI" cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc" colspan="3">PLATES</td>
-   </tr><tr>
-      <td class="tdr">I.</td>
-      <td class="tdl_ws1">Saturn</td>
-      <td class="tdr"><a href="#FRONTIS"><i>Frontispiece</i></a><br /><small>OPPOSITE<br />PAGE  </small></td>
-   </tr><tr>
-      <td class="tdr">II.</td>
-      <td class="tdl_ws1">The Moving Nebula surrounding Nova Persei, 1901-1902</td>
-      <td class="tdr"><a href="#I_014">14</a></td>
-   </tr><tr>
-      <td class="tdr">III.</td>
-      <td class="tdl_ws1">Representative Stellar Spectra</td>
-      <td class="tdr"><a href="#I_024">24</a></td>
-   </tr><tr>
-      <td class="tdr">IV.</td>
-      <td class="tdl_ws1">Spectra of the Major Planets</td>
-      <td class="tdr"><a href="#I_052">52</a></td>
-   </tr><tr>
-      <td class="tdr">V.</td>
-      <td class="tdl_ws1">Venus, 1896-1897</td>
-      <td class="tdr"><a href="#I_082">82</a></td>
-   </tr><tr>
-      <td class="tdr">VI.</td>
-      <td class="tdl_ws1">Asteroids: Major Axes of Orbits</td>
-      <td class="tdr"><a href="#I_098">98</a></td>
-   </tr><tr>
-      <td class="tdr">VII.</td>
-      <td class="tdl_ws1">Saturn—A Drawing showing Agglomerations</td>
-      <td class="tdr"><a href="#I_108">108</a></td>
-   </tr><tr>
-      <td class="tdr">VIII.</td>
-      <td class="tdl_ws1">Spectrogram of Jupiter, Moon Comparison</td>
-      <td class="tdr"><a href="#I_152">152</a></td>
-   </tr><tr>
-      <td class="tdr">IX.</td>
-      <td class="tdl_ws1">Spectrogram showing Water-vapor in Atmosphere of Mars</td>
-      <td class="tdr"><a href="#I_160">160</a></td>
-   </tr><tr>
-      <td class="tdr">X.</td>
-      <td class="tdl_ws1">Tree Fern</td>
-      <td class="tdr"><a href="#I_176">176</a></td>
-   </tr><tr>
-      <td class="tdr">XI.</td>
-      <td class="tdl_ws1">Ten Views of Mercury, showing Effect of Libration</td>
-      <td class="tdr"><a href="#I_222B">222</a></td>
-   </tr><tr>
-      <td class="tdr">XII.</td>
-      <td class="tdl_ws1">Spectrogram of Saturn</td>
-      <td class="tdr"><a href="#I_232">232</a></td>
-   </tr><tr>
-      <td class="tdc" colspan="3"><br />CUTS APPEARING IN TEXT</td>
-   </tr><tr>
-      <td class="tdr" colspan="3"><small>PAGE</small></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Algol and its Dark Companion</td>
-      <td class="tdr"><a href="#I_004"> 4</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Nova Persei</td>
-      <td class="tdr"><a href="#I_011">11</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Spectrum of Nova Persei</td>
-      <td class="tdr"><a href="#I_012">12</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">The Moving Nebula surrounding Nova Persei, 1901</td>
-      <td class="tdr"><a href="#I_013">13</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Great Nebula in Orion</td>
-      <td class="tdr"><a href="#I_017">17</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Great Nebula in Andromeda</td>
-      <td class="tdr"><a href="#I_018">18</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Nebula M. 100 Comæ</td>
-      <td class="tdr"><a href="#I_019">19</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Nebula ♅ I. 226 Ursæ Majoris</td>
-      <td class="tdr"><a href="#I_020">20</a>
-                <span class="pagenum"><a name="Page_xii" id="Page_xii">[Pg xii]</a></span></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Nebula ♅ V. 24 Comæ. Showing Globular Structure</td>
-      <td class="tdr"><a href="#I_021">21</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Nebula M. 101 Ursæ Majoris</td>
-      <td class="tdr"><a href="#I_023">23</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">The Radiant of a Meteoric Shower</td>
-      <td class="tdr"><a href="#I_037">37</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Diagram explaining Proportionate Visibility of Meteors</td>
-      <td class="tdr"><a href="#I_038">38</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">The Mart Iron</td>
-      <td class="tdr"><a href="#I_041">41</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Section of Meteorite showing Widmannstättian Lines</td>
-      <td class="tdr"><a href="#I_042">42</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Meteorite, Toluca</td>
-      <td class="tdr"><a href="#I_043">43</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Nebula ♅ V. 14 Cygni</td>
-      <td class="tdr"><a href="#I_045">45</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Nebula N.G.C. 1499 Persei</td>
-      <td class="tdr"><a href="#I_046">46</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Nebula N.G.C. 6960 in Cygnus</td>
-      <td class="tdr"><a href="#I_047">47</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Nebula M. 51 Canum Venaticorum</td>
-      <td class="tdr"><a href="#I_048">48</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Orbits of the Inner Planets</td>
-      <td class="tdr"><a href="#I_059">59</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Sulla Rotazione di Mercurio.—Di G. V. Schiaparelli</td>
-      <td class="tdr"><a href="#I_064">64</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Map of Mercury. Lowell</td>
-      <td class="tdr"><a href="#I_069">69</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Venus. October, 1896-March, 1897</td>
-      <td class="tdr"><a href="#I_078">78</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Venus. April 12, 1909.</td>
-      <td class="tdr"><a href="#I_079">79</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Diagram: Convection Currents in Atmosphere of Venus</td>
-      <td class="tdr"><a href="#I_081A">81</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Diagram: Shift in Central Barometric Depression</td>
-      <td class="tdr"><a href="#I_081B">81</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Spectrogram of Venus, showing its Long Day</td>
-      <td class="tdr"><a href="#I_087">87</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Spectrogram of Jupiter, giving the Length of its Day</td>
-      <td class="tdr"> </td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1"> &emsp;by the Tilt of its Spectral Lines</td>
-      <td class="tdr"><a href="#I_089">89</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Orbits of the Outer Planets</td>
-      <td class="tdr"><a href="#I_095">95</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Drawing of Jupiter showing its Ellipticity</td>
-      <td class="tdr"><a href="#I_103">103</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Two Drawings of Jupiter and its Wisps</td>
-      <td class="tdr"><a href="#I_105">105</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Photograph of Jupiter, 1909</td>
-      <td class="tdr"><a href="#I_107">107</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Diagram of Saturn’s Rings</td>
-      <td class="tdr"><a href="#I_113">113</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">The Tores of Saturn</td>
-      <td class="tdr"><a href="#I_114">114</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Chart showing increasing Tilts of the Major Planets</td>
-      <td class="tdr"><a href="#I_131">131</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Orbital Tilts and Eccentricities of Satellites</td>
-      <td class="tdr"><a href="#I_133">133</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Masses of Planets and Satellites</td>
-      <td class="tdr"><a href="#I_136">136</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Two Drawings of Jupiter and its “Great Red Spot”</td>
-      <td class="tdr"><a href="#I_164">164</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Sun Spots</td>
-      <td class="tdr"><a href="#I_165">165</a>
-             <span class="pagenum"><a name="Page_xiii" id="Page_xiii">[Pg xiii]</a></span></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Photograph of a Sun Spot</td>
-      <td class="tdr"><a href="#I_166">166</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">The Volcano Colima, Mexico, March 24, 1903</td>
-      <td class="tdr"><a href="#I_169">169</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Jukes Butte, a Denuded Laccolith, as seen from the Northwest</td>
-      <td class="tdr"><a href="#I_170A">170</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Ideal Section of a Laccolith</td>
-      <td class="tdr"><a href="#I_170B">170</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Earth as seen from above.—Photographed at an Altitude of 5500 Feet</td>
-      <td class="tdr"><a href="#I_183">183</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Tracks of Sauropus Primævus</td>
-      <td class="tdr"><a href="#I_188">188</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Adventures of a Heat Ray</td>
-      <td class="tdr"><a href="#I_193">193</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Polar Caps of Mars at their Maxima and Minima</td>
-      <td class="tdr"><a href="#I_198">198</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Glacial Map of Eurasia</td>
-      <td class="tdr"><a href="#I_200">200</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Map showing the Glaciated Area of North America</td>
-      <td class="tdr"><a href="#I_201">201</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Photograph of the Moon</td>
-      <td class="tdr"><a href="#I_205">205</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Petrified Bridge, Third Petrified Forest, near Adamana, Arizona</td>
-      <td class="tdr"><a href="#I_210">210</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Three Views of Venus, showing Agreement at Different Distances</td>
-      <td class="tdr"><a href="#I_220">220</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Diagram of Libration in Longitude due to Rotation</td>
-      <td class="tdr"><a href="#I_222A">222</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Moon,—Full and Half</td>
-      <td class="tdr"><a href="#I_225">225</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Diagram illustrating Molecular Motion in a Gas</td>
-      <td class="tdr"><a href="#I_227">227</a></td>
-   </tr><tr>
-      <td class="tdr"> </td>
-      <td class="tdl_ws1">Distribution of Molecular Velocities in a Gas</td>
-      <td class="tdr"><a href="#I_229">229</a></td>
-   </tr>
- </tbody>
-</table>
-
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_1" id="Page_1">[Pg 1]</a></span></p>
-<p class="f150"><b>THE EVOLUTION OF WORLDS</b></p>
-<hr class="chap" />
-
-<div class="chapter">
-<h2 class="nobreak">CHAPTER I<br /><span class="h_subtitle">BIRTH OF A SOLAR SYSTEM</span></h2>
-</div>
-
-<p class="drop-cap">ASTRONOMY is usually thought of as the study of
-the bodies visible in the sky. And such it largely is when the present
-state of the universe alone is considered. But when we attempt to peer
-into its past and to foresee its future, we find ourselves facing a
-new side of the heavens—the contemplation of the invisible there. For
-in the evolution of worlds not simply must the processes be followed
-by the mind’s eye, so short the span of human life, but they begin and
-end in what we cannot see. What the solar system sprang from, and what
-it will eventually become, is alike matter devoid of light. Out of
-darkness into darkness again: such are the bourns of cosmic action.</p>
-
-<p>The stars are suns; past, present, or potential. Each of those diamond
-points we mark studding the heavens on a winter’s night are globes
-comparable with, and in many cases greatly excelling, our own ruler of
-<span class="pagenum"><a name="Page_2" id="Page_2">[Pg 2]</a></span>
-the day. The telescope discloses myriads more. Yet these self-confessed
-denizens of space form but a fraction of its occupants. Quite as
-near, and perhaps much nearer, are orbs of which most of us have no
-suspicion. Unimpressing our senses and therefore ignored by our minds,
-bodies people it which, except for rare occurrences, remain forever
-invisible. For dark stars in countless numbers course hither and
-thither throughout the universe at speeds as stupendous as the lucent
-ones themselves.</p>
-
-<p>Had we no other knowledge of them, reasoning would suffice to
-demonstrate their existence. It is the logic of unlimited subtraction.
-Every self-shining star is continually giving out light and heat.
-Now such an expenditure cannot go on forever, as the source of its
-replenishing by contraction, accretion, or disintegration is finite.
-Long to our measures of time as the process may last, it must
-eventually have an end and the star finally become a cold dark body,
-pursuing as before its course, but in itself inert and dead; an orb
-grown <i>orbéd</i>, in the old French sense. So it must remain unless some
-cosmic catastrophe rekindle it to life. The chance of such occurrence
-in a given time compared with the duration of the star’s light-emitting
-career will determine the number of dark stars relative to the lucent
-ones. The chance is undoubtedly small, and the number of dark bodies in
-space proportionally large. Reasoning, then, informs us first that such
-<span class="pagenum"><a name="Page_3" id="Page_3">[Pg 3]</a></span>
-bodies must exist all about us, and second that their multitude must be
-great.</p>
-
-<p>Valid as this reasoning is, however, we are not left to inference
-for our knowledge of them. There is a certain star amid the polar
-constellations known as Algol,—el Ghoul, the Arabs called it, or The
-Dæmon. The name shows they noticed how it winked its eye and recognized
-something sarcastically sinister in its intent. For once in two days
-and twenty hours its light fades to one-third of its usual amount,
-remains thus for about twenty minutes, and then slowly regains its
-brightness. Seemingly unmoved itself, its steady blinking from the time
-man first observed it took on an uncanniness he felt. To untelescoped
-man it certainly seemed demoniacal, this punctual recurrent wink.
-Spectroscoped man has learnt its cause.</p>
-
-<p>Goodricke in 1795 divined it, and research since has confirmed his keen
-intuition. Its loss of light is occasioned by the passing in front of
-it of a dark companion almost of its own size revolving about it in
-a close elliptic orbit. That this is the explanation of its strange
-behavior, the shift of its spectral lines makes certain, by showing
-that the bright star is receding from us at twenty-seven miles a second
-seventeen hours before the eclipse and coming towards us at about the
-same rate seventeen hours after it; its dark companion, therefore,
-doing the reverse.
-<span class="pagenum"><a name="Page_4" id="Page_4">[Pg 4]</a></span></p>
-
-<p class="space-below2">Algol is no solitary specimen of a mind-seen
-invisible star. Many eclipsing binaries of the same class are now
-known; and considering that the phenomenon could not be disclosed
-unless the orbital plane of the pair traversed the observer’s eye, an
-unlikely chance in a fortuitous distribution, we perceive how many such
-in truth there must be which escape recognition for their tilt.</p>
-
-<div class="figcenter">
-   <a id="I_004" name="I_004"> </a>
-    <p class="center"><span class="smcap">Algol and its dark companion,</span></p>
-    <img src="images/i_004a.jpg" alt="" width="600" height="84" />
-    <p class="center space-below2"><span class="smcap">as seen from the Earth,</span></p>
-
-    <img src="images/i_004b.jpg" alt="" width="600" height="339" />
-    <p class="center"><span class="smcap">as seen from above orbit.</span></p>
-</div>
-<p><span class="pagenum"><a name="Page_5" id="Page_5">[Pg 5]</a></span>
-But if dark stars exist in connection with lucent ones, there must be
-many more that travel alone. Our own Sun is an instance in embryo. If
-he live long enough, he will become such a solitary shrouded tramp in
-his old age. For he has no companion to betray him. The only way in
-which we could become cognizant of these wanderers would be by their
-chance collision with some other star, dark or lucent as the case might
-be. The impact of the catastrophe would generate so much light and heat
-that the previously dark body would be converted into a blazing sun and
-a new star make its advent in the sky.</p>
-
-<p>Star births of the sort have actually been noted. Every now and then a
-new star suddenly appears in the firmament—a nova as it is technically
-called. These apparitions date from the dawn of astronomic history. The
-earliest chronicled is found in the Chinese Annals of 134 <span class="smcap">b.c.</span>
-It shone out in Scorpio and was probably the new star which Pliny
-tells us incited Hipparchus, “The Father of Astronomy,” to make his
-celebrated catalogue of stars. From this time down we have recorded
-instances of like character.</p>
-
-<p>One of the most famous was the “Pilgrim Star” of Tycho Brahe. That
-astronomer has left us a full account of it. “While I was living,” he
-tells us, “with my uncle in the monastery of Hearitzwadt, on quitting
-my chemical laboratory one evening, I raised my eyes to the well-known
-<span class="pagenum"><a name="Page_6" id="Page_6">[Pg 6]</a></span>
-vault of heaven and observed, with indescribable astonishment, near the
-zenith, in Cassiopeia, a radiant fixed star of a magnitude never before
-seen. In my amazement I doubted the evidence of my senses. However,
-to convince myself that it was no illusion, and to have the testimony
-of others, I summoned my assistants from the laboratory and inquired
-of them, and of all the country people that passed by, if they also
-observed the star that had thus suddenly burst forth. I subsequently
-heard that in Germany wagoners and other common people first called the
-attention of astronomers to this great phenomenon in the heavens,—a
-circumstance which, as in the case of non-predicted comets, furnished
-fresh occasion for the usual raillery at the expense of the learned.”</p>
-
-<p>The new star, he informs us, was just like all other fixed stars, but
-as bright as Venus at her brightest. Those gifted with keen sight could
-discern it in the daytime and even at noon. It soon began to wane.
-In December, 1572, it resembled Jupiter, and a year and three months
-later had sunk beyond recognition to the naked eye. It changed color
-as it did so, passing from white through yellow to red. In May, 1573,
-it returned to yellow (“the hue of Saturn,” he expressly states), and
-so remained till it disappeared from sight, scintillating strongly in
-proportion to its faintness.</p>
-
-<p>Thirty-two years later another stranger appeared and was seen by
-<span class="pagenum"><a name="Page_7" id="Page_7">[Pg 7]</a></span>
-Kepler, who wrote a paper about it entitled “The New Star in the Foot
-of the Serpent.” It shone out in the same sudden manner and faded in
-the same leisurely way.</p>
-
-<p>Since 1860 there have been several such apparitions, and since 1876
-it has been possible to study them with the spectroscope, which has
-immensely increased our knowledge of their constitution. Indeed, this
-instrument of research has really opened our eyes to what they are.
-Nova Cygni, in 1876, Nova Aurigæ, in 1892, and Nova Persei, in 1901,
-besides several others found by Mrs. Fleming on the Arequipa plates,
-were excellent examples, and all agreed in their main features,
-showing that novæ constitute a type of stars by themselves, whose
-appearing in the first place and whose behavior afterwards prove them
-to have started from like cause and to have pursued parallel lines of
-development.</p>
-
-<p>As a typical case we may review the history of Nova Aurigæ. On February
-1, 1892, an anonymous post-card was received by Dr. Copeland of the
-Royal Observatory, Edinburgh, that read as follows: “Nova in Aurigæ.
-In Milky Way, about 2° south of χ Aurigæ, preceding 26 Aurigæ. Fifth
-magnitude slightly brighter than χ.” The observatory staff at once
-looked for the nova and easily found it with an opera glass. They then
-examined it through a prism placed before their 24-inch reflector and
-found its spectrum. It proved to be that of a “blaze star.”
-<span class="pagenum"><a name="Page_8" id="Page_8">[Pg 8]</a></span></p>
-
-<p>Dr. Thomas D. Anderson turned out to be the writer of the anonymous
-post-card—his name modestly self-obliterated by the nova’s light.
-He had detected the star on January 24, but had only verified it as
-a new one on the 31st. Harvard College Observatory then looked up
-its archived plates. The plates showed that it had appeared sometime
-between December 1 and 10. Its maximum had been attained on December
-20, after which it declined, to record apparently another maximum on
-February 3 of the 3.5 magnitude. From this time its light steadily
-waned till on April 1 it was only of the 16th magnitude or ¹/₁₀₀₀₀₀ of
-what it had been. In August it brightened again and then waned once more.</p>
-
-<p>Meanwhile its spectrum underwent equally strange fluctuations. At
-first it exhibited the bright lines characteristic of the flaming red
-solar prominences, the calcium, hydrogen, and helium lines flanked by
-their dark correlatives upon a continuous background, showing that
-both glowing and cooler gases were here concerned. The sodium lines,
-too, appeared, like those that come out in comets as they approach the
-furnace of the Sun. An outburst such as occurs in miniature in the
-solar chromosphere or outermost gaseous layer of the Sun was here going
-on upon a gigantic scale. A veritable spectral chaos next supervened,
-<span class="pagenum"><a name="Page_9" id="Page_9">[Pg 9]</a></span>
-staying until the star had practically faded away. Then, on its
-reappearance, in August, Holden, Schaeberle, and Campbell discovered
-to their surprise not what had been at all, but something utterly new:
-the soberly bright lines only of a nebula. Finally, ten years later,
-January, 1902, Campbell found its spectrum had become continuous, the
-body having reverted to the condition of a star.</p>
-
-<p>Now how are we to interpret these grandiose vicissitudes, visually and
-spectrally revealed? That we witnessed some great catastrophe is clear.
-The sudden increase of light of many thousand fold from invisibility
-to prominence shows that a tremendous cataclysm occurred. The bright
-lines in the spectrum confirm it and imply that vast upheavals like
-those that shake the Sun were there in progress, but on so stupendous
-a scale that, if for no other reason, we must dismiss the idea that
-explosions alone can possibly be concerned. The dark correlatives of
-the bright lines have been interpreted as indicating that two bodies
-were concerned, each travelling at velocities of hundreds of miles a
-second. But in Nova Aurigæ shiftings of the spectral lines implying
-six bodies at least were recorded, if such be attributed to motion in
-the line of sight, and Vogel was minded to throw in a few planets as
-well—as Miss Clerke pithily puts it. There is not room for so many on
-the stage of the cosmic drama. Other causes, as we now know, may also
-<span class="pagenum"><a name="Page_10" id="Page_10">[Pg 10]</a></span>
-displace the spectral lines. Great pressure has been shown to do it,
-thanks to the labors of Humphreys and Mohler at Baltimore. “Anomalous
-refraction” may do it, as Professor Julius of Utrecht has found
-out. Finally, changes of density may produce it, as Michelson has
-discovered. To these causes we may confidently ascribe most of the
-shiftings in the stellar spectrum, for just such forces must be there
-at work.</p>
-
-<p>Mr. Monck suggested the idea that new stars are the result of old dark
-stars rushing through gaseous fields in space and rendered luminous
-by the encounter. Seeliger revived and developed this idea, which in
-certain cases is undoubtedly the truth. Probably this occurred to the
-new star of 1885 which suddenly blazed out almost in the centre of the
-great nebula in Andromeda. It behaved like a typical nova and in due
-course faded to indistinguishability. Something like it happened, too,
-in the nova of 1860, which suddenly flared up in the star cluster 80
-Messier, outdoing in lustre the cluster itself, and then, too, faded away.</p>
-
-<p>But just as psychology teaches us that not only do we cry because we
-are sorrowful, but that we are sorrowful because we cry, so while a
-nova may be made by a nebula, no less may a nebula be made by a star.</p>
-
-<p>Let us see how this might be brought about and what sign manuals it
-would present. Suppose that the two bodies actually grazed. Then the
-<span class="pagenum"><a name="Page_11" id="Page_11">[Pg 11]</a></span>
-disruption would affect the star’s cuticle, first raising the outer
-parts, consisting rather of carbon than of the metals, since that
-substance is the lighter, to intense heat and the gases about it at
-the same time. The glowing carbon would be intensely bright, and at
-first its light would overpower that from the gases, and not till its
-great glow had partially subsided would theirs be seen. Then the gases,
-hydrogen, helium, and so forth, would make themselves evident. Finally
-only the most tenuous ones, those peculiar to a nebula, would remain
-visible. After which the more solid particles due to the disruption
-would fall together and light up again by their individual collisions.
-Much the same would result if without striking the stars passed close.</p>
-
-<div class="figcenter">
-   <a id="I_011" name="I_011"> </a>
-    <img src="images/i_011.jpg" alt="" width="600" height="343" />
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">1901 February 20th</td>
-      <td class="tdc">1901 February 28th</td>
-   </tr><tr>
-      <td class="tdc">Before appearance of Nova</td>
-      <td class="tdc">The Nova</td>
-   </tr><tr>
-      <td class="tdc" colspan="2"><br />NOVA PERSEI.
-               Photographs by A. STANLEY WILLIAMS, Hove, Sussex.</td>
-   </tr>
- </tbody>
-</table>
-</div>
-<p class="space-above2"><span class="pagenum"><a name="Page_12" id="Page_12">[Pg 12]</a></span></p>
-<div class="figcenter">
-   <a id="I_012" name="I_012"> </a>
-    <img src="images/i_012.jpg" alt="" width="500" height="492" />
-    <p class="center space-below2"><span class="smcap">Spectrum of Nova Persei.</span>
-          (F. Ellerman, 40 in. Yerkes.)</p>
-</div>
-
-<p>Now to put this theory to the proof. In the early morning of the 22d of
-February, 1901, Dr. Anderson, the discoverer of Nova Aurigæ, perceived
-that Algol had a neighbor, a star as bright as itself, which had never
-been there before. Within twenty-four hours of its detection the
-newcomer rivalled Capella, and shortly after took rank as the premier
-star of the northern hemisphere. Its spectrum on the 22d was found at
-Harvard College Observatory to be like that of Rigel, a continuous one
-<span class="pagenum"><a name="Page_13" id="Page_13">[Pg 13]</a></span>
-crossed by some thirty faint dark lines. On the 24th, however, <i>so soon
-as it began to wane</i>, the bright lines of hydrogen were conspicuous
-with their dark correlatives, just as they had been with Nova Aurigæ
-and other novæ. At the same time each particular spectral line proved a
-law unto itself, some shifted more than others, thus negativing motion
-as their only cause and indicating change of pressure or density as
-concerned concomitants of the affair. Blue emissions like those of
-Wolf-Rayet stars next made their appearance; then a band, found by
-Wright at the Lick to characterize nebulæ, shone out, and finally in
-July the change to a nebular spectrum stood complete.</p>
-
-<div class="figcenter">
-   <a id="I_013" name="I_013"> </a>
-    <img src="images/i_013.jpg" alt="" width="600" height="355" />
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc" colspan="2">THE MOVING NEBULA SURROUNDING NOVA PERSEI.</td>
-
-   </tr><tr>
-      <td class="tdc"><br />1901, September 20th.</td>
-      <td class="tdc"><br />1901, November 13th.</td>
-   </tr><tr>
-      <td class="tdc" colspan="2"><br />Drawn by G. W. RITCHEY, from Photographs taken with the 24-in.
-                 Reflector,<br />YERKES OBSERVATORY.</td>
-   </tr>
- </tbody>
-</table>
-</div>
-
-<p>Then came what is the most suggestive feature in the whole event. On
-<span class="pagenum"><a name="Page_14" id="Page_14">[Pg 14]</a></span>
-August 22 and 23 Dr. Wolf at Königstahl took with his then new Bruce
-objective some long exposure plates of the nova, and on them found, to
-his surprise, wisps of nebulous matter to the southeast of the star. On
-September 20 Ritchey, with a two-foot mirror of his own constructing
-exposed for four hours, brought the whole formation to light. It turned
-out to be a spiral nebula encircling and apparently emanating from
-the star. Its connection with the nova was patent. But there was more
-to come. Later plates taken at the Lick on November 7 disclosed the
-startling fact that the nebula was visibly expanding, uncoiling outward
-from the star. A plate by Ritchey on November 13 confirmed this, and
-still later plates by him in December, January, and February showed the
-motion to be progressive. At the same time the star showed no parallax,
-and the speed of the motion seemed thus to be indicated as enormous.
-Kapteyn suggested to account for it that appearance, not reality, was
-here concerned; that the nebula had always existed, and was only shown
-up by the light from the conflagration travelling outward from the nova
-at the rate of one hundred and eighty-six thousand miles a second. This
-would make the catastrophe to have occurred as far back as the time of
-James I, of which the news more truthful but less timely than that of
-the morning papers had only just reached us.</p>
-
-<div class="figcontainer">
-  <div class="figsub">
-   <a id="I_014" name="I_014"> </a>
-   <img src="images/i_014a.jpg" alt="" width="250" height="260" />
-   <p class="center">December 14, 1901.</p>
-  </div>
-  <div class="figsub">
-   <img  src="images/i_014b.jpg" alt="" width="250" height="261" />
-   <p class="center">January 7 and 9, 1902.</p>
-   </div>
-</div>
-
-<div class="figcenter">
-   <img src="images/i_014c.jpg" alt="" width="250" height="270" />
-   <p class="center">1902. February 8, 1902.</p>
-   <p class="center space-below2"><span class="smcap">The Moving Nebula
-                    surrounding Nova Persei—after Ritchey.</span></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_15" id="Page_15">[Pg 15]</a></span>
-But a little of that simple reasoning by which Zadig recovered the
-lost horses of the Sultan, and which from its unaccustomedness in the
-affairs of men got him suspected of having stolen them and very nearly
-caused his death, will show the untenableness of this idea and help us
-to a solution. In the first place we note that the star holds the very
-centre of the nebular stage, a remarkable prominence if the star has
-no creative right to the position. Then the same knots and patches of
-the nebulous configuration are visible in all the photographs, in the
-same relative positions, turned through corresponding angles as one
-will see for himself, all having moved symmetrically from one date to
-another. At the truly marvellous mimicry implied if different objects
-were concerned common sense instinctively shies, and very properly,
-as the chances against it are millions to one. Clearly it was not a
-mere matter of ethereal motion, but a very material motion of matter,
-which was here concerned. Something corpuscular emanating from the nova
-spread outward into space.</p>
-
-<p>Clinching this conclusion is the result of a search by Perrine for
-traces of the nebula on earlier plates. For on one taken by him on
-March 29 (1901) he found the process already started in two close
-coils, its conception thus clearly dating from the time of the star’s
-outburst. In Nova Persei, then, we actually witnessed a spiral nebula
-evolved from a disrupted star.
-<span class="pagenum"><a name="Page_16" id="Page_16">[Pg 16]</a></span></p>
-
-<p>What was this ejectum and what drove it forth? Professor Very regarded
-it as composed of corpuscles such as give rise to cathode rays
-discharged from the star under the stress of light pressure or electric
-repulsion. But I think we may see in it something simpler still; to
-wit, gaseous molecules driven off by light pressure alone—the smoke,
-as one may say, of the catastrophe—akin exactly to the constituents of
-comet’s tails. The mere light of the conflagration pushed the hydrogen
-molecules away. This would explain their presence and their exceeding
-hurry at the same time. They were started on their travels by domestic
-jars and kept going by the vivid after-effects of that infelicity.</p>
-
-<p>The fairly steady rate of regression from the nova observed may be
-explained by the observed decrease in the light of the repellent
-source. Such combined with the retarding effect of gravity might make
-the regression equable. This is the more explanatory as the speed was
-certainly much less than that of light, though greatly exceeding any
-possible from the direct disruption. At the same time both the bright
-and the dark lines of hydrogen seen in the spectrum stand accounted
-for; the colliding molecules, at their starting on their travels
-from the star, shining through their sparser fellows farther out. An
-interesting biograph of the levity of light!
-<span class="pagenum"><a name="Page_17" id="Page_17">[Pg 17]</a></span></p>
-
-<p>Nova Persei thus introduces us at its birth to one of a class of most
-interesting objects comparatively recently discovered and of most
-pregnant import,—the spiral nebulæ.</p>
-
-<div class="figcenter">
-   <a id="I_017" name="I_017"> </a>
-   <img src="images/i_017.jpg" alt="" width="400" height="474" />
-   <p class="center space-below2"><span class="smcap">Great Nebula in
-                         Orion—after Ritchey.</span></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_18" id="Page_18">[Pg 18]</a></span></p>
-
-<div class="figcenter">
-   <a id="I_018" name="I_018"> </a>
-   <img src="images/i_018.jpg" alt="" width="400" height="485" />
-   <p class="center space-below2"><span class="smcap">Great Nebula in
-                         Andromeda—after Ritchey.</span></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_19" id="Page_19">[Pg 19]</a></span></p>
-
-<div class="figcenter">
-   <a id="I_019" name="I_019"> </a>
-   <img src="images/i_019.jpg" alt="" width="600" height="497" />
-   <p class="center space-below2"><span class="smcap">Nebula M. 100 Comæ—after Roberts.</span></p>
-</div>
-
-<p>In 1843 when Lord Rosse’s giant speculum, six feet across, was turned
-upon the sky, a nebula was brought to light which was unlike any ever
-before seen. It was neither irregular like the great nebula in Orion
-nor round like the so called planetary nebulæ,—the two great classes
-at that time known,—but exhibited a striking spiral structure. It
-proved the forerunner of a remarkable revelation. For the specimen
-thus disclosed has turned out to typify not only the most interesting
-form of those heavenly wreaths of light, but by far the commonest as
-well. As telescopic and especially photographic means improved, the
-number of such objects detected steadily increased until about thirteen
-years ago Keeler by his systematic discoveries of them came to the
-conclusion that a spiral structure pervaded the great majority of all
-the nebulæ visible. Their relative universality was outdone only by the
-invariability of their form. For they all represent spirals of one type:
-<span class="pagenum"><a name="Page_20" id="Page_20">[Pg 20]</a></span>
-two coiled arms radiating diametrically from a central nucleus and
-dilating outward. Even nebulæ not originally supposed spiral have
-disclosed on better revelation the dominant form. Thus the great nebula
-in Andromeda formerly thought lens-shaped proves to be a huge spiral
-coiled in a plane not many degrees inclined to the plane of sight.</p>
-
-<div class="figcenter">
-   <a id="I_020" name="I_020"> </a>
-   <img src="images/i_020.jpg" alt="" width="600" height="446" />
-   <p class="center space-below2"><span class="smcap">Nebula ♅ I. 226 Ursæ Majoris—after Roberts.</span></p>
-</div>
-
-<p>As should happen if the spirals are unrelated, left-handed and
-right-handed ones are about equally common. In Dr. Roberts’ great
-collection of those in which the structure is distinctly discernible,
-nine are right-handed, ten left-handed, showing that they partake of
-the ambidextrous impartiality of space.
-<span class="pagenum"><a name="Page_21" id="Page_21">[Pg 21]</a></span></p>
-
-<div class="figcenter">
-   <a id="I_021" name="I_021"> </a>
-   <img src="images/i_021.jpg" alt="" width="600" height="483" />
-   <p class="center"><span class="smcap">Nebula ♅ V. 24 Comæ—after Roberts.</span></p>
-   <p class="center space-below2">Showing globular structure.</p>
-</div>
-
-<p>Lastly the spirals are evidently thicker near the centre, thinning out
-at the edge, and when the central nucleus is pronounced, it seems to
-have a certain globularity not shared by the arms, and more or less
-detached from them. This appears in those cases where they are shown us
-edgewise, and it has been thought perceptible in the great nebula of
-Andromeda. The difficulty in establishing the phenomenon comes from the
-impossibility of both features showing at their best together. For the
-globularity to come out well, the spiral must be presented to us nearly
-in the plane of sight; for the spirality, in a plane at right angles to it.
-<span class="pagenum"><a name="Page_22" id="Page_22">[Pg 22]</a></span></p>
-
-<p>Much may be learnt by pondering on these peculiarities. The widespread
-character of the phenomenon points to some universal law. We are here
-clearly confronted by the embodiment of a great cosmic principle,
-causing the helices it is for us to uncoil. It is a problem in mechanics.</p>
-
-<p>In the first place, a spiral structure denotes action on the face
-of it. It implies a rotation combined with motion out or in. We are
-familiar with the fact in the sparks of pin-wheel pyrotechnics. Any
-rotating fluid urged by an outward or an inward impulse must take the
-spiral form. A common example occurs in the water let out of a basin
-through a hole in the centre when we draw out the plug. Here the
-force is inward, and because the bowl and orifice are not perfectly
-symmetric, a rotation is set up in the water trying to escape, and the
-two combine to give us a beautiful conchoidal swirl. In this case the
-particles seek the centre, but the same general shape is assumed when
-they seek to leave it.</p>
-
-<p>Another point to be noticed is that a spiral nebula could not develop
-of itself and subsist. To continue it must have outside help. For if
-it were due to internal explosive action in the pristine body, each
-ejectum must return to the point it started from, or else depart
-forever into space, for the orbit it would describe must either be
-closed or unclosed. If the former, it would revisit its starting-point;
-if the latter, it would never return. Explosion, therefore, of itself
-could not have produced the forms we see, unless they be ephemeral
-apparitions, a supposition their presence throughout the heavens seems
-effectually to exclude.
-<span class="pagenum"><a name="Page_23" id="Page_23">[Pg 23]</a></span></p>
-
-<div class="figcenter">
-   <a id="I_023" name="I_023"> </a>
-   <img src="images/i_023.jpg" alt="" width="400" height="492" />
-   <p class="center space-below2"><span class="smcap">Nebula M. 101 Ursæ Majoris—after Ritchey.</span></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_24" id="Page_24">[Pg 24]</a></span>
-The form of the spiral nebulæ proclaims their motion, but one of its
-particular features discloses more. For it implies the past cause which
-set this motion going. A distinctive detail of these spirals, which so
-far as we know is shared by all of them, are the two arms which leave
-the centre from diametrically opposite sides. This indicates that the
-outward driving force acted only in two places, the one the antipodes
-of the other. Now what kind of force is capable of this peculiar
-effect? If we think of the matter, we shall realize that tidal action
-would produce just this result. We see it daily in the case of the
-Moon; when it is high tide in the open ocean hereabouts, it is high
-tide also at the opposite end of the Earth. The reason is that the
-tideraising body pulls the fluid nearest it more strongly than it pulls
-the Earth as a whole, and pulls the Earth as a whole more than it pulls
-the fluid at the opposite extremity.</p>
-
-<p class="space-below2">Suppose, now, a stranger to approach a body
-in space near enough; it will inevitably raise tides in the other’s
-mass, and if the approach be very close, the tides will be so great
-as to tear the body in pieces along the line due to their action;
-that is, parts of the body will be separated from the main mass in
-two antipodal directions. This is precisely what we see in the spiral
-nebula. Nor is there any other action that we know of which would thus
-handle the body. If it were to disintegrate under increased speed
-of rotation due to contraction upon itself, parts of its periphery
-should be shed continually and a pin-wheel of matter, not a two-armed
-spiral, be thrown off. If explosion were the disintegrating cause,
-disruption would occur unsymmetrically in one or more directions, not
-symmetrically as here.</p>
-
-<div class="figcenter">
-   <a id="I_024" name="I_024"> </a>
-   <p class="f120"><b>REPRESENTATIVE STELLAR SPECTRA</b></p>
-   <p class="center"><i>Photographed, in 1907 and 1908, by</i> <span class="smcap">V. M. SLIPHER</span>,
-          <i>at</i> <span class="smcap">LOWELL OBSERVATORY</span><br /> <i>Flagstaff, Arizona,
-          with prism spectrograph.</i></p>
-   <img src="images/i_024.jpg" alt="" width="600" height="343" />
-</div>
-
-<p class="space-above2"><span class="pagenum"><a name="Page_25" id="Page_25">[Pg 25]</a></span>
-As the stranger passed on, his effect would diminish until his
-attraction no longer overbalanced that of the body for its disrupted
-portions. These might then be controlled and forced to move in elliptic
-orbits about the mass of which they had originally made part. Thence
-would come into being a solar system, the knots in the nebula going to
-form the planets that were to be.</p>
-
-<p>Before proceeding to what proof we have that it actually did occur in
-this way we may pause to consider some consequences of what we have
-already learned. Thus what brought about the beginning of the system
-may also compass its end. If one random encounter took place in the
-past, a second is as likely to occur in the future. Another celestial
-body may any day run into the Sun, and it is to a dark body that we
-must look for such destruction, because they are so much more numerous
-in space.</p>
-
-<p>That any of the lucent stars, the stars commonly so called, could
-collide with the Sun, or come near enough to amount to the same thing,
-is demonstrably impossible for æons of years. But this is far from the
-<span class="pagenum"><a name="Page_26" id="Page_26">[Pg 26]</a></span>
-case for a dark star. Such a body might well be within a hundredth of
-the distance of the nearest of our known neighbors, Alpha Centauri, at
-the present moment without our being aware of it at all. Our senses
-could only be cognizant of its proximity by the borrowed light it
-reflected from our own Sun. Dark in itself, our own head-lights alone
-would show it up when close upon us. It would loom out of the void thus
-suddenly before the crash.</p>
-
-<p>We can calculate how much warning we should have of the coming
-catastrophe. The Sun with its retinue is speeding through space at
-the rate of eleven miles a second toward a point near the bright star
-Vega. Since the tramp would probably also be in motion with a speed
-comparable with our own, it might hit us coming from any point in
-space, the likelihood depending upon the direction and amount of its
-own speed. So that at the present moment such a body may be in any
-part of the sky. But the chances are greatest if it be coming from the
-direction toward which the sun is travelling, since it would then be
-approaching us head on. If it were travelling itself as fast as the
-Sun, its relative speed of approach would be twenty-two miles a second.</p>
-
-<p>The previousness of the warning would depend upon the stranger’s size.
-The warning would be long according as the stranger was large. Let us
-<span class="pagenum"><a name="Page_27" id="Page_27">[Pg 27]</a></span>
-assume it the mass of the Sun, a most probable supposition. Being dark,
-it must have cooled to a solid, and its density therefore be much
-greater than the Sun’s, probably something like eight times as great,
-giving it a diameter about half his or four hundred and thirty thousand
-miles. Its apparent brightness would depend both upon its distance and
-upon its intrinsic brightness or albedo, and this last would itself
-vary according to its distance from the Sun. While it was still in the
-depths of space and its atmosphere lay inert, owing to the cold there,
-its intrinsic brightness might be that of the Moon or Mercury. As its
-own rotation would greatly affect the speed with which its sunward side
-was warmed, we can form no exact idea of the law of its increase in
-light. That the augmentation would be great we see from the behavior of
-comets as they approach the great hearth of our solar system. But we
-are not called upon to evaluate the question to that nicety. We shall
-assume, therefore, that its brilliancy would be only that of the Moon,
-remembering that the last stages of its fateful journey would be much
-more resplendently set off.</p>
-
-<p>With these data we can find how long it would be visible before
-the collision occurred. As a very small telescopic star it would
-undoubtedly escape detection. It is not likely that the stranger
-would be noticed simply from its appearance until it had attained
-the eleventh magnitude. It would then be one hundred and forty-nine
-<span class="pagenum"><a name="Page_28" id="Page_28">[Pg 28]</a></span>
-astronomical units from the Sun or at five times the distance of
-Neptune. But its detection would come about not through the eye of
-the body, but through the eye of the mind. Long before it could have
-attracted man’s attention to itself directly its effects would have
-betrayed it. Previous, indeed, to its possible showing in any telescope
-the behavior of the outer planets of the system would have revealed
-its presence. The far plummet of man’s analysis would have sounded
-the cause of their disturbance and pointed out the point from which
-that disturbance came. Celestial mechanics would have foretold, as
-once the discovery of another planet, so now the end of the world.
-Unexplained perturbations in the motions of the planets, the far
-tremors of its coming, would have spoken to astronomers as the first
-heralding of the stranger and of the destruction it was about to bring.
-Neptune and Uranus would begin to deviate from their prescribed paths
-in a manner not to be accounted for except by the action of some new
-force. Their perturbations would resemble those caused by an unknown
-exterior planet, but with this difference that the period of the
-disturbance would be exactly that of the disturbed planet’s own period
-of revolution round the Sun.</p>
-
-<p>Our exterior sentinels might fail thus to give us warning of the
-foreign body because of being at the time in the opposite parts of
-their orbits. We should then be first apprised of its coming by Saturn,
-which would give us less prefatory notice.
-<span class="pagenum"><a name="Page_29" id="Page_29">[Pg 29]</a></span></p>
-
-<p>It would be some twenty-seven years from the time it entered the range
-of vision of our present telescopes before it rose to that of the
-unarmed eye. It would then have reached forty-nine astronomical units’
-distance, or two-thirds as far again as Neptune. From here, however,
-its approach would be more rapid. Humanity by this time would have been
-made acquainted with its sinister intent from astronomic calculation,
-and would watch its slow gaining in conspicuousness with ever growing
-alarm. During the next three years it would have ominously increased
-to a first magnitude star, and two years and three months more have
-reached the distance of Jupiter and surpassed by far in lustre Venus at
-her brightest.</p>
-
-<p>Meanwhile the disturbance occasioned not simply in the outer planets
-but in our own Earth would have become very alarming indeed. The
-seasons would have been already greatly changed, and the year itself
-lengthened, and all these changes fraught with danger to everything
-upon the Earth’s face would momentarily grow worse. In one hundred and
-forty-five days from the time it passed the distance of Jupiter it
-would reach the distance of the Earth. Coming from Vega, it would not
-hit the Earth or any of the outer planets, as the Sun’s way is inclined
-to the planetary planes by some sixty degrees, but the effects would be
-none the less marked for that. Day and night alone of our astronomic
-<span class="pagenum"><a name="Page_30" id="Page_30">[Pg 30]</a></span>
-relations would remain. It would be like going mad and yet remaining
-conscious of the fact. Instead of following the Sun we should now in
-whole or part, according to the direction of its approach, obey the
-stranger. For nineteen more days this frightful chaos would continue;
-as like some comet glorified a thousand fold the tramp dropped silently
-upon the Sun. Toward the close of the nineteenth day the catastrophe
-would occur, and almost in merciful deliverance from the already
-chaotic cataclysm and the yet greater horror of its contemplation, we
-should know no more.</p>
-
-<p>Unless the universe is otherwise articulated than we have reason to
-suppose, such a catastrophe sometime seems certain. But we may bear
-ourselves with equanimity in its prospect for two mitigating details.
-One is that there is no sign whatever at the moment that any such
-stranger is near. The unaccounted-for errors in the planetary theories
-are not such as point to the advent of any tramp. Another is, that
-judged by any scale of time we know, the chance of such occurrence
-is immeasurably remote. Not only may each of us rest content in the
-thought that he will die from causes of his own choosing or neglect,
-but the Earth herself will cease to be a possible abode of life, and
-even the Sun will have become cold and dark and dead so long before
-that day arrives that when the final shock shall come, it will be quite
-ready for another resurrection.</p>
-
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_31" id="Page_31">[Pg 31]</a></span></p>
-<div class="chapter">
-<h2 class="nobreak">CHAPTER II<br /><span class="h_subtitle">EVIDENCE OF THE INITIAL CATASTROPHE<br />
-            IN OUR OWN CASE</span></h2>
-</div>
-
-<p class="drop-cap">BY quite another class of dark bodies than those we contemplated
-in the last chapter is the immediate space about us tenanted. For that,
-too, is anything but the void our senses give us to understand.
-Could we rise a hundred miles above the Earth’s surface we should be
-highly sorry we came, for we should incontinently be killed by flying
-brickbats. Instead of masses of a sunlike size we should have to do
-with bits of matter on the average smaller than ourselves but hardly on
-that account innocuous, as they would strike us with fifteen hundred
-times the speed of an express train. Only in one respect are the two
-classes of erratics alike, both remain invisible till they are upon
-us. Even so, the cause of their visibility is different. The one is
-announced by the light it reflects, the other by the glow it gives out
-on its destruction. These last are the meteorites or shooting-stars.
-They are as well known to every one for their commonness as,
-fortunately, the first are rare. On any starlight night one need not
-<span class="pagenum"><a name="Page_32" id="Page_32">[Pg 32]</a></span>
-tarry long before one of these visitants darts across the sky, a
-brilliant thread of fire gone almost ere it be descried.</p>
-
-<p>Usually this is all of which one is made aware. Silent, ghostlike, the
-apparition comes and goes, and nothing more of it is either seen or
-heard. But sometimes there is a good deal more. Occasionally a large
-ball of flame shoots through the air, a detonation like distant thunder
-startles the ear, and a luminous train, persisting for several seconds,
-floats slowly away. Finally if one be fortunate to be near,—but not
-too near,—one or more masses of stone are seen to fall swiftly and
-bury themselves in the ground. These are meteorites: far wanderers come
-at last to rest in graves they have dug themselves.</p>
-
-<p>A great revolution has taken place lately in our ideas concerning
-meteorites. Indeed, it was not so very long ago, since modern man
-admitted their astronomic character at all. He looked as askance at
-them as he did at fossils. It was the fall at Aigle, in Switzerland,
-April 26, 1803, that first opened men’s eyes to the fact that such
-falls actually occurred. It is more than a nine days’ wonder at times
-how long men, as well as puppies, can remain blind. To admit that
-stones fell from heaven, however, was not to see whence they came.
-Their paternity was imputed to nearly every body in the sky. They were
-at first supposed to have been ejected from earthly volcanic vents,
-<span class="pagenum"><a name="Page_33" id="Page_33">[Pg 33]</a></span>
-then from volcanoes in the Moon. That they are of domestic manufacture
-is, however, negatived by the paths they severally pursue. Nor can they
-for like reason have been ejected from the Sun.</p>
-
-<p>The Earth was not their birthplace. It is alien ground in which they
-lie at last and from which we transfer them to glass cases in our
-museums. This fact about their parentage they tell by the speed with
-which they enter our air. They become visible 100 miles up and explode
-at from 20 to 10, and their speed has been found to be from 10 to 40
-miles a second, which is that of cosmic bodies moving in large elliptic
-orbits about the Sun,—a speed greater than the Earth could ever have
-imparted.</p>
-
-<p>Four classes of such small celestial bodies tenant space where the
-planets move: sporadic shooting-stars, meteorites, meteor-streams, and
-comets. The discovery of the relation of each of these to the solar
-system and then to each other forms one of the latest chapters of
-astronomic history. For they turn out to be generically one.</p>
-
-<p>It was long, however, before this was perceived. The first step was
-taken simultaneously by Professor Olmstead of Yale and Twining in 1833
-from reasoning on the superb November meteor-shower of that year. All
-the shooting-stars, “thick as snowflakes in a storm,” had a common
-<span class="pagenum"><a name="Page_34" id="Page_34">[Pg 34]</a></span>
-radiant from which they seemed to come. Thus they argued that the
-meteors must all be travelling in parallel lines along an orbit which
-the previous shower, of 1799, showed to be periodic. This was the first
-recognition of a meteor-swarm.</p>
-
-<p>The next advance was when Schiaparelli, in 1862, pointed out the
-remarkable connection between meteor-swarms and comets. On calculation
-the August meteor-stream and the comet of 1862 proved to be pursuing
-exactly the same path. Soon other instances of like association were
-discovered, and we now know mathematically that meteor-streams can
-be, deductively that they must be, and observationally that they are,
-disintegrated comets. More than one comet has even been seen to split.</p>
-
-<p>Then came the recognition that comets are not visitors from space, as
-Sir Isaac Newton and Laplace supposed, but part and parcel of our own
-solar system. Without going into the history of the subject, which
-includes Gauss, Schiaparelli, and finally Fabry’s great Memoir, much
-too little known, the proof can, I think, be made comprehensible
-without too much technique, thanks to the fact that the Sun is speeding
-through space at the rate of eleven miles a second.</p>
-
-<p>Orbits described by bodies under the action of a central force are
-always conic sections, as Sir Isaac Newton proved. There are two
-classes of such curves: those which return into themselves, such as the
-<span class="pagenum"><a name="Page_35" id="Page_35">[Pg 35]</a></span>
-circle and ellipse, and those which do not, the hyperbolæ. If a body
-travel in the first or closed class about the Sun, it is clearly a
-member of his family; if in the second, it is a visitor who bows to
-him only in passing and never returns. Which orbit it shall pursue
-depends at a given distance solely upon the speed of the body; if that
-speed be one the Sun can control, the body will move in an ellipse;
-if greater, in an hyperbola. Obviously the Sun can control just the
-speed he can impart. Now a comet entering the system from without
-would already possess a motion of its own which, when compounded with
-the solar-acquired speed, would make one greater than the Sun could
-master. Comets, therefore, if visitors from space, should all move
-in hyperbolæ. None for certain do; and only six out of four hundred
-even hint at it. Comets, then, are all members of the solar family,
-excentric ones, but not to be denied recognition of kinship for such
-behavior.</p>
-
-<p>Still, admittance to the solar family circle was denied to meteorites
-and shooting-stars. Thus Professor Kirkwood, in 1861, had considered
-“that the motions of some luminous meteors (or cometoids, as perhaps
-they might be called) have been decidedly indicative of an origin
-beyond the limits of the solar system.” Here cometoid was an apt
-coinage, but when comets were later shown not to be of extra-solar
-<span class="pagenum"><a name="Page_36" id="Page_36">[Pg 36]</a></span>
-origin, the reasoning carried luminous meteors in its train.<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a>
-Finally Schiaparelli, in 1871, concluded an able Memoir on the subject with
-the decision that “a stellar origin for meteorites was the most likely and
-that meteorites were identifiable with shooting-stars.”<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a>
-A pregnant remark this, though not exactly as the author thought, for
-instead of proving both interstellar, as he intended, both have proved
-to be solar bound.</p>
-
-<p>It was Professor Newton, in 1889, who first showed that meteorites
-were pursuing, as a rule, small elliptic orbits about the Sun, and
-that their motion was direct. He, too, was the first to surmise that
-meteorites are but bigger shooting-stars.</p>
-
-<p>Now, as to their connection. Of direct evidence we have little. A
-few meteors have been observed to come from the known radiants of
-shooting-stars. Two instances we have of the fall of meteorites during
-star showers. One in 1095, when the Saxon Chronicle tells us stars
-fell “so thickly that no man could count them, one of which struck the
-ground and when a bystander cast water upon it steam was raised with a
-great noise of boiling.” The second case was the fall of a siderite,
-eight pounds’ worth of nickel-iron, at Mazapil during the Andromede
-<span class="pagenum"><a name="Page_37" id="Page_37">[Pg 37]</a></span>
-shower of 1885, which was by many supposed to be a part of the lost
-Biela comet. It contained graphite enough to pencil its own history,
-but unfortunately could not write. The direction from which it came was
-not recorded, and so the connection between it and the comet not made out.</p>
-
-<div class="figcenter">
-   <a id="I_037" name="I_037"> </a>
-   <img src="images/i_037.jpg" alt="" width="500" height="434" />
-   <div class="blockquot2">
-   <p class="neg-indent space-below2"><span class="smcap">The Radiant of a Meteoric Shower, showing also the Paths
-            of Three Meteors which do not belong to this Shower—after Denning.</span></p>
-</div></div>
-
-<p>If our direct knowledge is thus scanty, reasoning affords surer ground
-for belief. For at this point there steps in a bit of news about the
-family relations of shooting-stars from a source hardly to have been
-anticipated. Indeed, it arose from the thought to examine a qualitative
-statement in Young’s “Astronomy” quantitatively. Mathematics is simply
-<span class="pagenum"><a name="Page_38" id="Page_38">[Pg 38]</a></span>
-precise reasoning, applied usually to the discovery that a pet theory
-will not work. But sometimes it presents one with an unexpected find.
-This is what it did here.</p>
-
-<p>It is an interesting fact of observation that more meteors are visible
-at six o’clock in the morning than at six o’clock at night in the
-proportion of 3 to 1. This seeming preference for early rising is due
-to no matutinality on the part of the meteors, but to the matin aspect
-then presented by the Earth combined with its orbital motion round the
-Sun. For at six in the morning the observer stands on the advancing
-side of the Earth, at the bow of the airship; at six at night he is at
-the stern. He, therefore, runs into the meteors at sunrise and slips
-away from them at sunset. He is pelted in the morning in consequence.
-Just as a pedestrian facing a storm gets wetter in front than behind.</p>
-
-<div class="figcenter">
-   <a id="I_038" name="I_038"> </a>
-    <img src="images/i_038.jpg" alt="" width="600" height="247" />
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc" colspan="3">METEORS</td>
-   </tr><tr>
-      <td class="tdc" colspan="3">Diagram explaining their proportionate visibility.</td>
-   </tr><tr>
-      <td class="tdc" colspan="3"> </td>
-   </tr><tr>
-      <td class="tdc">———————</td>
-      <td class="tdc"><i>denotes</i></td>
-      <td class="tdl"><i>true paths.</i></td>
-   </tr><tr>
-      <td class="tdc">——— - ———</td>
-      <td class="tdc">”</td>
-      <td class="tdl"><i>apparent paths.</i></td>
-   </tr><tr>
-      <td class="tdc">---------- - - - - - -</td>
-      <td class="tdc">”</td>
-      <td class="tdl"><i>Earth’s path.</i></td>
-   </tr>
- </tbody>
-</table>
-</div>
-
-<p class="space-above2">So far the books. Now let us examine this quantitatively according
-<span class="pagenum"><a name="Page_39" id="Page_39">[Pg 39]</a></span>
-to the direction in which the meteors themselves may be moving before
-the encounter. Suppose, in the first place, that they were travelling
-in every possible direction, with the average velocity of the most
-erratic members of the family, the great comets. On this supposition
-calculation shows that we ought to meet 5.8 times as many at six in the
-morning as at six at night. If their orbits were smaller than this,
-say, something like those of the asteroids, we should find 7.6 to 1 for
-the ratio.</p>
-
-<p>Suppose, however, that they were all travelling in the same sense as
-the Earth, direct as it is called in contradistinction to retrograde,
-and let us calculate what proportion in that case we should meet
-at the two hours respectively. It turns out to be 2.4 to 1 for the
-parabolic ones, 3.3 to 1 for the smaller orbited, or almost precisely
-what observation shows to be the case [<a href="#NOTE_1">see NOTE 1</a>]. Here,
-then, a bit of abstract reasoning has apprized us of a most interesting family
-fact; to wit, that the great majority of shooting-stars are travelling
-in the same orderly sense as ourselves. Furthermore, as some must be
-moving in smaller orbits than the mean, others must be journeying in
-greater; or, in other words, shooting-stars are scattered throughout
-the system. In short, these little bodies are tiny planets themselves,
-as truly planets as the asteroids,—asteroids of a general instead of a
-localized habit.
-<span class="pagenum"><a name="Page_40" id="Page_40">[Pg 40]</a></span></p>
-
-<p>Thus meteorites and shooting-stars are kin, and from the fact that
-they are pursuing orbits not very unlike our own we get our initial
-hint of a community of origin. Indeed, they are the little bricks out
-of which the whole structure of our solar system was built up. What
-we encounter to-day are the left-over fragments of what once was, the
-fraction that has not as yet been swept up by the larger bodies. And
-this is why these latter-day survivors move, as a rule, direct. To run
-counter to the consensus of trend is to be subjected to greater chance
-of extermination. Those that did so have already been weeded out.</p>
-
-<div class="figright">
-   <a id="I_041" name="I_041"> </a>
-   <img src="images/i_041.jpg" alt="" width="200" height="322" />
-   <p class="center"><span class="smcap">The Mart Iron.</span></p>
-   <p class="center">(<i>Proc. Wash. Acad. of Sci.</i><br /> vol. II. Plate VI.)</p>
-</div>
-
-<p>From the behavior of meteorites we proceed to scan their appearance.
-And here we notice some further telltale facts about them. Their
-conduct informed us of their relationship, their character bespeaks
-their parentage.</p>
-
-<p>Most meteorites are stones, but one or two per cent are nearly pure
-iron mixed with nickel. When picked up, they are usually covered with
-a glossy thin black crust. This overcoat they have put on in coming
-through our air. Air-begotten, too, are the holes with which many of
-them are pitted. For entering our atmosphere with their speed in space
-is equivalent to immersing them suddenly in a blowpipe flame of several
-thousand degrees Fahrenheit. Thus their surface is burnt and fused to a
-<span class="pagenum"><a name="Page_41" id="Page_41">[Pg 41]</a></span>
-cinder. Yet in spite of being warm to the touch their hearts are still
-cosmically cold. The Dhurmsala meteorite falling into moist earth was
-found an hour afterwards coated with frost. Agassiz likened it to the
-Chinese culinary <i>chef d’œuvre</i> “fried ice.” It is the cold of space,
-200° or more Centigrade below zero, that they bear within, proof of
-their cosmic habitat.</p>
-
-<p>That they are bits of a once larger mass is
-evident on their face. Their shape shows that they are not wholes but
-parts, while their constitution bespeaks them anything but elementary.
-Diagnosis of it yields perhaps their most interesting bit of news. For
-it shows their origin. Their autopsy proves them to contain thirty
-known elements, and not one that is new. The list includes all the
-substances most common on the Earth’s surface, which is suggestive;
-but, what is still more instructive, these are combined into minerals
-<span class="pagenum"><a name="Page_42" id="Page_42">[Pg 42]</a></span>
-which largely differ from those with which we are superficially
-familiar. Professor Newton, whose specialty they were, has said: “In
-general they show no resemblance in their mechanical or mineralogical
-structure to the granitic and surface rocks of the Earth. One condition
-was certainly necessary in their formation, viz. the absence of free
-oxygen and of enough water to oxidize the iron.” Thus they are not of
-the Earth earthy; nor yet, poor little waifs, of the upper crust of any
-other body.</p>
-
-<div class="figcenter">
-   <a id="I_042" name="I_042"> </a>
-   <img src="images/i_042.jpg" alt="" width="500" height="338" />
-   <p class="center"><span class="smcap">Section of Meteorite showing Widmannstättian Lines.</span></p>
-   <p class="center space-below2">(Field Columbian Museum, Chicago.)</p>
-</div>
-
-<div class="figright">
-   <a id="I_043" name="I_043"> </a>
-   <img src="images/i_043.jpg" alt="" width="250" height="200" />
-   <p class="center"><span class="smcap">Meteorite, Toluca.</span><br />
-                 (Field Columbian Museum, Chicago.)</p>
-</div>
-
-<p><span class="pagenum"><a name="Page_43" id="Page_43">[Pg 43]</a></span>
-In them prove to be occluded gases, which can be got out by heating in
-the laboratory, and which must have got in when the meteorites were
-still subjected to great heat and pressure. For only thus could these
-gases have been absorbed. Both such heat and such pressure accuse some
-great solid body as origin of this flotsam of the sky. Fragments now,
-they owe to its disruption their present separate state. This parent
-mass must have been much larger and more massive than the Earth, as the
-grate amount of occluded hydrogen, sometimes one-third the volume at
-500° C., of the meteorite seems to testify.</p>
-
-<p>The two classes of meteorites, the stone and the iron, show this
-further by the very differences they exhibit between themselves. For
-both the amount and the proportions of the occluded gases in the two
-prove to be quite distinct. In the stones the quantity of gas is
-greater and the composition is diverse. In the stones carbonic acid gas
-is common, carbon monoxide rare; in the irons the ratio is just the
-other way. Thus Wright found in nine specimens of the iron meteorites:—</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc bigfont">CO₂</td>
-      <td class="tdc bigfont"> CO</td>
-      <td class="tdc bigfont">H</td>
-      <td class="tdl_ws1 bigfont">CH₄</td>
-   </tr><tr>
-      <td class="tdl">11.5%</td>
-      <td class="tdl_ws1">32.4%</td>
-      <td class="tdl_ws1">54.1%</td>
-      <td class="tdl_ws1">00% of the total;</td>
-   </tr>
- </tbody>
-</table>
-
-<p class="no-indent">in ten of stone:—</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc bigfont">CO₂</td>
-      <td class="tdc bigfont"> CO</td>
-      <td class="tdc bigfont">H</td>
-      <td class="tdl_ws1 bigfont">CH₄</td>
-   </tr><tr>
-      <td class="tdl">60.1%</td>
-      <td class="tdl_ws1">3.4%</td>
-      <td class="tdl_ws1">32.0%</td>
-      <td class="tdl_ws1">2.1%</td>
-   </tr>
- </tbody>
-</table>
-
-<p><span class="pagenum"><a name="Page_44" id="Page_44">[Pg 44]</a></span>
-The stones are much lighter than the iron, their specific gravities
-being as 3 to 7 or 8 for the metallic. The stones, therefore, came from
-a more superficial layer of the body torn apart than the iron, and the
-composition of their occluded gases bears this out. Those in the stones
-are such as we may conceive absorbed nearer the surface, those in the
-iron from regions deeper down.</p>
-
-<p>Here, then, the meteorites tell us of another, an earlier, stage of
-our solar system’s history, one that mounts back to before even the
-nebula arose to which we owe our birth. For the large body to whose
-dismemberment the meteorites were due can have been no other than the
-one whose cataclysmic shattering produced that very nebula which was
-for us the origin of things. The meteorites, by continuing unchanged,
-link the present to that far-off past. And they tell us, too, that
-this body must have been dark. For solid, they inform us, it was, and
-solidity in a heavenly body means deficiency of light.</p>
-
-<p>That such corroborative testimony to a cataclysmic origin is
-forthcoming in the sky we shall see by turning again to the spiral nebulæ.</p>
-
-<p>Of the two classes of nebulæ which we contemplated in the last chapter,
-the amorphous and the structural, there is more to be said than we
-touched on then.
-<span class="pagenum"><a name="Page_45" id="Page_45">[Pg 45]</a></span></p>
-
-<div class="figcenter">
-   <a id="I_045" name="I_045"> </a>
-   <img src="images/i_045.jpg" alt="" width="500" height="574" />
-   <p class="center space-below2"><span class="smcap">Nebula</span> ♅ <span class="smcap">V. 14 Cygni—after Roberts.</span></p>
-</div>
-
-<p>Not only in look are the two quite unlike, but the spectroscope shows
-that the difference in appearance is associated with dissimilarity of
-character. For the spectrum of the amorphous proves to consist of a
-few bright lines, due to hydrogen and nebulium chiefly, in the green,
-whence the name green nebulæ. That of the spirals, on the other hand,
-is continuous, and therefore white. The great nebula in Andromeda was
-one of the first in which this was recognized; and the perception was
-pregnant, for no nebula defies resolution more determinedly than it. We
-<span class="pagenum"><a name="Page_46" id="Page_46">[Pg 46]</a></span>
-may, therefore, infer that it is not made up of stars, certainly
-big enough for us to see. On the other hand, from the fact that its
-spectrum is continuous it must be solid or liquid. Young pointed out
-that this did not follow, because a gas under great pressure also
-gives a continuous spectrum. But he forgot that here no such pressure
-could exist. A nebula of compressed gas could not have an irregular
-form and would have, in the case of the Andromeda nebula, a mass so
-enormous as to preclude supposition. Continuity of spectrum here means
-discontinuity of mass. The spectral solidity of the nebula speaks of a
-<i>status quo ante</i>, not of a condition of condensation now going on.</p>
-
-<div class="figcenter">
-   <a id="I_046" name="I_046"> </a>
-   <img src="images/i_046.jpg" alt="" width="600" height="418" />
-   <p class="center space-below2"><span class="smcap">Nebula N. G. C. 1499 Persei—after Roberts.</span></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_47" id="Page_47">[Pg 47]</a></span></p>
-
-<div class="figcenter">
-   <a id="I_047" name="I_047"> </a>
-   <img src="images/i_047.jpg" alt="" width="500" height="586" />
-   <p class="center space-below2"><span class="smcap">Nebula N. G. C. 6960 in Cygnus—after Ritchey.</span></p>
-</div>
-
-<p>Advanced spectroscopic means reveals that the spectra of these “white”
-nebulæ are not simply continuous. Thus that of the Andromeda nebula
-shows very faint dark lines crossing it, apparently accordant with
-those of the solar spectrum and faint bright ones falling near and
-<span class="pagenum"><a name="Page_48" id="Page_48">[Pg 48]</a></span>
-probably coincident with those of the Wolf-Rayet stars, due to
-hydrogen, helium, and so forth. These later observations make
-practically certain what earlier ones permitted us just now only to
-infer: that it is not composed of stars, but of something subtler
-still; to wit, of meteorites. The reasoning is interesting, as showing
-that if one have hold of a true idea, the stars in their courses fight
-for him.</p>
-
-<div class="figcenter">
-   <a id="I_048" name="I_048"> </a>
-   <img src="images/i_048.jpg" alt="" width="500" height="531" />
-   <p class="center space-below2"><span class="smcap">Nebula M. 51 Canum Venaticorum—after Ritchey.</span></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_49" id="Page_49">[Pg 49]</a></span>
-Although Lockyer has long been of opinion that the nebulæ are composed
-of meteorites, the present argument differs from his. The way in which
-their spectra establish their constitution may be outlined as follows:
-the white nebulæ are from their structure evidently in process of
-evolution, and if they are in stable motion, as we suppose them to be,
-their parts are moving round their common centre of gravity. As the
-white nebulæ resist resolution as obstinately as the green, these parts
-must be not only solid but comminuted (composed of small particles).
-Now this would be the case were they flocks of meteorites such as we
-have seen composed our own system once upon a time. Though all are
-travelling round the centre of gravity of the flock, each is pursuing
-its own orbit slightly different from, and intersecting those of, its
-neighbors. Collisions between the meteors must therefore constantly
-occur, and the question is, are these shocks sufficient to cause light.
-Let us take our own system and consider two meteorites at our distance
-from the Sun, travelling in the same sense, the one in an ellipse,
-the other in a circle, with a major axis five per cent greater and
-meeting the other at aphelion. This would be no improper jostle for
-such heavenly bodies. If we calculate the speeds of both and deduct
-the elliptic from the circular, we shall have the relative speed of
-collision. It proves to be a half a mile a second or 30 times the speed
-<span class="pagenum"><a name="Page_50" id="Page_50">[Pg 50]</a></span>
-of an express train. As such a train brought up suddenly against a
-stone wall would certainly elicit sparks, we see that a speed 30 times
-as great, whose energy is 900 times greater, is quite competent to a
-shock sufficient to make us see stars <i>en masse</i>. But, indeed, there
-must be collisions much more violent than this; both because the
-central mass is often much greater and because the orbits differ much
-more, and the effect would increase as the square of the speed. The
-heat thus generated would cause the meteorites to glow, and at the same
-time raise the temperature of the gases in and about them. Furthermore,
-the light would come to us through other non-affected portions of gas
-between us and the scene of the collision. Thus all three peculiarities
-of the spectra stand explained: we have a continuous background of
-light due to heated solid meteorites, the bright lines of glowing
-gases, and dark lines due to other gases not ignited, lying in our line
-of sight.</p>
-
-<p>In addition we should perceive another result. Collisions would be both
-more numerous and more pronounced toward the centre of the nebula, for
-it must speedily grow denser toward its core owing to the falling in of
-meteorites, in consequence of shock. Being denser in the centre, the
-particles would there be thicker and be travelling at greater speed.
-The nebulæ, therefore, should be brightest at their centres, which is
-accordant with observation.
-<span class="pagenum"><a name="Page_51" id="Page_51">[Pg 51]</a></span></p>
-
-<p>Thus from having offered themselves exemplars of the way in which our
-own system came into being, the white nebulæ assert their present
-constitution to be that from which we know our system sprang.</p>
-
-<p>Another suggestive fact about the present members of our solar system
-which has something to say about a past collision is the densities of
-the different planets. The average density of the four inner planets,
-Mars, the Earth, Venus, and Mercury is nearly four times that of
-the four outer ones Neptune, Uranus, Saturn, and Jupiter[<a href="#NOTE_2">see NOTE
-2</a>]. The discrepancy is striking and cannot be explained by size, as
-the smallest are the most massive, and if all were primally of like
-constitution, should be the least compressed. Nor can it be explained
-simply by greater heat tending to expand them, for Neptune and Uranus
-show no signs of being very hot. The minor differences between members
-of each group are probably explicable in part by these two factors,
-mass and heat, but the great gulf between the two groups cannot so
-be spanned. We are then driven to the supposition that the materials
-composing the outer ones were originally lighter. Now this is precisely
-what should happen had all eight been formed by disruption of a
-previous body. For its cuticle would be its least dense portion, and on
-disruption would travel farthest away, not because of being lighter,
-but because of being on the outside. Parts coming from deeper down
-would remain near, and be denser intrinsically.
-<span class="pagenum"><a name="Page_52" id="Page_52">[Pg 52]</a></span></p>
-
-<p>What the present densities of the planets enable us to infer of the
-cataclysm from which they came, a remarkable set of spectrograms taken
-not long ago by Dr. V. M. Slipher, at Flagstaff, seems to confirm.</p>
-
-<p>The spectrograms in question were made possible by his production of a
-new kind of plate. His object was to obtain one which should combine
-sufficient speed with great photographic extension of the spectrum
-into the red. For it is in the red end that the absorption lines due
-to the planets’ atmospheres chiefly lie. With the plates heretofore
-used it was impossible to go much beyond the yellow, the C line marking
-the <i>Ultima Thule</i> of attent. Not only was it advisable to get more
-particularity in the parts previously explored, but it was imperative
-to go beyond into parts as yet unknown. After several attempts he
-succeeded, the plates when exposed showing the spectra beyond even the
-A band. Of their wealth of depiction it is only necessary to say that
-in the spectrum of Neptune 130 lines and bands can easily be counted
-between the wave-lengths 4600 µµ, 7600 µµ. Of these, 31 belong to the
-planet, which compares with 6 found by Huggins, 10 by Vogel, and 9 by
-Keeler in the part of its spectrum they were able to obtain.</p>
-
-<div class="figcenter">
-   <a id="I_052" name="I_052"> </a>
-   <p class="center bigfont space-above2">THE SPECTRA OF THE MAJOR PLANETS.</p>
-   <p class="center"><i>Photographed, in 1907, by V. M. Slipher, at THE LOWELL OBSERVATORY<br />
-                    Flagstaff, Arizona.</i></p>
-   <img src="images/i_052.jpg" alt="" width="600" height="368" />
-</div>
-
-<p class="space-above2"><span class="pagenum"><a name="Page_53" id="Page_53">[Pg 53]</a></span>
-The result was a revelation. The plates exposed a host of lines never
-previously seen; lines that do not appear in the spectrum of the Sun,
-nor yet in the added spectrum of the atmosphere of the Earth, but are
-due to the planets’ own envelopes. But this was only the starting-point
-of their disclosures. When in this manner he had taken the color
-signatures of Jupiter, Saturn, Uranus, and Neptune, an orderly sequence
-in their respective absorption bands stood strikingly confessed. In
-other words, their atmospheres proved not only peculiar to themselves
-and unlike what we have on Earth, but progressively so according to a
-definite law. That law was distance from the Sun. When the spectra were
-arranged vertically in ordered orbital relation outward from the Sun,
-with that of the lunar for comparison on top, a surprising progression
-showed down the column in the strange bands, an increase in number and
-a progressive deepening in tint. The lunar, of course, gives us the Sun
-and our own air. All else must therefore be of the individual planet’s
-own. Beginning, then, with Jupiter, we note, besides the reënforcement
-of what we know to be the great water-vapor bands ‘<i>a</i>,’ several new
-ones, which show still darker in the spectrum of Saturn. The strongest
-of these is apparently not identifiable with a band in the spectra
-of Mira Ceti in spite of falling near it. Passing on to Uranus, we
-perceive these bands still more accentuated, and with them others, some
-strangers, some solar lines enhanced. Thus the hydrogen lines stand out
-as in the Sirian stars. All deepen in Neptune, while further newcomers appear.
-<span class="pagenum"><a name="Page_54" id="Page_54">[Pg 54]</a></span></p>
-
-<p>Thus we are sure that free hydrogen exists in large quantities in
-the atmospheres of the two outermost planets and most so in the one
-farthest off. Helium, too, apparently is there, and other gases which
-in part may be those of long-period stars, decadent suns, in part
-substances we do not know.</p>
-
-<p>From the fact that these bands are not present in the Sun and
-apparently in no type of stars, we may perhaps infer that the
-substances occasioning them are not elements but compounds to us
-unknown. And from the fact that free hydrogen exists there alongside of
-them, and apparently helium, too, we may further conclude that they are
-of a lighter order than can be retained by the Earth.</p>
-
-<p>But now, we may ask, why should these lighter gases be found where they
-are? It cannot be in consequence simply of the kinetic theory of gases
-from which a corollary shows that the heaviest bodies would retain
-their gases longest, because the strange gases are not apportioned
-according to the sizes of their hosts. Jupiter, by all odds the biggest
-in mass, has the least, and Saturn, the next weightiest, the next
-in amount. Nor can title to such gaseous ownership be lodged in the
-planet’s present state. For though Jupiter is the hottest and Saturn
-the next so, the increased mass more than makes up in restraint what
-increased temperature adds in molecular volatility—as we perceive in
-the cases of the Sun and Earth.
-<span class="pagenum"><a name="Page_55" id="Page_55">[Pg 55]</a></span></p>
-
-<p>No; their envelopes are increasingly strange because their internal
-constituents are different, and as hydrogen is most abundant in
-Neptune, the lightest of all the gases, it is inferable that this
-planet’s material is lighter. As distance from the Sun determines
-their atmospheric clothing, so distance decides upon their bodies,
-too. It was all a case of primogeniture. The light strange matter
-that constitutes them was so because it came from the outer part of
-the dismembered parent orb. Neptune the outermost, Uranus the next,
-then Saturn and Jupiter came in that order from the several successive
-layers of the pristine body, while the inner planets came from parts of
-it deeper down. The major planets were of the skin of the dismembered
-body, we of its lower flesh.</p>
-
-<p>Very interesting the study of these curious spectral lines from the
-outer planets for themselves alone; even more so for what one would
-hardly have imagined: that they should actually tell us something of
-the genesis of our whole solar system. They corroborate in so far what
-the meteorites have to say.</p>
-
-<p>That the meteorites are solid and, except for their experiences in
-coming through our air, bear no marks of external heat, is a fact
-which is itself significant. It seems to hint not at a crash as their
-occasioning but at disruptive tidal strains. The parent body appears to
-<span class="pagenum"><a name="Page_56" id="Page_56">[Pg 56]</a></span>
-have been torn apart without much development of heat. Perhaps, then,
-we had no gloriously pyrotechnic birth, but a more modest coming into
-existence. But about this we must ourselves modestly be content to
-remain for the present in the dark.</p>
-
-<p>Not the least important feature of the theory I have thus outlined is
-that it finishes out the round of evolution. It becomes a conception
-<i>sapiens in se ipso totus, teres atque rotundus</i>. To frame a theory
-that carries one back into the past, to leave one there hung up in
-heaven, is for inconclusiveness as bad as the ancient fabulous support
-of the world, which Atlas carried standing on an elephant upheld by a
-tortoise. What supported the tortoise we were not told. So here, if
-meteorites were our occasioning, we must account for the meteorites,
-starting from our present state. This the present presentation does.</p>
-
-<p>Thus do the stones that fall from the sky inform us of two historic
-events in our solar system’s career. They tell us first and directly
-of a nebula made up of them, out of which the several planets were by
-agglomeration formed and of which material they are the last ungathered
-remains. And then they speak to us more remotely but with no less
-certainty of a time antedating that nebula itself, a time when the
-nebula’s constituents still lay enfolded in the womb of a former Sun.</p>
-
-<p>Man’s interest in them hitherto has been, as with other things, chiefly
-<span class="pagenum"><a name="Page_57" id="Page_57">[Pg 57]</a></span>
-proprietary. Greed of them has grown so keen that legal questions have
-been raised of the ownership of their finding, and our courts have
-solemnly declared them not “wild game” but “real estate,” and as such
-belonging to the owner of the land on which they fall.</p>
-
-<p>But to the scientific eye their estate is something more than “real,”
-for theirs is the oldest real estate in the solar system. They were
-what they are now when the Earth we pride ourselves in owning was but a
-molten mass.</p>
-
-<p>So that when in future you see these strange stones in rows upon a
-museum’s shelves, regard them not as rarities, in which each museum
-strives to outdo its neighbors by the quantity it can possess, but as
-rosetta stones telling us of an epoch in cosmic history long since
-passed away—of which they alone hold the key. Look at them as the
-literary do their books, for that which they contain, not as the
-bibliophile to whom a misprint copy outvalues a corrected one and by
-whom “uncuts” are the most prized of all.</p>
-
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_58" id="Page_58">[Pg 58]</a></span></p>
-<div class="chapter">
-<h2 class="nobreak">CHAPTER III<br /><span class="h_subtitle">THE INNER PLANETS</span></h2>
-</div>
-
-<p class="drop-cap">When we recall that the Ptolemaic system of
-the universe was once taught side by side with the Copernican at
-Harvard and at Yale, we are impressed, not so much with the age of
-our universities, as with the youth of modern astronomy and with the
-extraordinary vitality of old ideas. That the Ptolemaic system in its
-fundamental principle was antiquated at the start, the older Greeks
-having had juster conceptions, does not lessen our wonder at its
-tenacity. But the fact helps us to understand why so much fossil error
-holds its ground in many astronomic text-books to-day. That stale
-intellectual bread is deemed better for the digestion of the young, is
-one reason why it often seems to them so dry.
-<span class="pagenum"><a name="Page_59" id="Page_59">[Pg 59]</a></span></p>
-
-<div class="figcenter">
-   <a id="I_059" name="I_059"> </a>
-   <img src="images/i_059.jpg" alt="" width="600" height="441" />
-   <p class="center space-below2"><span class="smcap">Orbits of the Inner Planets.</span></p>
-</div>
-
-<p>Before entering upon the problem of the genesis and career of a world,
-it is essential to have acquaintance with the data upon which our
-deductions are to rest. To set forth, therefore, what is known of the
-several planets of our solar system, is a necessary preliminary to any
-understanding of how they came to be or whither they are tending; and
-as our knowledge has been vitally affected by modern discoveries about
-them, it is imperative that this exposition of the facts should be as
-near as possible abreast of the research itself. I shall, therefore,
-give the reader in this chapter a bird’s-eye view of the present state
-of planetary astronomy, which he will find almost a different part of
-speech from what it was thirty years ago. It is not so much in our
-knowledge of their paths as of their persons that our acquaintance with
-<span class="pagenum"><a name="Page_60" id="Page_60">[Pg 60]</a></span>
-the planets has been improved. And this knowledge it is which has made
-possible our study of their evolution as worlds.</p>
-
-<p>Could we get a cosmic view of the solar system by leaving the world we
-live on for some suitable vantage-point in space, two attributes of it
-would impose themselves upon us—the general symmetry of the whole, and
-the impressively graded proportions of its particular parts.</p>
-
-<p>Round a great central globular mass, the Sun, far exceeding in size
-any of his attendants, circle a series of bodies at distances from
-him quite vast, compared with their dimensions. These, his principal
-planets, are in their turn centres to satellite systems of like
-character, but on a correspondingly reduced scale. All of them travel
-substantially in one plane, a fact giving the system thus seen in its
-entirety a remarkably level appearance, as of an ideal surface passing
-through the centre of the Sun. Departing somewhat from this general
-uniformity in their directions of motion, and also deviating more
-from circularity in their paths, some much smaller bodies, a certain
-distance out, dart now up now down across it at different angles and
-from all the points of the compass, agreeing with the others only in
-having the centre of the Sun their seemingly never attained goal of
-<span class="pagenum"><a name="Page_61" id="Page_61">[Pg 61]</a></span>
-endeavor. These bodies are the asteroids. Surrounding the whole, and
-even penetrating within its orderly precincts, a third class would
-be visible which might be described for size as cosmic dust, and for
-display as heavenly pyrotechnics. Coming from all parts of space
-indifferently they would seem to seek the Sun in almost straight lines,
-bow to him in circuit, and then depart whence they came. For in such
-long ellipses do they journey that these seem to be parabolas. These
-visitants are the comets and their associates the meteor-streams.</p>
-
-<p>Although for purposes of discrimination we have labelled the several
-classes apart, an essential fact about the whole company is to be
-noted: that no hard and fast line can be drawn separating the several
-constituents from one another. In size the members of the one class
-merge insensibly into the other. Some of the planets are hardly larger
-than some of the satellites; some of the satellites than some of the
-asteroids; some of the asteroids than comets and shooting-stars. In
-path, too, we find every gradation from almost perfect circularity
-like the orbits of Io and Europa to the very threshold of where one
-step more would cease to leave the body a member of the Sun’s family
-by turning its ellipse into an hyperbola. Finally, in inclination we
-have every angle of departure from orthodox platitude to unconforming
-uprightness. This point, that heavenly bodies, like terrestrial ones,
-<span class="pagenum"><a name="Page_62" id="Page_62">[Pg 62]</a></span>
-show all possible grades of indistinction, is kin to that specific
-generalization by which Darwin revolutionized zoölogy a generation ago.
-It is as fundamental to planets as to plants. For it shows that the
-whole solar system is evolutionarily one.</p>
-
-<p>A second point to be noticed in passing is that undue inclination and
-excessive eccentricity go together. The bodies that have their paths
-least circular have them, as a rule, the most atilt. And with these two
-qualities goes lack of size. It is the smallest bodies that deviate
-most from the general consensus of the system. With so much by way of
-generic preface, the pregnancy of which will become apparent as we
-proceed, we come now to particular consideration of its members in turn.</p>
-
-<p>Nearest to the Sun of all the planets comes Mercury. So close is he to
-that luminary, and so far within the orbit of the earth, that he is
-not a very common object to the unaided eye. Copernicus is said never
-to have seen him, owing, doubtless, to the mists of the Vistula. By
-knowing when to look, however, he may be seen for a few days early in
-the spring in the west after sunset, or before sunrise in the east in
-autumn. He is then conspicuous, being about as bright as Capella, for
-which star or Arcturus he is easily mistaken by one not familiar with
-the constellations.
-<span class="pagenum"><a name="Page_63" id="Page_63">[Pg 63]</a></span></p>
-
-<p>His mean distance from the Sun is thirty-six million miles, but so
-eccentric is his orbit, the most so of any of the principal planets,
-that he is at times half as far off again as at others. Even his
-orbital behavior is the least understood of any in the solar system.
-His orbit swings round at a rate which so far has defied analysis. It
-may be a case of reflected perturbation, one, that is, of which the
-indirect effect from another body becomes more perceptible than would
-be the direct effect on the body itself. As yet it baffles geometers.</p>
-
-<p>As to his person, our ignorance until lately was profound. It is only
-recently that such fundamental facts about him as his size, his mass,
-and his density have been reached with any approach to precision. This
-was because he so closely hugs the Sun that observations upon his
-full, or nearly full, disk had never been attempted. When I say that
-his volume was not known to within a third of its amount, his mass not
-closer than one-half, while his received density was nearly double
-what we now have reason to suppose the fact, some idea of the depth
-of our nescience may be imagined. This, of course, did not prevent
-text-books from confidently misinstructing youth, or Nautical Almanacs
-from misguiding computers with figures that thus almost achieved
-immortality, so long had they passed current in spite of lacking that
-perfection which is usually assigned as its warrant.
-<span class="pagenum"><a name="Page_64" id="Page_64">[Pg 64]</a></span></p>
-
-<div class="figcenter">
-    <a name="I_064" id="I_064"> </a>
-    <img src="images/i_064.jpg" alt="" width="500" height="528" />
-    <p class="center space-below2"><span class="smcap">Sulla Rotazione di Mercurio—Di G. V. Schiaparelli.</span></p>
-</div>
-
-<p>Schiaparelli first put astronomy on the right track. By attempting
-daylight observations of the planet, not toward night, but actually
-at midday, he made some remarkable discoveries, and though he did
-not detect the hitherto erroneous values of the volume, the mass,
-or the density, his method of observation paved the way for their
-ascertainment. What he sought, and found, was evidence of markings upon
-<span class="pagenum"><a name="Page_65" id="Page_65">[Pg 65]</a></span>
-the disk by which the planet’s time of rotation might be determined.
-Up to then, Schroeter’s value of about twenty-four hours had been
-accepted, on very slender evidence indeed, and passed into all the
-books. But when the planet came to be observed by noon, very definite
-markings stood out on its face, which showed its rotation to take
-place, not in twenty-four hours, but in eighty-eight days. By a
-persistence equal to his able choice of observing time, he established
-this beyond dispute. He proved the revolutionizing fact that Mercury’s
-periods of rotation and of revolution were the same.</p>
-
-<p>He detected, too, the evidence in the position of the markings of the
-planet’s great libratory swing due to the eccentricity of its orbit, a result
-as remarkable as a feat of observation as it was conclusive as a proof.</p>
-
-<p>If Schiaparelli had never done any other astronomical work, this study
-of Mercury would have placed him as the first observer of his day. For
-the observations are so difficult that the planet not only baffled all
-his predecessors, but has foiled many since who are credited with being
-observers of eminence.</p>
-
-<p>In 1896 the study of Mercury was taken up at the Lowell Observatory
-in Arizona along the same lines that had proved so successful with
-Schiaparelli, but without using his observations as guide. Indeed,
-his papers had not then been read there. The two conclusions were,
-<span class="pagenum"><a name="Page_66" id="Page_66">[Pg 66]</a></span>
-therefore, independent of one another. The outcome was a complete
-corroboration and an extension of Schiaparelli’s work. We shall begin
-with the consideration of the most fundamental point. In the clear and
-steady air of Flagstaff, permitting of measurement of his disk up to
-within a few degrees of the Sun, Mercury was found to be much larger
-than previously thought.</p>
-
-<p>Instead of a diameter of three thousand miles he proved to have one
-of thirty-four hundred, making his volume nearly half as large again
-as had been credited him. These measures bore intrinsic evidence
-of their trustworthiness in an interesting manner, and at the same
-time produced internal testimony that accounted for the smallness of
-previous determinations. Measures heretofore had been made, usually if
-not invariably, either when the planet transited the Sun or when it
-exhibited a pronounced phase. Now in both these cases the planet looks
-smaller than it is. In the first case this is due to irradiation, the
-surrounding disk of the Sun encroaching both to the eye and to the
-camera upon the silhouette of Mercury. And this inevitable effect had
-not been allowed for in the measures. In the second case the horns of
-the planet never seem to extend quite to their true position. This was
-rendered evident by the Flagstaff series of measures, which began when
-<span class="pagenum"><a name="Page_67" id="Page_67">[Pg 67]</a></span>
-the planet was a half-moon and continued till it was almost full. As
-it did so, the values for the diameter steadily increased, even after
-irradiation was allowed for, although this against the brilliant
-background of the noonday sky must have been exceeding small, and
-tended in part to be diminished as the planet attained the full,
-because of its consequent nearing of the Sun. The measures thus
-explained themselves and vouched for their own accuracy.<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a></p>
-
-<p>Then came a curious bit of unexpected proof to corroborate them. In his
-“Astronomical Constants,”<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a>
-published but a short time before, Newcomb had detected a systematic
-error in the right ascensions of Mercury which he was not able to
-explain. By diligent mousing that eminent computer had discovered that
-Mercury was registered by observers too far from the Sun on whichever
-side of him it happened to be, and in proportion roughly not to its
-distance off but to the phase the planet exhibited. When the disk was
-a crescent the discrepancy between observation and theory was large,
-and thence decreased as the planet passed to the full. He suspected
-the cause, and would have found it had he not considered the diametral
-<span class="pagenum"><a name="Page_68" id="Page_68">[Pg 68]</a></span>
-measures of the planet too well assured to permit of doubt. As it was,
-he neglected a factor which has vitiated almost all the observations
-made on the planets up to within a few years, the correction for
-irradiation. This was the case here. The received measures, beginning
-with Bradley and ending with Todd, had almost without exception been
-made in transit, and, as no regard had been paid to the contracting
-effect of irradiation, had been invalidated in consequence. The new
-method supplied almost exactly the amount needed to explain the right
-ascensions, a second of arc, and in precise accordance with the place
-which the discrepancy demanded.</p>
-
-<p>About the mass there has been, and still is, great uncertainty. This
-is because it can only be found from the perturbing effect it has on
-Venus, the Earth, or Encke’s comet. Modern determinations, however, are
-smaller than the older ones; thus Backlund in 1894 got from the effect
-on Encke’s comet only one-half the mass that Encke had, fifty-three
-years before. Probably the most reliable information comes from Venus,
-which Tisserand found to give for Mercury ¹/₇₁₀₀₀₀₀ of the mass of the
-Sun, or ¹/₂₁ of the mass of the Earth. If we take ¹/₇₀₀₀₀₀₀ as the nearest
-round number, we find the planet’s density to be 0.66 that of the Earth.
-<span class="pagenum"><a name="Page_69" id="Page_69">[Pg 69]</a></span></p>
-
-<div class="figcenter">
-    <a name="I_069" id="I_069"> </a>
-    <p class="f120"><b>MAP of MERCURY</b></p>
-    <img src="images/i_069.jpg" alt="" width="500" height="396" />
-<p class="center space-below2">LOWELL OBSERVATORY 1896-97</p>
-</div>
-
-<p>The same observations that disclosed at Flagstaff the planet’s size
-revealed a set of markings on his face so definite as to make the
-rotation period unmistakable. It takes place, as Schiaparelli found,
-in eighty-eight days, or the time of the planet’s revolution round
-the Sun. The markings disclosed the fact, as Schiaparelli had also
-discovered, in a most interesting manner, for the ellipticity of the
-planet’s orbit stood reflected in the swing of the markings across the
-face of the disk, a definiteness in the proof of a really surprising
-kind. What this means we shall see in a subsequent chapter when we take
-up the mechanical problem of the tides. Another result that issued from
-the positions of the markings was the determination of the planet’s
-<span class="pagenum"><a name="Page_70" id="Page_70">[Pg 70]</a></span>
-pole. Except for the libration above noticed, the markings kept
-an invariable longitudinal position upon the illuminated disk,
-showing that the planet turned always the same face to the Sun; but
-latitudinally a difference was noticeable between their place in
-October-November, 1896, and in February-March, 1897, the latter being
-4° farther north. Now this is just what the orbital position should
-have caused, if the pole stood vertically to it. Thus a difference
-of 4° from perpendicularity should have been discernible, had it
-existed,—a very small amount in such a determination. We may,
-therefore, conclude that the axis stands plumb to the orbit, and this
-is what theory demands.</p>
-
-<p>The state of things this introduces to us upon that other world is
-to our ideas exceeding strange. It is not so much the slowness of
-the diurnal spin, eighty-eight times as long as our own, which is
-surprising, as the fact that this makes its day infinite in length.
-Two antipodal hemispheres divide the planet, the one of which frizzles
-under eternal sun, the other freezes amid everlasting night. The Sun
-does not, indeed, stand stock-still in the sky, but nods like some
-huge pendulum to and fro along a parallel of latitude. In consequence
-of libration the two great domains of day and night are sundered by a
-strip of debatable ground 23½° in breadth on either side, upon which
-<span class="pagenum"><a name="Page_71" id="Page_71">[Pg 71]</a></span>
-the Sun alternately rises and sets. Here there is a true day,
-eighty-eight of our days in length from one sunrise to the next. But
-its day and night are not apportioned alike. The eastern strip has its
-daylight briefer than its starlight hours; the western has them longer.
-Nor are different portions of the strips similarly circumstanced in
-their sunward regard. Only the edge next perpetual day has anything
-approaching an equal distribution of sunlight and shade. The farther
-one just peeps at the Sun for a moment every eighty-eight days, and
-then sinks back again into obscurity.</p>
-
-<p>The transition from day to night is equally instantaneous and profound.
-For little or no twilight here prolongs the light; since the air, if
-there be any at all, is too thin to bend it to service round the edge
-to illuminate the night. When the libratory Sun sets, darkness like
-a mantle falls swiftly over the face of the ground. No evidence of
-atmosphere has ever been perceived, and theory informs that it should
-be nearly, if not wholly, absent.</p>
-
-<p>In consequence of the rigid uprightness of the planet’s axis,
-seasons do not exist. Their nearest simulacrum comes from the
-seeming dilatation of the Sun during half the year, and its apparent
-contraction during the other half. It expands so much between its
-January and its July as to receive more heat in the ratio of nine to
-<span class="pagenum"><a name="Page_72" id="Page_72">[Pg 72]</a></span>
-four. A seasonless, dayless, and almost yearless planet, it is better
-to look at than to look from; but its study opens our eyes to the great
-diversity which even one of our nearest neighbors exhibits from what we
-take as matters of course on Earth.</p>
-
-<p>That what we take offhand to be purely astronomic phenomena should turn
-out to be so essentially of the particular world, worldly, clarifies
-vision of what these really are, and how dependent on and interwoven
-with everyday life astronomy is. Or, we may consider it turned about
-and realize how purely astronomic relations, such abstract mechanical
-matters as rotations and revolutions, result in completely changing the
-very face and character of the globe concerned. Mercury to-day stares
-forever at the Sun. The markings we see have stereotyped this stare
-to its inevitable result. For they seem to mark a globe sun-cracked.
-At such a condition the curious crisscross of dark, irregular lines
-certainly hints, accentuated and perfected as it is by a bounding curve
-where the mean sunward side terminates to the enclosing them as by the
-carapace of a tortoise. Though they cannot probably be actual cracks,
-however much they may resemble such, yet they may well owe their
-existence to that fundamental cause.</p>
-
-<p>In color the planet is ghastly white; of that wan hue that suggests a
-body from which all life has fled. Far whiter than Venus in point of
-<span class="pagenum"><a name="Page_73" id="Page_73">[Pg 73]</a></span>
-fact, the rosy tint with which it sparkles in the sunset glow is all
-borrowed of the dying day and vanishes when the planet is looked at in
-the uncompromising light of noon. Seen close together once at Flagstaff
-it was possible directly to compare the two; when Mercury, although lit
-by the Sun two and a half times as brilliantly as Venus, was, surface
-for surface, more than twice as faint. Müller has found its intrinsic
-brightness about that of our Moon, which in some respects it resembles,
-though it apparently departs widely from any similarity in others. The
-bleached bones of a world; that is what Mercury seems to be.</p>
-
-<p>Venus comes next in order outward from the Sun. To us her incomparable
-beauty is partly the result of propinquity: nearness to ourselves and
-nearness to the Sun. Relatively so close is she to both that she does
-not need the Sun’s withdrawal to appear, but may nearly always be seen
-in the daytime in clear air if one knows where to look for her. Situate
-about seven-tenths of our own distance from our common giver of light
-and heat, she gets about double the amount that falls to our lot, so
-that her surface is proportionately brilliantly illuminated. Being also
-relatively near us, she displays a correspondingly large surface.</p>
-
-<p>But though part of her lustre is due to her position, a part is her
-<span class="pagenum"><a name="Page_74" id="Page_74">[Pg 74]</a></span>
-own. Direct visual observation, as we remarked above, shows her
-intrinsic brightness to be more than five times that of Mercury,
-square mile to square mile of surface for the two. Now this has been
-determined very carefully photometrically by Müller at Potsdam. The
-result of his inquiry was to indicate that Mercury shines with 0.17 of
-absolute reflection, Venus with 0.92. So high a value has seemed to
-many astronomers impossible, because so far surpassing that which has
-tacitly been taken as the <i>ne plus ultra</i> of planetary brightness,
-that of cloud, 0.72.</p>
-
-<p>Now, one of the direct outcomes of the study of Venus at the Lowell
-Observatory was an explanation of this seemingly incredible phenomenon.
-When the planet came to be critically examined there under conditions
-of seeing which permitted discovery, markings very faint, but
-nevertheless assurable, stood presented on the planet’s face. These
-markings, of which we shall have more to say in a moment, had this of
-pertinency to our present point, that they kept an invariable position
-to one another. They thus betrayed themselves to be surface features.
-Furthermore, their dimness was as invariable an attribute of them as
-their place. They were not obscured on some occasions and revealed at
-others, but stayed, so far as one might judge, permanently the same.
-They were thus neither clouds themselves nor subject to the caprice of
-<span class="pagenum"><a name="Page_75" id="Page_75">[Pg 75]</a></span>
-cloud. The old idea that Venus was a cloud-wrapped planet and owed her
-splendor to this envelope, vanished literally into thin air.</p>
-
-<p>It is precisely because she is not cloud-covered that her lustre is so
-great. She “clothes herself with light as with a garment” by a physical
-process of some interest. As becomes the Mother of the Loves, this is
-gauze of the most attenuated character, and yet a wonderful heightener
-of effect. For it consists solely of the atmosphere that compasses her
-about. It is well known that a substance when comminuted reflects much
-more light than when condensed into a solid state. Now an atmosphere is
-itself such a comminuted affair, and, furthermore, holds in suspension
-a variety of dust. This would particularly be the case with the
-atmosphere of Venus, as we shall have reason to see when we consider
-the conditions upon that planet made evident by study of its surface
-markings. To her atmosphere, then, she owes four-fifths or more of her
-brilliancy. And this stands corroborated by the low albedo of both
-Mercury and the Moon, which have no atmosphere, and by the intermediate
-lustre of Mars, which has some, but little.<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a></p>
-
-<p>The rotation time of Venus, the determination, that is, of the planet’s
-day, is one of the fundamental astronomical acquisitions of recent
-<span class="pagenum"><a name="Page_76" id="Page_76">[Pg 76]</a></span>
-years. For upon it turns our whole knowledge of the planet’s physical
-condition. More than this, it adds something which must be reckoned
-with in the framing of any cosmogony. It is not a question of academic
-accuracy merely, of a little more or a little less in actual duration,
-but one which carries in its train a completely new outlook on Venus and
-sheds a valuable side-light upon the history of our whole planetary system.</p>
-
-<p>Unconsciously influenced, one is inclined to think, by terrestrial
-analogies, astronomers for more than a couple of centuries, ever since
-the time of the first Cassini in 1666, deemed the day of Venus to
-be just under twenty-four hours in length. So well attested was its
-determination, and so precisely figured to the minute, that it imposed
-itself upon text-books which stated it as an acquired fact down to the
-last second. Nevertheless, Schiaparelli was not so sure, and proceeded
-to look into the matter. He first looked for himself, and then looked
-up all the old observations. His chief observational departure was
-observing by day as near to noon as possible; because then the planet
-was highest, to say nothing of the taking off from its glare by the
-more brilliant sky. From certain dark markings around two bright spots
-near the southern cusp, of one of which spots the detection dates from
-<span class="pagenum"><a name="Page_77" id="Page_77">[Pg 77]</a></span>
-the time of Schroeter, and from a long, dark streak stretching thence
-well down the disk, he convinced himself that no such period as
-twenty-four hours could possibly be correct, inasmuch as whenever
-he looked, the markings were always there. His notes read, “Same
-appearance as yesterday,” day after day, until he would really have
-saved ink and penmanship had he had the phrase cut into a die and
-stamped. He concluded that the rotation was at least six months long,
-and was probably synchronous with the planet’s time of revolution. This
-was in 1889. In 1895 he became still more sure, and showed how the
-older observations were really compatible with what he had found.</p>
-
-<p>In 1896 the subject was taken up at Flagstaff. Very soon it became
-evident there that markings existed on the disk, most noticeable
-as fingerlike streaks pointing in from the terminator, faint but
-unmistakable from the identity of their successive presentation.
-Schroeter’s projection near the south cusp was also clearly discernible
-as well as two others, one in mid-terminator, one near the northern
-cusp. Schiaparelli’s dark markings also came out, developing into a
-sort of collar round the southern pole. Other spots and streaks also
-were discernible, and all proved permanent in place. By watching them
-assiduously it was possible to note that no change in position occurred
-in them, first through an interval of five hours, then through one
-of days, then of weeks. Care was taken to guard against illusion. It
-thus became evident that they bore always the same relation to the
-illuminated portion of the disk. This illuminated part, then, never
-changed. In other words, the planet turned always the same face to
-the Sun. The fact lay beyond a doubt, though of course not beyond a
-doubter.<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a></p>
-
-<p><span class="pagenum"><a name="Page_78" id="Page_78">[Pg 78]</a></span></p>
-
-<div class="figcenter">
-    <a name="I_078" id="I_078"> </a>
-    <img src="images/i_078.jpg" alt="" width="450" height="584" />
-<p class="center space-below2"><span class="smcap">Venus. October, 1896—March,
-                             1897—Drawings by Dr. Lowell.</span></p>
-</div>
-<p><span class="pagenum"><a name="Page_79" id="Page_79">[Pg 79]</a></span></p>
-<div class="figcenter">
-    <a name="I_079" id="I_079"> </a>
-    <img src="images/i_079.jpg" alt="" width="500" height="348" />
-<p class="center space-below2"><span class="smcap">Venus. April 12, 1909,
-                3h 26m—4h 22m—by Dr. Lowell.</span></p>
-</div>
-
-<p>The years that have passed since these observations were made have
-brought corroboration of them. Several observers at Flagstaff have
-seen and drawn them and added discoveries of their own, among whom are
-especially to be mentioned, of the observatory staff: Miss Leonard, Dr.
-Slipher, and Mr. E. C. Slipher.<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a></p>
-
-<p><span class="pagenum"><a name="Page_80" id="Page_80">[Pg 80]</a></span>
-In character these markings were peculiar and distinctive. In addition
-to some of more ordinary character were a set of spokelike streaks
-which started from the planet’s periphery and ran inwards to a point
-not very distant from the centre. The spokes started well-defined and
-broad at the edge, dwindling and growing fainter as they proceeded,
-requiring the best of definition for their following to their central hub.</p>
-
-<p>The peculiar symmetry thus displayed, a symmetry associated with the
-planet’s sunrise and sunset line, or, strictly speaking, what would be
-such did the Sun for Venus ever rise or set, would seem inexplicable,
-except for that very association. When we reflect, however, upon what
-this means, a very potent cause for them becomes apparent, so potent
-that surprise is turned into appreciation that nothing else could well
-exist. That Venus turns on her axis in the same time that she revolves
-about the Sun, in consequence of which she turns always the same face
-to him, must cause a state of things of which we can form but faint
-conception, from any earthly analogy. One face baked for countless
-æons, and still baking, backed by one chilled by everlasting night,
-while both are still surrounded by air, must produce indraughts from
-the cold to the hot side of tremendous power. A funnel-like rise must
-take place in the centre of the illuminated hemisphere, and the partial
-vacuum thus formed would be filled by air drawn from its periphery,
-which, in its turn, would draw from the regions of the night side.
-Such winds would sweep the surface as they entered, becoming less
-superficial as they advanced, and the marks of their inrush might well
-be discernible even at the distance we are off. Deltas of such inroad
-would thus seam the bounding circle of light and shade.
-<span class="pagenum"><a name="Page_81" id="Page_81">[Pg 81]</a></span></p>
-
-<div class="figcenter">
-    <a name="I_081A" id="I_081A"> </a>
-    <img src="images/i_081a.jpg" alt="" width="400" height="335" />
-   <p class="center space-below2"><big><b>I</b></big><br />Showing convection currents in the planet’s atmosphere.</p>
-    <a name="I_081B" id="I_081B"> </a>
-    <img src="images/i_081b.jpg" alt="" width="400" height="334" />
-   <p class="center space-below2"><big><b>II</b></big><br />
-      Showing shift in central barometric depression due<br />
-      to rotation of the planet affecting the winds.</p>
-   <p class="center space-below2"><span class="smcap">Venus.</span></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_82" id="Page_82">[Pg 82]</a></span>
-Another result of the aërial circulation would be the removal of all
-moisture from the sunward face, and its depositing in the form of
-ice upon the night one. For the heated air would be able to carry
-much water in suspension, which, on cooling, after it had reached the
-dark hemisphere would unload it there. In the low temperature there
-prevailing, this moisture would all be frozen, and so largely estopped
-from return. This process continuing for ages would finally deplete one
-side of all its water to heap it up in the form of ice upon the other.</p>
-
-<p>Now it is not a little odd that a phenomenon has been observed upon
-Venus which seems to display just this state of things. Many observers
-have noted an ashen light on the dark side of her disk. Some have
-tried to account for it as Earth shine, the same earth-reflected light
-that makes dimly visible the old moon in the new moon’s arms. But the
-Earth is too far away from Venus to permit of any such effect; nor is
-there any other body that could thus relieve its night. But if the
-night hemisphere of Venus be one vast polar sheet, we have there a
-substance able to mirror the stars to a ghostlike gleam which might be
-discernible even from our distant post.</p>
-
-<div class="figcenter">
-    <a name="I_082" id="I_082"> </a>
-    <p class="f150"><b><i>Venus</i></b></p>
-    <img src="images/i_082.jpg" alt="" width="600" height="243" />
-    <p class="f120 space-below2"><i>Rotation 225 days.</i></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_83" id="Page_83">[Pg 83]</a></span>
-Thus when we reason upon them we see that the peculiar markings of the
-planet lose their oddity, becoming the very pattern and prototype of
-what we should expect to view. Interpreted, they present us the picture
-of a plight more pitiable even than that of Mercury. For the nearly
-perfect circularity of Venus’ orbit prevents even that slight change
-from everlasting sameness which the libration of Mercury’s affords. To
-Venus the Sun stands substantially stock-still in the sky,—a fact which
-must prove highly reassuring to Ptolemaic astronomers there, if there
-be any still surviving from her past. No day, no seasons, practically
-no year, diversifies existence or records the flight of time. Monotony
-eternalized,—such is Venus’ lot.</p>
-
-<p>What visual observations have thus discovered of the rotation time of
-Venus, with all that follows from it, the spectroscope at Flagstaff has
-confirmed. At Dr. Slipher’s hands, spectrograms of the planet have told
-the same tale as the markings. It was with special reference to this
-point that the spectrograph there was constructed, and the first object
-to which it was directed was Venus.<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a></p>
-
-<p>The planet’s rotation time was to be investigated by means of the
-motion it brought about in the line of sight. Visual observation,
-<span class="pagenum"><a name="Page_84" id="Page_84">[Pg 84]</a></span>
-telescopically, reveals motion thwart-wise by the displacement it
-produces in the field of view; spectroscopic observation discloses
-motion to or from the observer by the shift it causes in the spectral
-lines due to a stretching or shortening of their wave-lengths.</p>
-
-<p>The spectroscope is an instrument for analyzing light. Ordinary light
-consists of light of various wave-lengths. By means of a prism or
-grating these are dispersed into a colored ribbon or band, the longer
-waves lying at the red end of the spectrum, as the ribbon is called,
-the shorter at the violet. Now the spectroscope is primarily such a
-prism or grating placed between the image and the observer, by means of
-which a series of colored images of the object are produced. In order
-that these may not overlap and so confuse one another, the light is
-allowed to enter the prism only through a narrow slit placed across the
-telescopic image of the object to be examined. Thus successive images
-of what is contained by the slit are presented arranged according to
-their wave-lengths. In practice the rays of light from the slit enter a
-small telescope called the collimator, and are there rendered parallel,
-in which condition they fall upon the prism. This spreads them out into
-the spectrum and another small telescope focusses them, each according
-to its kind, into a spectral image band which may then be viewed by the
-eye or caught upon a photographic plate.
-<span class="pagenum"><a name="Page_85" id="Page_85">[Pg 85]</a></span></p>
-
-<p>Now, if an object be coming toward the observer, emitting or reflecting
-light as it does so, each wave-length of its spectrum will be shortened
-in proportion to the relative speed of its approach as compared with
-the speed of light, because each new wave is given out by so much
-nearer the observer and in reflection the body may also meet it.
-Reversely it will be lengthened if the object be receding from the
-observer or he from it. This would change the color of the object were
-it not that while each hue moves into the place of the next, like the
-guests at Alice’s tea-party in Wonderland, some red rays pass off the
-visible spectrum, but new violet rays come up from the infraviolet
-and the spectrum is as complete as before. Fortunately, however, in
-all spectra are gaps where individual wave-lengths are absorbed or
-omitted, and these, the lines in the spectrum, tell the tale of shift.
-Now if a body be rotating, one side of it will be approaching the
-observer, while the opposite side is receding from him, and if the slit
-be placed perpendicular to the axis about which the spin takes place,
-each spectral line will appear not straight across the spectrum of the
-object, but skewed, the approaching side being tilted to the violet
-end, the receding side to the red.</p>
-
-<p>This was to be the procedure adopted for the rotation of Venus. By
-<span class="pagenum"><a name="Page_86" id="Page_86">[Pg 86]</a></span>
-placing the slit parallel to the ecliptic, or, more properly, to the
-orbit of Venus, which is practically the same thing, it found itself
-along what we have reason to suppose the equator of the planet. Even a
-considerable error on this point would make little difference in the
-rotational result. In order that there might be no question of illusion
-or personal bias, photographs instead of eye observations of the
-spectrum were made. For reference and check side by side with that of
-Venus were taken on either hand the spectra of iron, made by sparking a
-tube containing the vapor of that metal. The vapor, of course, had no
-motion with regard to the observer, and therefore its spectral lines
-could have no tilt, but must represent motional verticality.</p>
-
-<p>Dr. Slipher chose his time astutely. He selected the occasion when
-Venus was passing through superior conjunction, or the point in her
-orbit as regards us directly beyond the sun. At first sight this might
-seem to be the worst as well as the most impracticable of epochs,
-inasmuch as the planet is then not only at her farthest from the Earth,
-but in a line with the Sun, and so drowned in his glare. But in point
-of fact any tilt of the spectral lines is then, owing to phase, twice
-what it is at elongation, and exceeds still more what it is when Venus
-has her greatest lustre [<a href="#NOTE_3">see NOTE 3</a>]. In his purpose he was
-abetted by the Flagstaff air, which enabled the planet to be spectrographed much
-<span class="pagenum"><a name="Page_87" id="Page_87">[Pg 87]</a></span>
-nearer the sun than would otherwise have been the case. He thus
-selected the best possible opportunity. To guard against any subsequent
-bias on the part of the examiner of the plates, after the spectroscope
-had taken a plate it was then reversed, and the process repeated on
-another one, the iron being sparked as before. What had been the right
-side of Venus with regard to the red end of the spectrum thus became
-the left one, and <i>vice versa</i>. In this manner, when the plates came
-to be measured for tilt, the measurer would have no indication from
-the spectrum itself which way the lines might be expected to tilt; he
-could, therefore, not be influenced either consciously or unconsciously
-in his decision.</p>
-
-<div class="figcenter">
-    <a name="I_087" id="I_087"> </a>
-    <img src="images/i_087.jpg" alt="" width="600" height="67" />
-    <p class="center space-below2"><span class="smcap">Spectrogram of Venus, showing its long
-                     day—V. M. Slipher,<br />Lowell Observatory, 1903.</span></p>
-</div>
-
-<p>Eight plates with their comparison ferric spectra were thus secured;
-four with the spectroscope direct, four with it reversed. They
-were then shuffled, their numbers hidden, and given to Dr. Slipher
-to measure. The spectral lines told their own story, and without
-prompting. All the plates agreed within the margin of error accordant
-with their possible precision, a precision some thirty times that of
-Belopolski’s experiment on the same lines,—a result not derogatory of
-<span class="pagenum"><a name="Page_88" id="Page_88">[Pg 88]</a></span>
-that investigator, but merely illustrative of superior equipment. They
-showed conclusively that a rotation of anything like twenty-four hours
-was out of the question. They yielded, indeed, testimony to a negative
-rotation of three months, which, interpreted, means that so slow a spin
-as this was beyond their power to precise.</p>
-
-<p>For Dr. Slipher was at no less care to determine just what precision
-was possible in the case, although a speed corresponding to a spin
-of twenty-four hours on a globe the size of Venus is well known to
-be spectroscopically measurable. It would mean a motion toward us
-of one thousand miles an hour, or about a third of a mile a second.
-The tilt occasioned by this speed is well within the spectroscope’s
-ability to disclose. Not content with this, however, by two special
-investigations, he proved the spectroscope’s actual limits of
-performance to be far within the quantity concerned. One of them was
-the determination by the same means and in like manner of the rotation
-time of Mars, the length of that planet’s day, which in other ways we
-know to the hundredth of a second, and which is 24ʰ 37ᵐ 23.66ˢ Now Mars
-offers a test nearly twice as difficult as Venus, even supposing the
-apparent disks of the two the same, because his diameter being less in
-the proportion roughly of one-half, the actual speed of a particle at
-his edge is less for the same time of rotation in the like proportion,
-<span class="pagenum"><a name="Page_89" id="Page_89">[Pg 89]</a></span>
-and it is only with the speed in miles, not in angular amount, that the
-spectroscope is concerned. Nevertheless, when a like number of plates
-were tried on him, they indicated on measurement a rotation time within
-an hour of the true. This corresponds to half an hour on Venus. We see,
-therefore, that had Venus’ day been anywhere in the neighborhood of
-twenty-four hours, Dr. Slipher’s investigation would have disclosed it
-to within thirty-one minutes.</p>
-
-<div class="figcenter">
-    <a name="I_089" id="I_089"> </a>
-    <img src="images/i_089.jpg" alt="" width="600" height="118" />
-    <p class="center space-below2"><span class="smcap">Spectrogram of Jupiter,<br /> giving the length of its day
-            by the tilt of its spectral lines—<br />V. M. Slipher, Lowell Observatory.</span></p>
-</div>
-
-<p>This result was further borne out by a similar test made by him of
-Jupiter. Inasmuch as the diameter of Jupiter is twelve times that
-of Venus, while the rotation time is 9ʰ 50.4ᵐ at the equator,
-the precision attained on Venus should here have been about a
-minute. And this is what resulted. Slipher found the rotation time
-spectrographically 9ʰ 50ᵐ, or in accordance with the known facts,
-while previous determinations with the spectroscope had somehow fallen
-short of it.</p>
-
-<p>The care at Flagstaff with which the possibility of error was sought to
-be excluded in this investigation of the length of Venus’ day and the
-<span class="pagenum"><a name="Page_90" id="Page_90">[Pg 90]</a></span>
-concordant precision in the results are worthy of notice. For it is
-by thus being particular and systematic that the accuracy of the
-determinations made there, in other lines besides this, has been secured.</p>
-
-<p>In size, Venus of all the planets most nearly approaches the Earth.
-She is 7630 miles in diameter to the Earth’s 7918. Her density, too,
-is but just inferior to ours. And she stands next us in place, closest
-in condition and constitution in the primal nebula. Yet in her present
-state she could hardly be more diverse. This shows us how dangerous it
-is to dogmatize upon what can or cannot be, and how enlightening beyond
-expectation often is prolonged and systematic study of the facts.</p>
-
-<p>The next planet outward is our own abode. It is one of which most of
-us think we know considerable from experience and yet about which we
-often reason cosmically so ill. If we knew more, we should not deem
-ourselves nearly so unique. For we really differ from other members of
-our system not more than they do from one another. Much that appears
-to us fundamental is not so in fact. Thus many things which seem
-matters of course are merely accidents of size and position. Our very
-day and night upon which turn the habits of all animals and, even in a
-measure, those of plants, are, as we have seen, not the possession of
-our nearest of cosmic kin. Our seasons which both vegetally and vitally
-<span class="pagenum"><a name="Page_91" id="Page_91">[Pg 91]</a></span>
-mean so much are absent next door. And so the list of our globe’s
-peculiar attributes might be run through to the finding of diversity
-to our familiar ways at every turn. But, as we shall see later, these
-differences from one planet to the next are not only not incompatible
-with a certain oneness of the whole, but actually help to make the
-family relationship discoverable. Analogy alone is a dangerous guide,
-but analogy crossed with diversity is of all clews the most pregnant of
-understanding. The very fact that we can tell them apart when we see
-them together, as the Irishman remarked of two brothers he was in the
-habit of confusing, points to their brotherly relation.</p>
-
-<p>Proceeding still further, we come to Mars at a mean distance of one
-hundred and forty-one million miles. Smaller than ourselves, his
-diameter is but a little over half the Earth’s, or forty-two hundred
-miles, his mass one-ninth of ours, and his density about seven-tenths
-as much. Here, again, but in a different way, we find a planet unlike
-ourselves, and we know more about him than of any body outside the
-Earth and Moon. So much about him has been set forth elsewhere that
-it is enough to mention here that no oceans diversify his surface,
-no mountains relieve it, and but a thin air wraps it about,—an air
-containing water-vapor, but so clear that the surface itself is almost
-never veiled from view.
-<span class="pagenum"><a name="Page_92" id="Page_92">[Pg 92]</a></span></p>
-
-<p>About the satellites Mars possesses, Deimos and Phobos, we may perhaps
-say a word, as recent knowledge concerning them exemplifies the care
-now taken to such ascertainment and the importance of considering
-factors often overlooked. Soon after they were discovered in 1877, they
-were measured photometrically, with the result of giving a diameter
-of six miles to Deimos and one of seven miles to Phobos, and these
-values unchallenged entered the text-books. When the satellites came
-to be critically considered at Flagstaff, it was found that these
-determinations were markedly in error, Phobos being very much the
-larger of the two, the actual values reaching nearer ten miles for
-Deimos and thirty-six for Phobos.</p>
-
-<p>In getting the Flagstaff values, the size to the eye of the satellite
-was corrected for the background upon which it shone; for the
-background is all-important to the brilliancy of a star. In the case
-of a small star near a planet, the swamping glare of the planet is
-something like the inverse cube of its distance away. Furthermore, the
-Flagstaff observations indicated how the previous error had crept in.
-For before correction for the differing brilliancies of the field of
-view, the apparent size of the satellites judged by conspicuousness was
-about six to seven. The photometric values must have been taken just
-as they came out, no correction apparently having been made for the
-<span class="pagenum"><a name="Page_93" id="Page_93">[Pg 93]</a></span>
-background. Now the background is a fundamental factor in all
-photometric determinations, a factor somewhat too important in this
-case to neglect, since it affected the result 2500 per cent.</p>
-
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_94" id="Page_94">[Pg 94]</a></span></p>
-<div class="chapter">
-<h2 class="nobreak">CHAPTER IV<br /><span class="h_subtitle">THE OUTER PLANETS</span></h2>
-</div>
-
-<p class="drop-cap">BEYOND Mars lies the domain of the asteroids, a
-domain vast in extent, that, untenanted by any large planet, stretches
-out to Jupiter. Occupied solely by a host of little bodies agreeing
-only in lack of size, even this space seems too small to contain them,
-for recent research has shown some transgressing its bounds. One, Eros,
-discovered by De Witt, more than trenches on Mars’ territory, having
-an orbit smaller than that of the god of war, and may be considered
-perhaps the forerunner of more yet to be found between Mars and the
-Earth. On the other side, three recently detected by Max Wolf at
-Heidelberg have periods equal to that of Jupiter, and in their motions
-appear to exemplify an interesting case of celestial mechanics pointed
-out theoretically by Lagrange long before its corroboration in fact was
-so much as dreamt. Achilles, Patroclus, and Hector, as the triad are
-called, so move as always to keep their angular distance from Jupiter
-unaltered in their similar circuits of the Sun.
-<span class="pagenum"><a name="Page_95" id="Page_95">[Pg 95]</a></span></p>
-
-<div class="figcenter">
-    <a name="I_095" id="I_095"> </a>
-    <img src="images/i_095.jpg" alt="" width="600" height="468" />
-    <p class="center space-below2"><span class="smcap">Orbits of the Outer Planets.</span></p>
-</div>
-
-<p>Before considering these bodies individually, we may well look upon
-them <i>en bloc</i>, inasmuch as one attribute of the asteroids concerns
-them generically rather than specifically, and is of great interest
-both from a mechanical and an historical point of view. For, in fact,
-it is what led to their discovery. Titius of Wittenburg, about the
-middle of the eighteenth century, noticed a curious relation between
-the distances from the Sun of the then known planets. It consisted in a
-sort of regular progression, but with one significant gap. Bode was so
-<span class="pagenum"><a name="Page_96" id="Page_96">[Pg 96]</a></span>
-struck by the gap that he peopled it with a supposed planet, and so
-brought the relation into general regard in 1772. In consequence, it
-usually bears his name. It is this: if we take the geometrical series,
-3, 6, 12, 24, 48, 96 and add 4 to each term, we shall represent to
-a fair degree of precision the distances of the several planets,
-beginning with Mercury at 4 and ending with Saturn at 100, which was
-the outermost planet then known. All the terms were represented except
-24 + 4, or 28—a gap lying between Mars and Jupiter. When Uranus
-was discovered by Sir William Herschel in 1781 and was found to be
-travelling at what corresponded to the next outer term 192 + 4, or
-196, the opinion became quite general that the series represented a real law
-and that 28 must be occupied by a planet. Von Zach actually calculated
-what he called its analogical elements, and finally got up in 1800 a
-company to look for it which he jocularly described as his celestial
-police. Considering that Bode’s law is not a law at all, but a curious
-coincidence, as Gauss early showed in its lack of precision and in its
-failure to mark the place of Mercury with any approach to accuracy, and
-as the discovery of Neptune amply bore out, it was perhaps just in fate
-that the honor of filling the gap did not fall to any of the “celestial
-police,” but to an Italian astronomer, Piazzi, at the time engaged on
-a new star chart. An illness of Piazzi caused it to be lost almost as
-<span class="pagenum"><a name="Page_97" id="Page_97">[Pg 97]</a></span>
-soon as found. In this plight an appeal was made to the remarkable
-Gauss, just starting on his career. Gauss undertook the problem and
-devised formulæ by which its place was predicted and the planet itself
-recovered. It proved to fit admirably the gap. But it had hardly
-been recovered before another planet turned up equally filling the
-conditions. Ceres, the first, lay at 26.67 astronomical units from the
-Sun; Pallas, the second, at 27.72. Two claimants were one too many. But
-the inventive genius of Olbers came to the rescue. By a bold hypothesis
-he suggested that since two had appeared where only one was wanted,
-both must originally have formed parts of a single exploded planet. He
-predicted that others would be detected by watching the place where the
-explosion had occurred, to wit: where the orbits of Ceres and Pallas
-nearly intersected in the signs of the Virgin and the Whale.</p>
-
-<p>For in the case of an explosion the various parts, unless perturbed,
-must all return in time to the scene of the catastrophe. By following
-his precept, two more were in fact detected in the next two years, Juno
-and Vesta. His hypothesis seemed to be confirmed. No new planets were
-discovered, and the old fulfilled fairly what was required of them.
-Lagrange on calculation gave it his mathematical assent.</p>
-
-<p>Nevertheless, it was incorrect, as events eventually showed, though for
-<span class="pagenum"><a name="Page_98" id="Page_98">[Pg 98]</a></span>
-forty years it slept in peace, no new asteroids being found. We now
-know that this was because the rest were all much smaller, and for such
-nobody looked. It was not till 1845 that Hencke, an ex-postmaster of
-Driessen in Prussia, after fifteen years of search detected another,
-Astræa, of the 11th magnitude. After this discoveries of them came on
-apace, until now more than six hundred are known, and their real number
-seems to be legion. But those discovered are smaller each year on the
-average, showing that the larger have already been found. Their orbits
-are such that they cannot possibly ever have all formed part of a
-pristine whole. The idea, not the body, was exploded. For they are now
-recognized as having always been much as they are to-day.</p>
-
-<div class="figcenter">
-    <a name="I_098" id="I_098"> </a>
-    <p class="f150"><b>ASTEROIDS.</b></p>
-    <p class="f120"><i>MAJOR AXES OF ORBITS.</i></p>
-    <img src="images/i_098.jpg" alt="" width="600" height="238" />
-</div>
-
-<p class="space-above2"><span class="pagenum"><a name="Page_99" id="Page_99">[Pg 99]</a></span>
-They prove to be thickest at nearly the point where Bode’s law
-required, the spot where Ceres and Pallas were found. The mean of
-their distances is less, being 2.65 instead of 2.8 astronomical units,
-probably simply because the nearer ones are easier discovered. The
-fact that they are clustered most thickly just inside 2.8 astronomical
-units implies that there of all points within the space between Mars
-and Jupiter a planet would have formed if it could. A definite reason
-exists for its failure to do so—Jupiter’s disturbing presence.
-Throughout this whole region Jupiter’s influence is great; so great
-that his scattering effect upon the particles exceeds their own
-tendency to come together. We see this in the arrangement of the
-orbits. If we plot the orbits of the asteroids, we shall be struck by
-the emergence of certain blanks in the ribbon representing sections
-of their path. It is the woof of a plaid of Jupiter’s weaving. The
-gaps are where asteroids revolving about the Sun would have periods
-commensurate with his, ²/₅, ¹/₂, ³/₅, ⁴/₇, and the like. Such bodies
-would return after a few revolutions, five of theirs, for instance,
-to Jupiter’s two, into the same configurations with him at the
-same points of their orbits. Thus the same perturbation would be
-repeated over and over again until the asteroid’s path was so changed
-that commensurability ceased to exist. And it would be long before
-perturbation brought it back again. Thus the orbits are constantly
-swinging out and in, all of them within certain limits, but those
-are most disturbed which synchronize with his. In this manner he has
-fashioned their arrangement and even prevented any large planet from
-forming in the gap.</p>
-
-<p>Such restrictive action is not only at work to-day in the distribution
-of the asteroids and in the partitions of Saturn’s ring, but it must
-have operated still more in the past while the system was forming. To
-Professor Milham of Williamstown is due the brilliant suggestion that
-this was the force that fashioned the planetary orbits. For a planet
-<span class="pagenum"><a name="Page_100" id="Page_100">[Pg 100]</a></span>
-once given off from a central mass would exercise a prohibitive action
-upon any planet trying to form within. In certain places it would not
-allow it to collect at all. The evolution of the solar family would
-resemble that of some human ones in which each child brings up the next
-in turn. So that the planetary system made itself, as regards position,
-a steadily accumulative set of prohibitions combining to leave only
-certain places tenantable.</p>
-
-<p>In this manner we may perhaps be brought back to Bode’s law as
-representing within a certain degree of approximation a true mechanical
-result, although no such exact relation as the law demands exists.
-That a relation seemingly close to it is necessitated by the several
-successive inhibitions of each planet upon the next to form, is quite
-possible.</p>
-
-<p>One other general trait about their orbits is worth animadversion. In
-spite of being eccentric and inclined, they are all traversed in the
-same sense. Every one of the asteroids travels direct like the larger
-planets. In this they differ from cometary paths, which are as often
-retrograde as direct. Thus in more ways than one they hold a mid-course
-in regularity between the steady, even character of the planets proper
-and what was for long deemed the erratic behavior of the cometary class
-of cosmic bodies. Very telling this fact will be found with regard to
-the genesis of the solar family, as we shall see later.
-<span class="pagenum"><a name="Page_101" id="Page_101">[Pg 101]</a></span></p>
-
-<p>With regard now to their more individual characteristics, the asteroids
-may be said to agree in one point—their diversity, not only to all
-the larger members of the solar family, but to one another. For they
-travel in orbits ranging in ellipticity all the way from such as nearly
-approach circles to ellipses of cometary eccentricity. They voyage,
-too, without regard to the dynamical plane of the system, or, what is
-close to it, the ecliptic; departing from the general level often 30°
-and, in one instance, that of the little planet dubbed W. D., by as
-much as 48°. This eccentricity and inclination put them in a class by
-themselves. It is associated and unquestionably connected mechanically
-with another trait which likewise distinguishes them from the planets
-more particularly called—their diminutive size. Only four—Vesta,
-Ceres, Pallas, and Juno—out of the six hundred odd now known exceed
-a hundred miles in diameter, and the greater number are hardly over
-ten or twenty miles across. Very tiny worlds indeed they would seem,
-could we get near enough to them to discern their forms and features.
-Curiously enough, reasoning on certain light changes they exhibit has
-enabled us to divine something of their shapes, and even character.
-Thus it was soon perceived that Eros fluctuated in the light he sent
-us, being at times much brighter than at others. In February and March,
-1901, the changes were such that their maximum exceeded three times
-<span class="pagenum"><a name="Page_102" id="Page_102">[Pg 102]</a></span>
-their minimum two hours and a half later. Then in May the variation
-vanished. More than one explanation has been put forward, but the best
-so far, because the most simple, is that the body is not a sphere but
-a jagged mass, a mountain alone in space, and that as it turns upon
-its axis first one corner and then another is presented to our view or
-throws a shade upon its neighbor. When the pole directly faces us, no
-great change occurs, especially if it also nearly faces the Sun. Yet
-even this fails to explain all its vagaries.</p>
-
-<p>Eros is not alone in thus exhibiting variation. Sirona, Hertha, and
-Tercidina have also shown periodic variability, and it is suspected in
-others. Indeed, it would be surprising did they not show change. For
-they are too small to have drawn their contents into symmetry, and so
-remain as they were when launched in space. Mammoth meteorites they
-undoubtedly are.</p>
-
-<p>With the asteroids we leave the inner half of the Sun’s retinue and
-pass to the outer. Indeed, the asteroids not only mark in place the
-transition bound between the two, but stamp it such mechanically. In
-their own persons they witness that no large body was here allowed to
-form. The culmination of coalition was reached in Jupiter, and that
-very acme of accretion prevented through a long distance any other.</p>
-
-<div class="figright">
-    <a name="I_103" id="I_103"> </a>
-   <img src="images/i_103.jpg" alt="" width="250" height="225" />
-   <p class="center"><span class="smcap">Drawing of Jupiter by <br />Dr. Lowell. April 12, 1907.</span></p>
-</div>
-
-<p>In bulk, the major planets compared with the inner or terrestrial ones
-<span class="pagenum"><a name="Page_103" id="Page_103">[Pg 103]</a></span>
-form a class apart; and among the major Jupiter is by all odds first.
-His mass is 318 times the Earth’s and his volume nearly 1400 times
-hers. From this it appears that his density is very much less. Indeed,
-his substance is only fractionally denser than water. This and its
-tremendous spin, carrying a point at its equator two hundred and eighty
-thousand miles round in less than ten hours, flatten it to a very
-marked oval with an ellipticity of 1/15.5. Not the least beautiful of
-the revelations of astronomy are the geometrical shapes of the heavenly
-bodies, proceeding from nearly perfect spheres like the Sun or Moon to
-marked spheroids like Jupiter or Saturn. So enormous are the masses and
-the forces concerned that the forms assumed under them are mechanically
-regular. They are the visible expression of gravitation, and so delight
-the brain while they satisfy the eye.</p>
-
-<p>It is to appreciation of the detail visible on Jupiter’s disk that
-modern advance in the study of the planet is indebted. Examination has
-shown its features to be of great interest. To Mr. Stanley Williams of
-Brighton, England, much of our knowledge is due, and Mr. Scriven Bolton
-has also made some interesting contributions. The big print of the
-<span class="pagenum"><a name="Page_104" id="Page_104">[Pg 104]</a></span>
-subject, read long ago, is that the planet’s disk is noticeably banded
-by dark belts. Two characteristics of these belts are important. One
-is that they exhibit a regular secular progression with the lapse of
-years, the south tropical belt being broader and more salient for many
-years in succession, and then gradually fading out while the northern
-one increases in prominence. It has been suspected that the rhythm of
-their change is connected with that of sun spots. The second is that
-the belts do not preserve in their several features the same relation
-in longitude toward one another. They all rotate, but at different
-speeds. There could be no better proof that Jupiter is no solid, but
-a seething mass of heavy vapors boiling like a caldron. Tempered by
-distance we can form but a faint idea of the turmoil there going
-on. Further indication of it is furnished by its glow. For all the
-dark belts are a beautiful cherry red, a tint extending even to the
-darkish hoods over the planet’s caps. This hue comes out well in good
-seeing, and best, as with all planetary markings, in twilight, not at
-night, because the excessive brightness of the disk is then taken off,
-preventing the colors from being swamped.</p>
-
-<p>This brings us to the planet’s albedo, which Müller at Potsdam
-has found to be 75 per cent. Now the interest attaching to this
-determination is twofold, that it bespeaks cloud and that it seems to
-<span class="pagenum"><a name="Page_105" id="Page_105">[Pg 105]</a></span>
-imply something else. The albedo of cloud is 72 per cent of absolute
-whiteness. What looks like cloud, then, is such, on that distant disk.
-But Jupiter surpasses cloud in lustre, since his albedo exceeds 72
-per cent. Yet a large part of his surface is strikingly darker than
-that. The inference from this is that he shines by intrinsic light, in
-part at least. The fact may not be stated dogmatically, as there is
-no astronomic determination so uncertain as this one of determining
-albedoes, and therefore Herr Müller’s results must be accepted with
-every reserve, but they suggest that Jupiter is still a semi-sun, to be
-recognized as such by light as well as heat, though his self-luminosity,
-if it exist at all, can hardly exceed a dull red glow.</p>
-
-<div class="figcontainer">
-  <div class="figsub">
-   <a id="I_105" name="I_105"> </a>
-   <img src="images/i_105a.jpg" alt="" width="250" height="221" />
-   <p class="center">I.<br /><span class="smcap">Jupiter and its wisps.— A drawing<br />
-                     by Dr. Lowell, April 11, 1907.</span></p>
-  </div>
-  <div class="figsub">
-   <img  src="images/i_105b.jpg" alt="" width="260" height="222" />
-   <p class="center">II.<br /><span class="smcap">Jupiter and its wisps.—A drawing<br />
-                   by Dr. Lowell, April 11, 1907.</span></p>
-   </div>
-</div>
-
-<div class="figright">
-    <a name="I_107" id="I_107"> </a>
-   <p class="f150"><b>S.</b></p>
-   <img src="images/i_107.jpg" alt="" width="125" height="366" />
-   <p class="f150"><b>N.</b></p>
-   <p class="center"><span class="smcap">Photograph<br />of Jupiter,<br />1909.<br /> P. L.</span></p>
-</div>
-
-<p>A modern detection on Jupiter’s disk has been that of wisps or
-lacings across the bright equatorial belt, a detail of importance due to
-<span class="pagenum"><a name="Page_106" id="Page_106">[Pg 106]</a></span>
-Mr. Scriven Bolton. Requested to look for them, the observatory at
-Flagstaff was not long in corroborating this interesting phenomenon.
-The peculiarity about them pointed out by Mr. Bolton is that they
-traverse the belt at an angle of about 45° to the vertical, proceeding
-from caret-shaped dark spots projecting into the bright belt from the
-dark ones on either side. They exist all round the equator and are
-found indifferently dextrous or sinister—sometimes vertical. For there
-are others that go straight across. Nor are they confined to the bright
-equatorial belt, but are to be seen traversing all of the bright belts
-both north or south up to the polar hoods. From its sombreness it seems
-that we are here regarding a phenomenon in the negative; remarking it
-by what it has left behind, not by what it has accomplished. For the
-wisps are not wisps of cloud, since they are dark, not light, but gaps
-strung out in the clouds themselves.</p>
-
-<p>Recently photographs of Jupiter have been secured at Flagstaff, by the
-new methods there of planetary photography, showing a surprising amount
-of detail. The wisps come out with certainty, and the white spots,
-which are such a curious feature of the disk, have also left their
-impress on the plate. Not the least of the services thus rendered by
-the camera is the accurate positioning of the belts made possible by
-<span class="pagenum"><a name="Page_107" id="Page_107">[Pg 107]</a></span>
-it. Micrometric measures are all very well when nothing better is
-attainable, but any one who has made such upon a planet’s disk
-swinging like a lantern in the field of view under a variety of causes
-instrumental and optical, knows how encumbered they inevitably are
-with error. To have the disk caught and fixed on a plate where it
-may be measured at leisure and as often as one likes, is a distinct
-advance toward fundamental accuracy. Measures thus effected upon the
-Jupiter images of 1909 proved the bright equatorial belt to lie exactly
-upon the planet’s equator when allowance was made for the tilt of the
-planet’s axis toward the Earth. This showed that the aspect of the
-planet toward the Sun had no effect upon the position of the belt.
-Jupiter’s cloud formation, therefore, is not dependent, as all ours
-are, upon the solar heat.</p>
-
-<p>A like indifference to solar action is exhibited in the utter
-obliviousness of the belts to day or night. To them darkness and light
-are nugatory alike. They reappear round the sunrise edge of the disk
-just as they left it when they sank from sight round the sunset one,
-and they march across its sunlit face without so much as a flicker on
-their features.</p>
-
-<p><span class="pagenum"><a name="Page_108" id="Page_108">[Pg 108]</a></span>
-Yet this seeming immobility from moment to moment takes place in what
-is really a seething furnace, the fiery glow of which we catch below
-the vast ebullition of cloud in the cherry hue of its darker portions.
-Distance has merged the turmoil into the semblance of quiescence and
-left only its larger secular changes to show. Even so the Colorado
-River from the brink of the Grand Cañon is seen apparently at rest, the
-billows of its rapids so stereotyped to stability one takes the rippled
-sand bank for the river and the billows of the river for the ripple
-marks of its banks.</p>
-
-<p>At twice the distance of Jupiter we cross the orbit of Saturn. Here the
-ringed planet, with an annual sweep of twenty-nine and a half of our
-years, pursues his majestic circuit of the Sun. Diademed with three or
-more circlets of light and diamonded by ten satellites, he rivals in
-his cortège that of his own lord. In some ways his surpasses the Sun’s.
-For certainly his retinue is the more spectacular of the two; the more
-so that it is much of it fairly comprised within a single glance. Very
-impressive Saturn is as, attended thus, he sails into the field of view.</p>
-
-<div class="figcenter">
-    <a name="I_108" id="I_108"> </a>
-    <img src="images/i_108.jpg" alt="" width="600" height="253" />
-    <p class="center"><span class="smcap">Saturn—A drawing by Dr. Lowell,<br />
-               showing agglomerations.</span></p>
-</div>
-
-<p class="space-above2"><span class="pagenum"><a name="Page_109" id="Page_109">[Pg 109]</a></span>
-In our survey we may best begin with his globe. If Jupiter’s
-compression is striking, Saturn’s is positively startling when well
-displayed. This happens but at rare intervals. As the plane of his
-equator is almost exactly that of the rings, the flattening is
-conspicuous only on those occasions when the rings disappear because
-their plane passes through the line of sight. Seen at such times the
-effect of the discrowned orb is so strange as to suggest delusion.
-This occurred two years ago in 1907, and when the planet was picked up
-by its position and entered the field unheralded by its distinctive
-appendage, it was almost impossible to believe there had not been
-some mistake and a caricatured Jupiter had taken its place. For the
-flattening outdoes that of Jupiter as 3 to 2, being ⅒ of the equatorial
-diameter. Such a bulging almost suggests disruption and is due to
-the extreme lightness of the planet’s substance, which is actually
-only 0.72 of that of water. Like Jupiter, the disk exhibits belts,
-though very much fainter, and, like his, these are of a cherry red. As
-the planet’s albedo is even greater, 0.78 of absolute whiteness, as
-deduced from H. Struve’s measures of the diameter, the same suspicion
-of shining, at least in part, from inherent light, applies equally
-to him. But it is practically certain that in neither case does this
-light equal that of the planet’s clouds, or add anything to them. Both
-planets are red-hot, not white-hot. The determination of the albedo
-depends upon that of the diameter, and an increase in the latter would
-lower the albedo to that of cloud.</p>
-
-<p>His most unique possession are his rings. Broad, yet tenuous, they
-weigh next to nothing, being, as Struve has dubbed them, “Immaterial
-<span class="pagenum"><a name="Page_110" id="Page_110">[Pg 110]</a></span>
-light.” Nevertheless, it is not their lightness but their make-up that
-prevents from lying uneasy the head that wears this crown.</p>
-
-<p>The mechanical marvel was not appreciated by early astronomers, who
-took it for granted that they were what they seemed, solid, flat rings,
-all of a piece. Even Laplace considered it sufficient to divide them up
-concentrically to insure stability. To Edouard Roche of Montpellier,
-as retiringly modest as he was penetratingly profound, is due the
-mathematical detection that to subsist they must be composed of
-discrete particles,—brickbats, Clerk Maxwell called them, when, later,
-unaware of Roche’s work, he proved independently the same thing in his
-essay on Saturn’s rings. Peirce, too, in ignorance of Roche, had half
-taken the same step a little before, showing that they must at least
-be fluid. Then in 1895 Keeler ingeniously photographed the spectrum of
-both ball and rings to the revealing of velocities in the line of sight
-of the different portions of the spectrum exactly agreeing with the
-values mechanics demanded.</p>
-
-<p>The rings have usually been considered to be flat. At the time of
-their disappearance, however, knots have been seen upon them. It is
-as if their filament had suddenly been strung with beads. At the last
-occurrence of the sort in 1907, these beads were particularly well seen
-at several observatories, and were critically studied at Flagstaff. In
-<span class="pagenum"><a name="Page_111" id="Page_111">[Pg 111]</a></span>
-connection with a new phenomenon detected there, that of a dark core
-in the shadow the rings threw across the planet’s face, an explanation
-suggested itself to account for both them and it: to wit, that the
-rings were not really flat, but tores; rings, that is, like an anchor
-ring, any cross-section of which would be of the nature of an oval
-flattened on its inner side. The cogency of the explanation consisted
-in its solution not only of the appearances but of the cause competent
-to bring those appearances about.</p>
-
-<p>For measurement showed that the knots were permanent in position,
-which, since the ring revolved, indicated that they extended all round
-it in spite of their not seeming to do so, and that their distances
-from Saturn were just what this cause should produce.</p>
-
-<p>The action observed was a corollary from the important principle
-of commensurability of orbital period. As we saw in the case of
-the asteroids, if two bodies be travelling round a third and their
-respective periods of revolution be commensurate, they will constantly
-meet one another in such a manner that great perturbation will ensue
-and the bodies be thrown out of commensurability of period.</p>
-
-<p>What has happened to the asteroids has likewise occurred in Saturn’s
-rings. The disturber in this case has been, not Jupiter, as with them,
-<span class="pagenum"><a name="Page_112" id="Page_112">[Pg 112]</a></span>
-but one or other of Saturn’s own satellites. For when we calculate
-the problem, we find that Mimas, Enceladus, and Tethys have periods
-exactly commensurate with the divisions of the rings; in other words,
-these three inner satellites, whose action because of proximity is the
-greatest, have fashioned the rings into the three parts we know, called
-A, the outermost; B, the middle one; and C, the crêpe ring, nearest to
-the body of the planet. Mimas has been the chief actor, though helped
-by the two others, while Enceladus has further subdivided ring A by
-what is known as Encke’s division.</p>
-
-<p>Such has been the chief action of the satellites on the rings: it
-has made them into the system we see. But if we consider the matter,
-we shall realize that a secondary result must have ensued—when we
-remember that the particles composing the rings must be very crowded
-for the rings to show as bright as they do, and also that, though
-relatively thin, the rings are nevertheless some eighty miles through.</p>
-
-<p>Now it is evident that any disturbance in so closely packed a system
-of small bodies as that constituting Saturn’s rings must result in
-collisions between the bodies concerned. Particles pulled out or in
-must come in contact with others pursuing their own paths, and as at
-each collision some energy is lost by the blow, a general falling in
-toward the planet results. At the same time, as the blow will not
-<span class="pagenum"><a name="Page_113" id="Page_113">[Pg 113]</a></span>
-usually be exactly in the plane in which either particle was previously
-moving, both will be thrown more or less out of the general plane of
-their fellows, and the ring at that point, even if originally flat,
-will not remain so. For the ring, though very narrow relatively, has a
-real thickness, quite sufficient for slantwise collision, if the bodies
-impinge.</p>
-
-<div class="figcenter">
-    <a name="I_113" id="I_113"> </a>
-    <img src="images/i_113.jpg" alt="" width="400" height="508" />
-    <p class="f150"><i>Saturn’s Rings.</i></p>
-    <p class="center space-below2"><i>November 1907.</i></p>
-</div>
-
-<p>Now the knots or beads on the rings appeared exactly inside the points
-where the satellites’ disturbing action is greatest, or, in other
-words, in precisely their theoretic place. We can hardly doubt that
-such, then, was their origin.<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a></p>
-
-<p>The result must be gradually to force the particles as a rule nearer
-the planet, until they fall upon its surface, while a few are forced
-out to where they may coalesce into a satellite,—a result foreseen
-long ago by Maxwell. It is this process which in the knots we are
-actually witnessing take place, and which, like the corona about the
-<span class="pagenum"><a name="Page_114" id="Page_114">[Pg 114]</a></span>
-eclipsed Sun, only comes out to view when the obliterating brightness
-of the main body of the rings is withdrawn by their edgewise
-presentation.</p>
-
-<p>The reason the out-of-plane particles are most numerous just inside the
-point of disturbance is not only that there the action throwing them
-out is most violent, but that all the time a levelling action quite
-apart from disturbance is all the time tending to reduce them again to
-one plane, as we shall see further on when we come to the mechanical
-forces at work. Thus the tore is most pronounced on its outer edge, and
-falls to a uniform level at its inner boundary. The effect is somewhat
-as represented in the adjoining cut, in which the vertical scale is
-greatly magnified:—</p>
-
-<div class="figcenter">
-    <a name="I_114" id="I_114"> </a>
-    <img src="images/i_114.jpg" alt="" width="600" height="199" />
-    <p class="center space-below2"><span class="smcap">The Tores of Saturn.</span>&emsp;Not
-                     drawn to scale.</p>
-</div>
-
-<p>With Saturn ended the bounds of the solar system as known to the
-civilized world until 1781. On March 13 of that year Sir William
-Herschel in one of his telescopic voyages through space came upon a
-strange object which he at once saw was not a star, because of its very
-perceptible round disk, and which he therefore took for a peculiar kind
-<span class="pagenum"><a name="Page_115" id="Page_115">[Pg 115]</a></span>
-of comet. Nearly a year rolled by before Lexell showed by calculation
-of its motion that it was no comet, but undoubtedly a new planet beyond
-Saturn travelling at almost twice that body’s mean distance from the Sun.</p>
-
-<p>By reckoning backward, it was found to have been seen and mapped
-several times as a star,—no less than twelve times by Lemonnier
-alone,—and yet its planetary character had slipped through his
-fingers. It can even be seen with the naked eye as a star of the 6th
-magnitude, and its course is said to have been watched by savage tribes
-in Polynesia long before Sir William Herschel discovered it.</p>
-
-<p>Its greenish blue disk indicates that it is about thirty-two thousand
-miles in diameter, and its mass that its density is about 0.22 of
-the Earth’s or, like Jupiter’s, somewhat greater than water. Of its
-surface we probably see nothing. Indeed, it is very doubtful if it
-have any surface properly so called, being but a ball of vapors. Its
-flattening, ¹/₁₁ according to Schiaparelli, which is probably the best
-determination, agrees with the density given above, indicating its
-substance to be very light. Belts have faintly been descried traversing
-its disk after the analogy of Jupiter and Saturn. These would be much
-better known than they are but for the great tilt of the planet’s axis
-to the ecliptic, so that during a part of its immense annual sweep its
-<span class="pagenum"><a name="Page_116" id="Page_116">[Pg 116]</a></span>
-poles are pointed nearly at the Earth, and its tropical features, the
-places where the belts lie, are wholly hidden or greatly foreshortened
-from our point of view. As the planet’s year is eighty-four of our
-years long, it is only at intervals of forty odd years that the disk is
-well enough displayed to bring the belts into observable position.</p>
-
-<p>The planet is attended by four satellites,—Ariel, Umbriel, Titania,
-and Oberon,—a midsummer night’s dream to a watcher of the skies. They
-travel in a plane inclined 98° to the ecliptic, so that their motion is
-nearly up and down to that plane and even a little backward. Whether
-their plane is also the equatorial plane of the planet, we do not know
-for certain. The observations as yet are not conclusive one way or the
-other. If the two planes should turn out not to coincide, it will open
-up some new fields in celestial mechanics. The belts have been thought
-to indicate divergence, but the most recent observations by Perrotin on
-them minimize this. They suggest, too, a rotation period of about ten
-hours, which is what we should expect.</p>
-
-<p>Its albedo, or intrinsic brightness, is, according to Müller, 0.73,
-or almost exactly that of cloud. This tallies with the lack of
-pronouncement of the belts and is another argument against the reality
-of the recent diametral measurements, as all Müller’s values are got by
-dividing the amount of light received by the amount of surface sending
-<span class="pagenum"><a name="Page_117" id="Page_117">[Pg 117]</a></span>
-it. If the diameter were much less than thirty-two thousand miles, the
-resulting albedo would become impossibly high.</p>
-
-<p>If we know but little about the actual surface of Uranus, we know now a
-good deal about its atmosphere. And this partly because atmosphere is
-almost all that it is. The satellites are the only solid thing in the
-system. If we needed a telltale that the solar system had evolved, the
-gaseous constitution of its primaries and the condensed state of their
-attendants would sufficiently inform us. Probably all the major planets
-are nothing but gas. It has been debated whether Jupiter be almost all
-vapor with a solid kernel beneath, or vapor entirely. That he grows
-denser toward the core is doubtless the case, but that he is anywhere
-other than a gaseous fluid is very unlikely. For if he had really
-begun to condense, he must have contracted to far within his present
-dimensions. The same is true of Uranus.</p>
-
-<p>The surprising thing about Uranus is the enormous extent of his
-atmosphere. The earliest spectroscopists perceived this, but the more
-spectroscopy advances, the greater and more interesting it proves to
-be. By pushing inquiry into the red end of the spectrum, hitherto a
-terra incognita, Dr. Slipher has uncovered a mass of as yet unexplained
-revelation. Of these remarkable spectrograms we shall speak later. Here
-<span class="pagenum"><a name="Page_118" id="Page_118">[Pg 118]</a></span>
-it is sufficient to say that so great is the absorption in the red that
-only the blue and green in anything like their entirety get through;
-which accounts for the well-known sea-green look of the planet.
-Furthermore, the spectroscope shows that this atmosphere, or the great
-bulk of it, must lie above what we see as the contour of the disk.
-For the spectroscope is as incapable of seeing through opacity as the
-eye, though it distances the eye in seeing the invisible. It is not
-what is condensed into cloud, but what is not, of which it reveals the
-presence. We are thus made aware of a great shell of air enveloping the
-planet.</p>
-
-<p>In Uranus, then, we see a body in an early amorphous state, before the
-solid, the liquid, and the gaseous conditions of matter have become
-differentiate and settled each into distinctive place. Without even an
-embryo core its substance passes from viscosity to cloud.</p>
-
-<p>Neptune has proved a planet of surprises. Though its orbital revolution
-is performed direct, its rotation apparently takes place backward, in a
-plane tilted about 35° to its orbital course. Its satellite certainly
-travels in this retrograde manner. Then its appearance is unexpectedly
-bright, while its spectrum shows bands which as yet, for the most part,
-defy explanation, though they state positively the vast amount of its
-atmosphere and its very peculiar constitution. But first and not least
-of its surprises was its discovery,—a set of surprises, in fact. For
-<span class="pagenum"><a name="Page_119" id="Page_119">[Pg 119]</a></span>
-after owing recognition to one of the most brilliant mathematical
-triumphs, it turned out not to be the planet expected.</p>
-
-<p>“Neptune is much nearer the Sun than it ought to be,” is the
-authoritative way in which a popular historian puts the intruding
-planet in its place. For the planet failed to justify theory by not
-fulfilling Bode’s law, which Leverrier and Adams, in pointing out the
-disturber of Uranus, assumed “as they could do no otherwise.” Though
-not strictly correct, as not only did both geometers do otherwise, but
-neither did otherwise enough, the quotation may serve to bring Bode’s
-law into court, as it was at the bottom of one of the strangest and
-most generally misunderstood chapters in celestial mechanics.</p>
-
-<p>Very soon after Uranus was recognized as a planet, approximate
-ephemerides of its motion resulted in showing that it had several times
-previously been recorded as a fixed star. Bode himself discovered the
-first of these records, one by Mayer in 1756, and Bode and others
-found another made by Flamstead in 1690. These observations enabled an
-elliptic orbit to be calculated which satisfied them all. Subsequently
-others were detected. Lemonnier discovered that he had himself not
-discovered it several times, cataloguing it as a fixed star. Flamstead
-was spared a like mortification by being dead. For both these observers
-<span class="pagenum"><a name="Page_120" id="Page_120">[Pg 120]</a></span>
-had recorded it two or more nights running, from which it would seem
-almost incredible not to have suspected its character from its change
-of place.</p>
-
-<p>Sixteen of these pre-discovery observations were found (there are now
-nineteen known), which with those made upon it since gave a series
-running back a hundred and thirty years, when Alexis Bouvard prepared
-his tables of the planet, the best up to that time, published in 1821.
-In doing so, however, he stated that he had been unable to find any
-orbit which would satisfy both the new and the old observations. He
-therefore rejected the old as untrustworthy, forgetting that they had
-been satisfied thirty years before, and based his tables solely on
-the new, leaving it to posterity, he said, to decide whether the old
-observations were faulty or whether some unknown influence had acted
-on the planet. He had hardly made this invidious distinction against
-the accuracy of the ancient observers when his own tables began to be
-out and grew seriously more so, so that within eleven years they quite
-failed to represent the planet.</p>
-
-<p>The discrepancies between theory and observation attracted the
-attention of the astronomic world, and the idea of another planet
-began to be in the air. The great Bessel was the first to state
-definitely his conviction in a popular lecture at Königsberg in 1840,
-and thereupon encouraged his talented assistant Flemming to begin
-<span class="pagenum"><a name="Page_121" id="Page_121">[Pg 121]</a></span>
-reductions looking to its locating. Unfortunately, in the midst of his
-labors Flemming died, and shortly after Bessel himself, who had taken
-up the matter after Flemming’s death.</p>
-
-<p>Somewhat later Arago, then head of the Paris observatory, who had also
-been impressed with the existence of such a planet, requested one of
-his assistants, a remarkable young mathematician named Leverrier, to
-undertake its investigation. Leverrier, who had already evidenced
-his marked ability in celestial mechanics, proceeded to grapple with
-the problem in the most thorough manner. He began by looking into
-the perturbations of Uranus by Jupiter and Saturn. He started with
-Bouvard’s work, with the result of finding it very much the reverse
-of good. The farther he went, the more errors he found, until he was
-obliged to cast it aside entirely and recompute these perturbations
-himself. The catalogue of Bouvard’s errors he gave must have been an
-eye-opener generally, and it speaks for the ability and precision
-with which Leverrier conducted his investigation that neither Airy,
-Bessel, nor Adams had detected these errors, with the exception of
-one term noticed by Bessel and subsequently by Adams.<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a>
-The result of this recalculation of his was to show the more clearly that
-the irregularities in the motion of Uranus could not be explained except by
-the existence of another planet exterior to him. He next set himself to
-<span class="pagenum"><a name="Page_122" id="Page_122">[Pg 122]</a></span>
-locate this body. Influenced by Bode’s law, he began by assuming it
-to lie at twice Uranus’ distance from the Sun, and, expressing the
-observed discrepancies in longitude in equations, comprising the
-perturbations and possible errors in the elements of Uranus, proceeded
-to solve them. He could get no rational solution. He then gave the
-distance and the extreme observations a certain elasticity, and by this
-means was able to find a position for the disturber which sufficiently
-satisfied the conditions of the problem. Leverrier’s first memoir
-on the subject was presented to the French Academy on November 10,
-1845, that giving the place of the disturbing planet on June 1, 1846.
-There is no evidence that the slightest search in consequence was
-made by anybody, with the possible exception of the Naval Observatory
-at Washington. On August 31 he presented his third paper, giving an
-orbit, mass, and more precise place for the unknown. Still no search
-followed. Taking advantage of the acknowledging of a memoir, Leverrier,
-in September, wrote to Dr. Galle in Berlin asking him to look for
-the planet. The letter reached Galle on the 23d, and that very night
-he found a planet showing a disk just as Leverrier had foretold, and
-within 55′ of its predicted place.</p>
-
-<p>The planet had scarcely been found when, on October 1, a letter from
-Sir John Herschel appeared in the <i>London Athenæum</i> announcing that a
-<span class="pagenum"><a name="Page_123" id="Page_123">[Pg 123]</a></span>
-young Cambridge graduate, Mr. J. C. Adams, had been engaged on the
-same investigation as Leverrier, and with similar results. This was
-the first public announcement of Mr. Adams’ labors. It then appeared
-that he had started as early as 1843, and had communicated his
-results to Airy in October, 1845, a year before. Into the sad set of
-circumstances which prevented the brilliant young mathematician from
-reaping the fruit of what might have been his discovery, we need not
-go. It reflected no credit on any one concerned except Adams, who
-throughout his life maintained a dignified silence. Suffice it to say
-that Adams had found a place for the unknown within a few degrees of
-Leverrier’s; that he had communicated these results to Airy; that Airy
-had not considered them significant until Leverrier had published an
-almost identical place; that then Challis, the head of the Cambridge
-Observatory, had set to work to search for the planet but so routinely
-that he had actually mapped it several times without finding that he
-had done so, when word arrived of its discovery by Galle.</p>
-
-<p>But now came an even more interesting chapter in this whole
-strange story. Mr. Walker at Washington and Dr. Petersen of Altona
-independently came to the conclusion from a provisional circular orbit
-for the newcomer that Lalande had catalogued in the vicinity of its
-path. They therefore set to work to find out if any Lalande stars were
-<span class="pagenum"><a name="Page_124" id="Page_124">[Pg 124]</a></span>
-missing. Dr. Petersen compared a chart directly with the heavens to
-the finding a star absent, which his calculations showed was about
-where Neptune should have been at the time. Walker found that Lalande
-could only have swept in the neighborhood of Neptune on the 8th and
-10th of May, 1795. By assuming different eccentricities for Neptune’s
-orbit under two hypotheses for the place of its perihelion, he found
-a star catalogued on the latter date which sufficiently satisfied his
-computations. He predicted that on searching the sky this star would be
-found missing. On the next fine evening Professor Hubbard looked for
-it, and the star was gone. It had been Neptune.<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a></p>
-
-<p>This discovery enabled elliptic elements to be computed for it, when
-the surprising fact appeared that it was not moving in anything
-approaching the orbit either Leverrier or Adams had assigned. Instead
-of a mean distance of 36 astronomical units or more, the stranger was
-only at 30. The result so disconcerted Leverrier that he declared that
-“the small eccentricity which appeared to result from Mr. Walker’s
-computations would be incompatible with the nature of the perturbations
-of the planet Herschel,” as he called Uranus. In other words, he
-expressly denied that Neptune was his planet. For the newcomer
-<span class="pagenum"><a name="Page_125" id="Page_125">[Pg 125]</a></span>
-proceeded to follow the path Walker had computed. This was strikingly
-confirmed by Mauvais’ discovering that Lalande had observed the star on
-the 8th of May as well as on the 10th, but because the two places did
-not agree, he had rejected the first observation, and marked the second
-as doubtful, thus carefully avoiding a discovery that actually knocked
-at his door.</p>
-
-<p>Meanwhile Peirce had made a remarkable contribution to the whole
-subject. In a series of profound papers presented to the American
-Academy, he went into the matter more generally than either of the
-discoverers, to the startling conclusion “that the planet Neptune
-is not the planet to which geometrical analysis had directed the
-telescope, and that its discovery by Galle must be regarded as a happy
-accident.”<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a> He proved this first by showing that Leverrier’s two
-fundamental propositions,—</p>
-
-<div class="blockquot">
-<p>1. That the disturber’s mean distance must be between 35 and 37.9
-astronomical units;</p>
-
-<p>2. That its mean longitude for January 1, 1800, must have been
-between 243° and 252°,—</p>
-</div>
-
-<p class="no-indent">were incompatible with Neptune. Either alone might be reconciled with
-the observations, but not both.</p>
-
-<p>In justification of his assertion that the discovery was a happy
-accident, he showed that three solutions of the problem Leverrier had
-set himself were possible, all equally complete and decidedly different
-<span class="pagenum"><a name="Page_126" id="Page_126">[Pg 126]</a></span>
-from each other, the positions of the supposed planet being 120° apart.
-Had Leverrier and Adams fallen upon either of the other two, Neptune
-would not have been discovered.<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a></p>
-
-<p>He next showed that at 35.3 astronomical units, an important change
-takes place in the character of the perturbations because of the
-commensurability of period of a planet revolving there with that of
-Uranus. In consequence of which, a planet inside of this limit might
-equally account for the observed perturbations with the one outside
-of it supposed by Leverrier. This Neptune actually did. From not
-considering wide enough limits, Leverrier had found one solution,
-Neptune fulfilled the other.<a name="FNanchor_14_14" id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a>
-And Bode’s law was responsible for this. Had Bode’s law not been taken
-originally as basis for the disturber’s distance, those two great
-geometers, Leverrier and Adams, might have looked inside.</p>
-
-<p>This more general solution, as Peirce was careful to state, does not
-detract from the honor due either to Leverrier or to Adams. Their
-masterly calculations, the difficulty of which no one who has not
-had some experience of the subject can appreciate, remain as an
-imperishable monument to both, as does also Peirce’s to him.</p>
-
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_127" id="Page_127">[Pg 127]</a></span></p>
-<div class="chapter">
-<h2 class="nobreak">CHAPTER V<br /><span class="h_subtitle">FORMATION OF PLANETS</span></h2>
-</div>
-
-<p class="drop-cap">IN our first two chapters we saw what sign-posts in
-the sky there are pointing to the course evolution of a solar system
-probably follows, and secondly, what evidence there is that our system
-took this road. We now come to a question not so easy to precise,—the
-actual details of the journey. It is always difficult to descend from a
-glittering panoramic survey to particular path-finding. The obstacles
-loom so much larger on a near approach.</p>
-
-<p>Most men shy at decisions and shun self-committal to any positive
-course, but when it comes to constructing a cosmogony, few at all
-qualified hesitate to frame one if the old does not suit. The safety
-in so doing lies in the fact that nothing in particular happens if
-it refuses to work. Its absurdity is promptly shown up, it is true,
-by some one else. For there is almost as good a trade in exposing
-cosmogonies as in constructing them. But no special opprobrium attaches
-to failure, because everybody has failed, from Laplace down, or up, as
-you are pleased to consider it. Besides it is really not so easy to do,
-<span class="pagenum"><a name="Page_128" id="Page_128">[Pg 128]</a></span>
-as one is tempted to believe before his book is published. Then
-only does the difficulty dawn, with a speed and clarity inversely
-proportional to the previous relation of the critic to the author. For
-the author himself is apt to be blind. With the fatal fondness of a
-parent for his offspring it is rare for the defects to be so glaringly
-apparent to their perpetrator. At the worst he considers them venial
-faults which can be glossed away.</p>
-
-<p>Attacking the subject in this judicial spirit, the reader can hardly
-expect me to satisfy him with a cosmogony entirely home-made, but
-at best to pursue a happy middle course between creator and critic,
-advocating only such portions as happen to be my own, while sternly
-exposing the mistakes of others.</p>
-
-<p>In undertaking the hazardous climb toward the origin of things two
-qualities are necessary in the explorer: a quick eye for possibilities
-and a steady head in testing them. Without the discernment to perceive
-relations no ascent to first principles is possible; and without the
-support of quantitative criterion, one is in danger of becoming giddy
-from one’s own imagination. Congruities must first hint at a path;
-physical laws then determine its feasibility.</p>
-
-<p>An eye for congruities is the first essential. For congruity alone
-accuses an underlying law. It is the analogic that with logic leads to
-great generalizations. Certain concords of the sort in the motions of
-<span class="pagenum"><a name="Page_129" id="Page_129">[Pg 129]</a></span>
-the planets were what suggested to Laplace his system of the world.
-With the uncommon sense of a mathematician he perceived that such
-accordances were not necessitated by the law of gravitation, and on the
-other hand, could not be due to chance. The laws of probability showed
-millions to one against it. One of these happy harmonies was that all
-the large planets revolved about the Sun in substantially the same
-plane; another that they all travelled in the same sense (direction).
-Had they been unrelated bodies at the start, such agreement in motion
-was mathematically impossible. Their present consensus implied a common
-origin for all. In other words, the solar system must have grown to be
-what it is, not started so.</p>
-
-<p>This basic fact we may consider certain. But from it we would fain
-go on to find out how it evolved. To do so the same process must be
-followed. Considering, then, our solar system from this point of view,
-one cannot but be struck by some further congruities it presents. These
-are not quite those that inspired Laplace, because of discoveries
-since, and demand in consequence a theory different from his.</p>
-
-<p>The out about constructing a theory is that fresh facts will come
-along and knock for admission after the door is shut. They prove
-irreconcilables because they were not consulted in advance. The
-consequence is that since Laplace’s time new relations have come to
-<span class="pagenum"><a name="Page_130" id="Page_130">[Pg 130]</a></span>
-light, and some supposed concords have had to be given up; so that were
-he alive to-day he would himself have formulated some other scheme.
-Two, however, are still as true: that the planets all revolve in the
-same plane and in the same sense, and that sense that of the Sun’s
-rotation. But so general a congruity as this points only to an original
-common moment of momentum and is equally explicable however that motion
-was brought about. It seems quite compatible with an original shock. To
-say that it was caused by a disruption is simply to go one step farther
-back than Laplace. If, then, such a catastrophe did occur as the
-meteorites aver, we may perhaps draw some interesting inferences about
-it from the present state of the system. In a very close approach such
-as we must suppose for the disruption, one within Roches’ limit of 2.5
-diameters, the stranger, supposing him of equal size, would sweep from
-one side of the former Sun to the other in about two hours, and the
-brunt of the disrupting pull occur within that time. That the former
-Sun was rotating slowly seems established by the time, twenty-eight
-days, it now takes to go round. In which case the orbits of the
-masses which were to form the planets would all lie in about the same
-plane,—the plane of the tramp’s approach. If there were exceptions,
-they should be found in the innermost. For such should partake most
-<span class="pagenum"><a name="Page_131" id="Page_131">[Pg 131]</a></span>
-largely of the Sun’s own original rotation and travel therefore most
-nearly in its plane. And as a fact Mercury, the Benjamin, does differ
-from the others by revolving in a plane inclined some 7° to their mean,
-agreeing in this with the Sun’s own rotation, with whose plane it was
-probably originally coincident (digression from it now being due to
-secular retrogression of the planets’ nodes) [<a href="#NOTE_4">see NOTE 4</a>].</p>
-
-<p>From the relations which advance has left unchanged we pass to those
-phenomena which seemed to present congruities in Laplace’s day,
-but which have since proved void owing to subsequent detection of
-exceptions. Time prevents my making the catalogue complete, but the
-reader shall be shown enough to satisfy him of the problem’s complexity
-and to whet his desire for further research—on the part, preferably,
-of others.</p>
-
-<div class="figcenter">
-    <a name="I_131" id="I_131"> </a>
-    <img src="images/i_131.jpg" alt="" width="600" height="102" />
-    <p class="center space-below2"><span class="smcap">Chart showing increasing tilts of the major planets.</span></p>
-</div>
-
-<p>First comes, then, the rotations of the planets upon their axes, which
-Laplace supposed to be all in the same direction, counter to the hands
-of a clock; for the heavens mark time oppositely from us. All those
-within and including Saturn, the only ones he knew, turn, indeed, in
-the same sense that they travel round the Sun. But Uranus departs from
-that direction by a right angle, wallowing rather than spinning in his
-<span class="pagenum"><a name="Page_132" id="Page_132">[Pg 132]</a></span>
-orbit; while Neptune goes still farther in idiosyncratic departure
-and actually turns in the opposite direction. Here, then, Laplace’s
-congruity breaks down, but in its place a little attention will show
-that a new one has arisen. For Saturn’s tilt is 27° and Jupiter’s 3°,
-so that with the major planets there is revealed a systematic righting
-of the planetary axes from inversion through perpendicularity to
-directness as one proceeds inward toward the Sun.</p>
-
-<p>Another congruity supposed to exist a century ago was the exemplary
-agreement of all the satellites to follow in their planetary circuits
-the pattern set them by their primaries round the Sun. But as man has
-penetrated farther into space and photographic plates have come to be
-employed, satellites have been revealed which depart from this orderly
-arrangement. This is the case with the ninth, the outermost, satellite
-of Saturn and with the eighth, the outermost, of Jupiter. But, as
-before, the breaking down of one congruity seems but the establishing
-of another. It appears that only the most distant satellites are
-permitted such unconformity of demeanor. For departure from the
-supposed orthodoxy occurs in both instances where the distance is most,
-and does not occur in the case of all the other satellites found since
-Laplace’s day, eleven in number, nearer their planets.
-<span class="pagenum"><a name="Page_133" id="Page_133">[Pg 133]</a></span></p>
-
-<p>A third congruity formerly believed in has suffered a like fate; to
-wit, that satellites always moved in or near the equatorial plane of
-their primary. All those first discovered did; the four large ones of
-Jupiter, the main ones of Saturn, and probably those of Uranus and
-Neptune. Even the satellites of Mars conformed. Iapetus alone seemed to
-make exception, and that by a glossable amount. But this orderliness,
-too, has been disposed of, only, like the others, to experience a
-resurrection in a different form.</p>
-
-<div class="figcenter">
-    <a name="I_133" id="I_133"> </a>
-    <img src="images/i_133.jpg" alt="" width="600" height="458" />
-</div>
-
-<p>On examining more precisely the inclinations of these orbits some years
-ago, an interesting relation between them and the distances of the
-satellites from their primaries forced itself on my notice. The tilt
-<span class="pagenum"><a name="Page_134" id="Page_134">[Pg 134]</a></span>
-increased as the distance grew. The only exceptions were very tiny
-bodies occupying a sort of asteroidal relation to the rest.</p>
-
-<p>A diagram will make this clear. The kernel of it dates from the
-lectures then delivered before the Massachusetts Institute of
-Technology in 1901. The interesting thing now about it is that the
-congruity there pointed out has been conformed to by every satellite
-discovered since,—the sixth, seventh, and eighth of Jupiter and the
-ninth and tenth of Saturn. It is evident that we already know enough of
-the geniture of our system to prophesy something about it and have the
-prophecy come true.</p>
-
-<p>Closely connected with the previous relation is a fourth concordance
-clearly of mechanical origin, the relation of the orbital
-eccentricities of the satellites to their distances from their
-respective planets. The satellites pursue more and more eccentric
-orbits according as they stand removed from planetary proximity.</p>
-
-<p>A fifth congruity is no less striking. All the satellites of all the
-planets that we can observe well enough to judge of turn the same face
-always to their lords. That the Moon does so to the Earth is a fact of
-everyday knowledge, and the telescope hints that the same respectful
-regard is paid by Jupiter’s and Saturn’s retinues to them. What is
-still more remarkable, Mercury and Venus turn out to observe the like
-<span class="pagenum"><a name="Page_135" id="Page_135">[Pg 135]</a></span>
-vassal etiquette with reference to the Sun. And it will be noticed that
-they stand to him the nearest of his court. Here, then, is a law of
-proximity which points conclusively to some well-established force.</p>
-
-<p>Last is a remarkable congruity which study disclosed to me likewise
-some years ago, and which has received corroboration in discoveries
-since. This congruity is the peculiar arrangement of the masses in the
-solar system.</p>
-
-<p>Consider first the way in which the several planets, as respects size,
-stand ordered in distance from the sun. Nearest to him is Mercury, the
-smallest of all the principal ones. Venus and the Earth follow, each
-larger than the last; then comes Mars, of distinctly less bulk, and
-so to the asteroids, of almost none. After this the mass rises again
-to its maximum in Jupiter, and then subsequently falls through Saturn
-to Uranus and Neptune. Here we mark a more or less regular gradation
-between mass and position, a curve in which there are two ups and
-downs, the outer swell being much the larger, though the inner, too, is
-sufficiently pronounced.</p>
-
-<p>Now turn to Saturn and his family, which is the most numerous of the
-secondary systems and that having the greatest span. Under Saturn’s
-wing, as it were, is the ring, itself a congeries of tiny satellites.
-Then comes Mimas, the smallest of the principal ones; then Enceladus, a
-<span class="pagenum"><a name="Page_136" id="Page_136">[Pg 136]</a></span>
-little larger; then Tethys, the biggest of the three. Next stands
-Dione, smaller than Tethys. Then the mass increases with Rhea, reaching
-its culmination in Titan, after which it declines once more. Strangely
-reproductive this of the curve we marked in the arrangement of the
-planets themselves, even to the little inner rise and fall.</p>
-
-<div class="figcenter">
-    <a name="I_136" id="I_136"> </a>
-    <img src="images/i_136.jpg" alt="" width="600" height="354" />
-    <p class="center space-below2"><span class="smcap">Masses of planets and satellites.</span></p>
-</div>
-
-<p>Striking as such analogous ordering is, it is not all. For, scanning
-the Jovian system, we find the main curve here again; Ganymede, the
-Jupiter or Titan of the system, standing in the same medial position
-as they. Lastly, taking up Uranus and his family of satellites, the same
-order is observable there. Titania, the largest, is posted in the centre.</p>
-
-<p>Thus the order in which the little and the big are placed with
-<span class="pagenum"><a name="Page_137" id="Page_137">[Pg 137]</a></span>
-reference to their controlling orb is the same in the solar system
-and in that of every one of its satellite families. Method here is
-unmistakable. Nor is it easy to explain unless the cause in all was
-like. That the rule in the placing of the planets should be faithfully
-observed by them in the ordering of their own domestic retinues, is
-not the least strange feature of the arrangement. It argues a common
-principle for both. Not less significant is the secondary hump in
-their distribution, denoting recrudescence farther in of the primary
-procedure shown without.</p>
-
-<p>One point to be particularly noticed in these latter-day congruities
-is that they are not simply general concords like the older ones—the
-fact that the planets move in one plane or in the same sense in that
-plane—but detailed placings, ordered according to the distances of
-the planets from the Sun or of the satellites from the planets. They
-are thus not simply of the combinative but of the permutative order
-of probabilities, a much higher one; in other words, the chance that
-they can be due to chance is multiplicately small. Thus just as these
-analogies are by so much more remarkable, so are they by so much more
-cogent. They tell us not only of an evolution, but they speak of the
-very manner of its work. They do not simply generalize, they specify
-the mode of action. The difficulty is to understand their language. It
-is a case of celestial hieroglyphics to which we lack the key.
-<span class="pagenum"><a name="Page_138" id="Page_138">[Pg 138]</a></span></p>
-
-<p>In attempting now to discover how all this came about we notice first
-that the system could not have originated in the beautifully simple way
-suggested by Laplace, because of several impossibilities in the path.
-If rings were shed, as he supposed, from a symmetric contracting mass,
-they should have resulted in something even more symmetric than we
-observe to-day. In the next place they could not, it would appear, even
-if formed, have collected into planets.</p>
-
-<p>Nor could there have been an original “fire-mist” with which as a
-stock in trade Laplace thriftily endowed his nebula to start with—the
-necessity for which has been likened to our supposed descent from
-monkeys; but which in truth is as misty a conception of the facts in
-the one case as it is a monkeying with them in the other. Darwin’s
-theory distinctly avers that we were <i>not</i> descended from monkeys;
-and Laplace’s fire-mist under modern examination evaporates away.
-It is an interesting outcome of modern analysis that the very fact
-which suggested the annular genesis of planets to Laplace, the rings
-of Saturn, should now probably be deemed a striking instance of the
-reverse. Far from its being an exemplar in the heavens of the pristine
-state of the solar system, we may now see in it a shining pattern of
-how the devolution of bodies comes about. For instead of typifying an
-unfortunate set of particles which untoward circumstance has prevented
-<span class="pagenum"><a name="Page_139" id="Page_139">[Pg 139]</a></span>
-from coalescing into a single orb, it almost certainly represents the
-distraught state to which a once more compact congeries of them has
-been brought by planetary interference. For to just such fate must
-the stresses in it caused by Saturn have eventually led. Disruption
-inevitable to such a group the observation of comets demonstrates is
-daily taking place. When a comet passes round the Sun or near a planet,
-the partitive pulls of the body tend to dismember it, and the same is
-<i>a fortiori</i> true of matter circulating round a planet as relatively
-near as the meteoric particles that constitute Saturn’s rings. Starting
-as a congeries, it was pulled out more and more into a ring until it
-became practically even throughout. And the very action that produced
-it tends to keep it as surprisingly regular as we note to-day.</p>
-
-<p>No, the planets probably were otherwise generated and may have looked
-in their earlier stages as the knots in the spiral nebulæ do to-day.
-But this does not mean that we can detail the process [<a href="#NOTE_5">see NOTE 5</a>].</p>
-
-<p>Taking now the congruities for guide, we proceed to see what they
-affirm or negative. Laplace, when he ventured on his exposition of the
-system of the world, did so “with the mistrust which everything which
-is not the direct outcome of observation or calculation must inspire.”
-To all who know how even figures can lie this caution will seem well
-timed. The best we can do to keep our heads steady is to lay firm hold
-<span class="pagenum"><a name="Page_140" id="Page_140">[Pg 140]</a></span>
-at each step on the great underlying principles of physics. One of
-these is the conservation of the moment of momentum. This expression
-embodies one of the grandest generalizations of cosmic mechanics. The
-very phrase is fittingly sonorous, with something of that religious
-sublimity which the dear old lady said she found such a consolation in
-the biblical word Mesopotamia. Indeed the idea is grand for its very
-simplicity. Momentum means the quantity of motion in a body. It is the
-speed into the number of particles or the mass. Moment of momentum
-denotes the rotatory power of it round an axis. Now the curious and
-interesting thing about this quantity is that it can neither be
-diminished nor increased. It is an abstraction from which nothing
-can be abstracted—but results. It is the one unalterable thing in a
-universe of change. What it was in the beginning in a system, that it
-forever remains. Because of this unchangeableness we can use it very
-effectively for purposes of deduction. One of these is in connection
-with that other great principle of physics, the conservation of energy.
-By the mutual action of particles on one another, by contraction,
-by tidal pulls, and so on, some energy of motion is constantly
-being changed into heat and thus dissipated away. Energy of motion,
-therefore, is slowly being lost to the system, and the only stable
-state for the bodies composing it is when their energy of motion has
-<span class="pagenum"><a name="Page_141" id="Page_141">[Pg 141]</a></span>
-decreased to the minimum consistent with the initial moment of
-momentum. This principle we shall find very fecund in its application.
-It means that our whole system is evolving in a way to lessen its
-energy of motion while keeping its quantity of motion unchanged. The
-universe always does a thing with the least possible expenditure of
-force and gets rid of its superfluous energy by parting with it to
-space. Philosophers may wrangle over its being the best possible of
-worlds, but it is incontrovertibly mechanically the laziest, which a
-pessimistic friend of mine says proves it the best.</p>
-
-<p>Now this generalization finds immediate use in explaining certain
-features of the solar system. In looking over the congruities it will
-be seen that deviation from the principal plane of the system or
-departure from a circular orbit is always associated with smallness
-in size. The insignificant bodies are the erratic ones. Now it has
-been shown mathematically in several different ways that when small
-particles collect into a larger mass, the collisions tend to make
-the resultant orbit of the combination both more circular and more
-conformant to the general plane than its constituents. But we may see
-this more forthrightly by means of the general principle enunciated
-above. For in fact both results are direct outcomes of the conservation
-of moment of momentum. Given a certain moment of momentum for the
-<span class="pagenum"><a name="Page_142" id="Page_142">[Pg 142]</a></span>
-system, the total energy of the bodies is least when they all move in
-one plane. This is evident at once because the components of motion
-at right angles to the principal plane add nothing to the moment
-of momentum of the system. It is also least when the bodies all
-revolve in circles about the centre of gravity. The circle has some
-interesting properties which almost justify the regard paid to it by
-the ancients as the only perfect figure. It encloses the maximum area
-for a given periphery, so that according to the old legends, if one
-were given as much land as he could enclose with a certain bull’s
-hide, he should, after cutting the hide into strips, arrange these
-along the circumference of a circle. Now this property of the circle
-is intimately connected with the fact that a body revolving in a
-circle has the greatest moment of momentum for the least expenditure
-of energy. For under the same central force all ellipses of the same
-longest diameters—major axes these are technically called—are
-described in the same time, and with the same energy, and of all such,
-the circle encloses the greatest area, which area measures the moment
-of momentum [<a href="#NOTE_6">see NOTE 6</a>].</p>
-
-<p>Given a certain moment of momentum, then the energy is least when the
-bodies all move in one plane and all travel in circles in that plane.
-As energy is constantly being dissipated while any alteration among
-the bodies is going on, to coplanarity and circularity of path all the
-<span class="pagenum"><a name="Page_143" id="Page_143">[Pg 143]</a></span>
-bodies must tend, if by collision they be aggregated into larger
-masses. As in the present state of our system the small bodies travel
-out of the general plane in eccentric ellipses while the big ones
-travel in it in approximate circles, the facts indicate that the origin
-of the larger masses was due to development by aggregation out of
-smaller particles.</p>
-
-<p>The next principle is of a different character. Half a century ago
-celestial mechanics dealt with bodies chiefly as points. The Earth was
-treated as a weighted point, and so was the Sun. This was possible
-because a sphere acts upon outside bodies as if all its mass were
-collected at its centre, and the Sun and many of the planets are
-practically spheres. But when it came to nicer questions of their
-present behavior and especially of their past career, it grew necessary
-to take their shape into account in their mutual effects. One of the
-results was the discovery of the great rôle played in evolution by
-tidal action. Inasmuch as the planets are not perfectly rigid bodies,
-each is subject to tidal deformation by the other, the outside being
-pulled more than the centre on one side and less on the other. Bodily
-tides are thus raised in it analogous to the surface tides we see in
-the ocean, only vastly greater, and these in turn act as a brake on its
-rotation.</p>
-
-<p>Now the retrograde motions occurring in the outermost parts of all
-the systems, principal and subsidiary, only and always there: the
-<span class="pagenum"><a name="Page_144" id="Page_144">[Pg 144]</a></span>
-retrograde rotations of Neptune and Uranus, the retrograde revolutions
-of the ninth satellite of Saturn and of the eighth of Jupiter, point to
-something fundamental. For when we consider that it is precisely in its
-outer portions that any forces shaping the development of the system
-have had less time to produce their effect, we perceive that apparent
-abnormality now is really survival of the original normal state, only
-to be found at present in what has not been sufficiently forced to
-change. It suggests that the pristine motion of the constituents of the
-scattered agglomerations which went to form the planets was retrograde,
-and that their present direct rotations and the direct revolutions of
-most of their satellites have been imposed by some force acting since.
-Let us inquire if there be a force competent to this end, and what its
-mode of action.</p>
-
-<p>Let us see how tidal action would work. Tidal force would raise bulges,
-and these, not being carried round with the planet’s rotation except to
-a certain distance, due to viscosity, must necessarily act as brakes
-upon the planet’s spin. In consequence of the friction they would thus
-exert, energy of motion must be lost. So long, then, as tidal forces
-can come into play, the energy of the system is capable of decrease.
-According to the last principle we considered, the system cannot be in
-stable equilibrium until this superfluous energy is lost or until tidal
-<span class="pagenum"><a name="Page_145" id="Page_145">[Pg 145]</a></span>
-forces become inoperative, which cannot be till all the bodies in the
-system turn the same face to their respective centres of attraction.</p>
-
-<p>To see this more clearly, take the case of a retrograde spin of a
-planet as compared with a direct one. The energy of the planet’s spin
-is the same in both cases, because energy depends on the square of a
-quantity; to wit, that of the velocity, and is therefore independent
-of sign. Not so the moment of momentum. For this depends on the first
-power of the speed, and if positive in the one case, must be negative
-in the other. The moment of momentum of the whole system, then, is less
-in the former case, since the moment of momentum of the retrograde
-rotation must be subtracted from, that of the direct rotation be added
-to, that of the rest of the system. For a given initial moment of
-momentum with which the system was endowed at the start, there is,
-then, superfluous energy in the first state which can be got rid of
-through reduction to the second. Nature, according to her principles
-of least exertion, avails herself of the chance of dispensing with it,
-and a direct rotation results. Sir Robert Ball first suggested this
-argument.</p>
-
-<p>Tidal action accomplishes the end. In checking up a body rotating
-contrary to the general consensus of spin, its first effect is to
-start to turn the axis over. For the body is in dynamical unstable
-equilibrium with regard to the rest of the system. The righting would
-<span class="pagenum"><a name="Page_146" id="Page_146">[Pg 146]</a></span>
-continue, practically to the exclusion of any diminution at first of
-the spin, until the body had turned over in its plane so that the spin
-became direct. As the force increases greatly with nearness to the
-Sun, the effect would be most marked on the nearer, and most so on the
-biggest, bodies. This would account for the otherwise strange gradation
-from retrograde to direct in the tilts of the axes of the outer
-planets, and also for the present tilts of all the inner ones.</p>
-
-<p>Related to the initial retrograde rotations of the planets, and in a
-sense survivals from an earlier state of things, are two of the latest
-discoveries of motions in the solar system, the retrograde orbital
-movements of the ninth satellite of Saturn and the eighth of Jupiter.
-Considered so anomalous as scarcely at first to be believed, it has
-been stated that they directly contradict the theory of Laplace.
-This is true; in the same sense and no more in which they directly
-contradict the contradictor, one of the latest theories. For neither
-theory has anything to explain them as the result of law. That they
-cannot be the sport of indifferent chance seems evidenced by their
-occupying similar external positions in their respective systems. As
-the product of a law we must regard them, and to find that law we now
-turn. Suppose the planet originally to have been rotating backward, or
-in the direction of the hands of a clock. At this time the satellite,
-<span class="pagenum"><a name="Page_147" id="Page_147">[Pg 147]</a></span>
-which may never have formed a part of its mass, was travelling backward
-too, according to what we have said. Then under the friction of
-the tides raised on the planet by the Sun, the planet proceeded to
-turn over. It continued to do so until it spun direct. During this
-process there was no passage through zero of its moment of momentum
-<i>considered with regard to itself</i>, and therefore no difficulty on that
-score of supposing that it successively generated satellites at all
-degrees of inclination. That its children are of the nature of adopted
-waifs, Babinet’s criterion (1861) would seem to imply. But it must
-be remembered that the Sun has been slowing up the planet’s rotation
-now for æons. As it turned over, its tidal bulges tended to carry
-over with it such satellites as it already had. This effect was much
-greater on the nearer ones, both because they were nearer and because
-they were much larger than the outer. So that the nearer kept with the
-planet, the others lagged proportionately behind. This suggests itself
-to account for the facts, but the subject involves so much that is
-uncertain that I submit the hypothesis with the distrust which Laplace
-has so eminently bespoken. I advance in its favor only the three
-striking facts: that a steady progression in their tilts of rotation is
-observable from Neptune to Jupiter and a substantially accordant one
-from Mars to Mercury; secondly, that the satellites turn their faces to
-<span class="pagenum"><a name="Page_148" id="Page_148">[Pg 148]</a></span>
-their primaries, as likewise do Mercury and Venus to the Sun; and,
-thirdly, that the orbits of the satellites of all the planets are
-themselves tilted in accordance with what it would require [<a href="#NOTE_7">see NOTE 7</a>].</p>
-
-<p>After the axial spins have been made over to the same sense, the second
-consequence of tidal action in the case of two bodies revolving about
-their common centre of gravity is to slow down both spins until first
-the smaller and then the larger turn the same face to each other and
-remain thus constant ever after. Now such is precisely the pass to
-which we observe the satellites of the planets have come. All that we
-can be sure of now turn the same face always to their primary. The Moon
-was the first to betray her attitude, because the one we can best note.
-On scrutiny, however, Jupiter’s satellites, so far as we can make out,
-do the like; and Saturn’s, too. And a very proper attitude it is, this
-regard paid to compelling attraction. Thus one of the congruities we
-noticed stands accounted for. The satellites could hardly have been at
-first so observant; time has brought about this unfailing recognition
-of their lords.</p>
-
-<p>Of the peculiar massing of the bodies in the family of the Sun, and
-the still stranger copying of it in their own domestic circles, little
-can as yet be said in interpretation. That the planetary families and
-their ancestral group should agree is not the least strange part of the
-<span class="pagenum"><a name="Page_149" id="Page_149">[Pg 149]</a></span>
-affair. It shows that none of them was fortuitous, but that at the
-formation of all some common principle presided, apportioning the
-aggregations to their proper place. But it is such fine print of the
-system’s history as at present to preclude discernment.</p>
-
-<p>So much for the details we may deduce of the method of our birth. We
-perceive unmistakably that our solar system grew to be what it is,
-and that it developed by agglomeration of its previously shattered
-fragments into the planets we behold to-day, but exactly how the
-process progressed we are as yet unable to precise. We are, however,
-as what I have mentioned and tabled show, every day accumulating data
-which will enable an eventual determination probably to be reached.</p>
-
-<p>From the fact of agglomeration, the essence of the affair, we turn to
-the traces it has left upon its several offspring.</p>
-
-<p>Just as the continued existence to-day of meteorites <i>in statu quo</i>
-informs us of a previous body from which our nebula sprang; so a
-physical characteristic of our own earth at the present time shows it
-to have evolved from that nebula—even though we cannot make out all
-the steps. Of its having done so, we are far more sure than of how it
-did.</p>
-
-<p>That primitive man perceived that somewhere below him was a fiery
-region which was not an agreeable abode, is plain from his consigning
-to such Tophet those whose religious tenets did not square with his own.
-<span class="pagenum"><a name="Page_150" id="Page_150">[Pg 150]</a></span>
-That his conception of it was not strictly scientific is evidenced by
-his not realizing that to bury his enemies was the way to make them
-take the first step of the journey thither. Indeed, the vindictive
-venting of his notions clearly indicates their source as volcanic,
-rather than bred of a general disapproval of a downward descent either
-in silicates or sin.</p>
-
-<p>It was not till man began to bore into the Earth for metallic or
-potable purposes that he brought to light the generic fact that it
-was everywhere hotter as one went down. And this not only in a very
-regular, but in a most speedy, manner. The temperature increased in a
-really surprising way 1° F. for every sixty-five feet of descent. As
-the rise continued unabated to the limit of his borings, becoming very
-unpleasant at its end, it was clear that at a depth of thirty-five
-miles even so refractory a substance as platinum must melt, and
-practically all the Earth except a thin crust be molten or even gaseous.</p>
-
-<p>Now heat, like money, is easy to dissipate but hard to acquire, as
-primitive man was the first to realize. It does not come without
-cause. Being a mode of motion, other motion must have preceded it from
-which it sprang. So much the doctrine of the conservation of energy
-teaches us, a doctrine considered now to have been the great scientific
-heirloom of the nineteenth century to the twentieth, yet which in its
-<span class="pagenum"><a name="Page_151" id="Page_151">[Pg 151]</a></span>
-day caused the death of its first discoverer, Mayer, of a broken heart
-from non-recognition; its second, Helmholtz, was refused publication by
-the leading Berlin physical magazine of the time. So quick is man to
-delay his own advance.</p>
-
-<p>The only conceivable motion for thus heating the Earth as a whole was
-the falling together of its parts. The present heat of the Earth,
-then, accuses the concourse of particles in the past to its formation,
-or in other words proves that the Earth was evolved out of material
-originally more sparcely strewn. It does so not only in a generic but
-in a most particular manner, for the heat is distributed just where it
-would be by such a process. It is greater to-day within, increasingly,
-because when the globe began to cool, the surface necessarily cooled
-first and established a regular gradient of heat from core to cuticle.</p>
-
-<p>It is possible to test this qualitative inference quantitatively and
-see if the falling together of the meteorites was equal to the task.
-Knowing the mechanical equivalent of heat, what we do is to calculate
-the quantity of motion involved and then evaluate it in heat. As we are
-unaware of the exact law of density of the Earth, and are ignorant of
-how much was radiated away in the process, the problem is a little like
-estimating the fortune of a man when we do not know the stocks in which
-<span class="pagenum"><a name="Page_152" id="Page_152">[Pg 152]</a></span>
-he has invested, and ignore how much he has spent the while. We only
-know what he would have been worth had he followed our advice in the
-matter of investments and lived as frugally as we recommended. For
-here, too, we are obliged to make certain assumptions. Nevertheless
-the figure obtained in the case of the planets’ stores of heat is so
-enormous as to leave a most ample margin for dissipation. Had the Earth
-contracted from a fairly generous expansion to its present state under
-the probable law of density suggested by Laplace in another connection,
-the heat developed would have been enough to raise the whole globe to
-160,000° F. if of iron, 90,000° F. if of stone. As 10,000° F. would
-have sufficed for the Earth to have kept up its past, to say nothing of
-its present, state, we are justified of our deduction.</p>
-
-<p>Nor is the Earth the only body in the system which thus argues itself
-evolved by the falling together of its present constituents. In the
-larger planets Jupiter and Saturn we seem to see the heat, far as we
-are away. For the cherry hue they disclose between their brighter belts
-proves to come from greater absorption there of the green and blue rays
-of the spectrum, indicating a greater depth of atmosphere traversed.
-Thus these parts lie at a lower level, and their ruddy hue is just what
-they should show were they still glowing with a dull red heat.</p>
-
-<div class="figcenter">
-    <a name="I_152" id="I_152"> </a>
-    <img src="images/i_152.jpg" alt="" width="600" height="242" />
-    <p class="center"><span class="smcap">Spectrogram of Jupiter, Moon Comparison.</span></p>
-    <p class="center space-below2"><span class="smcap">Lowell Observatory. &emsp;
-                  V. M. Slipher.</span></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_153" id="Page_153">[Pg 153]</a></span>
-Heat is not only the end of the beginning, it is the beginning of the
-end as well. It is both the result of the evolving of definite bodies
-out of the agglomeration of matter-strewn space, and the cause of the
-higher evolution of those globes themselves. For the acquisition of
-heat is the necessary preface to all that follows. Heat is a body’s
-evolutionary capital whose wise expenditure through cooling down makes
-all further advance to higher products possible. A body too small
-to have acquired it must remain forever lifeless, as dead as the meteorites
-themselves that enter our air as mere inert bits of stone or iron.</p>
-
-<p>Curiously enough, heat both must have been and then must have been
-lost. Like the loss of fortune or of friends sometimes in the ennobling
-of character, it is through its passing away that its effects are
-realized. For in cooling down from a once heated condition, that train
-of events occurs which we most commonly particularize as evolution.
-So far in our survey the march of advance has been through masses of
-matter, a molar evolution; from this point on it passes into its minute
-constituents and becomes a molecular one. The one is the necessary
-prelude to the other. Up to this great turning-point in the history of
-each member of a solar system we have been busied with the acquisition
-of heat, though we may not have been aware of it the while. All
-the motions we have studied tended to that end. During these three
-<span class="pagenum"><a name="Page_154" id="Page_154">[Pg 154]</a></span>
-chapters, I, II, V, we have been gradually rising in our point of view
-until we stand at the temperature pinnacle of the whole process. In the
-next three we are to descend upon the other side. The slope we have
-come up was of necessity barren; the one we are to go down brings us to
-verdure and the haunts of men. Coming from the causes above, we reach
-at each step effects more and more related to ourselves which those
-causes will help us to explain.</p>
-
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_155" id="Page_155">[Pg 155]</a></span></p>
-<div class="chapter">
-<h2 class="nobreak">CHAPTER VI<br /><span class="h_subtitle">A PLANET’S HISTORY<br />
-<i>Self-sustained Stage</i></span></h2>
-</div>
-
-<p class="drop-cap">UP to this point in our retrospective survey
-the long course of evolution has taken one line, that of dynamical
-separation of the system’s parts with subsequent reunitement of
-them according to the laws of celestial mechanics. Of this action I
-have submitted the reader my brief: departing in it from common-law
-practice, in which the cause of action is short and the brief long. And
-I have, I trust, guarded against his appealing on exceptions.</p>
-
-<p>From this point on we have two kinds of development to follow: the one
-intrinsic, the chemical; the other incidental, the physical. Not that,
-in a way, the one is divorcible from the other. For the physical makes
-possible the chemical by furnishing it the conditions to act. But in
-another sense, and that which is most thrust upon our notice, the two
-are independent. Thus oceans and land, hills and valleys, clouds and
-blue sky, as we know them,—everything, pretty much, which we associate
-<span class="pagenum"><a name="Page_156" id="Page_156">[Pg 156]</a></span>
-with a world,—are not universal, inevitable, results of planetary
-evolution, but resultant, individual, characteristics of our
-particular abode. They are as much our own as the peculiar arithmetic
-of waiters is theirs, or as used to be the sobriety of the country
-doctor’s horse—his and no other’s. Our whole geologic career is
-essentially earthly. Not that its fundamental laws are not of universal
-application, but the kaleidoscopic patterns they produce depend on the
-little idiosyncrasies of the constituents and the mode in which these
-fall together. Our everyday experiences we should find quite changed,
-could we alight on Venus or on Mars.</p>
-
-<p>On the other hand, the chemical changes which follow a body’s
-acquisition of heat, setting in the moment that heat has reached its
-acme and starts to decline, are as universal as the universe itself.
-They are conditioned, it is true, by the body’s size and by the
-position that body occupied in the primal nebula, but they depend
-directly upon the degree of heat the body had attained. The larger
-the planet, the higher the temperature it reached and the fuller its
-possibilities. Even the planets are born to their estate. Thus the
-little meteorites live their whole waking life during the few seconds
-they spend rushing through our air. For then only does change affect
-their otherwise eternally inert careers. That the time is too short for
-any important experience is evident on their faces.
-<span class="pagenum"><a name="Page_157" id="Page_157">[Pg 157]</a></span></p>
-
-<p>Heat is most intimately associated with the very constitution of
-matter. It is, in fact, merely the motion of its ultimate particles,
-and plays an essential part in their chemical relations. Just as a
-certain discreet fervor and sufficient exposure for attraction to take,
-make for matrimony, so with the little molecules, a suitable degree of
-warmth and a propitious opportunity similarly conduce to conjunction;
-too fiery a temperament resulting in a vagabondage preventative of
-settled partnership and too cold a one in permanent celibacy. You may
-think the simile a touch too anthropomorphic, but it is a most sober
-statement of fact. Indeed, it is more than probable that in some dull
-sense they feel the impulse, though not the need of expressing it
-in verse. That metals can remember their past states seems to have
-been demonstrated by Bose, and is certainly in keeping with general
-principles as we know them to-day. For memory is the partial retention
-of past changes, rendering those changes more facile of repetition.</p>
-
-<p>A high degree of heat, then, makes chemical union impossible, because
-the great speeds at which the molecules are rushing past each other
-prevents any of them being caught. Lack of speed is equally deterrent.
-Nor is it wholly or even principally, perhaps, a movement of the whole
-which is here concerned, but a partitive throbbing of the molecule
-<span class="pagenum"><a name="Page_158" id="Page_158">[Pg 158]</a></span>
-itself. Certain it is that great cold is as prohibitive of chemic
-combination as great heat. Phosphorus, which evinces such avidity for
-oxygen at ordinary temperatures as to have got its name from the way it
-publishes the fact, at very low ones shows a coolness for its affinity
-amounting to absolute unconcern. Thus only within a certain range of
-temperature does chemical combination occur. To remain above or below
-this is to stay forever immortally dead. To get hot enough in the first
-place, and then subsequently to cool, are therefore essential processes
-to a body which is to know evolutionary advance.</p>
-
-<p>To pen the history of the solar system and leave out of it all mention
-of its most transcendentally wonderful result, the chemical evolution
-attendant upon cooling, would be to play “Hamlet” with Hamlet left out.
-For the thing which makes the second half of the great cosmic drama so
-inconceivably grand is the building up of the infinitely little into
-something far finer than the infinitely great. The mechanical action
-that first tore a sun apart, and then whirled the fragments into the
-beautifully symmetric system we behold to-day, is of a grandeur which
-is at least conceivable; the molecular one that, beginning where the
-other left off, built up first the diamond and then humanity is one
-that passes our power to imagine. That out of the aggregation of
-meteorites should come man, a being able to look back over his own
-<span class="pagenum"><a name="Page_159" id="Page_159">[Pg 159]</a></span>
-genesis, to be cognizant of it, as it were, from its first beginnings,
-is almost to prove him immanent in it from the start. Fortunate it is
-that his powers should seem more limited than his perceptions, and the
-more so as he goes farther, else he had been but the embodiment of conceit.</p>
-
-<p>We must sketch, therefore, the steps in this marvellous synthesis;
-hastily, for I have already spoken of it elsewhere in print and
-repetitions dull appreciation,—in the appreciative,—though we have
-the best of precedents for believing that, even in science, to be dull
-and iterative insures success; the dulness passing for wisdom and the
-iteration tiring opposition out.</p>
-
-<p>In the Sun all substances are in their elemental state. Though its
-materials are the same as the Earth’s, we should certainly not feel at
-home there, even if we waived the question of comfort, for we should
-recognize nothing we know. We talk glibly of elements as if we had
-personal acquaintance with them, man’s innate snobbery cropping out.
-For to the chemist alone are they observable entities. No one but he
-has ever beheld calcium or silicon, or magnesium, or manganese, and
-most of us would certainly not know these everyday elements if we met
-them on the street. Of all the substances composing the Earth’s crust,
-or the air above, or the water beneath, practically the only elements
-with which we are personally familiar are iron, copper, and carbon, and
-<span class="pagenum"><a name="Page_160" id="Page_160">[Pg 160]</a></span>
-these only in minute quantities and in that order of acquisition; which
-accounts for the stone, iron, and bronze ages of man, ending we may add
-with the graphite or lead-pencil age of early education.</p>
-
-<p>Yet that elementary substances once existed here we have evidence. We
-find such in volcanic vents. That the Earth was once as hot on its
-surface as it now is underneath, we know from the condition of the
-plutonic rocks where sedimentary strata have not covered them up.
-Volcanoes and geysers are our only avenues now to that earlier state of
-things. From these pathways to the past, and only from them, do we find
-elementary substances produced to-day,—hydrogen, sulphur, chlorine,
-oxygen, and carbon.<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a>
-We are thus made aware that once the Earth was simple, too, on the
-surface as well as deeper down. A side-light, this, to what we knew
-must have been the case.</p>
-
-<p>From its primordial state, the least complex compounds were evolved
-first. As the heat lessened, higher and higher combinations became
-possible. And this is why the more complex molecules are so unstable,
-the organic ones the most. Since they are not possible at all under
-much stir of their atomic constituents, it shows that the bond between
-them must be feeble—and, therefore, easily broken by other causes
-besides heat. To the instability of the organic molecule is due its
-power; and to cooling, the possibility of its expression.</p>
-
-<div class="figcenter">
-    <a name="I_160" id="I_160"> </a>
-    <img src="images/i_160.jpg" alt="" width="600" height="233" />
-    <p class="center space-below2"><span class="smcap">Lowell Observatory Spectrogram showing<br /> water-vapor
-                   in the atmosphere of Mars,<br /> January 1908.—V. M. Slipher.</span></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_161" id="Page_161">[Pg 161]</a></span>
-For the steps in the chemical process from Sun to habitable Earth
-we must look to the spectroscope; not in its older field, the blue
-end of the spectrum, but in that which is unfolding to our view in
-Dr. Slipher’s ingenious hands, the extension of the observable part
-of it into the red. For at that end lie the bands due to planetary
-absorption. Here we have already secured surprising results as to the
-atmospheres of the various planets. We have not only found positive
-evidence of water-vapor in the atmosphere of Mars, but we have detected
-strange envelopes in the major planets which show a constitution
-different from that of the Sun on the one hand, and of the Earth on
-the other. That size and position are for much in these peculiarities,
-I have already shown you; but something, too, is to be laid at the
-door of age. The major planets are not so advanced in their planetary
-history as is our Earth; and Dr. Slipher’s spectrograms of them
-disclose what is now going on in that prefatory, childish stage.</p>
-
-<p>These spectrograms are full of possibilities, and it is not too much to
-say that chemistry may yet be greatly indebted to the stars. Compounds,
-the strange unknown substances there revealed by their spectral
-lines, may be cryptic as yet to us. Some of the elements missing in
-<span class="pagenum"><a name="Page_162" id="Page_162">[Pg 162]</a></span>
-Mendeléeff’s table may be there, too. Helium was first found in the
-Sun; coronium still awaits detection elsewhere. So with these spectral
-lines of the outer planets. It looks as if chemistry had been a thought
-too previous in making free for others with what should have been their
-names, Zenon and Uranium. For we may yet have to speak of Dion and Varunium.</p>
-
-<p>From the chemical aspect of evolution we pass to its physical side;
-from the indirectly to the directly visible results. Here again, to
-learn what happened after the sunlike stage, we must turn to the major
-planets. For the cooling which induced both physical and chemical
-change has there progressed less far, inasmuch as a large globe takes
-longer to cool than a small one. To the largest planets, then, we
-should look for types of the early planetary stages to-day.</p>
-
-<p>Almost as soon as the telescope was directed to Jupiter, among the
-details it disclosed were the Jovian belts (in the year 1630), dark
-streaks ruling the planet’s disk parallel to its equator. They are
-of the first objects advertised as visible in small glasses to-day,
-vying with the craters in the Moon as purchasable wonders of the sky.
-As the belts were better and better seen, features came out in them
-which proved more and more interesting. Cassini, in 1692, noticed that
-the markings travelled round Jupiter and those nearest his equator the
-<span class="pagenum"><a name="Page_163" id="Page_163">[Pg 163]</a></span>
-quickest. Sir William Herschel thought them due to Jovian trade-winds,
-the planet’s swift rotation making up for deficiency of sun; why, does
-not appear.</p>
-
-<p>Modern study of the planet shows that the bright longitudinal layers
-between the dark belts are unquestionably belts of cloud. Their
-behavior indicates this, and their intrinsic brightness bears it out.
-For they are of almost exactly that albedo. Whether they are the kind
-of cloud with which we are familiar, clouds of water-vapor, we are not
-yet sure. But whatever their constitution, their conduct is quite other
-than is exhibited by our own.</p>
-
-<p>In the first place, they are of singular permanence for clouds. The
-fleeting forms we know as such assume in the Jovian air a stability
-worthy of Jove himself. In their general outlines, they remain the same
-for years at a time. “Constant as cloud” would be the proper poetic
-simile there. But while remaining true to themselves, they prove to be
-in slow, unequal shift with one another. Thus Jupiter’s official day
-differs according to the watch of the particular belt that times it.
-Spots in different latitudes drift round lazily in appearance, swiftly
-in fact, those near the equator as a rule the fastest. Nor is there any
-hard and fast latitudinal law; it is a go-as-they-please race in which
-one belt passes its neighbor at a rate sometimes of four hundred miles
-an hour. The mean day is 9ʰ 55ᵐ long.
-<span class="pagenum"><a name="Page_164" id="Page_164">[Pg 164]</a></span></p>
-
-<div class="figcontainer">
-  <div class="figsub">
-   <a id="I_164" name="I_164"> </a>
-   <img src="images/i_164a.jpg" alt="" width="250" height="223" />
-   <p class="center"><span class="smcap">Jupiter and its “great red spot”—
-                   a drawing<br /> by Dr. Lowell, April 12</span>, 7ʰ 0ᵐ-5ᵐ, 1907.</p>
-  </div>
-  <div class="figsub">
-   <img  src="images/i_164b.jpg" alt="" width="270" height="231" />
-   <p class="center"><span class="smcap">Jupiter and its “great red spot”—
-                  a drawing<br /> by Dr. Lowell, April 12</span>, 7ʰ 28ᵐ-42ᵐ, 1907.</p>
-   </div>
-</div>
-
-<p>A side-light is cast upon the Jovian state of things by the “great red
-spot,” which has been more or less visible for thirty years, and which
-takes five minutes longer than the equatorial band to travel round. Its
-tint bespoke interest in what might be its atmospheric horizon. Yet
-it betrayed no sign of being either depressed or exalted with regard
-to the rest of the surface. “In 1891,” as Miss Clerke puts it, “an
-opportunity was offered of determining its altitude relative to a small
-dark spot on the same parallel, by which, after months of pursuit,
-it was finally overtaken. An occultation appeared to be the only
-alternative from a transit; yet neither occurred. The dark spot chose a
-third. It coasted round the obstacle in its way, and got damaged beyond
-recognition in the process.” It thus astutely refused to testify.
-<span class="pagenum"><a name="Page_165" id="Page_165">[Pg 165]</a></span></p>
-
-<div class="figcenter">
-    <a name="I_165" id="I_165"> </a>
-    <img src="images/i_165.jpg" alt="" width="600" height="506" />
-    <p class="center space-below2"><span class="smcap">Sun spots—after Bond.</span></p>
-</div>
-
-<p>Now, this exclusiveness on the part of the “great red spot” really
-offers us an insight to its character. Clearly it was no void, but
-occupied space with more than ordinary persistency. As it was neither
-above nor below the dark spot and shattered that spot on approach,
-which its former surroundings had not done, its force must have been
-due to motion. This can be explained by its being formed of a vast
-uprush of heated vapor from the interior. In short, it was a sort of
-baby elephant of a volcano, or geyser, occurring as befits its youth in
-fluid, not solid, conditions, but fairly permanent, nevertheless—a bit
-of kindergarten Jovian geology. This estimate of it is concurred in by
-<span class="pagenum"><a name="Page_166" id="Page_166">[Pg 166]</a></span>
-Dr. Slipher’s spectrogram of the dark and light belts respectively. For
-in the spectrum of the dark one we see the distinctive Jovian bands
-intensified as if the light had traversed a greater depth of Jovian
-air. Its color, a cherry red, abets the conclusion—that in such places
-we look down into the fiery, chaotic turmoil so incessantly going on.</p>
-
-<div class="figcenter">
-    <a name="I_166" id="I_166"> </a>
-    <img src="images/i_166.jpg" alt="" width="600" height="403" />
-    <p class="center space-below2"><span class="smcap">Photograph of a sun spot—after the late M. Janssen.</span></p>
-</div>
-
-<p>It is of interest to note that we have prototypes of this sort of
-extraterrestrial cyclone in the Sun. His spots are probably local
-upsettings of atmospheric equilibrium, using the word atmospheric in
-the widest possible sense. Just as our storms are the mildest examples
-of the like expostulation at the impossibility of keeping up a too long
-<span class="pagenum"><a name="Page_167" id="Page_167">[Pg 167]</a></span>
-continued decorum. Only that with us the Earth is not so much to blame
-as the Sun; while both Jupiter and the Sun are themselves responsible
-for their condition.</p>
-
-<p>Thus we have, in the very depth of their negation, warrant from the
-dark belts of Jupiter that the bright ones are cloud. But also that
-they are not clouds ordered as ours. The Jovian clouds pay no sort
-of regard to the Sun. In orbital matters Jupiter obeys the ruler of
-the system; but he suffers no interference from him in his domestic
-affairs. His cloud-belts behave as if the Sun did not exist. Day and
-night cause no difference in them; nor does the Jovian year. They come
-when they will; last for months, years, decades; and disappear in like
-manner. They are <i>sui Jovis</i>, caused by vertical currents from the
-heated core and strung out in longitudinal procession by Jupiter’s
-spin. They are self-raised, not sun-raised, condensations of what is
-vaporized below. Jove is indeed the cloud-compeller his name implies.</p>
-
-<p>Yet Jupiter emits no light, unless the cherry red of his darker belts
-be considered its last lingering glow. He is thus on the road from Sun
-to world, and his present appearance informs us that this incubation
-takes place under cloud.</p>
-
-<p>The like is true of Saturn, in fainter replica, even to the cherry hue.
-In one way Saturn visibly asserts his independence beyond that possible
-<span class="pagenum"><a name="Page_168" id="Page_168">[Pg 168]</a></span>
-by Jupiter. For Jupiter’s equator lies almost in the plane of his
-orbit, and on a hasty view the Sun might be credited with the ordering
-of the belts, as was indeed long the case. But Saturn’s inclination to
-his orbital plane is 27°; yet his belts fit his figure as neatly as his
-rings, and never get displaced, no matter how his body be turned.</p>
-
-<p>Uranus and Neptune are in the same self-centred attitude at present as
-the faint traces of belts on their disk, otherwise of the same albedo
-as cloud, lead us to conclude. Yet both their densities and their
-situation give us to believe them further advanced than the giant
-planets, and still they lie wrapped in cloud.</p>
-
-<p>These planets, then, are quite unbeholden to the Sun for all their
-present internal economies. What goes on under that veil of clouds with
-which they discreetly hide their doings from the too curious astronomic
-eye—we can only conjecture. But we discern enough to know that it is
-no placid uneventfulness. That it will continue, too, we are assured.
-For whether these clouds are largely water-vapor now, or not, to
-watery ones they must come as the last of all the wrappers they will
-eventually put off.</p>
-
-<p>The major planets are the only ones at the present moment in this
-self-centred and self-sustained stage. Their great size has kept them
-young. In the smaller terrestrial planets we could not expect to
-witness any such condition to-day. If they experienced an ebullient
-<span class="pagenum"><a name="Page_169" id="Page_169">[Pg 169]</a></span>
-youth, they have long since outgrown it. Only by rummaging their
-past could we find evidence on the point, and this, distance both in
-time and space bars us from doing. There is but one body into whose
-foretime career we could hope to peer with the slightest prospect of
-success—our own Earth.</p>
-
-<div class="figcenter">
-    <a name="I_169" id="I_169"> </a>
-    <img src="images/i_169.jpg" alt="" width="450" height="581" />
-    <p class="center space-below2"><span class="smcap">The volcano Colima, Mexico, March 24, 1903—<br />
-              José Maria Arreola, per Frederick Starr.</span></p>
-</div>
-<p><span class="pagenum"><a name="Page_170" id="Page_170">[Pg 170]</a></span></p>
-<div class="figcenter">
-    <a name="I_170A" id="I_170A"> </a>
-    <img src="images/i_170a.jpg" alt="" width="600" height="215" />
-    <p class="center"><span class="smcap">Jukes Butte, a denuded laccolith,
-                        as seen from the northwest—Gilbert.</span></p>
-</div>
-<div class="figleft">
-    <a name="I_170B" id="I_170B"> </a>
-    <img src="images/i_170b.jpg" alt="" width="250" height="140" />
-    <p class="center"><span class="smcap">Ideal section of a laccolith—<br />Gilbert.</span></p>
-</div>
-
-<p>Whether our Earth was ever hot enough at the surface to vaporize
-those substances which now form the Jovian or Saturnian clouds, we do
-not know; but that it was once hot enough to vaporize water we are
-perfectly certain. And this from proof both of what did exist and
-of what did not. That the surface temperature was at onetime in the
-thousands of degrees Fahrenheit, the Plutonic magma underlying all the
-sedimentary rocks of the Earth amply shows. Reversely, the absence
-of any effect of water until we reach these sedimentary deposits,
-testifies that during all the earlier stages of the Earth’s career
-water as such was absent, and as water subsequently appeared, it is
-clear that the conditions did not at first allow it to form. We are
-sure, therefore, that there was a time when water existed only as
-steam, and very possibly a period still anterior to that when it did
-not exist at all, its constituent hydrogen and oxygen not having yet
-combined. There was certainly an era, then, in the morning of the ages,
-<span class="pagenum"><a name="Page_171" id="Page_171">[Pg 171]</a></span>
-when the Earth wore her cloud-wrapper much as Jupiter his now.</p>
-
-<p>That the seas were not once and yet are to-day, affords proof
-positive that at some intermediate period they began to be. Avery
-long intermediate one it must have been, too,—all the time it took
-the Earth to cool from about 2000° C. to 100° C. Not till after the
-temperature had fallen to the latter figure in the outer regions of
-the atmosphere could clouds form, and not till it had done so at the
-solid surface could the steam be deposited as water. Reasoning thus
-presents us with a picture of our Earth as a vast seething caldron from
-which steam condensing into cloud was precipitated upon a heated layer
-of rock, to rise in clouds of steam again. The solid surface had by
-this time formed, thickening slowly and more or less irregularly, and
-into its larger dimples the water settled as it grew, deepening them
-into the great ocean basins of to-day. We see the process with as much
-certainty and considerably more comfort than if, in the French sense,
-we had assisted at it. Presence of mind now thus amply makes up for
-absence of body then.</p>
-
-<p>Passing on evolutionarily we reach more and more tolerable conditions
-and solid ground in fact, as well as theory. Thus the crust hardened
-and cooled, while the oceans still remained uncomfortably hot. For
-<span class="pagenum"><a name="Page_172" id="Page_172">[Pg 172]</a></span>
-water requires much more heat to warm it to a given temperature than
-rock, about four and a half times as much. It has therefore by so much
-the more to lose, and is proportionally long in the losing. These
-hot seas must have produced a small universe of cloud, and as the
-conditions were the same all over the Earth, we can see easily with the
-mind’s eye that we could not have seen at all with the bodily one, had
-we occupied the land in those very early days. To be quite shut out
-from curious sight without, was hardly made up for by not being able
-to see more than dimly within. Any one who has stood on the edge of a
-not-extinct crater when the wind was blowing his way, will have as good
-a realization of the then state of things as he probably cares for.</p>
-
-<p>Now this astronomic drawing of the then Earth, which by its lack of
-detail allows of no doubt whatever, permits us to offer help in the
-elucidation of some of their phenomena to our geologic colleagues.
-We are the more emboldened to do so in that they have themselves
-appealed to astronomy for diagnosis, and accepted nostrums devised by
-themselves. It is always better in such cases to call in a regular
-practitioner. Not that he is necessarily more astute, but that he knows
-what will not work. It was in the matter of the paleologic climate that
-they were led to consult astronomy. The singular thing about paleologic
-times was the combination of much warmth with little light; and the not
-less singular fact that these conditions were roughly uniform over the
-<span class="pagenum"><a name="Page_173" id="Page_173">[Pg 173]</a></span>
-whole Earth. From this universality it was clear, as De Lapparent,
-their chief spokesman, puts it, that nothing local could explain the
-fact. It was something which demanded a cause common to the globe.</p>
-
-<p>It thus fell properly within the province of astronomy. For if we are
-to draw any line between the spheres of influence of the two sciences,
-it would seem to lie where totality ends and provincialism begins. I
-use this not as a pejorative, but simply to part local color from one
-universal drab. In the Earth’s general attributes,—its size, shape,
-and weight,—we must have recourse to astronomy to learn the facts.
-Not less so for those principal causes which have shaped its general
-career; we surrender it only at the point where everyday interest
-begins, when those causes that led it through its uninviting youth give
-way to effects which in the least concern humanity at large.</p>
-
-<p>Between the mere aggregation of matter into planetary bodies, of which
-nebular hypotheses treat, and the specific transformation of plants and
-animals upon their surfaces with which organic evolution is concerned,
-lies a long history of development, which, beginning at the time
-the body starts to cool, continues till it become, for one cause or
-another, again an inert mass. In this period is contained its career
-as a world. Planetology I have ventured to call the brand of astronomy
-<span class="pagenum"><a name="Page_174" id="Page_174">[Pg 174]</a></span>
-which deals with this evolution of worlds. It treats of what is general
-and cosmic in that evolution, as geology treats of what is terrestrial
-and specific in the history of one member of the class, our own Earth.
-The two do not interfere, as the one faces questions in time and space
-to which the other remains perforce a stranger. If the picture by the
-one be fuller of detail, the canvas of the other permits of the wider
-perspective. Certain events in the history of our Earth can only be
-explained by astronomy, as geologists have long since recognized. It is
-these that fall into our present province.</p>
-
-<p>Geologists, however, have applied astronomy according to their own
-ideas. Either they called in aurists, so to speak, when what they
-needed was an oculist, or they went to books for their drugs, which
-they then administered themselves—a somewhat dangerous practice.
-Thus they began by displacing the Earth’s axis in hope of effecting a
-result; not realizing that this would only shift the trouble, not cure
-it; in fact, make it rather worse. They next tried what De Lapparent,
-one of the most brilliant geologists of the age, calls “a variation
-in the eccentricity of the ecliptic<a name="FNanchor_16_16" id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a>
-joined to precession of the equinoxes,”—a startling condition unknown
-to astronomy which does not deal in eccentric planes, whatever such
-<span class="pagenum"><a name="Page_175" id="Page_175">[Pg 175]</a></span>
-geometric anomalies may be, but by which its coiner evidently means
-a change in the eccentricity of the orbit, as the context shows. Its
-effect on the Earth, as he wisely points out, would be to reduce its
-extremities to extremes. To get out of his quandary he then embraced
-a brilliant suggestion of a brother geologist, M. Blandet. M. Blandet
-conceived the idea, and brought it forth unaided, that all that was
-necessary was a sun big enough to look down on both poles of the
-Earth at once. To get this he travelled back to the time when, in
-Laplace’s cosmogony, the Sun filled the whole orbit of Mercury. This
-conception, which, De Lapparent remarks, “might, at the time of its
-apparition, have disconcerted spirits accustomed to consider our system
-as stable,”—an apparition which we may add would certainly continue
-to disconcert them,—he says seems to him quite in harmony with that
-system’s genesis. That it labors under two physical impossibilities,
-one on the score of the Sun, the other on that of the Earth, and that
-in this case two negatives do not make an affirmative, need not be
-repeated here, as the reader will find it set forth at length
-elsewhere,<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a>
-together with what I conceive to be the only explanation of
-paleothermal times which will work astronomically—presently to be
-mentioned. But before I do so, it is pertinent to record two things
-that have come to my notice since. One is that in rereading Faye’s
-<span class="pagenum"><a name="Page_176" id="Page_176">[Pg 176]</a></span>
-“Origine du Monde,” I came upon a passage in which it appears that M.
-Blandet had actually consulted Faye about his hypothesis, and that Faye
-had shown him its impossibility on much the same grounds as those above
-referred to; which, however, did not deter M. Blandet from giving it to
-the world nor De Lapparent from god-fathering the conception.</p>
-
-<p>Faye, meanwhile, developed his theory of the origin of the world,
-and by it explained the greater heat and lesser light of paleologic
-times compared with our own, thus: The Earth evolved before the
-Sun. In paleologic times the Sun was still of great extent,—an
-ungathered-up residue of nebula that had not yet fallen together enough
-to concentrate, not a contracting mass from which the planets had been
-detached,—and was in consequence but feebly luminous and of little
-heating effect; so that there were no seasons on Earth and no climatic
-zones. The Earth itself supplied the heat felt uniformly over its whole
-surface.</p>
-
-<p>This differs from my conception, as the reader will see presently, in
-one vital point—as to why the Earth was not heated by the Sun. In the
-first place Faye’s sun has no <i>raison d’être</i>; and in the second no
-visible means of existence. If its matter were not already within the
-orbit of the Earth at the time, there seems no reason why it should
-ever get there; and if there, why it should have been so loath to
-condense. We cannot admit, I think, any such juvenility in the Sun at
-the time the Earth was already so far advanced as geology shows it to
-have been in paleologic times. For the Earth had already cooled below
-the boiling-point of water.</p>
-
-<div class="figcenter">
-    <a name="I_176" id="I_176"> </a>
-    <img src="images/i_176.jpg" alt="" width="400" height="552" />
-    <p class="center"><span class="smcap">Tree fern.</span></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_177" id="Page_177">[Pg 177]</a></span>
-To understand the problem from the Earth’s point of view, let us
-review the facts with which geology presents us. The flora of
-paleologic times, as we see both at their advent in the Devonian and
-from their superb development in the Carboniferous era, consisted
-wholly of forms whose descendants now seek the shade.<a name="FNanchor_18_18" id="FNanchor_18_18"></a><a href="#Footnote_18_18" class="fnanchor">[18]</a>
-Tree ferns, sigillaria, equisetæ, and other gloom-seeking plants composed
-it. That some tree fern survivals to-day can bear the light does not
-invalidate the racial tendency. We have plenty of instances in nature
-of such adaptability to changed conditions. In fact, the dying out
-and deterioration of most of the order shows that the conditions have
-changed. And these plants, grown to the dimensions of trees, inhabited
-equally the tropic, the temperate, and the frigid zones as we know them
-now. Lastly, no annual rings of growth are to be found on them.<a name="FNanchor_19_19" id="FNanchor_19_19"></a><a href="#Footnote_19_19" class="fnanchor">[19]</a>
-In other words, they grew right on, day in, day out. The climate, then,
-was as continuous as it was widespread.</p>
-
-<p><span class="pagenum"><a name="Page_178" id="Page_178">[Pg 178]</a></span>
-On the other hand, astronomy and geology both assert that the seas
-were warm.<a name="FNanchor_20_20" id="FNanchor_20_20"></a><a href="#Footnote_20_20" class="fnanchor">[20]</a>
-From this it follows that a vastly greater evaporation must have gone
-on then than now, and that a welkin of cloud must thus inevitably have
-been formed.</p>
-
-<p>Now put the two facts together, and you have the solution. The climate
-was warm and equable over the whole globe because a thick cloud
-envelope shut off the Sun’s heat, the heat being wholly supplied from
-the steamy seas. At the same time, by the same means the light was
-necessarily so tempered as to produce exactly that half-light the ferns
-so dearly love. One and the same cause thus answers the double riddle
-of greater warmth and less light in those old days than is now the case.</p>
-
-<p>And here comes in the second find I spoke of above, in the person of
-some old trilobites who stepped in unexpectedly in corroboration. It
-has long been known—though its full significance seems to have escaped
-notice—that in 1872 M. Barrande made the discovery that many species
-of trilobites of the Cambrian and lower Silurian, the two lowest, and
-therefore the oldest, strata of paleozoic times, and distant relative
-of our horseshoe crabs, were blind. What is yet more significant,
-the most antediluvian were the least provided with eyes. Thus in the
-primordial strata, one-fourth of the whole number of species were
-eyeless, in the next above one-fifth, and in the latest of all one
-two-hundredth only.<a name="FNanchor_21_21" id="FNanchor_21_21"></a><a href="#Footnote_21_21" class="fnanchor">[21]</a>
-Furthermore, they testify to the difficulty of seeing, in two distinct
-ways, some by having no eyes and some colossal ones, strenuous
-<span class="pagenum"><a name="Page_179" id="Page_179">[Pg 179]</a></span>
-individuals increasing their equipment and the lazy letting it lapse.
-It seems more than questionable to attribute this blindness to a
-deep-sea habitat, as Suess does in describing them, for they lived in
-what geologists agree were shallow seas on the site of Bohemia to-day.
-Besides, trilobites never had abyssal proclivities; for they are found
-preserved in littoral deposits, not in deep-sea silt. Muddy water may
-have had some hand in this, but muddy water itself testifies to great
-commotion above and torrential rains. So the light in those seas was
-not what it became later, or would be now. Thus these trilobites were
-antelucan members of their brotherhood, and this accuses a lack of
-light in those earlier eras even greater than in Carboniferous times,
-which is just where it ought to be found if the theory is true.</p>
-
-<p>I trust this conception may prove acceptable to geologists, for it
-seems imperative from the astronomic side that something of the sort
-must have occurred. And it is just as well, if not better, to view it
-thus in the light of the dawn of geologic history as to remain in the
-dark about it altogether. Nescience is not science—whether hyphenized
-or apart; for the whole object of science is to synthesize and explain.
-Its body of learning is but the letter, coördination the spirit, of its
-<span class="pagenum"><a name="Page_180" id="Page_180">[Pg 180]</a></span>
-law. Nevertheless, the unpardonable impropriety of a new idea, I am
-aware, is as reprehensible as the atrocious crime of being a young man.
-Yet the world could not get on without both. Time is a sure reformer
-and will render the most hardened case of youth senile in the end. So
-even a new idea may grow respectable at last. And it is really as well
-to make its acquaintance while it still has vigor in it as to wait till
-it is old and may be embraced with impunity. Boasted conservatism is
-troglodytic, and usually proves a self-conferred euphuism for dull.
-For conservatism proceeds from slowness of apprehension. It may be
-necessary for certain minds to be in the rear of the procession, but it
-is of doubtful glory to find distinction in the fact.</p>
-
-<p>Thus the youth of a world, like the babyhood of an individual, is
-passed screened from immediate contact from without. That this is
-the only way that life can originate on a planet we cannot say, but
-that it is away in which it does occur, our own Earth attests, and
-that, moreover, it is the way with all planets of sufficient size,
-the present aspect of the major planets shows. It may well be that
-with celestial bodies as with earthly species, some swaddle their
-young, others cast them forth to take their chance, and that those
-that most protect them rear the higher progeny in the end. What
-glories in evolution thus await the giant planets when they shall have
-<span class="pagenum"><a name="Page_181" id="Page_181">[Pg 181]</a></span>
-sufficiently cooled down, we can only dimly imagine. But we can foresee
-enough to realize that we are not the sum of our solar system’s
-possibilities, and by studying the skies read there a future more
-wonderful than anything we know.</p>
-
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_182" id="Page_182">[Pg 182]</a></span></p>
-<div class="chapter">
-<h2 class="nobreak">CHAPTER VII<br /><span class="h_subtitle">A PLANET’S HISTORY<br />
-<i>Sun-sustained Stage</i></span></h2>
-</div>
-
-<p>Two stages have characterized the surface history of the Earth,—stages
-which may be likened to the career of the chick within and without the
-egg. In the first of them the Earth lay screened from outside influence
-under a thick shell of cloud, indifferently exclusive of the cold of
-space or of the heating beams of the Sun. Motherless, the warmth of
-its own body brooded over it, keeping its heat from dissipating too
-speedily into space, and so fostering the life that was quickening upon
-its surface.</p>
-
-<p>The second stage began when the egg-shell broke and the chick lay
-exposed to the universe about it, to get its living no longer from its
-little world within, but from the greater one without. One and the same
-event ended the old life to make possible the new. So soon as the cloud
-envelope was pierced, both the Earth’s own heat escaped and the Sun’s
-rays were permitted to come in.</p>
-
-<p>It is not surprising that under such changed conditions development
-<span class="pagenum"><a name="Page_183" id="Page_183">[Pg 183]</a></span>
-itself should have changed, too. In fact, the transformation was
-marked. That its epochal character has failed to impress itself
-generally on geologists, is perhaps because they look too closely,
-missing the march of events in the events themselves, and because, too,
-of the gradual nature of its processional change. We can recall only De
-Lapparent as having particularly signalled it; although not only in its
-cause, but for its effects, it should have delimited two great geologic
-divisions of time.</p>
-
-<div class="figcenter">
-    <a name="I_183" id="I_183"> </a>
-    <img src="images/i_183.jpg" alt="" width="600" height="355" />
-    <p class="center"><span class="smcap"><span class="smcap">Earth as seen from above—Photographed
-               by Dr. Lowell<br /> at an altitude of 5500 feet.</span></span></p>
-</div>
-
-<p>Astronomy and geology are each but part of one universal history. The
-tale each has to tell must prove in keeping with that of the other.
-If they seem at variance, it behooves us very carefully to scan their
-respective stories to find the flaw where the apparent incongruity
-slipped in. Each, too, fittingly supplements the other, and especially
-<span class="pagenum"><a name="Page_184" id="Page_184">[Pg 184]</a></span>
-must geology look to astronomy for its initial data, since astronomy
-deals with the beginning of our own Earth.</p>
-
-<p>That study of our Earth in its entirety falls properly within the
-province of astronomy, is not only deducible from its relationship to
-the other planets, but demonstrable from the cosmic causes that have
-been at work upon it, and the inadequacy of anything but cosmic laws
-to explain them. The ablest geologists to-day are becoming aware of
-this,—we have one of them at the head of the geology department of the
-Institute,—while from the curious astronomy at second hand which gets
-printed in geologic text-books, by eminent men at that, dating from
-some time before the flood,—of modern ideas,—it seems high time that
-the connection should be made clear.</p>
-
-<p>For, after all, our Earth too is a heavenly body, in spite of man’s
-doing his best to make it the reverse. It has some right to astronomic
-regard, even if it is our own mother. At the same time it is quite
-puerile to consider the universe as bounded by our terrestrial
-backyard. If man took himself a thought less importantly, he might
-perceive the humor of so circumscribed a view. Like children we play at
-being alone in the universe, and then go them one better by believing
-it too.</p>
-
-<p>I shall, of course, not touch on any matters purely geologic, for fear
-of committing the very excesses I deplore; mentioning only such points
-<span class="pagenum"><a name="Page_185" id="Page_185">[Pg 185]</a></span>
-as astronomy has information on, and which, by the sidelights it
-throws, may help to illuminate the subject.</p>
-
-<p>Thus it certainly is interesting and may to many be a new point of
-view, that the changes introduced when paleologic times passed into
-neologic ones were in their fundamental aspects essentially astronomic;
-which shows how truly astronomic causes are woven into the whole fabric
-of the Earth. For it was then only, terrestrially speaking, that the
-year began. The orbital period had existed, of course, from the time
-the Earth first made the circuit of the Sun. But the year was more a
-<i>succès d’estime</i> on the Sun’s part than one of popular appreciation.
-As the Sun could not be seen and worked no striking effects upon the
-Earth, the annual round had no recognizable parts, and one revolution
-lapsed into the next without demarcation. Only with the clearing of the
-sky did the seasons come in: to register time by stamping its record
-on the trees. Before that, summer and winter, spring and autumn, were
-unknown.</p>
-
-<p>Climate, too, made then its first appearance; climate, named after
-the sunward obliquity of the Earth, and seeming at times to live down
-to that characterization. Weather there had been before; pejoratively
-speaking, nothing but weather. For the downpours in paleologic times
-must have been exceeded in numbers only by their force. One dull
-<span class="pagenum"><a name="Page_186" id="Page_186">[Pg 186]</a></span>
-perpetual round of rain was the programme for the day, with absolutely
-no hope of a happy clearance to-morrow. It was the golden age only for
-weather prophets whose prognostications could hardly go wrong. With
-climate, however, it was a very different matter. With polyp corals
-building reefs almost to the pole (81° 50′),<a name="FNanchor_22_22" id="FNanchor_22_22"></a><a href="#Footnote_22_22" class="fnanchor">[22]</a>
-as far north nearly as man has yet by his utmost efforts succeeded in
-getting, while their fellows were busy at the like industry in the
-tropics, it is clear that latitude was laughed at and climate even
-lacked a name.</p>
-
-<p>Another astronomic feature, then for the first time disclosed, was the
-full significance of the day and the revelation of its cause. While
-the Earth brooded under perpetual cloud, there could have been but
-imperfect recognition of day and night. Or perhaps we may put it better
-by saying that the standard of both was greatly depressed, dull days
-alternating with nights black as pitch. But the moment the Sun was
-let in, all this changed, though not in a twinkling. The change came
-on most gradually. We can see in our mind’s eye the first openings in
-the great welkin permitting the Earth its initial peeps of the world
-beyond, and how quickly and tantalously they shut in again like a
-mid-storm morning which dreams of clearing only to find how drowsy it
-still is. But eventually the clouds parted afresh and farther, and the
-Earth began to open its eyes to the universe without.</p>
-
-<p><span class="pagenum"><a name="Page_187" id="Page_187">[Pg 187]</a></span>
-The cause of the clearing, of course, was the falling temperature of
-the seas. Evaporation went on much less fast as the heat of the water
-lessened. The whole round of aquatic travel from ocean to air, and back
-to ocean again, proceeded at an ever slackening pace. And here, if it
-so please geologists, may be found a reconciling of their demands for
-time to the relative pittance astronomy has been willing to dole them
-out, a paltry 50 or 100 millions of years, which like all framers of
-budgets they have declared utterly insufficient. For in early times the
-forces at work were greater, and by magnifying the means you quicken
-the process and contract the Earth’s earlier eras to reasonable limits.</p>
-
-<p>Upon these various astronomic novelties, the Earth on thus awakening
-looked for the first time. Such regard altered for good its own
-internal relations. The wider outlook made impossible the life of the
-narrower that preceded it. A totally changed set of animals and plants
-arose, to whom the cosmos bore a different aspect. The Earth ceased
-to be the self-centred spot it seemed before. As long ago as this had
-the idea that our globe was the centre of the universe been cosmically
-exploded. The Earth knew it if man did not.
-<span class="pagenum"><a name="Page_188" id="Page_188">[Pg 188]</a></span></p>
-
-<div class="figcenter">
-    <a name="I_188" id="I_188"> </a>
-    <img src="images/i_188.jpg" alt="" width="600" height="240" />
-    <p class="center space-below2"><span class="smcap">Tracks of Sauropus primævus (× ½). I. Lea.—Dana,<br />
-              “Manual of Geology.”</span></p>
-</div>
-
-<p>Its denizens responded. The organisms that already inhabited it
-proceeded to change their character and crawl out upon the land. For
-in Devonian times the Earth was the home of fishes. The land was not
-considered a fit abode by anything but insects, and not over-good
-by them. But it looked different when the Sun shone. Some maritime
-dwellers felt tempted to explore, and proceeded in the shape of
-amphibians to spy out the land. They have left very readable accounts
-of their travels in footnotes by the way. As one should always inspect
-the original documents, I will reproduce the footnotes of one early
-explorer. It is one of the few copies we have, as the type is worn out.
-But it tells a pretty full story as it stands. The ripple-marks show
-that a sea beach it was which the discoverer trod in his bold journey
-of a few feet from home and friends, and the pits in the sandstone that
-it was raining at the time of his excursion. No Columbus or Hakluyt
-could have left a record more precise or more eminently trustworthy.
-The pilgrims found it so good that their eventual collaterals, the
-great reptiles, actually took possession of the land and held it for
-many centuries by right of eminent domain. Yet throughout the time of
-<span class="pagenum"><a name="Page_189" id="Page_189">[Pg 189]</a></span>
-these bold adventurers, their skies were only clearing, as the pitting
-of the sandstone eloquently states.</p>
-
-<p>It was not till the chalk cliffs of Dover were being laid down that we
-have evidence that seasons had fully developed, in the shape of the
-first deciduous trees.<a name="FNanchor_23_23" id="FNanchor_23_23"></a><a href="#Footnote_23_23" class="fnanchor">[23]</a>
-Cryptogams, cycads, and, finally, conifers had in turn represented the
-highest attainments of vegetation, and the last of these had already
-recognized the seasons by a sort of half-hearted hibernation or annual
-moulting; deeming it wise not to be off with the old leaves before
-they were on with the new. But finally the most advanced among them
-decided unreservedly to accept the winter and go to sleep till spring.
-The larches and ginkgo trees are descendants of the leaders of this
-coniferous progressive party.</p>
-
-<p>At the same time color came in. We are not accustomed to realize that
-nature drew the Earth in grays and greens, and touched it up with color
-afterward. Only the tempered tints of the rocks and the leaden blue of
-the sea, subdued by the disheartening welkin overhead to a dull drab,
-enlivened their abode for the oldest inhabitants. But with Tertiary
-times entered the brilliantly petalled flowers. Beginning with yellow,
-these rose through a chromatic scale of beauty from white through red
-to blue.<a name="FNanchor_24_24" id="FNanchor_24_24"></a><a href="#Footnote_24_24" class="fnanchor">[24]</a>
-They decked themselves thus gaudily because the Sun was there to see
-by, as well as eyes to see. For without the Sun those unconscious
-horticulturists, the insects, could not have exercised their pictorial
-profession.</p>
-
-<p><span class="pagenum"><a name="Page_190" id="Page_190">[Pg 190]</a></span>
-To the entering of the Sun upon the scene this wondrous revolution was
-due; and once entered, it became the dominant factor in the Earth’s
-organic life. We are in the habit of apostrophizing the Sun as the
-source of all terrestrial existence. It is true enough to-day, and has
-been so since man entered on the scene. But it was not always thus.
-There was a time when the Sun played no part in the world’s affairs.</p>
-
-<p>As its heat is now all-important, it becomes an interesting matter to
-determine the laws governing its amount. That summer is hotter than
-winter we all know from experience, pleasurable or painful as the case
-may be. This is due to the fact that the Sun is above the horizon for
-a greater number of hours in summer and passes more directly overhead.
-But not so many people are aware that on midsummer day, so far as the
-Sun is concerned, the north pole should be the hottest place on earth.
-That Arctic explorers, who have got within speaking acquaintance of it,
-assure us it is not so, shows that something besides the direct rays
-of the Sun is involved. Indeed, we learn as much from the extensively
-advertised thermometers of winter resorts which, judiciously placed,
-<span class="pagenum"><a name="Page_191" id="Page_191">[Pg 191]</a></span>
-beguile the stranger to sojourn where it is just too cold for comfort.
-The factor in question is the blanketing character of our air. Now
-a blanket may keep heat out as well as keep it in. Our air acts in
-both capacities. It is by no means simply a storer of heat, as many
-people seem to suppose; it is a heat-stopper as well. What it really
-is is a temporizer, a buffer to ease the shocks of sudden change
-like those comfortable, phlegmatic souls who reduce all emotion to a
-level. For the heating power of the Sun, even at the Earth’s distance
-away, is much greater than appears. Knowledge of this we owe most to
-Langley, and then to Very, who continued his results to yet a finer
-determination, the best we have to-day. In consequence we have learnt
-that the amount of heat we should receive from the Sun, could we get
-above our air,—the solar constant, as it is called,—would be over
-three times what it is on the average in our latitude at the surface,
-and is rising still, so to speak. For as man has gone higher he has
-found his inferences rising too, and the limit would seem to be not
-yet. We see then that the air to which we thought ourselves so much
-indebted, actually begins its kindly offices by shutting off two-thirds
-of what was coming to us. As it plays, however, something of the same
-trick to what tries to escape, we are really somewhat beholden to it
-after all.</p>
-
-<p>But not so much as has been thought. We used to be told that the Moon’s
-<span class="pagenum"><a name="Page_192" id="Page_192">[Pg 192]</a></span>
-temperature even at midday hardly rose above freezing, but Very has
-found it about 350° F., which even the most chilly of souls might find
-warm. By the late afternoon, however, he would need his overcoat, and
-no end of blankets subsequently, for during the long lunar night of
-fourteen days the temperature must fall appallingly low, to -300° F. or
-less.</p>
-
-<p>As the determination of temperature is a vital one, not only to any
-organic existence, but even to inorganic conditions upon a planet,
-it behooves us to look carefully into the question of the effective
-heat received from the Sun. Until recently the only criterion in the
-case was assumed to be distance from the illuminating source, about
-as efficient a mode of computation as estimating a Russian army by
-its official roll. For as we saw in our own case, not all that ought
-to ever gets to the front, to say nothing of what is lost there. Yet
-on this worse than guesswork astronomic text-books still assert as
-a fact that the temperature of other bodies—the Moon and Mars, for
-example—must be excessively low.</p>
-
-<p>Let us now examine into this most interesting problem. It is intricate,
-of course, but I think you will find it more comprehensible than
-you imagine. Indeed, I shall be to blame if you do not. For if one
-knows his subject, he can always explain it, in untechnical language,
-technical terms being merely a sort of shorthand for the profession.
-The physical processes involved can be made clear without difficulty,
-<span class="pagenum"><a name="Page_193" id="Page_193">[Pg 193]</a></span>
-although their quantitative evaluation is less forthrightly
-demonstrable. Let me, then, give you an epitome of my investigation of
-the subject.</p>
-
-<div class="figcenter">
-    <a name="I_193" id="I_193"> </a>
-    <img src="images/i_193.jpg" alt="" width="500" height="493" />
-    <p class="center space-below2"><span class="smcap">Adventures of a heat ray.</span></p>
-</div>
-
-<p>Consider a ray of light falling on a surface from the Sun. A part of it
-is reflected; that is, is instantly thrown off again. By this part the
-body shines and makes its show in the world, but gets no good itself.
-Another part is absorbed; this alone goes to heat the body. Now if the
-visible rays were all that emanated from the Sun, it would be strictly
-<span class="pagenum"><a name="Page_194" id="Page_194">[Pg 194]</a></span>
-true, and a pretty paradox for believers in the efficacy of distance,
-that what heated the planet was precisely what seemed not to do so.
-Unfortunately there are also invisible rays, and these, too, are in
-part reflected and in part absorbed, and their ratio is different from
-that of the visible ones. To appreciate them, Langley invented the
-bolometer, in which heat falling on a strip of metal produces a current
-of electricity registered by a galvanometer. By thus recording the heat
-received at different parts of the spectrum and at different heights
-in our atmosphere, he was able to find how much the air cut off. Very
-has since determined this still more accurately. By thus determining
-the depletion in the invisible part of the spectrum joined to what
-astronomy tells us of the loss in the visible part, we have a value
-for the whole amount. By knowing, then, the immediate brightness of
-a planet and approximately the amount of atmosphere it owns, we are
-enabled to judge how much heat it actually receives. This proves to be,
-in the case of Mars, more than twice as much as distance alone would
-lead us to infer.</p>
-
-<p>The second question is how much of this it retains. The temperature of
-a body at any moment is the balance struck between what it receives and
-what it radiates. If it gets rid of a great deal of its income, it will
-clearly be less hot than if it is miserly retentive. To find how much
-<span class="pagenum"><a name="Page_195" id="Page_195">[Pg 195]</a></span>
-it radiates we may take the difference in temperature between sunset
-and sunrise, since during this interval the Earth receives no heat from
-the Sun. In the same way the efficacy of different atmospheric blankets
-may be judged. Thus the Earth parts with nine centigrade degrees’ worth
-of its store on clear nights, and only four degrees’ worth on cloudy
-ones, before morning. This is at sea-level. By going up a high mountain
-we get another set of depletions, and from this a relative scale for
-different atmospheric blankets. This is the principle, and we only have
-to fill out the skeleton of theory with appropriate numbers to find how
-warm the body is.</p>
-
-<p>In doing so, we light on some interesting facts. Thus clouds reflect 72
-per cent of the visible rays, letting through only 28 per cent of them.
-We feel chilly when a cloud passes over the Sun. On the other hand,
-slate reflects only 18 per cent of the visible rays, absorbing all the
-rest. This is why slate gets so much hotter in the Sun than chalk, and
-why men wear white in the tropics. White, indeed, is the best color to
-clothe one’s self in the year around, except for the cold effect it has
-on the imagination, for it keeps one’s own heat in as well as keeping
-the Sun’s out. The modest, self-obliterating, white winter habit of the
-polar hares not only enables them to keep still and escape notice, but
-keeps them warm while they wait.
-<span class="pagenum"><a name="Page_196" id="Page_196">[Pg 196]</a></span></p>
-
-<p>Astronomically, the effect is equally striking. Mars, for example,
-owing to being cloudless and of a duller hue, turns out to have a
-computed mean temperature nearly equal to the Earth’s,—a theoretic
-deduction which the aspect of the planet most obligingly corroborates.
-It thus enjoys a comparatively genial old age.</p>
-
-<p>For what is specially instructive in planetary economy is that, on
-the whole, clear skies add more by what they let in than they subtract
-by what they let out. If the Earth had no clouds at all, its mean
-temperature would be higher than it is to-day. Thus as a planet ages
-a beneficent compensation is brought about, the Sun’s heat increasing
-as its own gives out. Not that the foreign importation, however slight
-the duty levied on it by the air, ever fully makes up for the loss of
-the domestic article, but it tempers the refrigeration which inevitably
-occurs.</p>
-
-<p>The subject of refrigeration leads us to one of the most puzzling and
-vexed problems of geology: how to account for the great Ice Age of
-which the manifest sign manuals both in Europe and in America have so
-intrigued man since he began to read the riddle of the rocks. Upon
-this, also, planetology throws some light.</p>
-
-<p>If I needed an apology to the geologists for seeming again to
-trespass on their particular domain, I might refer to the astrocomico
-expositions put forward to account for the great Ice Age.
-<span class="pagenum"><a name="Page_197" id="Page_197">[Pg 197]</a></span></p>
-
-<p>We can all remember Croll’s “Climate and Time,” a book which can
-hardly be overpraised for its title and which had things worth reading
-inside, too. It had in consequence no inconsiderable vogue at one time.
-It undertook to account for glacial epochs on astronomic principles.
-It called in such grand cosmic conditions and dealt in such imposing
-periods of time that it fired fancy and almost compelled capitulation
-by the mere marshalling of its figurative array. Secular change in the
-eccentricity of the Earth’s orbit, combined with progression in the
-orbital place of the winter’s solstice, was supposed to have induced
-physical changes of climate which accentuated the snowfall in the
-northern hemisphere and so caused extensive and permanent glaciation
-there. In other words, long, cold winters followed by short, hot
-summers in one hemisphere were credited with accumulating a perpetual
-snow sheet, such as short, warm winters and long, cold summers could
-not effect.
-<span class="pagenum"><a name="Page_198" id="Page_198">[Pg 198]</a></span></p>
-
-<div class="figcenter">
-    <a name="I_198" id="I_198"> </a>
-    <img src="images/i_198.jpg" alt="" width="600" height="402" />
-    <p class="f120"><span class="smcap">Mars.</span></p>
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc" colspan="2">NORTH POLAR CAP.</td>
-      <td class="tdc" colspan="2">SOUTH POLAR CAP.</td>
-   </tr><tr>
-      <td class="tdl">At maximum</td>
-      <td class="tdl_ws1">full extent of white  </td>
-      <td class="tdl_ws1">At maximum</td>
-      <td class="tdl_ws1">white</td>
-   </tr><tr>
-      <td class="tdl">At minimum</td>
-      <td class="tdl_ws1">inner circle</td>
-      <td class="tdl_ws1">At minimum</td>
-      <td class="tdl_ws1">nothing</td>
-   </tr>
- </tbody>
-</table>
-</div>
-
-<p>Now it so happens that these astronomic conditions affecting the Earth
-several thousand years ago, are in process of action on one of our
-nearest planetary neighbors at the present time. The orbit of Mars
-is such that its present eccentricity is greater than what the Earth
-ever can have had, and the winter solstice of the planet’s southern
-hemisphere falls within 23° of its aphelion point. We have then the
-conditions for glaciation if these are the astronomic ones supposed,
-and we should expect a southern polar cap, larger at its maximum
-and still more so, relatively, at its minimum, than in the opposite
-hemisphere. Let us now look at the facts, for we have now a knowledge
-of the Martian polar caps exceeding in some respects what we know of
-our own. The accompanying diagrams exhibit the state of things at
-a glance, the maximum and minimum of each cap being represented in
-a single picture and the two being placed side by side. It will be
-observed that the southern cap outdoes its antipodal counterpart at its
-maximum, showing that the longer, colder winter has its effect in snow
-or hoar-frost deposition. But, on the other hand, instead of excelling
-<span class="pagenum"><a name="Page_199" id="Page_199">[Pg 199]</a></span>
-it at its minimum, which it should do to produce permanent glaciation,
-it so far falls short of its fellow that during the last opposition at
-which it could be well observed, it disappeared entirely. The short,
-hot summer, then, far exceeded in melting capacity that of the longer
-but colder one.</p>
-
-<p>Let us now suppose the precipitation to be increased, the winters and
-summers remaining both in length and temperature what they were before.
-The amount of snow which a summer of given length and warmth can
-dispose of is, roughly speaking, a definite quantity. For it depends
-to a great extent only on its amount of heat. The summer precipitation
-may be taken as offsetting itself in the two hemispheres alike. If,
-then, the snowfall in the winter be for any reason increased daily in
-both, a time will come when the deposition due the longer winter of
-the one will exceed what its summer can melt relatively to the other,
-and a permanent glaciation result in the hemisphere so circumstanced.
-Increased precipitation, then, not eccentricity of orbit, is the real
-cause of an Ice Age. And this astronomic deduction we owe not to
-theoretic conclusions, for which we lack the necessary quantitative
-data, but wholly to study of our neighbor in space. Had any one
-informed our geologic colleagues that they must look to the sky for
-definite information about the cause of an Ice Age, they would probably
-have been surprised.
-<span class="pagenum"><a name="Page_200" id="Page_200">[Pg 200]</a></span></p>
-
-<p>With this Martian information, received some years ago, it is pleasing
-now to see that Earthly knowledge is gradually catching up. For that
-increased precipitation could account for it, the evidence of pluvial
-eras in the equatorial regions, contemporaneous with glacial periods,
-indicates. But another and probably the chief factor involved was not
-a generally increased precipitation, potent as that would be, but an
-increased snow deposit due to temporary elevation of the ground.</p>
-
-<div class="figcenter">
-    <a name="I_200" id="I_200"> </a>
-    <img src="images/i_200.jpg" alt="" width="600" height="483" />
-    <p class="center space-below2"><span class="smcap">Glacial map of Eurasia—after James Geikie.</span></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_201" id="Page_201">[Pg 201]</a></span></p>
-
-<div class="figcenter">
-    <a name="I_201" id="I_201"> </a>
-    <img src="images/i_201.jpg" alt="" width="400" height="555" />
-    <p class="center"><span class="smcap">Map showing the glaciated area of North America—<br />the
-             arrows indicating the direction of ice movement—<br />Chamberlin and Salisbury.</span></p>
-</div>
-
-<p>For it now appears that there was no glacial <i>epoch</i>. Our early
-ideas inculcated by text-books at school received a rude shock when
-it appeared that the glacial <i>epoch</i> was not, as we had been led
-to believe, a polar phenomenon at all, but a local affair which on
-the face of it had nothing to do with the pole. For investigation
-has disclosed that instead of emanating from the pole southward, it
-proceeded from certain centres, descending thence in all directions,
-north as much as south. Thus there was a centre in Norway in 65°
-N. lat. and another in Scotland in 56° N. In North America there
-were three—the Labradorian in latitude 54° N., the Kerwatin to the
-northwest of Hudson’s Bay in latitude 62° N., and the Cordilleran
-along the Pacific coast in latitude 58° N. On the other hand, northern
-Siberia, the coldest region in the world, was not glaciated. That the
-ice flowed off these centres proves them to have been higher than the
-<span class="pagenum"><a name="Page_202" id="Page_202">[Pg 202]</a></span>
-sides. But we have further evidence of their then great height from
-the fact that dead littoral shells have been dredged from 1333 fathoms
-in the North Atlantic, and the prolongation under water of the fiords
-of Norway and of land valleys in North America witness to the same
-subsidence since.</p>
-
-<p>But evidence refuses to stop here. The Alps were then more glaciated
-than they are now. So was Kilimanjaro and Ruwenzori on the equator;
-and finally at the same time more ice and snow existed round about the
-south pole than is the case to-day. Now this is really going too far
-even for the most ardent believers in the force of eccentricity. For
-if the astronomic causes postulated were true, they must have produced
-just the opposite action at the antipodes, to say nothing of the crux
-of being equally effective at the equator. The theory lies down like
-the ass between two burdens. Whichever load it chooses to saddle, it
-must perforce abandon the other.</p>
-
-<p>So it turns out that the Ice Age was not an Ice Age at all but an
-untoward elevation of certain spots, and is to be relegated to the same
-limbo of exaggeration of a local incident into a world-wide cataclysm
-as the deluge. That some geologists will still cling to their former
-belief I doubt not; for as the philosophic old lady remarked: “There
-always have been two factions on every subject. Just as there are the
-<span class="pagenum"><a name="Page_203" id="Page_203">[Pg 203]</a></span>
-suffragists and anti-suffragists now, so there were slaveholders
-and the anti-slavery people in my time; and even in the days of
-the deluge, there were the diluvians who were in favor of a flood
-and the antediluvians who were opposed to it.” A tale which has a
-peculiarly scientific moral, as in science <i>anti</i> and <i>ante</i>
-seem often interchangeable terms.</p>
-
-<p>When I began the course of lectures that resulted in this volume, I
-labored under the apprehension that an account of cosmic physics might
-prove dull. It soon threatened to prove too startling. I therefore
-hasten to reassure the timid by saying that we are outgrowing ice ages
-and probably deluges. Elevations of the Earth’s crust are likely to be
-less and less pronounced in the future, and meanwhile such as exist are
-being slowly worn down. Secondly, the Sun is sure to continue of much
-the same efficiency for many æons to come. And lastly, the essential
-ingredient of both prodigies, water, is daily becoming more scarce. To
-this latter point we now turn, and perhaps when it is explained to him
-the reader may think that he has been rescued from one fate only to
-fall into the hands of another.</p>
-
-<p>Geology is necessarily limited in its scope to what has happened;
-planetology is not so circumscribed in its domain. It may indulge in
-prognostication of the future, and find countenance for its conclusions
-in the physiognomy of other worlds. Thus one of the things which it
-<span class="pagenum"><a name="Page_204" id="Page_204">[Pg 204]</a></span>
-foresees is the relative drying up of our abode. To those whose studies
-have never led them off this earth, the fact that the oceans are slowly
-evaporating into space may seem as incredible as would, to one marooned
-on a desert island, the march of mankind in the meantime. We live on
-an island in space, but can see something of the islands about us, and
-our conception of what is coming to our limited habitat can be judged
-most surely by what we note has happened to others more advanced than
-ourselves. Just as we look at Jupiter to perceive some likeness of
-what we once were, the real image of which has travelled by this time
-far into the depths of space beyond possibility of recall, so must we
-look to the Moon or Mars if we desire to see some faint adumbration
-of the pass to which we are likely to come. For from their lack of
-size they should have preceded us on the road we are bound to travel.
-Now, both these worlds to-day are water-lacking, in whole or part; the
-Moon practically absolutely so, Mars so far as any oceans or seas are
-concerned. We should do wisely then to take note. But we have more
-definite information than simply their present presentments. For both
-bear upon their faces marks of having held seas once upon a time. They
-were once, then, more as we are now. We cannot of course be sure, as
-we are unable to get near enough to scan their surfaces for signs of
-erosive action. But so far as we can make out, past seas best explain
-their appearance.
-<span class="pagenum"><a name="Page_205" id="Page_205">[Pg 205]</a></span></p>
-
-<div class="figcenter">
-    <a name="I_205" id="I_205"> </a>
-    <img src="images/i_205.jpg" alt="" width="400" height="396" />
-    <p class="center space-below2"><span class="smcap">The Moon—Photographed
-                           at the Lowell Observatory.</span></p>
-</div>
-
-<p>So sealike, indeed, was their look that the first astronomers to note
-them took them unhesitatingly for water expanses. Thus the moment the
-telescope brought the Moon near enough for map making of it we find the
-dark patches at once designated as seas. The Sea of Serenity, the Sea
-of Showers, the Bay of Rainbows, speak still of what once was supposed
-to be the nature of the dark, smooth, lunar surfaces they name.
-Suggestively, indeed, in an opera glass do they seem to lap the land.
-The Lake of Dreams fore-shadowed what was eventually to be thought of
-them. With increasing optical approach the substance evaporated, but
-the form remained. It was speedily evident that there was no water
-there; yet the semblance of its repository still lurked in those
-shadows and suggests itself to one scanning their surfaces to-day. If
-they be not old sea bottoms, they singularly mimic the reality in their
-smooth, sloping floors and their long, curving lines of beach. Their
-strange uniformity shows that something protected them from volcanic
-<span class="pagenum"><a name="Page_206" id="Page_206">[Pg 206]</a></span>
-fury while the rest of the lunar face was being corrugated. This
-preservative points to some superincumbent pressure which can have been
-no other than water. Lava-flows on such a scale seem inadmissible.
-What these surfaces show and what they do not show alike hint them sea
-bottoms once upon a time. In the strange chalk-like hue of the lunar
-landscape they look like plaster of Paris death-masks of the former seas.</p>
-
-<p>A like history fell to the lot of the surface features of Mars. There
-too, as soon as the telescope revealed them and their permanency of
-place, the dark patches upon the planet’s face were forthrightly taken
-for seas, and were so called: the Sea of the Sirens and the Great Red
-Sea. Such they long continued to be deemed. The seas of Mars held water
-in theory centuries after the idea of the lunar had vanished into
-air. At last, ruthless science pricked the pretty bubble analogy had
-pictured. Being so much farther off than the Moon, it was much later
-that their true character came out. Come out it has, though, within the
-last few years. Lines—some of the so called canals—have been detected
-crossing the seas, lines persistent in place. This has effectually
-disposed of any water in them. But here again something of semblance
-is left behind. They are still the darkest portions of the planet, and
-their tint changes in places with the progress of the planet’s year.
-That their color is that of vegetation, and that its change obeys the
-<span class="pagenum"><a name="Page_207" id="Page_207">[Pg 207]</a></span>
-seasons, stamp it for vegetation in fact. Thus these regions must be
-more humid than the rest of Mars. They must, therefore, be lower. That
-they are thus lower and possess a modicum of water to-day marks them
-out for the spots where seas would be, were there any seas to be. As we
-know of a <i>vera causa</i> which has for ages been tending to deplete them,
-extrapolation from what is now going on returns them the water they
-have lost and rehabilitates their ancient aquatic character. To the
-far-sight of inference, seas they again become in the morning of the
-ages long ago when Mars itself was young.</p>
-
-<p>Nor is this the end of the evidence. When we compare quantitatively the
-areas occupied by the quondam seas on Mars and on the Moon, we find
-reason to increase our confidence in our deduction. For the smaller
-body, the Moon, should have had less water relatively, at the time when
-the seas there were laid down, than the larger, Mars. Because from the
-moment its mass began to collect, it was in process of parting with its
-gases, water-vapor among the rest, and, as we shall see more in detail
-in the next chapter, it had from the start less hold on them than Mars.
-Its oceans, therefore, should have been less extensive than the Martian
-ones. This is what the present lunar Mare seem to attest. They are less
-extended than the dark areas of Mars. A fact which becomes the more
-<span class="pagenum"><a name="Page_208" id="Page_208">[Pg 208]</a></span>
-evident when we remember that the Moon has long turned the same face
-to the Earth. Her shape, therefore, has been that of an egg, with the
-apex pointing toward our world. Here the water would chiefly collect.
-The greater part of the seas she ever had should be on our side of her
-surface, the one she presents in perpetuity to our gaze.</p>
-
-<p>It is to the heavens that we must look for our surest information on
-such a cosmic point, because of the long perspective other bodies give
-us of our own career. Less conclusive, because dependent upon less
-time, is any evidence our globe can offer. Yet even from it we may
-learn something; if nothing else, that it does not contradict the story
-of the sky. To it, therefore, we return, quickened in apprehension by
-the sights we have elsewhere seen.</p>
-
-<p>The first thing our sharpened sense causes us to note is the spread
-of deserts even within historic times. Just as deserts show by their
-latitudinal girdling of the Earth their direct dependence upon the
-great system of planetary winds, as meteorologists recognizingly
-call them, so a study of the fringes of these belts discloses their
-encroachment upon formerly less arid lands. The southern borders of the
-Mediterranean reveal this all the way from Carthage to Palestine. The
-disappearance of their former peoples, leaving these lands but scantily
-<span class="pagenum"><a name="Page_209" id="Page_209">[Pg 209]</a></span>
-inhabited now, points to this; because other regions, as India, which
-still retain a waterful climate, are as populous as ever. Much of this
-is doubtless due to the overthrow of dynasties and the ensuing lapse
-of irrigation, but query: Is it all? For we have still more definite
-information in the drying up of the streams which have left the
-aqueducts of Carthage without continuation, as much to water on the one
-hand as to its drinkers on the other. Men may leave because of lack of
-water, but water does not leave because of dearth of men to drink.</p>
-
-<p>Recent search around the Caspian by Huntington has disclosed the
-like degeneration due to encroaching desertism there. Indeed, it
-is no chance coincidence that just where all the great nations
-thrived in the morning of the historic times should be precisely
-where populous peoples no longer exist. For neither increasing cold
-nor increasing heat is responsible for this, seeing that no general
-change has occurred in either. Nor were they particularly exposed to
-extermination by northern hordes of barbarians. Egypt as a world power
-died a natural death, and Babylonia too; but the common people died of
-thirst, indirect and unconscious and not wholly of their own choosing.
-Prehistoric records make this conclusion doubly sure, by lengthening
-the limit of our observation. Both extinct flora and extinct fauna tell
-the same tale. In the neighborhood of Cairo petrified forests attest
-<span class="pagenum"><a name="Page_210" id="Page_210">[Pg 210]</a></span>
-that Egypt was not always a wiped slate, while the unearthed animals of
-the Fayum bear witness to water where no water is to-day.</p>
-
-<div class="figcenter">
-    <a name="I_210" id="I_210"> </a>
-    <img src="images/i_210.jpg" alt="" width="600" height="355" />
-    <p class="center space-below2"><span class="smcap">Petrified bridge, third petrified forest, near<br />
-                    Adamana, Arizona—Photograph by Harvey.</span></p>
-</div>
-
-<p>Anywhere we wander along these girdling belts we find the same story
-written for us to read. The great deserts of New Mexico and Arizona
-show castellated structures far beyond the means of its present Indian
-population to inhabit. Yet this retrenchment occurred long before the
-white man came with his exterminating blight on everything he touched.
-Nor have we reason to suppose that it arose in consequence of invasion
-by other alien hordes. Individual communities may thus indeed have
-perished as the preservation of their domiciles intact leads us to
-infer, but all did not thus vanish from off the Earth. Here again
-humanity died or moved away because nature dried the sources of its
-<span class="pagenum"><a name="Page_211" id="Page_211">[Pg 211]</a></span>
-supply. And here, as elsewhere, we find prehistoric record in the rocks
-of a once more smiling state of things, strengthening the testimony we
-deduce from man. The forests, crowning now only the greater heights,
-are but the shrinking residues of what once clothed the land. The
-well-named Arid Zone is becoming more so every day.</p>
-
-<p>If from the land evidence of drying up we turn to the marine, we see
-the same shrinkage at work. It has even been discovered in a lowering
-of the ocean bed, but as this may so easily be disputed, we turn to
-one aspect of the situation which cannot so easily be gainsaid,—the
-bodies of water that have been cut off. That the Dead Sea, the Caspian,
-the Great Salt Lake, are slowly but surely giving way to land, is
-patent. If the climate at least were not more arid than before this
-could not occur; but more than this, if the ocean were not on the whole
-shrinking, there would be no tendency to leave such arms of itself
-behind to shrivel up. That the ocean basins are deepening is possible,
-but we know of one depletion which is not replaced—evaporation into
-space; and of another bound to come—withdrawal into fissures when the
-earth shall cease to be too hot.</p>
-
-<p>This gradual withdrawal of the water may seem an unpleasant one to
-contemplate, but like most things it has its silver lining in the hope
-it holds out that sometime there shall be no more sea. Those of us who
-<span class="pagenum"><a name="Page_212" id="Page_212">[Pg 212]</a></span>
-detest the constant going down to the sea in ships hardly more than the
-occasional going down with them, can take a crumb of comfort in the
-thought. Unfortunately it partakes of a somewhat far-off realization
-in our distant descendants, coming a little too late to be of material
-advantage to ourselves.</p>
-
-<p>But let me not leave the reader wholly disconsolate. For another
-thought we can take with us in closing our sketch of so much of the
-Earth’s life as brings it well down to to-day,—the thought that it has
-grown for us a steadily better place to contemplate from the earliest
-eras to the present time. Indeed, with innate prescience we forbore to
-appear till the prospect did prove pleasing. Finally, we may palliate
-prognostication by considering that if its future seem a thought less
-attractive, we, at least, shall not be there to see.</p>
-
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_213" id="Page_213">[Pg 213]</a></span></p>
-<div class="chapter">
-<h2 class="nobreak">CHAPTER VIII<br /><span class="h_subtitle">DEATH OF A WORLD</span></h2>
-</div>
-
-<p class="drop-cap">EVERYTHING around us on this Earth we see is
-subject to one inevitable cycle of birth, growth, decay. Nothing that
-begins but comes at last to end. Not less is this true of the Earth as
-a whole and of each of its sister planets. Though our own lives are too
-brief even to mark the slow nearing to that eventual goal, the past
-history of the Earth written in its rocks and the present aspects of
-the several planets that circle similarly round the Sun alike assure
-us of the course of aging as certainly as if time, with all it brings
-about, passed in one long procession before our very eyes.</p>
-
-<p>Death is a distressing thing to contemplate under any circumstances,
-and not less so to a philosopher when that of a whole world is
-concerned. To think that this fair globe with all it has brought forth
-must lapse in time to nothingness; that the generations of men shall
-cease to be, their very records obliterated, is something to strike a
-chill into the heart of the most callous and numb endeavor at its core.
-That æons must roll away before that final day is to the mind of the
-far-seeing no consolation for the end. Not only that we shall pass, but
-<span class="pagenum"><a name="Page_214" id="Page_214">[Pg 214]</a></span>
-that everything to show we ever were shall perish too, seems an
-extinction too overpowering for words.</p>
-
-<p>But vain regret avails not to change the universe’s course. What is
-concerns us and what will be too. From facing it we cannot turn away.
-We may alleviate its poignancy by the thought that our interest is
-after all remote, affecting chiefly descendants we shall never know,
-and commend to ourselves the altruistic example so nobly set us by
-doctors of medicine who, on the demise of others at which—and possibly
-to which—they have themselves assisted, show a fortitude not easily
-surpassed, a fortitude extending even to their bills. If they can act
-thus unshaken at sight of their contemporaries, we should not fall
-behind them in heroism toward posterity.</p>
-
-<p>Having in our last chapter run the gantlet of the geologists, we are
-in some sort fortified to face death—in a world—in this. The more so
-that we have some millenniums of respite before the execution of the
-decree. By the death of a planet we may designate that stage when all
-change on its surface, save disintegration, ceases. For then all we
-know as life in its manifold manifestations is at an end. To this it
-may come by many paths. For a planet, like a man, is exposed to death
-from a variety of untoward events.</p>
-
-<p>Of these the one least likely to occur is death by accident. This,
-<span class="pagenum"><a name="Page_215" id="Page_215">[Pg 215]</a></span>
-celestially speaking, is anything which may happen to the solar system
-from without, and is of the nature of an unforeseen catastrophe. Our
-Sun might, as we remarked, be run into. For so far as we know at
-present the stars are moving among themselves without any too careful
-regard for one another. The swarm may be circling a central Sun as
-André states, but the individual stars behave more like the random
-particles of a gas with licensed freedom to collide; whereas we may
-liken the members of the solar system to molecules in the solid state
-held to a centre from which they can never greatly depart. Their motions
-thus afford a sense of security lacking in the universe at large.</p>
-
-<p>Such an accident, a collision actual or virtual with another sun,
-would probably occur with some dark star; of which we sketched the
-ultimate results in our first chapter. The immediate ones would be of
-a most disastrous kind. For prefatory to the new birth would be the
-dissolution to make such resurrection possible. Destruction might come
-direct, or indirectly through the Sun. For though the Sun would be the
-tramp’s objective point, we might inadvertently find ourselves in the
-way. The choice would be purely academic; between being powdered, or
-deorbited and burnt up.</p>
-
-<p>So remote is this contingency that it need cause us no immediate
-alarm, as I carefully pointed out. But so strong is the instinct of
-<span class="pagenum"><a name="Page_216" id="Page_216">[Pg 216]</a></span>
-self-preservation and so pleasurable the sensation of spreading
-appalling news, that the press of America, and incidentally Europe,
-took fire, with the result, so I have been written, that by the time
-the pictured catastrophe reached the Pacific “it had assumed the
-dimensions of a first magnitude fact.”</p>
-
-<p>This is the first way in which our world may come by its death. It is
-possible, but unlikely. For our Earth, long before that, is morally
-certain to perish otherwise.</p>
-
-<p>The second mode is one, incident to the very constitution of our solar
-system. It follows as a direct outcome of that system’s mechanical
-evolution, and may be properly designated, therefore, as due to natural
-causes. It might be diagnosed as death by paralysis. For such it
-resembles in human beings, palsy of individual movement afflicting a
-planet instead of a man.</p>
-
-<p>Tidal friction is the slow undermining cause; a force which is
-constantly at work in the action of every body in the universe upon
-every other. As we previously explained, the pull of one mass upon
-another is inevitably differential. Not only is the second drawn in
-its entirety toward the first, falling literally as it circles round,
-but the nearer parts are drawn more than the centre and the centre
-more than those farthest away. We may liken the result to a stretched
-rotating rubber ball, with, however, one important difference,—that
-each layer is more or less free to shear over the others. The bulge,
-<span class="pagenum"><a name="Page_217" id="Page_217">[Pg 217]</a></span>
-solicited by the rotation to keep up, by the disturber to lag behind,
-is torn two ways, and the friction acts as a break upon the body’s
-rotation, tending first to turn it over if it be rotating backward
-and then to slow it down till the body presents the same face in
-perpetuity to its primary. The tides are the bulge, not simply those
-superficial ones which we observe in our oceans, and know to be so
-strong, but substantial ones of the whole body which we must conceive
-thus as egg-shaped through the action that goes on—the long diameter
-of the egg pointing somewhat ahead of the line joining its centre
-to the distorting mass. All the bodies in the solar system are thus
-really egg-shaped, though the deformation is so slight as to escape
-detection observationally. The knowledge is an instance of how much
-more perceptive the brain is than the eye. For we are certain of the
-fact, and yet to see it with our present means is impossible, and may
-long remain so.</p>
-
-<p>Two concomitant symptoms follow the friction of the tidal ansæ: a shift
-of the plane in which the rotation takes place, and a loss of speed in
-the spin itself. The first tends to bring the plane of rotation down
-to the orbital plane, with rotation and revolution in the same sense.
-This effect takes place quicker than the other, and in consequence
-different stages may be noted in the creeping paralysis by which the
-body is finally overcome. Loss of seasons characterizes the first. For
-the coincidence of the two planes means invariability in the Sun’s
-<span class="pagenum"><a name="Page_218" id="Page_218">[Pg 218]</a></span>
-declination throughout the year for a given latitude. This reduces
-all its days to one dead level in which summer and winter, spring and
-autumn, are always and everywhere the same. There is thus a return at
-the end of the planet’s career to an uneventful condition reminiscent of
-its start; a senility in planets comparable to second childhood in man.</p>
-
-<p>In large planets this outgrowing of seasons occurs before they have
-any, while the planet is yet cloud-wrapped. Such planets know nothing
-of some attributes of youth, like those unfortunate men who never
-were boys; just as reversely the meteorites are boys that never grew
-up. For if the planet be large, the action of the tidal forces is
-proportionately more powerful; while on the other hand the self-aging
-of the planet is greatly prolonged, and thus it may come about that
-the former process outstrips the latter to the missing of seasons
-entirely. This is sure to be the case with Jupiter, as the equator
-has already got down to within 3° of the orbit, and threatens to be
-the case with Saturn. These bodies, then, when they shall have put
-off their swaddling clothes of cloud, will wake to climates without
-seasons; globes where conditions are always the same on the same belts
-of latitude, and on which these alter progressively from equator to
-pole. Variety other than diurnal is thus excluded from their surfaces
-and from their skies. For the Sun and stars will rise always the same,
-<span class="pagenum"><a name="Page_219" id="Page_219">[Pg 219]</a></span>
-in punctual obedience only to the slowly shifting year.</p>
-
-<p>The next stage of deprivation is the parting with the day. Although the
-day disappears, the result is too much day or too little, depending on
-where you choose to consider yourself upon the afflicted orb. For tidal
-friction proceeds to lengthen the twenty-four or other hours first to
-weeks, then months, then years, and at last to infinity; thus bringing
-the sun to a stock-still on the meridian, to flood one side of the
-world with perpetual day and plunge the other in eternal night.</p>
-
-<p>Which of these two hemispheres would be the worse abode, is matter
-of personal predilection; dust or glacier, deserts both. Everlasting
-unshielded noon would cause a wind circulation from all points of the
-enlightened periphery to the centre, whence a funnel-shaped current
-would rise to overflow back into the antipodes, thence to return by the
-horizon again. As the night side would be several hundred degrees at
-least colder than the noon one, all the moisture would be evaporated
-on the sunlit hemisphere, to be carried round and deposited as ice on
-the other, there to stay. Life would be either toasted or <i>frappé</i>.
-A Sahara backed by polar regions would be the obverse and the reverse of
-the shield.
-<span class="pagenum"><a name="Page_220" id="Page_220">[Pg 220]</a></span></p>
-
-<div class="figcontainer">
-  <div class="figsub">
-   <a id="I_220" name="I_220"> </a>
-   <img src="images/i_220a.jpg" alt="" width="150" height="354" />
-   <p class="center">October 15, 1896.</p>
-  </div>
-  <div class="figsub">
-   <img  src="images/i_220b.jpg" alt="" width="175" height="352" />
-   <p class="center">February 12, 1897.</p>
-   </div>
-  <div class="figsub">
-   <img  src="images/i_220c.jpg" alt="" width="205" height="352" />
-   <p class="center">March 26, 1897.</p>
-   </div>
-    <p class="center space-below2"><span class="smcap">Venus—Drawings by Dr. Lowell
-               showing agreement at different distances.</span></p>
-</div>
-
-<p>The reader may deem the picture a fancy sketch which possibly may
-not appeal to him. Nevertheless, it not only is possible, but one
-which has overtaken our nearest of neighbors. To this pass the Mater
-Amorum, Venus herself, has already been brought. She betrays it by the
-wrinkles which modern observation has revealed upon her face. Innocent
-critics, with a gallantry one would hardly have credited them,—which
-shows how one may wrong even the humblest of creatures,—have denied
-the existence of these marks of age, on the chivalrous <i>a priori</i>
-assumption that it could not possibly be true because never seen
-before. Their negation, in naïve ignorance of the facts, partakes the
-logic of the gallant captain, who, when asked by a lady to guess her
-<span class="pagenum"><a name="Page_221" id="Page_221">[Pg 221]</a></span>
-age, replied: “’Pon my word, I haven’t the slightest idea,” hastily
-adding, “But you don’t look it!” Less commendable than this
-conventional nescience, but unfortunately more to the point, is the
-evidence of prying scientific curiosity. Shrewdly divined as much
-as detected by Schiaparelli, made more certain by the crow’s-feet
-disclosed at Flagstaff, and corroborated by the testimony of the
-spectroscope there, her isochronism of rotation and revolution lies
-beyond a doubt. Attraction to her lord has conquered at last her who
-was the cynosure of all. Venus, in her old age, stares forever at the
-Sun, and we all know how ill an aging beauty can support a garish light.</p>
-
-<p>Mercury has been brought to a like pass. This was evident even before
-the facts came out about Venus, for Venus, true to her instincts,
-shields herself with a veil of air which largely baffles man’s too
-curious gaze. Mercury, on the other hand, offers no objection to
-observation. When looked for at the proper time, his markings are
-quite distinct, dark, broken lines suggesting cracks. Schiaparelli,
-again, was the first to perceive the true state of the case, and his
-observations were independently confirmed and extended at Flagstaff in
-1896. In so doing the latter disclosed a very interesting fact. It was
-evident that the markings held in general a definite fixed position
-upon the illuminated part of the disk, showing that the planet kept the
-same face always to the Sun. But systematic observation, continued day
-<span class="pagenum"><a name="Page_222" id="Page_222">[Pg 222]</a></span>
-after day for weeks, disclosed a curious shift, which, though slight,
-was unmistakable. Upon thought the cause suggested itself, and on being
-subjected to calculation proved equal to such accounting. In this
-singular systematic sway stood revealed the libration in longitude
-caused by the eccentricity of the planet’s orbit.</p>
-
-<div class="figcenter">
-    <a name="I_222A" id="I_222A"> </a>
-    <img src="images/i_222a.jpg" alt="" width="500" height="411" />
-    <p class="center space-below2"><span class="smcap">Diagram of libration
-                       in longitude<br /> due to rotation.</span></p>
-
-    <a name="I_222B" id="I_222B"> </a>
-    <p class="f200"><b><i>Mercury.</i></b></p>
-    <img src="images/i_222b.jpg" alt="" width="600" height="430" />
-    <p class="f150"><i>Effect of Libration<br />Rotation 88 days.</i></p>
-</div>
-
-<p><span class="pagenum"><a name="Page_223" id="Page_223">[Pg 223]</a></span>
-Mercury revolves about the Sun in an ellipse more eccentric than that
-of any other principal planet. At times he is half as far off again
-from him as he is at others. When near, he travels faster than when
-far. For both reasons, nearness and speed, his angular revolution about
-the Sun varies greatly from point to point according to where he finds
-himself in his orbit. His rotation, however, is necessarily uniform.
-For even the Sun has no power at once to change the enormous moment
-of momentum of his axial spin. In consequence, at times his angular
-velocity of revolution gains on his rotation, at other times loses,
-both coming out together at the end of a complete Mercurial year. The
-result is a superb rhythmic oscillation, a true mercurial pendulum
-compensated by celestial laws to perfect isochronism of swing.</p>
-
-<p>The outward sign of this shows in the movement of the markings. To
-observers in space like ourselves, the planet seems to sway his head as
-he travels along his orbit. For weeks he turns his face, as shown by
-the markings on it, more and more over to the left; then turns it back
-again as far over to the right. It is as if he were looking furtively
-around as he hastens over his planetary path.</p>
-
-<p>Venus, of course, is equally subject to this law of distraction, but
-owing to the almost perfect circularity of her orbit she is less
-visibly affected. In fact, it is not possible to detect her lapse from
-a fixed regard to the Sun. At most it is no more than a glance out of
-the corner of her eyes—her slight deviation from perfect rectitude of
-<span class="pagenum"><a name="Page_224" id="Page_224">[Pg 224]</a></span>
-demeanor. Knowledge of the laws governing such action alone permits us
-to recognize its occurrence.</p>
-
-<p>Mercury and Venus are the only planets as yet that turn a constant
-face to their overruling lord. The reason for this appears when one
-goes into the matter analytically. The tidal force is not the direct
-pull of the Sun on a particle of the body, but the difference in the
-pulls upon a particle at the centre and one at the circumference. Being
-differential, it depends directly upon the radius of the distorted body
-and inversely upon the third power of its distance away. As the space
-through which the force acts is proportional to the force itself, the
-effect is as the squares of the quantities mentioned, or, inversely, as
-the sixth power of the distance and as the square of the body’s radius.
-The result thus proves greatest on the planets nearest to the Sun, and
-diminishes rapidly as we pass outward from him. If, then, the solar
-force had had time enough to produce its effects, it would be first in
-Mercury and then in Venus that it should be seen. And this is precisely
-where we observe it.</p>
-
-<p>The Moon presents us a well-known case of such filial regard, resulting
-in permanent incompetency of action on its own account. It turns always
-the same face to us, following us about with the mute attention of a
-dog to its master. Here again the libration may be detected, for no dog
-<span class="pagenum"><a name="Page_225" id="Page_225">[Pg 225]</a></span>
-but makes excursions on the road. This case differs from those of
-Mercury and Venus in that the body to which the regard is paid is not
-also the dispenser of light and warmth. In consequence, though the side
-of the Moon with which we are presented remains always the same, we do
-not always see it; the light creeping over it with the progress of the
-lunation, from new to full. On this account the worst that happens to
-our Moon in its old age is that its day becomes its month.</p>
-
-<div class="figcenter">
-    <a name="I_225" id="I_225"> </a>
-    <img src="images/i_225.jpg" alt="" width="600" height="317" />
-    <p class="center space-below2"><span class="smcap">Moon—full and half, photographed at the Lowell Observatory.</span></p>
-</div>
-
-<p>Our Moon is not peculiar in having its day and its month the same. On
-the contrary, it is now the rule with satellites thus to protract their
-days. So far as we can observe, all the large satellites of Jupiter
-turn the same face to him; those of Saturn pay him a like regard; while
-about those of Uranus and Neptune we are too far off to tell. Their
-direct respect for their primary, with only secondary recognition of
-<span class="pagenum"><a name="Page_226" id="Page_226">[Pg 226]</a></span>
-the Sun, keeps them from the full consequences of their fatal yielding
-to attraction. It is bad enough to have the day half a month long, but
-worse to have one that never ends, or, still worse, perpetual night.</p>
-
-<p>In our diagnosis of the cause of death in planets, we now pass from
-paralysis to heart failure. For so we may speak of the next affection
-which ends in their taking off, since it is due to want of circulation
-and lack of breath. It comes of a planet’s losing first its oceans and
-then its air.</p>
-
-<p>To understand how this distressing condition comes about, we must
-consider one of the interesting scientific legacies of the nineteenth
-century to the twentieth: the kinetic theory of gases.
-<span class="pagenum"><a name="Page_227" id="Page_227">[Pg 227]</a></span></p>
-
-<div class="figcenter">
-    <a name="I_227" id="I_227"> </a>
-    <img src="images/i_227.jpg" alt="" width="600" height="414" />
-    <p class="center space-below2"><span class="smcap">Illustrating molecular motion in a gas<br />
-                    (black molecules here considered at rest).</span></p>
-</div>
-
-<p>The kinetic theory of gases supposes them to be made up of minute
-particles all alike, which are perfectly elastic and are travelling
-hither and thither at great speeds in practically straight lines. In
-consequence, these are forever colliding among themselves, giving and
-taking velocities with bewildering rapidity, resulting in a state of
-confusion calculated to drive a computer mad. Somebody has likened a
-quiet bit of air to a boiler full of furious bees madly bent on getting
-out. The simile flatters the bees. To follow the vicissitudes of any
-one molecule in this hurly-burly would be out of the question; still
-more, it would seem, that of all of them at once. Yet no less Herculean
-a task confronts us. To find out about their motions, we are therefore
-driven to what is called the statistical method of inquiry,—which is
-simply a branch of the doctrine of probabilities. It is the method
-by which we learn how many people are going to catch cold in Boston
-next week when we know nothing about the people, or about colds, or
-about catching them. At first sight it might seem as if we could never
-discover anything in this hopelessly ignorant way, and as if we had
-almost better call in a doctor. But in the multitude of colds—not
-of counsellors—lies wisdom. So in other things not hygienic. As you
-cannot possibly divine, for instance, what each boy in town is going to
-<span class="pagenum"><a name="Page_228" id="Page_228">[Pg 228]</a></span>
-do during the year, nor what is his make of mind, how can you say
-whether he will accidentally discharge a firearm and shoot his playmate
-or not! And yet if you take all the boys of Boston, you can predict to
-a nicety how many will thus let off a gun and “not know that it was loaded.”</p>
-
-<p>In this only genuine method of prophecy, complete ignorance of all the
-actual facts, we are able without knowing anything whatever about each
-of the molecules to predicate a good deal about them all. To begin
-with, the pressure a gas exerts upon the sides of a vessel containing
-it must be the bombardment the sides receive from the little molecules;
-and the heating due this rain of blows, or the temperature to which the
-vessel is raised, must measure their energy of translation. On this
-supposition it is found that the laws of Avogadro and of Boyle are
-perfectly accounted for, besides many more properties of gases which
-the theory explains, and as nothing yet has been encountered seriously
-contradicting it, we may consider it as almost as surely correct as the
-theory of gravitation. To three great geniuses of the last century we
-owe this remarkable discovery—Clausius, Clerk Maxwell, and Boltzmann.</p>
-
-<p>By determining the density of a gas at a given temperature and under a
-given pressure, we can find by the statistical method the average speed
-of its molecules. It depends on the most probable distribution of their
-<span class="pagenum"><a name="Page_229" id="Page_229">[Pg 229]</a></span>
-energy. For hydrogen at the temperature of melting ice, and under
-atmospheric pressure, this speed proves to be a little over a mile a
-second—a speed, curiously enough, which is to that of light almost
-exactly as centimetres to miles. But some of the molecules are going at
-speeds much above the mean; fewer and fewer as the speed gets higher.
-Just how many there are for any assigned speed, we can calculate by the
-same ingenious application of unknown quantities.</p>
-
-<div class="figcenter">
-    <a name="I_229" id="I_229"> </a>
-    <img src="images/i_229.jpg" alt="" width="600" height="357" />
-    <p class="center space-below2"><span class="smcap">Distribution of molecular velocities in a gas.</span></p>
-</div>
-
-<p>These speeds have been found for a temperature of freezing, and as
-the speed varies as the square root of the absolute temperature, we
-might suppose that when an adventurous or lucky molecule arrived at
-practically the limit of the atmosphere, where the cold is intense, it
-<span class="pagenum"><a name="Page_230" id="Page_230">[Pg 230]</a></span>
-would become numbly sluggish. But let us consider this. When we enclose
-a gas in a cooler vessel, the molecules bombard the sides more than
-they are bombarded back. In consequence, they lose energy; as we say,
-are cooled. But in free air if a molecule be fortunate enough to elude
-its neighbors, there is nothing to take away its motion but the ether
-through radiation, and this is a very slow process. Thus the escaping
-fugitive must arrive at the confines of the air with the speed it had
-at its last encounter. We reach, then, this result: In space there is
-no such thing as temperature; temperature being simply the aggregate
-effect of molecular temperament. The reason we should consider it
-uncommonly cold up there is that fewer molecules would strike us.
-Quantity, therefore, in our estimation replaces quality,—a possible
-substitution which also accounts for some reputations, literary or
-otherwise. The only forces which could affect this lonely molecule would
-be the heating by the Sun, the repellent force of light, and gravity.</p>
-
-<p>Now the speed which gravity on the Earth can control is 6.9 miles a
-second. It can impart this to a body falling freely to it from infinite
-space, and can therefore annul it on the way up, and no more. If, then,
-any of the molecules reach the outer boundary of the air going at more
-than this speed, they will pass beyond the Earth’s power to restrain.
-They will become little rovers in space on their own account, and dart
-<span class="pagenum"><a name="Page_231" id="Page_231">[Pg 231]</a></span>
-off on interstellar travels of their own. This extension of the kinetic
-theory and of the consequent voyages of the molecules is due to Dr.
-Johnstone Stoney, who has since, humorously enough, tried to stop the
-very balls he set rolling. First thoughts are usually the best, after all.</p>
-
-<p>As among the molecules some are already travelling at speeds in excess
-of this critical velocity, molecules must constantly be attaining
-to this emancipation, and thus be leaving the Earth for good. In
-consequence there is a steady drain upon its gaseous covering.
-Furthermore, as we know from comets’ tails, the repellent power of the
-light-waves, what we may call the levity of light, much exceeds upon
-such volatile vagrants the heat excitement or even the gravity of the
-Sun, so that we arrive at this interesting conclusion—their escape is
-best effected under cover of the night.</p>
-
-<p>Again, the heavier the gas, the less its molecular speed at a given
-temperature, because its kinetic energy which measures that temperature
-is one-half the molecule’s mass into the square of its speed. Thus
-their ponderosity prevents as many of them from following their more
-agile cousins of a different constitution. So that the lighter gases
-are sooner gone. Water-vapor leaves before oxygen. Nor is there any
-escape from this escape of the gases. It may take excessively long, but
-<span class="pagenum"><a name="Page_232" id="Page_232">[Pg 232]</a></span>
-go they must until a solitary individual who happens to have had the
-wrong end of the last collision is alone left hopelessly behind.</p>
-
-<p>Another factor also is concerned. The smaller the planet, the lower the
-utmost velocity it can control, and the quicker, therefore, it must
-lose its atmosphere. For a greater number of molecules must at every
-instant reach the releasing speed. Thus those bodies that are little
-shall, perforce, have less to cover themselves withal.</p>
-
-<p>Now this inevitable depletion of their atmospheric envelopes, the
-aspects of the various planets strikingly attest. They do so in most
-exemplary fashion, according to law. The larger, the major planets, as
-we have already remarked, have a perfect plethora of atmosphere, more
-than we at least know what to do with in the way of cataloguing yet.
-The medium-sized, like our own Earth, have a very comfortable amount;
-Mars, an uncomfortable one, as we consider, and the smallest none at
-all. All the smaller bodies of our system are thus painfully deprived
-so far as we can discover. We are certain of it in the case of our
-Moon and Mercury, the only ones we can see well enough to be sure.
-In further evidence it has been shown at the Yerkes and at Flagstaff
-that no perceptible effect of air betrays itself in the spectroscopic
-imprint of the rings of Saturn, those tiny satellites of his, and very
-recently a spectrogram of Ganymede, Jupiter’s third moon, made at
-Flagstaff for the purpose by Mr. E. C. Slipher has proved equally void
-of atmospheric hint.</p>
-
-<div class="figcenter">
-    <a name="I_232" id="I_232"> </a>
-    <img src="images/i_232.jpg" alt="" width="600" height="216" />
-    <div class="blockquot">
-    <p class="neg-indent space-below2"><span class="smcap">Spectrogram of Saturn—Photographed by Dr. V. M.
-                Slipher, Lowell Observatory, October 11, 1904. Exposure</span> 4ʰ <span class="smcap">on
-                 “27” gilt edge plate. Long camera placed beneath the slit. Titanium
-                 comparison spectrum. Enlargement by Mr. C. O. Lampland.</span></p>
-</div></div>
-
-<p><span class="pagenum"><a name="Page_233" id="Page_233">[Pg 233]</a></span>
-With the loss of water and of air, all possibility of development
-departs. Not only must every organism die, but even the inorganic
-can no longer change its state. In the extinction thus not only of
-inhabitants but of the habitat that made them possible, occurs a
-curious inversion of the order we are familiar with in the life history
-of organisms. In planets it is the grandchildren that die first, then
-the children, and lastly their surviving parent. And this is not
-accidental, but inevitably consequent upon their respective origins.
-For the offspring, as we may spell it with a hyphen, of any cosmic mass
-is of necessity smaller than that from which it issued. Being smaller,
-it must age quicker. In the natural order of events, then, its end must
-be reached first.</p>
-
-<p>Such has been the course taken, or still taking, by the bodies of
-our solar family. The latest generation has already succumbed to
-this ebbing of vitality with time. Every one of the satellites of
-the planets—those of Neptune, Uranus, Saturn, Jupiter, and our own
-Moon—is practically dead; born so the smaller which never were alive.
-Our own Moon carries its decrepitude on its face. To all intents and
-purposes its life is past; and that it had at one time a very fiery
-existence, the great lunar craters amply testify. It is now, for all its
-<span class="pagenum"><a name="Page_234" id="Page_234">[Pg 234]</a></span>
-flooding with radiance our winter nights, the lifeless statue of its
-former self.</p>
-
-<p>The same inevitable end, in default of others, is now overtaking the
-planetary group. Its approach is stamped on the face of Mars. There
-we see a world dying of exhaustion. The signs of it are legible in
-the markings we descry. How long before its work is done, we ignore.
-But that it is a matter of time only, our study of the laws of the
-inexorable lead us to conclude. Mars has been spared the fate of
-Mercury and Venus to perish by this other form of planetary death.</p>
-
-<p>Last in our enumeration of the causes by which the end of a world may
-be brought about, because the last to occur in order of time, is the
-extinction of the Sun itself. Certain to come and conclude the solar
-system’s history as the abode of life, if all the others should by any
-chance fail to precede it, it fittingly forms the climax, grand in its
-very quietude, of all that went before.</p>
-
-<p>By the same physical laws that caused our Earth once to be hot, the
-Sun shines to-day. Only its greater size has given it a life and
-a brilliancy denied to smaller orbs. The falling together of the
-scattered particles of which it is composed, caused, and still is
-causing, the dazzling splendor it emits. And so long as it remains
-gaseous, its temperature must increase, in spite of its lavish
-expenditure of heat, as Homer Lane discovered forty years ago.
-<span class="pagenum"><a name="Page_235" id="Page_235">[Pg 235]</a></span></p>
-
-<p>But the Sun’s store of heat, immense as it is to-day, and continued as
-it is bound to be for untold æons by means of contraction of its globe
-upon itself, and possibly by other causes, must some day give out. From
-its present gaseous condition it must gradually but eventually contract
-to a solid one, and this in turn radiate all its heat into space.
-Slowly its lustre must dim as it becomes incapable of replenishing
-its supply of motive power by further shrinkage in size. Fitfully,
-probably, like Mira Ceti to-day, it will show temporary bursts of
-splendor as if striving to regain the brightness it had lost, only to
-sink after each effort into more and more impotent senility. At last
-some day must come, if we may talk of days at all when the great event
-occurs when all days shall be blotted out, that the last flicker shall
-grow extinct in the orb that for so long has made the hearth of the
-whole system. For, presciently enough, the Latin word <i>focus</i> means
-hearth, and the body which includes within it the focus about which all
-the planets revolve also constitutes the hearth from which they all are
-lighted and warmed.</p>
-
-<p>When this ultimate moment arrives and the last spark of solar energy
-goes out, the Sun will have reverted once more to what it was when the
-cataclysm of the foretime stranger awoke it into activity. It will
-again be the dark body it was when our peering into the past first
-<span class="pagenum"><a name="Page_236" id="Page_236">[Pg 236]</a></span>
-descries it down the far vista of unrecorded time. Ghostlike it will
-travel through space, unknown, unheralded, till another collision shall
-cause it to take a place again among the bright company of heaven.
-Thus, in our account of the career of a solar system, we began by
-seeing with the mind’s eye a dark body travelling incognito in space,
-and a dark body we find ourselves again contemplating at the end.</p>
-
-<p>In this kaleidoscopic biograph of the solar system’s life, each
-picture dissolves into its successor by the falling together of its
-parts to fresh adjustments of stability, as in that instrument of
-pleasure which so witched our childish wonder in early youth. Just as
-when a combination had proved so pretty, once gone, to our sorrow no
-turning of the handle could ever bring it back, so in the march of
-worlds no retrace is possible of steps that once are past. Inexorable
-permutations lead from one state to the next, till the last of all be
-reached.</p>
-
-<p>Yet, unlike our childhood’s toy, reasoning can conjure up beside the
-present picture far vistas of what preceded it and of what is yet
-to come. Hidden from thought only by the distraction of the day, as
-the universe to sight lies hid by the day’s overpowering glare, both
-come out on its withdrawal till we wonder we never gazed before. Our
-own surroundings shut out the glories that lie beyond. Our veil of
-atmosphere cloaks them from our view. But wait, as an astronomer, till
-<span class="pagenum"><a name="Page_237" id="Page_237">[Pg 237]</a></span>
-the Sun sinks behind the hills and his gorgeous gold of parting fades
-to amber amid the tender tapestry of trees. The very air takes on a
-meaning which the flood of day had swamped. Seen itself, no longer
-imperfectly seen through, it wakes to semi-sentient existence, a spirit
-come to life aloft to shield us from the too immediate vacancy of
-space. The perfumes of the soil, the trees, the flowers, steal out to
-it, as the twilight glow itself exhales to heaven. In the hushed quiet
-of the gloaming Earth holds her breath, prescient of a revelation to come.</p>
-
-<p>Then as the half-light deepens, the universe appears. One by one the
-company of heaven stand forth to human sight. Venus first in all her
-glory brightens amid the dying splendor of the west, growing in lustre
-as her setting fades. From mid-heaven the Moon lets fall a sheen of
-silvery light, the ghostly mantle of her ghostlike self, over the
-silent Earth. Eastward Jupiter, like some great lantern of the system’s
-central sweep, swings upward from the twilight bow to take possession
-of the night. Beyond lies Saturn, or Uranus perchance dim with
-distance, measuring still greater span. All in order in their several
-place the noble cortège of the Sun is exposed to view, seen now by
-the courtesy of his withdrawal, backgrounded against the immensity of
-space. Great worlds, these separate attendants, and yet as nothings in
-<span class="pagenum"><a name="Page_238" id="Page_238">[Pg 238]</a></span>
-the void where stare the silent stars, huge suns themselves with
-retinues unseen, so vast the distances ’twixt us and them.</p>
-
-<p>No less a revelation awaits the opening of the shutters of the mind.
-If night discloses glimpses of the great beyond, knowledge invests it
-with a meaning unfolding and extending as acquaintance grows. Sight
-is human; insight seems divine. To know those points of light for
-other worlds themselves, worlds the telescope approaches as the years
-advance, while study reconstructs their past and visions forth their
-future, is to be made free of the heritage of heaven. Time opens to
-us as space expands. We stand upon the Earth, but in the sky, a vital
-portion not only of our globe, but of all of which it, too, forms part.
-To feel it is to enter upon another life; and if to realization of
-its beauty, its grandeur, and its sublimity of thought these chapters
-of its history have proved in any wise the portal, they have not been
-penned in vain.
-<span class="pagenum"><a name="Page_239" id="Page_239">[Pg 239]</a></span></p>
-
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_240" id="Page_240">[Pg 240]</a></span></p>
-<div class="chapter">
-<h2 class="nobreak">NOTES</h2>
-</div>
-<p><span class="pagenum"><a name="Page_241" id="Page_241">[Pg 241]</a></span></p>
-
-<div><a name="NOTE_1" id="NOTE_1"> </a>
-<p class="f120"><b>1<br /><span class="smcap">Meteor Orbits</span></b></p></div>
-
-<p>If the space of the solar system be equally filled with meteors
-throughout, or if they diminish as one goes out from the Sun according
-to any rational law, their average speed of encounter with the Earth
-would be nearly parabolic.</p>
-
-<p>If they were travelling in orbits like those of the short-period
-comets, that is with their aphelia at Jupiter’s orbit and their
-perihelia at or within the Earth’s, their major axes would lie between
-6.2 and 5.2. If we suppose their perihelion distances to be equally
-distributed according to distance, we have for the mean a major axis of
-5.7. Their velocity, then, at the point where they cross the Earth’s
-track would be given by</p>
-
-<p class="center"><i>v</i>² = µ(2/1 - 1/2.85),</p>
-
-<p class="no-indent">in which µ = 18.5² in miles per second  = 342.25,<br />
-whence  <i>v</i>  = 23.76 in miles per second.</p>
-
-<p>Suppose them to be approaching the Earth indifferently from all
-directions.</p>
-
-<p>At sunset the zenith faces the Earth’s quit; at sunrise the Earth’s
-goal. Let θ be the real angle of the meteor’s approach reckoned from
-the Earth’s quit; θ₁ the apparent angle due to compounding the meteor’s
-<span class="pagenum"><a name="Page_242" id="Page_242">[Pg 242]</a></span>
-velocity-direction with that of the Earth. Then those approaching it
-at any angle 0 less than that which makes θ₁ = 90° will be visible at
-sunset; those at a greater angle, at sunrise. The angle 01 is given by
-the relation,</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-      <td class="tdc"> <i>a</i></td>
-   </tr><tr>
-      <td class="tdr">cos θ₁ </td>
-      <td class="tdl"> = </td>
-      <td class="tdr"> +   —— ,</td>
-   </tr><tr>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-      <td class="tdc"> <i>x</i></td>
-   </tr>
- </tbody>
-</table>
-
-<p class="no-indent">in which <i>a</i> is the Earth’s velocity, <i>x</i> the meteor’s, and θ₁
-is reckoned from the Earth’s quit.</p>
-
-<p>The portion of the celestial dome covered at sunset is, therefore,</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">⌠</td>
-      <td class="tdl">θ₁</td>
-      <td class="tdc">⌠</td>
-      <td class="tdl">360°</td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">⎮</td>
-      <td class="tdl"> </td>
-      <td class="tdc">⎮</td>
-      <td class="tdc"> </td>
-      <td class="tdc">sin θ·<i>d</i>θ·<i>d</i>φ,</td>
-   </tr><tr>
-      <td class="tdc">⌡</td>
-      <td class="tdl">0</td>
-      <td class="tdc">⌡</td>
-      <td class="tdl">0</td>
-      <td class="tdc"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p class="no-indent">where φ is the azimuth,</p>
-<p class="no-indent">that at sunrise,</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">⌠</td>
-      <td class="tdl">180°</td>
-      <td class="tdc">⌠</td>
-      <td class="tdl">360°</td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">⎮</td>
-      <td class="tdl"> </td>
-      <td class="tdc">⎮</td>
-      <td class="tdc"> </td>
-      <td class="tdc">sin θ·<i>d</i>θ·<i>d</i>φ.</td>
-   </tr><tr>
-      <td class="tdc">⌡</td>
-      <td class="tdl">θ₁</td>
-      <td class="tdc">⌡</td>
-      <td class="tdl">0</td>
-      <td class="tdc"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p>If the meteors have direct motion only, θ can never exceed 90°, and the
-limits become,</p>
-
-<p class="no-indent">for sunset,</p>
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">⌠</td>
-      <td class="tdl">θ₁</td>
-      <td class="tdc">⌠</td>
-      <td class="tdl">360°</td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">⎮</td>
-      <td class="tdl"> </td>
-      <td class="tdc">⎮</td>
-      <td class="tdc"> </td>
-      <td class="tdc">sin θ·<i>d</i>θ·<i>d</i>φ,</td>
-   </tr><tr>
-      <td class="tdc">⌡</td>
-      <td class="tdl">0</td>
-      <td class="tdc">⌡</td>
-      <td class="tdl">0</td>
-      <td class="tdc"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p class="no-indent">and for sunrise,</p>
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">⌠</td>
-      <td class="tdl">90°</td>
-      <td class="tdc">⌠</td>
-      <td class="tdl">360°</td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">⎮</td>
-      <td class="tdl"> </td>
-      <td class="tdc">⎮</td>
-      <td class="tdc"> </td>
-      <td class="tdc">sin θ·<i>d</i>θ·<i>d</i>φ.</td>
-   </tr><tr>
-      <td class="tdc">⌡</td>
-      <td class="tdl">θ₁</td>
-      <td class="tdc">⌡</td>
-      <td class="tdl">0</td>
-      <td class="tdc"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p>The mean inclination at sunset is</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">⌠</td>
-      <td class="tdl">θ₁</td>
-      <td class="tdc">⌠</td>
-      <td class="tdl">360°</td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">⎮</td>
-      <td class="tdl"> </td>
-      <td class="tdc">⎮</td>
-      <td class="tdc"> </td>
-      <td class="tdc">θ₁ · sin θ·<i>d</i>θ·<i>d</i>φ,</td>
-   </tr><tr>
-      <td class="tdc">⌡</td>
-      <td class="tdl">0</td>
-      <td class="tdc">⌡</td>
-      <td class="tdl">0</td>
-      <td class="tdc"> </td>
-   </tr>
- </tbody>
-</table>
-<p class="center">⸻⸻⸻⸻ ,</p>
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">⌠</td>
-      <td class="tdl">θ₁</td>
-      <td class="tdc">⌠</td>
-      <td class="tdl">360°</td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">⎮</td>
-      <td class="tdl"> </td>
-      <td class="tdc">⎮</td>
-      <td class="tdc"> </td>
-      <td class="tdc">sin θ·<i>d</i>θ·<i>d</i>φ,</td>
-   </tr><tr>
-      <td class="tdc">⌡</td>
-      <td class="tdl">0</td>
-      <td class="tdc">⌡</td>
-      <td class="tdl">0</td>
-      <td class="tdc"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p class="no-indent">in which θ₁ must be expressed in terms of θ, etc.</p>
-
-<p>From this it appears that the relative number of bodies, travelling in
-all directions and at parabolic speed, which the Earth would encounter
-at sunrise and sunset respectively would be:—</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdl">sunrise</td>
-      <td class="tdl_ws1">5.8</td>
-   </tr><tr>
-      <td class="tdl">sunset</td>
-      <td class="tdl_ws1">1.0</td>
-   </tr>
- </tbody>
-</table>
-<p class="no-indent"><span class="pagenum"><a name="Page_243" id="Page_243">[Pg 243]</a></span>
-and with the speed of the short-period comets,</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdl">sunrise</td>
-      <td class="tdl_ws1">8.0</td>
-   </tr><tr>
-      <td class="tdl">sunset</td>
-      <td class="tdl_ws1">1.0</td>
-   </tr>
- </tbody>
-</table>
-
-<p>If, however, the bodies were all moving in the same sense as the Earth,
-<i>i.e.</i> direct, the ratios would be:—</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" rules="cols">
-  <thead><tr>
-      <th class="tdc bb2" colspan="4"> </th>
-   </tr><tr>
-      <th class="tdc bb"> </th>
-      <th class="tdc bb"><span class="smcap"> Parabolic <br />Speed</span></th>
-      <th class="tdc bb"><span class="smcap"> Speed of Short <br />Period Comets</span></th>
-      <th class="tdc bb"><span class="smcap"> Speed of Actual Short-Period<br />Period Comets about Jupiter</span></th>
-   </tr>
-  </thead>
-  <tbody><tr>
-      <td class="tdl bb">Sunrise</td>
-      <td class="tdc bb">2.4</td>
-      <td class="tdc bb">3.5</td>
-      <td class="tdc bb">3.3</td>
-   </tr><tr>
-      <td class="tdl">Sunset</td>
-      <td class="tdc">1.0</td>
-      <td class="tdc">1.0</td>
-      <td class="tdc">1.0</td>
-   </tr><tr>
-      <td class="tdc bt" colspan="4"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p>As the actual number encountered is between 2 and 3 to 1, we see that
-the greater part must be travelling in the same sense as the Earth, since
-they come indifferently at all altitudes from the plane of her orbit.</p>
-
-<div><a name="NOTE_2" id="NOTE_2"> </a>
-<p class="f120"><b>2<br /><span class="smcap">Densities of the Planets</span></b></p></div>
-
-<p>The densities of the principal planets, so far as we can determine them
-at present, the density of water being unity, are:—</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdl">Mercury</td>
-      <td class="tdc">3.65</td>
-      <td class="tdl_ws1"> </td>
-   </tr><tr>
-      <td class="tdl">Venus</td>
-      <td class="tdc">5.36</td>
-      <td class="tdl_ws1"> </td>
-   </tr><tr>
-      <td class="tdl">Earth</td>
-      <td class="tdc">5.53</td>
-      <td class="tdl_ws1"> </td>
-   </tr><tr>
-      <td class="tdl">Moon</td>
-      <td class="tdc">3.32</td>
-      <td class="tdl_ws1"> </td>
-   </tr><tr>
-      <td class="tdl">Mars</td>
-      <td class="tdc">3.93</td>
-      <td class="tdl_ws1"> </td>
-   </tr><tr>
-      <td class="tdl"> </td>
-      <td class="tdc">———</td>
-      <td class="tdl_ws1">mean 4.36</td>
-   </tr><tr>
-      <td class="tdl">Jupiter</td>
-      <td class="tdc">1.33</td>
-      <td class="tdl_ws1"> </td>
-   </tr><tr>
-      <td class="tdl">Saturn</td>
-      <td class="tdc">0.72</td>
-      <td class="tdl_ws1"> </td>
-   </tr><tr>
-      <td class="tdl">Uranus</td>
-      <td class="tdc">1.22</td>
-      <td class="tdl_ws1"> </td>
-   </tr><tr>
-      <td class="tdl">Neptune</td>
-      <td class="tdc">1.11</td>
-      <td class="tdl_ws1"> </td>
-   </tr><tr>
-      <td class="tdl"> </td>
-      <td class="tdc">———</td>
-      <td class="tdl_ws1">mean 1.09</td>
-   </tr><tr>
-      <td class="tdl">Sun</td>
-      <td class="tdc">1.38</td>
-      <td class="tdl_ws1"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p>The second decimal place is not to be considered as anything but an indication.</p>
-
-<div><a name="NOTE_3" id="NOTE_3"> </a>
-<p class="f120"><b>3<br /><span class="smcap">Variation in Spectroscopic Shift</span></b></p></div>
-
-<p>In the case of a body reflecting light, the shift differs from that for
-<span class="pagenum"><a name="Page_244" id="Page_244">[Pg 244]</a></span>
-a body emitting it. If the planet be on the further side of the Sun,
-the approaching rim advances both toward the Sun and toward the Earth,
-thus doubling the shift. The receding rim recedes in like manner. At
-elongation the rims approach or recede with regard to the Earth, but
-not the Sun, and the shift is single as for emission. At inferior
-conjunction rotational approach to the Earth implies rotational
-recession from the Sun, and the two effects cancel.</p>
-
-<div><a name="NOTE_4" id="NOTE_4"> </a>
-<p class="f120"><b>4<br /><span class="smcap">On the Planets’ Orbital Tilts</span></b></p></div>
-
-<p>The tilts of the plane of rotation of the Sun and of the orbits of the
-several planets to the dynamical plane of the system tabulated are:—</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdl">Sun</td>
-      <td class="tdl_ws1">7°</td>
-   </tr><tr>
-      <td class="tdl">Mercury</td>
-      <td class="tdl_ws1">6° 14′</td>
-   </tr><tr>
-      <td class="tdl">Venus</td>
-      <td class="tdl_ws1">2°   4′</td>
-   </tr><tr>
-      <td class="tdl">Earth</td>
-      <td class="tdl_ws1">1° 41′</td>
-   </tr><tr>
-      <td class="tdl">Mars</td>
-      <td class="tdl_ws1">1° 38′</td>
-   </tr><tr>
-      <td class="tdl">Asteroids</td>
-      <td class="tdl_ws1">various</td>
-   </tr><tr>
-      <td class="tdl">Jupiter</td>
-      <td class="tdl_ws1">    20′</td>
-   </tr><tr>
-      <td class="tdl">Saturn</td>
-      <td class="tdl_ws1">    56′</td>
-   </tr><tr>
-      <td class="tdl">Uranus</td>
-      <td class="tdl_ws1">1°  2′</td>
-   </tr><tr>
-      <td class="tdl">Neptune</td>
-      <td class="tdl_ws1">    43′</td>
-   </tr>
- </tbody>
-</table>
-
-<p class="no-indent">where, in the determination of that plane, the latest values
-of the masses of the planets and the rotations of the Sun, Jupiter, and Saturn
-have been taken into account.</p>
-
-<p>These tilts suggest something, doubtless, but it is by no means clear
-what it is they suggest. They are just as compatible with a giving off
-from a slowly condensing nebula as with an origin by shock. The greater
-inclinations of Mercury and Venus may be due to their late birth from
-the central mass without the necessity of a cataclysm, the rotation
-of that central mass out of the general plane being caused by the
-consensus of the motions of the particles from which it was formed. The
-accordance of the larger planetary masses with the dynamical plane of
-the system would necessarily result from their great aggregations. So
-that this, too, is quite possible without shock.
-<span class="pagenum"><a name="Page_245" id="Page_245">[Pg 245]</a></span></p>
-
-<div><a name="NOTE_5" id="NOTE_5"> </a>
-<p class="f120"><b>5<br /><span class="smcap">Planets and their Satellite Systems</span></b></p></div>
-
-<p>If we compute the speeds of satellites about their primaries in the
-solar system and compare them with the velocities in their orbits of
-the planets themselves, a striking parallelism stands displayed between
-the several systems. This is shown in the following table of them:</p>
-
-<table border="0" cellspacing="0" summary="Planets" cellpadding="0" rules="cols">
-  <thead><tr>
-      <th class="tdc bb2" colspan="6"> </th>
-   </tr><tr>
-      <th class="tdc bb" rowspan="2" colspan="2"> </th>
-      <th class="tdc bb" colspan="2"><span class="smcap">Mean Speed,<br />Miles a Second</span></th>
-      <th class="tdc bb"><span class="smcap"> Parabolic <br />Speed at<br />Orbit</span></th>
-      <th class="tdc bb" rowspan="2"><span class="smcap">Ratio Speed<br />Sat. about<br />
-                                              Primary to<br />Planet’s<br /> Speed in Orbit</span></th>
-   </tr><tr>
-      <th class="tdc bb"> of Primary <br />in Orbit<br /><i>V</i></th>
-      <th class="tdc bb">of Satellite<br /> about Primary <br /><i>v</i></th>
-      <th class="tdc bb">Miles a<br />second</th>
-   </tr>
-  </thead>
-  <tbody><tr>
-      <td class="tdl" colspan="2">Jupiter<span class="ws2"> </span></td>
-      <td class="tdc">8.1</td>
-      <td class="tdc"> </td>
-      <td class="tdc">11.5</td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdr" colspan="2">Sat. 1 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">10.7  </td>
-      <td class="tdc"> </td>
-      <td class="tdc">1.32</td>
-   </tr><tr>
-      <td class="tdr" colspan="2">2 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">8.5</td>
-      <td class="tdc"> </td>
-      <td class="tdc">1.05</td>
-   </tr><tr>
-      <td class="tdr" colspan="2">3 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">6.7</td>
-      <td class="tdc"> </td>
-      <td class="tdc">0.83</td>
-   </tr><tr>
-      <td class="tdr bb" colspan="2">4 </td>
-      <td class="tdc bb"> </td>
-      <td class="tdc bb">5.1</td>
-      <td class="tdc bb"> </td>
-      <td class="tdc bb">0.63</td>
-   </tr><tr>
-      <td class="tdl" colspan="2">Saturn</td>
-      <td class="tdc">6.0</td>
-      <td class="tdc"> </td>
-      <td class="tdc">8.5</td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdr" colspan="2">1 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">9.0</td>
-      <td class="tdc"> </td>
-      <td class="tdc">1.50</td>
-   </tr><tr>
-      <td class="tdr" colspan="2">2 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">7.9</td>
-      <td class="tdc"> </td>
-      <td class="tdc">1.31</td>
-   </tr><tr>
-      <td class="tdr" colspan="2">3 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">8.2</td>
-      <td class="tdc"> </td>
-      <td class="tdc">1.36</td>
-   </tr><tr>
-      <td class="tdr" colspan="2">4 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">8.2</td>
-      <td class="tdc"> </td>
-      <td class="tdc">1.36</td>
-   </tr><tr>
-      <td class="tdr" colspan="2">5 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">5.3</td>
-      <td class="tdc"> </td>
-      <td class="tdc">0.89</td>
-   </tr><tr>
-      <td class="tdr" colspan="2">6 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">3.5</td>
-      <td class="tdc"> </td>
-      <td class="tdc">0.59</td>
-   </tr><tr>
-      <td class="tdr bb" colspan="2">8 </td>
-      <td class="tdc bb"> </td>
-      <td class="tdc bb">2.0</td>
-      <td class="tdc bb"> </td>
-      <td class="tdc bb">0.34</td>
-   </tr><tr>
-      <td class="tdl" colspan="2">Uranus</td>
-      <td class="tdc">4.2</td>
-      <td class="tdc"> </td>
-      <td class="tdc">5.9</td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdr" colspan="2">1 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">3.5</td>
-      <td class="tdc"> </td>
-      <td class="tdc">0.82</td>
-   </tr><tr>
-      <td class="tdr" colspan="2">2 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">2.9</td>
-      <td class="tdc"> </td>
-      <td class="tdc">0.70</td>
-   </tr><tr>
-      <td class="tdr" colspan="2">3 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">2.3</td>
-      <td class="tdc"> </td>
-      <td class="tdc">0.54</td>
-   </tr><tr>
-      <td class="tdr bb" colspan="2">4 </td>
-      <td class="tdc bb"> </td>
-      <td class="tdc bb">2.0</td>
-      <td class="tdc bb"> </td>
-      <td class="tdc bb">0.47</td>
-   </tr><tr>
-      <td class="tdl" colspan="2">Neptune</td>
-      <td class="tdc">3.4</td>
-      <td class="tdc"> </td>
-      <td class="tdc">4.8</td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdr" colspan="2">1 </td>
-      <td class="tdc"> </td>
-      <td class="tdc">2.7</td>
-      <td class="tdc"> </td>
-      <td class="tdc">0.81</td>
-   </tr><tr>
-      <td class="tdc bt" colspan="6"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p>The relations here disclosed are too systematic to be the result of chance.
-<span class="pagenum"><a name="Page_246" id="Page_246">[Pg 246]</a></span></p>
-
-<p>The orbits of all these satellites have no perceptible eccentricity
-independent of perturbation except Iapetus, of which the eccentricity
-is about .03.</p>
-
-<p>In view of the various cosmogonies which have been advanced for the
-genesis of the solar system it is interesting to note what these
-speeds imply as to the effect upon the satellites of the impact of
-particles circulating in the interplanetary spaces at the time the
-system evolved. To simplify the question we shall suppose—which is
-sufficiently near the truth—that the planets move in circles, the
-interplanetary particles in orbits of any eccentricity.</p>
-
-<p>Taking the Sun’s mass as unity, the distance <i>R</i> of any given planet
-from the Sun also as unity, let the planet’s mass be represented by <i>M</i>
-and the radius of its satellite’s orbit, supposed circular, as <i>r</i>. We
-have for the space velocity of the satellite on the sunward side of the
-planet, calling that of the planet in its orbit <i>V</i> and that of the
-satellite in its orbit round the planet <i>v</i>,</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-      <td class="tdl bb"> </td>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-      <td class="tdl bb"> </td>
-   </tr><tr>
-      <td class="tdl"><i>V</i> - <i>v</i> = </td>
-      <td class="tdl">√</td>
-      <td class="tdl">1/<i>R</i></td>
-      <td class="tdc"> - </td>
-      <td class="tdl">√</td>
-      <td class="tdl"><i>M</i>/<i>r</i>.</td>
-   </tr>
- </tbody>
-</table>
-
-<p>For a particle, the semi-major axis of whose orbit is <i>a₁</i> and which
-shall encounter the satellite, the velocity is</p>
-
-<p class="center"><i>v</i>₁ = <big>(</big>2/(<i>R</i>-<i>r</i>) - 1/<i>a₁</i><big>)</big><sup>½</sup>.</p>
-
-<p>That no effect shall be produced by the impact of these two bodies,
-their velocities must be equal, or</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc"> </td>
-      <td class="tdl bb"> </td>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-      <td class="tdl bb"> </td>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-      <td class="tdl bb"> </td>
-   </tr><tr>
-      <td class="tdl">√</td>
-      <td class="tdl">1/<i>R</i></td>
-      <td class="tdc"> - </td>
-      <td class="tdl">√</td>
-      <td class="tdl"><i>M</i>/<i>r</i>.</td>
-      <td class="tdc"> = </td>
-      <td class="tdl">√</td>
-      <td class="tdl">2/(<i>R</i>-<i>r</i>) - 1/<i>a₁</i></td>
-   </tr>
- </tbody>
-</table>
-
-<p>As  <i>R</i>-<i>r</i> = <i>a₁</i>(1 + <i>e</i>)  for the point of impact if the
-particle be wholly within the orbit of the planet and <i>e</i> the eccentricity of its
-orbit, we find</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-      <td class="tdl bb"> </td>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdl"><i>e</i> = </td>
-      <td class="tdl"> 2</td>
-      <td class="tdc">√</td>
-      <td class="tdl"><i>MR</i>/<i>r</i></td>
-      <td class="tdl"> - <i>RM</i>/<i>r</i></td>
-      <td class="tdl"> approx.</td>
-   </tr>
- </tbody>
-</table>
-
-<p class="no-indent"><span class="pagenum"><a name="Page_247" id="Page_247">[Pg 247]</a></span>
-for the case of no action, the other terms being insensible for the
-satellites in the table, since in all <i>r</i> &lt; <i>R</i>/400.</p>
-
-<p>Supposing, now, the particles within the orbit of the planet to be
-equally distributed according to their major axes, then as the velocity
-of any one of them, taking <i>R</i>-<i>r</i> = <i>R</i> approx. as unity, is</p>
-
-<p class="center"><i>v</i>₁ = (2/1 - 1/<i>a₁</i>)<sup>½</sup>,</p>
-
-<p class="no-indent">the mean velocity of all of those which may encounter the
-satellite is, at the point of collision,</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">⌠¹ </td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">⎮  </td>
-      <td class="tdc">((2<i>a₁</i> - 1)<sup>½</sup> / <i>a₁</i><sup>½</sup>) <i>da</i>₁</td>
-   </tr><tr>
-      <td class="tdc">⌡<sub>½</sub></td>
-      <td class="tdc"> </td>
-   </tr>
- </tbody>
-</table>
-<p class="center">⸻⸻⸻⸻⸻</p>
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">⌠¹ </td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">⎮  </td>
-      <td class="tdc"> <i>da</i>₁</td>
-   </tr><tr>
-      <td class="tdc">⌡<sub>½</sub></td>
-      <td class="tdc"> </td>
-   </tr>
- </tbody>
-</table>
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdr_bott">1</td>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">= 2 <span class="bigfont300">[</span></td>
-      <td class="tdc">(2<i>a₁</i>² - <i>a₁</i>)<sup>½</sup> - 1/√(2)
-            log<span class="bigfont200">{</span>(2<i>a₁</i> - 1)<sup>½</sup> + √(2<i>a₁</i>)<span class="bigfont200">}</span></td>
-      <td class="tdc bigfont300">]</td>
-   </tr><tr>
-      <td class="tdr">½</td>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc" colspan="3"> </td>
-   </tr><tr>
-      <td class="tdc">= 0.754;</td>
-      <td class="tdr"> </td>
-      <td class="tdc"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p class="no-indent">that is, just over three-quarters of the
-planet’s speed in its orbit.</p>
-
-<p>If we suppose the particles to be equally distributed in
-space, we shall have more with a given major axis in proportion
-to that axis, and our integral will become</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">⌠¹ </td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">⎮  </td>
-      <td class="tdc">(2<i>a₁</i> - 1)<sup>½</sup> <i>a₁</i><sup>½</sup> <i>da</i>₁</td>
-   </tr><tr>
-      <td class="tdc">⌡<sub>½</sub></td>
-      <td class="tdc"> </td>
-   </tr>
- </tbody>
-</table>
-<p class="center">⸻⸻⸻⸻⸻</p>
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">⌠¹ </td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">⎮  </td>
-      <td class="tdc"> <i>a₁ da</i>₁</td>
-   </tr><tr>
-      <td class="tdc">⌡<sub>½</sub></td>
-      <td class="tdc"> </td>
-   </tr>
- </tbody>
-</table>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdr_bott">1</td>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">= 8/3<span class="bigfont300">[</span></td>
-      <td class="tdc">(4<i>a₁</i>-1)/8 (2<i>a₁</i>² - <i>a₁</i>)<sup>½</sup>
-                    - (1/16√2 ) log<span class="bigfont200">[</span>(2<i>a₁</i>² - <i>a₁</i>)<sup>½</sup>
-                     + √2 · <i>a₁</i> - 1/2√2<span class="bigfont200">]</span></td>
-      <td class="tdc bigfont300">]</td>
-   </tr><tr>
-      <td class="tdr">½</td>
-      <td class="tdc"> </td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc" colspan="3"> </td>
-   </tr><tr>
-      <td class="tdl" colspan="3">= 0.792  of the planet’s orbital speed.</td>
-   </tr>
- </tbody>
-</table>
-
-<p><span class="pagenum"><a name="Page_248" id="Page_248">[Pg 248]</a></span>
-The speed <i>v</i>, then, at which a satellite must be moving round the
-planet to have the same velocity as the average particle within the
-planet’s orbit, is</p>
-
-<p class="center"><i>V</i> - <i>v</i>₁ = <i>v</i>.</p>
-
-<p>This velocity is, for the several planets:—</p>
-
-<table border="0" cellspacing="0" summary="Planet Velocities" cellpadding="0" rules="cols" >
-  <thead><tr>
-      <th class="tdc bb2" colspan="3"> </th>
-   </tr><tr>
-      <th class="tdc bb" rowspan="2"> </th>
-      <th class="tdc bb"><span class="smcap">Distribution of<br /> Particles as their <br />Major Axes</span></th>
-      <th class="tdc bb"><span class="smcap">Distribution of<br /> Particles Equal<br />in Space</span></th>
-   </tr><tr>
-      <th class="tdc bb">Miles a second</th>
-      <th class="tdc bb">Miles a second</th>
-    </tr>
-  </thead>
-  <tbody><tr>
-      <td class="tdl">Jupiter</td>
-      <td class="tdc">2.0</td>
-      <td class="tdc">1.6</td>
-   </tr><tr>
-      <td class="tdl">Saturn</td>
-      <td class="tdc">1.5</td>
-      <td class="tdc">1.2</td>
-   </tr><tr>
-      <td class="tdl">Uranus</td>
-      <td class="tdc">1.0</td>
-      <td class="tdc">0.9</td>
-   </tr><tr>
-      <td class="tdl">Neptune&emsp; </td>
-      <td class="tdc">0.8</td>
-      <td class="tdc">0.7</td>
-   </tr><tr>
-      <td class="tdc bt" colspan="3"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p>If the satellite be moving in its orbit less fast than this, its
-space-speed will exceed that of the average particle; it will strike
-the particle at its own rear and be accelerated by the collision. If
-faster, the particle will strike it in front and retard it in its
-motion round its primary.</p>
-
-<p>From the table it appears that all the large satellites of all the
-planets have an orbital speed round their primaries exceeding those
-in either column. In consequence, all of them must have been retarded
-during their formation by the impact of interplanetary particles and
-forced nearer their primaries than would otherwise have been the case;
-and this whether the particles were distributed more densely toward the
-Sun, as 1/<i>a₁</i>, or were equally strewn throughout.</p>
-
-<p>For interplanetary particles whose orbits lie without the particular
-planet’s path the mean speed is the parabolic at the planet’s distance,
-given in the third column of the table. This is the case on either
-<span class="pagenum"><a name="Page_249" id="Page_249">[Pg 249]</a></span>
-supposition of distribution. The orbital speed of the satellite which
-shall not be affected by collisions with them is, for the several planets:—</p>
-
-<table border="0" cellspacing="0" summary="Planet Velocities" cellpadding="0" rules="cols" >
-  <thead><tr>
-      <th class="tdc bb2" colspan="2"> </th>
-   </tr><tr>
-      <th class="tdc bb"> </th>
-      <th class="tdc bb"> Miles a second </th>
-    </tr>
-  </thead>
-  <tbody><tr>
-      <td class="tdl">Jupiter</td>
-      <td class="tdc">3.4</td>
-   </tr><tr>
-      <td class="tdl">Saturn</td>
-      <td class="tdc">2.5</td>
-   </tr><tr>
-      <td class="tdl">Uranus</td>
-      <td class="tdc">1.7</td>
-   </tr><tr>
-      <td class="tdl">Neptune&emsp; </td>
-      <td class="tdc">1.4</td>
-   </tr><tr>
-      <td class="tdc bt" colspan="2"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p>All the satellites but Iapetus have orbital speeds exceeding this, and
-consequently are retarded also by these particles.</p>
-
-<p>For particles crossing the orbit (2) the mean velocity would be
-practically parabolic, 1.4, even if the distribution were as 1/<i>r</i>′,
-<i>r</i>′ being the distance from the Sun. The effect would depend upon
-the angle of approach and in the mean give a greater velocity for the
-particle than for the satellite within the orbit, a less one without;
-retarding the satellite in both cases. Thus the total effect of all the
-particles encountering the large satellites is to retard them and to
-tend to make them hug their primary.</p>
-
-<p>For retrograde satellites the velocities of impact with inside and
-outside particles moving direct are respectively:</p>
-
-<table border="0" cellspacing="0" summary="Planet Velocities" cellpadding="0" rules="cols" >
-  <thead><tr>
-      <th class="tdc bb2" colspan="3"> </th>
-   </tr><tr>
-      <th class="tdc bb"> </th>
-      <th class="tdc bb">  <span class="smcap">Inside</span>  </th>
-      <th class="tdc bb">  <span class="smcap">Outside</span>  </th>
-    </tr>
-  </thead>
-  <tbody><tr>
-      <td class="tdl">Jupiter</td>
-      <td class="tdc">2.0 + <i>v</i></td>
-      <td class="tdc"><i>v</i> + 3.4</td>
-   </tr><tr>
-      <td class="tdl">Saturn</td>
-      <td class="tdc">1.5 + <i>v</i></td>
-      <td class="tdc"><i>v</i> + 2.5</td>
-   </tr><tr>
-      <td class="tdl">Uranus</td>
-      <td class="tdc">1.0 + <i>v</i></td>
-      <td class="tdc"><i>v</i> + 1.7</td>
-   </tr><tr>
-      <td class="tdl">Neptune&emsp; </td>
-      <td class="tdc">0.8 + <i>v</i></td>
-      <td class="tdc"><i>v</i> + 1.4</td>
-   </tr><tr>
-      <td class="tdc bt" colspan="3"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p>In both cases the impact tends to check the satellite.</p>
-
-<p>Comparing with these the velocities of impact for direct satellites in
-a direct plenum:—
-<span class="pagenum"><a name="Page_250" id="Page_250">[Pg 250]</a></span></p>
-
-<table border="0" cellspacing="0" summary="Planet Velocities" cellpadding="0" rules="cols" >
-  <thead><tr>
-      <th class="tdc bb2" colspan="3"> </th>
-   </tr><tr>
-      <th class="tdc bb"> </th>
-      <th class="tdc bb">  <span class="smcap">Inside</span>  </th>
-      <th class="tdc bb">  <span class="smcap">Outside</span>  </th>
-    </tr>
-  </thead>
-  <tbody><tr>
-      <td class="tdl">Jupiter</td>
-      <td class="tdc">2.0 - <i>v</i></td>
-      <td class="tdc">3.4 - <i>v</i></td>
-   </tr><tr>
-      <td class="tdl">Saturn</td>
-      <td class="tdc">1.5 - <i>v</i></td>
-      <td class="tdc">2.5 - <i>v</i></td>
-   </tr><tr>
-      <td class="tdl">Uranus</td>
-      <td class="tdc">1.0 - <i>v</i></td>
-      <td class="tdc">1.7 - <i>v</i></td>
-   </tr><tr>
-      <td class="tdl">Neptune&emsp; </td>
-      <td class="tdc">0.8 - <i>v</i></td>
-      <td class="tdc">1.4 - <i>v</i></td>
-   </tr><tr>
-      <td class="tdc bt" colspan="3"> </td>
-   </tr>
- </tbody>
-</table>
-
-<p class="no-indent">the signs being taken positive when the motion is
-direct, we see that retrograde satellites would be more arrested than
-direct ones with the same orbital speed round the primary.</p>
-
-<p>In a plenum of direct moving particles, then, the force tending to
-stop the satellite and bring it down upon the planet is greater for
-retrograde satellites than for direct ones.</p>
-
-<p>If, therefore, the positions of the satellites have been controlled
-by the impact of interplanetary particles, the retrograde satellites
-should be found nearer their planets than the direct ones.</p>
-
-<div><a name="NOTE_6" id="NOTE_6"> </a>
-<p class="f120"><b>6<br /><span class="smcap">On the Induced Circularity of Orbits<br /> through Collision</span></b></p></div>
-
-<p>Since the moment of momentum is the velocity into the perpendicular
-upon its direction, in the time <i>dt</i> it is:—</p>
-
-<p class="center"><i>vp dt</i> = <i>h dt</i> = <i>r</i>²<i>d</i>Θ.</p>
-
-<p>The whole moment of momentum from perihelion to perihelion is therefore:—</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">    ⌠<small>360°</small></td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc">⎮  </td>
-      <td class="tdc"><i>r</i>²<i>d</i>Θ = <i>a</i>²·(1-<i>e</i>²)² / 1-<i>e</i>²</td>
-   </tr><tr>
-      <td class="tdc">⌡<sub>0</sub></td>
-      <td class="tdc"> </td>
-   </tr><tr>
-      <td class="tdc" colspan="2"> </td>
-   </tr>
- </tbody>
-</table>
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdl" colspan="13"><small>360°</small></td>
-   </tr><tr>
-      <td class="tdc_big300" rowspan="2">[</td>
-      <td class="tdc">-<i>e</i> sin Θ</td>
-      <td class="tdc" rowspan="2"> + </td>
-      <td class="tdc">2</td>
-      <td class="tdc" rowspan="2">  tan⁻¹</td>
-      <td class="tdc_big200" rowspan="2">(</td>
-      <td class="tdc_big200" rowspan="2">√</td>
-      <td class="tdc">1-<i>e</i></td>
-      <td class="tdc" rowspan="2"> · tan </td>
-      <td class="tdc">Θ</td>
-      <td class="tdc_big200" rowspan="2">)</td>
-      <td class="tdc_big300" rowspan="2">]</td>
-   </tr><tr>
-      <td class="tdc bt">1+<i>e</i> cos Θ</td>
-      <td class="tdc bt">(1-<i>e</i>²)<sup>½</sup></td>
-      <td class="tdc bt">1+<i>e</i></td>
-      <td class="tdc bt"> 2 </td>
-   </tr><tr>
-      <td class="tdl" colspan="13"><small>0</small></td>
-   </tr><tr>
-      <td class="tdc" colspan="2"> </td>
-   </tr>
- </tbody>
-</table>
-<p class="center"><span class="pagenum"><a name="Page_251" id="Page_251">[Pg 251]</a></span>
-= 2π<i>a</i>² · (1 - <i>e</i>²)<sup>½</sup>,</p>
-
-<p class="no-indent">which is twice the area of the ellipse.</p>
-
-<p>The energy in the ellipse during an interval <i>dt</i> is</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">1</td>
-      <td class="tdc" rowspan="2"><i>mv</i>²<i>dt</i> = </td>
-      <td class="tdc">1</td>
-      <td class="tdc" rowspan="2"><i>m</i>µ</td>
-      <td class="tdc_big200" rowspan="2">(</td>
-      <td class="tdc">2</td>
-      <td class="tdc" rowspan="2"> - </td>
-      <td class="tdc">1</td>
-      <td class="tdc_big200" rowspan="2">)</td>
-      <td class="tdc" rowspan="2"> <i>dt</i>,</td>
-   </tr><tr>
-      <td class="tdc bt"> 2 </td>
-      <td class="tdc bt"> 2 </td>
-      <td class="tdc bt"> <i>r</i> </td>
-      <td class="tdc"> <i>a</i> </td>
-   </tr>
- </tbody>
-</table>
-
-<p class="no-indent">from the well-known equation for the velocity
-in a focal conic. The integral of this for the whole ellipse is</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">⌠<sup><small>T</small></sup></td>
-      <td class="tdc">1</td>
-      <td class="tdc" rowspan="2"><i>mv</i>²<i>dt</i> = </td>
-      <td class="tdc">⌠<sup><small>360°</small></sup></td>
-      <td class="tdc">1</td>
-      <td class="tdc"> </td>
-      <td class="tdc"><i>m</i>µ</td>
-      <td class="tdc_big200" rowspan="2">(</td>
-      <td class="tdc" rowspan="2">2<i>r</i> - </td>
-      <td class="tdc"><i>r</i>²</td>
-      <td class="tdc_big200" rowspan="2">)</td>
-     <td class="tdc" rowspan="2"> <i>d</i>Θ</td>
-   </tr><tr>
-      <td class="tdc">⌡<sub>0</sub></td>
-      <td class="tdc bt"> 2 </td>
-      <td class="tdc">⌡<sub>0</sub>    </td>
-      <td class="tdc bt"> 2 </td>
-      <td class="tdc"> </td>
-      <td class="tdc bt"> <i>h</i> </td>
-      <td class="tdc bt"> <i>a</i> </td>
-   </tr>
- </tbody>
-</table>
-<p class="center">= <i>m</i>µ<sup>½</sup>π<i>a</i><sup>½</sup>.</p>
-
-<p>Since</p>
-
-<table border="0" cellspacing="0" summary=" " cellpadding="0" >
-  <tbody><tr>
-      <td class="tdc">⌠ </td>
-      <td class="tdc" rowspan="2"><i>r d</i>Θ = </td>
-      <td class="tdc">⌠ </td>
-      <td class="tdc"><i>a</i> · 1 - <i>e</i>²</td>
-      <td class="tdc" rowspan="2"><i>d</i>Θ =  </td>
-      <td class="tdc bb">2<i>a</i> · 1 - <i>e</i>²</td>
-      <td class="tdc" rowspan="2"> tan⁻¹ </td>
-      <td class="tdc" rowspan="2">(</td>
-      <td class="tdc" rowspan="2"><big>√</big></td>
-      <td class="tdc">1 - <i>e</i></td>
-      <td class="tdc" rowspan="2"> tan </td>
-      <td class="tdc">Θ</td>
-      <td class="tdc" rowspan="2">)</td>
-   </tr><tr>
-      <td class="tdc">⌡ </td>
-
-      <td class="tdc">⌡ </td>
-      <td class="tdc bt">1 + <i>e</i> cos Θ</td>
-      <td class="tdc">(1 - <i>e</i>²)<sup>½</sup></td>
-      <td class="tdc bt">1 + <i>e</i></td>
-      <td class="tdc bt"> 2 </td>
-    </tr>
- </tbody>
-</table>
-
-<p class="no-indent">and   <big>∫<i>r² d</i>Θ</big>   is given above.</p>
-
-<p>By collision a part of this energy is lost, being converted into heat.
-The major axis, <i>a</i>, is, therefore, shortened. But from the expression
-2π<i>a</i>² · (1-<i>e</i>²)<sup>½</sup> for the moment of momentum we see
-that this is greatest when <i>e</i> is least. If, therefore, <i>a</i> is diminished,
-<i>e</i> must also be diminished, or the moment of momentum would be lessened,
-which is impossible.</p>
-
-<div><a name="NOTE_7" id="NOTE_7"> </a>
-<p class="f120"><b>7<br /><span class="smcap">Capture of Satellites</span></b></p></div>
-
-<p>See has recently shown (<i>Astr. Nach.</i> No. 4341-42) that a particle
-moving through a resisting medium under the attraction of two bodies
-revolving round one another in circles may eventually be captured
-by one of them though originally under the domination of both. The
-argument consists in introducing the effect of a resisting medium upon
-<span class="pagenum"><a name="Page_252" id="Page_252">[Pg 252]</a></span>
-the motion in the space permitted by Jacobi’s integral, following
-Darwin’s examination of this space. In the actual case of nature the
-effect is much more complicated, and at present is not capable of exact
-solution for masses other than indefinitely small, even supposing
-circular orbits for the chief bodies. It may, however, explain the
-curious relation shown in the arrangement of the direct and retrograde
-movement of satellites.
-<span class="pagenum"><a name="Page_253" id="Page_253">[Pg 253]</a></span></p>
-
-<hr class="chap" />
-<p><span class="pagenum"><a name="Page_254" id="Page_254">[Pg 254]</a></span></p>
-
-<div class="chapter">
-<h2 class="nobreak">INDEX</h2>
-</div>
-<p><span class="pagenum"><a name="Page_255" id="Page_255">[Pg 255]</a></span></p>
-
-<ul class="index">
-  <li class="ifrst isub10"><big><b>A</b></big></li>
-<li class="isub1">Abnormality, the survival of original state, <a href="#Page_144">144</a>,
-             <a href="#Page_146">146</a>.</li>
-<li class="isub1">Absorption in spectrum,</li>
-  <li class="isub3">planetary, <a href="#Page_52">52</a>, <a href="#Page_161">161</a>.</li>
-  <li class="isub3">of Uranus, <a href="#Page_118">118</a>.</li>
-  <li class="isub3">of Jupiter, <a href="#Page_152">152</a>.</li>
-  <li class="isub3">of Saturn, <a href="#Page_152">152</a>.</li>
-<li class="isub1">Achilles, <a href="#Page_94">94</a>.</li>
-<li class="isub1">Adams, <a href="#Page_119">119</a>, <a href="#Page_121">121</a>.</li>
-<li class="isub1">Adams, Mr. J. C., <a href="#Page_123">123-126</a>.</li>
-<li class="isub1">Agassiz, <a href="#Page_41">41</a>.</li>
-<li class="isub1">Airy, <a href="#Page_121">121</a>, <a href="#Page_123">123</a>.</li>
-<li class="isub1">Albedo,</li>
-  <li class="isub3">of dark star, <a href="#Page_27">27</a>.</li>
-  <li class="isub3">of Mercury, <a href="#Page_62">62</a>, <a href="#Page_73">73-75</a>.</li>
-  <li class="isub3">of Venus, <a href="#Page_73">73-75</a>.</li>
-  <li class="isub3">of Moon, <a href="#Page_75">75</a>.</li>
-  <li class="isub3">of Jupiter, <a href="#Page_104">104</a>, <a href="#Page_105">105</a>.</li>
-  <li class="isub3">of Saturn, <a href="#Page_109">109</a>.</li>
-  <li class="isub3">of Uranus, <a href="#Page_116">116</a>.</li>
-  <li class="isub3">of Neptune, <a href="#Page_168">168</a>.</li>
-  <li class="isub3">of clouds, <a href="#Page_195">195</a>.</li>
-<li class="isub1">Algol, <a href="#Page_3">3</a>.</li>
-<li class="isub1">American Academy, <a href="#Page_125">125</a>.</li>
-<li class="isub1">Amphibians, first record of, <a href="#Page_188">188</a>.</li>
-<li class="isub1">Anderson, Dr. Thomas D., <a href="#Page_8">8</a>, <a href="#Page_12">12</a>.</li>
-<li class="isub1">André, <a href="#Page_215">215</a>.</li>
-<li class="isub1">Andromeda, great nebula in, <a href="#Page_10">10</a>, <a href="#Page_20">20</a>, <a href="#Page_21">21</a>.</li>
-  <li class="isub3">constitution disclosed by spectroscope, <a href="#Page_45">45</a>, <a href="#Page_48">48</a>.</li>
-<li class="isub1">Apex of Sun’s way, <a href="#Page_26">26</a>.</li>
-<li class="isub1">Arago, <a href="#Page_121">121</a>.</li>
-<li class="isub1">Asteroids, <a href="#Page_39">39</a>, <a href="#Page_60">60</a>, <a href="#Page_61">61</a>,
-                <a href="#Page_94">94-102</a>.</li>
-  <li class="isub3">domain of, <a href="#Page_94">94</a>.</li>
-  <li class="isub3">diminutive size, <a href="#Page_94">94</a>, <a href="#Page_101">101</a>.</li>
-  <li class="isub3">number, <a href="#Page_94">94</a>, <a href="#Page_101">101</a>.</li>
-  <li class="isub3">peculiar discovery of, <a href="#Page_95">95-98</a>.</li>
-  <li class="isub3">never formed part of a pristine whole, <a href="#Page_98">98</a>.</li>
-  <li class="isub3">where thickest, <a href="#Page_98">98</a>.</li>
-  <li class="isub3">formation of large planet from, prevented, <a href="#Page_98">98</a>,
-          <a href="#Page_99">99</a>.</li>
-  <li class="isub3">mid-course between planets and comets, <a href="#Page_100">100</a>.</li>
-  <li class="isub3">shape of, <a href="#Page_101">101</a>, <a href="#Page_102">102</a>.</li>
-  <li class="isub3">mammoth meteorites, <a href="#Page_102">102</a>.</li>
-  <li class="isub3">mark transition between inner and outer planets, <a href="#Page_102">102</a>.</li>
-<li class="isub1">Atmosphere,</li>
-  <li class="isub3">spectrographic study of, <a href="#Page_53">53</a>, <a href="#Page_54">54</a>,
-                   <a href="#Page_161">161</a>.</li>
-  <li class="isub3">Mercury deprived of, <a href="#Page_71">71</a>, <a href="#Page_75">75</a>,
-               <a href="#Page_232">232</a>.</li>
-  <li class="isub3">reflecting power, <a href="#Page_75">75</a>.</li>
-  <li class="isub3">of Venus, <a href="#Page_75">75</a>.</li>
-  <li class="isub3">Moon deprived of, <a href="#Page_75">75</a>, <a href="#Page_232">232</a>.</li>
-  <li class="isub3">thin on Mars, <a href="#Page_75">75</a>, <a href="#Page_91">91</a>, <a href="#Page_232">232</a>.</li>
-  <li class="isub3">of Uranus, enormous, <a href="#Page_117">117</a>, <a href="#Page_118">118</a>,
-                     <a href="#Page_232">232</a>.</li>
-  <li class="isub3">of Neptune, vast, <a href="#Page_118">118</a>, <a href="#Page_232">232</a>.</li>
-  <li class="isub3">of Jupiter, <a href="#Page_166">166</a>, <a href="#Page_232">232</a>.</li>
-  <li class="isub3">depletion of, <a href="#Page_231">231-233</a>.</li>
-  <li class="isub3">none on Ganymede, <a href="#Page_232">232</a>, <a href="#Page_233">233</a>.</li>
-  <li class="isub3">of Saturn, <a href="#Page_232">232</a>.</li>
-  <li class="isub3">lacking in Saturn’s rings, <a href="#Page_232">232</a>.</li>
-<li class="isub1">Avogadro, <a href="#Page_228">228</a>.</li>
-<li class="isub1">Axes of planets,</li>
-  <li class="isub3">systematic righting of, <a href="#Page_132">132</a>.</li>
-  <li class="isub3">tilts accounted for, <a href="#Page_146">146</a>.</li>
-
-  <li class="ifrst isub10"><big><b>B</b></big></li>
-<li class="isub1">Babinet, <a href="#Page_147">147</a>.</li>
-<li class="isub1">Backland, <a href="#Page_68">68</a>.</li>
-<li class="isub1">Ball, Sir Robert, <a href="#Page_145">145</a>.</li>
-<li class="isub1">Barrande, M., <a href="#Page_178">178</a>.</li>
-<li class="isub1">Belopolski, <a href="#Page_87">87</a>.</li>
-<li class="isub1">Bessel, <a href="#Page_120">120</a>, <a href="#Page_121">121</a>.</li>
-<li class="isub1">Blandet, M., <a href="#Page_175">175</a>, <a href="#Page_176">176</a>.</li>
-<li class="isub1">Bode, <a href="#Page_95">95</a>, <a href="#Page_119">119</a>.</li>
-<li class="isub1">Bode’s law, <a href="#Page_96">96</a>, <a href="#Page_100">100</a>, <a href="#Page_119">119</a>,
-                <a href="#Page_122">122</a>, <a href="#Page_126">126</a>.</li>
-<li class="isub1">Bolometer, <a href="#Page_194">194</a>.</li>
-<li class="isub1">Bolton, Mr. Scriven, <a href="#Page_103">103</a>, <a href="#Page_105">105</a>,
-             <a href="#Page_106">106</a>.</li>
-<li class="isub1">Boltzmann, <a href="#Page_228">228</a>.</li>
-<li class="isub1">Bose, <a href="#Page_157">157</a>.</li>
-<li class="isub1">Bouvard, Alexis, <a href="#Page_120">120</a>, <a href="#Page_121">121</a>.</li>
-<li class="isub1">Boyle, <a href="#Page_228">228</a>.</li>
-<li class="isub1">Bradley, <a href="#Page_68">68</a>.</li>
-
-  <li class="ifrst isub10"><big><b>C</b></big></li>
-<li class="isub1">Cambrian era, <a href="#Page_178">178</a>.</li>
-<li class="isub1">Cambridge Observatory, <a href="#Page_123">123</a>.
-     <span class="pagenum"><a name="Page_295" id="Page_295">[Pg 295]</a></span></li>
-<li class="isub1">Campbell, <a href="#Page_9">9</a>.</li>
-<li class="isub1">Carboniferous period, <a href="#Page_179">179</a>.</li>
-<li class="isub1">Cassini, <a href="#Page_76">76</a>, <a href="#Page_162">162</a>.</li>
-<li class="isub1">Celestial mechanics, <a href="#Page_28">28</a>, <a href="#Page_94">94</a>, <a href="#Page_155">155</a>.</li>
-<li class="isub1">Ceres, <a href="#Page_101">101</a>.</li>
-<li class="isub1">Challis, <a href="#Page_123">123</a>.</li>
-<li class="isub1">Chemistry, indebted to the stars, <a href="#Page_160">160</a>.</li>
-<li class="isub1">Clausius, <a href="#Page_228">228</a>.</li>
-<li class="isub1">Clerke, Miss, <a href="#Page_9">9</a>, <a href="#Page_164">164</a>.</li>
-<li class="isub1">Climate, advent of, <a href="#Page_185">185</a>.</li>
-<li class="isub1">Clouds,</li>
-  <li class="isub3">none on Venus, <a href="#Page_75">75</a>.</li>
-  <li class="isub3">of Jupiter not ordered as ours, <a href="#Page_107">107</a>, <a href="#Page_163">163</a>,
-             <a href="#Page_167">167</a>.</li>
-  <li class="isub3">Uranus wrapped in, <a href="#Page_168">168</a>.</li>
-  <li class="isub3">Neptune wrapped in, <a href="#Page_168">168</a>.</li>
-  <li class="isub3">Earth once wrapped in, <a href="#Page_170">170</a>, <a href="#Page_171">171</a>, <a href="#Page_178">178</a>.</li>
-<li class="isub1">Collision of dark star with Sun, <a href="#Page_25">25</a>, <a href="#Page_215">215</a>.</li>
-  <li class="isub3">warning of, <a href="#Page_26">26-29</a>.</li>
-  <li class="isub3">disturbances previous to, <a href="#Page_29">29</a>, <a href="#Page_30">30</a>.</li>
-  <li class="isub3">rarity of event, <a href="#Page_30">30</a>.</li>
-<li class="isub1">Collisions between meteorites of a flock, <a href="#Page_11">11</a>, <a href="#Page_49">49</a>.</li>
-  <li class="isub3">causing light, <a href="#Page_49">49</a>, <a href="#Page_50">50</a>.</li>
-<li class="isub1">Columbus, <a href="#Page_188">188</a>.</li>
-<li class="isub1">Comets, <a href="#Page_33">33</a>, <a href="#Page_61">61</a>.</li>
-  <li class="isub3">members of solar system, <a href="#Page_34">34</a>, <a href="#Page_35">35</a>.</li>
-  <li class="isub3">orbits of, <a href="#Page_61">61</a>, <a href="#Page_100">100</a>.</li>
-<li class="isub1">Commensurability of orbital period, <a href="#Page_99">99</a>, <a href="#Page_111">111</a>.</li>
-<li class="isub1">Congruities of solar system, <a href="#Page_128">128-137</a>.</li>
-  <li class="isub3">deviations from, <a href="#Page_62">62</a>, <a href="#Page_100">100</a>, <a href="#Page_101">101</a>,
-                 <a href="#Page_130">130</a>, <a href="#Page_131">131</a>, <a href="#Page_141">141</a>.</li>
-  <li class="isub3">specify mode of evolution, <a href="#Page_137">137</a>.</li>
-<li class="isub1">Convection currents, <a href="#Page_219">219</a>.</li>
-  <li class="isub3">in atmosphere of Venus, <a href="#Page_80">80</a>.</li>
-<li class="isub1">Copeland, Dr. <a href="#Page_7">7</a>.</li>
-<li class="isub1">Copernican system, <a href="#Page_58">58</a>.</li>
-<li class="isub1">Copernicus, <a href="#Page_62">62</a>.</li>
-<li class="isub1">Cosmic action, <a href="#Page_1">1</a>, <a href="#Page_22">22</a>, <a href="#Page_184">184</a>.</li>
-<li class="isub1">Croll, <a href="#Page_196">196</a>.</li>
-<li class="isub1">Cuticle of star, effect of impact on, <a href="#Page_11">11</a>.</li>
-
-  <li class="ifrst isub10"><big><b>D</b></big></li>
-<li class="isub1">Dana, <a href="#Page_177">177</a>, <a href="#Page_186">186</a>, <a href="#Page_189">189</a>.</li>
-<li class="isub1">Dark stars,</li>
-  <li class="isub3">origin, <a href="#Page_2">2</a>.</li>
-  <li class="isub3">number, <a href="#Page_2">2</a>, <a href="#Page_25">25</a>.</li>
-  <li class="isub3">evidence of, <a href="#Page_3">3-5</a>.</li>
-  <li class="isub3">collision of, <a href="#Page_10">10</a>, <a href="#Page_11">11</a>.</li>
-  <li class="isub3">rendered visible, <a href="#Page_26">26</a>.</li>
-<li class="isub1">Darwin, <a href="#Page_62">62</a>, <a href="#Page_138">138</a>, Notes <a href="#Page_252">252</a>.</li>
-<li class="isub1">Day,</li>
-  <li class="isub3">lengthened to infinity, <a href="#Page_70">70</a>, <a href="#Page_219">219</a>.</li>
-  <li class="isub3">none on Venus, <a href="#Page_83">83</a>.</li>
-  <li class="isub3">Jovian, <a href="#Page_163">163</a>.</li>
-  <li class="isub3">first appreciation of, <a href="#Page_186">186</a>.</li>
-  <li class="isub3">coincides with month, on satellites, <a href="#Page_225">225</a>.</li>
-<li class="isub1">Death of a planet,</li>
-  <li class="isub3">defined, <a href="#Page_214">214</a>.</li>
-  <li class="isub3">catastrophic cause, <a href="#Page_215">215</a>, <a href="#Page_216">216</a>.</li>
-  <li class="isub3">due to tidal retardation of rotation, <a href="#Page_216">216-219</a>.</li>
-  <li class="isub3">outcome of loss of oceans and air, <a href="#Page_226">226</a>, <a href="#Page_233">233</a>.</li>
-  <li class="isub3">caused by extinction of Sun itself, <a href="#Page_234">234</a>.</li>
-<li class="isub1">Density,</li>
-  <li class="isub3">of dark star, <a href="#Page_27">27</a>.</li>
-  <li class="isub3">of planets, <a href="#Page_51">51</a>, Notes <a href="#Page_243">243</a>.</li>
-  <li class="isub3">of Mercury, <a href="#Page_63">63</a>, <a href="#Page_64">64</a>.</li>
-  <li class="isub3">of Venus, <a href="#Page_90">90</a>.</li>
-  <li class="isub3">of Jupiter, <a href="#Page_103">103</a>, <a href="#Page_117">117</a>.</li>
-  <li class="isub3">of Uranus, <a href="#Page_115">115</a>.</li>
-<li class="isub1">Deserts, increase of, on Earth, <a href="#Page_208">208-211</a>.</li>
-<li class="isub1">Devonian era, <a href="#Page_187">187</a>.</li>
-<li class="isub1">Dhurmsala meteorite, <a href="#Page_41">41</a>.</li>
-<li class="isub1">Diameter,</li>
-  <li class="isub3">of Mercury, <a href="#Page_63">63</a>, <a href="#Page_64">64</a>,
-            <a href="#Page_66">66</a>, <a href="#Page_67">67</a>.</li>
-  <li class="isub3">of Venus, <a href="#Page_90">90</a>.</li>
-  <li class="isub3">of Earth, <a href="#Page_90">90</a>.</li>
-  <li class="isub3">of Mars, <a href="#Page_91">91</a>.</li>
-  <li class="isub3">of satellites of Mars, <a href="#Page_92">92</a>.</li>
-  <li class="isub3">of Jupiter, <a href="#Page_103">103</a>.</li>
-  <li class="isub3">of Uranus, <a href="#Page_115">115-117</a>.</li>
-<li class="isub1">Dust, in atmosphere of Venus, <a href="#Page_75">75</a>.</li>
-
-  <li class="ifrst isub10"><big><b>E</b></big></li>
-<li class="isub1">Earth,</li>
-  <li class="isub3">characteristics, not universal, <a href="#Page_90">90</a>, <a href="#Page_91">91</a>,
-             <a href="#Page_155">155</a>.</li>
-  <li class="isub3">evolved from a nebula, <a href="#Page_149">149</a>.</li>
-  <li class="isub3">internal heat, <a href="#Page_150">150</a>.</li>
-  <li class="isub3">early surface temperature, <a href="#Page_160">160</a>, <a href="#Page_169">169</a>,
-                 <a href="#Page_170">170</a>.</li>
-  <li class="isub3">once cloud-wrapped, <a href="#Page_170">170</a>, <a href="#Page_171">171</a>,
-                 <a href="#Page_178">178</a>.</li>
-  <li class="isub3">solid surface formed, <a href="#Page_171">171</a>.</li>
-  <li class="isub3">hot seas of, <a href="#Page_171">171</a>, <a href="#Page_172">172</a>.</li>
-  <li class="isub3">self-sustained, <a href="#Page_182">182</a>.</li>
-  <li class="isub3">study of, within province of astronomy, <a href="#Page_184">184</a>.</li>
-  <li class="isub3">ceased to be self-centred, <a href="#Page_187">187</a>.</li>
-  <li class="isub3">Sun becomes dominant factor in organic life of, <a href="#Page_190">190</a>.</li>
-<li class="isub1">Earth shine, <a href="#Page_82">82</a>.</li>
-<li class="isub1">Eccentricity, orbital,</li>
-  <li class="isub3">of Mercury, <a href="#Page_63">63</a>, <a href="#Page_65">65</a>, <a href="#Page_69">69</a>,
-               <a href="#Page_222">222</a>.
-    <span class="pagenum"><a name="Page_257" id="Page_257">[Pg 257]</a></span></li>
-  <li class="isub3">of asteroids, erratic, <a href="#Page_100">100</a>, <a href="#Page_101">101</a>.</li>
-  <li class="isub3">of satellites, increases with distance from primary, <a href="#Page_134">134</a>.</li>
-<li class="isub1">Eclipsing binaries, <a href="#Page_3">3</a>, <a href="#Page_4">4</a>.</li>
-<li class="isub1">Ejectum from nova, <a href="#Page_5">5</a>, <a href="#Page_16">16</a>.</li>
-  <li class="isub3">rate of regression, <a href="#Page_16">16</a>.</li>
-<li class="isub1">Elemental substances, <a href="#Page_159">159</a>.</li>
-  <li class="isub3">in Sun, <a href="#Page_159">159</a>.</li>
-  <li class="isub3">once in Earth, <a href="#Page_160">160</a>.</li>
-  <li class="isub3">discovery of, in stars, <a href="#Page_161">161</a>, <a href="#Page_162">162</a>.</li>
-<li class="isub1">Ellipticity,</li>
-  <li class="isub3">of Jupiter, <a href="#Page_103">103</a>.</li>
-  <li class="isub3">of Saturn, <a href="#Page_109">109</a>.</li>
-  <li class="isub3">of Uranus, <a href="#Page_115">115</a>.</li>
-<li class="isub1">Encke, <a href="#Page_68">68</a>.</li>
-<li class="isub1">Energy,</li>
-  <li class="isub3">conservation of, <a href="#Page_140">140</a>, <a href="#Page_150">150</a>,
-                 <a href="#Page_151">151</a>.</li>
-  <li class="isub3">dissipation, <a href="#Page_140">140-142</a>.</li>
-  <li class="isub3">conditions for a minimum, <a href="#Page_142">142</a>.</li>
-<li class="isub1">Eros, fluctuation of light of, gives evidence of form, <a href="#Page_101">101</a>,
-                 <a href="#Page_102">102</a>.</li>
-<li class="isub1">Evolution, <a href="#Page_153">153</a>.</li>
-  <li class="isub3">white nebulæ in process of, <a href="#Page_49">49</a>.</li>
-  <li class="isub3">rounded out, <a href="#Page_56">56</a>.</li>
-  <li class="isub3">of solar family, <a href="#Page_100">100</a>.</li>
-  <li class="isub3">evidence of, in solar system, <a href="#Page_117">117</a>.</li>
-  <li class="isub3">manner of, lessens energy, <a href="#Page_141">141</a>.</li>
-<li class="isub1">Evolution, chemical, <a href="#Page_155">155</a>, <a href="#Page_173">173</a>.</li>
-  <li class="isub3">universal, <a href="#Page_156">156</a>.</li>
-  <li class="isub3">temperature conducive to, <a href="#Page_157">157</a>, <a href="#Page_158">158</a>.</li>
-  <li class="isub3">attendant upon cooling, <a href="#Page_158">158</a>, <a href="#Page_162">162</a>.</li>
-  <li class="isub3">steps in, shown by spectroscope, <a href="#Page_161">161</a>.</li>
-<li class="isub1">Evolution, physical, <a href="#Page_155">155</a>, <a href="#Page_162">162</a>.</li>
-  <li class="isub3">induced by cooling, <a href="#Page_162">162</a>.</li>
-
-  <li class="ifrst isub10"><big><b>F</b></big></li>
-<li class="isub1">Fabry, <a href="#Page_34">34</a>.</li>
-<li class="isub1">Fauna, <a href="#Page_178">178</a>, <a href="#Page_179">179</a>, <a href="#Page_187">187</a>.</li>
-<li class="isub1">Faye, <a href="#Page_175">175</a>, <a href="#Page_176">176</a>.</li>
-<li class="isub1">Flagstaff, Arizona, <a href="#Page_52">52</a>, <a href="#Page_66">66</a>, <a href="#Page_68">68</a>,
-       <a href="#Page_79">79</a>, <a href="#Page_83">83</a>, <a href="#Page_89">89</a>, <a href="#Page_92">92</a>,
-       <a href="#Page_106">106</a>, <a href="#Page_110">110</a>, <a href="#Page_221">221</a>, <a href="#Page_232">232</a>.</li>
-  <li class="isub3">clear and steady air of, <a href="#Page_66">66</a>, <a href="#Page_86">86</a>.</li>
-<li class="isub1">Flamstead, <a href="#Page_119">119</a>.</li>
-<li class="isub1">Fleming, Mrs., <a href="#Page_7">7</a>.</li>
-<li class="isub1">Flemming, <a href="#Page_120">120</a>, <a href="#Page_121">121</a>.</li>
-<li class="isub1">Flora, of paleologic times, <a href="#Page_177">177</a>.</li>
-<li class="isub1">French Academy, <a href="#Page_122">122</a>.</li>
-
-  <li class="ifrst isub10"><big><b>G</b></big></li>
-<li class="isub1">Galle, Dr., <a href="#Page_122">122</a>, <a href="#Page_123">123</a>, <a href="#Page_125">125</a>.</li>
-<li class="isub1">Gases,</li>
-  <li class="isub3">peculiar to nebulæ, <a href="#Page_11">11</a>, <a href="#Page_16">16</a>.</li>
-  <li class="isub3">occluded in meteorites, <a href="#Page_42">42</a>, <a href="#Page_43">43</a>.</li>
-  <li class="isub3">in atmospheres of planets, <a href="#Page_53">53-55</a>.</li>
-<li class="isub1">Gauss, <a href="#Page_34">34</a>, <a href="#Page_96">96</a>, <a href="#Page_97">97</a>.</li>
-<li class="isub1">Geikie, <a href="#Page_160">160</a>, <a href="#Page_177">177</a>, <a href="#Page_189">189</a>.</li>
-<li class="isub1">Geology,</li>
-  <li class="isub3">relation to astronomy, <a href="#Page_173">173</a>, <a href="#Page_174">174</a>,
-               <a href="#Page_183">183</a>, <a href="#Page_184">184</a>.</li>
-  <li class="isub3">scope of, <a href="#Page_174">174</a>, <a href="#Page_203">203</a>.</li>
-<li class="isub1">Geysers, avenues to earlier state, <a href="#Page_160">160</a>.</li>
-<li class="isub1">Goodricke, <a href="#Page_3">3</a>.</li>
-
-  <li class="ifrst isub10"><big><b>H</b></big></li>
-<li class="isub1">Hakluyt, <a href="#Page_188">188</a>.</li>
-<li class="isub1">Harvard College Observatory, <a href="#Page_8">8</a>, <a href="#Page_12">12</a>.</li>
-<li class="isub1">Heat,</li>
-  <li class="isub3">molecular motion, <a href="#Page_150">150</a>, <a href="#Page_157">157</a>, <a href="#Page_230">230</a>.</li>
-  <li class="isub3">the result of evolving, <a href="#Page_153">153</a>.</li>
-  <li class="isub3">the preface to higher evolution, <a href="#Page_153">153</a>, <a href="#Page_156">156</a>.</li>
-  <li class="isub3">laws governing amount of, <a href="#Page_190">190</a>.</li>
-  <li class="isub3">atmosphere keeps out, as well as stores, <a href="#Page_191">191</a>.</li>
-  <li class="isub3">effective, received from Sun, <a href="#Page_192">192-194</a>.</li>
-  <li class="isub3">invisible rays, <a href="#Page_194">194</a>.</li>
-  <li class="isub3">retained, <a href="#Page_194">194-196</a>.</li>
-  <li class="isub3">radiated, <a href="#Page_194">194-196</a>.</li>
-<li class="isub1">Heat of condensation of Earth,</li>
-  <li class="isub3">accuses concourse of particles, <a href="#Page_151">151</a>.</li>
-  <li class="isub3">evaluated, <a href="#Page_151">151</a>, <a href="#Page_152">152</a>.</li>
-  <li class="isub3">sufficient for geologic phenomena, <a href="#Page_152">152</a>.</li>
-<li class="isub1">Hector, <a href="#Page_94">94</a>.</li>
-<li class="isub1">Helmholtz, <a href="#Page_151">151</a>.</li>
-<li class="isub1">Hencke, <a href="#Page_98">98</a>.</li>
-<li class="isub1">Herschel, Sir John, <a href="#Page_122">122</a>.</li>
-<li class="isub1">Herschel, Sir William, <a href="#Page_96">96</a>, <a href="#Page_114">114</a>, <a href="#Page_162">162</a>.</li>
-<li class="isub1">Hertha, periodic variability, <a href="#Page_102">102</a>.</li>
-<li class="isub1">Hipparchus, <a href="#Page_5">5</a>.</li>
-<li class="isub1">Holden, <a href="#Page_9">9</a>.</li>
-<li class="isub1">Hubbard, Professor, <a href="#Page_124">124</a>.</li>
-<li class="isub1">Huggins, <a href="#Page_52">52</a>.</li>
-<li class="isub1">Humphreys, <a href="#Page_10">10</a>.</li>
-<li class="isub1">Huntington, <a href="#Page_209">209</a>.</li>
-
-  <li class="ifrst isub10"><big><b>I</b></big></li>
-<li class="isub1">Ice Age, <a href="#Page_196">196</a>.</li>
-  <li class="isub3">not of orbital occasioning, <a href="#Page_197">197-199</a>.</li>
-  <li class="isub3">increased precipitation, the cause, <a href="#Page_199">199</a>, <a href="#Page_200">200</a>.</li>
-  <li class="isub3">a local affair, <a href="#Page_200">200-202</a>.</li>
-<li class="isub1">Irradiation, affecting diameter of Mercury, <a href="#Page_66">66</a>, <a href="#Page_68">68</a>.</li>
-
-  <li class="ifrst isub10"><big><b>J</b></big></li>
-<li class="isub1">Jacobi, Notes <a href="#Page_252">252</a>.</li>
-<li class="isub1">Julius, Professor, <a href="#Page_10">10</a>.
-    <span class="pagenum"><a name="Page_258" id="Page_258">[Pg 258]</a></span></li>
-<li class="isub1">Juno, <a href="#Page_101">101</a>.</li>
-<li class="isub1">Jupiter, <a href="#Page_103">103-108</a>.</li>
-  <li class="isub3">not solid, <a href="#Page_104">104</a>, <a href="#Page_107">107</a>.</li>
-  <li class="isub3">a semi-sun, <a href="#Page_105">105</a>, <a href="#Page_108">108</a>, <a href="#Page_152">152</a>,
-              <a href="#Page_166">166</a>, <a href="#Page_167">167</a>.</li>
-  <li class="isub3">white spots of, <a href="#Page_106">106</a>.</li>
-<li class="isub1">Jupiter, “great red spot” of, <a href="#Page_164">164</a>.</li>
-  <li class="isub3">time of rotation, <a href="#Page_164">164</a>.</li>
-  <li class="isub3">a vast uprush of heated vapor, <a href="#Page_165">165</a>, <a href="#Page_166">166</a>.</li>
-<li class="isub1">Jupiter’s belts,</li>
-  <li class="isub3">secular progression, <a href="#Page_104">104</a>.</li>
-  <li class="isub3">rotate at different speeds, <a href="#Page_104">104</a>, <a href="#Page_162">162</a>, <a href="#Page_163">163</a>.</li>
-  <li class="isub3">color, <a href="#Page_104">104</a>.</li>
-  <li class="isub3">wisps across, <a href="#Page_105">105</a>, <a href="#Page_106">106</a>.</li>
-  <li class="isub3">bright ones, cloud, <a href="#Page_163">163</a>, <a href="#Page_167">167</a>.</li>
-  <li class="isub3">spectrographic study of, <a href="#Page_166">166</a>.</li>
-
-  <li class="ifrst isub10"><big><b>K</b></big></li>
-<li class="isub1">Kapteyn, <a href="#Page_14">14</a>.</li>
-<li class="isub1">Keeler, <a href="#Page_19">19</a>, <a href="#Page_52">52</a>, <a href="#Page_110">110</a>.</li>
-<li class="isub1">Kepler, <a href="#Page_6">6</a>.</li>
-<li class="isub1">Kinetic theory of gases, <a href="#Page_226">226</a>, <a href="#Page_228">228</a>.</li>
-  <li class="isub3">corollary of, <a href="#Page_54">54</a>.</li>
-  <li class="isub3">extension of, <a href="#Page_230">230</a>, <a href="#Page_231">231</a>.</li>
-<li class="isub1">Kirkwood, Professor, <a href="#Page_35">35</a>.</li>
-
-  <li class="ifrst isub10"><big><b>L</b></big></li>
-<li class="isub1">Lagrange, <a href="#Page_94">94</a>, <a href="#Page_97">97</a>.</li>
-<li class="isub1">Lalande, <a href="#Page_123">123</a>, <a href="#Page_124">124</a>.</li>
-<li class="isub1">Lane, Homer, <a href="#Page_234">234</a>.</li>
-<li class="isub1">Langley, <a href="#Page_191">191</a>, <a href="#Page_194">194</a>.</li>
-<li class="isub1">Laplace, <a href="#Page_34">34</a>, <a href="#Page_110">110</a>, <a href="#Page_127">127</a>, <a href="#Page_129">129</a>,
-                   <a href="#Page_131">131</a>, <a href="#Page_132">132</a>, <a href="#Page_138">138</a>, <a href="#Page_139">139</a>,
-                   <a href="#Page_147">147</a>, <a href="#Page_152">152</a>, <a href="#Page_175">175</a>.</li>
-<li class="isub1">Laplacian cosmos, <a href="#Page_129">129</a>, <a href="#Page_130">130</a>.</li>
-  <li class="isub3">false congruities of, <a href="#Page_131">131-133</a>.</li>
-  <li class="isub3">annular genesis, disproved, <a href="#Page_138">138</a>, <a href="#Page_139">139</a>.</li>
-  <li class="isub3">original “fire-mist” of, impossible, <a href="#Page_138">138</a>.</li>
-<li class="isub1">Lapparent, de, <a href="#Page_173">173-176</a>, <a href="#Page_183">183</a>, <a href="#Page_189">189</a>.</li>
-<li class="isub1">Lemonnier, <a href="#Page_115">115</a>, <a href="#Page_119">119</a>.</li>
-<li class="isub1">Leonard, Miss, <a href="#Page_79">79</a>.</li>
-<li class="isub1">Leverrier, <a href="#Page_119">119</a>, <a href="#Page_121">121-126</a>.</li>
-<li class="isub1">Lexell, <a href="#Page_115">115</a>.</li>
-<li class="isub1">Libration in longitude,</li>
-  <li class="isub3">of Mercury, <a href="#Page_65">65</a>, <a href="#Page_69">69</a>, <a href="#Page_70">70</a>,
-           <a href="#Page_222">222</a>, <a href="#Page_223">223</a>.</li>
-  <li class="isub3">causes true day, <a href="#Page_70">70</a>, <a href="#Page_71">71</a>.</li>
-  <li class="isub3">of Venus, inappreciable, <a href="#Page_83">83</a>, <a href="#Page_223">223</a>.</li>
-  <li class="isub3">of Moon, <a href="#Page_224">224</a>.</li>
-<li class="isub1">Lick Observatory, <a href="#Page_13">13</a>, <a href="#Page_14">14</a>.</li>
-<li class="isub1">Lockyer, <a href="#Page_48">48</a>.</li>
-<li class="isub1">Lowell Observatory, <a href="#Page_65">65</a>, <a href="#Page_74">74</a>.</li>
-
-  <li class="ifrst isub10"><big><b>M</b></big></li>
-<li class="isub1">Major planets,</li>
-  <li class="isub3">gaseous, <a href="#Page_117">117</a>.</li>
-  <li class="isub3">constitution of, differs from Sun or Earth, <a href="#Page_161">161</a>.</li>
-  <li class="isub3">types of early planetary stages, <a href="#Page_162">162</a>.</li>
-  <li class="isub3">self-centred and self-sustained, <a href="#Page_168">168</a>.</li>
-<li class="isub1">Man, immanent, <a href="#Page_159">159</a>.</li>
-<li class="isub1">Mars,</li>
-  <li class="isub3">polar caps, <a href="#Page_198">198</a>.</li>
-  <li class="isub3">canals in dark regions, <a href="#Page_206">206</a>, <a href="#Page_207">207</a>.</li>
-  <li class="isub3">dying of exhaustion, <a href="#Page_234">234</a>.</li>
-<li class="isub1">Mass,</li>
-  <li class="isub3">of Mercury, <a href="#Page_63">63</a>, <a href="#Page_64">64</a>, <a href="#Page_68">68</a>.</li>
-  <li class="isub3">of Mars, <a href="#Page_91">91</a>.</li>
-  <li class="isub3">of Jupiter, <a href="#Page_103">103</a>.</li>
-  <li class="isub3">arrangement of, in solar system, <a href="#Page_135">135-137</a>, <a href="#Page_148">148</a>.</li>
-<li class="isub1">Massachusetts Institute of Technology, <a href="#Page_134">134</a>, <a href="#Page_184">184</a>.</li>
-<li class="isub1">Mauvais, <a href="#Page_125">125</a>.</li>
-<li class="isub1">Maxwell, Clerk, <a href="#Page_110">110</a>, <a href="#Page_113">113</a>, <a href="#Page_228">228</a>.</li>
-<li class="isub1">Mayer, <a href="#Page_119">119</a>, <a href="#Page_151">151</a>.</li>
-<li class="isub1">Mendeléeff, <a href="#Page_161">161</a>.</li>
-<li class="isub1">Mercury, 62-<a href="#Page_73">73</a>.</li>
-  <li class="isub3">time of rotation and revolution the same, <a href="#Page_65">65</a>, <a href="#Page_69">69</a>.</li>
-  <li class="isub3">axis stands plumb to orbit, <a href="#Page_70">70</a>.</li>
-  <li class="isub3">turns same face to the Sun, <a href="#Page_70">70</a>, <a href="#Page_72">72</a>, <a href="#Page_134">134</a>,
-               <a href="#Page_221">221</a>.</li>
-  <li class="isub3">surface markings, <a href="#Page_72">72</a>, <a href="#Page_221">221</a>.</li>
-  <li class="isub3">color, <a href="#Page_72">72</a>.</li>
-<li class="isub1">Meteorites, <a href="#Page_31">31</a>, <a href="#Page_35">35</a>, <a href="#Page_36">36</a>.</li>
-  <li class="isub3">cosmic bodies, <a href="#Page_32">32</a>, <a href="#Page_33">33</a>.</li>
-  <li class="isub3">relation to shooting-stars, <a href="#Page_36">36</a>.</li>
-  <li class="isub3">members of solar system, <a href="#Page_36">36</a>.</li>
-  <li class="isub3">composition, <a href="#Page_40">40-44</a>, <a href="#Page_55">55</a>.</li>
-  <li class="isub3">fused by friction with atmosphere, <a href="#Page_40">40</a>.</li>
-  <li class="isub3">temperature, <a href="#Page_41">41</a>, <a href="#Page_55">55</a>.</li>
-  <li class="isub3">fragments of a dark body, <a href="#Page_44">44</a>.</li>
-  <li class="isub3">link past to present, <a href="#Page_44">44</a>, <a href="#Page_56">56</a>, <a href="#Page_57">57</a>,
-                  <a href="#Page_130">130</a>.</li>
-<li class="isub1">Meteors,</li>
-  <li class="isub3">orbits of, <a href="#Page_36">36</a>, <a href="#Page_39">39</a>, Notes <a href="#Page_241">241-243</a>.</li>
-  <li class="isub3">visibility of, <a href="#Page_38">38</a>.</li>
-<li class="isub1">Meteor-streams, <a href="#Page_33">33</a>, <a href="#Page_61">61</a>.</li>
-  <li class="isub3">first recognition of, <a href="#Page_34">34</a>.</li>
-  <li class="isub3">disintegrated comets, <a href="#Page_34">34</a>.</li>
-<li class="isub1">Michelson, <a href="#Page_10">10</a>.</li>
-<li class="isub1">Milham, Professor, <a href="#Page_99">99</a>.</li>
-<li class="isub1">Mira Ceti, <a href="#Page_235">235</a>.</li>
-<li class="isub1">Mohler, <a href="#Page_10">10</a>.</li>
-<li class="isub1">Molecular speeds, gaseous, <a href="#Page_228">228-231</a>.
-    <span class="pagenum"><a name="Page_259" id="Page_259">[Pg 259]</a></span></li>
-  <li class="isub3">critical velocity, <a href="#Page_230">230</a>, <a href="#Page_231">231</a>.</li>
-<li class="isub1">Molecule, organic, power in its instability, <a href="#Page_160">160</a>.</li>
-<li class="isub1">Moment of momentum, <a href="#Page_140">140</a>, Notes <a href="#Page_250">250</a>.</li>
-  <li class="isub3">cause of original, <a href="#Page_130">130</a>.</li>
-<li class="isub1">Moment of momentum, conservation of, <a href="#Page_140">140</a>.</li>
-  <li class="isub3">applied to solar system, <a href="#Page_141">141-143</a>.</li>
-<li class="isub1">Momentum, <a href="#Page_140">140</a>.</li>
-<li class="isub1">Monck, Mr., <a href="#Page_10">10</a>.</li>
-<li class="isub1">Moon,</li>
-  <li class="isub3">turns same face to Earth, <a href="#Page_134">134</a>, <a href="#Page_208">208</a>,
-                 <a href="#Page_224">224</a>, <a href="#Page_225">225</a>.</li>
-  <li class="isub3">once fiery, now dead, <a href="#Page_233">233</a>, <a href="#Page_234">234</a>.</li>
-<li class="isub1">Mountains, none on Mars, <a href="#Page_91">91</a>.</li>
-<li class="isub1">Müller, <a href="#Page_73">73</a>, <a href="#Page_74">74</a>, <a href="#Page_104">104</a>,
-              <a href="#Page_105">105</a>, <a href="#Page_116">116</a>.</li>
-
-  <li class="ifrst isub10"><big><b>N</b></big></li>
-<li class="isub1">Naval Observatory at Washington, <a href="#Page_122">122</a>.</li>
-<li class="isub1">Nebulæ,</li>
-  <li class="isub3">origin of, <a href="#Page_10">10</a>, <a href="#Page_11">11</a>.</li>
-  <li class="isub3">amorphous, <a href="#Page_18">18</a>, <a href="#Page_44">44</a>.</li>
-  <li class="isub3">planetary, <a href="#Page_18">18</a>.</li>
-  <li class="isub3">spectrum of amorphous, <a href="#Page_45">45</a>.</li>
-<li class="isub1">Nebulæ, spiral, <a href="#Page_17">17-25</a>, <a href="#Page_44">44</a>.</li>
-  <li class="isub3">evolved from disrupted stars, <a href="#Page_10">10-15</a>.</li>
-  <li class="isub3">relation to novæ, <a href="#Page_14">14-16</a>.</li>
-  <li class="isub3">corpuscular character of, <a href="#Page_15">15</a>, <a href="#Page_16">16</a>.</li>
-  <li class="isub3">knots and patches of, <a href="#Page_15">15</a>.</li>
-  <li class="isub3">most common, <a href="#Page_19">19</a>, <a href="#Page_20">20</a>.</li>
-  <li class="isub3">two-armed, <a href="#Page_20">20</a>, <a href="#Page_25">25</a>.</li>
-  <li class="isub3">central nucleus, globular, <a href="#Page_21">21</a>.</li>
-  <li class="isub3">not due to explosive action, <a href="#Page_22">22</a>, <a href="#Page_23">23</a>, <a href="#Page_25">25</a>.</li>
-  <li class="isub3">not caused by disintegration, <a href="#Page_24">24</a>, <a href="#Page_25">25</a>.</li>
-  <li class="isub3">cause of development, <a href="#Page_24">24</a>, <a href="#Page_25">25</a>.</li>
-  <li class="isub3">spectrum of, <a href="#Page_45">45-48</a>.</li>
-  <li class="isub3">composed of flocks of meteorites, <a href="#Page_48">48</a>, <a href="#Page_49">49</a>.</li>
-  <li class="isub3">constitution established by spectroscope, <a href="#Page_49">49</a>, <a href="#Page_50">50</a>.</li>
-<li class="isub1">Nebular hypotheses, <a href="#Page_173">173</a>.</li>
-<li class="isub1">Neologic times, clearing of sky in, <a href="#Page_185">185</a>.</li>
-<li class="isub1">Neptune, <a href="#Page_118">118</a>.</li>
-  <li class="isub3">rotates backward, <a href="#Page_118">118</a>.</li>
-  <li class="isub3">owes discovery to mathematical triumph, <a href="#Page_119">119-126</a>.</li>
-  <li class="isub3">faint belts on, <a href="#Page_168">168</a>.</li>
-  <li class="isub3">further advanced than giant planets, <a href="#Page_168">168</a>.</li>
-<li class="isub1">Newcomb, <a href="#Page_67">67</a>.</li>
-<li class="isub1">Newton, Professor, <a href="#Page_36">36</a>, <a href="#Page_42">42</a>.</li>
-<li class="isub1">Newton, Sir Isaac, <a href="#Page_34">34</a>.</li>
-<li class="isub1">Nova Aurigæ, <a href="#Page_7">7</a>, <a href="#Page_8">8</a>, <a href="#Page_12">12</a>.</li>
-  <li class="isub3">history chronicled by its spectrum, <a href="#Page_8">8</a>, <a href="#Page_9">9</a>.</li>
-<li class="isub1">Nova Cygni, <a href="#Page_7">7</a>.</li>
-<li class="isub1">Novæ, <a href="#Page_6">6</a>, <a href="#Page_7">7</a>.</li>
-  <li class="isub3">origin <a href="#Page_5">5</a>, <a href="#Page_10">10</a>.</li>
-  <li class="isub3">first chronicled, <a href="#Page_5">5</a>.</li>
-  <li class="isub3">spectroscopic study of, <a href="#Page_7">7</a>.</li>
-<li class="isub1">Nova Persei, <a href="#Page_7">7</a>.</li>
-  <li class="isub3">history of, <a href="#Page_12">12-15</a>.</li>
-
-  <li class="ifrst isub10"><big><b>O</b></big></li>
-<li class="isub1">Oceans,</li>
-  <li class="isub3">none on Mars, <a href="#Page_91">91</a>.</li>
-  <li class="isub3">evaporation of, <a href="#Page_204">204</a>.</li>
-  <li class="isub3">basins of, on Moon, <a href="#Page_204">204-208</a>.</li>
-  <li class="isub3">basins of, on Mars, <a href="#Page_206">206</a>, <a href="#Page_207">207</a>.</li>
-<li class="isub1">Olbers, <a href="#Page_97">97</a>.</li>
-<li class="isub1">Olmstead, Professor, <a href="#Page_33">33</a>.</li>
-<li class="isub1">Orbital distance,</li>
-  <li class="isub3">of Mercury, <a href="#Page_62">62</a>.</li>
-  <li class="isub3">of Venus, <a href="#Page_73">73</a>.</li>
-  <li class="isub3">of Mars, <a href="#Page_91">91</a>.</li>
-  <li class="isub3">of Eros, <a href="#Page_94">94</a>.</li>
-  <li class="isub3">of Saturn, <a href="#Page_108">108</a>.</li>
-<li class="isub1">Orbital tilts,</li>
-  <li class="isub3">of asteroids, erratic, <a href="#Page_100">100</a>, <a href="#Page_101">101</a>.</li>
-  <li class="isub3">of satellites of Uranus, <a href="#Page_116">116</a>.</li>
-  <li class="isub3">of planets, substantially the same, <a href="#Page_129">129-131</a>, Notes <a href="#Page_244">244</a>.</li>
-  <li class="isub3">deviation from rule, by Mercury, <a href="#Page_131">131</a>.</li>
-  <li class="isub3">of satellites, increase with distance from primary, <a href="#Page_133">133</a>, <a href="#Page_134">134</a>.</li>
-<li class="isub1">Orbits,</li>
-  <li class="isub3">determining factors, <a href="#Page_35">35</a>.</li>
-  <li class="isub3">rendered more circular by collisions, <a href="#Page_141">141-143</a>, Notes <a href="#Page_250">250</a>,
-                <a href="#Page_251">251</a>.</li>
-  <li class="isub3">made more conformant to general plane by collisions, <a href="#Page_141">141-143</a>.</li>
-<li class="isub1">Orion, great nebula in, <a href="#Page_18">18</a>.</li>
-
-  <li class="ifrst isub10"><big><b>P</b></big></li>
-<li class="isub1">Paleologic times,</li>
-  <li class="isub3">much warmth and little light in, <a href="#Page_172">172</a>.</li>
-  <li class="isub3">fallacies in geologists’ expositions of, <a href="#Page_174">174-176</a>.</li>
-  <li class="isub3">climate continuous, <a href="#Page_177">177</a>, <a href="#Page_186">186</a>.</li>
-  <li class="isub3">seas warm, <a href="#Page_177">177</a>, <a href="#Page_178">178</a>.</li>
-  <li class="isub3">explained by cloud envelope, <a href="#Page_178">178</a>.</li>
-  <li class="isub3">corroboration of explanation, <a href="#Page_187">187</a>, <a href="#Page_179">179</a>.</li>
-  <li class="isub3">excessive rain in, <a href="#Page_185">185</a>, <a href="#Page_186">186</a>.</li>
-  <li class="isub3">passage into Neologic, essentially astronomic, <a href="#Page_185">185</a>.</li>
-<li class="isub1">Pallas, <a href="#Page_101">101</a>.
-    <span class="pagenum"><a name="Page_260" id="Page_260">[Pg 260]</a></span></li>
-<li class="isub1">Parabolic speed at orbit, Notes <a href="#Page_245">245</a>.</li>
-<li class="isub1">Patroclus, <a href="#Page_94">94</a>.</li>
-<li class="isub1">Peirce, <a href="#Page_110">110</a>, <a href="#Page_125">125</a>, <a href="#Page_126">126</a>.</li>
-<li class="isub1">Perrine, <a href="#Page_15">15</a>.</li>
-<li class="isub1">Perrotin, <a href="#Page_116">116</a>.</li>
-<li class="isub1">Perturbations,</li>
-  <li class="isub3">in motion of planets, heralding a catastrophe, <a href="#Page_28">28</a>, <a href="#Page_30">30</a>.</li>
-  <li class="isub3">reflected, <a href="#Page_63">63</a>.</li>
-  <li class="isub3">mass of planet determined by, <a href="#Page_68">68</a>.</li>
-  <li class="isub3">of asteroids by Jupiter, <a href="#Page_98">98</a>, <a href="#Page_99">99</a>.</li>
-  <li class="isub3">restrictive action of, <a href="#Page_99">99</a>.</li>
-  <li class="isub3">the fashioning force of planetary orbits, <a href="#Page_99">99</a>, <a href="#Page_100">100</a>.</li>
-  <li class="isub3">of rings of Saturn by satellites, <a href="#Page_111">111</a>, <a href="#Page_112">112</a>.</li>
-  <li class="isub3">of Uranus lead to discovery of Neptune, <a href="#Page_121">121-126</a>.</li>
-<li class="isub1">Petersen, Dr., <a href="#Page_123">123</a>.</li>
-<li class="isub1">Photometric determinations, <a href="#Page_92">92</a>, <a href="#Page_93">93</a>.</li>
-  <li class="isub3">background, the fundamental factor in, <a href="#Page_92">92</a>, <a href="#Page_93">93</a>.</li>
-<li class="isub1">Piazzi, <a href="#Page_96">96</a>.</li>
-<li class="isub1">Pilgrim Star, <a href="#Page_5">5</a>, <a href="#Page_6">6</a>.</li>
-<li class="isub1">Planetary astronomy, advance in, <a href="#Page_59">59</a>, <a href="#Page_60">60</a>.</li>
-<li class="isub1">Planetology, <a href="#Page_203">203</a>.</li>
-  <li class="isub3">defined, <a href="#Page_173">173</a>, <a href="#Page_174">174</a>.</li>
-<li class="isub1">Planets, <a href="#Page_61">61</a>.</li>
-  <li class="isub3">knots in spiral nebulæ, <a href="#Page_25">25</a>, <a href="#Page_139">139</a>.</li>
-  <li class="isub3">developed by agglomeration, <a href="#Page_143">143</a>, <a href="#Page_149">149</a>,
-           <a href="#Page_151">151</a>, <a href="#Page_152">152</a>.</li>
-<li class="isub1">Pliny, <a href="#Page_5">5</a>.</li>
-<li class="isub1">Plutonic rocks, <a href="#Page_160">160</a>.</li>
-<li class="isub1">Pluvial eras, contemporaneous with glacial, <a href="#Page_200">200</a>.</li>
-<li class="isub1">Polyp corals, in paleologic times, <a href="#Page_186">186</a>.</li>
-<li class="isub1">Pristine motion of planetary particles,</li>
-  <li class="isub3">retrograde, <a href="#Page_144">144</a>.</li>
-  <li class="isub3">superfluous energy in, <a href="#Page_145">145</a>.</li>
-  <li class="isub3">unstable, <a href="#Page_145">145</a>.</li>
-<li class="isub1">Ptolemaic system, <a href="#Page_58">58</a>.</li>
-
-  <li class="ifrst isub10"><big><b>R</b></big></li>
-<li class="isub1">Refrigeration, tempered by loss of cloud, <a href="#Page_196">196</a>.</li>
-<li class="isub1">Revolutions,</li>
-  <li class="isub3">of shooting-stars, <a href="#Page_39">39</a>.</li>
-  <li class="isub3">of asteroids, direct like planets, <a href="#Page_100">100</a>.</li>
-  <li class="isub3">planetary, in same sense, <a href="#Page_129">129</a>, <a href="#Page_130">130</a>.</li>
-  <li class="isub3">outermost satellites, retrograde, <a href="#Page_132">132</a>.</li>
-  <li class="isub3">of satellites explained, <a href="#Page_146">146</a>, <a href="#Page_147">147</a>,
-                 Notes <a href="#Page_252">252</a>.</li>
-<li class="isub1">Ritchey, <a href="#Page_14">14</a>.</li>
-<li class="isub1">Roberts, Dr., <a href="#Page_20">20</a>.</li>
-<li class="isub1">Roche, Edouard, <a href="#Page_110">110</a>.</li>
-<li class="isub1">Rosse, Lord, <a href="#Page_17">17</a>.</li>
-<li class="isub1">Rotation of planets, <a href="#Page_131">131</a>, <a href="#Page_132">132</a>.</li>
-  <li class="isub3">systematic righting of axes, <a href="#Page_132">132</a>.</li>
-  <li class="isub3">initially, retrograde, <a href="#Page_146">146</a>.</li>
-<li class="isub1">Rotation period,</li>
-  <li class="isub3">of Venus, spectrographically determined, <a href="#Page_83">83</a>, <a href="#Page_85">85-90</a>.</li>
-  <li class="isub3">of Mars, spectrographically determined, <a href="#Page_88">88</a>, <a href="#Page_89">89</a>.</li>
-  <li class="isub3">of Jupiter, spectrographically determined, <a href="#Page_89">89</a>.</li>
-  <li class="isub3">of Uranus, <a href="#Page_116">116</a>.</li>
-<li class="isub1">Royal Observatory, Edinburgh, <a href="#Page_7">7</a>.</li>
-
-  <li class="ifrst isub10"><big><b>S</b></big></li>
-<li class="isub1">Satellites, <a href="#Page_61">61</a>.</li>
-  <li class="isub3">of Mars, <a href="#Page_92">92</a>.</li>
-  <li class="isub3">of Saturn, <a href="#Page_108">108</a>, <a href="#Page_112">112</a>.</li>
-  <li class="isub3">of Uranus, <a href="#Page_116">116</a>.</li>
-  <li class="isub3">solid, <a href="#Page_117">117</a>.</li>
-  <li class="isub3">of Neptune, <a href="#Page_118">118</a>.</li>
-  <li class="isub3">turn same face to primaries, <a href="#Page_134">134</a>, <a href="#Page_147">147</a>,
-                <a href="#Page_148">148</a>, <a href="#Page_225">225</a>.</li>
-  <li class="isub3">latest discoveries in regard to motions of, <a href="#Page_146">146</a>.</li>
-  <li class="isub3">origin of, <a href="#Page_147">147</a>.</li>
-  <li class="isub3">death of, before planet, <a href="#Page_233">233</a>.</li>
-  <li class="isub3">impact of interplanetary particles on, Notes <a href="#Page_246">246-250</a>.</li>
-  <li class="isub3">capture of, Notes <a href="#Page_251">251</a>, <a href="#Page_252">252</a>.</li>
-<li class="isub1">Saturn, <a href="#Page_108">108-114</a>.</li>
-  <li class="isub3">belts of, <a href="#Page_109">109</a>, <a href="#Page_168">168</a>.</li>
-  <li class="isub3">inherent light, <a href="#Page_109">109</a>, <a href="#Page_152">152</a>.</li>
-<li class="isub1">Saturn’s rings, <a href="#Page_109">109-114</a>.</li>
-  <li class="isub3">mechanical marvel of, not early appreciated, <a href="#Page_110">110</a>.</li>
-  <li class="isub3">discrete particles, <a href="#Page_110">110</a>, <a href="#Page_135">135</a>.</li>
-  <li class="isub3">knots upon, <a href="#Page_110">110-113</a>.</li>
-  <li class="isub3">not flat, but tores, <a href="#Page_111">111-114</a>.</li>
-  <li class="isub3">show devolution—not pristine state of solar system, <a href="#Page_138">138</a>,
-                 <a href="#Page_139">139</a>.</li>
-  <li class="isub3">once a congeries, <a href="#Page_139">139</a>.</li>
-<li class="isub1">Schaeberle, <a href="#Page_9">9</a>.</li>
-<li class="isub1">Schiaparelli, <a href="#Page_34">34</a>, <a href="#Page_36">36</a>, <a href="#Page_64">64-66</a>,
-             <a href="#Page_69">69</a>, <a href="#Page_76">76</a>, <a href="#Page_77">77</a>, <a href="#Page_221">221</a>.</li>
-<li class="isub1">Schroeter, <a href="#Page_65">65</a>, <a href="#Page_77">77</a>.</li>
-<li class="isub1">Seasons,</li>
-  <li class="isub3">loss of, <a href="#Page_71">71</a>, <a href="#Page_83">83</a>, <a href="#Page_217">217</a>,
-              <a href="#Page_218">218</a>.</li>
-  <li class="isub3">begin with clearing of sky, <a href="#Page_185">185</a>.</li>
-  <li class="isub3">fully developed, <a href="#Page_189">189</a>.
-    <span class="pagenum"><a name="Page_261" id="Page_261">[Pg 261]</a></span></li>
-<li class="isub1">See, Notes <a href="#Page_251">251</a>.</li>
-<li class="isub1">Seeliger, <a href="#Page_10">10</a>.</li>
-<li class="isub1">Shooting-stars, <a href="#Page_33">33</a>, <a href="#Page_35">35</a>.</li>
-  <li class="isub3">radiant of, <a href="#Page_33">33</a>, <a href="#Page_36">36</a>.</li>
-  <li class="isub3">members of solar system, <a href="#Page_36">36-40</a>.</li>
-  <li class="isub3">tiny planets, <a href="#Page_39">39</a>.</li>
-<li class="isub1">Siderite, <a href="#Page_36">36</a>.</li>
-<li class="isub1">Silurian era, <a href="#Page_178">178</a>.</li>
-<li class="isub1">Sirona, periodic variability of, <a href="#Page_102">102</a>.</li>
-<li class="isub1">Sky, cause of clearing, <a href="#Page_187">187</a>.</li>
-<li class="isub1">Slipher, Dr. V. M., <a href="#Page_52">52</a>, <a href="#Page_79">79</a>,
-       <a href="#Page_83">83</a>, <a href="#Page_86">86</a>,
-      <a href="#Page_88">88</a>, <a href="#Page_89">89</a>, <a href="#Page_117">117</a>,
-      <a href="#Page_161">161</a>, <a href="#Page_166">166</a>.</li>
-<li class="isub1">Slipher, Mr. E. C., <a href="#Page_79">79</a>, <a href="#Page_233">233</a>.</li>
-<li class="isub1">Solar constant, <a href="#Page_191">191</a>.</li>
-<li class="isub1">Solar system,</li>
-  <li class="isub3">evolved from a dark star, <a href="#Page_44">44</a>.</li>
-  <li class="isub3">evidence of origin, <a href="#Page_51">51</a>, <a href="#Page_130">130</a>.</li>
-  <li class="isub3">characteristics of, <a href="#Page_60">60-62</a>.</li>
-  <li class="isub3">evolutionarily one, <a href="#Page_62">62</a>.</li>
-  <li class="isub3">gap in progression of orbital distances, <a href="#Page_95">95-100</a>.</li>
-  <li class="isub3">bodies of, egg-shaped, <a href="#Page_217">217</a>.</li>
-<li class="isub1">Specific gravity, of stone and iron, <a href="#Page_44">44</a>.</li>
-<li class="isub1">Spectroscope, <a href="#Page_7">7</a>, <a href="#Page_84">84</a>.</li>
-<li class="isub1">Spectroscopic shift, <a href="#Page_84">84</a>.</li>
-  <li class="isub3">determining velocity, <a href="#Page_3">3</a>.</li>
-  <li class="isub3">in Nova Aurigæ, <a href="#Page_9">9</a>.</li>
-  <li class="isub3">produced by great pressure, <a href="#Page_10">10</a>, <a href="#Page_13">13</a>.</li>
-  <li class="isub3">produced by anomalous refraction, <a href="#Page_10">10</a>.</li>
-  <li class="isub3">produced by change of density, <a href="#Page_10">10</a>, <a href="#Page_13">13</a>.</li>
-  <li class="isub3">explained, <a href="#Page_85">85</a>.</li>
-  <li class="isub3">variation in, Notes <a href="#Page_243">243</a>, <a href="#Page_244">244</a>.</li>
-<li class="isub1">Spectrum,</li>
-  <li class="isub3">of Nova Persei, <a href="#Page_12">12</a>, <a href="#Page_13">13</a>.</li>
-  <li class="isub3">nebular, <a href="#Page_13">13</a>, <a href="#Page_16">16</a>, <a href="#Page_45">45-48</a>.</li>
-  <li class="isub3">peculiarities of nebular, explained, <a href="#Page_50">50</a>.</li>
-  <li class="isub3">photographic extension of, <a href="#Page_52">52</a>, <a href="#Page_117">117</a>,
-             <a href="#Page_161">161</a>.</li>
-  <li class="isub3">of major planets, <a href="#Page_52">52</a>, <a href="#Page_53">53</a>, <a href="#Page_161">161</a>.</li>
-  <li class="isub3">of belts of Jupiter, <a href="#Page_166">166</a>.</li>
-<li class="isub1">Spiral structure, implies rotation combined with motion out or in, <a href="#Page_22">22</a>.</li>
-<li class="isub1">Stability of a system, condition for, <a href="#Page_140">140</a>, <a href="#Page_141">141</a>.</li>
-<li class="isub1">Stoney, Dr. Johnstone, <a href="#Page_231">231</a>.</li>
-<li class="isub1">Struve, <a href="#Page_109">109</a>.</li>
-<li class="isub1">Suess, <a href="#Page_179">179</a>.</li>
-<li class="isub1">Sun,</li>
-  <li class="isub3">original slow rotation of the, <a href="#Page_130">130</a>.</li>
-  <li class="isub3">heat of, <a href="#Page_234">234</a>, <a href="#Page_235">235</a>.</li>
-  <li class="isub3">reversion to a dark star, <a href="#Page_235">235</a>, <a href="#Page_236">236</a>.</li>
-<li class="isub1">Sun spots, <a href="#Page_104">104</a>, <a href="#Page_166">166</a>.</li>
-
-  <li class="ifrst isub10"><big><b>T</b></big></li>
-<li class="isub1">Temperature,</li>
-  <li class="isub3">of Moon, <a href="#Page_191">191</a>, <a href="#Page_192">192</a>.</li>
-  <li class="isub3">of Mars, <a href="#Page_192">192</a>, <a href="#Page_194">194</a>, <a href="#Page_196">196</a>.</li>
-  <li class="isub3">defined, <a href="#Page_230">230</a>.</li>
-  <li class="isub3">no such thing as, in space, <a href="#Page_230">230</a>.</li>
-<li class="isub1">Tercidina, periodic variability of, <a href="#Page_102">102</a>.</li>
-<li class="isub1">Tertiary times, entrance of color with, <a href="#Page_189">189</a>, <a href="#Page_190">190</a>.</li>
-<li class="isub1">Tidal action, <a href="#Page_143">143-147</a>, <a href="#Page_216">216-218</a>.</li>
-  <li class="isub3">causes loss of energy, <a href="#Page_144">144</a>.</li>
-  <li class="isub3">inoperative, <a href="#Page_144">144</a>, <a href="#Page_145">145</a>,
-                 <a href="#Page_147">147</a>.</li>
-  <li class="isub3">changes retrograde rotation of planet to direct, <a href="#Page_145">145-147</a>,
-                    <a href="#Page_217">217</a>.</li>
-  <li class="isub3">on satellites, <a href="#Page_147">147</a>.</li>
-  <li class="isub3">slows down spin, <a href="#Page_148">148</a>, <a href="#Page_217">217</a>.</li>
-  <li class="isub3">brings plane of rotation down to orbital plane, <a href="#Page_217">217</a>.</li>
-  <li class="isub3">lengthens day to infinity, <a href="#Page_219">219</a>.</li>
-  <li class="isub3">analytically expressed, <a href="#Page_224">224</a>.</li>
-  <li class="isub3">greatest on planets near Sun, <a href="#Page_135">135</a>, <a href="#Page_224">224</a>.</li>
-<li class="isub1">Tidal action, disruptive, <a href="#Page_130">130</a>.</li>
-  <li class="isub3">exemplified by spiral nebulæ, <a href="#Page_24">24</a>, <a href="#Page_25">25</a>.</li>
-  <li class="isub3">hinted at, by meteorites, <a href="#Page_55">55</a>.</li>
-  <li class="isub3">theory corroborated by densities of planets, <a href="#Page_51">51</a>.</li>
-  <li class="isub3">theory corroborated by atmospheres of planets, <a href="#Page_52">52-55</a>.</li>
-  <li class="isub3">on comets, <a href="#Page_139">139</a>.</li>
-  <li class="isub3">cause of Saturn’s rings, <a href="#Page_139">139</a>.</li>
-<li class="isub1">Tisserand, <a href="#Page_68">68</a>.</li>
-<li class="isub1">Titius, <a href="#Page_95">95</a>.</li>
-<li class="isub1">Todd, <a href="#Page_68">68</a>.</li>
-<li class="isub1">Trees, deciduous, first appearance of, <a href="#Page_189">189</a>.</li>
-<li class="isub1">Trilobites, blindness of, <a href="#Page_178">178</a>, <a href="#Page_179">179</a>.</li>
-<li class="isub1">Twining, <a href="#Page_33">33</a>.</li>
-<li class="isub1">Tycho Brahe, <a href="#Page_5">5</a>.</li>
-
-  <li class="ifrst isub10"><big><b>U</b></big></li>
-<li class="isub1">Uranus, <a href="#Page_114">114-118</a>.</li>
-  <li class="isub3">history of discovery, <a href="#Page_114">114</a>, <a href="#Page_115">115</a>,
-            <a href="#Page_119">119</a>.</li>
-  <li class="isub3">a ball of vapor, <a href="#Page_115">115</a>, <a href="#Page_117">117</a>.</li>
-  <li class="isub3">belts of, <a href="#Page_115">115</a>, <a href="#Page_116">116</a>, <a href="#Page_168">168</a>.</li>
-  <li class="isub3">tilt of axis to ecliptic, great, <a href="#Page_115">115</a>.</li>
-  <li class="isub3">spectroscopic revelations of, <a href="#Page_117">117</a>, <a href="#Page_118">118</a>.</li>
-  <li class="isub3">in an early amorphous state, <a href="#Page_118">118</a>.
-     <span class="pagenum"><a name="Page_262" id="Page_262">[Pg 262]</a></span></li>
-  <li class="isub3">further advanced than the giant planets, <a href="#Page_168">168</a>.</li>
-
-  <li class="ifrst isub10"><big><b>V</b></big></li>
-<li class="isub1">Velocity,</li>
-  <li class="isub3">of Mercury in orbit, <a href="#Page_63">63</a>.</li>
-  <li class="isub3">of satellites about primary, Notes <a href="#Page_245">245</a>.</li>
-  <li class="isub3">of major planets, in orbit, Notes <a href="#Page_245">245</a>.</li>
-<li class="isub1">Venus, <a href="#Page_73">73-90</a>.</li>
-  <li class="isub3">surface markings, <a href="#Page_74">74</a>, <a href="#Page_77">77</a>,
-              <a href="#Page_79">79</a>, <a href="#Page_80">80</a>,
-      <a href="#Page_83">83</a>, <a href="#Page_220">220</a>, <a href="#Page_221">221</a>.</li>
-  <li class="isub3">brilliancy due to cloudless atmosphere, <a href="#Page_75">75</a>.</li>
-  <li class="isub3">importance of rotation period, <a href="#Page_75">75</a>, <a href="#Page_76">76</a>.</li>
-  <li class="isub3">turns same face to the Sun, <a href="#Page_77">77-80</a>, <a href="#Page_134">134</a>,
-                   <a href="#Page_220">220</a>, <a href="#Page_221">221</a>.</li>
-  <li class="isub3">ice on the night side, causes ashen light, <a href="#Page_82">82</a>.</li>
-<li class="isub1">Very, Professor, <a href="#Page_16">16</a>, <a href="#Page_191">191</a>,
-                 <a href="#Page_192">192</a>, <a href="#Page_194">194</a>.</li>
-<li class="isub1">Vesta, <a href="#Page_101">101</a>.</li>
-<li class="isub1">Vogel, <a href="#Page_52">52</a>.</li>
-<li class="isub1">Volcanoes, avenues to earlier state, <a href="#Page_160">160</a>.</li>
-<li class="isub1">Von Zach, <a href="#Page_96">96</a>.</li>
-
-  <li class="ifrst isub10"><big><b>W</b></big></li>
-<li class="isub1">Walker, Mr., <a href="#Page_123">123</a>, <a href="#Page_124">124</a>.</li>
-<li class="isub1">Water,</li>
-  <li class="isub3">becoming more scarce, <a href="#Page_203">203</a>, <a href="#Page_204">204</a>,
-              <a href="#Page_211">211</a>.</li>
-  <li class="isub3">lacking on Moon, <a href="#Page_204">204</a>.</li>
-<li class="isub1">Water-vapor,</li>
-  <li class="isub3">in atmosphere of Jupiter, <a href="#Page_53">53</a>.</li>
-  <li class="isub3">in atmosphere of Mars, <a href="#Page_91">91</a>, <a href="#Page_161">161</a>.</li>
-  <li class="isub3">smaller planet has less hold on, <a href="#Page_207">207</a>.</li>
-<li class="isub1">Williams, Mr. Stanley, <a href="#Page_103">103</a>.</li>
-<li class="isub1">Witt, de, <a href="#Page_94">94</a>.</li>
-<li class="isub1">Wolf, Dr., <a href="#Page_13">13</a>.</li>
-<li class="isub1">Wolf, Max, <a href="#Page_94">94</a>.</li>
-<li class="isub1">Wolf-Rayet stars, <a href="#Page_13">13</a>, <a href="#Page_48">48</a>.</li>
-<li class="isub1">Wright, <a href="#Page_13">13</a>, <a href="#Page_43">43</a>.</li>
-
-  <li class="ifrst isub10"><big><b>Y</b></big></li>
-<li class="isub1">Year, of Uranus, <a href="#Page_116">116</a>.</li>
-<li class="isub1">Yerkes Observatory, <a href="#Page_232">232</a>.</li>
-<li class="isub1">Young, <a href="#Page_46">46</a>.</li>
-</ul>
-<hr class="full" />
-
-<div class="blockquot">
-<p class="f200"><b>PERCIVAL LOWELL’S</b></p>
-<p class="f150"><b>Mars and Its Canals</b></p>
-
-<p class="author"><i>Illustrated, 8vo, $2.50 net</i></p>
-
-<p>“The book makes fascinating reading and is intended for the average
-man of intelligence and scientific curiosity. It represents mature
-reflection, patient investigation and observation, and eleven years’
-additional work and verification.... It is the work of a scientist who
-has found inspiration and joy in his work; it is full of enthusiasm,
-but the enthusiasm is not allowed to influence unduly a single
-conclusion.”—<i>Chicago Evening Post.</i></p>
-
-<p>“It seems impossible that Mr. Lowell can raise another girder more
-grandly impressive and expressive of the whole fabric or take another
-step in his scientific syllogism that will hold us any tighter in his
-logic. He has practically reached already his ‘Q. E. D.’ The thing
-is done, apparently, except for filling in the detail. But with his
-racy, epigrammatic brilliancy of style, his delicate, quiet humor,
-his daring scientific imagination—all held in check by instructive
-modesty of good breeding, gayly throwing to the winds all professional
-airs and mere rhetorical bounce—his course will be no doubt as
-charming to the end as it has been steadily illuminating even for the
-illuminati.”—<i>Boston Transcript.</i></p>
-
-<p>“Whether or not we choose to follow the author of this book to his
-ultimate inferences, he at least opens up a field of fascinating
-conjecture. The work is written in a style as popular as the precise
-enumeration of the ascertained facts permits, and if the narrative
-is not in all its details as entrancing as a novel, it nevertheless
-transports us into a region of superlatively romantic interest.”—<i>New
-York Tribune.</i></p>
-
-<p>“No doubt the highest living authority on Mars and things Martian
-is Prof. Percival Lowell, director of the observatory at Flagstaff,
-Arizona, an astronomical investigator and writer known over the
-entire world. Professor Lowell’s book, ‘Mars and Its Canals,’ is the
-final word, up to the present, on the planet and what we know of
-it.”—<i>Review of Reviews.</i></p>
-
-<hr class="r5" />
-<p class="center">PUBLISHED BY</p>
-<p class="f120">THE MACMILLAN COMPANY</p>
-<p class="center">64-66 Fifth Avenue, New York</p>
-</div>
-
-<hr class="full" />
-<div class="blockquot">
-<p class="f200"><b>PERCIVAL LOWELL’S</b></p>
-<p class="f150"><b>Mars as the Abode of Life</b></p>
-
-<p class="author"><i>Illustrated, 8vo, $2.50 net</i></p>
-
-<p>The book is based on a course of lectures delivered at the Lowell
-Institute in 1906, supplemented by the results of later observations.
-It is, in the large, the presentation of the results of the author’s
-research into the genesis and development of what we call a world; not
-the mere aggregating of matter, but the process by which that matter
-comes to be individual as we find it. He bridges with the new science
-of planetology the evolutionary gap between the nebular hypothesis and
-the Darwinian theory.</p>
-
-<p>“It is not only as an astronomer but as a writer that Professor Lowell
-charms the reader in this work. The beguilement of the theme is well
-matched by the grace and literary finish of the style in which it is
-presented. The subject is one to beget enthusiasm in its advocates, and
-the author certainly is not devoid of it. The warmth and earnestness of
-the true lover of his theme shine through the entire work so that in
-its whole style and illustrations it is a charming production.”—<i>St.
-Louis Globe Democrat.</i></p>
-
-<p>“Mr. Lowell approaches the subject by outlining the now generally
-accepted theory of the formation of planets and the solar system. He
-describes the stages in the life history of a planet three of which are
-illustrated in the present state of the earth, Mars, and the moon. He
-tells what conditions we would expect to find on a planet in what we
-may call the Martian age, and proceeds to show how the facts revealed
-by observation square with the theories. The book is fascinatingly
-readable.”—<i>The Outlook.</i></p>
-
-<p>“So attractive are the style and the illustrations that the work will
-doubtless draw the attention of many new readers to its fascinating
-subject. Professor Lowell has fairly preëmpted that portion of the
-field of astronomy which interests the widest readers, for there
-is no doubt that speculation regarding the possibility of life on
-other planets than our own has a peculiar attraction for the average
-human mind.... For the convenience of the non-technical reader,
-the body of the book has been made as simple and understandable as
-possible.”—<i>Philadelphia Press.</i></p>
-
-<hr class="r5" />
-<p class="center">PUBLISHED BY</p>
-<p class="f120">THE MACMILLAN COMPANY</p>
-<p class="center space-below2">64-66 Fifth Avenue, New York</p>
-</div>
-
-<div class="footnotes space-below2">
-<p class="f150"><b>Footnotes:</b></p>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a>
-“Mem. del Reale Inst. Lombardo,” Vol. XII. III della serie III.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a>
-Quoted in “Luminous Meteors,” Committee’s Report for 1870-1871, p. 48.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a>
-New Observations of the Planet Mercury, <i>Memoirs Amer. Acad.</i> 1897. Vol. XII, No. 4.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a>
-“Astronomical Constants,” 1895, pp. 67, 68.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span></a>
-<i>Astr. Nach.</i> No. 3406. Monthly Notices R. A. S., March, 1897.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_6_6" id="Footnote_6_6"></a><a href="#FNanchor_6_6"><span class="label">[6]</span></a>
-Monthly notices R. A. S., March, 1897.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_7_7" id="Footnote_7_7"></a><a href="#FNanchor_7_7"><span class="label">[7]</span></a>
-Lowell Observatory Bulletin 6.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_8_8" id="Footnote_8_8"></a><a href="#FNanchor_8_8"><span class="label">[8]</span></a>
-Lowell Observatory Bulletin No. 3.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_9_9" id="Footnote_9_9"></a><a href="#FNanchor_9_9"><span class="label">[9]</span></a>
-Paper by the writer in the <i>Phil. Mag.</i>, April, 1908.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_10_10" id="Footnote_10_10"></a><a href="#FNanchor_10_10"><span class="label">[10]</span></a>
-Adams, “Explanation of the Motion of Uranus,” 1846.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_11_11" id="Footnote_11_11"></a><a href="#FNanchor_11_11"><span class="label">[11]</span></a>
-Proc. Amer. Acad., Vol. I, p. 64.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_12_12" id="Footnote_12_12"></a><a href="#FNanchor_12_12"><span class="label">[12]</span></a>
-Proc. Amer. Acad., Vol. I, p. 65 <i>et seq.</i></p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_13_13" id="Footnote_13_13"></a><a href="#FNanchor_13_13"><span class="label">[13]</span></a>
-Proc. Amer. Acad., Vol. I, p. 144.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_14_14" id="Footnote_14_14"></a><a href="#FNanchor_14_14"><span class="label">[14]</span></a>
-Proc. Amer. Acad., Vol. I, p. 332.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_15_15" id="Footnote_15_15"></a><a href="#FNanchor_15_15"><span class="label">[15]</span></a>
-Geikie, “Geology,” pages 85, 86, and 131-136.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_16_16" id="Footnote_16_16"></a><a href="#FNanchor_16_16"><span class="label">[16]</span></a>
-“Abrégé de Geologie,” De Lapparent.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_17_17" id="Footnote_17_17"></a><a href="#FNanchor_17_17"><span class="label">[17]</span></a>
-“Mars as the Abode of Life,” Macmillan, 1908.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_18_18" id="Footnote_18_18"></a><a href="#FNanchor_18_18"><span class="label">[18]</span></a>
-De Lapparent, Dana, Geikie, <i>passim</i>.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_19_19" id="Footnote_19_19"></a><a href="#FNanchor_19_19"><span class="label">[19]</span></a>
-De Lapparent.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_20_20" id="Footnote_20_20"></a><a href="#FNanchor_20_20"><span class="label">[20]</span></a>
-De Lapparent, Dana, Geikie, <i>passim</i>.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_21_21" id="Footnote_21_21"></a><a href="#FNanchor_21_21"><span class="label">[21]</span></a>
-Suess, “The Face of the Earth,” p. 213.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_22_22" id="Footnote_22_22"></a><a href="#FNanchor_22_22"><span class="label">[22]</span></a>
-Dana. “Geology.”</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_23_23" id="Footnote_23_23"></a><a href="#FNanchor_23_23"><span class="label">[23]</span></a>
-Dana, Geikie, De Lapparent.</p></div>
-
-<div class="footnote"><p class="no-indent">
-<a name="Footnote_24_24" id="Footnote_24_24"></a><a href="#FNanchor_24_24"><span class="label">[24]</span></a>
-Cf. Grant Allen.</p></div>
-</div>
-<div class="transnote bbox">
-<p class="f120 space-above1">Transcriber’s Notes:</p>
-<hr class="r5" />
-<p class="indent">The illustrations have been moved so that they do not break up
-                  paragraphs and so that they are next to the text they illustrate.</p>
-<p class="indent">Typographical errors have been silently corrected.</p>
-<p class="indent">The numerical reference to the NOTES, i. e. [1] have been changed to
-    "(see NOTE x)" in order to avoid confusion with the true footnotes.</p>
-</div>
-
-
-
-
-
-
-
-<pre>
-
-
-
-
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