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+The Project Gutenberg EBook of Astronomy of To-day, by Cecil G. Dolmage
+
+This eBook is for the use of anyone anywhere 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
+
+
+Title: Astronomy of To-day
+       A Popular Introduction in Non-Technical Language
+
+Author: Cecil G. Dolmage
+
+Release Date: April 21, 2009 [EBook #28570]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK ASTRONOMY OF TO-DAY ***
+
+
+
+
+Produced by Brenda Lewis, Scott Marusak, Greg Bergquist
+and the Online Distributed Proofreading Team at
+https://www.pgdp.net (This file was produced from images
+generously made available by The Internet Archive/American
+Libraries.)
+
+
+
+
+
+
+Transcriber's Note
+
+The punctuation and spelling from the original text have been faithfully
+preserved. Only obvious typographical errors have been corrected. The
+advertisement from the beginning of the book has been joined with the
+other advertisements near the end of the book.
+
+Greek words are spelled out and represented as [word]. Greek letters are
+represented as [a] "for alpha".
+
+
+
+
+ASTRONOMY OF TO-DAY
+
+[Illustration: THE TOTAL ECLIPSE OF THE SUN OF AUGUST 30TH, 1905.
+
+The Corona; from a water-colour sketch, made at Burgos, in Spain, during
+the total phase, by the French Artist, Mdlle. ANDR�E MOCH.]
+
+
+
+
+  ASTRONOMY OF
+  TO-DAY
+
+  _A POPULAR INTRODUCTION IN
+  NON-TECHNICAL LANGUAGE_
+
+  By
+
+  CECIL G. DOLMAGE, M.A., LL.D., D.C.L.
+
+  Fellow of the Royal Astronomical Society; Member of
+  the British Astronomical Association; Member of
+  the Astronomical Society of the Pacific; Membre
+  de la Soci�t� Astronomique de France;
+  Membre de la Soci�t� Belge
+  d'Astronomie
+
+
+
+  With a Frontispiece in Colour
+  and 45 Illustrations & Diagrams
+
+
+  _THIRD EDITION_
+
+
+  LONDON
+  SEELEY AND CO. LIMITED
+  38 GREAT RUSSELL STREET
+  1910
+
+
+
+
+PREFACE
+
+
+The object of this book is to give an account of the science of
+Astronomy, as it is known at the present day, in a manner acceptable to
+the _general reader_.
+
+It is too often supposed that it is impossible to acquire any useful
+knowledge of Astronomy without much laborious study, and without
+adventuring into quite a new world of thought. The reasoning applied to
+the study of the celestial orbs is, however, of no different order from
+that which is employed in the affairs of everyday life. The science of
+mathematics is perhaps responsible for the idea that some kind of
+difference does exist; but mathematical processes are, in effect, no
+more than ordinary logic in concentrated form, the _shorthand of
+reasoning_, so to speak. I have attempted in the following pages to take
+the main facts and theories of Astronomy out of those mathematical forms
+which repel the general reader, and to present them in the _ordinary
+language of our workaday world_.
+
+The few diagrams introduced are altogether supplementary, and are not
+connected with the text by any wearying cross-references. Each diagram
+is complete in itself, being intended to serve as a pictorial aid, in
+case the wording of the text should not have perfectly conveyed the
+desired meaning. The full page illustrations are also described as
+adequately as possible at the foot of each.
+
+As to the coloured frontispiece, this must be placed in a category by
+itself. It is the work of the _artist_ as distinct from the scientist.
+
+The book itself contains incidentally a good deal of matter concerned
+with the Astronomy of the past, the introduction of which has been found
+necessary in order to make clearer the Astronomy of our time.
+
+It would be quite impossible for me to enumerate here the many sources
+from which information has been drawn. But I acknowledge my especial
+indebtedness to Professor F.R. Moulton's _Introduction to Astronomy_
+(Macmillan, 1906), to the works on Eclipses of the late Rev. S.J.
+Johnson and of Mr. W.T. Lynn, and to the excellent _Journals of the
+British Astronomical Association_. Further, for those grand questions
+concerned with the Stellar Universe at large, I owe a very deep debt to
+the writings of the famous American astronomer, Professor Simon Newcomb,
+and of our own countryman, Mr. John Ellard Gore; to the latter of whom I
+am under an additional obligation for much valuable information
+privately rendered.
+
+In my search for suitable illustrations, I have been greatly aided by
+the kindly advice of Mr. W. H. Wesley, the Assistant Secretary of the
+Royal Astronomical Society. To those who have been so good as to permit
+me to reproduce pictures and photographs, I desire to record my best
+thanks as follows:--To the French Artist, Mdlle. Andr�e Moch; to the
+Astronomer Royal; to Sir David Gill, K.C.B., LL.D., F.R.S.; to the
+Council of the Royal Astronomical Society; to Professor E.B. Frost,
+Director of the Yerkes Observatory; to M.P. Puiseux, of the Paris
+Observatory; to Dr. Max Wolf, of Heidelberg; to Professor Percival
+Lowell; to the Rev. Theodore E.R. Phillips, M.A., F.R.A.S.; to Mr. W.H.
+Wesley; to the Warner and Swasey Co., of Cleveland, Ohio, U.S.A.; to the
+publishers of _Knowledge_, and to Messrs. Sampson, Low & Co. For
+permission to reproduce the beautiful photograph of the Spiral Nebula in
+Canes Venatici (Plate XXII.), I am indebted to the distinguished
+astronomer, the late Dr. W.E. Wilson, D.Sc., F.R.S., whose untimely
+death, I regret to state, occurred in the early part of this year.
+
+Finally, my best thanks are due to Mr. John Ellard Gore, F.R.A.S.,
+M.R.I.A., to Mr. W.H. Wesley, and to Mr. John Butler Burke, M.A., of
+Cambridge, for their kindness in reading the proof-sheets.
+
+CECIL G. DOLMAGE.
+
+LONDON, S.W.,
+_August 4, 1908._
+
+
+
+
+PREFATORY NOTE TO THE SECOND EDITION
+
+
+The author of this book lived only long enough to hear of the favour
+with which it had been received, and to make a few corrections in view
+of the second edition which it has so soon reached.
+
+_December 1908._
+
+
+
+
+CONTENTS
+
+
+  CHAPTER I
+                                            PAGE
+  THE ANCIENT VIEW                            17
+
+  CHAPTER II
+  THE MODERN VIEW                             20
+
+  CHAPTER III
+  THE SOLAR SYSTEM                            29
+
+  CHAPTER IV
+  CELESTIAL MECHANISM                         38
+
+  CHAPTER V
+  CELESTIAL DISTANCES                         46
+
+  CHAPTER VI
+  CELESTIAL MEASUREMENT                       55
+
+  CHAPTER VII
+  ECLIPSES AND KINDRED PHENOMENA              61
+
+  CHAPTER VIII
+  FAMOUS ECLIPSES OF THE SUN                  83
+
+  CHAPTER IX
+  FAMOUS ECLIPSES OF THE MOON                101
+
+  CHAPTER X
+  THE GROWTH OF OBSERVATION                  105
+
+  CHAPTER XI
+  SPECTRUM ANALYSIS                          121
+
+  CHAPTER XII
+  THE SUN                                    127
+
+  CHAPTER XIII
+  THE SUN--_continued_                       134
+
+  CHAPTER XIV
+  THE INFERIOR PLANETS                       146
+
+  CHAPTER XV
+  THE EARTH                                  158
+
+  CHAPTER XVI
+  THE MOON                                   183
+
+  CHAPTER XVII
+  THE SUPERIOR PLANETS                       209
+
+  CHAPTER XVIII
+  THE SUPERIOR PLANETS--_continued_          229
+
+  CHAPTER XIX
+  COMETS                                     247
+
+  CHAPTER XX
+  REMARKABLE COMETS                          259
+
+  CHAPTER XXI
+  METEORS OR SHOOTING STARS                  266
+
+  CHAPTER XXII
+  THE STARS                                  278
+
+  CHAPTER XXIII
+  THE STARS--_continued_                     287
+
+  CHAPTER XXIV
+  SYSTEMS OF STARS                           300
+
+  CHAPTER XXV
+  THE STELLAR UNIVERSE                       319
+
+  CHAPTER XXVI
+  THE STELLAR UNIVERSE--_continued_          329
+
+  CHAPTER XXVII
+  THE BEGINNING OF THINGS                    333
+
+  CHAPTER XXVIII
+  THE END OF THINGS                          342
+
+  INDEX                                      351
+
+
+
+
+LIST OF ILLUSTRATIONS
+
+
+LIST OF PLATES
+
+  PLATE
+  THE TOTAL ECLIPSE OF THE SUN
+  OF AUGUST 30, 1905                       _Frontispiece_
+
+  I. THE TOTAL ECLIPSE OF THE SUN
+     OF MAY 17, 1882                   _To face page_  96
+
+  II. GREAT TELESCOPE OF HEVELIUS        "    "       110
+
+  III. A TUBELESS, OR "AERIAL" TELESCOPE "    "       112
+
+  IV. THE GREAT YERKES TELESCOPE         "    "       118
+
+  V. THE SUN, SHOWING SEVERAL
+     GROUPS OF SPOTS                     "    "       134
+
+  VI. PHOTOGRAPH OF A SUNSPOT            "    "       136
+
+  VII. FORMS OF THE SOLAR CORONA
+       AT THE EPOCHS OF SUNSPOT
+       MAXIMUM AND SUNSPOT
+       MINIMUM RESPECTIVELY.
+      (A) THE TOTAL ECLIPSE OF
+      THE SUN OF DECEMBER 22, 1870.
+      (B) THE TOTAL ECLIPSE OF
+      THE SUN OF MAY 28, 1900            "    "       142
+
+  VIII. THE MOON                       _To face page_ 196
+
+  IX. MAP OF THE MOON, SHOWING
+      THE PRINCIPAL "CRATERS,"
+      MOUNTAIN RANGES AND
+      "SEAS"                             "    "       198
+
+  X. ONE OF THE MOST INTERESTING
+     REGIONS ON THE MOON                 "    "       200
+
+  XI. THE MOON (SHOWING SYSTEMS
+      OF "RAYS")                         "    "       204
+
+  XII. A MAP OF THE PLANET MARS          "    "       216
+
+  XIII. MINOR PLANET TRAILS              "    "       226
+
+  XIV. THE PLANET JUPITER                "    "       230
+
+  XV. THE PLANET SATURN                  "    "       236
+
+  XVI. EARLY REPRESENTATIONS OF
+       SATURN                            "    "       242
+
+  XVII. DONATI'S COMET                   "    "       256
+
+  XVIII. DANIEL'S COMET OF 1907          "    "       258
+
+  XIX. THE SKY AROUND THE NORTH
+       POLE                              "    "       292
+
+  XX. ORION AND HIS NEIGHBOURS           "    "       296
+
+  XXI. THE GREAT GLOBULAR CLUSTER
+       IN THE SOUTHERN CONSTELLATION
+       OF CENTAURUS                      "    "       306
+
+  XXII. SPIRAL NEBULA IN THE CONSTELLATION
+        OF CANES VENATICI                "    "       314
+
+  XXIII. THE GREAT NEBULA IN THE
+         CONSTELLATION OF ANDROMEDA    _To face page_ 316
+
+  XXIV. THE GREAT NEBULA IN THE
+        CONSTELLATION OF ORION           "    "       318
+
+
+
+
+LIST OF DIAGRAMS
+
+
+  FIG.                                         PAGE
+  1. THE PTOLEMAIC IDEA OF THE UNIVERSE          19
+
+  2. THE COPERNICAN THEORY OF THE SOLAR
+     SYSTEM                                      21
+
+  3. TOTAL AND PARTIAL ECLIPSES OF THE MOON      64
+
+  4. TOTAL AND PARTIAL ECLIPSES OF THE SUN       67
+
+  5. "BAILY'S BEADS"                             70
+
+  6. MAP OF THE WORLD ON MERCATOR'S PROJECTION,
+     SHOWING A PORTION OF THE PROGRESS
+     OF THE TOTAL SOLAR ECLIPSE OF
+     AUGUST 30, 1905, ACROSS THE SURFACE
+     OF THE EARTH                                81
+
+  7. THE "RING WITH WINGS"                       87
+
+  8. THE VARIOUS TYPES OF TELESCOPE             113
+
+  9. THE SOLAR SPECTRUM                         123
+
+  10. A SECTION THROUGH THE SUN, SHOWING HOW
+      THE PROMINENCES RISE FROM THE
+      CHROMOSPHERE                              131
+
+  11. ORBIT AND PHASES OF AN INFERIOR PLANET    148
+
+  12. THE "BLACK DROP"                          153
+
+  13. SUMMER AND WINTER                         176
+
+  14. ORBIT AND PHASES OF THE MOON              184
+
+  15. THE ROTATION OF THE MOON ON HER AXIS      187
+
+  16. LAPLACE'S "PERENNIAL FULL MOON"           191
+
+  17. ILLUSTRATING THE AUTHOR'S EXPLANATION OF
+      THE APPARENT ENLARGEMENT OF CELESTIAL
+      OBJECTS                                   195
+
+  18. SHOWING HOW THE TAIL OF A COMET IS
+      DIRECTED AWAY FROM THE SUN                248
+
+  19. THE COMET OF 1066, AS REPRESENTED IN THE
+      BAYEUX TAPESTRY                           263
+
+  20. PASSAGE OF THE EARTH THROUGH THE THICKEST
+      PORTION OF A METEOR SWARM                 269
+
+
+
+
+ASTRONOMY OF TO-DAY
+
+
+
+
+CHAPTER I
+
+THE ANCIENT VIEW
+
+
+It is never safe, as we know, to judge by appearances, and this is
+perhaps more true of astronomy than of anything else.
+
+For instance, the idea which one would most naturally form of the earth
+and heaven is that the solid earth on which we live and move extends to
+a great distance in every direction, and that the heaven is an immense
+dome upon the inner surface of which the stars are fixed. Such must
+needs have been the idea of the universe held by men in the earliest
+times. In their view the earth was of paramount importance. The sun and
+moon were mere lamps for the day and for the night; and these, if not
+gods themselves, were at any rate under the charge of special deities,
+whose task it was to guide their motions across the vaulted sky.
+
+Little by little, however, this simple estimate of nature began to be
+overturned. Difficult problems agitated the human mind. On what, for
+instance, did the solid earth rest, and what prevented the vaulted
+heaven from falling in upon men and crushing them out of existence?
+Fantastic myths sprang from the vain attempts to solve these riddles.
+The Hindoos, for example, imagined the earth as supported by four
+elephants which stood upon the back of a gigantic tortoise, which, in
+its turn, floated on the surface of an elemental ocean. The early
+Western civilisations conceived the fable of the Titan Atlas, who, as a
+punishment for revolt against the Olympian gods, was condemned to hold
+up the expanse of sky for ever and ever.
+
+Later on glimmerings of the true light began to break in upon men. The
+Greek philosophers, who busied themselves much with such matters,
+gradually became convinced that the earth was spherical in shape, that
+is to say, round like a ball. In this opinion we now know that they were
+right; but in their other important belief, viz. that the earth was
+placed at the centre of all things, they were indeed very far from the
+truth.
+
+By the second century of the Christian era, the ideas of the early
+philosophers had become hardened into a definite theory, which, though
+it appears very incorrect to us to-day, nevertheless demands exceptional
+notice from the fact that it was everywhere accepted as the true
+explanation until so late as some four centuries ago. This theory of the
+universe is known by the name of the Ptolemaic System, because it was
+first set forth in definite terms by one of the most famous of the
+astronomers of antiquity, Claudius Ptolem�us Pelusinensis (100-170
+A.D.), better known as Ptolemy of Alexandria.
+
+In his system the Earth occupied the centre; while around it circled in
+order outwards the Moon, the planets Mercury and Venus, the Sun, and
+then the planets Mars, Jupiter, and Saturn. Beyond these again revolved
+the background of the heaven, upon which it was believed that the stars
+were fixed--
+
+     "Stellis ardentibus aptum,"
+
+as Virgil puts it (see Fig. 1).
+
+[Illustration: FIG. 1.--The Ptolemaic idea of the Universe.]
+
+The Ptolemaic system persisted unshaken for about fourteen hundred years
+after the death of its author. Clearly men were flattered by the notion
+that their earth was the most important body in nature, that it stood
+still at the centre of the universe, and was the pivot upon which all
+things revolved.
+
+
+
+
+CHAPTER II
+
+THE MODERN VIEW
+
+
+It is still well under four hundred years since the modern, or
+Copernican, theory of the universe supplanted the Ptolemaic, which had
+held sway during so many centuries. In this new theory, propounded
+towards the middle of the sixteenth century by Nicholas Copernicus
+(1473-1543), a Prussian astronomer, the earth was dethroned from its
+central position and considered merely as one of a number of planetary
+bodies which revolve around the sun. As it is not a part of our purpose
+to follow in detail the history of the science, it seems advisable to
+begin by stating in a broad fashion the conception of the universe as
+accepted and believed in to-day.
+
+The Sun, the most important of the celestial bodies so far as we are
+concerned, occupies the central position; not, however, in the whole
+universe, but only in that limited portion which is known as the Solar
+System. Around it, in the following order outwards, circle the planets
+Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
+(see Fig. 2, p. 21). At an immense distance beyond the solar system, and
+scattered irregularly through the depth of space, lie the stars. The two
+first-mentioned members of the solar system, Mercury and Venus, are
+known as the Inferior Planets; and in their courses about the sun, they
+always keep well inside the path along which our earth moves. The
+remaining members (exclusive of the earth) are called Superior Planets,
+and their paths lie all outside that of the earth.
+
+[Illustration: FIG. 2.--The Copernican theory of the Solar System.]
+
+The five planets, Mercury, Venus, Mars, Jupiter, and Saturn, have been
+known from all antiquity. Nothing then can bring home to us more
+strongly the immense advance which has taken place in astronomy during
+modern times than the fact that it is only 127 years since observation
+of the skies first added a planet to that time-honoured number. It was
+indeed on the 13th of March 1781, while engaged in observing the
+constellation of the Twins, that the justly famous Sir William Herschel
+caught sight of an object which he did not recognise as having met with
+before. He at first took it for a comet, but observations of its
+movements during a few days showed it to be a planet. This body, which
+the power of the telescope alone had thus shown to belong to the solar
+family, has since become known to science under the name of Uranus. By
+its discovery the hitherto accepted limits of the solar system were at
+once pushed out to twice their former extent, and the hope naturally
+arose that other planets would quickly reveal themselves in the
+immensities beyond.
+
+For a number of years prior to Herschel's great discovery, it had been
+noticed that the distances at which the then known planets circulated
+appeared to be arranged in a somewhat orderly progression outwards from
+the sun. This seeming plan, known to astronomers by the name of Bode's
+Law, was closely confirmed by the distance of the new planet Uranus.
+There still lay, however, a broad gap between the planets Mars and
+Jupiter. Had another planet indeed circuited there, the solar system
+would have presented an appearance of almost perfect order. But the void
+between Mars and Jupiter was unfilled; the space in which one would
+reasonably expect to find another planet circling was unaccountably
+empty.
+
+On the first day of the nineteenth century the mystery was however
+explained, a body being discovered[1] which revolved in the space that
+had hitherto been considered planetless. But it was a tiny globe hardly
+worthy of the name of planet. In the following year a second body was
+discovered revolving in the same space; but it was even smaller in size
+than the first. During the ensuing five years two more of these little
+planets were discovered. Then came a pause, no more such bodies being
+added to the system until half-way through the century, when suddenly
+the discovery of these so-called "minor planets" began anew. Since then
+additions to this portion of our system have rained thick and fast. The
+small bodies have received the name of Asteroids or Planetoids; and up
+to the present time some six hundred of them are known to exist, all
+revolving in the previously unfilled space between Mars and Jupiter.
+
+In the year 1846 the outer boundary of the solar system was again
+extended by the discovery that a great planet circulated beyond Uranus.
+The new body, which received the name of Neptune, was brought to light
+as the result of calculations made at the same time, though quite
+independently, by the Cambridge mathematician Adams, and the French
+astronomer Le Verrier. The discovery of Neptune differed, however, from
+that of Uranus in the following respect. Uranus was found merely in the
+course of ordinary telescopic survey of the heavens. The position of
+Neptune, on the other hand, was predicted as the result of rigorous
+mathematical investigations undertaken with the object of fixing the
+position of an unseen and still more distant body, the attraction of
+which, in passing by, was disturbing the position of Uranus in its
+revolution around the sun. Adams actually completed his investigation
+first; but a delay at Cambridge in examining that portion of the sky,
+where he announced that the body ought just then to be, allowed France
+to snatch the honour of discovery, and the new planet was found by the
+observer Galle at Berlin, very near the place in the heavens which Le
+Verrier had mathematically predicted for it.
+
+Nearly fifty years later, that is to say, in our own time, another
+important planetary discovery was made. One of the recent additions to
+the numerous and constantly increasing family of the asteroids, a tiny
+body brought to light in 1898, turned out after all not to be
+circulating in the customary space between Mars and Jupiter, but
+actually in that between our earth and Mars. This body is very small,
+not more than about twenty miles across. It has received the name of
+Eros (the Greek equivalent for Cupid), in allusion to its insignificant
+size as compared with the other leading members of the system.
+
+This completes the list of the planets which, so far, have revealed
+themselves to us. Whether others exist time alone will show. Two or
+three have been suspected to revolve beyond the path of Neptune; and it
+has even been asserted, on more than one occasion, that a planet
+circulates nearer to the sun than Mercury. This supposed body, to which
+the name of "Vulcan" was provisionally given, is said to have been
+"discovered" in 1859 by a French doctor named Lescarbault, of Org�res
+near Orleans; but up to the present there has been no sufficient
+evidence of its existence. The reason why such uncertainty should exist
+upon this point is easy enough to understand, when we consider the
+overpowering glare which fills our atmosphere all around the sun's place
+in the sky. Mercury, the nearest known planet to the sun, is for this
+reason always very difficult to see; and even when, in its course, it
+gets sufficiently far from the sun to be left for a short time above the
+horizon after sunset, it is by no means an easy object to observe on
+account of the mists which usually hang about low down near the earth.
+One opportunity, however, offers itself from time to time to solve the
+riddle of an "intra-Mercurial" planet, that is to say, of a planet which
+circulates within the path followed by Mercury. The opportunity in
+question is furnished by a total eclipse of the sun; for when, during an
+eclipse of that kind, the body of the moon for a few minutes entirely
+hides the sun's face, and the dazzling glare is thus completely cut off,
+astronomers are enabled to give an unimpeded, though all too hurried,
+search to the region close around. A goodly number of total eclipses of
+the sun have, however, come and gone since the days of Lescarbault, and
+no planet, so far, has revealed itself in the intra-Mercurial space. It
+seems, therefore, quite safe to affirm that no globe of sufficient size
+to be seen by means of our modern telescopes circulates nearer to the
+sun than the planet Mercury.
+
+Next in importance to the planets, as permanent members of the solar
+system, come the relatively small and secondary bodies known by the name
+of Satellites. The name _satellite_ is derived from a Latin word
+signifying _an attendant_; for the bodies so-called move along always in
+close proximity to their respective "primaries," as the planets which
+they accompany are technically termed.
+
+The satellites cannot be considered as allotted with any particular
+regularity among the various members of the system; several of the
+planets, for instance, having a goodly number of these bodies
+accompanying them, while others have but one or two, and some again have
+none at all. Taking the planets in their order of distance outward from
+the Sun, we find that neither Mercury nor Venus are provided with
+satellites; the Earth has only one, viz. our neighbour the Moon; while
+Mars has but two tiny ones, so small indeed that one might imagine them
+to be merely asteroids, which had wandered out of their proper region
+and attached themselves to that planet. For the rest, so far as we at
+present know, Jupiter possesses seven,[2] Saturn ten, Uranus four, and
+Neptune one. It is indeed possible, nay more, it is extremely probable,
+that the two last-named planets have a greater number of these secondary
+bodies revolving around them; but, unfortunately, the Uranian and
+Neptunian systems are at such immense distances from us, that even the
+magnificent telescopes of to-day can extract very little information
+concerning them.
+
+From the distribution of the satellites, the reader will notice that the
+planets relatively near to the sun are provided with few or none, while
+the more distant planets are richly endowed. The conclusion, therefore,
+seems to be that nearness to the sun is in some way unfavourable either
+to the production, or to the continued existence, of satellites.
+
+A planet and its satellites form a repetition of the solar system on a
+tiny scale. Just as the planets revolve around the sun, so do these
+secondary bodies revolve around their primaries. When Galileo, in 1610,
+turned his newly invented telescope upon Jupiter, he quickly recognised
+in the four circling moons which met his gaze, a miniature edition of
+the solar system.
+
+Besides the planets and their satellites, there are two other classes of
+bodies which claim membership of the solar system. These are Comets and
+Meteors. Comets differ from the bodies which we have just been
+describing in that they appear filmy and transparent, whereas the others
+are solid and opaque. Again, the paths of the planets around the sun and
+of the satellites around their primaries are not actually circles; they
+are ovals, but their ovalness is not of a marked degree. The paths of
+comets on the other hand are usually _very_ oval; so that in their
+courses many of them pass out as far as the known limits of the solar
+system, and even far beyond. It should be mentioned that nowadays the
+tendency is to consider comets as permanent members of the system,
+though this was formerly not by any means an article of faith with
+astronomers.
+
+Meteors are very small bodies, as a rule perhaps no larger than pebbles,
+which move about unseen in space, and of which we do not become aware
+until they arrive very close to the earth. They are then made visible to
+us for a moment or two in consequence of being heated to a white heat by
+the friction of rushing through the atmosphere, and are thus usually
+turned into ashes and vapour long before they reach the surface of our
+globe. Though occasionally a meteoric body survives the fiery ordeal,
+and reaches the earth more or less in a solid state to bury itself deep
+in the soil, the majority of these celestial visitants constitute no
+source of danger whatever for us. Any one who will take the trouble to
+gaze at the sky for a short time on a clear night, is fairly certain to
+be rewarded with the view of a meteor. The impression received is as if
+one of the stars had suddenly left its accustomed place, and dashed
+across the heavens, leaving in its course a trail of light. It is for
+this reason that meteors are popularly known under the name of "shooting
+stars."
+
+
+[1] By the Italian astronomer, Piazzi, at Palermo.
+
+[2] Probably eight. (See note, page 232.)
+
+
+
+
+CHAPTER III
+
+THE SOLAR SYSTEM
+
+
+We have seen, in the course of the last chapter, that the solar system
+is composed as follows:--there is a central body, the sun, around which
+revolve along stated paths a number of important bodies known as
+planets. Certain of these planets, in their courses, carry along in
+company still smaller bodies called satellites, which revolve around
+them. With regard, however, to the remaining members of the system, viz.
+the comets and the meteors, it is not advisable at this stage to add
+more to what has been said in the preceding chapter. For the time being,
+therefore, we will devote our attention merely to the sun, the planets,
+and the satellites.
+
+Of what shape then are these bodies? Of one shape, and that one alone
+which appears to characterise all solid objects in the celestial spaces:
+they are spherical, which means _round like a ball_.
+
+Each of these spherical bodies rotates; that is to say, turns round and
+round, as a top does when it is spinning. This rotation is said to take
+place "upon an axis," a statement which may be explained as
+follows:--Imagine a ball with a knitting-needle run right through its
+centre. Then imagine this needle held pointing in one fixed direction
+while the ball is turned round and round. Well, it is the same thing
+with the earth. As it journeys about the sun, it keeps turning round and
+round continually as if pivoted upon a mighty knitting needle
+transfixing it from North Pole to South Pole. In reality, however, there
+is no such material axis to regulate the constant direction of the
+rotation, just as there are no actual supports to uphold the earth
+itself in space. The causes which keep the celestial spheres poised, and
+which control their motions, are far more wonderful than any mechanical
+device.
+
+At this juncture it will be well to emphasise the sharp distinction
+between the terms _rotation_ and _revolution_. The term "rotation" is
+invariably used by astronomers to signify the motion which a celestial
+body has upon an axis; the term "revolution," on the other hand, is used
+for the movement of one celestial body around another. Speaking of the
+earth, for instance, we say, that it _rotates_ on its axis, and that it
+_revolves_ around the sun.
+
+So far, nothing has been said about the sizes of the members of our
+system. Is there any stock size, any pattern according to which they may
+be judged? None whatever! They vary enormously. Very much the largest of
+all is the Sun, which is several hundred times larger than all the
+planets and satellites of the system rolled together. Next comes Jupiter
+and afterwards the other planets in the following order of
+size:--Saturn, Uranus, Neptune, the Earth, Venus, Mars, and Mercury.
+Very much smaller than any of these are the asteroids, of which Ceres,
+the largest, is less than 500 miles in diameter. It is, by the way, a
+strange fact that the zone of asteroids should mark the separation of
+the small planets from the giant ones. The following table, giving
+roughly the various diameters of the sun and the principal planets in
+miles, will clearly illustrate the great discrepancy in size which
+prevails in the system.
+
+Sun          866,540 miles
+Mercury        2,765  "
+Venus          7,826  "
+Earth          7,918  "
+Mars           4,332  "
+
+ZONE OF ASTEROIDS
+
+Jupiter       87,380  "
+Saturn        73,125  "
+Uranus[3]     34,900  "
+Neptune[3]    32,900  "
+
+It does not seem possible to arrive at any generalisation from the above
+data, except it be to state that there is a continuous increase in size
+from Mercury to the earth, and a similar decrease in size from Jupiter
+outwards. Were Mars greater than the earth, the planets could then with
+truth be said to increase in size up to Jupiter, and then to decrease.
+But the zone of asteroids, and the relative smallness of Mars, negative
+any attempt to regard the dimensions of the planets as an orderly
+sequence.
+
+Next with respect to relative distance from the sun, Venus circulates
+nearly twice as far from it as Mercury, the earth nearly three times as
+far, and Mars nearly four times. After this, just as we found a sudden
+increase in size, so do we meet with a sudden increase in distance.
+Jupiter, for instance, is about thirteen times as far as Mercury, Saturn
+about twenty-five times, Uranus about forty-nine times, and Neptune
+about seventy-seven. (See Fig. 2, p. 21.)
+
+It will thus be seen how enormously the solar system was enlarged in
+extent by the discovery of the outermost planets. The finding of Uranus
+plainly doubled its breadth; the finding of Neptune made it more than
+half as broad again. Nothing indeed can better show the import of these
+great discoveries than to take a pair of compasses and roughly set out
+the above relative paths in a series of concentric circles upon a large
+sheet of paper, and then to consider that the path of Saturn was the
+supposed boundary of our solar system prior to the year 1781.
+
+We have seen that the usual shape of celestial bodies themselves is
+spherical. Of what form then are their paths, or _orbits_, as these are
+called? One might be inclined at a venture to answer "circular," but
+this is not the case. The orbits of the planets cannot be regarded as
+true circles. They are ovals, or, to speak in technical language,
+"ellipses." Their ovalness or "ellipticity" is, however, in each case
+not by any means of the same degree. Some orbits--for instance, that of
+the earth--differ only slightly from circles; while others--those of
+Mars or Mercury, for example--are markedly elliptic. The orbit of the
+tiny planet Eros is, however, far and away the most elliptic of all, as
+we shall see when we come to deal with that little planet in detail.
+
+It has been stated that the sun and planets are always rotating. The
+various rates at which they do so will, however, be best appreciated by
+a comparison with the rate at which the earth itself rotates.
+
+But first of all, let us see what ground we have, if any, for asserting
+that the earth rotates at all?
+
+If we carefully watch the heavens we notice that the background of the
+sky, with all the brilliant objects which sparkle in it, appears to turn
+once round us in about twenty-four hours; and that the pivot upon which
+this movement takes place is situated somewhere near what is known to us
+as the _Pole Star_. This was one of the earliest facts noted with regard
+to the sky; and to the men of old it therefore seems as if the heavens
+and all therein were always revolving around the earth. It was natural
+enough for them to take this view, for they had not the slightest idea
+of the immense distance of the celestial bodies, and in the absence of
+any knowledge of the kind they were inclined to imagine them
+comparatively near. It was indeed only after the lapse of many
+centuries, when men had at last realised the enormous gulf which
+separated them from even the nearest object in the sky, that a more
+reasonable opinion began to prevail. It was then seen that this
+revolution of the heavens about the earth could be more easily and more
+satisfactorily explained by supposing a mere rotation of the solid earth
+about a fixed axis, pointed in the direction of the polar star. The
+probability of such a rotation on the part of the earth itself was
+further strengthened by the observations made with the telescope. When
+the surfaces of the sun and planets were carefully studied these bodies
+were seen to be rotating. This being the case, there could not surely
+be much hesitation in granting that the earth rotated also; particularly
+when it so simply explained the daily movement of the sky, and saved men
+from the almost inconceivable notion that the whole stupendous vaulted
+heaven was turning about them once in every twenty-four hours.
+
+If the sun be regularly observed through a telescope, it will gradually
+be gathered from the slow displacement of sunspots across its face,
+their disappearance at one edge and their reappearance again at the
+other edge, that it is rotating on an axis in a period of about
+twenty-six days. The movement, too, of various well-known markings on
+the surfaces of the planets Mars, Jupiter, and Saturn proves to us that
+these bodies are rotating in periods, which are about twenty-four hours
+for the first, and about ten hours for each of the other two. With
+regard, however, to Uranus and Neptune there is much more uncertainty,
+as these planets are at such great distances that even our best
+telescopes give but a confused view of the markings which they display;
+still a period of rotation of from ten to twelve hours appears to be
+accepted for them. On the other hand the constant blaze of sunlight in
+the neighbourhood of Mercury and Venus equally hampers astronomers in
+this quest. The older telescopic observers considered that the rotation
+periods of these two planets were about the same as that of the earth;
+but of recent years the opinion has been gaining ground that they turn
+round on their axes in exactly the same time as they revolve about the
+sun. This question is, however, a very doubtful one, and will be again
+referred to later on; but, putting it on one side, it will be seen from
+what we have said above, that the rotation periods of the other planets
+of our system are usually about twenty-four hours, or under. The fact
+that the rotation period of the sun should run into _days_ need not seem
+extraordinary when one considers its enormous size.
+
+The periods taken by the various planets to revolve around the sun is
+the next point which has to be considered. Here, too, it is well to
+start with the earth's period of revolution as the standard, and to see
+how the periods taken by the other planets compare with it.
+
+The earth takes about 365-1/4 days to revolve around the sun. This
+period of time is known to us as a "year." The following table shows in
+days and years the periods taken by each of the other planets to make a
+complete revolution round the sun:--
+
+Mercury  about  88 days.
+Venus     "    226   "
+Mars      "      1 year and 321 days.
+Jupiter   "     11 years and 313 days.
+Saturn    "     29 years and 167 days.
+Uranus    "     84 years and 7 days.
+Neptune   "    164 years and 284 days.
+
+From these periods we gather an important fact, namely, that the nearer
+a planet is to the sun the faster it revolves.
+
+Compared with one of our years what a long time does an Uranian, or
+Neptunian, "year" seem? For instance, if a "year" had commenced in
+Neptune about the middle of the reign of George II., that "year" would
+be only just coming to a close; for the planet is but now arriving back
+to the position, with regard to the sun, which it then occupied. Uranus,
+too, has only completed a little more than 1-1/2 of its "years" since
+Herschel discovered it.
+
+Having accepted the fact that the planets are revolving around the sun,
+the next point to be inquired into is:--What are the positions of their
+orbits, or paths, relatively to each other?
+
+Suppose, for instance, the various planetary orbits to be represented by
+a set of hoops of different sizes, placed one within the other, and the
+sun by a small ball in the middle of the whole; in what positions will
+these hoops have to be arranged so as to imitate exactly the true
+condition of things?
+
+First of all let us suppose the entire arrangement, ball and hoops, to
+be on one level, so to speak. This may be easily compassed by imagining
+the hoops as floating, one surrounding the other, with the ball in the
+middle of all, upon the surface of still water. Such a set of objects
+would be described in astronomical parlance as being _in the same
+plane_. Suppose, on the other hand, that some of these floating hoops
+are tilted with regard to the others, so that one half of a hoop rises
+out of the water and the other half consequently sinks beneath the
+surface. This indeed is the actual case with regard to the planetary
+orbits. They do not by any means lie all exactly in the same plane. Each
+one of them is tilted, or _inclined_, a little with respect to the plane
+of the earth's orbit, which astronomers, for convenience, regard as the
+_level_ of the solar system. This tilting, or "inclination," is, in the
+larger planets, greatest for the orbit of Mercury, least for that of
+Uranus. Mercury's orbit is inclined to that of the earth at an angle of
+about 7�, that of Venus at a little over 3�, that of Saturn 2-1/2�;
+while in those of Mars, Neptune, and Jupiter the inclination is less
+than 2�. But greater than any of these is the inclination of the orbit
+of the tiny planet Eros, viz. nearly 11�.
+
+The systems of satellites revolving around their respective planets
+being, as we have already pointed out, mere miniature editions of the
+solar system, the considerations so far detailed, which regulate the
+behaviour of the planets in their relations to the sun, will of
+necessity apply to the satellites very closely. In one respect, however,
+a system of satellites differs materially from a system of planets. The
+central body around which planets are in motion is self-luminous,
+whereas the planetary body around which a satellite revolves is not.
+True, planets shine, and shine very brightly too; as, for instance,
+Venus and Jupiter. But they do not give forth any light of their own, as
+the sun does; they merely reflect the sunlight which they receive from
+him. Putting this one fact aside, the analogy between the planetary
+system and a satellite system is remarkable. The satellites are
+spherical in form, and differ markedly in size; they rotate, so far as
+we know, upon their axes in varying times; they revolve around their
+governing planets in orbits, not circular, but elliptic; and these
+orbits, furthermore, do not of necessity lie in the same plane. Last of
+all the satellites revolve around their primaries at rates which are
+directly comparable with those at which the planets revolve around the
+sun, the rule in fact holding good that the nearer a satellite is to its
+primary the faster it revolves.
+
+
+[3] As there seems to be much difference of opinion concerning the
+diameters of Uranus and Neptune, it should here be mentioned that the
+above figures are taken from Professor F.R. Moulton's _Introduction to
+Astronomy_ (1906). They are there stated to be given on the authority of
+"Barnard's many measures at the Lick Observatory."
+
+
+
+
+CHAPTER IV
+
+CELESTIAL MECHANISM
+
+
+As soon as we begin to inquire closely into the actual condition of the
+various members of the solar system we are struck with a certain
+distinction. We find that there are two quite different points of view
+from which these bodies can be regarded. For instance, we may make our
+estimates of them either as regards _volume_--that is to say, the mere
+room which they take up; or as regards _mass_--that is to say, the
+amount of matter which they contain.
+
+Let us imagine two globes of equal volume; in other words, which take up
+an equal amount of space. One of these globes, however, may be composed
+of material much more tightly put together than in the other; or of
+greater _density_, as the term goes. That globe is said to be the
+greater of the two in mass. Were such a pair of globes to be weighed in
+scales, one globe in each pan, we should see at once, by its weighing
+down the other, which of the two was composed of the more tightly packed
+materials; and we should, in astronomical parlance, say of this one that
+it had the greater mass.
+
+Volume being merely another word for size, the order of the members of
+the solar system, with regard to their volumes, will be as follows,
+beginning with the greatest:--the Sun, Jupiter, Saturn, Uranus,
+Neptune, the Earth, Venus, Mars, and Mercury.
+
+With regard to mass the same order strangely enough holds good. The
+actual densities of the bodies in question are, however, very different.
+The densest or closest packed body of all is the Earth, which is about
+five and a half times as dense as if it were composed entirely of water.
+Venus follows next, then Mars, and then Mercury. The remaining bodies,
+on the other hand, are relatively loose in structure. Saturn is the
+least dense of all, less so than water. The density of the Sun is a
+little greater than that of water.
+
+This method of estimating is, however, subject to a qualification. It
+must be remembered that in speaking of the Sun, for instance, as being
+only a little denser than water, we are merely treating the question
+from the point of view of an average. Certain parts of it in fact will
+be ever so much denser than water: those are the parts in the centre.
+Other portions, for instance, the outside portions, will be very much
+less dense. It will easily be understood that in all such bodies the
+densest or most compressed portions are to be found towards the centre;
+while the portions towards the exterior being less pressed upon, will be
+less dense.
+
+We now reach a very important point, the question of Gravitation.
+_Gravitation_, or _gravity_, as it is often called, is the attractive
+force which, for instance, causes objects to fall to the earth. Now it
+seems rather strange that one should say that it is owing to a certain
+force that things fall towards the earth. All things seem to us to fall
+so of their own accord, as if it were quite natural, or rather most
+unnatural if they did not. Why then require a "force" to make them fall?
+
+The story goes that the great Sir Isaac Newton was set a-thinking on
+this subject by seeing an apple fall from a tree to the earth. He then
+carried the train of thought further; and, by studying the movements of
+the moon, he reached the conclusion that a body even so far off as our
+satellite would be drawn towards the earth in the same manner. This
+being the case, one will naturally ask why the moon herself does not
+fall in upon the earth. The answer is indeed found to be that the moon
+is travelling round and round the earth at a certain rapid pace, and it
+is this very same rapid pace which keeps her from falling in upon us.
+Any one can test this simple fact for himself. If we tie a stone to the
+end of a string, and keep whirling it round and round fast enough, there
+will be a strong pull from the stone in an outward direction, and the
+string will remain tight all the time that the stone is being whirled.
+If, however, we gradually slacken the speed at which we are making the
+stone whirl, a moment will come at length when the string will become
+limp, and the stone will fall back towards our hand.
+
+It seems, therefore, that there are two causes which maintain the stone
+at a regular distance all the time it is being steadily whirled. One of
+these is the continual pull inward towards our hand by means of the
+string. The other is the continual pull away from us caused by the rate
+at which the stone is travelling. When the rate of whirling is so
+regulated that these pulls exactly balance each other, the stone travels
+comfortably round and round, and shows no tendency either to fall back
+upon our hand or to break the string and fly away into the air. It is
+indeed precisely similar with regard to the moon. The continual pull of
+the earth's gravitation takes the place of the string. If the moon were
+to go round and round slower than it does, it would tend to fall in
+towards the earth; if, on the other hand, it were to go faster, it would
+tend to rush away into space.
+
+The same kind of pull which the earth exerts upon the objects at its
+surface, or upon its satellite, the moon, exists through space so far as
+we know. Every particle of matter in the universe is found in fact to
+attract every other particle. The moon, for instance, attracts the earth
+also, but the controlling force is on the side of the much greater mass
+of the earth. This force of gravity or attraction of gravitation, as it
+is also called, is perfectly regular in its action. Its power depends
+first of all exactly upon the mass of the body which exerts it. The
+gravitational pull of the sun, for instance, reaches out to an enormous
+distance, controlling perhaps, in their courses, unseen planets circling
+far beyond the orbit of Neptune. Again, the strength with which the
+force of gravity acts depends upon distance in a regularly diminishing
+proportion. Thus, the nearer an object is to the earth, for instance,
+the stronger is the gravitational pull which it gets from it; the
+farther off it is, the weaker is this pull. If then the moon were to be
+brought nearer to the earth, the gravitational pull of the latter would
+become so much stronger that the moon's rate of motion would have also
+to increase in due proportion to prevent her from being drawn into the
+earth. Last of all, the point in a body from which the attraction of
+gravitation acts, is not necessarily the centre of the body, but rather
+what is known as its _centre of gravity_, that is to say, the balancing
+point of all the matter which the body contains.
+
+It should here be noted that the moon does not actually revolve around
+the centre of gravity of the earth. What really happens is that both
+orbs revolve around their _common_ centre of gravity, which is a point
+within the body of the earth, and situated about three thousand miles
+from its centre. In the same manner the planets and the sun revolve
+around the centre of gravity of the solar system, which is a point
+within the body of the sun.
+
+The neatly poised movements of the planets around the sun, and of the
+satellites around their respective planets, will therefore be readily
+understood to result from a nice balance between gravitation and speed
+of motion.
+
+The mass of the earth is ascertained to be about eighty times that of
+the moon. Our knowledge of the mass of a planet is learned from
+comparing the revolutions of its satellite or satellites around it, with
+those of the moon around the earth. We are thus enabled to deduce what
+the mass of such a planet would be compared to the earth's mass; that is
+to say, a study, for instance, of Jupiter's satellite system shows that
+Jupiter must have a mass nearly three hundred and eighteen times that of
+our earth. In the same manner we can argue out the mass of the sun from
+the movements of the planets and other bodies of the system around it.
+With regard, however, to Venus and Mercury, the problem is by no means
+such an easy one, as these bodies have no satellites. For information in
+this latter case we have to rely upon such uncertain evidence as, for
+instance, the slight disturbances caused in the motion of the earth by
+the attraction of these planets when they pass closest to us, or their
+observed effect upon the motions of such comets as may happen to pass
+near to them.
+
+Mass and weight, though often spoken of as one and the same thing, are
+by no means so. Mass, as we have seen, merely means the amount of matter
+which a body contains. The weight of a body, on the other hand, depends
+entirely upon the gravitational pull which it receives. The force of
+gravity at the surface of the earth is, for instance, about six times as
+great as that at the surface of the moon. All bodies, therefore, weigh
+about six times as much on the earth as they would upon the moon; or,
+rather, a body transferred to the moon's surface would weigh only about
+one-sixth of what it did on the terrestrial surface. It will therefore
+be seen that if a body of given _mass_ were to be placed upon planet
+after planet in turn, its _weight_ would regularly alter according to
+the force of gravity at each planet's surface.
+
+Gravitation is indeed one of the greatest mysteries of nature. What it
+is, the means by which it acts, or why such a force should exist at all,
+are questions to which so far we have not had even the merest hint of an
+answer. Its action across space appears to be instantaneous.
+
+The intensity of gravitation is said in mathematical parlance "to vary
+inversely with the square of the distance." This means that at _twice_
+the distance the pull will become only _one-quarter_ as strong, and not
+one-half as otherwise might be expected. At _four_ times the distance,
+therefore, it will be _one-sixteenth_ as strong. At the earth's surface
+a body is pulled by the earth's gravitation, or "falls," as we
+ordinarily term it, through 16 feet in one _second_ of time; whereas at
+the distance of the moon the attraction of the earth is so very much
+weakened that a body would take as long as one _minute_ to fall through
+the same space.
+
+Newton's investigations showed that if a body were to be placed _at
+rest_ in space entirely away from the attraction of any other body it
+would remain always in a motionless condition, because there would
+plainly be no reason why it should move in any one direction rather than
+in another. And, similarly, if a body were to be projected in a certain
+direction and at a certain speed, it would move always in the same
+direction and at the same speed so long as it did not come within the
+gravitational attraction of any other body.
+
+The possibility of an interaction between the celestial orbs had
+occurred to astronomers before the time of Newton; for instance, in the
+ninth century to the Arabian Musa-ben-Shakir, to Camillus Agrippa in
+1553, and to Kepler, who suspected its existence from observation of the
+tides. Horrox also, writing in 1635, spoke of the moon as moved by an
+_emanation_ from the earth. But no one prior to Newton attempted to
+examine the question from a mathematical standpoint.
+
+Notwithstanding the acknowledged truth and far-reaching scope of the law
+of gravitation--for we find its effects exemplified in every portion of
+the universe--there are yet some minor movements which it does not
+account for. For instance, there are small irregularities in the
+movement of Mercury which cannot be explained by the influence of
+possible intra-Mercurial planets, and similarly there are slight
+unaccountable deviations in the motions of our neighbour the Moon.
+
+
+
+
+CHAPTER V
+
+CELESTIAL DISTANCES
+
+
+Up to this we have merely taken a general view of the solar system--a
+bird's-eye view, so to speak, from space.
+
+In the course of our inquiry we noted in a rough way the _relative_
+distances at which the various planets move around the sun. But we have
+not yet stated what these distances _actually_ are, and it were
+therefore well now to turn our attention to this important matter.
+
+Each of us has a fair idea of what a mile is. It is a quarter of an
+hour's sharp walk, for instance; or yonder village or building, we know,
+lies such and such a number of miles away.
+
+The measurements which have already been given of the diameters of the
+various bodies of the solar system appear very great to us, who find
+that a walk of a few miles at a time taxes our strength; but they are a
+mere nothing when we consider the distances from the sun at which the
+various planets revolve in their orbits.
+
+The following table gives these distances in round numbers. As here
+stated they are what are called "mean" distances; for, as the orbits are
+oval, the planets vary in their distances from the sun, and we are
+therefore obliged to strike a kind of average for each case:--
+
+Mercury  about   36,000,000 miles.
+Venus      "     67,200,000  "
+Earth      "     92,900,000  "
+Mars       "    141,500,000  "
+Jupiter    "    483,300,000  "
+Saturn     "    886,000,000  "
+Uranus     "  1,781,900,000  "
+Neptune    "  2,791,600,000  "
+
+From the above it will be seen at a glance that we have entered upon a
+still greater scale of distance than in dealing with the diameters of
+the various bodies of the system. In that case the distances were
+limited to thousands of miles; in this, however, we have to deal with
+millions. A million being ten hundred thousand, it will be noticed that
+even the diameter of the huge sun is well under a million miles.
+
+How indeed are we to get a grasp of such distances, when those to which
+we are ordinarily accustomed--the few miles' walk, the little stretch of
+sea or land which we gaze upon around us--are so utterly minute in
+comparison? The fact is, that though men may think that they can picture
+in their minds such immense distances, they actually can not. In matters
+like these we unconsciously employ a kind of convention, and we estimate
+a thing as being two or three or more times the size of another. More
+than this we are unable to do. For instance, our ordinary experience of
+a mile enables us to judge, in a way, of a stretch of several miles,
+such as one can take in with a glance; but in our estimation of a
+thousand miles, or even of one hundred, we are driven back upon a mental
+trick, so to speak.
+
+In our attempts to realise such immense distances as those in the solar
+system we are obliged to have recourse to analogies; to comparisons with
+other and simpler facts, though this is at the best a mere self-cheating
+device. The analogy which seems most suited to our purpose here, and one
+which has often been employed by writers, is borrowed from the rate at
+which an express train travels.
+
+Let us imagine, for instance, that we possess an express train which is
+capable of running anywhere, never stops, never requires fuel, and
+always goes along at sixty miles an hour. Suppose we commence by
+employing it to gauge the size of our own planet, the earth. Let us send
+it on a trip around the equator, the span of which is about 24,000
+miles. At its sixty-miles-an-hour rate of going, this journey will take
+nearly 17 days. Next let us send it from the earth to the moon. This
+distance, 240,000 miles, being ten times as great as the last, will of
+course take ten times as long to cover, namely, 170 days; that is to
+say, nearly half a year. Again, let us send it still further afield, to
+the sun, for example. Here, however, it enters upon a journey which is
+not to be measured in thousands of miles, as the others were, but in
+millions. The distance from the earth to the sun, as we have seen in the
+foregoing table, is about 93 millions of miles. Our express train would
+take about 178 _years_ to traverse this.
+
+Having arrived at the sun, let us suppose that our train makes a tour
+right round it. This will take more than five years.
+
+Supposing, finally, that our train were started from the sun, and made
+to run straight out to the known boundaries of the solar system, that is
+to say, as far as the orbit of Neptune, it would take over 5000 years to
+traverse the distance.
+
+That sixty miles an hour is a very great speed any one, I think, will
+admit who has stood upon the platform of a country station while one of
+the great mail trains has dashed past. But are not the immensities of
+space appalling to contemplate, when one realises that a body moving
+incessantly at such a rate would take so long as 10,000 years to
+traverse merely the breadth of our solar system? Ten thousand years!
+Just try to conceive it. Why, it is only a little more than half that
+time since the Pyramids were built, and they mark for us the Dawn of
+History. And since then half-a-dozen mighty empires have come and gone!
+
+Having thus concluded our general survey of the appearance and
+dimensions of the solar system, let us next inquire into its position
+and size in relation to what we call the Universe.
+
+A mere glance at the night sky, when it is free from clouds, shows us
+that in every direction there are stars; and this holds good, no matter
+what portion of the globe we visit. The same is really true of the sky
+by day, though in that case we cannot actually see the stars, for their
+light is quite overpowered by the dazzling light of the sun.
+
+We thus reach the conclusion that our earth, that our solar system in
+fact, lies plunged within the midst of a great tangle of stars. What
+position, by the way, do we occupy in this mighty maze? Are we at the
+centre, or anywhere near the centre, or where?
+
+It has been indeed amply proved by astronomical research that the stars
+are bodies giving off a light of their own, just as our sun does; that
+they are in fact suns, and that our sun is merely one, perhaps indeed a
+very unimportant member, of this great universe of stars. Each of these
+stars, or suns, besides, may be the centre of a system similar to what
+we call our solar system, comprising planets and satellites, comets and
+meteors;--or perchance indeed some further variety of attendant bodies
+of which we have no example in our tiny corner of space. But as to
+whether one is right in a conjecture of this kind, there is up to the
+present no proof whatever. No telescope has yet shown a planet in
+attendance upon one of these distant suns; for such bodies, even if they
+do exist, are entirely out of the range of our mightiest instruments. On
+what then can we ground such an assumption? Merely upon analogy; upon
+the common-sense deduction that as the stars have characteristics
+similar to our particular star, the sun, it would seem unlikely that
+ours should be the only such body in the whole of space which is
+attended by a planetary system.
+
+"The Stars," using that expression in its most general sense, do not lie
+at one fixed distance from us, set here and there upon a background of
+sky. There is in fact no background at all. The brilliant orbs are all
+around us in space, at different distances from us and from each other;
+and we can gaze between them out into the blackness of the void which,
+perhaps, continues to extend unceasingly long after the very outposts of
+the stellar universe has been left behind. Shall we then start our
+imaginary express train once more, and send it out towards the nearest
+of the stars? This would, however, be a useless experiment. Our
+express-train method of gauging space would fail miserably in the
+attempt to bring home to us the mighty gulf by which we are now faced.
+Let us therefore halt for a moment and look back upon the orders of
+distance with which we have been dealing. First of all we dealt with
+thousands of miles. Next we saw how they shrank into insignificance when
+we embarked upon millions. We found, indeed, that our sixty-mile-an-hour
+train, rushing along without ceasing, would consume nearly the whole of
+historical time in a journey from the sun to Neptune.
+
+In the spaces beyond the solar system we are faced, however, by a new
+order of distance. From sun to planets is measured in millions of miles,
+but from sun to sun is measured in billions. But does the mere stating
+of this fact convey anything? I fear not. For the word "billion" runs as
+glibly off the tongue as "million," and both are so wholly unrealisable
+by us that the actual difference between them might easily pass
+unnoticed.
+
+Let us, however, make a careful comparison. What is a million? It is a
+thousand thousands. But what is a billion? It is a million millions.
+Consider for a moment! A million of millions. That means a million, each
+unit of which is again a million. In fact every separate "1" in this
+million is itself a million. Here is a way of trying to realise this
+gigantic number. A million seconds make only eleven and a half days and
+nights. But a billion seconds will make actually more than thirty
+thousand years!
+
+Having accepted this, let us try and probe with our express train even a
+little of the new gulf which now lies before us. At our old rate of
+going it took almost two years to cover a million miles. To cover a
+billion miles--that is to say, a million times this distance--would thus
+take of course nearly two million years. Alpha Centauri, the nearest
+star to our earth, is some twenty-five billions of miles away. Our
+express train would thus take about fifty millions of years to reach it!
+
+This shows how useless our illustration, appropriate though it seemed
+for interplanetary space, becomes when applied to the interstellar
+spaces. It merely gives us millions in return for billions; and so the
+mind, driven in upon itself, whirls round and round like a squirrel in
+its revolving cage. There is, however, a useful illustration still left
+us, and it is the one which astronomers usually employ in dealing with
+the distances of the stars. The illustration in question is taken from
+the velocity of light.
+
+Light travels at the tremendous speed of about 186,000 miles a second.
+It therefore takes only about a second and a quarter to come to us from
+the moon. It traverses the 93,000,000 of miles which separate us from
+the sun in about eight minutes. It travels from the sun out to Neptune
+in about four hours, which means that it would cross the solar system
+from end to end in eight. To pass, however, across the distance which
+separates us from Alpha Centauri it would take so long as about four
+and a quarter years!
+
+Astronomers, therefore, agree in estimating the distances of the stars
+from the point of view of the time which light would take to pass from
+them to our earth. They speak of that distance which light takes a year
+to traverse as a "light year." According to this notation, Alpha
+Centauri is spoken of as being about four and a quarter light years
+distant from us.
+
+Now as the rays of light coming from Alpha Centauri to us are chasing
+one another incessantly across the gulf of space, and as each ray left
+that star some four years before it reaches us, our view of the star
+itself must therefore be always some four years old. Were then this star
+to be suddenly removed from the universe at any moment, we should
+continue to see it still in its place in the sky for some four years
+more, after which it would suddenly disappear. The rays which had
+already started upon their journey towards our earth must indeed
+continue travelling, and reaching us in their turn until the last one
+had arrived; after which no more would come.
+
+We have drawn attention to Alpha Centauri as the nearest of the stars.
+The majority of the others indeed are ever so much farther. We can only
+hazard a guess at the time it takes for the rays from many of them to
+reach our globe. Suppose, for instance, we see a sudden change in the
+light of any of these remote stars, we are inclined to ask ourselves
+when that change did actually occur. Was it in the days of Queen
+Elizabeth, or at the time of the Norman Conquest; or was it when Rome
+was at the height of her glory, or perhaps ages before that when the
+Pyramids of Egypt were being built? Even the last of these suppositions
+cannot be treated lightly. We have indeed no real knowledge of the
+distance from us of those stars which our giant telescopes have brought
+into view out of the depths of the celestial spaces.
+
+
+
+
+CHAPTER VI
+
+CELESTIAL MEASUREMENT
+
+
+Had the telescope never been invented our knowledge of astronomy would
+be trifling indeed.
+
+Prior to the year 1610, when Galileo first turned the new instrument
+upon the sky, all that men knew of the starry realms was gathered from
+observation with their own eyes unaided by any artificial means. In such
+researches they had been very much at a disadvantage. The sun and moon,
+in their opinion, were no doubt the largest bodies in the heavens, for
+the mere reason that they looked so! The mighty solar disturbances,
+which are now such common-places to us, were then quite undreamed of.
+The moon displayed a patchy surface, and that was all; her craters and
+ring-mountains were surprises as yet in store for men. Nothing of course
+was known about the surfaces of the planets. These objects had indeed no
+particular characteristics to distinguish them from the great host of
+the stars, except that they continually changed their positions in the
+sky while the rest did not. The stars themselves were considered as
+fixed inalterably upon the vault of heaven. The sun, moon, and planets
+apparently moved about in the intermediate space, supported in their
+courses by strange and fanciful devices. The idea of satellites was as
+yet unknown. Comets were regarded as celestial portents, and meteors as
+small conflagrations taking place in the upper air.
+
+In the entire absence of any knowledge with regard to the actual sizes
+and distances of the various celestial bodies, men naturally considered
+them as small; and, concluding that they were comparatively near,
+assigned to them in consequence a permanent connection with terrestrial
+affairs. Thus arose the quaint and erroneous beliefs of astrology,
+according to which the events which took place upon our earth were
+considered to depend upon the various positions in which the planets,
+for instance, found themselves from time to time.
+
+It must, however, be acknowledged that the study of astrology,
+fallacious though its conclusions were, indirectly performed a great
+service to astronomy by reason of the accurate observations and diligent
+study of the stars which it entailed.
+
+We will now inquire into the means by which the distances and sizes of
+the celestial orbs have been ascertained, and see how it was that the
+ancients were so entirely in the dark in this matter.
+
+There are two distinct methods of finding out the distance at which any
+object happens to be situated from us.
+
+One method is by actual measurement.
+
+The other is by moving oneself a little to the right or left, and
+observing whether the distant object appears in any degree altered in
+position by our own change of place.
+
+One of the best illustrations of this relative change of position which
+objects undergo as a result of our own change of place, is to observe
+the landscape from the window of a moving railway carriage. As we are
+borne rapidly along we notice that the telegraph posts which are set
+close to the line appear to fly past us in the contrary direction; the
+trees, houses, and other things beyond go by too, but not so fast;
+objects a good way off displace slowly; while some spire, or tall
+landmark, in the far distance appears to remain unmoved during a
+comparatively long time.
+
+Actual change of position on our own part is found indeed to be
+invariably accompanied by an apparent displacement of the objects about
+us, such apparent displacement as a result of our own change of position
+being known as "parallax." The dependence between the two is so
+mathematically exact, that if we know the amount of our own change of
+place, and if we observe the amount of the consequent displacement of
+any object, we are enabled to calculate its precise distance from us.
+Thus it comes to pass that distances can be measured without the
+necessity of moving over them; and the breadth of a river, for instance,
+or the distance from us of a ship at sea, can be found merely by such
+means.
+
+It is by the application of this principle to the wider field of the sky
+that we are able to ascertain the distance of celestial bodies. We have
+noted that it requires a goodly change of place on our own part to shift
+the position in which some object in the far distance is seen by us. To
+two persons separated by, say, a few hundred yards, a ship upon the
+horizon will appear pretty much in the same direction. They would
+require, in fact, to be much farther apart in order to displace it
+sufficiently for the purpose of estimating their distance from it. It
+is the same with regard to the moon. Two observers, standing upon our
+earth, will require to be some thousands of miles apart in order to see
+the position of our satellite sufficiently altered with regard to the
+starry background, to give the necessary data upon which to ground their
+calculations.
+
+The change of position thus offered by one side of the earth's surface
+at a time is, however, not sufficient to displace any but the nearest
+celestial bodies. When we have occasion to go farther afield we have to
+seek a greater change of place. This we can get as a consequence of the
+earth's movement around the sun. Observations, taken several days apart,
+will show the effect of the earth's change of place during the interval
+upon the positions of the other bodies of our system. But when we desire
+to sound the depths of space beyond, and to reach out to measure the
+distance of the nearest star, we find ourselves at once thrown upon the
+greatest change of place which we can possibly hope for; and this we get
+during the long journey of many millions of miles which our earth
+performs around the sun during the course of each year. But even this
+last change of place, great as it seems in comparison with terrestrial
+measurements, is insufficient to show anything more than the tiniest
+displacements in a paltry forty-three out of the entire host of the
+stars.
+
+We can thus realise at what a disadvantage the ancients were. The
+measuring instruments at their command were utterly inadequate to detect
+such small displacements. It was reserved for the telescope to reveal
+them; and even then it required the great telescopes of recent times to
+show the slight changes in the position of the nearer stars, which were
+caused by the earth's being at one time at one end of its orbit, and
+some six months later at the other end--stations separated from each
+other by a gulf of about one hundred and eighty-six millions of miles.
+
+The actual distances of certain celestial bodies being thus
+ascertainable, it becomes a matter of no great difficulty to determine
+the actual sizes of the measurable ones. It is a matter of everyday
+experience that the size which any object appears to have, depends
+exactly upon the distance it is from us. The farther off it is the
+smaller it looks; the nearer it is the bigger. If, then, an object which
+lies at a known distance from us looks such and such a size, we can of
+course ascertain its real dimensions. Take the moon, for instance. As we
+have already shown, we are able to ascertain its distance. We observe
+also that it looks a certain size. It is therefore only a matter of
+calculation to find what its actual dimensions should be, in order that
+it may look that size at that distance away. Similarly we can ascertain
+the real dimensions of the sun. The planets, appearing to us as points
+of light, seem at first to offer a difficulty; but, by means of the
+telescope, we can bring them, as it were, so much nearer to us, that
+their broad expanses may be seen. We fail, however, signally with regard
+to the stars; for they are so very distant, and therefore such tiny
+points of light, that our mightiest telescopes cannot magnify them
+sufficiently to show any breadth of surface.
+
+Instead of saying that an object looks a certain breadth across, such
+as a yard or a foot, a statement which would really mean nothing,
+astronomers speak of it as measuring a certain angle. Such angles are
+estimated in what are called "degrees of arc"; each degree being divided
+into sixty minutes, and each minute again into sixty seconds. Popularly
+considered the moon and sun _look_ about the same size, or, as an
+astronomer would put it, they measure about the same angle. This is an
+angle, roughly, of thirty-two minutes of arc; that is to say, slightly
+more than half a degree. The broad expanse of surface which a celestial
+body shows to us, whether to the naked eye, as in the case of the sun
+and moon, or in the telescope, as in the case of other members of our
+system, is technically known as its "disc."
+
+
+
+
+CHAPTER VII
+
+ECLIPSES AND KINDRED PHENOMENA
+
+
+Since some members of the solar system are nearer to us than others, and
+all are again much nearer than any of the stars, it must often happen
+that one celestial body will pass between us and another, and thus
+intercept its light for a while. The moon, being the nearest object in
+the universe, will, of course, during its motion across the sky,
+temporarily blot out every one of the others which happen to lie in its
+path. When it passes in this manner across the face of the sun, it is
+said to _eclipse_ it. When it thus hides a planet or star, it is said to
+_occult_ it. The reason why a separate term is used for what is merely a
+case of obscuring light in exactly the same way, will be plain when one
+considers that the disc of the sun is almost of the same apparent size
+as that of the moon, and so the complete hiding of the sun can last but
+a few minutes at the most; whereas a planet or a star looks so very
+small in comparison, that it is always _entirely swallowed up for some
+time_ when it passes behind the body of our satellite.
+
+The sun, of course, occults planets and stars in exactly the same manner
+as the moon does, but we cannot see these occultations on account of the
+blaze of sunlight.
+
+By reason of the small size which the planets look when viewed with the
+naked eye, we are not able to note them in the act of passing over stars
+and so blotting them out; but such occurrences may be seen in the
+telescope, for the planetary bodies then display broad discs.
+
+There is yet another occurrence of the same class which is known as a
+_transit_. This takes place when an apparently small body passes across
+the face of an apparently large one, the phenomenon being in fact the
+exact reverse of an occultation. As there is no appreciable body nearer
+to us than the moon, we can never see anything in transit across her
+disc. But since the planets Venus and Mercury are both nearer to us than
+the sun, they will occasionally be seen to pass across his face, and
+thus we get the well-known phenomena called Transits of Venus and
+Transits of Mercury.
+
+As the satellites of Jupiter are continually revolving around him, they
+will often pass behind or across his disc. Such occultations and
+transits of satellites can be well observed in the telescope.
+
+There is, however, a way in which the light of a celestial body may be
+obscured without the necessity of its being hidden from us by one
+nearer. It will no doubt be granted that any opaque object casts a
+shadow when a strong light falls directly upon it. Thus the earth, under
+the powerful light which is directed upon it from the sun, casts an
+extensive shadow, though we are not aware of the existence of this
+shadow until it falls upon something. The shadow which the earth casts
+is indeed not noticeable to us until some celestial body passes into it.
+As the sun is very large, and the earth in comparison very small, the
+shadow thrown by the earth is comparatively short, and reaches out in
+space for only about a million miles. There is no visible object except
+the moon, which circulates within that distance from our globe, and
+therefore she is the only body which can pass into this shadow. Whenever
+such a thing happens, her surface at once becomes dark, for the reason
+that she never emits any light of her own, but merely reflects that of
+the sun. As the moon is continually revolving around the earth, one
+would be inclined to imagine that once in every month, namely at what is
+called _full moon_, when she is on the other side of the earth with
+respect to the sun, she ought to pass through the shadow in question.
+But this does not occur every time, because the moon's orbit is not
+quite _upon the same plane_ with the earth's. It thus happens that time
+after time the moon passes clear of the earth's shadow, sometimes above
+it, and sometimes below it. It is indeed only at intervals of about six
+months that the moon can be thus obscured. This darkening of her light
+is known as an _eclipse of the moon_. It seems a great pity that custom
+should oblige us to employ the one term "eclipse" for this and also for
+the quite different occurrence, an eclipse of the sun; in which the
+sun's face is hidden as a consequence of the moon's body coming directly
+_between_ it and our eyes.
+
+The popular mind seems always to have found it more difficult to grasp
+the causes of an eclipse of the moon than an eclipse of the sun. As Mr.
+J.E. Gore[4] puts it: "The darkening of the sun's light by the
+interposition of the moon's body seems more obvious than the passing of
+the moon through the earth's shadow."
+
+Eclipses of the moon furnish striking spectacles, but really add little
+to our knowledge. They exhibit, however, one of the most remarkable
+evidences of the globular shape of our earth; for the outline of its
+shadow when seen creeping over the moon's surface is always circular.
+
+[Illustration: FIG. 3.--Total and Partial Eclipses of the Moon. The Moon
+is here shown in two positions; i.e. _entirely_ plunged in the earth's
+shadow and therefore totally eclipsed, and only _partly_ plunged in it
+or partially eclipsed.]
+
+_Eclipses of the Moon_, or Lunar Eclipses, as they are also called, are
+of two kinds--_Total_, and _Partial_. In a total lunar eclipse the moon
+passes entirely into the earth's shadow, and the whole of her surface is
+consequently darkened. This darkening lasts for about two hours. In a
+partial lunar eclipse, a portion only of the moon passes through the
+shadow, and so only _part_ of her surface is darkened (see Fig. 3). A
+very striking phenomenon during a total eclipse of the moon, is that the
+darkening of the lunar surface is usually by no means so intense as one
+would expect, when one considers that the sunlight at that time should
+be _wholly_ cut off from it. The occasions indeed upon which the moon
+has completely disappeared from view during the progress of a total
+lunar eclipse are very rare. On the majority of these occasions she has
+appeared of a coppery-red colour, while sometimes she has assumed an
+ashen hue. The explanations of these variations of colour is to be found
+in the then state of the atmosphere which surrounds our earth. When
+those portions of our earth's atmosphere through which the sun's rays
+have to filter on their way towards the moon are free from watery
+vapour, the lunar surface will be tinged with a reddish light, such as
+we ordinarily experience at sunset when our air is dry. The ashen colour
+is the result of our atmosphere being laden with watery vapour, and is
+similar to what we see at sunset when rain is about. Lastly, when the
+air around the earth is thickly charged with cloud, no light at all can
+pass; and on such occasions the moon disappears altogether for the time
+being from the night sky.
+
+_Eclipses of the Sun_, otherwise known as Solar Eclipses, are divided
+into _Total_, _Partial_, and _Annular_. A total eclipse of the sun takes
+place when the moon comes between the sun and the earth, in such a
+manner that it cuts off the sunlight _entirely_ for the time being from
+a _portion_ of the earth's surface. A person situated in the region in
+question will, therefore, at that moment find the sun temporarily
+blotted out from his view by the body of the moon. Since the moon is a
+very much smaller body than the sun, and also very much the nearer to us
+of the two, it will readily be understood that the portion of the earth
+from which the sun is seen thus totally eclipsed will be of small
+extent. In places not very distant from this region, the moon will
+appear so much shifted in the sky that the sun will be seen only
+partially eclipsed. The moon being in constant movement round the earth,
+the portion of the earth's surface from which an eclipse is seen as
+total will be always a comparatively narrow band lying roughly from west
+to east. This band, known as the _track of totality_, can, at the
+utmost, never be more than about 165 miles in width, and as a rule is
+very much less. For about 2000 miles on either side of it the sun is
+seen partially eclipsed. Outside these limits no eclipse of any kind is
+visible, as from such regions the moon is not seen to come in the way of
+the sun (see Fig. 4 (i.), p. 67).
+
+It may occur to the reader that eclipses can also take place in the
+course of which the positions, where the eclipse would ordinarily be
+seen as total, will lie outside the surface of the earth. Such an
+eclipse is thus not dignified with the name of total eclipse, but is
+called a partial eclipse, because from the earth's surface the sun is
+only seen _partly eclipsed at the utmost_ (see Fig. 4 (ii.), p. 67).
+
+[Illustration: (i.) Total Eclipse of the Sun.]
+
+[Illustration: (ii.) Partial Eclipse of the Sun.
+
+FIG. 4.--Total and Partial Eclipses of the Sun. From the position A the
+Sun cannot be seen, as it is entirely blotted out by the Moon. From B it
+is seen partially blotted out, because the Moon is to a certain degree
+in the way. From C no eclipse is seen, because the Moon does not come in
+the way.
+
+It is to be noted that in a Partial Eclipse of the Sun, the position A
+lies _outside_ the surface of the Earth.]
+
+An _Annular eclipse_ is an eclipse which just fails to become total for
+yet another reason. We have pointed out that the orbits of the various
+members of the solar system are not circular, but oval. Such oval
+figures, it will be remembered, are technically known as ellipses. In an
+elliptic orbit the controlling body is situated not in the middle of the
+figure, but rather towards one of the ends; the actual point which it
+occupies being known as the _focus_. The sun being at the focus of the
+earth's orbit, it follows that the earth is, at times, a little nearer
+to him than at others. The sun will therefore appear to us to vary a
+little in size, looking sometimes slightly larger than at other times.
+It is so, too, with the moon, at the focus of whose orbit the earth is
+situated. She therefore also appears to us at times to vary slightly in
+size. The result is that when the sun is eclipsed by the moon, and the
+moon at the time appears the larger of the two, she is able to blot out
+the sun completely, and so we can get a total eclipse. But when, on the
+other hand, the sun appears the larger, the eclipse will not be quite
+total, for a portion of the sun's disc will be seen protruding all
+around the moon like a ring of light. This is what is known as an
+annular eclipse, from the Latin word _annulus_, which means a ring. The
+term is consecrated by long usage, but it seems an unfortunate one on
+account of its similarity to the word "annual." The Germans speak of
+this kind of eclipse as "ring-formed," which is certainly much more to
+the point.
+
+There can never be a year without an eclipse of the sun. Indeed there
+must be always two such eclipses _at least_ during that period, though
+there need be no eclipse of the moon at all. On the other hand, the
+greatest number of eclipses which can ever take place during a year are
+seven; that is to say, either five solar eclipses and two lunar, or four
+solar and three lunar. This general statement refers merely to eclipses
+in their broadest significance, and informs us in no way whether they
+will be total or partial.
+
+Of all the phenomena which arise from the hiding of any celestial body
+by one nearer coming in the way, a total eclipse of the sun is far the
+most important. It is, indeed, interesting to consider how much poorer
+modern astronomy would be but for the extraordinary coincidence which
+makes a total solar eclipse just possible. The sun is about 400 times
+farther off from us than the moon, and enormously greater than her in
+bulk. Yet the two are relatively so distanced from us as to look about
+the same size. The result of this is that the moon, as has been seen,
+can often blot out the sun entirely from our view for a short time. When
+this takes place the great blaze of sunlight which ordinarily dazzles
+our eyes is completely cut off, and we are thus enabled, unimpeded, to
+note what is going on in the immediate vicinity of the sun itself.
+
+In a total solar eclipse, the time which elapses from the moment when
+the moon's disc first begins to impinge upon that of the sun at his
+western edge until the eclipse becomes total, lasts about an hour.
+During all this time the black lunar disc may be watched making its way
+steadily across the solar face. Notwithstanding the gradual obscuration
+of the sun, one does not notice much diminution of light until about
+three-quarters of his disc are covered. Then a wan, unearthly appearance
+begins to pervade all things, the temperature falls noticeably, and
+nature seems to halt in expectation of the coming of something unusual.
+The decreasing portion of sun becomes more and more narrow, until at
+length it is reduced to a crescent-shaped strip of exceeding fineness.
+Strange, ill-defined, flickering shadows (known as "Shadow Bands") may
+at this moment be seen chasing each other across any white expanse such
+as a wall, a building, or a sheet stretched upon the ground. The western
+side of the sky has now assumed an appearance dark and lowering, as if a
+rainstorm of great violence were approaching. This is caused by the
+mighty mass of the lunar shadow sweeping rapidly along. It flies onward
+at the terrific velocity of about half a mile a second.
+
+If the gradually diminishing crescent of sun be now watched through a
+telescope, the observer will notice that it does not eventually vanish
+all at once, as he might have expected. Rather, it breaks up first of
+all along its length into a series of brilliant dots, known as "Baily's
+Beads." The reason of this phenomenon is perhaps not entirely agreed
+upon, but the majority of astronomers incline to the opinion that the
+so-called "beads" are merely the last remnants of sunlight peeping
+between those lunar mountain peaks which happen at the moment to fringe
+the advancing edge of the moon. The beads are no sooner formed than they
+rapidly disappear one after the other, after which no portion of the
+solar surface is left to view, and the eclipse is now total (see Fig.
+5).
+
+[Illustration: _In a total Eclipse_ _In an annular Eclipse_
+
+FIG. 5.--"Baily's Beads."]
+
+But with the disappearance of the sun there springs into view a new and
+strange appearance, ordinarily unseen because of the blaze of sunlight.
+It is a kind of aureole, or halo, pearly white in colour, which is seen
+to surround the black disc of the moon. This white radiance is none
+other than the celebrated phenomenon widely known as the _Solar Corona_.
+It was once upon a time thought to belong to the moon, and to be perhaps
+a lunar atmosphere illuminated by the sunlight shining through it from
+behind. But the suddenness with which the moon always blots out stars
+when occulting them, has amply proved that she possesses no atmosphere
+worth speaking about. It is now, however, satisfactorily determined that
+the corona belongs to the sun, for during the short time that it remains
+in view the black body of the moon can be seen creeping across it.
+
+All the time that the _total phase_ (as it is called) lasts, the corona
+glows with its pale unearthly light, shedding upon the earth's surface
+an illumination somewhat akin to full moonlight. Usually the planet
+Venus and a few stars shine out the while in the darkened heaven.
+Meantime around the observer animal and plant life behave as at
+nightfall. Birds go to roost, bats fly out, worms come to the surface of
+the ground, flowers close up. In the Norwegian eclipse of 1896 fish were
+seen rising to the surface of the water. When the total phase at length
+is over, and the moon in her progress across the sky has allowed the
+brilliant disc of the sun to spring into view once more at the other
+side, the corona disappears.
+
+There is another famous accompaniment of the sun which partly reveals
+itself during total solar eclipses. This is a layer of red flame which
+closely envelops the body of the sun and lies between it and the corona.
+This layer is known by the name of the _Chromosphere_. Just as at
+ordinary times we cannot see the corona on account of the blaze of
+sunlight, so are we likewise unable to see the chromosphere because of
+the dazzling white light which shines through from the body of the sun
+underneath and completely overpowers it. When, however, during a solar
+eclipse, the lunar disc has entirely hidden the brilliant face of the
+sun, we are still able for a few moments to see an edgewise portion of
+the chromosphere in the form of a narrow red strip, fringing the
+advancing border of the moon. Later on, just before the moon begins to
+uncover the face of the sun from the other side, we may again get a view
+of a strip of chromosphere.
+
+The outer surface of the chromosphere is not by any means even. It is
+rough and billowy, like the surface of a storm-tossed sea. Portions of
+it, indeed, rise at times to such heights that they may be seen standing
+out like blood-red points around the black disc of the moon, and remain
+thus during a good part of the total phase. These projections are known
+as the _Solar Prominences_. In the same way as the corona, the
+chromosphere and prominences were for a time supposed to belong to the
+moon. This, however, was soon found not to be the case, for the lunar
+disc was noticed to creep slowly across them also.
+
+The total phase, or "totality," as it is also called, lasts for
+different lengths of time in different eclipses. It is usually of about
+two or three minutes' duration, and at the utmost it can never last
+longer than about eight minutes.
+
+When totality is over and the corona has faded away, the moon's disc
+creeps little by little from the face of the sun, light and heat returns
+once more to the earth, and nature recovers gradually from the gloom in
+which she has been plunged. About an hour after totality, the last
+remnant of moon draws away from the solar disc, and the eclipse is
+entirely at an end.
+
+The corona, the chromosphere, and the prominences are the most important
+of these accompaniments of the sun which a total eclipse reveals to us.
+Our further consideration of them must, however, be reserved for a
+subsequent chapter, in which the sun will be treated of at length.
+
+Every one who has had the good fortune to see a total eclipse of the sun
+will, the writer feels sure, agree with the verdict of Sir Norman
+Lockyer that it is at once one of the "grandest and most awe-inspiring
+sights" which man can witness. Needless to say, such an occurrence used
+to cause great consternation in less civilised ages; and that it has not
+in modern times quite parted with its terrors for some persons, is shown
+by the fact that in Iowa, in the United States, a woman died from fright
+during the eclipse of 1869.
+
+To the serious observer of a total solar eclipse every instant is
+extremely precious. Many distinct observations have to be crowded into a
+time all too limited, and this in an eclipse-party necessitates constant
+rehearsals in order that not a moment may be wasted when the longed-for
+totality arrives. Such preparation is very necessary; for the rarity and
+uncommon nature of a total eclipse of the sun, coupled with its
+exceeding short duration, tends to flurry the mind, and to render it
+slow to seize upon salient points of detail. And, even after every
+precaution has been taken, weather possibilities remain to be reckoned
+with, so that success is rather a lottery.
+
+Above all things, therefore, a total solar eclipse is an occurrence for
+the proper utilisation of which personal experience is of absolute
+necessity. It was manifestly out of the question that such experience
+could be gained by any individual in early times, as the imperfection
+of astronomical theory and geographical knowledge rendered the
+predicting of the exact position of the track of totality well-nigh
+impossible. Thus chance alone would have enabled one in those days to
+witness a total phase, and the probabilities, of course, were much
+against a second such experience in the span of a life-time. And even in
+more modern times, when the celestial motions had come to be better
+understood, the difficulties of foreign travel still were in the way;
+for it is, indeed, a notable fact that during many years following the
+invention of the telescope the tracks were placed for the most part in
+far-off regions of the earth, and Europe was visited by singularly few
+total solar eclipses. Thus it came to pass that the building up of a
+body of organised knowledge upon this subject was greatly delayed.
+
+Nothing perhaps better shows the soundness of modern astronomical theory
+than the almost exact agreement of the time predicted for an eclipse
+with its actual occurrence. Similarly, by calculating backwards,
+astronomers have discovered the times and seasons at which many ancient
+eclipses took place, and valuable opportunities have thus arisen for
+checking certain disputed dates in history.
+
+It should not be omitted here that the ancients were actually able, _in
+a rough way_, to predict eclipses. The Chaldean astronomers had indeed
+noticed very early a curious circumstance, _i.e._ that eclipses tend to
+repeat themselves after a lapse of slightly more than eighteen years.
+
+In this connection it must, however, be pointed out, in the first
+instance, that the eclipses which occur in any particular year are in
+no way associated with those which occurred in the previous year. In
+other words, the mere fact that an eclipse takes place upon a certain
+day this year will not bring about a repetition of it at the same time
+next year. However, the nicely balanced behaviour of the solar system,
+an equilibrium resulting from �ons of orbital ebb and flow, naturally
+tends to make the members which compose that family repeat their ancient
+combinations again and again; so that after definite lapses of time the
+same order of things will _almost exactly_ recur. Thus, as a consequence
+of their beautifully poised motions, the sun, the moon, and the earth
+tend, after a period of 18 years and 10-1/3 days,[5] to occupy very
+nearly the same positions with regard to each other. The result of this
+is that, during each recurring period, the eclipses comprised within it
+will be repeated in their order.
+
+To give examples:--
+
+The total solar eclipse of August 30, 1905, was a repetition of that of
+August 19, 1887.
+
+The partial solar eclipse of February 23, 1906, corresponded to that
+which took place on February 11, 1888.
+
+The annular eclipse of July 10, 1907, was a recurrence of that of June
+28, 1889.
+
+In this way we can go on until the eighteen year cycle has run out, and
+we come upon a total solar eclipse predicted for September 10, 1923,
+which will repeat the above-mentioned ones of 1905 and 1887; and so on
+too with the others.
+
+From mere observation alone, extending no doubt over many ages, those
+time-honoured watchers of the sky, the early Chaldeans, had arrived at
+this remarkable generalisation; and they used it for the rough
+prediction of eclipses. To the period of recurrence they give the name
+of "Saros."
+
+And here we find ourselves led into one of the most interesting and
+fascinating by-paths in astronomy, to which writers, as a rule, pay all
+too little heed.
+
+In order not to complicate matters unduly, the recurrence of solar
+eclipses alone will first be dealt with. This limitation will, however,
+not affect the arguments in the slightest, and it will be all the more
+easy in consequence to show their application to the case of eclipses of
+the moon.
+
+The reader will perhaps have noticed that, with regard to the repetition
+of an eclipse, it has been stated that the conditions which bring it on
+at each recurrence are reproduced _almost exactly_. Here, then, lies the
+_crux_ of the situation. For it is quite evident that were the
+conditions _exactly_ reproduced, the recurrences of each eclipse would
+go on for an indefinite period. For instance, if the lapse of a saros
+period found the sun, moon, and earth again in the precise relative
+situations which they had previously occupied, the recurrences of a
+solar eclipse would tend to duplicate its forerunner with regard to the
+position of the shadow upon the terrestrial surface. But the conditions
+_not_ being exactly reproduced, the shadow-track does not pass across
+the earth in quite the same regions. It is shifted a little, so to
+speak; and each time the eclipse comes round it is found to be shifted a
+little farther. Every solar eclipse has therefore a definite "life" of
+its own upon the earth, lasting about 1150 years, or 64 saros returns,
+and working its way little by little across our globe from north to
+south, or from south to north, as the case may be. Let us take an
+imaginary example. A _partial_ eclipse occurs, say, somewhere near the
+North Pole, the edge of the "partial" shadow just grazing the earth, and
+the "track of totality" being as yet cast into space. Here we have the
+beginning of a series. At each saros recurrence the partial shadow
+encroaches upon a greater extent of earth-surface. At length, in its
+turn, the track of totality begins to impinge upon the earth. This track
+streaks across our globe at each return of the eclipse, repeating itself
+every time in a slightly more southerly latitude. South and south it
+moves, passing in turn the Tropic of Cancer, the Equator, the Tropic of
+Capricorn, until it reaches the South Pole; after which it touches the
+earth no longer, but is cast into space. The rear portion of the partial
+shadow, in its turn, grows less and less in extent; and it too in time
+finally passes off. Our imaginary eclipse series is now no more--its
+"life" has ended.
+
+We have taken, as an example, an eclipse series moving from north to
+south. We might have taken one moving from south to north, for they
+progress in either direction.
+
+From the description just given the reader might suppose that, if the
+tracks of totality of an eclipse series were plotted upon a chart of the
+world, they would lie one beneath another like a set of steps. This is,
+however, _not_ the case, and the reason is easily found. It depends upon
+the fact that the saros does not comprise an exact number of days, but
+includes, as we have seen, one-third of a day in addition.
+
+It will be granted, of course, that if the number of days was exact, the
+_same_ parts of the earth would always be brought round by the axial
+rotation _to front the sun_ at the moment of the recurrence of the
+eclipse. But as there is still one-third of a day to complete the saros
+period, the earth has yet to make one-third of a rotation upon its axis
+before the eclipse takes place. Thus at every recurrence the track of
+totality finds itself placed one-third of the earth's circumference to
+the _westward_. Three of the recurrences will, of course, complete the
+circuit of the globe; and so the fourth recurrence will duplicate the
+one which preceded it, three saros returns, or 54 years and 1 month
+before. This duplication, as we have already seen, will, however, be
+situated in a latitude to the south or north of its predecessor,
+according as the eclipse series is progressing in a southerly or
+northerly direction.
+
+Lastly, every eclipse series, after working its way across the earth,
+will return again to go through the same process after some 12,000
+years; so that, at the end of that great lapse of time, the entire
+"life" of every eclipse should repeat itself, provided that the
+conditions of the solar system have not altered appreciably during the
+interval.
+
+We are now in a position to consider this gradual southerly or
+northerly progress of eclipse recurrences in its application to the case
+of eclipses of the moon. It should be evident that, just as in solar
+eclipses the lunar shadow is lowered or raised (as the case may be) each
+time it strikes the terrestrial surface, so in lunar eclipses will the
+body of the moon shift its place at each recurrence relatively to the
+position of the earth's shadow. Every lunar eclipse, therefore, will
+commence on our satellite's disc as a partial eclipse at the northern or
+southern extremity, as the case may be. Let us take, as an example, an
+imaginary series of eclipses of the moon progressing from north to
+south. At each recurrence the partial phase will grow greater, its
+boundary encroaching more and more to the southward, until eventually
+the whole disc is enveloped by the shadow, and the eclipse becomes
+total. It will then repeat itself as total during a number of
+recurrences, until the entire breadth of the shadow has been passed
+through, and the northern edge of the moon at length springs out into
+sunlight. This illuminated portion will grow more and more extensive at
+each succeeding return, the edge of the shadow appearing to recede from
+it until it finally passes off at the south. Similarly, when a lunar
+eclipse commences as partial at the south of the moon, the edge of the
+shadow at each subsequent recurrence finds itself more and more to the
+northward. In due course the total phase will supervene, and will
+persist during a number of recurrences until the southerly trend of the
+moon results in the uncovering of the lunar surface at the south. Thus,
+as the boundary of the shadow is left more and more to the northward,
+the illuminated portion on the southern side of the moon becomes at each
+recurrence greater and the darkened portion on the northern side less,
+until the shadow eventually passes off at the north.
+
+The "life" of an eclipse of the moon happens, for certain reasons, to be
+much shorter than that of an eclipse of the sun. It lasts during only
+about 860 years, or 48 saros returns.
+
+Fig. 6, p. 81, is a map of the world on Mercator's Projection, showing a
+portion of the march of the total solar eclipse of August 30, 1905,
+across the surface of the earth. The projection in question has been
+employed because it is the one with which people are most familiar. This
+eclipse began by striking the neighbourhood of the North Pole in the
+guise of a partial eclipse during the latter part of the reign of Queen
+Elizabeth, and became total on the earth for the first time on the 24th
+of June 1797. Its next appearance was on the 6th of July 1815. It has
+not been possible to show the tracks of totality of these two early
+visitations on account of the distortion of the polar regions consequent
+on the _fiction_ of Mercator's Projection. It is therefore made to
+commence with the track of its third appearance, viz. on July 17, 1833.
+In consequence of those variations in the apparent sizes of the sun and
+moon, which result, as we have seen, from the variations in their
+distances from the earth, this eclipse will change from a total into an
+annular eclipse towards the end of the twenty-first century. By that
+time the track will have passed to the southern side of the equator. The
+track will eventually leave the earth near the South Pole about the
+beginning of the twenty-sixth century, and the rear portion of the
+partial shadow will in its turn be clear of the terrestrial surface by
+about 2700 A.D., when the series comes to an end.
+
+[Illustration: FIG. 6.--Map of the World on Mercator's Projection,
+showing a portion of the progress of the Total Solar Eclipse of August
+30, 1905, across the surface of the earth.]
+
+
+[4] Astronomical Essays (p. 40), London, 1907.
+
+[5] In some cases the periods between the dates of the corresponding
+eclipses _appear_ to include a greater number of days than ten; but this
+is easily explained when allowance is made for intervening _leap_ years
+(in each of which an _extra_ day has of course been added), and also for
+variations in local time.
+
+
+
+
+CHAPTER VIII
+
+FAMOUS ECLIPSES OF THE SUN
+
+
+What is thought to be the earliest reference to an eclipse comes down to
+us from the ancient Chinese records, and is over four thousand years
+old. The eclipse in question was a solar one, and occurred, so far as
+can be ascertained, during the twenty-second century B.C. The story runs
+that the two state astronomers, Ho and Hi by name, being exceedingly
+intoxicated, were unable to perform their required duties, which
+consisted in superintending the customary rites of beating drums,
+shooting arrows, and the like, in order to frighten away the mighty
+dragon which it was believed was about to swallow up the Lord of Day.
+This eclipse seems to have been only partial; nevertheless a great
+turmoil ensued, and the two astronomers were put to death, no doubt with
+the usual _celestial_ cruelty.
+
+The next eclipse mentioned in the Chinese annals is also a solar
+eclipse, and appears to have taken place more than a thousand years
+later, namely in 776 B.C. Records of similar eclipses follow from the
+same source; but as they are mere notes of the events, and do not enter
+into any detail, they are of little interest. Curiously enough the
+Chinese have taken practically no notice of eclipses of the moon, but
+have left us a comparatively careful record of comets, which has been
+of value to modern astronomy.
+
+The earliest mention of a _total_ eclipse of the sun (for it should be
+noted that the ancient Chinese eclipse above-mentioned was merely
+partial) was deciphered in 1905, on a very ancient Babylonian tablet, by
+Mr. L.W. King of the British Museum. This eclipse took place in the year
+1063 B.C.
+
+Assyrian tablets record three solar eclipses which occurred between
+three and four hundred years later than this. The first of these was in
+763 B.C.; the total phase being visible near Nineveh.
+
+The next record of an eclipse of the sun comes to us from a Grecian
+source. This eclipse took place in 585 B.C., and has been the subject of
+much investigation. Herodotus, to whom we are indebted for the account,
+tells us that it occurred during a battle in a war which had been waging
+for some years between the Lydians and Medes. The sudden coming on of
+darkness led to a termination of the contest, and peace was afterwards
+made between the combatants. The historian goes on to state that the
+eclipse had been foretold by Thales, who is looked upon as the Founder
+of Grecian astronomy. This eclipse is in consequence known as the
+"Eclipse of Thales." It would seem as if that philosopher were
+acquainted with the Chaldean saros.
+
+The next solar eclipse worthy of note was an annular one, and occurred
+in 431 B.C., the first year of the Peloponnesian War. Plutarch relates
+that the pilot of the ship, which was about to convey Pericles to the
+Peloponnesus, was very much frightened by it; but Pericles calmed him by
+holding up a cloak before his eyes, and saying that the only difference
+between this and the eclipse was that something larger than the cloak
+prevented his seeing the sun for the time being.
+
+An eclipse of great historical interest is that known as the "Eclipse of
+Agathocles," which occurred on the morning of the 15th of August, 310
+B.C. Agathocles, Tyrant of Syracuse, had been blockaded in the harbour
+of that town by the Carthaginian fleet, but effected the escape of his
+squadron under cover of night, and sailed for Africa in order to invade
+the enemy's territory. During the following day he and his vessels
+experienced a total eclipse, in which "day wholly put on the appearance
+of night, and the stars were seen in all parts of the sky."
+
+A few solar eclipses are supposed to be referred to in early Roman
+history, but their identity is very doubtful in comparison with those
+which the Greeks have recorded. Additional doubt is cast upon them by
+the fact that they are usually associated with famous events. The birth
+and death of Romulus, and the Passage of the Rubicon by Julius C�sar,
+are stated indeed to have been accompanied by these marks of the
+approval or disapproval of the gods!
+
+Reference to our subject in the Bible is scanty. Amos viii. 9 is thought
+to refer to the Nineveh eclipse of 763 B.C., to which allusion has
+already been made; while the famous episode of Hezekiah and the shadow
+on the dial of Ahaz has been connected with an eclipse which was partial
+at Jerusalem in 689 B.C.
+
+The first solar eclipse, recorded during the Christian Era, is known as
+the "Eclipse of Phlegon," from the fact that we are indebted for the
+account to a pagan writer of that name. This eclipse took place in A.D.
+29, and the total phase was visible a little to the north of Palestine.
+It has sometimes been confounded with the "darkness of the Crucifixion,"
+which event took place near the date in question; but it is sufficient
+here to say that the Crucifixion is well known to have occurred during
+the Passover of the Jews, which is always celebrated at the _full_ moon,
+whereas an eclipse of the sun can only take place at _new_ moon.
+
+Dion Cassius, commenting on the Emperor Claudius about the year A.D. 45,
+writes as follows:--
+
+"As there was going to be an eclipse on his birthday, through fear of a
+disturbance, as there had been other prodigies, he put forth a public
+notice, not only that the obscuration would take place, and about the
+time and magnitude of it, but also about the causes that produce such an
+event."
+
+This is a remarkable piece of information; for the Romans, an
+essentially military nation, appear hitherto to have troubled themselves
+very little about astronomical matters, and were content, as we have
+seen, to look upon phenomena, like eclipses, as mere celestial
+prodigies.
+
+What is thought to be the first definite mention of the solar corona
+occurs in a passage of Plutarch. The eclipse to which he refers is
+probably one which took place in A.D. 71. He says that the obscuration
+caused by the moon "has no time to last and no extensiveness, but some
+light shows itself round the sun's circumference, which does not allow
+the darkness to become deep and complete." No further reference to this
+phenomenon occurs until near the end of the sixteenth century. It
+should, however, be here mentioned that Mr. E.W. Maunder has pointed
+out the probability[6] that we have a very ancient symbolic
+representation of the corona in the "winged circle," "winged disc," or
+"ring with wings," as it is variously called, which appears so often
+upon Assyrian and Egyptian monuments, as the symbol of the Deity (Fig.
+7).
+
+[Illustration: FIG. 7.--The "Ring with Wings." The upper is the Assyrian
+form of the symbol, the lower the Egyptian. (From _Knowledge_.) Compare
+the form of the corona on Plate VII. (B), p. 142.]
+
+The first solar eclipse recorded to have been seen in England is that of
+A.D. 538, mention of which is found in the _Anglo-Saxon Chronicle_. The
+track of totality did not, however, come near our islands, for only
+two-thirds of the sun's disc were eclipsed at London.
+
+In 840 a great eclipse took place in Europe, which was total for more
+than five minutes across what is now Bavaria. Terror at this eclipse is
+said to have hastened the death of Louis le Debonnaire, Emperor of the
+West, who lay ill at Worms.
+
+In 878--_temp._ King Alfred--an eclipse of the sun took place which was
+total at London. From this until 1715 no other eclipse was total at
+London itself; though this does not apply to other portions of England.
+
+An eclipse, generally known as the "Eclipse of Stiklastad," is said to
+have taken place in 1030, during the sea-fight in which Olaf of Norway
+is supposed to have been slain. Longfellow, in his _Saga of King Olaf_,
+has it that
+
+"The Sun hung red
+As a drop of blood,"
+
+but, as in the case of most poets, the dramatic value of an eclipse
+seems to have escaped his notice.
+
+In the year 1140 there occurred a total eclipse of the sun, the last to
+be visible in England for more than five centuries. Indeed there have
+been only two such since--namely, those of 1715 and 1724, to which we
+shall allude in due course. The eclipse of 1140 took place on the 20th
+March, and is thus referred to in the _Anglo-Saxon Chronicle_:--
+
+"In the Lent, the sun and the day darkened, about the noon-tide of the
+day, when men were eating, and they lighted candles to eat by. That was
+the 13th day before the calends of April. Men were very much struck with
+wonder."
+
+Several of the older historians speak of a "fearful eclipse" as having
+taken place on the morning of the Battle of Crecy, 1346. Lingard, for
+instance, in his _History of England_, has as follows:--
+
+"Never, perhaps, were preparations for battle made under circumstances
+so truly awful. On that very day the sun suffered a partial eclipse:
+birds, in clouds, the precursors of a storm, flew screaming over the two
+armies, and the rain fell in torrents, accompanied by incessant thunder
+and lightning. About five in the afternoon the weather cleared up; the
+sun in full splendour darted his rays in the eyes of the enemy."
+
+Calculations, however, show that no eclipse of the sun took place in
+Europe during that year. This error is found to have arisen from the
+mistranslation of an obsolete French word _esclistre_ (lightning), which
+is employed by Froissart in his description of the battle.
+
+In 1598 an eclipse was total over Scotland and part of North Germany. It
+was observed at Torgau by Jessenius, an Hungarian physician, who noticed
+a bright light around the moon during the time of totality. This is said
+to be the first reference to the corona since that of Plutarch, to which
+we have already drawn attention.
+
+Mention of Scotland recalls the fact that an unusual number of eclipses
+happen to have been visible in that country, and the occult bent natural
+to the Scottish character has traditionalised a few of them in such
+terms as the "Black Hour" (an eclipse of 1433), "Black Saturday" (the
+eclipse of 1598 which has been alluded to above), and "Mirk Monday"
+(1652). The track of the last-named also passed over Carrickfergus in
+Ireland, where it was observed by a certain Dr. Wybord, in whose account
+the term "corona" is first employed. This eclipse is the last which has
+been total in Scotland, and it is calculated that there will not be
+another eclipse seen as total there until the twenty-second century.
+
+An eclipse of the sun which took place on May 30, 1612, is recorded as
+having been seen "through a tube." This probably refers to the then
+recent invention--the telescope.
+
+The eclipses which we have been describing are chiefly interesting from
+an historical point of view. The old mystery and confusion to the
+beholders seem to have lingered even into comparatively enlightened
+times, for we see how late it is before the corona attracts definite
+attention for the sake of itself alone.
+
+It is not a far cry from notice of the corona to that of other
+accompaniments of a solar eclipse. Thus the eclipse of 1706, the total
+phase of which was visible in Switzerland, is of great interest; for it
+was on this occasion that the famous red prominences seem first to have
+been noted. A certain Captain Stannyan observed this eclipse from Berne
+in Switzerland, and described it in a letter to Flamsteed, the then
+Astronomer Royal. He says the sun's "getting out of his eclipse was
+preceded by a blood-red streak of light from its left limb, which
+continued not longer than six or seven seconds of time; then part of the
+Sun's disc appeared all of a sudden, as bright as Venus was ever seen in
+the night, nay brighter; and in that very instant gave a Light and
+Shadow to things as strong as Moonlight uses to do." How little was then
+expected of the sun is, however, shown by Flamsteed's words, when
+communicating this information to the Royal Society:--
+
+"The Captain is the first man I ever heard of that took notice of a Red
+Streak of Light preceding the Emersion of the Sun's body from a total
+Eclipse. And I take notice of it to you because it infers that _the Moon
+has an atmosphere_; and its short continuance of only six or seven
+seconds of time, tells us that _its height is not more than the five or
+six hundredth part of her diameter_."
+
+What a change has since come over the ideas of men! The sun has proved a
+veritable mine of discovery, while the moon has yielded up nothing new.
+
+The eclipse of 1715, the first total at London since that of 878, was
+observed by the famous astronomer, Edmund Halley, from the rooms of the
+Royal Society, then in Crane Court, Fleet Street. On this occasion both
+the corona and a red projection were noted. Halley further makes
+allusion to that curious phenomenon, which later on became celebrated
+under the name of "Baily's beads." It was also on the occasion of this
+eclipse that the _earliest recorded drawings of the corona_ were made.
+Cambridge happened to be within the track of totality; and a certain
+Professor Cotes of that University, who is responsible for one of the
+drawings in question, forwarded them to Sir Isaac Newton together with a
+letter describing his observations.
+
+In 1724 there occurred an eclipse, the total phase of which was visible
+from the south-west of England, but not from London. The weather was
+unfavourable, and the eclipse consequently appears to have been seen by
+only one person, a certain Dr. Stukeley, who observed it from Haraden
+Hill near Salisbury Plain. This is the last eclipse of which the total
+phase was seen in any part of England. The next will not be until June
+29, 1927, and will be visible along a line across North Wales and
+Lancashire. The discs of the sun and moon will just then be almost of
+the same apparent size, and so totality will be of extremely short
+duration; in fact only a few seconds. London itself will not see a
+totality until the year 2151--a circumstance which need hardly distress
+any of us personally!
+
+It is only from the early part of the nineteenth century that serious
+scientific attention to eclipses of the sun can be dated. An _annular_
+eclipse, visible in 1836 in the south of Scotland, drew the careful
+notice of Francis Baily of Jedburgh in Roxburghshire to that curious
+phenomenon which we have already described, and which has ever since
+been known by the name of "Baily's beads." Spurred by his observation,
+the leading astronomers of the day determined to pay particular
+attention to a total eclipse, which in the year 1842 was to be visible
+in the south of France and the north of Italy. The public interest
+aroused on this occasion was also very great, for the region across
+which the track of totality was to pass was very populous, and inhabited
+by races of a high degree of culture.
+
+This eclipse occurred on the morning of the 8th July, and from it may be
+dated that great enthusiasm with which total eclipses of the sun have
+ever since been received. Airy, our then Astronomer Royal, observed it
+from Turin; Arago, the celebrated director of the Paris Observatory,
+from Perpignan in the south of France; Francis Baily from Pavia; and Sir
+John Herschel from Milan. The corona and three large red prominences
+were not only well observed by the astronomers, but drew tremendous
+applause from the watching multitudes.
+
+The success of the observations made during this eclipse prompted
+astronomers to pay similar attention to that of July 28, 1851, the total
+phase of which was to be visible in the south of Norway and Sweden, and
+across the east of Prussia. This eclipse was also a success, and it was
+now ascertained that the red prominences belonged to the sun and not to
+the moon; for the lunar disc, as it moved onward, was seen to cover and
+to uncover them in turn. It was also noted that these prominences were
+merely uprushes from a layer of glowing gaseous matter, which was seen
+closely to envelop the sun.
+
+The total eclipse of July 18, 1860, was observed in Spain, and
+photography was for the first time _systematically_ employed in its
+observation.[7] In the photographs taken the stationary appearance of
+both the corona and prominences with respect to the moving moon,
+definitely confirmed the view already put forward that they were actual
+appendages of the sun.
+
+The eclipse of August 18, 1868, the total phase of which lasted nearly
+six minutes, was visible in India, and drew thither a large concourse of
+astronomers. In this eclipse the spectroscope came to the front, and
+showed that both the prominences, and the chromospheric layer from which
+they rise, are composed of glowing vapours--chief among which is the
+vapour of hydrogen. The direct result of the observations made on this
+occasion was the spectroscopic method of examining prominences at any
+time in full daylight, and without a total eclipse. This method, which
+has given such an immense impetus to the study of the sun, was the
+outcome of independent and simultaneous investigation on the part of the
+French astronomer, the late M. Janssen, and the English astronomer,
+Professor (now Sir Norman) Lockyer, a circumstance strangely reminiscent
+of the discovery of Neptune. The principles on which the method was
+founded seem, however, to have occurred to Dr. (now Sir William) Huggins
+some time previously.
+
+The eclipse of December 22, 1870, was total for a little more than two
+minutes, and its track passed across the Mediterranean. M. Janssen, of
+whom mention has just been made, escaped in a balloon from then besieged
+Paris, taking his instruments with him, and made his way to Oran, in
+Algeria, in order to observe it; but his expectations were disappointed
+by cloudy weather. The expedition sent out from England had the
+misfortune to be shipwrecked off the coast of Sicily. But the occasion
+was redeemed by a memorable observation made by the American astronomer,
+the late Professor Young, which revealed the existence of what is now
+known as the "Reversing Layer." This is a shallow layer of gases which
+lies immediately beneath the chromosphere. An illustration of the
+corona, as it was seen during the above eclipse, will be found on Plate
+VII. (A), p. 142.
+
+In the eclipse of December 12, 1871, total across Southern India, the
+photographs of the corona obtained by Mr. Davis, assistant to Lord
+Lindsay (now the Earl of Crawford), displayed a wealth of detail
+hitherto unapproached.
+
+The eclipse of July 29, 1878, total across the western states of North
+America, was a remarkable success, and a magnificent view of the corona
+was obtained by the well-known American astronomer and physicist, the
+late Professor Langley, from the summit of Pike's Peak, Colorado, over
+14,000 feet above the level of the sea. The coronal streamers were seen
+to extend to a much greater distance at this altitude than at points
+less elevated, and the corona itself remained visible during more than
+four minutes after the end of totality. It was, however, not entirely a
+question of altitude; the coronal streamers were actually very much
+longer on this occasion than in most of the eclipses which had
+previously been observed.
+
+The eclipse of May 17, 1882, observed in Upper Egypt, is notable from
+the fact that, in one of the photographs taken by Dr. Schuster at Sohag,
+a bright comet appeared near the outer limit of the corona (see Plate
+I., p. 96). The comet in question had not been seen before the eclipse,
+and was never seen afterwards. This is the third occasion on which
+attention has been drawn to a comet _merely_ by a total eclipse. The
+first is mentioned by Seneca; and the second by Philostorgius, in an
+account of an eclipse observed at Constantinople in A.D. 418. A fourth
+case of the kind occurred in 1893, when faint evidences of one of these
+filmy objects were found on photographs of the corona taken by the
+American astronomer, Professor Schaeberle, during the total eclipse of
+April 16 of that year.
+
+The eclipse of May 6, 1883, had a totality of over five minutes, but
+the central track unfortunately passed across the Pacific Ocean, and the
+sole point of land available for observing it from was one of the
+Marquesas Group, Caroline Island, a coral atoll seven and a half miles
+long by one and a half broad. Nevertheless astronomers did not hesitate
+to take up their posts upon that little spot, and were rewarded with
+good weather.
+
+The next eclipse of importance was that of April 16, 1893. It stretched
+from Chili across South America and the Atlantic Ocean to the West Coast
+of Africa, and, as the weather was fine, many good results were
+obtained. Photographs were taken at both ends of the track, and these
+showed that the appearance of the corona remained unchanged during the
+interval of time occupied by the passage of the shadow across the earth.
+It was on the occasion of this eclipse that Professor Schaeberle found
+upon his photographs those traces of the presence of a comet, to which
+allusion has already been made.
+
+Extensive preparations were made to observe the eclipse of August 9,
+1896. Totality lasted from two to three minutes, and the track stretched
+from Norway to Japan. Bad weather disappointed the observers, with the
+exception of those taken to Nova Zembla by Sir George Baden Powell in
+his yacht _Otaria_.
+
+The eclipse of January 22, 1898, across India _vi�_ Bombay and Benares,
+was favoured with good weather, and is notable for a photograph obtained
+by Mrs. E.W. Maunder, which showed a ray of the corona extending to a
+most unusual distance.
+
+[Illustration: PLATE I. THE TOTAL ECLIPSE OF THE SUN OF MAY 17TH, 1882
+
+A comet is here shown in the immediate neighbourhood of the corona.
+
+Drawn by Mr. W.H. Wesley from the photographs.
+
+(Page 95)]
+
+Of very great influence in the growth of our knowledge with regard to
+the sun, is the remarkable piece of good fortune by which the countries
+around the Mediterranean, so easy of access, have been favoured with a
+comparatively large number of total eclipses during the past sixty
+years. Tracks of totality have, for instance, traversed the Spanish
+peninsula on no less than five occasions during that period. Two of
+these are among the most notable eclipses of recent years, namely, those
+of May 28, 1900, and of August 30, 1905. In the former the track of
+totality stretched from the western seaboard of Mexico, through the
+Southern States of America, and across the Atlantic Ocean, after which
+it passed over Portugal and Spain into North Africa. The total phase
+lasted for about a minute and a half, and the eclipse was well observed
+from a great many points along the line. A representation of the corona,
+as it appeared on this occasion, will be found on Plate VII. (B), p.
+142.
+
+The track of the other eclipse to which we have alluded, _i.e._ that of
+August 30, 1905, crossed Spain about 200 miles to the northward of that
+of 1900. It stretched from Winnipeg in Canada, through Labrador, and
+over the Atlantic; then traversing Spain, it passed across the Balearic
+Islands, North Africa, and Egypt, and ended in Arabia (see Fig. 6, p.
+81). Much was to be expected from a comparison between the photographs
+taken in Labrador and Egypt on the question as to whether the corona
+would show any alteration in shape during the time that the shadow was
+traversing the intervening space--some 6000 miles. The duration of the
+total phase in this eclipse was nearly four minutes. Bad weather,
+however, interfered a good deal with the observations. It was not
+possible, for instance, to do anything at all in Labrador. In Spain the
+weather conditions were by no means favourable; though at Burgos, where
+an immense number of people had assembled, the total phase was,
+fortunately, well seen. On the whole, the best results were obtained at
+Guelma in Algeria. The corona on the occasion of this eclipse was a very
+fine one, and some magnificent groups of prominences were plainly
+visible to the naked eye (see the Frontispiece).
+
+The next total eclipse after that of 1905 was one which occurred on
+January 14, 1907. It passed across Central Asia and Siberia, and had a
+totality lasting two and a half minutes at most; but it was not observed
+as the weather was extremely bad, a circumstance not surprising with
+regard to those regions at that time of year.
+
+The eclipse of January 3, 1908, passed across the Pacific Ocean. Only
+two small coral islands--Hull Island in the Phoenix Group, and Flint
+Island about 400 miles north of Tahiti--lay in the track. Two
+expeditions set out to observe it, _i.e._ a combined American party from
+the Lick Observatory and the Smithsonian Institution of Washington, and
+a private one from England under Mr. F.K. McClean. As Hull Island
+afforded few facilities, both parties installed their instruments on
+Flint Island, although it was very little better. The duration of the
+total phase was fairly long--about four minutes, and the sun very
+favourably placed, being nearly overhead. Heavy rain and clouds,
+however, marred observation during the first minute of totality, but the
+remaining three minutes were successfully utilised, good photographs of
+the corona being obtained.
+
+The next few years to come are unfortunately by no means favourable
+from the point of view of the eclipse observer. An eclipse will take
+place on June 17, 1909, the track stretching from Greenland across the
+North Polar regions into Siberia. The geographical situation is,
+however, a very awkward one, and totality will be extremely short--only
+six seconds in Greenland and twenty-three seconds in Siberia.
+
+The eclipse of May 9, 1910, will be visible in Tasmania. Totality will
+last so long as four minutes, but the sun will be at the time much too
+low in the sky for good observation.
+
+The eclipse of the following year, April 28, 1911, will also be
+confined, roughly speaking, to the same quarter of the earth, the track
+passing across the old convict settlement of Norfolk Island, and then
+out into the Pacific.
+
+The eclipse of April 17, 1912, will stretch from Portugal, through
+France and Belgium into North Germany. It will, however, be of
+practically no service to astronomy. Totality, for instance, will last
+for only three seconds in Portugal; and, though Paris lies in the
+central track, the eclipse, which begins as barely total, will have
+changed into an _annular_ one by the time it passes over that city.
+
+The first really favourable eclipse in the near future will be that of
+August 21, 1914. Its track will stretch from Greenland across Norway,
+Sweden, and Russia. This eclipse is a return, after one saros, of the
+eclipse of August 9, 1896.
+
+The last solar eclipse which we will touch upon is that predicted for
+June 29, 1927. It has been already alluded to as the first of those in
+the future to be _total_ in England. The central line will stretch from
+Wales in a north-easterly direction. Stonyhurst Observatory, in
+Lancashire, will lie in the track; but totality there will be very
+short, only about twenty seconds in duration.
+
+
+[6] _Knowledge_, vol. xx. p. 9, January 1897.
+
+[7] The _first photographic representation of the corona_ had, however,
+been made during the eclipse of 1851. This was a daguerreotype taken by
+Dr. Busch at K�nigsberg in Prussia.
+
+
+
+
+CHAPTER IX
+
+FAMOUS ECLIPSES OF THE MOON
+
+
+The earliest lunar eclipse, of which we have any trustworthy
+information, was a total one which took place on the 19th March, 721
+B.C., and was observed from Babylon. For our knowledge of this eclipse
+we are indebted to Ptolemy, the astronomer, who copied it, along with
+two others, from the records of the reign of the Chaldean king,
+Merodach-Baladan.
+
+The next eclipse of the moon worth noting was a total one, which took
+place some three hundred years later, namely, in 425 B.C. This eclipse
+was observed at Athens, and is mentioned by Aristophanes in his play,
+_The Clouds_.
+
+Plutarch relates that a total eclipse of the moon, which occurred in 413
+B.C., so greatly frightened Nicias, the general of the Athenians, then
+warring in Sicily, as to cause a delay in his retreat from Syracuse
+which led to the destruction of his whole army.
+
+Seven years later--namely, in 406 B.C., the twenty-sixth year of the
+Peloponnesian War--there took place another total lunar eclipse of which
+mention is made by Xenophon.
+
+Omitting a number of other eclipses alluded to by ancient writers, we
+come to one recorded by Josephus as having occurred a little before the
+death of Herod the Great. It is probable that the eclipse in question
+was the total lunar one, which calculation shows to have taken place on
+the 15th September 5 B.C., and to have been visible in Western Asia.
+This is very important, for we are thus enabled to fix that year as the
+date of the birth of Christ, for Herod is known to have died in the
+early part of the year following the Nativity.
+
+In those accounts of total lunar eclipses, which have come down to us
+from the Dark and Middle Ages, the colour of the moon is nearly always
+likened to "blood." On the other hand, in an account of the eclipse of
+January 23, A.D. 753, our satellite is described as "covered with a
+horrid black shield." We thus have examples of the two distinct
+appearances alluded to in Chapter VII., _i.e._ when the moon appears of
+a coppery-red colour, and when it is entirely darkened.
+
+It appears, indeed, that, in the majority of lunar eclipses on record,
+the moon has appeared of a ruddy, or rather of a coppery hue, and the
+details on its surface have been thus rendered visible. One of the best
+examples of a _bright_ eclipse of this kind is that of the 19th March
+1848, when the illumination of our satellite was so great that many
+persons could not believe that an eclipse was actually taking place. A
+certain Mr. Foster, who observed this eclipse from Bruges, states that
+the markings on the lunar disc were almost as visible as on an "ordinary
+dull moonlight night." He goes on to say that the British Consul at
+Ghent, not knowing that there had been any eclipse, wrote to him for an
+explanation of the red colour of the moon on that evening.
+
+Out of the _dark_ eclipses recorded, perhaps the best example is that
+of May 18, 1761, observed by Wargentin at Stockholm. On this occasion
+the lunar disc is said to have disappeared so completely, that it could
+not be discovered even with the telescope. Another such instance is the
+eclipse of June 10, 1816, observed from London. The summer of that year
+was particularly wet--a point worthy of notice in connection with the
+theory that these different appearances are due to the varying state of
+our earth's atmosphere.
+
+Sometimes, indeed, it has happened that an eclipse of the moon has
+partaken of both appearances, part of the disc being visible and part
+invisible. An instance of this occurred in the eclipse of July 12, 1870,
+when the late Rev. S.J. Johnson, one of the leading authorities on
+eclipses, who observed it, states that he found one-half the moon's
+surface quite invisible, both with the naked eye and with the telescope.
+
+In addition to the examples given above, there are three total lunar
+eclipses which deserve especial mention.
+
+1. A.D. 755, November 23. During the progress of this eclipse the moon
+occulted the star Aldebaran in the constellation of Taurus.
+
+2. A.D. 1493, April 2. This is the celebrated eclipse which is said to
+have so well served the purposes of Christopher Columbus. Certain
+natives having refused to supply him with provisions when in sore
+straits, he announced to them that the moon would be darkened as a sign
+of the anger of heaven. When the event duly came to pass, the savages
+were so terrified that they brought him provisions as much as he needed.
+
+3. A.D. 1610, July 6. The eclipse in question is notable as having been
+seen through the telescope, then a recent invention. It was without
+doubt the first so observed, but unfortunately the name of the observer
+has not come down to us.
+
+
+
+
+CHAPTER X
+
+THE GROWTH OF OBSERVATION
+
+
+The earliest astronomical observations must have been made in the Dawn
+of Historic Time by the men who tended their flocks upon the great
+plains. As they watched the clear night sky they no doubt soon noticed
+that, with the exception of the moon and those brilliant wandering
+objects known to us as the planets, the individual stars in the heaven
+remained apparently fixed with reference to each other. These seemingly
+changeless points of light came in time to be regarded as sign-posts to
+guide the wanderer across the trackless desert, or the voyager upon the
+wide sea.
+
+Just as when looking into the red coals of a fire, or when watching the
+clouds, our imagination conjures up strange and grotesque forms, so did
+the men of old see in the grouping of the stars the outlines of weird
+and curious shapes. Fed with mythological lore, they imagined these to
+be rough representations of ancient heroes and fabled beasts, whom they
+supposed to have been elevated to the heavens as a reward for great
+deeds done upon the earth. We know these groupings of stars to-day under
+the name of the Constellations. Looking up at them we find it extremely
+difficult to fit in the majority with the figures which the ancients
+believed them to represent. Nevertheless, astronomy has accepted the
+arrangement, for want of a better method of fixing the leading stars in
+the memory.
+
+Our early ancestors lived the greater part of their lives in the open
+air, and so came to pay more attention in general to the heavenly orbs
+than we do. Their clock and their calendar was, so to speak, in the
+celestial vault. They regulated their hours, their days, and their
+nights by the changing positions of the sun, the moon, and the stars;
+and recognised the periods of seed-time and harvest, of calm and stormy
+weather, by the rising or setting of certain well-known constellations.
+Students of the classics will recall many allusions to this, especially
+in the Odes of Horace.
+
+As time went on and civilisation progressed, men soon devised measuring
+instruments, by means of which they could note the positions of the
+celestial bodies in the sky with respect to each other; and, from
+observations thus made, they constructed charts of the stars. The
+earliest complete survey of this kind, of which we have a record, is the
+great Catalogue of stars which was made, in the second century B.C., by
+the celebrated Greek astronomer, Hipparchus, and in which he is said to
+have noted down about 1080 stars.
+
+It is unnecessary to follow in detail the tedious progress of
+astronomical discovery prior to the advent of the telescope. Certain it
+is that, as time went on, the measuring instruments to which we have
+alluded had become greatly improved; but, had they even been perfect,
+they would have been utterly inadequate to reveal those minute
+displacements, from which we have learned the actual distance of the
+nearest of the celestial orbs. From the early times, therefore, until
+the medi�val period of our own era, astronomy grew up upon a faulty
+basis, for the earth ever seemed so much the largest body in the
+universe, that it continued from century to century to be regarded as
+the very centre of things.
+
+To the Arabians is due the credit of having kept alive the study of the
+stars during the dark ages of European history. They erected some fine
+observatories, notably in Spain and in the neighbourhood of Bagdad.
+Following them, some of the Oriental peoples embraced the science in
+earnest; Ulugh Beigh, grandson of the famous Tamerlane, founding, for
+instance, a great observatory at Samarcand in Central Asia. The Mongol
+emperors of India also established large astronomical instruments in the
+chief cities of their empire. When the revival of learning took place in
+the West, the Europeans came to the front once more in science, and
+rapidly forged ahead of those who had so assiduously kept alight the
+lamp of knowledge through the long centuries.
+
+The dethronement of the older theories by the Copernican system, in
+which the earth was relegated to its true place, was fortunately soon
+followed by an invention of immense import, the invention of the
+Telescope. It is to this instrument, indeed, that we are indebted for
+our knowledge of the actual scale of the celestial distances. It
+penetrated the depths of space; it brought the distant orbs so near,
+that men could note the detail on the planets, or measure the small
+changes in their positions in the sky which resulted from the movement
+of our own globe.
+
+It was in the year 1609 that the telescope was first constructed. A
+year or so previous to this a spectacle-maker of Middleburgh in Holland,
+one Hans Lippershey, had, it appears, hit upon the fact that distant
+objects, when viewed through certain glass lenses suitably arranged,
+looked nearer.[8] News of this discovery reached the ears of Galileo
+Galilei, of Florence, the foremost philosopher of the day, and he at
+once applied his great scientific attainments to the construction of an
+instrument based upon this principle. The result was what was called an
+"optick tube," which magnified distant objects some few times. It was
+not much larger than what we nowadays contemptuously refer to as a
+"spy-glass," yet its employment upon the leading celestial objects
+instantly sent astronomical science onward with a bound. In rapid
+succession Galileo announced world-moving discoveries; large spots upon
+the face of the sun; crater-like mountains upon the moon; four
+subordinate bodies, or satellites, circling around the planet Jupiter;
+and a strange appearance in connection with Saturn, which later
+telescopic observers found to be a broad flat ring encircling that
+planet. And more important still, the magnified image of Venus showed
+itself in the telescope at certain periods in crescent and other forms;
+a result which Copernicus is said to have announced should of necessity
+follow if his system were the true one.
+
+The discoveries made with the telescope produced, as time went on, a
+great alteration in the notions of men with regard to the universe at
+large. It must have been, indeed, a revelation to find that those points
+of light which they called the planets, were, after all, globes of a
+size comparable with the earth, and peopled perchance with sentient
+beings. Even to us, who have been accustomed since our early youth to
+such an idea, it still requires a certain stretch of imagination to
+enlarge, say, the Bright Star of Eve, into a body similar in size to our
+earth. The reader will perhaps recollect Tennyson's allusion to this in
+_Locksley Hall, Sixty Years After_:--
+
+"Hesper--Venus--were we native to that splendour or in Mars,
+We should see the Globe we groan in, fairest of their evening stars.
+
+"Could we dream of wars and carnage, craft and madness, lust and spite,
+Roaring London, raving Paris, in that point of peaceful light?"
+
+The form of instrument as devised by Galileo is called the Refracting
+Telescope, or "Refractor." As we know it to-day it is the same in
+principle as his "optick tube," but it is not quite the same in
+construction. The early _object-glass_, or large glass at the end, was a
+single convex lens (see Fig. 8, p. 113, "Galilean"); the modern one is,
+on the other hand, composed of two lenses fitted together. The attempts
+to construct large telescopes of the Galilean type met in course of time
+with a great difficulty. The magnified image of the object observed was
+not quite pure; its edges, indeed, were fringed with rainbow-like
+colours. This defect was found to be aggravated with increase in the
+size of object-glasses. A method was, however, discovered of
+diminishing this colouration, or _chromatic aberration_ as it is called
+from the Greek word [chr�ma] (_chroma_), which means colour, viz. by
+making telescopes of great length and only a few inches in width. But
+the remedy was, in a way, worse than the disease; for telescopes thus
+became of such huge proportions as to be too unwieldy for use. Attempts
+were made to evade this unwieldiness by constructing them with skeleton
+tubes (see Plate II., p. 110), or, indeed, even without tubes at all;
+the object-glass in the tubeless or "aerial" telescope being fixed at
+the top of a high post, and the _eye-piece_, that small lens or
+combination of lenses, which the eye looks directly into, being kept in
+line with it by means of a string and manoeuvred about near the ground
+(Plate III., p. 112). The idea of a telescope without a tube may appear
+a contradiction in terms; but it is not really so, for the tube adds
+nothing to the magnifying power of the instrument, and is, in fact, no
+more than a mere device for keeping the object-glass and eye-piece in a
+straight line, and for preventing the observer from being hindered by
+stray lights in his neighbourhood. It goes without saying, of course,
+that the image of a celestial object will be more clear and defined when
+examined in the darkness of a tube.
+
+The ancients, though they knew nothing of telescopes, had, however,
+found out the merit of a tube in this respect; for they employed simple
+tubes, blackened on the inside, in order to obtain a clearer view of
+distant objects. It is said that Julius C�sar, before crossing the
+Channel, surveyed the opposite coast of Britain through a tube of this
+kind.
+
+[Illustration: PLATE II. GREAT TELESCOPE OF HEVELIUS
+
+This instrument, 150 feet in length, with a _skeleton_ tube, was
+constructed by the celebrated seventeenth century astronomer, Hevelius
+of Danzig. From an illustration in the _Machina Celestis_.
+
+(Page 110)]
+
+A few of the most famous of the immensely long telescopes above alluded
+to are worthy of mention. One of these, 123 feet in length, was
+presented to the Royal Society of London by the Dutch astronomer
+Huyghens. Hevelius of Danzig constructed a skeleton one of 150 feet in
+length (see Plate II., p. 110). Bradley used a tubeless one 212 feet
+long to measure the diameter of Venus in 1722; while one of 600 feet is
+said to have been constructed, but to have proved quite unworkable!
+
+Such difficulties, however, produced their natural result. They set men
+at work to devise another kind of telescope. In the new form, called the
+Reflecting Telescope, or "Reflector," the light coming from the object
+under observation was _reflected_ into the eye-piece from the surface of
+a highly polished concave metallic mirror, or _speculum_, as it was
+called. It is to Sir Isaac Newton that the world is indebted for the
+reflecting telescope in its best form. That philosopher had set himself
+to investigate the causes of the rainbow-like, or prismatic colours
+which for a long time had been such a source of annoyance to telescopic
+observers; and he pointed out that, as the colours were produced in the
+passage of the rays of light _through_ the glass, they would be entirely
+absent if the light were reflected from the _surface_ of a mirror
+instead.
+
+The reflecting telescope, however, had in turn certain drawbacks of its
+own. A mirror, for instance, can plainly never be polished to such a
+high degree as to reflect as much light as a piece of transparent glass
+will let through. Further, the position of the eye-piece is by no means
+so convenient. It cannot, of course, be pointed directly towards the
+mirror, for the observer would then have to place his head right in the
+way of the light coming from the celestial object, and would thus, of
+course, cut it off. In order to obviate this difficulty, the following
+device was employed by Newton in his telescope, of which he constructed
+his first example in 1668. A small, flat mirror was fixed by thin wires
+in the centre of the tube of the telescope, and near to its open end. It
+was set slant-wise, so that it reflected the rays of light directly into
+the eye-piece, which was screwed into a hole at the side of the tube
+(see Fig. 8, p. 113, "Newtonian").
+
+Although the Newtonian form of telescope had the immense advantage of
+doing away with the prismatic colours, yet it wasted a great deal of
+light; for the objection in this respect with regard to loss of light by
+reflection from the large mirror applied, of course, to the small mirror
+also. In addition, the position of the "flat," as the small mirror is
+called, had the further effect of excluding from the great mirror a
+certain proportion of light. But the reflector had the advantage, on the
+other hand, of costing less to make than the refractor, as it was not
+necessary to procure flawless glass for the purpose. A disc of a certain
+metallic composition, an alloy of copper and tin, known in consequence
+as _speculum metal_, had merely to be cast; and this had to be ground
+and polished _upon one side only_, whereas a lens has to be thus treated
+_upon both its sides_. It was, therefore, possible to make a much larger
+instrument at a great deal less labour and expense.
+
+[Illustration: PLATE III. A TUBELESS, OR "AERIAL" TELESCOPE
+
+From an illustration in the _Opera Varia_ of Christian Huyghens.
+
+(Page 110)]
+
+[Illustration: FIG. 8.--The various types of Telescope. All the above
+telescopes are _pointed_ in the same direction; that is to say, the rays
+of light from the object are coming from the left-hand side.]
+
+We have given the Newtonian form as an example of the principle of the
+reflecting telescope. A somewhat similar instrument had, however, been
+projected, though not actually constructed, by James Gregory a few years
+earlier than Newton's, _i.e._ in 1663. In this form of reflector, known
+as the "Gregorian" telescope, a hole was made in the big concave mirror;
+and a small mirror, also concave, which faced it at a certain distance,
+received the reflected rays, and reflected them back again through the
+hole in question into the eye-piece, which was fixed just behind (see
+Fig. 8, p. 113, "Gregorian"). The Gregorian had thus the sentimental
+advantage of being _pointed directly at the object_. The hole in the big
+mirror did not cause any loss of light, for the central portion in which
+it was made was anyway unable to receive light through the small mirror
+being directly in front of it. An adaptation of the Gregorian was the
+"Cassegrainian" telescope, devised by Cassegrain in 1672, which differed
+from it chiefly in the small mirror being convex instead of concave (see
+Fig. 8, p. 113, "Cassegrainian"). These _direct-view_ forms of the
+reflecting telescope were much in vogue about the middle of the
+eighteenth century, when many beautiful examples of Gregorians were made
+by the famous optician, James Short, of Edinburgh.
+
+An adaptation of the Newtonian type of telescope is known as the
+"Herschelian," from being the kind favoured by Sir William Herschel. It
+is, however, only suitable in immense instruments, such as Herschel was
+in the habit of employing. In this form the object-glass is set at a
+slight slant, so that the light coming from the object is reflected
+straight into the eye-piece, which is fixed facing it in the side of the
+tube (see Fig. 8, p. 113, "Herschelian"). This telescope has an
+advantage over the other forms of reflector through the saving of light
+consequent on doing away with the _second_ reflection. There is,
+however, the objection that the slant of the object-glass is productive
+of some distortion in the appearance of the object observed; but this
+slant is of necessity slight when the length of the telescope is very
+great.
+
+The principle of this type of telescope had been described to the
+French Academy of Sciences as early as 1728 by Le Maire, but no one
+availed himself of the idea until 1776, when Herschel tried it. At
+first, however, he rejected it; but in 1786 he seems to have found that
+it suited the huge instruments which he was then making. Herschel's
+largest telescope, constructed in 1789, was about four feet in diameter
+and forty feet in length. It is generally spoken of as the "Forty-foot
+Telescope," though all other instruments have been known by their
+_diameters_, rather than by their lengths.
+
+To return to the refracting telescope. A solution of the colour
+difficulty was arrived at in 1729 (two years after Newton's death) by an
+Essex gentleman named Chester Moor Hall. He discovered that by making a
+double object-glass, composed of an outer convex lens and an inner
+concave lens, made respectively of different kinds of glass, _i.e._
+_crown_ glass and _flint_ glass, the troublesome colour effects could
+be, _to a very great extent_, removed. Hall's investigations appear to
+have been rather of an academic nature; and, although he is believed to
+have constructed a small telescope upon these lines, yet he seems to
+have kept the matter so much to himself that it was not until the year
+1758 that the first example of the new instrument was given to the
+world. This was done by John Dollond, founder of the well-known optical
+firm of Dollond, of Ludgate Hill, London, who had, quite independently,
+re-discovered the principle.
+
+This "Achromatic" telescope, or telescope "free from colour effects," is
+the kind ordinarily in use at present, whether for astronomical or for
+terrestrial purposes (see Fig. 8, p. 113, "Achromatic"). The expense of
+making large instruments of this type is very great, for, in the
+object-glass alone, no less than _four_ surfaces have to be ground and
+polished to the required curves; and, usually, the two lenses of which
+it is composed have to fit quite close together.
+
+With the object of evading the expense referred to, and of securing
+_complete_ freedom from colour effects, telescopes have even been made,
+the object-glasses of which were composed of various transparent liquids
+placed between thin lenses; but leakages, and currents set up within
+them by changes of temperature, have defeated the ingenuity of those who
+devised these substitutes.
+
+The solution of the colour difficulty by means of Dollond's achromatic
+refractor has not, however, ousted the reflecting telescope in its best,
+or Newtonian form, for which great concave mirrors made of glass,
+covered with a thin coating of silver and highly polished, have been
+used since about 1870 instead of metal mirrors. They are very much
+lighter in weight and cheaper to make than the old specula; and though
+the silvering, needless to say, deteriorates with time, it can be
+renewed at a comparatively trifling cost. Also these mirrors reflect
+much more light, and give a clearer view, than did the old metallic
+ones.
+
+When an object is viewed through the type of astronomical telescope
+ordinarily in use, it is seen _upside down_. This is, however, a matter
+of very small moment in dealing with celestial objects; for, as they are
+usually round, it is really not of much consequence which part we regard
+as top and which as bottom. Such an inversion would, of course, be most
+inconvenient when viewing terrestrial objects. In order to observe the
+latter we therefore employ what is called a terrestrial telescope, which
+is merely a refractor with some extra lenses added in the eye portion
+for the purpose of turning the inverted image the right way up again.
+These extra lenses, needless to say, absorb a certain amount of light;
+wherefore it is better in astronomical observation to save light by
+doing away with them, and putting up with the slight inconvenience of
+seeing the object inverted.
+
+This inversion of images by the astronomical telescope must be specially
+borne in mind with regard to the photographs of the moon in Chapter XVI.
+
+In the year 1825 the largest achromatic refractor in existence was one
+of nine and a half inches in diameter constructed by Fraunhofer for the
+Observatory of Dorpat in Russia. The largest refractors in the world
+to-day are in the United States, _i.e._ the forty-inch of the Yerkes
+Observatory (see Plate IV., p. 118), and the thirty-six inch of the
+Lick. The object-glasses of these and of the thirty-inch telescope of
+the Observatory of Pulkowa, in Russia, were made by the great optical
+house of Alvan Clark & Sons, of Cambridge, Massachusetts, U.S.A. The
+tubes and other portions of the Yerkes and Lick telescopes were,
+however, constructed by the Warner and Swasey Co., of Cleveland, Ohio.
+
+The largest reflector, and so the largest telescope in the world, is
+still the six-foot erected by the late Lord Rosse at Parsonstown in
+Ireland, and completed in the year 1845. It is about fifty-six feet in
+length. Next come two of five feet, with mirrors of silver on glass;
+one of them made by the late Dr. Common, of Ealing, and the other by the
+American astronomer, Professor G.W. Ritchey. The latter of these is
+installed in the Solar Observatory belonging to Carnegie Institution of
+Washington, which is situated on Mount Wilson in California. The former
+is now at the Harvard College Observatory, and is considered by
+Professor Moulton to be probably the most efficient reflector in use at
+present. Another large reflector is the three-foot made by Dr. Common.
+It came into the possession of Mr. Crossley of Halifax, who presented it
+to the Lick Observatory, where it is now known as the "Crossley
+Reflector."
+
+Although to the house of Clark belongs, as we have seen, the credit of
+constructing the object-glasses of the largest refracting telescopes of
+our time, it has nevertheless keen competitors in Sir Howard Grubb, of
+Dublin, and such well-known firms as Cooke of York and Steinheil of
+Munich. In the four-foot reflector, made in 1870 for the Observatory of
+Melbourne by the firm of Grubb, the Cassegrainian principle was
+employed.
+
+With regard to the various merits of refractors and reflectors much
+might be said. Each kind of instrument has, indeed, its special
+advantages; though perhaps, on the whole, the most perfect type of
+telescope is the achromatic refractor.
+
+[Illustration: PLATE IV. THE GREAT YERKES TELESCOPE
+
+Great telescope at the Yerkes Observatory of the University of Chicago,
+Williams Bay, Wisconsin, U.S.A. It was erected in 1896-7, and is the
+largest refracting telescope in the world. Diameter of object-glass, 40
+inches; length of telescope, about 60 feet. The object-glass was made by
+the firm of Alvan Clark and Sons, of Cambridge, Massachusetts; the other
+portions of the instrument by the Warner and Swasey Co., of Cleveland,
+Ohio.
+
+(Page 117)]
+
+In connection with telescopes certain devices have from time to time
+been introduced, but these merely aim at the _convenience_ of the
+observer and do not supplant the broad principles upon which are based
+the various types of instrument above described. Such, for instance, are
+the "Siderostat," and another form of it called the "Coelostat," in
+which a plane mirror is made to revolve in a certain manner, so as to
+reflect those portions of the sky which are to be observed, into the
+tube of a telescope kept fixed. Such too are the "Equatorial Coud�" of
+the late M. Loewy, Director of the Paris Observatory, and the
+"Sheepshanks Telescope" of the Observatory of Cambridge, in which a
+telescope is separated into two portions, the eye-piece portion being
+fixed upon a downward slant, and the object-glass portion jointed to it
+at an angle and pointed up at the sky. In these two instruments (which,
+by the way, differ materially) an arrangement of slanting mirrors in the
+tubes directs the journey of the rays of light from the object-glass to
+the eye-piece. The observer can thus sit at the eye-end of his telescope
+in the warmth and comfort of his room, and observe the stars in the same
+unconstrained manner as if he were merely looking down into a
+microscope.
+
+Needless to say, devices such as these are subject to the drawback that
+the mirrors employed sap a certain proportion of the rays of light. It
+will be remembered that we made allusion to loss of light in this way,
+when pointing out the advantage in light grasp of the Herschelian form
+of telescope, where only _one_ reflection takes place, over the
+Newtonian in which there are _two_.
+
+It is an interesting question as to whether telescopes can be made much
+larger. The American astronomer, Professor G.E. Hale, concludes that the
+limit of refractors is about five feet in diameter, but he thinks that
+reflectors as large as nine feet in diameter might now be made. As
+regards refractors there are several strong reasons against augmenting
+their proportions. First of all comes the great cost. Secondly, since
+the lenses are held in position merely round their rims, they will bend
+by their weight in the centres if they are made much larger. On the
+other hand, attempts to obviate this, by making the lenses thicker,
+would cause a decrease in the amount of light let through.
+
+But perhaps the greatest stumbling-block to the construction of larger
+telescopes is the fact that the unsteadiness of the air will be
+increasingly magnified. And further, the larger the tubes become, the
+more difficult will it be to keep the air within them at one constant
+temperature throughout their lengths.
+
+It would, indeed, seem as if telescopes are not destined greatly to
+increase in size, but that the means of observation will break out in
+some new direction, as it has already done in the case of photography
+and the spectroscope. The direct use of the eye is gradually giving
+place to indirect methods. We are, in fact, now _feeling_ rather than
+seeing our way about the universe. Up to the present, for instance, we
+have not the slightest proof that life exists elsewhere than upon our
+earth. But who shall say that the twentieth century has not that in
+store for us, by which the presence of life in other orbs may be
+perceived through some form of vibration transmitted across illimitable
+space? There is no use speaking of the impossible or the inconceivable.
+After the extraordinary revelations of the spectroscope--nay, after the
+astounding discovery of R�ntgen--the word impossible should be cast
+aside, and inconceivability cease to be regarded as any criterion.
+
+
+[8] The principle upon which the telescope is based appears to have been
+known _theoretically_ for a long time previous to this. The monk Roger
+Bacon, who lived in the thirteenth century, describes it very clearly;
+and several writers of the sixteenth century have also dealt with the
+idea. Even Lippershey's claims to a practical solution of the question
+were hotly contested at the time by two of his own countrymen, _i.e._ a
+certain Jacob Metius, and another spectacle-maker of Middleburgh, named
+Jansen.
+
+
+
+
+CHAPTER XI
+
+SPECTRUM ANALYSIS
+
+
+If white light (that of the sun, for instance) be passed through a glass
+prism, namely, a piece of glass of triangular shape, it will issue from
+it in rainbow-tinted colours. It is a common experience with any of us
+to notice this when the sunlight shines through cut-glass, as in the
+pendant of a chandelier, or in the stopper of a wine-decanter.
+
+The same effect may be produced when light passes through water. The
+Rainbow, which we all know so well, is merely the result of the sunlight
+passing through drops of falling rain.
+
+White light is composed of rays of various colours. Red, orange, yellow,
+green, blue, indigo, and violet, taken all together, go, in fact, to
+make up that effect which we call white.
+
+It is in the course of the _refraction_, or bending of a beam of light,
+when it passes in certain conditions through a transparent and denser
+medium, such as glass or water, that the constituent rays are sorted out
+and spread in a row according to their various colours. This production
+of colour takes place usually near the edges of a lens; and, as will be
+recollected, proved very obnoxious to the users of the old form of
+refracting telescope.
+
+It is, indeed, a strange irony of fate that this very same production
+of colour, which so hindered astronomy in the past, should have aided it
+in recent years to a remarkable degree. If sunlight, for instance, be
+admitted through a narrow slit before it falls upon a glass prism, it
+will issue from the latter in the form of a band of variegated colour,
+each colour blending insensibly with the next. The colours arrange
+themselves always in the order which we have mentioned. This seeming
+band is, in reality, an array of countless coloured images of the
+original slit ranged side by side; the colour of each image being the
+slightest possible shade different from that next to it. This strip of
+colour when produced by sunlight is called the "Solar Spectrum" (see
+Fig. 9, p. 123). A similar strip, or _spectrum_, will be produced by any
+other light; but the appearance of the strip, with regard to
+preponderance of particular colours, will depend upon the character of
+that light. Electric light and gas light yield spectra not unlike that
+of sunlight; but that of gas is less rich in blue and violet than that
+of the sun.
+
+The Spectroscope, an instrument devised for the examination of spectra,
+is, in its simplest form, composed of a small tube with a narrow slit
+and prism at one end, and an eye-piece at the other. If we drop ordinary
+table salt into the flame of a gas light, the flame becomes strongly
+yellow. If, then, we observe this yellow flame with the spectroscope, we
+find that its spectrum consists almost entirely of two bright yellow
+transverse lines. Chemically considered ordinary table salt is sodium
+chloride; that is to say, a compound of the metal sodium and the gas
+chlorine. Now if other compounds of sodium be experimented with in the
+same manner, it will soon be found that these two yellow lines are
+characteristic of sodium when turned into vapour by great heat. In the
+same manner it can be ascertained that every element, when heated to a
+condition of vapour, gives as its spectrum a set of lines peculiar to
+itself. Thus the spectroscope enables us to find out the composition of
+substances when they are reduced to vapour in the laboratory.
+
+[Illustration: FIG. 9.--The Solar Spectrum.]
+
+In order to increase the power of a spectroscope, it is necessary to
+add to the number of prisms. Each extra prism has the effect of
+lengthening the coloured strip still more, so that lines, which at first
+appeared to be single merely through being crowded together, are
+eventually drawn apart and become separately distinguishable.
+
+On this principle it has gradually been determined that the sun is
+composed of elements similar to those which go to make up our earth.
+Further, the composition of the stars can be ascertained in the same
+manner; and we find them formed on a like pattern, though with certain
+elements in greater or less proportion as the case may be. It is in
+consequence of our thus definitely ascertaining that the stars are
+self-luminous, and of a sun-like character, that we are enabled to speak
+of them as _suns_, or to call the sun a _star_.
+
+In endeavouring to discover the elements of which the planets and
+satellites of our system are composed, we, however, find ourselves
+baffled, for the simple reason that these bodies emit no real light of
+their own. The light which reaches us from them, being merely reflected
+sunlight, gives only the ordinary solar spectrum when examined with the
+spectroscope. But in certain cases we find that the solar spectrum thus
+viewed shows traces of being weakened, or rather of suffering
+absorption; and it is concluded that this may be due to the sunlight
+having had to pass through an atmosphere on its way to and from the
+surface of the planet from which it is reflected to us.
+
+Since the sun is found to be composed of elements similar to those which
+go to make up our earth, we need not be disheartened at this failure of
+the spectroscope to inform us of the composition of the planets and
+satellites. We are justified, indeed, in assuming that more or less the
+same constituents run through our solar system; and that the elements of
+which these bodies are composed are similar to those which are found
+upon our earth and in the sun.
+
+The spectroscope supplies us with even more information. It tells us,
+indeed, whether the sun-like body which we are observing is moving away
+from us or towards us. A certain slight shifting of the lines towards
+the red or violet end of the spectrum respectively, is found to follow
+such movement. This method of observation is known by the name of
+_Doppler's Method_,[9] and by it we are enabled to confirm the evidence
+which the sunspots give us of the rotation of the sun; for we find thus
+that one edge of that body is continually approaching us, and the other
+edge is continually receding from us. Also, we can ascertain in the same
+manner that certain of the stars are moving towards us, and certain of
+them away from us.
+
+
+[9] The idea, initiated by Christian Doppler at Prague in 1842, was
+originally applied to sound. The approach or recession of a source from
+which sound is coming is invariably accompanied by alterations of pitch,
+as the reader has no doubt noticed when a whistling railway-engine has
+approached him or receded from him. It is to Sir William Huggins,
+however, that we are indebted for the application of the principle to
+spectroscopy. This he gave experimental proof of in the year 1868.
+
+
+
+
+CHAPTER XII
+
+THE SUN
+
+
+The sun is the chief member of our system. It controls the motions of
+the planets by its immense gravitative power. Besides this it is the
+most important body in the entire universe, so far as we are concerned;
+for it pours out continually that flood of light and heat, without which
+life, as we know it, would quickly become extinct upon our globe.
+
+Light and heat, though not precisely the same thing, may be regarded,
+however, as next-door neighbours. The light rays are those which
+directly affect the eye and are comprised in the visible spectrum. We
+_feel_ the heat rays, the chief of which are beyond the red portion of
+the spectrum. They may be investigated with the _bolometer_, an
+instrument invented by the late Professor Langley. Chemical rays--for
+instance, those radiations which affect the photographic plate--are for
+the most part also outside the visible spectrum. They are, however, at
+the other end of it, namely, beyond the violet.
+
+Such a scale of radiations may be compared to the keyboard of an
+imaginary piano, the sound from only one of whose octaves is audible to
+us.
+
+The brightest light we know on the earth is dull compared with the light
+of the sun. It would, indeed, look quite dark if held up against it.
+
+It is extremely difficult to arrive at a precise notion of the
+temperature of the body of the sun. However, it is far in excess of any
+temperature which we can obtain here, even in the most powerful electric
+furnace.
+
+A rough idea of the solar heat may be gathered from the calculation that
+if the sun's surface were coated all over with a layer of ice 4000 feet
+thick, it would melt through this completely in one hour.
+
+The sun cannot be a hot body merely cooling; for the rate at which it is
+at present giving off heat could not in such circumstances be kept up,
+according to Professor Moulton, for more than 3000 years. Further, it is
+not a mere burning mass, like a coal fire, for instance; as in that case
+about a thousand years would show a certain drop in temperature. No
+perceptible diminution of solar heat having taken place within historic
+experience, so far as can be ascertained, we are driven to seek some
+more abstruse explanation.
+
+The theory which seems to have received most acceptance is that put
+forward by Helmholtz in 1854. His idea was that gravitation produces
+continual contraction, or falling in of the outer parts of the sun; and
+that this falling in, in its turn, generates enough heat to compensate
+for what is being given off. The calculations of Helmholtz showed that a
+contraction of about 100 feet a year from the surface towards the centre
+would suffice for the purpose. In recent years, however, this estimate
+has been extended to about 180 feet. Nevertheless, even with this
+increased figure, the shrinkage required is so slight in comparison with
+the immense girth of the sun, that it would take a continual
+contraction at this rate for about 6000 years, to show even in our
+finest telescopes that any change in the size of that body was taking
+place at all. Upon this assumption of continuous contraction, a time
+should, however, eventually be reached when the sun will have shrunk to
+such a degree of solidity, that it will not be able to shrink any
+further. Then, the loss of heat not being made up for any longer, the
+body of the sun should begin to grow cold. But we need not be distressed
+on this account; for it will take some 10,000,000 years, according to
+the above theory, before the solar orb becomes too cold to support life
+upon our earth.
+
+Since the discovery of radium it has, on the other hand, been suggested,
+and not unreasonably, that radio-active matter may possibly play an
+important part in keeping up the heat of the sun. But the body of
+scientific opinion appears to consider the theory of contraction as a
+result of gravitation, which has been outlined above, to be of itself
+quite a sound explanation. Indeed, the late Lord Kelvin is said to have
+held to the last that it was amply sufficient to account for the
+underground heat of the earth, the heat of the sun, and that of all the
+stars in the universe.
+
+One great difficulty in forming theories with regard to the sun, is the
+fact that the temperature and gravitation there are enormously in excess
+of anything we meet with upon our earth. The force of gravity at the
+sun's surface is, indeed, about twenty-seven times that at the surface
+of our globe.
+
+The earth's atmosphere appears to absorb about one-half of the
+radiations which come to us from the sun. This absorptive effect is very
+noticeable when the solar orb is low down in our sky, for its light and
+heat are then clearly much reduced. Of the light rays, the blue ones are
+the most easily absorbed in this way; which explains why the sun looks
+red when near the horizon. It has then, of course, to shine through a
+much greater thickness of atmosphere than when high up in the heavens.
+
+What astonishes one most about the solar radiation, is the immense
+amount of it that is apparently wasted into space in comparison with
+what falls directly upon the bodies of the solar system. Only about the
+one-hundred-millionth is caught by all the planets together. What
+becomes of the rest we cannot tell.
+
+That brilliant white body of the sun, which we see, is enveloped by
+several layers of gases and vaporous matter, in the same manner as our
+globe is enveloped by its atmosphere (see Fig. 10, p. 131). These are
+transparent, just as our atmosphere is transparent; and so we see the
+white bright body of the sun right through them.
+
+This white bright portion is called the _Photosphere_. From it comes
+most of that light and heat which we see and feel. We do not know what
+lies under the photosphere, but, no doubt, the more solid portions of
+the sun are there situated. Just above the photosphere, and lying close
+upon it, is a veil of smoke-like haze.
+
+Next upon this is what is known as the _Reversing Layer_, which is
+between 500 and 1000 miles in thickness. It is cooler than the
+underlying photosphere, and is composed of glowing gases. Many of the
+elements which go to make up our earth are present in the reversing
+layer in the form of vapour.
+
+The _Chromosphere_, of which especial mention has already been made in
+dealing with eclipses of the sun, is another layer lying immediately
+upon the last one. It is between 5000 and 10,000 miles in thickness.
+Like the reversing layer, it is composed of glowing gases, chief among
+which is the vapour of hydrogen. The colour of the chromosphere is, in
+reality, a brilliant scarlet; but, as we have already said, the
+intensely white light of the photosphere shines through it from behind,
+and entirely overpowers its redness. The upper portion of the
+chromosphere is in violent agitation, like the waves of a stormy sea,
+and from it rise those red prominences which, it will be recollected,
+are such a notable feature in total solar eclipses.
+
+[Illustration: FIG. 10.--A section through the Sun, showing how the
+prominences rise from the chromosphere.]
+
+The _Corona_ lies next in order outside the chromosphere, and is, so
+far as we know, the outermost of the accompaniments of the sun. This
+halo of pearly-white light is irregular in outline, and fades away into
+the surrounding sky. It extends outwards from the sun to several
+millions of miles. As has been stated, we can never see the corona
+unless, when during a total solar eclipse, the moon has, for the time
+being, hidden the brilliant photosphere completely from our view.
+
+The solar spectrum is really composed of three separate spectra
+commingled, _i.e._ those of the photosphere, of the reversing layer, and
+of the chromosphere respectively.
+
+If, therefore, the photosphere could be entirely removed, or covered up,
+we should see only the spectra of those layers which lie upon it. Such a
+state of things actually occurs in a total eclipse of the sun. When the
+moon's body has crept across the solar disc, and hidden the last piece
+of photosphere, the solar spectrum suddenly becomes what is technically
+called "reversed,"--the dark lines crossing it changing into bright
+lines. This occurs because a strip of those layers which lie immediately
+upon the photosphere remains still uncovered. The lower of these layers
+has therefore been called the "reversing layer," for want of a better
+name. After a second or two this reversed spectrum mostly vanishes, and
+an altered spectrum is left to view. Taking into consideration the rate
+at which the moon is moving across the face of the sun, and the very
+short time during which the spectrum of the reversing layer lasts, the
+thickness of that layer is estimated to be not more than a few hundred
+miles. In the same way the last of the three spectra--namely, that of
+the chromosphere--remains visible for such a time as allows us to
+estimate its depth at about ten times that of the reversing layer, or
+several thousand miles.
+
+When the chromosphere, in its turn during a total eclipse, has been
+covered by the moon, the corona alone is left. This has a distinct
+spectrum of its own also; wherein is seen a strange line in the green
+portion, which does not tally with that of any element we are acquainted
+with upon the earth. This unknown element has received for the time
+being the name of "Coronium."
+
+
+
+
+CHAPTER XIII
+
+THE SUN--_continued_
+
+
+The various parts of the Sun will now be treated of in detail.
+
+
+I. PHOTOSPHERE.
+
+The photosphere, or "light-sphere," from the Greek [ph�s] (_phos_),
+which means _light_, is, as we have already said, the innermost portion
+of the sun which can be seen. Examined through a good telescope it shows
+a finely mottled structure, as of brilliant granules, somewhat like rice
+grains, with small dark spaces lying in between them. It has been
+supposed that we have here the process of some system of circulation by
+which the sun keeps sending forth its radiations. In the bright granules
+we perhaps see masses of intensely heated matter, rising from the
+interior of the sun. The dark interspaces may represent matter which has
+become cooled and darkened through having parted with its heat and
+light, and is falling back again into the solar furnace.
+
+The _sun spots_, so familiar to every one nowadays, are dark patches
+which are often seen to break out in the photosphere (see Plate V., p.
+134). They last during various periods of time; sometimes only for a few
+days, sometimes so long as a month or more. A spot is usually composed
+of a dark central portion called the _umbra_, and a less dark fringe
+around this called the _penumbra_ (see Plate VI., p. 136). The umbra
+ordinarily has the appearance of a deep hole in the photosphere; but,
+that it is a hole at all, has by no means been definitely proved.
+
+[Illustration: PLATE V. THE SUN, SHOWING SEVERAL GROUPS OF SPOTS
+
+From a photograph taken at the Royal Observatory, Greenwich. The
+cross-lines seen on the disc are in no way connected with the Sun, but
+belong to the telescope through which the photograph was taken.
+
+(Page 134)]
+
+Sun spots are, as a rule, some thousands of miles across. The umbra of
+a good-sized spot could indeed engulf at once many bodies the size of
+our earth.
+
+Sun spots do not usually appear singly, but in groups. The total area of
+a group of this kind may be of immense extent; even so great as to cover
+the one-hundredth part of the whole surface of the sun. Very large
+spots, when such are present, may be seen without any telescope; either
+through a piece of smoked glass, or merely with the naked eye when the
+air is misty, or the sun low on the horizon.
+
+The umbra of a spot is not actually dark. It only appears so in contrast
+with the brilliant photosphere around.
+
+Spots form, grow to a large size in comparatively short periods of time,
+and then quickly disappear. They seem to shrink away as a consequence of
+the photosphere closing in upon them.
+
+That the sun is rotating upon an axis, is shown by the continual change
+of position of all spots in one constant direction across his disc. The
+time in which a spot is carried completely round depends, however, upon
+the position which it occupies upon the sun's surface. A spot situated
+near the equator of the sun goes round once in about twenty-five days.
+The further a spot is situated from this equator, the longer it takes.
+About twenty-seven days is the time taken by a spot situated midway
+between the equator and the solar poles. Spots occur to the north of
+the sun's equator, as well as to the south; though, since regular
+observations have been made--that is to say, during the past fifty years
+or so--they appear to have broken out a little more frequently in the
+southern parts.
+
+From these considerations it will be seen that the sun does not rotate
+as the earth does, but that different portions appear to move at
+different speeds. Whether in the neighbourhood of the solar poles the
+time of rotation exceeds twenty-seven days we are unable to ascertain,
+for spots are not seen in those regions. No explanation has yet been
+given of this peculiar rotation; and the most we can say on the subject
+is that the sun is not by any means a solid body.
+
+_Facul�_ (Latin, little torches) are brilliant patches which appear here
+and there upon the sun's surface, and are in some way associated with
+spots. Their displacement, too, across the solar face confirms the
+evidence which the spots give us of the sun's rotation.
+
+Our proofs of this rotation are still further strengthened by the
+Doppler spectroscopic method of observation alluded to in Chapter XI. As
+was then stated, one edge of the sun is thus found to be continually
+approaching us, and the other side continually receding from us. The
+varying rates of rotation, which the spots and facul� give us, are duly
+confirmed by this method.
+
+[Illustration: PLATE VI. PHOTOGRAPH OF A SUNSPOT
+
+This fine picture was taken by the late M. Janssen. The granular
+structure of the Sun's surface is here well represented. (From
+_Knowledge_.)
+
+(Page 135)]
+
+The first attempt to bring some regularity into the question of
+sunspots was the discovery by Schwabe, in 1852, that they were subject
+to a regular variation. As a matter of fact they wax and wane in their
+number, and the total area which they cover, in the course of a period,
+or cycle, of on an average about 11-1/4 years; being at one part of this
+period large and abundant, and at another few and small. This period of
+11-1/4 years is known as the sun spot cycle. No explanation has yet been
+given of the curious round of change, but the period in question seems
+to govern most of the phenomena connected with the sun.
+
+
+II. REVERSING LAYER.
+
+This is a layer of relatively cool gases lying immediately upon the
+photosphere. We never see it directly; and the only proof we have of its
+presence is that remarkable reversal of the spectrum already described,
+when during an instant or two in a total eclipse, the advancing edge of
+the moon, having just hidden the brilliant photosphere, is moving across
+the fine strip which the layer then presents edgewise towards us. The
+fleeting moments during which this reversed spectrum lasts, informs us
+that the layer is comparatively shallow; little more indeed than about
+500 miles in depth.
+
+The spectrum of the reversing layer, or "flash spectrum," as it is
+sometimes called on account of the instantaneous character with which
+the change takes place, was, as we have seen, first noticed by Young in
+1870; and has been successfully photographed since then during several
+eclipses. The layer itself appears to be in a fairly quiescent state; a
+marked contrast to the seething photosphere beneath, and the agitated
+chromosphere above.
+
+
+III. THE CHROMOSPHERE.
+
+The Chromosphere--so called from the Greek [chr�ma] (_chroma_), which
+signifies _colour_--is a layer of gases lying immediately upon the
+preceding one. Its thickness is, however, plainly much the greater of
+the two; for whereas the reversing layer is only revealed to us
+_indirectly_ by the spectroscope, a portion of the chromosphere may
+clearly be _seen_ in a total eclipse in the form of a strip of scarlet
+light. The time which the moon's edge takes to traverse it tells us that
+it must be about ten times as deep as the reversing layer, namely, from
+5000 to 10,000 miles in depth. Its spectrum shows that it is composed
+chiefly of hydrogen, calcium and helium, in the state of vapour. Its red
+colour is mainly due to glowing hydrogen. The element helium, which it
+also contains, has received its appellation from [h�lios] (_helios_),
+the Greek name for the sun; because, at the time when it first attracted
+attention, there appeared to be no element corresponding to it upon our
+earth, and it was consequently imagined to be confined to the sun alone.
+Sir William Ramsay, however, discovered it to be also a terrestrial
+element in 1895, and since then it has come into much prominence as one
+of the products given off by radium.
+
+Taking into consideration the excessive force of gravity on the sun, one
+would expect to find the chromosphere and reversing layer growing
+gradually thicker in the direction of the photosphere. This, however, is
+not the case. Both these layers are strangely enough of the same
+densities all through; which makes it suspected that, in these regions,
+the force of gravity may be counteracted by some other force or forces,
+exerting a powerful pressure outwards from the sun.
+
+
+IV. THE PROMINENCES.
+
+We have already seen, in dealing with total eclipses, that the exterior
+surface of the chromosphere is agitated like a stormy sea, and from it
+billows of flame are tossed up to gigantic heights. These flaming jets
+are known under the name of prominences, because they were first noticed
+in the form of brilliant points projecting from behind the rim of the
+moon when the sun was totally eclipsed. Prominences are of two kinds,
+_eruptive_ and _quiescent_. The eruptive prominences spurt up directly
+from the chromosphere with immense speeds, and change their shape with
+great rapidity. Quiescent prominences, on the other hand, have a form
+somewhat like trees, and alter their shape but slowly. In the eruptive
+prominences glowing masses of gas are shot up to altitudes sometimes as
+high as 300,000 miles,[10] with velocities even so great as from 500 to
+600 miles a second. It has been noticed that the eruptive prominences
+are mostly found in those portions of the sun where spots usually
+appear, namely, in the regions near the solar equator. The quiescent
+prominences, on the other hand, are confined, as a rule, to the
+neighbourhood of the sun's poles.
+
+Prominences were at first never visible except during total eclipses of
+the sun. But in the year 1868, as we have already seen, a method of
+employing the spectroscope was devised, by means of which they could be
+observed and studied at any time, without the necessity of waiting for
+an eclipse.
+
+A still further development of the spectroscope, the
+_Spectroheliograph_, an instrument invented almost simultaneously by
+Professor Hale and the French astronomer, M. Deslandres, permits of
+photographs being taken of the sun, with the light emanating from _only
+one_ of its glowing gases at a time. For instance, we can thus obtain a
+record of what the glowing hydrogen alone is doing on the solar body at
+any particular moment. With this instrument it is also possible to
+obtain a series of photographs, showing what is taking place upon the
+sun at various levels. This is very useful in connection with the study
+of the spots; for we are, in consequence, enabled to gather more
+evidence on the subject of their actual form than is given us by their
+highly foreshortened appearances when observed directly in the
+telescope.
+
+
+V. CORONA. (Latin, _a Crown_.)
+
+This marvellous halo of pearly-white light, which displays itself to our
+view only during the total phase of an eclipse of the sun, is by no
+means a layer like those other envelopments of the sun of which we have
+just been treating. It appears, on the other hand, to be composed of
+filmy matter, radiating outwards in every direction, and fading away
+gradually into space. Its structure is noted to bear a strong
+resemblance to the tails of comets, or the streamers of the aurora
+borealis.
+
+Our knowledge concerning the corona has, however, advanced very slowly.
+We have not, so far, been as fortunate with regard to it as with regard
+to the prominences; and, for all we can gather concerning it, we are
+still entirely dependent upon the changes and chances of total solar
+eclipses. All attempts, in fact, to apply the spectroscopic method, so
+as to observe the corona at leisure in full sunlight in the way in which
+the prominences can be observed, have up to the present met with
+failure.
+
+The general form under which the corona appears to our eyes varies
+markedly at different eclipses. Sometimes its streamers are many, and
+radiate all round; at other times they are confined only to the middle
+portions of the sun, and are very elongated, with short feathery-looking
+wisps adorning the solar poles. It is noticed that this change of shape
+varies in close accordance with that 11-1/4 year period during which the
+sun spots wax and wane; the many-streamered regular type corresponding
+to the time of great sunspot activity, while the irregular type with the
+long streamers is present only when the spots are few (see Plate VII.,
+p. 142). Streamers have often been noted to issue from those regions of
+the sun where active prominences are at the moment in existence; but it
+cannot be laid down that this is always the case.
+
+No hypothesis has yet been formulated which will account for the
+structure of the corona, or for its variation in shape. The great
+difficulty with regard to theorising upon this subject, is the fact
+that we see so much of the corona under conditions of marked
+foreshortening. Assuming, what indeed seems natural, that the rays of
+which it is composed issue in every direction from the solar body, in a
+manner which may be roughly imitated by sticking pins all over a ball;
+it is plainly impossible to form any definite idea concerning streamers,
+which actually may owe most of the shape they present to us, to the
+mixing up of multitudes of rays at all kinds of angles to the line of
+sight. In a word, we have to try and form an opinion concerning an
+arrangement which, broadly speaking, is _spherical_, but which, on
+account of its distance, must needs appear to us as absolutely _flat_.
+
+The most known about the composition of the corona is that it is made up
+of particles of matter, mingled with a glowing gas. It is an element in
+the composition of this gas which, as has been stated, is not found to
+tally with any known terrestrial element, and has, therefore, received
+the name of coronium for want of a better designation.
+
+One definite conclusion appears to be reached with regard to the corona,
+_i.e._ that the matter of which it is composed, must be exceedingly
+rarefied; as it is not found, for instance, to retard appreciably the
+speed of comets, on occasions when these bodies pass very close to the
+sun. A calculation has indeed been made which would tend to show that
+the particles composing the coronal matter, are separated from each
+other by a distance of perhaps between two and three yards! The density
+of the corona is found not to increase inwards towards the sun. This is
+what has already been noted with regard to the layers lying beneath it.
+Powerful forces, acting in opposition to gravity, must hold sway here
+also.
+
+[Illustration: (A.) THE TOTAL ECLIPSE OF THE SUN OF DECEMBER 22ND, 1870
+
+Drawn by Mr. W.H. Wesley from a photograph taken at Syracuse by Mr.
+Brothers. This is the type of corona seen at the time of _greatest_
+sunspot activity. The coronas of 1882 (Plate I., p. 96) and of 1905
+(Frontispiece) are of the same type.
+
+(B.) THE TOTAL ECLIPSE OF THE SUN OF MAY 28TH, 1900
+
+Drawn by Mr. W.H. Wesley from photographs taken by Mr. E.W. Maunder.
+This is the type of corona seen when the sunspots are _least_ active.
+Compare the "Ring with Wings," Fig. 7, p. 87.
+
+PLATE VII. FORMS OF THE SOLAR CORONA AT THE EPOCHS OF SUNSPOT MAXIMUM
+AND SUNSPOT MINIMUM, RESPECTIVELY
+
+(Page 141)]
+
+The 11-1/4 year period, during which the sun spots vary in number and
+size, appears to govern the activities of the sun much in the same way
+that our year does the changing seasonal conditions of our earth. Not
+only, as we have seen, does the corona vary its shape in accordance with
+the said period, but the activity of the prominences, and of the facul�,
+follow suit. Further, this constant round of ebb and flow is not
+confined to the sun itself, but, strangely enough, affects the earth
+also. The displays of the aurora borealis, which we experience here,
+coincide closely with it, as does also the varying state of the earth's
+magnetism. The connection may be still better appreciated when a great
+spot, or group of spots, has made its appearance upon the sun. It has,
+for example, often been noted that when the solar rotation carries a
+spot, or group of spots, across the middle of the visible surface of the
+sun, our magnetic and electrical arrangements are disturbed for the time
+being. The magnetic needles in our observatories are, for instance, seen
+to oscillate violently, telegraphic communication is for a while upset,
+and magnificent displays of the aurora borealis illumine our night
+skies. Mr. E.W. Maunder, of Greenwich Observatory, who has made a very
+careful investigation of this subject, suspects that, when elongated
+coronal streamers are whirled round in our direction by the solar
+rotation, powerful magnetic impulses may be projected upon us at the
+moments when such streamers are pointing towards the earth.
+
+Some interesting investigations with regard to sunspots have recently
+been published by Mrs. E.W. Maunder. In an able paper, communicated to
+the Royal Astronomical Society on May 10, 1907, she reviews the
+Greenwich Observatory statistics dealing with the number and extent of
+the spots which have appeared during the period from 1889 to 1901--a
+whole sunspot cycle. From a detailed study of the dates in question, she
+finds that the number of those spots which are formed on the side of the
+sun turned away from us, and die out upon the side turned towards us, is
+much greater than the number of those which are formed on the side
+turned towards us and die out upon the side turned away. It used, for
+instance, to be considered that the influence of a planet might
+_produce_ sunspots; but these investigations make it look rather as if
+some influence on the part of the earth tends, on the contrary, to
+_extinguish_ them. Mrs. Maunder, so far, prefers to call the influence
+thus traced an _apparent_ influence only, for, as she very fairly points
+out, it seems difficult to attribute a real influence in this matter to
+the earth, which is so small a thing in comparison not only with the
+sun, but even with many individual spots.
+
+The above investigation was to a certain degree anticipated by Mr. Henry
+Corder in 1895; but Mrs. Maunder's researches cover a much longer
+period, and the conclusions deduced are of a wider and more defined
+nature.
+
+With regard to its chemical composition, the spectroscope shows us that
+thirty-nine of the elements which are found upon our earth are also to
+be found in the sun. Of these the best known are hydrogen, oxygen,
+helium, carbon, calcium, aluminium, iron, copper, zinc, silver, tin, and
+lead. Some elements of the metallic order have, however, not been found
+there, as, for instance, gold and mercury; while a few of the other
+class of element, such as nitrogen, chlorine, and sulphur, are also
+absent. It must not, indeed, be concluded that the elements apparently
+missing do not exist at all in the solar body. Gold and mercury have, in
+consequence of their great atomic weight, perhaps sunk away into the
+centre. Again, the fact that we cannot find traces of certain other
+elements, is no real proof of their entire absence. Some of them may,
+for instance, be resolved into even simpler forms, under the unusual
+conditions which exist in the sun; and so we are unable to trace them
+with the spectroscope, the experience of which rests on laboratory
+experiments conducted, at best, in conditions which obtain upon the
+earth.
+
+
+[10] On November 15, 1907, Dr. A. Rambaut, Radcliffe Observer at Oxford
+University, noted a prominence which rose to a height of 324,600 miles.
+
+
+
+
+CHAPTER XIV
+
+THE INFERIOR PLANETS
+
+
+Starting from the centre of the solar system, the first body we meet
+with is the planet Mercury. It circulates at an average distance from
+the sun of about thirty-six millions of miles. The next body to it is
+the planet Venus, at about sixty-seven millions of miles, namely, about
+double the distance of Mercury from the sun. Since our earth comes next
+again, astronomers call those planets which circulate within its orbit,
+_i.e._ Mercury and Venus, the Inferior Planets, while those which
+circulate outside it they call the Superior Planets.[11]
+
+In studying the inferior planets, the circumstances in which we make our
+observations are so very similar with regard to each, that it is best to
+take them together. Let us begin by considering the various positions of
+an inferior planet, as seen from the earth, during the course of its
+journeys round the sun. When furthest from us it is at the other side of
+the sun, and cannot then be seen owing to the blaze of light. As it
+continues its journey it passes to the left of the sun, and is then
+sufficiently away from the glare to be plainly seen. It next draws in
+again towards the sun, and is once more lost to view in the blaze at
+the time of its passing nearest to us. Then it gradually comes out to
+view on the right hand, separates from the sun up to a certain distance
+as before, and again recedes beyond the sun, and is for the time being
+once more lost to view.
+
+To these various positions technical names are given. When the inferior
+planet is on the far side of the sun from us, it is said to be in
+_Superior Conjunction_. When it has drawn as far as it can to the left
+hand, and is then as east as possible of the sun, it is said to be at
+its _Greatest Eastern Elongation_. Again, when it is passing nearest to
+us, it is said to be in _Inferior Conjunction_; and, finally, when it
+has drawn as far as it can to the right hand, it is spoken of as being
+at its _Greatest Western Elongation_ (see Fig. 11, p. 148).
+
+The continual variation in the distance of an interior planet from us,
+during its revolution around the sun, will of course be productive of
+great alterations in its apparent size. At superior conjunction it
+ought, being then farthest away, to show the smallest disc; while at
+inferior conjunction, being the nearest, it should look much larger.
+When at greatest elongation, whether eastern or western, it should
+naturally present an appearance midway in size between the two.
+
+[Illustration: Various positions, and illumination by the Sun, of an
+Inferior Planet in the course of its orbit.
+
+Corresponding views of the same situations of an Inferior Planet as seen
+from the Earth, showing consequent phases and alterations in apparent
+size.
+
+FIG. 11.--Orbit and Phases of an Inferior Planet.]
+
+From the above considerations one would be inclined to assume that the
+best time for studying the surface of an interior planet with the
+telescope is when it is at inferior conjunction, or, nearest to us. But
+that this is not the case will at once appear if we consider that the
+sunlight is then falling upon the side away from us, leaving the side
+which is towards us unillumined. In superior conjunction, on the other
+hand, the light falls full upon the side of the planet facing us; but
+the disc is then so small-looking, and our view besides is so dazzled by
+the proximity of the sun, that observations are of little avail. In the
+elongations, however, the sunlight comes from the side, and so we see
+one half of the planet lit up; the right half at eastern elongation, and
+the left half at western elongation. Piecing together the results given
+us at these more favourable views, we are enabled, bit by bit, to gather
+some small knowledge concerning the surface of an inferior planet.
+
+From these considerations it will be seen at once that the inferior
+planets show various phases comparable to the waxing and waning of our
+moon in its monthly round. Superior conjunction is, in fact, similar to
+full moon, and inferior conjunction to new moon; while the eastern and
+western elongations may be compared respectively to the moon's first and
+last quarters. It will be recollected how, when these phases were first
+seen by the early telescopic observers, the Copernican theory was felt
+to be immensely strengthened; for it had been pointed out that if this
+system were the correct one, the planets Venus and Mercury, were it
+possible to see them more distinctly, would of necessity present phases
+like these when viewed from the earth. It should here be noted that the
+telescope was not invented until nearly seventy years after the death of
+Copernicus.
+
+The apparent swing of an inferior planet from side to side of the sun,
+at one time on the east side, then passing into and lost in the sun's
+rays to appear once more on the west side, is the explanation of what is
+meant when we speak of an _evening_ or a _morning star_. An inferior
+planet is called an evening star when it is at its eastern elongation,
+that is to say, on the left-hand of the sun; for, being then on the
+eastern side, it will set after the sun sets, as both sink in their turn
+below the western horizon at the close of day. Similarly, when such a
+planet is at its western elongation, that is to say, to the right-hand
+of the sun, it will go in advance of him, and so will rise above the
+eastern horizon before the sun rises, receiving therefore the
+designation of morning star. In very early times, however, before any
+definite ideas had been come to with regard to the celestial motions, it
+was generally believed that the morning and evening stars were quite
+distinct bodies. Thus Venus, when a morning star, was known to the
+ancients under the name of Phosphorus, or Lucifer; whereas they called
+it Hesperus when it was an evening star.
+
+Since an inferior planet circulates between us and the sun, one would be
+inclined to expect that such a body, each time it passed on the side
+nearest to the earth, should be seen as a black spot against the bright
+solar disc. Now this would most certainly be the case were the orbit of
+an inferior planet in the same plane with the orbit of the earth. But we
+have already seen how the orbits in the solar system, whether those of
+planets or of satellites, are by no means in the one plane; and that it
+is for this very reason that the moon is able to pass time after time in
+the direction of the sun, at the epoch known as new moon, and yet not to
+eclipse him save after the lapse of several such passages. Transits,
+then, as the passages of an inferior planet across the sun's disc are
+called, take place, for the same reason, only after certain regular
+lapses of time; and, as regards the circumstances of their occurrence,
+are on a par with eclipses of the sun. The latter, however, happen much
+more frequently, because the moon passes in the neighbourhood of the
+sun, roughly speaking, once a month, whereas Venus comes to each
+inferior conjunction at intervals so long apart as a year and a half,
+and Mercury only about every four months. From this it will be further
+gathered that transits of Mercury take place much oftener than transits
+of Venus.
+
+Until recent years _Transits of Venus_ were phenomena of great
+importance to astronomers, for they furnished the best means then
+available of calculating the distance of the sun from the earth. This
+was arrived at through comparing the amount of apparent displacement in
+the planet's path across the solar disc, when the transit was observed
+from widely separated stations on the earth's surface. The last transit
+of Venus took place in 1882, and there will not be another until the
+year 2004.
+
+_Transits of Mercury_, on the other hand, are not of much scientific
+importance. They are of no interest as a popular spectacle; for the
+dimensions of the planet are so small, that it can be seen only with the
+aid of a telescope when it is in the act of crossing the sun's disc. The
+last transit of Mercury took place on November 14, 1907, and there will
+be another on November 6, 1914.
+
+The first person known to have observed a transit of an inferior planet
+was the celebrated French philosopher, Gassendi. This was the transit of
+Mercury which took place on the 7th of December 1631.
+
+The first time a transit of Venus was ever seen, so far as is known, was
+on the 24th of November 1639. The observer was a certain Jeremiah
+Horrox, curate of Hoole, near Preston, in Lancashire. The transit in
+question commenced shortly before sunset, and his observations in
+consequence were limited to only about half-an-hour. Horrox happened to
+have a great friend, one William Crabtree, of Manchester, whom he had
+advised by letter to be on the look out for the phenomenon. The weather
+in Crabtree's neighbourhood was cloudy, with the result that he only got
+a view of the transit for about ten minutes before the sun set.
+
+That this transit was observed at all is due entirely to the remarkable
+ability of Horrox. According to the calculations of the great Kepler, no
+transit could take place that year (1639), as the planet would just pass
+clear of the lower edge of the sun. Horrox, however, not being satisfied
+with this, worked the question out for himself, and came to the
+conclusion that the planet would _actually_ traverse the lower portion
+of the sun's disc. The event, as we have seen, proved him to be quite in
+the right. Horrox is said to have been a veritable prodigy of
+astronomical skill; and had he lived longer would, no doubt, have become
+very famous. Unfortunately he died about two years after his celebrated
+transit, in his _twenty-second_ year only, according to the accounts.
+His friend Crabtree, who was then also a young man, is said to have been
+killed at the battle of Naseby in 1645.
+
+There is an interesting phenomenon in connection with transits which is
+known as the "Black Drop." When an inferior planet has just made its way
+on to the face of the sun, it is usually seen to remain for a short time
+as if attached to the sun's edge by what looks like a dark ligament (see
+Fig. 12, p. 153). This gives to the planet for the time being an
+elongated appearance, something like that of a pear; but when the
+ligament, which all the while keeps getting thinner and thinner, has at
+last broken, the black body of the planet is seen to stand out round
+against the solar disc.
+
+[Illustration: FIG. 12.--The "Black Drop."]
+
+This appearance may be roughly compared to the manner in which a drop of
+liquid (or, preferably, of some glutinous substance) tends for a while
+to adhere to an object from which it is falling.
+
+When the planet is in turn making its way off the face of the sun, the
+ligament is again seen to form and to attach it to the sun's edge before
+its due time.
+
+The phenomenon of the black drop, or ligament, is entirely an illusion,
+and, broadly speaking, of an optical origin. Something very similar will
+be noticed if one brings one's thumb and forefinger _slowly_ together
+against a very bright background.
+
+This peculiar phenomenon has proved one of the greatest drawbacks to the
+proper observation of transits, for it is quite impossible to note the
+exact instant of the planet's entrance upon and departure from the solar
+disc in conditions such as these.
+
+The black drop seems to bear a family resemblance, so to speak, to the
+phenomenon of Baily's beads. In the latter instance the lunar peaks, as
+they approach the sun's edge, appear to lengthen out in a similar manner
+and bridge the intervening space before their time, thus giving
+prominence to an effect which otherwise should scarcely be noticeable.
+
+The last transit of Mercury, which, as has been already stated, took
+place on November 14, 1907, was not successfully observed by astronomers
+in England, on account of the cloudiness of the weather. In France,
+however, Professor Moye, of Montpellier, saw it under good conditions,
+and mentions that the black drop remained very conspicuous for fully a
+minute. The transit was also observed in the United States, the reports
+from which speak of the black drop as very "troublesome."
+
+Before leaving the subject of transits it should be mentioned that it
+was in the capacity of commander of an expedition to Otaheite, in the
+Pacific, to observe the transit of Venus of June 3, 1769, that Captain
+Cook embarked upon the first of his celebrated voyages.
+
+In studying the surfaces of Venus and Mercury with the telescope,
+observers are, needless to say, very much hindered by the proximity of
+the sun. Venus, when at the greatest elongations, certainly draws some
+distance out of the glare; but her surface is, even then, so dazzlingly
+bright, that the markings upon it are difficult to see. Mercury, on the
+other hand, is much duller in contrast, but the disc it shows in the
+telescope is exceedingly small; and, in addition, when that planet is
+left above the horizon for a short time after sunset, as necessarily
+happens after certain intervals, the mists near the earth's surface
+render observation of it very difficult.
+
+Until about twenty-five years ago, it was generally believed that both
+these planets rotated on their axes in about twenty-four hours, a
+notion, no doubt, originally founded upon an unconscious desire to bring
+them into some conformity with our earth. But Schiaparelli, observing in
+Italy, and Percival Lowell, in the clear skies of Arizona and Mexico,
+have lately come to the conclusion that both planets rotate upon their
+axes in the same time as they revolve in their orbits,[12] the result
+being that they turn one face ever towards the sun in the same manner
+that the moon turns one face ever towards the earth--a curious state of
+things, which will be dealt with more fully when we come to treat of our
+satellite.
+
+The marked difference in the brightness between the two planets has
+already been alluded to. The surface of Venus is, indeed, about five
+times as bright as that of Mercury. The actual brightness of Mercury is
+about equivalent to that of our moon, and astronomers are, therefore,
+inclined to think that it may resemble her in having a very rugged
+surface and practically no atmosphere. This probable lack of atmosphere
+is further corroborated by two circumstances. One of these is that when
+Mercury is just about to transit the face of the sun, no ring of
+diffused light is seen to encircle its disc as would be the case if it
+possessed an atmosphere. Such a lack of atmosphere is, indeed, only to
+be expected from what is known as the _Kinetic Theory of Gases_.
+According to this theory, which is based upon the behaviour of various
+kinds of gas, it is found that these elements tend to escape into space
+from the surface of bodies whose force of gravitation is weak. Hydrogen
+gas, for example, tends to fly away from our earth, as any one may see
+for himself when a balloon rises into the air. The gravitation of the
+earth seems, however, powerful enough to hold down other gases, as, for
+instance, those of which the air is chiefly composed, namely, oxygen and
+nitrogen. In due accordance with the Kinetic theory, we find the moon
+and Mercury, which are much about the same size, destitute of
+atmospheres. Mars, too, whose diameter is only about double that of the
+moon, has very little atmosphere. We find, on the other hand, that
+Venus, which is about the same size as our earth, clearly possesses an
+atmosphere, as just before the planet is in transit across the sun, the
+outline of its dark body is seen to be surrounded by a bright ring of
+light.
+
+The results of telescopic observation show that more markings are
+visible on Mercury than on Venus. The intense brilliancy of Venus is,
+indeed, about the same as that of our white clouds when the sun is
+shining directly upon them. It has, therefore, been supposed that the
+planet is thickly enveloped in cloud, and that we do not ever see any
+part of its surface, except perchance the summit of some lofty mountain
+projecting through the fleecy mass.
+
+With regard to the great brilliancy of Venus, it may be mentioned that
+she has frequently been seen in England, with the naked eye in full
+sunshine, when at the time of her greatest brightness. The writer has
+seen her thus at noonday. Needless to say, the sky at the moment was
+intensely blue and clear.
+
+The orbit of Mercury is very oval, and much more so than that of any
+other planet. The consequence is that, when Mercury is nearest to the
+sun, the heat which it receives is twice as great as when it is farthest
+away. The orbit of Venus, on the other hand, is in marked contrast with
+that of Mercury, and is, besides, more nearly of a circular shape than
+that of any of the other planets. Venus, therefore, always keeps about
+the same distance from the sun, and so the heat which she receives
+during the course of her year can only be subject to very slight
+variations.
+
+
+[11] In employing the terms Inferior and Superior the writer bows to
+astronomical custom, though he cannot help feeling that, in the
+circumstances, Interior and Exterior would be much more appropriate.
+
+[12] This question is, however, uncertain, for some very recent
+spectroscopic observations of Venus seem to show a rotation period of
+about twenty-four hours.
+
+
+
+
+CHAPTER XV
+
+THE EARTH
+
+
+We have already seen (in Chapter I.) how, in very early times, men
+naturally enough considered the earth to be a flat plane extending to a
+very great distance in every direction; but that, as years went on,
+certain of the Greek philosophers suspected it to be a sphere. One or
+two of the latter are, indeed, said to have further believed in its
+rotation about an axis, and even in its revolution around the sun; but,
+as the ideas in question were founded upon fancy, rather than upon any
+direct evidence, they did not generally attract attention. The small
+effect, therefore, which these theories had upon astronomy, may well be
+gathered from the fact that in the Ptolemaic system the earth was
+considered as fixed and at the centre of things; and this belief, as we
+have seen, continued unaltered down to the days of Copernicus. It was,
+indeed, quite impossible to be certain of the real shape of the earth or
+the reality of its motions until knowledge became more extended and
+scientific instruments much greater in precision.
+
+We will now consider in detail a few of the more obvious arguments which
+can be put forward to show that our earth is a sphere.
+
+If, for instance, the earth were a plane surface, a ship sailing away
+from us over the sea would appear to grow smaller and smaller as it
+receded into the distance, becoming eventually a tiny speck, and fading
+gradually from our view. This, however, is not at all what actually
+takes place. As we watch a vessel receding, its hull appears bit by bit
+to slip gently down over the horizon, leaving the masts alone visible.
+Then, in their turn, the masts are seen to slip down in the same manner,
+until eventually every trace of the vessel is gone. On the other hand,
+when a ship comes into view, the masts are the first portions to appear.
+They gradually rise up from below the horizon, and the hull follows in
+its turn, until the whole vessel is visible. Again, when one is upon a
+ship at sea, a set of masts will often be seen sticking up alone above
+the horizon, and these may shorten and gradually disappear from view
+without the body of the ship to which they belong becoming visible at
+all. Since one knows from experience that there is no _edge_ at the
+horizon over which a vessel can drop down, the appearance which we have
+been describing can only be explained by supposing that the surface of
+the earth is always curving gradually in every direction.
+
+The distance at which what is known as the _horizon_ lies away from us
+depends entirely upon the height above the earth's surface where we
+happen at the moment to be. A ship which has appeared to sink below the
+horizon for a person standing on the beach, will be found to come back
+again into view if he at once ascends a high hill. Experiment shows that
+the horizon line lies at about three miles away for a person standing at
+the water's edge. The curving of the earth's surface is found, indeed,
+to be at the rate of eight inches in every mile. Now it can be
+ascertained, by calculation, that a body curving at this rate in every
+direction must be a globe about 8000 miles in diameter.
+
+Again, the fact that, if not stopped by such insuperable obstacles as
+the polar ice and snow, those who travel continually in any one
+direction upon the earth's surface always find themselves back again at
+the regions from which they originally set out, is additional ground for
+concluding that the earth is a globe.
+
+We can find still further evidence. For instance, in an eclipse of the
+moon the earth's shadow, when seen creeping across the moon's face, is
+noted to be _always_ circular in shape. One cannot imagine how such a
+thing could take place unless the earth were a sphere.
+
+Also, it is found from observation that the sun, the planets, and the
+satellites are, all of them, round. This roundness cannot be the
+roundness of a flat plate, for instance, for then the objects in
+question would sometimes present their thin sides to our view. It
+happens, also, that upon the discs which these bodies show, we see
+certain markings shifting along continually in one direction, to
+disappear at one side and to reappear again at the other. Such bodies
+must, indeed, be spheres in rotation.
+
+The crescent and other phases, shown by the moon and the inferior
+planets, should further impress the truth of the matter upon us, as such
+appearances can only be caused by the sunlight falling from various
+directions upon the surfaces of spherical bodies.
+
+Another proof, perhaps indeed the weightiest of all, is the continuous
+manner in which the stars overhead give place to others as one travels
+about the surface of the earth. When in northern regions the Pole Star
+and its neighbours--the stars composing the Plough, for instance--are
+over our heads. As one journeys south these gradually sink towards the
+northern horizon, while other stars take their place, and yet others are
+uncovered to view from the south. The regularity with which these
+changes occur shows that every point on the earth's surface faces a
+different direction of the sky, and such an arrangement would only be
+possible if the earth were a sphere. The celebrated Greek philosopher,
+Aristotle, is known to have believed in the globular shape of the earth,
+and it was by this very argument that he had convinced himself that it
+was so.
+
+The idea of the sphericity of the earth does not appear, however, to
+have been generally accepted until the voyages of the great navigators
+showed that it could be sailed round.
+
+The next point we have to consider is the rotation of the earth about
+its axis. From the earliest times men noticed that the sky and
+everything in it appeared to revolve around the earth in one fixed
+direction, namely, towards what is called the West, and that it made one
+complete revolution in the period of time which we know as twenty-four
+hours. The stars were seen to come up, one after another, from below the
+eastern horizon, to mount the sky, and then to sink in turn below the
+western horizon. The sun was seen to perform exactly the same journey,
+and the moon, too, whenever she was visible. One or two of the ancient
+Greek philosophers perceived that this might be explained, either by a
+movement of the entire heavens around the earth, or by a turning motion
+on the part of the earth itself. Of these diverse explanations, that
+which supposed an actual movement of the heavens appealed to them the
+most, for they could hardly conceive that the earth should continually
+rotate and men not be aware of its movement. The question may be
+compared to what we experience when borne along in a railway train. We
+see the telegraph posts and the trees and buildings near the line fly
+past us one after another in the contrary direction. Either these must
+be moving, or we must be moving; and as we happen to _know_ that it is,
+indeed, we who are moving, there can be no question therefore about the
+matter. But it would not be at all so easy to be sure of this movement
+were one unable to see the objects close at hand displacing themselves.
+For instance, if one is shut up in a railway carriage at night with the
+blinds down, there is really nothing to show that one is moving, except
+the jolting of the train. And even then it is hard to be sure in which
+direction one is actually travelling.
+
+The way we are situated upon the earth is therefore as follows. There
+are no other bodies sufficiently near to be seen flying past us in turn;
+our earth spins without a jolt; we and all things around us, including
+the atmosphere itself, are borne along together with precisely the same
+impetus, just as all the objects scattered about a railway carriage
+share in the forward movement of the train. Such being the case, what
+wonder that we are unconscious of the earth's rotation, of which we
+should know nothing at all, were it not for that slow displacement of
+the distant objects in the heavens, as we are borne past them in turn.
+
+If the night sky be watched, it will be soon found that its apparent
+turning movement seems to take place around a certain point, which
+appears as if fixed. This point is known as the north pole of the
+heavens; and a rather bright star, which is situated very close to this
+hub of movement, is in consequence called the Pole Star. For the
+dwellers in southern latitudes there is also a point in their sky which
+appears to remain similarly fixed, and this is known as the south pole
+of the heavens. Since, however, the heavens do not turn round at all,
+but the earth does, it will easily be seen that these apparently
+stationary regions in the sky are really the points towards which the
+axis of the earth is directed. The positions on the earth's surface
+itself, known as the North and South Poles, are merely the places where
+the earth's axis, if there were actually such a thing, would be expected
+to jut out. The north pole of the earth will thus be situated exactly
+beneath the north pole of the heavens, and the south pole of the earth
+exactly beneath the south pole of the heavens.
+
+We have seen that the earth rotates upon its imaginary axis once in
+about every twenty-four hours. This means that everything upon the
+surface of the earth is carried round once during that time. The
+measurement around the earth's equator is about 24,000 miles; and,
+therefore, an object situated at the equator must be carried round
+through a distance of about 24,000 miles in each twenty-four hours.
+Everything at the equator is thus moving along at the rapid rate of
+about 1000 miles an hour, or between sixteen and seventeen times as
+fast as an express train. If, however, one were to take measurements
+around the earth parallel to the equator, one would find these
+measurements becoming less and less, according as the poles were
+approached. It is plain, therefore, that the speed with which any point
+moves, in consequence of the earth's rotation, will be greatest at the
+equator, and less and less in the direction of the poles; while at the
+poles themselves there will be practically no movement, and objects
+there situated will merely turn round.
+
+The considerations above set forth, with regard to the different speeds
+at which different portions of a rotating globe will necessarily be
+moving, is the foundation of an interesting experiment, which gives us
+further evidence of the rotation of our earth. The measurement around
+the earth at any distance below the surface, say, for instance, at the
+depth of a mile, will clearly be less than a similar measurement at the
+surface itself. The speed of a point at the bottom of a mine, which
+results from the actual rotation of the earth, must therefore be less
+than the speed of a point at the surface overhead. This can be
+definitely proved by dropping a heavy object down a mine shaft. The
+object, which starts with the greater speed of the surface, will, when
+it reaches the bottom of the mine, be found, as might be indeed
+expected, to be a little ahead (_i.e._ to the east) of the point which
+originally lay exactly underneath it. The distance by which the object
+gains upon this point is, however, very small. In our latitudes it
+amounts to about an inch in a fall of 500 feet.
+
+The great speed at which, as we have seen, the equatorial regions of
+the earth are moving, should result in giving to the matter there
+situated a certain tendency to fly outwards. Sir Isaac Newton was the
+first to appreciate this point, and he concluded from it that the earth
+must be _bulged_ a little all round the equator. This is, indeed, found
+to be the case, the diameter at the equator being nearly twenty-seven
+miles greater than it is from pole to pole. The reader will, no doubt,
+be here reminded of the familiar comparison in geographies between the
+shape of the earth and that of an orange.
+
+In this connection it is interesting to consider that, were the earth to
+rotate seventeen times as fast as it does (_i.e._ in one hour
+twenty-five minutes, instead of twenty-four hours), bodies at the
+equator would have such a strong tendency to fly outwards that the force
+of terrestrial gravity acting upon them would just be counterpoised, and
+they would virtually have _no weight_. And, further, were the earth to
+rotate a little faster still, objects lying loose upon its surface would
+be shot off into space.
+
+The earth is, therefore, what is technically known as an _oblate
+spheroid_; that is, a body of spherical shape flattened at the poles. It
+follows of course from this, that objects at the polar regions are
+slightly nearer to the earth's centre than objects at the equatorial
+regions. We have already seen that gravitation acts from the central
+parts of a body, and that its force is greater the nearer are those
+central parts. The result of this upon our earth will plainly be that
+objects in the polar regions will be pulled with a slightly stronger
+pull, and will therefore _weigh_ a trifle more than objects in the
+equatorial regions. This is, indeed, found by actual experiment to be
+the case. As an example of the difference in question, Professor Young,
+in his _Manual of Astronomy_, points out that a man who weighs 190
+pounds at the equator would weigh 191 at the pole. In such an experiment
+the weighing would, however, have to be made with a _spring balance_,
+and _not with scales_; for, in the latter case, the "weights" used would
+alter in their weight in exactly the same degree as the objects to be
+weighed.
+
+It used to be thought that the earth was composed of a relatively thin
+crust, with a molten interior. Scientific men now believe, on the other
+hand, that such a condition cannot after all prevail, and that the earth
+must be more or less solid all through, except perhaps in certain
+isolated places where collections of molten matter may exist.
+
+The _atmosphere_, or air which we breathe, is in the form of a layer of
+limited depth which closely envelops the earth. Actually, it is a
+mixture of several gases, the most important being nitrogen and oxygen,
+which between them practically make up the air, for the proportion of
+the other gases, the chief of which is carbonic acid gas, is exceedingly
+small.
+
+It is hard to picture our earth, as we know it, without this atmosphere.
+Deprived of it, men at once would die; but even if they could be made to
+go on living without it by any miraculous means, they would be like unto
+deaf beings, for they would never hear any sound. What we call _sounds_
+are merely vibrations set up in the air, which travel along and strike
+upon the drum of the ear.
+
+The atmosphere is densest near the surface of the earth, and becomes
+less and less dense away from it, as a result of diminishing pressure of
+air from above. The greater portion of it is accumulated within four or
+five miles of the earth's surface.
+
+It is impossible to determine exactly at what distance from the earth's
+surface the air ceases altogether, for it grows continually more and
+more rarefied. There are, however, two distinct methods of ascertaining
+the distance beyond which it can be said practically not to exist. One
+of these methods we get from twilight. Twilight is, in fact, merely
+light reflected to us from those upper regions of the air, which still
+continue to be illuminated by the sun after it has disappeared from our
+view below the horizon. The time during which twilight lasts, shows us
+that the atmosphere must be at least fifty miles high.
+
+But the most satisfactory method of ascertaining the height to which the
+atmosphere extends is from the observation of meteors. It is found that
+these bodies become ignited, by the friction of passing into the
+atmosphere, at a height of about 100 miles above the surface of the
+earth. We thus gather that the atmosphere has a certain degree of
+density even at this height. It may, indeed, extend as far as about 150
+miles.
+
+The layer of atmosphere surrounding our earth acts somewhat in the
+manner of the glass covering of a greenhouse, bottling in the sun's
+rays, and thus storing up their warmth for our benefit. Were this not
+so, the heat which we get from the sun would, after falling upon the
+earth, be quickly radiated again into space.
+
+It is owing to the unsteadiness of the air that stars are seen to
+twinkle. A night when this takes place, though it may please the average
+person, is worse than useless to the astronomer, for the unsteadiness is
+greatly magnified in the telescope. This twinkling is, no doubt, in a
+great measure responsible for the conventional "points" with which Art
+has elected to embellish stars, and which, of course, have no existence
+in fact.
+
+The phenomena of _Refraction_,[13] namely, that bending which rays of
+light undergo, when passing _slant-wise_ from a rare into a dense
+transparent medium, are very marked with regard to the atmosphere. The
+denser the medium into which such rays pass, the greater is this bending
+found to be. Since the layer of air around us becomes denser and denser
+towards the surface of the earth, it will readily be granted that the
+rays of light reaching our eyes from a celestial object, will suffer the
+greater bending the lower the object happens to be in the sky. Celestial
+objects, unless situated directly overhead, are thus not seen in their
+true places, and when nearest to the horizon are most out of place. The
+bending alluded to is upwards. Thus the sun and the moon, for instance,
+when we see them resting upon the horizon, are actually _entirely_
+beneath it.
+
+When the sun, too, is sinking towards the horizon, the lower edge of its
+disc will, for the above reason, look somewhat more raised than the
+upper. The result is a certain appearance of flattening; which may
+plainly be seen by any one who watches the orb at setting.
+
+In observations to determine the exact positions of celestial objects
+correction has to be made for the effects of refraction, according to
+the apparent elevation of these objects in the sky. Such effects are
+least when the objects in question are directly overhead, for then the
+rays of light, coming from them to the eye, enter the atmosphere
+perpendicularly, and not at any slant.
+
+A very curious effect, due to refraction, has occasionally been observed
+during a total eclipse of the moon. To produce an eclipse of this kind,
+_the earth must, of course, lie directly between the sun and the moon_.
+Therefore, when we see the shadow creeping over the moon's surface, the
+sun should actually be well below the horizon. But when a lunar eclipse
+happens to come on just about sunset, the sun, although really sunk
+below the horizon, appears still above it through refraction, and the
+eclipsed moon, situated, of course, exactly opposite to it in the sky,
+is also lifted up above the horizon by the same cause. Pliny, writing in
+the first century of the Christian era, describes an eclipse of this
+kind, and refers to it as a "prodigy." The phenomenon is known as a
+"horizontal eclipse." It was, no doubt, partly owing to it that the
+ancients took so long to decide that an eclipse of the moon was really
+caused by the shadow cast by the earth. Plutarch, indeed, remarks that
+it was easy enough to understand that a solar eclipse was caused by the
+interposition of the moon, but that one could not imagine by the
+interposition _of what body_ the moon itself could be eclipsed.
+
+In that apparent movement of the heavens about the earth, which men now
+know to be caused by the mere rotation of the earth itself, a slight
+change is observed to be continually taking place. The stars, indeed,
+are always found to be gradually drawing westward, _i.e._ towards the
+sun, and losing themselves one after the other in the blaze of his
+light, only to reappear, however, on the other side of him after a
+certain lapse of time. This is equivalent to saying that the sun itself
+seems always creeping slowly _eastward_ in the heaven. The rate at which
+this appears to take place is such that the sun finds itself back again
+to its original position, with regard to the starry background, at the
+end of a year's time. In other words, the sun seems to make a complete
+tour of the heavens in the course of a year. Here, however, we have
+another illusion, just as the daily movement of the sky around the earth
+was an illusion. The truth indeed is, that this apparent movement of the
+sun eastward among the stars during a year, arises merely from a
+_continuous displacement of his position_ caused by an actual motion of
+the earth itself around him in that very time. In a word, it is the
+earth which really moves around the sun, and not the sun around the
+earth.
+
+The stress laid upon this fundamental point by Copernicus, marks the
+separation of the modern from the ancient view. Not that Copernicus,
+indeed, had obtained any real proof that the earth is merely a planet
+revolving around the sun; but it seemed to his profound intellect that a
+movement of this kind on the part of our globe was the more likely
+explanation of the celestial riddle. The idea was not new; for, as we
+have already seen, certain of the ancient Greeks (Aristarchus of Samos,
+for example) had held such a view; but their notions on the subject were
+very fanciful, and unsupported by any good argument.
+
+What Copernicus, however, really seems to have done was to _insist_ upon
+the idea that the sun occupied the _centre_, as being more consonant
+with common sense. No doubt, he was led to take up this position by the
+fact that the sun appeared entirely of a different character from the
+other members of the system. The one body in the scheme, which performed
+the important function of dispenser of light and heat, would indeed be
+more likely to occupy a position apart from the rest; and what position
+more appropriate for its purposes than the centre!
+
+But here Copernicus only partially solved the difficult question. He
+unfortunately still clung to an ancient belief, which as yet remained
+unquestioned; _i.e._ the great virtue, one might almost say, the
+_divineness_, of circular motion. The ancients had been hag-ridden, so
+to speak, by the circle; and it appeared to them that such a perfectly
+formed curve was alone fitted for the celestial motions. Ptolemy
+employed it throughout his system. According to him the "planets" (which
+included, under the ancient view, both the sun and the moon), moved
+around the earth in circles; but, as their changing positions in the sky
+could not be altogether accounted for in this way, it was further
+supposed that they performed additional circular movements, around
+peculiarly placed centres, during the course of their orbital
+revolutions. Thus the Ptolemaic system grew to be extremely
+complicated; for astronomers did not hesitate to add new circular
+movements whenever the celestial positions calculated for the planets
+were found not to tally with the positions observed. In this manner,
+indeed, they succeeded in doctoring the theory, so that it fairly
+satisfied the observations made with the rough instruments of
+pre-telescopic times.
+
+Although Copernicus performed the immense service to astronomy of boldly
+directing general attention to the central position of the sun, he
+unfortunately took over for the new scheme the circular machinery of the
+Ptolemaic system. It therefore remained for the famous Kepler, who lived
+about a century after him, to find the complete solution. Just as
+Copernicus, for instance, had broken free from tradition with regard to
+the place of the sun; so did Kepler, in turn, break free from the spell
+of circular motion, and thus set the coping-stone to the new
+astronomical edifice. This astronomer showed, in fact, that if the paths
+of the planets around the sun, and of the moon around the earth, were
+not circles, but _ellipses_, the movements of these bodies about the sky
+could be correctly accounted for. The extreme simplicity of such an
+arrangement was far more acceptable than the bewildering intricacy of
+movement required by the Ptolemaic theory. The Copernican system, as
+amended by Kepler, therefore carried the day; and was further
+strengthened, as we have already seen, by the telescopic observations of
+Galileo and the researches of Newton into the effects of gravitation.
+
+And here a word on the circle, now fallen from its high estate. The
+ancients were in error in supposing that it stood entirely apart--the
+curve of curves. As a matter of fact it is merely _a special kind of
+ellipse_. To put it paradoxically, it is an ellipse which has no
+ellipticity, an oval without any ovalness!
+
+Notwithstanding all this, astronomy had to wait yet a long time for a
+definite proof of the revolution of the earth around the sun. The
+leading argument advanced by Aristotle, against the reality of any
+movement of the earth, still held good up to about seventy years ago.
+That philosopher had pointed out that the earth could not move about in
+space to any great extent, or the stars would be found to alter their
+apparent places in the sky, a thing which had never been observed to
+happen. Centuries ran on, and instruments became more and more perfect,
+yet no displacements of stars were noted. In accepting the Copernican
+theory men were therefore obliged to suppose these objects as
+immeasurably distant. At length, however, between the years 1835 and
+1840, it was discovered by the Prussian astronomer, Bessel, that a star
+known as 61 Cygni--that is to say, the star marked in celestial atlases
+as No. 61 in the constellation of the Swan--appeared, during the course
+of a year, to perform a tiny circle in the heavens, such as would result
+from a movement on our own part around the sun. Since then about
+forty-three stars have been found to show minute displacements of a
+similar kind, which cannot be accounted for upon any other supposition
+than that of a continuous revolution of the earth around the sun. The
+triumph of the Copernican system is now at last supreme.
+
+If the axis of the earth stood "straight up," so to speak, while the
+earth revolved in its orbit, the sun would plainly keep always on a
+level with the equator. This is equivalent to stating that, in such
+circumstances, a person at the equator would see it rise each morning
+exactly in the east, pass through the _zenith_, that is, the point
+directly overhead of him, at midday, and set in the evening due in the
+west. As this would go on unchangingly at the equator every day
+throughout the year, it should be clear that, at any particular place
+upon the earth, the sun would in these conditions always be seen to move
+in an unvarying manner across the sky at a certain altitude depending
+upon the latitude of the place. Thus the more north one went upon the
+earth's surface, the more southerly in the sky would the sun's path lie;
+while at the north pole itself, the sun would always run round and round
+the horizon. Similarly, the more south one went from the equator the
+more northerly would the path of the sun lie, while at the south pole it
+would be seen to skirt the horizon in the same manner as at the north
+pole. The result of such an arrangement would be, that each place upon
+the earth would always have one unvarying climate; in which case there
+would not exist any of those beneficial changes of season to which we
+owe so much.
+
+The changes of season, which we fortunately experience, are due,
+however, to the fact that the sun does not appear to move across the sky
+each day at one unvarying altitude, but is continually altering the
+position of its path; so that at one period of the year it passes across
+the sky low down, and remains above the horizon for a short time only,
+while at another it moves high up across the heavens, and is above the
+horizon for a much longer time. Actually, the sun seems little by little
+to creep up the sky during one half of the year, namely, from mid-winter
+to mid-summer, and then, just as gradually, to slip down it again during
+the other half, namely, from mid-summer to mid-winter. It will therefore
+be clear that every region of the earth is much more thoroughly warmed
+during one portion of the year than during another, _i.e._ when the
+sun's path is high in the heavens than when it is low down.
+
+Once more we find appearances exactly the contrary from the truth. The
+earth is in this case the real cause of the deception, just as it was in
+the other cases. The sun does not actually creep slowly up the sky, and
+then slowly dip down it again, but, owing to the earth's axis being set
+aslant, different regions of the earth's surface are presented to the
+sun at different times. Thus, in one portion of its orbit, the northerly
+regions of the earth are presented to the sun, and in the other portion
+the southerly. It follows of course from this, that when it is summer in
+the northern hemisphere it is winter in the southern, and _vice vers�_
+(see Fig. 13, p. 176).
+
+[Illustration: FIG. 13.--Summer and Winter.]
+
+The fact that, in consequence of this slant of the earth's axis, the sun
+is for part of the year on the north side of the equator and part of the
+year on the south side, leads to a very peculiar result. The path of the
+moon around the earth is nearly on the same plane with the earth's path
+around the sun. The moon, therefore, always keeps to the same regions of
+the sky as the sun. The slant of the earth's axis thus regularly
+displaces the position of both the sun and the moon to the north and
+south sides of the equator respectively in the manner we have been
+describing. Were the earth, however, a perfect sphere, such change of
+position would not produce any effect. We have shown, however, that the
+earth is not a perfect sphere, but that it is bulged out all round the
+equator. The result is that this bulged-out portion swings slowly under
+the pulls of solar and lunar gravitation, in response to the
+displacements of the sun and moon to the north and to the south of it.
+This slow swing of the equatorial regions results, of course, in a
+certain slow change of the direction of the earth's axis, so that the
+north pole does not go on pointing continually to the same region of the
+sky. The change in the direction of the axis is, however, so extremely
+slight, that it shows up only after the lapse of ages. The north pole of
+the heavens, that is, the region of the sky towards which the north pole
+of the earth's axis points, displaces therefore extremely slowly,
+tracing out a wide circle, and arriving back again to the same position
+in the sky only after a period of about 25,000 years. At present the
+north pole of the heavens is quite close to a bright star in the tail of
+the constellation of the Little Bear, which is consequently known as the
+Pole Star; but in early Greek times it was at least ten times as far
+away from this star as it is now. After some 12,000 years the pole will
+point to the constellation of Lyra, and Vega, the most brilliant star in
+that constellation, will then be considered as the pole star. This slow
+twisting of the earth's axis is technically known as _Precession_, or
+the _Precession of the Equinoxes_ (see Plate XIX., p. 292).
+
+The slow displacement of the celestial pole appears to have attracted
+the attention of men in very early times, but it was not until the
+second century B.C. that precession was established as a fact by the
+celebrated Greek astronomer, Hipparchus. For the ancients this strange
+cyclical movement had a mystic significance; and they looked towards the
+end of the period as the end, so to speak, of a "dispensation," after
+which the life of the universe would begin anew:--
+
+"Magnus ab integro s�clorum nascitur ordo.
+Jam redit et Virgo, redeunt Saturnia regna;
+   .      .      .      .      .      .
+Alter erit tum Tiphys, et altera qu� vehat Argo
+Delectos heroas; erunt etiam altera bella,
+Atque iterum ad Trojam magnus mittetur Achilles."
+
+We have seen that the orbit of the earth is an ellipse, and that the sun
+is situated at what is called the _focus_, a point not in the middle of
+the ellipse, but rather towards one of its ends. Therefore, during the
+course of the year the distance of the earth from the sun varies. The
+sun, in consequence of this, is about 3,000,000 miles _nearer_ to us in
+our northern _winter_ than it is in our northern summer, a statement
+which sounds somewhat paradoxical. This variation in distance, large as
+it appears in figures, can, however, not be productive of much
+alteration in the amount of solar heat which we receive, for during the
+first week in January, when the distance is least, the sun only looks
+about _one-eighteenth_ broader than at the commencement of July, when
+the distance is greatest. The great disparity in temperature between
+winter and summer depends, as we have seen, upon causes of quite another
+kind, and varies between such wide limits that the effects of this
+slight alteration in the distance of the sun from the earth may be
+neglected for practical purposes.
+
+The Tides are caused by the gravitational pull of the sun and moon upon
+the water of the earth's surface. Of the two, the moon, being so much
+the nearer, exerts the stronger pull, and therefore may be regarded as
+the chief cause of the tides. This pull always draws that portion of the
+water, which happens to be right underneath the moon at the time, into a
+heap; and there is also a _second_ heaping of water at the same moment
+_at the contrary side of the earth_, the reasons for which can be shown
+mathematically, but cannot be conveniently dealt with here.
+
+As the earth rotates on its axis each portion of its surface passes
+beneath the moon, and is swelled up by this pull; the watery portions
+being, however, the only ones to yield visibly. A similar swelling up,
+as we have seen, takes place at the point exactly away from the moon.
+Thus each portion of our globe is borne by the rotation through two
+"tide-areas" every day, and this is the reason why there are two tides
+during every twenty-four hours.
+
+The crest of the watery swelling is known as high tide. The journey of
+the moon around the earth takes about a month, and this brings her past
+each place in turn by about fifty minutes later each day, which is the
+reason why high tide is usually about twenty-five minutes later each
+time.
+
+The moon is, however, not the sole cause of the tides, but the sun, as
+we have said, has a part in the matter also. When it is new moon the
+gravitational attractions of both sun and moon are clearly acting
+together from precisely the same direction, and, therefore, the tide
+will be pulled up higher than at other times. At full moon, too, the
+same thing happens; for, although the bodies are now acting from
+opposite directions, they do not neutralise each other's pulls as one
+might imagine, since the sun, in the same manner as the moon, produces a
+tide both under it and also at the opposite side of the earth. Thus both
+these tides are actually increased in height. The exceptionally high
+tides which we experience at new and full moons are known as _Spring
+Tides_, in contradistinction to the minimum high tides, which are known
+as _Neap Tides_.
+
+The ancients appear to have had some idea of the cause of the tides. It
+is said that as early as 1000 B.C. the Chinese noticed that the moon
+exerted an influence upon the waters of the sea. The Greeks and Romans,
+too, had noticed the same thing; and C�sar tells us that when he was
+embarking his troops for Britain the tide was high _because_ the moon
+was full. Pliny went even further than this, in recognising a similar
+connection between the waters and the sun.
+
+From casual observation one is inclined to suppose that the high tide
+always rises many feet. But that this is not the case is evidenced by
+the fact that the tides in the midst of the great oceans are only from
+three to four feet high. However, in the seas and straits around our
+Isles, for instance, the tides rise very many feet indeed, but this is
+merely owing to the extra heaping up which the large volumes of water
+undergo in forcing their passage through narrow channels.
+
+As the earth, in rotating, is continually passing through these
+tide-areas, one might expect that the friction thus set up would tend to
+slow down the rotation itself. Such a slowing down, or "tidal drag," as
+it is called, is indeed continually going on; but the effects produced
+are so exceedingly minute that it will take many millions of years to
+make the rotation appreciably slower, and so to lengthen the day.
+
+Recently it has been proved that the axis of the earth is subject to a
+very small displacement, or rather, "wobbling," in the course of a
+period of somewhat over a year. As a consequence of this, the pole
+shifts its place through a circle of, roughly, a few yards in width
+during the time in question. This movement is, perhaps, the combined
+result of two causes. One of these is the change of place during the
+year of large masses of material upon our earth; such as occurs, for
+instance, when ice and snow melt, or when atmospheric and ocean
+currents transport from place to place great bodies of air and water.
+The other cause is supposed to be the fact that the earth is not
+absolutely rigid, and so yields to certain strains upon it. In the
+course of investigation of this latter point the interesting conclusion
+has been reached by the famous American astronomer, Professor Simon
+Newcomb, that our globe as a whole is _a little more rigid than steel_.
+
+We will bring this chapter to a close by alluding briefly to two strange
+appearances which are sometimes seen in our night skies. These are known
+respectively as the Zodiacal Light and the Gegenschein.
+
+The _Zodiacal Light_ is a faint cone-shaped illumination which is seen
+to extend upwards from the western horizon after evening twilight has
+ended, and from the eastern horizon before morning twilight has begun.
+It appears to rise into the sky from about the position where the sun
+would be at that time. The proper season of the year for observing it
+during the evening is in the spring, while in autumn it is best seen in
+the early morning. In our latitudes its light is not strong enough to
+render it visible when the moon is full, but in the tropics it is
+reported to be very bright, and easily seen in full moonlight. One
+theory regards it as the reflection of light from swarms of meteors
+revolving round the sun; another supposes it to be a very rarefied
+extension of the corona.
+
+The _Gegenschein_ (German for "counter-glow") is a faint oval patch of
+light, seen in the sky exactly opposite to the place of the sun. It is
+usually treated of in connection with the zodiacal light, and one theory
+regards it similarly as of meteoric origin. Another theory,
+however--that of Mr. Evershed--considers it a sort of _tail_ to the
+earth (like a comet's tail) composed of hydrogen and helium--the two
+_lightest_ gases we know--driven off from our planet in the direction
+contrary to the sun.
+
+
+[13] Every one knows the simple experiment in which a coin lying at the
+bottom of an empty basin, and hidden from the eye by its side, becomes
+visible when a certain quantity of water has been poured in. This is an
+example of refraction. The rays of light coming from the coin ought
+_not_ to reach the eye, on account of the basin's side being in the way;
+yet by the action of the water they are _refracted_, or bent over its
+edge, in such a manner that they do.
+
+
+
+
+CHAPTER XVI
+
+THE MOON
+
+
+What we call the moon's "phases" are merely the various ways in which we
+see the sun shining upon her surface during the course of her monthly
+revolutions around the earth (see Fig. 14, p. 184). When she passes in
+the neighbourhood of the sun all his light falls upon that side which is
+turned away from us, and so the side which is turned towards us is
+unillumined, and therefore invisible. When in this position the moon is
+spoken of as _new_.
+
+As she continues her motion around the earth, she draws gradually to the
+east of the sun's place in the sky. The sunlight then comes somewhat
+from the side; and so we see a small portion of the right side of the
+lunar disc illuminated. This is the phase known as the _crescent_ moon.
+
+As she moves on in her orbit more and more of her illuminated surface is
+brought into view; and so the crescent of light becomes broader and
+broader, until we get what is called half-moon, or _first quarter_, when
+we see exactly one-half of her surface lit up by the sun's rays. As she
+draws still further round yet more of her illuminated surface is brought
+into view, until three-quarters of the disc appear lighted up. She is
+then said to be _gibbous_.
+
+Eventually she moves round so that she faces the sun completely, and
+the whole of her disc appears illuminated. She is then spoken of as
+_full_. When in this position it is clear that she is on the contrary
+side of the earth to the sun, and therefore rises about the same time
+that he is setting. She is now, in fact, at her furthest from the sun.
+
+[Illustration: Direction from which the sun's rays are coming.
+
+Various positions and illumination of the mooon by the sun during her
+revolution around the earth.
+
+The corresponding positions as viewed from the earth, showing the
+consequent phases.
+
+FIG. 14.--Orbit and Phases of the Moon.]
+
+After this, the motion of the moon in her orbit carries her on back
+again in the direction of the sun. She thus goes through her phases as
+before, only these of course are _in the reverse order_. The full phase
+is seen to give place to the gibbous, and this in turn to the half-moon
+and to the crescent; after which her motion carries her into the
+neighbourhood of the sun, and she is once more new, and lost to our
+sight in the solar glare. Following this she draws away to the east of
+the sun again, and the old order of phases repeat themselves as before.
+
+The early Babylonians imagined that the moon had a bright and a dark
+side, and that her phases were caused by the bright side coming more and
+more into view during her movement around the sky. The Greeks, notably
+Aristotle, set to work to examine the question from a mathematical
+standpoint, and came to the conclusion that the crescent and other
+appearances were such as would necessarily result if the moon were a
+dark body of spherical shape illumined merely by the light of the sun.
+
+Although the true explanation of the moon's phases has thus been known
+for centuries, it is unfortunately not unusual to see
+pictures--advertisement posters, for instance--in which stars appear
+_within_ the horns of a crescent moon! Can it be that there are to-day
+educated persons who believe that the moon is a thing which _grows_ to a
+certain size and then wastes away again; who, in fact, do not know that
+the entire body of the moon is there all the while?
+
+When the moon shows a very thin crescent, we are able dimly to see her
+still dark portion standing out against the sky. This appearance is
+popularly known as the "old moon in the new moon's arms." The dark part
+of her surface must, indeed, be to some degree illumined, or we should
+not be able to see it at all. Whence then comes the light which
+illumines it, since it clearly cannot come from the sun? The riddle is
+easily solved, if we consider what kind of view of our earth an observer
+situated on this darkened part of the moon would at that moment get. He
+would, as a matter of fact, just then see nearly the whole disc of the
+earth brightly lit up by sunlight. The lunar landscape all around would,
+therefore, be bathed in what to _him_ would be "earthlight," which of
+course takes the place there of what _we_ call moonlight. If, then, we
+recollect how much greater in size the earth is than the moon, it should
+not surprise us that this earthlight will be many times brighter than
+moonlight. It is considered, indeed, to be some twenty times brighter.
+It is thus not at all astonishing that we can see the dark portion of
+the moon illumined merely by sunlight reflected upon it from our earth.
+
+The ancients were greatly exercised in their minds to account for this
+"earthlight," or "earthshine," as it is also called. Posidonius (135-51
+B.C.) tried to explain it by supposing that the moon was partially
+transparent, and that some sunlight consequently filtered through from
+the other side. It was not, however, until the fifteenth century that
+the correct solution was arrived at.
+
+[Illustration: One side of the moon only is ever presented to the
+earth. This side is here indicated by the letters S.F.E. (side facing
+earth).
+
+By placing the above positions in a row, we can see at once that the
+moon makes one complete rotation on her axis in exactly the same time as
+she revolves around the earth.
+
+FIG. 15.--The Rotation of the Moon on her Axis.]
+
+Perhaps the most remarkable thing which one notices about the moon is
+that she always turns the same side towards us, and so we never see her
+other side. One might be led from this to jump to the conclusion that
+she does not rotate upon an axis, as do the other bodies which we see;
+but, paradoxical as it may appear, the fact that she turns one face
+always towards the earth, is actually a proof that she _does_ rotate
+upon an axis. The rotation, however, takes place with such slowness,
+that she turns round but once during the time in which she revolves
+around the earth (see Fig. 15). In order to understand the matter
+clearly, let the reader place an object in the centre of a room and walk
+around it once, _keeping his face turned towards it the whole time_,
+While he is doing this, it is evident that he will face every one of the
+four walls of the room in succession. Now in order to face each of the
+four walls of a room in succession one would be obliged _to turn oneself
+entirely round_. Therefore, during the act of walking round an object
+with his face turned directly towards it, a person at the same time
+turns his body once entirely round.
+
+In the long, long past the moon must have turned round much faster than
+this. Her rate of rotation has no doubt been slowed down by the action
+of some force. It will be recollected how, in the course of the previous
+chapter, we found that the tides were tending, though exceedingly
+gradually, to slow down the rotation of the earth upon its axis. But, on
+account of the earth's much greater mass, the force of gravitation
+exercised by it upon the surface of the moon is, of course, much more
+powerful than that which the moon exercises upon the surface of the
+earth. The tendency to tidal action on the moon itself must, therefore,
+be much in excess of anything which we here experience. It is, in
+consequence, probable that such a tidal drag, extending over a very long
+period of time, has resulted in slowing down the moon's rotation to its
+present rate.
+
+The fact that we never see but one side of the moon has given rise from
+time to time to fantastic speculations with regard to the other side.
+Some, indeed, have wished to imagine that our satellite is shaped like
+an egg, the more pointed end being directed away from us. We are here,
+of course, faced with a riddle, which is all the more tantalising from
+its appearing for ever insoluble to men, chained as they are to the
+earth. However, it seems going too far to suppose that any abnormal
+conditions necessarily exist at the other side of the moon. As a matter
+of fact, indeed, small portions of that side are brought into our view
+from time to time in consequence of slight irregularities in the moon's
+movement; and these portions differ in no way from those which we
+ordinarily see. On the whole, we obtain a view of about 60 per cent. of
+the entire lunar surface; that is to say, a good deal more than
+one-half.
+
+The actual diameter of the moon is about 2163 miles, which is somewhat
+more than one-quarter the diameter of the earth. For a satellite,
+therefore, she seems very large compared with her primary, the earth;
+when we consider that Jupiter's greatest satellite, although nearly
+twice as broad as our moon, has a diameter only one twenty-fifth that of
+Jupiter. Furthermore, the moon moves around the earth comparatively
+slowly, making only about thirteen revolutions during the entire year.
+Seen from space, therefore, she would not give the impression of a
+circling body, as other satellites do. Her revolutions are, indeed,
+relatively so very slow that she would appear rather like a smaller
+planet accompanying the earth in its orbit. In view of all this, some
+astronomers are inclined to regard the earth and moon rather as a
+"double planet" than as a system of planet and satellite.
+
+When the moon is full she attracts more attention perhaps than in any of
+her other phases. The moon, in order to be full, must needs be in that
+region of the heavens exactly opposite to the sun. The sun _appears_ to
+go once entirely round the sky in the course of a year, and the moon
+performs the same journey in the space of about a month. The moon, when
+full, having got half-way round this journey, occupies, therefore, that
+region of the sky which the sun itself will occupy half a year later.
+Thus in winter the full moon will be found roughly to occupy the sun's
+summer position in the sky, and in summer the sun's winter position. It
+therefore follows that the full moon in winter time is high up in the
+heavens, while in summer time it is low down. We thus get the greatest
+amount of full moonlight when it is the most needed.
+
+The great French astronomer, Laplace, being struck by the fact that the
+"lesser light" did not rule the night to anything like the same extent
+that the "greater light" ruled the day, set to work to examine the
+conditions under which it might have been made to do so. The result of
+his speculations showed that if the moon were removed to such a distance
+that she took a year instead of a month to revolve around the earth; and
+if she were started off in her orbit at full moon, she would always
+continue to remain full--a great advantage for us. Whewell, however,
+pointed out that in order to get the moon to move with the requisite
+degree of slowness, she would have to revolve so far from the earth that
+she would only look one-sixteenth as large as she does at present, which
+rather militates against the advantage Laplace had in mind! Finally,
+however, it was shown by M. Liouville, in 1845, that the position of a
+_perennial full moon_, such as Laplace dreamed of, would be
+unstable--that is to say, the body in question could not for long remain
+undisturbed in the situation suggested (see Fig. 16, p. 191).
+
+[Illustration: Various positions of Laplace's "Moon" with regard to the
+earth and sun during the course of a year.
+
+The same positions of Laplace's "Moon," arranged around the earth, show
+that it would make only one revolution in a year.
+
+FIG. 16.--Laplace's "Perennial Full Moon."]
+
+There is a well-known phenomenon called _harvest moon_, concerning the
+nature of which there seems to be much popular confusion. An idea in
+fact appears to prevail among a good many people that the moon is a
+harvest moon when, at rising, it looks bigger and redder than usual.
+Such an appearance has, however, nothing at all to say to the matter;
+for the moon always _looks_ larger when low down in the sky, and,
+furthermore, it usually looks red in the later months of the year, when
+there is more mist and fog about than there is in summer. What
+astronomers actually term the harvest moon is, indeed, something
+entirely different from this. About the month of September the slant at
+which the full moon comes up from below the horizon happens to be such
+that, during several evenings together, she _rises almost at the same
+hour_, instead of some fifty minutes later, as is usually the case. As
+the harvest is being gathered in about that time, it has come to be
+popularly considered that this is a provision of nature, according to
+which the sunlight is, during several evenings, replaced without delay
+by more or less full-moonlight, in order that harvesters may continue
+their work straight on into the night, and not be obliged to break off
+after sunset to wait until the moon rises. The same phenomenon is almost
+exactly repeated a month later, but by reason of the pursuits then
+carried on it is known as the "hunter's moon."
+
+In this connection should be mentioned that curious phenomenon above
+alluded to, and which seems to attract universal notice, namely, that
+the moon _looks much larger when near the horizon_--at its rising, for
+instance, than when higher up in the sky. This seeming enlargement is,
+however, by no means confined to the moon. That the sun also looks much
+larger when low down in the sky than when high up, seems to strike even
+the most casual watcher of a sunset. The same kind of effect will,
+indeed, be noted if close attention be paid to the stars themselves. A
+constellation, for instance, appears more spread out when low down in
+the sky than when high up. This enlargement of celestial objects when in
+the neighbourhood of the horizon is, however, only _apparent_ and not
+real. It must be entirely an _illusion_; for the most careful
+measurements of the discs of the sun and of the moon fail to show that
+the bodies are any larger when near the horizon than when high up in the
+sky. In fact, if there be any difference in measurements with regard to
+the moon, it will be found to be the other way round; for her disc, when
+carefully measured, is actually the slightest degree _greater_ when
+_high_ in the sky, than when low down. The reason for this is that, on
+account of the rotundity of the earth's surface, she is a trifle nearer
+the observer when overhead of him.
+
+This apparent enlargement of celestial objects, when low down in the
+sky, is granted on all sides to be an illusion; but although the
+question has been discussed with animation time out of mind, none of the
+explanations proposed can be said to have received unreserved
+acceptance. The one which usually figures in text-books is that we
+unconsciously compare the sun and moon, when low down in the sky, with
+the terrestrial objects in the same field of view, and are therefore
+inclined to exaggerate the size of these orbs. Some persons, on the
+other hand, imagine the illusion to have its source in the structure of
+the human eye; while others, again, put it down to the atmosphere,
+maintaining that the celestial objects in question _loom_ large in the
+thickened air near the horizon, in the same way that they do when viewed
+through fog or mist.
+
+The writer[14] ventures, however, to think that the illusion has its
+origin in our notion of the shape of the celestial vault. One would be
+inclined, indeed, to suppose that this vault ought to appear to us as
+the half of a hollow sphere; but he maintains that it does not so
+appear, as a consequence of the manner in which the eyes of men are set
+quite close together in their heads. If one looks, for instance, high up
+in the sky, the horizon cannot come within the field of view, and so
+there is nothing to make one think that the expanse then gazed upon is
+other than quite _flat_--let us say like the ceiling of a room. But, as
+the eyes are lowered, a portion of the _encircling_ horizon comes
+gradually into the field of view, and the region of the sky then gazed
+upon tends in consequence to assume a _hollowed-out_ form. From this it
+would seem that our idea of the shape of the celestial vault is, that it
+is _flattened down over our heads and hollowed out all around in the
+neighbourhood of the horizon_ (see Fig. 17, p. 195). Now, as a
+consequence of their very great distance, all the objects in the heavens
+necessarily appear to us to move as if they were placed on the
+background of the vault; the result being that the mind is obliged to
+conceive them as expanded or contracted, in its unconscious attempts to
+make them always fill their due proportion of space in the various parts
+of this abnormally shaped sky.
+
+From such considerations the writer concludes that the apparent
+enlargement in question is merely the natural consequence of the idea we
+have of the shape of the celestial vault--an idea gradually built up in
+childhood, to become later on what is called "second nature." And in
+support of this contention, he would point to the fact that the
+enlargement is not by any means confined to the sun and moon, but is
+every whit as marked in the case of the constellations. To one who has
+not noticed this before, it is really quite a revelation to compare the
+appearance of one of the large constellations (Orion, for instance) when
+high up in the sky and when low down. The widening apart of the various
+stars composing the group, when in the latter position, is very
+noticeable indeed.
+
+[Illustration: FIG. 17.--Illustrating the author's explanation of the
+apparent enlargement of celestial objects.]
+
+Further, if a person were to stand in the centre of a large dome, he
+would be exactly situated as if he were beneath the vaulted heaven, and
+one would consequently expect him to suffer the same illusion as to the
+shape of the dome. Objects fixed upon its background would therefore
+appear to him under the same conditions as objects in the sky, and the
+illusions as to their apparent enlargement should hold good here also.
+
+Some years ago a Belgian astronomer, M. Stroobant, in an investigation
+of the matter at issue, chanced to make a series of experiments under
+the very conditions just detailed. To various portions of the inner
+surface of a large dome he attached pairs of electric lights; and on
+placing himself at the centre of the building, he noticed that, in every
+case, those pairs which were high up appeared closer together than those
+which were low down! He does not, however, seem to have sought for the
+cause in the vaulted expanse. On the contrary, he attributed the effect
+to something connected with our upright stature, to some physiological
+reason which regularly makes us estimate objects as larger when in front
+than when overhead.
+
+In connection with this matter, it may be noted that it always appears
+extremely difficult to estimate with the eye the exact height above the
+horizon at which any object (say a star) happens to be. Even skilled
+observers find themselves in error in attempting to do so. This seems to
+bear out the writer's contention that the form under which the celestial
+vault really appears to us is a peculiar one, and tends to give rise to
+false judgments.
+
+Before leaving this question, it should also be mentioned that nothing
+perhaps is more deceptive than the size which objects in the sky appear
+to present. The full moon looks so like a huge plate, that it astonishes
+one to find that a threepenny bit held at arm's length will a long way
+more than cover its disc.
+
+[Illustration: PLATE VIII. THE MOON
+
+From a photograph taken at the Paris Observatory by M.P. Puiseux.
+
+(Page 197)]
+
+The moon is just too far off to allow us to see the actual detail on
+her surface with the naked eye. When thus viewed she merely displays a
+patchy appearance,[15] and the imaginary forms which her darker markings
+suggest to the fancy are popularly expressed by the term "Man in the
+Moon." An examination of her surface with very moderate optical aid is,
+however, quite a revelation, and the view we then get is not easily
+comparable to what we see with the unaided eye.
+
+Even with an ordinary opera-glass, an observer will be able to note a
+good deal of detail upon the lunar disc. If it be his first observation
+of the kind, he cannot fail to be struck by the fact to which we have
+just made allusion, namely, the great change which the moon appears to
+undergo when viewed with magnifying power. "Cain and his Dog," the "Man
+in the Moon gathering sticks," or whatever indeed his fancy was wont to
+conjure up from the lights and shades upon the shining surface, have now
+completely disappeared; and he sees instead a silvery globe marked here
+and there with extensive dark areas, and pitted all over with
+crater-like formations (see Plate VIII., p. 196). The dark areas retain
+even to the present day their ancient name of "seas," for Galileo and
+the early telescopic observers believed them to be such, and they are
+still catalogued under the mystic appellations given to them in the long
+ago; as, for instance, "Sea of Showers," "Bay of Rainbows," "Lake of
+Dreams."[16] The improved telescopes of later times showed, however,
+that they were not really seas (there is no water on the moon), but
+merely areas of darker material.
+
+The crater-like formations above alluded to are the "lunar mountains." A
+person examining the moon for the first time with telescopic aid, will
+perhaps not at once grasp the fact that his view of lunar mountains must
+needs be what is called a "bird's-eye" one, namely, a view from above,
+like that from a balloon and that he cannot, of course, expect to see
+them from the side, as he does the mountains upon the earth. But once he
+has realised this novel point of view, he will no doubt marvel at the
+formations which lie scattered as it were at his feet. The type of lunar
+mountain is indeed in striking contrast to the terrestrial type. On our
+earth the range-formation is supreme; on the moon the crater-formation
+is the rule, and is so-called from analogy to our volcanoes. A typical
+lunar crater may be described as a circular wall, enclosing a central
+plain, or "floor," which is often much depressed below the level of the
+surface outside. These so-called "craters," or "ring-mountains," as they
+are also termed, are often of gigantic proportions. For instance, the
+central plain of one of them, known as Ptolem�us,[17] is about 115 miles
+across, while that of Plato is about 60. The walls of craters often rise
+to great heights; which, in proportion to the small size of the moon,
+are very much in excess of our highest terrestrial elevations.
+Nevertheless, a person posted at the centre of one of the larger craters
+might be surprised to find that he could not see the encompassing
+crater-walls, which would in every direction be below his horizon. This
+would arise not alone from the great breadth of the crater itself, but
+also from the fact that the curving of the moon's surface is very sharp
+compared with that of our earth.
+
+[Illustration: PLATE IX. MAP OF THE MOON, SHOWING THE PRINCIPAL
+"CRATERS," MOUNTAIN RANGES, AND "SEAS"
+
+In this, as in the other plates of the Moon, the _South_ will be found
+at the top of the picture; such being the view given by the ordinary
+astronomical telescope, in which all objects are seen _inverted_.
+
+(Page 199)]
+
+We have mentioned Ptolem�us as among the very large craters, or
+ring-mountains, on the moon. Its encompassing walls rise to nearly
+13,000 feet, and it has the further distinction of being almost in the
+centre of the lunar disc. There are, however, several others much wider,
+but they are by no means in such a conspicuous position. For instance,
+Schickard, close to the south-eastern border, is nearly 130 miles in
+diameter, and its wall rises in one point to over 10,000 feet. Grimaldi,
+almost exactly at the east point, is nearly as large as Schickard.
+Another crater, Clavius, situated near the south point, is about 140
+miles across; while its neighbour Bailly--named after a famous French
+astronomer of the eighteenth century--is 180, and the largest of those
+which we can see (see Plate IX., p. 198).
+
+Many of the lunar craters encroach upon one another; in fact there is
+not really room for them all upon the visible hemisphere of the moon.
+About 30,000 have been mapped; but this is only a small portion, for
+according to the American astronomer, Professor W.H. Pickering, there
+are more than 200,000 in all.
+
+Notwithstanding the fact that the crater is the type of mountain
+associated in the mind with the moon, it must not be imagined that upon
+our satellite there are no mountains at all of the terrestrial type.
+There are indeed many isolated peaks, but strangely enough they are
+nearly always to be found in the centres of craters. Some of these peaks
+are of great altitude, that in the centre of the crater Copernicus being
+over 11,000 feet high. A few mountain ranges also exist; the best known
+of which are styled, the Lunar Alps and Lunar Apennines (see Plate X.,
+p. 200).
+
+Since the _mass_ of the moon is only about one-eightieth that of the
+earth, it will be understood that the force of gravity which she
+exercises is much less. It is calculated that, at her surface, this is
+only about one-sixth of what we experience. A man transported to the
+moon would thus be able to jump _six times as high_ as he can here. A
+building could therefore be six times as tall as upon our earth, without
+causing any more strain upon its foundations. It should not, then, be
+any subject for wonder, that the highest peaks in the Lunar Apennines
+attain to such heights as 22,000 feet. Such a height, upon a
+comparatively small body like the moon, for her _volume_ is only
+one-fiftieth that of the earth, is relatively very much in excess of the
+29,000 feet of Himalayan structure, Mount Everest, the boast of our
+planet, 8000 miles across!
+
+High as are the Lunar Apennines, the highest peaks on the moon are yet
+not found among them. There is, for instance, on the extreme southern
+edge of the lunar disc, a range known as the Leibnitz Mountains; several
+peaks of which rise to a height of nearly 30,000 feet, one peak in
+particular being said to attain to 36,000 feet (see Plate IX., p. 198).
+
+[Illustration: PLATE X. ONE OF THE MOST INTERESTING REGIONS ON THE MOON
+
+We have here (see "Map," Plate IX., p. 198) the mountain ranges of the
+Apennines, the Caucasus and the Alps; also the craters Plato, Aristotle,
+Eudoxus, Cassini, Aristillus, Autolycus, Archimedes and Linn�. The
+crater Linn� is the very bright spot in the dark area at the upper left
+hand side of the picture. From a photograph taken at the Paris
+Observatory by M.M. Loewy and Puiseux.
+
+(Page 200)]
+
+But the reader will surely ask the question: "How is it possible to
+determine the actual height of a lunar mountain, if one cannot go upon
+the moon to measure it?" The answer is, that we can calculate its height
+from noting the length of the shadow which it casts. Any one will allow
+that the length of a shadow cast by the sun depends upon two things:
+firstly, upon the height of the object which causes the shadow, and
+secondly, upon the elevation of the sun at the moment in the sky. The
+most casual observer of nature upon our earth can scarcely have failed
+to notice that shadows are shortest at noonday, when the sun is at its
+highest in the sky; and that they lengthen out as the sun declines
+towards its setting. Here, then, we have the clue. To ascertain,
+therefore, the height of a lunar mountain, we have first to consider at
+what elevation the sun is at that moment above the horizon of the place
+where the mountain in question is situated. Then, having measured the
+actual length in miles of the shadow extended before us, all that is
+left is to ask ourselves the question: "What height must an object be
+whose shadow cast by the sun, when at that elevation in the sky, will
+extend to this length?"
+
+There is no trace whatever of water upon the moon. The opinion, indeed,
+which seems generally held, is that water has never existed upon its
+surface. Erosions, sedimentary deposits, and all those marks which point
+to a former occupation by water are notably absent.
+
+Similarly there appears to be no atmosphere on the moon; or, at any
+rate, such an excessively rare one, as to be quite inappreciable. Of
+this there are several proofs. For instance, in a solar eclipse the
+moon's disc always stands out quite clear-cut against that of the sun.
+Again during occultations, stars disappear behind the moon with a
+suddenness, which could not be the case were there any appreciable
+atmosphere. Lastly, we see no traces of twilight upon the lunar surface,
+nor any softening at the edges of shadows; both which effects would be
+apparent if there were an atmosphere.
+
+The moon's surface is rough and rocky, and displays no marks of the
+"weathering" that would necessarily follow, had it possessed anything of
+an atmosphere in the past. This makes us rather inclined to doubt that
+it ever had one at all. Supposing, however, that it did possess an
+atmosphere in the past, it is interesting to inquire what may have
+become of it. In the first place it might have gradually disappeared, in
+consequence of the gases which composed it uniting chemically with the
+materials of which the lunar body is constructed; or, again, its
+constituent gases may have escaped into space, in accordance with the
+principles of that kinetic theory of which we have already spoken. The
+latter solution seems, indeed, the most reasonable of the two, for the
+force of gravity at the lunar surface appears too weak to hold down any
+known gases. This argument seems also to dispose of the question of
+absence of water; for Dr. George Johnstone Stoney, in a careful
+investigation of the subject, has shown that the liquid in question,
+when in the form of vapour, will escape from a planet if its mass is
+less than _one-fourth_ that of our earth. And the mass of the moon is
+very much less than this; indeed only the _one-eightieth_, as we have
+already stated.
+
+In consequence of this lack of atmosphere, the condition of things upon
+the moon will be in marked contrast to what we experience upon the
+earth. The atmosphere here performs a double service in shielding us
+from the direct rays of the sun, and in bottling the heat as a
+glass-house does. On the moon, however, the sun beats down in the
+day-time with a merciless force; but its rays are reflected away from
+the surface as quickly as they are received, and so the cold of the
+lunar night is excessive. It has been calculated that the day
+temperature on the moon may, indeed, be as high as our boiling-point,
+while the night temperature may be more than twice as low as the
+greatest cold known in our arctic regions.
+
+That a certain amount of solar heat is reflected to us from the moon is
+shown by the sharp drop in temperature which certain heat-measuring
+instruments record when the moon becomes obscured in a lunar eclipse.
+The solar heat which is thus reflected to us by the moon is, however, on
+the whole extremely small; more light and heat, indeed, reach us
+_direct_ from the sun in half a minute than we get by _reflection_ from
+the moon during the entire course of the year.
+
+With regard to the origin of the lunar craters there has been much
+discussion. Some have considered them to be evidence of violent volcanic
+action in the dim past; others, again, as the result of the impact of
+meteorites upon the lunar surface, when the moon was still in a plastic
+condition; while a third theory holds that they were formed by the
+bursting of huge bubbles during the escape into space of gases from the
+interior. The question is, indeed, a very difficult one. Though
+volcanic action, such as would result in craters of the size of
+Ptolem�us, is hard for us to picture, and though the lone peaks which
+adorn the centres of many craters have nothing reminiscent of them in
+our terrestrial volcanoes, nevertheless the volcanic theory seems to
+receive more favour than the others.
+
+In addition to the craters there are two more features which demand
+notice, namely, what are known as _rays_ and _rills_. The rays are long,
+light-coloured streaks which radiate from several of the large craters,
+and extend to a distance of some hundreds of miles. That they are mere
+markings on the surface is proved by the fact that they cast no shadows
+of any kind. One theory is, that they were originally great cracks which
+have been filled with lighter coloured material, welling up from
+beneath. The rills, on the other hand, are actually fissures, about a
+mile or so in width and about a quarter of a mile in depth.
+
+The rays are seen to the best advantage in connection with the craters
+Tycho and Copernicus (see Plate XI., p. 204). In consequence of its
+fairly forward position on the lunar disc, and of the remarkable system
+of rays which issue from it like spokes from the axle of a wheel, Tycho
+commands especial attention. The late Rev. T.W. Webb, a famous observer,
+christened it, very happily, the "metropolitan crater of the moon."
+
+[Illustration: PLATE XI. THE MOON
+
+The systems of rays from the craters Tycho, Copernicus and Kepler are
+well shown here. From a photograph taken at the Paris Observatory by
+M.P. Puiseux.
+
+(Page 204)]
+
+A great deal of attention is, and has been, paid by certain astronomers
+to the moon, in the hope of finding out if any changes are actually in
+progress at present upon her surface. Sir William Herschel, indeed, once
+thought that he saw a lunar volcano in eruption, but this proved to be
+merely the effect of the sunlight striking the top of the crater
+Aristarchus, while the region around it was still in shadow--sunrise
+upon Aristarchus, in fact! No change of any real importance has,
+however, been noted, although it is suspected that some minor
+alterations have from time to time taken place. For instance, slight
+variations of tint have been noticed in certain areas of the lunar
+surface. Professor W.H. Pickering puts forward the conjecture that these
+may be caused by the growth and decay of some low form of vegetation,
+brought into existence by vapours of water, or carbonic acid gas, making
+their way out from the interior through cracks near at hand.
+
+Again, during the last hundred years one small crater known as Linn�
+(Linn�us), situated in the Mare Serenitatis (Sea of Serenity), has
+appeared to undergo slight changes, and is even said to have been
+invisible for a while (see Plate X., p. 200). It is, however, believed
+that the changes in question may be due to the varying angles at which
+the sunlight falls upon the crater; for it is an understood fact that
+the irregularities of the moon's motion give us views of her surface
+which always differ slightly.
+
+The suggestion has more than once been put forward that the surface of
+the moon is covered with a thick layer of ice. This is generally
+considered improbable, and consequently the idea has received very
+little support. It first originated with the late Mr. S.E. Peal, an
+English observer of the moon, and has recently been resuscitated by the
+German observer, Herr Fauth.
+
+The most unfavourable time for telescopic study of the moon is when she
+is full. The sunlight is then falling directly upon her visible
+hemisphere, and so the mountains cast no shadows. We thus do not get
+that impression of hill and hollow which is so very noticeable in the
+other phases.
+
+The first map of the moon was constructed by Galileo. Tobias Mayer
+published another in 1775; while during the nineteenth century greatly
+improved ones were made by Beer and M�dler, Schmidt, Neison and others.
+In 1903, Professor W.H. Pickering brought out a complete photographic
+lunar atlas; and a similar publication has recently appeared, the work
+of MM. Loewy and Puiseux of the Observatory of Paris.
+
+The so-called "seas" of the moon are, as we have seen, merely dark
+areas, and there appears to be no proof that they were ever occupied by
+any liquid. They are for the most part found in the _northern_ portion
+of the moon; a striking contrast to our seas and oceans, which take up
+so much of the _southern_ hemisphere of the earth.
+
+There are many erroneous ideas popularly held with regard to certain
+influences which the moon is supposed to exercise upon the earth. For
+instance, a change in the weather is widely believed to depend upon a
+change in the moon. But the word "change" as here used is meaningless,
+for the moon is continually changing her phase during the whole of her
+monthly round. Besides, the moon is visible over a great portion of the
+earth _at the same moment_, and certainly all the places from which it
+can then be seen do not get the same weather! Further, careful
+observations, and records extending over the past one hundred years and
+more, fail to show any reliable connection between the phases of the
+moon and the condition of the weather.
+
+It has been stated, on very good authority, that no telescope ever shows
+the surface of the moon as clearly as we could see it with the naked eye
+were it only 240 miles distant from us.
+
+Supposing, then, that we were able to approach our satellite, and view
+it without optical aid at such comparatively close quarters, it is
+interesting to consider what would be the smallest detail which our eye
+could take in. The question of the limit of what can be appreciated with
+the naked eye is somewhat uncertain, but it appears safe to say that at
+a distance of 240 miles the _minutest speck_ visible would have to be
+_at least_ some 60 yards across.
+
+Atmosphere and liquid both wanting, the lunar surface must be the seat
+of an eternal calm; where no sound breaks the stillness and where
+change, as we know it, does not exist. The sun beats down upon the arid
+rocks, and inky shadows lie athwart the valleys. There is no mellowing
+of the harsh contrasts.
+
+We cannot indeed absolutely affirm that Life has no place at all upon
+this airless and waterless globe, since we know not under what strange
+conditions it may manifest its presence; and our most powerful
+telescopes, besides, do not bring the lunar surface sufficiently near to
+us to disprove the existence there of even such large creatures as
+disport themselves upon our planet. Still, we find it hard to rid
+ourselves of the feeling that we are in the presence of a dead world. On
+she swings around the earth month after month, with one face ever
+turned towards us, leaving a certain mystery to hang around that hidden
+side, the greater part of which men can never hope to see. The rotation
+of the moon upon her axis--the lunar day--has become, as we have seen,
+equal to her revolution around the earth. An epoch may likewise
+eventually be reached in the history of our own planet, when the length
+of the terrestrial day has been so slowed down by tidal friction that it
+will be equal to the year. Then will the earth revolve around the
+central orb, with one side plunged in eternal night and the other in
+eternal sunshine. But such a vista need not immediately distress us. It
+is millions of years forward in time.
+
+
+[14] _Journal of the British Astronomical Association_, vol. x.
+(1899-1900), Nos. 1 and 3.
+
+[15] Certain of the ancient Greeks thought the markings on the moon to
+be merely the reflection of the seas and lands of our earth, as in a
+badly polished mirror.
+
+[16] Mare Imbrium, Sinus Iridum, Lacus Somniorum.
+
+[17] The lunar craters have, as a rule, received their names from
+celebrated persons, usually men of science. This system of nomenclature
+was originated by Riccioli, in 1651.
+
+
+
+
+CHAPTER XVII
+
+THE SUPERIOR PLANETS
+
+
+Having, in a previous chapter, noted the various aspects which an
+inferior planet presents to our view, in consequence of its orbit being
+nearer to the sun than the orbit of the earth, it will be well here to
+consider in the same way the case of a superior planet, and to mark
+carefully the difference.
+
+To begin with, it should be quite evident that we cannot ever have a
+transit of a superior planet. The orbit of such a body being entirely
+_outside_ that of the earth, the body itself can, of course, never pass
+between us and the sun.
+
+A superior planet will be at its greatest distance from us when on the
+far side of the sun. It is said then to be in _conjunction_. As it comes
+round in its orbit it eventually passes, so to speak, at the _back_ of
+us. It is then at its nearest, or in _opposition_, as this is
+technically termed, and therefore in the most favourable position for
+telescopic observation of its surface. Being, besides, seen by us at
+that time in the direction of the heavens exactly opposite to where the
+sun is, it will thus at midnight be high up in the south side of the
+sky, a further advantage to the observer.
+
+Last of all, a superior planet cannot show crescent shapes like an
+interior; for whether it be on the far side of the sun, or behind us,
+or again to our right or left, the sunlight must needs appear to fall
+more or less full upon its face.
+
+
+THE PLANETOID EROS
+
+The nearest to us of the superior planets is the tiny body, Eros, which,
+as has been already stated, was discovered so late as the year 1898. In
+point of view, however, of its small size, it can hardly be considered
+as a true planet, and the name "planetoid" seems much more appropriate
+to it.
+
+Eros was not discovered, like Uranus, in the course of telescopic
+examination of the heavens, nor yet, like Neptune, as the direct result
+of difficult calculations, but was revealed by the impress of its light
+upon a photographic plate, which had been exposed for some length of
+time to the starry sky. Since many of the more recent additions to the
+asteroids have been discovered in the same manner, we shall have
+somewhat more to say about this special employment of photography when
+we come to deal with those bodies later on.
+
+The path of Eros around the sun is so very elliptical, or, to use the
+exact technical term, so very "eccentric," that the planetoid does not
+keep all the time entirely in the space between our orbit and that of
+Mars, which latter happens to be the next body in the order of planetary
+succession outwards. In portions of its journey Eros, indeed, actually
+goes outside the Martian orbit. The paths of the planetoid and of Mars
+are, however, _not upon the same plane_, so the bodies always pass clear
+of each other, and there is thus as little chance of their dashing
+together as there would be of trains which run across a bridge at an
+upper level, colliding with those which pass beneath it at a lower
+level.
+
+When Eros is in opposition, it comes within about 13-1/2 million miles
+of our earth, and, after the moon, is therefore by a long way our
+nearest neighbour in space. It is, however, extremely small, not more,
+perhaps, than twenty miles in diameter, and is subject to marked
+variations in brightness, which do not appear up to the present to meet
+with a satisfactory explanation. But, insignificant as is this little
+body, it is of great importance to astronomy; for it happens to furnish
+the best method known of calculating the sun's distance from our
+earth--a method which Galle, in 1872, and Sir David Gill, in 1877,
+suggested that asteroids might be employed for, and which has in
+consequence supplanted the old one founded upon transits of Venus. The
+sun's distance is now an ascertained fact to within 100,000 miles, or
+less than half the distance of the moon.
+
+
+THE PLANET MARS
+
+We next come to the planet Mars. This body rotates in a period of
+slightly more than twenty-four hours. The inclination, or slant, of its
+axis is about the same as that of the earth, so that, putting aside its
+greater distance from the sun, the variations of season which it
+experiences ought to be very much like ours.
+
+The first marking detected upon Mars was the notable one called the
+Syrtis Major, also known, on account of its shape, as the Hour-Glass
+Sea. This observation was made by the famous Huyghens in 1659; and, from
+the movement of the marking in question across the disc, he inferred
+that the planet rotated on its axis in a period of about twenty-four
+hours.
+
+There appears to be very little atmosphere upon Mars, the result being
+that we almost always obtain a clear view of the detail on its surface.
+Indeed, it is only to be expected from the kinetic theory that Mars
+could not retain much of an atmosphere, as the force of gravity at its
+surface is less than one-half of what we experience upon the earth. It
+should here be mentioned that recent researches with the spectroscope
+seem to show that, whatever atmosphere there may be upon Mars, its
+density at the surface of the planet cannot be more than the one-fourth
+part of the density of the air at the surface of the earth. Professor
+Lowell, indeed, thinks it may be more rarefied than that upon our
+highest mountain-tops.
+
+Seen with the naked eye, Mars appears of a red colour. Viewed in the
+telescope, its surface is found to be in general of a ruddy hue, varied
+here and there with darker patches of a bluish-green colour. These
+markings are permanent, and were supposed by the early telescopic
+observers to imply a distribution of the planet's surface into land and
+water, the ruddy portions being considered as continental areas (perhaps
+sandy deserts), and the bluish-green as seas. The similarity to our
+earth thus suggested was further heightened by the fact that broad white
+caps, situated at the poles, were seen to vary with the planet's
+seasons, diminishing greatly in extent during the Martian summer (the
+southern cap in 1894 even disappearing altogether), and developing again
+in the Martian winter.[18] Readers of Oliver Wendell Holmes will no
+doubt recollect that poet's striking lines:--
+
+"The snows that glittered on the disc of Mars
+Have melted, and the planet's fiery orb
+Rolls in the crimson summer of its year."
+
+A state of things so strongly analogous to what we experience here,
+naturally fired the imaginations of men, and caused them to look on Mars
+as a world like ours, only upon a much smaller scale. Being smaller, it
+was concluded to have cooled quicker, and to be now long past its prime;
+and its "inhabitants" were, therefore, pictured as at a later stage of
+development than the inhabitants of our earth.
+
+Notwithstanding the strong temptation to assume that the whiteness of
+the Martian polar caps is due to fallen snow, such a solution is,
+however, by no means so simple as it looks. The deposition of water in
+the form of snow, or even of hoar frost, would at least imply that the
+atmosphere of Mars should now and then display traces of aqueous vapour,
+which it does not appear to do.[19] It has, indeed, been suggested that
+the whiteness may not after all be due to this cause, but to carbonic
+acid gas (carbon dioxide), which is known to freeze at a _very low_
+temperature. The suggestion is plainly based upon the assumption that,
+as Mars is so much further from the sun than we are, it would receive
+much less heat, and that the little thus received would be quickly
+radiated away into space through lack of atmosphere to bottle it in.
+
+We now come to those well-known markings, popularly known as the
+"canals" of Mars, which have been the subject of so much discussion
+since their discovery thirty years ago.
+
+It was, in fact, in the year 1877, when Mars was in opposition, and thus
+at its nearest to us, that the famous Italian astronomer, Schiaparelli,
+announced to the world that he had found that the ruddy areas, thought
+to be continents, were intersected by a network of straight dark lines.
+These lines, he reported, appeared in many cases to be of great length,
+so long, indeed, as several thousands of miles, and from about twenty to
+sixty miles in width. He christened the lines _channels_, the Italian
+word for which, "canali," was unfortunately translated into English as
+"canals." The analogy, thus accidentally suggested, gave rise to the
+idea that they might be actual waterways.[20]
+
+In the winter of 1881-1882, when Mars was again in opposition,
+Schiaparelli further announced that he had found some of these lines
+doubled; that is to say, certain of them were accompanied by similar
+lines running exactly parallel at no great distance away. There was at
+first a good deal of scepticism on the subject of Schiaparelli's
+discoveries, but gradually other observers found themselves seeing both
+the lines and their doublings. We have in this a good example of a
+curious circumstance in astronomical observation, namely, the fact that
+when fine detail has once been noted by a competent observer, it is not
+long before other observers see the same detail with ease.
+
+An immense amount of close attention has been paid to the planet Mars
+during recent years by the American observer, Professor Percival Lowell,
+at his famous observatory, 7300 feet above the sea, near the town of
+Flagstaff, Arizona, U.S.A. His observations have not, like those of most
+astronomers, been confined merely to "oppositions," but he has
+systematically kept the planet in view, so far as possible, since the
+year 1894.
+
+The instrumental equipment of his observatory is of the very best, and
+the "seeing" at Flagstaff is described as excellent. In support of the
+latter statement, Mr. Lampland, of the Lowell Observatory, maintains
+that the faintest stars shown on charts made at the Lick Observatory
+with the 36-inch telescope there, are _perfectly visible_ with the
+24-inch telescope at Flagstaff.
+
+Professor Lowell is, indeed, generally at issue with the other observers
+of Mars. He finds the canals extremely narrow and sharply defined, and
+he attributes the blurred and hazy appearance, which they have presented
+to other astronomers, to the unsteady and imperfect atmospheric
+conditions in which their observations have been made. He assigns to the
+thinnest a width of two or three miles, and from fifteen to twenty to
+the larger. Relatively to their width, however, he finds their length
+enormous. Many of them are 2000 miles long, while one is even as much
+as 3540. Such lengths as these are very great in comparison with the
+smallness of the planet. He considers that the canals stand in some
+peculiar relation to the polar cap, for they crowd together in its
+neighbourhood. In place, too, of ill-defined condensations, he sees
+sharp black spots where the canals meet and intersect, and to these he
+gives the name of "Oases." He further lays particular stress upon a dark
+band of a blue tint, which is always seen closely to surround the edges
+of the polar caps all the time that they are disappearing; and this he
+takes to be a proof that the white material is something which actually
+_melts_. Of all substances which we know, water alone, he affirms, would
+act in such a manner.
+
+The question of melting at all may seem strange in a planet which is
+situated so far from the sun, and possesses such a rarefied atmosphere.
+But Professor Lowell considers that this very thinness of the atmosphere
+allows the direct solar rays to fall with great intensity upon the
+planet's surface, and that this heating effect is accentuated by the
+great length of the Martian summer. In consequence he concludes that,
+although the general climate of Mars is decidedly cold, it is above the
+freezing point of water.
+
+The observations at Flagstaff appear to do away with the old idea that
+the darkish areas are seas, for numerous lines belonging to the
+so-called "canal system" are seen to traverse them. Again, there is no
+star-like image of the sun reflected from them, as there would be, of
+course, from the surface of a great sheet of water. Lastly, they are
+observed to vary in tone and colour with the changing Martian seasons,
+the blue-green changing into ochre, and later on back again into
+blue-green. Professor Lowell regards these areas as great tracts of
+vegetation, which are brought into activity as the liquid reaches them
+from the melting snows.
+
+[Illustration: PLATE XII. A MAP OF THE PLANET MARS
+
+We see here the Syrtis Major (or "Hour-Glass Sea"), the polar caps,
+several "oases," and a large number of "canals," some of which are
+double. The South is at the top of the picture, in accordance with the
+_inverted_ view given by an astronomical telescope. From a drawing by
+Professor Percival Lowell.
+
+(Page 216)]
+
+With respect to the canals, the Lowell observations further inform us
+that these are invisible during the Martian winter, but begin to appear
+in the spring when the polar cap is disappearing. Professor Lowell,
+therefore, inclines to the view that in the middle of the so-called
+canals there exist actual waterways which serve the purposes of
+irrigation, and that what we see is not the waterways themselves, for
+they are too narrow, but the fringe of vegetation which springs up along
+the banks as the liquid is borne through them from the melting of the
+polar snows. He supports this by his observation that the canals begin
+to appear in the neighbourhood of the polar caps, and gradually grow, as
+it were, in the direction of the planet's equator.
+
+It is the idea of life on Mars which has given this planet such a
+fascination in the eyes of men. A great deal of nonsense has, however,
+been written in newspapers upon the subject, and many persons have thus
+been led to think that we have obtained some actual evidence of the
+existence of living beings upon Mars. It must be clearly understood,
+however, that Professor Lowell's advocacy of the existence of life upon
+that planet is by no means of this wild order. At the best he merely
+indulges in such theories as his remarkable observations naturally call
+forth. His views are as follows:--He considers that the planet has
+reached a time when "water" has become so scarce that the "inhabitants"
+are obliged to employ their utmost skill to make their scanty supply
+suffice for purposes of irrigation. The changes of tone and colour upon
+the Martian surface, as the irrigation produces its effects, are similar
+to what a telescopic observer--say, upon Venus--would notice on our
+earth when the harvest ripens over huge tracts of country; that is, of
+course, if the earth's atmosphere allowed a clear view of the
+terrestrial surface--a very doubtful point indeed. Professor Lowell
+thinks that the perfect straightness of the lines, and the geometrical
+manner in which they are arranged, are clear evidences of artificiality.
+On a globe, too, there is plainly no reason why the liquid which results
+from the melting of the polar caps should trend at all in the direction
+of the equator. Upon our earth, for instance, the transference of water,
+as in rivers, merely follows the slope of the ground, and nothing else.
+The Lowell observations show, however, that the Martian liquid is
+apparently carried from one pole towards the equator, and then past it
+to the other pole, where it once more freezes, only to melt again in due
+season, and to reverse the process towards and across the equator as
+before. Professor Lowell therefore holds, and it seems a strong point in
+favour of his theory, that the liquid must, in some artificial manner,
+as by pumping, for instance, be _helped_ in its passage across the
+surface of the planet.
+
+A number of attempts have been made to explain the _doubling_ of the
+canals merely as effects of refraction or reflection; and it has even
+been suggested that it may arise from the telescope not being accurately
+focussed.
+
+The actual doubling of the canals once having been doubted, it was an
+easy step to the casting of doubt on the reality of the canals
+themselves. The idea, indeed, was put forward that the human eye, in
+dealing with detail so very close to the limit of visibility, may
+unconsciously treat as an actual line several point-like markings which
+merely happen to lie in a line. In order to test this theory,
+experiments were carried out in 1902 by Mr. E.W. Maunder of Greenwich
+Observatory, and Mr. J.E. Evans of the Royal Hospital School at
+Greenwich, in which certain schoolboys were set to make drawings of a
+white disc with some faint markings upon it. The boys were placed at
+various distances from the disc in question; and it was found that the
+drawings made by those who were just too far off to see distinctly, bore
+out the above theory in a remarkable manner. Recently, however, the
+plausibility of the _illusion_ view has been shaken by photographs of
+Mars taken during the opposition of 1905 by Mr. Lampland at the Lowell
+Observatory, in which a number of the more prominent canals come out as
+straight dark lines. Further still, in some photographs made there quite
+lately, several canals are said to appear visibly double.
+
+Following up the idea alluded to in Chapter XVI., that the moon may be
+covered with a layer of ice, Mr. W.T. Lynn has recently suggested that
+this may be the case on Mars; and that, at certain seasons, the water
+may break through along definite lines, and even along lines parallel to
+these. This, he maintains, would account for the canals becoming
+gradually visible across the disc, without the necessity of Professor
+Lowell's "pumping" theory.
+
+And now for the views of Professor Lowell himself with regard to the
+doubling of the canals. From his observations, he considers that no
+pairs of railway lines could apparently be laid down with greater
+parallelism. He draws attention to the fact that the doubling does not
+take place by any means in every canal; indeed, out of 400 canals seen
+at Flagstaff, only fifty-one--or, roughly, one-eighth--have at any time
+been seen double. He lays great stress upon this, which he considers
+points strongly against the duplication being an optical phenomenon. He
+finds that the distance separating pairs of canals is much less in some
+doubles than in others, and varies on the whole from 75 to 200 miles.
+According to him, the double canals appear to be confined to within 40
+degrees of the equator: or, to quote his own words, they are "an
+equatorial feature of the planet, confined to the tropic and temperate
+belts." Finally, he points out that they seem to _avoid_ the blue-green
+areas. But, strangely enough, Professor Lowell does not so far attempt
+to fit in the doubling with his body of theory. He makes the obvious
+remark that they may be "channels and return channels," and with that he
+leaves us.
+
+The conclusions of Professor Lowell have recently been subjected to
+strenuous criticism by Professor W.H. Pickering and Dr. Alfred Russel
+Wallace. It was Professor Pickering who discovered the "oases," and who
+originated the idea that we did not see the so-called "canals"
+themselves, but only the growth of vegetation along their borders. He
+holds that the oases are craterlets, and that the canals are cracks
+which radiate from them, as do the rifts and streaks from craters upon
+the moon. He goes on to suggest that vapours of water, or of carbonic
+acid gas, escaping from the interior, find their way out through these
+cracks, and promote the growth of a low form of vegetation on either
+side of them. In support of this view he draws attention to the
+existence of long "steam-cracks," bordered by vegetation, in the deserts
+of the highly volcanic island of Hawaii. We have already seen, in an
+earlier chapter, how he has applied this idea to the explanation of
+certain changes which are suspected to be taking place upon the moon.
+
+In dealing with the Lowell canal system, Professor Pickering points out
+that under such a slight atmospheric pressure as exists on Mars, the
+evaporation of the polar caps--supposing them to be formed of
+snow--would take place with such extraordinary rapidity that the
+resulting water could never be made to travel along open channels, but
+that a system of gigantic tubes or water-mains would have to be
+employed!
+
+As will be gathered from his theories regarding vegetation, Professor
+Pickering does not deny the existence of a form of life upon Mars. But
+he will not hear of civilisation, or of anything even approaching it. He
+thinks, however, that as Mars is intermediate physically between the
+moon and earth, the form of life which it supports may be higher than
+that on the moon and lower than that on the earth.
+
+In a small book published in the latter part of 1907, and entitled _Is
+Mars Habitable?_ Dr. Alfred Russel Wallace sets himself, among other
+things, to combat the idea of a comparatively high temperature, such as
+Professor Lowell has allotted to Mars. He shows the immense service
+which the water-vapour in our atmosphere exercises, through keeping the
+solar heat from escaping from the earth's surface. He then draws
+attention to the fact that there is no spectroscopic evidence of
+water-vapour on Mars[21]; and points out that its absence is only to be
+expected, as Dr. George Johnstone Stoney has shown that it will escape
+from a body whose mass is less than one-quarter the mass of the earth.
+The mass of Mars is, in fact, much less than this, _i.e._ only
+one-ninth. Dr. Wallace considers, therefore, that the temperature of
+Mars ought to be extremely low, unless the constitution of its
+atmosphere is very different from ours. With regard to the latter
+statement, it should be mentioned that the Swedish physicist, Arrhenius,
+has recently shown that the carbonic acid gas in our atmosphere has an
+important influence upon climate. The amount of it in our air is, as we
+have seen, extremely small; but Arrhenius shows that, if it were
+doubled, the temperature would be more uniform and much higher. We thus
+see how futile it is, with our present knowledge, to dogmatise on the
+existence or non-existence of life in other celestial orbs.
+
+As to the canals Dr. Wallace puts forward a theory of his own. He
+contends that after Mars had cooled to a state of solidity, a great
+swarm of meteorites and small asteroids fell in upon it, with the result
+that a thin molten layer was formed all over the planet. As this layer
+cooled, the imprisoned gases escaped, producing vents or craterlets; and
+as it attempted to contract further upon the solid interior, it split in
+fissures radiating from points of weakness, such, for instance, as the
+craterlets. And he goes on to suggest that the two tiny Martian
+satellites, with which we shall deal next, are the last survivors of his
+hypothetical swarm. Finally, with regard to the habitability of Mars,
+Dr. Wallace not only denies it, but asserts that the planet is
+"absolutely uninhabitable."
+
+For a long time it was supposed that Mars did not possess any
+satellites. In 1877, however, during that famous opposition in which
+Schiaparelli first saw the canals, two tiny satellites were discovered
+at the Washington Observatory by an American astronomer, Professor Asaph
+Hall. These satellites are so minute, and so near to the planet, that
+they can only be seen with very large telescopes; and even then the
+bright disc of the planet must be shielded off. They have been
+christened Phobos and Deimos (Fear and Dread); these being the names of
+the two subordinate deities who, according to Homer, attended upon Mars,
+the god of war.
+
+It is impossible to measure the exact sizes of these satellites, as they
+are too small to show any discs, but an estimate has been formed from
+their brightness. The diameter of Phobos was at first thought to be six
+miles, and that of Deimos, seven. As later estimates, however,
+considerably exceed this, it will, perhaps, be not far from the truth to
+state that they are each roughly about the size of the planetoid Eros.
+Phobos revolves around Mars in about 7-1/2 hours, at a distance of about
+only 4000 miles from the planet's surface, and Deimos in about 30 hours,
+at a distance of about 12,000 miles. As Mars rotates on its axis in
+about 24 hours, it will be seen that Phobos makes more than three
+revolutions while the planet is rotating once--a very interesting
+condition of things.
+
+A strange foreshadowing of the discovery of the satellites of Mars will
+be familiar to readers of _Gulliver's Travels_. According to Dean
+Swift's hero, the astronomers on the Flying Island of Laputa had found
+two tiny satellites to Mars, one of which revolved around the planet in
+ten hours. The correctness of this guess is extraordinarily close,
+though at best it is, of course, nothing more than a pure coincidence.
+
+It need not be at all surprising that much uncertainty should exist with
+regard to the actual condition of the surface of Mars. The circumstances
+in which we are able to see that planet at the best are, indeed, hardly
+sufficient to warrant us in propounding any hard and fast theories. One
+of the most experienced of living observers, the American astronomer,
+Professor E.E. Barnard, considers that the view we get of Mars with the
+best telescope may be fairly compared with our naked eye view of the
+moon. Since we have seen that a view with quite a small telescope
+entirely alters our original idea of the lunar surface, a slight
+magnification revealing features of whose existence we had not
+previously the slightest conception, it does not seem too much to say
+that a further improvement in optical power might entirely subvert the
+present notions with regard to the Martian canals. Therefore, until we
+get a still nearer view of these strange markings, it seems somewhat
+futile to theorise. The lines which we see are perhaps, indeed, a
+foreshortened and all too dim view of some type of formation entirely
+novel to us, and possibly peculiar to Mars. Differences of gravity and
+other conditions, such as obtain upon different planets, may perhaps
+produce very diverse results. The earth, the moon, and Mars differ
+greatly from one another in size, gravitation, and other such
+characteristics. Mountain-ranges so far appear typical of our globe, and
+ring-mountains typical of the moon. May not the so-called "canals" be
+merely some special formation peculiar to Mars, though quite a natural
+result of its particular conditions and of its past history?
+
+
+THE ASTEROIDS (OR MINOR PLANETS)
+
+We now come to that belt of small planets which are known by the name of
+asteroids. In the general survey of the solar system given in Chapter
+II., we saw how it was long ago noticed that the distances of the
+planetary orbits from the sun would have presented a marked appearance
+of orderly sequence, were it not for a gap between the orbits of Mars
+and Jupiter where no large planet was known to circulate. The suspicion
+thus aroused that some planet might, after all, be moving in this
+seemingly empty space, gave rise to the gradual discovery of a great
+number of small bodies; the largest of which, Ceres, is less than 500
+miles in diameter. Up to the present day some 600 of these bodies have
+been discovered; the four leading ones, in order of size, being named
+Ceres, Pallas, Juno, and Vesta. All the asteroids are invisible to the
+naked eye, with the exception of Vesta, which, though by no means the
+largest, happens to be the brightest. It is, however, only just visible
+to the eye under favourable conditions. No trace of an atmosphere has
+been noted upon any of the asteroids, but such a state of things is only
+to be expected from the kinetic theory.
+
+For a good many years the discoveries of asteroids were made by means of
+the telescope. When, in the course of searching the heavens, an object
+was noticed which did not appear upon any of the recognised star charts,
+it was kept under observation for several nights to see whether it
+changed its place in the sky. Since asteroids move around the sun in
+orbits, just as planets do, they, of course, quickly reveal themselves
+by their change of position against the starry background.
+
+The year 1891 started a new era in the discovery of asteroids. It
+occurred to the Heidelberg observer, Dr. Max Wolf, one of the most
+famous of the hunters of these tiny planets, that photography might be
+employed in the quest with success. This photographic method, to which
+allusion has already been made in dealing with Eros, is an extremely
+simple one. If a photograph of a portion of the heavens be taken through
+an "equatorial"--that is, a telescope, moving by machinery, so as to
+keep the stars, at which it is pointed, always exactly in the field of
+view during their apparent movement across the sky--the images of these
+stars will naturally come out in the photograph as sharply defined
+points. If, however, there happens to be an asteroid, or other planetary
+body, in the same field of view, its image will come out as a short
+white streak; because the body has a comparatively rapid motion of its
+own, and will, during the period of exposure, have moved sufficiently
+against the background of the stars to leave a short trail, instead of a
+dot, upon the photographic plate. By this method Wolf himself has
+succeeded in discovering more than a hundred asteroids (see Plate XIII.,
+p. 226). It was, indeed, a little streak of this kind, appearing upon a
+photograph taken by the astronomer Witt, at Berlin, in 1898, which first
+informed the world of the existence of Eros.
+
+[Illustration: PLATE XIII. MINOR PLANET TRAILS
+
+Two trails of minor planets (asteroids) imprinted _at the same time_
+upon one photographic plate. In the white streak on the left-hand side
+of the picture we witness the _discovery_ of a new minor planet. The
+streak on the right was made by a body already known--the minor planet
+"Fiducia." This photograph was taken by Dr. Max Wolf, at Heidelberg, on
+the 4th of November, 1901, with the aid of a 16-inch telescope. The time
+of exposure was two hours.
+
+(Page 227)]
+
+It has been calculated that the total mass of the asteroids must be
+much less than one-quarter that of the earth. They circulate as a rule
+within a space of some 30,000,000 miles in breadth, lying about midway
+between the paths of Mars and Jupiter. Two or three, however, of the
+most recently discovered of these small bodies have been found to pass
+quite close to Jupiter. The orbits of the asteroids are by no means in
+the one plane, that of Pallas being the most inclined to the plane of
+the earth's orbit. It is actually three times as much inclined as that
+of Eros.
+
+Two notable theories have been put forward to account for the origin of
+the asteroids. The first is that of the celebrated German astronomer,
+Olbers, who was the discoverer of Pallas and Vesta. He suggested that
+they were the fragments of an exploded planet. This theory was for a
+time generally accepted, but has now been abandoned in consequence of
+certain definite objections. The most important of these objections is
+that, in accordance with the theory of gravitation, the orbits of such
+fragments would all have to pass through the place where the explosion
+originally occurred. But the wide area over which the asteroids are
+spread points rather against the notion that they all set out originally
+from one particular spot. Another objection is that it does not appear
+possible that, within a planet already formed, forces could originate
+sufficiently powerful to tear the body asunder.
+
+The second theory is that for some reason a planet here failed in the
+making. Possibly the powerful gravitational action of the huge body of
+Jupiter hard by, disturbed this region so much that the matter
+distributed through it was never able to collect itself into a single
+mass.
+
+
+[18] Sir William Herschel was the first to note these polar changes.
+
+[19] Quite recently, however, Professor Lowell has announced that his
+observer, Mr. E.C. Slipher, finds with the spectroscope faint traces of
+water vapour in the Martian atmosphere.
+
+[20] In a somewhat similar manner the term "crater," as applied to the
+ring-mountain formation on the moon, has evidently given a bias in
+favour of the volcanic theory as an explanation of that peculiar
+structure.
+
+[21] Mr. Slipher's results (see note 2, page 213) were not then known.
+
+
+
+
+CHAPTER XVIII
+
+THE SUPERIOR PLANETS--_continued_
+
+
+The planets, so far, have been divided into inferior and superior. Such
+a division, however, refers merely to the situation of their orbits with
+regard to that of our earth. There is, indeed, another manner in which
+they are often classed, namely, according to size. On this principle
+they are divided into two groups; one group called the _Terrestrial
+Planets_, or those which have characteristics like our earth, and the
+other called the _Major Planets_, because they are all of very great
+size. The terrestrial planets are Mercury, Venus, the earth, and Mars.
+The major planets are the remainder, namely, Jupiter, Saturn, Uranus,
+and Neptune. As the earth's orbit is the boundary which separates the
+inferior from the superior planets, so does the asteroidal belt divide
+the terrestrial from the major planets. We found the division into
+inferior and superior useful for emphasising the marked difference in
+aspect which those two classes present as seen from our earth; the
+inferior planets showing phases like the moon when viewed in the
+telescope, whereas the superior planets do not. But the division into
+terrestrial and major planets is the more far-reaching classification of
+the two, for it includes the whole number of planets, whereas the other
+arrangement necessarily excludes the earth. The members of each of
+these classes have many definite characteristics in common. The
+terrestrial planets are all of them relatively small in size,
+comparatively near together, and have few or no satellites. They are,
+moreover, rather dense in structure. The major planets, on the other
+hand, are huge bodies, circulating at great distances from each other,
+and are, as a rule, provided with a number of satellites. With respect
+to structure, they may be fairly described as being loosely put
+together. Further, the markings on the surfaces of the terrestrial
+planets are permanent, whereas those on the major planets are
+continually shifting.
+
+
+THE PLANET JUPITER
+
+Jupiter is the greatest of the major planets. It has been justly called
+the "Giant" planet, for both in volume and in mass it exceeds all the
+other planets put together. When seen through the telescope it exhibits
+a surface plentifully covered with markings, the most remarkable being a
+series of broad parallel belts. The chief belt lies in the central parts
+of the planet, and is at present about 10,000 miles wide. It is bounded
+on either side by a reddish brown belt of about the same width. Bright
+spots also appear upon the surface of the planet, last for a while, and
+then disappear. The most notable of the latter is one known as the
+"Great Red Spot." This is situated a little beneath the southern red
+belt, and appeared for the first time about thirty years ago. It has
+undergone a good many changes in colour and brightness, and is still
+faintly visible. This spot is the most permanent marking which has yet
+been seen upon Jupiter. In general, the markings change so often that
+the surface which we see is evidently not solid, but of a fleeting
+nature akin to cloud (see Plate XIV., p. 230).
+
+[Illustration: PLATE XIV. THE PLANET JUPITER
+
+The Giant Planet as seen at 11.30 p.m., on the 11th of January, 1908,
+with a 12-1/2-inch reflecting telescope. The extensive oval marking in
+the upper portion of the disc is the "Great Red Spot." The South is at
+the top of the picture, the view being the _inverted_ one given by an
+astronomical telescope. From a drawing by the Rev. Theodore E.R.
+Phillips, M.A., F.R.A.S., Director of the Jupiter Section of the British
+Astronomical Association.
+
+(Page 231)]
+
+Observations of Jupiter's markings show that on an average the planet
+rotates on its axis in a period of about 9 hours 54 minutes. The mention
+here of _an average_ with reference to the rotation will, no doubt,
+recall to the reader's mind the similar case of the sun, the different
+portions of which rotate with different velocities. The parts of Jupiter
+which move quickest take 9 hours 50 minutes to go round, while those
+which move slowest take 9 hours 57 minutes. The middle portions rotate
+the fastest, a phenomenon which the reader will recollect was also the
+case with regard to the sun.
+
+Jupiter is a very loosely packed body. Its density is on an average only
+about 1-1/2 times that of water, or about one-fourth the density of the
+earth; but its bulk is so great that the gravitation at that surface
+which we see is about 2-1/2 times what it is on the surface of the
+earth. In accordance, therefore, with the kinetic theory, we may expect
+the planet to retain an extensive layer of gases around it; and this is
+confirmed by the spectroscope, which gives evidence of the presence of a
+dense atmosphere.
+
+All things considered, it may be safely inferred that the interior of
+Jupiter is very hot, and that what we call its surface is not the actual
+body of the planet, but a voluminous layer of clouds and vapours driven
+upwards from the heated mass underneath. The planet was indeed formerly
+thought to be self-luminous; but this can hardly be the case, for those
+portions of the surface which happen to lie at any moment in the
+shadows cast by the satellites appear to be quite black. Again, when a
+satellite passes into the great shadow cast by the planet it becomes
+entirely invisible, which would not be the case did the planet emit any
+perceptible light of its own.
+
+In its revolutions around the sun, Jupiter is attended, so far as we
+know, by seven[22] satellites. Four of these were among the first
+celestial objects which Galileo discovered with his "optick tube," and
+he named them the "Medicean Stars" in honour of his patron, Cosmo de
+Medici. Being comparatively large bodies they might indeed just be seen
+with the naked eye, were it not for the overpowering glare of the
+planet.
+
+It was only in quite recent times, namely, in 1892, that a fifth
+satellite was added to the system of Jupiter. This body, discovered by
+Professor E.E. Barnard, is very small. It circulates nearer to the
+planet than the innermost of Galileo's moons; and, on account of the
+glare, is a most difficult object to obtain a glimpse of, even in the
+best of telescopes. In December 1904 and January 1905 respectively, two
+more moons were added to the system, these being found by _photography_,
+by the American astronomer, Professor C.D. Perrine. Both the bodies in
+question revolve at a greater distance from the planet than the
+outermost of the older known satellites.
+
+Galileo's moons, though the largest bodies of Jupiter's satellite
+system, are, as we have already pointed out, very small indeed when
+compared with the planet itself. The diameters of two of them, Europa
+and Io, are, however, about the same as that of our moon, while those of
+the other two, Callisto and Ganymede, are more than half as large again.
+The recently discovered satellites are, on the other hand,
+insignificant; that found by Barnard, for example, being only about 100
+miles in diameter.
+
+Of the four original satellites Io is the nearest to Jupiter, and, seen
+from the planet, it would show a disc somewhat larger than that of our
+moon. The others would appear somewhat smaller. However, on account of
+the great distance of the sun, the entire light reflected to Jupiter by
+all the satellites should be very much less than what we get from our
+moon.
+
+Barnard's satellite circles around Jupiter at a distance less than our
+moon is from us, and in a period of about 12 hours. Galileo's four
+satellites revolve in periods of about 2, 3-1/2, 7, and 16-1/2 days
+respectively, at distances lying roughly between a quarter of a million
+and one million miles. Perrine's two satellites are at a distance of
+about seven million miles, and take about nine months to complete their
+revolutions.
+
+The larger satellites, when viewed in the telescope, exhibit certain
+defined markings; but the bodies are so far away from us, that only
+those details which are of great extent can be seen. The satellite Io,
+according to Professor Barnard, shows a darkish disc, with a broad white
+belt across its middle regions. Mr. Douglass, one of the observers at
+the Lowell Observatory, has noted upon Ganymede a number of markings
+somewhat resembling those seen on Mars, and he concludes, from their
+movement, that this satellite rotates on its axis in about seven days.
+Professor Barnard, on the other hand, does not corroborate this, though
+he claims to have discovered bright polar caps on both Ganymede and
+Callisto.
+
+In an earlier chapter we dealt at length with eclipses, occultations,
+and transits, and endeavoured to make clear the distinction between
+them. The system of Jupiter's satellites furnishes excellent examples of
+all these phenomena. The planet casts a very extensive shadow, and the
+satellites are constantly undergoing obscuration by passing through it.
+Such occurrences are plainly comparable to our lunar eclipses. Again,
+the satellites may, at one time, be occulted by the huge disc of the
+planet, and at another time seen in transit over its face. A fourth
+phenomenon is what is known as an _eclipse of the planet by a
+satellite_, which is the exact equivalent of what we style on the earth
+an eclipse of the sun. In this last case the shadow, cast by the
+satellite, appears as a round black spot in movement across the planet's
+surface.
+
+In the passages of these attendant bodies behind the planet, into its
+shadow, or across its face, respectively, it occasionally happens that
+Galileo's four satellites all disappear from view, and the planet is
+then seen for a while in the unusual condition of being apparently
+without its customary attendants. An instance of this phenomenon took
+place on the 3rd of October 1907. On that occasion, the satellites known
+as I. and III. (_i.e._ Io and Ganymede) were eclipsed, that is to say,
+obscured by passing into the planet's shadow; Satellite IV. (Callisto)
+was occulted by the planet's disc; while Satellite II. (Europa), being
+at the same moment in transit across the planet's face, was invisible
+against that brilliant background. A number of instances of this kind of
+occurrence are on record. Galileo, for example, noted one on the 15th of
+March 1611, while Herschel observed another on the 23rd of May 1802.
+
+It was indirectly to Jupiter's satellites that the world was first
+indebted for its knowledge of the velocity of light. When the periods of
+revolution of the satellites were originally determined, Jupiter
+happened, at the time, to be at his nearest to us. From the periods thus
+found tables were made for the prediction of the moments at which the
+eclipses and other phenomena of the satellites should take place. As
+Jupiter, in the course of his orbit, drew further away from the earth,
+it was noticed that the disappearances of the satellites into the shadow
+of the planet occurred regularly later than the time predicted. In the
+year 1675, Roemer, a Danish astronomer, inferred from this, not that the
+predictions were faulty, but that light did not travel instantaneously.
+It appeared, in fact, to take longer to reach us, the greater the
+distance it had to traverse. Thus, when the planet was far from the
+earth, the last ray given out by the satellite, before its passage into
+the shadow, took a longer time to cross the intervening space, than when
+the planet was near. Modern experiments in physics have quite confirmed
+this, and have proved for us that light does not travel across space in
+the twinkling of an eye, as might hastily be supposed, but actually
+moves, as has been already stated, at the rate of about 186,000 miles
+per second.
+
+
+THE PLANET SATURN
+
+Seen in the telescope the planet Saturn is a wonderful and very
+beautiful object. It is distinguished from all the other planets, in
+fact from all known celestial bodies, through being girt around its
+equator by what looks like a broad, flat ring of exceeding thinness.
+This, however, upon closer examination, is found to be actually composed
+of three concentric rings. The outermost of these is nearly of the same
+brightness as the body of the planet itself. The ring which comes
+immediately within it is also bright, and is separated from the outer
+one all the way round by a relatively narrow space, known as "Cassini's
+division," because it was discovered by the celebrated French
+astronomer, J.D. Cassini, in the year 1675. Inside the second ring, and
+merging insensibly into it, is a third one, known as the "crape ring,"
+because it is darker in hue than the others and partly transparent, the
+body of Saturn being visible through it. The inner boundary of this
+third and last ring does not adjoin the planet, but is everywhere
+separated from it by a definite space. This ring was discovered
+_independently_[23] in 1850 by Bond in America and Dawes in England.
+
+[Illustration: PLATE XV. THE PLANET SATURN
+
+From a drawing made by Professor Barnard with the Great Lick Telescope.
+The black band fringing the outer ring, where it crosses the disc, is
+portion of the _shadow which the rings cast upon the planet_. The black
+wedge-shaped mark, where the rings disappear behind the disc at the
+left-hand side, is portion of the _shadow which the planet casts upon
+the rings_.
+
+(Page 237)]
+
+As distinguished from the crape ring, the bright rings must have a
+considerable closeness of texture; for the shadow of the planet may be
+seen projected upon them, and their shadows in turn projected upon the
+surface of the planet (see Plate XV., p. 236).
+
+According to Professor Barnard, the entire breadth of the ring system,
+that is to say, from one side to the other of the outer ring, is 172,310
+miles, or somewhat more than double the planet's diameter.
+
+In the varying views which we get of Saturn, the system of the rings is
+presented to us at very different angles. Sometimes we are enabled to
+gaze upon its broad expanse; at other times, however, its thin edge is
+turned exactly towards us, an occurrence which takes place after
+intervals of about fifteen years. When this happened in 1892 the rings
+are said to have disappeared entirely from view in the great Lick
+telescope. We thus get an idea of their small degree of thickness, which
+would appear to be only about 50 miles. The last time the system of
+rings was exactly edgewise to the earth was on the 3rd of October 1907.
+
+The question of the composition of these rings has given rise to a good
+deal of speculation. It was formerly supposed that they were either
+solid or liquid, but in 1857 it was proved by Clerk Maxwell that a
+structure of this kind would not be able to stand. He showed, however,
+that they could be fully explained by supposing them to consist of an
+immense number of separate solid particles, or, as one might otherwise
+put it, extremely small satellites, circling in dense swarms around the
+middle portions of the planet. It is therefore believed that we have
+here the materials ready for the formation of a satellite or satellites;
+but that the powerful gravitative action, arising through the planet's
+being so near at hand, is too great ever to allow these materials to
+aggregate themselves into a solid mass. There is, as a matter of fact, a
+minimum distance from the body of any planet within which it can be
+shown that a satellite will be unable to form on account of
+gravitational stress. This is known as "Roche's limit," from the name of
+a French astronomer who specially investigated the question.
+
+There thus appears to be a certain degree of analogy between Saturn's
+rings and the asteroids. Empty spaces, too, exist in the asteroidal
+zone, the relative position of one of which bears a striking resemblance
+to that of "Cassini's division." It is suggested, indeed, that this
+division had its origin in gravitational disturbances produced by the
+attraction of the larger satellites, just as the empty spaces in the
+asteroidal zone are supposed to be the result of perturbations caused by
+the Giant Planet hard by.
+
+It has long been understood that the system of the rings must be
+rotating around Saturn, for if they were not in motion his intense
+gravitational attraction would quickly tear them in pieces. This was at
+length proved to be the fact by the late Professor Keeler, Director of
+the Lick Observatory, who from spectroscopic observations found that
+those portions of the rings situated near to the planet rotated faster
+than those farther from it. This directly supports the view that the
+rings are composed of satellites; for, as we have already seen, the
+nearer a satellite is to its primary the faster it will revolve. On the
+other hand, were the rings solid, their outer portions would move the
+fastest; as we have seen takes place in the body of the earth, for
+example. The mass of the ring system, however, must be exceedingly
+small, for it does not appear to produce any disturbances in the
+movements of Saturn's satellites. From the kinetic theory, therefore,
+one would not expect to find any atmosphere on the rings, and the
+absence of it is duly shown by spectroscopic observations.
+
+The diameter of Saturn, roughly speaking, is about one-fifth less than
+that of Jupiter. The planet is very flattened at the poles, this
+flattening being quite noticeable in a good telescope. For instance, the
+diameter across the equator is about 76,470 miles, while from pole to
+pole it is much less, namely, 69,770.
+
+The surface of Saturn bears a strong resemblance to that of Jupiter. Its
+markings, though not so well defined, are of the same belt-like
+description; and from observation of them it appears that the planet
+rotates _on an average_ in a little over ten hours. The rotation is in
+fact of the same peculiar kind as that of the sun and Jupiter; but the
+difference of speed at which the various portions of Saturn go round are
+even more marked than in the case of the Giant Planet. The density of
+Saturn is less than that of Jupiter; so that it must be largely in a
+condition of vapour, and in all probability at a still earlier stage of
+planetary evolution.
+
+Up to the present we know of as many as ten satellites circling around
+Saturn, which is more than any other planet of the solar system can lay
+claim to. Two of these, however, are very recent discoveries; one,
+Phoebe, having been found by photography in August 1898, and the other,
+Themis, in 1904, also by the same means. For both of these we are
+indebted to Professor W.H. Pickering. Themis is said to be _the faintest
+object in the solar system_. It cannot be _seen_, even with the largest
+telescope in existence; a fact which should hardly fail to impress upon
+one the great advantage the photographic plate possesses in these
+researches over the human eye.
+
+The most important of the whole Saturnian family of satellites are the
+two known as Titan and Japetus. These were discovered respectively by
+Huyghens in 1655 and by Cassini in 1671. Japetus is about the same size
+as our moon; while the diameter of Titan, the largest of the satellites,
+is about half as much again. Titan takes about sixteen days to revolve
+around Saturn, while Japetus takes more than two months and a half. The
+former is about three-quarters of a million miles distant from the
+planet, and the latter about two and a quarter millions. To Sir William
+Herschel we are indebted for the discovery of two more satellites, one
+of which he found on the evening that he used his celebrated 40-foot
+telescope for the first time. The ninth satellite, Phoebe, one of the
+two discovered by Professor Pickering, is perhaps the most remarkable
+body in the solar system, for all the other known members of that system
+perform their revolutions in one fixed direction, whereas this satellite
+revolves in the _contrary_ direction.
+
+In consequence of the great distance of Saturn, the sun, as seen from
+the planet, would appear so small that it would scarcely show any disc.
+The planet, indeed, only receives from the sun about one-ninetieth of
+the heat and light which the earth receives. Owing to this diminished
+intensity of illumination, the combined light reflected to Saturn by the
+whole of its satellites must be very small.
+
+With the sole exception of Jupiter, not one of the planets circulating
+nearer to the sun could be seen from Saturn, as they would be entirely
+lost in the solar glare. For an observer upon Saturn, Jupiter would,
+therefore, fill much the same position as Venus does for us, regularly
+displaying phases and being alternately a morning and an evening star.
+
+It is rather interesting to consider the appearances which would be
+produced in our skies were the earth embellished with a system of rings
+similar to those of Saturn. In consequence of the curving of the
+terrestrial surface, they would not be seen at all from within the
+Arctic or Antarctic circles, as they would be always below the horizon.
+From the equator they would be continually seen edgewise, and so would
+appear merely as line of light stretching right across the heaven and
+passing through the zenith. But the dwellers in the remaining regions
+would find them very objectionable, for they would cut off the light of
+the sun during lengthy periods of time.
+
+Saturn was a sore puzzle to the early telescopic observers. They did not
+for a long time grasp the fact that it was surrounded by a ring--so slow
+is the human mind to seek for explanations out of the ordinary course of
+things. The protrusions of the ring on either side of the planet, at
+first looked to Galileo like two minor globes placed on opposite sides
+of it, and slightly overlapping the disc. He therefore informed Kepler
+that "Saturn consists of three stars in contact with one another." Yet
+he was genuinely puzzled by the fact that the two attendant bodies (as
+he thought them) always retained the same position with regard to the
+planet's disc, and did not appear to revolve around it, nor to be in any
+wise shifted as a consequence of the movements of our earth.
+
+About a year and a half elapsed before he again examined Saturn; and, if
+he was previously puzzled, he was now thoroughly amazed. It happened
+just then to be one of those periods when the ring is edgewise towards
+the earth, and of course he only saw a round disc like that of Jupiter.
+What, indeed, had become of the attendant orbs? Was some demon mocking
+him? Had Saturn devoured his own children? He was, however, fated to be
+still more puzzled, for soon the minor orbs reappeared, and, becoming
+larger and larger as time went on, they ended by losing their globular
+appearance and became like two pairs of arms clasping the planet from
+each side! (see Plate XVI., p. 242).
+
+Galileo went to his grave with the riddle still unsolved, and it
+remained for the famous Dutch astronomer, Huyghens, to clear up the
+matter. It was, however, some little time before he hit upon the real
+explanation. Having noticed that there were dark spaces between the
+strange appendages and the body of the planet, he imagined Saturn to be
+a globe fitted with handles at each side; "ans�" these came to be
+called, from the Latin _ansa_, which means a handle. At length, in the
+year 1656, he solved the problem, and this he did by means of that
+123-foot tubeless telescope, of which mention has already been made. The
+ring happened then to be at its edgewise period, and a careful study of
+the behaviour of the ans� when disappearing and reappearing soon
+revealed to Huyghens the true explanation.
+
+[Illustration: PLATE XVI. EARLY REPRESENTATIONS OF SATURN
+
+From an illustration in the _Systema Saturnium_ of Christian Huyghens.
+
+(Page 242)]
+
+
+THE PLANETS URANUS AND NEPTUNE
+
+We have already explained (in Chapter II.) the circumstances in which
+both Uranus and Neptune were discovered. It should, however, be added
+that after the discovery of Uranus, that planet was found to have been
+already noted upon several occasions by different observers, but always
+without the least suspicion that it was other than a mere faint star.
+Again, with reference to the discovery of Neptune, it may here be
+mentioned that the apparent amount by which that planet had pulled
+Uranus out of its place upon the starry background was exceedingly
+small--so small, indeed, that no eye could have detected it without the
+aid of a telescope!
+
+Of the two predictions of the place of Neptune in the sky, that of Le
+Verrier was the nearer. Indeed, the position calculated by Adams was
+more than twice as far out. But Adams was by a long way the first in the
+field with his results, and only for unfortunate delays the prize would
+certainly have fallen to him. For instance, there was no star-map at
+Cambridge, and Professor Challis, the director of the observatory there,
+was in consequence obliged to make a laborious examination of the stars
+in the suspected region. On the other hand, all that Galle had to do was
+to compare that part of the sky where Le Verrier told him to look with
+the Berlin star-chart which he had by him. This he did on September 23,
+1846, with the result that he quickly noted an eighth magnitude star
+which did not figure in that chart. By the next night this star had
+altered its position in the sky, thus disclosing the fact that it was
+really a planet.
+
+Six days later Professor Challis succeeded in finding the planet, but of
+course he was now too late. On reviewing his labours he ascertained that
+he had actually noted down its place early in August, and had he only
+been able to sift his observations as he made them, the discovery would
+have been made then.
+
+Later on it was found that Neptune had only just missed being discovered
+about fifty years earlier. In certain observations made during 1795, the
+famous French astronomer, Lalande, found that a star, which he had
+mapped in a certain position on the 8th of May of that year, was in a
+different position two days later. The idea of a planet does not appear
+to have entered his mind, and he merely treated the first observation as
+an error!
+
+The reader will, no doubt, recollect how the discovery of the asteroids
+was due in effect to an apparent break in the seemingly regular sequence
+of the planetary orbits outwards from the sun. This curious sequence of
+relative distances is usually known as "Bode's Law," because it was
+first brought into general notice by an astronomer of that name. It had,
+however, previously been investigated mathematically by Titius in 1772.
+Long before this, indeed, the unnecessarily wide space between the
+orbits of Mars and Jupiter had attracted the attention of the great
+Kepler to such a degree, that he predicted that a planet would some day
+be found to fill the void. Notwithstanding the service which the
+so-called Law of Bode has indirectly rendered to astronomy, it has
+strangely enough been found after all not to rest upon any scientific
+foundation. It will not account for the distance from the sun of the
+orbit of Neptune, and the very sequence seems on the whole to be in the
+nature of a mere coincidence.
+
+Neptune is invisible to the naked eye; Uranus is just at the limit of
+visibility. Both planets are, however, so far from us that we can get
+but the poorest knowledge of their condition and surroundings. Uranus,
+up to the present, is known to be attended by four satellites, and
+Neptune by one. The planets themselves are about equal in size; their
+diameters, roughly speaking, being about one-half that of Saturn. Some
+markings have, indeed, been seen upon the disc of Uranus, but they are
+very indistinct and fleeting. From observation of them, it is assumed
+that the planet rotates on its axis in a period of some ten to twelve
+hours. No definite markings have as yet been seen upon Neptune, which
+body is described by several observers as resembling a faint planetary
+nebula.
+
+With regard to their physical condition, the most that can be said about
+these two planets is that they are probably in much the same vaporous
+state as Jupiter and Saturn. On account of their great distance from the
+sun they can receive but little solar heat and light. Seen from
+Neptune, in fact, the sun would appear only about the size of Venus at
+her best, though of a brightness sufficiently intense to illumine the
+Neptunian landscape with about seven hundred times our full moonlight.
+
+
+[22] Mr. P. Melotte, of Greenwich Observatory, while examining a
+photograph taken there on February 28, 1908, discovered upon it a very
+faint object which it is firmly believed will prove to be an _eighth_
+satellite of Jupiter. This object was afterwards found on plates exposed
+as far back as January 27. It has since been photographed several times
+at Greenwich, and also at Heidelberg (by Dr. Max Wolf) and at the Lick
+Observatory. Its movement is probably _retrograde_, like that of Phoebe
+(p. 240).
+
+[23] In the history of astronomy two salient points stand out.
+
+The first of these is the number of "independent" discoveries which have
+taken place; such, for instance, as in the cases of Le Verrier and Adams
+with regard to Neptune, and of Lockyer and Janssen in the matter of the
+spectroscopic method of observing solar prominences.
+
+The other is the great amount of "anticipation." Copernicus, as we have
+seen, was anticipated by the Greeks; Kepler was not actually the first
+who thought of elliptic orbits; others before Newton had imagined an
+attractive force.
+
+Both these points furnish much food for thought!
+
+
+
+
+CHAPTER XIX
+
+COMETS
+
+
+The reader has, no doubt, been struck by the marked uniformity which
+exists among those members of the solar system with which we have dealt
+up to the present. The sun, the planets, and their satellites are all
+what we call solid bodies. The planets move around the sun, and the
+satellites around the planets, in orbits which, though strictly
+speaking, ellipses, are yet not in any instance of a very oval form. Two
+results naturally follow from these considerations. Firstly, the bodies
+in question hide the light coming to us from those further off, when
+they pass in front of them. Secondly, the planets never get so far from
+the sun that we lose sight of them altogether.
+
+With the objects known as Comets it is, however, quite the contrary.
+These objects do not conform to our notions of solidity. They are so
+transparent that they can pass across the smallest star without dimming
+its light in the slightest degree. Again, they are only visible to us
+during a portion of their orbits. A comet may be briefly described as an
+illuminated filmy-looking object, made up usually of three portions--a
+head, a nucleus, or brighter central portion within this head, and a
+tail. The heads of comets vary greatly in size; some, indeed, appear
+quite small, like stars, while others look even as large as the moon.
+Occasionally the nucleus is wanting, and sometimes the tail also.
+
+[Illustration: FIG. 18.--Showing how the Tail of a Comet is directed
+away from the Sun.]
+
+These mysterious visitors to our skies come up into view out of the
+immensities beyond, move towards the sun at a rapidly increasing speed,
+and, having gone around it, dash away again into the depths of space. As
+a comet approaches the sun, its body appears to grow smaller and
+smaller, while, at the same time, it gradually throws out behind it an
+appendage like a tail. As the comet moves round the central orb this
+tail is always directed _away_ from the sun; and when it departs again
+into space the tail goes in advance. As the comet's distance from the
+sun increases, the tail gradually shrinks away and the head once more
+grows in size (see Fig. 18). In consequence of these changes, and of the
+fact that we lose sight of comets comparatively quickly, one is much
+inclined to wonder what further changes may take place after the bodies
+have passed beyond our ken.
+
+The orbits of comets are, as we have seen, very elliptic. In some
+instances this ellipticity is so great as to take the bodies out into
+space to nearly six times the distance of Neptune from the sun. For a
+long time, indeed, it was considered that comets were of two kinds,
+namely, those which actually _belonged_ to the solar system, and those
+which were merely _visitors_ to it for the first and only time--rushing
+in from the depths of space, rapidly circuiting the sun, and finally
+dashing away into space again, never to return. On the contrary,
+nowadays, astronomers are generally inclined to regard comets as
+permanent members of the solar system.
+
+The difficulty, however, of deciding absolutely whether the orbits of
+comets are really always _closed_ curves, that is to say, curves which
+must sooner or later bring the bodies back again towards the sun, is,
+indeed, very great. Comets, in the first place, are always so diffuse,
+that it is impossible to determine their exact position, or, rather, the
+exact position of that important point within them, known as the centre
+of gravity. Secondly, that stretch of its orbit along which we can
+follow a comet, is such a very small portion of the whole path, that the
+slightest errors of observation which we make will result in
+considerably altering our estimate of the actual shape of the orbit.
+
+Comets have been described as so transparent that they can pass across
+the sky without dimming the lustre of the smallest stars, which the
+thinnest fog or mist would do. This is, indeed, true of every portion
+of a comet except the nucleus, which is, as its name implies, the
+densest part. And yet, in contrast to this ghostlike character, is the
+strange fact that when comets are of a certain brightness they may
+actually be seen in full daylight.
+
+As might be gathered from their extreme tenuity, comets are so
+exceedingly small in mass that they do not appear to exert any
+gravitational attraction upon the other bodies of our system. It is,
+indeed, a known fact that in the year 1886 a comet passed right amidst
+the satellites of Jupiter without disturbing them in the slightest
+degree. The attraction of the planet, on the other hand, so altered the
+comet's orbit, as to cause it to revolve around the sun in a period of
+seven years, instead of twenty-seven, as had previously been the case.
+Also, in 1779, the comet known as Lexell's passed quite close to
+Jupiter, and its orbit was so changed by that planet's attraction that
+it has never been seen since. The density of comets must, as a rule, be
+very much less than the one-thousandth part of that of the air at the
+surface of our globe; for, if the density of the comet were even so
+small as this, its mass would _not_ be inappreciable.
+
+If comets are really undoubted members of the solar system, the
+circumstances in which they were evolved must have been different from
+those which produced the planets and satellites. The axial rotations of
+both the latter, and also their revolutions, take place in one certain
+direction;[24] their orbits, too, are ellipses which do not differ much
+from circles, and which, furthermore, are situated fairly in the one
+plane. Comets, on the other hand, do not necessarily travel round the
+sun in the same fixed direction as the planets. Their orbits, besides,
+are exceedingly elliptic; and, far from keeping to one plane, or even
+near it, they approach the sun from all directions.
+
+Broadly speaking, comets may be divided into two distinct classes, or
+"families." In the first class, the same orbit appears to be shared in
+common by a series of comets which travel along it, one following the
+other. The comets which appeared in the years 1668, 1843, 1880, 1882,
+and 1887 are instances of a number of different bodies pursuing the same
+path around the sun. The members of a comet family of this kind are
+observed to have similar characteristics. The idea is that such comets
+are merely portions of one much larger cometary body, which became
+broken up by the gravitational action of other bodies in the system, or
+through violent encounter with the sun's surroundings.
+
+The second class is composed of comets which are supposed to have been
+seized by the gravitative action of certain planets, and thus forced to
+revolve in short ellipses around the sun, well within the limits of the
+solar system. These comets are, in consequence, spoken of as "captures."
+They move around the sun in the same direction as the planets do.
+Jupiter has a fairly large comet family of this kind attached to him. As
+a result of his overpowering gravitation, it is imagined that during the
+ages he must have attracted a large number of these bodies on his own
+account, and, perhaps, have robbed other planets of their captures. His
+family at present numbers about thirty. Of the other planets, so far as
+we know, Saturn possesses a comet family of two, Uranus three, and
+Neptune six. There are, indeed, a few comets which appear as if under
+the influence of some force situated outside the known bounds of the
+solar system, a circumstance which goes to strengthen the idea that
+other planets may revolve beyond the orbit of Neptune. The terrestrial
+planets, on the other hand, cannot have comet families; because the
+enormous gravitative action of the sun in their vicinity entirely
+overpowers the attractive force which they exert upon those comets which
+pass close to them. Besides this, a comet, when in the inner regions of
+the solar system, moves with such rapidity, that the gravitational pull
+of the planets there situated is not powerful enough to deflect it to
+any extent. It must not be presumed, however, that a comet once captured
+should always remain a prisoner. Further disturbing causes might
+unsettle its newly acquired orbit, and send it out again into the
+celestial spaces.
+
+With regard to the matter of which comets are composed, the spectroscope
+shows the presence in them of hydrocarbon compounds (a notable
+characteristic of these bodies), and at times, also, of sodium and iron.
+Some of the light which we get from comets is, however, merely reflected
+sunlight.
+
+The fact that the tails of comets are always directed away from the sun,
+has given rise to the idea that this is caused by some repelling action
+emanating from the sun itself, which is continually driving off the
+smallest particles. Two leading theories have been formulated to account
+for the tails themselves upon the above assumption. One of these, first
+suggested by Olbers in 1812, and now associated with the name of the
+Russian astronomer, the late Professor Br�dikhine, who carefully worked
+it out, presumes an electrical action emanating from the sun; the other,
+that of Arrhenius, supposes a pressure exerted by the solar light in its
+radiation outwards into space. It is possible, indeed, that repelling
+forces of both these kinds may be at work together. Minute particles are
+probably being continually produced by friction and collisions among the
+more solid parts in the heads of comets. Supposing that such particles
+are driven off altogether, one may therefore assume that the so-called
+captured comets are disintegrating at a comparatively rapid rate. Kepler
+long ago maintained that "comets die," and this actually appears to be
+the case. The ordinary periodic ones, such, for instance, as Encke's
+Comet, are very faint, and becoming fainter at each return. Certain of
+these comets have, indeed, failed altogether to reappear. It is notable
+that the members of Jupiter's comet family are not very conspicuous
+objects. They have small tails, and even in some cases have none at all.
+The family, too, does not contain many members, and yet one cannot but
+suppose that Jupiter, on account of his great mass, has had many
+opportunities for making captures adown the ages.
+
+Of the two theories to which allusion has above been made, that of
+Br�dikhine has been worked out so carefully, and with such a show of
+plausibility, that it here calls for a detailed description. It appears
+besides to explain the phenomena of comets' tails so much more
+satisfactorily than that of Arrhenius, that astronomers are inclined to
+accept it the more readily of the two. According to Br�dikhine's theory
+the electrical repulsive force, which he assumes for the purposes of his
+argument, will drive the minutest particles of the comet in a direction
+away from the sun much more readily than the gravitative action of that
+body will pull them towards it. This may be compared to the ease with
+which fine dust may be blown upwards, although the earth's gravitation
+is acting upon it all the time.
+
+The researches of Br�dikhine, which began seriously with his
+investigation of Coggia's Comet of 1874, led him to classify the tails
+of comets in _three types_. Presuming that the repulsive force emanating
+from the sun did not vary, he came to the conclusion that the different
+forms assumed by cometary tails must be ascribed to the special action
+of this force upon the various elements which happen to be present in
+the comet. The tails which he classes as of the first type, are those
+which are long and straight and point directly away from the sun.
+Examples of such tails are found in the comets of 1811, 1843, and 1861.
+Tails of this kind, he thinks, are in all probability formed of
+_hydrogen_. His second type comprises those which are pointed away from
+the sun, but at the same time are considerably curved, as was seen in
+the comets of Donati and Coggia. These tails are formed of _hydrocarbon
+gas_. The third type of tail is short, brush-like, and strongly bent,
+and is formed of the _vapour of iron_, mixed with that of sodium and
+other elements. It should, however, be noted that comets have
+occasionally been seen which possess several tails of these various
+types.
+
+We will now touch upon a few of the best known comets of modern times.
+
+The comet of 1680 was the first whose orbit was calculated according to
+the laws of gravitation. This was accomplished by Newton, and he found
+that the comet in question completed its journey round the sun in a
+period of about 600 years.
+
+In 1682 there appeared a great comet, which has become famous under the
+name of Halley's Comet, in consequence of the profound investigations
+made into its motion by the great astronomer, Edmund Halley. He fixed
+its period of revolution around the sun at about seventy-five years, and
+predicted that it would reappear in the early part of 1759. He did not,
+however, live to see this fulfilled, but the comet duly returned--_the
+first body of the kind to verify such a prediction_--and was detected on
+Christmas Day, 1758, by George Palitzch, an amateur observer living near
+Dresden. Halley also investigated the past history of the comet, and
+traced it back to the year 1456. The orbit of Halley's comet passes out
+slightly beyond the orbit of Neptune. At its last visit in 1835, this
+comet passed comparatively close to us, namely, within five million
+miles of the earth. According to the calculations of Messrs P.H. Cowell
+and A.C.D. Crommelin of Greenwich Observatory, its next return will be
+in the spring of 1910; the nearest approach to the earth taking place
+about May 12.
+
+On the 26th of March, 1811, a great comet appeared, which remained
+visible for nearly a year and a half. It was a magnificent object; the
+tail being about 100 millions of miles in length, and the head about
+127,000 miles in diameter. A detailed study which he gave to this comet
+prompted Olbers to put forward that theory of electrical repulsion
+which, as we have seen, has since been so carefully worked out by
+Br�dikhine. Olbers had noticed that the particles expelled from the head
+appeared to travel to the end of the tail in about eleven minutes, thus
+showing a velocity per second very similar to that of light.
+
+The discovery in 1819 of the comet known as Encke's, because its orbit
+was determined by an astronomer of that name, drew attention for the
+first time to Jupiter's comet family, and, indeed, to short-period
+comets in general. This comet revolves around the sun in the shortest
+known period of any of these bodies, namely, 3-1/3 years. Encke
+predicted that it would return in 1822. This duly occurred, the comet
+passing at its nearest to the sun within three hours of the time
+indicated; being thus the second instance of the fulfilment of a
+prediction of the kind. A certain degree of irregularity which Encke's
+Comet displays in the dates of its returns to the sun, has been supposed
+to indicate that it passes in the course of its orbit through some
+retarding medium, but no definite conclusions have so far been arrived
+at in this matter.
+
+A comet, which appeared in 1826, goes by the name of Biela's Comet,
+because of its discovery by an Austrian military officer, Wilhelm von
+Biela. This comet was found to have a period of between six and seven
+years. Certain calculations made by Olbers showed that, at its return in
+1832, it would pass _through the earth's orbit_. The announcement of
+this gave rise to a panic; for people did not wait to inquire whether
+the earth would be anywhere near that part of its orbit when the comet
+passed. The panic, however, subsided when the French astronomer, Arago,
+showed that at the moment in question the earth would be some 50
+millions of miles away from the point indicated!
+
+[Illustration: PLATE XVII. DONATI'S COMET
+
+From a drawing made on October 9th, 1858, by G.P. Bond, of Harvard
+College Observatory, U.S.A. A good illustration of Br�dikhine's theory:
+note the straight tails of his _first_ type, and the curved tail of his
+_second_.
+
+(Page 257)]
+
+In 1846, shortly after one of its returns, Biela's Comet divided into
+two portions. At its next appearance (1852) these portions had separated
+to a distance of about 1-1/2 millions of miles from each other. This
+comet, or rather its constituents, have never since been seen.
+
+Perhaps the most remarkable comet of recent times was that of 1858,
+known as Donati's, it having been discovered at Florence by the Italian
+astronomer, G.B. Donati. This comet, a magnificent object, was visible
+for more than three months with the naked eye. Its tail was then 54
+millions of miles in length. It was found to revolve around the sun in a
+period of over 2000 years, and to go out in its journey to about 5-1/2
+times the distance of Neptune. Its motion is retrograde, that is to say,
+in the contrary direction to the usual movement in the solar system. A
+number of beautiful drawings of Donati's Comet were made by the American
+astronomer, G.P. Bond. One of the best of these is reproduced on Plate
+XVII., p. 256.
+
+In 1861 there appeared a great comet. On the 30th of June of that year
+the earth and moon actually passed through its tail; but no effects were
+noticed, other than a peculiar luminosity in the sky.
+
+In the year 1881 there appeared another large comet, known as Tebbutt's
+Comet, from the name of its discoverer. This was the _first comet of
+which a satisfactory photograph was obtained_. The photograph in
+question was taken by the late M. Janssen.
+
+The comet of 1882 was of vast size and brilliance. It approached so
+close to the sun that it passed through some 100,000 miles of the solar
+corona. Though its orbit was not found to have been altered by this
+experience, its nucleus displayed signs of breaking up. Some very fine
+photographs of this comet were obtained at the Cape of Good Hope by Mr.
+(now Sir David) Gill.
+
+The comet of 1889 was followed with the telescope nearly up to the orbit
+of Saturn, which seems to be the greatest distance at which a comet has
+ever been seen.
+
+The _first discovery of a comet by photographic means_[25] was made by
+Professor Barnard in 1892; and, since then, photography has been
+employed with marked success in the detection of small periodic comets.
+
+The best comet seen in the Northern hemisphere since that of 1882,
+appears to have been Daniel's Comet of 1907 (see Plate XVIII., p. 258).
+This comet was discovered on June 9, 1907, by Mr. Z. Daniel, at
+Princeton Observatory, New Jersey, U.S.A. It became visible to the naked
+eye about mid-July of that year, and reached its greatest brilliancy
+about the end of August. It did not, however, attract much popular
+attention, as its position in the sky allowed it to be seen only just
+before dawn.
+
+
+[24] With the exception, of course, of such an anomaly as the retrograde
+motion of the ninth satellite of Saturn.
+
+[25] If we except the case of the comet which was photographed near the
+solar corona in the eclipse of 1882.
+
+[Illustration: PLATE XVIII. DANIEL'S COMET OF 1907
+
+From a photograph taken, on August 11th, 1907, by Dr. Max Wolf, at the
+Astrophysical Observatory, Heidelberg. The instrument used was a 28-inch
+reflecting telescope, and the time of exposure was fifteen minutes. As
+the telescope was guided to follow the moving comet, the stars have
+imprinted themselves upon the photographic plate as short trails. This
+is clearly the opposite to what is depicted on Plate XIII.
+
+(Page 258)]
+
+
+
+
+CHAPTER XX
+
+REMARKABLE COMETS
+
+
+If eclipses were a cause of terror in past ages, comets appear to have
+been doubly so. Their much longer continuance in the sight of men had no
+doubt something to say to this, and also the fact that they arrived
+without warning; it not being then possible to give even a rough
+prediction of their return, as in the case of eclipses. As both these
+phenomena were occasional, and out of the ordinary course of things,
+they drew exceptional attention as unusual events always do; for it must
+be allowed that quite as wonderful things exist, but they pass unnoticed
+merely because men have grown accustomed to them.
+
+For some reason the ancients elected to class comets along with meteors,
+the aurora borealis, and other phenomena of the atmosphere, rather than
+with the planets and the bodies of the spaces beyond. The sudden
+appearance of these objects led them to be regarded as signs sent by the
+gods to announce remarkable events, chief among these being the deaths
+of monarchs. Shakespeare has reminded us of this in those celebrated
+lines in _Julius C�sar_:--
+
+"When beggars die there are no comets seen,
+The heavens themselves blaze forth the death of princes."
+
+Numbed by fear, the men of old blindly accepted these presages of fate;
+and did not too closely question whether the threatened danger was to
+their own nation or to some other, to their ruler or to his enemy. Now
+and then, as in the case of the Roman Emperor Vespasian, there was a
+cynical attempt to apply some reasoning to the portent. That emperor, in
+alluding to the comet of A.D. 79, is reported to have said: "This hairy
+star does not concern me; it menaces rather the King of the Parthians,
+for he is hairy and I am bald." Vespasian, all the same, died shortly
+afterwards!
+
+Pliny, in his natural history, gives several instances of the terrible
+significance which the ancients attached to comets. "A comet," he says,
+"is ordinarily a very fearful star; it announces no small effusion of
+blood. We have seen an example of this during the civil commotion of
+Octavius."
+
+A very brilliant comet appeared in 371 B.C., and about the same time an
+earthquake caused Helic� and Bura, two towns in Achaia, to be swallowed
+up by the sea. The following remark made by Seneca concerning it shows
+that the ancients did not consider comets merely as precursors, but even
+as actual _causes_ of fatal events: "This comet, so anxiously observed
+by every one, _because of the great catastrophe which it produced as
+soon as it appeared_, the submersion of Bura and Helic�."
+
+Comets are by no means rare visitors to our skies, and very few years
+have elapsed in historical times without such objects making their
+appearance. In the Dark and Middle Ages, when Europe was split up into
+many small kingdoms and principalities, it was, of course, hardly
+possible for a comet to appear without the death of some ruler occurring
+near the time. Critical situations, too, were continually arising in
+those disturbed days. The end of Louis le Debonnaire was hastened, as
+the reader will, no doubt, recollect, by the great eclipse of 840; but
+it was firmly believed that a comet which had appeared a year or two
+previously presaged his death. The comet of 1556 is reported to have
+_influenced_ the abdication of the Emperor Charles V.; but curiously
+enough, this event had already taken place before the comet made its
+appearance! Such beliefs, no doubt, had a very real effect upon rulers
+of a superstitious nature, or in a weak state of health. For instance,
+Gian Galeazzo Visconti, Duke of Milan, was sick when the comet of 1402
+appeared. After seeing it, he is said to have exclaimed: "I render
+thanks to God for having decreed that my death should be announced to
+men by this celestial sign." His malady then became worse, and he died
+shortly afterwards.
+
+It is indeed not improbable that such superstitious fears in monarchs
+were fanned by those who would profit by their deaths, and yet did not
+wish to stain their own hands with blood.
+
+Evil though its effects may have been, this morbid interest which past
+ages took in comets has proved of the greatest service to our science.
+Had it not been believed that the appearance of these objects was
+attended with far-reaching effects, it is very doubtful whether the old
+chroniclers would have given themselves the trouble of alluding to them
+at all; and thus the modern investigators of cometary orbits would have
+lacked a great deal of important material.
+
+We will now mention a few of the most notable comets which historians
+have recorded.
+
+A comet which appeared in 344 B.C. was thought to betoken the success
+of the expedition undertaken in that year by Timoleon of Corinth against
+Sicily. "The gods by an extraordinary prodigy announced his success and
+future greatness: a burning torch appeared in the heavens throughout the
+night and preceded the fleet of Timoleon until it arrived off the coast
+of Sicily."
+
+The comet of 43 B.C. was generally believed to be the soul of C�sar on
+its way to heaven.
+
+Josephus tells us that in A.D. 69 several prodigies, and amongst them a
+comet in the shape of a sword, announced the destruction of Jerusalem.
+This comet is said to have remained over the city for the space of a
+year!
+
+A comet which appeared in A.D. 336 was considered to have announced the
+death of the Emperor Constantine.
+
+But perhaps the most celebrated comet of early times was the one which
+appeared in A.D. 1000. That year was, in more than one way, big with
+portent, for there had long been a firm belief that the Christian era
+could not possibly run into four figures. Men, indeed, steadfastly
+believed that when the thousand years had ended, the millennium would
+immediately begin. Therefore they did not reap neither did they sow,
+they toiled not, neither did they spin, and the appearance of the comet
+strengthened their convictions. The fateful year, however, passed by
+without anything remarkable taking place; but the neglect of husbandry
+brought great famine and pestilence over Europe in the years which
+followed.
+
+In April 1066, that year fraught with such immense consequences for
+England, a comet appeared. No one doubted but that it was a presage of
+the success of the Conquest, and perhaps, indeed, it had its due weight
+in determining the minds and actions of the men who took part in the
+expedition. _Nova stella, novus rex_ ("a new star, a new sovereign") was
+a favourite proverb of the time. The chroniclers, with one accord, have
+delighted to relate that the Normans, "guided by a comet," invaded
+England. A representation of this object appears in the Bayeux Tapestry
+(see Fig. 19, p. 263).[26]
+
+[Illustration: FIG. 19.--The comet of 1066, as represented in the Bayeux
+Tapestry.
+
+(From the _World of Comets_.)]
+
+We have mentioned Halley's Comet of 1682, and how it revisits the
+neighbourhood of the earth at intervals of seventy-six years. The comet
+of 1066 has for many years been supposed to be Halley's Comet on one of
+its visits. The identity of these two, however, was only quite recently
+placed beyond all doubt by the investigations of Messrs Cowell and
+Crommelin. This comet appeared also in 1456, when John Huniades was
+defending Belgrade against the Turks led by Mahomet II., the conqueror
+of Constantinople, and is said to have paralysed both armies with fear.
+
+The Middle Ages have left us descriptions of comets, which show only too
+well how the imagination will run riot under the stimulus of terror. For
+instance, the historian, Nicetas, thus describes the comet of the year
+1182: "After the Romans were driven from Constantinople a prognostic was
+seen of the excesses and crimes to which Andronicus was to abandon
+himself. A comet appeared in the heavens similar to a writhing serpent;
+sometimes it extended itself, sometimes it drew itself in; sometimes, to
+the great terror of the spectators, it opened a huge mouth; it seemed
+that, as if thirsting for human blood, it was upon the point of
+satiating itself." And, again, the celebrated Ambrose Par�, the father
+of surgery, has left us the following account of the comet of 1528,
+which appeared in his own time: "This comet," said he, "was so horrible,
+so frightful, and it produced such great terror in the vulgar, that some
+died of fear, and others fell sick. It appeared to be of excessive
+length, and was of the colour of blood. At the summit of it was seen the
+figure of a bent arm, holding in its hand a great sword, as if about to
+strike. At the end of the point there were three stars. On both sides of
+the rays of this comet were seen a great number of axes, knives,
+blood-coloured swords, among which were a great number of hideous human
+faces, with beards and bristling hair." Par�, it is true, was no
+astronomer; yet this shows the effect of the phenomenon, even upon a man
+of great learning, as undoubtedly he was. It should here be mentioned
+that nothing very remarkable happened at or near the year 1528.
+
+Concerning the comet of 1680, the extraordinary story got about that, at
+Rome, a hen had laid an egg on which appeared a representation of the
+comet!
+
+But the superstitions with regard to comets were now nearing their end.
+The last blow was given by Halley, who definitely proved that they
+obeyed the laws of gravitation, and circulated around the sun as planets
+do; and further announced that the comet of 1682 had a period of
+seventy-six years, which would cause it to reappear in the year 1759. We
+have seen how this prediction was duly verified. We have seen, too, how
+this comet appeared again in 1835, and how it is due to return in the
+early part of 1910.
+
+
+[26] With regard to the words "Isti mirant stella" in the figure, Mr.
+W.T. Lynn suggests that they may not, after all, be the grammatically
+bad Latin which they appear, but that the legend is really "Isti
+mirantur stellam," the missing letters being supposed to be hidden by
+the building and the comet.
+
+
+
+
+CHAPTER XXI
+
+METEORS OR SHOOTING STARS
+
+
+Any one who happens to gaze at the sky for a short time on a clear night
+is pretty certain to be rewarded with a view of what is popularly known
+as a "shooting star." Such an object, however, is not a star at all, but
+has received its appellation from an analogy; for the phenomenon gives
+to the inexperienced in these matters an impression as if one of the
+many points of light, which glitter in the vaulted heaven, had suddenly
+become loosened from its place, and was falling towards the earth. In
+its passage across the sky the moving object leaves behind a trail of
+light which usually lasts for a few moments. Shooting stars, or meteors,
+as they are technically termed, are for the most part very small bodies,
+perhaps no larger than peas or pebbles, which, dashing towards our earth
+from space beyond, are heated to a white heat, and reduced to powder by
+the friction resulting from their rapid passage into our atmosphere.
+This they enter at various degrees of speed, in some cases so great as
+45 miles a second. The speed, of course, will depend greatly upon
+whether the earth and the meteors are rushing towards each other, or
+whether the latter are merely overtaking the earth. In the first of
+these cases the meteors will naturally collide with the atmosphere with
+great force; in the other case they will plainly come into it with much
+less rapidity. As has been already stated, it is from observations of
+such bodies that we are enabled to estimate, though very imperfectly,
+the height at which the air around our globe practically ceases, and
+this height is imagined to be somewhere about 100 miles. Fortunate,
+indeed, is it for us that there is a goodly layer of atmosphere over our
+heads, for, were this not so, these visitors from space would strike
+upon the surface of our earth night and day, and render existence still
+more unendurable than many persons choose to consider it. To what a
+bombardment must the moon be continually subject, destitute as she is of
+such an atmospheric shield!
+
+It is only in the moment of their dissolution that we really learn
+anything about meteors, for these bodies are much too small to be seen
+before they enter our atmosphere. The d�bris arising from their
+destruction is wafted over the earth, and, settling down eventually upon
+its surface, goes to augment the accumulation of that humble domestic
+commodity which men call dust. This continual addition of material
+tends, of course, to increase the mass of the earth, though the effect
+thus produced will be on an exceedingly small scale.
+
+The total number of meteors moving about in space must be practically
+countless. The number which actually dash into the earth's atmosphere
+during each year is, indeed, very great. Professor Simon Newcomb, the
+well-known American astronomer, has estimated that, of the latter, those
+large enough to be seen with the naked eye cannot be in all less than
+146,000,000,000 per annum. Ten times more numerous still are thought to
+be those insignificant ones which are seen to pass like mere sparks of
+light across the field of an observer's telescope.
+
+Until comparatively recent times, perhaps up to about a hundred years
+ago, it was thought that meteors were purely terrestrial phenomena which
+had their origin in the upper regions of the air. It, however, began to
+be noticed that at certain periods of the year these moving objects
+appeared to come from definite areas of the sky. Considerations,
+therefore, respecting their observed velocities, directions, and
+altitudes, gave rise to the theory that they are swarms of small bodies
+travelling around the sun in elongated elliptical orbits, all along the
+length of which they are scattered, and that the earth, in its annual
+revolution, rushing through the midst of such swarms at the same epoch
+each year, naturally entangles many of them in its atmospheric net.
+
+The dates at which the earth is expected to pass through the principal
+meteor-swarms are now pretty well known. These swarms are distinguished
+from one another by the direction of the sky from which the meteors seem
+to arrive. Many of the swarms are so wide that the earth takes days, and
+even weeks, to pass through them. In some of these swarms, or streams,
+as they are also called, the meteors are distributed with fair evenness
+along the entire length of their orbits, so that the earth is greeted
+with a somewhat similar shower at each yearly encounter. In others, the
+chief portions are bunched together, so that, in certain years, the
+display is exceptional (see Fig. 20, p. 269). That part of the heavens
+from which a shower of meteors is seen to emanate is called the
+"radiant," or radiant point, because the foreshortened view we get of
+the streaks of light makes it appear as if they radiated outwards from
+this point. In observations of these bodies the attention of astronomers
+is directed to registering the path and speed of each meteor, and to
+ascertaining the position of the radiant. It is from data such as these
+that computations concerning the swarms and their orbits are made.
+
+[Illustration: FIG. 20.--Passage of the Earth through the thickest
+portion of a Meteor Swarm. The Earth and the Meteors are here
+represented as approaching each other from opposite directions.]
+
+For the present state of knowledge concerning meteors, astronomy is
+largely indebted to the researches of Mr. W.F. Denning, of Bristol, and
+of the late Professor A.S. Herschel.
+
+During the course of each year the earth encounters a goodly number of
+meteor-swarms. Three of these, giving rise to fine displays, are very
+well known--the "Perseids," or August Meteors, and the "Leonids" and
+"Bielids," which appear in November.
+
+Of the above three the _Leonid_ display is by far the most important,
+and the high degree of attention paid to it has laid the foundation of
+meteoric astronomy in much the same way that the study of the
+fascinating corona has given such an impetus to our knowledge of the
+sun. The history of this shower of meteors may be traced back as far as
+A.D. 902, which was known as the "Year of the Stars." It is related that
+in that year, on the night of October 12th--the shower now comes about a
+month later--whilst the Moorish King, Ibrahim Ben Ahmed, lay dying
+before Cosenza, in Calabria, "a multitude of falling stars scattered
+themselves across the sky like rain," and the beholders shuddered at
+what they considered a dread celestial portent. We have, however, little
+knowledge of the subsequent history of the Leonids until 1698, since
+which time the maximum shower has appeared with considerable regularity
+at intervals of about thirty-three years. But it was not until 1799 that
+they sprang into especial notice. On the 11th November in that year a
+splendid display was witnessed at Cumana, in South America, by the
+celebrated travellers, Humboldt and Bonpland. Finer still, and
+surpassing all displays of the kind ever seen, was that of November 12,
+1833, when the meteors fell thick as snowflakes, 240,000 being estimated
+to have appeared during seven hours. Some of them were even so bright as
+to be seen in full daylight. The radiant from which the meteors seem to
+diverge was ascertained to be situated in the head of the constellation
+of the Lion, or "Sickle of Leo," as it is popularly termed, whence
+their name--Leonids. It was from a discussion of the observations then
+made that the American astronomer, Olmsted, concluded that these meteors
+sprang upon us from interplanetary space, and were not, as had been
+hitherto thought, born of our atmosphere. Later on, in 1837, Olbers
+formulated the theory that the bodies in question travelled around the
+sun in an elliptical orbit, and at the same time he established the
+periodicity of the maximum shower.
+
+The periodic time of recurrence of this maximum, namely, about
+thirty-three years, led to eager expectancy as 1866 drew near. Hopes
+were then fulfilled, and another splendid display took place, of which
+Sir Robert Ball, who observed it, has given a graphic description in his
+_Story of the Heavens_. The display was repeated upon a smaller scale in
+the two following years. The Leonids were henceforth deemed to hold an
+anomalous position among meteor swarms. According to theory the earth
+cut through their orbit at about the same date each year, and so a
+certain number were then seen to issue from the radiant. But, in
+addition, after intervals of thirty-three years, as has been seen, an
+exceptional display always took place; and this state of things was not
+limited to one year alone, but was repeated at each meeting for about
+three years running. The further assumption was, therefore, made that
+the swarm was much denser in one portion of the orbit than
+elsewhere,[27] and that this congested part was drawn out to such an
+extent that the earth could pass through the crossing place during
+several annual meetings, and still find it going by like a long
+procession (see Fig. 20, p. 269).
+
+In accordance with this ascertained period of thirty-three years, the
+recurrence of the great Leonid shower was timed to take place on the
+15th of November 1899. But there was disappointment then, and the
+displays which occurred during the few years following were not of much
+importance. A good deal of comment was made at the time, and theories
+were accordingly put forward to account for the failure of the great
+shower. The most probable explanation seems to be, that the attraction
+of one of the larger planets--Jupiter perhaps--has diverted the orbit
+somewhat from its old position, and the earth does not in consequence
+cut through the swarm in the same manner as it used to do.
+
+The other November display alluded to takes place between the 23rd and
+27th of that month. It is called the _Andromedid_ Shower, because the
+meteors appear to issue from the direction of the constellation of
+Andromeda, which at that period of the year is well overhead during the
+early hours of the night. These meteors are also known by the name of
+_Bielids_, from a connection which the orbit assigned to them appears to
+have with that of the well-known comet of Biela.
+
+M. Egenitis, Director of the Observatory of Athens, accords to the
+Bielids a high antiquity. He traces the shower back to the days of the
+Emperor Justinian. Theophanes, the Chronicler of that epoch, writing of
+the famous revolt of Nika in the year A.D. 532, says:--"During the same
+year a great fall of stars came from the evening till the dawn." M.
+Egenitis notes another early reference to these meteors in A.D. 752,
+during the reign of the Eastern Emperor, Constantine Copronymous.
+Writing of that year, Nicephorus, a Patriarch of Constantinople, has as
+follows:--"All the stars appeared to be detached from the sky, and to
+fall upon the earth."
+
+The Bielids, however, do not seem to have attracted particular notice
+until the nineteenth century. Attention first began to be riveted upon
+them on account of their suspected connection with Biela's comet. It
+appeared that the same orbit was shared both by that comet and the
+Bielid swarm. It will be remembered that the comet in question was not
+seen after its appearance in 1852. Since that date, however, the Bielid
+shower has shown an increased activity; which was further noticed to be
+especially great in those years in which the comet, had it still
+existed, would be due to pass near the earth.
+
+The third of these great showers to which allusion has above been made,
+namely, the _Perseids_, strikes the earth about the 10th of August; for
+which reason it is known on the Continent under the name of the "tears
+of St. Lawrence," the day in question being sacred to that Saint. This
+shower is traceable back many centuries, even as far as the year A.D.
+811. The name given to these meteors, "Perseids," arises from the fact
+that their radiant point is situated in the constellation of Perseus.
+This shower is, however, not by any means limited to the particular
+night of August 10th, for meteors belonging to the swarm may be observed
+to fall in more or less varying quantities from about July 8th to August
+22nd. The Perseid meteors sometimes fall at the rate of about sixty per
+hour. They are noted for their great rapidity of motion, and their
+trails besides often persist for a minute or two before being
+disseminated. Unlike the other well-known showers, the radiants of which
+are stationary, that of the Perseids shifts each night a little in an
+easterly direction.
+
+The orbit of the Perseids cuts that of the earth almost perpendicularly.
+The bodies are generally supposed to be the result of the disintegration
+of an ancient comet which travelled in the same orbit. Tuttle's Comet,
+which passed close to the earth in 1862, also belongs to this orbit; and
+its period of revolution is calculated to be 131 years. The Perseids
+appear to be disseminated all along this great orbit, for we meet them
+in considerable quantities each year. The bodies in question are in
+general particularly small. The swarm has, however, like most others, a
+somewhat denser portion, and through this the earth passed in 1848. The
+_aphelion_, or point where the far end of the orbit turns back again
+towards the sun, is situated right away beyond the path of Neptune, at a
+distance of forty-eight times that of the earth from the sun. The comet
+of 1532 also belongs to the Perseid orbit. It revisited the
+neighbourhood of the earth in 1661, and should have returned in 1789.
+But we have no record of it in that year; for which omission the then
+politically disturbed state of Europe may account. If not already
+disintegrated, this comet is due to return in 1919.
+
+This supposed connection between comets and meteor-swarms must be also
+extended to the case of the Leonids. These meteors appear to travel
+along the same track as Tempel's Comet of 1866.
+
+It is considered that the attractions of the various bodies of the
+solar system upon a meteor swarm must eventually result in breaking up
+the "bunched" portion, so that in time the individual meteors should
+become distributed along the whole length of the orbit. Upon this
+assumption the Perseid swarm, in which the meteors are fairly well
+scattered along its path, should be of greater age than the Leonid. As
+to the Leonid swarm itself, Le Verrier held that it was first brought
+into the solar system in A.D. 126, having been captured from outer space
+by the gravitative action of the planet Uranus.
+
+The acknowledged theory of meteor swarms has naturally given rise to an
+idea, that the sunlight shining upon such a large collection of
+particles ought to render a swarm visible before its collision with the
+earth's atmosphere. Several attempts have therefore been made to search
+for approaching swarms by photography, but, so far, it appears without
+success. It has also been proposed, by Mr. W.H.S. Monck, that the stars
+in those regions from which swarms are due, should be carefully watched,
+to see if their light exhibits such temporary diminutions as would be
+likely to arise from the momentary interposition of a cloud of moving
+particles.
+
+Between ten and fifteen years ago it happened that several well-known
+observers, employed in telescopic examination of the sun and moon,
+reported that from time to time they had seen small dark bodies,
+sometimes singly, sometimes in numbers, in passage across the discs of
+the luminaries. It was concluded that these were meteors moving in space
+beyond the atmosphere of the earth. The bodies were called "dark
+meteors," to emphasise the fact that they were seen in their natural
+condition, and not in that momentary one in which they had hitherto been
+always seen; _i.e._ when heated to white heat, and rapidly vaporised, in
+the course of their passage through the upper regions of our air. This
+"discovery" gave promise of such assistance to meteor theories, that
+calculations were made from the directions in which they had been seen
+to travel, and the speeds at which they had moved, in the hope that some
+information concerning their orbits might be revealed. But after a while
+some doubt began to be thrown upon their being really meteors, and
+eventually an Australian observer solved the mystery. He found that they
+were merely tiny particles of dust, or of the black coating on the inner
+part of the tube of the telescope, becoming detached from the sides of
+the eye-piece and falling across the field of view. He was led to this
+conclusion by having noted that a gentle tapping of his instrument
+produced the "dark" bodies in great numbers! Thus the opportunity of
+observing meteors beyond our atmosphere had once more failed.
+
+_Meteorites_, also known as �rolites and fireballs, are usually placed
+in quite a separate category from meteors. They greatly exceed the
+latter in size, are comparatively rare, and do not appear in any way
+connected with the various showers of meteors. The friction of their
+passage through the atmosphere causes them to shine with a great light;
+and if not shattered to pieces by internal explosions, they reach the
+ground to bury themselves deep in it with a great rushing and noise.
+When found by uncivilised peoples, or savages, they are, on account of
+their celestial origin, usually regarded as objects of wonder and of
+worship, and thus have arisen many mythological legends and deifications
+of blackened stones. On the other hand, when they get into the
+possession of the civilised, they are subjected to careful examinations
+and tests in chemical laboratories. The bodies are, as a rule, composed
+of stone, in conjunction with iron, nickel, and such elements as exist
+in abundance upon our earth; though occasionally specimens are found
+which are practically pure metal. In the museums of the great capitals
+of both Continents are to be seen some fine collections of meteorites.
+Several countries--Greenland and Mexico, for instance--contain in the
+soil much meteoric iron, often in masses so large as to baffle all
+attempts at removal. Blocks of this kind have been known to furnish the
+natives in their vicinity for many years with sources of workable iron.
+
+The largest meteorite in the world is one known as the Anighito
+meteorite. It was brought to the United States by the explorer Peary,
+who found it at Cape York in Greenland. He estimates its weight at from
+90 to 100 tons. One found in Mexico, called the Bacubirito, comes next,
+with an estimated weight of 27-1/2 tons. The third in size is the
+Willamette meteorite, found at Willamette in Oregon in 1902. It measures
+10 � 6-1/2 � 4-1/2 feet, and weighs about 15-1/2 tons.
+
+
+[27] The "gem" of the meteor ring, as it has been termed.
+
+
+
+
+CHAPTER XXII
+
+THE STARS
+
+
+In the foregoing chapters we have dealt at length with those celestial
+bodies whose nearness to us brings them into our especial notice. The
+entire room, however, taken up by these bodies, is as a mere point in
+the immensities of star-filled space. The sun, too, is but an ordinary
+star; perhaps quite an insignificant one[28] in comparison with the
+majority of those which stud that background of sky against which the
+planets are seen to perform their wandering courses.
+
+Dropping our earth and the solar system behind, let us go afield and
+explore the depths of space.
+
+We have seen how, in very early times, men portioned out the great mass
+of the so-called "fixed stars" into divisions known as constellations.
+The various arrangements, into which the brilliant points of light fell
+as a result of perspective, were noticed and roughly compared with such
+forms as were familiar to men upon the earth. Imagination quickly saw in
+them the semblances of heroes and of mighty fabled beasts; and, around
+these monstrous shapes, legends were woven, which told how the great
+deeds done in the misty dawn of historical time had been enshrined by
+the gods in the sky as an example and a memorial for men. Though the
+centuries have long outlived such fantasies, yet the constellation
+figures and their ancient names have been retained to this day, pretty
+well unaltered for want of any better arrangement. The Great and Little
+Bears, Cassiopeia, Perseus, and Andromeda, Orion and the rest, glitter
+in our night skies just as they did centuries and centuries ago.
+
+Many persons seem to despair of gaining any real knowledge of astronomy,
+merely because they are not versed in recognising the constellations.
+For instance, they will say:--"What is the use of my reading anything
+about the subject? Why, I believe I couldn't even point out the Great
+Bear, were I asked to do so!" But if such persons will only consider for
+a moment that what we call the Great Bear has no existence in fact, they
+need not be at all disheartened. Could we but view this familiar
+constellation from a different position in space, we should perhaps be
+quite unable to recognise it. Mountain masses, for instance, when seen
+from new directions, are often unrecognisable.
+
+It took, as we have seen, a very long time for men to acknowledge the
+immense distances of the stars from our earth. Their seeming
+unchangeableness of position was, as we have seen, largely responsible
+for the idea that the earth was immovable in space. It is a wonder that
+the Copernican system ever gained the day in the face of this apparent
+fixity of the stars. As time went on, it became indeed necessary to
+accord to these objects an almost inconceivable distance, in order to
+account for the fact that they remained apparently quite undisplaced,
+notwithstanding the journey of millions of miles which the earth was now
+acknowledged to make each year around the sun. In the face of the
+gradual and immense improvement in telescopes, this apparent immobility
+of the stars was, however, not destined to last. The first ascertained
+displacement of a star, namely that of 61 Cygni, noted by Bessel in the
+year 1838, definitely proved to men the truth of the Copernican system.
+Since then some forty more stars have been found to show similar tiny
+displacements. We are, therefore, in possession of the fact, that the
+actual distances of a few out of the great host can be calculated.
+
+To mention some of these. The nearest star to the earth, so far as we
+yet know, is Alpha Centauri, which is distant from us about 25 billions
+of miles. The light from this star, travelling at the stupendous rate of
+about 186,000 miles per second, takes about 4-1/4 years to reach our
+earth, or, to speak astronomically, Alpha Centauri is about 4-1/4 "light
+years" distant from us. Sirius--the brightest star in the whole sky--is
+at twice this distance, _i.e._ about 8-1/2 light years. Vega is about 30
+light years distant from us, Capella about 32, and Arcturus about 100.
+
+The displacements, consequent on the earth's movement, have, however,
+plainly nothing to say to any real movements on the part of the stars
+themselves. The old idea was that the stars were absolutely fixed; hence
+arose the term "fixed stars"--a term which, though inaccurate, has not
+yet been entirely banished from the astronomical vocabulary. But careful
+observations extending over a number of years have shown slight changes
+of position among these bodies; and such alterations cannot be ascribed
+to the revolution of the earth in its orbit, for they appear to take
+place in every direction. These evidences of movement are known as
+"proper motions," that is to say, actual motions in space proper to the
+stars themselves. Stars which are comparatively near to us show, as a
+rule, greater proper motions than those which are farther off. It must
+not, however, be concluded that these proper motions are of any very
+noticeable amounts. They are, as a matter of fact, merely upon the same
+apparently minute scale as other changes in the heavens; and would
+largely remain unnoticed were it not for the great precision of modern
+astronomical instruments.
+
+One of the swiftest moving of the stars is a star of the sixth magnitude
+in the constellation of the Great Bear; which is known as "1830
+Groombridge," because this was the number assigned to it in a catalogue
+of stars made by an astronomer of that name. It is popularly known as
+the "Runaway Star," a name given to it by Professor Newcomb. Its speed
+is estimated to be at least 138 miles per second. It may be actually
+moving at a much greater rate, for it is possible that we see its path
+somewhat foreshortened.
+
+A still greater proper motion--the greatest, in fact, known--is that of
+an eighth magnitude star in the southern hemisphere, in the
+constellation of Pictor. Nothing, indeed, better shows the enormous
+distance of the stars from us, and the consequent inability of even such
+rapid movements to alter the appearance of the sky during the course of
+ages, than the fact that it would take more than two centuries for the
+star in question to change its position in the sky by a space equal to
+the apparent diameter of the moon; a statement which is equivalent to
+saying that, were it possible to see this star with the naked eye, which
+it is not, at least twenty-five years would have to elapse before one
+would notice that it had changed its place at all!
+
+Both the stars just mentioned are very faint. That in Pictor is, as has
+been said, not visible to the naked eye. It appears besides to be a very
+small body, for Sir David Gill finds a parallax which makes it only as
+far off from us as Sirius. The Groombridge star, too, is just about the
+limit of ordinary visibility. It is, indeed, a curious fact that the
+fainter stars seem, on the average, to be moving more rapidly than the
+brighter.
+
+Investigations into proper motions lead us to think that every one of
+the stars must be moving in space in some particular direction. To take
+a few of the best known. Sirius and Vega are both approaching our system
+at a rate of about 10 miles per second, Arcturus at about 5 miles per
+second, while Capella is receding from us at about 15 miles per second.
+Of the twin brethren, Castor and Pollux, Castor is moving away from us
+at about 4-1/2 miles per second, while Pollux is coming towards us at
+about 33 miles per second.
+
+Much of our knowledge of proper motions has been obtained indirectly by
+means of the spectroscope, on the Doppler principle already treated of,
+by which we are enabled to ascertain whether a source from which light
+is coming is approaching or receding.
+
+The sun being, after all, a mere star, it will appear only natural for
+it also to have a proper motion of its own. This is indeed the case; and
+it is rushing along in space at a rate of between ten and twelve miles
+per second, carrying with it its whole family of planets and satellites,
+of comets and meteors. The direction in which it is advancing is towards
+a point in the constellation of Lyra, not far from its chief star Vega.
+This is shown by the fact that the stars about the region in question
+appear to be opening out slightly, while those in the contrary portion
+of the sky appear similarly to be closing together.
+
+Sir William Herschel was the first to discover this motion of the sun
+through space; though in the idea that such a movement might take place
+he seems to have been anticipated by Mayer in 1760, by Michell in 1767,
+and by Lalande in 1776.
+
+A suggestion has been made that our solar system, in its motion through
+the celestial spaces, may occasionally pass through regions where
+abnormal magnetic conditions prevail, in consequence of which
+disturbances may manifest themselves throughout the system at the same
+instant. Thus the sun may be getting the credit of _producing_ what it
+merely reacts to in common with the rest of its family. But this
+suggestion, plausible though it may seem, will not explain why the
+magnetic disturbances experienced upon our earth show a certain
+dependence upon such purely local facts, as the period of the sun's
+rotation, for instance.
+
+One would very much like to know whether the movement of the sun is
+along a straight line, or in an enormous orbit around some centre. The
+idea has been put forward that it may be moving around the centre of
+gravity of the whole visible stellar universe. M�dler, indeed,
+propounded the notion that Alcyone--the chief star in the group known as
+the Pleiades--occupied this centre, and that everything revolved around
+it. He went even further to proclaim that here was the Place of the
+Almighty, the Mansion of the Eternal! But M�dler's ideas upon this point
+have long been shelved.
+
+To return to the general question of the proper motion of stars.
+
+In several instances these motions appear to take place in groups, as if
+certain stars were in some way associated together. For example, a large
+number of the stars composing the Pleiades appear to be moving through
+space in the same direction. Also, of the seven stars composing the
+Plough, all but two--the star at the end of its "handle," and that one
+of the "pointers," as they are called, which is the nearer to the pole
+star--have a common proper motion, _i.e._ are moving in the same
+direction and nearly at the same rate.
+
+Further still, the well-known Dutch astronomer, Professor Kapteyn, of
+Groningen, has lately reached the astonishing conclusion that a great
+part of the visible universe is occupied by two vast streams of stars
+travelling in opposite directions. In both these great streams, the
+individual bodies are found, besides, to be alike in design, alike in
+chemical constitution, and alike in the stage of their development.
+
+A fable related by the Persian astronomer, Al Sufi (tenth century, A.D.)
+shows well the changes in the face of the sky which proper motions are
+bound to produce after great lapses of time. According to this fable the
+stars Sirius and Procyon were the sisters of the star Canopus. Canopus
+married Rigel (another star,) but, having murdered her, he fled towards
+the South Pole, fearing the anger of his sisters. The fable goes on to
+relate, among other things, that Sirius followed him across the Milky
+Way. Mr. J. E. Gore, in commenting on the story, thinks that it may be
+based upon a tradition of Sirius having been seen by the men of the
+Stone Age on the opposite side of the Milky Way to that on which it now
+is.
+
+Sirius is in that portion of the heavens _from_ which the sun is
+advancing. Its proper motion is such that it is gaining upon the earth
+at the rate of about ten miles per second, and so it must overtake the
+sun after the lapse of great ages. Vega, on the other hand, is coming
+towards us from that part of the sky _towards_ which the sun is
+travelling. It should be about half a million years before the sun and
+Vega pass by one another. Those who have specially investigated this
+question say that, as regards the probability of a near approach, it is
+much more likely that Vega will be then so far to one side of the sun,
+that her brightness will not be much greater than it is at this moment.
+
+Considerations like these call up the chances of stellar collisions.
+Such possibilities need not, however, give rise to alarm; for the stars,
+as a rule, are at such great distances from each other, that the
+probability of relatively near approaches is slight.
+
+We thus see that the constellations do not in effect exist, and that
+there is in truth no real background to the sky. We find further that
+the stars are strewn through space at immense distances from each other,
+and are moving in various directions hither and thither. The sun, which
+is merely one of them, is moving also in a certain direction, carrying
+the solar system along with it. It seems, therefore, but natural to
+suppose that many a star may be surrounded by some planetary system in a
+way similar to ours, which accompanies it through space in the course of
+its celestial journeyings.
+
+
+[28] Vega, for instance, shines one hundred times more brightly than the
+sun would do, were it to be removed to the distance at which that star
+is from us.
+
+
+
+
+CHAPTER XXIII
+
+THE STARS--_continued_
+
+
+The stars appear to us to be scattered about the sky without any orderly
+arrangement. Further, they are of varying degrees of brightness; some
+being extremely brilliant, whilst others can but barely be seen. The
+brightness of a star may arise from either of two causes. On the one
+hand, the body may be really very bright in itself; on the other hand,
+it may be situated comparatively near to us. Sometimes, indeed, both
+these circumstances may come into play together.
+
+Since variation in brightness is the most noticeable characteristic of
+the stars, men have agreed to class them in divisions called
+"magnitudes." This term, it must be distinctly understood, is employed
+in such classification without any reference whatever to actual size,
+being merely taken to designate roughly the amount of light which we
+receive from a star. The twenty brightest stars in the sky are usually
+classed in the first magnitude. In descending the scale, each magnitude
+will be noticed to contain, broadly speaking, three times as many stars
+as the one immediately above it. Thus the second magnitude contains 65,
+the third 190, the fourth 425, the fifth 1100, and the sixth 3200. The
+last of these magnitudes is about the limit of the stars which we are
+able to see with the naked eye. Adding, therefore, the above numbers
+together, we find that, without the aid of the telescope, we cannot see
+more than about 5000 stars in the entire sky--northern and southern
+hemispheres included. Quite a small telescope will, however, allow us to
+see down to the ninth magnitude, so that the total number of stars
+visible to us with such very moderate instrumental means will be well
+over 100,000.
+
+It must not, however, be supposed that the stars included within each
+magnitude are all of exactly the same brightness. In fact, it would be
+difficult to say if there exist in the whole sky two stars which send us
+precisely the same amount of light. In arranging the magnitudes, all
+that was done was to make certain broad divisions, and to class within
+them such stars as were much on a par with regard to brightness. It may
+here be noted that a standard star of the first magnitude gives us about
+one hundred times as much light as a star of the sixth magnitude, and
+about one million times as much as one of the sixteenth magnitude--which
+is near the limit of what we can see with the very best telescope.
+
+Though the first twenty stars in the sky are popularly considered as
+being of the first magnitude, yet several of them are much brighter than
+an average first magnitude star would be. For instance, Sirius--the
+brightest star in the whole sky--is equal to about eleven first
+magnitude stars, like, say, Aldebaran. In consequence of such
+differences, astronomers are agreed in classifying the brightest of them
+as _brighter_ than the standard first magnitude star. On this principle
+Sirius would be about two and a half magnitudes _above_ the first. This
+notation is usefully employed in making comparisons between the amount
+of light which we receive from the sun, and that which we get from an
+individual star. Thus the sun will be about twenty-seven and a half
+magnitudes _above_ the first magnitude. The range, therefore, between
+the light which we receive from the sun (considered merely as a very
+bright star) and the first magnitude stars is very much greater than
+that between the latter and the faintest star which can be seen with the
+telescope, or even registered upon the photographic plate.
+
+To classify stars merely by their magnitudes, without some definite note
+of their relative position in the sky, would be indeed of little avail.
+We must have some simple method of locating them in the memory, and the
+constellations of the ancients here happily come to our aid. A system
+combining magnitudes with constellations was introduced by Bayer in
+1603, and is still adhered to. According to this the stars in each
+constellation, beginning with the brightest star, are designated by the
+letters of the Greek alphabet taken in their usual order. For example,
+in the constellation of Canis Major, or the Greater Dog, the brightest
+star is the well-known Sirius, called by the ancients the "Dog Star";
+and this star, in accordance with Bayer's method, has received the Greek
+letter [a] (alpha), and is consequently known as Alpha Canis
+Majoris.[29] As soon as the Greek letters are used up in this way the
+Roman alphabet is brought into requisition, after which recourse is had
+to ordinary numbers.
+
+Notwithstanding this convenient arrangement, some of the brightest
+stars are nearly always referred to by certain proper names given to
+them in old times. For instance, it is more usual to speak of Sirius,
+Arcturus, Vega, Capella, Procyon, Aldebaran, Regulus, and so on, than of
+[a] Canis Majoris, [a] Bo�tis, [a] Lyr�, [a] Aurig�, [a] Canis Minoris,
+[a] Tauri, [a] Leonis, &c. &c.
+
+In order that future generations might be able to ascertain what changes
+were taking place in the face of the sky, astronomers have from time to
+time drawn up catalogues of stars. These lists have included stars of a
+certain degree of brightness, their positions in the sky being noted
+with the utmost accuracy possible at the period. The earliest known
+catalogue of this kind was made, as we have seen, by the celebrated
+Greek astronomer, Hipparchus, about the year 125 B.C. It contained 1080
+stars. It was revised and brought up to date by Ptolemy in A.D. 150.
+Another celebrated list was that drawn up by the Persian astronomer, Al
+Sufi, about the year A.D. 964. In it 1022 stars were noted down. A
+catalogue of 1005 stars was made in 1580 by the famous Danish
+astronomer, Tycho Brahe. Among modern catalogues that of Argelander
+(1799-1875) contained as many as 324,198 stars. It was extended by
+Sch�nfeld so as to include a portion of the Southern Hemisphere, in
+which way 133,659 more stars were added.
+
+In recent years a project was placed on foot of making a photographic
+survey of the sky, the work to be portioned out among various nations. A
+great part of this work has already been brought to a conclusion. About
+15,000,000 stars will appear upon the plates; but, so far, it has been
+proposed to catalogue only about a million and a quarter of the
+brightest of them. This idea of surveying the face of the sky by
+photography sprang indirectly from the fine photographs which Sir David
+Gill took, when at the Cape of Good Hope, of the Comet of 1882. The
+immense number of star-images which had appeared upon his plates
+suggested the idea that photography could be very usefully employed to
+register the relative positions of the stars.
+
+The arrangement of seven stars known as the "Plough" is perhaps the most
+familiar configuration in the sky (see Plate XIX., p. 292). In the
+United States it is called the "Dipper," on account of its likeness to
+the outline of a saucepan, or ladle. "Charles' Wain" was the old English
+name for it, and readers of C�sar will recollect it under
+_Septentriones_, or the "Seven Stars," a term which that writer uses as
+a synonym for the North. Though identified in most persons' minds with
+_Ursa Major_, or the Great Bear, the Plough is actually only a small
+portion of that famous constellation. Six out of the seven stars which
+go to make up the well-known figure are of the second magnitude, while
+the remaining one, which is the middle star of the group, is of the
+third.
+
+The Greek letters, as borne by the individual stars of the Plough, are a
+plain transgression of Bayer's method as above described, for they have
+certainly not been allotted here in accordance with the proper order of
+brightness. For instance, the third magnitude star, just alluded to as
+being in the middle of the group, has been marked with the Greek letter
+[d] (Delta); and so is made to take rank _before_ the stars composing
+what is called the "handle" of the Plough, which are all of the second
+magnitude. Sir William Herschel long ago drew attention to the irregular
+manner in which Bayer's system had been applied. It is, indeed, a great
+pity that this notation was not originally worked out with greater care
+and correctness; for, were it only reliable, it would afford great
+assistance to astronomers in judging of what changes in relative
+brightness have taken place among the stars.
+
+Though we may speak of using the constellations as a method of finding
+our way about the sky, it is, however, to certain marked groupings in
+them of the brighter stars that we look for our sign-posts.
+
+Most of the constellations contain a group or so of noticeable stars,
+whose accidental arrangement dimly recalls the outline of some familiar
+geometrical figure and thus arrests the attention.[30] For instance, in
+an almost exact line with the two front stars of the Plough, or
+"pointers" as they are called,[31] and at a distance about five times as
+far away as the interval between them, there will be found a third star
+of the second magnitude. This is known as Polaris, or the Pole Star, for
+it very nearly occupies that point of the heaven towards which the north
+pole of the earth's axis is _at present_ directed (see Plate XIX., p.
+292). Thus during the apparently daily rotation of the heavens, this
+star looks always practically stationary. It will, no doubt, be
+remembered how Shakespeare has put into the mouth of Julius C�sar these
+memorable words:--
+
+"But I am constant as the northern star,
+Of whose true-fix'd and resting quality
+There is no fellow in the firmament."
+
+[Illustration: PLATE XIX. THE SKY AROUND THE NORTH POLE
+
+We see here the Plough, the Pole Star, Ursa Minor, Auriga, Cassiopeia's
+Chair, and Lyra. Also the Circle of Precession, along which the Pole
+makes a complete revolution in a period of 25,868 years, and the
+Temporary Star discovered by Tycho Brahe in the year 1572.
+
+(Page 291)]
+
+On account of the curvature of the earth's surface, the height at which
+the Pole Star is seen above the horizon at any place depends regularly
+upon the latitude; that is to say, the distance of the place in question
+from the equator. For instance, at the north pole of the earth, where
+the latitude is greatest, namely, 90�, the Pole Star will appear
+directly overhead; whereas in England, where the latitude is about 50�,
+it will be seen a little more than half way up the northern sky. At the
+equator, where the latitude is _nil_, the Pole Star will be on the
+horizon due north.
+
+In consequence of its unique position, the Pole Star is of very great
+service in the study of the constellations. It is a kind of centre
+around which to hang our celestial ideas--a starting point, so to speak,
+in our voyages about the sky.
+
+According to the constellation figures, the Pole Star is in _Ursa
+Minor_, or the Little Bear, and is situated at the end of the tail of
+that imaginary figure (see Plate XIX., p. 292). The chief stars of this
+constellation form a group not unlike the Plough, except that the
+"handle" is turned in the contrary direction. The Americans, in
+consequence, speak of it as the "Little Dipper."
+
+Before leaving this region of the sky, it will be well to draw attention
+to the second magnitude star [z] in the Great Bear (Zeta Urs� Majoris),
+which is the middle star in the "handle" of the Plough. This star is
+usually known as Mizar, a name given to it by the Arabians. A person
+with good eyesight can see quite near to it a fifth magnitude star,
+known under the name of Alcor. We have here a very good example of that
+deception in the estimation of objects in the sky, which has been
+alluded to in an earlier chapter. Alcor is indeed distant from Mizar by
+about one-third the apparent diameter of the moon, yet no one would
+think so!
+
+On the other side of Polaris from the Plough, and at about an equal
+apparent distance, will be found a figure in the form of an irregular
+"W", made up of second and third magnitude stars. This is the well-known
+"Cassiopeia's Chair"--portion of the constellation of _Cassiopeia_ (see
+Plate XIX., p. 292).
+
+On either side of the Pole Star, about midway between the Plough and
+Cassiopeia's Chair, but a little further off from it than these, are the
+constellations of _Auriga_ and _Lyra_ (see Plate XIX., p. 292). The
+former constellation will be easily recognised, because its chief
+features are a brilliant yellowish first magnitude star, with one of the
+second magnitude not far from it. The first magnitude star is Capella,
+the other is [b] Aurig�. Lyra contains only one first magnitude
+star--Vega, pale blue in colour. This star has a certain interest for us
+from the fact that, as a consequence of that slow shift of direction of
+the earth's axis known as Precession, it will be very near the north
+pole of the heavens in some 12,000 years, and so will then be considered
+the pole star (see Plate XIX., p. 292). The constellation of Lyra
+itself, it must also be borne in mind, occupies that region of the
+heavens towards which the solar system is travelling.
+
+The handle of the Plough points roughly towards the constellation of
+_Bo�tes_, in which is the brilliant first magnitude star Arcturus. This
+star is of an orange tint.
+
+Between Bo�tes and Lyra lie the constellations of _Corona Borealis_ (or
+the Northern Crown) and _Hercules_. The chief feature of Corona
+Borealis, which is a small constellation, is a semicircle of six small
+stars, the brightest of which is of the second magnitude. The
+constellation of Hercules is very extensive, but contains no star
+brighter than the third magnitude.
+
+Near to Lyra, on the side away from Hercules, are the constellations of
+_Cygnus_ and _Aquila_. Of the two, the former is the nearer to the Pole
+Star, and will be recognised by an arrangement of stars widely set in
+the form of a cross, or perhaps indeed more like the framework of a
+boy's kite. The position of Aquila will be found through the fact that
+three of its brightest stars are almost in a line and close together.
+The middle of these is Altair, a yellowish star of the first magnitude.
+
+At a little distance from Ursa Major, on the side away from the Pole
+Star, is the constellation of _Leo_, or the Lion. Its chief feature is a
+series of seven stars, supposed to form the head of that animal. The
+arrangement of these stars is, however, much more like a sickle,
+wherefore this portion of the constellation is usually known as the
+"Sickle of Leo." At the end of the handle of the sickle is a white first
+magnitude star--Regulus.
+
+The reader will, no doubt, recollect that it is from a point in the
+Sickle of Leo that the Leonid meteors appear to radiate.
+
+The star second in brightness in the constellation of Leo is known as
+Denebola. This star, now below the second magnitude, seems to have been
+very much brighter in the past. It is noted, indeed, as a brilliant
+first magnitude star by Al Sufi, that famous Persian astronomer who
+lived, as we have seen, in the tenth century. Ptolemy also notes it as
+of the first magnitude.
+
+In the neighbourhood of Auriga, and further than it from the Pole Star,
+are several remarkable constellations--Taurus, Orion, Gemini, Canis
+Minor, and Canis Major (see Plate XX., p. 296).
+
+The first of these, _Taurus_ (or the Bull), contains two conspicuous
+star groups--the Pleiades and the Hyades. The Pleiades are six or seven
+small stars quite close together, the majority of which are of the
+fourth magnitude. This group is sometimes occulted by the moon. The way
+in which the stars composing it are arranged is somewhat similar to that
+in the Plough, though of course on a scale ever so much smaller. The
+impression which the group itself gives to the casual glance is thus
+admirably pictured in Tennyson's _Locksley Hall_:--
+
+"Many a night I saw the Pleiads, rising through the mellow shade,
+Glitter like a swarm of fire-flies tangled in a silver braid."
+
+[Illustration: PLATE XX. ORION AND HIS NEIGHBOURS
+
+We see here that magnificent region of the sky which contains the
+brightest star of all--Sirius. Note also especially the Milky Way, the
+Pleiades, the Hyades, and the "Belt" and "Sword" of Orion.
+
+(Page 296)]
+
+The group of the Hyades occupies the "head" of the Bull, and is much
+more spread out than that of the Pleiades. It is composed besides of
+brighter stars, the brightest being one of the first magnitude,
+Aldebaran. This star is of a red colour, and is sometimes known as the
+"Eye of the Bull."
+
+The constellation of _Orion_ is easily recognised as an irregular
+quadrilateral formed of four bright stars, two of which, Betelgeux
+(reddish) and Rigel (brilliant white), are of the first magnitude. In
+the middle of the quadrilateral is a row of three second magnitude
+stars, known as the "Belt" of Orion. Jutting off from this is another
+row of stars called the "Sword" of Orion.
+
+The constellation of _Gemini_, or the Twins, contains two bright
+stars--Castor and Pollux--close to each other. Pollux, though marked
+with the Greek letter [b], is the brighter of the two, and nearly of the
+standard first magnitude.
+
+Just further from the Pole than Gemini, is the constellation of _Canis
+Minor_, or the Lesser Dog. Its chief star is a white first magnitude
+one--Procyon.
+
+Still further again from the Pole than Canis Minor is the constellation
+of _Canis Major_, or the Greater Dog. It contains the brightest star in
+the whole sky, the first magnitude star Sirius, bluish-white in colour,
+also known as the "Dog Star." This star is almost in line with the stars
+forming the Belt of Orion, and is not far from that constellation.
+
+Taken in the following order, the stars Capella, [b] Aurig�, Castor,
+Pollux, Procyon, and Sirius, when they are all above the horizon at the
+same time, form a beautiful curve stretching across the heaven.
+
+The groups of stars visible in the southern skies have by no means the
+same fascination for us as those in the northern. The ancients were in
+general unacquainted with the regions beyond the equator, and so their
+scheme of constellations did not include the sky around the South Pole
+of the heavens. In modern times, however, this part of the celestial
+expanse was also portioned out into constellations for the purpose of
+easy reference; but these groupings plainly lack that simplicity of
+conception and legendary interest which are so characteristic of the
+older ones.
+
+The brightest star in the southern skies is found in the constellation
+of _Argo_, and is known as Canopus. In brightness it comes next to
+Sirius, and so is second in that respect in the entire heaven. It does
+not, however, rise above the English horizon.
+
+Of the other southern constellations, two call for especial notice, and
+these adjoin each other. One is _Centaurus_ (or the Centaur), which
+contains the two first magnitude stars, [a] and [b] Centauri. The first
+of these, Alpha Centauri, comes next in brightness to Canopus, and is
+notable as being the nearest of all the stars to our earth. The other
+constellation is called _Crux_, and contains five stars set in the form
+of a rough cross, known as the "Southern Cross." The brightest of these,
+[a] Crucis, is of the first magnitude.
+
+Owing to the Precession of the Equinoxes, which, as we have seen,
+gradually shifts the position of the Pole among the stars, certain
+constellations used to be visible in ancient times in more northerly
+latitudes than at present. For instance, some five thousand years ago
+the Southern Cross rose above the English horizon, and was just visible
+in the latitude of London. It has, however, long ago even ceased to be
+seen in the South of Europe. The constellation of Crux happens to be
+situated in that remarkable region of the southern skies, in which are
+found the stars Canopus and Alpha Centauri, and also the most brilliant
+portion of the Milky Way. It is believed to be to this grand celestial
+region that allusion is made in the Book of Job (ix. 9), under the title
+of the "Chambers of the South." The "Cross" must have been still a
+notable feature in the sky of Palestine in the days when that ancient
+poem was written.
+
+There is no star near enough to the southern pole of the heavens to earn
+the distinction of South Polar Star.
+
+The Galaxy, or _Milky Way_ (see Plate XX., p. 296), is a broad band of
+diffused light which is seen to stretch right around the sky. The
+telescope, however, shows it to be actually composed of a great host of
+very faint stars--too faint, indeed, to be separately distinguished with
+the naked eye. Along a goodly stretch of its length it is cleft in two;
+while near the south pole of the heavens it is entirely cut across by a
+dark streak.
+
+In this rapid survey of the face of the sky, we have not been able to do
+more than touch in the broadest manner upon some of the most noticeable
+star groups and a few of the most remarkable stars. To go any further is
+not a part of our purpose; our object being to deal with celestial
+bodies as they actually are, and not in those groupings under which they
+display themselves to us as a mere result of perspective.
+
+
+[29] Attention must here be drawn to the fact that the name of the
+constellation is always put in the genitive case.
+
+[30] The early peoples, as we have seen, appear to have been attracted
+by those groupings of the stars which reminded them in a way of the
+figures of men and animals. We moderns, on the other hand, seek almost
+instinctively for geometrical arrangements. This is, perhaps,
+symptomatic of the evolution of the race. In the growth of the
+individual we find, for example, something analogous. A child, who has
+been given pencil and paper, is almost certain to produce grotesque
+drawings of men and animals; whereas the idle and half-conscious
+scribblings which a man may make upon his blotting-paper are usually of
+a geometrical character.
+
+[31] Because the line joining them _points_ in the direction of the Pole
+Star.
+
+
+
+
+CHAPTER XXIV
+
+SYSTEMS OF STARS
+
+
+Many stars are seen comparatively close together. This may plainly arise
+from two reasons. Firstly, the stars may happen to be almost in the same
+line of sight; that is to say, seen in nearly the same direction; and
+though one star may be ever so much nearer to us than the other, the
+result will give all the appearance of a related pair. A seeming
+arrangement of two stars in this way is known as a "double," or double
+star; or, indeed, to be very precise, an "optical double." Secondly, in
+a pair of stars, both bodies may be about the same distance from us, and
+actually connected as a system like, for instance, the moon and the
+earth. A pairing of stars in this way, though often casually alluded to
+as a double star, is properly termed a "binary," or binary system.
+
+But collocations of stars are by no means limited to two. We find,
+indeed, all over the sky such arrangements in which there are three or
+more stars; and these are technically known as "triple" or "multiple"
+stars respectively. Further, groups are found in which a great number of
+stars are closely massed together, such a massing together of stars
+being known as a "cluster."
+
+The Pole Star (Polaris) is a double star, one of the components being of
+a little below the second magnitude, and the other a little below the
+ninth. They are so close together that they appear as one star to the
+naked eye, but they may be seen separate with a moderately sized
+telescope. The brighter star is yellowish, and the faint one white. This
+brighter star is found _by means of the spectroscope_ to be actually
+composed of three stars so very close together that they cannot be seen
+separately even with a telescope. It is thus a triple star, and the
+three bodies of which it is composed are in circulation about each
+other. Two of them are darker than the third.
+
+The method of detecting binary stars by means of the spectroscope is an
+application of Doppler's principle. It will, no doubt, be remembered
+that, according to the principle in question, we are enabled, from
+certain shiftings of the lines in the spectrum of a luminous body, to
+ascertain whether that body is approaching us or receding from us. Now
+there are certain stars which always appear single even in the largest
+telescopes, but when the spectroscope is directed to them a spectrum
+_with two sets of lines_ is seen. Such stars must, therefore, be double.
+Further, if the shiftings of the lines, in a spectrum like this, tell us
+that the component stars are making small movements to and from us which
+go on continuously, we are therefore justified in concluding that these
+are the orbital revolutions of a binary system greatly compressed by
+distance. Such connected pairs of stars, since they cannot be seen
+separately by means of any telescope, no matter how large, are known as
+"spectroscopic binaries."
+
+In observations of spectroscopic binaries we do not always get a double
+spectrum. Indeed, if one of the components be below a certain
+magnitude, its spectrum will not appear at all; and so we are left in
+the strange uncertainty as to whether this component is merely faint or
+actually dark. It is, however, from the shiftings of the lines in the
+spectrum of the other component that we see that an orbital movement is
+going on, and are thus enabled to conclude that two bodies are here
+connected into a system, although one of these bodies resolutely refuses
+directly to reveal itself even to the all-conquering spectroscope.
+
+Mizar, that star in the handle of the Plough to which we have already
+drawn attention, will be found with a small telescope to be a fine
+double, one of the components being white and the other greenish.
+Actually, however, as the American astronomer, Professor F.R. Moulton,
+points out, these stars are so far from each other that if we could be
+transferred to one of them we should see the other merely as an ordinary
+bright star. The spectroscope shows that the brighter of these stars is
+again a binary system of two huge suns, the components revolving around
+each other in a period of about twenty days. This discovery made by
+Professor E.C. Pickering, the _first_ of the kind by means of the
+spectroscope, was announced in 1889 from the Harvard Observatory in the
+United States.
+
+A star close to Vega, known as [e] (Epsilon) Lyr� (see Plate XIX., p.
+292), is a double, the components of which may be seen separately with
+the naked eye by persons with very keen eyesight. If this star, however,
+be viewed with the telescope, the two companions will be seen far apart;
+and it will be noticed that each of them is again a double.
+
+By means of the spectroscope Capella is shown to be really composed of
+two stars (one about twice as bright as the other) situated very close
+together and forming a binary system. Sirius is also a binary system;
+but it is what is called a "visual" one, for its component stars may be
+_seen_ separately in very large telescopes. Its double, or rather
+binary, nature, was discovered in 1862 by the celebrated optician Alvan
+G. Clark, while in the act of testing the 18-inch refracting telescope,
+then just constructed by his firm, and now at the Dearborn Observatory,
+Illinois, U.S.A. The companion is only of the tenth magnitude, and
+revolves around Sirius in a period of about fifty years, at a mean
+distance equal to about that of Uranus from the sun. Seen from Sirius,
+it would shine only something like our full moon. It must be
+self-luminous and not a mere planet; for Mr. Gore has shown that if it
+shone only by the light reflected from Sirius, it would be quite
+invisible even in the Great Yerkes Telescope.
+
+Procyon is also a binary, its companion having been discovered by
+Professor J.M. Schaeberle at the Lick Observatory in 1896. The period of
+revolution in this system is about forty years. Observations by Mr. T.
+Lewis of Greenwich seem, however, to point to the companion being a
+small nebula rather than a star.
+
+The star [�] (Eta) Cassiopei� (see Plate XIX., p. 292), is easily seen
+as a fine double in telescopes of moderate size. It is a binary system,
+the component bodies revolving around their common centre of gravity in
+a period of about two hundred years. This system is comparatively near
+to us, _i.e._ about nine light years, or a little further off than
+Sirius.
+
+In a small telescope the star Castor will be found double, the
+components, one of which is brighter than the other, forming a binary
+system. The fainter of these was found by Belopolsky, with the
+spectroscope, to be composed of a system of two stars, one bright and
+the other either dark or not so bright, revolving around each other in a
+period of about three days. The brighter component of Castor is also a
+spectroscopic binary, with a period of about nine days; so that the
+whole of what we see with the naked eye as Castor, is in reality a
+remarkable system of four stars in mutual orbital movement.
+
+Alpha Centauri--the nearest star to the earth--is a visual binary, the
+component bodies revolving around each other in a period of about
+eighty-one years. The extent of this system is about the same as that of
+Sirius. Viewed from each other, the bodies would shine only like our sun
+as seen from Neptune.
+
+Among the numerous binary stars the orbits of some fifty have been
+satisfactorily determined. Many double stars, for which this has not yet
+been done, are, however, believed to be, without doubt, binary. In some
+cases a parallax has been found; so that we are enabled to estimate in
+miles the actual extent of such systems, and the masses of the bodies in
+terms of the sun's mass.
+
+Most of the spectroscopic binaries appear to be upon a smaller scale
+than the telescopic ones. Some are, indeed, comparatively speaking,
+quite small. For instance, the component stars forming [b] Aurig� are
+about eight million miles apart, while in [z] Geminorum, the distance
+between the bodies is only a little more than a million miles.
+
+Spectroscopic binaries are probably very numerous. Professor W.W.
+Campbell, Director of the Lick Observatory, estimates, for instance,
+that, out of about every half-a-dozen stars, one is a spectroscopic
+binary.
+
+It is only in the case of binary systems that we can discover the masses
+of stars at all. These are ascertained from their movements with regard
+to each other under the influence of their mutual gravitative
+attractions. In the case of simple stars we have clearly nothing of the
+kind to judge by; though, if we can obtain a parallax, we may hazard a
+guess from their brightness.
+
+Binary stars were incidentally discovered by Sir William Herschel. In
+his researches to get a stellar parallax he had selected a number of
+double stars for test purposes, on the assumption that, if one of such a
+pair were much nearer than the other, it might show a displacement with
+regard to its neighbour as a direct consequence of the earth's orbital
+movement around the sun. He, however, failed entirely to obtain any
+parallaxes, the triumph in this being, as we have seen, reserved for
+Bessel. But in some of the double stars which he had selected, he found
+certain alterations in the relative positions of the bodies, which
+plainly were not a consequence of the earth's motion, but showed rather
+that there was an actual circling movement of the bodies themselves
+under their mutual attractions. It is to be noted that the existence of
+such connected pairs had been foretold as probable by the Rev. John
+Michell, who lived a short time before Herschel.
+
+The researches into binary systems--both those which can be seen with
+the eye and those which can be observed by means of the spectroscope,
+ought to impress upon us very forcibly the wide sway of the law of
+gravitation.
+
+Of star clusters about 100 are known, and such systems often contain
+several thousand stars. They usually cover an area of sky somewhat
+smaller than the moon appears to fill. In most clusters the stars are
+very faint, and, as a rule, are between the twelfth and sixteenth
+magnitudes. It is difficult to say whether these are actually small
+bodies, or whether their faintness is due merely to their great distance
+from us, since they are much too far off to show any appreciable
+parallactic displacement. Mr. Gore, however, thinks there is good
+evidence to show that the stars in clusters are really close, and that
+the clusters themselves fill a comparatively small space.
+
+One of the finest examples of a cluster is the great globular one, in
+the constellation of Hercules, discovered by Halley in 1714. It contains
+over 5000 stars, and upon a clear, dark night is visible to the naked
+eye as a patch of light. In the telescope, however, it is a wonderful
+object. There are also fine clusters in the constellations of Auriga,
+Pegasus, and Canes Venatici. In the southern heavens there are some
+magnificent examples of globular clusters. This hemisphere seems,
+indeed, to be richer in such objects than the northern. For instance,
+there is a great one in the constellation of the Centaur, containing
+some 6000 stars (see Plate XXI., p. 306).
+
+[Illustration: PLATE XXI. THE GREAT GLOBULAR CLUSTER IN THE SOUTHERN
+CONSTELLATION OF CENTAURUS
+
+From a photograph taken at the Cape Observatory, on May 24th, 1903. Time
+of exposure, 1 hour.
+
+(Page 306)]
+
+Certain remarkable groups of stars, of a nature similar to clusters,
+though not containing such faint or densely packed stars as those we
+have just alluded to, call for a mention in this connection. The best
+example of such star groups are the Pleiades and the Hyades (see Plate
+XX., p. 296), Coma Berenices, and Pr�sepe (or the Beehive), the
+last-named being in the constellation of Cancer.
+
+Stars which alter in their brightness are called _Variable Stars_, or
+"variables." The first star whose variability attracted attention is
+that known as Omicron Ceti, namely, the star marked with the Greek
+letter [o] (Omicron) in the constellation of Cetus, or the Whale, a
+constellation situated not far from Taurus. This star, the variability
+of which was discovered by Fabricius in 1596, is also known as Mira, or
+the "Wonderful," on account of the extraordinary manner in which its
+light varies from time to time. The star known by the name of Algol,[32]
+popularly called the "Demon Star"--whose astronomical designation is [b]
+(Beta) Persei, or the star second in brightness in the constellation of
+Perseus--was discovered by Goodricke, in the year 1783, to be a variable
+star. In the following year [b] Lyr�, the star in Lyra next in order of
+brightness after Vega, was also found by the same observer to be a
+variable. It may be of interest to the reader to know that Goodricke was
+deaf and dumb, and that he died in 1786 at the early age of twenty-one
+years!
+
+It was not, however, until the close of the nineteenth century that much
+attention was paid to variable stars. Now several hundreds of these are
+known, thanks chiefly to the observations of, amongst others, Professor
+S.C. Chandler of Boston, U.S.A., Mr. John Ellard Gore of Dublin, and Dr.
+A.W. Roberts of South Africa. This branch of astronomy has not, indeed,
+attracted as much popular attention as it deserves, no doubt because the
+nature of the work required does not call for the glamour of an
+observatory or a large telescope.
+
+The chief discoveries with regard to variable stars have been made by
+the naked eye, or with a small binocular. The amount of variation is
+estimated by a comparison with other stars. As in many other branches of
+astronomy, photography is now employed in this quest with marked
+success; and lately many variable stars have been found to exist in
+clusters and nebul�.
+
+It was at one time considered that a variable star was in all
+probability a body, a portion of whose surface had been relatively
+darkened in some manner akin to that in which sun spots mar the face of
+the sun; and that when its axial rotation brought the less illuminated
+portions in turn towards us, we witnessed a consequent diminution in the
+star's general brightness. Herschel, indeed, inclined to this
+explanation, for his belief was that all the stars bore spots like those
+of the sun. It appears preferably thought nowadays that disturbances
+take place periodically in the atmosphere or surroundings of certain
+stars, perhaps through the escape of imprisoned gases, and that this may
+be a fruitful cause of changes of brilliancy. The theory in question
+will, however, apparently account for only one class of variable star,
+namely, that of which Mira Ceti is the best-known example. The scale on
+which it varies in brightness is very great, for it changes from the
+second to the ninth magnitude. For the other leading type of variable
+star, Algol, of which mention has already been made, is the best
+instance. The shortness of the period in which the changes of brightness
+in such stars go their round, is the chief characteristic of this latter
+class. The period of Algol is a little under three days. This star when
+at its brightest is of about the second magnitude, and when least bright
+is reduced to below the third magnitude; from which it follows that its
+light, when at the minimum, is only about one-third of what it is when
+at the maximum. It seems definitely proved by means of the spectroscope
+that variables of this kind are merely binary stars, too close to be
+separated by the telescope, which, as a consequence of their orbits
+chancing to be edgewise towards us, eclipse each other in turn time
+after time. If, for instance, both components of such a pair are bright,
+then when one of them is right behind the other, we will not, of course,
+get the same amount of light as when they are side by side. If, on the
+other hand, one of the components happens to be dark or less luminous
+and the other bright, the manner in which the light of the bright star
+will be diminished when the darker star crosses its face should easily
+be understood. It is to the second of these types that Algol is supposed
+to belong. The Algol system appears to be composed of a body about as
+broad as our sun, which regularly eclipses a brighter body which has a
+diameter about half as great again.
+
+Since the companion of Algol is often spoken of as a _dark_ body, it
+were well here to point out that we have no evidence at all that it is
+entirely devoid of light. We have already found, in dealing with
+spectroscopic binaries, that when one of the component stars is below a
+certain magnitude[33] its spectrum will not be seen; so one is left in
+the glorious uncertainty as to whether the body in question is
+absolutely dark, or darkish, or faint, or indeed only just out of range
+of the spectroscope.
+
+It is thought probable by good authorities that the companion of Algol
+is not quite dark, but has some inherent light of its own. It is, of
+course, much too near Algol to be seen with the largest telescope. There
+is in fact a distance of only from two to three millions of miles
+between the bodies, from which Mr. Gore infers that they would probably
+remain unseparated even in the largest telescope which could ever be
+constructed by man.
+
+The number of known variables of the Algol type is, so far, small; not
+much indeed over thirty. In some of them the components are believed to
+revolve touching each other, or nearly so. An extreme example of this is
+found in the remarkable star V. Puppis, an Algol variable of the
+southern hemisphere. Both its components are bright, and the period of
+light variation is about one and a half days. Dr. A. W. Roberts finds
+that the bodies are revolving around each other in actual contact.
+
+_Temporary stars_ are stars which have suddenly blazed out in regions of
+the sky where no star was previously seen, and have faded away more or
+less gradually.
+
+It was the appearance of such a star, in the year 134 B.C., which
+prompted Hipparchus to make his celebrated catalogue, with the object of
+leaving a record by which future observers could note celestial changes.
+In 1572 another star of this kind flashed out in the constellation of
+Cassiopeia (see Plate XIX., p. 292), and was detected by Tycho Brahe. It
+became as bright as the planet Venus, and eventually was visible in the
+day-time. Two years later, however, it disappeared, and has never since
+been seen. In 1604 Kepler recorded a similar star in the constellation
+of Ophiuchus which grew to be as bright as Jupiter. It also lasted for
+about two years, and then faded away, leaving no trace behind. It is
+rarely, however, that temporary stars attain to such a brilliance; and
+so possibly in former times a number of them may have appeared, but not
+have risen to a sufficient magnitude to attract attention. Even now,
+unless such a star becomes clearly visible to the naked eye, it runs a
+good chance of not being detected. A curious point, worth noting, with
+regard to temporary stars is that the majority of them have appeared in
+the Milky Way.
+
+These sudden visitations have in our day received the name of _Nov�_;
+that is to say, "New" Stars. Two, in recent years, attracted a good deal
+of attention. The first of these, known as Nova Aurig�, or the New Star
+in the constellation of Auriga, was discovered by Dr. T.D. Anderson at
+Edinburgh in January 1892. At its greatest brightness it attained to
+about the fourth magnitude. By April it had sunk to the twelfth, but
+during August it recovered to the ninth magnitude. After this last
+flare-up it gradually faded away.
+
+The startling suddenness with which temporary stars usually spring into
+being is the groundwork upon which theories to account for their origin
+have been erected. That numbers of dark stars, extinguished suns, so to
+speak, may exist in space, there is a strong suspicion; and it is just
+possible that we have an instance of one dark stellar body in the
+companion of Algol. That such dark stars might be in rapid motion is
+reasonable to assume from the already known movements of bright stars.
+Two dark bodies might, indeed, collide together, or a collision might
+take place between a dark star and a star too faint to be seen even with
+the most powerful telescope. The conflagration produced by the impact
+would thus appear where nothing had been seen previously. Again, a
+similar effect might be produced by a dark body, or a star too faint to
+be seen, being heated to incandescence by plunging in its course through
+a nebulous mass of matter, of which there are many examples lying about
+in space.
+
+The last explanation, which is strongly reminiscent of what takes place
+in shooting stars, appears more probable than the collision theory. The
+flare-up of new stars continues, indeed, only for a comparatively short
+time; whereas a collision between two bodies would, on the other hand,
+produce an enormous nebula which might take even millions of years to
+cool down. We have, indeed, no record of any such sudden appearance of a
+lasting nebula.
+
+The other temporary star, known as Nova Persei, or the new star in the
+constellation of Perseus, was discovered early in the morning of
+February 22, 1901, also by Dr. Anderson. A day later it had grown to be
+brighter than Capella. Photographs which had been taken, some three days
+previous to its discovery, of the very region of the sky in which it had
+burst forth, were carefully examined, and it was not found in these. At
+the end of two days after its discovery Nova Persei had lost one-third
+of its light. During the ensuing six months it passed through a series
+of remarkable fluctuations, varying in brightness between the third and
+fifth magnitudes. In the month of August it was seen to be surrounded by
+luminous matter in the form of a nebula, which appeared to be gradually
+spreading to some distance around. Taking into consideration the great
+way off at which all this was taking place, it looked as if the new star
+had ejected matter which was travelling outward with a velocity
+equivalent to that of light. The remarkable theory was, however, put
+forward by Professor Kapteyn and the late Dr. W.E. Wilson that there
+might be after all no actual transmission of matter; but that perhaps
+the real explanation was the gradual _illumination_ of hitherto
+invisible nebulous matter, as a consequence of the flare-up which had
+taken place about six months before. It was, therefore, imagined that
+some dark body moving through space at a very rapid rate had plunged
+through a mass of invisible nebulous matter, and had consequently become
+heated to incandescence in its passage, very much like what happens to a
+meteor when moving through our atmosphere. The illumination thus set up
+temporarily in one point, being transmitted through the nebulous wastes
+around with the ordinary velocity of light, had gradually rendered this
+surrounding matter visible. On the assumptions required to fit in with
+such a theory, it was shown that Nova Persei must be at a distance from
+which light would take about three hundred years in coming to us. The
+actual outburst of illumination, which gave rise to this temporary star,
+would therefore have taken place about the beginning of the reign of
+James I.
+
+Some recent investigations with regard to Nova Persei have, however,
+greatly narrowed down the above estimate of its distance from us. For
+instance, Bergstrand proposes a distance of about ninety-nine light
+years; while the conclusions of Mr. F.W. Very would bring it still
+nearer, _i.e._ about sixty-five light years.
+
+The last celestial objects with which we have here to deal are the
+_Nebul�_. These are masses of diffused shining matter scattered here and
+there through the depths of space. Nebul� are of several kinds, and have
+been classified under the various headings of Spiral, Planetary, Ring,
+and Irregular.
+
+A typical _spiral_ nebula is composed of a disc-shaped central portion,
+with long curved arms projecting from opposite sides of it, which give
+an impression of rapid rotatory movement.
+
+The discovery of spiral nebul� was made by Lord Rosse with his great
+6-foot reflector. Two good examples of these objects will be found in
+Ursa Major, while there is another fine one in Canes Venatici (see Plate
+XXII., p. 314), a constellation which lies between Ursa Major and
+Bo�tes. But the finest spiral of all, perhaps the most remarkable nebula
+known to us, is the Great Nebula in the constellation of Andromeda, (see
+Plate XXIII., p. 316)--a constellation just further from the pole than
+Cassiopeia. When the moon is absent and the night clear this nebula can
+be easily seen with the naked eye as a small patch of hazy light. It is
+referred to by Al Sufi.
+
+[Illustration: PLATE XXII. SPIRAL NEBULA IN THE CONSTELLATION OF CANES
+VENATICI
+
+From a photograph by the late Dr. W.E. Wilson, D.Sc., F.R.S.
+
+(Page 314)]
+
+Spiral nebul� are white in colour, whereas the other kinds of nebula
+have a greenish tinge. They are also by far the most numerous; and the
+late Professor Keeler, who considered this the normal type of nebula,
+estimated that there were at least 120,000 of such spirals within the
+reach of the Crossley reflector of the Lick Observatory. Professor
+Perrine has indeed lately raised this estimate to half a million, and
+thinks that with more sensitive photographic plates and longer exposures
+the number of spirals would exceed a million. The majority of these
+objects are very small, and appear to be distributed over the sky in a
+fairly uniform manner.
+
+_Planetary_ nebul� are small faint roundish objects which, when seen in
+the telescope, recall the appearance of a planet, hence their name. One
+of these nebul�, known astronomically as G.C. 4373, has recently been
+found to be rushing through space towards the earth at a rate of between
+thirty and forty miles per second. It seems strange, indeed, that any
+gaseous mass should move at such a speed!
+
+What are known as _ring_ nebul� were until recently believed to form a
+special class. These objects have the appearance of mere rings of
+nebulous matter. Much doubt has, however, been thrown upon their being
+rings at all; and the best authorities regard them merely as spiral
+nebul�, of which we happen to get a foreshortened view. Very few
+examples are known, the most famous being one in the constellation of
+Lyra, usually known as the Annular Nebula in Lyra. This object is so
+remote from us as to be entirely invisible to the naked eye. It contains
+a star of the fifteenth magnitude near to its centre. From photographs
+taken with the Crossley reflector, Professor Schaeberle finds in this
+nebula evidences of spiral structure. It may here be mentioned that the
+Great Nebula in Andromeda, which has now turned out to be a spiral, had
+in earlier photographs the appearance of a ring.
+
+There also exist nebul� of _irregular_ form, the most notable being the
+Great Nebula in the constellation of Orion (see Plate XXIV., p. 318). It
+is situated in the centre of the "Sword" of Orion (see Plate XX., p.
+296). In large telescopes it appears as a magnificent object, and in
+actual dimensions it must be much on the same scale as the Andromeda
+Nebula. The spectroscope tells us that it is a mass of glowing gas.
+
+The Trifid Nebula, situated in the constellation of Sagittarius, is an
+object of very strange shape. Three dark clefts radiate from its centre,
+giving it an appearance as if it had been torn into shreds.
+
+The Dumb-bell Nebula, a celebrated object, so called from its likeness
+to a dumb-bell, turns out, from recent photographs taken by Professor
+Schaeberle, which bring additional detail into view, to be after all a
+great spiral.
+
+There is a nest, or rather a cluster of nebul� in the constellation of
+Coma Berenices; over a hundred of these objects being here gathered into
+a space of sky about the size of our full moon.
+
+[Illustration: PLATE XXIII. THE GREAT NEBULA IN THE CONSTELLATION OF
+ANDROMEDA
+
+From a photograph taken at the Yerkes Observatory.
+
+(Page 314)]
+
+The spectroscope informs us that spiral nebul� are composed of
+partially-cooled matter. Their colour, as we have seen, is white. Nebul�
+of a greenish tint are, on the other hand, found to be entirely in a
+gaseous condition. Just as the solar corona contains an unknown element,
+which for the time being has been called "Coronium," so do the gaseous
+nebul� give evidence of the presence of another unknown element. To this
+Sir William Huggins has given the provisional name of "Nebulium."
+
+The _Magellanic Clouds_ are two patches of nebulous-looking light, more
+or less circular in form, which are situated in the southern hemisphere
+of the sky. They bear a certain resemblance to portions of the Milky
+Way, but are, however, not connected with it. They have received their
+name from the celebrated navigator, Magellan, who seems to have been one
+of the first persons to draw attention to them. "Nubecul�" is another
+name by which they are known, the larger cloud being styled _nubecula
+major_ and the smaller one _nubecula minor_. They contain within them
+stars, clusters, and gaseous nebul�. No parallax has yet been found for
+any object which forms part of the nubecul�, so it is very difficult to
+estimate at what distance from us they may lie. They are, however,
+considered to be well within our stellar universe.
+
+Having thus brought to a conclusion our all too brief review of the
+stars and the nebul�--of the leading objects in fine which the celestial
+spaces have revealed to man--we will close this chapter with a recent
+summation by Sir David Gill of the relations which appear to obtain
+between these various bodies. "Huggins's spectroscope," he says, "has
+shown that many nebul� are not stars at all; that many well-condensed
+nebul�, as well as vast patches of nebulous light in the sky, are but
+inchoate masses of luminous gas. Evidence upon evidence has accumulated
+to show that such nebul� consist of the matter out of which stars
+(_i.e._ suns) have been and are being evolved. The different types of
+star spectra form such a complete and gradual sequence (from simple
+spectra resembling those of nebul� onwards through types of gradually
+increasing complexity) as to suggest that we have before us, written in
+the cryptograms of these spectra, the complete story of the evolution of
+suns from the inchoate nebula onwards to the most active sun (like our
+own), and then downward to the almost heatless and invisible ball. The
+period during which human life has existed upon our globe is probably
+too short--even if our first parents had begun the work--to afford
+observational proof of such a cycle of change in any particular star;
+but the fact of such evolution, with the evidence before us, can hardly
+be doubted."[34]
+
+[32] The name Al g�l, meaning the Demon, was what the old Arabian
+astronomers called it, which looks very much as if they had already
+noticed its rapid fluctuations in brightness.
+
+[33] Mr. Gore thinks that the companion of Algol may be a star of the
+sixth magnitude.
+
+[34] Presidential Address to the British Association for the Advancement
+of Science (Leicester, 1907), by Sir David Gill, K.C.B., LL.D., F.R.S.,
+&c. &c.
+
+[Illustration: PLATE XXIV. THE GREAT NEBULA IN THE CONSTELLATION OF
+ORION
+
+From a photograph taken at the Yerkes Observatory.
+
+(Page 316)]
+
+
+
+
+CHAPTER XXV
+
+THE STELLAR UNIVERSE
+
+
+The stars appear fairly evenly distributed all around us, except in one
+portion of the sky where they seem very crowded, and so give one an
+impression of being very distant. This portion, known as the Milky Way,
+stretches, as we have already said, in the form of a broad band right
+round the entire heavens. In those regions of the sky most distant from
+the Milky Way the stars appear to be thinly sown, but become more and
+more closely massed together as the Milky Way is approached.
+
+This apparent distribution of the stars in space has given rise to a
+theory which was much favoured by Sir William Herschel, and which is
+usually credited to him, although it was really suggested by one Thomas
+Wright of Durham in 1750; that is to say, some thirty years or more
+before Herschel propounded it. According to this, which is known as the
+"Disc" or "Grindstone" Theory, the stars are considered as arranged in
+space somewhat in the form of a thick disc, or grindstone, close to the
+_central_ parts of which our solar system is situated.[35] Thus we
+should see a greater number of stars when we looked out through the
+_length_ of such a disc in any direction, than when we looked out
+through its _breadth_. This theory was, for a time, supposed to account
+quite reasonably for the Milky Way, and for the gradual increase in the
+number of stars in its vicinity.
+
+It is quite impossible to verify directly such a theory, for we know the
+actual distance of only about forty-three stars. We are unable,
+therefore, definitely to assure ourselves whether, as the grindstone
+theory presupposes, the stellar universe actually reaches out very much
+further from us in the direction of the Milky Way than in the other
+parts of the sky. The theory is clearly founded upon the supposition
+that the stars are more or less equal in size, and are scattered through
+space at fairly regular distances from each other.
+
+Brightness, therefore, had been taken as implying nearness to us, and
+faintness great distance. But we know to-day that this is not the case,
+and that the stars around us are, on the other hand, of various degrees
+of brightness and of all orders of size. Some of the faint stars--for
+instance, the galloping star in Pictor--are indeed nearer to us than
+many of the brighter ones. Sirius, on the other hand, is twice as far
+off from us as [a] Centauri, and yet it is very much brighter; while
+Canopus, which in brightness is second only to Sirius out of the whole
+sky, is too far off for its distance to be ascertained! It must be
+remembered that no parallax had yet been found for any star in the days
+of Herschel, and so his estimations of stellar distances were
+necessarily of a very circumstantial kind. He did not, however, continue
+always to build upon such uncertain ground; but, after some further
+examination of the Milky Way, he gave up his idea that the stars were
+equally disposed in space, and eventually abandoned the grindstone
+theory.
+
+Since we have no means of satisfactorily testing the matter, through
+finding out the various distances from us at which the stars are really
+placed, one might just as well go to the other extreme, and assume that
+the thickening of stars in the region of the Milky Way is not an effect
+of perspective at all, but that the stars in that part of the sky are
+actually more crowded together than elsewhere--a thing which astronomers
+now believe to be the case. Looked at in this way, the shape of the
+stellar universe might be that of a globe-shaped aggregation of stars,
+in which the individuals are set at fairly regular distances from each
+other; the whole being closely encircled by a belt of densely packed
+stars. It must, however, be allowed that the gradual increase in the
+number of stars towards the Milky Way appears a strong argument in
+favour of the grindstone theory; yet the belt theory, as above detailed,
+seems to meet with more acceptance.
+
+There is, in fact, one marked circumstance which is remarkably difficult
+of explanation by means of the grindstone theory. This is the existence
+of vacant spaces--holes, so to speak, in the groundwork of the Milky
+Way. For instance, there is a cleft running for a good distance along
+its length, and there is also a starless gap in its southern portion. It
+seems rather improbable that such a great number of stars could have
+arranged themselves so conveniently, as to give us a clear view right
+out into empty space through such a system in its greatest thickness;
+as if, in fact, holes had been bored, and clefts made, from the boundary
+of the disc clean up to where our solar system lies. Sir John Herschel
+long ago drew attention to this point very forcibly. It is plain that
+such vacant spaces can, on the other hand, be more simply explained as
+mere holes in a belt; and the best authorities maintain that the
+appearance of the Milky Way confirms a view of this kind.
+
+Whichever theory be indeed the correct one, it appears at any rate that
+the stars do not stretch out in every direction to an infinite distance;
+but that _the stellar system is of limited extent_, and has in fact a
+boundary.
+
+In the first place, Science has no grounds for supposing that light is
+in any way absorbed or destroyed merely by its passage through the
+"ether," that imponderable medium which is believed to transmit the
+luminous radiations through space. This of course is tantamount to
+saying that all the direct light from all the stars should reach us,
+excepting that little which is absorbed in its passage through our own
+atmosphere. If stars, and stars, and stars existed in every direction
+outwards without end, it can be proved mathematically that in such
+circumstances there could not remain the tiniest space in the sky
+without a star to fill it, and that therefore the heavens would always
+blaze with light, and the night would be as bright as the noonday.[36]
+How very far indeed this is from being the case, may be gathered from an
+estimate which has been made of the general amount of light which we
+receive from the stars. According to this estimate the sky is considered
+as more or less dark, the combined illumination sent to us by all the
+stars being only about the one-hundreth part of what we get from the
+full moon.[37]
+
+Secondly, it has been suggested that although light may not suffer any
+extinction or diminution from the ether itself, still a great deal of
+illumination may be prevented from reaching us through myriads of
+extinguished suns, or dark meteoric matter lying about in space. The
+idea of such extinguished suns, dark stars in fact, seems however to be
+merely founded upon the sole instance of the invisible companion of
+Algol; but, as we have seen, there is no proof whatever that it is a
+dark body. Again, some astronomers have thought that the dark holes in
+the Milky Way, "Coal Sacks," as they are called, are due to masses of
+cool, or partially cooled matter, which cuts off the light of the stars
+beyond. The most remarkable of these holes is one in the neighbourhood
+of the Southern Cross, known as the "Coal Sack in Crux." But Mr. Gore
+thinks that the cause of the holes is to be sought for rather in what
+Sir William Herschel termed "clustering power," _i.e._ a tendency on the
+part of stars to accumulate in certain places, thus leaving others
+vacant; and the fact that globular and other clusters are to be found
+very near to such holes certainly seems corroborative of this theory. In
+summing up the whole question, Professor Newcomb maintains that there
+does not appear any evidence of the light from the Milky Way stars,
+which are apparently the furthest bodies we see, being intercepted by
+dark bodies or dark matter. As far as our telescopes can penetrate, he
+holds that we see the stars _just as they are_.
+
+Also, if there did exist an infinite number of stars, one would expect
+to find evidence in some direction of an overpoweringly great
+force,--the centre of gravity of all these bodies.
+
+It is noticed, too, that although the stars increase in number with
+decrease in magnitude, so that as we descend in the scale we find three
+times as many stars in each magnitude as in the one immediately above
+it, yet this progression does not go on after a while. There is, in
+fact, a rapid falling off in numbers below the twelfth magnitude; which
+looks as if, at a certain distance from us, the stellar universe were
+beginning to _thin out_.
+
+Again, it is estimated, by Mr. Gore and others, that only about 100
+millions of stars are to be seen in the whole of the sky with the best
+optical aids. This shows well the limited extent of the stellar system,
+for the number is not really great. For instance, there are from fifteen
+to sixteen times as many persons alive upon the earth at this moment!
+
+Last of all, there appears to be strong photographic evidence that our
+sidereal system is limited in extent. Two photographs taken by the late
+Dr. Isaac Roberts of a region rich in stellar objects in the
+constellation of Cygnus, clearly show what has been so eloquently called
+the "darkness behind the stars." One of these photographs was taken in
+1895, and the other in 1898. On both occasions the state of the
+atmosphere was practically the same, and the sensitiveness of the films
+was of the same degree. The exposure in the first case was only one
+hour; in the second it was about two hours and a half. And yet both
+photographs show _exactly the same stars, even down to the faintest_.
+From this one would gather that the region in question, which is one of
+the most thickly star-strewn in the Milky Way, is _penetrable right
+through_ with the means at our command. Dr. Roberts himself in
+commenting upon the matter drew attention to the fact, that many
+astronomers seemed to have tacitly adopted the assumption that the stars
+extend indefinitely through space.
+
+From considerations such as these the foremost astronomical authorities
+of our time consider themselves justified in believing that the
+collection of stars around us is _finite_; and that although our best
+telescopes may not yet be powerful enough to penetrate to the final
+stars, still the rapid decrease in numbers as space is sounded with
+increasing telescopic power, points strongly to the conclusion that the
+boundaries of the stellar system may not lie very far beyond the
+uttermost to which we can at present see.
+
+Is it possible then to make an estimate of the extent of this stellar
+system?
+
+Whatever estimates we may attempt to form cannot however be regarded as
+at all exact, for we know the actual distances of such a very few only
+of the nearest of the stars. But our knowledge of the distances even of
+these few, permits us to assume that the stars close around us may be
+situated, on an average, at about eight light-years from each other; and
+that this holds good of the stellar spaces, with the exception of the
+encircling girdle of the Milky Way, where the stars seem actually to be
+more closely packed together. This girdle further appears to contain the
+greater number of the stars. Arguing along these lines, Professor
+Newcomb reaches the conclusion that the farthest stellar bodies which we
+see are situated at about between 3000 and 4000 light-years from us.
+
+Starting our inquiry from another direction, we can try to form an
+estimate by considering the question of proper motions.
+
+It will be noticed that such motions do not depend entirely upon the
+actual speed of the stars themselves, but that some of the apparent
+movement arises indirectly from the speed of our own sun. The part in a
+proper motion which can be ascribed to the movement of our solar system
+through space is clearly a displacement in the nature of a parallax--Sir
+William Herschel called it "_Systematic_ Parallax"; so that knowing the
+distance which we move over in a certain lapse of time, we are able to
+hazard a guess at the distances of a good many of the stars. An inquiry
+upon such lines must needs be very rough, and is plainly based upon the
+assumption that the stars whose distances we attempt to estimate are
+moving at an average speed much like that of our own sun, and that they
+are not "runaway stars" of the 1830 Groombridge order. Be that as it
+may, the results arrived at by Professor Newcomb from this method of
+reasoning are curiously enough very much on a par with those founded on
+the few parallaxes which we are really certain about; with the exception
+that they point to somewhat closer intervals between the individual
+stars, and so tend to narrow down our previous estimate of the extent of
+the stellar system.
+
+Thus far we get, and no farther. Our solar system appears to lie
+somewhere near the centre of a great collection of stars, separated each
+one from the other, on an average, by some 40 billions of miles; the
+whole being arranged in the form of a mighty globular cluster. Light
+from the nearest of these stars takes some four years to come to us. It
+takes about 1000 times as long to reach us from the confines of the
+system. This globe of stars is wrapt around closely by a stellar girdle,
+the individual stars in which are set together more densely than those
+in the globe itself. The entire arrangement appears to be constructed
+upon a very regular plan. Here and there, as Professor Newcomb points
+out, the aspect of the heavens differs in small detail; but generally it
+may be laid down that the opposite portions of the sky, whether in the
+Milky Way itself, or in those regions distant from it, show a marked
+degree of symmetry. The proper motions of stars in corresponding
+portions of the sky reveal the same kind of harmony, a harmony which may
+even be extended to the various colours of the stars. The stellar
+system, which we see disposed all around us, appears in fine to bear all
+the marks of an _organised whole_.
+
+The older astronomers, to take Sir William Herschel as an example,
+supposed some of the nebul� to be distant "universes." Sir William was
+led to this conclusion by the idea he had formed that, when his
+telescopes failed to show the separate stars of which he imagined these
+objects to be composed, he must put down the failure to their stupendous
+distance from us. For instance, he thought the Orion Nebula, which is
+now known to be made up of glowing gas, to be an external stellar
+system. Later on, however, he changed his mind upon this point, and came
+to the conclusion that "shining fluid" would better account both for
+this nebula, and for others which his telescopes had failed to separate
+into component stars.
+
+The old ideas with regard to external systems and distant universes have
+been shelved as a consequence of recent research. All known clusters and
+nebul� are now firmly believed to lie _within_ our stellar system.
+
+This view of the universe of stars as a sort of island in the
+immensities, does not, however, give us the least idea about the actual
+extent of space itself. Whether what is called space is really infinite,
+that is to say, stretches out unendingly in every direction, or whether
+it has eventually a boundary somewhere, are alike questions which the
+human mind seems utterly unable to picture to itself.
+
+
+[35] The Ptolemaic idea dies hard!
+
+[36] Even the Milky Way itself is far from being a blaze of light, which
+shows that the stars composing it do not extend outwards indefinitely.
+
+[37] Mr. Gore has recently made some remarkable deductions, with regard
+to the amount of light which we get from the stars. He considers that
+most of this light comes from stars below the sixth magnitude; and
+consequently, if all the stars visible to the naked eye were to be
+blotted out, the glow of the night sky would remain practically the same
+as it is at present. Going to the other end of the scale, he thinks also
+that the combined light which we get from all the stars below the
+seventeenth magnitude is so very small, that it may be neglected in such
+an estimation. He finds, indeed, that if there are stars so low as the
+twentieth magnitude, one hundred millions of them would only be equal in
+brightness to a single first-magnitude star like Vega. On the other
+hand, it is possible that the light of the sky at night is not entirely
+due to starlight, but that some of it may be caused by phosphorescent
+glow.
+
+
+
+
+CHAPTER XXVI
+
+THE STELLAR UNIVERSE--_continued_
+
+
+It is very interesting to consider the proper motions of stars with
+reference to such an isolated stellar system as has been pictured in the
+previous chapter. These proper motions are so minute as a rule, that we
+are quite unable to determine whether the stars which show them are
+moving along in straight lines, or in orbits of immense extent. It
+would, in fact, take thousands of years of careful observation to
+determine whether the paths in question showed any degree of curving. In
+the case of the more distant stars, the accurate observations which have
+been conducted during the last hundred years have not so far revealed
+any proper motions with regard to them; but one cannot escape the
+conclusion that these stars move as the others do.
+
+If space outside our stellar system is infinite in extent, and if all
+the stars within that system are moving unchecked in every conceivable
+direction, the result must happen that after immense ages these stars
+will have drawn apart to such a distance from each other, that the
+system will have entirely disintegrated, and will cease to exist as a
+connected whole. Eventually, indeed, as Professor Newcomb points out,
+the stars will have separated so far from each other that each will be
+left by itself in the midst of a black and starless sky. If, however, a
+certain proportion of stars have a speed sufficiently slow, they will
+tend under mutual attraction to be brought to rest by collisions, or
+forced to move in orbits around each other. But those stars which move
+at excessive speeds, such, for instance, as 1830 Groombridge, or the
+star in the southern constellation of Pictor, seem utterly incapable of
+being held back in their courses by even the entire gravitative force of
+our stellar system acting as a whole. These stars must, therefore, move
+eventually right through the system and pass out again into the empty
+spaces beyond. Add to this; certain investigations, made into the speed
+of 1830 Groombridge, furnish a remarkable result. It is calculated,
+indeed, that had this star been _falling through infinite space for
+ever_, pulled towards us by the combined gravitative force of our entire
+system of stars, it could not have gathered up anything like the speed
+with which it is at present moving. No force, therefore, which we can
+conjure out of our visible universe, seems powerful enough either to
+have impressed upon this runaway star the motion which it now has, or to
+stay it in its wild course. What an astounding condition of things!
+
+Speculations like this call up a suspicion that there may yet exist
+other universes, other centres of force, notwithstanding the apparent
+solitude of our stellar system in space. It will be recollected that the
+idea of this isolation is founded upon such facts as, that the heavens
+do not blaze with light, and that the stars gradually appear to thin out
+as we penetrate the system with increasing telescopic power. But
+perchance there is something which hinders us from seeing out into space
+beyond our cluster of stars; which prevents light, in fact, from
+reaching us from other possible systems scattered through the depths
+beyond. It has, indeed, been suggested by Mr. Gore[38] that the
+light-transmitting ether may be after all merely a kind of "atmosphere"
+of the stars; and that it may, therefore, thin off and cease a little
+beyond the confines of our stellar system, just as the air thins off and
+practically ceases at a comparatively short distance from the earth. A
+clashing together of solid bodies outside our atmosphere could plainly
+send us no sound, for there is no air extending the whole way to bear to
+our ears the vibrations thus set up; so light emitted from any body
+lying beyond our system of stars, would not be able to come to us if the
+ether, whose function it is to convey the rays of light, ceased at or
+near the confines of that system.
+
+Perchance we have in this suggestion the key to the mystery of how our
+sun and the other stellar bodies maintain their functions of temperature
+and illumination. The radiations of heat and light arriving at the
+limits of this ether, and unable to pass any further, may be thrown back
+again into the system in some altered form of energy.
+
+But these, at best, are mere airy and fascinating speculations. We have,
+indeed, no evidence whatever that the luminiferous ether ceases at the
+boundary of the stellar system. If, therefore, it extends outwards
+infinitely in every direction, and if it has no absorbing or weakening
+effect on the vibrations which it transmits, we cannot escape from the
+conclusion that practically all the rays of light ever emitted by all
+the stars must chase one another eternally through the never-ending
+abysses of space.
+
+
+[38] _Planetary and Stellar Studies_, by John Ellard Gore, F.R.A.S.,
+M.R.I.A., London, 1888.
+
+
+
+
+CHAPTER XXVII
+
+THE BEGINNING OF THINGS
+
+
+LAPLACE'S NEBULAR HYPOTHESIS
+
+Dwelling upon the fact that all the motions of revolution and rotation
+in the solar system, as known in his day, took place in the same
+direction and nearly in the same plane, the great French astronomer,
+Laplace, about the year 1796, put forward a theory to account for the
+origin and evolution of that system. He conceived that it had come into
+being as a result of the gradual contraction, through cooling, of an
+intensely heated gaseous lens-shaped mass, which had originally occupied
+its place, and had extended outwards beyond the orbit of the furthest
+planet. He did not, however, attempt to explain how such a mass might
+have originated! He went on to suppose that this mass, _in some manner_,
+perhaps by mutual gravitation among its parts, had acquired a motion of
+rotation in the same direction as the planets now revolve. As this
+nebulous mass parted with its heat by radiation, it contracted towards
+the centre. Becoming smaller and smaller, it was obliged to rotate
+faster and faster in order to preserve its equilibrium. Meanwhile, in
+the course of contraction, rings of matter became separated from the
+nucleus of the mass, and were left behind at various intervals. These
+rings were swept up into subordinate masses similar to the original
+nebula. These subordinate masses also contracted in the same manner,
+leaving rings behind them which, in turn, were swept up to form
+satellites. Saturn's ring was considered, by Laplace, as the only
+portion of the system left which still showed traces of this
+evolutionary process. It is even probable that it may have suggested the
+whole of the idea to him.
+
+Laplace was, however, not the first philosopher who had speculated along
+these lines concerning the origin of the world.
+
+Nearly fifty years before, in 1750 to be exact, Thomas Wright, of
+Durham, had put forward a theory to account for the origin of the whole
+sidereal universe. In his theory, however, the birth of our solar system
+was treated merely as an incident. Shortly afterwards the subject was
+taken up by the famous German philosopher, Kant, who dealt with the
+question in a still more ambitious manner, and endeavoured to account in
+detail for the origin of the solar system as well as of the sidereal
+universe. Something of the trend of such theories may be gathered from
+the remarkable lines in Tennyson's _Princess_:--
+
+"This world was once a fluid haze of light,
+Till toward the centre set the starry tides,
+And eddied into suns, that wheeling cast
+The planets."
+
+The theory, as worked out by Kant, was, however, at the best merely a
+_tour de force_ of philosophy. Laplace's conception was much less
+ambitious, for it did not attempt to explain the origin of the entire
+universe, but only of the solar system. Being thus reasonably limited in
+its scope, it more easily obtained credence. The arguments of Laplace
+were further founded upon a mathematical basis. The great place which
+he occupied among the astronomers of that time caused his theory to
+exert a preponderating influence on scientific thought during the
+century which followed.
+
+A modification of Laplace's theory is the Meteoritic Hypothesis of Sir
+Norman Lockyer. According to the views of that astronomer, the material
+of which the original nebula was composed is presumed to have been in
+the meteoric, rather than in the gaseous, state. Sir Norman Lockyer
+holds, indeed, that nebul� are, in reality, vast swarms of meteors, and
+the light they emit results from continual collisions between the
+constituent particles. The French astronomer, Faye, also proposed to
+modify Laplace's theory by assuming that the nebula broke up into rings
+all at once, and not in detail, as Laplace had wished to suppose.
+
+The hypothesis of Laplace fits in remarkably well with the theory put
+forward in later times by Helmholtz, that the heat of the sun is kept up
+by the continual contraction of its mass. It could thus have only
+contracted to its present size from one very much larger.
+
+Plausible, however, as Laplace's great hypothesis appears on the
+surface, closer examination shows several vital objections, a few of
+those set forth by Professor Moulton being here enumerated--
+
+Although Laplace held that the orbits of the planets were sufficiently
+near to being in the one plane to support his views, yet later
+investigators consider that their very deviations from this plane are a
+strong argument against the hypothesis.
+
+Again, it is thought that if the theory were the correct explanation,
+the various orbits of the planets would be much more nearly circular
+than they are.
+
+It is also thought that such interlaced paths, as those in which the
+asteroids and the little planet Eros move, are most unlikely to have
+been produced as a result of Laplace's nebula.
+
+Further, while each of the rings was sweeping up its matter into a body
+of respectable dimensions, its gravitative power would have been for the
+time being so weak, through being thus spread out, that any lighter
+elements, as, for instance, those of the gaseous order, would have
+escaped into space in accordance with the principles of the kinetic
+theory.
+
+_The idea that rings would at all be left behind at certain intervals
+during the contraction of the nebula is, perhaps, one of the weakest
+points in Laplace's hypothesis._
+
+Mathematical investigation does not go to show that the rings, presuming
+they could be left behind during the contraction of the mass, would have
+aggregated into planetary bodies. Indeed, it rather points to the
+reverse.
+
+Lastly, such a discovery as that the ninth satellite of Saturn revolves
+in a _retrograde_ direction--that is to say, in a direction contrary to
+the other revolutions and rotations in our solar system--appears
+directly to contradict the hypothesis.
+
+Although Laplace's hypothesis seems to break down under the keen
+criticism to which it has been subjected, yet astronomers have not
+relinquished the idea that our solar system has probably had its origin
+from a nebulous mass. But the apparent failure of the Laplacian theory
+is emphasised by the fact, that _not a single example of a nebula, in
+the course of breaking up into concentric rings, is known to exist in
+the entire heaven_. Indeed, as we saw in Chapter XXIV., there seems to
+be no reliable example of even a "ring" nebula at all. Mr. Gore has
+pointed this out very succinctly in his recently published work,
+_Astronomical Essays_, where he says:--"To any one who still persists in
+maintaining the hypothesis of ring formation in nebul�, it may be said
+that the whole heavens are against him."
+
+The conclusions of Keeler already alluded to, that the spiral is the
+normal type of nebula, has led during the past few years to a new theory
+by the American astronomers, Professors Chamberlin and Moulton. In the
+detailed account of it which they have set forth, they show that those
+anomalies which were stumbling-blocks to Laplace's theory do not
+contradict theirs. To deal at length with this theory, to which the name
+of "Planetesimal Hypothesis" has been given, would not be possible in a
+book of this kind. But it may be of interest to mention that the authors
+of the theory in question remount the stream of time still further than
+did Laplace, and seek to explain the _origin_ of the spiral nebul�
+themselves in the following manner:--
+
+Having begun by assuming that the stars are moving apparently in every
+direction with great velocities, they proceed to point out that sooner
+or later, although the lapse of time may be extraordinarily long,
+collisions or near approaches between stars are bound to occur. In the
+case of collisions the chances are against the bodies striking together
+centrally, it being very much more likely that they will hit each other
+rather towards the side. The nebulous mass formed as a result of the
+disintegration of the bodies through their furious impact would thus
+come into being with a spinning movement, and a spiral would ensue.
+Again, the stars may not actually collide, but merely approach near to
+each other. If very close, the interaction of gravitation will give rise
+to intense strains, or tides, which will entirely disintegrate the
+bodies, and a spiral nebula will similarly result. As happens upon our
+earth, two such tides would rise opposite to each other; and,
+consequently, it is a noticeable fact that spiral nebul� have almost
+invariably two opposite branches (see Plate XXII., p 314). Even if not
+so close, the gravitational strains set up would produce tremendous
+eruptions of matter; and in this case, a spiral movement would also be
+generated. On such an assumption the various bodies of the solar system
+may be regarded as having been ejected from parent masses.
+
+The acceptance of the Planetesimal Hypothesis in the place of the
+Hypothesis of Laplace will not, as we have seen, by any means do away
+with the probability that our solar system, and similar systems, have
+originated from a nebulous mass. On the contrary it puts that idea on a
+firmer footing than before. The spiral nebul� which we see in the
+heavens are on a vast scale, and may represent the formation of stellar
+systems and globular clusters. Our solar system may have arisen from a
+small spiral.
+
+We will close these speculations concerning the origin of things with a
+short sketch of certain investigations made in recent years by Sir
+George H. Darwin, of Cambridge University, into the question of the
+probable birth of our moon. He comes to the conclusion that at least
+fifty-four millions of years ago the earth and moon formed one body,
+which had a diameter of a little over 8000 miles. This body rotated on
+an axis in about five hours, namely, about five times as fast as it does
+at present. The rapidity of the rotation caused such a tremendous strain
+that the mass was in a condition of, what is called, unstable
+equilibrium; very little more, in fact, being required to rend it
+asunder. The gravitational pull of the sun, which, as we have already
+seen, is in part the cause of our ordinary tides, supplied this extra
+strain, and a portion of the mass consequently broke off, which receded
+gradually from the rest and became what we now know as the moon. Sir
+George Darwin holds that the gravitational action of the sun will in
+time succeed in also disturbing the present apparent harmony of the
+earth-moon system, and will eventually bring the moon back towards the
+earth, so that after the lapse of great ages they will re-unite once
+again.
+
+In support of this theory of the terrestrial origin of the moon,
+Professor W.H. Pickering has put forward a bold hypothesis that our
+satellite had its origin in the great basin of the Pacific. This ocean
+is roughly circular, and contains no large land masses, except the
+Australian Continent. He supposes that, prior to the moon's birth, our
+globe was already covered with a slight crust. In the tearing away of
+that portion which was afterwards destined to become the moon the
+remaining area of the crust was rent in twain by the shock; and thus
+were formed the two great continental masses of the Old and New Worlds.
+These masses floated apart across the fiery ocean, and at last settled
+in the positions which they now occupy. In this way Professor Pickering
+explains the remarkable parallelism which exists between the opposite
+shores of the Atlantic. The fact of this parallelism had, however, been
+noticed before; as, for example, by the late Rev. S.J. Johnson, in his
+book _Eclipses, Past and Future_, where we find the following passage:--
+
+"If we look at our maps we shall see the parts of one Continent that jut
+out agree with the indented portions of another. The prominent coast of
+Africa would fit in the opposite opening between North and South
+America, and so in numerous other instances. A general rending asunder
+of the World would seem to have taken place when the foundations of the
+great deep were broken up."
+
+Although Professor Pickering's theory is to a certain degree anticipated
+in the above words, still he has worked out the idea much more fully,
+and given it an additional fascination by connecting it with the birth
+of the moon. He points out, in fact, that there is a remarkable
+similarity between the lunar volcanoes and those in the immediate
+neighbourhood of the Pacific Ocean. He goes even further to suggest that
+Australia is another portion of the primal crust which was detached out
+of the region now occupied by the Indian Ocean, where it was originally
+connected with the south of India or the east of Africa.
+
+Certain objections to the theory have been put forward, one of which is
+that the parallelism noticed between the opposite shores of the Atlantic
+is almost too perfect to have remained through some sixty millions of
+years down to our own day, in the face of all those geological movements
+of upheaval and submergence, which are perpetually at work upon our
+globe. Professor Pickering, however, replies to this objection by
+stating that many geologists believe that the main divisions of land and
+water on the earth are permanent, and that the geological alterations
+which have taken place since these were formed have been merely of a
+temporary and superficial nature.
+
+
+
+
+CHAPTER XXVIII
+
+THE END OF THINGS
+
+
+We have been trying to picture the beginning of things. We will now try
+to picture the end.
+
+In attempting this, we find that our theories must of necessity be
+limited to the earth, or at most to the solar system. The time-honoured
+expression "End of the World" really applies to very little beyond the
+end of our own earth. To the people of past ages it, of course, meant
+very much more. For them, as we have seen, the earth was the centre of
+everything; and the heavens and all around were merely a kind of minor
+accompaniment, created, as they no doubt thought, for their especial
+benefit. In the ancient view, therefore, the beginning of the earth
+meant the beginning of the universe, and the end of the earth the
+extinction of all things. The belief, too, was general that this end
+would be accomplished through fire. In the modern view, however, the
+birth and death of the earth, or indeed of the solar system, might pass
+as incidents almost unnoticed in space. They would be but mere links in
+the chain of cosmic happenings.
+
+A number of theories have been forward from time to time prognosticating
+the end of the earth, and consequently of human life. We will conclude
+with a recital of a few of them, though which, if any, is the true one,
+the Last Men alone can know.
+
+Just as a living creature may at any moment die in the fulness of
+strength through sudden malady or accident, or, on the other hand, may
+meet with death as a mere consequence of old age, so may our globe be
+destroyed by some sudden cataclysm, or end in slow processes of decay.
+Barring accidents, therefore, it would seem probable that the growing
+cold of the earth, or the gradual extinction of the sun, should after
+many millions of years close the chapter of life, as we know it. On the
+former of these suppositions, the decrease of temperature on our globe
+might perhaps be accelerated by the thinning of the atmosphere, through
+the slow escape into space of its constituent gases, or their gradual
+chemical combination with the materials of the earth. The subterranean
+heat entirely radiated away, there would no longer remain any of those
+volcanic elevating forces which so far have counteracted the slow
+wearing down of the land surface of our planet, and thus what water
+remained would in time wash over all. If this preceded the growing cold
+of the sun, certain strange evolutions of marine forms of life would be
+the last to endure, but these, too, would have to go in the end.
+
+Should, however, the actual process be the reverse of this, and the sun
+cool down the quicker, then man would, as a consequence of his
+scientific knowledge, tend in all probability to outlive the other forms
+of terrestrial life. In such a vista we can picture the regions of the
+earth towards the north and south becoming gradually more and more
+uninhabitable through cold, and human beings withdrawing before the
+slow march of the icy boundary, until the only regions capable of
+habitation would lie within the tropics. In such a struggle between man
+and destiny science would be pressed to the uttermost, in the devising
+of means to counteract the slow diminution of the solar heat and the
+gradual disappearance of air and water. By that time the axial rotation
+of our globe might possibly have been slowed down to such an extent that
+one side alone of its surface would be turned ever towards the fast
+dying sun. And the mind's eye can picture the last survivors of the
+human race, huddled together for warmth in a glass-house somewhere on
+the equator, waiting for the end to come.
+
+The mere idea of the decay and death of the solar system almost brings
+to one a cold shudder. All that sun's light and heat, which means so
+much to us, entirely a thing of the past. A dark, cold ball rushing
+along in space, accompanied by several dark, cold balls circling
+ceaselessly around it. One of these a mere cemetery, in which there
+would be no longer any recollection of the mighty empires, the loves and
+hates, and all that teeming play of life which we call History.
+Tombstones of men and of deeds, whirling along forgotten in the darkness
+and silence. _Sic transit gloria mundi._
+
+In that brilliant flight of scientific fancy, the _Time Machine_, Mr.
+H.G. Wells has pictured the closing years of the earth in some such
+long-drawn agony as this. He has given us a vision of a desolate beach
+by a salt and almost motionless sea. Foul monsters of crab-like form
+crawl slowly about, beneath a huge hull of sun, red and fixed in the
+sky. The rocks around are partly coated with an intensely green
+vegetation, like the lichen in caves, or the plants which grow in a
+perpetual twilight. And the air is now of an exceeding thinness.
+
+He dips still further into the future, and thus predicts the final form
+of life:--
+
+"I saw again the moving thing upon the shoal--there was no mistake now
+that it was a moving thing--against the red water of the sea. It was a
+round thing, the size of a football perhaps, or it may be bigger, and
+tentacles trailed down from it; it seemed black against the weltering
+blood-red water, and it was hopping fitfully about."
+
+What a description of the "Heir of all the Ages!"
+
+To picture the end of our world as the result of a cataclysm of some
+kind, is, on the other hand, a form of speculation as intensely dramatic
+as that with which we have just been dealing is unutterably sad.
+
+It is not so many years ago, for instance, that men feared a sudden
+catastrophe from the possible collision of a comet with our earth. The
+unreasoning terror with which the ancients were wont to regard these
+mysterious visitants to our skies had, indeed, been replaced by an
+apprehension of quite another kind. For instance, as we have seen, the
+announcement in 1832 that Biela's Comet, then visible, would cut through
+the orbit of the earth on a certain date threw many persons into a
+veritable panic. They did not stop to find out the real facts of the
+case, namely, that, at the time mentioned, the earth would be nearly a
+month's journey from the point indicated!
+
+It is, indeed, very difficult to say what form of damage the earth
+would suffer from such a collision. In 1861 it passed, as we have seen,
+through the tail of the comet without any noticeable result. But the
+head of a comet, on the other hand, may, for aught we know, contain
+within it elements of peril for us. A collision with this part might,
+for instance, result in a violent bombardment of meteors. But these
+meteors could not be bodies of any great size, for the masses of comets
+are so very minute that one can hardly suppose them to contain any large
+or dense constituent portions.
+
+The danger, however, from a comet's head might after all be a danger to
+our atmosphere. It might precipitate, into the air, gases which would
+asphyxiate us or cause a general conflagration. It is scarcely necessary
+to point out that dire results would follow upon any interference with
+the balance of our atmosphere. For instance, the well-known French
+astronomer, M. Camille Flammarion,[39] has imagined the absorption of
+the nitrogen of the air in this way; and has gone on to picture men and
+animals reduced to breathing only oxygen, first becoming excited, then
+mad, and finally ending in a perfect saturnalia of delirium.
+
+Lastly, though we have no proof that stars eventually become dark and
+cold, for human time has so far been all too short to give us even the
+smallest evidence as to whether heat and light are diminishing in our
+own sun, yet it seems natural to suppose that such bodies must at last
+cease their functions, like everything else which we know of. We may,
+therefore, reasonably presume that there are dark bodies scattered in
+the depths of space. We have, indeed, a suspicion of at least one,
+though perhaps it partakes rather of a planetary nature, namely, that
+"dark" body which continually eclipses Algol, and so causes the
+temporary diminution of its light. As the sun rushes towards the
+constellation of Lyra such an extinguished sun may chance to find itself
+in his path; just as a derelict hulk may loom up out of the darkness
+right beneath the bows of a vessel sailing the great ocean.
+
+Unfortunately a collision between the sun and a body of this kind could
+not occur with such merciful suddenness. A tedious warning of its
+approach would be given from that region of the heavens whither our
+system is known to be tending. As the dark object would become visible
+only when sufficiently near our sun to be in some degree illuminated by
+his rays, it might run the chance at first of being mistaken for a new
+planet. If such a body were as large, for instance, as our own sun, it
+should, according to Mr. Gore's calculations, reveal itself to the
+telescope some fifteen years before the great catastrophe. Steadily its
+disc would appear to enlarge, so that, about nine years after its
+discovery, it would become visible to the naked eye. At length the
+doomed inhabitants of the earth, paralysed with terror, would see their
+relentless enemy shining like a second moon in the northern skies.
+Rapidly increasing in apparent size, as the gravitational attractions of
+the solar orb and of itself interacted more powerfully with diminishing
+distance, it would at last draw quickly in towards the sun and disappear
+in the glare.
+
+It is impossible for us to conceive anything more terrible than these
+closing days, for no menace of catastrophe which we can picture could
+bear within it such a certainty of fulfilment. It appears, therefore,
+useless to speculate on the probable actions of men in their now
+terrestrial prison. Hope, which so far had buoyed them up in the direst
+calamities, would here have no place. Humanity, in the fulness of its
+strength, would await a wholesale execution from which there could be no
+chance at all of a reprieve. Observations of the approaching body would
+have enabled astronomers to calculate its path with great exactness, and
+to predict the instant and character of the impact. Eight minutes after
+the moment allotted for the collision the resulting tide of flame would
+surge across the earth's orbit, and our globe would quickly pass away in
+vapour.
+
+And what then?
+
+A nebula, no doubt; and after untold ages the formation possibly from it
+of a new system, rising phoenix-like from the vast crematorium and
+filling the place of the old one. A new central sun, perhaps, with its
+attendant retinue of planets and satellites. And teeming life,
+perchance, appearing once more in the fulness of time, when temperature
+in one or other of these bodies had fallen within certain limits, and
+other predisposing conditions had supervened.
+
+  "The world's great age begins anew,
+      The golden years return,
+  The earth doth like a snake renew
+      Her winter weeds outworn:
+  Heaven smiles, and faiths and empires gleam
+  Like wrecks of a dissolving dream.
+
+  A brighter Hellas rears its mountains
+      From waves serener far;
+  A new Peneus rolls his fountains
+      Against the morning star;
+  Where fairer Tempes bloom, there sleep
+  Young Cyclads on a sunnier deep.
+
+  A loftier Argo cleaves the main,
+      Fraught with a later prize;
+  Another Orpheus sings again,
+      And loves, and weeps, and dies;
+  A new Ulysses leaves once more
+  Calypso for his native shore.
+
+       *       *       *       *       *
+
+Oh cease! must hate and death return?
+    Cease! must men kill and die?
+Cease! drain not to its dregs the urn
+    Of bitter prophecy!
+The world is weary of the past,--
+Oh might it die or rest at last!"
+
+
+[39] See his work, _La Fin du Monde_, wherein the various ways by which
+our world may come to an end are dealt with at length, and in a
+profoundly interesting manner.
+
+
+
+
+INDEX
+
+
+  Achromatic telescope, 115, 116
+
+  Adams, 24, 236, 243
+
+  Aerial telescopes, 110, 111
+
+  Agathocles, Eclipse of, 85
+
+  Agrippa, Camillus, 44
+
+  Ahaz, dial of, 85
+
+  Air, 166
+
+  Airy, Sir G.B., 92
+
+  Al g�l, 307
+
+  Al Sufi, 284, 290, 296, 315
+
+  Alcor, 294
+
+  Alcyone, 284
+
+  Aldebaran, 103, 288, 290, 297
+
+  Algol, 307, 309-310, 312, 323, 347
+
+  Alpha, Centauri, 52-53, 280, 298-299, 304, 320
+
+  Alpha Crucis, 298
+
+  Alps, Lunar, 200
+
+  Altair, 295
+
+  Altitude of objects in sky, 196
+
+  Aluminium, 145
+
+  Amos viii. 9, 85
+
+  Anderson, T.D., 311-312
+
+  Andromeda (constellation), 279, 314;
+    Great Nebula in, 314, 316
+
+  Andromedid meteors, 272
+
+  Anglo-Saxon Chronicle, 87-88
+
+  Anighito meteorite, 277
+
+  Annular eclipse, 65-68, 80, 92, 99
+
+  Annular Nebula in Lyra, 315-316
+
+  Annulus, 68
+
+  Ans�, 242-243
+
+  Anticipation in discovery, 236-237
+
+  Apennines, Lunar, 200
+
+  Aphelion, 274
+
+  Apparent enlargement of celestial objects, 192-196
+
+  Apparent size of celestial objects deceptive, 196, 294
+
+  Apparent sizes of sun and moon, variations in, 67, 80, 178
+
+  Aquila (constellation), 295
+
+  Arabian astronomers, 107, 307
+
+  Arago, 92, 257
+
+  Arc, degrees minutes and seconds of, 60
+
+  Arcturus, 280, 282, 290, 295
+
+  Argelander, 290
+
+  Argo (constellation), 298
+
+  Aristarchus of Samos, 171
+
+  Aristarchus (lunar crater), 205
+
+  Aristophanes, 101
+
+  Aristotle, 161, 173, 185
+
+  Arrhenius 222, 253-254
+
+  Assyrian tablet, 84
+
+  Asteroidal zone, analogy of, to Saturn's rings, 238
+
+  Asteroids (or minor planets), 30-31, 225-228, 336;
+    discovery of the, 23, 244;
+    Wolf's method of discovering, 226-227
+
+  Astrology, 56
+
+  _Astronomical Essays_, 63, 337
+
+  Astronomical Society, Royal, 144
+
+  _Astronomy, Manual of_, 166
+
+  Atlantic Ocean, parallelism of opposite shores, 340-341
+
+  Atlas, the Titan, 18
+
+  Atmosphere, absorption by earth's, 129-130;
+    ascertainment of, by spectroscope, 124-125, 212;
+    height of earth's, 167, 267;
+    of asteroids, 226;
+    of earth, 129, 130, 166-169, 218, 222, 267, 346;
+    of Mars, 156, 212, 216;
+    of Mercury, 156;
+    of moon, 70-71, 156, 201-203;
+    of Jupiter, 231;
+    of planets, 125;
+    of Saturn's rings, 239
+
+  "Atmosphere" of the stars, 331
+
+  Atmospheric layer and "glass-house" compared, 167, 203
+
+  August Meteors (Perseids), 270
+
+  Auriga (constellation), 294-296, 306, 311;
+    New Star in, 311
+
+  Aurig�, [b] (Beta), 294, 297, 304
+
+  Aurora Borealis, 141, 143, 259
+
+  Australia, suggested origin of, 340
+
+  Axis, 29-30;
+    of earth, 163, 180;
+    small movement of earth's, 180-181
+
+
+  Babylonian tablet, 84
+
+  Babylonian idea of the moon, 185
+
+  Bacon, Roger, 108
+
+  Bacubirito meteorite, 277
+
+  Bagdad, 107
+
+  Baily, Francis, 92
+
+  "Baily's Beads," 69, 70, 91-92, 154
+
+  Bailly (lunar crater), 199
+
+  Ball, Sir Robert, 271
+
+  Barnard, E.E., 31, 224, 232-234, 237, 258
+
+  "Bay of Rainbows," 197
+
+  Bayer's classification of stars, 289, 291-292
+
+  Bayeux Tapestry, 263
+
+  Bear, Great (constellation). _See_ Ursa Major;
+    Little, _see_ Ursa Minor
+
+  Beehive (Pr�sepe), 307
+
+  Beer, 206
+
+  Belopolsky, 304
+
+  "Belt" of Orion, 297
+
+  Belt theory of Milky Way, 321
+
+  Belts of Jupiter, 230
+
+  Bergstrand, 314
+
+  Berlin star chart, 244
+
+  Bessel, 173, 280, 305
+
+  Beta ([b]) Lyr�, 307
+
+  Beta ([b]) Persei. _See_ Algol
+
+  Betelgeux, 297
+
+  Bible, eclipses in, 85
+
+  Biela's Comet, 256-257, 272-273, 345
+
+  Bielids, 270, 272-273
+
+  Billion, 51-52
+
+  Binary stars, spectroscopic, 301-306, 309;
+    visual, 300, 303-306
+
+  "Black Drop," 152-154
+
+  "Black Hour," 89
+
+  "Black Saturday," 89
+
+  Blood, moon in eclipse like, 102
+
+  Blue (rays of light), 121, 130
+
+  Bode's Law, 22-23, 244-245
+
+  Bolometer, 127
+
+  Bond, G.P., 236, 257
+
+  Bonpland, 270
+
+  Bo�tes (constellation), 295, 314
+
+  Bradley, 111
+
+  Brahe, Tycho, 290, 311
+
+  Br�dikhine's theory of comets' tails, 253-254, 256
+
+  Bright eclipses of moon, 65, 102
+
+  British Association for the Advancement of Science, 318
+
+  _British Astronomical Association, Journal of_, 194
+
+  British Museum, 84
+
+  Bull (constellation). _See_ Taurus;
+    "Eye" of the, 297;
+    "Head" of the, 297
+
+  Burgos, 98
+
+  Busch, 93
+
+
+  C�sar, Julius, 85, 110, 180, 259, 262, 291, 293
+
+  Calcium, 138, 145
+
+  Callisto, 233-234
+
+  Cambridge, 24, 91, 119, 243
+
+  Campbell, 305
+
+  Canali, 214
+
+  "Canals" of Mars, 214-222, 224-225
+
+  Cancer (constellation), 307
+
+  Canes Venatici (constellation), 306, 314
+
+  Canis Major (constellation), 289, 296-297;
+    Minor, 296-297
+
+  Canopus, 285, 298-299, 320
+
+  Capella, 280, 282, 290, 294, 297, 303, 313
+
+  Carbon, 145
+
+  Carbon dioxide. _See_ Carbonic acid gas
+
+  Carbonic acid gas, 166, 213, 221-222
+
+  Carnegie Institution, Solar Observatory of, 118
+
+  Cassegrainian telescope, 114, 118
+
+  Cassini, J.D., 236, 240
+
+  "Cassini's Division" in Saturn's ring, 236, 238
+
+  Cassiopeia (constellation), 279, 294, 311, 314
+
+  Cassiopei�, [�] (Eta), 303
+
+  Cassiopeia's Chair, 294
+
+  Cassius, Dion, 86
+
+  Castor, 282, 297, 304
+
+  Catalogues of stars, 106, 290-291, 311
+
+  Centaur. _See_ Centaurus
+
+  Centaurus (constellation), 298, 306
+
+  Centre of gravity, 42, 283-284, 324
+
+  Ceres, diameter of, 30, 225
+
+  Ceti, Omicron (or Mira), 307-308
+
+  Cetus, or the Whale (constellation), 307
+
+  Chaldean astronomers, 74, 76
+
+  Challis, 243-244
+
+  Chamberlin, 337
+
+  "Chambers of the South," 299
+
+  Chandler, 308
+
+  Charles V., 261
+
+  "Charles' Wain," 291
+
+  Chemical rays, 127
+
+  Chinese and eclipses, 83
+
+  Chloride of sodium, 122
+
+  Chlorine, 122, 145
+
+  Christ, Birth of, 102
+
+  Christian Era, first recorded solar eclipse in, 85
+
+  Chromatic aberration, 110
+
+  Chromosphere, 71-72, 93-94, 130-132, 138-139
+
+  Circle, 171-173
+
+  Clark, Alvan, & Sons, 117-118, 303
+
+  Claudius, Emperor, 86
+
+  Clavius (lunar crater), 199
+
+  Clerk Maxwell, 237
+
+  "Clouds" (of Aristophanes), 101
+
+  Clustering power, 325
+
+  Clusters of stars, 300, 306, 314, 328
+
+  Coal Sacks. _See_ Holes in Milky Way
+
+  Coelostat, 119
+
+  Coggia's Comet, 254
+
+  Colour, production of, in telescopes, 109-111, 115, 121
+
+  Collision of comet with earth, 345-346;
+    of dark star with sun, 346-348;
+    of stars, 285, 312
+
+  Columbus, 103
+
+  Coma Berenices (constellation), 307, 316
+
+  Comet, first discovery of by photography, 258;
+    first orbit calculated, 255;
+    first photograph of, 257-258;
+    furthest distance seen, 258;
+    passage of among satellites of Jupiter, 250;
+    passage of earth and moon through tail of, 257, 346
+
+  Comet of 1000 A.D., 262;
+    1066, 262-264;
+    1680, 255, 265;
+    1811, 254-255;
+    1861, 254, 257, 346;
+    1881, 257-258;
+    1882, 251, 258, 291;
+    1889, 258;
+    1907, 258
+
+  Comets, 27-28, 58, Chaps. XIX. and XX., 345-346;
+    ancient view of, 259-261;
+    captured, 251-253;
+    Chinese records of, 83-84;
+    composition of, 252;
+    contrasted with planets, 247;
+    families of, 251-252, 256;
+    meteor swarms and, 274;
+    revealed by solar eclipses, 95-96;
+    tails of, 141, 182, 248, 252-254
+
+  Common, telescopes of Dr. A.A., 118
+
+  Conjunction, 209
+
+  Constellations, 105, 278-279, 285, 289
+
+  Contraction theory of sun's heat, 128-129, 335
+
+  Cook, Captain, 154
+
+  Cooke, 118
+
+  Copernican system, 20, 107, 149, 170-173, 279, 280
+
+  Copernicus, 20, 108, 149, 158, 170-172, 236
+
+  Copernicus (lunar crater), 200, 204
+
+  Copper, 145
+
+  Corder, H., 144
+
+  Corona, 70-72, 90, 92-97, 132, 140-141, 270;
+    earliest drawing of, 91;
+    earliest employment of term, 90;
+    earliest mention of, 86;
+    earliest photograph of, 93;
+    illumination given by, 71;
+    possible change in shape of during eclipse, 96-98;
+    structure of, 142-143;
+    variations in shape of, 141
+
+  Corona Borealis (constellation), 295
+
+  Coronal matter, 142;
+    streamers, 95-96, 141-143
+
+  Coronium, 133, 142, 317
+
+  Cotes, 91
+
+  Coud�, equatorial, 119
+
+  Cowell, P.H., 255, 264
+
+  Crabtree, 152
+
+  Crape ring of Saturn, 236-237
+
+  Craterlets on Mars, 220
+
+  Craters (ring-mountains) on moon, 197-205, 214, 340;
+    suggested origin of, 203-204, 214
+
+  Crawford, Earl of, 94
+
+  Crecy, supposed eclipse at battle of, 88-89
+
+  Crescent moon, 183, 185
+
+  Crommelin, A.C.D., 255, 264
+
+  Crossley Reflector, 118, 315-316
+
+  Crown glass, 115
+
+  Crucifixion, darkness of, 86
+
+  Crucis, [a] (Alpha), 298
+
+  Crux, or "Southern Cross" (constellation), 298-299, 323
+
+  Cycle, sunspot, 136-137, 141, 143-144
+
+  Cygni, 61, 173, 280
+
+  Cygnus, or the Swan (constellation), 295, 325
+
+
+  Daniel's Comet of 1897, 258
+
+  Danzig, 111
+
+  Dark Ages, 102, 107, 260
+
+  Dark eclipses of moon, 65, 102-103
+
+  Dark matter in space, 323
+
+  Dark meteors, 275-276
+
+  Dark stars, 309-310, 312, 323, 346-347
+
+  "Darkness behind the stars," 325
+
+  Darwin, Sir G.H., 339
+
+  Davis, 94
+
+  Dawes, 236
+
+  Dearborn Observatory, 303
+
+  Death from fright at eclipse, 73
+
+  Debonnaire, Louis le, 88, 261
+
+  Deimos, 223
+
+  Deity, symbol of the, 87
+
+  "Demon star." _See_ Algol
+
+  Denebola, 296
+
+  Denning, W.F., 269
+
+  Densities of sun and planets, 39
+
+  Density, 38
+
+  Deslandres, 140
+
+  Diameters of sun and planets, 31
+
+  Disappearance of moon in lunar eclipse, 65, 102-103
+
+  Disc, 60
+
+  "Disc" theory. _See_ "Grindstone" theory
+
+  Discoveries, independent, 236
+
+  Discovery, anticipation in, 236-237;
+    indirect methods of, 120
+
+  "Dipper," the, 291;
+    the "Little," 294
+
+  Distance of a celestial body, how ascertained, 56-58;
+    of sun from earth, how determined, 151, 211
+
+  Distances of planets from sun, 47
+
+  Distances of sun and moon, relative, 68
+
+  Dog, the Greater. _See_ Canis Major;
+    the Lesser, _see_ Canis Minor
+
+  "Dog Star," 289, 297
+
+  Dollond, John, 115-116
+
+  Donati's Comet, 254, 257
+
+  Doppler's method, 125, 136, 282, 301-302
+
+  Dorpat, 117
+
+  Double canals of Mars, 214-215, 218-220
+
+  Double planet, earth and moon a, 189
+
+  Double stars, 300
+
+  Douglass, 233
+
+  "Dreams, Lake of," 197
+
+  Dumb-bell Nebula, 316
+
+
+  Earth, 20, 22, 31, 39, 48, 64, Chap. XV., 267;
+    cooling of, 343;
+    diameter of, 31;
+    interior of, 166;
+    mean distance of from sun, 47;
+    rigidity of, 181;
+    rotation of, 30, 33, 161-165, 170;
+    shape of, 165;
+    "tail" to, 182
+
+  "Earthlight," or "Earthshine," 186
+
+  Earth's axis, Precessional movement of, 175-177, 295, 298-299
+
+  Earth's shadow, circular shape of, 64, 160
+
+  Eclipse, 61
+
+  Eclipse knowledge, delay of, 74
+
+  Eclipse party, work of, 73
+
+  Eclipse of sun, advance of shadow in total, 69;
+    animal and plant life during, 71;
+    earliest record of total, 84;
+    description of total, 69-73;
+    duration of total, 69, 72;
+    importance of total, 68
+
+  Eclipses, ascertainment of dates of past, 74;
+    experience a necessity in solar, 73-74;
+    of moon, 63-65, Chap. IX., 203;
+    photography in, 93;
+    prediction of future, 74;
+    recurrence of, 74-80
+
+  Eclipses of sun, 25, 65-74, Chap. VIII., 201-202, 234;
+    1612 A.D., 90;
+    1715, 88, 91;
+    1724, 88, 91;
+    1836, 92;
+    1842, 92-93;
+    1851, 81, 93;
+    1868, 93;
+    1870, 94;
+    1871, 94;
+    1878, 95;
+    1882, 95;
+    1883, 95-96;
+    1893, 95-96;
+    1896, 96, 99;
+    1898, 96, 98;
+    1900, 97;
+    1905, 75-76, 80-81, 97-98;
+    1907, 98;
+    1908, 98;
+    1914, 99;
+    1927, 92, 99-100
+
+  _Eclipses, Past and Future_, 340
+
+  Egenitis, 272
+
+  Electric furnace, 128
+
+  Electric light, spectrum of, 122
+
+  Elements composing sun, 144-145
+
+  Ellipses, 32, 66, 172-173, 177-178
+
+  Elliptic orbit, 66, 177
+
+  Ellipticity, 32
+
+  Elongation, Eastern, 147, 149;
+    Western, 147, 149
+
+  Encke's Comet, 253, 256
+
+  "End of the World," 342
+
+  England, solar eclipses visible in, 87-88, 91-92
+
+  Epsilon, ([e]) Lyr�, 302
+
+  Equator, 48
+
+  Equatorial telescope, 226
+
+  Equinoxes. _See_ Precession of
+
+  Eros, 210-211, 223, 226-227;
+    discovery of, 24, 210, 227;
+    importance of, 211;
+    orbit of, 32, 37, 210, 336
+
+  Eruptive prominences, 139
+
+  _Esclistre_, 89
+
+  Ether, 322-323, 331-332
+
+  Europa, 233, 235
+
+  Evans, J.E., 219
+
+  Evening star, 149-150, 241
+
+  Everest, Mount, 200
+
+  Evershed, 182
+
+  Eye-piece, 110
+
+
+  Fabricius, 307
+
+  Facul�, 136, 143
+
+  Fauth, 205
+
+  Faye, 335
+
+  _Fin du Monde_, 346
+
+  First quarter, 183
+
+  "Fixed stars," 280
+
+  Flagstaff, 215-216, 220
+
+  Flammarion, Camille, 346
+
+  Flamsteed, 90
+
+  "Flash spectrum," 137
+
+  "Flat," 112
+
+  Flint glass, 115
+
+  Focus, 66, 177
+
+  "Forty-foot Telescope," 115
+
+  Foster, 102
+
+  Fraunhofer, 117
+
+  French Academy of Sciences, 115
+
+  Froissart, 89
+
+  "Full moon" of Laplace, 190
+
+
+  Galaxy. _See_ Milky Way.
+
+  Galilean telescope, 109
+
+  Galileo, 55, 109, 172, 197, 206, 232-235, 242
+
+  Galle, 24, 211, 244
+
+  Ganymede, 233-234
+
+  Gas light, spectrum of, 122
+
+  Gegenschein, 181-182
+
+  "Gem" of meteor ring, 271
+
+  Gemini, or the Twins (constellation), 22, 296-297
+
+  Geminorum, [z] (Zeta), 304
+
+  Geometrical groupings of stars, 292
+
+  "Giant" planet, 230, 238-239
+
+  Gibbous, 183, 185
+
+  Gill, Sir David, 211, 258, 291, 317-318
+
+  Gold, 145
+
+  Goodricke, 307
+
+  Gore, J.E., 63, 285, 303, 307-308, 310, 323-324, 331, 337, 347
+
+  Granulated structure of photosphere, 134
+
+  Gravitation (or gravity), 39, 41-45, 128, 306
+
+  Greek ideas, 18, 158, 161-162, 171, 186, 197
+
+  Green (rays of light), 121
+
+  Greenwich Observatory, 143-144, 232, 255, 303
+
+  Gregorian telescope, 113-114
+
+  Grimaldi (lunar crater), 199
+
+  "Grindstone" theory, 319-322
+
+  "Groombridge, 1830," 281-282, 326, 330
+
+  Groups of stars, 306-307
+
+  Grubb, Sir Howard, 118
+
+  _Gulliver's Travels_, 224
+
+
+  Hale, G.E., 119, 140
+
+  Half moon, 183, 185
+
+  Hall, Asaph, 223
+
+  Hall, Chester Moor, 115
+
+  Halley, Edmund, 91, 255, 264-265, 306
+
+  Halley's Comet, 255, 264-265
+
+  Haraden Hill, 91
+
+  Harvard, 118, 302
+
+  Harvest moon, 190-192
+
+  Hawaii, 221
+
+  Heat rays, 127
+
+  Heidelberg, 226, 232
+
+  Height of lunar mountains, how determined, 201
+
+  Height of objects in sky, estimation of, 196
+
+  Helium, 138, 145, 182
+
+  Helmholtz, 128, 335
+
+  Hercules (constellation), 295
+
+  Herod the Great, 101-102
+
+  Herodotus, 84
+
+  Herschel, A.S., 269
+
+  Herschel, Sir John, 92, 322
+
+  Herschel, Sir William, 22, 36, 114-115, 204, 213, 235, 283, 292, 308,
+    319-320, 326-328
+
+  Herschelian telescope, 114, 119
+
+  Hesper, 109
+
+  Hesperus, 150
+
+  Hevelius, 111
+
+  Hezekiah, 85
+
+  Hi, 83
+
+  Hindoos, 18
+
+  Hipparchus, 106, 177, 290, 311
+
+  Ho, 83
+
+  Holes in Milky Way, 321-323
+
+  Holmes, Oliver Wendell, 213
+
+  Homer, 223
+
+  Horace, Odes of, 106
+
+  Horizon, 159
+
+  Horizontal eclipse, 169
+
+  Horrox, 44, 151-152
+
+  Hour Glass Sea, 212
+
+  Huggins, Sir William, 94, 125, 317
+
+  Humboldt, 270
+
+  "Hunter's moon," 192
+
+  Huyghens, 111-112, 240, 242-243
+
+  Hyades, 296-297, 307
+
+  Hydrocarbon gas, 254
+
+  Hydrogen, 94, 131, 138, 140, 144, 156, 182, 254
+
+
+  Ibrahim ben Ahmed, 270
+
+  Ice-layer theory:
+    Mars, 219;
+    moon, 205, 219
+
+  Illusion theory of Martian canals, 219
+
+  Imbrium, Mare, 197
+
+  Inclination of orbits, 36-37
+
+  Indigo (rays of light), 121
+
+  Inferior conjunction, 147, 149
+
+  Inferior planets, 20, 22, Chap. XIV., 229
+
+  Instruments, pre-telescopic, 106-107, 172
+
+  International photographic survey of sky, 290-291
+
+  Intra-Mercurial planet, 25-26
+
+  _Introduction to Astronomy_, 31
+
+  Inverted view in astronomical telescope, 116-117
+
+  Io, 233-234
+
+  Iridum, Sinus, 197
+
+  Iron, 145, 254
+
+  _Is Mars Habitable?_ 221
+
+
+  Jansen, 108
+
+  Janssen, 94, 236, 258
+
+  Japetus, 240
+
+  Jessenius, 89
+
+  Job, Book of, 299
+
+  Johnson, S.J., 103, 340
+
+  Josephus, 101, 262
+
+  Juno, 225
+
+  Jupiter, 20, 22-23, 31, 34, 37, 42, 227-228, 230-236, 241, 272, 311;
+    comet family of, 251-253, 256;
+    discovery of eighth satellite, 26, 232;
+    eclipse of, by satellite, 234;
+    without satellites, 234-235
+
+  Jupiter, satellites of, 26, 62, 108, 189, 232-235;
+    their eclipses, 234-235;
+    their occultations, 62, 234;
+    their transits, 62, 234
+
+
+  Kant, 334
+
+  Kapteyn, 284, 313
+
+  Keeler, 315, 337
+
+  Kelvin, Lord, 129
+
+  Kepler, 44, 152, 172, 237, 242, 245, 253, 311
+
+  Kinetic theory, 156, 202, 212, 226, 231, 239, 336
+
+  King, L.W., 84
+
+  _Knowledge_, 87
+
+
+  Labrador, 97
+
+  Lacus Somniorum, 197
+
+  "Lake of Dreams," 197
+
+  Lalande, 244, 283
+
+  Lampland, 215, 219
+
+  Langley, 95, 127
+
+  Laplace, 190, 333
+
+  Laputa, 224
+
+  Le Maire, 115
+
+  Le Verrier, 24, 236, 243-244, 275
+
+  Lead, 145
+
+  Leibnitz Mountains (lunar), 200
+
+  Leo (constellation), 270, 295-296
+
+  Leonids, 270-272, 274-275
+
+  Lescarbault, 25
+
+  Lewis, T., 303
+
+  Lexell's Comet, 250
+
+  Lick Observatory, 31, 98, 117-118, 215, 232, 303, 305, 315;
+    Great Telescope of, 117, 215, 237
+
+  "Life" of an eclipse of the moon, 80;
+    of the sun, 77-78
+
+  Life on Mars, Lowell's views, 217-218;
+    Pickering's, 221;
+    Wallace's, 221-223
+
+  Light, no extinction of, 322-324;
+    rays of, 127;
+    velocity of, 52, 235-236;
+    white, 121
+
+  "Light year," 53, 280
+
+  Lindsay, Lord, 94
+
+  Linn� (lunar crater), 205
+
+  Liouville, 190
+
+  Lippershey, 108
+
+  Liquid-filled lenses, 116
+
+  _Locksley Hall_, 296;
+    _Sixty Years After_, 109
+
+  Lockyer, Sir Norman, 73, 94, 236, 335
+
+  Loewy, 119, 206
+
+  London, eclipses visible at, 87-88, 91-92
+
+  Longfellow, 88
+
+  Lowell Observatory, 215, 219, 233-234
+
+  Lowell, Percival, 155, 212-213, 215-221
+
+  Lucifer, 150
+
+  Lynn, W.T., 219, 263
+
+  Lyra (constellation), 177, 283, 294-295, 307, 315, 347
+
+
+  M�dler, 206, 284
+
+  Magellanic Clouds, 317
+
+  Magnetism, disturbances of terrestrial, 143, 283
+
+  Magnitudes of stars, 287-289
+
+  Major planets, 229-230
+
+  "Man in the Moon," 197
+
+  _Manual of Astronomy_, 166
+
+  Maps of the moon, 206
+
+  Mare Imbrium, 197
+
+  Mare Serenitatis, 205
+
+  Mars, 20, 22-23, 31-32, 34, 37, 109, 155, 210-225, 234;
+    compared with earth and moon, 221, 225;
+    polar caps of, 212-214, 216;
+    satellites of, 26, 223-224;
+    temperature of, 213, 216, 221-222
+
+  Mass, 38;
+    of a star, how determined, 305
+
+  Masses of celestial bodies, how ascertained, 42;
+    of earth and moon compared, 42;
+    of sun and planets compared, 39
+
+  Maunder, E.W., 87, 143, 219
+
+  Maunder, Mrs., E.W., 96, 144
+
+  Maxwell, Clerk, 237
+
+  Mayer, Tobias, 206, 283
+
+  McClean, F.K., 98
+
+  Mean distance, 46
+
+  "Medicean Stars," 232
+
+  Mediterranean, eclipse tracks across, 94, 97
+
+  Melbourne telescope, 118
+
+  Melotte, P., 232
+
+  Mercator's Projection, 80-81
+
+  Mercury (the metal), 145
+
+  Mercury (the planet), 20, 22, 25-26, 31-32, 34, 37, Chap. XIV.;
+    markings on, 156;
+    possible planets within orbit of, 25-26;
+    transit of, 62, 151, 154
+
+  Metals in sun, 145
+
+  Meteor swarms, 268-269, 271, 274-275
+
+  Meteors, 28, 56, 167, 259, Chap. XXI.
+
+  Meteors beyond earth's atmosphere, 275-276
+
+  Meteorites, 276-277
+
+  Meteoritic Hypothesis, 335
+
+  Metius, Jacob, 108
+
+  Michell, 283, 305
+
+  Middle Ages, 102, 260, 264
+
+  Middleburgh, 108
+
+  Milky Way (or Galaxy), 285, 299, 311, 317, 319-327;
+    penetration of, by photography, 325
+
+  Million, 47, 51-52
+
+  Minor planets. _See_ Asteroids.
+
+  Mira Ceti, 307-308
+
+  "Mirk Monday," 89
+
+  Mirror (speculum), 111, 116
+
+  Mizar, 294, 302
+
+  Monck, W.H.S., 275
+
+  Mongol Emperors of India, 107
+
+  Moon, 26, Chap. XVI.;
+    appearance of, in lunar eclipse, 65, 102-103;
+    diameter of, 189;
+    distance of, how ascertained, 58;
+    distance of, from earth, 48;
+    full, 63, 86, 149, 184, 189, 190, 206;
+    mass of, 200, 202;
+    mountains on, 197-205;
+    how their height is determined, 201;
+    movement of, 40-42;
+    new, 86, 149, 183, 185;
+    origin of, 339-341;
+    plane of orbit of, 63;
+    possible changes on, 204-205, 221;
+    "seas" of, 197, 206;
+    smallest detail visible on, 207;
+    volume of, 200
+
+  Morning star, 149-150, 241
+
+  Moulton, F.R., 31, 118, 128, 302, 335, 337
+
+  Moye, 154
+
+  Multiple stars, 300
+
+  Musa-ben-Shakir, 44
+
+  Mythology, 105
+
+
+  Neap-tides, 179
+
+  Nebul�, 314-318, 328, 335, 345;
+    evolution of stars from, 317-318
+
+  Nebular Hypothesis of Laplace, 333-338
+
+  Nebular hypotheses, Chap. XXVII.
+
+  Nebulium, 317
+
+  Neison, 206
+
+  Neptune, 20, 25, 31, 34, 37, 243-246, 249, 252, 274, 304;
+    discovery of, 23-24, 94, 210, 236, 243-244;
+    Lalande and, 244;
+    possible planets beyond, 25, 252;
+    satellite of, 26, 245;
+    "year" in, 35-36
+
+  "New" (or temporary), stars, 310-314
+
+  Newcomb, Simon, 181, 267, 281, 324, 326-327, 329
+
+  Newton, Sir Isaac, 40, 44, 91, 111-113, 115, 165, 172, 237, 255
+
+  Newtonian telescope, 112, 114, 116, 119
+
+  Nineveh Eclipse, 84-85
+
+  Nitrogen, 145, 156, 166, 346
+
+  Northern Crown, 295
+
+  Nova Aurig�, 311
+
+  Nova Persei, 312-314
+
+  Nov�. _See_ New (or temporary) stars
+
+  Nubecul�, 317
+
+
+  "Oases" of Mars, 216, 220
+
+  Object-glass, 109
+
+  Oblate spheroid, 165
+
+  Occultation, 61-62, 202, 296
+
+  _Olaf, Saga of King_, 88
+
+  Olbers, 227, 253, 256, 271
+
+  "Old moon in new moon's arms," 185
+
+  Olmsted, 271
+
+  Omicron (or "Mira") Ceti, 307-308
+
+  Opposition, 209
+
+  "Optick tube," 108-109, 232
+
+  Orange (rays of light), 121
+
+  Orbit of moon, plane of, 63
+
+  Orbits, 32, 36-37, 66, 150, 157
+
+  Oriental astronomy, 107
+
+  Orion (constellation), 195, 279, 296-297, 316;
+    Great Nebula in, 316, 328
+
+  Oxford, 139
+
+  Oxygen, 145, 156, 166, 346
+
+
+  Pacific Ocean, origin of moon in, 339
+
+  Palitzch, 255
+
+  Pallas, 225, 227
+
+  Parallax, 57, 173, 280, 305, 320, 326
+
+  Par�, Ambrose, 264-265
+
+  Peal, S.E., 205
+
+  Peary, 277
+
+  Pegasus (constellation), 306
+
+  Penumbra of sunspot, 135
+
+  Perennial full moon of Laplace, 190
+
+  Pericles, 84
+
+  Perrine, C.D., 232-233, 315
+
+  Perseids, 270, 273-275
+
+  Perseus (constellation), 273, 279, 307, 312
+
+  Phases of an inferior planet, 149, 160;
+    of the moon, 149, 160, 183-185
+
+  Phlegon, Eclipse of, 85-86
+
+  Phobos, 223
+
+  Phoebe, retrograde motion of, 240, 250, 336
+
+  Phosphorescent glow in sky, 323
+
+  Phosphorus (Venus), 150
+
+  Photographic survey of sky, international, 290-291
+
+  Photosphere, 130-131, 134
+
+  Piazzi, 23
+
+  Pickering, E.C., 302
+
+  Pickering, W.H., 199, 205-206, 220-221, 240, 339-341
+
+  Pictor, "runaway star" in constellation of, 281-282, 320, 330
+
+  Plane of orbit, 36, 150
+
+  Planetary nebul�, 245, 315
+
+  _Planetary and Stellar Studies_, 331
+
+  Planetesimal hypothesis, 337-338
+
+  Planetoids. _See_ Asteroids
+
+  Planets, classification of, 229;
+    contrasted with comets, 247;
+    in Ptolemaic scheme, 171;
+    relative distances of, from sun, 31-32
+
+  Plato (lunar crater), 198
+
+  Pleiades, 284, 296-297, 307
+
+  Pliny, 169, 260
+
+  Plough, 284, 291-296, 302
+
+  Plutarch, 86, 89, 169, 181
+
+  "Pointers," 292
+
+  Polaris. _See_ Pole Star
+
+  Pole of earth, Precessional movement of, 176-177, 295, 298-299
+
+  Pole Star, 33, 163, 177, 292-296, 300-301
+
+  Poles, 30, 163-164;
+    of earth, speed of point at, 164
+
+  Pollux, 282, 297
+
+  Posidonius, 186
+
+  Powell, Sir George Baden, 96
+
+  Pr�sepe (the Beehive), 307
+
+  Precession of the Equinoxes, 177, 295, 298-299
+
+  Pre-telescopic notions, 55
+
+  Primaries, 26
+
+  _Princess, The_ (Tennyson), 334
+
+  Princeton Observatory, 258
+
+  Prism, 121
+
+  Prismatic colours, 111, 121
+
+  Procyon, 284, 290, 297, 303
+
+  Prominences, Solar, 72, 93, 131, 139-140, 143;
+    first observation of, with spectroscope, 94, 140, 236
+
+  Proper motions of stars, 126, 281-285, 326, 329-330
+
+  Ptolem�us (lunar crater), 198-199, 204
+
+  Ptolemaic idea, 319;
+    system, 18, 19, 158, 171-172
+
+  Ptolemy, 18, 101, 171, 290, 296
+
+  Puiseux, P., 206
+
+  Pulkowa telescope, 117
+
+  Puppis, V., 310
+
+
+  Quiescent prominences, 139
+
+
+  Radcliffe Observer, 139
+
+  "Radiant," or radiant point, 269
+
+  Radiation from sun, 130, 134
+
+  Radium, 129, 138
+
+  Rainbow, 121
+
+  "Rainbows, Bay of," 197
+
+  Rambaut, A., 139
+
+  Ramsay, Sir William, 138
+
+  Rays (on moon), 204
+
+  Recurrence of eclipses, 74-80
+
+  Red (rays of light), 121, 125, 127, 130
+
+  Red Spot, the Great, 230
+
+  Reflecting telescope, 111-116;
+    future of, 119
+
+  Reflector. _See_ Reflecting telescope
+
+  Refracting and reflecting telescopes contrasted, 118
+
+  Refracting telescope, 109-111, 115-117;
+    limits to size of, 119-120
+
+  Refraction, 121, 168-169
+
+  Refractor, _See_ Refracting telescope
+
+  Regulus, 290, 296
+
+  Retrograde motion of Phoebe, 240, 250, 336
+
+  "Reversing Layer," 94, 130, 132, 137-138
+
+  Revival of learning, 107
+
+  Revolution, 30;
+    of earth around sun, 170-173;
+    periods of sun and planets, 35
+
+  Riccioli, 198
+
+  Rice-grain structure of photosphere, 134
+
+  Rigel, 285, 297
+
+  Rills (on moon), 204
+
+  Ring-mountains of moon. _See_ Craters
+
+  "Ring" nebul�, 315, 337
+
+  "Ring with wings," 87
+
+  Rings of Saturn, 108, 236-239, 241-243, 334
+
+  Ritchey, G.W., 118
+
+  Roberts, A.W., 308, 310
+
+  Roberts, Isaac, 325
+
+  "Roche's limit," 238
+
+  Roemer, 235
+
+  Roman history, eclipses in, 85-86
+
+  Romulus, 85
+
+  R�ntgen, 120
+
+  Rosse, great telescope of Lord, 117, 314
+
+  Rotation, 30;
+    of earth, 33, 161-165, 170;
+    of sun, 34, 125, 135-136, 231;
+    periods of sun and planets, 35
+
+  Royal Society of London, 90-91, 111
+
+  Rubicon, Passage of the, 85
+
+  "Runaway" stars, 281, 326, 330
+
+
+  Sagittarius (constellation), 316
+
+  Salt, spectrum of table, 122
+
+  Samarcand, 107
+
+  "Saros," Chaldean, 76-78, 84
+
+  Satellites, 26-27, 37
+
+  Saturn, 20, 22, 34, 37, 108, 236-243, 258;
+    comet family of, 252;
+    a puzzle to the early telescope observers, 241-243;
+    retrograde motion of satellite Phoebe, 240, 250, 336;
+    ring system of, 241;
+    satellites of, 36, 239-240;
+    shadows of planet on rings and of rings on planet, 237
+
+  Schaeberle, 95-96, 303, 316
+
+  Schiaparelli, 155, 214, 223
+
+  Schickhard (lunar crater), 199
+
+  Schmidt, 206
+
+  Sch�nfeld, 290
+
+  Schuster, 95
+
+  Schwabe, 136
+
+  Scotland, solar eclipses visible in, 89-90, 92
+
+  Sea of Serenity, 205
+
+  "Sea of Showers," 197
+
+  "Seas" of moon, 197, 206
+
+  Seasons on earth, 174-175;
+    on Mars, 211
+
+  Secondary bodies, 26
+
+  Seneca, 95, 260
+
+  _Septentriones_, 291
+
+  Serenitatis, Mare, 205
+
+  "Seven Stars," 291
+
+  "Shadow Bands," 69
+
+  Shadow of earth, circular shape of, 62-64
+
+  Shadows on moon, inky blackness of, 202
+
+  Shakespeare, 259, 293
+
+  Sheepshanks Telescope, 119
+
+  "Shining fluid" of Sir W. Herschel, 328
+
+  "Shooting Stars." _See_ Meteors
+
+  Short (of Edinburgh), 114
+
+  "Showers, Sea of," 197
+
+  Sickle of Leo, 270-271, 296
+
+  Siderostat, 118
+
+  Silver, 145
+
+  Silvered mirrors for reflecting telescopes, 116
+
+  Sinus Iridum, 197
+
+  Sirius, 280, 282, 284-285, 288-290, 297, 303-304, 320;
+    companion of, 303;
+    stellar magnitude of, 289
+
+  Size of celestial bodies, how ascertained, 59
+
+  Skeleton telescopes, 110
+
+  Sky, international photographic survey of, 290-291;
+    light of the, 323
+
+  Slipher, E.C., 213, 222
+
+  Smithsonian Institution of Washington, 98
+
+  Snow on Mars, 213
+
+  Sodium, 122, 124, 254
+
+  Sohag, 95
+
+  Solar system, 20-21, 29-31;
+    centre of gravity of, 42;
+    decay and death of, 344
+
+  Somniorum, Lacus, 197
+
+  Sound, 125, 166, 331
+
+  South pole of heavens, 163, 285, 298-299
+
+  Southern constellations, 298-299
+
+  Southern Cross. _See_ Crux
+
+  Space, 328
+
+  Spain, early astronomy in, 107;
+    eclipse tracks across 93, 97-98
+
+  Spectroheliograph, 140
+
+  Spectroscope, 120, 122, 124-125, 144-145, 212, 231;
+    prominences first observed with, 94, 140, 236
+
+  Spectrum of chromosphere, 132-133;
+    of corona, 133;
+    of photosphere, 132;
+    of reversing layer, 132, 137;
+    solar, 122-123, 127, 132
+
+  Speculum, 111, 116;
+    metal, 112
+
+  Spherical bodies, 29
+
+  Spherical shape of earth, proofs of, 158-161
+
+  Spherical shapes of sun, planets, and satellites, 160
+
+  Spiral nebul�, 314-316, 337-338
+
+  Spring balance, 166
+
+  Spring tides, 192
+
+  Spy-glass, 108
+
+  "Square of the distance," 43-44
+
+  Stannyan, Captain, 90
+
+  Star, mass of, how determined, 305;
+    parallax of, first ascertained, 173, 280
+
+  Stars, the, 20, 124, 126, 278 _et seq._;
+    brightness of, 287, 320;
+    distances between, 326-327;
+    distances of some, 173, 280, 320;
+    diminution of, below twelfth magnitude, 324;
+    evolution of, from nebul�, 317-318;
+    faintest magnitude of, 288;
+    number of those visible altogether, 324;
+    number of those visible to naked eye, 288
+
+  "Steam cracks," 221
+
+  Steinheil, 118
+
+  Stellar system, estimated extent of, 325-327;
+    an organised whole, 327;
+    limited extent of, 322-328, 330;
+    possible disintegration of, 329
+
+  Stiklastad, eclipse of, 88
+
+  Stone Age, 285
+
+  Stoney, G.J., 202, 222
+
+  Stonyhurst Observatory, 100
+
+  _Story of the Heavens_, 271
+
+  Streams of stars, Kapteyn's two, 284
+
+  Stroobant, 196
+
+  Stukeley, 91
+
+  Sulphur, 145
+
+  Summer, 175, 178
+
+  Sun, Chaps XII. and XIII.;
+    as a star, 124, 278, 289;
+    as seen from Neptune, 246, 304;
+    chemical composition of, 144-145;
+    distance of, how ascertained, 151, 211;
+    equator of, 135-136, 139;
+    gravitation at surface of, 129, 138-139;
+    growing cold of, 343-344;
+    mean distance of, from earth, 47, 211;
+    motion of, through space, 282-286, 326;
+    not a solid body, 136;
+    poles of, 136;
+    radiations from, 130;
+    revolution of earth around, 170-173;
+    stellar magnitude of, 288-289;
+    variation in distance of, 66, 178
+
+  Sunspots, 34, 125, 134-137, 140-141, 143-144, 308;
+    influence of earth on, 144
+
+  Suns and possible systems, 50, 286
+
+  Superior conjunction, 147-149
+
+  Superior planets, 22, 146, 209-210, 229
+
+  Swan (constellation). _See_ Cygnus
+
+  Swift, Dean, 224
+
+  "Sword" of Orion, 297, 316
+
+  Syrtis Major. _See_ Hour Glass Sea
+
+  "_Systematic_ Parallax," 326
+
+  Systems, other possible, 50, 286
+
+
+  Tails of comets, 182
+
+  Tamerlane, 107
+
+  Taurus (constellation), 103, 296-297, 307
+
+  "Tears of St. Lawrence," 273
+
+  Tebbutt's Comet, 257-258
+
+  Telescope, 33, 55, 107-108, 149;
+    first eclipse of moon seen through, 104;
+    of sun, 90
+
+  Telescopes, direct view reflecting, 114;
+    gigantic, 111;
+    great constructors of, 117-118;
+    great modern, 117-118
+
+  Tempel's Comet, 274
+
+  Temperature on moon, 203;
+    of sun, 128
+
+  Temporary (or new) stars, 310-314
+
+  Tennyson, Lord, 109, 296, 334
+
+  Terrestrial planets, 229-230
+
+  Terrestrial telescope, 117
+
+  Thales, Eclipse of, 84
+
+  Themis, 240
+
+  "Tidal drag," 180, 188, 208, 344
+
+  Tide areas, 179-180
+
+  Tides, 178-180, 338-339
+
+  _Time Machine_, 344
+
+  Tin, 145
+
+  Titan, 240
+
+  Titius, 245
+
+  Total phase, 71-72
+
+  Totality, 72;
+    track of, 66
+
+  Trail of a minor planet, 226-227
+
+  Transit, 62, 150-154;
+    of Mercury, 62, 151, 154;
+    of Venus, 62, 151-152, 154, 211
+
+  Trifid Nebula, 316
+
+  Triple stars, 300
+
+  Tubeless telescopes, 110-111, 243
+
+  Tubes used by ancients, 110
+
+  Tuttle's Comet, 274
+
+  Twilight, 167, 202
+
+  Twinkling of stars, 168
+
+  Twins (constellation). _See_ Gemini
+
+  Tycho Brahe, 290, 311
+
+  Tycho (lunar crater), 204
+
+
+  Ulugh Beigh, 107
+
+  Umbra of sunspot, 134-135
+
+  Universe, early ideas concerning, 17-18, 158, 177, 342
+
+  Universes, possibility of other, 330-331
+
+  Uranus, 22-24, 31, 210, 243, 245, 275;
+    comet family of, 252;
+    discovery of, 22, 210, 243;
+    rotation period of 34, 245;
+    satellites of, 26, 245;
+    "year" in, 35-36
+
+  Ursa Major (constellation), 279, 281, 291, 295, 314;
+    minor, 177, 279, 293-294
+
+  Urs� Majoris, ([z]) Zeta. _See_ Mizar
+
+
+  Variable stars, 307-310
+
+  Variations in apparent sizes of sun and moon, 67, 80, 178
+
+  Vault, shape of the celestial, 194-196
+
+  Vega, 177, 278, 280, 282-283, 285, 290, 294, 302, 307, 323
+
+  Vegetation on Mars, 221, 217-218;
+    on moon, 205
+
+  Venus, 20, 22, 31, 71, 90, 108-109, 111, Chap. XIV., 246, 311;
+    rotation period of, 34, 155
+
+  Very, F.W., 314
+
+  Vesta, 225, 227
+
+  Violet (rays of light), 121-122, 125
+
+  Virgil, 19
+
+  Volcanic theory of lunar craters, 203-204, 214
+
+  Volume, 38
+
+  Volumes of sun and planets compared, 38-39
+
+  "Vulcan," 25
+
+
+  Wallace, A.R., on Mars, 220-223
+
+  Water, lack of, on moon, 201-202
+
+  Water vapour, 202, 213, 222
+
+  Wargentin, 103
+
+  Warner and Swasey Co., 117
+
+  Weather, moon and, 206-207
+
+  Weathering, 202
+
+  Webb, Rev. T.W., 204
+
+  Weight, 43, 165-166
+
+  Wells, H.G., 344
+
+  Whale (constellation). _See_ Cetus
+
+  Whewell, 190
+
+  Willamette meteorite, 277
+
+  Wilson, Mount, 118
+
+  Wilson, W.E., 313
+
+  "Winged circle" (or "disc"), 87
+
+  Winter, 175, 178
+
+  Witt, 227
+
+  Wolf, Max, 226-227, 232
+
+  Wright, Thomas, 319, 334
+
+  Wybord, 89
+
+
+  Xenophon, 101
+
+
+  Year, 35
+
+  "Year" in Uranus and Neptune, 35-36
+
+  Year, number of eclipses in a, 68
+
+  "Year of the Stars," 270
+
+  Yellow (rays of light), 121-122, 124
+
+  Yerkes Telescope Great, 117, 303
+
+  Young, 94, 137, 166
+
+
+  Zenith, 174
+
+  Zinc, 145
+
+  Zodiacal light, 181
+
+  Zone of asteroids, 30-31, 227
+
+
+THE END
+
+Printed by BALLANTYNE, HANSON & CO.
+
+Edinburgh & London
+
+
+
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+MACHINERY, MECHANICAL. DEVICES, & MARVELLOUSLY DELICATE SCIENTIFIC
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+
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+     cannot stop till you come to the end of the
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+
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+
+SEELEY & CO., LTD., 88 GREAT RUSSELL STREET.
+
+
+A Catalogue of Books on Art, History, and General Literature Published
+by Seeley, Service & Co Ltd. 38 Great Russell St. London
+
+
+_Some of the Contents_
+
+Crown Library, The                       4
+
+Elzevir Library, The                     5
+
+Events of Our Own Times Series           6
+
+Illuminated Series, The                  8
+
+Miniature Library of Devotion, The       9
+
+Miniature Portfolio Monographs, The      9
+
+Missions, The Library of                10
+
+New Art Library, The                    11
+
+Portfolio Monographs                    11
+
+Science of To-Day Series, The           14
+
+Seeley's Illustrated Pocket Library     14
+
+Seeley's Standard Library               15
+
+Story Series, The                       15
+
+"Things Seen" Series, The               16
+
+
+_The Publishers will be pleased to post their complete Catalogue or
+their Illustrated Miniature Catalogue on receipt of a post-card_
+
+
+
+
+CATALOGUE OF BOOKS
+
+_Arranged alphabetically under the names of Authors and Series_
+
+
+ABBOTT, Rev. E.A., D.D.
+
+     How to Parse. An English Grammar. Fcap. 8vo, 3s. 6d.
+
+     How to Tell the Parts of Speech. An Introduction to English
+     Grammar. Fcap. 8vo, 2s.
+
+     How to Write Clearly. Rules and Exercises on English Composition.
+     1s. 6d.
+
+     Latin Gate, The. A First Latin Translation Book. Crown 8vo, 3s. 6d.
+
+     Via Latina. A First Latin Grammar. Crown 8vo, 3s. 6d.
+
+
+ABBOTT, Rev. E.A., and Sir J.R. SEELEY.
+
+     English Lessons for English People. Crown 8vo, 4s. 6d.
+
+     ADY, Mrs. _See_ CARTWRIGHT, JULIA.
+
+
+� KEMPIS, THOMAS.
+
+     Of the Imitation of Christ. With Illuminated Frontispiece and Title
+     Page, and Illuminated Sub-Titles to each book. In white or blue
+     cloth, with inset miniatures. Gilt top; crown 8vo, 6s. nett; also
+     bound in same manner in real classic vellum. Each copy in a box,
+     10s. 6d. nett; Antique leather with clasps, 10s. 6d. nett.
+
+     "It may well be questioned whether the great work of Thomas �
+     Kempis has ever been presented to better advantage."--_The
+     Guardian._
+
+
+ANDERSON, Prof. W.
+
+     Japanese Wood Engravings. Coloured Illustrations. Super-royal 8vo,
+     sewed, 2s. 6d. nett; half-linen, 3s. 6d. nett; also small 4to,
+     cloth, 2s. nett; lambskin, 3s. nett.
+
+
+ARMSTRONG, Sir WALTER.
+
+     The Art of Velazquez. Illustrated. Super-royal 8vo, 3s. 6d. nett.
+
+     The Life of Velazquez. Illustrated. Super-royal 8vo, 3s. 6d. nett.
+
+     Velazquez. A Study of his Life and Art. With Eight Copper Plates
+     and many minor Illustrations. Super-royal 8vo, cloth, 9s. nett.
+
+     Thomas Gainsborough. Illustrated. Super-royal 8vo, half-linen, 3s.
+     6d. nett. Also new edition small 4to, cloth, 2s. nett; leather, 3s.
+     nett and 5s. nett.
+
+     The Peel Collection and the Dutch School of Painting. With
+     Illustrations in Photogravure and Half-tone. Super-royal 8vo,
+     sewed, 5s. nett; cloth, 7s. nett.
+
+     W.Q. Orchardson. Super-royal 8vo, sewed, 2s. 6d.; half-linen, 3s.
+     6d. nett.
+
+
+AUGUSTINE, S.
+
+     Confessions of S. Augustine. With Illuminated pages. In white or
+     blue cloth, gilt top, crown 8vo, 6s. nett; also in vellum, 10s. 6d.
+     nett.
+
+
+BAKER, Captain B. GRANVILLE
+
+     The Passing of the Turkish Empire in Europe. With Thirty-two
+     Illustrations. Demy 8vo, 16s. nett.
+
+
+BARING-GOULD, Rev. S.
+
+     Family Names and their Story. Demy 8vo, 7s. 6d. nett. 5s. nett.
+
+
+BEDFORD, Rev. W.K.R.
+
+     Malta and the Knights Hospitallers. Super-royal 8vo, sewed, 2s. 6d.
+     nett; half-linen, 3s. 6d. nett.
+
+
+BENHAM, Rev. Canon D.D., F.S.A.
+
+     The Tower of London. With Four Plates in Colours and many other
+     Illustrations. Super-royal 8vo, sewed, 5s. nett; cloth, 7s. nett.
+
+     Medi�val London. With a Frontispiece in Photogravure, Four Plates
+     in Colour, and many other Illustrations. Super-royal 8vo, sewed,
+     5s. nett; cloth, gilt top, 7s. nett. Also extra crown 8vo, 3s. 6d.
+     nett.
+
+     Old St. Paul's Cathedral. With a Frontispiece in Photogravure, Four
+     Plates printed in Colour, and many other Illustrations. Super-royal
+     8vo, sewed, 5s. nett, or cloth, gilt top, 7s. nett.
+
+
+BENNETT, EDWARD.
+
+     The Post Office and its Story. An interesting account of the
+     activities of a great Government department. With Twenty-five
+     Illustrations. Ex. crn. 8vo, 5s. nett.
+
+
+BICKERSTETH, Rev. E.
+
+     Family Prayers for Six Weeks. Crown 8vo, 3s. 6d.
+
+     A Companion to the Holy Communion. 32mo, cloth, 1s.
+
+
+BINYON, LAURENCE.
+
+     Dutch Etchers of the Seventeenth Century. Illustrated. Super-royal
+     8vo, sewed, 2s. 6d.; half-linen, 3s. 6d. nett.
+
+     John Crome and John Sell Cotman. Illustrated. Super-royal 8vo
+     sewed, 3s. 6d. nett.
+
+
+BIRCH, G.H.
+
+     London on Thames in Bygone Days. With Four Plates printed in Colour
+     and many other Illustrations. Super-royal 8vo, sewed, 5s. nett;
+     cloth, 7s. nett.
+
+
+BRIDGES, Rev. C.
+
+     An Exposition of Psalm CXIX. Crown 8vo, 5s.
+
+
+BUTCHER, E.L.
+
+     Things Seen in Egypt. With Fifty Illustrations. Small 4to, cloth,
+     2s. nett; lambskin, 3s. nett; velvet leather, in box, 5s. nett.
+
+     Poems, 1s. 6d. nett.
+
+
+CACHEMAILLE, Rev. E.P., M.A.
+
+     XXVI Present-Day Papers on Prophecy. An explanation of the visions
+     of Daniel and of the Revelation, on the continuous historic system.
+     With Maps and Diagrams. 700 pp. 6s. nett.
+
+
+CARTWRIGHT, JULIA.
+
+     Jules Bastien-Lepage. Super-royal 8vo, sewed, 2s. 6d.; cloth, 3s.
+     6d. nett.
+
+     Sacharissa. Some Account of Dorothy Sidney, Countess of Sunderland,
+     her Family and Friends. With Five Portraits. Demy 8vo, 7s. 6d.
+
+     Raphael in Rome. Illustrated. Super-royal 8vo, sewed, 2s. 6d.;
+     half-linen, 3s. 6d. nett; also in small 4to, cloth, 2s. nett;
+     leather, 3s. nett and 5s. nett.
+
+     The Early Work of Raphael. Illustrated. Super-royal 8vo, sewed 2s.
+     6d.; half-linen, 3s. 6d. Also new edition, revised, in small 4to,
+     in cloth, 2s. nett; leather, 3s. nett.
+
+     Raphael: A Study of his Life and Work. With Eight Copper Plates and
+     many other Illustrations. Super-royal 8vo, 7s. 6d. nett.
+
+
+CESARESCO, The Countess MARTINENGO
+
+     The Liberation of Italy. With Portraits on Copper. Crown 8vo, 5s.
+
+
+CHATTERTON, E. KEBLE.
+
+     Fore and Aft. The Story of the Fore and Aft Rig from the Earliest
+     Times to the Present Day. Sq. ex. royal 8vo. With 150 Illustrations
+     and Coloured Frontispiece by C. Dixon, R.I. 16s. nett.
+
+     Through Holland in the "Vivette." The Cruise of a 4-Tonner from the
+     Solent to the Zuyder Zee, through the Dutch Waterways. With Sixty
+     Illustrations and Charts, 6s. nett.
+
+
+CHITTY, J.R.
+
+     Things Seen in China. With Fifty Illustrations. Small 4to; cloth,
+     2s.; leather, 3s.; velvet leather in a box, 5s. nett.
+
+
+CHORAL SERVICE-BOOK FOR PARISH CHURCHES, THE.
+
+Compiled and Edited by J.W. ELLIOTT, Organist and Choirmaster of St.
+Mark's, Hamilton Terrace, London. With some Practical Counsels taken by
+permission from "Notes on the Church Service," by Bishop WALSHAM HOW.
+
+A. Royal 8vo, sewed, 1s.; cloth, 1s. 6d.
+B. 16mo, sewed, 6d.; cloth, 8d.
+
+
+_The following portions may be had separately:_--
+
+     The Ferial and Festal Responses and the Litany. Arranged by J.W.
+     ELLIOTT. Sewed, 4d.
+
+     The Communion Service, Kyrie, Credo, Sanctus, and Gloria in
+     Excelsis. Set to Music by Dr. J. NAYLOR, Organist of York Minster.
+     Sewed, 4d.
+
+
+CHURCH, Sir ARTHUR H., F.R.S.
+
+     Josiah Wedgwood, Master Potter. With many Illustrations.
+     Super-royal 8vo, sewed, 5s. nett; cloth, 7s. nett; also small 4to,
+     cloth, 2s. nett; leather, 3s. and 5s. nett.
+
+     The Chemistry of Paints and Painting. Third Edition. Crown 8vo, 6s.
+
+
+CHURCH, Rev. A.J.
+
+     Nicias, and the Sicilian Expedition. Crown 8vo, 1s. 6d.
+
+For other books by Professor CHURCH see Complete Catalogue.
+
+
+CLARK, J.W., M.A.
+
+     Cambridge. With a coloured Frontispiece and many other
+     Illustrations by A. BRUNET-DEBAINES and H. TOUSSAINT &c. Extra
+     crown 8vo, 6s.; also crown 8vo, cloth, 2s. nett; leather, 3s.;
+     special leather, in box, 5s. nett.
+
+
+CODY, Rev. H.A.
+
+     An Apostle of the North. The Biography of the late Bishop BOMPAS,
+     First Bishop of Athabasca, and with an Introduction by the
+     ARCHBISHOP of RUPERTS-LAND. With 42 Illustrations. Demy 8vo, 7s.
+     6d. nett. 5s. nett.
+
+
+CORBIN, T.W.
+
+     Engineering of To-day. With Seventy-three Illustrations and
+     Diagrams. Extra crown 8vo, 5s. nett.
+
+     Mechanical Inventions of To-Day. Ex. Crown 8vo; with Ninety-four
+     Illustrations, 5s. nett.
+
+
+CORNISH, C.J.
+
+     Animals of To-day: Their Life and Conversation. With Illustrations
+     from Photographs by C. REID of Wishaw. Crown 8vo, 6s.
+
+     The Isle of Wight. Illustrated. Super-royal 8vo, sewed, 2s. 6d.
+     nett; half-linen, 3s. 6d. nett; also a new edition, small 4to,
+     cloth, 2s.; leather, 3s. and 5s.
+
+     Life at the Zoo. Notes and Traditions of the Regent's Park Gardens.
+     Illustrated from Photographs by GAMBIER BOLTON. Fifth Edition.
+     Crown 8vo, 6s.
+
+     The Naturalist on the Thames. Many Illustrations. Demy 8vo, 7s. 6d.
+
+     The New Forest. Super-royal 8vo, sewed, 2s. 6d. nett; half-linen,
+     3s. 6d. nett; also new edition, small 4to, cloth, 2s.; leather, 3s.
+     nett; and special velvet leather, each copy in a box, 5s.
+
+     The New Forest and the Isle of Wight. With Eight Plates and many
+     other Illustrations. Super-royal 8vo, 7s. 6d. nett.
+
+     Nights with an Old Gunner, and other Studies of Wild Life. With
+     Sixteen Illustrations by LANCELOT SPEED, CHARLES WHYMPER, and from
+     Photographs. Crown 8vo, 6s.
+
+       *       *       *       *       *
+
+THE CROWN LIBRARY
+
+A series of notable copyright books issued in uniform binding. Extra
+crown 8vo. With many illustrations, 5s. nett.
+
+
+_JUST ISSUED. SECOND AND CHEAPER EDITION._
+
+
+SWANN, A.J.
+
+     Fighting the Slave Hunters in Central Africa. A Record of
+     Twenty-six Years of Travel and Adventure round the Great Lakes, and
+     of the overthrow of Tip-pu-Tib, Rumaliza, and other great Slave
+     Traders. With 45 Illustrations and a Map, 5s. nett.
+
+
+_RECENTLY ISSUED._
+
+
+GRUBB, W. BARBROOKE.
+
+     An Unknown People in an Unknown Land. An Account of the Life and
+     Customs of the Lengua Indians of the Paraguayan Chaco, with
+     Adventures and Experiences met with during Twenty Years' Pioneering
+     and Exploration amongst them. With Twenty-four Illustrations and a
+     Map. Extra crown 8vo, 5s. nett.
+
+
+FRASER, Sir A.H.L., K.C.S.I., M.A., LL.D., Litt.D.,
+ex-Lieutenant-Governor of Bengal.
+
+     Among Indian Rajahs and Ryots. A Civil Servants' Recollections and
+     Impressions of Thirty-seven Years of Work and Sport in the Central
+     Provinces and Bengal. Third Edition, 5s. nett.
+
+
+CODY, Rev. H.A.
+
+     An Apostle of the North. The Story of Bishop Bompas's Life amongst
+     the Red Indians & Eskimo. Third Edition, 5s. nett.
+
+
+PENNELL, T.L., M.D., B.Sc.
+
+     Among the Wild Tribes of the Afghan Frontier. A Record of Sixteen
+     Years' close intercourse with the natives of Afghanistan and the
+     North-West Frontier. Introduction by EARL ROBERTS. Extra crown 8vo.
+     Twenty-six Illustrations and Map. Fifth Edition, 5s. net.
+
+       *       *       *       *       *
+
+CUST, LIONEL.
+
+     The Engravings of Albert D�rer. Illustrated. Super-royal 8vo,
+     half-linen, 3s. 6d. nett.
+
+     Paintings and Drawings of Albert D�rer. Illustrated. Super-royal
+     8vo, sewed, 3s. 6d. nett.
+
+     Albrecht D�rer. A Study of his Life and Work. With Eight Copper
+     Plates and many other Illustrations. Super-royal 8vo, 7s. 6d.
+
+
+DAVENPORT, CYRIL.
+
+     Cameos. With examples in Colour and many other Illustrations.
+     Super-royal 8vo, sewed, 5s. nett; cloth, 7s. nett.
+
+     Royal English Bookbindings. With Coloured Plates and many other
+     Illustrations. Super-royal 8vo, sewed, 3s. 6d.; cloth, 4s. 6d.
+
+
+DAVIES, RANDALL, F.S.A.
+
+     English Society of the Eighteenth Century in Contemporary Art. With
+     Four Coloured and many other Illustrations. Super royal 8vo, sewed,
+     5s. nett; cloth, 7s. nett.
+
+
+DAWSON, Rev. E.C.
+
+     The Life of Bishop Hannington. Crown 8vo, paper boards, 2s. 6d.; or
+     with Map and Illustrations, cloth, 3s. 6d.
+
+
+DESTR�E, O.G.
+
+     The Renaissance of Sculpture in Belgium. Illustrated. Super-royal
+     8vo, sewed, 2s. 6d. nett; half-linen, 3s. 6d. nett.
+
+
+DOLMAGE, CECIL G., M.A., D.C.L., LL.D., F.R.A.S.
+
+     Astronomy of To-Day. A popular account in non-technical language.
+     With Forty-six Illustrations and Diagrams. Extra crown 8vo, 5s.
+     nett.
+
+
+DOMVILLE-FIFE, CHARLES W.
+
+     Submarine Engineering of To-Day. Extra crown 8vo, 5s. nett.
+
+
+ELZEVIR LIBRARY, THE.
+
+     Selections from the choicest English Writers. Exquisitely
+     Illustrated, with Frontispiece and Title-page in Colours by H.M.
+     BROCK, and many other Illustrations. Half bound in cloth, coloured
+     top, 1s. nett; full leather, 1s. 6d. nett; velvet leather, gilt
+     edges, in a box, 2s. 6d. nett.
+
+Volume I. Fancy & Humour of Lamb.
+  "   II. Wit & Imagination of Disraeli.
+  "  III. Vignettes from Oliver Goldsmith.
+  "   IV. Wit & Sagacity of Dr. Johnson.
+  "    V. Insight & Imagination of John Ruskin.
+  "   VI. Vignettes of London Life from Dickens.
+  "  VII. XVIIIth Century Vignettes from Thackeray.
+  " VIII. Vignettes of Country Life from Dickens.
+  "   IX. Wisdom & Humour of Carlyle.
+
+"Decidedly natty and original in get-up."--_The Saturday Review._
+
+
+EVANS, WILLMOTT, M.D.
+
+     Medical Science of To-Day. Ex. crn. 8vo; 24 Illustrations, 5s.
+     nett.
+
+
+WILMOT, EARDLEY, Rear-Admiral S.
+
+     Our Fleet To-day and its Development during the last Half Century.
+     With many Illustrations. Crown 8vo, 5s.
+
+
+EVENTS OF OUR OWN TIMES
+
+Crown 8vo. With Illustrations, 5s. each.
+
+     The War in the Crimea. By General Sir E. HAMLEY, K.C.B.
+
+     The Indian Mutiny. By Colonel MALLESON, C.S.I.
+
+     The Afghan Wars, 1839-42, and 1878-80. By ARCHIBALD FORBES.
+
+     Our Fleet To-Day and its Development during the last Half-Century.
+     By Rear-Admiral S. EARDLEY WILMOT.
+
+     The Refounding of the German Empire. By Colonel MALLESON, C.S.I.
+
+     The Liberation of Italy. By the Countess MARTINENGO CESARESCO.
+
+     Great Britain in Modern Africa. By EDGAR SANDERSON, M.A.
+
+     The War in the Peninsula. By A. INNES SHAND.
+
+
+FLETCHER, W.Y.
+
+     Bookbinding in France. Coloured Plates. Super-royal, sewed, 2s. 6d.
+     nett; half-linen, 3s. 6d. nett.
+
+
+FORBES, ARCHIBALD.
+
+     The Afghan Wars of 1839-1842 and 1878-1880. With Four Portraits on
+     Copper, and Maps and Plans. Crown 8vo, 5s.
+
+
+FRASER, Sir ANDREW H.L.
+
+     Among Indian Rajahs and Ryots. With 34 Illustrations and a Map.
+     Demy 8vo, 18s. nett. Third and Cheaper Edition, 5s. nett.
+
+
+FRASER, DONALD.
+
+     Winning a Primitive People. Illustrated. Extra crown 8vo, 5s. nett.
+
+
+FRIPP, Sir ALFRED D., K.C.V.O., & R. THOMPSON, F.R.C.S.
+
+     Human Anatomy for Art Students. Profusely Illustrated with
+     Photographs and Drawings by INNES FRIPP, A.R.C.A. Square extra
+     crown 8vo, 7s. 6d. nett.
+
+
+FROBENIUS, LEO.
+
+     The Childhood of Man. A Popular Account of the Lives and Thoughts
+     of Primitive Races. Translated by Prof. A.H. KEANE, LL.D. With 416
+     Illustrations. Demy 8vo, 16s. nett.
+
+
+FRY, ROGER.
+
+     Discourses Delivered to the Students of the Royal Academy by Sir
+     Joshua Reynolds. With an Introduction and Notes by ROGER FRY. With
+     Thirty-three Illustrations. Square Crown 8vo, 7s. 6d. nett.
+
+
+GARDNER, J. STARKIE.
+
+     Armour in England. With Eight Coloured Plates and many other
+     Illustrations. Super-royal 8vo, sewed, 3s. 6d. nett.
+
+     Foreign Armour in England. With Eight Coloured Plates and many
+     other Illustrations. Super-royal 8vo, sewed, 3s. 6d. nett.
+
+     Armour in England. With Sixteen Coloured Plates and many other
+     Illustrations. The two parts in one volume. Super-royal 8vo, cloth,
+     gilt top, 9s. nett.
+
+
+GARNETT, R., LL.D.
+
+     Richmond on Thames. Illustrated. Super-royal 8vo, sewed, 3s. 6d.
+     nett.
+
+
+GIBERNE, AGNES.
+
+     Beside the Waters of Comfort. Crown 8vo, 3s. 6d.
+
+
+GIBSON, CHARLES R., F.R.S.E.
+
+     Electricity of To-Day. Its Works and Mysteries described in
+     non-technical language. With 30 Illustrations. Extra crown 8vo, 5s.
+     nett.
+
+     Scientific Ideas of To-day. A Popular Account in non-technical
+     language of the Nature of Matter, Electricity, Light, Heat, &c.,
+     &c. With 25 Illustrations. Extra crown 8vo, 5s. nett.
+
+     How Telegraphs and Telephones Work. With many Illustrations. Crown
+     8vo, 1s. 6d. nett.
+
+     The Autobiography of an Electron. With 8 Illustrations. Long 8vo,
+     3s. 6d. nett.
+
+     Wireless Telegraphy. With many Illustrations. Ex. crn. 8vo, 2s.
+     nett.
+
+
+GODLEY, A.D.
+
+     Socrates and Athenian Society in his Day. Crown 8vo, 4s. 6d.
+
+     Aspects of Modern Oxford. With many Illustrations. Crown 8vo,
+     cloth, 2s. nett; lambskin, 3s. nett; velvet leather, in box, 5s.
+     nett.
+
+
+GOLDEN RECITER (_See_ JAMES, Prof. CAIRNS.)
+
+
+GOMES, EDWIN H., M.A.
+
+     Seventeen Years among the Sea Dyaks of Borneo. With 40
+     Illustrations and a Map. Demy 8vo, 16s. nett.
+
+
+GRAHAME, GEORGE.
+
+     Claude Lorrain. Illustrated. Super-royal 8vo, 2s. 6d. nett;
+     half-linen, 3s. 6d. nett.
+
+
+GRIFFITH, M.E. HUME.
+
+     Behind the Veil in Persia and Turkish Arabia. An Account of an
+     Englishwoman's Eight Years' Residence amongst the Women of the
+     East. With 37 Illustrations and a Map. Demy 8vo, 16s. nett.
+
+
+GRINDON, LEO.
+
+     Lancashire. Brief Historical and Descriptive Notes. With many
+     Illustrations. Crown 8vo, 6s.
+
+
+GRUBB, W. BARBROOKE (Pioneer and Explorer of the Chaco).
+
+     An Unknown People in an Unknown Land. With Sixty Illustrations and
+     a Map. Demy 8vo, 16s. nett. Third and Cheaper Edition, 5s.
+
+     A Church in the Wilds. Illustrated. Extra crown 8vo, 5s. nett.
+
+
+HADOW, W.H.
+
+     A Croatian Composer. Notes toward the Study of Joseph Haydn. Crown
+     8vo, 2s. 6d. nett.
+
+     Studies in Modern Music. First Series. Berlioz, Schumann, Wagner.
+     With an Essay on Music and Musical Criticism. With Five Portraits.
+     Crown 8vo, 7s. 6d.
+
+     Studies in Modern Music. Second Series. Chopin, Dvor�k, Brahms.
+     With an Essay on Musical Form. With Four Portraits. Crown 8vo, 7s.
+     6d.
+
+
+HAMERTON, P.G.
+
+     The Etchings of Rembrandt, and Dutch Etchers of the Seventeenth
+     Century. By P.G. HAMERTON and LAURENCE BINTON. With Eight Copper
+     Plates and many other Illustrations. Super-royal 8vo, 7s. 6d. nett.
+
+     The Mount. Narrative of a Visit to the Site of a Gaulish City on
+     Mount Beuvray. With a Description of the neighbouring City of
+     Autun. Crown 8vo, 3s. 6d.
+
+     Round my House. Notes on Rural Life in Peace and War. Crown 8vo,
+     with Illustrations, 2s. 6d. nett. Cheaper edition, 2s. nett.
+
+     Paris. Illustrated. New edition. Cloth, 2s. nett; leather, 3s. nett
+     in special leather, full gilt, in box, 5s. nett.
+
+
+HAMLEY, Gen. Sir E.
+
+     The War in the Crimea. With Copper Plates and other Illustrations.
+     Crown 8vo, 5s.
+
+
+HANOUM ZEYNEB (Heroine of Pierre Loti's Novel "Les D�senchant�es.")
+
+     A Turkish Woman's European Impressions. Edited by GRACE ELLISON.
+     With a portrait by AUGUSTE RODIN and 23 other Illustrations from
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+HERBERT, GEORGE.
+
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+JAMES, CAIRNS.
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+       *       *       *       *       *
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+
+       *       *       *       *       *
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+
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+
+
+THE NEW ART LIBRARY
+
+EDITED BY M.H. SPIELMANN, F.S.A., & P.G. KONODY.
+
+     "The admirable New Art Library.... Thoroughly practical."--_The
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+
+
+THE PRACTICE AND SCIENCE OF DRAWING.
+
+     By HAROLD SPEED, Associ� de la Soci�t� Nationale des Beaux-Arts;
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+
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+HUMAN ANATOMY FOR ART STUDENTS.
+
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+
+
+MODELLING AND SCULPTURE.
+
+     By ALBERT TOFT, A.R.C.A., M.S.B.S. With 119 Photographs and
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+
+       *       *       *       *       *
+
+PAGE, J. Ll. WARDEN.
+
+     Exmoor, An Exploration of. With Maps, Etchings, and other
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+
+
+PENNELL, T.L., M.D., B.Sc., F.R.C.S.
+
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+     With an Introduction by Field-Marshal LORD ROBERTS, V.C. Extra
+     Crown 8vo. With 26 Illustrations and a Map. 5s. nett. Fourth and
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+
+     Things Seen in Northern India. With 50 Illustrations. 2s., 3s., 5s.
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+
+
+PHILLIPS, CLAUDE.
+
+     The Earlier Work of Titian. With many Illustrations. Super-royal
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+
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+
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+POLLARD, A.W.
+
+     Italian Book Illustrations. Super-royal 8vo, sewed, 2s. 6d. nett;
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+       *       *       *       *       *
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+       *       *       *       *       *
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+       *       *       *       *       *
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+     Photography of To-day. By H. CHAPMAN JONES, F.I.C., F.C.S.,
+     F.R.P.S.
+
+     Aerial Navigation of To-day. By CHARLES C. TURNER.
+
+     Astronomy of To-Day. By C.G. DOLMAGE, M.A., LL.D., D.C.L., F.R.A.S.
+
+     Botany of To-day. By Prof. G.F. SCOTT-ELLIOT, M.A., B.Sc.
+
+     Electricity of To-Day. By CHARLES R. GIBSON, F.R.S.E.
+
+     Engineering of To-day. By THOMAS W. CORBIN.
+
+     Mechanical Inventions of To-Day. By T.W. CORBIN.
+
+     Medical Science of To-Day. By WILLMOTT EVANS, M.D.
+
+     Scientific Ideas of To-Day. By CHARLES R. GIBSON, F.R.S.E.
+
+       *       *       *       *       *
+
+SEELEY'S ILLUSTRATED POCKET LIBRARY
+
+Crown 8vo, cloth, gilt edge, 2s. nett; also in leather, 3s. nett; and
+yapp leather in box at 5s.
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+ADDISON and STEELE.
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+     The Spectator in London. With Fifty-six Illustrations by RALPH
+     CLEAVER, and Headpieces by W.A. ATKIN BERRY, CLOUGH BROMLEY, &c.
+
+
+CLARK, J.W., Registrary of the University of Cambridge.
+
+     Cambridge. With many Illustrations.
+
+
+GODLEY, A.D.
+
+     Aspects of Modern Oxford. With many Illustrations.
+
+
+HAMERTON, P.G.
+
+     Paris. With many Illustrations.
+
+
+LEE, Sir SIDNEY.
+
+     Stratford-on-Avon. From the Earliest Times to the Death of
+     Shakespeare. With 52 Illustrations by HERBERT RAILTON and E. HULL.
+
+
+MITFORD, MARY RUSSELL.
+
+     Country Stories. With 68 Illustrations by GEORGE MORROW.
+
+
+HERBERT, GEORGE.
+
+     The Temple. Sacred Poems and Ejaculations. The Text reprinted from
+     the first edition. With 76 Illustrations after D�RER, HOLBEIN, and
+     other Masters.
+
+
+LANG, ANDREW.
+
+     Oxford. With 40 Illustrations by various artists.
+
+
+LEFROY, W. CHAMBERS.
+
+     The Ruined Abbeys of Yorkshire. With many Illustrations.
+
+
+LEYLAND, JOHN.
+
+     The Peak of Derbyshire: its Scenery and Antiquities.
+
+
+LOFTIE, W.J.
+
+     The Inns of Court. With 60 Illustrations.
+
+
+RUSSELL, W. CLARK.
+
+     British Seas. With 50 Illustrations by J.C. HOOK, R.A., HAMILTON
+     MACCALLUM, COLIN HUNTER, &c.
+
+
+STEVENSON, R.L.
+
+     Edinburgh. With many Illustrations by T. HAMILTON CRAWFORD, R.S.A.
+     (This volume is only to be had in this series in leather, 5s. nett.
+     For other editions of this book, see below.)
+
+       *       *       *       *       *
+
+SOLOMON, SOLOMON J., R.A.
+
+     The Practice of Oil Painting and Drawing. With 80 Illustrations.
+     6s. nett.
+
+
+SPEED, HAROLD.
+
+     The Practice and Science of Drawing. With Ninety-six Illustrations
+     and Diagrams. Square extra crown 8vo, 6s.
+
+       *       *       *       *       *
+
+THE STANDARD LIBRARY
+
+Extra Crown 8vo, With many Illustrations. Price 2s. 6d. nett.
+
+
+     Lady Mary Wortley Montagu. By A.R. ROPES.
+
+     Mrs. Thrale. By L.B. SEELEY.
+
+     Round My House. By P.G. HAMERTON.
+
+     Fanny Burney & Her Friends. By L.B. SEELEY.
+
+
+STORY SERIES, THE. Extra crown 8vo, 5s. nett.
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+     The Post Office and its Story. By EDWARD BENNETT. With 31 Illus.
+
+     Family Names and their Story. By the Rev. S. BARING GOULD.
+
+     The Press and its Story. By JAMES D. SYMON.
+
+
+SEELEY, Sir J.R.
+
+     Goethe Reviewed after Sixty Years. With Portrait. Crown 8vo, 3s.
+     6d.
+
+     A Short History of Napoleon the First. With Portrait. Crown 8vo,
+     5s.
+
+
+SEELEY, Sir J.R., and Dr. ABBOTT.
+
+     English Lessons for English People. Crown 8vo, 4s. 6d.
+
+
+SEELEY, L.B.
+
+     Mrs. Thrale, afterwards Mrs. Piozzi. With Eight Illustrations.
+     Crown 8vo, 2s. 6d nett.
+
+     Fanny Burney and her Friends. With Eight Illustrations. Crown 8vo,
+     2s. 6d nett.
+
+
+SHAND, A. INNES.
+
+     The War in the Peninsula. With Portraits and Plans. 5s.
+
+
+SHARP, WILLIAM.
+
+     Fair Women. Illustrated. Super-royal 8vo, sewed, 2s. 6d. nett;
+     half-linen, 3s. 6d. nett. Also new edition, small 4to, cloth, 2s.
+     nett; leather, 3s. and 5s. nett.
+
+
+SHIPLEY, M.E.
+
+     Daily Help for Daily Need. A Selection of Scripture Verses and
+     Poetry for Every Day in the Year. Crown 8vo, 4s. 6d.
+
+
+STEPHENS, F.G.
+
+     Rossetti, D.G. Super-royal 8vo, sewed, 2s. 6d. nett; also small
+     4to, cloth, 2s. nett; leather, 3s. nett; velvet leather, in a box,
+     5s. nett.
+
+
+STEVENSON, R.L.
+
+     Edinburgh. Fcap. 8vo, with Frontispiece, gilt top, cloth, 2s. nett;
+     leather, 3s. nett. Crown 8vo, Illustrated, cloth, 3s. 6d. Roxburgh,
+     gilt top, 5s. Library Edition. Crown 8vo, buckram, dark blue, gilt
+     top, Sixteen Full-page Illustrations, 6s. Presentation Edition.
+     Extra crown 8vo, with Sixty-four Illustrations, 6s. nett; also
+     People's Edition, demy 8vo, 6d. nett. With Twenty-four
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+     Messrs. T. & A. Constable, of Edinburgh. Ordinary Edition, 12s. 6d.
+     nett. EDITION DE LUXE, limited to 385 copies, of which only 375 are
+     for sale, printed on unbleached Arnold handmade paper, and bound in
+     buckram, with paper label, each copy numbered, 25s. nett.
+
+
+STEVENSON, R.A.M.
+
+     Rubens, Peter Paul. Illustrated. Super-royal 8vo, 3s. 6d. nett,
+     sewed. Also small 4to, cloth, 2s. nett; leather, 3s. nett and 5s.
+     nett.
+
+
+STIGAND, Captain C.H., F.R.G.S., F.Z.S.
+
+     To Abyssinia Through an Unknown Land. With Thirty-six Illustrations
+     and Two Maps. Demy 8vo, 16s. nett.
+
+
+SWANN, ALFRED J.
+
+     Fighting the Slave Hunters in Central Africa. With Forty-five
+     Illustrations and a Map. Demy 8vo, 16s. nett. Extra crown 8vo. 5s.
+     nett.
+
+
+TALBOT, F.A.
+
+     The Makings of a Great Canadian Railway. Demy 8vo. With Forty-one
+     Illustrations and a Map. 16s. nett.
+
+       *       *       *       *       *
+
+THE THINGS SEEN SERIES
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+Each volume with 50 Illustrations. Small 4to, cloth, 2s.; leather, 3s.;
+and velvet leather, in a box, 5s. nett.
+
+     Things Seen in Oxford. By N.J. DAVIDSON, B.A. (Oxon.)
+
+     Things Seen in Russia. By W. BARNES STEVENI.
+
+     Things Seen in Palestine. By A. GOODRICH FREER.
+
+     Things Seen in Japan. By CLIVE HOLLAND.
+
+     Things Seen in China. By J.R. CHITTY.
+
+     Things Seen in Egypt. By E.L. BUTCHER.
+
+     Things Seen in Holland. By C.E. ROCHE.
+
+     Things Seen in Spain. By C. GASQUOINE HARTLEY.
+
+     Things Seen in Northern India. By T.L. PENNELL, M.D., B.Sc.
+
+     Things Seen in Venice. By LONSDALE RAGG, B.D. (Oxon.)
+
+
+TOFT, ALBERT, Hon., A.R.C.A., M.S.B.S.
+
+     Modelling and Sculpture. Profusely Illustrated with 119 Photographs
+     and Drawings. Square extra crown 8vo, 6s. nett.
+
+
+TORDAY, E.
+
+     Camp and Tramp in African Wilds. Demy 8vo. With Forty-five
+     Illustrations and a Map, 16s. nett.
+
+
+TOWNSHEND, Captain A.T.
+
+     A Military Consul in Turkey. With 29 Illustrations. Demy 8vo, 16s.
+     nett.
+
+
+TREMEARNE, Major A.J.N.
+
+     The Tailed Head-Hunters of Nigeria. Demy 8vo, with 38 Illustrations
+     and a Map. 16s. nett.
+
+
+TURNER, CHARLES C.
+
+     Aerial Navigation of To-day. With Seventy Illustrations and
+     Diagrams. Extra crown 8vo, 5s. nett.
+
+
+WAERN, C.
+
+     John La Farge. Illustrated. Super-royal 8vo, sewed, 3s. 6d. nett.
+
+
+WEALE, W.H. JAMES.
+
+     Gerard David, Painter and Illuminator. Illustrated. Super-royal
+     8vo, sewed, 2s. 6d. nett; half-linen, 3s. 6d. nett.
+
+
+WEEKS, JOHN H.
+
+     Among Congo Cannibals. Demy 8vo. With Fifty-four Illustrations and
+     a Map, 16s. nett.
+
+
+WELCH, C., and Canon BENHAM.
+
+     Medi�val London. With a Frontispiece in Photogravure, Four Plates
+     in Colour, and many other Illustrations. Super-royal 8vo, sewed,
+     5s. nett; cloth, gilt top, 7s. nett. Also extra crown 8vo, 3s. 6d.
+     nett.
+
+
+WICKS, MARK.
+
+     To Mars via the Moon. An Astronomical Story. With Sixteen
+     Illustrations and Diagrams. Extra crown 8vo, 5s.
+
+
+
+
+
+End of the Project Gutenberg EBook of Astronomy of To-day, by Cecil G. Dolmage
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+      The Project Gutenberg eBook of Astronomy Of To-Day, by Cecil G. Dolmage.
+    </title>
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+<pre>
+
+The Project Gutenberg EBook of Astronomy of To-day, by Cecil G. Dolmage
+
+This eBook is for the use of anyone anywhere 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
+
+
+Title: Astronomy of To-day
+       A Popular Introduction in Non-Technical Language
+
+Author: Cecil G. Dolmage
+
+Release Date: April 21, 2009 [EBook #28570]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK ASTRONOMY OF TO-DAY ***
+
+
+
+
+Produced by Brenda Lewis, Scott Marusak, Greg Bergquist
+and the Online Distributed Proofreading Team at
+https://www.pgdp.net (This file was produced from images
+generously made available by The Internet Archive/American
+Libraries.)
+
+
+
+
+
+
+</pre>
+
+
+
+<div class="tn">
+
+<p class="center"><big><b>Transcriber&#8217;s Note</b></big></p>
+
+<p class="noin">The punctuation and spelling from the original text have been faithfully preserved. Only obvious
+typographical errors have been corrected. The advertisement from the beginning of the book has been joined with
+the other advertisements near the end of the book.</p>
+
+</div>
+<hr />
+<div class="figcenter" style="width: 500px;">
+<img src="images/frontcover.jpg" width="500" height="766" alt="" title="Front Cover" />
+</div>
+<hr />
+
+
+
+<p class='t1'><br /><br /><br />ASTRONOMY OF TO-DAY<br /><br /><br /></p>
+<hr />
+<div class="figcenter" style="width: 500px;"><a name="Frontispiece" id="Frontispiece"></a>
+<img src="images/frontisplate.jpg" width="500" height="319" alt="The Total Eclipse of the Sun of August 30th, 1905." title="" />
+<span class="caption"><span class="smcap">The Total Eclipse of the Sun of August 30th, 1905.</span><br />
+The Corona; from a water-colour sketch, made at Burgos, in Spain, during
+the total phase, by the French Artist, Mdlle. <span class="smcap">Andr&eacute;e Moch.</span></span>
+</div>
+
+<hr />
+
+
+
+<h1>
+ASTRONOMY OF<br />
+TO-DAY<br />
+</h1>
+
+<p class="t2"><i>A POPULAR INTRODUCTION IN<br />
+NON-TECHNICAL LANGUAGE</i></p>
+
+<p class="t3"><br />By<br />
+
+CECIL G. DOLMAGE, M.A., LL.D., D.C.L.</p>
+
+<p class="center">Fellow of the Royal Astronomical Society; Member of<br />
+the British Astronomical Association; Member of<br />
+the Astronomical Society of the Pacific; Membre<br />
+de la Soci&eacute;t&eacute; Astronomique de France;<br />
+Membre de la Soci&eacute;t&eacute; Belge<br />
+d'Astronomie<br />
+<br />
+<br />
+<br />
+<big>With a Frontispiece in Colour<br />
+and 45 Illustrations &amp; Diagrams</big><br />
+<br />
+<br />
+<i>THIRD EDITION</i><br />
+<br />
+<br />
+</p>
+<p class="center"><big>LONDON</big><br />
+<big>SEELEY AND CO. LIMITED</big><br />
+<big><span class="smcap">38 Great Russell Street</span><br />
+1910</big><br />
+</p>
+
+
+
+<hr />
+<h2>PREFACE</h2>
+
+
+<p class="noin"><span class="smcap">The</span> object of this book is to give an account of the science of
+Astronomy, as it is known at the present day, in a manner acceptable to
+the <i>general reader</i>.</p>
+
+<p>It is too often supposed that it is impossible to acquire any useful
+knowledge of Astronomy without much laborious study, and without
+adventuring into quite a new world of thought. The reasoning applied to
+the study of the celestial orbs is, however, of no different order from
+that which is employed in the affairs of everyday life. The science of
+mathematics is perhaps responsible for the idea that some kind of
+difference does exist; but mathematical processes are, in effect, no
+more than ordinary logic in concentrated form, the <i>shorthand of
+reasoning</i>, so to speak. I have attempted in the following pages to take
+the main facts and theories of Astronomy out of those mathematical forms
+which repel the general reader, and to present them in the <i>ordinary
+language of our workaday world</i>.</p>
+
+<p>The few diagrams introduced are altogether supplementary, and are not
+connected with the text by any wearying cross-references. Each diagram
+is complete in itself, being intended to serve as a pictorial aid, in
+case the wording of the text should not have perfectly conveyed the
+desired meaning. The full page illustrations are also described as
+adequately as possible at the foot of each.</p>
+
+<p>As to the coloured frontispiece, this must be placed in a category by
+itself. It is the work of the <i>artist</i> as distinct from the scientist.</p>
+
+<p>The book itself contains incidentally a good deal of matter concerned
+with the Astronomy of the past, the introduction of which has been found
+necessary in order to make clearer the Astronomy of our time.</p>
+
+<p>It would be quite impossible for me to enumerate here the many sources
+from which information has been drawn. But I acknowledge my especial
+indebtedness to Professor F.R. Moulton's <i>Introduction to Astronomy</i>
+(Macmillan, 1906), to the works on Eclipses of the late Rev. S.J.
+Johnson and of Mr. W.T. Lynn, and to the excellent <i>Journals of the
+British Astronomical Association</i>. Further, for those grand questions
+concerned with the Stellar Universe at large, I owe a very deep debt to
+the writings of the famous American astronomer, Professor Simon Newcomb,
+and of our own countryman, Mr. John Ellard Gore; to the latter of whom I
+am under an additional obligation for much valuable information
+privately rendered.</p>
+
+<p>In my search for suitable illustrations, I have been greatly aided by
+the kindly advice of Mr. W. H. Wesley, the Assistant Secretary of the
+Royal Astronomical Society. To those who have been so good as to permit
+me to reproduce pictures and photographs, I desire to record my best
+thanks as follows:&mdash;To the French Artist, Mdlle. Andr&eacute;e Moch; to the
+Astronomer Royal; to Sir David Gill, K.C.B., LL.D., F.R.S.; to the
+Council of the Royal Astronomical Society; to Professor E.B. Frost,
+Director of the Yerkes Observatory; to M.P. Puiseux, of the Paris
+Observatory; to Dr. Max Wolf, of Heidelberg; to Professor Percival
+Lowell; to the Rev. Theodore E.R. Phillips, M.A., F.R.A.S.; to Mr. W.H.
+Wesley; to the Warner and Swasey Co., of Cleveland, Ohio, U.S.A.; to the
+publishers of <i>Knowledge</i>, and to Messrs. Sampson, Low &amp; Co. For
+permission to reproduce the beautiful photograph of the Spiral Nebula in
+Canes Venatici (<a href="#Plate_XXII">Plate XXII.</a>), I am indebted to the distinguished
+astronomer, the late Dr. W.E. Wilson, D.Sc., F.R.S., whose untimely
+death, I regret to state, occurred in the early part of this year.</p>
+
+<p>Finally, my best thanks are due to Mr. John Ellard Gore, F.R.A.S.,
+M.R.I.A., to Mr. W.H. Wesley, and to Mr. John Butler Burke, M.A., of
+Cambridge, for their kindness in reading the proof-sheets.</p>
+
+<p class="right">
+<big>CECIL G. DOLMAGE.</big></p>
+
+<p><span class="smcap">London, S.W.</span>,<br />
+<span style="margin-left: 3em;"><i>August 4, 1908.</i></span>
+</p>
+
+
+
+<hr />
+<p class="t3">PREFATORY NOTE TO THE<br /> SECOND EDITION</p>
+
+
+<p class="noin"><span class="smcap">The</span> author of this book lived only long enough to hear of the favour
+with which it had been received, and to make a few corrections in view
+of the second edition which it has so soon reached.</p>
+
+<p>
+<i>December 1908.</i><br />
+</p>
+
+
+
+<hr />
+<h2>CONTENTS</h2>
+
+
+
+<div class='toc'>
+<table border="0" width="65%" cellpadding="4" cellspacing="0" summary="Contents">
+<tr class='tr1'>
+ <td align='center' colspan='2'><a href="#CHAPTER_I">CHAPTER I</a></td>
+</tr>
+<tr>
+ <td align='right' colspan='2'><small>PAGE</small></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Ancient View</span></td>
+ <td align='right'>17</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_II">CHAPTER II</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Modern View</span></td>
+ <td align='right'>20</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_III">CHAPTER III</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Solar System</span></td>
+ <td align='right'>29</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_IV">CHAPTER IV</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">Celestial Mechanism</span></td>
+ <td align='right'>38</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_V">CHAPTER V</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">Celestial Distances</span></td>
+ <td align='right'>46</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_VI">CHAPTER VI</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">Celestial Measurement</span></td>
+ <td align='right'>55</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_VII">CHAPTER VII</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">Eclipses and Kindred Phenomena</span></td>
+ <td align='right'>61</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_VIII">CHAPTER VIII</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">Famous Eclipses of the Sun</span></td>
+ <td align='right'>83</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_IX">CHAPTER IX</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">Famous Eclipses of the Moon</span></td>
+ <td align='right'>101</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_X">CHAPTER X</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Growth of Observation</span></td>
+ <td align='right'>105</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XI">CHAPTER XI</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">Spectrum Analysis</span></td>
+ <td align='right'>121</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XII">CHAPTER XII</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Sun</span></td>
+ <td align='right'>127</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XIII">CHAPTER XIII</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Sun</span>&mdash;<i>continued</i></td>
+ <td align='right'>134</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XIV">CHAPTER XIV</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Inferior Planets</span></td>
+ <td align='right'>146</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XV">CHAPTER XV</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Earth</span></td>
+ <td align='right'>158</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XVI">CHAPTER XVI</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Moon</span></td>
+ <td align='right'>183</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XVII">CHAPTER XVII</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Superior Planets</span></td>
+ <td align='right'>209</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XVIII">CHAPTER XVIII</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Superior Planets</span>&mdash;<i>continued</i></td>
+ <td align='right'>229</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XIX">CHAPTER XIX</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">Comets</span></td>
+ <td align='right'>247</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XX">CHAPTER XX</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">Remarkable Comets</span></td>
+ <td align='right'>259</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XXI">CHAPTER XXI</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">Meteors Or Shooting Stars</span></td>
+ <td align='right'>266</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XXII">CHAPTER XXII</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Stars</span></td>
+ <td align='right'>278</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XXIII">CHAPTER XXIII</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Stars</span>&mdash;<i>continued</i></td>
+ <td align='right'>287</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XXIV">CHAPTER XXIV</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">Systems of Stars</span></td>
+ <td align='right'>300</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XXV">CHAPTER XXV</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Stellar Universe</span></td>
+ <td align='right'>319</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XXVI">CHAPTER XXVI</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Stellar Universe</span>&mdash;<i>continued</i></td>
+ <td align='right'>329</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XXVII">CHAPTER XXVII</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The Beginning of Things</span></td>
+ <td align='right'>333</td>
+</tr>
+<tr class='tr1'>
+<td align='center' colspan='2'><a href="#CHAPTER_XXVIII">CHAPTER XXVIII</a></td>
+</tr>
+<tr>
+ <td align='left'><span class="smcap">The End of Things</span></td>
+ <td align='right'>342</td>
+</tr>
+<tr class='tr1'>
+ <td align='left'><span class="smcap"><a href="#INDEX">Index</a></span></td>
+ <td align='right'>351</td>
+</tr>
+</table></div>
+
+
+
+<hr />
+<h2>LIST OF ILLUSTRATIONS</h2>
+
+
+<p class='t3'>LIST OF PLATES</p>
+
+<div class='toc'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Plates">
+
+<tr>
+ <td align='left'><small>PLATE</small></td>
+</tr>
+<tr class='tr2'>
+ <td align='right'>&nbsp;</td>
+ <td align='left'><span class="smcap">The Total Eclipse of the Sun of August 30, 1905</span></td>
+ <td align='center'>&nbsp;</td>
+ <td align='right'><a href="#Frontispiece"><i>Frontispiece</i></a></td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_I">I.</a></td>
+ <td align='left'><span class="smcap">The Total Eclipse of the Sun of May 17, 1882</span></td>
+ <td align='center'><i>To face page</i></td>
+ <td align='right'>96</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_II">II.</a></td>
+ <td align='left'><span class="smcap">Great Telescope of Hevelius</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>110</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_III">III.</a></td>
+ <td align='left'><span class="smcap">A Tubeless, or "Aerial" Telescope</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>112</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_IV">IV.</a></td>
+ <td align='left'><span class="smcap">The Great Yerkes Telescope</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>118</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_V">V.</a></td>
+ <td align='left'><span class="smcap">The Sun, showing several groups of Spots</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>134</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_VI">VI.</a></td>
+ <td align='left'><span class="smcap">Photograph of a Sunspot</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>136</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_VII">VII.</a></td>
+ <td align='left'><span class="smcap">Forms of the Solar Corona at the epochs of Sunspot Maximum<br /> and Sunspot Minimum respectively.<br /><br />
+(<i>A</i>) The Total Eclipse of the Sun of December 22, 1870.<br /><br />
+(<i>B</i>) The Total Eclipse of the Sun of May 28, 1900</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>142</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_VIII">VIII.</a></td>
+ <td align='left'><span class="smcap">The Moon</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>196</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_IX">IX.</a></td>
+ <td align='left'><span class="smcap">Map of the Moon, showing the principal "Craters," Mountain<br /> Ranges And "Seas"</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>198</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_X">X.</a></td>
+ <td align='left'><span class="smcap">One of the most interesting Regions on the Moon</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>200</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XI">XI.</a></td>
+ <td align='left'><span class="smcap">The Moon (showing systems of "Rays")</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>204</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XII">XII.</a></td>
+ <td align='left'><span class="smcap">A Map of the Planet Mars</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>216</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XIII">XIII.</a></td>
+ <td align='left'><span class="smcap">Minor Planet Trails</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>226</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XIV">XIV.</a></td>
+ <td align='left'><span class="smcap">The Planet Jupiter</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>230</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XV">XV.</a></td>
+ <td align='left'><span class="smcap">The Planet Saturn</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>236</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XVI">XVI.</a></td>
+ <td align='left'><span class="smcap">Early Representations of Saturn</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>242</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XVII">XVII.</a></td>
+ <td align='left'><span class="smcap">Donati's Comet</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>256</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XVIII">XVIII.</a></td>
+ <td align='left'><span class="smcap">Daniel's Comet of 1907</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>258</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XIX">XIX.</a></td>
+ <td align='left'><span class="smcap">The Sky around the North Pole</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>292</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XX">XX.</a></td>
+ <td align='left'><span class="smcap">Orion and his Neighbours</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>296</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XXI">XXI.</a></td>
+ <td align='left'><span class="smcap">The Great Globular Cluster in the Southern Constellation<br /> of Centaurus</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>306</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XXII">XXII.</a></td>
+ <td align='left'><span class="smcap">Spiral Nebula in the Constellation of Canes Venatici</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>314</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XXIII">XXIII.</a></td>
+ <td align='left'><span class="smcap">The Great Nebula in the Constellation of Andromeda</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>316</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Plate_XXIV">XXIV.</a></td>
+ <td align='left'><span class="smcap">The Great Nebula in the Constellation of Orion</span></td>
+ <td align='center'>"&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+ <td align='right'>318</td>
+</tr>
+</table></div>
+
+
+
+<hr />
+<h2>LIST OF DIAGRAMS</h2>
+
+
+<div class='toc'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Diagrams">
+
+<tr class='tr2'>
+ <td align='right'><small>FIG.</small></td>
+ <td align='left'>&nbsp;</td>
+ <td align='right'><small>PAGE</small></td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_1">1.</a></td>
+ <td align='left'><span class="smcap">The Ptolemaic Idea of the Universe</span></td>
+ <td align='right'>19</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_2">2.</a></td>
+ <td align='left'><span class="smcap">The Copernican Theory of the Solar System</span></td>
+ <td align='right'>21</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_3">3.</a></td>
+ <td align='left'><span class="smcap">Total and Partial Eclipses of the Moon</span></td>
+ <td align='right'>64</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_4">4.</a></td>
+ <td align='left'><span class="smcap">Total and Partial Eclipses of the Sun</span></td>
+ <td align='right'>67</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_5">5.</a></td>
+ <td align='left'><span class="smcap">"Baily's Beads"</span></td>
+ <td align='right'>70</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_6">6.</a></td>
+ <td align='left'><span class="smcap">Map of the World on Mercator's Projection,
+ showing a portion of the progress of the Total Solar Eclipse Of August 30, 1905, across the surface of the Earth</span></td>
+ <td align='right'>81</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_7">7.</a></td>
+ <td align='left'><span class="smcap">The "Ring with Wings"</span></td>
+ <td align='right'>87</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_8">8.</a></td>
+ <td align='left'><span class="smcap">The Various Types of Telescope</span></td>
+ <td align='right'>113</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_9">9.</a></td>
+ <td align='left'><span class="smcap">The Solar Spectrum</span></td>
+ <td align='right'>123</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_10">10.</a></td>
+ <td align='left'><span class="smcap">A Section through the Sun, showing how the Prominences rise from the Chromosphere</span></td>
+ <td align='right'>131</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_11">11.</a></td>
+ <td align='left'><span class="smcap">Orbit and Phases of an Inferior Planet</span></td>
+ <td align='right'>148</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_12">12.</a></td>
+ <td align='left'><span class="smcap">The "Black Drop"</span></td>
+ <td align='right'>153</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_13">13.</a></td>
+ <td align='left'><span class="smcap">Summer and Winter</span></td>
+ <td align='right'>176</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_14">14.</a></td>
+ <td align='left'><span class="smcap">Orbit and Phases of the Moon</span></td>
+ <td align='right'>184</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_15">15.</a></td>
+ <td align='left'><span class="smcap">The Rotation of the Moon on her Axis</span></td>
+ <td align='right'>187</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_16">16.</a></td>
+ <td align='left'><span class="smcap">Laplace's "Perennial Full Moon"</span></td>
+ <td align='right'>191</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_17">17.</a></td>
+ <td align='left'><span class="smcap">Illustrating the Author's explanation of the apparent Enlargement of Celestial Objects</span></td>
+ <td align='right'>195</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_18">18.</a></td>
+ <td align='left'><span class="smcap">Showing how the Tail of a Comet is directed away from the Sun</span></td>
+ <td align='right'>248</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_19">19.</a></td>
+ <td align='left'><span class="smcap">The Comet of 1066, as represented in the Bayeux Tapestry</span></td>
+ <td align='right'>263</td>
+</tr>
+<tr class='tr2'>
+ <td align='right'><a href="#Fig_20">20.</a></td>
+ <td align='left'><span class="smcap">Passage of the Earth through the thickest portion of a Meteor Swarm</span></td>
+ <td align='right'>269</td>
+</tr>
+</table></div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_17" id="Page_17">[Pg 17]</a></span></p>
+<h2>ASTRONOMY OF TO-DAY</h2>
+
+
+
+<hr />
+<h3><a name="CHAPTER_I" id="CHAPTER_I"></a>CHAPTER I</h3>
+
+<h4>THE ANCIENT VIEW</h4>
+
+
+<p class="noin"><span class="smcap">It</span> is never safe, as we know, to judge by appearances, and this is
+perhaps more true of astronomy than of anything else.</p>
+
+<p>For instance, the idea which one would most naturally form of the earth
+and heaven is that the solid earth on which we live and move extends to
+a great distance in every direction, and that the heaven is an immense
+dome upon the inner surface of which the stars are fixed. Such must
+needs have been the idea of the universe held by men in the earliest
+times. In their view the earth was of paramount importance. The sun and
+moon were mere lamps for the day and for the night; and these, if not
+gods themselves, were at any rate under the charge of special deities,
+whose task it was to guide their motions across the vaulted sky.</p>
+
+<p>Little by little, however, this simple estimate of nature began to be
+overturned. Difficult problems agitated the human mind. On what, for
+instance, did the solid earth rest, and what prevented the vaulted
+heaven from falling in upon men and crushing them out of existence?<span class='pagenum'><a name="Page_18" id="Page_18">[Pg 18]</a></span>
+Fantastic myths sprang from the vain attempts to solve these riddles.
+The Hindoos, for example, imagined the earth as supported by four
+elephants which stood upon the back of a gigantic tortoise, which, in
+its turn, floated on the surface of an elemental ocean. The early
+Western civilisations conceived the fable of the Titan Atlas, who, as a
+punishment for revolt against the Olympian gods, was condemned to hold
+up the expanse of sky for ever and ever.</p>
+
+<p>Later on glimmerings of the true light began to break in upon men. The
+Greek philosophers, who busied themselves much with such matters,
+gradually became convinced that the earth was spherical in shape, that
+is to say, round like a ball. In this opinion we now know that they were
+right; but in their other important belief, viz. that the earth was
+placed at the centre of all things, they were indeed very far from the
+truth.</p>
+
+<p>By the second century of the Christian era, the ideas of the early
+philosophers had become hardened into a definite theory, which, though
+it appears very incorrect to us to-day, nevertheless demands exceptional
+notice from the fact that it was everywhere accepted as the true
+explanation until so late as some four centuries ago. This theory of the
+universe is known by the name of the Ptolemaic System, because it was
+first set forth in definite terms by one of the most famous of the
+astronomers of antiquity, Claudius Ptolem&aelig;us Pelusinensis (100&ndash;170
+<span class="ampm">A.D.</span>), better known as Ptolemy of Alexandria.</p>
+
+<p>In his system the Earth occupied the centre; while around it circled in
+order outwards the Moon, the planets Mercury and Venus, the Sun, and
+then the planets Mars, Jupiter, and Saturn. Beyond<span class='pagenum'><a name="Page_19" id="Page_19">[Pg 19]</a></span> these again revolved
+the background of the heaven, upon which it was believed that the stars
+were fixed&mdash;</p>
+
+<p class="poem">"Stellis ardentibus aptum,"</p>
+
+<p class="noin">as Virgil puts it (<a href="#Fig_1">see Fig. 1</a>).</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_1" id="Fig_1"></a>
+<img src="images/figure1.jpg" width="500" height="488" alt="Fig. 1." title="" />
+<span class="caption"><span class="smcap">Fig. 1.</span>&mdash;The Ptolemaic idea of the Universe.</span>
+</div>
+
+<p>The Ptolemaic system persisted unshaken for about fourteen hundred years
+after the death of its author. Clearly men were flattered by the notion
+that their earth was the most important body in nature, that it stood
+still at the centre of the universe, and was the pivot upon which all
+things revolved.</p>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_20" id="Page_20">[Pg 20]</a></span></p>
+<h3><a name="CHAPTER_II" id="CHAPTER_II"></a>CHAPTER II</h3>
+
+<h4>THE MODERN VIEW</h4>
+
+
+<p class="noin"><span class="smcap">It</span> is still well under four hundred years since the modern, or
+Copernican, theory of the universe supplanted the Ptolemaic, which had
+held sway during so many centuries. In this new theory, propounded
+towards the middle of the sixteenth century by Nicholas Copernicus
+(1473&ndash;1543), a Prussian astronomer, the earth was dethroned from its
+central position and considered merely as one of a number of planetary
+bodies which revolve around the sun. As it is not a part of our purpose
+to follow in detail the history of the science, it seems advisable to
+begin by stating in a broad fashion the conception of the universe as
+accepted and believed in to-day.</p>
+
+<p>The Sun, the most important of the celestial bodies so far as we are
+concerned, occupies the central position; not, however, in the whole
+universe, but only in that limited portion which is known as the Solar
+System. Around it, in the following order outwards, circle the planets
+Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
+(<a href="#Fig_2">see Fig. 2</a>, p. 21). At an immense distance beyond the solar system, and
+scattered irregularly through the depth of space, lie the stars. The two
+first-mentioned members of the solar system, Mercury and Venus, are
+known as the Inferior Planets; and in their courses about the sun, they
+always keep well inside the path along which our earth moves. The
+remaining members (exclusive of the earth) are called Superior Planets,
+and their paths lie all outside that of the earth.</p>
+
+<p><span class='pagenum'><a name="Page_21" id="Page_21">[Pg 21]</a></span></p>
+<div class="figcenter" style="width: 500px;"><a name="Fig_2" id="Fig_2"></a>
+<img src="images/figure2.jpg" width="500" height="568" alt="Fig. 2." title="" />
+<span class="caption"><span class="smcap">Fig. 2.</span>&mdash;The Copernican theory of the Solar System.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_22" id="Page_22">[Pg 22]</a></span></p><p>The five planets, Mercury, Venus, Mars, Jupiter, and Saturn, have been
+known from all antiquity. Nothing then can bring home to us more
+strongly the immense advance which has taken place in astronomy during
+modern times than the fact that it is only 127 years since observation
+of the skies first added a planet to that time-honoured number. It was
+indeed on the 13th of March 1781, while engaged in observing the
+constellation of the Twins, that the justly famous Sir William Herschel
+caught sight of an object which he did not recognise as having met with
+before. He at first took it for a comet, but observations of its
+movements during a few days showed it to be a planet. This body, which
+the power of the telescope alone had thus shown to belong to the solar
+family, has since become known to science under the name of Uranus. By
+its discovery the hitherto accepted limits of the solar system were at
+once pushed out to twice their former extent, and the hope naturally
+arose that other planets would quickly reveal themselves in the
+immensities beyond.</p>
+
+<p>For a number of years prior to Herschel's great discovery, it had been
+noticed that the distances at which the then known planets circulated
+appeared to be arranged in a somewhat orderly progression outwards from
+the sun. This seeming plan, known to astronomers by the name of Bode's
+Law, was<span class='pagenum'><a name="Page_23" id="Page_23">[Pg 23]</a></span> closely confirmed by the distance of the new planet Uranus.
+There still lay, however, a broad gap between the planets Mars and
+Jupiter. Had another planet indeed circuited there, the solar system
+would have presented an appearance of almost perfect order. But the void
+between Mars and Jupiter was unfilled; the space in which one would
+reasonably expect to find another planet circling was unaccountably
+empty.</p>
+
+<p>On the first day of the nineteenth century the mystery was however
+explained, a body being discovered<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a> which revolved in the space that
+had hitherto been considered planetless. But it was a tiny globe hardly
+worthy of the name of planet. In the following year a second body was
+discovered revolving in the same space; but it was even smaller in size
+than the first. During the ensuing five years two more of these little
+planets were discovered. Then came a pause, no more such bodies being
+added to the system until half-way through the century, when suddenly
+the discovery of these so-called "minor planets" began anew. Since then
+additions to this portion of our system have rained thick and fast. The
+small bodies have received the name of Asteroids or Planetoids; and up
+to the present time some six hundred of them are known to exist, all
+revolving in the previously unfilled space between Mars and Jupiter.</p>
+
+<p>In the year 1846 the outer boundary of the solar system was again
+extended by the discovery that a great planet circulated beyond Uranus.
+The new body, which received the name of Neptune, was<span class='pagenum'><a name="Page_24" id="Page_24">[Pg 24]</a></span> brought to light
+as the result of calculations made at the same time, though quite
+independently, by the Cambridge mathematician Adams, and the French
+astronomer Le Verrier. The discovery of Neptune differed, however, from
+that of Uranus in the following respect. Uranus was found merely in the
+course of ordinary telescopic survey of the heavens. The position of
+Neptune, on the other hand, was predicted as the result of rigorous
+mathematical investigations undertaken with the object of fixing the
+position of an unseen and still more distant body, the attraction of
+which, in passing by, was disturbing the position of Uranus in its
+revolution around the sun. Adams actually completed his investigation
+first; but a delay at Cambridge in examining that portion of the sky,
+where he announced that the body ought just then to be, allowed France
+to snatch the honour of discovery, and the new planet was found by the
+observer Galle at Berlin, very near the place in the heavens which Le
+Verrier had mathematically predicted for it.</p>
+
+<p>Nearly fifty years later, that is to say, in our own time, another
+important planetary discovery was made. One of the recent additions to
+the numerous and constantly increasing family of the asteroids, a tiny
+body brought to light in 1898, turned out after all not to be
+circulating in the customary space between Mars and Jupiter, but
+actually in that between our earth and Mars. This body is very small,
+not more than about twenty miles across. It has received the name of
+Eros (the Greek equivalent for Cupid), in allusion to its insignificant
+size as compared with the other leading members of the system.</p>
+
+<p>This completes the list of the planets which, so<span class='pagenum'><a name="Page_25" id="Page_25">[Pg 25]</a></span> far, have revealed
+themselves to us. Whether others exist time alone will show. Two or
+three have been suspected to revolve beyond the path of Neptune; and it
+has even been asserted, on more than one occasion, that a planet
+circulates nearer to the sun than Mercury. This supposed body, to which
+the name of "Vulcan" was provisionally given, is said to have been
+"discovered" in 1859 by a French doctor named Lescarbault, of Org&egrave;res
+near Orleans; but up to the present there has been no sufficient
+evidence of its existence. The reason why such uncertainty should exist
+upon this point is easy enough to understand, when we consider the
+overpowering glare which fills our atmosphere all around the sun's place
+in the sky. Mercury, the nearest known planet to the sun, is for this
+reason always very difficult to see; and even when, in its course, it
+gets sufficiently far from the sun to be left for a short time above the
+horizon after sunset, it is by no means an easy object to observe on
+account of the mists which usually hang about low down near the earth.
+One opportunity, however, offers itself from time to time to solve the
+riddle of an "intra-Mercurial" planet, that is to say, of a planet which
+circulates within the path followed by Mercury. The opportunity in
+question is furnished by a total eclipse of the sun; for when, during an
+eclipse of that kind, the body of the moon for a few minutes entirely
+hides the sun's face, and the dazzling glare is thus completely cut off,
+astronomers are enabled to give an unimpeded, though all too hurried,
+search to the region close around. A goodly number of total eclipses of
+the sun have, however, come and gone since the days of Lescarbault,<span class='pagenum'><a name="Page_26" id="Page_26">[Pg 26]</a></span> and
+no planet, so far, has revealed itself in the intra-Mercurial space. It
+seems, therefore, quite safe to affirm that no globe of sufficient size
+to be seen by means of our modern telescopes circulates nearer to the
+sun than the planet Mercury.</p>
+
+<p>Next in importance to the planets, as permanent members of the solar
+system, come the relatively small and secondary bodies known by the name
+of Satellites. The name <i>satellite</i> is derived from a Latin word
+signifying <i>an attendant</i>; for the bodies so-called move along always in
+close proximity to their respective "primaries," as the planets which
+they accompany are technically termed.</p>
+
+<p>The satellites cannot be considered as allotted with any particular
+regularity among the various members of the system; several of the
+planets, for instance, having a goodly number of these bodies
+accompanying them, while others have but one or two, and some again have
+none at all. Taking the planets in their order of distance outward from
+the Sun, we find that neither Mercury nor Venus are provided with
+satellites; the Earth has only one, viz. our neighbour the Moon; while
+Mars has but two tiny ones, so small indeed that one might imagine them
+to be merely asteroids, which had wandered out of their proper region
+and attached themselves to that planet. For the rest, so far as we at
+present know, Jupiter possesses seven,<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a> Saturn ten, Uranus four, and
+Neptune one. It is indeed possible, nay more, it is extremely probable,
+that the two last-named planets have a greater number of these secondary
+bodies revolving around them; but, unfortunately, the Uranian and<span class='pagenum'><a name="Page_27" id="Page_27">[Pg 27]</a></span>
+Neptunian systems are at such immense distances from us, that even the
+magnificent telescopes of to-day can extract very little information
+concerning them.</p>
+
+<p>From the distribution of the satellites, the reader will notice that the
+planets relatively near to the sun are provided with few or none, while
+the more distant planets are richly endowed. The conclusion, therefore,
+seems to be that nearness to the sun is in some way unfavourable either
+to the production, or to the continued existence, of satellites.</p>
+
+<p>A planet and its satellites form a repetition of the solar system on a
+tiny scale. Just as the planets revolve around the sun, so do these
+secondary bodies revolve around their primaries. When Galileo, in 1610,
+turned his newly invented telescope upon Jupiter, he quickly recognised
+in the four circling moons which met his gaze, a miniature edition of
+the solar system.</p>
+
+<p>Besides the planets and their satellites, there are two other classes of
+bodies which claim membership of the solar system. These are Comets and
+Meteors. Comets differ from the bodies which we have just been
+describing in that they appear filmy and transparent, whereas the others
+are solid and opaque. Again, the paths of the planets around the sun and
+of the satellites around their primaries are not actually circles; they
+are ovals, but their ovalness is not of a marked degree. The paths of
+comets on the other hand are usually <i>very</i> oval; so that in their
+courses many of them pass out as far as the known limits of the solar
+system, and even far beyond. It should be mentioned that nowadays the
+tendency is to consider comets as permanent members of the system,
+though<span class='pagenum'><a name="Page_28" id="Page_28">[Pg 28]</a></span> this was formerly not by any means an article of faith with
+astronomers.</p>
+
+<p>Meteors are very small bodies, as a rule perhaps no larger than pebbles,
+which move about unseen in space, and of which we do not become aware
+until they arrive very close to the earth. They are then made visible to
+us for a moment or two in consequence of being heated to a white heat by
+the friction of rushing through the atmosphere, and are thus usually
+turned into ashes and vapour long before they reach the surface of our
+globe. Though occasionally a meteoric body survives the fiery ordeal,
+and reaches the earth more or less in a solid state to bury itself deep
+in the soil, the majority of these celestial visitants constitute no
+source of danger whatever for us. Any one who will take the trouble to
+gaze at the sky for a short time on a clear night, is fairly certain to
+be rewarded with the view of a meteor. The impression received is as if
+one of the stars had suddenly left its accustomed place, and dashed
+across the heavens, leaving in its course a trail of light. It is for
+this reason that meteors are popularly known under the name of "shooting
+stars."</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a> By the Italian astronomer, Piazzi, at Palermo.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a> Probably eight. (<a href="#Page_232">See note, page 232.</a>)</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_29" id="Page_29">[Pg 29]</a></span></p>
+<h3><a name="CHAPTER_III" id="CHAPTER_III"></a>CHAPTER III</h3>
+
+<h4>THE SOLAR SYSTEM</h4>
+
+
+<p class="noin"><span class="smcap">We</span> have seen, in the course of the last chapter, that the solar system
+is composed as follows:&mdash;there is a central body, the sun, around which
+revolve along stated paths a number of important bodies known as
+planets. Certain of these planets, in their courses, carry along in
+company still smaller bodies called satellites, which revolve around
+them. With regard, however, to the remaining members of the system, viz.
+the comets and the meteors, it is not advisable at this stage to add
+more to what has been said in the preceding chapter. For the time being,
+therefore, we will devote our attention merely to the sun, the planets,
+and the satellites.</p>
+
+<p>Of what shape then are these bodies? Of one shape, and that one alone
+which appears to characterise all solid objects in the celestial spaces:
+they are spherical, which means <i>round like a ball</i>.</p>
+
+<p>Each of these spherical bodies rotates; that is to say, turns round and
+round, as a top does when it is spinning. This rotation is said to take
+place "upon an axis," a statement which may be explained as
+follows:&mdash;Imagine a ball with a knitting-needle run right through its
+centre. Then imagine this needle held pointing in one fixed direction
+while the ball is turned round and round. Well, it is the same<span class='pagenum'><a name="Page_30" id="Page_30">[Pg 30]</a></span> thing
+with the earth. As it journeys about the sun, it keeps turning round and
+round continually as if pivoted upon a mighty knitting needle
+transfixing it from North Pole to South Pole. In reality, however, there
+is no such material axis to regulate the constant direction of the
+rotation, just as there are no actual supports to uphold the earth
+itself in space. The causes which keep the celestial spheres poised, and
+which control their motions, are far more wonderful than any mechanical
+device.</p>
+
+<p>At this juncture it will be well to emphasise the sharp distinction
+between the terms <i>rotation</i> and <i>revolution</i>. The term "rotation" is
+invariably used by astronomers to signify the motion which a celestial
+body has upon an axis; the term "revolution," on the other hand, is used
+for the movement of one celestial body around another. Speaking of the
+earth, for instance, we say, that it <i>rotates</i> on its axis, and that it
+<i>revolves</i> around the sun.</p>
+
+<p>So far, nothing has been said about the sizes of the members of our
+system. Is there any stock size, any pattern according to which they may
+be judged? None whatever! They vary enormously. Very much the largest of
+all is the Sun, which is several hundred times larger than all the
+planets and satellites of the system rolled together. Next comes Jupiter
+and afterwards the other planets in the following order of
+size:&mdash;Saturn, Uranus, Neptune, the Earth, Venus, Mars, and Mercury.
+Very much smaller than any of these are the asteroids, of which Ceres,
+the largest, is less than 500 miles in diameter. It is, by the way, a
+strange fact that the zone of asteroids should mark the separation of
+the small planets from the giant<span class='pagenum'><a name="Page_31" id="Page_31">[Pg 31]</a></span> ones. The following table, giving
+roughly the various diameters of the sun and the principal planets in
+miles, will clearly illustrate the great discrepancy in size which
+prevails in the system.</p>
+
+
+<div class='table1'>
+<table border="0" width="30%" cellpadding="4" cellspacing="0" summary="Sun and Planet Diamters">
+<tr>
+ <td align='left'>Sun</td>
+ <td align='right'>866,540</td>
+ <td align='center'>miles</td>
+</tr>
+<tr>
+ <td align='left'>Mercury</td>
+ <td align='right'>2,765</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='left'>Venus</td>
+ <td align='right'>7,826</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='left'>Earth</td>
+ <td align='right'>7,918</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='left'>Mars</td>
+ <td align='right'>4,332</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='center' colspan='3'>ZONE OF ASTEROIDS</td>
+</tr>
+<tr>
+ <td align='left'>Jupiter</td>
+ <td align='right'>87,380</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='left'>Saturn</td>
+ <td align='right'>73,125</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='left'>Uranus<a href="#Footnote_3_3" class="fnanchor">[3]</a></td>
+ <td align='right'>34,900</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='left'>Neptune<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a></td>
+ <td align='right'>32,900</td>
+ <td align='center'>"</td>
+</tr>
+</table></div>
+
+<p>It does not seem possible to arrive at any generalisation from the above
+data, except it be to state that there is a continuous increase in size
+from Mercury to the earth, and a similar decrease in size from Jupiter
+outwards. Were Mars greater than the earth, the planets could then with
+truth be said to increase in size up to Jupiter, and then to decrease.
+But the zone of asteroids, and the relative smallness of Mars, negative
+any attempt to regard the dimensions of the planets as an orderly
+sequence.</p>
+
+<p>Next with respect to relative distance from the sun, Venus circulates
+nearly twice as far from it as Mercury, the earth nearly three times as
+far, and<span class='pagenum'><a name="Page_32" id="Page_32">[Pg 32]</a></span> Mars nearly four times. After this, just as we found a sudden
+increase in size, so do we meet with a sudden increase in distance.
+Jupiter, for instance, is about thirteen times as far as Mercury, Saturn
+about twenty-five times, Uranus about forty-nine times, and Neptune
+about seventy-seven. (<a href="#Fig_2">See Fig. 2</a>, p. 21.)</p>
+
+<p>It will thus be seen how enormously the solar system was enlarged in
+extent by the discovery of the outermost planets. The finding of Uranus
+plainly doubled its breadth; the finding of Neptune made it more than
+half as broad again. Nothing indeed can better show the import of these
+great discoveries than to take a pair of compasses and roughly set out
+the above relative paths in a series of concentric circles upon a large
+sheet of paper, and then to consider that the path of Saturn was the
+supposed boundary of our solar system prior to the year 1781.</p>
+
+<p>We have seen that the usual shape of celestial bodies themselves is
+spherical. Of what form then are their paths, or <i>orbits</i>, as these are
+called? One might be inclined at a venture to answer "circular," but
+this is not the case. The orbits of the planets cannot be regarded as
+true circles. They are ovals, or, to speak in technical language,
+"ellipses." Their ovalness or "ellipticity" is, however, in each case
+not by any means of the same degree. Some orbits&mdash;for instance, that of
+the earth&mdash;differ only slightly from circles; while others&mdash;those of
+Mars or Mercury, for example&mdash;are markedly elliptic. The orbit of the
+tiny planet Eros is, however, far and away the most elliptic of all, as
+we shall see when we come to deal with that little planet in detail.</p>
+
+<p>It has been stated that the sun and planets are<span class='pagenum'><a name="Page_33" id="Page_33">[Pg 33]</a></span> always rotating. The
+various rates at which they do so will, however, be best appreciated by
+a comparison with the rate at which the earth itself rotates.</p>
+
+<p>But first of all, let us see what ground we have, if any, for asserting
+that the earth rotates at all?</p>
+
+<p>If we carefully watch the heavens we notice that the background of the
+sky, with all the brilliant objects which sparkle in it, appears to turn
+once round us in about twenty-four hours; and that the pivot upon which
+this movement takes place is situated somewhere near what is known to us
+as the <i>Pole Star</i>. This was one of the earliest facts noted with regard
+to the sky; and to the men of old it therefore seems as if the heavens
+and all therein were always revolving around the earth. It was natural
+enough for them to take this view, for they had not the slightest idea
+of the immense distance of the celestial bodies, and in the absence of
+any knowledge of the kind they were inclined to imagine them
+comparatively near. It was indeed only after the lapse of many
+centuries, when men had at last realised the enormous gulf which
+separated them from even the nearest object in the sky, that a more
+reasonable opinion began to prevail. It was then seen that this
+revolution of the heavens about the earth could be more easily and more
+satisfactorily explained by supposing a mere rotation of the solid earth
+about a fixed axis, pointed in the direction of the polar star. The
+probability of such a rotation on the part of the earth itself was
+further strengthened by the observations made with the telescope. When
+the surfaces of the sun and planets were carefully studied these bodies
+were seen to be rotating. This<span class='pagenum'><a name="Page_34" id="Page_34">[Pg 34]</a></span> being the case, there could not surely
+be much hesitation in granting that the earth rotated also; particularly
+when it so simply explained the daily movement of the sky, and saved men
+from the almost inconceivable notion that the whole stupendous vaulted
+heaven was turning about them once in every twenty-four hours.</p>
+
+<p>If the sun be regularly observed through a telescope, it will gradually
+be gathered from the slow displacement of sunspots across its face,
+their disappearance at one edge and their reappearance again at the
+other edge, that it is rotating on an axis in a period of about
+twenty-six days. The movement, too, of various well-known markings on
+the surfaces of the planets Mars, Jupiter, and Saturn proves to us that
+these bodies are rotating in periods, which are about twenty-four hours
+for the first, and about ten hours for each of the other two. With
+regard, however, to Uranus and Neptune there is much more uncertainty,
+as these planets are at such great distances that even our best
+telescopes give but a confused view of the markings which they display;
+still a period of rotation of from ten to twelve hours appears to be
+accepted for them. On the other hand the constant blaze of sunlight in
+the neighbourhood of Mercury and Venus equally hampers astronomers in
+this quest. The older telescopic observers considered that the rotation
+periods of these two planets were about the same as that of the earth;
+but of recent years the opinion has been gaining ground that they turn
+round on their axes in exactly the same time as they revolve about the
+sun. This question is, however, a very doubtful one, and will be again
+referred<span class='pagenum'><a name="Page_35" id="Page_35">[Pg 35]</a></span> to later on; but, putting it on one side, it will be seen from
+what we have said above, that the rotation periods of the other planets
+of our system are usually about twenty-four hours, or under. The fact
+that the rotation period of the sun should run into <i>days</i> need not seem
+extraordinary when one considers its enormous size.</p>
+
+<p>The periods taken by the various planets to revolve around the sun is
+the next point which has to be considered. Here, too, it is well to
+start with the earth's period of revolution as the standard, and to see
+how the periods taken by the other planets compare with it.</p>
+
+<p>The earth takes about 365&frac14; days to revolve around the sun. This
+period of time is known to us as a "year." The following table shows in
+days and years the periods taken by each of the other planets to make a
+complete revolution round the sun:&mdash;</p>
+
+
+<div class='table1'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Planet Rotation around Sun Period">
+<tr>
+ <td align='left'>Mercury</td>
+ <td align='center'>about</td>
+ <td align='right'>88</td>
+ <td align='left'>days.</td>
+</tr>
+<tr>
+ <td align='left'>Venus</td>
+ <td align='center'>"</td>
+ <td align='right'>226</td>
+ <td align='left'>&nbsp;&nbsp;&nbsp;&nbsp;"</td>
+</tr>
+<tr>
+ <td align='left'>Mars</td>
+ <td align='center'>"</td>
+ <td align='right'>1</td>
+ <td align='left'>year and 321 days.</td>
+</tr>
+<tr>
+ <td align='left'>Jupiter</td>
+ <td align='center'>"</td>
+ <td align='right'>11</td>
+ <td align='left'>years and 313 days.</td>
+</tr>
+<tr>
+ <td align='left'>Saturn</td>
+ <td align='center'>"</td>
+ <td align='right'>29</td>
+ <td align='left'>years and 167 days.</td>
+</tr>
+<tr>
+ <td align='left'>Uranus</td>
+ <td align='center'>"</td>
+ <td align='right'>84</td>
+ <td align='left'>years and 7 days.</td>
+</tr>
+<tr>
+ <td align='left'>Neptune</td>
+ <td align='center'>"</td>
+ <td align='right'>164</td>
+ <td align='left'>years and 284 days.</td>
+</tr>
+</table></div>
+
+<p>From these periods we gather an important fact, namely, that the nearer
+a planet is to the sun the faster it revolves.</p>
+
+<p>Compared with one of our years what a long time does an Uranian, or
+Neptunian, "year" seem? For instance, if a "year" had commenced in
+Neptune about the middle of the reign of George II., that "year"<span class='pagenum'><a name="Page_36" id="Page_36">[Pg 36]</a></span> would
+be only just coming to a close; for the planet is but now arriving back
+to the position, with regard to the sun, which it then occupied. Uranus,
+too, has only completed a little more than 1&frac12; of its "years" since
+Herschel discovered it.</p>
+
+<p>Having accepted the fact that the planets are revolving around the sun,
+the next point to be inquired into is:&mdash;What are the positions of their
+orbits, or paths, relatively to each other?</p>
+
+<p>Suppose, for instance, the various planetary orbits to be represented by
+a set of hoops of different sizes, placed one within the other, and the
+sun by a small ball in the middle of the whole; in what positions will
+these hoops have to be arranged so as to imitate exactly the true
+condition of things?</p>
+
+<p>First of all let us suppose the entire arrangement, ball and hoops, to
+be on one level, so to speak. This may be easily compassed by imagining
+the hoops as floating, one surrounding the other, with the ball in the
+middle of all, upon the surface of still water. Such a set of objects
+would be described in astronomical parlance as being <i>in the same
+plane</i>. Suppose, on the other hand, that some of these floating hoops
+are tilted with regard to the others, so that one half of a hoop rises
+out of the water and the other half consequently sinks beneath the
+surface. This indeed is the actual case with regard to the planetary
+orbits. They do not by any means lie all exactly in the same plane. Each
+one of them is tilted, or <i>inclined</i>, a little with respect to the plane
+of the earth's orbit, which astronomers, for convenience, regard as the
+<i>level</i> of the solar system. This tilting, or "inclination," is, in the
+larger planets, greatest for the orbit of Mercury,<span class='pagenum'><a name="Page_37" id="Page_37">[Pg 37]</a></span> least for that of
+Uranus. Mercury's orbit is inclined to that of the earth at an angle of
+about 7&deg;, that of Venus at a little over 3&deg;, that of Saturn 2&frac12;&deg;;
+while in those of Mars, Neptune, and Jupiter the inclination is less
+than 2&deg;. But greater than any of these is the inclination of the orbit
+of the tiny planet Eros, viz. nearly 11&deg;.</p>
+
+<p>The systems of satellites revolving around their respective planets
+being, as we have already pointed out, mere miniature editions of the
+solar system, the considerations so far detailed, which regulate the
+behaviour of the planets in their relations to the sun, will of
+necessity apply to the satellites very closely. In one respect, however,
+a system of satellites differs materially from a system of planets. The
+central body around which planets are in motion is self-luminous,
+whereas the planetary body around which a satellite revolves is not.
+True, planets shine, and shine very brightly too; as, for instance,
+Venus and Jupiter. But they do not give forth any light of their own, as
+the sun does; they merely reflect the sunlight which they receive from
+him. Putting this one fact aside, the analogy between the planetary
+system and a satellite system is remarkable. The satellites are
+spherical in form, and differ markedly in size; they rotate, so far as
+we know, upon their axes in varying times; they revolve around their
+governing planets in orbits, not circular, but elliptic; and these
+orbits, furthermore, do not of necessity lie in the same plane. Last of
+all the satellites revolve around their primaries at rates which are
+directly comparable with those at which the planets revolve around the
+sun, the rule in fact holding good that the nearer a satellite is to its
+primary the faster it revolves.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a> As there seems to be much difference of opinion concerning
+the diameters of Uranus and Neptune, it should here be mentioned that
+the above figures are taken from Professor F.R. Moulton's <i>Introduction
+to Astronomy</i> (1906). They are there stated to be given on the authority
+of "Barnard's many measures at the Lick Observatory."</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_38" id="Page_38">[Pg 38]</a></span></p>
+<h3><a name="CHAPTER_IV" id="CHAPTER_IV"></a>CHAPTER IV</h3>
+
+<h4>CELESTIAL MECHANISM</h4>
+
+
+<p class="noin"><span class="smcap">As</span> soon as we begin to inquire closely into the actual condition of the
+various members of the solar system we are struck with a certain
+distinction. We find that there are two quite different points of view
+from which these bodies can be regarded. For instance, we may make our
+estimates of them either as regards <i>volume</i>&mdash;that is to say, the mere
+room which they take up; or as regards <i>mass</i>&mdash;that is to say, the
+amount of matter which they contain.</p>
+
+<p>Let us imagine two globes of equal volume; in other words, which take up
+an equal amount of space. One of these globes, however, may be composed
+of material much more tightly put together than in the other; or of
+greater <i>density</i>, as the term goes. That globe is said to be the
+greater of the two in mass. Were such a pair of globes to be weighed in
+scales, one globe in each pan, we should see at once, by its weighing
+down the other, which of the two was composed of the more tightly packed
+materials; and we should, in astronomical parlance, say of this one that
+it had the greater mass.</p>
+
+<p>Volume being merely another word for size, the order of the members of
+the solar system, with regard to their volumes, will be as follows,
+beginning with the greatest:&mdash;the Sun, Jupiter, Saturn,<span class='pagenum'><a name="Page_39" id="Page_39">[Pg 39]</a></span> Uranus,
+Neptune, the Earth, Venus, Mars, and Mercury.</p>
+
+<p>With regard to mass the same order strangely enough holds good. The
+actual densities of the bodies in question are, however, very different.
+The densest or closest packed body of all is the Earth, which is about
+five and a half times as dense as if it were composed entirely of water.
+Venus follows next, then Mars, and then Mercury. The remaining bodies,
+on the other hand, are relatively loose in structure. Saturn is the
+least dense of all, less so than water. The density of the Sun is a
+little greater than that of water.</p>
+
+<p>This method of estimating is, however, subject to a qualification. It
+must be remembered that in speaking of the Sun, for instance, as being
+only a little denser than water, we are merely treating the question
+from the point of view of an average. Certain parts of it in fact will
+be ever so much denser than water: those are the parts in the centre.
+Other portions, for instance, the outside portions, will be very much
+less dense. It will easily be understood that in all such bodies the
+densest or most compressed portions are to be found towards the centre;
+while the portions towards the exterior being less pressed upon, will be
+less dense.</p>
+
+<p>We now reach a very important point, the question of Gravitation.
+<i>Gravitation</i>, or <i>gravity</i>, as it is often called, is the attractive
+force which, for instance, causes objects to fall to the earth. Now it
+seems rather strange that one should say that it is owing to a certain
+force that things fall towards the earth. All things seem to us to fall
+so of their own accord, as if it<span class='pagenum'><a name="Page_40" id="Page_40">[Pg 40]</a></span> were quite natural, or rather most
+unnatural if they did not. Why then require a "force" to make them fall?</p>
+
+<p>The story goes that the great Sir Isaac Newton was set a-thinking on
+this subject by seeing an apple fall from a tree to the earth. He then
+carried the train of thought further; and, by studying the movements of
+the moon, he reached the conclusion that a body even so far off as our
+satellite would be drawn towards the earth in the same manner. This
+being the case, one will naturally ask why the moon herself does not
+fall in upon the earth. The answer is indeed found to be that the moon
+is travelling round and round the earth at a certain rapid pace, and it
+is this very same rapid pace which keeps her from falling in upon us.
+Any one can test this simple fact for himself. If we tie a stone to the
+end of a string, and keep whirling it round and round fast enough, there
+will be a strong pull from the stone in an outward direction, and the
+string will remain tight all the time that the stone is being whirled.
+If, however, we gradually slacken the speed at which we are making the
+stone whirl, a moment will come at length when the string will become
+limp, and the stone will fall back towards our hand.</p>
+
+<p>It seems, therefore, that there are two causes which maintain the stone
+at a regular distance all the time it is being steadily whirled. One of
+these is the continual pull inward towards our hand by means of the
+string. The other is the continual pull away from us caused by the rate
+at which the stone is travelling. When the rate of whirling is so
+regulated that these pulls exactly balance each other, the stone travels
+comfortably round and round, and shows no tendency<span class='pagenum'><a name="Page_41" id="Page_41">[Pg 41]</a></span> either to fall back
+upon our hand or to break the string and fly away into the air. It is
+indeed precisely similar with regard to the moon. The continual pull of
+the earth's gravitation takes the place of the string. If the moon were
+to go round and round slower than it does, it would tend to fall in
+towards the earth; if, on the other hand, it were to go faster, it would
+tend to rush away into space.</p>
+
+<p>The same kind of pull which the earth exerts upon the objects at its
+surface, or upon its satellite, the moon, exists through space so far as
+we know. Every particle of matter in the universe is found in fact to
+attract every other particle. The moon, for instance, attracts the earth
+also, but the controlling force is on the side of the much greater mass
+of the earth. This force of gravity or attraction of gravitation, as it
+is also called, is perfectly regular in its action. Its power depends
+first of all exactly upon the mass of the body which exerts it. The
+gravitational pull of the sun, for instance, reaches out to an enormous
+distance, controlling perhaps, in their courses, unseen planets circling
+far beyond the orbit of Neptune. Again, the strength with which the
+force of gravity acts depends upon distance in a regularly diminishing
+proportion. Thus, the nearer an object is to the earth, for instance,
+the stronger is the gravitational pull which it gets from it; the
+farther off it is, the weaker is this pull. If then the moon were to be
+brought nearer to the earth, the gravitational pull of the latter would
+become so much stronger that the moon's rate of motion would have also
+to increase in due proportion to prevent her from being drawn into the
+earth. Last of all,<span class='pagenum'><a name="Page_42" id="Page_42">[Pg 42]</a></span> the point in a body from which the attraction of
+gravitation acts, is not necessarily the centre of the body, but rather
+what is known as its <i>centre of gravity</i>, that is to say, the balancing
+point of all the matter which the body contains.</p>
+
+<p>It should here be noted that the moon does not actually revolve around
+the centre of gravity of the earth. What really happens is that both
+orbs revolve around their <i>common</i> centre of gravity, which is a point
+within the body of the earth, and situated about three thousand miles
+from its centre. In the same manner the planets and the sun revolve
+around the centre of gravity of the solar system, which is a point
+within the body of the sun.</p>
+
+<p>The neatly poised movements of the planets around the sun, and of the
+satellites around their respective planets, will therefore be readily
+understood to result from a nice balance between gravitation and speed
+of motion.</p>
+
+<p>The mass of the earth is ascertained to be about eighty times that of
+the moon. Our knowledge of the mass of a planet is learned from
+comparing the revolutions of its satellite or satellites around it, with
+those of the moon around the earth. We are thus enabled to deduce what
+the mass of such a planet would be compared to the earth's mass; that is
+to say, a study, for instance, of Jupiter's satellite system shows that
+Jupiter must have a mass nearly three hundred and eighteen times that of
+our earth. In the same manner we can argue out the mass of the sun from
+the movements of the planets and other bodies of the system around it.
+With regard, however, to Venus and Mercury, the problem is by<span class='pagenum'><a name="Page_43" id="Page_43">[Pg 43]</a></span> no means
+such an easy one, as these bodies have no satellites. For information in
+this latter case we have to rely upon such uncertain evidence as, for
+instance, the slight disturbances caused in the motion of the earth by
+the attraction of these planets when they pass closest to us, or their
+observed effect upon the motions of such comets as may happen to pass
+near to them.</p>
+
+<p>Mass and weight, though often spoken of as one and the same thing, are
+by no means so. Mass, as we have seen, merely means the amount of matter
+which a body contains. The weight of a body, on the other hand, depends
+entirely upon the gravitational pull which it receives. The force of
+gravity at the surface of the earth is, for instance, about six times as
+great as that at the surface of the moon. All bodies, therefore, weigh
+about six times as much on the earth as they would upon the moon; or,
+rather, a body transferred to the moon's surface would weigh only about
+one-sixth of what it did on the terrestrial surface. It will therefore
+be seen that if a body of given <i>mass</i> were to be placed upon planet
+after planet in turn, its <i>weight</i> would regularly alter according to
+the force of gravity at each planet's surface.</p>
+
+<p>Gravitation is indeed one of the greatest mysteries of nature. What it
+is, the means by which it acts, or why such a force should exist at all,
+are questions to which so far we have not had even the merest hint of an
+answer. Its action across space appears to be instantaneous.</p>
+
+<p>The intensity of gravitation is said in mathematical parlance "to vary
+inversely with the square of the distance." This means that at <i>twice</i>
+the distance the<span class='pagenum'><a name="Page_44" id="Page_44">[Pg 44]</a></span> pull will become only <i>one-quarter</i> as strong, and not
+one-half as otherwise might be expected. At <i>four</i> times the distance,
+therefore, it will be <i>one-sixteenth</i> as strong. At the earth's surface
+a body is pulled by the earth's gravitation, or "falls," as we
+ordinarily term it, through 16 feet in one <i>second</i> of time; whereas at
+the distance of the moon the attraction of the earth is so very much
+weakened that a body would take as long as one <i>minute</i> to fall through
+the same space.</p>
+
+<p>Newton's investigations showed that if a body were to be placed <i>at
+rest</i> in space entirely away from the attraction of any other body it
+would remain always in a motionless condition, because there would
+plainly be no reason why it should move in any one direction rather than
+in another. And, similarly, if a body were to be projected in a certain
+direction and at a certain speed, it would move always in the same
+direction and at the same speed so long as it did not come within the
+gravitational attraction of any other body.</p>
+
+<p>The possibility of an interaction between the celestial orbs had
+occurred to astronomers before the time of Newton; for instance, in the
+ninth century to the Arabian Musa-ben-Shakir, to Camillus Agrippa in
+1553, and to Kepler, who suspected its existence from observation of the
+tides. Horrox also, writing in 1635, spoke of the moon as moved by an
+<i>emanation</i> from the earth. But no one prior to Newton attempted to
+examine the question from a mathematical standpoint.</p>
+
+<p>Notwithstanding the acknowledged truth and far-reaching scope of the law
+of gravitation&mdash;for we find its effects exemplified in every portion of
+the universe&mdash;there<span class='pagenum'><a name="Page_45" id="Page_45">[Pg 45]</a></span> are yet some minor movements which it does not
+account for. For instance, there are small irregularities in the
+movement of Mercury which cannot be explained by the influence of
+possible intra-Mercurial planets, and similarly there are slight
+unaccountable deviations in the motions of our neighbour the Moon.</p>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_46" id="Page_46">[Pg 46]</a></span></p>
+<h3><a name="CHAPTER_V" id="CHAPTER_V"></a>CHAPTER V</h3>
+
+<h4>CELESTIAL DISTANCES</h4>
+
+
+<p class="noin"><span class="smcap">Up</span> to this we have merely taken a general view of the solar system&mdash;a
+bird's-eye view, so to speak, from space.</p>
+
+<p>In the course of our inquiry we noted in a rough way the <i>relative</i>
+distances at which the various planets move around the sun. But we have
+not yet stated what these distances <i>actually</i> are, and it were
+therefore well now to turn our attention to this important matter.</p>
+
+<p>Each of us has a fair idea of what a mile is. It is a quarter of an
+hour's sharp walk, for instance; or yonder village or building, we know,
+lies such and such a number of miles away.</p>
+
+<p>The measurements which have already been given of the diameters of the
+various bodies of the solar system appear very great to us, who find
+that a walk of a few miles at a time taxes our strength; but they are a
+mere nothing when we consider the distances from the sun at which the
+various planets revolve in their orbits.</p>
+
+<p>The following table gives these distances in round numbers. As here
+stated they are what are called "mean" distances; for, as the orbits are
+oval, the planets vary in their distances from the sun, and<span class='pagenum'><a name="Page_47" id="Page_47">[Pg 47]</a></span> we are
+therefore obliged to strike a kind of average for each case:&mdash;</p>
+
+
+<div class='table1'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Planet Distance to Sun">
+<tr>
+ <td align='left'>Mercury</td>
+ <td align='center'>about</td>
+ <td align='right'>36,000,000</td>
+ <td align='center'>miles.</td>
+</tr>
+<tr>
+ <td align='left'>Venus</td>
+ <td align='center'>"</td>
+ <td align='right'>67,200,000</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='left'>Earth</td>
+ <td align='center'>"</td>
+ <td align='right'>92,900,000</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='left'>Mars</td>
+ <td align='center'>"</td>
+ <td align='right'>141,500,000</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='left'>Jupiter</td>
+ <td align='center'>"</td>
+ <td align='right'>483,300,000</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='left'>Saturn</td>
+ <td align='center'>"</td>
+ <td align='right'>886,000,000</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='left'>Uranus</td>
+ <td align='center'>"</td>
+ <td align='right'>1,781,900,000</td>
+ <td align='center'>"</td>
+</tr>
+<tr>
+ <td align='left'>Neptune</td>
+ <td align='center'>"</td>
+ <td align='right'>2,791,600,000</td>
+ <td align='center'>"</td>
+</tr>
+</table></div>
+
+<p>From the above it will be seen at a glance that we have entered upon a
+still greater scale of distance than in dealing with the diameters of
+the various bodies of the system. In that case the distances were
+limited to thousands of miles; in this, however, we have to deal with
+millions. A million being ten hundred thousand, it will be noticed that
+even the diameter of the huge sun is well under a million miles.</p>
+
+<p>How indeed are we to get a grasp of such distances, when those to which
+we are ordinarily accustomed&mdash;the few miles' walk, the little stretch of
+sea or land which we gaze upon around us&mdash;are so utterly minute in
+comparison? The fact is, that though men may think that they can picture
+in their minds such immense distances, they actually can not. In matters
+like these we unconsciously employ a kind of convention, and we estimate
+a thing as being two or three or more times the size of another. More
+than this we are unable to do. For instance, our ordinary experience of
+a mile enables us to judge, in a way, of a stretch of several miles,
+such<span class='pagenum'><a name="Page_48" id="Page_48">[Pg 48]</a></span> as one can take in with a glance; but in our estimation of a
+thousand miles, or even of one hundred, we are driven back upon a mental
+trick, so to speak.</p>
+
+<p>In our attempts to realise such immense distances as those in the solar
+system we are obliged to have recourse to analogies; to comparisons with
+other and simpler facts, though this is at the best a mere self-cheating
+device. The analogy which seems most suited to our purpose here, and one
+which has often been employed by writers, is borrowed from the rate at
+which an express train travels.</p>
+
+<p>Let us imagine, for instance, that we possess an express train which is
+capable of running anywhere, never stops, never requires fuel, and
+always goes along at sixty miles an hour. Suppose we commence by
+employing it to gauge the size of our own planet, the earth. Let us send
+it on a trip around the equator, the span of which is about 24,000
+miles. At its sixty-miles-an-hour rate of going, this journey will take
+nearly 17 days. Next let us send it from the earth to the moon. This
+distance, 240,000 miles, being ten times as great as the last, will of
+course take ten times as long to cover, namely, 170 days; that is to
+say, nearly half a year. Again, let us send it still further afield, to
+the sun, for example. Here, however, it enters upon a journey which is
+not to be measured in thousands of miles, as the others were, but in
+millions. The distance from the earth to the sun, as we have seen in the
+foregoing table, is about 93 millions of miles. Our express train would
+take about 178 <i>years</i> to traverse this.</p>
+
+<p>Having arrived at the sun, let us suppose that our<span class='pagenum'><a name="Page_49" id="Page_49">[Pg 49]</a></span> train makes a tour
+right round it. This will take more than five years.</p>
+
+<p>Supposing, finally, that our train were started from the sun, and made
+to run straight out to the known boundaries of the solar system, that is
+to say, as far as the orbit of Neptune, it would take over 5000 years to
+traverse the distance.</p>
+
+<p>That sixty miles an hour is a very great speed any one, I think, will
+admit who has stood upon the platform of a country station while one of
+the great mail trains has dashed past. But are not the immensities of
+space appalling to contemplate, when one realises that a body moving
+incessantly at such a rate would take so long as 10,000 years to
+traverse merely the breadth of our solar system? Ten thousand years!
+Just try to conceive it. Why, it is only a little more than half that
+time since the Pyramids were built, and they mark for us the Dawn of
+History. And since then half-a-dozen mighty empires have come and gone!</p>
+
+<p>Having thus concluded our general survey of the appearance and
+dimensions of the solar system, let us next inquire into its position
+and size in relation to what we call the Universe.</p>
+
+<p>A mere glance at the night sky, when it is free from clouds, shows us
+that in every direction there are stars; and this holds good, no matter
+what portion of the globe we visit. The same is really true of the sky
+by day, though in that case we cannot actually see the stars, for their
+light is quite overpowered by the dazzling light of the sun.</p>
+
+<p>We thus reach the conclusion that our earth, that our solar system in
+fact, lies plunged within the midst<span class='pagenum'><a name="Page_50" id="Page_50">[Pg 50]</a></span> of a great tangle of stars. What
+position, by the way, do we occupy in this mighty maze? Are we at the
+centre, or anywhere near the centre, or where?</p>
+
+<p>It has been indeed amply proved by astronomical research that the stars
+are bodies giving off a light of their own, just as our sun does; that
+they are in fact suns, and that our sun is merely one, perhaps indeed a
+very unimportant member, of this great universe of stars. Each of these
+stars, or suns, besides, may be the centre of a system similar to what
+we call our solar system, comprising planets and satellites, comets and
+meteors;&mdash;or perchance indeed some further variety of attendant bodies
+of which we have no example in our tiny corner of space. But as to
+whether one is right in a conjecture of this kind, there is up to the
+present no proof whatever. No telescope has yet shown a planet in
+attendance upon one of these distant suns; for such bodies, even if they
+do exist, are entirely out of the range of our mightiest instruments. On
+what then can we ground such an assumption? Merely upon analogy; upon
+the common-sense deduction that as the stars have characteristics
+similar to our particular star, the sun, it would seem unlikely that
+ours should be the only such body in the whole of space which is
+attended by a planetary system.</p>
+
+<p>"The Stars," using that expression in its most general sense, do not lie
+at one fixed distance from us, set here and there upon a background of
+sky. There is in fact no background at all. The brilliant orbs are all
+around us in space, at different distances from us and from each other;
+and we can gaze between them out into the blackness of the void<span class='pagenum'><a name="Page_51" id="Page_51">[Pg 51]</a></span> which,
+perhaps, continues to extend unceasingly long after the very outposts of
+the stellar universe has been left behind. Shall we then start our
+imaginary express train once more, and send it out towards the nearest
+of the stars? This would, however, be a useless experiment. Our
+express-train method of gauging space would fail miserably in the
+attempt to bring home to us the mighty gulf by which we are now faced.
+Let us therefore halt for a moment and look back upon the orders of
+distance with which we have been dealing. First of all we dealt with
+thousands of miles. Next we saw how they shrank into insignificance when
+we embarked upon millions. We found, indeed, that our sixty-mile-an-hour
+train, rushing along without ceasing, would consume nearly the whole of
+historical time in a journey from the sun to Neptune.</p>
+
+<p>In the spaces beyond the solar system we are faced, however, by a new
+order of distance. From sun to planets is measured in millions of miles,
+but from sun to sun is measured in billions. But does the mere stating
+of this fact convey anything? I fear not. For the word "billion" runs as
+glibly off the tongue as "million," and both are so wholly unrealisable
+by us that the actual difference between them might easily pass
+unnoticed.</p>
+
+<p>Let us, however, make a careful comparison. What is a million? It is a
+thousand thousands. But what is a billion? It is a million millions.
+Consider for a moment! A million of millions. That means a million, each
+unit of which is again a million. In fact every separate "1" in this
+million is itself a million. Here is a way of trying to realise this<span class='pagenum'><a name="Page_52" id="Page_52">[Pg 52]</a></span>
+gigantic number. A million seconds make only eleven and a half days and
+nights. But a billion seconds will make actually more than thirty
+thousand years!</p>
+
+<p>Having accepted this, let us try and probe with our express train even a
+little of the new gulf which now lies before us. At our old rate of
+going it took almost two years to cover a million miles. To cover a
+billion miles&mdash;that is to say, a million times this distance&mdash;would thus
+take of course nearly two million years. Alpha Centauri, the nearest
+star to our earth, is some twenty-five billions of miles away. Our
+express train would thus take about fifty millions of years to reach it!</p>
+
+<p>This shows how useless our illustration, appropriate though it seemed
+for interplanetary space, becomes when applied to the interstellar
+spaces. It merely gives us millions in return for billions; and so the
+mind, driven in upon itself, whirls round and round like a squirrel in
+its revolving cage. There is, however, a useful illustration still left
+us, and it is the one which astronomers usually employ in dealing with
+the distances of the stars. The illustration in question is taken from
+the velocity of light.</p>
+
+<p>Light travels at the tremendous speed of about 186,000 miles a second.
+It therefore takes only about a second and a quarter to come to us from
+the moon. It traverses the 93,000,000 of miles which separate us from
+the sun in about eight minutes. It travels from the sun out to Neptune
+in about four hours, which means that it would cross the solar system
+from end to end in eight. To pass, however, across the distance which
+separates us from Alpha Centauri<span class='pagenum'><a name="Page_53" id="Page_53">[Pg 53]</a></span> it would take so long as about four
+and a quarter years!</p>
+
+<p>Astronomers, therefore, agree in estimating the distances of the stars
+from the point of view of the time which light would take to pass from
+them to our earth. They speak of that distance which light takes a year
+to traverse as a "light year." According to this notation, Alpha
+Centauri is spoken of as being about four and a quarter light years
+distant from us.</p>
+
+<p>Now as the rays of light coming from Alpha Centauri to us are chasing
+one another incessantly across the gulf of space, and as each ray left
+that star some four years before it reaches us, our view of the star
+itself must therefore be always some four years old. Were then this star
+to be suddenly removed from the universe at any moment, we should
+continue to see it still in its place in the sky for some four years
+more, after which it would suddenly disappear. The rays which had
+already started upon their journey towards our earth must indeed
+continue travelling, and reaching us in their turn until the last one
+had arrived; after which no more would come.</p>
+
+<p>We have drawn attention to Alpha Centauri as the nearest of the stars.
+The majority of the others indeed are ever so much farther. We can only
+hazard a guess at the time it takes for the rays from many of them to
+reach our globe. Suppose, for instance, we see a sudden change in the
+light of any of these remote stars, we are inclined to ask ourselves
+when that change did actually occur. Was it in the days of Queen
+Elizabeth, or at the time of the Norman Conquest; or was it when Rome
+was at the height of her glory, or perhaps ages before that when the
+Pyramids<span class='pagenum'><a name="Page_54" id="Page_54">[Pg 54]</a></span> of Egypt were being built? Even the last of these suppositions
+cannot be treated lightly. We have indeed no real knowledge of the
+distance from us of those stars which our giant telescopes have brought
+into view out of the depths of the celestial spaces.</p>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_55" id="Page_55">[Pg 55]</a></span></p>
+<h3><a name="CHAPTER_VI" id="CHAPTER_VI"></a>CHAPTER VI</h3>
+
+<h4>CELESTIAL MEASUREMENT</h4>
+
+
+<p class="noin"><span class="smcap">Had</span> the telescope never been invented our knowledge of astronomy would
+be trifling indeed.</p>
+
+<p>Prior to the year 1610, when Galileo first turned the new instrument
+upon the sky, all that men knew of the starry realms was gathered from
+observation with their own eyes unaided by any artificial means. In such
+researches they had been very much at a disadvantage. The sun and moon,
+in their opinion, were no doubt the largest bodies in the heavens, for
+the mere reason that they looked so! The mighty solar disturbances,
+which are now such common-places to us, were then quite undreamed of.
+The moon displayed a patchy surface, and that was all; her craters and
+ring-mountains were surprises as yet in store for men. Nothing of course
+was known about the surfaces of the planets. These objects had indeed no
+particular characteristics to distinguish them from the great host of
+the stars, except that they continually changed their positions in the
+sky while the rest did not. The stars themselves were considered as
+fixed inalterably upon the vault of heaven. The sun, moon, and planets
+apparently moved about in the intermediate space, supported in their
+courses by strange and fanciful devices. The idea of satellites was as
+yet unknown. Comets were regarded as<span class='pagenum'><a name="Page_56" id="Page_56">[Pg 56]</a></span> celestial portents, and meteors as
+small conflagrations taking place in the upper air.</p>
+
+<p>In the entire absence of any knowledge with regard to the actual sizes
+and distances of the various celestial bodies, men naturally considered
+them as small; and, concluding that they were comparatively near,
+assigned to them in consequence a permanent connection with terrestrial
+affairs. Thus arose the quaint and erroneous beliefs of astrology,
+according to which the events which took place upon our earth were
+considered to depend upon the various positions in which the planets,
+for instance, found themselves from time to time.</p>
+
+<p>It must, however, be acknowledged that the study of astrology,
+fallacious though its conclusions were, indirectly performed a great
+service to astronomy by reason of the accurate observations and diligent
+study of the stars which it entailed.</p>
+
+<p>We will now inquire into the means by which the distances and sizes of
+the celestial orbs have been ascertained, and see how it was that the
+ancients were so entirely in the dark in this matter.</p>
+
+<p>There are two distinct methods of finding out the distance at which any
+object happens to be situated from us.</p>
+
+<p>One method is by actual measurement.</p>
+
+<p>The other is by moving oneself a little to the right or left, and
+observing whether the distant object appears in any degree altered in
+position by our own change of place.</p>
+
+<p>One of the best illustrations of this relative change of position which
+objects undergo as a result of our own change of place, is to observe
+the landscape from the<span class='pagenum'><a name="Page_57" id="Page_57">[Pg 57]</a></span> window of a moving railway carriage. As we are
+borne rapidly along we notice that the telegraph posts which are set
+close to the line appear to fly past us in the contrary direction; the
+trees, houses, and other things beyond go by too, but not so fast;
+objects a good way off displace slowly; while some spire, or tall
+landmark, in the far distance appears to remain unmoved during a
+comparatively long time.</p>
+
+<p>Actual change of position on our own part is found indeed to be
+invariably accompanied by an apparent displacement of the objects about
+us, such apparent displacement as a result of our own change of position
+being known as "parallax." The dependence between the two is so
+mathematically exact, that if we know the amount of our own change of
+place, and if we observe the amount of the consequent displacement of
+any object, we are enabled to calculate its precise distance from us.
+Thus it comes to pass that distances can be measured without the
+necessity of moving over them; and the breadth of a river, for instance,
+or the distance from us of a ship at sea, can be found merely by such
+means.</p>
+
+<p>It is by the application of this principle to the wider field of the sky
+that we are able to ascertain the distance of celestial bodies. We have
+noted that it requires a goodly change of place on our own part to shift
+the position in which some object in the far distance is seen by us. To
+two persons separated by, say, a few hundred yards, a ship upon the
+horizon will appear pretty much in the same direction. They would
+require, in fact, to be much farther apart in order to displace it
+sufficiently for the purpose of estimating their distance from it. It<span class='pagenum'><a name="Page_58" id="Page_58">[Pg 58]</a></span>
+is the same with regard to the moon. Two observers, standing upon our
+earth, will require to be some thousands of miles apart in order to see
+the position of our satellite sufficiently altered with regard to the
+starry background, to give the necessary data upon which to ground their
+calculations.</p>
+
+<p>The change of position thus offered by one side of the earth's surface
+at a time is, however, not sufficient to displace any but the nearest
+celestial bodies. When we have occasion to go farther afield we have to
+seek a greater change of place. This we can get as a consequence of the
+earth's movement around the sun. Observations, taken several days apart,
+will show the effect of the earth's change of place during the interval
+upon the positions of the other bodies of our system. But when we desire
+to sound the depths of space beyond, and to reach out to measure the
+distance of the nearest star, we find ourselves at once thrown upon the
+greatest change of place which we can possibly hope for; and this we get
+during the long journey of many millions of miles which our earth
+performs around the sun during the course of each year. But even this
+last change of place, great as it seems in comparison with terrestrial
+measurements, is insufficient to show anything more than the tiniest
+displacements in a paltry forty-three out of the entire host of the
+stars.</p>
+
+<p>We can thus realise at what a disadvantage the ancients were. The
+measuring instruments at their command were utterly inadequate to detect
+such small displacements. It was reserved for the telescope to reveal
+them; and even then it required the great telescopes of recent times to
+show the<span class='pagenum'><a name="Page_59" id="Page_59">[Pg 59]</a></span> slight changes in the position of the nearer stars, which were
+caused by the earth's being at one time at one end of its orbit, and
+some six months later at the other end&mdash;stations separated from each
+other by a gulf of about one hundred and eighty-six millions of miles.</p>
+
+<p>The actual distances of certain celestial bodies being thus
+ascertainable, it becomes a matter of no great difficulty to determine
+the actual sizes of the measurable ones. It is a matter of everyday
+experience that the size which any object appears to have, depends
+exactly upon the distance it is from us. The farther off it is the
+smaller it looks; the nearer it is the bigger. If, then, an object which
+lies at a known distance from us looks such and such a size, we can of
+course ascertain its real dimensions. Take the moon, for instance. As we
+have already shown, we are able to ascertain its distance. We observe
+also that it looks a certain size. It is therefore only a matter of
+calculation to find what its actual dimensions should be, in order that
+it may look that size at that distance away. Similarly we can ascertain
+the real dimensions of the sun. The planets, appearing to us as points
+of light, seem at first to offer a difficulty; but, by means of the
+telescope, we can bring them, as it were, so much nearer to us, that
+their broad expanses may be seen. We fail, however, signally with regard
+to the stars; for they are so very distant, and therefore such tiny
+points of light, that our mightiest telescopes cannot magnify them
+sufficiently to show any breadth of surface.</p>
+
+<p>Instead of saying that an object looks a certain<span class='pagenum'><a name="Page_60" id="Page_60">[Pg 60]</a></span> breadth across, such
+as a yard or a foot, a statement which would really mean nothing,
+astronomers speak of it as measuring a certain angle. Such angles are
+estimated in what are called "degrees of arc"; each degree being divided
+into sixty minutes, and each minute again into sixty seconds. Popularly
+considered the moon and sun <i>look</i> about the same size, or, as an
+astronomer would put it, they measure about the same angle. This is an
+angle, roughly, of thirty-two minutes of arc; that is to say, slightly
+more than half a degree. The broad expanse of surface which a celestial
+body shows to us, whether to the naked eye, as in the case of the sun
+and moon, or in the telescope, as in the case of other members of our
+system, is technically known as its "disc."</p>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_61" id="Page_61">[Pg 61]</a></span></p>
+<h3><a name="CHAPTER_VII" id="CHAPTER_VII"></a>CHAPTER VII</h3>
+
+<h4>ECLIPSES AND KINDRED PHENOMENA</h4>
+
+
+<p class="noin"><span class="smcap">Since</span> some members of the solar system are nearer to us than others, and
+all are again much nearer than any of the stars, it must often happen
+that one celestial body will pass between us and another, and thus
+intercept its light for a while. The moon, being the nearest object in
+the universe, will, of course, during its motion across the sky,
+temporarily blot out every one of the others which happen to lie in its
+path. When it passes in this manner across the face of the sun, it is
+said to <i>eclipse</i> it. When it thus hides a planet or star, it is said to
+<i>occult</i> it. The reason why a separate term is used for what is merely a
+case of obscuring light in exactly the same way, will be plain when one
+considers that the disc of the sun is almost of the same apparent size
+as that of the moon, and so the complete hiding of the sun can last but
+a few minutes at the most; whereas a planet or a star looks so very
+small in comparison, that it is always <i>entirely swallowed up for some
+time</i> when it passes behind the body of our satellite.</p>
+
+<p>The sun, of course, occults planets and stars in exactly the same manner
+as the moon does, but we cannot see these occultations on account of the
+blaze of sunlight.</p>
+
+<p>By reason of the small size which the planets look<span class='pagenum'><a name="Page_62" id="Page_62">[Pg 62]</a></span> when viewed with the
+naked eye, we are not able to note them in the act of passing over stars
+and so blotting them out; but such occurrences may be seen in the
+telescope, for the planetary bodies then display broad discs.</p>
+
+<p>There is yet another occurrence of the same class which is known as a
+<i>transit</i>. This takes place when an apparently small body passes across
+the face of an apparently large one, the phenomenon being in fact the
+exact reverse of an occultation. As there is no appreciable body nearer
+to us than the moon, we can never see anything in transit across her
+disc. But since the planets Venus and Mercury are both nearer to us than
+the sun, they will occasionally be seen to pass across his face, and
+thus we get the well-known phenomena called Transits of Venus and
+Transits of Mercury.</p>
+
+<p>As the satellites of Jupiter are continually revolving around him, they
+will often pass behind or across his disc. Such occultations and
+transits of satellites can be well observed in the telescope.</p>
+
+<p>There is, however, a way in which the light of a celestial body may be
+obscured without the necessity of its being hidden from us by one
+nearer. It will no doubt be granted that any opaque object casts a
+shadow when a strong light falls directly upon it. Thus the earth, under
+the powerful light which is directed upon it from the sun, casts an
+extensive shadow, though we are not aware of the existence of this
+shadow until it falls upon something. The shadow which the earth casts
+is indeed not noticeable to us until some celestial body passes into it.
+As the sun is very large, and the earth in comparison very<span class='pagenum'><a name="Page_63" id="Page_63">[Pg 63]</a></span> small, the
+shadow thrown by the earth is comparatively short, and reaches out in
+space for only about a million miles. There is no visible object except
+the moon, which circulates within that distance from our globe, and
+therefore she is the only body which can pass into this shadow. Whenever
+such a thing happens, her surface at once becomes dark, for the reason
+that she never emits any light of her own, but merely reflects that of
+the sun. As the moon is continually revolving around the earth, one
+would be inclined to imagine that once in every month, namely at what is
+called <i>full moon</i>, when she is on the other side of the earth with
+respect to the sun, she ought to pass through the shadow in question.
+But this does not occur every time, because the moon's orbit is not
+quite <i>upon the same plane</i> with the earth's. It thus happens that time
+after time the moon passes clear of the earth's shadow, sometimes above
+it, and sometimes below it. It is indeed only at intervals of about six
+months that the moon can be thus obscured. This darkening of her light
+is known as an <i>eclipse of the moon</i>. It seems a great pity that custom
+should oblige us to employ the one term "eclipse" for this and also for
+the quite different occurrence, an eclipse of the sun; in which the
+sun's face is hidden as a consequence of the moon's body coming directly
+<i>between</i> it and our eyes.</p>
+
+<p>The popular mind seems always to have found it more difficult to grasp
+the causes of an eclipse of the moon than an eclipse of the sun. As Mr.
+J.E. Gore<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a> puts it: "The darkening of the sun's light by the
+interposition of the moon's body seems more<span class='pagenum'><a name="Page_64" id="Page_64">[Pg 64]</a></span> obvious than the passing of
+the moon through the earth's shadow."</p>
+
+<p>Eclipses of the moon furnish striking spectacles, but really add little
+to our knowledge. They exhibit, however, one of the most remarkable
+evidences of the globular shape of our earth; for the outline of its
+shadow when seen creeping over the moon's surface is always circular.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_3" id="Fig_3"></a>
+<img src="images/figure3.jpg" width="500" height="166" alt="Fig. 3." title="" />
+<div class="caption1"><span class="smcap">Fig. 3.</span>&mdash;Total and Partial Eclipses of the Moon. The Moon
+is here shown in two positions; i.e. <i>entirely</i> plunged in the earth's
+shadow and therefore totally eclipsed, and only <i>partly</i> plunged in it
+or partially eclipsed.</div>
+</div>
+
+<p><i>Eclipses of the Moon</i>, or Lunar Eclipses, as they are also called, are
+of two kinds&mdash;<i>Total</i>, and <i>Partial</i>. In a total lunar eclipse the moon
+passes entirely into the earth's shadow, and the whole of her surface is
+consequently darkened. This darkening lasts for about two hours. In a
+partial lunar eclipse, a portion only of the moon passes through the
+shadow, and so only <i>part</i> of her surface is darkened (<a href="#Fig_3">see Fig. 3</a>). A
+very striking phenomenon during a total eclipse of the moon, is that the
+darkening of the lunar surface is usually by no means so intense as one
+would expect, when one considers that the sunlight at that time should
+be <i>wholly</i> cut off from it. The occasions indeed upon which the moon
+has completely<span class='pagenum'><a name="Page_65" id="Page_65">[Pg 65]</a></span> disappeared from view during the progress of a total
+lunar eclipse are very rare. On the majority of these occasions she has
+appeared of a coppery-red colour, while sometimes she has assumed an
+ashen hue. The explanations of these variations of colour is to be found
+in the then state of the atmosphere which surrounds our earth. When
+those portions of our earth's atmosphere through which the sun's rays
+have to filter on their way towards the moon are free from watery
+vapour, the lunar surface will be tinged with a reddish light, such as
+we ordinarily experience at sunset when our air is dry. The ashen colour
+is the result of our atmosphere being laden with watery vapour, and is
+similar to what we see at sunset when rain is about. Lastly, when the
+air around the earth is thickly charged with cloud, no light at all can
+pass; and on such occasions the moon disappears altogether for the time
+being from the night sky.</p>
+
+<p><i>Eclipses of the Sun</i>, otherwise known as Solar Eclipses, are divided
+into <i>Total</i>, <i>Partial</i>, and <i>Annular</i>. A total eclipse of the sun takes
+place when the moon comes between the sun and the earth, in such a
+manner that it cuts off the sunlight <i>entirely</i> for the time being from
+a <i>portion</i> of the earth's surface. A person situated in the region in
+question will, therefore, at that moment find the sun temporarily
+blotted out from his view by the body of the moon. Since the moon is a
+very much smaller body than the sun, and also very much the nearer to us
+of the two, it will readily be understood that the portion of the earth
+from which the sun is seen thus totally eclipsed will be of small
+extent. In places not very distant<span class='pagenum'><a name="Page_66" id="Page_66">[Pg 66]</a></span> from this region, the moon will
+appear so much shifted in the sky that the sun will be seen only
+partially eclipsed. The moon being in constant movement round the earth,
+the portion of the earth's surface from which an eclipse is seen as
+total will be always a comparatively narrow band lying roughly from west
+to east. This band, known as the <i>track of totality</i>, can, at the
+utmost, never be more than about 165 miles in width, and as a rule is
+very much less. For about 2000 miles on either side of it the sun is
+seen partially eclipsed. Outside these limits no eclipse of any kind is
+visible, as from such regions the moon is not seen to come in the way of
+the sun (<a href="#Fig_4">see Fig. 4</a> (i.), p. 67).</p>
+
+<p>It may occur to the reader that eclipses can also take place in the
+course of which the positions, where the eclipse would ordinarily be
+seen as total, will lie outside the surface of the earth. Such an
+eclipse is thus not dignified with the name of total eclipse, but is
+called a partial eclipse, because from the earth's surface the sun is
+only seen <i>partly eclipsed at the utmost</i> (<a href="#Fig_4">see Fig. 4</a> (ii.), p. 67).</p>
+
+<p><span class='pagenum'><a name="Page_67" id="Page_67">[Pg 67]</a></span></p>
+<div class="figcenter" style="width: 500px;"><a name="Fig_4" id="Fig_4"></a>
+<img src="images/figure4a.jpg" width="500" height="119" alt="(i.) Total Eclipse of the Sun." title="" />
+<span class="caption">(i.) Total Eclipse of the Sun.</span>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/figure4b.jpg" width="500" height="128" alt="(ii.) Partial Eclipse of the Sun." title="" />
+<span class="caption">(ii.) Partial Eclipse of the Sun.</span><br /><br />
+<div class="caption1"><span class="smcap">Fig. 4.</span>&mdash;Total and Partial Eclipses of the Sun. From the position A the
+Sun cannot be seen, as it is entirely blotted out by the Moon. From B it
+is seen partially blotted out, because the Moon is to a certain degree
+in the way. From C no eclipse is seen, because the Moon does not come in
+the way.<br /><br />
+It is to be noted that in a Partial Eclipse of the Sun, the position A
+lies <i>outside</i> the surface of the Earth.</div>
+</div>
+
+<p>An <i>Annular eclipse</i> is an eclipse which just fails to become total for
+yet another reason. We have pointed out that the orbits of the various
+members of the solar system are not circular, but oval. Such oval
+figures, it will be remembered, are technically known as ellipses. In an
+elliptic orbit the controlling body is situated not in the middle of the
+figure, but rather towards one of the ends; the actual point which it
+occupies being known as the <i>focus</i>. The sun being at the focus of the
+earth's orbit, it follows that the earth is, at times, a little nearer
+to him than at others. The sun will therefore appear to us to vary a
+little in size, looking sometimes slightly larger than at other times.
+It is so, too, with the moon, at the focus of whose orbit the earth is
+situated. She therefore also appears to us at times to vary slightly in
+size. The result is that when the sun is eclipsed by the moon, and the
+moon at the time appears the larger of the two, she is able to blot out
+the sun completely, and so we can get a total eclipse. But when, on the
+other hand, the sun appears the larger, the eclipse will not be quite
+total, for a portion of the sun's disc will be seen protruding all
+around the moon like a ring of light. This is what is known as<span class='pagenum'><a name="Page_68" id="Page_68">[Pg 68]</a></span> an
+annular eclipse, from the Latin word <i>annulus</i>, which means a ring. The
+term is consecrated by long usage, but it seems an unfortunate one on
+account of its similarity to the word "annual." The Germans speak of
+this kind of eclipse as "ring-formed," which is certainly much more to
+the point.</p>
+
+<p>There can never be a year without an eclipse of the sun. Indeed there
+must be always two such eclipses <i>at least</i> during that period, though
+there need be no eclipse of the moon at all. On the other hand, the
+greatest number of eclipses which can ever take place during a year are
+seven; that is to say, either five solar eclipses and two lunar, or four
+solar and three lunar. This general statement refers merely to eclipses
+in their broadest significance, and informs us in no way whether they
+will be total or partial.</p>
+
+<p>Of all the phenomena which arise from the hiding of any celestial body
+by one nearer coming in the way, a total eclipse of the sun is far the
+most important. It is, indeed, interesting to consider how much poorer
+modern astronomy would be but for the extraordinary coincidence which
+makes a total solar eclipse just possible. The sun is about 400 times
+farther off from us than the moon, and enormously greater than her in
+bulk. Yet the two are relatively so distanced from us as to look about
+the same size. The result of this is that the moon, as has been seen,
+can often blot out the sun entirely from our view for a short time. When
+this takes place the great blaze of sunlight which ordinarily dazzles
+our eyes is completely cut off, and we are thus enabled, unimpeded, to
+note what is going on in the immediate vicinity of the sun itself.</p>
+
+<p><span class='pagenum'><a name="Page_69" id="Page_69">[Pg 69]</a></span></p><p>In a total solar eclipse, the time which elapses from the moment when
+the moon's disc first begins to impinge upon that of the sun at his
+western edge until the eclipse becomes total, lasts about an hour.
+During all this time the black lunar disc may be watched making its way
+steadily across the solar face. Notwithstanding the gradual obscuration
+of the sun, one does not notice much diminution of light until about
+three-quarters of his disc are covered. Then a wan, unearthly appearance
+begins to pervade all things, the temperature falls noticeably, and
+nature seems to halt in expectation of the coming of something unusual.
+The decreasing portion of sun becomes more and more narrow, until at
+length it is reduced to a crescent-shaped strip of exceeding fineness.
+Strange, ill-defined, flickering shadows (known as "Shadow Bands") may
+at this moment be seen chasing each other across any white expanse such
+as a wall, a building, or a sheet stretched upon the ground. The western
+side of the sky has now assumed an appearance dark and lowering, as if a
+rainstorm of great violence were approaching. This is caused by the
+mighty mass of the lunar shadow sweeping rapidly along. It flies onward
+at the terrific velocity of about half a mile a second.</p>
+
+<p>If the gradually diminishing crescent of sun be now watched through a
+telescope, the observer will notice that it does not eventually vanish
+all at once, as he might have expected. Rather, it breaks up first of
+all along its length into a series of brilliant dots, known as "Baily's
+Beads." The reason of this phenomenon is perhaps not entirely agreed
+upon, but the majority of astronomers incline to the opinion<span class='pagenum'><a name="Page_70" id="Page_70">[Pg 70]</a></span> that the
+so-called "beads" are merely the last remnants of sunlight peeping
+between those lunar mountain peaks which happen at the moment to fringe
+the advancing edge of the moon. The beads are no sooner formed than they
+rapidly disappear one after the other, after which no portion of the
+solar surface is left to view, and the eclipse is now total (<a href="#Fig_5">see Fig.
+5</a>).</p>
+
+<div class='center'><a name="Fig_5" id="Fig_5"></a>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 5a and 5b">
+<tr class='tr3'><td align='center'>
+<div class="figcenter" style="width: 250px;">
+<img src="images/figure5a.jpg" width="250" height="246" alt="In a total Eclipse" title="" />
+<span class="caption"><i>In a total Eclipse</i></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 250px;">
+<img src="images/figure5b.jpg" width="250" height="246" alt="In an annular Eclipse" title="" />
+<span class="caption"><i>In an annular Eclipse</i></span>
+</div></td></tr>
+<tr>
+<td align='center' colspan='2'><span class="caption"><span class="smcap">Fig. 5.</span>&mdash;"Baily's Beads."</span>
+</td></tr>
+</table></div>
+
+<p>But with the disappearance of the sun there springs into view a new and
+strange appearance, ordinarily unseen because of the blaze of sunlight.
+It is a kind of aureole, or halo, pearly white in colour, which is seen
+to surround the black disc of the moon. This white radiance is none
+other than the celebrated phenomenon widely known as the <i>Solar Corona</i>.
+It was once upon a time thought to belong to the moon, and to be perhaps
+a lunar atmosphere illuminated by the sunlight shining through it from
+behind. But the suddenness with which the moon always blots out stars
+when occulting them, has amply<span class='pagenum'><a name="Page_71" id="Page_71">[Pg 71]</a></span> proved that she possesses no atmosphere
+worth speaking about. It is now, however, satisfactorily determined that
+the corona belongs to the sun, for during the short time that it remains
+in view the black body of the moon can be seen creeping across it.</p>
+
+<p>All the time that the <i>total phase</i> (as it is called) lasts, the corona
+glows with its pale unearthly light, shedding upon the earth's surface
+an illumination somewhat akin to full moonlight. Usually the planet
+Venus and a few stars shine out the while in the darkened heaven.
+Meantime around the observer animal and plant life behave as at
+nightfall. Birds go to roost, bats fly out, worms come to the surface of
+the ground, flowers close up. In the Norwegian eclipse of 1896 fish were
+seen rising to the surface of the water. When the total phase at length
+is over, and the moon in her progress across the sky has allowed the
+brilliant disc of the sun to spring into view once more at the other
+side, the corona disappears.</p>
+
+<p>There is another famous accompaniment of the sun which partly reveals
+itself during total solar eclipses. This is a layer of red flame which
+closely envelops the body of the sun and lies between it and the corona.
+This layer is known by the name of the <i>Chromosphere</i>. Just as at
+ordinary times we cannot see the corona on account of the blaze of
+sunlight, so are we likewise unable to see the chromosphere because of
+the dazzling white light which shines through from the body of the sun
+underneath and completely overpowers it. When, however, during a solar
+eclipse, the lunar disc has entirely hidden the brilliant face of the
+sun, we are still able for a few moments to see an edgewise portion of
+the<span class='pagenum'><a name="Page_72" id="Page_72">[Pg 72]</a></span> chromosphere in the form of a narrow red strip, fringing the
+advancing border of the moon. Later on, just before the moon begins to
+uncover the face of the sun from the other side, we may again get a view
+of a strip of chromosphere.</p>
+
+<p>The outer surface of the chromosphere is not by any means even. It is
+rough and billowy, like the surface of a storm-tossed sea. Portions of
+it, indeed, rise at times to such heights that they may be seen standing
+out like blood-red points around the black disc of the moon, and remain
+thus during a good part of the total phase. These projections are known
+as the <i>Solar Prominences</i>. In the same way as the corona, the
+chromosphere and prominences were for a time supposed to belong to the
+moon. This, however, was soon found not to be the case, for the lunar
+disc was noticed to creep slowly across them also.</p>
+
+<p>The total phase, or "totality," as it is also called, lasts for
+different lengths of time in different eclipses. It is usually of about
+two or three minutes' duration, and at the utmost it can never last
+longer than about eight minutes.</p>
+
+<p>When totality is over and the corona has faded away, the moon's disc
+creeps little by little from the face of the sun, light and heat returns
+once more to the earth, and nature recovers gradually from the gloom in
+which she has been plunged. About an hour after totality, the last
+remnant of moon draws away from the solar disc, and the eclipse is
+entirely at an end.</p>
+
+<p>The corona, the chromosphere, and the prominences are the most important
+of these accompaniments of the sun which a total eclipse reveals to us.<span class='pagenum'><a name="Page_73" id="Page_73">[Pg 73]</a></span>
+Our further consideration of them must, however, be reserved for a
+subsequent chapter, in which the sun will be treated of at length.</p>
+
+<p>Every one who has had the good fortune to see a total eclipse of the sun
+will, the writer feels sure, agree with the verdict of Sir Norman
+Lockyer that it is at once one of the "grandest and most awe-inspiring
+sights" which man can witness. Needless to say, such an occurrence used
+to cause great consternation in less civilised ages; and that it has not
+in modern times quite parted with its terrors for some persons, is shown
+by the fact that in Iowa, in the United States, a woman died from fright
+during the eclipse of 1869.</p>
+
+<p>To the serious observer of a total solar eclipse every instant is
+extremely precious. Many distinct observations have to be crowded into a
+time all too limited, and this in an eclipse-party necessitates constant
+rehearsals in order that not a moment may be wasted when the longed-for
+totality arrives. Such preparation is very necessary; for the rarity and
+uncommon nature of a total eclipse of the sun, coupled with its
+exceeding short duration, tends to flurry the mind, and to render it
+slow to seize upon salient points of detail. And, even after every
+precaution has been taken, weather possibilities remain to be reckoned
+with, so that success is rather a lottery.</p>
+
+<p>Above all things, therefore, a total solar eclipse is an occurrence for
+the proper utilisation of which personal experience is of absolute
+necessity. It was manifestly out of the question that such experience
+could be gained by any individual in early times,<span class='pagenum'><a name="Page_74" id="Page_74">[Pg 74]</a></span> as the imperfection
+of astronomical theory and geographical knowledge rendered the
+predicting of the exact position of the track of totality well-nigh
+impossible. Thus chance alone would have enabled one in those days to
+witness a total phase, and the probabilities, of course, were much
+against a second such experience in the span of a life-time. And even in
+more modern times, when the celestial motions had come to be better
+understood, the difficulties of foreign travel still were in the way;
+for it is, indeed, a notable fact that during many years following the
+invention of the telescope the tracks were placed for the most part in
+far-off regions of the earth, and Europe was visited by singularly few
+total solar eclipses. Thus it came to pass that the building up of a
+body of organised knowledge upon this subject was greatly delayed.</p>
+
+<p>Nothing perhaps better shows the soundness of modern astronomical theory
+than the almost exact agreement of the time predicted for an eclipse
+with its actual occurrence. Similarly, by calculating backwards,
+astronomers have discovered the times and seasons at which many ancient
+eclipses took place, and valuable opportunities have thus arisen for
+checking certain disputed dates in history.</p>
+
+<p>It should not be omitted here that the ancients were actually able, <i>in
+a rough way</i>, to predict eclipses. The Chaldean astronomers had indeed
+noticed very early a curious circumstance, <i>i.e.</i> that eclipses tend to
+repeat themselves after a lapse of slightly more than eighteen years.</p>
+
+<p>In this connection it must, however, be pointed out, in the first
+instance, that the eclipses which<span class='pagenum'><a name="Page_75" id="Page_75">[Pg 75]</a></span> occur in any particular year are in
+no way associated with those which occurred in the previous year. In
+other words, the mere fact that an eclipse takes place upon a certain
+day this year will not bring about a repetition of it at the same time
+next year. However, the nicely balanced behaviour of the solar system,
+an equilibrium resulting from &aelig;ons of orbital ebb and flow, naturally
+tends to make the members which compose that family repeat their ancient
+combinations again and again; so that after definite lapses of time the
+same order of things will <i>almost exactly</i> recur. Thus, as a consequence
+of their beautifully poised motions, the sun, the moon, and the earth
+tend, after a period of 18 years and 10&#8531; days,<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a> to occupy very
+nearly the same positions with regard to each other. The result of this
+is that, during each recurring period, the eclipses comprised within it
+will be repeated in their order.</p>
+
+<p>To give examples:&mdash;</p>
+
+<p>The total solar eclipse of August 30, 1905, was a repetition of that of
+August 19, 1887.</p>
+
+<p>The partial solar eclipse of February 23, 1906, corresponded to that
+which took place on February 11, 1888.</p>
+
+<p>The annular eclipse of July 10, 1907, was a recurrence of that of June
+28, 1889.</p>
+
+<p>In this way we can go on until the eighteen year cycle has run out, and
+we come upon a total solar<span class='pagenum'><a name="Page_76" id="Page_76">[Pg 76]</a></span> eclipse predicted for September 10, 1923,
+which will repeat the above-mentioned ones of 1905 and 1887; and so on
+too with the others.</p>
+
+<p>From mere observation alone, extending no doubt over many ages, those
+time-honoured watchers of the sky, the early Chaldeans, had arrived at
+this remarkable generalisation; and they used it for the rough
+prediction of eclipses. To the period of recurrence they give the name
+of "Saros."</p>
+
+<p>And here we find ourselves led into one of the most interesting and
+fascinating by-paths in astronomy, to which writers, as a rule, pay all
+too little heed.</p>
+
+<p>In order not to complicate matters unduly, the recurrence of solar
+eclipses alone will first be dealt with. This limitation will, however,
+not affect the arguments in the slightest, and it will be all the more
+easy in consequence to show their application to the case of eclipses of
+the moon.</p>
+
+<p>The reader will perhaps have noticed that, with regard to the repetition
+of an eclipse, it has been stated that the conditions which bring it on
+at each recurrence are reproduced <i>almost exactly</i>. Here, then, lies the
+<i>crux</i> of the situation. For it is quite evident that were the
+conditions <i>exactly</i> reproduced, the recurrences of each eclipse would
+go on for an indefinite period. For instance, if the lapse of a saros
+period found the sun, moon, and earth again in the precise relative
+situations which they had previously occupied, the recurrences of a
+solar eclipse would tend to duplicate its forerunner with regard to the
+position of the shadow upon the terrestrial surface. But the conditions
+<i>not</i> being exactly reproduced, the<span class='pagenum'><a name="Page_77" id="Page_77">[Pg 77]</a></span> shadow-track does not pass across
+the earth in quite the same regions. It is shifted a little, so to
+speak; and each time the eclipse comes round it is found to be shifted a
+little farther. Every solar eclipse has therefore a definite "life" of
+its own upon the earth, lasting about 1150 years, or 64 saros returns,
+and working its way little by little across our globe from north to
+south, or from south to north, as the case may be. Let us take an
+imaginary example. A <i>partial</i> eclipse occurs, say, somewhere near the
+North Pole, the edge of the "partial" shadow just grazing the earth, and
+the "track of totality" being as yet cast into space. Here we have the
+beginning of a series. At each saros recurrence the partial shadow
+encroaches upon a greater extent of earth-surface. At length, in its
+turn, the track of totality begins to impinge upon the earth. This track
+streaks across our globe at each return of the eclipse, repeating itself
+every time in a slightly more southerly latitude. South and south it
+moves, passing in turn the Tropic of Cancer, the Equator, the Tropic of
+Capricorn, until it reaches the South Pole; after which it touches the
+earth no longer, but is cast into space. The rear portion of the partial
+shadow, in its turn, grows less and less in extent; and it too in time
+finally passes off. Our imaginary eclipse series is now no more&mdash;its
+"life" has ended.</p>
+
+<p>We have taken, as an example, an eclipse series moving from north to
+south. We might have taken one moving from south to north, for they
+progress in either direction.</p>
+
+<p>From the description just given the reader might<span class='pagenum'><a name="Page_78" id="Page_78">[Pg 78]</a></span> suppose that, if the
+tracks of totality of an eclipse series were plotted upon a chart of the
+world, they would lie one beneath another like a set of steps. This is,
+however, <i>not</i> the case, and the reason is easily found. It depends upon
+the fact that the saros does not comprise an exact number of days, but
+includes, as we have seen, one-third of a day in addition.</p>
+
+<p>It will be granted, of course, that if the number of days was exact, the
+<i>same</i> parts of the earth would always be brought round by the axial
+rotation <i>to front the sun</i> at the moment of the recurrence of the
+eclipse. But as there is still one-third of a day to complete the saros
+period, the earth has yet to make one-third of a rotation upon its axis
+before the eclipse takes place. Thus at every recurrence the track of
+totality finds itself placed one-third of the earth's circumference to
+the <i>westward</i>. Three of the recurrences will, of course, complete the
+circuit of the globe; and so the fourth recurrence will duplicate the
+one which preceded it, three saros returns, or 54 years and 1 month
+before. This duplication, as we have already seen, will, however, be
+situated in a latitude to the south or north of its predecessor,
+according as the eclipse series is progressing in a southerly or
+northerly direction.</p>
+
+<p>Lastly, every eclipse series, after working its way across the earth,
+will return again to go through the same process after some 12,000
+years; so that, at the end of that great lapse of time, the entire
+"life" of every eclipse should repeat itself, provided that the
+conditions of the solar system have not altered appreciably during the
+interval.</p>
+
+<p><span class='pagenum'><a name="Page_79" id="Page_79">[Pg 79]</a></span></p><p>We are now in a position to consider this gradual southerly or
+northerly progress of eclipse recurrences in its application to the case
+of eclipses of the moon. It should be evident that, just as in solar
+eclipses the lunar shadow is lowered or raised (as the case may be) each
+time it strikes the terrestrial surface, so in lunar eclipses will the
+body of the moon shift its place at each recurrence relatively to the
+position of the earth's shadow. Every lunar eclipse, therefore, will
+commence on our satellite's disc as a partial eclipse at the northern or
+southern extremity, as the case may be. Let us take, as an example, an
+imaginary series of eclipses of the moon progressing from north to
+south. At each recurrence the partial phase will grow greater, its
+boundary encroaching more and more to the southward, until eventually
+the whole disc is enveloped by the shadow, and the eclipse becomes
+total. It will then repeat itself as total during a number of
+recurrences, until the entire breadth of the shadow has been passed
+through, and the northern edge of the moon at length springs out into
+sunlight. This illuminated portion will grow more and more extensive at
+each succeeding return, the edge of the shadow appearing to recede from
+it until it finally passes off at the south. Similarly, when a lunar
+eclipse commences as partial at the south of the moon, the edge of the
+shadow at each subsequent recurrence finds itself more and more to the
+northward. In due course the total phase will supervene, and will
+persist during a number of recurrences until the southerly trend of the
+moon results in the uncovering of the lunar surface at the south. Thus,
+as the boundary of the shadow is left<span class='pagenum'><a name="Page_80" id="Page_80">[Pg 80]</a></span> more and more to the northward,
+the illuminated portion on the southern side of the moon becomes at each
+recurrence greater and the darkened portion on the northern side less,
+until the shadow eventually passes off at the north.</p>
+
+<p>The "life" of an eclipse of the moon happens, for certain reasons, to be
+much shorter than that of an eclipse of the sun. It lasts during only
+about 860 years, or 48 saros returns.</p>
+
+<p>Fig. 6, p. 81, is a map of the world on Mercator's Projection, showing a
+portion of the march of the total solar eclipse of August 30, 1905,
+across the surface of the earth. The projection in question has been
+employed because it is the one with which people are most familiar. This
+eclipse began by striking the neighbourhood of the North Pole in the
+guise of a partial eclipse during the latter part of the reign of Queen
+Elizabeth, and became total on the earth for the first time on the 24th
+of June 1797. Its next appearance was on the 6th of July 1815. It has
+not been possible to show the tracks of totality of these two early
+visitations on account of the distortion of the polar regions consequent
+on the <i>fiction</i> of Mercator's Projection. It is therefore made to
+commence with the track of its third appearance, viz. on July 17, 1833.
+In consequence of those variations in the apparent sizes of the sun and
+moon, which result, as we have seen, from the variations in their
+distances from the earth, this eclipse will change from a total into an
+annular eclipse towards the end of the twenty-first century. By that
+time the track will have passed to the southern side of the equator. The
+track will eventually leave the earth near the South Pole about the
+beginning of the twenty-sixth century, and the rear portion of the
+partial shadow will in its turn be clear of the terrestrial surface by
+about 2700 <span class="ampm">A.D.</span>, when the series comes to an end.</p>
+
+<p><span class='pagenum'><a name="Page_81" id="Page_81">[Pg 81]</a></span></p>
+<div class="figcenter" style="width: 600px;"><a name="Fig_6" id="Fig_6"></a>
+<img src="images/figure6.jpg" width="600" height="416" alt="Fig. 6." title="" />
+<span class="caption"><span class="smcap">Fig. 6.</span>&mdash;Map of the World on Mercator's Projection,
+showing a portion of the progress of the Total Solar Eclipse of August
+30, 1905, across the surface of the earth.</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_82" id="Page_82">[Pg 82]</a></span></p>
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a> Astronomical Essays (p. 40), London, 1907.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span></a> In some cases the periods between the dates of the
+corresponding eclipses <i>appear</i> to include a greater number of days than
+ten; but this is easily explained when allowance is made for intervening
+<i>leap</i> years (in each of which an <i>extra</i> day has of course been added),
+and also for variations in local time.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_83" id="Page_83">[Pg 83]</a></span></p>
+<h3><a name="CHAPTER_VIII" id="CHAPTER_VIII"></a>CHAPTER VIII</h3>
+
+<h4>FAMOUS ECLIPSES OF THE SUN</h4>
+
+
+<p class="noin"><span class="smcap">What</span> is thought to be the earliest reference to an eclipse comes down to
+us from the ancient Chinese records, and is over four thousand years
+old. The eclipse in question was a solar one, and occurred, so far as
+can be ascertained, during the twenty-second century <span class="ampm">B.C.</span> The story runs
+that the two state astronomers, Ho and Hi by name, being exceedingly
+intoxicated, were unable to perform their required duties, which
+consisted in superintending the customary rites of beating drums,
+shooting arrows, and the like, in order to frighten away the mighty
+dragon which it was believed was about to swallow up the Lord of Day.
+This eclipse seems to have been only partial; nevertheless a great
+turmoil ensued, and the two astronomers were put to death, no doubt with
+the usual <i>celestial</i> cruelty.</p>
+
+<p>The next eclipse mentioned in the Chinese annals is also a solar
+eclipse, and appears to have taken place more than a thousand years
+later, namely in 776 <span class="ampm">B.C.</span> Records of similar eclipses follow from the
+same source; but as they are mere notes of the events, and do not enter
+into any detail, they are of little interest. Curiously enough the
+Chinese have taken practically no notice of eclipses of the moon, but
+have left us a comparatively careful record of<span class='pagenum'><a name="Page_84" id="Page_84">[Pg 84]</a></span> comets, which has been
+of value to modern astronomy.</p>
+
+<p>The earliest mention of a <i>total</i> eclipse of the sun (for it should be
+noted that the ancient Chinese eclipse above-mentioned was merely
+partial) was deciphered in 1905, on a very ancient Babylonian tablet, by
+Mr. L.W. King of the British Museum. This eclipse took place in the year
+1063 <span class="ampm">B.C.</span></p>
+
+<p>Assyrian tablets record three solar eclipses which occurred between
+three and four hundred years later than this. The first of these was in
+763 <span class="ampm">B.C.</span>; the total phase being visible near Nineveh.</p>
+
+<p>The next record of an eclipse of the sun comes to us from a Grecian
+source. This eclipse took place in 585 <span class="ampm">B.C.</span>, and has been the subject of
+much investigation. Herodotus, to whom we are indebted for the account,
+tells us that it occurred during a battle in a war which had been waging
+for some years between the Lydians and Medes. The sudden coming on of
+darkness led to a termination of the contest, and peace was afterwards
+made between the combatants. The historian goes on to state that the
+eclipse had been foretold by Thales, who is looked upon as the Founder
+of Grecian astronomy. This eclipse is in consequence known as the
+"Eclipse of Thales." It would seem as if that philosopher were
+acquainted with the Chaldean saros.</p>
+
+<p>The next solar eclipse worthy of note was an annular one, and occurred
+in 431 <span class="ampm">B.C.</span>, the first year of the Peloponnesian War. Plutarch relates
+that the pilot of the ship, which was about to convey Pericles to the
+Peloponnesus, was very much frightened by it; but Pericles calmed him by
+holding up a cloak<span class='pagenum'><a name="Page_85" id="Page_85">[Pg 85]</a></span> before his eyes, and saying that the only difference
+between this and the eclipse was that something larger than the cloak
+prevented his seeing the sun for the time being.</p>
+
+<p>An eclipse of great historical interest is that known as the "Eclipse of
+Agathocles," which occurred on the morning of the 15th of August, 310
+<span class="ampm">B.C.</span> Agathocles, Tyrant of Syracuse, had been blockaded in the harbour
+of that town by the Carthaginian fleet, but effected the escape of his
+squadron under cover of night, and sailed for Africa in order to invade
+the enemy's territory. During the following day he and his vessels
+experienced a total eclipse, in which "day wholly put on the appearance
+of night, and the stars were seen in all parts of the sky."</p>
+
+<p>A few solar eclipses are supposed to be referred to in early Roman
+history, but their identity is very doubtful in comparison with those
+which the Greeks have recorded. Additional doubt is cast upon them by
+the fact that they are usually associated with famous events. The birth
+and death of Romulus, and the Passage of the Rubicon by Julius C&aelig;sar,
+are stated indeed to have been accompanied by these marks of the
+approval or disapproval of the gods!</p>
+
+<p>Reference to our subject in the Bible is scanty. Amos viii. 9 is thought
+to refer to the Nineveh eclipse of 763 <span class="ampm">B.C.</span>, to which allusion has
+already been made; while the famous episode of Hezekiah and the shadow
+on the dial of Ahaz has been connected with an eclipse which was partial
+at Jerusalem in 689 <span class="ampm">B.C.</span></p>
+
+<p>The first solar eclipse, recorded during the Christian Era, is known as
+the "Eclipse of Phlegon," from the fact that we are indebted for the
+account to a pagan writer<span class='pagenum'><a name="Page_86" id="Page_86">[Pg 86]</a></span> of that name. This eclipse took place in <span class="ampm">A.D.</span>
+29, and the total phase was visible a little to the north of Palestine.
+It has sometimes been confounded with the "darkness of the Crucifixion,"
+which event took place near the date in question; but it is sufficient
+here to say that the Crucifixion is well known to have occurred during
+the Passover of the Jews, which is always celebrated at the <i>full</i> moon,
+whereas an eclipse of the sun can only take place at <i>new</i> moon.</p>
+
+<p>Dion Cassius, commenting on the Emperor Claudius about the year <span class="ampm">A.D.</span> 45,
+writes as follows:&mdash;</p>
+
+<p>"As there was going to be an eclipse on his birthday, through fear of a
+disturbance, as there had been other prodigies, he put forth a public
+notice, not only that the obscuration would take place, and about the
+time and magnitude of it, but also about the causes that produce such an
+event."</p>
+
+<p>This is a remarkable piece of information; for the Romans, an
+essentially military nation, appear hitherto to have troubled themselves
+very little about astronomical matters, and were content, as we have
+seen, to look upon phenomena, like eclipses, as mere celestial
+prodigies.</p>
+
+<p>What is thought to be the first definite mention of the solar corona
+occurs in a passage of Plutarch. The eclipse to which he refers is
+probably one which took place in <span class="ampm">A.D.</span> 71. He says that the obscuration
+caused by the moon "has no time to last and no extensiveness, but some
+light shows itself round the sun's circumference, which does not allow
+the darkness to become deep and complete." No further reference to this
+phenomenon occurs until near the end of the sixteenth century. It
+should, however, be here<span class='pagenum'><a name="Page_87" id="Page_87">[Pg 87]</a></span> mentioned that Mr. E.W. Maunder has pointed
+out the probability<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a> that we have a very ancient symbolic
+representation of the corona in the "winged circle," "winged disc," or
+"ring with wings," as it is variously called, which appears so often
+upon Assyrian and Egyptian monuments, as the symbol of the Deity (Fig.
+7).</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_7" id="Fig_7"></a>
+<img src="images/figure7.jpg" width="500" height="406" alt="Fig. 7." title="" />
+<div class="caption1"><span class="smcap">Fig. 7.</span>&mdash;The "Ring with Wings." The upper is the Assyrian
+form of the symbol, the lower the Egyptian. (From <i>Knowledge</i>.) Compare
+the form of the corona on <a href="#Plate_VII">Plate VII.</a> (B), p. 142.</div>
+</div>
+
+<p>The first solar eclipse recorded to have been seen in England is that of
+<span class="ampm">A.D.</span> 538, mention of which is found in the <i>Anglo-Saxon Chronicle</i>. The
+track of totality did not, however, come near our islands, for only
+two-thirds of the sun's disc were eclipsed at London.</p>
+
+<p><span class='pagenum'><a name="Page_88" id="Page_88">[Pg 88]</a></span></p><p>In 840 a great eclipse took place in Europe, which was total for more
+than five minutes across what is now Bavaria. Terror at this eclipse is
+said to have hastened the death of Louis le Debonnaire, Emperor of the
+West, who lay ill at Worms.</p>
+
+<p>In 878&mdash;<i>temp.</i> King Alfred&mdash;an eclipse of the sun took place which was
+total at London. From this until 1715 no other eclipse was total at
+London itself; though this does not apply to other portions of England.</p>
+
+<p>An eclipse, generally known as the "Eclipse of Stiklastad," is said to
+have taken place in 1030, during the sea-fight in which Olaf of Norway
+is supposed to have been slain. Longfellow, in his <i>Saga of King Olaf</i>,
+has it that</p>
+
+<p class="poem">
+"The Sun hung red<br />
+As a drop of blood,"<br />
+</p>
+
+<p class="noin">but, as in the case of most poets, the dramatic value of an eclipse
+seems to have escaped his notice.</p>
+
+<p>In the year 1140 there occurred a total eclipse of the sun, the last to
+be visible in England for more than five centuries. Indeed there have
+been only two such since&mdash;namely, those of 1715 and 1724, to which we
+shall allude in due course. The eclipse of 1140 took place on the 20th
+March, and is thus referred to in the <i>Anglo-Saxon Chronicle</i>:&mdash;</p>
+
+<p>"In the Lent, the sun and the day darkened, about the noon-tide of the
+day, when men were eating, and they lighted candles to eat by. That was
+the 13th day before the calends of April. Men were very much struck with
+wonder."</p>
+
+<p>Several of the older historians speak of a "fearful eclipse" as having
+taken place on the morning of the<span class='pagenum'><a name="Page_89" id="Page_89">[Pg 89]</a></span> Battle of Crecy, 1346. Lingard, for
+instance, in his <i>History of England</i>, has as follows:&mdash;</p>
+
+<p>"Never, perhaps, were preparations for battle made under circumstances
+so truly awful. On that very day the sun suffered a partial eclipse:
+birds, in clouds, the precursors of a storm, flew screaming over the two
+armies, and the rain fell in torrents, accompanied by incessant thunder
+and lightning. About five in the afternoon the weather cleared up; the
+sun in full splendour darted his rays in the eyes of the enemy."</p>
+
+<p>Calculations, however, show that no eclipse of the sun took place in
+Europe during that year. This error is found to have arisen from the
+mistranslation of an obsolete French word <i>esclistre</i> (lightning), which
+is employed by Froissart in his description of the battle.</p>
+
+<p>In 1598 an eclipse was total over Scotland and part of North Germany. It
+was observed at Torgau by Jessenius, an Hungarian physician, who noticed
+a bright light around the moon during the time of totality. This is said
+to be the first reference to the corona since that of Plutarch, to which
+we have already drawn attention.</p>
+
+<p>Mention of Scotland recalls the fact that an unusual number of eclipses
+happen to have been visible in that country, and the occult bent natural
+to the Scottish character has traditionalised a few of them in such
+terms as the "Black Hour" (an eclipse of 1433), "Black Saturday" (the
+eclipse of 1598 which has been alluded to above), and "Mirk Monday"
+(1652). The track of the last-named also passed over Carrickfergus in
+Ireland, where it was observed by a certain Dr. Wybord, in whose account
+the<span class='pagenum'><a name="Page_90" id="Page_90">[Pg 90]</a></span> term "corona" is first employed. This eclipse is the last which has
+been total in Scotland, and it is calculated that there will not be
+another eclipse seen as total there until the twenty-second century.</p>
+
+<p>An eclipse of the sun which took place on May 30, 1612, is recorded as
+having been seen "through a tube." This probably refers to the then
+recent invention&mdash;the telescope.</p>
+
+<p>The eclipses which we have been describing are chiefly interesting from
+an historical point of view. The old mystery and confusion to the
+beholders seem to have lingered even into comparatively enlightened
+times, for we see how late it is before the corona attracts definite
+attention for the sake of itself alone.</p>
+
+<p>It is not a far cry from notice of the corona to that of other
+accompaniments of a solar eclipse. Thus the eclipse of 1706, the total
+phase of which was visible in Switzerland, is of great interest; for it
+was on this occasion that the famous red prominences seem first to have
+been noted. A certain Captain Stannyan observed this eclipse from Berne
+in Switzerland, and described it in a letter to Flamsteed, the then
+Astronomer Royal. He says the sun's "getting out of his eclipse was
+preceded by a blood-red streak of light from its left limb, which
+continued not longer than six or seven seconds of time; then part of the
+Sun's disc appeared all of a sudden, as bright as Venus was ever seen in
+the night, nay brighter; and in that very instant gave a Light and
+Shadow to things as strong as Moonlight uses to do." How little was then
+expected of the sun is, however, shown by Flamsteed's words, when
+communicating this information to the Royal Society:&mdash;</p>
+
+<p><span class='pagenum'><a name="Page_91" id="Page_91">[Pg 91]</a></span></p><p>"The Captain is the first man I ever heard of that took notice of a Red
+Streak of Light preceding the Emersion of the Sun's body from a total
+Eclipse. And I take notice of it to you because it infers that <i>the Moon
+has an atmosphere</i>; and its short continuance of only six or seven
+seconds of time, tells us that <i>its height is not more than the five or
+six hundredth part of her diameter</i>."</p>
+
+<p>What a change has since come over the ideas of men! The sun has proved a
+veritable mine of discovery, while the moon has yielded up nothing new.</p>
+
+<p>The eclipse of 1715, the first total at London since that of 878, was
+observed by the famous astronomer, Edmund Halley, from the rooms of the
+Royal Society, then in Crane Court, Fleet Street. On this occasion both
+the corona and a red projection were noted. Halley further makes
+allusion to that curious phenomenon, which later on became celebrated
+under the name of "Baily's beads." It was also on the occasion of this
+eclipse that the <i>earliest recorded drawings of the corona</i> were made.
+Cambridge happened to be within the track of totality; and a certain
+Professor Cotes of that University, who is responsible for one of the
+drawings in question, forwarded them to Sir Isaac Newton together with a
+letter describing his observations.</p>
+
+<p>In 1724 there occurred an eclipse, the total phase of which was visible
+from the south-west of England, but not from London. The weather was
+unfavourable, and the eclipse consequently appears to have been seen by
+only one person, a certain Dr. Stukeley, who observed it from Haraden
+Hill near Salisbury Plain. This is the last eclipse of which the total<span class='pagenum'><a name="Page_92" id="Page_92">[Pg 92]</a></span>
+phase was seen in any part of England. The next will not be until June
+29, 1927, and will be visible along a line across North Wales and
+Lancashire. The discs of the sun and moon will just then be almost of
+the same apparent size, and so totality will be of extremely short
+duration; in fact only a few seconds. London itself will not see a
+totality until the year 2151&mdash;a circumstance which need hardly distress
+any of us personally!</p>
+
+<p>It is only from the early part of the nineteenth century that serious
+scientific attention to eclipses of the sun can be dated. An <i>annular</i>
+eclipse, visible in 1836 in the south of Scotland, drew the careful
+notice of Francis Baily of Jedburgh in Roxburghshire to that curious
+phenomenon which we have already described, and which has ever since
+been known by the name of "Baily's beads." Spurred by his observation,
+the leading astronomers of the day determined to pay particular
+attention to a total eclipse, which in the year 1842 was to be visible
+in the south of France and the north of Italy. The public interest
+aroused on this occasion was also very great, for the region across
+which the track of totality was to pass was very populous, and inhabited
+by races of a high degree of culture.</p>
+
+<p>This eclipse occurred on the morning of the 8th July, and from it may be
+dated that great enthusiasm with which total eclipses of the sun have
+ever since been received. Airy, our then Astronomer Royal, observed it
+from Turin; Arago, the celebrated director of the Paris Observatory,
+from Perpignan in the south of France; Francis Baily from Pavia; and Sir
+John Herschel from Milan. The corona<span class='pagenum'><a name="Page_93" id="Page_93">[Pg 93]</a></span> and three large red prominences
+were not only well observed by the astronomers, but drew tremendous
+applause from the watching multitudes.</p>
+
+<p>The success of the observations made during this eclipse prompted
+astronomers to pay similar attention to that of July 28, 1851, the total
+phase of which was to be visible in the south of Norway and Sweden, and
+across the east of Prussia. This eclipse was also a success, and it was
+now ascertained that the red prominences belonged to the sun and not to
+the moon; for the lunar disc, as it moved onward, was seen to cover and
+to uncover them in turn. It was also noted that these prominences were
+merely uprushes from a layer of glowing gaseous matter, which was seen
+closely to envelop the sun.</p>
+
+<p>The total eclipse of July 18, 1860, was observed in Spain, and
+photography was for the first time <i>systematically</i> employed in its
+observation.<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a> In the photographs taken the stationary appearance of
+both the corona and prominences with respect to the moving moon,
+definitely confirmed the view already put forward that they were actual
+appendages of the sun.</p>
+
+<p>The eclipse of August 18, 1868, the total phase of which lasted nearly
+six minutes, was visible in India, and drew thither a large concourse of
+astronomers. In this eclipse the spectroscope came to the front, and
+showed that both the prominences, and the chromospheric layer from which
+they rise, are composed of glowing vapours&mdash;chief among which is the<span class='pagenum'><a name="Page_94" id="Page_94">[Pg 94]</a></span>
+vapour of hydrogen. The direct result of the observations made on this
+occasion was the spectroscopic method of examining prominences at any
+time in full daylight, and without a total eclipse. This method, which
+has given such an immense impetus to the study of the sun, was the
+outcome of independent and simultaneous investigation on the part of the
+French astronomer, the late M. Janssen, and the English astronomer,
+Professor (now Sir Norman) Lockyer, a circumstance strangely reminiscent
+of the discovery of Neptune. The principles on which the method was
+founded seem, however, to have occurred to Dr. (now Sir William) Huggins
+some time previously.</p>
+
+<p>The eclipse of December 22, 1870, was total for a little more than two
+minutes, and its track passed across the Mediterranean. M. Janssen, of
+whom mention has just been made, escaped in a balloon from then besieged
+Paris, taking his instruments with him, and made his way to Oran, in
+Algeria, in order to observe it; but his expectations were disappointed
+by cloudy weather. The expedition sent out from England had the
+misfortune to be shipwrecked off the coast of Sicily. But the occasion
+was redeemed by a memorable observation made by the American astronomer,
+the late Professor Young, which revealed the existence of what is now
+known as the "Reversing Layer." This is a shallow layer of gases which
+lies immediately beneath the chromosphere. An illustration of the
+corona, as it was seen during the above eclipse, will be found on <a href="#Plate_VII">Plate
+VII.</a> (A), p. 142.</p>
+
+<p>In the eclipse of December 12, 1871, total across Southern India, the
+photographs of the corona obtained by Mr. Davis, assistant to Lord
+Lindsay (now<span class='pagenum'><a name="Page_95" id="Page_95">[Pg 95]</a></span> the Earl of Crawford), displayed a wealth of detail
+hitherto unapproached.</p>
+
+<p>The eclipse of July 29, 1878, total across the western states of North
+America, was a remarkable success, and a magnificent view of the corona
+was obtained by the well-known American astronomer and physicist, the
+late Professor Langley, from the summit of Pike's Peak, Colorado, over
+14,000 feet above the level of the sea. The coronal streamers were seen
+to extend to a much greater distance at this altitude than at points
+less elevated, and the corona itself remained visible during more than
+four minutes after the end of totality. It was, however, not entirely a
+question of altitude; the coronal streamers were actually very much
+longer on this occasion than in most of the eclipses which had
+previously been observed.</p>
+
+<p>The eclipse of May 17, 1882, observed in Upper Egypt, is notable from
+the fact that, in one of the photographs taken by Dr. Schuster at Sohag,
+a bright comet appeared near the outer limit of the corona (<a href="#Plate_I">see Plate
+I.</a>, p. 96). The comet in question had not been seen before the eclipse,
+and was never seen afterwards. This is the third occasion on which
+attention has been drawn to a comet <i>merely</i> by a total eclipse. The
+first is mentioned by Seneca; and the second by Philostorgius, in an
+account of an eclipse observed at Constantinople in <span class="ampm">A.D.</span> 418. A fourth
+case of the kind occurred in 1893, when faint evidences of one of these
+filmy objects were found on photographs of the corona taken by the
+American astronomer, Professor Schaeberle, during the total eclipse of
+April 16 of that year.</p>
+
+<p>The eclipse of May 6, 1883, had a totality of over<span class='pagenum'><a name="Page_96" id="Page_96">[Pg 96]</a></span> five minutes, but
+the central track unfortunately passed across the Pacific Ocean, and the
+sole point of land available for observing it from was one of the
+Marquesas Group, Caroline Island, a coral atoll seven and a half miles
+long by one and a half broad. Nevertheless astronomers did not hesitate
+to take up their posts upon that little spot, and were rewarded with
+good weather.</p>
+
+<p>The next eclipse of importance was that of April 16, 1893. It stretched
+from Chili across South America and the Atlantic Ocean to the West Coast
+of Africa, and, as the weather was fine, many good results were
+obtained. Photographs were taken at both ends of the track, and these
+showed that the appearance of the corona remained unchanged during the
+interval of time occupied by the passage of the shadow across the earth.
+It was on the occasion of this eclipse that Professor Schaeberle found
+upon his photographs those traces of the presence of a comet, to which
+allusion has already been made.</p>
+
+<p>Extensive preparations were made to observe the eclipse of August 9,
+1896. Totality lasted from two to three minutes, and the track stretched
+from Norway to Japan. Bad weather disappointed the observers, with the
+exception of those taken to Nova Zembla by Sir George Baden Powell in
+his yacht <i>Otaria</i>.</p>
+
+<p>The eclipse of January 22, 1898, across India <i>vi&acirc;</i> Bombay and Benares,
+was favoured with good weather, and is notable for a photograph obtained
+by Mrs. E.W. Maunder, which showed a ray of the corona extending to a
+most unusual distance.</p>
+
+<div class="figcenter" style="width: 600px;"><a name="Plate_I" id="Plate_I"></a>
+<img src="images/plate1.jpg" width="500" height="702" alt="Plate I." title="" /><br />
+<span class="caption"><span class="smcap">Plate I. The Total Eclipse of the Sun of May 17th, 1882</span><br />
+A comet is here shown in the immediate neighbourhood of the corona.<br />
+Drawn by Mr. W.H. Wesley from the photographs.<br />(<a href="#Page_95"><small>Page 95</small></a>)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_97" id="Page_97">[Pg 97]</a></span></p><p>Of very great influence in the growth of our knowledge with regard to
+the sun, is the remarkable piece of good fortune by which the countries
+around the Mediterranean, so easy of access, have been favoured with a
+comparatively large number of total eclipses during the past sixty
+years. Tracks of totality have, for instance, traversed the Spanish
+peninsula on no less than five occasions during that period. Two of
+these are among the most notable eclipses of recent years, namely, those
+of May 28, 1900, and of August 30, 1905. In the former the track of
+totality stretched from the western seaboard of Mexico, through the
+Southern States of America, and across the Atlantic Ocean, after which
+it passed over Portugal and Spain into North Africa. The total phase
+lasted for about a minute and a half, and the eclipse was well observed
+from a great many points along the line. A representation of the corona,
+as it appeared on this occasion, will be found on <a href="#Plate_VII">Plate VII.</a> (B), p.
+142.</p>
+
+<p>The track of the other eclipse to which we have alluded, <i>i.e.</i> that of
+August 30, 1905, crossed Spain about 200 miles to the northward of that
+of 1900. It stretched from Winnipeg in Canada, through Labrador, and
+over the Atlantic; then traversing Spain, it passed across the Balearic
+Islands, North Africa, and Egypt, and ended in Arabia (<a href="#Fig_6">see Fig. 6</a>, p.
+81). Much was to be expected from a comparison between the photographs
+taken in Labrador and Egypt on the question as to whether the corona
+would show any alteration in shape during the time that the shadow was
+traversing the intervening space&mdash;some 6000 miles. The duration of the
+total phase in this eclipse was nearly four minutes. Bad weather,
+however, interfered a good deal with the observations. It was not
+possible, for instance, to do anything at all<span class='pagenum'><a name="Page_98" id="Page_98">[Pg 98]</a></span> in Labrador. In Spain the
+weather conditions were by no means favourable; though at Burgos, where
+an immense number of people had assembled, the total phase was,
+fortunately, well seen. On the whole, the best results were obtained at
+Guelma in Algeria. The corona on the occasion of this eclipse was a very
+fine one, and some magnificent groups of prominences were plainly
+visible to the naked eye (<a href="#Frontispiece">see the Frontispiece</a>).</p>
+
+<p>The next total eclipse after that of 1905 was one which occurred on
+January 14, 1907. It passed across Central Asia and Siberia, and had a
+totality lasting two and a half minutes at most; but it was not observed
+as the weather was extremely bad, a circumstance not surprising with
+regard to those regions at that time of year.</p>
+
+<p>The eclipse of January 3, 1908, passed across the Pacific Ocean. Only
+two small coral islands&mdash;Hull Island in the Ph&#339;nix Group, and Flint
+Island about 400 miles north of Tahiti&mdash;lay in the track. Two
+expeditions set out to observe it, <i>i.e.</i> a combined American party from
+the Lick Observatory and the Smithsonian Institution of Washington, and
+a private one from England under Mr. F.K. McClean. As Hull Island
+afforded few facilities, both parties installed their instruments on
+Flint Island, although it was very little better. The duration of the
+total phase was fairly long&mdash;about four minutes, and the sun very
+favourably placed, being nearly overhead. Heavy rain and clouds,
+however, marred observation during the first minute of totality, but the
+remaining three minutes were successfully utilised, good photographs of
+the corona being obtained.</p>
+
+<p><span class='pagenum'><a name="Page_99" id="Page_99">[Pg 99]</a></span></p><p>The next few years to come are unfortunately by no means favourable
+from the point of view of the eclipse observer. An eclipse will take
+place on June 17, 1909, the track stretching from Greenland across the
+North Polar regions into Siberia. The geographical situation is,
+however, a very awkward one, and totality will be extremely short&mdash;only
+six seconds in Greenland and twenty-three seconds in Siberia.</p>
+
+<p>The eclipse of May 9, 1910, will be visible in Tasmania. Totality will
+last so long as four minutes, but the sun will be at the time much too
+low in the sky for good observation.</p>
+
+<p>The eclipse of the following year, April 28, 1911, will also be
+confined, roughly speaking, to the same quarter of the earth, the track
+passing across the old convict settlement of Norfolk Island, and then
+out into the Pacific.</p>
+
+<p>The eclipse of April 17, 1912, will stretch from Portugal, through
+France and Belgium into North Germany. It will, however, be of
+practically no service to astronomy. Totality, for instance, will last
+for only three seconds in Portugal; and, though Paris lies in the
+central track, the eclipse, which begins as barely total, will have
+changed into an <i>annular</i> one by the time it passes over that city.</p>
+
+<p>The first really favourable eclipse in the near future will be that of
+August 21, 1914. Its track will stretch from Greenland across Norway,
+Sweden, and Russia. This eclipse is a return, after one saros, of the
+eclipse of August 9, 1896.</p>
+
+<p>The last solar eclipse which we will touch upon is that predicted for
+June 29, 1927. It has been already alluded to as the first of those in
+the future<span class='pagenum'><a name="Page_100" id="Page_100">[Pg 100]</a></span> to be <i>total</i> in England. The central line will stretch from
+Wales in a north-easterly direction. Stonyhurst Observatory, in
+Lancashire, will lie in the track; but totality there will be very
+short, only about twenty seconds in duration.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_6_6" id="Footnote_6_6"></a><a href="#FNanchor_6_6"><span class="label">[6]</span></a> <i>Knowledge</i>, vol. xx. p. 9, January 1897.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_7" id="Footnote_7_7"></a><a href="#FNanchor_7_7"><span class="label">[7]</span></a> The <i>first photographic representation of the corona</i> had,
+however, been made during the eclipse of 1851. This was a daguerreotype
+taken by Dr. Busch at K&ouml;nigsberg in Prussia.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_101" id="Page_101">[Pg 101]</a></span></p>
+<h3><a name="CHAPTER_IX" id="CHAPTER_IX"></a>CHAPTER IX</h3>
+
+<h4>FAMOUS ECLIPSES OF THE MOON</h4>
+
+
+<p class="noin"><span class="smcap">The</span> earliest lunar eclipse, of which we have any trustworthy
+information, was a total one which took place on the 19th March, 721
+<span class="ampm">B.C.</span>, and was observed from Babylon. For our knowledge of this eclipse
+we are indebted to Ptolemy, the astronomer, who copied it, along with
+two others, from the records of the reign of the Chaldean king,
+Merodach-Baladan.</p>
+
+<p>The next eclipse of the moon worth noting was a total one, which took
+place some three hundred years later, namely, in 425 <span class="ampm">B.C.</span> This eclipse
+was observed at Athens, and is mentioned by Aristophanes in his play,
+<i>The Clouds</i>.</p>
+
+<p>Plutarch relates that a total eclipse of the moon, which occurred in 413
+<span class="ampm">B.C.</span>, so greatly frightened Nicias, the general of the Athenians, then
+warring in Sicily, as to cause a delay in his retreat from Syracuse
+which led to the destruction of his whole army.</p>
+
+<p>Seven years later&mdash;namely, in 406 <span class="ampm">B.C.</span>, the twenty-sixth year of the
+Peloponnesian War&mdash;there took place another total lunar eclipse of which
+mention is made by Xenophon.</p>
+
+<p>Omitting a number of other eclipses alluded to by ancient writers, we
+come to one recorded by Josephus as having occurred a little before the
+death of Herod the Great. It is probable that the eclipse in question<span class='pagenum'><a name="Page_102" id="Page_102">[Pg 102]</a></span>
+was the total lunar one, which calculation shows to have taken place on
+the 15th September 5 <span class="ampm">B.C.</span>, and to have been visible in Western Asia.
+This is very important, for we are thus enabled to fix that year as the
+date of the birth of Christ, for Herod is known to have died in the
+early part of the year following the Nativity.</p>
+
+<p>In those accounts of total lunar eclipses, which have come down to us
+from the Dark and Middle Ages, the colour of the moon is nearly always
+likened to "blood." On the other hand, in an account of the eclipse of
+January 23, <span class="ampm">A.D.</span> 753, our satellite is described as "covered with a
+horrid black shield." We thus have examples of the two distinct
+appearances alluded to in <a href="#CHAPTER_VII">Chapter VII.</a>, <i>i.e.</i> when the moon appears of
+a coppery-red colour, and when it is entirely darkened.</p>
+
+<p>It appears, indeed, that, in the majority of lunar eclipses on record,
+the moon has appeared of a ruddy, or rather of a coppery hue, and the
+details on its surface have been thus rendered visible. One of the best
+examples of a <i>bright</i> eclipse of this kind is that of the 19th March
+1848, when the illumination of our satellite was so great that many
+persons could not believe that an eclipse was actually taking place. A
+certain Mr. Foster, who observed this eclipse from Bruges, states that
+the markings on the lunar disc were almost as visible as on an "ordinary
+dull moonlight night." He goes on to say that the British Consul at
+Ghent, not knowing that there had been any eclipse, wrote to him for an
+explanation of the red colour of the moon on that evening.</p>
+
+<p>Out of the <i>dark</i> eclipses recorded, perhaps the<span class='pagenum'><a name="Page_103" id="Page_103">[Pg 103]</a></span> best example is that
+of May 18, 1761, observed by Wargentin at Stockholm. On this occasion
+the lunar disc is said to have disappeared so completely, that it could
+not be discovered even with the telescope. Another such instance is the
+eclipse of June 10, 1816, observed from London. The summer of that year
+was particularly wet&mdash;a point worthy of notice in connection with the
+theory that these different appearances are due to the varying state of
+our earth's atmosphere.</p>
+
+<p>Sometimes, indeed, it has happened that an eclipse of the moon has
+partaken of both appearances, part of the disc being visible and part
+invisible. An instance of this occurred in the eclipse of July 12, 1870,
+when the late Rev. S.J. Johnson, one of the leading authorities on
+eclipses, who observed it, states that he found one-half the moon's
+surface quite invisible, both with the naked eye and with the telescope.</p>
+
+<p>In addition to the examples given above, there are three total lunar
+eclipses which deserve especial mention.</p>
+
+<p>1. <span class="ampm">A.D.</span> 755, November 23. During the progress of this eclipse the moon
+occulted the star Aldebaran in the constellation of Taurus.</p>
+
+<p>2. <span class="ampm">A.D.</span> 1493, April 2. This is the celebrated eclipse which is said to
+have so well served the purposes of Christopher Columbus. Certain
+natives having refused to supply him with provisions when in sore
+straits, he announced to them that the moon would be darkened as a sign
+of the anger of heaven. When the event duly came to pass, the savages
+were so terrified that they brought him provisions as much as he needed.</p>
+
+<p><span class='pagenum'><a name="Page_104" id="Page_104">[Pg 104]</a></span></p><p>3. <span class="ampm">A.D.</span> 1610, July 6. The eclipse in question is notable as having been
+seen through the telescope, then a recent invention. It was without
+doubt the first so observed, but unfortunately the name of the observer
+has not come down to us.</p>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_105" id="Page_105">[Pg 105]</a></span></p>
+<h3><a name="CHAPTER_X" id="CHAPTER_X"></a>CHAPTER X</h3>
+
+<h4>THE GROWTH OF OBSERVATION</h4>
+
+
+<p class="noin"><span class="smcap">The</span> earliest astronomical observations must have been made in the Dawn
+of Historic Time by the men who tended their flocks upon the great
+plains. As they watched the clear night sky they no doubt soon noticed
+that, with the exception of the moon and those brilliant wandering
+objects known to us as the planets, the individual stars in the heaven
+remained apparently fixed with reference to each other. These seemingly
+changeless points of light came in time to be regarded as sign-posts to
+guide the wanderer across the trackless desert, or the voyager upon the
+wide sea.</p>
+
+<p>Just as when looking into the red coals of a fire, or when watching the
+clouds, our imagination conjures up strange and grotesque forms, so did
+the men of old see in the grouping of the stars the outlines of weird
+and curious shapes. Fed with mythological lore, they imagined these to
+be rough representations of ancient heroes and fabled beasts, whom they
+supposed to have been elevated to the heavens as a reward for great
+deeds done upon the earth. We know these groupings of stars to-day under
+the name of the Constellations. Looking up at them we find it extremely
+difficult to fit in the majority with the figures which the ancients
+believed them to represent.<span class='pagenum'><a name="Page_106" id="Page_106">[Pg 106]</a></span> Nevertheless, astronomy has accepted the
+arrangement, for want of a better method of fixing the leading stars in
+the memory.</p>
+
+<p>Our early ancestors lived the greater part of their lives in the open
+air, and so came to pay more attention in general to the heavenly orbs
+than we do. Their clock and their calendar was, so to speak, in the
+celestial vault. They regulated their hours, their days, and their
+nights by the changing positions of the sun, the moon, and the stars;
+and recognised the periods of seed-time and harvest, of calm and stormy
+weather, by the rising or setting of certain well-known constellations.
+Students of the classics will recall many allusions to this, especially
+in the Odes of Horace.</p>
+
+<p>As time went on and civilisation progressed, men soon devised measuring
+instruments, by means of which they could note the positions of the
+celestial bodies in the sky with respect to each other; and, from
+observations thus made, they constructed charts of the stars. The
+earliest complete survey of this kind, of which we have a record, is the
+great Catalogue of stars which was made, in the second century <span class="ampm">B.C.</span>, by
+the celebrated Greek astronomer, Hipparchus, and in which he is said to
+have noted down about 1080 stars.</p>
+
+<p>It is unnecessary to follow in detail the tedious progress of
+astronomical discovery prior to the advent of the telescope. Certain it
+is that, as time went on, the measuring instruments to which we have
+alluded had become greatly improved; but, had they even been perfect,
+they would have been utterly inadequate to reveal those minute
+displacements, from which we have learned the actual distance of the
+nearest of the<span class='pagenum'><a name="Page_107" id="Page_107">[Pg 107]</a></span> celestial orbs. From the early times, therefore, until
+the medi&aelig;val period of our own era, astronomy grew up upon a faulty
+basis, for the earth ever seemed so much the largest body in the
+universe, that it continued from century to century to be regarded as
+the very centre of things.</p>
+
+<p>To the Arabians is due the credit of having kept alive the study of the
+stars during the dark ages of European history. They erected some fine
+observatories, notably in Spain and in the neighbourhood of Bagdad.
+Following them, some of the Oriental peoples embraced the science in
+earnest; Ulugh Beigh, grandson of the famous Tamerlane, founding, for
+instance, a great observatory at Samarcand in Central Asia. The Mongol
+emperors of India also established large astronomical instruments in the
+chief cities of their empire. When the revival of learning took place in
+the West, the Europeans came to the front once more in science, and
+rapidly forged ahead of those who had so assiduously kept alight the
+lamp of knowledge through the long centuries.</p>
+
+<p>The dethronement of the older theories by the Copernican system, in
+which the earth was relegated to its true place, was fortunately soon
+followed by an invention of immense import, the invention of the
+Telescope. It is to this instrument, indeed, that we are indebted for
+our knowledge of the actual scale of the celestial distances. It
+penetrated the depths of space; it brought the distant orbs so near,
+that men could note the detail on the planets, or measure the small
+changes in their positions in the sky which resulted from the movement
+of our own globe.</p>
+
+<p>It was in the year 1609 that the telescope was first<span class='pagenum'><a name="Page_108" id="Page_108">[Pg 108]</a></span> constructed. A
+year or so previous to this a spectacle-maker of Middleburgh in Holland,
+one Hans Lippershey, had, it appears, hit upon the fact that distant
+objects, when viewed through certain glass lenses suitably arranged,
+looked nearer.<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a> News of this discovery reached the ears of Galileo
+Galilei, of Florence, the foremost philosopher of the day, and he at
+once applied his great scientific attainments to the construction of an
+instrument based upon this principle. The result was what was called an
+"optick tube," which magnified distant objects some few times. It was
+not much larger than what we nowadays contemptuously refer to as a
+"spy-glass," yet its employment upon the leading celestial objects
+instantly sent astronomical science onward with a bound. In rapid
+succession Galileo announced world-moving discoveries; large spots upon
+the face of the sun; crater-like mountains upon the moon; four
+subordinate bodies, or satellites, circling around the planet Jupiter;
+and a strange appearance in connection with Saturn, which later
+telescopic observers found to be a broad flat ring encircling that
+planet. And more important still, the magnified image of Venus showed
+itself in the telescope at certain periods in crescent and other forms;
+a result which Copernicus is said to have announced should of necessity
+follow if his system were the true one.</p>
+
+<p><span class='pagenum'><a name="Page_109" id="Page_109">[Pg 109]</a></span></p><p>The discoveries made with the telescope produced, as time went on, a
+great alteration in the notions of men with regard to the universe at
+large. It must have been, indeed, a revelation to find that those points
+of light which they called the planets, were, after all, globes of a
+size comparable with the earth, and peopled perchance with sentient
+beings. Even to us, who have been accustomed since our early youth to
+such an idea, it still requires a certain stretch of imagination to
+enlarge, say, the Bright Star of Eve, into a body similar in size to our
+earth. The reader will perhaps recollect Tennyson's allusion to this in
+<i>Locksley Hall, Sixty Years After</i>:&mdash;</p>
+
+<p class="poem">
+"Hesper&mdash;Venus&mdash;were we native to that splendour or in Mars,<br />
+We should see the Globe we groan in, fairest of their evening stars.<br />
+<br />
+"Could we dream of wars and carnage, craft and madness, lust and spite,<br />
+Roaring London, raving Paris, in that point of peaceful light?"<br />
+</p>
+
+<p>The form of instrument as devised by Galileo is called the Refracting
+Telescope, or "Refractor." As we know it to-day it is the same in
+principle as his "optick tube," but it is not quite the same in
+construction. The early <i>object-glass</i>, or large glass at the end, was a
+single convex lens (<a href="#Fig_8">see Fig. 8</a>, p. 113, "Galilean"); the modern one is,
+on the other hand, composed of two lenses fitted together. The attempts
+to construct large telescopes of the Galilean type met in course of time
+with a great difficulty. The magnified image of the object observed was
+not quite pure; its edges, indeed, were fringed with rainbow-like
+colours. This defect was found to be aggravated with increase in the
+size of object-glasses. A method was, however,<span class='pagenum'><a name="Page_110" id="Page_110">[Pg 110]</a></span> discovered of
+diminishing this colouration, or <i>chromatic aberration</i> as it is called
+from the Greek word &#967;&#961;&#8182;&#956;&#945; (<i>chroma</i>), which means colour, viz.
+by making telescopes of great length and only a few inches in width. But
+the remedy was, in a way, worse than the disease; for telescopes thus
+became of such huge proportions as to be too unwieldy for use. Attempts
+were made to evade this unwieldiness by constructing them with skeleton
+tubes (<a href="#Plate_II">see Plate II.</a>, p. 110), or, indeed, even without tubes at all;
+the object-glass in the tubeless or "aerial" telescope being fixed at
+the top of a high post, and the <i>eye-piece</i>, that small lens or
+combination of lenses, which the eye looks directly into, being kept in
+line with it by means of a string and man&#339;uvred about near the ground
+(<a href="#Plate_III">Plate III.</a>, p. 112). The idea of a telescope without a tube may appear
+a contradiction in terms; but it is not really so, for the tube adds
+nothing to the magnifying power of the instrument, and is, in fact, no
+more than a mere device for keeping the object-glass and eye-piece in a
+straight line, and for preventing the observer from being hindered by
+stray lights in his neighbourhood. It goes without saying, of course,
+that the image of a celestial object will be more clear and defined when
+examined in the darkness of a tube.</p>
+
+<p>The ancients, though they knew nothing of telescopes, had, however,
+found out the merit of a tube in this respect; for they employed simple
+tubes, blackened on the inside, in order to obtain a clearer view of
+distant objects. It is said that Julius C&aelig;sar, before crossing the
+Channel, surveyed the opposite coast of Britain through a tube of this
+kind.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Plate_II" id="Plate_II"></a>
+<img src="images/plate2.jpg" width="500" height="420" alt="Plate II." title="" />
+<span class="caption"><span class="smcap">Plate II. Great Telescope of Hevelius</span><br />
+This instrument, 150 feet in length, with a <i>skeleton</i> tube, was
+constructed by the celebrated seventeenth century astronomer, Hevelius
+of Danzig. From an illustration in the <i>Machina Celestis</i>.<br />(<a href="#Page_110"><small>Page 110</small></a>)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_111" id="Page_111">[Pg 111]</a></span></p><p>A few of the most famous of the immensely long telescopes above alluded
+to are worthy of mention. One of these, 123 feet in length, was
+presented to the Royal Society of London by the Dutch astronomer
+Huyghens. Hevelius of Danzig constructed a skeleton one of 150 feet in
+length (<a href="#Plate_II">see Plate II.</a>, p. 110). Bradley used a tubeless one 212 feet
+long to measure the diameter of Venus in 1722; while one of 600 feet is
+said to have been constructed, but to have proved quite unworkable!</p>
+
+<p>Such difficulties, however, produced their natural result. They set men
+at work to devise another kind of telescope. In the new form, called the
+Reflecting Telescope, or "Reflector," the light coming from the object
+under observation was <i>reflected</i> into the eye-piece from the surface of
+a highly polished concave metallic mirror, or <i>speculum</i>, as it was
+called. It is to Sir Isaac Newton that the world is indebted for the
+reflecting telescope in its best form. That philosopher had set himself
+to investigate the causes of the rainbow-like, or prismatic colours
+which for a long time had been such a source of annoyance to telescopic
+observers; and he pointed out that, as the colours were produced in the
+passage of the rays of light <i>through</i> the glass, they would be entirely
+absent if the light were reflected from the <i>surface</i> of a mirror
+instead.</p>
+
+<p>The reflecting telescope, however, had in turn certain drawbacks of its
+own. A mirror, for instance, can plainly never be polished to such a
+high degree as to reflect as much light as a piece of transparent glass
+will let through. Further, the position of the eye-piece is by no means
+so convenient. It cannot, of course, be pointed directly towards the
+mirror, for the observer would then have to place his head right<span class='pagenum'><a name="Page_112" id="Page_112">[Pg 112]</a></span> in the
+way of the light coming from the celestial object, and would thus, of
+course, cut it off. In order to obviate this difficulty, the following
+device was employed by Newton in his telescope, of which he constructed
+his first example in 1668. A small, flat mirror was fixed by thin wires
+in the centre of the tube of the telescope, and near to its open end. It
+was set slant-wise, so that it reflected the rays of light directly into
+the eye-piece, which was screwed into a hole at the side of the tube
+(<a href="#Fig_8">see Fig. 8</a>, p. 113, "Newtonian").</p>
+
+<p>Although the Newtonian form of telescope had the immense advantage of
+doing away with the prismatic colours, yet it wasted a great deal of
+light; for the objection in this respect with regard to loss of light by
+reflection from the large mirror applied, of course, to the small mirror
+also. In addition, the position of the "flat," as the small mirror is
+called, had the further effect of excluding from the great mirror a
+certain proportion of light. But the reflector had the advantage, on the
+other hand, of costing less to make than the refractor, as it was not
+necessary to procure flawless glass for the purpose. A disc of a certain
+metallic composition, an alloy of copper and tin, known in consequence
+as <i>speculum metal</i>, had merely to be cast; and this had to be ground
+and polished <i>upon one side only</i>, whereas a lens has to be thus treated
+<i>upon both its sides</i>. It was, therefore, possible to make a much larger
+instrument at a great deal less labour and expense.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Plate_III" id="Plate_III"></a>
+<img src="images/plate3.jpg" width="500" height="581" alt="Plate III." title="" />
+<span class="caption"><span class="smcap">Plate III. A Tubeless, or &quot;Aerial&quot; Telescope</span><br />
+From an illustration in the <i>Opera Varia</i> of Christian Huyghens.<br />(<a href="#Page_110"><small>Page 110</small></a>)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_113" id="Page_113">[Pg 113]</a></span></p>
+<div class="figcenter" style="width: 500px;"><br /><a name="Fig_8" id="Fig_8"></a>
+<img src="images/figure8.jpg" width="500" height="581" alt="Fig. 8." title="" />
+<div class="caption1"><span class="smcap">Fig. 8.</span>&mdash;The various types of Telescope. All the above
+telescopes are <i>pointed</i> in the same direction; that is to say, the rays
+of light from the object are coming from the left-hand side.</div>
+</div>
+
+<p>We have given the Newtonian form as an example of the principle of the
+reflecting telescope. A somewhat similar instrument had, however, been
+projected, though not actually constructed, by James Gregory a few years
+earlier than Newton's, <i>i.e.</i> in 1663. In this form of reflector, known
+as the "Gregorian" telescope, a hole was made in the big concave mirror;
+and a small mirror, also concave, which faced it at a<span class='pagenum'><a name="Page_114" id="Page_114">[Pg 114]</a></span> certain distance,
+received the reflected rays, and reflected them back again through the
+hole in question into the eye-piece, which was fixed just behind (<a href="#Fig_8">see
+Fig. 8</a>, p. 113, "Gregorian"). The Gregorian had thus the sentimental
+advantage of being <i>pointed directly at the object</i>. The hole in the big
+mirror did not cause any loss of light, for the central portion in which
+it was made was anyway unable to receive light through the small mirror
+being directly in front of it. An adaptation of the Gregorian was the
+"Cassegrainian" telescope, devised by Cassegrain in 1672, which differed
+from it chiefly in the small mirror being convex instead of concave (<a href="#Fig_8">see
+Fig. 8</a>, p. 113, "Cassegrainian"). These <i>direct-view</i> forms of the
+reflecting telescope were much in vogue about the middle of the
+eighteenth century, when many beautiful examples of Gregorians were made
+by the famous optician, James Short, of Edinburgh.</p>
+
+<p>An adaptation of the Newtonian type of telescope is known as the
+"Herschelian," from being the kind favoured by Sir William Herschel. It
+is, however, only suitable in immense instruments, such as Herschel was
+in the habit of employing. In this form the object-glass is set at a
+slight slant, so that the light coming from the object is reflected
+straight into the eye-piece, which is fixed facing it in the side of the
+tube (<a href="#Fig_8">see Fig. 8</a>, p. 113, "Herschelian"). This telescope has an
+advantage over the other forms of reflector through the saving of light
+consequent on doing away with the <i>second</i> reflection. There is,
+however, the objection that the slant of the object-glass is productive
+of some distortion in the appearance of the object observed; but this
+slant is of necessity slight when the length of the telescope is very
+great.</p>
+
+<p><span class='pagenum'><a name="Page_115" id="Page_115">[Pg 115]</a></span></p><p>The principle of this type of telescope had been described to the
+French Academy of Sciences as early as 1728 by Le Maire, but no one
+availed himself of the idea until 1776, when Herschel tried it. At
+first, however, he rejected it; but in 1786 he seems to have found that
+it suited the huge instruments which he was then making. Herschel's
+largest telescope, constructed in 1789, was about four feet in diameter
+and forty feet in length. It is generally spoken of as the "Forty-foot
+Telescope," though all other instruments have been known by their
+<i>diameters</i>, rather than by their lengths.</p>
+
+<p>To return to the refracting telescope. A solution of the colour
+difficulty was arrived at in 1729 (two years after Newton's death) by an
+Essex gentleman named Chester Moor Hall. He discovered that by making a
+double object-glass, composed of an outer convex lens and an inner
+concave lens, made respectively of different kinds of glass, <i>i.e.</i>
+<i>crown</i> glass and <i>flint</i> glass, the troublesome colour effects could
+be, <i>to a very great extent</i>, removed. Hall's investigations appear to
+have been rather of an academic nature; and, although he is believed to
+have constructed a small telescope upon these lines, yet he seems to
+have kept the matter so much to himself that it was not until the year
+1758 that the first example of the new instrument was given to the
+world. This was done by John Dollond, founder of the well-known optical
+firm of Dollond, of Ludgate Hill, London, who had, quite independently,
+re-discovered the principle.</p>
+
+<p>This "Achromatic" telescope, or telescope "free from colour effects," is
+the kind ordinarily in use at present, whether for astronomical or for
+terrestrial<span class='pagenum'><a name="Page_116" id="Page_116">[Pg 116]</a></span> purposes (<a href="#Fig_8">see Fig. 8</a>, p. 113, "Achromatic"). The expense of
+making large instruments of this type is very great, for, in the
+object-glass alone, no less than <i>four</i> surfaces have to be ground and
+polished to the required curves; and, usually, the two lenses of which
+it is composed have to fit quite close together.</p>
+
+<p>With the object of evading the expense referred to, and of securing
+<i>complete</i> freedom from colour effects, telescopes have even been made,
+the object-glasses of which were composed of various transparent liquids
+placed between thin lenses; but leakages, and currents set up within
+them by changes of temperature, have defeated the ingenuity of those who
+devised these substitutes.</p>
+
+<p>The solution of the colour difficulty by means of Dollond's achromatic
+refractor has not, however, ousted the reflecting telescope in its best,
+or Newtonian form, for which great concave mirrors made of glass,
+covered with a thin coating of silver and highly polished, have been
+used since about 1870 instead of metal mirrors. They are very much
+lighter in weight and cheaper to make than the old specula; and though
+the silvering, needless to say, deteriorates with time, it can be
+renewed at a comparatively trifling cost. Also these mirrors reflect
+much more light, and give a clearer view, than did the old metallic
+ones.</p>
+
+<p>When an object is viewed through the type of astronomical telescope
+ordinarily in use, it is seen <i>upside down</i>. This is, however, a matter
+of very small moment in dealing with celestial objects; for, as they are
+usually round, it is really not of much consequence which part we regard
+as top and which as bottom. Such an inversion would, of course, be<span class='pagenum'><a name="Page_117" id="Page_117">[Pg 117]</a></span> most
+inconvenient when viewing terrestrial objects. In order to observe the
+latter we therefore employ what is called a terrestrial telescope, which
+is merely a refractor with some extra lenses added in the eye portion
+for the purpose of turning the inverted image the right way up again.
+These extra lenses, needless to say, absorb a certain amount of light;
+wherefore it is better in astronomical observation to save light by
+doing away with them, and putting up with the slight inconvenience of
+seeing the object inverted.</p>
+
+<p>This inversion of images by the astronomical telescope must be specially
+borne in mind with regard to the photographs of the moon in <a href="#CHAPTER_XVI">Chapter XVI</a>.</p>
+
+<p>In the year 1825 the largest achromatic refractor in existence was one
+of nine and a half inches in diameter constructed by Fraunhofer for the
+Observatory of Dorpat in Russia. The largest refractors in the world
+to-day are in the United States, <i>i.e.</i> the forty-inch of the Yerkes
+Observatory (<a href="#Plate_IV">see Plate IV.</a>, p. 118), and the thirty-six inch of the
+Lick. The object-glasses of these and of the thirty-inch telescope of
+the Observatory of Pulkowa, in Russia, were made by the great optical
+house of Alvan Clark &amp; Sons, of Cambridge, Massachusetts, U.S.A. The
+tubes and other portions of the Yerkes and Lick telescopes were,
+however, constructed by the Warner and Swasey Co., of Cleveland, Ohio.</p>
+
+<p>The largest reflector, and so the largest telescope in the world, is
+still the six-foot erected by the late Lord Rosse at Parsonstown in
+Ireland, and completed in the year 1845. It is about fifty-six feet in
+length. Next come two of five feet, with mirrors of silver on<span class='pagenum'><a name="Page_118" id="Page_118">[Pg 118]</a></span> glass;
+one of them made by the late Dr. Common, of Ealing, and the other by the
+American astronomer, Professor G.W. Ritchey. The latter of these is
+installed in the Solar Observatory belonging to Carnegie Institution of
+Washington, which is situated on Mount Wilson in California. The former
+is now at the Harvard College Observatory, and is considered by
+Professor Moulton to be probably the most efficient reflector in use at
+present. Another large reflector is the three-foot made by Dr. Common.
+It came into the possession of Mr. Crossley of Halifax, who presented it
+to the Lick Observatory, where it is now known as the "Crossley
+Reflector."</p>
+
+<p>Although to the house of Clark belongs, as we have seen, the credit of
+constructing the object-glasses of the largest refracting telescopes of
+our time, it has nevertheless keen competitors in Sir Howard Grubb, of
+Dublin, and such well-known firms as Cooke of York and Steinheil of
+Munich. In the four-foot reflector, made in 1870 for the Observatory of
+Melbourne by the firm of Grubb, the Cassegrainian principle was
+employed.</p>
+
+<p>With regard to the various merits of refractors and reflectors much
+might be said. Each kind of instrument has, indeed, its special
+advantages; though perhaps, on the whole, the most perfect type of
+telescope is the achromatic refractor.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Plate_IV" id="Plate_IV"></a>
+<img src="images/plate4.jpg" width="500" height="785" alt="Plate IV." title="" />
+<span class="caption"><span class="smcap">Plate IV. The Great Yerkes Telescope</span></span><br />
+<div class="caption2">Great telescope at the Yerkes Observatory of the University of Chicago,
+Williams Bay, Wisconsin, U.S.A. It was erected in 1896&ndash;7, and is the
+largest refracting telescope in the world. Diameter of object-glass, 40
+inches; length of telescope, about 60 feet. The object-glass was made by
+the firm of Alvan Clark and Sons, of Cambridge, Massachusetts; the other
+portions of the instrument by the Warner and Swasey Co., of Cleveland,
+Ohio.<br />(<a href="#Page_117"><small>Page 117</small></a>)</div>
+</div>
+
+<p><span class='pagenum'><a name="Page_119" id="Page_119">[Pg 119]</a></span></p><p>In connection with telescopes certain devices have from time to time
+been introduced, but these merely aim at the <i>convenience</i> of the
+observer and do not supplant the broad principles upon which are based
+the various types of instrument above described. Such, for instance, are
+the "Siderostat," and another form of it called the "C&#339;lostat," in
+which a plane mirror is made to revolve in a certain manner, so as to
+reflect those portions of the sky which are to be observed, into the
+tube of a telescope kept fixed. Such too are the "Equatorial Coud&eacute;" of
+the late M. Loewy, Director of the Paris Observatory, and the
+"Sheepshanks Telescope" of the Observatory of Cambridge, in which a
+telescope is separated into two portions, the eye-piece portion being
+fixed upon a downward slant, and the object-glass portion jointed to it
+at an angle and pointed up at the sky. In these two instruments (which,
+by the way, differ materially) an arrangement of slanting mirrors in the
+tubes directs the journey of the rays of light from the object-glass to
+the eye-piece. The observer can thus sit at the eye-end of his telescope
+in the warmth and comfort of his room, and observe the stars in the same
+unconstrained manner as if he were merely looking down into a
+microscope.</p>
+
+<p>Needless to say, devices such as these are subject to the drawback that
+the mirrors employed sap a certain proportion of the rays of light. It
+will be remembered that we made allusion to loss of light in this way,
+when pointing out the advantage in light grasp of the Herschelian form
+of telescope, where only <i>one</i> reflection takes place, over the
+Newtonian in which there are <i>two</i>.</p>
+
+<p>It is an interesting question as to whether telescopes can be made much
+larger. The American astronomer, Professor G.E. Hale, concludes that the
+limit of refractors is about five feet in diameter, but he thinks that
+reflectors as large as nine feet in diameter might now be made. As
+regards refractors<span class='pagenum'><a name="Page_120" id="Page_120">[Pg 120]</a></span> there are several strong reasons against augmenting
+their proportions. First of all comes the great cost. Secondly, since
+the lenses are held in position merely round their rims, they will bend
+by their weight in the centres if they are made much larger. On the
+other hand, attempts to obviate this, by making the lenses thicker,
+would cause a decrease in the amount of light let through.</p>
+
+<p>But perhaps the greatest stumbling-block to the construction of larger
+telescopes is the fact that the unsteadiness of the air will be
+increasingly magnified. And further, the larger the tubes become, the
+more difficult will it be to keep the air within them at one constant
+temperature throughout their lengths.</p>
+
+<p>It would, indeed, seem as if telescopes are not destined greatly to
+increase in size, but that the means of observation will break out in
+some new direction, as it has already done in the case of photography
+and the spectroscope. The direct use of the eye is gradually giving
+place to indirect methods. We are, in fact, now <i>feeling</i> rather than
+seeing our way about the universe. Up to the present, for instance, we
+have not the slightest proof that life exists elsewhere than upon our
+earth. But who shall say that the twentieth century has not that in
+store for us, by which the presence of life in other orbs may be
+perceived through some form of vibration transmitted across illimitable
+space? There is no use speaking of the impossible or the inconceivable.
+After the extraordinary revelations of the spectroscope&mdash;nay, after the
+astounding discovery of R&ouml;ntgen&mdash;the word impossible should be cast
+aside, and inconceivability cease to be regarded as any criterion.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_8_8" id="Footnote_8_8"></a><a href="#FNanchor_8_8"><span class="label">[8]</span></a> The principle upon which the telescope is based appears to
+have been known <i>theoretically</i> for a long time previous to this. The
+monk Roger Bacon, who lived in the thirteenth century, describes it very
+clearly; and several writers of the sixteenth century have also dealt
+with the idea. Even Lippershey's claims to a practical solution of the
+question were hotly contested at the time by two of his own countrymen,
+<i>i.e.</i> a certain Jacob Metius, and another spectacle-maker of
+Middleburgh, named Jansen.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_121" id="Page_121">[Pg 121]</a></span></p>
+<h3><a name="CHAPTER_XI" id="CHAPTER_XI"></a>CHAPTER XI</h3>
+
+<h4>SPECTRUM ANALYSIS</h4>
+
+
+<p class="noin"><span class="smcap">If</span> white light (that of the sun, for instance) be passed through a glass
+prism, namely, a piece of glass of triangular shape, it will issue from
+it in rainbow-tinted colours. It is a common experience with any of us
+to notice this when the sunlight shines through cut-glass, as in the
+pendant of a chandelier, or in the stopper of a wine-decanter.</p>
+
+<p>The same effect may be produced when light passes through water. The
+Rainbow, which we all know so well, is merely the result of the sunlight
+passing through drops of falling rain.</p>
+
+<p>White light is composed of rays of various colours. Red, orange, yellow,
+green, blue, indigo, and violet, taken all together, go, in fact, to
+make up that effect which we call white.</p>
+
+<p>It is in the course of the <i>refraction</i>, or bending of a beam of light,
+when it passes in certain conditions through a transparent and denser
+medium, such as glass or water, that the constituent rays are sorted out
+and spread in a row according to their various colours. This production
+of colour takes place usually near the edges of a lens; and, as will be
+recollected, proved very obnoxious to the users of the old form of
+refracting telescope.</p>
+
+<p>It is, indeed, a strange irony of fate that this very<span class='pagenum'><a name="Page_122" id="Page_122">[Pg 122]</a></span> same production
+of colour, which so hindered astronomy in the past, should have aided it
+in recent years to a remarkable degree. If sunlight, for instance, be
+admitted through a narrow slit before it falls upon a glass prism, it
+will issue from the latter in the form of a band of variegated colour,
+each colour blending insensibly with the next. The colours arrange
+themselves always in the order which we have mentioned. This seeming
+band is, in reality, an array of countless coloured images of the
+original slit ranged side by side; the colour of each image being the
+slightest possible shade different from that next to it. This strip of
+colour when produced by sunlight is called the "Solar Spectrum" (<a href="#Fig_9">see
+Fig. 9</a>, p. 123). A similar strip, or <i>spectrum</i>, will be produced by any
+other light; but the appearance of the strip, with regard to
+preponderance of particular colours, will depend upon the character of
+that light. Electric light and gas light yield spectra not unlike that
+of sunlight; but that of gas is less rich in blue and violet than that
+of the sun.</p>
+
+<p>The Spectroscope, an instrument devised for the examination of spectra,
+is, in its simplest form, composed of a small tube with a narrow slit
+and prism at one end, and an eye-piece at the other. If we drop ordinary
+table salt into the flame of a gas light, the flame becomes strongly
+yellow. If, then, we observe this yellow flame with the spectroscope, we
+find that its spectrum consists almost entirely of two bright yellow
+transverse lines. Chemically considered ordinary table salt is sodium
+chloride; that is to say, a compound of the metal sodium and the gas
+chlorine. Now if other compounds of sodium be experimented with in the
+same manner, it will soon be found that these two yellow lines are
+characteristic of sodium when turned into vapour by great heat. In the
+same manner it can be ascertained that every element, when heated to a
+condition of vapour, gives as its spectrum a set of lines peculiar to
+itself. Thus the spectroscope enables us to find out the composition of
+substances when they are reduced to vapour in the laboratory.</p>
+
+<p><span class='pagenum'><a name="Page_123" id="Page_123">[Pg 123]</a></span></p>
+<div class="figcenter" style="width: 500px;"><a name="Fig_9" id="Fig_9"></a>
+<img src="images/figure9.jpg" width="500" height="109" alt="Fig. 9." title="" />
+<span class="caption"><span class="smcap">Fig. 9.</span>&mdash;The Solar Spectrum.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_124" id="Page_124">[Pg 124]</a></span></p><p>In order to increase the power of a spectroscope, it is necessary to
+add to the number of prisms. Each extra prism has the effect of
+lengthening the coloured strip still more, so that lines, which at first
+appeared to be single merely through being crowded together, are
+eventually drawn apart and become separately distinguishable.</p>
+
+<p>On this principle it has gradually been determined that the sun is
+composed of elements similar to those which go to make up our earth.
+Further, the composition of the stars can be ascertained in the same
+manner; and we find them formed on a like pattern, though with certain
+elements in greater or less proportion as the case may be. It is in
+consequence of our thus definitely ascertaining that the stars are
+self-luminous, and of a sun-like character, that we are enabled to speak
+of them as <i>suns</i>, or to call the sun a <i>star</i>.</p>
+
+<p>In endeavouring to discover the elements of which the planets and
+satellites of our system are composed, we, however, find ourselves
+baffled, for the simple reason that these bodies emit no real light of
+their own. The light which reaches us from them, being merely reflected
+sunlight, gives only the ordinary<span class='pagenum'><a name="Page_125" id="Page_125">[Pg 125]</a></span> solar spectrum when examined with the
+spectroscope. But in certain cases we find that the solar spectrum thus
+viewed shows traces of being weakened, or rather of suffering
+absorption; and it is concluded that this may be due to the sunlight
+having had to pass through an atmosphere on its way to and from the
+surface of the planet from which it is reflected to us.</p>
+
+<p>Since the sun is found to be composed of elements similar to those which
+go to make up our earth, we need not be disheartened at this failure of
+the spectroscope to inform us of the composition of the planets and
+satellites. We are justified, indeed, in assuming that more or less the
+same constituents run through our solar system; and that the elements of
+which these bodies are composed are similar to those which are found
+upon our earth and in the sun.</p>
+
+<p>The spectroscope supplies us with even more information. It tells us,
+indeed, whether the sun-like body which we are observing is moving away
+from us or towards us. A certain slight shifting of the lines towards
+the red or violet end of the spectrum respectively, is found to follow
+such movement. This method of observation is known by the name of
+<i>Doppler's Method</i>,<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a> and by it we are enabled to confirm the evidence
+which the sunspots give us of the rotation of the sun; for we find thus
+that one edge<span class='pagenum'><a name="Page_126" id="Page_126">[Pg 126]</a></span> of that body is continually approaching us, and the other
+edge is continually receding from us. Also, we can ascertain in the same
+manner that certain of the stars are moving towards us, and certain of
+them away from us.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_9_9" id="Footnote_9_9"></a><a href="#FNanchor_9_9"><span class="label">[9]</span></a> The idea, initiated by Christian Doppler at Prague in 1842,
+was originally applied to sound. The approach or recession of a source
+from which sound is coming is invariably accompanied by alterations of
+pitch, as the reader has no doubt noticed when a whistling
+railway-engine has approached him or receded from him. It is to Sir
+William Huggins, however, that we are indebted for the application of
+the principle to spectroscopy. This he gave experimental proof of in the
+year 1868.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_127" id="Page_127">[Pg 127]</a></span></p>
+<h3><a name="CHAPTER_XII" id="CHAPTER_XII"></a>CHAPTER XII</h3>
+
+<h4>THE SUN</h4>
+
+
+<p class="noin"><span class="smcap">The</span> sun is the chief member of our system. It controls the motions of
+the planets by its immense gravitative power. Besides this it is the
+most important body in the entire universe, so far as we are concerned;
+for it pours out continually that flood of light and heat, without which
+life, as we know it, would quickly become extinct upon our globe.</p>
+
+<p>Light and heat, though not precisely the same thing, may be regarded,
+however, as next-door neighbours. The light rays are those which
+directly affect the eye and are comprised in the visible spectrum. We
+<i>feel</i> the heat rays, the chief of which are beyond the red portion of
+the spectrum. They may be investigated with the <i>bolometer</i>, an
+instrument invented by the late Professor Langley. Chemical rays&mdash;for
+instance, those radiations which affect the photographic plate&mdash;are for
+the most part also outside the visible spectrum. They are, however, at
+the other end of it, namely, beyond the violet.</p>
+
+<p>Such a scale of radiations may be compared to the keyboard of an
+imaginary piano, the sound from only one of whose octaves is audible to
+us.</p>
+
+<p>The brightest light we know on the earth is dull compared with the light
+of the sun. It would, indeed, look quite dark if held up against it.</p>
+
+<p><span class='pagenum'><a name="Page_128" id="Page_128">[Pg 128]</a></span></p><p>It is extremely difficult to arrive at a precise notion of the
+temperature of the body of the sun. However, it is far in excess of any
+temperature which we can obtain here, even in the most powerful electric
+furnace.</p>
+
+<p>A rough idea of the solar heat may be gathered from the calculation that
+if the sun's surface were coated all over with a layer of ice 4000 feet
+thick, it would melt through this completely in one hour.</p>
+
+<p>The sun cannot be a hot body merely cooling; for the rate at which it is
+at present giving off heat could not in such circumstances be kept up,
+according to Professor Moulton, for more than 3000 years. Further, it is
+not a mere burning mass, like a coal fire, for instance; as in that case
+about a thousand years would show a certain drop in temperature. No
+perceptible diminution of solar heat having taken place within historic
+experience, so far as can be ascertained, we are driven to seek some
+more abstruse explanation.</p>
+
+<p>The theory which seems to have received most acceptance is that put
+forward by Helmholtz in 1854. His idea was that gravitation produces
+continual contraction, or falling in of the outer parts of the sun; and
+that this falling in, in its turn, generates enough heat to compensate
+for what is being given off. The calculations of Helmholtz showed that a
+contraction of about 100 feet a year from the surface towards the centre
+would suffice for the purpose. In recent years, however, this estimate
+has been extended to about 180 feet. Nevertheless, even with this
+increased figure, the shrinkage required is so slight in comparison with
+the immense girth of the sun, that it<span class='pagenum'><a name="Page_129" id="Page_129">[Pg 129]</a></span> would take a continual
+contraction at this rate for about 6000 years, to show even in our
+finest telescopes that any change in the size of that body was taking
+place at all. Upon this assumption of continuous contraction, a time
+should, however, eventually be reached when the sun will have shrunk to
+such a degree of solidity, that it will not be able to shrink any
+further. Then, the loss of heat not being made up for any longer, the
+body of the sun should begin to grow cold. But we need not be distressed
+on this account; for it will take some 10,000,000 years, according to
+the above theory, before the solar orb becomes too cold to support life
+upon our earth.</p>
+
+<p>Since the discovery of radium it has, on the other hand, been suggested,
+and not unreasonably, that radio-active matter may possibly play an
+important part in keeping up the heat of the sun. But the body of
+scientific opinion appears to consider the theory of contraction as a
+result of gravitation, which has been outlined above, to be of itself
+quite a sound explanation. Indeed, the late Lord Kelvin is said to have
+held to the last that it was amply sufficient to account for the
+underground heat of the earth, the heat of the sun, and that of all the
+stars in the universe.</p>
+
+<p>One great difficulty in forming theories with regard to the sun, is the
+fact that the temperature and gravitation there are enormously in excess
+of anything we meet with upon our earth. The force of gravity at the
+sun's surface is, indeed, about twenty-seven times that at the surface
+of our globe.</p>
+
+<p>The earth's atmosphere appears to absorb about one-half of the
+radiations which come to us from the sun. This absorptive effect is very
+noticeable when<span class='pagenum'><a name="Page_130" id="Page_130">[Pg 130]</a></span> the solar orb is low down in our sky, for its light and
+heat are then clearly much reduced. Of the light rays, the blue ones are
+the most easily absorbed in this way; which explains why the sun looks
+red when near the horizon. It has then, of course, to shine through a
+much greater thickness of atmosphere than when high up in the heavens.</p>
+
+<p>What astonishes one most about the solar radiation, is the immense
+amount of it that is apparently wasted into space in comparison with
+what falls directly upon the bodies of the solar system. Only about the
+one-hundred-millionth is caught by all the planets together. What
+becomes of the rest we cannot tell.</p>
+
+<p>That brilliant white body of the sun, which we see, is enveloped by
+several layers of gases and vaporous matter, in the same manner as our
+globe is enveloped by its atmosphere (<a href="#Fig_10">see Fig. 10</a>, p. 131). These are
+transparent, just as our atmosphere is transparent; and so we see the
+white bright body of the sun right through them.</p>
+
+<p>This white bright portion is called the <i>Photosphere</i>. From it comes
+most of that light and heat which we see and feel. We do not know what
+lies under the photosphere, but, no doubt, the more solid portions of
+the sun are there situated. Just above the photosphere, and lying close
+upon it, is a veil of smoke-like haze.</p>
+
+<p>Next upon this is what is known as the <i>Reversing Layer</i>, which is
+between 500 and 1000 miles in thickness. It is cooler than the
+underlying photosphere, and is composed of glowing gases. Many of the
+elements which go to make up our earth are present in the reversing
+layer in the form of vapour.</p>
+
+<p>The <i>Chromosphere</i>, of which especial mention has<span class='pagenum'><a name="Page_131" id="Page_131">[Pg 131]</a></span> already been made in
+dealing with eclipses of the sun, is another layer lying immediately
+upon the last one. It is between 5000 and 10,000 miles in thickness.
+Like the reversing layer, it is composed of glowing gases, chief among
+which is the vapour of hydrogen. The colour of the chromosphere is, in
+reality, a brilliant scarlet; but, as we have already said, the
+intensely white light of the photosphere shines through it from behind,
+and entirely overpowers its redness. The upper portion of the
+chromosphere is in violent agitation, like the waves of a stormy sea,
+and from it rise those red prominences which, it will be recollected,
+are such a notable feature in total solar eclipses.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_10" id="Fig_10"></a>
+<img src="images/figure10.jpg" width="500" height="367" alt="Fig. 10." title="" />
+<span class="caption"><span class="smcap">Fig. 10.</span>&mdash;A section through the Sun, showing how the
+prominences rise from the chromosphere.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_132" id="Page_132">[Pg 132]</a></span></p><p>The <i>Corona</i> lies next in order outside the chromosphere, and is, so
+far as we know, the outermost of the accompaniments of the sun. This
+halo of pearly-white light is irregular in outline, and fades away into
+the surrounding sky. It extends outwards from the sun to several
+millions of miles. As has been stated, we can never see the corona
+unless, when during a total solar eclipse, the moon has, for the time
+being, hidden the brilliant photosphere completely from our view.</p>
+
+<p>The solar spectrum is really composed of three separate spectra
+commingled, <i>i.e.</i> those of the photosphere, of the reversing layer, and
+of the chromosphere respectively.</p>
+
+<p>If, therefore, the photosphere could be entirely removed, or covered up,
+we should see only the spectra of those layers which lie upon it. Such a
+state of things actually occurs in a total eclipse of the sun. When the
+moon's body has crept across the solar disc, and hidden the last piece
+of photosphere, the solar spectrum suddenly becomes what is technically
+called "reversed,"&mdash;the dark lines crossing it changing into bright
+lines. This occurs because a strip of those layers which lie immediately
+upon the photosphere remains still uncovered. The lower of these layers
+has therefore been called the "reversing layer," for want of a better
+name. After a second or two this reversed spectrum mostly vanishes, and
+an altered spectrum is left to view. Taking into consideration the rate
+at which the moon is moving across the face of the sun, and the very
+short time during which the spectrum of the reversing layer lasts, the
+thickness of that layer is estimated to be<span class='pagenum'><a name="Page_133" id="Page_133">[Pg 133]</a></span> not more than a few hundred
+miles. In the same way the last of the three spectra&mdash;namely, that of
+the chromosphere&mdash;remains visible for such a time as allows us to
+estimate its depth at about ten times that of the reversing layer, or
+several thousand miles.</p>
+
+<p>When the chromosphere, in its turn during a total eclipse, has been
+covered by the moon, the corona alone is left. This has a distinct
+spectrum of its own also; wherein is seen a strange line in the green
+portion, which does not tally with that of any element we are acquainted
+with upon the earth. This unknown element has received for the time
+being the name of "Coronium."</p>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_134" id="Page_134">[Pg 134]</a></span></p>
+<h3><a name="CHAPTER_XIII" id="CHAPTER_XIII"></a>CHAPTER XIII</h3>
+
+<h4>THE SUN&mdash;<i>continued</i></h4>
+
+
+<p class="noin"><span class="smcap">The</span> various parts of the Sun will now be treated of in detail.</p>
+
+
+<p class="center"><br /><span class="smcap">I. Photosphere.</span></p>
+
+<p>The photosphere, or "light-sphere," from the Greek &#966;&#8182;&#962;
+(<i>phos</i>), which means <i>light</i>, is, as we have already said, the
+innermost portion of the sun which can be seen. Examined through a good
+telescope it shows a finely mottled structure, as of brilliant granules,
+somewhat like rice grains, with small dark spaces lying in between them.
+It has been supposed that we have here the process of some system of
+circulation by which the sun keeps sending forth its radiations. In the
+bright granules we perhaps see masses of intensely heated matter, rising
+from the interior of the sun. The dark interspaces may represent matter
+which has become cooled and darkened through having parted with its heat
+and light, and is falling back again into the solar furnace.</p>
+
+<p>The <i>sun spots</i>, so familiar to every one nowadays, are dark patches
+which are often seen to break out in the photosphere (<a href="#Plate_V">see Plate V.</a>, p.
+134). They last during various periods of time; sometimes only for a few
+days, sometimes so long as a month or more. A spot is usually composed
+of a dark central portion called the <i>umbra</i>, and a less dark fringe
+around this called the <i>penumbra</i> (<a href="#Plate_VI">see Plate VI.</a>, p. 136). The umbra
+ordinarily has the appearance of a deep hole in the photosphere; but,
+that it is a hole at all, has by no means been definitely proved.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Plate_V" id="Plate_V"></a>
+<img src="images/plate5.jpg" width="500" height="501" alt="Plate V." title="" />
+<span class="caption"><span class="smcap">Plate V. The Sun, showing several groups of Spots</span></span>
+<div class="caption2">From a photograph taken at the Royal Observatory, Greenwich. The
+cross-lines seen on the disc are in no way connected with the Sun, but
+belong to the telescope through which the photograph was taken.<br />(<a href="#Page_134"><small>Page 134</small></a>)</div>
+</div>
+
+<p><span class='pagenum'><a name="Page_135" id="Page_135">[Pg 135]</a></span></p><p>Sun spots are, as a rule, some thousands of miles across. The umbra of
+a good-sized spot could indeed engulf at once many bodies the size of
+our earth.</p>
+
+<p>Sun spots do not usually appear singly, but in groups. The total area of
+a group of this kind may be of immense extent; even so great as to cover
+the one-hundredth part of the whole surface of the sun. Very large
+spots, when such are present, may be seen without any telescope; either
+through a piece of smoked glass, or merely with the naked eye when the
+air is misty, or the sun low on the horizon.</p>
+
+<p>The umbra of a spot is not actually dark. It only appears so in contrast
+with the brilliant photosphere around.</p>
+
+<p>Spots form, grow to a large size in comparatively short periods of time,
+and then quickly disappear. They seem to shrink away as a consequence of
+the photosphere closing in upon them.</p>
+
+<p>That the sun is rotating upon an axis, is shown by the continual change
+of position of all spots in one constant direction across his disc. The
+time in which a spot is carried completely round depends, however, upon
+the position which it occupies upon the sun's surface. A spot situated
+near the equator of the sun goes round once in about twenty-five days.
+The further a spot is situated from this equator, the longer it takes.
+About twenty-seven days is the time taken by a spot situated midway
+between the equator and<span class='pagenum'><a name="Page_136" id="Page_136">[Pg 136]</a></span> the solar poles. Spots occur to the north of
+the sun's equator, as well as to the south; though, since regular
+observations have been made&mdash;that is to say, during the past fifty years
+or so&mdash;they appear to have broken out a little more frequently in the
+southern parts.</p>
+
+<p>From these considerations it will be seen that the sun does not rotate
+as the earth does, but that different portions appear to move at
+different speeds. Whether in the neighbourhood of the solar poles the
+time of rotation exceeds twenty-seven days we are unable to ascertain,
+for spots are not seen in those regions. No explanation has yet been
+given of this peculiar rotation; and the most we can say on the subject
+is that the sun is not by any means a solid body.</p>
+
+<p><i>Facul&aelig;</i> (Latin, little torches) are brilliant patches which appear here
+and there upon the sun's surface, and are in some way associated with
+spots. Their displacement, too, across the solar face confirms the
+evidence which the spots give us of the sun's rotation.</p>
+
+<p>Our proofs of this rotation are still further strengthened by the
+Doppler spectroscopic method of observation alluded to in <a href="#CHAPTER_XI">Chapter XI</a>. As
+was then stated, one edge of the sun is thus found to be continually
+approaching us, and the other side continually receding from us. The
+varying rates of rotation, which the spots and facul&aelig; give us, are duly
+confirmed by this method.</p>
+
+<div class="figcenter" style="width: 600px;"><a name="Plate_VI" id="Plate_VI"></a>
+<img src="images/plate6.jpg" width="500" height="765" alt="Plate VI." title="" /><br />
+<span class="caption"><span class="smcap">Plate VI. Photograph of a Sunspot</span><br />
+This fine picture was taken by the late M. Janssen. The granular
+structure of the Sun's surface is here well represented. (From
+<i>Knowledge</i>.)<br />(<a href="#Page_135"><small>Page 135</small></a>)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_137" id="Page_137">[Pg 137]</a></span></p><p>The first attempt to bring some regularity into the question of
+sunspots was the discovery by Schwabe, in 1852, that they were subject
+to a regular variation. As a matter of fact they wax and wane in their
+number, and the total area which they cover, in the course of a period,
+or cycle, of on an average about 11&frac14; years; being at one part of this
+period large and abundant, and at another few and small. This period of
+11&frac14; years is known as the sun spot cycle. No explanation has yet been
+given of the curious round of change, but the period in question seems
+to govern most of the phenomena connected with the sun.</p>
+
+
+<p class="center"><br /><span class="smcap">II. Reversing Layer.</span></p>
+
+<p>This is a layer of relatively cool gases lying immediately upon the
+photosphere. We never see it directly; and the only proof we have of its
+presence is that remarkable reversal of the spectrum already described,
+when during an instant or two in a total eclipse, the advancing edge of
+the moon, having just hidden the brilliant photosphere, is moving across
+the fine strip which the layer then presents edgewise towards us. The
+fleeting moments during which this reversed spectrum lasts, informs us
+that the layer is comparatively shallow; little more indeed than about
+500 miles in depth.</p>
+
+<p>The spectrum of the reversing layer, or "flash spectrum," as it is
+sometimes called on account of the instantaneous character with which
+the change takes place, was, as we have seen, first noticed by Young in
+1870; and has been successfully photographed since then during several
+eclipses. The layer itself appears to be in a fairly quiescent state; a
+marked contrast to the seething photosphere beneath, and the agitated
+chromosphere above.</p>
+
+
+<p><span class='pagenum'><a name="Page_138" id="Page_138">[Pg 138]</a></span></p>
+<p class="center"><br /><span class="smcap">III. The Chromosphere.</span></p>
+
+<p>The Chromosphere&mdash;so called from the Greek &#967;&#961;&#8182;&#956;&#945; (<i>chroma</i>),
+which signifies <i>colour</i>&mdash;is a layer of gases lying immediately upon the
+preceding one. Its thickness is, however, plainly much the greater of
+the two; for whereas the reversing layer is only revealed to us
+<i>indirectly</i> by the spectroscope, a portion of the chromosphere may
+clearly be <i>seen</i> in a total eclipse in the form of a strip of scarlet
+light. The time which the moon's edge takes to traverse it tells us that
+it must be about ten times as deep as the reversing layer, namely, from
+5000 to 10,000 miles in depth. Its spectrum shows that it is composed
+chiefly of hydrogen, calcium and helium, in the state of vapour. Its red
+colour is mainly due to glowing hydrogen. The element helium, which it
+also contains, has received its appellation from &#7973;&#955;&#953;&#959;&#962;
+(<i>helios</i>), the Greek name for the sun; because, at the time when it
+first attracted attention, there appeared to be no element corresponding
+to it upon our earth, and it was consequently imagined to be confined to
+the sun alone. Sir William Ramsay, however, discovered it to be also a
+terrestrial element in 1895, and since then it has come into much
+prominence as one of the products given off by radium.</p>
+
+<p>Taking into consideration the excessive force of gravity on the sun, one
+would expect to find the chromosphere and reversing layer growing
+gradually thicker in the direction of the photosphere. This, however, is
+not the case. Both these layers are strangely enough of the same
+densities all through;<span class='pagenum'><a name="Page_139" id="Page_139">[Pg 139]</a></span> which makes it suspected that, in these regions,
+the force of gravity may be counteracted by some other force or forces,
+exerting a powerful pressure outwards from the sun.</p>
+
+
+<p class="center"><br /><span class="smcap">IV. The Prominences.</span></p>
+
+<p>We have already seen, in dealing with total eclipses, that the exterior
+surface of the chromosphere is agitated like a stormy sea, and from it
+billows of flame are tossed up to gigantic heights. These flaming jets
+are known under the name of prominences, because they were first noticed
+in the form of brilliant points projecting from behind the rim of the
+moon when the sun was totally eclipsed. Prominences are of two kinds,
+<i>eruptive</i> and <i>quiescent</i>. The eruptive prominences spurt up directly
+from the chromosphere with immense speeds, and change their shape with
+great rapidity. Quiescent prominences, on the other hand, have a form
+somewhat like trees, and alter their shape but slowly. In the eruptive
+prominences glowing masses of gas are shot up to altitudes sometimes as
+high as 300,000 miles,<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a> with velocities even so great as from 500 to
+600 miles a second. It has been noticed that the eruptive prominences
+are mostly found in those portions of the sun where spots usually
+appear, namely, in the regions near the solar equator. The quiescent
+prominences, on the other hand, are confined, as a rule, to the
+neighbourhood of the sun's poles.</p>
+
+<p><span class='pagenum'><a name="Page_140" id="Page_140">[Pg 140]</a></span></p><p>Prominences were at first never visible except during total eclipses of
+the sun. But in the year 1868, as we have already seen, a method of
+employing the spectroscope was devised, by means of which they could be
+observed and studied at any time, without the necessity of waiting for
+an eclipse.</p>
+
+<p>A still further development of the spectroscope, the
+<i>Spectroheliograph</i>, an instrument invented almost simultaneously by
+Professor Hale and the French astronomer, M. Deslandres, permits of
+photographs being taken of the sun, with the light emanating from <i>only
+one</i> of its glowing gases at a time. For instance, we can thus obtain a
+record of what the glowing hydrogen alone is doing on the solar body at
+any particular moment. With this instrument it is also possible to
+obtain a series of photographs, showing what is taking place upon the
+sun at various levels. This is very useful in connection with the study
+of the spots; for we are, in consequence, enabled to gather more
+evidence on the subject of their actual form than is given us by their
+highly foreshortened appearances when observed directly in the
+telescope.</p>
+
+
+<p class="center"><br /><span class="smcap">V. Corona.</span> (Latin, <i>a Crown</i>.)</p>
+
+<p>This marvellous halo of pearly-white light, which displays itself to our
+view only during the total phase of an eclipse of the sun, is by no
+means a layer like those other envelopments of the sun of which we have
+just been treating. It appears, on the other hand, to be composed of
+filmy matter, radiating outwards in every direction, and fading<span class='pagenum'><a name="Page_141" id="Page_141">[Pg 141]</a></span> away
+gradually into space. Its structure is noted to bear a strong
+resemblance to the tails of comets, or the streamers of the aurora
+borealis.</p>
+
+<p>Our knowledge concerning the corona has, however, advanced very slowly.
+We have not, so far, been as fortunate with regard to it as with regard
+to the prominences; and, for all we can gather concerning it, we are
+still entirely dependent upon the changes and chances of total solar
+eclipses. All attempts, in fact, to apply the spectroscopic method, so
+as to observe the corona at leisure in full sunlight in the way in which
+the prominences can be observed, have up to the present met with
+failure.</p>
+
+<p>The general form under which the corona appears to our eyes varies
+markedly at different eclipses. Sometimes its streamers are many, and
+radiate all round; at other times they are confined only to the middle
+portions of the sun, and are very elongated, with short feathery-looking
+wisps adorning the solar poles. It is noticed that this change of shape
+varies in close accordance with that 11&frac14; year period during which the
+sun spots wax and wane; the many-streamered regular type corresponding
+to the time of great sunspot activity, while the irregular type with the
+long streamers is present only when the spots are few (<a href="#Plate_VII">see Plate VII.</a>,
+p. 142). Streamers have often been noted to issue from those regions of
+the sun where active prominences are at the moment in existence; but it
+cannot be laid down that this is always the case.</p>
+
+<p>No hypothesis has yet been formulated which will account for the
+structure of the corona, or for its variation in shape. The great
+difficulty with regard<span class='pagenum'><a name="Page_142" id="Page_142">[Pg 142]</a></span> to theorising upon this subject, is the fact
+that we see so much of the corona under conditions of marked
+foreshortening. Assuming, what indeed seems natural, that the rays of
+which it is composed issue in every direction from the solar body, in a
+manner which may be roughly imitated by sticking pins all over a ball;
+it is plainly impossible to form any definite idea concerning streamers,
+which actually may owe most of the shape they present to us, to the
+mixing up of multitudes of rays at all kinds of angles to the line of
+sight. In a word, we have to try and form an opinion concerning an
+arrangement which, broadly speaking, is <i>spherical</i>, but which, on
+account of its distance, must needs appear to us as absolutely <i>flat</i>.</p>
+
+<p>The most known about the composition of the corona is that it is made up
+of particles of matter, mingled with a glowing gas. It is an element in
+the composition of this gas which, as has been stated, is not found to
+tally with any known terrestrial element, and has, therefore, received
+the name of coronium for want of a better designation.</p>
+
+<p>One definite conclusion appears to be reached with regard to the corona,
+<i>i.e.</i> that the matter of which it is composed, must be exceedingly
+rarefied; as it is not found, for instance, to retard appreciably the
+speed of comets, on occasions when these bodies pass very close to the
+sun. A calculation has indeed been made which would tend to show that
+the particles composing the coronal matter, are separated from each
+other by a distance of perhaps between two and three yards! The density
+of the corona is found not to increase inwards towards the sun. This is
+what has already been noted with regard to the layers lying beneath it.
+Powerful forces, acting in opposition to gravity, must hold sway here
+also.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Plate_VII" id="Plate_VII"></a>
+<img src="images/plate7a.jpg" width="500" height="427" alt="(A.) The Total Eclipse of the Sun of December 22nd, 1870" title="" />
+<span class="caption"><span class="smcap">(A.) The Total Eclipse of the Sun of December 22nd, 1870</span></span>
+<div class="caption2">Drawn by Mr. W.H. Wesley from a photograph taken at Syracuse by Mr.
+Brothers. This is the type of corona seen at the time of <i>greatest</i>
+sunspot activity. The coronas of 1882 (<a href="#Plate_I">Plate I.</a>, p. 96) and of 1905
+(<a href="#Frontispiece">Frontispiece</a>) are of the same type.</div>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/plate7b.jpg" width="500" height="310" alt="(B.) The Total Eclipse of the Sun of May 28th, 1900" title="" />
+<span class="caption"><span class="smcap">(B.) The Total Eclipse of the Sun of May 28th, 1900</span></span>
+<div class="caption2">Drawn by Mr. W.H. Wesley from photographs taken by Mr. E.W. Maunder.
+This is the type of corona seen when the sunspots are <i>least</i> active.
+Compare the "Ring with Wings," <a href="#Fig_7">Fig. 7</a>, p. 87.<br /></div>
+<span class="caption"><br /><span class="smcap">Plate VII. Forms of the Solar Corona at the Epochs Of Sunspot Maximum
+and Sunspot Minimum, respectively</span><br />(<a href="#Page_141"><small>Page 141</small></a>)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_143" id="Page_143">[Pg 143]</a></span></p><p>The 11&frac14; year period, during which the sun spots vary in number and
+size, appears to govern the activities of the sun much in the same way
+that our year does the changing seasonal conditions of our earth. Not
+only, as we have seen, does the corona vary its shape in accordance with
+the said period, but the activity of the prominences, and of the facul&aelig;,
+follow suit. Further, this constant round of ebb and flow is not
+confined to the sun itself, but, strangely enough, affects the earth
+also. The displays of the aurora borealis, which we experience here,
+coincide closely with it, as does also the varying state of the earth's
+magnetism. The connection may be still better appreciated when a great
+spot, or group of spots, has made its appearance upon the sun. It has,
+for example, often been noted that when the solar rotation carries a
+spot, or group of spots, across the middle of the visible surface of the
+sun, our magnetic and electrical arrangements are disturbed for the time
+being. The magnetic needles in our observatories are, for instance, seen
+to oscillate violently, telegraphic communication is for a while upset,
+and magnificent displays of the aurora borealis illumine our night
+skies. Mr. E.W. Maunder, of Greenwich Observatory, who has made a very
+careful investigation of this subject, suspects that, when elongated
+coronal streamers are whirled round in our direction by the solar
+rotation, powerful magnetic impulses may be projected upon us at the
+moments when such streamers are pointing towards the earth.</p>
+
+<p><span class='pagenum'><a name="Page_144" id="Page_144">[Pg 144]</a></span></p><p>Some interesting investigations with regard to sunspots have recently
+been published by Mrs. E.W. Maunder. In an able paper, communicated to
+the Royal Astronomical Society on May 10, 1907, she reviews the
+Greenwich Observatory statistics dealing with the number and extent of
+the spots which have appeared during the period from 1889 to 1901&mdash;a
+whole sunspot cycle. From a detailed study of the dates in question, she
+finds that the number of those spots which are formed on the side of the
+sun turned away from us, and die out upon the side turned towards us, is
+much greater than the number of those which are formed on the side
+turned towards us and die out upon the side turned away. It used, for
+instance, to be considered that the influence of a planet might
+<i>produce</i> sunspots; but these investigations make it look rather as if
+some influence on the part of the earth tends, on the contrary, to
+<i>extinguish</i> them. Mrs. Maunder, so far, prefers to call the influence
+thus traced an <i>apparent</i> influence only, for, as she very fairly points
+out, it seems difficult to attribute a real influence in this matter to
+the earth, which is so small a thing in comparison not only with the
+sun, but even with many individual spots.</p>
+
+<p>The above investigation was to a certain degree anticipated by Mr. Henry
+Corder in 1895; but Mrs. Maunder's researches cover a much longer
+period, and the conclusions deduced are of a wider and more defined
+nature.</p>
+
+<p>With regard to its chemical composition, the spectroscope shows us that
+thirty-nine of the elements which are found upon our earth are also to
+be found in the sun. Of these the best known are hydrogen,<span class='pagenum'><a name="Page_145" id="Page_145">[Pg 145]</a></span> oxygen,
+helium, carbon, calcium, aluminium, iron, copper, zinc, silver, tin, and
+lead. Some elements of the metallic order have, however, not been found
+there, as, for instance, gold and mercury; while a few of the other
+class of element, such as nitrogen, chlorine, and sulphur, are also
+absent. It must not, indeed, be concluded that the elements apparently
+missing do not exist at all in the solar body. Gold and mercury have, in
+consequence of their great atomic weight, perhaps sunk away into the
+centre. Again, the fact that we cannot find traces of certain other
+elements, is no real proof of their entire absence. Some of them may,
+for instance, be resolved into even simpler forms, under the unusual
+conditions which exist in the sun; and so we are unable to trace them
+with the spectroscope, the experience of which rests on laboratory
+experiments conducted, at best, in conditions which obtain upon the
+earth.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_10_10" id="Footnote_10_10"></a><a href="#FNanchor_10_10"><span class="label">[10]</span></a> On November 15, 1907, Dr. A. Rambaut, Radcliffe Observer
+at Oxford University, noted a prominence which rose to a height of
+324,600 miles.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_146" id="Page_146">[Pg 146]</a></span></p>
+<h3><a name="CHAPTER_XIV" id="CHAPTER_XIV"></a>CHAPTER XIV</h3>
+
+<h4>THE INFERIOR PLANETS</h4>
+
+
+<p class="noin"><span class="smcap">Starting</span> from the centre of the solar system, the first body we meet
+with is the planet Mercury. It circulates at an average distance from
+the sun of about thirty-six millions of miles. The next body to it is
+the planet Venus, at about sixty-seven millions of miles, namely, about
+double the distance of Mercury from the sun. Since our earth comes next
+again, astronomers call those planets which circulate within its orbit,
+<i>i.e.</i> Mercury and Venus, the Inferior Planets, while those which
+circulate outside it they call the Superior Planets.<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a></p>
+
+<p>In studying the inferior planets, the circumstances in which we make our
+observations are so very similar with regard to each, that it is best to
+take them together. Let us begin by considering the various positions of
+an inferior planet, as seen from the earth, during the course of its
+journeys round the sun. When furthest from us it is at the other side of
+the sun, and cannot then be seen owing to the blaze of light. As it
+continues its journey it passes to the left of the sun, and is then
+sufficiently away from the glare to be plainly seen. It next draws in
+again towards<span class='pagenum'><a name="Page_147" id="Page_147">[Pg 147]</a></span> the sun, and is once more lost to view in the blaze at
+the time of its passing nearest to us. Then it gradually comes out to
+view on the right hand, separates from the sun up to a certain distance
+as before, and again recedes beyond the sun, and is for the time being
+once more lost to view.</p>
+
+<p>To these various positions technical names are given. When the inferior
+planet is on the far side of the sun from us, it is said to be in
+<i>Superior Conjunction</i>. When it has drawn as far as it can to the left
+hand, and is then as east as possible of the sun, it is said to be at
+its <i>Greatest Eastern Elongation</i>. Again, when it is passing nearest to
+us, it is said to be in <i>Inferior Conjunction</i>; and, finally, when it
+has drawn as far as it can to the right hand, it is spoken of as being
+at its <i>Greatest Western Elongation</i> (<a href="#Fig_11">see Fig. 11</a>, p. 148).</p>
+
+<p>The continual variation in the distance of an interior planet from us,
+during its revolution around the sun, will of course be productive of
+great alterations in its apparent size. At superior conjunction it
+ought, being then farthest away, to show the smallest disc; while at
+inferior conjunction, being the nearest, it should look much larger.
+When at greatest elongation, whether eastern or western, it should
+naturally present an appearance midway in size between the two.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_11" id="Fig_11"></a><span class="caption">Various positions, and illumination by the Sun, of an
+Inferior Planet in the course of its orbit.</span>
+<img src="images/figure11.jpg" width="500" height="470" alt="Fig. 11." title="" />
+<div class="caption1">Corresponding views of the same situations of an Inferior Planet as seen
+from the Earth, showing consequent phases and alterations in apparent
+size.</div>
+<span class="caption"><span class="smcap">Fig. 11.</span>&mdash;Orbit and Phases of an Inferior Planet.</span>
+</div>
+
+<p>From the above considerations one would be inclined to assume that the
+best time for studying the surface of an interior planet with the
+telescope is when it is at inferior conjunction, or, nearest to us. But
+that this is not the case will at once appear if we consider that the
+sunlight is then falling upon the side away from us, leaving the side
+which is towards us unillumined. In superior conjunction, on the other
+hand, the light falls full upon the side of the planet facing us; but
+the disc is then so small-looking, and our view besides is so dazzled by
+the proximity of the sun, that observations are of little avail. In the
+elongations, however, the sunlight comes from the side,<span class='pagenum'><a name="Page_149" id="Page_149">[Pg 149]</a></span> and so we see
+one half of the planet lit up; the right half at eastern elongation, and
+the left half at western elongation. Piecing together the results given
+us at these more favourable views, we are enabled, bit by bit, to gather
+some small knowledge concerning the surface of an inferior planet.</p>
+
+<p>From these considerations it will be seen at once that the inferior
+planets show various phases comparable to the waxing and waning of our
+moon in its monthly round. Superior conjunction is, in fact, similar to
+full moon, and inferior conjunction to new moon; while the eastern and
+western elongations may be compared respectively to the moon's first and
+last quarters. It will be recollected how, when these phases were first
+seen by the early telescopic observers, the Copernican theory was felt
+to be immensely strengthened; for it had been pointed out that if this
+system were the correct one, the planets Venus and Mercury, were it
+possible to see them more distinctly, would of necessity present phases
+like these when viewed from the earth. It should here be noted that the
+telescope was not invented until nearly seventy years after the death of
+Copernicus.</p>
+
+<p>The apparent swing of an inferior planet from side to side of the sun,
+at one time on the east side, then passing into and lost in the sun's
+rays to appear once more on the west side, is the explanation of what is
+meant when we speak of an <i>evening</i> or a <i>morning star</i>. An inferior
+planet is called an evening star when it is at its eastern elongation,
+that is to say, on the left-hand of the sun; for, being then on the
+eastern side, it will set after the sun sets, as both sink in their turn
+below the western horizon at the close of<span class='pagenum'><a name="Page_150" id="Page_150">[Pg 150]</a></span> day. Similarly, when such a
+planet is at its western elongation, that is to say, to the right-hand
+of the sun, it will go in advance of him, and so will rise above the
+eastern horizon before the sun rises, receiving therefore the
+designation of morning star. In very early times, however, before any
+definite ideas had been come to with regard to the celestial motions, it
+was generally believed that the morning and evening stars were quite
+distinct bodies. Thus Venus, when a morning star, was known to the
+ancients under the name of Phosphorus, or Lucifer; whereas they called
+it Hesperus when it was an evening star.</p>
+
+<p>Since an inferior planet circulates between us and the sun, one would be
+inclined to expect that such a body, each time it passed on the side
+nearest to the earth, should be seen as a black spot against the bright
+solar disc. Now this would most certainly be the case were the orbit of
+an inferior planet in the same plane with the orbit of the earth. But we
+have already seen how the orbits in the solar system, whether those of
+planets or of satellites, are by no means in the one plane; and that it
+is for this very reason that the moon is able to pass time after time in
+the direction of the sun, at the epoch known as new moon, and yet not to
+eclipse him save after the lapse of several such passages. Transits,
+then, as the passages of an inferior planet across the sun's disc are
+called, take place, for the same reason, only after certain regular
+lapses of time; and, as regards the circumstances of their occurrence,
+are on a par with eclipses of the sun. The latter, however, happen much
+more frequently, because the moon passes in the neighbourhood of the
+sun, roughly speaking, once<span class='pagenum'><a name="Page_151" id="Page_151">[Pg 151]</a></span> a month, whereas Venus comes to each
+inferior conjunction at intervals so long apart as a year and a half,
+and Mercury only about every four months. From this it will be further
+gathered that transits of Mercury take place much oftener than transits
+of Venus.</p>
+
+<p>Until recent years <i>Transits of Venus</i> were phenomena of great
+importance to astronomers, for they furnished the best means then
+available of calculating the distance of the sun from the earth. This
+was arrived at through comparing the amount of apparent displacement in
+the planet's path across the solar disc, when the transit was observed
+from widely separated stations on the earth's surface. The last transit
+of Venus took place in 1882, and there will not be another until the
+year 2004.</p>
+
+<p><i>Transits of Mercury</i>, on the other hand, are not of much scientific
+importance. They are of no interest as a popular spectacle; for the
+dimensions of the planet are so small, that it can be seen only with the
+aid of a telescope when it is in the act of crossing the sun's disc. The
+last transit of Mercury took place on November 14, 1907, and there will
+be another on November 6, 1914.</p>
+
+<p>The first person known to have observed a transit of an inferior planet
+was the celebrated French philosopher, Gassendi. This was the transit of
+Mercury which took place on the 7th of December 1631.</p>
+
+<p>The first time a transit of Venus was ever seen, so far as is known, was
+on the 24th of November 1639. The observer was a certain Jeremiah
+Horrox, curate of Hoole, near Preston, in Lancashire. The transit in
+question commenced shortly before sunset, and his<span class='pagenum'><a name="Page_152" id="Page_152">[Pg 152]</a></span> observations in
+consequence were limited to only about half-an-hour. Horrox happened to
+have a great friend, one William Crabtree, of Manchester, whom he had
+advised by letter to be on the look out for the phenomenon. The weather
+in Crabtree's neighbourhood was cloudy, with the result that he only got
+a view of the transit for about ten minutes before the sun set.</p>
+
+<p>That this transit was observed at all is due entirely to the remarkable
+ability of Horrox. According to the calculations of the great Kepler, no
+transit could take place that year (1639), as the planet would just pass
+clear of the lower edge of the sun. Horrox, however, not being satisfied
+with this, worked the question out for himself, and came to the
+conclusion that the planet would <i>actually</i> traverse the lower portion
+of the sun's disc. The event, as we have seen, proved him to be quite in
+the right. Horrox is said to have been a veritable prodigy of
+astronomical skill; and had he lived longer would, no doubt, have become
+very famous. Unfortunately he died about two years after his celebrated
+transit, in his <i>twenty-second</i> year only, according to the accounts.
+His friend Crabtree, who was then also a young man, is said to have been
+killed at the battle of Naseby in 1645.</p>
+
+<p>There is an interesting phenomenon in connection with transits which is
+known as the "Black Drop." When an inferior planet has just made its way
+on to the face of the sun, it is usually seen to remain for a short time
+as if attached to the sun's edge by what looks like a dark ligament (<a href="#Fig_12">see
+Fig. 12</a>, p. 153). This gives to the planet for the time being an
+elongated<span class='pagenum'><a name="Page_153" id="Page_153">[Pg 153]</a></span> appearance, something like that of a pear; but when the
+ligament, which all the while keeps getting thinner and thinner, has at
+last broken, the black body of the planet is seen to stand out round
+against the solar disc.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_12" id="Fig_12"></a>
+<img src="images/figure12.jpg" width="500" height="351" alt="Fig. 12." title="" />
+<span class="caption"><span class="smcap">Fig. 12.</span>&mdash;The "Black Drop."</span>
+</div>
+
+<p>This appearance may be roughly compared to the manner in which a drop of
+liquid (or, preferably, of some glutinous substance) tends for a while
+to adhere to an object from which it is falling.</p>
+
+<p>When the planet is in turn making its way off the face of the sun, the
+ligament is again seen to form and to attach it to the sun's edge before
+its due time.</p>
+
+<p>The phenomenon of the black drop, or ligament, is entirely an illusion,
+and, broadly speaking, of an optical origin. Something very similar will
+be noticed if one<span class='pagenum'><a name="Page_154" id="Page_154">[Pg 154]</a></span> brings one's thumb and forefinger <i>slowly</i> together
+against a very bright background.</p>
+
+<p>This peculiar phenomenon has proved one of the greatest drawbacks to the
+proper observation of transits, for it is quite impossible to note the
+exact instant of the planet's entrance upon and departure from the solar
+disc in conditions such as these.</p>
+
+<p>The black drop seems to bear a family resemblance, so to speak, to the
+phenomenon of Baily's beads. In the latter instance the lunar peaks, as
+they approach the sun's edge, appear to lengthen out in a similar manner
+and bridge the intervening space before their time, thus giving
+prominence to an effect which otherwise should scarcely be noticeable.</p>
+
+<p>The last transit of Mercury, which, as has been already stated, took
+place on November 14, 1907, was not successfully observed by astronomers
+in England, on account of the cloudiness of the weather. In France,
+however, Professor Moye, of Montpellier, saw it under good conditions,
+and mentions that the black drop remained very conspicuous for fully a
+minute. The transit was also observed in the United States, the reports
+from which speak of the black drop as very "troublesome."</p>
+
+<p>Before leaving the subject of transits it should be mentioned that it
+was in the capacity of commander of an expedition to Otaheite, in the
+Pacific, to observe the transit of Venus of June 3, 1769, that Captain
+Cook embarked upon the first of his celebrated voyages.</p>
+
+<p>In studying the surfaces of Venus and Mercury with the telescope,
+observers are, needless to say, very much hindered by the proximity of
+the sun. Venus,<span class='pagenum'><a name="Page_155" id="Page_155">[Pg 155]</a></span> when at the greatest elongations, certainly draws some
+distance out of the glare; but her surface is, even then, so dazzlingly
+bright, that the markings upon it are difficult to see. Mercury, on the
+other hand, is much duller in contrast, but the disc it shows in the
+telescope is exceedingly small; and, in addition, when that planet is
+left above the horizon for a short time after sunset, as necessarily
+happens after certain intervals, the mists near the earth's surface
+render observation of it very difficult.</p>
+
+<p>Until about twenty-five years ago, it was generally believed that both
+these planets rotated on their axes in about twenty-four hours, a
+notion, no doubt, originally founded upon an unconscious desire to bring
+them into some conformity with our earth. But Schiaparelli, observing in
+Italy, and Percival Lowell, in the clear skies of Arizona and Mexico,
+have lately come to the conclusion that both planets rotate upon their
+axes in the same time as they revolve in their orbits,<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a> the result
+being that they turn one face ever towards the sun in the same manner
+that the moon turns one face ever towards the earth&mdash;a curious state of
+things, which will be dealt with more fully when we come to treat of our
+satellite.</p>
+
+<p>The marked difference in the brightness between the two planets has
+already been alluded to. The surface of Venus is, indeed, about five
+times as bright as that of Mercury. The actual brightness of Mercury is
+about equivalent to that of our moon, and astronomers are, therefore,
+inclined to think that it may<span class='pagenum'><a name="Page_156" id="Page_156">[Pg 156]</a></span> resemble her in having a very rugged
+surface and practically no atmosphere. This probable lack of atmosphere
+is further corroborated by two circumstances. One of these is that when
+Mercury is just about to transit the face of the sun, no ring of
+diffused light is seen to encircle its disc as would be the case if it
+possessed an atmosphere. Such a lack of atmosphere is, indeed, only to
+be expected from what is known as the <i>Kinetic Theory of Gases</i>.
+According to this theory, which is based upon the behaviour of various
+kinds of gas, it is found that these elements tend to escape into space
+from the surface of bodies whose force of gravitation is weak. Hydrogen
+gas, for example, tends to fly away from our earth, as any one may see
+for himself when a balloon rises into the air. The gravitation of the
+earth seems, however, powerful enough to hold down other gases, as, for
+instance, those of which the air is chiefly composed, namely, oxygen and
+nitrogen. In due accordance with the Kinetic theory, we find the moon
+and Mercury, which are much about the same size, destitute of
+atmospheres. Mars, too, whose diameter is only about double that of the
+moon, has very little atmosphere. We find, on the other hand, that
+Venus, which is about the same size as our earth, clearly possesses an
+atmosphere, as just before the planet is in transit across the sun, the
+outline of its dark body is seen to be surrounded by a bright ring of
+light.</p>
+
+<p>The results of telescopic observation show that more markings are
+visible on Mercury than on Venus. The intense brilliancy of Venus is,
+indeed, about the same as that of our white clouds when the sun is<span class='pagenum'><a name="Page_157" id="Page_157">[Pg 157]</a></span>
+shining directly upon them. It has, therefore, been supposed that the
+planet is thickly enveloped in cloud, and that we do not ever see any
+part of its surface, except perchance the summit of some lofty mountain
+projecting through the fleecy mass.</p>
+
+<p>With regard to the great brilliancy of Venus, it may be mentioned that
+she has frequently been seen in England, with the naked eye in full
+sunshine, when at the time of her greatest brightness. The writer has
+seen her thus at noonday. Needless to say, the sky at the moment was
+intensely blue and clear.</p>
+
+<p>The orbit of Mercury is very oval, and much more so than that of any
+other planet. The consequence is that, when Mercury is nearest to the
+sun, the heat which it receives is twice as great as when it is farthest
+away. The orbit of Venus, on the other hand, is in marked contrast with
+that of Mercury, and is, besides, more nearly of a circular shape than
+that of any of the other planets. Venus, therefore, always keeps about
+the same distance from the sun, and so the heat which she receives
+during the course of her year can only be subject to very slight
+variations.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_11_11" id="Footnote_11_11"></a><a href="#FNanchor_11_11"><span class="label">[11]</span></a> In employing the terms Inferior and Superior the writer
+bows to astronomical custom, though he cannot help feeling that, in the
+circumstances, Interior and Exterior would be much more appropriate.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_12" id="Footnote_12_12"></a><a href="#FNanchor_12_12"><span class="label">[12]</span></a> This question is, however, uncertain, for some very recent
+spectroscopic observations of Venus seem to show a rotation period of
+about twenty-four hours.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_158" id="Page_158">[Pg 158]</a></span></p>
+<h3><a name="CHAPTER_XV" id="CHAPTER_XV"></a>CHAPTER XV</h3>
+
+<h4>THE EARTH</h4>
+
+
+<p class="noin"><span class="smcap">We</span> have already seen (<a href="#CHAPTER_I">in Chapter I.</a>) how, in very early times, men
+naturally enough considered the earth to be a flat plane extending to a
+very great distance in every direction; but that, as years went on,
+certain of the Greek philosophers suspected it to be a sphere. One or
+two of the latter are, indeed, said to have further believed in its
+rotation about an axis, and even in its revolution around the sun; but,
+as the ideas in question were founded upon fancy, rather than upon any
+direct evidence, they did not generally attract attention. The small
+effect, therefore, which these theories had upon astronomy, may well be
+gathered from the fact that in the Ptolemaic system the earth was
+considered as fixed and at the centre of things; and this belief, as we
+have seen, continued unaltered down to the days of Copernicus. It was,
+indeed, quite impossible to be certain of the real shape of the earth or
+the reality of its motions until knowledge became more extended and
+scientific instruments much greater in precision.</p>
+
+<p>We will now consider in detail a few of the more obvious arguments which
+can be put forward to show that our earth is a sphere.</p>
+
+<p>If, for instance, the earth were a plane surface, a ship sailing away
+from us over the sea would appear<span class='pagenum'><a name="Page_159" id="Page_159">[Pg 159]</a></span> to grow smaller and smaller as it
+receded into the distance, becoming eventually a tiny speck, and fading
+gradually from our view. This, however, is not at all what actually
+takes place. As we watch a vessel receding, its hull appears bit by bit
+to slip gently down over the horizon, leaving the masts alone visible.
+Then, in their turn, the masts are seen to slip down in the same manner,
+until eventually every trace of the vessel is gone. On the other hand,
+when a ship comes into view, the masts are the first portions to appear.
+They gradually rise up from below the horizon, and the hull follows in
+its turn, until the whole vessel is visible. Again, when one is upon a
+ship at sea, a set of masts will often be seen sticking up alone above
+the horizon, and these may shorten and gradually disappear from view
+without the body of the ship to which they belong becoming visible at
+all. Since one knows from experience that there is no <i>edge</i> at the
+horizon over which a vessel can drop down, the appearance which we have
+been describing can only be explained by supposing that the surface of
+the earth is always curving gradually in every direction.</p>
+
+<p>The distance at which what is known as the <i>horizon</i> lies away from us
+depends entirely upon the height above the earth's surface where we
+happen at the moment to be. A ship which has appeared to sink below the
+horizon for a person standing on the beach, will be found to come back
+again into view if he at once ascends a high hill. Experiment shows that
+the horizon line lies at about three miles away for a person standing at
+the water's edge. The curving of the earth's surface is found, indeed,
+to be at the rate<span class='pagenum'><a name="Page_160" id="Page_160">[Pg 160]</a></span> of eight inches in every mile. Now it can be
+ascertained, by calculation, that a body curving at this rate in every
+direction must be a globe about 8000 miles in diameter.</p>
+
+<p>Again, the fact that, if not stopped by such insuperable obstacles as
+the polar ice and snow, those who travel continually in any one
+direction upon the earth's surface always find themselves back again at
+the regions from which they originally set out, is additional ground for
+concluding that the earth is a globe.</p>
+
+<p>We can find still further evidence. For instance, in an eclipse of the
+moon the earth's shadow, when seen creeping across the moon's face, is
+noted to be <i>always</i> circular in shape. One cannot imagine how such a
+thing could take place unless the earth were a sphere.</p>
+
+<p>Also, it is found from observation that the sun, the planets, and the
+satellites are, all of them, round. This roundness cannot be the
+roundness of a flat plate, for instance, for then the objects in
+question would sometimes present their thin sides to our view. It
+happens, also, that upon the discs which these bodies show, we see
+certain markings shifting along continually in one direction, to
+disappear at one side and to reappear again at the other. Such bodies
+must, indeed, be spheres in rotation.</p>
+
+<p>The crescent and other phases, shown by the moon and the inferior
+planets, should further impress the truth of the matter upon us, as such
+appearances can only be caused by the sunlight falling from various
+directions upon the surfaces of spherical bodies.</p>
+
+<p>Another proof, perhaps indeed the weightiest of all, is the continuous
+manner in which the stars overhead<span class='pagenum'><a name="Page_161" id="Page_161">[Pg 161]</a></span> give place to others as one travels
+about the surface of the earth. When in northern regions the Pole Star
+and its neighbours&mdash;the stars composing the Plough, for instance&mdash;are
+over our heads. As one journeys south these gradually sink towards the
+northern horizon, while other stars take their place, and yet others are
+uncovered to view from the south. The regularity with which these
+changes occur shows that every point on the earth's surface faces a
+different direction of the sky, and such an arrangement would only be
+possible if the earth were a sphere. The celebrated Greek philosopher,
+Aristotle, is known to have believed in the globular shape of the earth,
+and it was by this very argument that he had convinced himself that it
+was so.</p>
+
+<p>The idea of the sphericity of the earth does not appear, however, to
+have been generally accepted until the voyages of the great navigators
+showed that it could be sailed round.</p>
+
+<p>The next point we have to consider is the rotation of the earth about
+its axis. From the earliest times men noticed that the sky and
+everything in it appeared to revolve around the earth in one fixed
+direction, namely, towards what is called the West, and that it made one
+complete revolution in the period of time which we know as twenty-four
+hours. The stars were seen to come up, one after another, from below the
+eastern horizon, to mount the sky, and then to sink in turn below the
+western horizon. The sun was seen to perform exactly the same journey,
+and the moon, too, whenever she was visible. One or two of the ancient
+Greek philosophers perceived that this might be explained, either by a
+movement<span class='pagenum'><a name="Page_162" id="Page_162">[Pg 162]</a></span> of the entire heavens around the earth, or by a turning motion
+on the part of the earth itself. Of these diverse explanations, that
+which supposed an actual movement of the heavens appealed to them the
+most, for they could hardly conceive that the earth should continually
+rotate and men not be aware of its movement. The question may be
+compared to what we experience when borne along in a railway train. We
+see the telegraph posts and the trees and buildings near the line fly
+past us one after another in the contrary direction. Either these must
+be moving, or we must be moving; and as we happen to <i>know</i> that it is,
+indeed, we who are moving, there can be no question therefore about the
+matter. But it would not be at all so easy to be sure of this movement
+were one unable to see the objects close at hand displacing themselves.
+For instance, if one is shut up in a railway carriage at night with the
+blinds down, there is really nothing to show that one is moving, except
+the jolting of the train. And even then it is hard to be sure in which
+direction one is actually travelling.</p>
+
+<p>The way we are situated upon the earth is therefore as follows. There
+are no other bodies sufficiently near to be seen flying past us in turn;
+our earth spins without a jolt; we and all things around us, including
+the atmosphere itself, are borne along together with precisely the same
+impetus, just as all the objects scattered about a railway carriage
+share in the forward movement of the train. Such being the case, what
+wonder that we are unconscious of the earth's rotation, of which we
+should know nothing at all, were it not for that slow displacement<span class='pagenum'><a name="Page_163" id="Page_163">[Pg 163]</a></span> of
+the distant objects in the heavens, as we are borne past them in turn.</p>
+
+<p>If the night sky be watched, it will be soon found that its apparent
+turning movement seems to take place around a certain point, which
+appears as if fixed. This point is known as the north pole of the
+heavens; and a rather bright star, which is situated very close to this
+hub of movement, is in consequence called the Pole Star. For the
+dwellers in southern latitudes there is also a point in their sky which
+appears to remain similarly fixed, and this is known as the south pole
+of the heavens. Since, however, the heavens do not turn round at all,
+but the earth does, it will easily be seen that these apparently
+stationary regions in the sky are really the points towards which the
+axis of the earth is directed. The positions on the earth's surface
+itself, known as the North and South Poles, are merely the places where
+the earth's axis, if there were actually such a thing, would be expected
+to jut out. The north pole of the earth will thus be situated exactly
+beneath the north pole of the heavens, and the south pole of the earth
+exactly beneath the south pole of the heavens.</p>
+
+<p>We have seen that the earth rotates upon its imaginary axis once in
+about every twenty-four hours. This means that everything upon the
+surface of the earth is carried round once during that time. The
+measurement around the earth's equator is about 24,000 miles; and,
+therefore, an object situated at the equator must be carried round
+through a distance of about 24,000 miles in each twenty-four hours.
+Everything at the equator is thus moving along at the rapid rate of
+about 1000 miles an hour, or between sixteen<span class='pagenum'><a name="Page_164" id="Page_164">[Pg 164]</a></span> and seventeen times as
+fast as an express train. If, however, one were to take measurements
+around the earth parallel to the equator, one would find these
+measurements becoming less and less, according as the poles were
+approached. It is plain, therefore, that the speed with which any point
+moves, in consequence of the earth's rotation, will be greatest at the
+equator, and less and less in the direction of the poles; while at the
+poles themselves there will be practically no movement, and objects
+there situated will merely turn round.</p>
+
+<p>The considerations above set forth, with regard to the different speeds
+at which different portions of a rotating globe will necessarily be
+moving, is the foundation of an interesting experiment, which gives us
+further evidence of the rotation of our earth. The measurement around
+the earth at any distance below the surface, say, for instance, at the
+depth of a mile, will clearly be less than a similar measurement at the
+surface itself. The speed of a point at the bottom of a mine, which
+results from the actual rotation of the earth, must therefore be less
+than the speed of a point at the surface overhead. This can be
+definitely proved by dropping a heavy object down a mine shaft. The
+object, which starts with the greater speed of the surface, will, when
+it reaches the bottom of the mine, be found, as might be indeed
+expected, to be a little ahead (<i>i.e.</i> to the east) of the point which
+originally lay exactly underneath it. The distance by which the object
+gains upon this point is, however, very small. In our latitudes it
+amounts to about an inch in a fall of 500 feet.</p>
+
+<p>The great speed at which, as we have seen, the<span class='pagenum'><a name="Page_165" id="Page_165">[Pg 165]</a></span> equatorial regions of
+the earth are moving, should result in giving to the matter there
+situated a certain tendency to fly outwards. Sir Isaac Newton was the
+first to appreciate this point, and he concluded from it that the earth
+must be <i>bulged</i> a little all round the equator. This is, indeed, found
+to be the case, the diameter at the equator being nearly twenty-seven
+miles greater than it is from pole to pole. The reader will, no doubt,
+be here reminded of the familiar comparison in geographies between the
+shape of the earth and that of an orange.</p>
+
+<p>In this connection it is interesting to consider that, were the earth to
+rotate seventeen times as fast as it does (<i>i.e.</i> in one hour
+twenty-five minutes, instead of twenty-four hours), bodies at the
+equator would have such a strong tendency to fly outwards that the force
+of terrestrial gravity acting upon them would just be counterpoised, and
+they would virtually have <i>no weight</i>. And, further, were the earth to
+rotate a little faster still, objects lying loose upon its surface would
+be shot off into space.</p>
+
+<p>The earth is, therefore, what is technically known as an <i>oblate
+spheroid</i>; that is, a body of spherical shape flattened at the poles. It
+follows of course from this, that objects at the polar regions are
+slightly nearer to the earth's centre than objects at the equatorial
+regions. We have already seen that gravitation acts from the central
+parts of a body, and that its force is greater the nearer are those
+central parts. The result of this upon our earth will plainly be that
+objects in the polar regions will be pulled with a slightly stronger
+pull, and will therefore <i>weigh</i> a trifle more than objects in the
+equatorial regions. This is,<span class='pagenum'><a name="Page_166" id="Page_166">[Pg 166]</a></span> indeed, found by actual experiment to be
+the case. As an example of the difference in question, Professor Young,
+in his <i>Manual of Astronomy</i>, points out that a man who weighs 190
+pounds at the equator would weigh 191 at the pole. In such an experiment
+the weighing would, however, have to be made with a <i>spring balance</i>,
+and <i>not with scales</i>; for, in the latter case, the "weights" used would
+alter in their weight in exactly the same degree as the objects to be
+weighed.</p>
+
+<p>It used to be thought that the earth was composed of a relatively thin
+crust, with a molten interior. Scientific men now believe, on the other
+hand, that such a condition cannot after all prevail, and that the earth
+must be more or less solid all through, except perhaps in certain
+isolated places where collections of molten matter may exist.</p>
+
+<p>The <i>atmosphere</i>, or air which we breathe, is in the form of a layer of
+limited depth which closely envelops the earth. Actually, it is a
+mixture of several gases, the most important being nitrogen and oxygen,
+which between them practically make up the air, for the proportion of
+the other gases, the chief of which is carbonic acid gas, is exceedingly
+small.</p>
+
+<p>It is hard to picture our earth, as we know it, without this atmosphere.
+Deprived of it, men at once would die; but even if they could be made to
+go on living without it by any miraculous means, they would be like unto
+deaf beings, for they would never hear any sound. What we call <i>sounds</i>
+are merely vibrations set up in the air, which travel along and strike
+upon the drum of the ear.</p>
+
+<p>The atmosphere is densest near the surface of the<span class='pagenum'><a name="Page_167" id="Page_167">[Pg 167]</a></span> earth, and becomes
+less and less dense away from it, as a result of diminishing pressure of
+air from above. The greater portion of it is accumulated within four or
+five miles of the earth's surface.</p>
+
+<p>It is impossible to determine exactly at what distance from the earth's
+surface the air ceases altogether, for it grows continually more and
+more rarefied. There are, however, two distinct methods of ascertaining
+the distance beyond which it can be said practically not to exist. One
+of these methods we get from twilight. Twilight is, in fact, merely
+light reflected to us from those upper regions of the air, which still
+continue to be illuminated by the sun after it has disappeared from our
+view below the horizon. The time during which twilight lasts, shows us
+that the atmosphere must be at least fifty miles high.</p>
+
+<p>But the most satisfactory method of ascertaining the height to which the
+atmosphere extends is from the observation of meteors. It is found that
+these bodies become ignited, by the friction of passing into the
+atmosphere, at a height of about 100 miles above the surface of the
+earth. We thus gather that the atmosphere has a certain degree of
+density even at this height. It may, indeed, extend as far as about 150
+miles.</p>
+
+<p>The layer of atmosphere surrounding our earth acts somewhat in the
+manner of the glass covering of a greenhouse, bottling in the sun's
+rays, and thus storing up their warmth for our benefit. Were this not
+so, the heat which we get from the sun would, after falling upon the
+earth, be quickly radiated again into space.</p>
+
+<p>It is owing to the unsteadiness of the air that stars<span class='pagenum'><a name="Page_168" id="Page_168">[Pg 168]</a></span> are seen to
+twinkle. A night when this takes place, though it may please the average
+person, is worse than useless to the astronomer, for the unsteadiness is
+greatly magnified in the telescope. This twinkling is, no doubt, in a
+great measure responsible for the conventional "points" with which Art
+has elected to embellish stars, and which, of course, have no existence
+in fact.</p>
+
+<p>The phenomena of <i>Refraction</i>,<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a> namely, that bending which rays of
+light undergo, when passing <i>slant-wise</i> from a rare into a dense
+transparent medium, are very marked with regard to the atmosphere. The
+denser the medium into which such rays pass, the greater is this bending
+found to be. Since the layer of air around us becomes denser and denser
+towards the surface of the earth, it will readily be granted that the
+rays of light reaching our eyes from a celestial object, will suffer the
+greater bending the lower the object happens to be in the sky. Celestial
+objects, unless situated directly overhead, are thus not seen in their
+true places, and when nearest to the horizon are most out of place. The
+bending alluded to is upwards. Thus the sun and the moon, for instance,
+when we see them resting upon the horizon, are actually <i>entirely</i>
+beneath it.</p>
+
+<p>When the sun, too, is sinking towards the horizon, the lower edge of its
+disc will, for the above reason,<span class='pagenum'><a name="Page_169" id="Page_169">[Pg 169]</a></span> look somewhat more raised than the
+upper. The result is a certain appearance of flattening; which may
+plainly be seen by any one who watches the orb at setting.</p>
+
+<p>In observations to determine the exact positions of celestial objects
+correction has to be made for the effects of refraction, according to
+the apparent elevation of these objects in the sky. Such effects are
+least when the objects in question are directly overhead, for then the
+rays of light, coming from them to the eye, enter the atmosphere
+perpendicularly, and not at any slant.</p>
+
+<p>A very curious effect, due to refraction, has occasionally been observed
+during a total eclipse of the moon. To produce an eclipse of this kind,
+<i>the earth must, of course, lie directly between the sun and the moon</i>.
+Therefore, when we see the shadow creeping over the moon's surface, the
+sun should actually be well below the horizon. But when a lunar eclipse
+happens to come on just about sunset, the sun, although really sunk
+below the horizon, appears still above it through refraction, and the
+eclipsed moon, situated, of course, exactly opposite to it in the sky,
+is also lifted up above the horizon by the same cause. Pliny, writing in
+the first century of the Christian era, describes an eclipse of this
+kind, and refers to it as a "prodigy." The phenomenon is known as a
+"horizontal eclipse." It was, no doubt, partly owing to it that the
+ancients took so long to decide that an eclipse of the moon was really
+caused by the shadow cast by the earth. Plutarch, indeed, remarks that
+it was easy enough to understand that a solar eclipse was caused by the
+interposition of the moon, but that one could not<span class='pagenum'><a name="Page_170" id="Page_170">[Pg 170]</a></span> imagine by the
+interposition <i>of what body</i> the moon itself could be eclipsed.</p>
+
+<p>In that apparent movement of the heavens about the earth, which men now
+know to be caused by the mere rotation of the earth itself, a slight
+change is observed to be continually taking place. The stars, indeed,
+are always found to be gradually drawing westward, <i>i.e.</i> towards the
+sun, and losing themselves one after the other in the blaze of his
+light, only to reappear, however, on the other side of him after a
+certain lapse of time. This is equivalent to saying that the sun itself
+seems always creeping slowly <i>eastward</i> in the heaven. The rate at which
+this appears to take place is such that the sun finds itself back again
+to its original position, with regard to the starry background, at the
+end of a year's time. In other words, the sun seems to make a complete
+tour of the heavens in the course of a year. Here, however, we have
+another illusion, just as the daily movement of the sky around the earth
+was an illusion. The truth indeed is, that this apparent movement of the
+sun eastward among the stars during a year, arises merely from a
+<i>continuous displacement of his position</i> caused by an actual motion of
+the earth itself around him in that very time. In a word, it is the
+earth which really moves around the sun, and not the sun around the
+earth.</p>
+
+<p>The stress laid upon this fundamental point by Copernicus, marks the
+separation of the modern from the ancient view. Not that Copernicus,
+indeed, had obtained any real proof that the earth is merely a planet
+revolving around the sun; but it seemed to his profound intellect that a
+movement of this kind<span class='pagenum'><a name="Page_171" id="Page_171">[Pg 171]</a></span> on the part of our globe was the more likely
+explanation of the celestial riddle. The idea was not new; for, as we
+have already seen, certain of the ancient Greeks (Aristarchus of Samos,
+for example) had held such a view; but their notions on the subject were
+very fanciful, and unsupported by any good argument.</p>
+
+<p>What Copernicus, however, really seems to have done was to <i>insist</i> upon
+the idea that the sun occupied the <i>centre</i>, as being more consonant
+with common sense. No doubt, he was led to take up this position by the
+fact that the sun appeared entirely of a different character from the
+other members of the system. The one body in the scheme, which performed
+the important function of dispenser of light and heat, would indeed be
+more likely to occupy a position apart from the rest; and what position
+more appropriate for its purposes than the centre!</p>
+
+<p>But here Copernicus only partially solved the difficult question. He
+unfortunately still clung to an ancient belief, which as yet remained
+unquestioned; <i>i.e.</i> the great virtue, one might almost say, the
+<i>divineness</i>, of circular motion. The ancients had been hag-ridden, so
+to speak, by the circle; and it appeared to them that such a perfectly
+formed curve was alone fitted for the celestial motions. Ptolemy
+employed it throughout his system. According to him the "planets" (which
+included, under the ancient view, both the sun and the moon), moved
+around the earth in circles; but, as their changing positions in the sky
+could not be altogether accounted for in this way, it was further
+supposed that they performed additional circular movements, around
+peculiarly placed centres, during the course of their orbital
+revolutions.<span class='pagenum'><a name="Page_172" id="Page_172">[Pg 172]</a></span> Thus the Ptolemaic system grew to be extremely
+complicated; for astronomers did not hesitate to add new circular
+movements whenever the celestial positions calculated for the planets
+were found not to tally with the positions observed. In this manner,
+indeed, they succeeded in doctoring the theory, so that it fairly
+satisfied the observations made with the rough instruments of
+pre-telescopic times.</p>
+
+<p>Although Copernicus performed the immense service to astronomy of boldly
+directing general attention to the central position of the sun, he
+unfortunately took over for the new scheme the circular machinery of the
+Ptolemaic system. It therefore remained for the famous Kepler, who lived
+about a century after him, to find the complete solution. Just as
+Copernicus, for instance, had broken free from tradition with regard to
+the place of the sun; so did Kepler, in turn, break free from the spell
+of circular motion, and thus set the coping-stone to the new
+astronomical edifice. This astronomer showed, in fact, that if the paths
+of the planets around the sun, and of the moon around the earth, were
+not circles, but <i>ellipses</i>, the movements of these bodies about the sky
+could be correctly accounted for. The extreme simplicity of such an
+arrangement was far more acceptable than the bewildering intricacy of
+movement required by the Ptolemaic theory. The Copernican system, as
+amended by Kepler, therefore carried the day; and was further
+strengthened, as we have already seen, by the telescopic observations of
+Galileo and the researches of Newton into the effects of gravitation.</p>
+
+<p>And here a word on the circle, now fallen from<span class='pagenum'><a name="Page_173" id="Page_173">[Pg 173]</a></span> its high estate. The
+ancients were in error in supposing that it stood entirely apart&mdash;the
+curve of curves. As a matter of fact it is merely <i>a special kind of
+ellipse</i>. To put it paradoxically, it is an ellipse which has no
+ellipticity, an oval without any ovalness!</p>
+
+<p>Notwithstanding all this, astronomy had to wait yet a long time for a
+definite proof of the revolution of the earth around the sun. The
+leading argument advanced by Aristotle, against the reality of any
+movement of the earth, still held good up to about seventy years ago.
+That philosopher had pointed out that the earth could not move about in
+space to any great extent, or the stars would be found to alter their
+apparent places in the sky, a thing which had never been observed to
+happen. Centuries ran on, and instruments became more and more perfect,
+yet no displacements of stars were noted. In accepting the Copernican
+theory men were therefore obliged to suppose these objects as
+immeasurably distant. At length, however, between the years 1835 and
+1840, it was discovered by the Prussian astronomer, Bessel, that a star
+known as 61 Cygni&mdash;that is to say, the star marked in celestial atlases
+as No. 61 in the constellation of the Swan&mdash;appeared, during the course
+of a year, to perform a tiny circle in the heavens, such as would result
+from a movement on our own part around the sun. Since then about
+forty-three stars have been found to show minute displacements of a
+similar kind, which cannot be accounted for upon any other supposition
+than that of a continuous revolution of the earth around the sun. The
+triumph of the Copernican system is now at last supreme.</p>
+
+<p><span class='pagenum'><a name="Page_174" id="Page_174">[Pg 174]</a></span></p><p>If the axis of the earth stood "straight up," so to speak, while the
+earth revolved in its orbit, the sun would plainly keep always on a
+level with the equator. This is equivalent to stating that, in such
+circumstances, a person at the equator would see it rise each morning
+exactly in the east, pass through the <i>zenith</i>, that is, the point
+directly overhead of him, at midday, and set in the evening due in the
+west. As this would go on unchangingly at the equator every day
+throughout the year, it should be clear that, at any particular place
+upon the earth, the sun would in these conditions always be seen to move
+in an unvarying manner across the sky at a certain altitude depending
+upon the latitude of the place. Thus the more north one went upon the
+earth's surface, the more southerly in the sky would the sun's path lie;
+while at the north pole itself, the sun would always run round and round
+the horizon. Similarly, the more south one went from the equator the
+more northerly would the path of the sun lie, while at the south pole it
+would be seen to skirt the horizon in the same manner as at the north
+pole. The result of such an arrangement would be, that each place upon
+the earth would always have one unvarying climate; in which case there
+would not exist any of those beneficial changes of season to which we
+owe so much.</p>
+
+<p>The changes of season, which we fortunately experience, are due,
+however, to the fact that the sun does not appear to move across the sky
+each day at one unvarying altitude, but is continually altering the
+position of its path; so that at one period of the year it passes across
+the sky low down, and remains<span class='pagenum'><a name="Page_175" id="Page_175">[Pg 175]</a></span> above the horizon for a short time only,
+while at another it moves high up across the heavens, and is above the
+horizon for a much longer time. Actually, the sun seems little by little
+to creep up the sky during one half of the year, namely, from mid-winter
+to mid-summer, and then, just as gradually, to slip down it again during
+the other half, namely, from mid-summer to mid-winter. It will therefore
+be clear that every region of the earth is much more thoroughly warmed
+during one portion of the year than during another, <i>i.e.</i> when the
+sun's path is high in the heavens than when it is low down.</p>
+
+<p>Once more we find appearances exactly the contrary from the truth. The
+earth is in this case the real cause of the deception, just as it was in
+the other cases. The sun does not actually creep slowly up the sky, and
+then slowly dip down it again, but, owing to the earth's axis being set
+aslant, different regions of the earth's surface are presented to the
+sun at different times. Thus, in one portion of its orbit, the northerly
+regions of the earth are presented to the sun, and in the other portion
+the southerly. It follows of course from this, that when it is summer in
+the northern hemisphere it is winter in the southern, and <i>vice vers&acirc;</i>
+(<a href="#Fig_13">see Fig. 13</a>, p. 176).</p>
+
+<p><span class='pagenum'><a name="Page_176" id="Page_176">[Pg 176]</a></span></p>
+<div class="figcenter" style="width: 500px;"><a name="Fig_13" id="Fig_13"></a>
+<img src="images/figure13.jpg" width="500" height="251" alt="Fig. 13." title="" />
+<span class="caption"><span class="smcap">Fig. 13.</span>&mdash;Summer and Winter.</span>
+</div>
+
+<p>The fact that, in consequence of this slant of the earth's axis, the sun
+is for part of the year on the north side of the equator and part of the
+year on the south side, leads to a very peculiar result. The path of the
+moon around the earth is nearly on the same plane with the earth's path
+around the sun. The moon, therefore, always keeps to the same regions of
+the sky as the sun. The slant of the earth's axis thus regularly
+displaces the position of both the sun and the moon to the north and
+south sides of the equator respectively in the manner we have been
+describing. Were the earth, however, a perfect sphere, such change of
+position would not produce any effect. We have shown, however, that the
+earth is not a perfect sphere, but that it is bulged out all round the
+equator. The result is that this bulged-out portion swings slowly under
+the pulls of solar and lunar gravitation, in response to the
+displacements of the sun and moon to the north and to the south of it.
+This slow swing of the equatorial regions results, of course, in a
+certain slow change of the direction of the earth's axis, so that the
+north pole does not go on pointing continually to the same region of the
+sky. The change in the direction of the axis is, however, so extremely
+slight, that it shows up only after the lapse of ages. The north pole of
+the heavens, that is, the region of the sky towards which the north pole
+of the earth's axis points, displaces<span class='pagenum'><a name="Page_177" id="Page_177">[Pg 177]</a></span> therefore extremely slowly,
+tracing out a wide circle, and arriving back again to the same position
+in the sky only after a period of about 25,000 years. At present the
+north pole of the heavens is quite close to a bright star in the tail of
+the constellation of the Little Bear, which is consequently known as the
+Pole Star; but in early Greek times it was at least ten times as far
+away from this star as it is now. After some 12,000 years the pole will
+point to the constellation of Lyra, and Vega, the most brilliant star in
+that constellation, will then be considered as the pole star. This slow
+twisting of the earth's axis is technically known as <i>Precession</i>, or
+the <i>Precession of the Equinoxes</i> (<a href="#Plate_XIX">see Plate XIX.</a>, p. 292).</p>
+
+<p>The slow displacement of the celestial pole appears to have attracted
+the attention of men in very early times, but it was not until the
+second century <span class="ampm">B.C.</span> that precession was established as a fact by the
+celebrated Greek astronomer, Hipparchus. For the ancients this strange
+cyclical movement had a mystic significance; and they looked towards the
+end of the period as the end, so to speak, of a "dispensation," after
+which the life of the universe would begin anew:&mdash;</p>
+
+<p class="poem">
+"Magnus ab integro s&aelig;clorum nascitur ordo.<br />
+Jam redit et Virgo, redeunt Saturnia regna;<br />
+<span class="dots">������</span><br />
+Alter erit tum Tiphys, et altera qu&aelig; vehat Argo<br />
+Delectos heroas; erunt etiam altera bella,<br />
+Atque iterum ad Trojam magnus mittetur Achilles."<br />
+</p>
+
+<p>We have seen that the orbit of the earth is an ellipse, and that the sun
+is situated at what is called the <i>focus</i>, a point not in the middle of
+the ellipse,<span class='pagenum'><a name="Page_178" id="Page_178">[Pg 178]</a></span> but rather towards one of its ends. Therefore, during the
+course of the year the distance of the earth from the sun varies. The
+sun, in consequence of this, is about 3,000,000 miles <i>nearer</i> to us in
+our northern <i>winter</i> than it is in our northern summer, a statement
+which sounds somewhat paradoxical. This variation in distance, large as
+it appears in figures, can, however, not be productive of much
+alteration in the amount of solar heat which we receive, for during the
+first week in January, when the distance is least, the sun only looks
+about <i>one-eighteenth</i> broader than at the commencement of July, when
+the distance is greatest. The great disparity in temperature between
+winter and summer depends, as we have seen, upon causes of quite another
+kind, and varies between such wide limits that the effects of this
+slight alteration in the distance of the sun from the earth may be
+neglected for practical purposes.</p>
+
+<p>The Tides are caused by the gravitational pull of the sun and moon upon
+the water of the earth's surface. Of the two, the moon, being so much
+the nearer, exerts the stronger pull, and therefore may be regarded as
+the chief cause of the tides. This pull always draws that portion of the
+water, which happens to be right underneath the moon at the time, into a
+heap; and there is also a <i>second</i> heaping of water at the same moment
+<i>at the contrary side of the earth</i>, the reasons for which can be shown
+mathematically, but cannot be conveniently dealt with here.</p>
+
+<p>As the earth rotates on its axis each portion of its surface passes
+beneath the moon, and is swelled up by this pull; the watery portions
+being, however, the<span class='pagenum'><a name="Page_179" id="Page_179">[Pg 179]</a></span> only ones to yield visibly. A similar swelling up,
+as we have seen, takes place at the point exactly away from the moon.
+Thus each portion of our globe is borne by the rotation through two
+"tide-areas" every day, and this is the reason why there are two tides
+during every twenty-four hours.</p>
+
+<p>The crest of the watery swelling is known as high tide. The journey of
+the moon around the earth takes about a month, and this brings her past
+each place in turn by about fifty minutes later each day, which is the
+reason why high tide is usually about twenty-five minutes later each
+time.</p>
+
+<p>The moon is, however, not the sole cause of the tides, but the sun, as
+we have said, has a part in the matter also. When it is new moon the
+gravitational attractions of both sun and moon are clearly acting
+together from precisely the same direction, and, therefore, the tide
+will be pulled up higher than at other times. At full moon, too, the
+same thing happens; for, although the bodies are now acting from
+opposite directions, they do not neutralise each other's pulls as one
+might imagine, since the sun, in the same manner as the moon, produces a
+tide both under it and also at the opposite side of the earth. Thus both
+these tides are actually increased in height. The exceptionally high
+tides which we experience at new and full moons are known as <i>Spring
+Tides</i>, in contradistinction to the minimum high tides, which are known
+as <i>Neap Tides</i>.</p>
+
+<p>The ancients appear to have had some idea of the cause of the tides. It
+is said that as early as 1000 <span class="ampm">B.C.</span> the Chinese noticed that the moon
+exerted an influence upon the waters of the sea. The Greeks and<span class='pagenum'><a name="Page_180" id="Page_180">[Pg 180]</a></span> Romans,
+too, had noticed the same thing; and C&aelig;sar tells us that when he was
+embarking his troops for Britain the tide was high <i>because</i> the moon
+was full. Pliny went even further than this, in recognising a similar
+connection between the waters and the sun.</p>
+
+<p>From casual observation one is inclined to suppose that the high tide
+always rises many feet. But that this is not the case is evidenced by
+the fact that the tides in the midst of the great oceans are only from
+three to four feet high. However, in the seas and straits around our
+Isles, for instance, the tides rise very many feet indeed, but this is
+merely owing to the extra heaping up which the large volumes of water
+undergo in forcing their passage through narrow channels.</p>
+
+<p>As the earth, in rotating, is continually passing through these
+tide-areas, one might expect that the friction thus set up would tend to
+slow down the rotation itself. Such a slowing down, or "tidal drag," as
+it is called, is indeed continually going on; but the effects produced
+are so exceedingly minute that it will take many millions of years to
+make the rotation appreciably slower, and so to lengthen the day.</p>
+
+<p>Recently it has been proved that the axis of the earth is subject to a
+very small displacement, or rather, "wobbling," in the course of a
+period of somewhat over a year. As a consequence of this, the pole
+shifts its place through a circle of, roughly, a few yards in width
+during the time in question. This movement is, perhaps, the combined
+result of two causes. One of these is the change of place during the
+year of large masses of material upon our earth; such as occurs, for
+instance, when ice and snow<span class='pagenum'><a name="Page_181" id="Page_181">[Pg 181]</a></span> melt, or when atmospheric and ocean
+currents transport from place to place great bodies of air and water.
+The other cause is supposed to be the fact that the earth is not
+absolutely rigid, and so yields to certain strains upon it. In the
+course of investigation of this latter point the interesting conclusion
+has been reached by the famous American astronomer, Professor Simon
+Newcomb, that our globe as a whole is <i>a little more rigid than steel</i>.</p>
+
+<p>We will bring this chapter to a close by alluding briefly to two strange
+appearances which are sometimes seen in our night skies. These are known
+respectively as the Zodiacal Light and the Gegenschein.</p>
+
+<p>The <i>Zodiacal Light</i> is a faint cone-shaped illumination which is seen
+to extend upwards from the western horizon after evening twilight has
+ended, and from the eastern horizon before morning twilight has begun.
+It appears to rise into the sky from about the position where the sun
+would be at that time. The proper season of the year for observing it
+during the evening is in the spring, while in autumn it is best seen in
+the early morning. In our latitudes its light is not strong enough to
+render it visible when the moon is full, but in the tropics it is
+reported to be very bright, and easily seen in full moonlight. One
+theory regards it as the reflection of light from swarms of meteors
+revolving round the sun; another supposes it to be a very rarefied
+extension of the corona.</p>
+
+<p>The <i>Gegenschein</i> (German for "counter-glow") is a faint oval patch of
+light, seen in the sky exactly opposite to the place of the sun. It is
+usually treated of in connection with the zodiacal light, and one theory
+regards it similarly as of meteoric origin.<span class='pagenum'><a name="Page_182" id="Page_182">[Pg 182]</a></span> Another theory,
+however&mdash;that of Mr. Evershed&mdash;considers it a sort of <i>tail</i> to the
+earth (like a comet's tail) composed of hydrogen and helium&mdash;the two
+<i>lightest</i> gases we know&mdash;driven off from our planet in the direction
+contrary to the sun.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_13_13" id="Footnote_13_13"></a><a href="#FNanchor_13_13"><span class="label">[13]</span></a> Every one knows the simple experiment in which a coin
+lying at the bottom of an empty basin, and hidden from the eye by its
+side, becomes visible when a certain quantity of water has been poured
+in. This is an example of refraction. The rays of light coming from the
+coin ought <i>not</i> to reach the eye, on account of the basin's side being
+in the way; yet by the action of the water they are <i>refracted</i>, or bent
+over its edge, in such a manner that they do.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_183" id="Page_183">[Pg 183]</a></span></p>
+<h3><a name="CHAPTER_XVI" id="CHAPTER_XVI"></a>CHAPTER XVI</h3>
+
+<h4>THE MOON</h4>
+
+
+<p class="noin"><span class="smcap">What</span> we call the moon's "phases" are merely the various ways in which we
+see the sun shining upon her surface during the course of her monthly
+revolutions around the earth (<a href="#Fig_14">see Fig. 14</a>, p. 184). When she passes in
+the neighbourhood of the sun all his light falls upon that side which is
+turned away from us, and so the side which is turned towards us is
+unillumined, and therefore invisible. When in this position the moon is
+spoken of as <i>new</i>.</p>
+
+<p>As she continues her motion around the earth, she draws gradually to the
+east of the sun's place in the sky. The sunlight then comes somewhat
+from the side; and so we see a small portion of the right side of the
+lunar disc illuminated. This is the phase known as the <i>crescent</i> moon.</p>
+
+<p>As she moves on in her orbit more and more of her illuminated surface is
+brought into view; and so the crescent of light becomes broader and
+broader, until we get what is called half-moon, or <i>first quarter</i>, when
+we see exactly one-half of her surface lit up by the sun's rays. As she
+draws still further round yet more of her illuminated surface is brought
+into view, until three-quarters of the disc appear lighted up. She is
+then said to be <i>gibbous</i>.</p>
+
+<p>Eventually she moves round so that she faces the<span class='pagenum'><a name="Page_184" id="Page_184">[Pg 184]</a></span> sun completely, and
+the whole of her disc appears illuminated. She is then spoken of as
+<i>full</i>. When in this position it is clear that she is on the contrary
+side of the earth to the sun, and therefore rises about the same time
+that he is setting. She is now, in fact, at her furthest from the sun.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_14" id="Fig_14"></a><span class="caption">Direction from which the sun's rays are coming.</span>
+<img src="images/figure14a.jpg" width="500" height="339" alt="Fig. 14a" title="" />
+<span class="caption">Various positions and illumination of the mooon by the sun during her
+revolution around the earth.</span>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/figure14b.jpg" width="500" height="100" alt="Fig. 14b" title="" />
+<span class="caption"><br />The corresponding positions as viewed from the earth, showing the
+consequent phases.<br />
+<span class="smcap"><br />Fig. 14.</span>&mdash;Orbit and Phases of the Moon.</span>
+</div>
+
+<p>After this, the motion of the moon in her orbit carries her on back
+again in the direction of the sun.<span class='pagenum'><a name="Page_185" id="Page_185">[Pg 185]</a></span> She thus goes through her phases as
+before, only these of course are <i>in the reverse order</i>. The full phase
+is seen to give place to the gibbous, and this in turn to the half-moon
+and to the crescent; after which her motion carries her into the
+neighbourhood of the sun, and she is once more new, and lost to our
+sight in the solar glare. Following this she draws away to the east of
+the sun again, and the old order of phases repeat themselves as before.</p>
+
+<p>The early Babylonians imagined that the moon had a bright and a dark
+side, and that her phases were caused by the bright side coming more and
+more into view during her movement around the sky. The Greeks, notably
+Aristotle, set to work to examine the question from a mathematical
+standpoint, and came to the conclusion that the crescent and other
+appearances were such as would necessarily result if the moon were a
+dark body of spherical shape illumined merely by the light of the sun.</p>
+
+<p>Although the true explanation of the moon's phases has thus been known
+for centuries, it is unfortunately not unusual to see
+pictures&mdash;advertisement posters, for instance&mdash;in which stars appear
+<i>within</i> the horns of a crescent moon! Can it be that there are to-day
+educated persons who believe that the moon is a thing which <i>grows</i> to a
+certain size and then wastes away again; who, in fact, do not know that
+the entire body of the moon is there all the while?</p>
+
+<p>When the moon shows a very thin crescent, we are able dimly to see her
+still dark portion standing out against the sky. This appearance is
+popularly known as the "old moon in the new moon's arms." The dark part
+of her surface must, indeed, be to some<span class='pagenum'><a name="Page_186" id="Page_186">[Pg 186]</a></span> degree illumined, or we should
+not be able to see it at all. Whence then comes the light which
+illumines it, since it clearly cannot come from the sun? The riddle is
+easily solved, if we consider what kind of view of our earth an observer
+situated on this darkened part of the moon would at that moment get. He
+would, as a matter of fact, just then see nearly the whole disc of the
+earth brightly lit up by sunlight. The lunar landscape all around would,
+therefore, be bathed in what to <i>him</i> would be "earthlight," which of
+course takes the place there of what <i>we</i> call moonlight. If, then, we
+recollect how much greater in size the earth is than the moon, it should
+not surprise us that this earthlight will be many times brighter than
+moonlight. It is considered, indeed, to be some twenty times brighter.
+It is thus not at all astonishing that we can see the dark portion of
+the moon illumined merely by sunlight reflected upon it from our earth.</p>
+
+<p>The ancients were greatly exercised in their minds to account for this
+"earthlight," or "earthshine," as it is also called. Posidonius (135&ndash;51
+<span class="ampm">B.C.</span>) tried to explain it by supposing that the moon was partially
+transparent, and that some sunlight consequently filtered through from
+the other side. It was not, however, until the fifteenth century that
+the correct solution was arrived at.</p>
+
+<p><span class='pagenum'><a name="Page_187" id="Page_187">[Pg 187]</a></span></p>
+<div class="figcenter" style="width: 500px;"><a name="Fig_15" id="Fig_15"></a>
+<img src="images/figure15a.jpg" width="500" height="330" alt="Fig. 15a" title="" />
+<div class="caption1">One side of the moon only is ever presented to the
+earth. This side is here indicated by the letters S.F.E. (side facing
+earth).</div>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/figure15b.jpg" width="500" height="97" alt="Fig. 15" title="" />
+<div class="caption1">By placing the above positions in a row, we can see at once that the
+moon makes one complete rotation on her axis in exactly the same time as
+she revolves around the earth.</div>
+<span class="caption"><br /><span class="smcap">Fig. 15.</span>&mdash;The Rotation of the Moon on her Axis.</span>
+</div>
+
+<p>Perhaps the most remarkable thing which one notices about the moon is
+that she always turns the same side towards us, and so we never see her
+other side. One might be led from this to jump to the conclusion that
+she does not rotate upon an axis, as do the other bodies which we see;
+but, paradoxical as it may appear, the fact that she turns one face
+always towards the earth, is actually a proof that she <i>does</i> rotate
+upon an axis. The rotation, however, takes place with such slowness,
+that she turns round but once during the time in which she revolves
+around the earth (<a href="#Fig_15">see Fig. 15</a>). In order to understand the matter
+clearly, let the reader place an object in the centre of a room and walk
+around it once, <i>keeping his face turned towards it the whole time</i>,
+While he is doing this, it is evident that he will face every one of the
+four walls of the room in succession.<span class='pagenum'><a name="Page_188" id="Page_188">[Pg 188]</a></span> Now in order to face each of the
+four walls of a room in succession one would be obliged <i>to turn oneself
+entirely round</i>. Therefore, during the act of walking round an object
+with his face turned directly towards it, a person at the same time
+turns his body once entirely round.</p>
+
+<p>In the long, long past the moon must have turned round much faster than
+this. Her rate of rotation has no doubt been slowed down by the action
+of some force. It will be recollected how, in the course of the previous
+chapter, we found that the tides were tending, though exceedingly
+gradually, to slow down the rotation of the earth upon its axis. But, on
+account of the earth's much greater mass, the force of gravitation
+exercised by it upon the surface of the moon is, of course, much more
+powerful than that which the moon exercises upon the surface of the
+earth. The tendency to tidal action on the moon itself must, therefore,
+be much in excess of anything which we here experience. It is, in
+consequence, probable that such a tidal drag, extending over a very long
+period of time, has resulted in slowing down the moon's rotation to its
+present rate.</p>
+
+<p>The fact that we never see but one side of the moon has given rise from
+time to time to fantastic speculations with regard to the other side.
+Some, indeed, have wished to imagine that our satellite is shaped like
+an egg, the more pointed end being directed away from us. We are here,
+of course, faced with a riddle, which is all the more tantalising from
+its appearing for ever insoluble to men, chained as they are to the
+earth. However, it seems going too far to suppose that any abnormal
+conditions necessarily<span class='pagenum'><a name="Page_189" id="Page_189">[Pg 189]</a></span> exist at the other side of the moon. As a matter
+of fact, indeed, small portions of that side are brought into our view
+from time to time in consequence of slight irregularities in the moon's
+movement; and these portions differ in no way from those which we
+ordinarily see. On the whole, we obtain a view of about 60 per cent. of
+the entire lunar surface; that is to say, a good deal more than
+one-half.</p>
+
+<p>The actual diameter of the moon is about 2163 miles, which is somewhat
+more than one-quarter the diameter of the earth. For a satellite,
+therefore, she seems very large compared with her primary, the earth;
+when we consider that Jupiter's greatest satellite, although nearly
+twice as broad as our moon, has a diameter only one twenty-fifth that of
+Jupiter. Furthermore, the moon moves around the earth comparatively
+slowly, making only about thirteen revolutions during the entire year.
+Seen from space, therefore, she would not give the impression of a
+circling body, as other satellites do. Her revolutions are, indeed,
+relatively so very slow that she would appear rather like a smaller
+planet accompanying the earth in its orbit. In view of all this, some
+astronomers are inclined to regard the earth and moon rather as a
+"double planet" than as a system of planet and satellite.</p>
+
+<p>When the moon is full she attracts more attention perhaps than in any of
+her other phases. The moon, in order to be full, must needs be in that
+region of the heavens exactly opposite to the sun. The sun <i>appears</i> to
+go once entirely round the sky in the course of a year, and the moon
+performs the same journey in the space of about a month. The moon,<span class='pagenum'><a name="Page_190" id="Page_190">[Pg 190]</a></span> when
+full, having got half-way round this journey, occupies, therefore, that
+region of the sky which the sun itself will occupy half a year later.
+Thus in winter the full moon will be found roughly to occupy the sun's
+summer position in the sky, and in summer the sun's winter position. It
+therefore follows that the full moon in winter time is high up in the
+heavens, while in summer time it is low down. We thus get the greatest
+amount of full moonlight when it is the most needed.</p>
+
+<p>The great French astronomer, Laplace, being struck by the fact that the
+"lesser light" did not rule the night to anything like the same extent
+that the "greater light" ruled the day, set to work to examine the
+conditions under which it might have been made to do so. The result of
+his speculations showed that if the moon were removed to such a distance
+that she took a year instead of a month to revolve around the earth; and
+if she were started off in her orbit at full moon, she would always
+continue to remain full&mdash;a great advantage for us. Whewell, however,
+pointed out that in order to get the moon to move with the requisite
+degree of slowness, she would have to revolve so far from the earth that
+she would only look one-sixteenth as large as she does at present, which
+rather militates against the advantage Laplace had in mind! Finally,
+however, it was shown by M. Liouville, in 1845, that the position of a
+<i>perennial full moon</i>, such as Laplace dreamed of, would be
+unstable&mdash;that is to say, the body in question could not for long remain
+undisturbed in the situation suggested (<a href="#Fig_16">see Fig. 16</a>, p. 191).</p>
+
+<p><span class='pagenum'><a name="Page_191" id="Page_191">[Pg 191]</a></span></p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_16" id="Fig_16"></a>
+<img src="images/figure16a.jpg" width="500" height="354" alt="Fig. 16a" title="" />
+<span class="caption">Various positions of Laplace's "Moon" with regard to the
+earth and sun during the course of a year.</span>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/figure16b.jpg" width="500" height="167" alt="Fig. 16." title="" />
+<span class="caption"><br />The same positions of Laplace's "Moon," arranged around the earth, show
+that it would make only one revolution in a year.<br /><br />
+<span class="smcap">Fig. 16.</span>&mdash;Laplace's "Perennial Full Moon."</span>
+</div>
+
+<p>There is a well-known phenomenon called <i>harvest moon</i>, concerning the
+nature of which there seems to be much popular confusion. An idea in
+fact appears to prevail among a good many people that the moon is a
+harvest moon when, at rising, it looks bigger and redder than usual.
+Such an appearance has, however, nothing at all to say to the matter;
+for the<span class='pagenum'><a name="Page_192" id="Page_192">[Pg 192]</a></span> moon always <i>looks</i> larger when low down in the sky, and,
+furthermore, it usually looks red in the later months of the year, when
+there is more mist and fog about than there is in summer. What
+astronomers actually term the harvest moon is, indeed, something
+entirely different from this. About the month of September the slant at
+which the full moon comes up from below the horizon happens to be such
+that, during several evenings together, she <i>rises almost at the same
+hour</i>, instead of some fifty minutes later, as is usually the case. As
+the harvest is being gathered in about that time, it has come to be
+popularly considered that this is a provision of nature, according to
+which the sunlight is, during several evenings, replaced without delay
+by more or less full-moonlight, in order that harvesters may continue
+their work straight on into the night, and not be obliged to break off
+after sunset to wait until the moon rises. The same phenomenon is almost
+exactly repeated a month later, but by reason of the pursuits then
+carried on it is known as the "hunter's moon."</p>
+
+<p>In this connection should be mentioned that curious phenomenon above
+alluded to, and which seems to attract universal notice, namely, that
+the moon <i>looks much larger when near the horizon</i>&mdash;at its rising, for
+instance, than when higher up in the sky. This seeming enlargement is,
+however, by no means confined to the moon. That the sun also looks much
+larger when low down in the sky than when high up, seems to strike even
+the most casual watcher of a sunset. The same kind of effect will,
+indeed, be noted if close attention be paid to the stars themselves. A
+constellation, for instance, appears more<span class='pagenum'><a name="Page_193" id="Page_193">[Pg 193]</a></span> spread out when low down in
+the sky than when high up. This enlargement of celestial objects when in
+the neighbourhood of the horizon is, however, only <i>apparent</i> and not
+real. It must be entirely an <i>illusion</i>; for the most careful
+measurements of the discs of the sun and of the moon fail to show that
+the bodies are any larger when near the horizon than when high up in the
+sky. In fact, if there be any difference in measurements with regard to
+the moon, it will be found to be the other way round; for her disc, when
+carefully measured, is actually the slightest degree <i>greater</i> when
+<i>high</i> in the sky, than when low down. The reason for this is that, on
+account of the rotundity of the earth's surface, she is a trifle nearer
+the observer when overhead of him.</p>
+
+<p>This apparent enlargement of celestial objects, when low down in the
+sky, is granted on all sides to be an illusion; but although the
+question has been discussed with animation time out of mind, none of the
+explanations proposed can be said to have received unreserved
+acceptance. The one which usually figures in text-books is that we
+unconsciously compare the sun and moon, when low down in the sky, with
+the terrestrial objects in the same field of view, and are therefore
+inclined to exaggerate the size of these orbs. Some persons, on the
+other hand, imagine the illusion to have its source in the structure of
+the human eye; while others, again, put it down to the atmosphere,
+maintaining that the celestial objects in question <i>loom</i> large in the
+thickened air near the horizon, in the same way that they do when viewed
+through fog or mist.</p>
+
+<p><span class='pagenum'><a name="Page_194" id="Page_194">[Pg 194]</a></span></p><p>The writer<a name="FNanchor_14_14" id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a> ventures, however, to think that the illusion has its
+origin in our notion of the shape of the celestial vault. One would be
+inclined, indeed, to suppose that this vault ought to appear to us as
+the half of a hollow sphere; but he maintains that it does not so
+appear, as a consequence of the manner in which the eyes of men are set
+quite close together in their heads. If one looks, for instance, high up
+in the sky, the horizon cannot come within the field of view, and so
+there is nothing to make one think that the expanse then gazed upon is
+other than quite <i>flat</i>&mdash;let us say like the ceiling of a room. But, as
+the eyes are lowered, a portion of the <i>encircling</i> horizon comes
+gradually into the field of view, and the region of the sky then gazed
+upon tends in consequence to assume a <i>hollowed-out</i> form. From this it
+would seem that our idea of the shape of the celestial vault is, that it
+is <i>flattened down over our heads and hollowed out all around in the
+neighbourhood of the horizon</i> (<a href="#Fig_17">see Fig. 17</a>, p. 195). Now, as a
+consequence of their very great distance, all the objects in the heavens
+necessarily appear to us to move as if they were placed on the
+background of the vault; the result being that the mind is obliged to
+conceive them as expanded or contracted, in its unconscious attempts to
+make them always fill their due proportion of space in the various parts
+of this abnormally shaped sky.</p>
+
+<p>From such considerations the writer concludes that the apparent
+enlargement in question is merely the natural consequence of the idea we
+have of the shape of the celestial vault&mdash;an idea gradually built up in<span class='pagenum'><a name="Page_195" id="Page_195">[Pg 195]</a></span>
+childhood, to become later on what is called "second nature." And in
+support of this contention, he would point to the fact that the
+enlargement is not by any means confined to the sun and moon, but is
+every whit as marked in the case of the constellations. To one who has
+not noticed this before, it is really quite a revelation to compare the
+appearance of one of the large constellations (Orion, for instance) when
+high up in the sky and when low down. The widening apart of the various
+stars composing the group, when in the latter position, is very
+noticeable indeed.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_17" id="Fig_17"></a>
+<img src="images/figure17.jpg" width="500" height="285" alt="Fig. 17." title="" />
+<span class="caption"><span class="smcap">Fig. 17.</span>&mdash;Illustrating the author's explanation of the
+apparent enlargement of celestial objects.</span>
+</div>
+
+<p>Further, if a person were to stand in the centre of a large dome, he
+would be exactly situated as if he were beneath the vaulted heaven, and
+one would consequently expect him to suffer the same illusion as to the
+shape of the dome. Objects fixed upon its background would therefore
+appear to him under the<span class='pagenum'><a name="Page_196" id="Page_196">[Pg 196]</a></span> same conditions as objects in the sky, and the
+illusions as to their apparent enlargement should hold good here also.</p>
+
+<p>Some years ago a Belgian astronomer, M. Stroobant, in an investigation
+of the matter at issue, chanced to make a series of experiments under
+the very conditions just detailed. To various portions of the inner
+surface of a large dome he attached pairs of electric lights; and on
+placing himself at the centre of the building, he noticed that, in every
+case, those pairs which were high up appeared closer together than those
+which were low down! He does not, however, seem to have sought for the
+cause in the vaulted expanse. On the contrary, he attributed the effect
+to something connected with our upright stature, to some physiological
+reason which regularly makes us estimate objects as larger when in front
+than when overhead.</p>
+
+<p>In connection with this matter, it may be noted that it always appears
+extremely difficult to estimate with the eye the exact height above the
+horizon at which any object (say a star) happens to be. Even skilled
+observers find themselves in error in attempting to do so. This seems to
+bear out the writer's contention that the form under which the celestial
+vault really appears to us is a peculiar one, and tends to give rise to
+false judgments.</p>
+
+<p>Before leaving this question, it should also be mentioned that nothing
+perhaps is more deceptive than the size which objects in the sky appear
+to present. The full moon looks so like a huge plate, that it astonishes
+one to find that a threepenny bit held at arm's length will a long way
+more than cover its disc.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Plate_VIII" id="Plate_VIII"></a>
+<img src="images/plate8.jpg" width="500" height="788" alt="Plate VIII." title="" />
+<span class="caption"><span class="smcap">Plate VIII. The Moon</span><br />
+From a photograph taken at the Paris Observatory by M.P. Puiseux.<br />(<a href="#Page_197"><small>Page 197</small></a>)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_197" id="Page_197">[Pg 197]</a></span></p><p>The moon is just too far off to allow us to see the actual detail on
+her surface with the naked eye. When thus viewed she merely displays a
+patchy appearance,<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a> and the imaginary forms which her darker markings
+suggest to the fancy are popularly expressed by the term "Man in the
+Moon." An examination of her surface with very moderate optical aid is,
+however, quite a revelation, and the view we then get is not easily
+comparable to what we see with the unaided eye.</p>
+
+<p>Even with an ordinary opera-glass, an observer will be able to note a
+good deal of detail upon the lunar disc. If it be his first observation
+of the kind, he cannot fail to be struck by the fact to which we have
+just made allusion, namely, the great change which the moon appears to
+undergo when viewed with magnifying power. "Cain and his Dog," the "Man
+in the Moon gathering sticks," or whatever indeed his fancy was wont to
+conjure up from the lights and shades upon the shining surface, have now
+completely disappeared; and he sees instead a silvery globe marked here
+and there with extensive dark areas, and pitted all over with
+crater-like formations (<a href="#Plate_VIII">see Plate VIII.</a>, p. 196). The dark areas retain
+even to the present day their ancient name of "seas," for Galileo and
+the early telescopic observers believed them to be such, and they are
+still catalogued under the mystic appellations given to them in the long
+ago; as, for instance, "Sea of Showers," "Bay of Rainbows," "Lake of
+Dreams."<a name="FNanchor_16_16" id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a> The improved telescopes of later<span class='pagenum'><a name="Page_198" id="Page_198">[Pg 198]</a></span> times showed, however,
+that they were not really seas (there is no water on the moon), but
+merely areas of darker material.</p>
+
+<p>The crater-like formations above alluded to are the "lunar mountains." A
+person examining the moon for the first time with telescopic aid, will
+perhaps not at once grasp the fact that his view of lunar mountains must
+needs be what is called a "bird's-eye" one, namely, a view from above,
+like that from a balloon and that he cannot, of course, expect to see
+them from the side, as he does the mountains upon the earth. But once he
+has realised this novel point of view, he will no doubt marvel at the
+formations which lie scattered as it were at his feet. The type of lunar
+mountain is indeed in striking contrast to the terrestrial type. On our
+earth the range-formation is supreme; on the moon the crater-formation
+is the rule, and is so-called from analogy to our volcanoes. A typical
+lunar crater may be described as a circular wall, enclosing a central
+plain, or "floor," which is often much depressed below the level of the
+surface outside. These so-called "craters," or "ring-mountains," as they
+are also termed, are often of gigantic proportions. For instance, the
+central plain of one of them, known as Ptolem&aelig;us,<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a> is about 115 miles
+across, while that of Plato is about 60. The walls of craters often rise
+to great heights; which, in proportion to the small size of the moon,
+are very much in excess of our highest terrestrial elevations.
+Nevertheless, a person posted at the centre of one of the larger craters
+might be surprised to find that he could not see the encompassing
+crater-walls, which would in every direction be below his horizon. This
+would arise not alone from the great breadth of the crater itself, but
+also from the fact that the curving of the moon's surface is very sharp
+compared with that of our earth.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Plate_IX" id="Plate_IX"></a>
+<img src="images/plate9.jpg" width="500" height="501" alt="Plate IX." title="" />
+<span class="caption"><span class="smcap">Plate IX. Map of the Moon, showing the principal
+&quot;Craters,&quot; Mountain Ranges, and &quot;Seas&quot;</span></span>
+<div class="caption2"><br />In this, as in the other plates of the Moon, the <i>South</i> will be found
+at the top of the picture; such being the view given by the ordinary
+astronomical telescope, in which all objects are seen <i>inverted</i>.<br />(<a href="#Page_199"><small>Page 199</small></a>)</div>
+</div>
+
+<p><span class='pagenum'><a name="Page_199" id="Page_199">[Pg 199]</a></span></p><p>We have mentioned Ptolem&aelig;us as among the very large craters, or
+ring-mountains, on the moon. Its encompassing walls rise to nearly
+13,000 feet, and it has the further distinction of being almost in the
+centre of the lunar disc. There are, however, several others much wider,
+but they are by no means in such a conspicuous position. For instance,
+Schickard, close to the south-eastern border, is nearly 130 miles in
+diameter, and its wall rises in one point to over 10,000 feet. Grimaldi,
+almost exactly at the east point, is nearly as large as Schickard.
+Another crater, Clavius, situated near the south point, is about 140
+miles across; while its neighbour Bailly&mdash;named after a famous French
+astronomer of the eighteenth century&mdash;is 180, and the largest of those
+which we can see (<a href="#Plate_IX">see Plate IX.</a>, p. 198).</p>
+
+<p>Many of the lunar craters encroach upon one another; in fact there is
+not really room for them all upon the visible hemisphere of the moon.
+About 30,000 have been mapped; but this is only a small portion, for
+according to the American astronomer, Professor W.H. Pickering, there
+are more than 200,000 in all.</p>
+
+<p>Notwithstanding the fact that the crater is the type of mountain
+associated in the mind with the moon, it must not be imagined that upon
+our satellite there<span class='pagenum'><a name="Page_200" id="Page_200">[Pg 200]</a></span> are no mountains at all of the terrestrial type.
+There are indeed many isolated peaks, but strangely enough they are
+nearly always to be found in the centres of craters. Some of these peaks
+are of great altitude, that in the centre of the crater Copernicus being
+over 11,000 feet high. A few mountain ranges also exist; the best known
+of which are styled, the Lunar Alps and Lunar Apennines (<a href="#Plate_X">see Plate X.</a>,
+p. 200).</p>
+
+<p>Since the <i>mass</i> of the moon is only about one-eightieth that of the
+earth, it will be understood that the force of gravity which she
+exercises is much less. It is calculated that, at her surface, this is
+only about one-sixth of what we experience. A man transported to the
+moon would thus be able to jump <i>six times as high</i> as he can here. A
+building could therefore be six times as tall as upon our earth, without
+causing any more strain upon its foundations. It should not, then, be
+any subject for wonder, that the highest peaks in the Lunar Apennines
+attain to such heights as 22,000 feet. Such a height, upon a
+comparatively small body like the moon, for her <i>volume</i> is only
+one-fiftieth that of the earth, is relatively very much in excess of the
+29,000 feet of Himalayan structure, Mount Everest, the boast of our
+planet, 8000 miles across!</p>
+
+<p>High as are the Lunar Apennines, the highest peaks on the moon are yet
+not found among them. There is, for instance, on the extreme southern
+edge of the lunar disc, a range known as the Leibnitz Mountains; several
+peaks of which rise to a height of nearly 30,000 feet, one peak in
+particular being said to attain to 36,000 feet (<a href="#Plate_IX">see Plate IX.</a>, p. 198).</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Plate_X" id="Plate_X"></a>
+<img src="images/plate10.jpg" width="500" height="755" alt="Plate X." title="" />
+<span class="caption"><span class="smcap">Plate X. One of the most interesting regions on the Moon</span></span>
+<div class="caption2">We have here (<a href="#Plate_IX">see "Map," Plate IX.</a>, p. 198) the mountain ranges of the
+Apennines, the Caucasus and the Alps; also the craters Plato, Aristotle,
+Eudoxus, Cassini, Aristillus, Autolycus, Archimedes and Linn&eacute;. The
+crater Linn&eacute; is the very bright spot in the dark area at the upper left
+hand side of the picture. From a photograph taken at the Paris
+Observatory by M.M. Loewy and Puiseux.<br />(<a href="#Page_200"><small>Page 200</small></a>)</div>
+</div>
+
+<p><span class='pagenum'><a name="Page_201" id="Page_201">[Pg 201]</a></span></p><p>But the reader will surely ask the question: "How is it possible to
+determine the actual height of a lunar mountain, if one cannot go upon
+the moon to measure it?" The answer is, that we can calculate its height
+from noting the length of the shadow which it casts. Any one will allow
+that the length of a shadow cast by the sun depends upon two things:
+firstly, upon the height of the object which causes the shadow, and
+secondly, upon the elevation of the sun at the moment in the sky. The
+most casual observer of nature upon our earth can scarcely have failed
+to notice that shadows are shortest at noonday, when the sun is at its
+highest in the sky; and that they lengthen out as the sun declines
+towards its setting. Here, then, we have the clue. To ascertain,
+therefore, the height of a lunar mountain, we have first to consider at
+what elevation the sun is at that moment above the horizon of the place
+where the mountain in question is situated. Then, having measured the
+actual length in miles of the shadow extended before us, all that is
+left is to ask ourselves the question: "What height must an object be
+whose shadow cast by the sun, when at that elevation in the sky, will
+extend to this length?"</p>
+
+<p>There is no trace whatever of water upon the moon. The opinion, indeed,
+which seems generally held, is that water has never existed upon its
+surface. Erosions, sedimentary deposits, and all those marks which point
+to a former occupation by water are notably absent.</p>
+
+<p>Similarly there appears to be no atmosphere on the moon; or, at any
+rate, such an excessively rare one, as to be quite inappreciable. Of
+this there are several proofs. For instance, in a solar eclipse the<span class='pagenum'><a name="Page_202" id="Page_202">[Pg 202]</a></span>
+moon's disc always stands out quite clear-cut against that of the sun.
+Again during occultations, stars disappear behind the moon with a
+suddenness, which could not be the case were there any appreciable
+atmosphere. Lastly, we see no traces of twilight upon the lunar surface,
+nor any softening at the edges of shadows; both which effects would be
+apparent if there were an atmosphere.</p>
+
+<p>The moon's surface is rough and rocky, and displays no marks of the
+"weathering" that would necessarily follow, had it possessed anything of
+an atmosphere in the past. This makes us rather inclined to doubt that
+it ever had one at all. Supposing, however, that it did possess an
+atmosphere in the past, it is interesting to inquire what may have
+become of it. In the first place it might have gradually disappeared, in
+consequence of the gases which composed it uniting chemically with the
+materials of which the lunar body is constructed; or, again, its
+constituent gases may have escaped into space, in accordance with the
+principles of that kinetic theory of which we have already spoken. The
+latter solution seems, indeed, the most reasonable of the two, for the
+force of gravity at the lunar surface appears too weak to hold down any
+known gases. This argument seems also to dispose of the question of
+absence of water; for Dr. George Johnstone Stoney, in a careful
+investigation of the subject, has shown that the liquid in question,
+when in the form of vapour, will escape from a planet if its mass is
+less than <i>one-fourth</i> that of our earth. And the mass of the moon is
+very much less than this; indeed only the <i>one-eightieth</i>, as we have
+already stated.</p>
+
+<p><span class='pagenum'><a name="Page_203" id="Page_203">[Pg 203]</a></span></p><p>In consequence of this lack of atmosphere, the condition of things upon
+the moon will be in marked contrast to what we experience upon the
+earth. The atmosphere here performs a double service in shielding us
+from the direct rays of the sun, and in bottling the heat as a
+glass-house does. On the moon, however, the sun beats down in the
+day-time with a merciless force; but its rays are reflected away from
+the surface as quickly as they are received, and so the cold of the
+lunar night is excessive. It has been calculated that the day
+temperature on the moon may, indeed, be as high as our boiling-point,
+while the night temperature may be more than twice as low as the
+greatest cold known in our arctic regions.</p>
+
+<p>That a certain amount of solar heat is reflected to us from the moon is
+shown by the sharp drop in temperature which certain heat-measuring
+instruments record when the moon becomes obscured in a lunar eclipse.
+The solar heat which is thus reflected to us by the moon is, however, on
+the whole extremely small; more light and heat, indeed, reach us
+<i>direct</i> from the sun in half a minute than we get by <i>reflection</i> from
+the moon during the entire course of the year.</p>
+
+<p>With regard to the origin of the lunar craters there has been much
+discussion. Some have considered them to be evidence of violent volcanic
+action in the dim past; others, again, as the result of the impact of
+meteorites upon the lunar surface, when the moon was still in a plastic
+condition; while a third theory holds that they were formed by the
+bursting of huge bubbles during the escape into space of gases from the
+interior. The question is, indeed, a very difficult<span class='pagenum'><a name="Page_204" id="Page_204">[Pg 204]</a></span> one. Though
+volcanic action, such as would result in craters of the size of
+Ptolem&aelig;us, is hard for us to picture, and though the lone peaks which
+adorn the centres of many craters have nothing reminiscent of them in
+our terrestrial volcanoes, nevertheless the volcanic theory seems to
+receive more favour than the others.</p>
+
+<p>In addition to the craters there are two more features which demand
+notice, namely, what are known as <i>rays</i> and <i>rills</i>. The rays are long,
+light-coloured streaks which radiate from several of the large craters,
+and extend to a distance of some hundreds of miles. That they are mere
+markings on the surface is proved by the fact that they cast no shadows
+of any kind. One theory is, that they were originally great cracks which
+have been filled with lighter coloured material, welling up from
+beneath. The rills, on the other hand, are actually fissures, about a
+mile or so in width and about a quarter of a mile in depth.</p>
+
+<p>The rays are seen to the best advantage in connection with the craters
+Tycho and Copernicus (<a href="#Plate_XI">see Plate XI.</a>, p. 204). In consequence of its
+fairly forward position on the lunar disc, and of the remarkable system
+of rays which issue from it like spokes from the axle of a wheel, Tycho
+commands especial attention. The late Rev. T.W. Webb, a famous observer,
+christened it, very happily, the "metropolitan crater of the moon."</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Plate_XI" id="Plate_XI"></a>
+<img src="images/plate11.jpg" width="500" height="654" alt="Plate XI." title="" />
+<span class="caption"><span class="smcap">Plate XI. The Moon</span><br /><br />
+The systems of rays from the craters Tycho, Copernicus and Kepler are
+well shown here. From a photograph taken at the Paris Observatory by
+M.P. Puiseux.<br />(<a href="#Page_204"><small>Page 204</small></a>)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_205" id="Page_205">[Pg 205]</a></span></p><p>A great deal of attention is, and has been, paid by certain astronomers
+to the moon, in the hope of finding out if any changes are actually in
+progress at present upon her surface. Sir William Herschel, indeed, once
+thought that he saw a lunar volcano in eruption, but this proved to be
+merely the effect of the sunlight striking the top of the crater
+Aristarchus, while the region around it was still in shadow&mdash;sunrise
+upon Aristarchus, in fact! No change of any real importance has,
+however, been noted, although it is suspected that some minor
+alterations have from time to time taken place. For instance, slight
+variations of tint have been noticed in certain areas of the lunar
+surface. Professor W.H. Pickering puts forward the conjecture that these
+may be caused by the growth and decay of some low form of vegetation,
+brought into existence by vapours of water, or carbonic acid gas, making
+their way out from the interior through cracks near at hand.</p>
+
+<p>Again, during the last hundred years one small crater known as Linn&eacute;
+(Linn&aelig;us), situated in the Mare Serenitatis (Sea of Serenity), has
+appeared to undergo slight changes, and is even said to have been
+invisible for a while (<a href="#Plate_X">see Plate X.</a>, p. 200). It is, however, believed
+that the changes in question may be due to the varying angles at which
+the sunlight falls upon the crater; for it is an understood fact that
+the irregularities of the moon's motion give us views of her surface
+which always differ slightly.</p>
+
+<p>The suggestion has more than once been put forward that the surface of
+the moon is covered with a thick layer of ice. This is generally
+considered improbable, and consequently the idea has received very
+little support. It first originated with the late Mr. S.E. Peal, an
+English observer of the moon, and has recently been resuscitated by the
+German observer, Herr Fauth.</p>
+
+<p><span class='pagenum'><a name="Page_206" id="Page_206">[Pg 206]</a></span></p><p>The most unfavourable time for telescopic study of the moon is when she
+is full. The sunlight is then falling directly upon her visible
+hemisphere, and so the mountains cast no shadows. We thus do not get
+that impression of hill and hollow which is so very noticeable in the
+other phases.</p>
+
+<p>The first map of the moon was constructed by Galileo. Tobias Mayer
+published another in 1775; while during the nineteenth century greatly
+improved ones were made by Beer and M&auml;dler, Schmidt, Neison and others.
+In 1903, Professor W.H. Pickering brought out a complete photographic
+lunar atlas; and a similar publication has recently appeared, the work
+of MM. Loewy and Puiseux of the Observatory of Paris.</p>
+
+<p>The so-called "seas" of the moon are, as we have seen, merely dark
+areas, and there appears to be no proof that they were ever occupied by
+any liquid. They are for the most part found in the <i>northern</i> portion
+of the moon; a striking contrast to our seas and oceans, which take up
+so much of the <i>southern</i> hemisphere of the earth.</p>
+
+<p>There are many erroneous ideas popularly held with regard to certain
+influences which the moon is supposed to exercise upon the earth. For
+instance, a change in the weather is widely believed to depend upon a
+change in the moon. But the word "change" as here used is meaningless,
+for the moon is continually changing her phase during the whole of her
+monthly round. Besides, the moon is visible over a great portion of the
+earth <i>at the same moment</i>, and certainly all the places from which it
+can then be seen do not get the same weather! Further, careful
+observations,<span class='pagenum'><a name="Page_207" id="Page_207">[Pg 207]</a></span> and records extending over the past one hundred years and
+more, fail to show any reliable connection between the phases of the
+moon and the condition of the weather.</p>
+
+<p>It has been stated, on very good authority, that no telescope ever shows
+the surface of the moon as clearly as we could see it with the naked eye
+were it only 240 miles distant from us.</p>
+
+<p>Supposing, then, that we were able to approach our satellite, and view
+it without optical aid at such comparatively close quarters, it is
+interesting to consider what would be the smallest detail which our eye
+could take in. The question of the limit of what can be appreciated with
+the naked eye is somewhat uncertain, but it appears safe to say that at
+a distance of 240 miles the <i>minutest speck</i> visible would have to be
+<i>at least</i> some 60 yards across.</p>
+
+<p>Atmosphere and liquid both wanting, the lunar surface must be the seat
+of an eternal calm; where no sound breaks the stillness and where
+change, as we know it, does not exist. The sun beats down upon the arid
+rocks, and inky shadows lie athwart the valleys. There is no mellowing
+of the harsh contrasts.</p>
+
+<p>We cannot indeed absolutely affirm that Life has no place at all upon
+this airless and waterless globe, since we know not under what strange
+conditions it may manifest its presence; and our most powerful
+telescopes, besides, do not bring the lunar surface sufficiently near to
+us to disprove the existence there of even such large creatures as
+disport themselves upon our planet. Still, we find it hard to rid
+ourselves of the feeling that we are in the presence of a dead world. On
+she swings around the earth month<span class='pagenum'><a name="Page_208" id="Page_208">[Pg 208]</a></span> after month, with one face ever
+turned towards us, leaving a certain mystery to hang around that hidden
+side, the greater part of which men can never hope to see. The rotation
+of the moon upon her axis&mdash;the lunar day&mdash;has become, as we have seen,
+equal to her revolution around the earth. An epoch may likewise
+eventually be reached in the history of our own planet, when the length
+of the terrestrial day has been so slowed down by tidal friction that it
+will be equal to the year. Then will the earth revolve around the
+central orb, with one side plunged in eternal night and the other in
+eternal sunshine. But such a vista need not immediately distress us. It
+is millions of years forward in time.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_14_14" id="Footnote_14_14"></a><a href="#FNanchor_14_14"><span class="label">[14]</span></a> <i>Journal of the British Astronomical Association</i>, vol. x.
+(1899&ndash;1900), Nos. 1 and 3.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_15" id="Footnote_15_15"></a><a href="#FNanchor_15_15"><span class="label">[15]</span></a> Certain of the ancient Greeks thought the markings on the
+moon to be merely the reflection of the seas and lands of our earth, as
+in a badly polished mirror.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_16" id="Footnote_16_16"></a><a href="#FNanchor_16_16"><span class="label">[16]</span></a> Mare Imbrium, Sinus Iridum, Lacus Somniorum.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_17" id="Footnote_17_17"></a><a href="#FNanchor_17_17"><span class="label">[17]</span></a> The lunar craters have, as a rule, received their names
+from celebrated persons, usually men of science. This system of
+nomenclature was originated by Riccioli, in 1651.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_209" id="Page_209">[Pg 209]</a></span></p>
+<h3><a name="CHAPTER_XVII" id="CHAPTER_XVII"></a>CHAPTER XVII</h3>
+
+<h4>THE SUPERIOR PLANETS</h4>
+
+
+<p class="noin"><span class="smcap">Having</span>, in a previous chapter, noted the various aspects which an
+inferior planet presents to our view, in consequence of its orbit being
+nearer to the sun than the orbit of the earth, it will be well here to
+consider in the same way the case of a superior planet, and to mark
+carefully the difference.</p>
+
+<p>To begin with, it should be quite evident that we cannot ever have a
+transit of a superior planet. The orbit of such a body being entirely
+<i>outside</i> that of the earth, the body itself can, of course, never pass
+between us and the sun.</p>
+
+<p>A superior planet will be at its greatest distance from us when on the
+far side of the sun. It is said then to be in <i>conjunction</i>. As it comes
+round in its orbit it eventually passes, so to speak, at the <i>back</i> of
+us. It is then at its nearest, or in <i>opposition</i>, as this is
+technically termed, and therefore in the most favourable position for
+telescopic observation of its surface. Being, besides, seen by us at
+that time in the direction of the heavens exactly opposite to where the
+sun is, it will thus at midnight be high up in the south side of the
+sky, a further advantage to the observer.</p>
+
+<p>Last of all, a superior planet cannot show crescent shapes like an
+interior; for whether it be on the far<span class='pagenum'><a name="Page_210" id="Page_210">[Pg 210]</a></span> side of the sun, or behind us,
+or again to our right or left, the sunlight must needs appear to fall
+more or less full upon its face.</p>
+
+
+<p class="center"><br /><span class="smcap">The Planetoid Eros</span></p>
+
+<p>The nearest to us of the superior planets is the tiny body, Eros, which,
+as has been already stated, was discovered so late as the year 1898. In
+point of view, however, of its small size, it can hardly be considered
+as a true planet, and the name "planetoid" seems much more appropriate
+to it.</p>
+
+<p>Eros was not discovered, like Uranus, in the course of telescopic
+examination of the heavens, nor yet, like Neptune, as the direct result
+of difficult calculations, but was revealed by the impress of its light
+upon a photographic plate, which had been exposed for some length of
+time to the starry sky. Since many of the more recent additions to the
+asteroids have been discovered in the same manner, we shall have
+somewhat more to say about this special employment of photography when
+we come to deal with those bodies later on.</p>
+
+<p>The path of Eros around the sun is so very elliptical, or, to use the
+exact technical term, so very "eccentric," that the planetoid does not
+keep all the time entirely in the space between our orbit and that of
+Mars, which latter happens to be the next body in the order of planetary
+succession outwards. In portions of its journey Eros, indeed, actually
+goes outside the Martian orbit. The paths of the planetoid and of Mars
+are, however, <i>not upon the same plane</i>, so the bodies always pass clear
+of each other,<span class='pagenum'><a name="Page_211" id="Page_211">[Pg 211]</a></span> and there is thus as little chance of their dashing
+together as there would be of trains which run across a bridge at an
+upper level, colliding with those which pass beneath it at a lower
+level.</p>
+
+<p>When Eros is in opposition, it comes within about 13&frac12; million miles
+of our earth, and, after the moon, is therefore by a long way our
+nearest neighbour in space. It is, however, extremely small, not more,
+perhaps, than twenty miles in diameter, and is subject to marked
+variations in brightness, which do not appear up to the present to meet
+with a satisfactory explanation. But, insignificant as is this little
+body, it is of great importance to astronomy; for it happens to furnish
+the best method known of calculating the sun's distance from our
+earth&mdash;a method which Galle, in 1872, and Sir David Gill, in 1877,
+suggested that asteroids might be employed for, and which has in
+consequence supplanted the old one founded upon transits of Venus. The
+sun's distance is now an ascertained fact to within 100,000 miles, or
+less than half the distance of the moon.</p>
+
+
+<p class="center"><br /><span class="smcap">The Planet Mars</span></p>
+
+<p>We next come to the planet Mars. This body rotates in a period of
+slightly more than twenty-four hours. The inclination, or slant, of its
+axis is about the same as that of the earth, so that, putting aside its
+greater distance from the sun, the variations of season which it
+experiences ought to be very much like ours.</p>
+
+<p>The first marking detected upon Mars was the notable one called the
+Syrtis Major, also known, on<span class='pagenum'><a name="Page_212" id="Page_212">[Pg 212]</a></span> account of its shape, as the Hour-Glass
+Sea. This observation was made by the famous Huyghens in 1659; and, from
+the movement of the marking in question across the disc, he inferred
+that the planet rotated on its axis in a period of about twenty-four
+hours.</p>
+
+<p>There appears to be very little atmosphere upon Mars, the result being
+that we almost always obtain a clear view of the detail on its surface.
+Indeed, it is only to be expected from the kinetic theory that Mars
+could not retain much of an atmosphere, as the force of gravity at its
+surface is less than one-half of what we experience upon the earth. It
+should here be mentioned that recent researches with the spectroscope
+seem to show that, whatever atmosphere there may be upon Mars, its
+density at the surface of the planet cannot be more than the one-fourth
+part of the density of the air at the surface of the earth. Professor
+Lowell, indeed, thinks it may be more rarefied than that upon our
+highest mountain-tops.</p>
+
+<p>Seen with the naked eye, Mars appears of a red colour. Viewed in the
+telescope, its surface is found to be in general of a ruddy hue, varied
+here and there with darker patches of a bluish-green colour. These
+markings are permanent, and were supposed by the early telescopic
+observers to imply a distribution of the planet's surface into land and
+water, the ruddy portions being considered as continental areas (perhaps
+sandy deserts), and the bluish-green as seas. The similarity to our
+earth thus suggested was further heightened by the fact that broad white
+caps, situated at the poles, were seen to vary with the planet's
+seasons, diminishing greatly in extent during the<span class='pagenum'><a name="Page_213" id="Page_213">[Pg 213]</a></span> Martian summer (the
+southern cap in 1894 even disappearing altogether), and developing again
+in the Martian winter.<a name="FNanchor_18_18" id="FNanchor_18_18"></a><a href="#Footnote_18_18" class="fnanchor">[18]</a> Readers of Oliver Wendell Holmes will no
+doubt recollect that poet's striking lines:&mdash;</p>
+
+<p class="poem">
+"The snows that glittered on the disc of Mars<br />
+Have melted, and the planet's fiery orb<br />
+Rolls in the crimson summer of its year."<br />
+</p>
+
+<p>A state of things so strongly analogous to what we experience here,
+naturally fired the imaginations of men, and caused them to look on Mars
+as a world like ours, only upon a much smaller scale. Being smaller, it
+was concluded to have cooled quicker, and to be now long past its prime;
+and its "inhabitants" were, therefore, pictured as at a later stage of
+development than the inhabitants of our earth.</p>
+
+<p>Notwithstanding the strong temptation to assume that the whiteness of
+the Martian polar caps is due to fallen snow, such a solution is,
+however, by no means so simple as it looks. The deposition of water in
+the form of snow, or even of hoar frost, would at least imply that the
+atmosphere of Mars should now and then display traces of aqueous vapour,
+which it does not appear to do.<a name="FNanchor_19_19" id="FNanchor_19_19"></a><a href="#Footnote_19_19" class="fnanchor">[19]</a> It has, indeed, been suggested that
+the whiteness may not after all be due to this cause, but to carbonic
+acid gas (carbon dioxide), which is known to freeze at a <i>very low</i>
+temperature. The suggestion is plainly<span class='pagenum'><a name="Page_214" id="Page_214">[Pg 214]</a></span> based upon the assumption that,
+as Mars is so much further from the sun than we are, it would receive
+much less heat, and that the little thus received would be quickly
+radiated away into space through lack of atmosphere to bottle it in.</p>
+
+<p>We now come to those well-known markings, popularly known as the
+"canals" of Mars, which have been the subject of so much discussion
+since their discovery thirty years ago.</p>
+
+<p>It was, in fact, in the year 1877, when Mars was in opposition, and thus
+at its nearest to us, that the famous Italian astronomer, Schiaparelli,
+announced to the world that he had found that the ruddy areas, thought
+to be continents, were intersected by a network of straight dark lines.
+These lines, he reported, appeared in many cases to be of great length,
+so long, indeed, as several thousands of miles, and from about twenty to
+sixty miles in width. He christened the lines <i>channels</i>, the Italian
+word for which, "canali," was unfortunately translated into English as
+"canals." The analogy, thus accidentally suggested, gave rise to the
+idea that they might be actual waterways.<a name="FNanchor_20_20" id="FNanchor_20_20"></a><a href="#Footnote_20_20" class="fnanchor">[20]</a></p>
+
+<p>In the winter of 1881&ndash;1882, when Mars was again in opposition,
+Schiaparelli further announced that he had found some of these lines
+doubled; that is to say, certain of them were accompanied by similar
+lines running exactly parallel at no great distance away. There was at
+first a good deal of scepticism on the subject of Schiaparelli's
+discoveries, but gradually other observers found themselves seeing both
+the<span class='pagenum'><a name="Page_215" id="Page_215">[Pg 215]</a></span> lines and their doublings. We have in this a good example of a
+curious circumstance in astronomical observation, namely, the fact that
+when fine detail has once been noted by a competent observer, it is not
+long before other observers see the same detail with ease.</p>
+
+<p>An immense amount of close attention has been paid to the planet Mars
+during recent years by the American observer, Professor Percival Lowell,
+at his famous observatory, 7300 feet above the sea, near the town of
+Flagstaff, Arizona, U.S.A. His observations have not, like those of most
+astronomers, been confined merely to "oppositions," but he has
+systematically kept the planet in view, so far as possible, since the
+year 1894.</p>
+
+<p>The instrumental equipment of his observatory is of the very best, and
+the "seeing" at Flagstaff is described as excellent. In support of the
+latter statement, Mr. Lampland, of the Lowell Observatory, maintains
+that the faintest stars shown on charts made at the Lick Observatory
+with the 36&ndash;inch telescope there, are <i>perfectly visible</i> with the
+24&ndash;inch telescope at Flagstaff.</p>
+
+<p>Professor Lowell is, indeed, generally at issue with the other observers
+of Mars. He finds the canals extremely narrow and sharply defined, and
+he attributes the blurred and hazy appearance, which they have presented
+to other astronomers, to the unsteady and imperfect atmospheric
+conditions in which their observations have been made. He assigns to the
+thinnest a width of two or three miles, and from fifteen to twenty to
+the larger. Relatively to their width, however, he finds their length
+enormous.<span class='pagenum'><a name="Page_216" id="Page_216">[Pg 216]</a></span> Many of them are 2000 miles long, while one is even as much
+as 3540. Such lengths as these are very great in comparison with the
+smallness of the planet. He considers that the canals stand in some
+peculiar relation to the polar cap, for they crowd together in its
+neighbourhood. In place, too, of ill-defined condensations, he sees
+sharp black spots where the canals meet and intersect, and to these he
+gives the name of "Oases." He further lays particular stress upon a dark
+band of a blue tint, which is always seen closely to surround the edges
+of the polar caps all the time that they are disappearing; and this he
+takes to be a proof that the white material is something which actually
+<i>melts</i>. Of all substances which we know, water alone, he affirms, would
+act in such a manner.</p>
+
+<p>The question of melting at all may seem strange in a planet which is
+situated so far from the sun, and possesses such a rarefied atmosphere.
+But Professor Lowell considers that this very thinness of the atmosphere
+allows the direct solar rays to fall with great intensity upon the
+planet's surface, and that this heating effect is accentuated by the
+great length of the Martian summer. In consequence he concludes that,
+although the general climate of Mars is decidedly cold, it is above the
+freezing point of water.</p>
+
+<p>The observations at Flagstaff appear to do away with the old idea that
+the darkish areas are seas, for numerous lines belonging to the
+so-called "canal system" are seen to traverse them. Again, there is no
+star-like image of the sun reflected from them, as there would be, of
+course, from the surface of a great sheet of water. Lastly, they are
+observed to vary in tone and colour with the changing Martian seasons,
+the blue-green changing into ochre, and later on back again into
+blue-green. Professor Lowell regards these areas as great tracts of
+vegetation, which are brought into activity as the liquid reaches them
+from the melting snows.</p>
+
+<div class="figcenter" style="width: 600px;"><a name="Plate_XII" id="Plate_XII"></a>
+<img src="images/plate12.jpg" width="600" height="408" alt="Plate XII." title="" />
+<span class="caption"><span class="smcap">Plate XII. A Map of the Planet Mars</span></span>
+<div class="caption2">We see here the Syrtis Major (or "Hour-Glass Sea"), the polar caps,
+several "oases," and a large number of "canals," some of which are
+double. The South is at the top of the picture, in accordance with the
+<i>inverted</i> view given by an astronomical telescope. From a drawing by
+Professor Percival Lowell.<br />(<a href="#Page_216"><small>Page 216</small></a>)</div>
+</div>
+
+<p><span class='pagenum'><a name="Page_217" id="Page_217">[Pg 217]</a></span></p><p>With respect to the canals, the Lowell observations further inform us
+that these are invisible during the Martian winter, but begin to appear
+in the spring when the polar cap is disappearing. Professor Lowell,
+therefore, inclines to the view that in the middle of the so-called
+canals there exist actual waterways which serve the purposes of
+irrigation, and that what we see is not the waterways themselves, for
+they are too narrow, but the fringe of vegetation which springs up along
+the banks as the liquid is borne through them from the melting of the
+polar snows. He supports this by his observation that the canals begin
+to appear in the neighbourhood of the polar caps, and gradually grow, as
+it were, in the direction of the planet's equator.</p>
+
+<p>It is the idea of life on Mars which has given this planet such a
+fascination in the eyes of men. A great deal of nonsense has, however,
+been written in newspapers upon the subject, and many persons have thus
+been led to think that we have obtained some actual evidence of the
+existence of living beings upon Mars. It must be clearly understood,
+however, that Professor Lowell's advocacy of the existence of life upon
+that planet is by no means of this wild order. At the best he merely
+indulges in such theories as his remarkable observations naturally call
+forth. His views are as follows:&mdash;He considers that the planet has
+reached a time when "water" has become so scarce that the<span class='pagenum'><a name="Page_218" id="Page_218">[Pg 218]</a></span> "inhabitants"
+are obliged to employ their utmost skill to make their scanty supply
+suffice for purposes of irrigation. The changes of tone and colour upon
+the Martian surface, as the irrigation produces its effects, are similar
+to what a telescopic observer&mdash;say, upon Venus&mdash;would notice on our
+earth when the harvest ripens over huge tracts of country; that is, of
+course, if the earth's atmosphere allowed a clear view of the
+terrestrial surface&mdash;a very doubtful point indeed. Professor Lowell
+thinks that the perfect straightness of the lines, and the geometrical
+manner in which they are arranged, are clear evidences of artificiality.
+On a globe, too, there is plainly no reason why the liquid which results
+from the melting of the polar caps should trend at all in the direction
+of the equator. Upon our earth, for instance, the transference of water,
+as in rivers, merely follows the slope of the ground, and nothing else.
+The Lowell observations show, however, that the Martian liquid is
+apparently carried from one pole towards the equator, and then past it
+to the other pole, where it once more freezes, only to melt again in due
+season, and to reverse the process towards and across the equator as
+before. Professor Lowell therefore holds, and it seems a strong point in
+favour of his theory, that the liquid must, in some artificial manner,
+as by pumping, for instance, be <i>helped</i> in its passage across the
+surface of the planet.</p>
+
+<p>A number of attempts have been made to explain the <i>doubling</i> of the
+canals merely as effects of refraction or reflection; and it has even
+been suggested that it may arise from the telescope not being accurately
+focussed.</p>
+
+<p><span class='pagenum'><a name="Page_219" id="Page_219">[Pg 219]</a></span></p><p>The actual doubling of the canals once having been doubted, it was an
+easy step to the casting of doubt on the reality of the canals
+themselves. The idea, indeed, was put forward that the human eye, in
+dealing with detail so very close to the limit of visibility, may
+unconsciously treat as an actual line several point-like markings which
+merely happen to lie in a line. In order to test this theory,
+experiments were carried out in 1902 by Mr. E.W. Maunder of Greenwich
+Observatory, and Mr. J.E. Evans of the Royal Hospital School at
+Greenwich, in which certain schoolboys were set to make drawings of a
+white disc with some faint markings upon it. The boys were placed at
+various distances from the disc in question; and it was found that the
+drawings made by those who were just too far off to see distinctly, bore
+out the above theory in a remarkable manner. Recently, however, the
+plausibility of the <i>illusion</i> view has been shaken by photographs of
+Mars taken during the opposition of 1905 by Mr. Lampland at the Lowell
+Observatory, in which a number of the more prominent canals come out as
+straight dark lines. Further still, in some photographs made there quite
+lately, several canals are said to appear visibly double.</p>
+
+<p>Following up the idea alluded to in <a href="#CHAPTER_XVI">Chapter XVI.</a>, that the moon may be
+covered with a layer of ice, Mr. W.T. Lynn has recently suggested that
+this may be the case on Mars; and that, at certain seasons, the water
+may break through along definite lines, and even along lines parallel to
+these. This, he maintains, would account for the canals becoming
+gradually visible across the disc, without the necessity of Professor
+Lowell's "pumping" theory.</p>
+
+<p><span class='pagenum'><a name="Page_220" id="Page_220">[Pg 220]</a></span></p><p>And now for the views of Professor Lowell himself with regard to the
+doubling of the canals. From his observations, he considers that no
+pairs of railway lines could apparently be laid down with greater
+parallelism. He draws attention to the fact that the doubling does not
+take place by any means in every canal; indeed, out of 400 canals seen
+at Flagstaff, only fifty-one&mdash;or, roughly, one-eighth&mdash;have at any time
+been seen double. He lays great stress upon this, which he considers
+points strongly against the duplication being an optical phenomenon. He
+finds that the distance separating pairs of canals is much less in some
+doubles than in others, and varies on the whole from 75 to 200 miles.
+According to him, the double canals appear to be confined to within 40
+degrees of the equator: or, to quote his own words, they are "an
+equatorial feature of the planet, confined to the tropic and temperate
+belts." Finally, he points out that they seem to <i>avoid</i> the blue-green
+areas. But, strangely enough, Professor Lowell does not so far attempt
+to fit in the doubling with his body of theory. He makes the obvious
+remark that they may be "channels and return channels," and with that he
+leaves us.</p>
+
+<p>The conclusions of Professor Lowell have recently been subjected to
+strenuous criticism by Professor W.H. Pickering and Dr. Alfred Russel
+Wallace. It was Professor Pickering who discovered the "oases," and who
+originated the idea that we did not see the so-called "canals"
+themselves, but only the growth of vegetation along their borders. He
+holds that the oases are craterlets, and that the canals are cracks
+which radiate from them, as do the rifts and streaks<span class='pagenum'><a name="Page_221" id="Page_221">[Pg 221]</a></span> from craters upon
+the moon. He goes on to suggest that vapours of water, or of carbonic
+acid gas, escaping from the interior, find their way out through these
+cracks, and promote the growth of a low form of vegetation on either
+side of them. In support of this view he draws attention to the
+existence of long "steam-cracks," bordered by vegetation, in the deserts
+of the highly volcanic island of Hawaii. We have already seen, in an
+earlier chapter, how he has applied this idea to the explanation of
+certain changes which are suspected to be taking place upon the moon.</p>
+
+<p>In dealing with the Lowell canal system, Professor Pickering points out
+that under such a slight atmospheric pressure as exists on Mars, the
+evaporation of the polar caps&mdash;supposing them to be formed of
+snow&mdash;would take place with such extraordinary rapidity that the
+resulting water could never be made to travel along open channels, but
+that a system of gigantic tubes or water-mains would have to be
+employed!</p>
+
+<p>As will be gathered from his theories regarding vegetation, Professor
+Pickering does not deny the existence of a form of life upon Mars. But
+he will not hear of civilisation, or of anything even approaching it. He
+thinks, however, that as Mars is intermediate physically between the
+moon and earth, the form of life which it supports may be higher than
+that on the moon and lower than that on the earth.</p>
+
+<p>In a small book published in the latter part of 1907, and entitled <i>Is
+Mars Habitable?</i> Dr. Alfred Russel Wallace sets himself, among other
+things, to combat the idea of a comparatively high temperature, such as
+Professor Lowell has allotted to Mars. He shows<span class='pagenum'><a name="Page_222" id="Page_222">[Pg 222]</a></span> the immense service
+which the water-vapour in our atmosphere exercises, through keeping the
+solar heat from escaping from the earth's surface. He then draws
+attention to the fact that there is no spectroscopic evidence of
+water-vapour on Mars<a name="FNanchor_21_21" id="FNanchor_21_21"></a><a href="#Footnote_21_21" class="fnanchor">[21]</a>; and points out that its absence is only to be
+expected, as Dr. George Johnstone Stoney has shown that it will escape
+from a body whose mass is less than one-quarter the mass of the earth.
+The mass of Mars is, in fact, much less than this, <i>i.e.</i> only
+one-ninth. Dr. Wallace considers, therefore, that the temperature of
+Mars ought to be extremely low, unless the constitution of its
+atmosphere is very different from ours. With regard to the latter
+statement, it should be mentioned that the Swedish physicist, Arrhenius,
+has recently shown that the carbonic acid gas in our atmosphere has an
+important influence upon climate. The amount of it in our air is, as we
+have seen, extremely small; but Arrhenius shows that, if it were
+doubled, the temperature would be more uniform and much higher. We thus
+see how futile it is, with our present knowledge, to dogmatise on the
+existence or non-existence of life in other celestial orbs.</p>
+
+<p>As to the canals Dr. Wallace puts forward a theory of his own. He
+contends that after Mars had cooled to a state of solidity, a great
+swarm of meteorites and small asteroids fell in upon it, with the result
+that a thin molten layer was formed all over the planet. As this layer
+cooled, the imprisoned gases escaped, producing vents or craterlets; and
+as it attempted to contract further upon the solid interior, it split in
+fissures radiating from points of weakness,<span class='pagenum'><a name="Page_223" id="Page_223">[Pg 223]</a></span> such, for instance, as the
+craterlets. And he goes on to suggest that the two tiny Martian
+satellites, with which we shall deal next, are the last survivors of his
+hypothetical swarm. Finally, with regard to the habitability of Mars,
+Dr. Wallace not only denies it, but asserts that the planet is
+"absolutely uninhabitable."</p>
+
+<p>For a long time it was supposed that Mars did not possess any
+satellites. In 1877, however, during that famous opposition in which
+Schiaparelli first saw the canals, two tiny satellites were discovered
+at the Washington Observatory by an American astronomer, Professor Asaph
+Hall. These satellites are so minute, and so near to the planet, that
+they can only be seen with very large telescopes; and even then the
+bright disc of the planet must be shielded off. They have been
+christened Phobos and Deimos (Fear and Dread); these being the names of
+the two subordinate deities who, according to Homer, attended upon Mars,
+the god of war.</p>
+
+<p>It is impossible to measure the exact sizes of these satellites, as they
+are too small to show any discs, but an estimate has been formed from
+their brightness. The diameter of Phobos was at first thought to be six
+miles, and that of Deimos, seven. As later estimates, however,
+considerably exceed this, it will, perhaps, be not far from the truth to
+state that they are each roughly about the size of the planetoid Eros.
+Phobos revolves around Mars in about 7&frac12; hours, at a distance of about
+only 4000 miles from the planet's surface, and Deimos in about 30 hours,
+at a distance of about 12,000 miles. As Mars rotates on its axis in
+about 24 hours, it will be seen that Phobos makes<span class='pagenum'><a name="Page_224" id="Page_224">[Pg 224]</a></span> more than three
+revolutions while the planet is rotating once&mdash;a very interesting
+condition of things.</p>
+
+<p>A strange foreshadowing of the discovery of the satellites of Mars will
+be familiar to readers of <i>Gulliver's Travels</i>. According to Dean
+Swift's hero, the astronomers on the Flying Island of Laputa had found
+two tiny satellites to Mars, one of which revolved around the planet in
+ten hours. The correctness of this guess is extraordinarily close,
+though at best it is, of course, nothing more than a pure coincidence.</p>
+
+<p>It need not be at all surprising that much uncertainty should exist with
+regard to the actual condition of the surface of Mars. The circumstances
+in which we are able to see that planet at the best are, indeed, hardly
+sufficient to warrant us in propounding any hard and fast theories. One
+of the most experienced of living observers, the American astronomer,
+Professor E.E. Barnard, considers that the view we get of Mars with the
+best telescope may be fairly compared with our naked eye view of the
+moon. Since we have seen that a view with quite a small telescope
+entirely alters our original idea of the lunar surface, a slight
+magnification revealing features of whose existence we had not
+previously the slightest conception, it does not seem too much to say
+that a further improvement in optical power might entirely subvert the
+present notions with regard to the Martian canals. Therefore, until we
+get a still nearer view of these strange markings, it seems somewhat
+futile to theorise. The lines which we see are perhaps, indeed, a
+foreshortened and all too dim view of some type of formation entirely
+novel to us, and possibly<span class='pagenum'><a name="Page_225" id="Page_225">[Pg 225]</a></span> peculiar to Mars. Differences of gravity and
+other conditions, such as obtain upon different planets, may perhaps
+produce very diverse results. The earth, the moon, and Mars differ
+greatly from one another in size, gravitation, and other such
+characteristics. Mountain-ranges so far appear typical of our globe, and
+ring-mountains typical of the moon. May not the so-called "canals" be
+merely some special formation peculiar to Mars, though quite a natural
+result of its particular conditions and of its past history?</p>
+
+
+<p class="center"><br /><span class="smcap">The Asteroids (or Minor Planets)</span></p>
+
+<p>We now come to that belt of small planets which are known by the name of
+asteroids. In the general survey of the solar system given in <a href="#CHAPTER_II">Chapter
+II.</a>, we saw how it was long ago noticed that the distances of the
+planetary orbits from the sun would have presented a marked appearance
+of orderly sequence, were it not for a gap between the orbits of Mars
+and Jupiter where no large planet was known to circulate. The suspicion
+thus aroused that some planet might, after all, be moving in this
+seemingly empty space, gave rise to the gradual discovery of a great
+number of small bodies; the largest of which, Ceres, is less than 500
+miles in diameter. Up to the present day some 600 of these bodies have
+been discovered; the four leading ones, in order of size, being named
+Ceres, Pallas, Juno, and Vesta. All the asteroids are invisible to the
+naked eye, with the exception of Vesta, which, though by no means the
+largest, happens to be the brightest. It is, however, only just visible
+to the eye under favourable conditions.<span class='pagenum'><a name="Page_226" id="Page_226">[Pg 226]</a></span> No trace of an atmosphere has
+been noted upon any of the asteroids, but such a state of things is only
+to be expected from the kinetic theory.</p>
+
+<p>For a good many years the discoveries of asteroids were made by means of
+the telescope. When, in the course of searching the heavens, an object
+was noticed which did not appear upon any of the recognised star charts,
+it was kept under observation for several nights to see whether it
+changed its place in the sky. Since asteroids move around the sun in
+orbits, just as planets do, they, of course, quickly reveal themselves
+by their change of position against the starry background.</p>
+
+<p>The year 1891 started a new era in the discovery of asteroids. It
+occurred to the Heidelberg observer, Dr. Max Wolf, one of the most
+famous of the hunters of these tiny planets, that photography might be
+employed in the quest with success. This photographic method, to which
+allusion has already been made in dealing with Eros, is an extremely
+simple one. If a photograph of a portion of the heavens be taken through
+an "equatorial"&mdash;that is, a telescope, moving by machinery, so as to
+keep the stars, at which it is pointed, always exactly in the field of
+view during their apparent movement across the sky&mdash;the images of these
+stars will naturally come out in the photograph as sharply defined
+points. If, however, there happens to be an asteroid, or other planetary
+body, in the same field of view, its image will come out as a short
+white streak; because the body has a comparatively rapid motion of its
+own, and will, during the period of exposure, have moved sufficiently
+against the background of the stars to leave a short trail, instead of a
+dot, upon the photographic plate. By this method Wolf himself has
+succeeded in discovering more than a hundred asteroids (<a href="#Plate_XIII">see Plate XIII.</a>,
+p. 226). It was, indeed, a little streak of this kind, appearing upon a
+photograph taken by the astronomer Witt, at Berlin, in 1898, which first
+informed the world of the existence of Eros.</p>
+
+<div class="figcenter" style="width: 600px;"><a name="Plate_XIII" id="Plate_XIII"></a>
+<img src="images/plate13.jpg" width="600" height="383" alt="Plate XIII." title="" />
+<span class="caption"><span class="smcap">Plate XIII. Minor Planet Trails</span></span>
+<div class="caption2">Two trails of minor planets (asteroids) imprinted <i>at the same time</i>
+upon one photographic plate. In the white streak on the left-hand side
+of the picture we witness the <i>discovery</i> of a new minor planet. The
+streak on the right was made by a body already known&mdash;the minor planet
+"Fiducia." This photograph was taken by Dr. Max Wolf, at Heidelberg, on
+the 4th of November, 1901, with the aid of a 16&ndash;inch telescope. The time
+of exposure was two hours.<br />(<a href="#Page_227"><small>Page 227</small></a>)</div>
+</div>
+
+<p><span class='pagenum'><a name="Page_227" id="Page_227">[Pg 227]</a></span></p><p>It has been calculated that the total mass of the asteroids must be
+much less than one-quarter that of the earth. They circulate as a rule
+within a space of some 30,000,000 miles in breadth, lying about midway
+between the paths of Mars and Jupiter. Two or three, however, of the
+most recently discovered of these small bodies have been found to pass
+quite close to Jupiter. The orbits of the asteroids are by no means in
+the one plane, that of Pallas being the most inclined to the plane of
+the earth's orbit. It is actually three times as much inclined as that
+of Eros.</p>
+
+<p>Two notable theories have been put forward to account for the origin of
+the asteroids. The first is that of the celebrated German astronomer,
+Olbers, who was the discoverer of Pallas and Vesta. He suggested that
+they were the fragments of an exploded planet. This theory was for a
+time generally accepted, but has now been abandoned in consequence of
+certain definite objections. The most important of these objections is
+that, in accordance with the theory of gravitation, the orbits of such
+fragments would all have to pass through the place where the explosion
+originally occurred. But the wide area over which the asteroids are
+spread points rather against the notion that they all set out originally
+from one particular spot. Another objection is that it does<span class='pagenum'><a name="Page_228" id="Page_228">[Pg 228]</a></span> not appear
+possible that, within a planet already formed, forces could originate
+sufficiently powerful to tear the body asunder.</p>
+
+<p>The second theory is that for some reason a planet here failed in the
+making. Possibly the powerful gravitational action of the huge body of
+Jupiter hard by, disturbed this region so much that the matter
+distributed through it was never able to collect itself into a single
+mass.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_18_18" id="Footnote_18_18"></a><a href="#FNanchor_18_18"><span class="label">[18]</span></a> Sir William Herschel was the first to note these polar
+changes.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_19" id="Footnote_19_19"></a><a href="#FNanchor_19_19"><span class="label">[19]</span></a> Quite recently, however, Professor Lowell has announced
+that his observer, Mr. E.C. Slipher, finds with the spectroscope faint
+traces of water vapour in the Martian atmosphere.</p></div>
+
+<div class="footnote"><p><a name="Footnote_20_20" id="Footnote_20_20"></a><a href="#FNanchor_20_20"><span class="label">[20]</span></a> In a somewhat similar manner the term "crater," as applied
+to the ring-mountain formation on the moon, has evidently given a bias
+in favour of the volcanic theory as an explanation of that peculiar
+structure.</p></div>
+
+<div class="footnote"><p><a name="Footnote_21_21" id="Footnote_21_21"></a><a href="#FNanchor_21_21"><span class="label">[21]</span></a> Mr. Slipher's results (<a href="#Page_213">see note 2, page 213</a>) were not then
+known.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_229" id="Page_229">[Pg 229]</a></span></p>
+<h3><a name="CHAPTER_XVIII" id="CHAPTER_XVIII"></a>CHAPTER XVIII</h3>
+
+<h4>THE SUPERIOR PLANETS&mdash;<i>continued</i></h4>
+
+
+<p class="noin"><span class="smcap">The</span> planets, so far, have been divided into inferior and superior. Such
+a division, however, refers merely to the situation of their orbits with
+regard to that of our earth. There is, indeed, another manner in which
+they are often classed, namely, according to size. On this principle
+they are divided into two groups; one group called the <i>Terrestrial
+Planets</i>, or those which have characteristics like our earth, and the
+other called the <i>Major Planets</i>, because they are all of very great
+size. The terrestrial planets are Mercury, Venus, the earth, and Mars.
+The major planets are the remainder, namely, Jupiter, Saturn, Uranus,
+and Neptune. As the earth's orbit is the boundary which separates the
+inferior from the superior planets, so does the asteroidal belt divide
+the terrestrial from the major planets. We found the division into
+inferior and superior useful for emphasising the marked difference in
+aspect which those two classes present as seen from our earth; the
+inferior planets showing phases like the moon when viewed in the
+telescope, whereas the superior planets do not. But the division into
+terrestrial and major planets is the more far-reaching classification of
+the two, for it includes the whole number of planets, whereas the other
+arrangement necessarily<span class='pagenum'><a name="Page_230" id="Page_230">[Pg 230]</a></span> excludes the earth. The members of each of
+these classes have many definite characteristics in common. The
+terrestrial planets are all of them relatively small in size,
+comparatively near together, and have few or no satellites. They are,
+moreover, rather dense in structure. The major planets, on the other
+hand, are huge bodies, circulating at great distances from each other,
+and are, as a rule, provided with a number of satellites. With respect
+to structure, they may be fairly described as being loosely put
+together. Further, the markings on the surfaces of the terrestrial
+planets are permanent, whereas those on the major planets are
+continually shifting.</p>
+
+
+<p class="center"><br /><span class="smcap">The Planet Jupiter</span></p>
+
+<p>Jupiter is the greatest of the major planets. It has been justly called
+the "Giant" planet, for both in volume and in mass it exceeds all the
+other planets put together. When seen through the telescope it exhibits
+a surface plentifully covered with markings, the most remarkable being a
+series of broad parallel belts. The chief belt lies in the central parts
+of the planet, and is at present about 10,000 miles wide. It is bounded
+on either side by a reddish brown belt of about the same width. Bright
+spots also appear upon the surface of the planet, last for a while, and
+then disappear. The most notable of the latter is one known as the
+"Great Red Spot." This is situated a little beneath the southern red
+belt, and appeared for the first time about thirty years ago. It has
+undergone a good many changes in colour and brightness, and is still
+faintly visible. This spot is the most permanent marking which has yet
+been seen upon Jupiter. In general, the markings change so often that
+the surface which we see is evidently not solid, but of a fleeting
+nature akin to cloud (<a href="#Plate_XIV">see Plate XIV.</a>, p. 230).</p>
+
+<div class="figcenter" style="width: 600px;"><a name="Plate_XIV" id="Plate_XIV"></a>
+<img src="images/plate14.jpg" width="600" height="390" alt="Plate XIV." title="" />
+<span class="caption"><span class="smcap">Plate XIV. The Planet Jupiter</span></span>
+<div class="caption2">The Giant Planet as seen at 11.30 p.m., on the 11th of January, 1908,
+with a 12&frac12;-inch reflecting telescope. The extensive oval marking in
+the upper portion of the disc is the "Great Red Spot." The South is at
+the top of the picture, the view being the <i>inverted</i> one given by an
+astronomical telescope. From a drawing by the Rev. Theodore E.R.
+Phillips, M.A., F.R.A.S., Director of the Jupiter Section of the British
+Astronomical Association.<br />(<a href="#Page_231"><small>Page 231</small></a>)</div>
+</div>
+
+<p><span class='pagenum'><a name="Page_231" id="Page_231">[Pg 231]</a></span></p><p>Observations of Jupiter's markings show that on an average the planet
+rotates on its axis in a period of about 9 hours 54 minutes. The mention
+here of <i>an average</i> with reference to the rotation will, no doubt,
+recall to the reader's mind the similar case of the sun, the different
+portions of which rotate with different velocities. The parts of Jupiter
+which move quickest take 9 hours 50 minutes to go round, while those
+which move slowest take 9 hours 57 minutes. The middle portions rotate
+the fastest, a phenomenon which the reader will recollect was also the
+case with regard to the sun.</p>
+
+<p>Jupiter is a very loosely packed body. Its density is on an average only
+about 1&frac12; times that of water, or about one-fourth the density of the
+earth; but its bulk is so great that the gravitation at that surface
+which we see is about 2&frac12; times what it is on the surface of the
+earth. In accordance, therefore, with the kinetic theory, we may expect
+the planet to retain an extensive layer of gases around it; and this is
+confirmed by the spectroscope, which gives evidence of the presence of a
+dense atmosphere.</p>
+
+<p>All things considered, it may be safely inferred that the interior of
+Jupiter is very hot, and that what we call its surface is not the actual
+body of the planet, but a voluminous layer of clouds and vapours driven
+upwards from the heated mass underneath. The planet was indeed formerly
+thought to be self-luminous; but this can hardly be the case, for those
+portions of the<span class='pagenum'><a name="Page_232" id="Page_232">[Pg 232]</a></span> surface which happen to lie at any moment in the
+shadows cast by the satellites appear to be quite black. Again, when a
+satellite passes into the great shadow cast by the planet it becomes
+entirely invisible, which would not be the case did the planet emit any
+perceptible light of its own.</p>
+
+<p>In its revolutions around the sun, Jupiter is attended, so far as we
+know, by seven<a name="FNanchor_22_22" id="FNanchor_22_22"></a><a href="#Footnote_22_22" class="fnanchor">[22]</a> satellites. Four of these were among the first
+celestial objects which Galileo discovered with his "optick tube," and
+he named them the "Medicean Stars" in honour of his patron, Cosmo de
+Medici. Being comparatively large bodies they might indeed just be seen
+with the naked eye, were it not for the overpowering glare of the
+planet.</p>
+
+<p>It was only in quite recent times, namely, in 1892, that a fifth
+satellite was added to the system of Jupiter. This body, discovered by
+Professor E.E. Barnard, is very small. It circulates nearer to the
+planet than the innermost of Galileo's moons; and, on account of the
+glare, is a most difficult object to obtain a glimpse of, even in the
+best of telescopes. In December 1904 and January 1905 respectively, two
+more moons were added to the system, these being found by <i>photography</i>,
+by the American astronomer, Professor C.D. Perrine. Both the bodies in
+question revolve at a greater distance from the planet than the
+outermost of the older known satellites.</p>
+
+<p><span class='pagenum'><a name="Page_233" id="Page_233">[Pg 233]</a></span></p><p>Galileo's moons, though the largest bodies of Jupiter's satellite
+system, are, as we have already pointed out, very small indeed when
+compared with the planet itself. The diameters of two of them, Europa
+and Io, are, however, about the same as that of our moon, while those of
+the other two, Callisto and Ganymede, are more than half as large again.
+The recently discovered satellites are, on the other hand,
+insignificant; that found by Barnard, for example, being only about 100
+miles in diameter.</p>
+
+<p>Of the four original satellites Io is the nearest to Jupiter, and, seen
+from the planet, it would show a disc somewhat larger than that of our
+moon. The others would appear somewhat smaller. However, on account of
+the great distance of the sun, the entire light reflected to Jupiter by
+all the satellites should be very much less than what we get from our
+moon.</p>
+
+<p>Barnard's satellite circles around Jupiter at a distance less than our
+moon is from us, and in a period of about 12 hours. Galileo's four
+satellites revolve in periods of about 2, 3&frac12;, 7, and 16&frac12; days
+respectively, at distances lying roughly between a quarter of a million
+and one million miles. Perrine's two satellites are at a distance of
+about seven million miles, and take about nine months to complete their
+revolutions.</p>
+
+<p>The larger satellites, when viewed in the telescope, exhibit certain
+defined markings; but the bodies are so far away from us, that only
+those details which are of great extent can be seen. The satellite Io,
+according to Professor Barnard, shows a darkish disc, with a broad white
+belt across its middle regions. Mr. Douglass, one of the observers at
+the Lowell Observatory,<span class='pagenum'><a name="Page_234" id="Page_234">[Pg 234]</a></span> has noted upon Ganymede a number of markings
+somewhat resembling those seen on Mars, and he concludes, from their
+movement, that this satellite rotates on its axis in about seven days.
+Professor Barnard, on the other hand, does not corroborate this, though
+he claims to have discovered bright polar caps on both Ganymede and
+Callisto.</p>
+
+<p>In an earlier chapter we dealt at length with eclipses, occultations,
+and transits, and endeavoured to make clear the distinction between
+them. The system of Jupiter's satellites furnishes excellent examples of
+all these phenomena. The planet casts a very extensive shadow, and the
+satellites are constantly undergoing obscuration by passing through it.
+Such occurrences are plainly comparable to our lunar eclipses. Again,
+the satellites may, at one time, be occulted by the huge disc of the
+planet, and at another time seen in transit over its face. A fourth
+phenomenon is what is known as an <i>eclipse of the planet by a
+satellite</i>, which is the exact equivalent of what we style on the earth
+an eclipse of the sun. In this last case the shadow, cast by the
+satellite, appears as a round black spot in movement across the planet's
+surface.</p>
+
+<p>In the passages of these attendant bodies behind the planet, into its
+shadow, or across its face, respectively, it occasionally happens that
+Galileo's four satellites all disappear from view, and the planet is
+then seen for a while in the unusual condition of being apparently
+without its customary attendants. An instance of this phenomenon took
+place on the 3rd of October 1907. On that occasion, the satellites known
+as I. and III. (<i>i.e.</i> Io and Ganymede) were<span class='pagenum'><a name="Page_235" id="Page_235">[Pg 235]</a></span> eclipsed, that is to say,
+obscured by passing into the planet's shadow; Satellite IV. (Callisto)
+was occulted by the planet's disc; while Satellite II. (Europa), being
+at the same moment in transit across the planet's face, was invisible
+against that brilliant background. A number of instances of this kind of
+occurrence are on record. Galileo, for example, noted one on the 15th of
+March 1611, while Herschel observed another on the 23rd of May 1802.</p>
+
+<p>It was indirectly to Jupiter's satellites that the world was first
+indebted for its knowledge of the velocity of light. When the periods of
+revolution of the satellites were originally determined, Jupiter
+happened, at the time, to be at his nearest to us. From the periods thus
+found tables were made for the prediction of the moments at which the
+eclipses and other phenomena of the satellites should take place. As
+Jupiter, in the course of his orbit, drew further away from the earth,
+it was noticed that the disappearances of the satellites into the shadow
+of the planet occurred regularly later than the time predicted. In the
+year 1675, Roemer, a Danish astronomer, inferred from this, not that the
+predictions were faulty, but that light did not travel instantaneously.
+It appeared, in fact, to take longer to reach us, the greater the
+distance it had to traverse. Thus, when the planet was far from the
+earth, the last ray given out by the satellite, before its passage into
+the shadow, took a longer time to cross the intervening space, than when
+the planet was near. Modern experiments in physics have quite confirmed
+this, and have proved for us that light does not travel across space in
+the twinkling of an eye, as might hastily be supposed,<span class='pagenum'><a name="Page_236" id="Page_236">[Pg 236]</a></span> but actually
+moves, as has been already stated, at the rate of about 186,000 miles
+per second.</p>
+
+
+<p class="center"><br /><span class="smcap">The Planet Saturn</span></p>
+
+<p>Seen in the telescope the planet Saturn is a wonderful and very
+beautiful object. It is distinguished from all the other planets, in
+fact from all known celestial bodies, through being girt around its
+equator by what looks like a broad, flat ring of exceeding thinness.
+This, however, upon closer examination, is found to be actually composed
+of three concentric rings. The outermost of these is nearly of the same
+brightness as the body of the planet itself. The ring which comes
+immediately within it is also bright, and is separated from the outer
+one all the way round by a relatively narrow space, known as "Cassini's
+division," because it was discovered by the celebrated French
+astronomer, J.D. Cassini, in the year 1675. Inside the second ring, and
+merging insensibly into it, is a third one, known as the "crape ring,"
+because it is darker in hue than the others and partly transparent, the
+body of Saturn being visible through it. The inner boundary of this
+third and last ring does not adjoin the planet, but is everywhere
+separated from it by a definite space. This ring was discovered
+<i>independently</i><a name="FNanchor_23_23" id="FNanchor_23_23"></a><a href="#Footnote_23_23" class="fnanchor">[23]</a> in 1850 by Bond in America and Dawes in England.</p>
+
+<div class="figcenter" style="width: 600px;"><a name="Plate_XV" id="Plate_XV"></a>
+<img src="images/plate15.jpg" width="600" height="391" alt="Plate XV." title="" />
+<span class="caption"><span class="smcap">Plate XV. The Planet Saturn</span></span>
+<div class="caption2">From a drawing made by Professor Barnard with the Great Lick Telescope.
+The black band fringing the outer ring, where it crosses the disc, is
+portion of the <i>shadow which the rings cast upon the planet</i>. The black
+wedge-shaped mark, where the rings disappear behind the disc at the
+left-hand side, is portion of the <i>shadow which the planet casts upon
+the rings</i>.<br />(<a href="#Page_237"><small>Page 237</small></a>)</div>
+</div>
+
+<p><span class='pagenum'><a name="Page_237" id="Page_237">[Pg 237]</a></span></p><p>As distinguished from the crape ring, the bright rings must have a
+considerable closeness of texture; for the shadow of the planet may be
+seen projected upon them, and their shadows in turn projected upon the
+surface of the planet (<a href="#Plate_XV">see Plate XV.</a>, p. 236).</p>
+
+<p>According to Professor Barnard, the entire breadth of the ring system,
+that is to say, from one side to the other of the outer ring, is 172,310
+miles, or somewhat more than double the planet's diameter.</p>
+
+<p>In the varying views which we get of Saturn, the system of the rings is
+presented to us at very different angles. Sometimes we are enabled to
+gaze upon its broad expanse; at other times, however, its thin edge is
+turned exactly towards us, an occurrence which takes place after
+intervals of about fifteen years. When this happened in 1892 the rings
+are said to have disappeared entirely from view in the great Lick
+telescope. We thus get an idea of their small degree of thickness, which
+would appear to be only about 50 miles. The last time the system of
+rings was exactly edgewise to the earth was on the 3rd of October 1907.</p>
+
+<p>The question of the composition of these rings has given rise to a good
+deal of speculation. It was formerly supposed that they were either
+solid or liquid, but in 1857 it was proved by Clerk Maxwell that a
+structure of this kind would not be able to stand. He showed, however,
+that they could be fully explained by supposing them to consist of an
+immense number of<span class='pagenum'><a name="Page_238" id="Page_238">[Pg 238]</a></span> separate solid particles, or, as one might otherwise
+put it, extremely small satellites, circling in dense swarms around the
+middle portions of the planet. It is therefore believed that we have
+here the materials ready for the formation of a satellite or satellites;
+but that the powerful gravitative action, arising through the planet's
+being so near at hand, is too great ever to allow these materials to
+aggregate themselves into a solid mass. There is, as a matter of fact, a
+minimum distance from the body of any planet within which it can be
+shown that a satellite will be unable to form on account of
+gravitational stress. This is known as "Roche's limit," from the name of
+a French astronomer who specially investigated the question.</p>
+
+<p>There thus appears to be a certain degree of analogy between Saturn's
+rings and the asteroids. Empty spaces, too, exist in the asteroidal
+zone, the relative position of one of which bears a striking resemblance
+to that of "Cassini's division." It is suggested, indeed, that this
+division had its origin in gravitational disturbances produced by the
+attraction of the larger satellites, just as the empty spaces in the
+asteroidal zone are supposed to be the result of perturbations caused by
+the Giant Planet hard by.</p>
+
+<p>It has long been understood that the system of the rings must be
+rotating around Saturn, for if they were not in motion his intense
+gravitational attraction would quickly tear them in pieces. This was at
+length proved to be the fact by the late Professor Keeler, Director of
+the Lick Observatory, who from spectroscopic observations found that
+those portions of the rings situated near to the planet rotated faster<span class='pagenum'><a name="Page_239" id="Page_239">[Pg 239]</a></span>
+than those farther from it. This directly supports the view that the
+rings are composed of satellites; for, as we have already seen, the
+nearer a satellite is to its primary the faster it will revolve. On the
+other hand, were the rings solid, their outer portions would move the
+fastest; as we have seen takes place in the body of the earth, for
+example. The mass of the ring system, however, must be exceedingly
+small, for it does not appear to produce any disturbances in the
+movements of Saturn's satellites. From the kinetic theory, therefore,
+one would not expect to find any atmosphere on the rings, and the
+absence of it is duly shown by spectroscopic observations.</p>
+
+<p>The diameter of Saturn, roughly speaking, is about one-fifth less than
+that of Jupiter. The planet is very flattened at the poles, this
+flattening being quite noticeable in a good telescope. For instance, the
+diameter across the equator is about 76,470 miles, while from pole to
+pole it is much less, namely, 69,770.</p>
+
+<p>The surface of Saturn bears a strong resemblance to that of Jupiter. Its
+markings, though not so well defined, are of the same belt-like
+description; and from observation of them it appears that the planet
+rotates <i>on an average</i> in a little over ten hours. The rotation is in
+fact of the same peculiar kind as that of the sun and Jupiter; but the
+difference of speed at which the various portions of Saturn go round are
+even more marked than in the case of the Giant Planet. The density of
+Saturn is less than that of Jupiter; so that it must be largely in a
+condition of vapour, and in all probability at a still earlier stage of
+planetary evolution.</p>
+
+<p>Up to the present we know of as many as ten<span class='pagenum'><a name="Page_240" id="Page_240">[Pg 240]</a></span> satellites circling around
+Saturn, which is more than any other planet of the solar system can lay
+claim to. Two of these, however, are very recent discoveries; one,
+Ph&#339;be, having been found by photography in August 1898, and the
+other, Themis, in 1904, also by the same means. For both of these we are
+indebted to Professor W.H. Pickering. Themis is said to be <i>the faintest
+object in the solar system</i>. It cannot be <i>seen</i>, even with the largest
+telescope in existence; a fact which should hardly fail to impress upon
+one the great advantage the photographic plate possesses in these
+researches over the human eye.</p>
+
+<p>The most important of the whole Saturnian family of satellites are the
+two known as Titan and Japetus. These were discovered respectively by
+Huyghens in 1655 and by Cassini in 1671. Japetus is about the same size
+as our moon; while the diameter of Titan, the largest of the satellites,
+is about half as much again. Titan takes about sixteen days to revolve
+around Saturn, while Japetus takes more than two months and a half. The
+former is about three-quarters of a million miles distant from the
+planet, and the latter about two and a quarter millions. To Sir William
+Herschel we are indebted for the discovery of two more satellites, one
+of which he found on the evening that he used his celebrated 40&ndash;foot
+telescope for the first time. The ninth satellite, Ph&#339;be, one of the
+two discovered by Professor Pickering, is perhaps the most remarkable
+body in the solar system, for all the other known members of that system
+perform their revolutions in one fixed direction, whereas this satellite
+revolves in the <i>contrary</i> direction.</p>
+
+<p><span class='pagenum'><a name="Page_241" id="Page_241">[Pg 241]</a></span></p><p>In consequence of the great distance of Saturn, the sun, as seen from
+the planet, would appear so small that it would scarcely show any disc.
+The planet, indeed, only receives from the sun about one-ninetieth of
+the heat and light which the earth receives. Owing to this diminished
+intensity of illumination, the combined light reflected to Saturn by the
+whole of its satellites must be very small.</p>
+
+<p>With the sole exception of Jupiter, not one of the planets circulating
+nearer to the sun could be seen from Saturn, as they would be entirely
+lost in the solar glare. For an observer upon Saturn, Jupiter would,
+therefore, fill much the same position as Venus does for us, regularly
+displaying phases and being alternately a morning and an evening star.</p>
+
+<p>It is rather interesting to consider the appearances which would be
+produced in our skies were the earth embellished with a system of rings
+similar to those of Saturn. In consequence of the curving of the
+terrestrial surface, they would not be seen at all from within the
+Arctic or Antarctic circles, as they would be always below the horizon.
+From the equator they would be continually seen edgewise, and so would
+appear merely as line of light stretching right across the heaven and
+passing through the zenith. But the dwellers in the remaining regions
+would find them very objectionable, for they would cut off the light of
+the sun during lengthy periods of time.</p>
+
+<p>Saturn was a sore puzzle to the early telescopic observers. They did not
+for a long time grasp the fact that it was surrounded by a ring&mdash;so slow
+is the human mind to seek for explanations out of the ordinary course of
+things. The protrusions of the<span class='pagenum'><a name="Page_242" id="Page_242">[Pg 242]</a></span> ring on either side of the planet, at
+first looked to Galileo like two minor globes placed on opposite sides
+of it, and slightly overlapping the disc. He therefore informed Kepler
+that "Saturn consists of three stars in contact with one another." Yet
+he was genuinely puzzled by the fact that the two attendant bodies (as
+he thought them) always retained the same position with regard to the
+planet's disc, and did not appear to revolve around it, nor to be in any
+wise shifted as a consequence of the movements of our earth.</p>
+
+<p>About a year and a half elapsed before he again examined Saturn; and, if
+he was previously puzzled, he was now thoroughly amazed. It happened
+just then to be one of those periods when the ring is edgewise towards
+the earth, and of course he only saw a round disc like that of Jupiter.
+What, indeed, had become of the attendant orbs? Was some demon mocking
+him? Had Saturn devoured his own children? He was, however, fated to be
+still more puzzled, for soon the minor orbs reappeared, and, becoming
+larger and larger as time went on, they ended by losing their globular
+appearance and became like two pairs of arms clasping the planet from
+each side! (<a href="#Plate_XVI">see Plate XVI.</a>, p. 242).</p>
+
+<p>Galileo went to his grave with the riddle still unsolved, and it
+remained for the famous Dutch astronomer, Huyghens, to clear up the
+matter. It was, however, some little time before he hit upon the real
+explanation. Having noticed that there were dark spaces between the
+strange appendages and the body of the planet, he imagined Saturn to be
+a globe fitted with handles at each side; "ans&aelig;" these came to be
+called, from the Latin <i>ansa</i>, which means a handle. At length, in the
+year 1656, he solved the problem, and this he did by means of that
+123&ndash;foot tubeless telescope, of which mention has already been made. The
+ring happened then to be at its edgewise period, and a careful study of
+the behaviour of the ans&aelig; when disappearing and reappearing soon
+revealed to Huyghens the true explanation.</p>
+
+<div class="figcenter" style="width: 600px;"><a name="Plate_XVI" id="Plate_XVI"></a>
+<img src="images/plate16.jpg" width="500" height="484" alt="Plate XVI." title="" /><br />
+<span class="caption"><span class="smcap">Plate XVI. Early Representations of Saturn</span><br />
+From an illustration in the <i>Systema Saturnium</i> of Christian Huyghens.<br />
+(<a href="#Page_242"><small>Page 242</small></a>)</span>
+</div>
+
+
+
+<p><span class='pagenum'><a name="Page_243" id="Page_243">[Pg 243]</a></span></p>
+<p class="center"><br /><span class="smcap">The Planets Uranus and Neptune</span></p>
+
+<p>We have already explained (<a href="#CHAPTER_II">in Chapter II.</a>) the circumstances in which
+both Uranus and Neptune were discovered. It should, however, be added
+that after the discovery of Uranus, that planet was found to have been
+already noted upon several occasions by different observers, but always
+without the least suspicion that it was other than a mere faint star.
+Again, with reference to the discovery of Neptune, it may here be
+mentioned that the apparent amount by which that planet had pulled
+Uranus out of its place upon the starry background was exceedingly
+small&mdash;so small, indeed, that no eye could have detected it without the
+aid of a telescope!</p>
+
+<p>Of the two predictions of the place of Neptune in the sky, that of Le
+Verrier was the nearer. Indeed, the position calculated by Adams was
+more than twice as far out. But Adams was by a long way the first in the
+field with his results, and only for unfortunate delays the prize would
+certainly have fallen to him. For instance, there was no star-map at
+Cambridge, and Professor Challis, the director of the observatory there,
+was in consequence obliged to make a laborious<span class='pagenum'><a name="Page_244" id="Page_244">[Pg 244]</a></span> examination of the stars
+in the suspected region. On the other hand, all that Galle had to do was
+to compare that part of the sky where Le Verrier told him to look with
+the Berlin star-chart which he had by him. This he did on September 23,
+1846, with the result that he quickly noted an eighth magnitude star
+which did not figure in that chart. By the next night this star had
+altered its position in the sky, thus disclosing the fact that it was
+really a planet.</p>
+
+<p>Six days later Professor Challis succeeded in finding the planet, but of
+course he was now too late. On reviewing his labours he ascertained that
+he had actually noted down its place early in August, and had he only
+been able to sift his observations as he made them, the discovery would
+have been made then.</p>
+
+<p>Later on it was found that Neptune had only just missed being discovered
+about fifty years earlier. In certain observations made during 1795, the
+famous French astronomer, Lalande, found that a star, which he had
+mapped in a certain position on the 8th of May of that year, was in a
+different position two days later. The idea of a planet does not appear
+to have entered his mind, and he merely treated the first observation as
+an error!</p>
+
+<p>The reader will, no doubt, recollect how the discovery of the asteroids
+was due in effect to an apparent break in the seemingly regular sequence
+of the planetary orbits outwards from the sun. This curious sequence of
+relative distances is usually known as "Bode's Law," because it was
+first brought into general notice by an astronomer of that name. It had,
+however, previously been investigated mathematically<span class='pagenum'><a name="Page_245" id="Page_245">[Pg 245]</a></span> by Titius in 1772.
+Long before this, indeed, the unnecessarily wide space between the
+orbits of Mars and Jupiter had attracted the attention of the great
+Kepler to such a degree, that he predicted that a planet would some day
+be found to fill the void. Notwithstanding the service which the
+so-called Law of Bode has indirectly rendered to astronomy, it has
+strangely enough been found after all not to rest upon any scientific
+foundation. It will not account for the distance from the sun of the
+orbit of Neptune, and the very sequence seems on the whole to be in the
+nature of a mere coincidence.</p>
+
+<p>Neptune is invisible to the naked eye; Uranus is just at the limit of
+visibility. Both planets are, however, so far from us that we can get
+but the poorest knowledge of their condition and surroundings. Uranus,
+up to the present, is known to be attended by four satellites, and
+Neptune by one. The planets themselves are about equal in size; their
+diameters, roughly speaking, being about one-half that of Saturn. Some
+markings have, indeed, been seen upon the disc of Uranus, but they are
+very indistinct and fleeting. From observation of them, it is assumed
+that the planet rotates on its axis in a period of some ten to twelve
+hours. No definite markings have as yet been seen upon Neptune, which
+body is described by several observers as resembling a faint planetary
+nebula.</p>
+
+<p>With regard to their physical condition, the most that can be said about
+these two planets is that they are probably in much the same vaporous
+state as Jupiter and Saturn. On account of their great distance from the
+sun they can receive but little solar<span class='pagenum'><a name="Page_246" id="Page_246">[Pg 246]</a></span> heat and light. Seen from
+Neptune, in fact, the sun would appear only about the size of Venus at
+her best, though of a brightness sufficiently intense to illumine the
+Neptunian landscape with about seven hundred times our full moonlight.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_22_22" id="Footnote_22_22"></a><a href="#FNanchor_22_22"><span class="label">[22]</span></a> Mr. P. Melotte, of Greenwich Observatory, while examining
+a photograph taken there on February 28, 1908, discovered upon it a very
+faint object which it is firmly believed will prove to be an <i>eighth</i>
+satellite of Jupiter. This object was afterwards found on plates exposed
+as far back as January 27. It has since been photographed several times
+at Greenwich, and also at Heidelberg (by Dr. Max Wolf) and at the Lick
+Observatory. Its movement is probably <i>retrograde</i>, like that of
+Ph&#339;be (p. 240).</p></div>
+
+<div class="footnote"><p><a name="Footnote_23_23" id="Footnote_23_23"></a><a href="#FNanchor_23_23"><span class="label">[23]</span></a> In the history of astronomy two salient points stand out.
+</p><p>
+The first of these is the number of "independent" discoveries which have
+taken place; such, for instance, as in the cases of Le Verrier and Adams
+with regard to Neptune, and of Lockyer and Janssen in the matter of the
+spectroscopic method of observing solar prominences.
+</p><p>
+The other is the great amount of "anticipation." Copernicus, as we have
+seen, was anticipated by the Greeks; Kepler was not actually the first
+who thought of elliptic orbits; others before Newton had imagined an
+attractive force.
+</p><p>
+Both these points furnish much food for thought!</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_247" id="Page_247">[Pg 247]</a></span></p>
+<h3><a name="CHAPTER_XIX" id="CHAPTER_XIX"></a>CHAPTER XIX</h3>
+
+<h4>COMETS</h4>
+
+
+<p class="noin"><span class="smcap">The</span> reader has, no doubt, been struck by the marked uniformity which
+exists among those members of the solar system with which we have dealt
+up to the present. The sun, the planets, and their satellites are all
+what we call solid bodies. The planets move around the sun, and the
+satellites around the planets, in orbits which, though strictly
+speaking, ellipses, are yet not in any instance of a very oval form. Two
+results naturally follow from these considerations. Firstly, the bodies
+in question hide the light coming to us from those further off, when
+they pass in front of them. Secondly, the planets never get so far from
+the sun that we lose sight of them altogether.</p>
+
+<p>With the objects known as Comets it is, however, quite the contrary.
+These objects do not conform to our notions of solidity. They are so
+transparent that they can pass across the smallest star without dimming
+its light in the slightest degree. Again, they are only visible to us
+during a portion of their orbits. A comet may be briefly described as an
+illuminated filmy-looking object, made up usually of three portions&mdash;a
+head, a nucleus, or brighter central portion within this head, and a
+tail. The heads of comets vary greatly in size; some, indeed, appear
+quite small, like stars, while others look even as large as the<span class='pagenum'><a name="Page_248" id="Page_248">[Pg 248]</a></span> moon.
+Occasionally the nucleus is wanting, and sometimes the tail also.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_18" id="Fig_18"></a>
+<img src="images/figure18.jpg" width="500" height="334" alt="Fig. 18." title="" />
+<span class="caption"><span class="smcap">Fig. 18.</span>&mdash;Showing how the Tail of a Comet is directed
+away from the Sun.</span>
+</div>
+
+<p>These mysterious visitors to our skies come up into view out of the
+immensities beyond, move towards the sun at a rapidly increasing speed,
+and, having gone around it, dash away again into the depths of space. As
+a comet approaches the sun, its body appears to grow smaller and
+smaller, while, at the same time, it gradually throws out behind it an
+appendage like a tail. As the comet moves round the central orb this
+tail is always directed <i>away</i> from the sun; and when it departs again
+into space the tail goes in advance. As the comet's distance from the
+sun increases, the tail gradually shrinks away and the head once more
+grows in size (<a href="#Fig_18">see Fig. 18</a>). In consequence of these changes, and of the
+fact that we lose sight of comets comparatively quickly, one<span class='pagenum'><a name="Page_249" id="Page_249">[Pg 249]</a></span> is much
+inclined to wonder what further changes may take place after the bodies
+have passed beyond our ken.</p>
+
+<p>The orbits of comets are, as we have seen, very elliptic. In some
+instances this ellipticity is so great as to take the bodies out into
+space to nearly six times the distance of Neptune from the sun. For a
+long time, indeed, it was considered that comets were of two kinds,
+namely, those which actually <i>belonged</i> to the solar system, and those
+which were merely <i>visitors</i> to it for the first and only time&mdash;rushing
+in from the depths of space, rapidly circuiting the sun, and finally
+dashing away into space again, never to return. On the contrary,
+nowadays, astronomers are generally inclined to regard comets as
+permanent members of the solar system.</p>
+
+<p>The difficulty, however, of deciding absolutely whether the orbits of
+comets are really always <i>closed</i> curves, that is to say, curves which
+must sooner or later bring the bodies back again towards the sun, is,
+indeed, very great. Comets, in the first place, are always so diffuse,
+that it is impossible to determine their exact position, or, rather, the
+exact position of that important point within them, known as the centre
+of gravity. Secondly, that stretch of its orbit along which we can
+follow a comet, is such a very small portion of the whole path, that the
+slightest errors of observation which we make will result in
+considerably altering our estimate of the actual shape of the orbit.</p>
+
+<p>Comets have been described as so transparent that they can pass across
+the sky without dimming the lustre of the smallest stars, which the
+thinnest fog<span class='pagenum'><a name="Page_250" id="Page_250">[Pg 250]</a></span> or mist would do. This is, indeed, true of every portion
+of a comet except the nucleus, which is, as its name implies, the
+densest part. And yet, in contrast to this ghostlike character, is the
+strange fact that when comets are of a certain brightness they may
+actually be seen in full daylight.</p>
+
+<p>As might be gathered from their extreme tenuity, comets are so
+exceedingly small in mass that they do not appear to exert any
+gravitational attraction upon the other bodies of our system. It is,
+indeed, a known fact that in the year 1886 a comet passed right amidst
+the satellites of Jupiter without disturbing them in the slightest
+degree. The attraction of the planet, on the other hand, so altered the
+comet's orbit, as to cause it to revolve around the sun in a period of
+seven years, instead of twenty-seven, as had previously been the case.
+Also, in 1779, the comet known as Lexell's passed quite close to
+Jupiter, and its orbit was so changed by that planet's attraction that
+it has never been seen since. The density of comets must, as a rule, be
+very much less than the one-thousandth part of that of the air at the
+surface of our globe; for, if the density of the comet were even so
+small as this, its mass would <i>not</i> be inappreciable.</p>
+
+<p>If comets are really undoubted members of the solar system, the
+circumstances in which they were evolved must have been different from
+those which produced the planets and satellites. The axial rotations of
+both the latter, and also their revolutions, take place in one certain
+direction;<a name="FNanchor_24_24" id="FNanchor_24_24"></a><a href="#Footnote_24_24" class="fnanchor">[24]</a> their orbits, too, are<span class='pagenum'><a name="Page_251" id="Page_251">[Pg 251]</a></span> ellipses which do not differ much
+from circles, and which, furthermore, are situated fairly in the one
+plane. Comets, on the other hand, do not necessarily travel round the
+sun in the same fixed direction as the planets. Their orbits, besides,
+are exceedingly elliptic; and, far from keeping to one plane, or even
+near it, they approach the sun from all directions.</p>
+
+<p>Broadly speaking, comets may be divided into two distinct classes, or
+"families." In the first class, the same orbit appears to be shared in
+common by a series of comets which travel along it, one following the
+other. The comets which appeared in the years 1668, 1843, 1880, 1882,
+and 1887 are instances of a number of different bodies pursuing the same
+path around the sun. The members of a comet family of this kind are
+observed to have similar characteristics. The idea is that such comets
+are merely portions of one much larger cometary body, which became
+broken up by the gravitational action of other bodies in the system, or
+through violent encounter with the sun's surroundings.</p>
+
+<p>The second class is composed of comets which are supposed to have been
+seized by the gravitative action of certain planets, and thus forced to
+revolve in short ellipses around the sun, well within the limits of the
+solar system. These comets are, in consequence, spoken of as "captures."
+They move around the sun in the same direction as the planets do.
+Jupiter has a fairly large comet family of this kind attached to him. As
+a result of his overpowering gravitation, it is imagined that during the
+ages he must have attracted a large number of these bodies on his own
+account, and, perhaps, have robbed other planets of their captures.<span class='pagenum'><a name="Page_252" id="Page_252">[Pg 252]</a></span> His
+family at present numbers about thirty. Of the other planets, so far as
+we know, Saturn possesses a comet family of two, Uranus three, and
+Neptune six. There are, indeed, a few comets which appear as if under
+the influence of some force situated outside the known bounds of the
+solar system, a circumstance which goes to strengthen the idea that
+other planets may revolve beyond the orbit of Neptune. The terrestrial
+planets, on the other hand, cannot have comet families; because the
+enormous gravitative action of the sun in their vicinity entirely
+overpowers the attractive force which they exert upon those comets which
+pass close to them. Besides this, a comet, when in the inner regions of
+the solar system, moves with such rapidity, that the gravitational pull
+of the planets there situated is not powerful enough to deflect it to
+any extent. It must not be presumed, however, that a comet once captured
+should always remain a prisoner. Further disturbing causes might
+unsettle its newly acquired orbit, and send it out again into the
+celestial spaces.</p>
+
+<p>With regard to the matter of which comets are composed, the spectroscope
+shows the presence in them of hydrocarbon compounds (a notable
+characteristic of these bodies), and at times, also, of sodium and iron.
+Some of the light which we get from comets is, however, merely reflected
+sunlight.</p>
+
+<p>The fact that the tails of comets are always directed away from the sun,
+has given rise to the idea that this is caused by some repelling action
+emanating from the sun itself, which is continually driving off the
+smallest particles. Two leading theories have been formulated to account
+for the tails themselves upon the<span class='pagenum'><a name="Page_253" id="Page_253">[Pg 253]</a></span> above assumption. One of these, first
+suggested by Olbers in 1812, and now associated with the name of the
+Russian astronomer, the late Professor Br&eacute;dikhine, who carefully worked
+it out, presumes an electrical action emanating from the sun; the other,
+that of Arrhenius, supposes a pressure exerted by the solar light in its
+radiation outwards into space. It is possible, indeed, that repelling
+forces of both these kinds may be at work together. Minute particles are
+probably being continually produced by friction and collisions among the
+more solid parts in the heads of comets. Supposing that such particles
+are driven off altogether, one may therefore assume that the so-called
+captured comets are disintegrating at a comparatively rapid rate. Kepler
+long ago maintained that "comets die," and this actually appears to be
+the case. The ordinary periodic ones, such, for instance, as Encke's
+Comet, are very faint, and becoming fainter at each return. Certain of
+these comets have, indeed, failed altogether to reappear. It is notable
+that the members of Jupiter's comet family are not very conspicuous
+objects. They have small tails, and even in some cases have none at all.
+The family, too, does not contain many members, and yet one cannot but
+suppose that Jupiter, on account of his great mass, has had many
+opportunities for making captures adown the ages.</p>
+
+<p>Of the two theories to which allusion has above been made, that of
+Br&eacute;dikhine has been worked out so carefully, and with such a show of
+plausibility, that it here calls for a detailed description. It appears
+besides to explain the phenomena of comets' tails so much more
+satisfactorily than that of Arrhenius,<span class='pagenum'><a name="Page_254" id="Page_254">[Pg 254]</a></span> that astronomers are inclined to
+accept it the more readily of the two. According to Br&eacute;dikhine's theory
+the electrical repulsive force, which he assumes for the purposes of his
+argument, will drive the minutest particles of the comet in a direction
+away from the sun much more readily than the gravitative action of that
+body will pull them towards it. This may be compared to the ease with
+which fine dust may be blown upwards, although the earth's gravitation
+is acting upon it all the time.</p>
+
+<p>The researches of Br&eacute;dikhine, which began seriously with his
+investigation of Coggia's Comet of 1874, led him to classify the tails
+of comets in <i>three types</i>. Presuming that the repulsive force emanating
+from the sun did not vary, he came to the conclusion that the different
+forms assumed by cometary tails must be ascribed to the special action
+of this force upon the various elements which happen to be present in
+the comet. The tails which he classes as of the first type, are those
+which are long and straight and point directly away from the sun.
+Examples of such tails are found in the comets of 1811, 1843, and 1861.
+Tails of this kind, he thinks, are in all probability formed of
+<i>hydrogen</i>. His second type comprises those which are pointed away from
+the sun, but at the same time are considerably curved, as was seen in
+the comets of Donati and Coggia. These tails are formed of <i>hydrocarbon
+gas</i>. The third type of tail is short, brush-like, and strongly bent,
+and is formed of the <i>vapour of iron</i>, mixed with that of sodium and
+other elements. It should, however, be noted that comets have
+occasionally been seen which possess several tails of these various
+types.</p>
+
+<p><span class='pagenum'><a name="Page_255" id="Page_255">[Pg 255]</a></span></p><p>We will now touch upon a few of the best known comets of modern times.</p>
+
+<p>The comet of 1680 was the first whose orbit was calculated according to
+the laws of gravitation. This was accomplished by Newton, and he found
+that the comet in question completed its journey round the sun in a
+period of about 600 years.</p>
+
+<p>In 1682 there appeared a great comet, which has become famous under the
+name of Halley's Comet, in consequence of the profound investigations
+made into its motion by the great astronomer, Edmund Halley. He fixed
+its period of revolution around the sun at about seventy-five years, and
+predicted that it would reappear in the early part of 1759. He did not,
+however, live to see this fulfilled, but the comet duly returned&mdash;<i>the
+first body of the kind to verify such a prediction</i>&mdash;and was detected on
+Christmas Day, 1758, by George Palitzch, an amateur observer living near
+Dresden. Halley also investigated the past history of the comet, and
+traced it back to the year 1456. The orbit of Halley's comet passes out
+slightly beyond the orbit of Neptune. At its last visit in 1835, this
+comet passed comparatively close to us, namely, within five million
+miles of the earth. According to the calculations of Messrs P.H. Cowell
+and A.C.D. Crommelin of Greenwich Observatory, its next return will be
+in the spring of 1910; the nearest approach to the earth taking place
+about May 12.</p>
+
+<p>On the 26th of March, 1811, a great comet appeared, which remained
+visible for nearly a year and a half. It was a magnificent object; the
+tail being about 100 millions of miles in length, and the head about
+127,000 miles in diameter. A detailed study which<span class='pagenum'><a name="Page_256" id="Page_256">[Pg 256]</a></span> he gave to this comet
+prompted Olbers to put forward that theory of electrical repulsion
+which, as we have seen, has since been so carefully worked out by
+Br&eacute;dikhine. Olbers had noticed that the particles expelled from the head
+appeared to travel to the end of the tail in about eleven minutes, thus
+showing a velocity per second very similar to that of light.</p>
+
+<p>The discovery in 1819 of the comet known as Encke's, because its orbit
+was determined by an astronomer of that name, drew attention for the
+first time to Jupiter's comet family, and, indeed, to short-period
+comets in general. This comet revolves around the sun in the shortest
+known period of any of these bodies, namely, 3&#8531; years. Encke
+predicted that it would return in 1822. This duly occurred, the comet
+passing at its nearest to the sun within three hours of the time
+indicated; being thus the second instance of the fulfilment of a
+prediction of the kind. A certain degree of irregularity which Encke's
+Comet displays in the dates of its returns to the sun, has been supposed
+to indicate that it passes in the course of its orbit through some
+retarding medium, but no definite conclusions have so far been arrived
+at in this matter.</p>
+
+<p>A comet, which appeared in 1826, goes by the name of Biela's Comet,
+because of its discovery by an Austrian military officer, Wilhelm von
+Biela. This comet was found to have a period of between six and seven
+years. Certain calculations made by Olbers showed that, at its return in
+1832, it would pass <i>through the earth's orbit</i>. The announcement of
+this gave rise to a panic; for people did not wait to inquire whether
+the earth would be anywhere near that part of its orbit when the comet
+passed. The panic, however, subsided when the French astronomer, Arago,
+showed that at the moment in question the earth would be some 50
+millions of miles away from the point indicated!</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Plate_XVII" id="Plate_XVII"></a>
+<img src="images/plate17.jpg" width="500" height="771" alt="Plate XVII." title="" />
+<span class="caption"><span class="smcap">Plate XVII. Donati&#39;s Comet</span><br />
+From a drawing made on October 9th, 1858, by G.P. Bond, of Harvard
+College Observatory, U.S.A. A good illustration of Br&eacute;dikhine's theory:
+note the straight tails of his <i>first</i> type, and the curved tail of his
+<i>second</i>.<br />(<a href="#Page_257"><small>Page 257</small></a>)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_257" id="Page_257">[Pg 257]</a></span></p><p>In 1846, shortly after one of its returns, Biela's Comet divided into
+two portions. At its next appearance (1852) these portions had separated
+to a distance of about 1&frac12; millions of miles from each other. This
+comet, or rather its constituents, have never since been seen.</p>
+
+<p>Perhaps the most remarkable comet of recent times was that of 1858,
+known as Donati's, it having been discovered at Florence by the Italian
+astronomer, G.B. Donati. This comet, a magnificent object, was visible
+for more than three months with the naked eye. Its tail was then 54
+millions of miles in length. It was found to revolve around the sun in a
+period of over 2000 years, and to go out in its journey to about 5&frac12;
+times the distance of Neptune. Its motion is retrograde, that is to say,
+in the contrary direction to the usual movement in the solar system. A
+number of beautiful drawings of Donati's Comet were made by the American
+astronomer, G.P. Bond. One of the best of these is reproduced on <a href="#Plate_XVII">Plate
+XVII.</a>, p. 256.</p>
+
+<p>In 1861 there appeared a great comet. On the 30th of June of that year
+the earth and moon actually passed through its tail; but no effects were
+noticed, other than a peculiar luminosity in the sky.</p>
+
+<p>In the year 1881 there appeared another large comet, known as Tebbutt's
+Comet, from the name of its discoverer. This was the <i>first comet of
+which a<span class='pagenum'><a name="Page_258" id="Page_258">[Pg 258]</a></span> satisfactory photograph was obtained</i>. The photograph in
+question was taken by the late M. Janssen.</p>
+
+<p>The comet of 1882 was of vast size and brilliance. It approached so
+close to the sun that it passed through some 100,000 miles of the solar
+corona. Though its orbit was not found to have been altered by this
+experience, its nucleus displayed signs of breaking up. Some very fine
+photographs of this comet were obtained at the Cape of Good Hope by Mr.
+(now Sir David) Gill.</p>
+
+<p>The comet of 1889 was followed with the telescope nearly up to the orbit
+of Saturn, which seems to be the greatest distance at which a comet has
+ever been seen.</p>
+
+<p>The <i>first discovery of a comet by photographic means</i><a name="FNanchor_25_25" id="FNanchor_25_25"></a><a href="#Footnote_25_25" class="fnanchor">[25]</a> was made by
+Professor Barnard in 1892; and, since then, photography has been
+employed with marked success in the detection of small periodic comets.</p>
+
+<p>The best comet seen in the Northern hemisphere since that of 1882,
+appears to have been Daniel's Comet of 1907 (<a href="#Plate_XVIII">see Plate XVIII.</a>, p. 258).
+This comet was discovered on June 9, 1907, by Mr. Z. Daniel, at
+Princeton Observatory, New Jersey, U.S.A. It became visible to the naked
+eye about mid-July of that year, and reached its greatest brilliancy
+about the end of August. It did not, however, attract much popular
+attention, as its position in the sky allowed it to be seen only just
+before dawn.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Plate_XVIII" id="Plate_XVIII"></a>
+<img src="images/plate18.jpg" width="500" height="771" alt="Plate XVIII." title="" />
+<span class="caption"><span class="smcap">Plate XVIII. Daniel&#39;s Comet of 1907</span></span>
+<div class="caption2">From a photograph taken, on August 11th, 1907, by Dr. Max Wolf, at the
+Astrophysical Observatory, Heidelberg. The instrument used was a 28&ndash;inch
+reflecting telescope, and the time of exposure was fifteen minutes. As
+the telescope was guided to follow the moving comet, the stars have
+imprinted themselves upon the photographic plate as short trails. This
+is clearly the opposite to what is depicted on <a href="#Plate_XIII">Plate XIII</a>.<br />
+(<a href="#Page_258"><small>Page 258</small></a>)</div>
+</div>
+
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_24_24" id="Footnote_24_24"></a><a href="#FNanchor_24_24"><span class="label">[24]</span></a> With the exception, of course, of such an anomaly as the
+retrograde motion of the ninth satellite of Saturn.</p></div>
+
+<div class="footnote"><p><a name="Footnote_25_25" id="Footnote_25_25"></a><a href="#FNanchor_25_25"><span class="label">[25]</span></a> If we except the case of the comet which was photographed
+near the solar corona in the eclipse of 1882.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_259" id="Page_259">[Pg 259]</a></span></p>
+<h3><a name="CHAPTER_XX" id="CHAPTER_XX"></a>CHAPTER XX</h3>
+
+<h4>REMARKABLE COMETS</h4>
+
+
+<p class="noin"><span class="smcap">If</span> eclipses were a cause of terror in past ages, comets appear to have
+been doubly so. Their much longer continuance in the sight of men had no
+doubt something to say to this, and also the fact that they arrived
+without warning; it not being then possible to give even a rough
+prediction of their return, as in the case of eclipses. As both these
+phenomena were occasional, and out of the ordinary course of things,
+they drew exceptional attention as unusual events always do; for it must
+be allowed that quite as wonderful things exist, but they pass unnoticed
+merely because men have grown accustomed to them.</p>
+
+<p>For some reason the ancients elected to class comets along with meteors,
+the aurora borealis, and other phenomena of the atmosphere, rather than
+with the planets and the bodies of the spaces beyond. The sudden
+appearance of these objects led them to be regarded as signs sent by the
+gods to announce remarkable events, chief among these being the deaths
+of monarchs. Shakespeare has reminded us of this in those celebrated
+lines in <i>Julius C&aelig;sar</i>:&mdash;</p>
+
+<p class="poem">
+"When beggars die there are no comets seen,<br />
+The heavens themselves blaze forth the death of princes."<br />
+</p>
+
+<p class="noin">Numbed by fear, the men of old blindly accepted these presages of fate;
+and did not too closely<span class='pagenum'><a name="Page_260" id="Page_260">[Pg 260]</a></span> question whether the threatened danger was to
+their own nation or to some other, to their ruler or to his enemy. Now
+and then, as in the case of the Roman Emperor Vespasian, there was a
+cynical attempt to apply some reasoning to the portent. That emperor, in
+alluding to the comet of <span class="ampm">A.D.</span> 79, is reported to have said: "This hairy
+star does not concern me; it menaces rather the King of the Parthians,
+for he is hairy and I am bald." Vespasian, all the same, died shortly
+afterwards!</p>
+
+<p>Pliny, in his natural history, gives several instances of the terrible
+significance which the ancients attached to comets. "A comet," he says,
+"is ordinarily a very fearful star; it announces no small effusion of
+blood. We have seen an example of this during the civil commotion of
+Octavius."</p>
+
+<p>A very brilliant comet appeared in 371 <span class="ampm">B.C.</span>, and about the same time an
+earthquake caused Helic&egrave; and Bura, two towns in Achaia, to be swallowed
+up by the sea. The following remark made by Seneca concerning it shows
+that the ancients did not consider comets merely as precursors, but even
+as actual <i>causes</i> of fatal events: "This comet, so anxiously observed
+by every one, <i>because of the great catastrophe which it produced as
+soon as it appeared</i>, the submersion of Bura and Helic&egrave;."</p>
+
+<p>Comets are by no means rare visitors to our skies, and very few years
+have elapsed in historical times without such objects making their
+appearance. In the Dark and Middle Ages, when Europe was split up into
+many small kingdoms and principalities, it was, of course, hardly
+possible for a comet to appear without the death of some ruler occurring
+near the time.<span class='pagenum'><a name="Page_261" id="Page_261">[Pg 261]</a></span> Critical situations, too, were continually arising in
+those disturbed days. The end of Louis le Debonnaire was hastened, as
+the reader will, no doubt, recollect, by the great eclipse of 840; but
+it was firmly believed that a comet which had appeared a year or two
+previously presaged his death. The comet of 1556 is reported to have
+<i>influenced</i> the abdication of the Emperor Charles V.; but curiously
+enough, this event had already taken place before the comet made its
+appearance! Such beliefs, no doubt, had a very real effect upon rulers
+of a superstitious nature, or in a weak state of health. For instance,
+Gian Galeazzo Visconti, Duke of Milan, was sick when the comet of 1402
+appeared. After seeing it, he is said to have exclaimed: "I render
+thanks to God for having decreed that my death should be announced to
+men by this celestial sign." His malady then became worse, and he died
+shortly afterwards.</p>
+
+<p>It is indeed not improbable that such superstitious fears in monarchs
+were fanned by those who would profit by their deaths, and yet did not
+wish to stain their own hands with blood.</p>
+
+<p>Evil though its effects may have been, this morbid interest which past
+ages took in comets has proved of the greatest service to our science.
+Had it not been believed that the appearance of these objects was
+attended with far-reaching effects, it is very doubtful whether the old
+chroniclers would have given themselves the trouble of alluding to them
+at all; and thus the modern investigators of cometary orbits would have
+lacked a great deal of important material.</p>
+
+<p>We will now mention a few of the most notable comets which historians
+have recorded.</p>
+
+<p><span class='pagenum'><a name="Page_262" id="Page_262">[Pg 262]</a></span></p><p>A comet which appeared in 344 <span class="ampm">B.C.</span> was thought to betoken the success
+of the expedition undertaken in that year by Timoleon of Corinth against
+Sicily. "The gods by an extraordinary prodigy announced his success and
+future greatness: a burning torch appeared in the heavens throughout the
+night and preceded the fleet of Timoleon until it arrived off the coast
+of Sicily."</p>
+
+<p>The comet of 43 <span class="ampm">B.C.</span> was generally believed to be the soul of C&aelig;sar on
+its way to heaven.</p>
+
+<p>Josephus tells us that in <span class="ampm">A.D.</span> 69 several prodigies, and amongst them a
+comet in the shape of a sword, announced the destruction of Jerusalem.
+This comet is said to have remained over the city for the space of a
+year!</p>
+
+<p>A comet which appeared in <span class="ampm">A.D.</span> 336 was considered to have announced the
+death of the Emperor Constantine.</p>
+
+<p>But perhaps the most celebrated comet of early times was the one which
+appeared in <span class="ampm">A.D.</span> 1000. That year was, in more than one way, big with
+portent, for there had long been a firm belief that the Christian era
+could not possibly run into four figures. Men, indeed, steadfastly
+believed that when the thousand years had ended, the millennium would
+immediately begin. Therefore they did not reap neither did they sow,
+they toiled not, neither did they spin, and the appearance of the comet
+strengthened their convictions. The fateful year, however, passed by
+without anything remarkable taking place; but the neglect of husbandry
+brought great famine and pestilence over Europe in the years which
+followed.</p>
+
+<p>In April 1066, that year fraught with such immense<span class='pagenum'><a name="Page_263" id="Page_263">[Pg 263]</a></span> consequences for
+England, a comet appeared. No one doubted but that it was a presage of
+the success of the Conquest, and perhaps, indeed, it had its due weight
+in determining the minds and actions of the men who took part in the
+expedition. <i>Nova stella, novus rex</i> ("a new star, a new sovereign") was
+a favourite proverb of the time. The chroniclers, with one accord, have
+delighted to relate that the Normans, "guided by a comet," invaded
+England. A representation of this object appears in the Bayeux Tapestry
+(<a href="#Fig_19">see Fig. 19</a>, p. 263).<a name="FNanchor_26_26" id="FNanchor_26_26"></a><a href="#Footnote_26_26" class="fnanchor">[26]</a></p>
+
+<div class="figcenter" style="width: 600px;"><a name="Fig_19" id="Fig_19"></a>
+<img src="images/figure19.jpg" width="500" height="412" alt="Fig. 19." title="" /><br />
+<span class="caption"><span class="smcap">Fig. 19.</span>&mdash;The comet of 1066, as represented in the Bayeux
+Tapestry.<br />(From the <i>World of Comets</i>.)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_264" id="Page_264">[Pg 264]</a></span></p><p>We have mentioned Halley's Comet of 1682, and how it revisits the
+neighbourhood of the earth at intervals of seventy-six years. The comet
+of 1066 has for many years been supposed to be Halley's Comet on one of
+its visits. The identity of these two, however, was only quite recently
+placed beyond all doubt by the investigations of Messrs Cowell and
+Crommelin. This comet appeared also in 1456, when John Huniades was
+defending Belgrade against the Turks led by Mahomet II., the conqueror
+of Constantinople, and is said to have paralysed both armies with fear.</p>
+
+<p>The Middle Ages have left us descriptions of comets, which show only too
+well how the imagination will run riot under the stimulus of terror. For
+instance, the historian, Nicetas, thus describes the comet of the year
+1182: "After the Romans were driven from Constantinople a prognostic was
+seen of the excesses and crimes to which Andronicus was to abandon
+himself. A comet appeared in the heavens similar to a writhing serpent;
+sometimes it extended itself, sometimes it drew itself in; sometimes, to
+the great terror of the spectators, it opened a huge mouth; it seemed
+that, as if thirsting for human blood, it was upon the point of
+satiating itself." And, again, the celebrated Ambrose Par&eacute;, the father
+of surgery, has left us the following account of the comet of 1528,
+which appeared in his own time: "This comet," said he, "was so horrible,
+so frightful, and it produced such great terror in the vulgar, that some
+died of fear, and others fell sick. It appeared to be of excessive
+length, and was of the colour of blood. At the summit of it was seen the
+figure of a bent arm, holding in its hand a<span class='pagenum'><a name="Page_265" id="Page_265">[Pg 265]</a></span> great sword, as if about to
+strike. At the end of the point there were three stars. On both sides of
+the rays of this comet were seen a great number of axes, knives,
+blood-coloured swords, among which were a great number of hideous human
+faces, with beards and bristling hair." Par&eacute;, it is true, was no
+astronomer; yet this shows the effect of the phenomenon, even upon a man
+of great learning, as undoubtedly he was. It should here be mentioned
+that nothing very remarkable happened at or near the year 1528.</p>
+
+<p>Concerning the comet of 1680, the extraordinary story got about that, at
+Rome, a hen had laid an egg on which appeared a representation of the
+comet!</p>
+
+<p>But the superstitions with regard to comets were now nearing their end.
+The last blow was given by Halley, who definitely proved that they
+obeyed the laws of gravitation, and circulated around the sun as planets
+do; and further announced that the comet of 1682 had a period of
+seventy-six years, which would cause it to reappear in the year 1759. We
+have seen how this prediction was duly verified. We have seen, too, how
+this comet appeared again in 1835, and how it is due to return in the
+early part of 1910.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_26_26" id="Footnote_26_26"></a><a href="#FNanchor_26_26"><span class="label">[26]</span></a> With regard to the words "Isti mirant stella" in the
+figure, Mr. W.T. Lynn suggests that they may not, after all, be the
+grammatically bad Latin which they appear, but that the legend is really
+"Isti mirantur stellam," the missing letters being supposed to be hidden
+by the building and the comet.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_266" id="Page_266">[Pg 266]</a></span></p>
+<h3><a name="CHAPTER_XXI" id="CHAPTER_XXI"></a>CHAPTER XXI</h3>
+
+<h4>METEORS OR SHOOTING STARS</h4>
+
+
+<p class="noin"><span class="smcap">Any</span> one who happens to gaze at the sky for a short time on a clear night
+is pretty certain to be rewarded with a view of what is popularly known
+as a "shooting star." Such an object, however, is not a star at all, but
+has received its appellation from an analogy; for the phenomenon gives
+to the inexperienced in these matters an impression as if one of the
+many points of light, which glitter in the vaulted heaven, had suddenly
+become loosened from its place, and was falling towards the earth. In
+its passage across the sky the moving object leaves behind a trail of
+light which usually lasts for a few moments. Shooting stars, or meteors,
+as they are technically termed, are for the most part very small bodies,
+perhaps no larger than peas or pebbles, which, dashing towards our earth
+from space beyond, are heated to a white heat, and reduced to powder by
+the friction resulting from their rapid passage into our atmosphere.
+This they enter at various degrees of speed, in some cases so great as
+45 miles a second. The speed, of course, will depend greatly upon
+whether the earth and the meteors are rushing towards each other, or
+whether the latter are merely overtaking the earth. In the first of
+these cases the meteors will naturally collide with<span class='pagenum'><a name="Page_267" id="Page_267">[Pg 267]</a></span> the atmosphere with
+great force; in the other case they will plainly come into it with much
+less rapidity. As has been already stated, it is from observations of
+such bodies that we are enabled to estimate, though very imperfectly,
+the height at which the air around our globe practically ceases, and
+this height is imagined to be somewhere about 100 miles. Fortunate,
+indeed, is it for us that there is a goodly layer of atmosphere over our
+heads, for, were this not so, these visitors from space would strike
+upon the surface of our earth night and day, and render existence still
+more unendurable than many persons choose to consider it. To what a
+bombardment must the moon be continually subject, destitute as she is of
+such an atmospheric shield!</p>
+
+<p>It is only in the moment of their dissolution that we really learn
+anything about meteors, for these bodies are much too small to be seen
+before they enter our atmosphere. The d&eacute;bris arising from their
+destruction is wafted over the earth, and, settling down eventually upon
+its surface, goes to augment the accumulation of that humble domestic
+commodity which men call dust. This continual addition of material
+tends, of course, to increase the mass of the earth, though the effect
+thus produced will be on an exceedingly small scale.</p>
+
+<p>The total number of meteors moving about in space must be practically
+countless. The number which actually dash into the earth's atmosphere
+during each year is, indeed, very great. Professor Simon Newcomb, the
+well-known American astronomer, has estimated that, of the latter, those
+large enough to be seen with the naked eye cannot be in<span class='pagenum'><a name="Page_268" id="Page_268">[Pg 268]</a></span> all less than
+146,000,000,000 per annum. Ten times more numerous still are thought to
+be those insignificant ones which are seen to pass like mere sparks of
+light across the field of an observer's telescope.</p>
+
+<p>Until comparatively recent times, perhaps up to about a hundred years
+ago, it was thought that meteors were purely terrestrial phenomena which
+had their origin in the upper regions of the air. It, however, began to
+be noticed that at certain periods of the year these moving objects
+appeared to come from definite areas of the sky. Considerations,
+therefore, respecting their observed velocities, directions, and
+altitudes, gave rise to the theory that they are swarms of small bodies
+travelling around the sun in elongated elliptical orbits, all along the
+length of which they are scattered, and that the earth, in its annual
+revolution, rushing through the midst of such swarms at the same epoch
+each year, naturally entangles many of them in its atmospheric net.</p>
+
+<p>The dates at which the earth is expected to pass through the principal
+meteor-swarms are now pretty well known. These swarms are distinguished
+from one another by the direction of the sky from which the meteors seem
+to arrive. Many of the swarms are so wide that the earth takes days, and
+even weeks, to pass through them. In some of these swarms, or streams,
+as they are also called, the meteors are distributed with fair evenness
+along the entire length of their orbits, so that the earth is greeted
+with a somewhat similar shower at each yearly encounter. In others, the
+chief portions are bunched together, so that, in certain years, the
+display<span class='pagenum'><a name="Page_269" id="Page_269">[Pg 269]</a></span> is exceptional (<a href="#Fig_20">see Fig. 20</a>, p. 269). That part of the heavens
+from which a shower of meteors is seen to emanate is called the
+"radiant," or radiant point, because the foreshortened view we get of
+the streaks of light makes it appear as if they radiated outwards from
+this point. In observations of these bodies the attention of astronomers
+is directed to registering the path and speed of each meteor, and to
+ascertaining the position of the radiant. It is from data such as these
+that computations concerning the swarms and their orbits are made.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_20" id="Fig_20"></a>
+<img src="images/figure20.jpg" width="500" height="288" alt="Fig. 20." title="" />
+<div class="caption1"><span class="smcap">Fig. 20.</span>&mdash;Passage of the Earth through the thickest
+portion of a Meteor Swarm. The Earth and the Meteors are here
+represented as approaching each other from opposite directions.</div>
+</div>
+
+<p>For the present state of knowledge concerning meteors, astronomy is
+largely indebted to the researches of Mr. W.F. Denning, of Bristol, and
+of the late Professor A.S. Herschel.</p>
+
+<p>During the course of each year the earth encounters a goodly number of
+meteor-swarms. Three of these,<span class='pagenum'><a name="Page_270" id="Page_270">[Pg 270]</a></span> giving rise to fine displays, are very
+well known&mdash;the "Perseids," or August Meteors, and the "Leonids" and
+"Bielids," which appear in November.</p>
+
+<p>Of the above three the <i>Leonid</i> display is by far the most important,
+and the high degree of attention paid to it has laid the foundation of
+meteoric astronomy in much the same way that the study of the
+fascinating corona has given such an impetus to our knowledge of the
+sun. The history of this shower of meteors may be traced back as far as
+<span class="ampm">A.D.</span> 902, which was known as the "Year of the Stars." It is related that
+in that year, on the night of October 12th&mdash;the shower now comes about a
+month later&mdash;whilst the Moorish King, Ibrahim Ben Ahmed, lay dying
+before Cosenza, in Calabria, "a multitude of falling stars scattered
+themselves across the sky like rain," and the beholders shuddered at
+what they considered a dread celestial portent. We have, however, little
+knowledge of the subsequent history of the Leonids until 1698, since
+which time the maximum shower has appeared with considerable regularity
+at intervals of about thirty-three years. But it was not until 1799 that
+they sprang into especial notice. On the 11th November in that year a
+splendid display was witnessed at Cumana, in South America, by the
+celebrated travellers, Humboldt and Bonpland. Finer still, and
+surpassing all displays of the kind ever seen, was that of November 12,
+1833, when the meteors fell thick as snowflakes, 240,000 being estimated
+to have appeared during seven hours. Some of them were even so bright as
+to be seen in full daylight. The radiant from which the meteors seem to
+diverge was ascertained to be situated in the head of the constellation
+of the Lion, or "Sickle<span class='pagenum'><a name="Page_271" id="Page_271">[Pg 271]</a></span> of Leo," as it is popularly termed, whence
+their name&mdash;Leonids. It was from a discussion of the observations then
+made that the American astronomer, Olmsted, concluded that these meteors
+sprang upon us from interplanetary space, and were not, as had been
+hitherto thought, born of our atmosphere. Later on, in 1837, Olbers
+formulated the theory that the bodies in question travelled around the
+sun in an elliptical orbit, and at the same time he established the
+periodicity of the maximum shower.</p>
+
+<p>The periodic time of recurrence of this maximum, namely, about
+thirty-three years, led to eager expectancy as 1866 drew near. Hopes
+were then fulfilled, and another splendid display took place, of which
+Sir Robert Ball, who observed it, has given a graphic description in his
+<i>Story of the Heavens</i>. The display was repeated upon a smaller scale in
+the two following years. The Leonids were henceforth deemed to hold an
+anomalous position among meteor swarms. According to theory the earth
+cut through their orbit at about the same date each year, and so a
+certain number were then seen to issue from the radiant. But, in
+addition, after intervals of thirty-three years, as has been seen, an
+exceptional display always took place; and this state of things was not
+limited to one year alone, but was repeated at each meeting for about
+three years running. The further assumption was, therefore, made that
+the swarm was much denser in one portion of the orbit than
+elsewhere,<a name="FNanchor_27_27" id="FNanchor_27_27"></a><a href="#Footnote_27_27" class="fnanchor">[27]</a> and that this congested part was drawn out to such an
+extent that the earth could pass through the crossing place during<span class='pagenum'><a name="Page_272" id="Page_272">[Pg 272]</a></span>
+several annual meetings, and still find it going by like a long
+procession (<a href="#Fig_20">see Fig. 20</a>, p. 269).</p>
+
+<p>In accordance with this ascertained period of thirty-three years, the
+recurrence of the great Leonid shower was timed to take place on the
+15th of November 1899. But there was disappointment then, and the
+displays which occurred during the few years following were not of much
+importance. A good deal of comment was made at the time, and theories
+were accordingly put forward to account for the failure of the great
+shower. The most probable explanation seems to be, that the attraction
+of one of the larger planets&mdash;Jupiter perhaps&mdash;has diverted the orbit
+somewhat from its old position, and the earth does not in consequence
+cut through the swarm in the same manner as it used to do.</p>
+
+<p>The other November display alluded to takes place between the 23rd and
+27th of that month. It is called the <i>Andromedid</i> Shower, because the
+meteors appear to issue from the direction of the constellation of
+Andromeda, which at that period of the year is well overhead during the
+early hours of the night. These meteors are also known by the name of
+<i>Bielids</i>, from a connection which the orbit assigned to them appears to
+have with that of the well-known comet of Biela.</p>
+
+<p>M. Egenitis, Director of the Observatory of Athens, accords to the
+Bielids a high antiquity. He traces the shower back to the days of the
+Emperor Justinian. Theophanes, the Chronicler of that epoch, writing of
+the famous revolt of Nika in the year <span class="ampm">A.D.</span> 532, says:&mdash;"During the same
+year a great fall of stars came from the evening till the dawn." M.
+Egenitis notes another early reference to these meteors in <span class="ampm">A.D.</span> 752,
+during<span class='pagenum'><a name="Page_273" id="Page_273">[Pg 273]</a></span> the reign of the Eastern Emperor, Constantine Copronymous.
+Writing of that year, Nicephorus, a Patriarch of Constantinople, has as
+follows:&mdash;"All the stars appeared to be detached from the sky, and to
+fall upon the earth."</p>
+
+<p>The Bielids, however, do not seem to have attracted particular notice
+until the nineteenth century. Attention first began to be riveted upon
+them on account of their suspected connection with Biela's comet. It
+appeared that the same orbit was shared both by that comet and the
+Bielid swarm. It will be remembered that the comet in question was not
+seen after its appearance in 1852. Since that date, however, the Bielid
+shower has shown an increased activity; which was further noticed to be
+especially great in those years in which the comet, had it still
+existed, would be due to pass near the earth.</p>
+
+<p>The third of these great showers to which allusion has above been made,
+namely, the <i>Perseids</i>, strikes the earth about the 10th of August; for
+which reason it is known on the Continent under the name of the "tears
+of St. Lawrence," the day in question being sacred to that Saint. This
+shower is traceable back many centuries, even as far as the year <span class="ampm">A.D.</span>
+811. The name given to these meteors, "Perseids," arises from the fact
+that their radiant point is situated in the constellation of Perseus.
+This shower is, however, not by any means limited to the particular
+night of August 10th, for meteors belonging to the swarm may be observed
+to fall in more or less varying quantities from about July 8th to August
+22nd. The Perseid meteors sometimes fall at the rate of about sixty per
+hour. They are noted for their great rapidity of<span class='pagenum'><a name="Page_274" id="Page_274">[Pg 274]</a></span> motion, and their
+trails besides often persist for a minute or two before being
+disseminated. Unlike the other well-known showers, the radiants of which
+are stationary, that of the Perseids shifts each night a little in an
+easterly direction.</p>
+
+<p>The orbit of the Perseids cuts that of the earth almost perpendicularly.
+The bodies are generally supposed to be the result of the disintegration
+of an ancient comet which travelled in the same orbit. Tuttle's Comet,
+which passed close to the earth in 1862, also belongs to this orbit; and
+its period of revolution is calculated to be 131 years. The Perseids
+appear to be disseminated all along this great orbit, for we meet them
+in considerable quantities each year. The bodies in question are in
+general particularly small. The swarm has, however, like most others, a
+somewhat denser portion, and through this the earth passed in 1848. The
+<i>aphelion</i>, or point where the far end of the orbit turns back again
+towards the sun, is situated right away beyond the path of Neptune, at a
+distance of forty-eight times that of the earth from the sun. The comet
+of 1532 also belongs to the Perseid orbit. It revisited the
+neighbourhood of the earth in 1661, and should have returned in 1789.
+But we have no record of it in that year; for which omission the then
+politically disturbed state of Europe may account. If not already
+disintegrated, this comet is due to return in 1919.</p>
+
+<p>This supposed connection between comets and meteor-swarms must be also
+extended to the case of the Leonids. These meteors appear to travel
+along the same track as Tempel's Comet of 1866.</p>
+
+<p>It is considered that the attractions of the various<span class='pagenum'><a name="Page_275" id="Page_275">[Pg 275]</a></span> bodies of the
+solar system upon a meteor swarm must eventually result in breaking up
+the "bunched" portion, so that in time the individual meteors should
+become distributed along the whole length of the orbit. Upon this
+assumption the Perseid swarm, in which the meteors are fairly well
+scattered along its path, should be of greater age than the Leonid. As
+to the Leonid swarm itself, Le Verrier held that it was first brought
+into the solar system in <span class="ampm">A.D.</span> 126, having been captured from outer space
+by the gravitative action of the planet Uranus.</p>
+
+<p>The acknowledged theory of meteor swarms has naturally given rise to an
+idea, that the sunlight shining upon such a large collection of
+particles ought to render a swarm visible before its collision with the
+earth's atmosphere. Several attempts have therefore been made to search
+for approaching swarms by photography, but, so far, it appears without
+success. It has also been proposed, by Mr. W.H.S. Monck, that the stars
+in those regions from which swarms are due, should be carefully watched,
+to see if their light exhibits such temporary diminutions as would be
+likely to arise from the momentary interposition of a cloud of moving
+particles.</p>
+
+<p>Between ten and fifteen years ago it happened that several well-known
+observers, employed in telescopic examination of the sun and moon,
+reported that from time to time they had seen small dark bodies,
+sometimes singly, sometimes in numbers, in passage across the discs of
+the luminaries. It was concluded that these were meteors moving in space
+beyond the atmosphere of the earth. The bodies were called "dark
+meteors," to emphasise the fact that they were seen<span class='pagenum'><a name="Page_276" id="Page_276">[Pg 276]</a></span> in their natural
+condition, and not in that momentary one in which they had hitherto been
+always seen; <i>i.e.</i> when heated to white heat, and rapidly vaporised, in
+the course of their passage through the upper regions of our air. This
+"discovery" gave promise of such assistance to meteor theories, that
+calculations were made from the directions in which they had been seen
+to travel, and the speeds at which they had moved, in the hope that some
+information concerning their orbits might be revealed. But after a while
+some doubt began to be thrown upon their being really meteors, and
+eventually an Australian observer solved the mystery. He found that they
+were merely tiny particles of dust, or of the black coating on the inner
+part of the tube of the telescope, becoming detached from the sides of
+the eye-piece and falling across the field of view. He was led to this
+conclusion by having noted that a gentle tapping of his instrument
+produced the "dark" bodies in great numbers! Thus the opportunity of
+observing meteors beyond our atmosphere had once more failed.</p>
+
+<p><i>Meteorites</i>, also known as &aelig;rolites and fireballs, are usually placed
+in quite a separate category from meteors. They greatly exceed the
+latter in size, are comparatively rare, and do not appear in any way
+connected with the various showers of meteors. The friction of their
+passage through the atmosphere causes them to shine with a great light;
+and if not shattered to pieces by internal explosions, they reach the
+ground to bury themselves deep in it with a great rushing and noise.
+When found by uncivilised peoples, or savages, they are, on account of
+their celestial origin, usually regarded as objects of wonder<span class='pagenum'><a name="Page_277" id="Page_277">[Pg 277]</a></span> and of
+worship, and thus have arisen many mythological legends and deifications
+of blackened stones. On the other hand, when they get into the
+possession of the civilised, they are subjected to careful examinations
+and tests in chemical laboratories. The bodies are, as a rule, composed
+of stone, in conjunction with iron, nickel, and such elements as exist
+in abundance upon our earth; though occasionally specimens are found
+which are practically pure metal. In the museums of the great capitals
+of both Continents are to be seen some fine collections of meteorites.
+Several countries&mdash;Greenland and Mexico, for instance&mdash;contain in the
+soil much meteoric iron, often in masses so large as to baffle all
+attempts at removal. Blocks of this kind have been known to furnish the
+natives in their vicinity for many years with sources of workable iron.</p>
+
+<p>The largest meteorite in the world is one known as the Anighito
+meteorite. It was brought to the United States by the explorer Peary,
+who found it at Cape York in Greenland. He estimates its weight at from
+90 to 100 tons. One found in Mexico, called the Bacubirito, comes next,
+with an estimated weight of 27&frac12; tons. The third in size is the
+Willamette meteorite, found at Willamette in Oregon in 1902. It measures
+10 &times; 6&frac12; &times; 4&frac12; feet, and weighs about 15&frac12; tons.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_27_27" id="Footnote_27_27"></a><a href="#FNanchor_27_27"><span class="label">[27]</span></a> The "gem" of the meteor ring, as it has been termed.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_278" id="Page_278">[Pg 278]</a></span></p>
+<h3><a name="CHAPTER_XXII" id="CHAPTER_XXII"></a>CHAPTER XXII</h3>
+
+<h4>THE STARS</h4>
+
+
+<p class="noin"><span class="smcap">In</span> the foregoing chapters we have dealt at length with those celestial
+bodies whose nearness to us brings them into our especial notice. The
+entire room, however, taken up by these bodies, is as a mere point in
+the immensities of star-filled space. The sun, too, is but an ordinary
+star; perhaps quite an insignificant one<a name="FNanchor_28_28" id="FNanchor_28_28"></a><a href="#Footnote_28_28" class="fnanchor">[28]</a> in comparison with the
+majority of those which stud that background of sky against which the
+planets are seen to perform their wandering courses.</p>
+
+<p>Dropping our earth and the solar system behind, let us go afield and
+explore the depths of space.</p>
+
+<p>We have seen how, in very early times, men portioned out the great mass
+of the so-called "fixed stars" into divisions known as constellations.
+The various arrangements, into which the brilliant points of light fell
+as a result of perspective, were noticed and roughly compared with such
+forms as were familiar to men upon the earth. Imagination quickly saw in
+them the semblances of heroes and of mighty fabled beasts; and, around
+these monstrous shapes, legends were woven, which told how the great
+deeds done in the misty dawn of historical time had been<span class='pagenum'><a name="Page_279" id="Page_279">[Pg 279]</a></span> enshrined by
+the gods in the sky as an example and a memorial for men. Though the
+centuries have long outlived such fantasies, yet the constellation
+figures and their ancient names have been retained to this day, pretty
+well unaltered for want of any better arrangement. The Great and Little
+Bears, Cassiopeia, Perseus, and Andromeda, Orion and the rest, glitter
+in our night skies just as they did centuries and centuries ago.</p>
+
+<p>Many persons seem to despair of gaining any real knowledge of astronomy,
+merely because they are not versed in recognising the constellations.
+For instance, they will say:&mdash;"What is the use of my reading anything
+about the subject? Why, I believe I couldn't even point out the Great
+Bear, were I asked to do so!" But if such persons will only consider for
+a moment that what we call the Great Bear has no existence in fact, they
+need not be at all disheartened. Could we but view this familiar
+constellation from a different position in space, we should perhaps be
+quite unable to recognise it. Mountain masses, for instance, when seen
+from new directions, are often unrecognisable.</p>
+
+<p>It took, as we have seen, a very long time for men to acknowledge the
+immense distances of the stars from our earth. Their seeming
+unchangeableness of position was, as we have seen, largely responsible
+for the idea that the earth was immovable in space. It is a wonder that
+the Copernican system ever gained the day in the face of this apparent
+fixity of the stars. As time went on, it became indeed necessary to
+accord to these objects an almost inconceivable distance, in order to
+account for the fact that they remained<span class='pagenum'><a name="Page_280" id="Page_280">[Pg 280]</a></span> apparently quite undisplaced,
+notwithstanding the journey of millions of miles which the earth was now
+acknowledged to make each year around the sun. In the face of the
+gradual and immense improvement in telescopes, this apparent immobility
+of the stars was, however, not destined to last. The first ascertained
+displacement of a star, namely that of 61 Cygni, noted by Bessel in the
+year 1838, definitely proved to men the truth of the Copernican system.
+Since then some forty more stars have been found to show similar tiny
+displacements. We are, therefore, in possession of the fact, that the
+actual distances of a few out of the great host can be calculated.</p>
+
+<p>To mention some of these. The nearest star to the earth, so far as we
+yet know, is Alpha Centauri, which is distant from us about 25 billions
+of miles. The light from this star, travelling at the stupendous rate of
+about 186,000 miles per second, takes about 4&frac14; years to reach our
+earth, or, to speak astronomically, Alpha Centauri is about 4&frac14; "light
+years" distant from us. Sirius&mdash;the brightest star in the whole sky&mdash;is
+at twice this distance, <i>i.e.</i> about 8&frac12; light years. Vega is about 30
+light years distant from us, Capella about 32, and Arcturus about 100.</p>
+
+<p>The displacements, consequent on the earth's movement, have, however,
+plainly nothing to say to any real movements on the part of the stars
+themselves. The old idea was that the stars were absolutely fixed; hence
+arose the term "fixed stars"&mdash;a term which, though inaccurate, has not
+yet been entirely banished from the astronomical vocabulary. But careful
+observations extending over a number of years have shown slight changes
+of position among these bodies; and<span class='pagenum'><a name="Page_281" id="Page_281">[Pg 281]</a></span> such alterations cannot be ascribed
+to the revolution of the earth in its orbit, for they appear to take
+place in every direction. These evidences of movement are known as
+"proper motions," that is to say, actual motions in space proper to the
+stars themselves. Stars which are comparatively near to us show, as a
+rule, greater proper motions than those which are farther off. It must
+not, however, be concluded that these proper motions are of any very
+noticeable amounts. They are, as a matter of fact, merely upon the same
+apparently minute scale as other changes in the heavens; and would
+largely remain unnoticed were it not for the great precision of modern
+astronomical instruments.</p>
+
+<p>One of the swiftest moving of the stars is a star of the sixth magnitude
+in the constellation of the Great Bear; which is known as "1830
+Groombridge," because this was the number assigned to it in a catalogue
+of stars made by an astronomer of that name. It is popularly known as
+the "Runaway Star," a name given to it by Professor Newcomb. Its speed
+is estimated to be at least 138 miles per second. It may be actually
+moving at a much greater rate, for it is possible that we see its path
+somewhat foreshortened.</p>
+
+<p>A still greater proper motion&mdash;the greatest, in fact, known&mdash;is that of
+an eighth magnitude star in the southern hemisphere, in the
+constellation of Pictor. Nothing, indeed, better shows the enormous
+distance of the stars from us, and the consequent inability of even such
+rapid movements to alter the appearance of the sky during the course of
+ages, than the fact that it would take more than two centuries for the
+star in question to change its position in the sky by a space<span class='pagenum'><a name="Page_282" id="Page_282">[Pg 282]</a></span> equal to
+the apparent diameter of the moon; a statement which is equivalent to
+saying that, were it possible to see this star with the naked eye, which
+it is not, at least twenty-five years would have to elapse before one
+would notice that it had changed its place at all!</p>
+
+<p>Both the stars just mentioned are very faint. That in Pictor is, as has
+been said, not visible to the naked eye. It appears besides to be a very
+small body, for Sir David Gill finds a parallax which makes it only as
+far off from us as Sirius. The Groombridge star, too, is just about the
+limit of ordinary visibility. It is, indeed, a curious fact that the
+fainter stars seem, on the average, to be moving more rapidly than the
+brighter.</p>
+
+<p>Investigations into proper motions lead us to think that every one of
+the stars must be moving in space in some particular direction. To take
+a few of the best known. Sirius and Vega are both approaching our system
+at a rate of about 10 miles per second, Arcturus at about 5 miles per
+second, while Capella is receding from us at about 15 miles per second.
+Of the twin brethren, Castor and Pollux, Castor is moving away from us
+at about 4&frac12; miles per second, while Pollux is coming towards us at
+about 33 miles per second.</p>
+
+<p>Much of our knowledge of proper motions has been obtained indirectly by
+means of the spectroscope, on the Doppler principle already treated of,
+by which we are enabled to ascertain whether a source from which light
+is coming is approaching or receding.</p>
+
+<p>The sun being, after all, a mere star, it will appear only natural for
+it also to have a proper motion of its own. This is indeed the case; and
+it is rushing<span class='pagenum'><a name="Page_283" id="Page_283">[Pg 283]</a></span> along in space at a rate of between ten and twelve miles
+per second, carrying with it its whole family of planets and satellites,
+of comets and meteors. The direction in which it is advancing is towards
+a point in the constellation of Lyra, not far from its chief star Vega.
+This is shown by the fact that the stars about the region in question
+appear to be opening out slightly, while those in the contrary portion
+of the sky appear similarly to be closing together.</p>
+
+<p>Sir William Herschel was the first to discover this motion of the sun
+through space; though in the idea that such a movement might take place
+he seems to have been anticipated by Mayer in 1760, by Michell in 1767,
+and by Lalande in 1776.</p>
+
+<p>A suggestion has been made that our solar system, in its motion through
+the celestial spaces, may occasionally pass through regions where
+abnormal magnetic conditions prevail, in consequence of which
+disturbances may manifest themselves throughout the system at the same
+instant. Thus the sun may be getting the credit of <i>producing</i> what it
+merely reacts to in common with the rest of its family. But this
+suggestion, plausible though it may seem, will not explain why the
+magnetic disturbances experienced upon our earth show a certain
+dependence upon such purely local facts, as the period of the sun's
+rotation, for instance.</p>
+
+<p>One would very much like to know whether the movement of the sun is
+along a straight line, or in an enormous orbit around some centre. The
+idea has been put forward that it may be moving around the centre of
+gravity of the whole visible stellar<span class='pagenum'><a name="Page_284" id="Page_284">[Pg 284]</a></span> universe. M&auml;dler, indeed,
+propounded the notion that Alcyone&mdash;the chief star in the group known as
+the Pleiades&mdash;occupied this centre, and that everything revolved around
+it. He went even further to proclaim that here was the Place of the
+Almighty, the Mansion of the Eternal! But M&auml;dler's ideas upon this point
+have long been shelved.</p>
+
+<p>To return to the general question of the proper motion of stars.</p>
+
+<p>In several instances these motions appear to take place in groups, as if
+certain stars were in some way associated together. For example, a large
+number of the stars composing the Pleiades appear to be moving through
+space in the same direction. Also, of the seven stars composing the
+Plough, all but two&mdash;the star at the end of its "handle," and that one
+of the "pointers," as they are called, which is the nearer to the pole
+star&mdash;have a common proper motion, <i>i.e.</i> are moving in the same
+direction and nearly at the same rate.</p>
+
+<p>Further still, the well-known Dutch astronomer, Professor Kapteyn, of
+Groningen, has lately reached the astonishing conclusion that a great
+part of the visible universe is occupied by two vast streams of stars
+travelling in opposite directions. In both these great streams, the
+individual bodies are found, besides, to be alike in design, alike in
+chemical constitution, and alike in the stage of their development.</p>
+
+<p>A fable related by the Persian astronomer, Al Sufi (tenth century, <span class="ampm">A.D.</span>)
+shows well the changes in the face of the sky which proper motions are
+bound to produce after great lapses of time. According to this fable the
+stars Sirius and Procyon were the sisters of<span class='pagenum'><a name="Page_285" id="Page_285">[Pg 285]</a></span> the star Canopus. Canopus
+married Rigel (another star,) but, having murdered her, he fled towards
+the South Pole, fearing the anger of his sisters. The fable goes on to
+relate, among other things, that Sirius followed him across the Milky
+Way. Mr. J. E. Gore, in commenting on the story, thinks that it may be
+based upon a tradition of Sirius having been seen by the men of the
+Stone Age on the opposite side of the Milky Way to that on which it now
+is.</p>
+
+<p>Sirius is in that portion of the heavens <i>from</i> which the sun is
+advancing. Its proper motion is such that it is gaining upon the earth
+at the rate of about ten miles per second, and so it must overtake the
+sun after the lapse of great ages. Vega, on the other hand, is coming
+towards us from that part of the sky <i>towards</i> which the sun is
+travelling. It should be about half a million years before the sun and
+Vega pass by one another. Those who have specially investigated this
+question say that, as regards the probability of a near approach, it is
+much more likely that Vega will be then so far to one side of the sun,
+that her brightness will not be much greater than it is at this moment.</p>
+
+<p>Considerations like these call up the chances of stellar collisions.
+Such possibilities need not, however, give rise to alarm; for the stars,
+as a rule, are at such great distances from each other, that the
+probability of relatively near approaches is slight.</p>
+
+<p>We thus see that the constellations do not in effect exist, and that
+there is in truth no real background to the sky. We find further that
+the stars are strewn through space at immense distances from each other,
+and are moving in various directions hither and<span class='pagenum'><a name="Page_286" id="Page_286">[Pg 286]</a></span> thither. The sun, which
+is merely one of them, is moving also in a certain direction, carrying
+the solar system along with it. It seems, therefore, but natural to
+suppose that many a star may be surrounded by some planetary system in a
+way similar to ours, which accompanies it through space in the course of
+its celestial journeyings.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_28_28" id="Footnote_28_28"></a><a href="#FNanchor_28_28"><span class="label">[28]</span></a> Vega, for instance, shines one hundred times more brightly
+than the sun would do, were it to be removed to the distance at which
+that star is from us.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_287" id="Page_287">[Pg 287]</a></span></p>
+<h3><a name="CHAPTER_XXIII" id="CHAPTER_XXIII"></a>CHAPTER XXIII</h3>
+
+<h4>THE STARS&mdash;<i>continued</i></h4>
+
+
+<p class="noin"><span class="smcap">The</span> stars appear to us to be scattered about the sky without any orderly
+arrangement. Further, they are of varying degrees of brightness; some
+being extremely brilliant, whilst others can but barely be seen. The
+brightness of a star may arise from either of two causes. On the one
+hand, the body may be really very bright in itself; on the other hand,
+it may be situated comparatively near to us. Sometimes, indeed, both
+these circumstances may come into play together.</p>
+
+<p>Since variation in brightness is the most noticeable characteristic of
+the stars, men have agreed to class them in divisions called
+"magnitudes." This term, it must be distinctly understood, is employed
+in such classification without any reference whatever to actual size,
+being merely taken to designate roughly the amount of light which we
+receive from a star. The twenty brightest stars in the sky are usually
+classed in the first magnitude. In descending the scale, each magnitude
+will be noticed to contain, broadly speaking, three times as many stars
+as the one immediately above it. Thus the second magnitude contains 65,
+the third 190, the fourth 425, the fifth 1100, and the sixth 3200. The
+last of these magnitudes is about the limit of the stars which we are
+able to see with the<span class='pagenum'><a name="Page_288" id="Page_288">[Pg 288]</a></span> naked eye. Adding, therefore, the above numbers
+together, we find that, without the aid of the telescope, we cannot see
+more than about 5000 stars in the entire sky&mdash;northern and southern
+hemispheres included. Quite a small telescope will, however, allow us to
+see down to the ninth magnitude, so that the total number of stars
+visible to us with such very moderate instrumental means will be well
+over 100,000.</p>
+
+<p>It must not, however, be supposed that the stars included within each
+magnitude are all of exactly the same brightness. In fact, it would be
+difficult to say if there exist in the whole sky two stars which send us
+precisely the same amount of light. In arranging the magnitudes, all
+that was done was to make certain broad divisions, and to class within
+them such stars as were much on a par with regard to brightness. It may
+here be noted that a standard star of the first magnitude gives us about
+one hundred times as much light as a star of the sixth magnitude, and
+about one million times as much as one of the sixteenth magnitude&mdash;which
+is near the limit of what we can see with the very best telescope.</p>
+
+<p>Though the first twenty stars in the sky are popularly considered as
+being of the first magnitude, yet several of them are much brighter than
+an average first magnitude star would be. For instance, Sirius&mdash;the
+brightest star in the whole sky&mdash;is equal to about eleven first
+magnitude stars, like, say, Aldebaran. In consequence of such
+differences, astronomers are agreed in classifying the brightest of them
+as <i>brighter</i> than the standard first magnitude star. On this principle
+Sirius would be about two and a half magnitudes<span class='pagenum'><a name="Page_289" id="Page_289">[Pg 289]</a></span> <i>above</i> the first. This
+notation is usefully employed in making comparisons between the amount
+of light which we receive from the sun, and that which we get from an
+individual star. Thus the sun will be about twenty-seven and a half
+magnitudes <i>above</i> the first magnitude. The range, therefore, between
+the light which we receive from the sun (considered merely as a very
+bright star) and the first magnitude stars is very much greater than
+that between the latter and the faintest star which can be seen with the
+telescope, or even registered upon the photographic plate.</p>
+
+<p>To classify stars merely by their magnitudes, without some definite note
+of their relative position in the sky, would be indeed of little avail.
+We must have some simple method of locating them in the memory, and the
+constellations of the ancients here happily come to our aid. A system
+combining magnitudes with constellations was introduced by Bayer in
+1603, and is still adhered to. According to this the stars in each
+constellation, beginning with the brightest star, are designated by the
+letters of the Greek alphabet taken in their usual order. For example,
+in the constellation of Canis Major, or the Greater Dog, the brightest
+star is the well-known Sirius, called by the ancients the "Dog Star";
+and this star, in accordance with Bayer's method, has received the Greek
+letter &#945; (alpha), and is consequently known as Alpha Canis
+Majoris.<a name="FNanchor_29_29" id="FNanchor_29_29"></a><a href="#Footnote_29_29" class="fnanchor">[29]</a> As soon as the Greek letters are used up in this way the
+Roman alphabet is brought into requisition, after which recourse is had
+to ordinary numbers.</p>
+
+<p><span class='pagenum'><a name="Page_290" id="Page_290">[Pg 290]</a></span></p><p>Notwithstanding this convenient arrangement, some of the brightest
+stars are nearly always referred to by certain proper names given to
+them in old times. For instance, it is more usual to speak of Sirius,
+Arcturus, Vega, Capella, Procyon, Aldebaran, Regulus, and so on, than of
+&#945; Canis Majoris, &#945; Bo&ouml;tis, &#945; Lyr&aelig;, &#945;
+Aurig&aelig;, &#945; Canis Minoris, &#945; Tauri, &#945; Leonis,
+&amp;c. &amp;c.</p>
+
+<p>In order that future generations might be able to ascertain what changes
+were taking place in the face of the sky, astronomers have from time to
+time drawn up catalogues of stars. These lists have included stars of a
+certain degree of brightness, their positions in the sky being noted
+with the utmost accuracy possible at the period. The earliest known
+catalogue of this kind was made, as we have seen, by the celebrated
+Greek astronomer, Hipparchus, about the year 125 <span class="ampm">B.C.</span> It contained 1080
+stars. It was revised and brought up to date by Ptolemy in <span class="ampm">A.D.</span> 150.
+Another celebrated list was that drawn up by the Persian astronomer, Al
+Sufi, about the year <span class="ampm">A.D.</span> 964. In it 1022 stars were noted down. A
+catalogue of 1005 stars was made in 1580 by the famous Danish
+astronomer, Tycho Brahe. Among modern catalogues that of Argelander
+(1799&ndash;1875) contained as many as 324,198 stars. It was extended by
+Sch&ouml;nfeld so as to include a portion of the Southern Hemisphere, in
+which way 133,659 more stars were added.</p>
+
+<p>In recent years a project was placed on foot of making a photographic
+survey of the sky, the work to be portioned out among various nations. A
+great part of this work has already been brought to a conclusion. About
+15,000,000 stars will appear upon the plates; but, so far, it has been
+proposed to catalogue<span class='pagenum'><a name="Page_291" id="Page_291">[Pg 291]</a></span> only about a million and a quarter of the
+brightest of them. This idea of surveying the face of the sky by
+photography sprang indirectly from the fine photographs which Sir David
+Gill took, when at the Cape of Good Hope, of the Comet of 1882. The
+immense number of star-images which had appeared upon his plates
+suggested the idea that photography could be very usefully employed to
+register the relative positions of the stars.</p>
+
+<p>The arrangement of seven stars known as the "Plough" is perhaps the most
+familiar configuration in the sky (<a href="#Plate_XIX">see Plate XIX.</a>, p. 292). In the
+United States it is called the "Dipper," on account of its likeness to
+the outline of a saucepan, or ladle. "Charles' Wain" was the old English
+name for it, and readers of C&aelig;sar will recollect it under
+<i>Septentriones</i>, or the "Seven Stars," a term which that writer uses as
+a synonym for the North. Though identified in most persons' minds with
+<i>Ursa Major</i>, or the Great Bear, the Plough is actually only a small
+portion of that famous constellation. Six out of the seven stars which
+go to make up the well-known figure are of the second magnitude, while
+the remaining one, which is the middle star of the group, is of the
+third.</p>
+
+<p>The Greek letters, as borne by the individual stars of the Plough, are a
+plain transgression of Bayer's method as above described, for they have
+certainly not been allotted here in accordance with the proper order of
+brightness. For instance, the third magnitude star, just alluded to as
+being in the middle of the group, has been marked with the Greek letter
+&#948; (Delta); and so is made to take rank <i>before</i> the stars
+composing what is called the "handle" of the Plough,<span class='pagenum'><a name="Page_292" id="Page_292">[Pg 292]</a></span> which are all of
+the second magnitude. Sir William Herschel long ago drew attention to
+the irregular manner in which Bayer's system had been applied. It is,
+indeed, a great pity that this notation was not originally worked out
+with greater care and correctness; for, were it only reliable, it would
+afford great assistance to astronomers in judging of what changes in
+relative brightness have taken place among the stars.</p>
+
+<p>Though we may speak of using the constellations as a method of finding
+our way about the sky, it is, however, to certain marked groupings in
+them of the brighter stars that we look for our sign-posts.</p>
+
+<p>Most of the constellations contain a group or so of noticeable stars,
+whose accidental arrangement dimly recalls the outline of some familiar
+geometrical figure and thus arrests the attention.<a name="FNanchor_30_30" id="FNanchor_30_30"></a><a href="#Footnote_30_30" class="fnanchor">[30]</a> For instance, in
+an almost exact line with the two front stars of the Plough, or
+"pointers" as they are called,<a name="FNanchor_31_31" id="FNanchor_31_31"></a><a href="#Footnote_31_31" class="fnanchor">[31]</a> and at a distance about five times as
+far away as the interval between them, there will be found a third star
+of the second magnitude. This is known as Polaris, or the Pole Star, for
+it very nearly occupies that point of the heaven towards which the north
+pole of the earth's axis is <i>at present</i> directed (<a href="#Plate_XIX">see Plate XIX.</a>, p.
+292). Thus during the apparently daily rotation of the heavens, this
+star looks always practically stationary. It will, no doubt, be
+remembered how Shakespeare has put into the mouth of Julius C&aelig;sar these
+memorable words:&mdash;</p>
+
+<p class="poem">
+"But I am constant as the northern star,<br />
+Of whose true-fix'd and resting quality<br />
+There is no fellow in the firmament."<br />
+</p>
+
+<div class="figcenter" style="width: 600px;"><a name="Plate_XIX" id="Plate_XIX"></a>
+<img src="images/plate19.jpg" width="600" height="456" alt="Plate XIX." title="" />
+<span class="caption"><span class="smcap">Plate XIX. The Sky around the North Pole</span></span>
+<div class="caption2">We see here the Plough, the Pole Star, Ursa Minor, Auriga, Cassiopeia's
+Chair, and Lyra. Also the Circle of Precession, along which the Pole
+makes a complete revolution in a period of 25,868 years, and the
+Temporary Star discovered by Tycho Brahe in the year 1572.<br />(<a href="#Page_291"><small>Page 291</small></a>)</div>
+</div>
+
+<p><span class='pagenum'><a name="Page_293" id="Page_293">[Pg 293]</a></span></p><p>On account of the curvature of the earth's surface, the height at which
+the Pole Star is seen above the horizon at any place depends regularly
+upon the latitude; that is to say, the distance of the place in question
+from the equator. For instance, at the north pole of the earth, where
+the latitude is greatest, namely, 90&deg;, the Pole Star will appear
+directly overhead; whereas in England, where the latitude is about 50&deg;,
+it will be seen a little more than half way up the northern sky. At the
+equator, where the latitude is <i>nil</i>, the Pole Star will be on the
+horizon due north.</p>
+
+<p>In consequence of its unique position, the Pole Star is of very great
+service in the study of the constellations. It is a kind of centre
+around which to hang our celestial ideas&mdash;a starting point, so to speak,
+in our voyages about the sky.</p>
+
+<p>According to the constellation figures, the Pole Star is in <i>Ursa
+Minor</i>, or the Little Bear, and is situated at the end of the tail of
+that imaginary figure (<a href="#Plate_XIX">see Plate XIX.</a>, p. 292). The chief stars of this
+constellation form a group not unlike the Plough, except that the
+"handle" is turned in the contrary direction.<span class='pagenum'><a name="Page_294" id="Page_294">[Pg 294]</a></span> The Americans, in
+consequence, speak of it as the "Little Dipper."</p>
+
+<p>Before leaving this region of the sky, it will be well to draw attention
+to the second magnitude star &#950; in the Great Bear (Zeta Urs&aelig;
+Majoris), which is the middle star in the "handle" of the Plough. This
+star is usually known as Mizar, a name given to it by the Arabians. A
+person with good eyesight can see quite near to it a fifth magnitude
+star, known under the name of Alcor. We have here a very good example of
+that deception in the estimation of objects in the sky, which has been
+alluded to in an earlier chapter. Alcor is indeed distant from Mizar by
+about one-third the apparent diameter of the moon, yet no one would
+think so!</p>
+
+<p>On the other side of Polaris from the Plough, and at about an equal
+apparent distance, will be found a figure in the form of an irregular
+"W", made up of second and third magnitude stars. This is the well-known
+"Cassiopeia's Chair"&mdash;portion of the constellation of <i>Cassiopeia</i> (<a href="#Plate_XIX">see
+Plate XIX.</a>, p. 292).</p>
+
+<p>On either side of the Pole Star, about midway between the Plough and
+Cassiopeia's Chair, but a little further off from it than these, are the
+constellations of <i>Auriga</i> and <i>Lyra</i> (<a href="#Plate_XIX">see Plate XIX.</a>, p. 292). The
+former constellation will be easily recognised, because its chief
+features are a brilliant yellowish first magnitude star, with one of the
+second magnitude not far from it. The first magnitude star is Capella,
+the other is &#946; Aurig&aelig;. Lyra contains only one first magnitude
+star&mdash;Vega, pale blue in colour. This star has a certain interest for us
+from the fact that, as a consequence of that slow shift of direction of
+the<span class='pagenum'><a name="Page_295" id="Page_295">[Pg 295]</a></span> earth's axis known as Precession, it will be very near the north
+pole of the heavens in some 12,000 years, and so will then be considered
+the pole star (<a href="#Plate_XIX">see Plate XIX.</a>, p. 292). The constellation of Lyra
+itself, it must also be borne in mind, occupies that region of the
+heavens towards which the solar system is travelling.</p>
+
+<p>The handle of the Plough points roughly towards the constellation of
+<i>Bo&ouml;tes</i>, in which is the brilliant first magnitude star Arcturus. This
+star is of an orange tint.</p>
+
+<p>Between Bo&ouml;tes and Lyra lie the constellations of <i>Corona Borealis</i> (or
+the Northern Crown) and <i>Hercules</i>. The chief feature of Corona
+Borealis, which is a small constellation, is a semicircle of six small
+stars, the brightest of which is of the second magnitude. The
+constellation of Hercules is very extensive, but contains no star
+brighter than the third magnitude.</p>
+
+<p>Near to Lyra, on the side away from Hercules, are the constellations of
+<i>Cygnus</i> and <i>Aquila</i>. Of the two, the former is the nearer to the Pole
+Star, and will be recognised by an arrangement of stars widely set in
+the form of a cross, or perhaps indeed more like the framework of a
+boy's kite. The position of Aquila will be found through the fact that
+three of its brightest stars are almost in a line and close together.
+The middle of these is Altair, a yellowish star of the first magnitude.</p>
+
+<p>At a little distance from Ursa Major, on the side away from the Pole
+Star, is the constellation of <i>Leo</i>, or the Lion. Its chief feature is a
+series of seven stars, supposed to form the head of that animal. The
+arrangement of these stars is, however, much more like<span class='pagenum'><a name="Page_296" id="Page_296">[Pg 296]</a></span> a sickle,
+wherefore this portion of the constellation is usually known as the
+"Sickle of Leo." At the end of the handle of the sickle is a white first
+magnitude star&mdash;Regulus.</p>
+
+<p>The reader will, no doubt, recollect that it is from a point in the
+Sickle of Leo that the Leonid meteors appear to radiate.</p>
+
+<p>The star second in brightness in the constellation of Leo is known as
+Denebola. This star, now below the second magnitude, seems to have been
+very much brighter in the past. It is noted, indeed, as a brilliant
+first magnitude star by Al Sufi, that famous Persian astronomer who
+lived, as we have seen, in the tenth century. Ptolemy also notes it as
+of the first magnitude.</p>
+
+<p>In the neighbourhood of Auriga, and further than it from the Pole Star,
+are several remarkable constellations&mdash;Taurus, Orion, Gemini, Canis
+Minor, and Canis Major (<a href="#Plate_XX">see Plate XX.</a>, p. 296).</p>
+
+<p>The first of these, <i>Taurus</i> (or the Bull), contains two conspicuous
+star groups&mdash;the Pleiades and the Hyades. The Pleiades are six or seven
+small stars quite close together, the majority of which are of the
+fourth magnitude. This group is sometimes occulted by the moon. The way
+in which the stars composing it are arranged is somewhat similar to that
+in the Plough, though of course on a scale ever so much smaller. The
+impression which the group itself gives to the casual glance is thus
+admirably pictured in Tennyson's <i>Locksley Hall</i>:&mdash;</p>
+
+<p class="poem">
+"Many a night I saw the Pleiads, rising through the mellow shade,<br />
+Glitter like a swarm of fire-flies tangled in a silver braid."<br />
+</p>
+
+<div class="figcenter" style="width: 600px;"><a name="Plate_XX" id="Plate_XX"></a>
+<img src="images/plate20.jpg" width="600" height="403" alt="Plate XX." title="" />
+<span class="caption"><span class="smcap">Plate XX. Orion and his Neighbours</span><br />
+We see here that magnificent region of the sky which contains the
+brightest star of all&mdash;Sirius. Note also especially the Milky Way, the
+Pleiades, the Hyades, and the "Belt" and "Sword" of Orion.<br />(<a href="#Page_296"><small>Page 296</small></a>)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_297" id="Page_297">[Pg 297]</a></span></p><p>The group of the Hyades occupies the "head" of the Bull, and is much
+more spread out than that of the Pleiades. It is composed besides of
+brighter stars, the brightest being one of the first magnitude,
+Aldebaran. This star is of a red colour, and is sometimes known as the
+"Eye of the Bull."</p>
+
+<p>The constellation of <i>Orion</i> is easily recognised as an irregular
+quadrilateral formed of four bright stars, two of which, Betelgeux
+(reddish) and Rigel (brilliant white), are of the first magnitude. In
+the middle of the quadrilateral is a row of three second magnitude
+stars, known as the "Belt" of Orion. Jutting off from this is another
+row of stars called the "Sword" of Orion.</p>
+
+<p>The constellation of <i>Gemini</i>, or the Twins, contains two bright
+stars&mdash;Castor and Pollux&mdash;close to each other. Pollux, though marked
+with the Greek letter &#946;, is the brighter of the two, and nearly
+of the standard first magnitude.</p>
+
+<p>Just further from the Pole than Gemini, is the constellation of <i>Canis
+Minor</i>, or the Lesser Dog. Its chief star is a white first magnitude
+one&mdash;Procyon.</p>
+
+<p>Still further again from the Pole than Canis Minor is the constellation
+of <i>Canis Major</i>, or the Greater Dog. It contains the brightest star in
+the whole sky, the first magnitude star Sirius, bluish-white in colour,
+also known as the "Dog Star." This star is almost in line with the stars
+forming the Belt of Orion, and is not far from that constellation.</p>
+
+<p>Taken in the following order, the stars Capella, &#946; Aurig&aelig;,
+Castor, Pollux, Procyon, and Sirius, when they are all above the horizon
+at the same time, form a beautiful curve stretching across the heaven.</p>
+
+<p><span class='pagenum'><a name="Page_298" id="Page_298">[Pg 298]</a></span></p><p>The groups of stars visible in the southern skies have by no means the
+same fascination for us as those in the northern. The ancients were in
+general unacquainted with the regions beyond the equator, and so their
+scheme of constellations did not include the sky around the South Pole
+of the heavens. In modern times, however, this part of the celestial
+expanse was also portioned out into constellations for the purpose of
+easy reference; but these groupings plainly lack that simplicity of
+conception and legendary interest which are so characteristic of the
+older ones.</p>
+
+<p>The brightest star in the southern skies is found in the constellation
+of <i>Argo</i>, and is known as Canopus. In brightness it comes next to
+Sirius, and so is second in that respect in the entire heaven. It does
+not, however, rise above the English horizon.</p>
+
+<p>Of the other southern constellations, two call for especial notice, and
+these adjoin each other. One is <i>Centaurus</i> (or the Centaur), which
+contains the two first magnitude stars, &#945; and &#946;
+Centauri. The first of these, Alpha Centauri, comes next in brightness
+to Canopus, and is notable as being the nearest of all the stars to our
+earth. The other constellation is called <i>Crux</i>, and contains five stars
+set in the form of a rough cross, known as the "Southern Cross." The
+brightest of these, &#945; Crucis, is of the first magnitude.</p>
+
+<p>Owing to the Precession of the Equinoxes, which, as we have seen,
+gradually shifts the position of the Pole among the stars, certain
+constellations used to be visible in ancient times in more northerly
+latitudes than at present. For instance, some five thousand years ago
+the Southern Cross rose above the English<span class='pagenum'><a name="Page_299" id="Page_299">[Pg 299]</a></span> horizon, and was just visible
+in the latitude of London. It has, however, long ago even ceased to be
+seen in the South of Europe. The constellation of Crux happens to be
+situated in that remarkable region of the southern skies, in which are
+found the stars Canopus and Alpha Centauri, and also the most brilliant
+portion of the Milky Way. It is believed to be to this grand celestial
+region that allusion is made in the Book of Job (ix. 9), under the title
+of the "Chambers of the South." The "Cross" must have been still a
+notable feature in the sky of Palestine in the days when that ancient
+poem was written.</p>
+
+<p>There is no star near enough to the southern pole of the heavens to earn
+the distinction of South Polar Star.</p>
+
+<p>The Galaxy, or <i>Milky Way</i> (<a href="#Plate_XX">see Plate XX.</a>, p. 296), is a broad band of
+diffused light which is seen to stretch right around the sky. The
+telescope, however, shows it to be actually composed of a great host of
+very faint stars&mdash;too faint, indeed, to be separately distinguished with
+the naked eye. Along a goodly stretch of its length it is cleft in two;
+while near the south pole of the heavens it is entirely cut across by a
+dark streak.</p>
+
+<p>In this rapid survey of the face of the sky, we have not been able to do
+more than touch in the broadest manner upon some of the most noticeable
+star groups and a few of the most remarkable stars. To go any further is
+not a part of our purpose; our object being to deal with celestial
+bodies as they actually are, and not in those groupings under which they
+display themselves to us as a mere result of perspective.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_29_29" id="Footnote_29_29"></a><a href="#FNanchor_29_29"><span class="label">[29]</span></a> Attention must here be drawn to the fact that the name of
+the constellation is always put in the genitive case.</p></div>
+
+<div class="footnote"><p><a name="Footnote_30_30" id="Footnote_30_30"></a><a href="#FNanchor_30_30"><span class="label">[30]</span></a> The early peoples, as we have seen, appear to have been
+attracted by those groupings of the stars which reminded them in a way
+of the figures of men and animals. We moderns, on the other hand, seek
+almost instinctively for geometrical arrangements. This is, perhaps,
+symptomatic of the evolution of the race. In the growth of the
+individual we find, for example, something analogous. A child, who has
+been given pencil and paper, is almost certain to produce grotesque
+drawings of men and animals; whereas the idle and half-conscious
+scribblings which a man may make upon his blotting-paper are usually of
+a geometrical character.</p></div>
+
+<div class="footnote"><p><a name="Footnote_31_31" id="Footnote_31_31"></a><a href="#FNanchor_31_31"><span class="label">[31]</span></a> Because the line joining them <i>points</i> in the direction of
+the Pole Star.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_300" id="Page_300">[Pg 300]</a></span></p>
+<h3><a name="CHAPTER_XXIV" id="CHAPTER_XXIV"></a>CHAPTER XXIV</h3>
+
+<h4>SYSTEMS OF STARS</h4>
+
+
+<p class="noin"><span class="smcap">Many</span> stars are seen comparatively close together. This may plainly arise
+from two reasons. Firstly, the stars may happen to be almost in the same
+line of sight; that is to say, seen in nearly the same direction; and
+though one star may be ever so much nearer to us than the other, the
+result will give all the appearance of a related pair. A seeming
+arrangement of two stars in this way is known as a "double," or double
+star; or, indeed, to be very precise, an "optical double." Secondly, in
+a pair of stars, both bodies may be about the same distance from us, and
+actually connected as a system like, for instance, the moon and the
+earth. A pairing of stars in this way, though often casually alluded to
+as a double star, is properly termed a "binary," or binary system.</p>
+
+<p>But collocations of stars are by no means limited to two. We find,
+indeed, all over the sky such arrangements in which there are three or
+more stars; and these are technically known as "triple" or "multiple"
+stars respectively. Further, groups are found in which a great number of
+stars are closely massed together, such a massing together of stars
+being known as a "cluster."</p>
+
+<p>The Pole Star (Polaris) is a double star, one of the components being of
+a little below the second magnitude,<span class='pagenum'><a name="Page_301" id="Page_301">[Pg 301]</a></span> and the other a little below the
+ninth. They are so close together that they appear as one star to the
+naked eye, but they may be seen separate with a moderately sized
+telescope. The brighter star is yellowish, and the faint one white. This
+brighter star is found <i>by means of the spectroscope</i> to be actually
+composed of three stars so very close together that they cannot be seen
+separately even with a telescope. It is thus a triple star, and the
+three bodies of which it is composed are in circulation about each
+other. Two of them are darker than the third.</p>
+
+<p>The method of detecting binary stars by means of the spectroscope is an
+application of Doppler's principle. It will, no doubt, be remembered
+that, according to the principle in question, we are enabled, from
+certain shiftings of the lines in the spectrum of a luminous body, to
+ascertain whether that body is approaching us or receding from us. Now
+there are certain stars which always appear single even in the largest
+telescopes, but when the spectroscope is directed to them a spectrum
+<i>with two sets of lines</i> is seen. Such stars must, therefore, be double.
+Further, if the shiftings of the lines, in a spectrum like this, tell us
+that the component stars are making small movements to and from us which
+go on continuously, we are therefore justified in concluding that these
+are the orbital revolutions of a binary system greatly compressed by
+distance. Such connected pairs of stars, since they cannot be seen
+separately by means of any telescope, no matter how large, are known as
+"spectroscopic binaries."</p>
+
+<p>In observations of spectroscopic binaries we do not always get a double
+spectrum. Indeed, if one of the<span class='pagenum'><a name="Page_302" id="Page_302">[Pg 302]</a></span> components be below a certain
+magnitude, its spectrum will not appear at all; and so we are left in
+the strange uncertainty as to whether this component is merely faint or
+actually dark. It is, however, from the shiftings of the lines in the
+spectrum of the other component that we see that an orbital movement is
+going on, and are thus enabled to conclude that two bodies are here
+connected into a system, although one of these bodies resolutely refuses
+directly to reveal itself even to the all-conquering spectroscope.</p>
+
+<p>Mizar, that star in the handle of the Plough to which we have already
+drawn attention, will be found with a small telescope to be a fine
+double, one of the components being white and the other greenish.
+Actually, however, as the American astronomer, Professor F.R. Moulton,
+points out, these stars are so far from each other that if we could be
+transferred to one of them we should see the other merely as an ordinary
+bright star. The spectroscope shows that the brighter of these stars is
+again a binary system of two huge suns, the components revolving around
+each other in a period of about twenty days. This discovery made by
+Professor E.C. Pickering, the <i>first</i> of the kind by means of the
+spectroscope, was announced in 1889 from the Harvard Observatory in the
+United States.</p>
+
+<p>A star close to Vega, known as &#949; (Epsilon) Lyr&aelig; (<a href="#Plate_XIX">see Plate
+XIX.</a>, p. 292), is a double, the components of which may be seen
+separately with the naked eye by persons with very keen eyesight. If
+this star, however, be viewed with the telescope, the two companions
+will be seen far apart; and it will be noticed that each of them is
+again a double.</p>
+
+<p><span class='pagenum'><a name="Page_303" id="Page_303">[Pg 303]</a></span></p><p>By means of the spectroscope Capella is shown to be really composed of
+two stars (one about twice as bright as the other) situated very close
+together and forming a binary system. Sirius is also a binary system;
+but it is what is called a "visual" one, for its component stars may be
+<i>seen</i> separately in very large telescopes. Its double, or rather
+binary, nature, was discovered in 1862 by the celebrated optician Alvan
+G. Clark, while in the act of testing the 18&ndash;inch refracting telescope,
+then just constructed by his firm, and now at the Dearborn Observatory,
+Illinois, U.S.A. The companion is only of the tenth magnitude, and
+revolves around Sirius in a period of about fifty years, at a mean
+distance equal to about that of Uranus from the sun. Seen from Sirius,
+it would shine only something like our full moon. It must be
+self-luminous and not a mere planet; for Mr. Gore has shown that if it
+shone only by the light reflected from Sirius, it would be quite
+invisible even in the Great Yerkes Telescope.</p>
+
+<p>Procyon is also a binary, its companion having been discovered by
+Professor J.M. Schaeberle at the Lick Observatory in 1896. The period of
+revolution in this system is about forty years. Observations by Mr. T.
+Lewis of Greenwich seem, however, to point to the companion being a
+small nebula rather than a star.</p>
+
+<p>The star &#951; (Eta) Cassiopei&aelig; (<a href="#Plate_XIX">see Plate XIX.</a>, p. 292), is easily
+seen as a fine double in telescopes of moderate size. It is a binary
+system, the component bodies revolving around their common centre of
+gravity in a period of about two hundred years. This system is
+comparatively near to us, <i>i.e.</i> about nine light years, or a little
+further off than Sirius.</p>
+
+<p><span class='pagenum'><a name="Page_304" id="Page_304">[Pg 304]</a></span></p><p>In a small telescope the star Castor will be found double, the
+components, one of which is brighter than the other, forming a binary
+system. The fainter of these was found by Belopolsky, with the
+spectroscope, to be composed of a system of two stars, one bright and
+the other either dark or not so bright, revolving around each other in a
+period of about three days. The brighter component of Castor is also a
+spectroscopic binary, with a period of about nine days; so that the
+whole of what we see with the naked eye as Castor, is in reality a
+remarkable system of four stars in mutual orbital movement.</p>
+
+<p>Alpha Centauri&mdash;the nearest star to the earth&mdash;is a visual binary, the
+component bodies revolving around each other in a period of about
+eighty-one years. The extent of this system is about the same as that of
+Sirius. Viewed from each other, the bodies would shine only like our sun
+as seen from Neptune.</p>
+
+<p>Among the numerous binary stars the orbits of some fifty have been
+satisfactorily determined. Many double stars, for which this has not yet
+been done, are, however, believed to be, without doubt, binary. In some
+cases a parallax has been found; so that we are enabled to estimate in
+miles the actual extent of such systems, and the masses of the bodies in
+terms of the sun's mass.</p>
+
+<p>Most of the spectroscopic binaries appear to be upon a smaller scale
+than the telescopic ones. Some are, indeed, comparatively speaking,
+quite small. For instance, the component stars forming &#946; Aurig&aelig;
+are about eight million miles apart, while in &#950; Geminorum, the
+distance between the bodies is only a little more than a million miles.</p>
+
+<p><span class='pagenum'><a name="Page_305" id="Page_305">[Pg 305]</a></span></p><p>Spectroscopic binaries are probably very numerous. Professor W.W.
+Campbell, Director of the Lick Observatory, estimates, for instance,
+that, out of about every half-a-dozen stars, one is a spectroscopic
+binary.</p>
+
+<p>It is only in the case of binary systems that we can discover the masses
+of stars at all. These are ascertained from their movements with regard
+to each other under the influence of their mutual gravitative
+attractions. In the case of simple stars we have clearly nothing of the
+kind to judge by; though, if we can obtain a parallax, we may hazard a
+guess from their brightness.</p>
+
+<p>Binary stars were incidentally discovered by Sir William Herschel. In
+his researches to get a stellar parallax he had selected a number of
+double stars for test purposes, on the assumption that, if one of such a
+pair were much nearer than the other, it might show a displacement with
+regard to its neighbour as a direct consequence of the earth's orbital
+movement around the sun. He, however, failed entirely to obtain any
+parallaxes, the triumph in this being, as we have seen, reserved for
+Bessel. But in some of the double stars which he had selected, he found
+certain alterations in the relative positions of the bodies, which
+plainly were not a consequence of the earth's motion, but showed rather
+that there was an actual circling movement of the bodies themselves
+under their mutual attractions. It is to be noted that the existence of
+such connected pairs had been foretold as probable by the Rev. John
+Michell, who lived a short time before Herschel.</p>
+
+<p>The researches into binary systems&mdash;both those<span class='pagenum'><a name="Page_306" id="Page_306">[Pg 306]</a></span> which can be seen with
+the eye and those which can be observed by means of the spectroscope,
+ought to impress upon us very forcibly the wide sway of the law of
+gravitation.</p>
+
+<p>Of star clusters about 100 are known, and such systems often contain
+several thousand stars. They usually cover an area of sky somewhat
+smaller than the moon appears to fill. In most clusters the stars are
+very faint, and, as a rule, are between the twelfth and sixteenth
+magnitudes. It is difficult to say whether these are actually small
+bodies, or whether their faintness is due merely to their great distance
+from us, since they are much too far off to show any appreciable
+parallactic displacement. Mr. Gore, however, thinks there is good
+evidence to show that the stars in clusters are really close, and that
+the clusters themselves fill a comparatively small space.</p>
+
+<p>One of the finest examples of a cluster is the great globular one, in
+the constellation of Hercules, discovered by Halley in 1714. It contains
+over 5000 stars, and upon a clear, dark night is visible to the naked
+eye as a patch of light. In the telescope, however, it is a wonderful
+object. There are also fine clusters in the constellations of Auriga,
+Pegasus, and Canes Venatici. In the southern heavens there are some
+magnificent examples of globular clusters. This hemisphere seems,
+indeed, to be richer in such objects than the northern. For instance,
+there is a great one in the constellation of the Centaur, containing
+some 6000 stars (<a href="#Plate_XXI">see Plate XXI.</a>, p. 306).</p>
+
+<div class="figcenter" style="width: 600px;"><a name="Plate_XXI" id="Plate_XXI"></a>
+<img src="images/plate21.jpg" width="600" height="518" alt="Plate XXI." title="" />
+<span class="caption"><span class="smcap">Plate XXI. The Great Globular Cluster in the Southern
+Constellation of Centaurus</span><br />
+From a photograph taken at the Cape Observatory, on May 24th, 1903. Time
+of exposure, 1 hour.<br />(<a href="#Page_306"><small>Page 306</small></a>)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_307" id="Page_307">[Pg 307]</a></span></p><p>Certain remarkable groups of stars, of a nature similar to clusters,
+though not containing such faint or densely packed stars as those we
+have just alluded to, call for a mention in this connection. The best
+example of such star groups are the Pleiades and the Hyades (<a href="#Plate_XX">see Plate
+XX.</a>, p. 296), Coma Berenices, and Pr&aelig;sepe (or the Beehive), the
+last-named being in the constellation of Cancer.</p>
+
+<p>Stars which alter in their brightness are called <i>Variable Stars</i>, or
+"variables." The first star whose variability attracted attention is
+that known as Omicron Ceti, namely, the star marked with the Greek
+letter &#959; (Omicron) in the constellation of Cetus, or the Whale,
+a constellation situated not far from Taurus. This star, the variability
+of which was discovered by Fabricius in 1596, is also known as Mira, or
+the "Wonderful," on account of the extraordinary manner in which its
+light varies from time to time. The star known by the name of Algol,<a name="FNanchor_32_32" id="FNanchor_32_32"></a><a href="#Footnote_32_32" class="fnanchor">[32]</a>
+popularly called the "Demon Star"&mdash;whose astronomical designation is
+&#946; (Beta) Persei, or the star second in brightness in the
+constellation of Perseus&mdash;was discovered by Goodricke, in the year 1783,
+to be a variable star. In the following year &#946; Lyr&aelig;, the star
+in Lyra next in order of brightness after Vega, was also found by the
+same observer to be a variable. It may be of interest to the reader to
+know that Goodricke was deaf and dumb, and that he died in 1786 at the
+early age of twenty-one years!</p>
+
+<p>It was not, however, until the close of the nineteenth century that much
+attention was paid to variable stars. Now several hundreds of these are
+known, thanks chiefly to the observations of, amongst others,<span class='pagenum'><a name="Page_308" id="Page_308">[Pg 308]</a></span> Professor
+S.C. Chandler of Boston, U.S.A., Mr. John Ellard Gore of Dublin, and Dr.
+A.W. Roberts of South Africa. This branch of astronomy has not, indeed,
+attracted as much popular attention as it deserves, no doubt because the
+nature of the work required does not call for the glamour of an
+observatory or a large telescope.</p>
+
+<p>The chief discoveries with regard to variable stars have been made by
+the naked eye, or with a small binocular. The amount of variation is
+estimated by a comparison with other stars. As in many other branches of
+astronomy, photography is now employed in this quest with marked
+success; and lately many variable stars have been found to exist in
+clusters and nebul&aelig;.</p>
+
+<p>It was at one time considered that a variable star was in all
+probability a body, a portion of whose surface had been relatively
+darkened in some manner akin to that in which sun spots mar the face of
+the sun; and that when its axial rotation brought the less illuminated
+portions in turn towards us, we witnessed a consequent diminution in the
+star's general brightness. Herschel, indeed, inclined to this
+explanation, for his belief was that all the stars bore spots like those
+of the sun. It appears preferably thought nowadays that disturbances
+take place periodically in the atmosphere or surroundings of certain
+stars, perhaps through the escape of imprisoned gases, and that this may
+be a fruitful cause of changes of brilliancy. The theory in question
+will, however, apparently account for only one class of variable star,
+namely, that of which Mira Ceti is the best-known example. The scale on
+which it varies in brightness is very<span class='pagenum'><a name="Page_309" id="Page_309">[Pg 309]</a></span> great, for it changes from the
+second to the ninth magnitude. For the other leading type of variable
+star, Algol, of which mention has already been made, is the best
+instance. The shortness of the period in which the changes of brightness
+in such stars go their round, is the chief characteristic of this latter
+class. The period of Algol is a little under three days. This star when
+at its brightest is of about the second magnitude, and when least bright
+is reduced to below the third magnitude; from which it follows that its
+light, when at the minimum, is only about one-third of what it is when
+at the maximum. It seems definitely proved by means of the spectroscope
+that variables of this kind are merely binary stars, too close to be
+separated by the telescope, which, as a consequence of their orbits
+chancing to be edgewise towards us, eclipse each other in turn time
+after time. If, for instance, both components of such a pair are bright,
+then when one of them is right behind the other, we will not, of course,
+get the same amount of light as when they are side by side. If, on the
+other hand, one of the components happens to be dark or less luminous
+and the other bright, the manner in which the light of the bright star
+will be diminished when the darker star crosses its face should easily
+be understood. It is to the second of these types that Algol is supposed
+to belong. The Algol system appears to be composed of a body about as
+broad as our sun, which regularly eclipses a brighter body which has a
+diameter about half as great again.</p>
+
+<p>Since the companion of Algol is often spoken of as a <i>dark</i> body, it
+were well here to point out that<span class='pagenum'><a name="Page_310" id="Page_310">[Pg 310]</a></span> we have no evidence at all that it is
+entirely devoid of light. We have already found, in dealing with
+spectroscopic binaries, that when one of the component stars is below a
+certain magnitude<a name="FNanchor_33_33" id="FNanchor_33_33"></a><a href="#Footnote_33_33" class="fnanchor">[33]</a> its spectrum will not be seen; so one is left in
+the glorious uncertainty as to whether the body in question is
+absolutely dark, or darkish, or faint, or indeed only just out of range
+of the spectroscope.</p>
+
+<p>It is thought probable by good authorities that the companion of Algol
+is not quite dark, but has some inherent light of its own. It is, of
+course, much too near Algol to be seen with the largest telescope. There
+is in fact a distance of only from two to three millions of miles
+between the bodies, from which Mr. Gore infers that they would probably
+remain unseparated even in the largest telescope which could ever be
+constructed by man.</p>
+
+<p>The number of known variables of the Algol type is, so far, small; not
+much indeed over thirty. In some of them the components are believed to
+revolve touching each other, or nearly so. An extreme example of this is
+found in the remarkable star V. Puppis, an Algol variable of the
+southern hemisphere. Both its components are bright, and the period of
+light variation is about one and a half days. Dr. A. W. Roberts finds
+that the bodies are revolving around each other in actual contact.</p>
+
+<p><i>Temporary stars</i> are stars which have suddenly blazed out in regions of
+the sky where no star was previously seen, and have faded away more or
+less gradually.</p>
+
+<p>It was the appearance of such a star, in the year<span class='pagenum'><a name="Page_311" id="Page_311">[Pg 311]</a></span> 134 <span class="ampm">B.C.</span>, which
+prompted Hipparchus to make his celebrated catalogue, with the object of
+leaving a record by which future observers could note celestial changes.
+In 1572 another star of this kind flashed out in the constellation of
+Cassiopeia (<a href="#Plate_XIX">see Plate XIX.</a>, p. 292), and was detected by Tycho Brahe. It
+became as bright as the planet Venus, and eventually was visible in the
+day-time. Two years later, however, it disappeared, and has never since
+been seen. In 1604 Kepler recorded a similar star in the constellation
+of Ophiuchus which grew to be as bright as Jupiter. It also lasted for
+about two years, and then faded away, leaving no trace behind. It is
+rarely, however, that temporary stars attain to such a brilliance; and
+so possibly in former times a number of them may have appeared, but not
+have risen to a sufficient magnitude to attract attention. Even now,
+unless such a star becomes clearly visible to the naked eye, it runs a
+good chance of not being detected. A curious point, worth noting, with
+regard to temporary stars is that the majority of them have appeared in
+the Milky Way.</p>
+
+<p>These sudden visitations have in our day received the name of <i>Nov&aelig;</i>;
+that is to say, "New" Stars. Two, in recent years, attracted a good deal
+of attention. The first of these, known as Nova Aurig&aelig;, or the New Star
+in the constellation of Auriga, was discovered by Dr. T.D. Anderson at
+Edinburgh in January 1892. At its greatest brightness it attained to
+about the fourth magnitude. By April it had sunk to the twelfth, but
+during August it recovered to the ninth magnitude. After this last
+flare-up it gradually faded away.</p>
+
+<p><span class='pagenum'><a name="Page_312" id="Page_312">[Pg 312]</a></span></p><p>The startling suddenness with which temporary stars usually spring into
+being is the groundwork upon which theories to account for their origin
+have been erected. That numbers of dark stars, extinguished suns, so to
+speak, may exist in space, there is a strong suspicion; and it is just
+possible that we have an instance of one dark stellar body in the
+companion of Algol. That such dark stars might be in rapid motion is
+reasonable to assume from the already known movements of bright stars.
+Two dark bodies might, indeed, collide together, or a collision might
+take place between a dark star and a star too faint to be seen even with
+the most powerful telescope. The conflagration produced by the impact
+would thus appear where nothing had been seen previously. Again, a
+similar effect might be produced by a dark body, or a star too faint to
+be seen, being heated to incandescence by plunging in its course through
+a nebulous mass of matter, of which there are many examples lying about
+in space.</p>
+
+<p>The last explanation, which is strongly reminiscent of what takes place
+in shooting stars, appears more probable than the collision theory. The
+flare-up of new stars continues, indeed, only for a comparatively short
+time; whereas a collision between two bodies would, on the other hand,
+produce an enormous nebula which might take even millions of years to
+cool down. We have, indeed, no record of any such sudden appearance of a
+lasting nebula.</p>
+
+<p>The other temporary star, known as Nova Persei, or the new star in the
+constellation of Perseus, was discovered early in the morning of
+February 22, 1901, also by Dr. Anderson. A day later it had<span class='pagenum'><a name="Page_313" id="Page_313">[Pg 313]</a></span> grown to be
+brighter than Capella. Photographs which had been taken, some three days
+previous to its discovery, of the very region of the sky in which it had
+burst forth, were carefully examined, and it was not found in these. At
+the end of two days after its discovery Nova Persei had lost one-third
+of its light. During the ensuing six months it passed through a series
+of remarkable fluctuations, varying in brightness between the third and
+fifth magnitudes. In the month of August it was seen to be surrounded by
+luminous matter in the form of a nebula, which appeared to be gradually
+spreading to some distance around. Taking into consideration the great
+way off at which all this was taking place, it looked as if the new star
+had ejected matter which was travelling outward with a velocity
+equivalent to that of light. The remarkable theory was, however, put
+forward by Professor Kapteyn and the late Dr. W.E. Wilson that there
+might be after all no actual transmission of matter; but that perhaps
+the real explanation was the gradual <i>illumination</i> of hitherto
+invisible nebulous matter, as a consequence of the flare-up which had
+taken place about six months before. It was, therefore, imagined that
+some dark body moving through space at a very rapid rate had plunged
+through a mass of invisible nebulous matter, and had consequently become
+heated to incandescence in its passage, very much like what happens to a
+meteor when moving through our atmosphere. The illumination thus set up
+temporarily in one point, being transmitted through the nebulous wastes
+around with the ordinary velocity of light, had gradually rendered this
+surrounding matter visible. On the assumptions required to fit in<span class='pagenum'><a name="Page_314" id="Page_314">[Pg 314]</a></span> with
+such a theory, it was shown that Nova Persei must be at a distance from
+which light would take about three hundred years in coming to us. The
+actual outburst of illumination, which gave rise to this temporary star,
+would therefore have taken place about the beginning of the reign of
+James I.</p>
+
+<p>Some recent investigations with regard to Nova Persei have, however,
+greatly narrowed down the above estimate of its distance from us. For
+instance, Bergstrand proposes a distance of about ninety-nine light
+years; while the conclusions of Mr. F.W. Very would bring it still
+nearer, <i>i.e.</i> about sixty-five light years.</p>
+
+<p>The last celestial objects with which we have here to deal are the
+<i>Nebul&aelig;</i>. These are masses of diffused shining matter scattered here and
+there through the depths of space. Nebul&aelig; are of several kinds, and have
+been classified under the various headings of Spiral, Planetary, Ring,
+and Irregular.</p>
+
+<p>A typical <i>spiral</i> nebula is composed of a disc-shaped central portion,
+with long curved arms projecting from opposite sides of it, which give
+an impression of rapid rotatory movement.</p>
+
+<p>The discovery of spiral nebul&aelig; was made by Lord Rosse with his great
+6&ndash;foot reflector. Two good examples of these objects will be found in
+Ursa Major, while there is another fine one in Canes Venatici (<a href="#Plate_XXII">see Plate
+XXII.</a>, p. 314), a constellation which lies between Ursa Major and
+Bo&ouml;tes. But the finest spiral of all, perhaps the most remarkable nebula
+known to us, is the Great Nebula in the constellation of Andromeda, (<a href="#Plate_XXIII">see
+Plate XXIII.</a>, p. 316)&mdash;a constellation just further from the pole than
+Cassiopeia. When the moon is absent and the night clear this nebula can
+be easily seen with the naked eye as a small patch of hazy light. It is
+referred to by Al Sufi.</p>
+
+<div class="figcenter" style="width: 600px;"><a name="Plate_XXII" id="Plate_XXII"></a>
+<img src="images/plate22.jpg" width="500" height="795" alt="Plate XXII." title="" /><br />
+<span class="caption"><span class="smcap">Plate XXII. Spiral Nebula in the Constellation of Canes
+Venatici</span><br />From a photograph by the late Dr. W.E. Wilson, D.Sc., F.R.S.<br />
+(<a href="#Page_314"><small>Page 314</small></a>)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_315" id="Page_315">[Pg 315]</a></span></p><p>Spiral nebul&aelig; are white in colour, whereas the other kinds of nebula
+have a greenish tinge. They are also by far the most numerous; and the
+late Professor Keeler, who considered this the normal type of nebula,
+estimated that there were at least 120,000 of such spirals within the
+reach of the Crossley reflector of the Lick Observatory. Professor
+Perrine has indeed lately raised this estimate to half a million, and
+thinks that with more sensitive photographic plates and longer exposures
+the number of spirals would exceed a million. The majority of these
+objects are very small, and appear to be distributed over the sky in a
+fairly uniform manner.</p>
+
+<p><i>Planetary</i> nebul&aelig; are small faint roundish objects which, when seen in
+the telescope, recall the appearance of a planet, hence their name. One
+of these nebul&aelig;, known astronomically as G.C. 4373, has recently been
+found to be rushing through space towards the earth at a rate of between
+thirty and forty miles per second. It seems strange, indeed, that any
+gaseous mass should move at such a speed!</p>
+
+<p>What are known as <i>ring</i> nebul&aelig; were until recently believed to form a
+special class. These objects have the appearance of mere rings of
+nebulous matter. Much doubt has, however, been thrown upon their being
+rings at all; and the best authorities regard them merely as spiral
+nebul&aelig;, of which we happen to get a foreshortened view. Very few
+examples are known, the most famous being one in the constellation of
+Lyra, usually known as the Annular Nebula<span class='pagenum'><a name="Page_316" id="Page_316">[Pg 316]</a></span> in Lyra. This object is so
+remote from us as to be entirely invisible to the naked eye. It contains
+a star of the fifteenth magnitude near to its centre. From photographs
+taken with the Crossley reflector, Professor Schaeberle finds in this
+nebula evidences of spiral structure. It may here be mentioned that the
+Great Nebula in Andromeda, which has now turned out to be a spiral, had
+in earlier photographs the appearance of a ring.</p>
+
+<p>There also exist nebul&aelig; of <i>irregular</i> form, the most notable being the
+Great Nebula in the constellation of Orion (<a href="#Plate_XXIV">see Plate XXIV.</a>, p. 318). It
+is situated in the centre of the "Sword" of Orion (<a href="#Plate_XX">see Plate XX.</a>, p.
+296). In large telescopes it appears as a magnificent object, and in
+actual dimensions it must be much on the same scale as the Andromeda
+Nebula. The spectroscope tells us that it is a mass of glowing gas.</p>
+
+<p>The Trifid Nebula, situated in the constellation of Sagittarius, is an
+object of very strange shape. Three dark clefts radiate from its centre,
+giving it an appearance as if it had been torn into shreds.</p>
+
+<p>The Dumb-bell Nebula, a celebrated object, so called from its likeness
+to a dumb-bell, turns out, from recent photographs taken by Professor
+Schaeberle, which bring additional detail into view, to be after all a
+great spiral.</p>
+
+<p>There is a nest, or rather a cluster of nebul&aelig; in the constellation of
+Coma Berenices; over a hundred of these objects being here gathered into
+a space of sky about the size of our full moon.</p>
+
+<div class="figcenter" style="width: 600px;"><a name="Plate_XXIII" id="Plate_XXIII"></a>
+<img src="images/plate23.jpg" width="500" height="638" alt="Plate XXIII." title="" /><br />
+<span class="caption"><span class="smcap">Plate XXIII. The Great Nebula in the Constellation of
+Andromeda</span><br />From a photograph taken at the Yerkes Observatory.<br />(<a href="#Page_314"><small>Page 314</small></a>)</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_317" id="Page_317">[Pg 317]</a></span></p><p>The spectroscope informs us that spiral nebul&aelig; are composed of
+partially-cooled matter. Their colour, as we have seen, is white. Nebul&aelig;
+of a greenish tint are, on the other hand, found to be entirely in a
+gaseous condition. Just as the solar corona contains an unknown element,
+which for the time being has been called "Coronium," so do the gaseous
+nebul&aelig; give evidence of the presence of another unknown element. To this
+Sir William Huggins has given the provisional name of "Nebulium."</p>
+
+<p>The <i>Magellanic Clouds</i> are two patches of nebulous-looking light, more
+or less circular in form, which are situated in the southern hemisphere
+of the sky. They bear a certain resemblance to portions of the Milky
+Way, but are, however, not connected with it. They have received their
+name from the celebrated navigator, Magellan, who seems to have been one
+of the first persons to draw attention to them. "Nubecul&aelig;" is another
+name by which they are known, the larger cloud being styled <i>nubecula
+major</i> and the smaller one <i>nubecula minor</i>. They contain within them
+stars, clusters, and gaseous nebul&aelig;. No parallax has yet been found for
+any object which forms part of the nubecul&aelig;, so it is very difficult to
+estimate at what distance from us they may lie. They are, however,
+considered to be well within our stellar universe.</p>
+
+<p>Having thus brought to a conclusion our all too brief review of the
+stars and the nebul&aelig;&mdash;of the leading objects in fine which the celestial
+spaces have revealed to man&mdash;we will close this chapter with a recent
+summation by Sir David Gill of the relations which appear to obtain
+between these various bodies. "Huggins's spectroscope," he says, "has
+shown that many nebul&aelig; are not stars at all; that many well-condensed
+nebul&aelig;, as well as vast patches of nebulous light in the sky, are but
+inchoate masses of luminous<span class='pagenum'><a name="Page_318" id="Page_318">[Pg 318]</a></span> gas. Evidence upon evidence has accumulated
+to show that such nebul&aelig; consist of the matter out of which stars
+(<i>i.e.</i> suns) have been and are being evolved. The different types of
+star spectra form such a complete and gradual sequence (from simple
+spectra resembling those of nebul&aelig; onwards through types of gradually
+increasing complexity) as to suggest that we have before us, written in
+the cryptograms of these spectra, the complete story of the evolution of
+suns from the inchoate nebula onwards to the most active sun (like our
+own), and then downward to the almost heatless and invisible ball. The
+period during which human life has existed upon our globe is probably
+too short&mdash;even if our first parents had begun the work&mdash;to afford
+observational proof of such a cycle of change in any particular star;
+but the fact of such evolution, with the evidence before us, can hardly
+be doubted."<a name="FNanchor_34_34" id="FNanchor_34_34"></a><a href="#Footnote_34_34" class="fnanchor">[34]</a></p>
+
+<div class="figcenter" style="width: 500px;"><a name="Plate_XXIV" id="Plate_XXIV"></a>
+<img src="images/plate24.jpg" width="500" height="750" alt="Plate XXIV." title="" />
+<span class="caption"><span class="smcap">Plate XXIV. The Great Nebula in the Constellation of
+Orion</span><br />From a photograph taken at the Yerkes Observatory.<br />(<a href="#Page_316"><small>Page 316</small></a>)</span>
+</div>
+
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_32_32" id="Footnote_32_32"></a><a href="#FNanchor_32_32"><span class="label">[32]</span></a> The name Al g&ucirc;l, meaning the Demon, was what the old
+Arabian astronomers called it, which looks very much as if they had
+already noticed its rapid fluctuations in brightness.</p></div>
+
+<div class="footnote"><p><a name="Footnote_33_33" id="Footnote_33_33"></a><a href="#FNanchor_33_33"><span class="label">[33]</span></a> Mr. Gore thinks that the companion of Algol may be a star
+of the sixth magnitude.</p></div>
+
+<div class="footnote"><p><a name="Footnote_34_34" id="Footnote_34_34"></a><a href="#FNanchor_34_34"><span class="label">[34]</span></a> Presidential Address to the British Association for the
+Advancement of Science (Leicester, 1907), by Sir David Gill, K.C.B.,
+LL.D., F.R.S., &amp;c. &amp;c.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_319" id="Page_319">[Pg 319]</a></span></p>
+<h3><a name="CHAPTER_XXV" id="CHAPTER_XXV"></a>CHAPTER XXV</h3>
+
+<h4>THE STELLAR UNIVERSE</h4>
+
+
+<p class="noin"><span class="smcap">The</span> stars appear fairly evenly distributed all around us, except in one
+portion of the sky where they seem very crowded, and so give one an
+impression of being very distant. This portion, known as the Milky Way,
+stretches, as we have already said, in the form of a broad band right
+round the entire heavens. In those regions of the sky most distant from
+the Milky Way the stars appear to be thinly sown, but become more and
+more closely massed together as the Milky Way is approached.</p>
+
+<p>This apparent distribution of the stars in space has given rise to a
+theory which was much favoured by Sir William Herschel, and which is
+usually credited to him, although it was really suggested by one Thomas
+Wright of Durham in 1750; that is to say, some thirty years or more
+before Herschel propounded it. According to this, which is known as the
+"Disc" or "Grindstone" Theory, the stars are considered as arranged in
+space somewhat in the form of a thick disc, or grindstone, close to the
+<i>central</i> parts of which our solar system is situated.<a name="FNanchor_35_35" id="FNanchor_35_35"></a><a href="#Footnote_35_35" class="fnanchor">[35]</a> Thus we
+should see a greater number of stars when we looked out through the
+<i>length</i> of such a disc in<span class='pagenum'><a name="Page_320" id="Page_320">[Pg 320]</a></span> any direction, than when we looked out
+through its <i>breadth</i>. This theory was, for a time, supposed to account
+quite reasonably for the Milky Way, and for the gradual increase in the
+number of stars in its vicinity.</p>
+
+<p>It is quite impossible to verify directly such a theory, for we know the
+actual distance of only about forty-three stars. We are unable,
+therefore, definitely to assure ourselves whether, as the grindstone
+theory presupposes, the stellar universe actually reaches out very much
+further from us in the direction of the Milky Way than in the other
+parts of the sky. The theory is clearly founded upon the supposition
+that the stars are more or less equal in size, and are scattered through
+space at fairly regular distances from each other.</p>
+
+<p>Brightness, therefore, had been taken as implying nearness to us, and
+faintness great distance. But we know to-day that this is not the case,
+and that the stars around us are, on the other hand, of various degrees
+of brightness and of all orders of size. Some of the faint stars&mdash;for
+instance, the galloping star in Pictor&mdash;are indeed nearer to us than
+many of the brighter ones. Sirius, on the other hand, is twice as far
+off from us as &#945; Centauri, and yet it is very much brighter;
+while Canopus, which in brightness is second only to Sirius out of the
+whole sky, is too far off for its distance to be ascertained! It must be
+remembered that no parallax had yet been found for any star in the days
+of Herschel, and so his estimations of stellar distances were
+necessarily of a very circumstantial kind. He did not, however, continue
+always to build upon such uncertain ground;<span class='pagenum'><a name="Page_321" id="Page_321">[Pg 321]</a></span> but, after some further
+examination of the Milky Way, he gave up his idea that the stars were
+equally disposed in space, and eventually abandoned the grindstone
+theory.</p>
+
+<p>Since we have no means of satisfactorily testing the matter, through
+finding out the various distances from us at which the stars are really
+placed, one might just as well go to the other extreme, and assume that
+the thickening of stars in the region of the Milky Way is not an effect
+of perspective at all, but that the stars in that part of the sky are
+actually more crowded together than elsewhere&mdash;a thing which astronomers
+now believe to be the case. Looked at in this way, the shape of the
+stellar universe might be that of a globe-shaped aggregation of stars,
+in which the individuals are set at fairly regular distances from each
+other; the whole being closely encircled by a belt of densely packed
+stars. It must, however, be allowed that the gradual increase in the
+number of stars towards the Milky Way appears a strong argument in
+favour of the grindstone theory; yet the belt theory, as above detailed,
+seems to meet with more acceptance.</p>
+
+<p>There is, in fact, one marked circumstance which is remarkably difficult
+of explanation by means of the grindstone theory. This is the existence
+of vacant spaces&mdash;holes, so to speak, in the groundwork of the Milky
+Way. For instance, there is a cleft running for a good distance along
+its length, and there is also a starless gap in its southern portion. It
+seems rather improbable that such a great number of stars could have
+arranged themselves so conveniently, as to give us a clear view right
+out into empty space<span class='pagenum'><a name="Page_322" id="Page_322">[Pg 322]</a></span> through such a system in its greatest thickness;
+as if, in fact, holes had been bored, and clefts made, from the boundary
+of the disc clean up to where our solar system lies. Sir John Herschel
+long ago drew attention to this point very forcibly. It is plain that
+such vacant spaces can, on the other hand, be more simply explained as
+mere holes in a belt; and the best authorities maintain that the
+appearance of the Milky Way confirms a view of this kind.</p>
+
+<p>Whichever theory be indeed the correct one, it appears at any rate that
+the stars do not stretch out in every direction to an infinite distance;
+but that <i>the stellar system is of limited extent</i>, and has in fact a
+boundary.</p>
+
+<p>In the first place, Science has no grounds for supposing that light is
+in any way absorbed or destroyed merely by its passage through the
+"ether," that imponderable medium which is believed to transmit the
+luminous radiations through space. This of course is tantamount to
+saying that all the direct light from all the stars should reach us,
+excepting that little which is absorbed in its passage through our own
+atmosphere. If stars, and stars, and stars existed in every direction
+outwards without end, it can be proved mathematically that in such
+circumstances there could not remain the tiniest space in the sky
+without a star to fill it, and that therefore the heavens would always
+blaze with light, and the night would be as bright as the noonday.<a name="FNanchor_36_36" id="FNanchor_36_36"></a><a href="#Footnote_36_36" class="fnanchor">[36]</a>
+How very far indeed this is from being the case, may be gathered from an
+estimate which has been made of the general<span class='pagenum'><a name="Page_323" id="Page_323">[Pg 323]</a></span> amount of light which we
+receive from the stars. According to this estimate the sky is considered
+as more or less dark, the combined illumination sent to us by all the
+stars being only about the one-hundreth part of what we get from the
+full moon.<a name="FNanchor_37_37" id="FNanchor_37_37"></a><a href="#Footnote_37_37" class="fnanchor">[37]</a></p>
+
+<p>Secondly, it has been suggested that although light may not suffer any
+extinction or diminution from the ether itself, still a great deal of
+illumination may be prevented from reaching us through myriads of
+extinguished suns, or dark meteoric matter lying about in space. The
+idea of such extinguished suns, dark stars in fact, seems however to be
+merely founded upon the sole instance of the invisible companion of
+Algol; but, as we have seen, there is no proof whatever that it is a
+dark body. Again, some astronomers have thought that the dark holes in
+the Milky Way, "Coal Sacks," as they are called, are due to masses of
+cool, or partially cooled matter, which cuts off the light of the stars
+beyond. The most remarkable of these holes is one in the neighbourhood
+of the Southern Cross, known as the "Coal Sack in Crux." But Mr. Gore
+thinks that the cause of the holes is to be sought for rather in<span class='pagenum'><a name="Page_324" id="Page_324">[Pg 324]</a></span> what
+Sir William Herschel termed "clustering power," <i>i.e.</i> a tendency on the
+part of stars to accumulate in certain places, thus leaving others
+vacant; and the fact that globular and other clusters are to be found
+very near to such holes certainly seems corroborative of this theory. In
+summing up the whole question, Professor Newcomb maintains that there
+does not appear any evidence of the light from the Milky Way stars,
+which are apparently the furthest bodies we see, being intercepted by
+dark bodies or dark matter. As far as our telescopes can penetrate, he
+holds that we see the stars <i>just as they are</i>.</p>
+
+<p>Also, if there did exist an infinite number of stars, one would expect
+to find evidence in some direction of an overpoweringly great
+force,&mdash;the centre of gravity of all these bodies.</p>
+
+<p>It is noticed, too, that although the stars increase in number with
+decrease in magnitude, so that as we descend in the scale we find three
+times as many stars in each magnitude as in the one immediately above
+it, yet this progression does not go on after a while. There is, in
+fact, a rapid falling off in numbers below the twelfth magnitude; which
+looks as if, at a certain distance from us, the stellar universe were
+beginning to <i>thin out</i>.</p>
+
+<p>Again, it is estimated, by Mr. Gore and others, that only about 100
+millions of stars are to be seen in the whole of the sky with the best
+optical aids. This shows well the limited extent of the stellar system,
+for the number is not really great. For instance, there are from fifteen
+to sixteen times as many persons alive upon the earth at this moment!</p>
+
+<p>Last of all, there appears to be strong photographic<span class='pagenum'><a name="Page_325" id="Page_325">[Pg 325]</a></span> evidence that our
+sidereal system is limited in extent. Two photographs taken by the late
+Dr. Isaac Roberts of a region rich in stellar objects in the
+constellation of Cygnus, clearly show what has been so eloquently called
+the "darkness behind the stars." One of these photographs was taken in
+1895, and the other in 1898. On both occasions the state of the
+atmosphere was practically the same, and the sensitiveness of the films
+was of the same degree. The exposure in the first case was only one
+hour; in the second it was about two hours and a half. And yet both
+photographs show <i>exactly the same stars, even down to the faintest</i>.
+From this one would gather that the region in question, which is one of
+the most thickly star-strewn in the Milky Way, is <i>penetrable right
+through</i> with the means at our command. Dr. Roberts himself in
+commenting upon the matter drew attention to the fact, that many
+astronomers seemed to have tacitly adopted the assumption that the stars
+extend indefinitely through space.</p>
+
+<p>From considerations such as these the foremost astronomical authorities
+of our time consider themselves justified in believing that the
+collection of stars around us is <i>finite</i>; and that although our best
+telescopes may not yet be powerful enough to penetrate to the final
+stars, still the rapid decrease in numbers as space is sounded with
+increasing telescopic power, points strongly to the conclusion that the
+boundaries of the stellar system may not lie very far beyond the
+uttermost to which we can at present see.</p>
+
+<p>Is it possible then to make an estimate of the extent of this stellar
+system?</p>
+
+<p>Whatever estimates we may attempt to form cannot<span class='pagenum'><a name="Page_326" id="Page_326">[Pg 326]</a></span> however be regarded as
+at all exact, for we know the actual distances of such a very few only
+of the nearest of the stars. But our knowledge of the distances even of
+these few, permits us to assume that the stars close around us may be
+situated, on an average, at about eight light-years from each other; and
+that this holds good of the stellar spaces, with the exception of the
+encircling girdle of the Milky Way, where the stars seem actually to be
+more closely packed together. This girdle further appears to contain the
+greater number of the stars. Arguing along these lines, Professor
+Newcomb reaches the conclusion that the farthest stellar bodies which we
+see are situated at about between 3000 and 4000 light-years from us.</p>
+
+<p>Starting our inquiry from another direction, we can try to form an
+estimate by considering the question of proper motions.</p>
+
+<p>It will be noticed that such motions do not depend entirely upon the
+actual speed of the stars themselves, but that some of the apparent
+movement arises indirectly from the speed of our own sun. The part in a
+proper motion which can be ascribed to the movement of our solar system
+through space is clearly a displacement in the nature of a parallax&mdash;Sir
+William Herschel called it "<i>Systematic</i> Parallax"; so that knowing the
+distance which we move over in a certain lapse of time, we are able to
+hazard a guess at the distances of a good many of the stars. An inquiry
+upon such lines must needs be very rough, and is plainly based upon the
+assumption that the stars whose distances we attempt to estimate are
+moving at an average speed much like that of our own sun, and that they
+are not "runaway stars" of the 1830 Groombridge<span class='pagenum'><a name="Page_327" id="Page_327">[Pg 327]</a></span> order. Be that as it
+may, the results arrived at by Professor Newcomb from this method of
+reasoning are curiously enough very much on a par with those founded on
+the few parallaxes which we are really certain about; with the exception
+that they point to somewhat closer intervals between the individual
+stars, and so tend to narrow down our previous estimate of the extent of
+the stellar system.</p>
+
+<p>Thus far we get, and no farther. Our solar system appears to lie
+somewhere near the centre of a great collection of stars, separated each
+one from the other, on an average, by some 40 billions of miles; the
+whole being arranged in the form of a mighty globular cluster. Light
+from the nearest of these stars takes some four years to come to us. It
+takes about 1000 times as long to reach us from the confines of the
+system. This globe of stars is wrapt around closely by a stellar girdle,
+the individual stars in which are set together more densely than those
+in the globe itself. The entire arrangement appears to be constructed
+upon a very regular plan. Here and there, as Professor Newcomb points
+out, the aspect of the heavens differs in small detail; but generally it
+may be laid down that the opposite portions of the sky, whether in the
+Milky Way itself, or in those regions distant from it, show a marked
+degree of symmetry. The proper motions of stars in corresponding
+portions of the sky reveal the same kind of harmony, a harmony which may
+even be extended to the various colours of the stars. The stellar
+system, which we see disposed all around us, appears in fine to bear all
+the marks of an <i>organised whole</i>.</p>
+
+<p>The older astronomers, to take Sir William Herschel<span class='pagenum'><a name="Page_328" id="Page_328">[Pg 328]</a></span> as an example,
+supposed some of the nebul&aelig; to be distant "universes." Sir William was
+led to this conclusion by the idea he had formed that, when his
+telescopes failed to show the separate stars of which he imagined these
+objects to be composed, he must put down the failure to their stupendous
+distance from us. For instance, he thought the Orion Nebula, which is
+now known to be made up of glowing gas, to be an external stellar
+system. Later on, however, he changed his mind upon this point, and came
+to the conclusion that "shining fluid" would better account both for
+this nebula, and for others which his telescopes had failed to separate
+into component stars.</p>
+
+<p>The old ideas with regard to external systems and distant universes have
+been shelved as a consequence of recent research. All known clusters and
+nebul&aelig; are now firmly believed to lie <i>within</i> our stellar system.</p>
+
+<p>This view of the universe of stars as a sort of island in the
+immensities, does not, however, give us the least idea about the actual
+extent of space itself. Whether what is called space is really infinite,
+that is to say, stretches out unendingly in every direction, or whether
+it has eventually a boundary somewhere, are alike questions which the
+human mind seems utterly unable to picture to itself.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_35_35" id="Footnote_35_35"></a><a href="#FNanchor_35_35"><span class="label">[35]</span></a> The Ptolemaic idea dies hard!</p></div>
+
+<div class="footnote"><p><a name="Footnote_36_36" id="Footnote_36_36"></a><a href="#FNanchor_36_36"><span class="label">[36]</span></a> Even the Milky Way itself is far from being a blaze of
+light, which shows that the stars composing it do not extend outwards
+indefinitely.</p></div>
+
+<div class="footnote"><p><a name="Footnote_37_37" id="Footnote_37_37"></a><a href="#FNanchor_37_37"><span class="label">[37]</span></a> Mr. Gore has recently made some remarkable deductions,
+with regard to the amount of light which we get from the stars. He
+considers that most of this light comes from stars below the sixth
+magnitude; and consequently, if all the stars visible to the naked eye
+were to be blotted out, the glow of the night sky would remain
+practically the same as it is at present. Going to the other end of the
+scale, he thinks also that the combined light which we get from all the
+stars below the seventeenth magnitude is so very small, that it may be
+neglected in such an estimation. He finds, indeed, that if there are
+stars so low as the twentieth magnitude, one hundred millions of them
+would only be equal in brightness to a single first-magnitude star like
+Vega. On the other hand, it is possible that the light of the sky at
+night is not entirely due to starlight, but that some of it may be
+caused by phosphorescent glow.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_329" id="Page_329">[Pg 329]</a></span></p>
+<h3><a name="CHAPTER_XXVI" id="CHAPTER_XXVI"></a>CHAPTER XXVI</h3>
+
+<h4>THE STELLAR UNIVERSE&mdash;<i>continued</i></h4>
+
+
+<p class="noin"><span class="smcap">It</span> is very interesting to consider the proper motions of stars with
+reference to such an isolated stellar system as has been pictured in the
+previous chapter. These proper motions are so minute as a rule, that we
+are quite unable to determine whether the stars which show them are
+moving along in straight lines, or in orbits of immense extent. It
+would, in fact, take thousands of years of careful observation to
+determine whether the paths in question showed any degree of curving. In
+the case of the more distant stars, the accurate observations which have
+been conducted during the last hundred years have not so far revealed
+any proper motions with regard to them; but one cannot escape the
+conclusion that these stars move as the others do.</p>
+
+<p>If space outside our stellar system is infinite in extent, and if all
+the stars within that system are moving unchecked in every conceivable
+direction, the result must happen that after immense ages these stars
+will have drawn apart to such a distance from each other, that the
+system will have entirely disintegrated, and will cease to exist as a
+connected whole. Eventually, indeed, as Professor Newcomb points out,
+the stars will have separated so far from each other<span class='pagenum'><a name="Page_330" id="Page_330">[Pg 330]</a></span> that each will be
+left by itself in the midst of a black and starless sky. If, however, a
+certain proportion of stars have a speed sufficiently slow, they will
+tend under mutual attraction to be brought to rest by collisions, or
+forced to move in orbits around each other. But those stars which move
+at excessive speeds, such, for instance, as 1830 Groombridge, or the
+star in the southern constellation of Pictor, seem utterly incapable of
+being held back in their courses by even the entire gravitative force of
+our stellar system acting as a whole. These stars must, therefore, move
+eventually right through the system and pass out again into the empty
+spaces beyond. Add to this; certain investigations, made into the speed
+of 1830 Groombridge, furnish a remarkable result. It is calculated,
+indeed, that had this star been <i>falling through infinite space for
+ever</i>, pulled towards us by the combined gravitative force of our entire
+system of stars, it could not have gathered up anything like the speed
+with which it is at present moving. No force, therefore, which we can
+conjure out of our visible universe, seems powerful enough either to
+have impressed upon this runaway star the motion which it now has, or to
+stay it in its wild course. What an astounding condition of things!</p>
+
+<p>Speculations like this call up a suspicion that there may yet exist
+other universes, other centres of force, notwithstanding the apparent
+solitude of our stellar system in space. It will be recollected that the
+idea of this isolation is founded upon such facts as, that the heavens
+do not blaze with light, and that the stars gradually appear to thin out
+as we penetrate the system with increasing telescopic power. But<span class='pagenum'><a name="Page_331" id="Page_331">[Pg 331]</a></span>
+perchance there is something which hinders us from seeing out into space
+beyond our cluster of stars; which prevents light, in fact, from
+reaching us from other possible systems scattered through the depths
+beyond. It has, indeed, been suggested by Mr. Gore<a name="FNanchor_38_38" id="FNanchor_38_38"></a><a href="#Footnote_38_38" class="fnanchor">[38]</a> that the
+light-transmitting ether may be after all merely a kind of "atmosphere"
+of the stars; and that it may, therefore, thin off and cease a little
+beyond the confines of our stellar system, just as the air thins off and
+practically ceases at a comparatively short distance from the earth. A
+clashing together of solid bodies outside our atmosphere could plainly
+send us no sound, for there is no air extending the whole way to bear to
+our ears the vibrations thus set up; so light emitted from any body
+lying beyond our system of stars, would not be able to come to us if the
+ether, whose function it is to convey the rays of light, ceased at or
+near the confines of that system.</p>
+
+<p>Perchance we have in this suggestion the key to the mystery of how our
+sun and the other stellar bodies maintain their functions of temperature
+and illumination. The radiations of heat and light arriving at the
+limits of this ether, and unable to pass any further, may be thrown back
+again into the system in some altered form of energy.</p>
+
+<p>But these, at best, are mere airy and fascinating speculations. We have,
+indeed, no evidence whatever that the luminiferous ether ceases at the
+boundary of the stellar system. If, therefore, it extends outwards
+infinitely in every direction, and if it has no absorbing<span class='pagenum'><a name="Page_332" id="Page_332">[Pg 332]</a></span> or weakening
+effect on the vibrations which it transmits, we cannot escape from the
+conclusion that practically all the rays of light ever emitted by all
+the stars must chase one another eternally through the never-ending
+abysses of space.</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_38_38" id="Footnote_38_38"></a><a href="#FNanchor_38_38"><span class="label">[38]</span></a> <i>Planetary and Stellar Studies</i>, by John Ellard Gore,
+F.R.A.S., M.R.I.A., London, 1888.</p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_333" id="Page_333">[Pg 333]</a></span></p>
+<h3><a name="CHAPTER_XXVII" id="CHAPTER_XXVII"></a>CHAPTER XXVII</h3>
+
+<h4>THE BEGINNING OF THINGS</h4>
+
+
+<p class="center"><br /><span class="smcap">Laplace's Nebular Hypothesis</span></p>
+
+<p class="noin"><span class="smcap">Dwelling</span> upon the fact that all the motions of revolution and rotation
+in the solar system, as known in his day, took place in the same
+direction and nearly in the same plane, the great French astronomer,
+Laplace, about the year 1796, put forward a theory to account for the
+origin and evolution of that system. He conceived that it had come into
+being as a result of the gradual contraction, through cooling, of an
+intensely heated gaseous lens-shaped mass, which had originally occupied
+its place, and had extended outwards beyond the orbit of the furthest
+planet. He did not, however, attempt to explain how such a mass might
+have originated! He went on to suppose that this mass, <i>in some manner</i>,
+perhaps by mutual gravitation among its parts, had acquired a motion of
+rotation in the same direction as the planets now revolve. As this
+nebulous mass parted with its heat by radiation, it contracted towards
+the centre. Becoming smaller and smaller, it was obliged to rotate
+faster and faster in order to preserve its equilibrium. Meanwhile, in
+the course of contraction, rings of matter became separated from the
+nucleus of the mass, and were left behind at various intervals. These
+rings were swept up into subordinate masses similar<span class='pagenum'><a name="Page_334" id="Page_334">[Pg 334]</a></span> to the original
+nebula. These subordinate masses also contracted in the same manner,
+leaving rings behind them which, in turn, were swept up to form
+satellites. Saturn's ring was considered, by Laplace, as the only
+portion of the system left which still showed traces of this
+evolutionary process. It is even probable that it may have suggested the
+whole of the idea to him.</p>
+
+<p>Laplace was, however, not the first philosopher who had speculated along
+these lines concerning the origin of the world.</p>
+
+<p>Nearly fifty years before, in 1750 to be exact, Thomas Wright, of
+Durham, had put forward a theory to account for the origin of the whole
+sidereal universe. In his theory, however, the birth of our solar system
+was treated merely as an incident. Shortly afterwards the subject was
+taken up by the famous German philosopher, Kant, who dealt with the
+question in a still more ambitious manner, and endeavoured to account in
+detail for the origin of the solar system as well as of the sidereal
+universe. Something of the trend of such theories may be gathered from
+the remarkable lines in Tennyson's <i>Princess</i>:&mdash;</p>
+
+<p class="poem">
+"This world was once a fluid haze of light,<br />
+Till toward the centre set the starry tides,<br />
+And eddied into suns, that wheeling cast<br />
+The planets."<br />
+</p>
+
+<p>The theory, as worked out by Kant, was, however, at the best merely a
+<i>tour de force</i> of philosophy. Laplace's conception was much less
+ambitious, for it did not attempt to explain the origin of the entire
+universe, but only of the solar system. Being thus reasonably limited in
+its scope, it more easily obtained credence. The arguments of Laplace
+were further<span class='pagenum'><a name="Page_335" id="Page_335">[Pg 335]</a></span> founded upon a mathematical basis. The great place which
+he occupied among the astronomers of that time caused his theory to
+exert a preponderating influence on scientific thought during the
+century which followed.</p>
+
+<p>A modification of Laplace's theory is the Meteoritic Hypothesis of Sir
+Norman Lockyer. According to the views of that astronomer, the material
+of which the original nebula was composed is presumed to have been in
+the meteoric, rather than in the gaseous, state. Sir Norman Lockyer
+holds, indeed, that nebul&aelig; are, in reality, vast swarms of meteors, and
+the light they emit results from continual collisions between the
+constituent particles. The French astronomer, Faye, also proposed to
+modify Laplace's theory by assuming that the nebula broke up into rings
+all at once, and not in detail, as Laplace had wished to suppose.</p>
+
+<p>The hypothesis of Laplace fits in remarkably well with the theory put
+forward in later times by Helmholtz, that the heat of the sun is kept up
+by the continual contraction of its mass. It could thus have only
+contracted to its present size from one very much larger.</p>
+
+<p>Plausible, however, as Laplace's great hypothesis appears on the
+surface, closer examination shows several vital objections, a few of
+those set forth by Professor Moulton being here enumerated&mdash;</p>
+
+<p>Although Laplace held that the orbits of the planets were sufficiently
+near to being in the one plane to support his views, yet later
+investigators consider that their very deviations from this plane are a
+strong argument against the hypothesis.<span class='pagenum'><a name="Page_336" id="Page_336">[Pg 336]</a></span></p>
+
+<p>Again, it is thought that if the theory were the correct explanation,
+the various orbits of the planets would be much more nearly circular
+than they are.</p>
+
+<p>It is also thought that such interlaced paths, as those in which the
+asteroids and the little planet Eros move, are most unlikely to have
+been produced as a result of Laplace's nebula.</p>
+
+<p>Further, while each of the rings was sweeping up its matter into a body
+of respectable dimensions, its gravitative power would have been for the
+time being so weak, through being thus spread out, that any lighter
+elements, as, for instance, those of the gaseous order, would have
+escaped into space in accordance with the principles of the kinetic
+theory.</p>
+
+<p><i>The idea that rings would at all be left behind at certain intervals
+during the contraction of the nebula is, perhaps, one of the weakest
+points in Laplace's hypothesis.</i></p>
+
+<p>Mathematical investigation does not go to show that the rings, presuming
+they could be left behind during the contraction of the mass, would have
+aggregated into planetary bodies. Indeed, it rather points to the
+reverse.</p>
+
+<p>Lastly, such a discovery as that the ninth satellite of Saturn revolves
+in a <i>retrograde</i> direction&mdash;that is to say, in a direction contrary to
+the other revolutions and rotations in our solar system&mdash;appears
+directly to contradict the hypothesis.</p>
+
+<p>Although Laplace's hypothesis seems to break down under the keen
+criticism to which it has been subjected, yet astronomers have not
+relinquished the idea that our solar system has probably had its<span class='pagenum'><a name="Page_337" id="Page_337">[Pg 337]</a></span> origin
+from a nebulous mass. But the apparent failure of the Laplacian theory
+is emphasised by the fact, that <i>not a single example of a nebula, in
+the course of breaking up into concentric rings, is known to exist in
+the entire heaven</i>. Indeed, as we saw in <a href="#CHAPTER_XXIV">Chapter XXIV.</a>, there seems to
+be no reliable example of even a "ring" nebula at all. Mr. Gore has
+pointed this out very succinctly in his recently published work,
+<i>Astronomical Essays</i>, where he says:&mdash;"To any one who still persists in
+maintaining the hypothesis of ring formation in nebul&aelig;, it may be said
+that the whole heavens are against him."</p>
+
+<p>The conclusions of Keeler already alluded to, that the spiral is the
+normal type of nebula, has led during the past few years to a new theory
+by the American astronomers, Professors Chamberlin and Moulton. In the
+detailed account of it which they have set forth, they show that those
+anomalies which were stumbling-blocks to Laplace's theory do not
+contradict theirs. To deal at length with this theory, to which the name
+of "Planetesimal Hypothesis" has been given, would not be possible in a
+book of this kind. But it may be of interest to mention that the authors
+of the theory in question remount the stream of time still further than
+did Laplace, and seek to explain the <i>origin</i> of the spiral nebul&aelig;
+themselves in the following manner:&mdash;</p>
+
+<p>Having begun by assuming that the stars are moving apparently in every
+direction with great velocities, they proceed to point out that sooner
+or later, although the lapse of time may be extraordinarily long,
+collisions or near approaches between stars are bound to occur. In the
+case of collisions the<span class='pagenum'><a name="Page_338" id="Page_338">[Pg 338]</a></span> chances are against the bodies striking together
+centrally, it being very much more likely that they will hit each other
+rather towards the side. The nebulous mass formed as a result of the
+disintegration of the bodies through their furious impact would thus
+come into being with a spinning movement, and a spiral would ensue.
+Again, the stars may not actually collide, but merely approach near to
+each other. If very close, the interaction of gravitation will give rise
+to intense strains, or tides, which will entirely disintegrate the
+bodies, and a spiral nebula will similarly result. As happens upon our
+earth, two such tides would rise opposite to each other; and,
+consequently, it is a noticeable fact that spiral nebul&aelig; have almost
+invariably two opposite branches (<a href="#Plate_XXII">see Plate XXII.</a>, p 314). Even if not
+so close, the gravitational strains set up would produce tremendous
+eruptions of matter; and in this case, a spiral movement would also be
+generated. On such an assumption the various bodies of the solar system
+may be regarded as having been ejected from parent masses.</p>
+
+<p>The acceptance of the Planetesimal Hypothesis in the place of the
+Hypothesis of Laplace will not, as we have seen, by any means do away
+with the probability that our solar system, and similar systems, have
+originated from a nebulous mass. On the contrary it puts that idea on a
+firmer footing than before. The spiral nebul&aelig; which we see in the
+heavens are on a vast scale, and may represent the formation of stellar
+systems and globular clusters. Our solar system may have arisen from a
+small spiral.</p>
+
+<p>We will close these speculations concerning the<span class='pagenum'><a name="Page_339" id="Page_339">[Pg 339]</a></span> origin of things with a
+short sketch of certain investigations made in recent years by Sir
+George H. Darwin, of Cambridge University, into the question of the
+probable birth of our moon. He comes to the conclusion that at least
+fifty-four millions of years ago the earth and moon formed one body,
+which had a diameter of a little over 8000 miles. This body rotated on
+an axis in about five hours, namely, about five times as fast as it does
+at present. The rapidity of the rotation caused such a tremendous strain
+that the mass was in a condition of, what is called, unstable
+equilibrium; very little more, in fact, being required to rend it
+asunder. The gravitational pull of the sun, which, as we have already
+seen, is in part the cause of our ordinary tides, supplied this extra
+strain, and a portion of the mass consequently broke off, which receded
+gradually from the rest and became what we now know as the moon. Sir
+George Darwin holds that the gravitational action of the sun will in
+time succeed in also disturbing the present apparent harmony of the
+earth-moon system, and will eventually bring the moon back towards the
+earth, so that after the lapse of great ages they will re-unite once
+again.</p>
+
+<p>In support of this theory of the terrestrial origin of the moon,
+Professor W.H. Pickering has put forward a bold hypothesis that our
+satellite had its origin in the great basin of the Pacific. This ocean
+is roughly circular, and contains no large land masses, except the
+Australian Continent. He supposes that, prior to the moon's birth, our
+globe was already covered with a slight crust. In the tearing away of
+that portion which was afterwards destined<span class='pagenum'><a name="Page_340" id="Page_340">[Pg 340]</a></span> to become the moon the
+remaining area of the crust was rent in twain by the shock; and thus
+were formed the two great continental masses of the Old and New Worlds.
+These masses floated apart across the fiery ocean, and at last settled
+in the positions which they now occupy. In this way Professor Pickering
+explains the remarkable parallelism which exists between the opposite
+shores of the Atlantic. The fact of this parallelism had, however, been
+noticed before; as, for example, by the late Rev. S.J. Johnson, in his
+book <i>Eclipses, Past and Future</i>, where we find the following passage:&mdash;</p>
+
+<p>"If we look at our maps we shall see the parts of one Continent that jut
+out agree with the indented portions of another. The prominent coast of
+Africa would fit in the opposite opening between North and South
+America, and so in numerous other instances. A general rending asunder
+of the World would seem to have taken place when the foundations of the
+great deep were broken up."</p>
+
+<p>Although Professor Pickering's theory is to a certain degree anticipated
+in the above words, still he has worked out the idea much more fully,
+and given it an additional fascination by connecting it with the birth
+of the moon. He points out, in fact, that there is a remarkable
+similarity between the lunar volcanoes and those in the immediate
+neighbourhood of the Pacific Ocean. He goes even further to suggest that
+Australia is another portion of the primal crust which was detached out
+of the region now occupied by the Indian Ocean, where it was originally
+connected with the south of India or the east of Africa.</p>
+
+<p><span class='pagenum'><a name="Page_341" id="Page_341">[Pg 341]</a></span></p><p>Certain objections to the theory have been put forward, one of which is
+that the parallelism noticed between the opposite shores of the Atlantic
+is almost too perfect to have remained through some sixty millions of
+years down to our own day, in the face of all those geological movements
+of upheaval and submergence, which are perpetually at work upon our
+globe. Professor Pickering, however, replies to this objection by
+stating that many geologists believe that the main divisions of land and
+water on the earth are permanent, and that the geological alterations
+which have taken place since these were formed have been merely of a
+temporary and superficial nature.</p>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_342" id="Page_342">[Pg 342]</a></span></p>
+<h3><a name="CHAPTER_XXVIII" id="CHAPTER_XXVIII"></a>CHAPTER XXVIII</h3>
+
+<h4>THE END OF THINGS</h4>
+
+
+<p class="noin"><span class="smcap">We</span> have been trying to picture the beginning of things. We will now try
+to picture the end.</p>
+
+<p>In attempting this, we find that our theories must of necessity be
+limited to the earth, or at most to the solar system. The time-honoured
+expression "End of the World" really applies to very little beyond the
+end of our own earth. To the people of past ages it, of course, meant
+very much more. For them, as we have seen, the earth was the centre of
+everything; and the heavens and all around were merely a kind of minor
+accompaniment, created, as they no doubt thought, for their especial
+benefit. In the ancient view, therefore, the beginning of the earth
+meant the beginning of the universe, and the end of the earth the
+extinction of all things. The belief, too, was general that this end
+would be accomplished through fire. In the modern view, however, the
+birth and death of the earth, or indeed of the solar system, might pass
+as incidents almost unnoticed in space. They would be but mere links in
+the chain of cosmic happenings.</p>
+
+<p>A number of theories have been forward from time to time prognosticating
+the end of the earth, and consequently of human life. We will conclude
+with<span class='pagenum'><a name="Page_343" id="Page_343">[Pg 343]</a></span> a recital of a few of them, though which, if any, is the true one,
+the Last Men alone can know.</p>
+
+<p>Just as a living creature may at any moment die in the fulness of
+strength through sudden malady or accident, or, on the other hand, may
+meet with death as a mere consequence of old age, so may our globe be
+destroyed by some sudden cataclysm, or end in slow processes of decay.
+Barring accidents, therefore, it would seem probable that the growing
+cold of the earth, or the gradual extinction of the sun, should after
+many millions of years close the chapter of life, as we know it. On the
+former of these suppositions, the decrease of temperature on our globe
+might perhaps be accelerated by the thinning of the atmosphere, through
+the slow escape into space of its constituent gases, or their gradual
+chemical combination with the materials of the earth. The subterranean
+heat entirely radiated away, there would no longer remain any of those
+volcanic elevating forces which so far have counteracted the slow
+wearing down of the land surface of our planet, and thus what water
+remained would in time wash over all. If this preceded the growing cold
+of the sun, certain strange evolutions of marine forms of life would be
+the last to endure, but these, too, would have to go in the end.</p>
+
+<p>Should, however, the actual process be the reverse of this, and the sun
+cool down the quicker, then man would, as a consequence of his
+scientific knowledge, tend in all probability to outlive the other forms
+of terrestrial life. In such a vista we can picture the regions of the
+earth towards the north and south becoming gradually more and more
+uninhabitable<span class='pagenum'><a name="Page_344" id="Page_344">[Pg 344]</a></span> through cold, and human beings withdrawing before the
+slow march of the icy boundary, until the only regions capable of
+habitation would lie within the tropics. In such a struggle between man
+and destiny science would be pressed to the uttermost, in the devising
+of means to counteract the slow diminution of the solar heat and the
+gradual disappearance of air and water. By that time the axial rotation
+of our globe might possibly have been slowed down to such an extent that
+one side alone of its surface would be turned ever towards the fast
+dying sun. And the mind's eye can picture the last survivors of the
+human race, huddled together for warmth in a glass-house somewhere on
+the equator, waiting for the end to come.</p>
+
+<p>The mere idea of the decay and death of the solar system almost brings
+to one a cold shudder. All that sun's light and heat, which means so
+much to us, entirely a thing of the past. A dark, cold ball rushing
+along in space, accompanied by several dark, cold balls circling
+ceaselessly around it. One of these a mere cemetery, in which there
+would be no longer any recollection of the mighty empires, the loves and
+hates, and all that teeming play of life which we call History.
+Tombstones of men and of deeds, whirling along forgotten in the darkness
+and silence. <i>Sic transit gloria mundi.</i></p>
+
+<p>In that brilliant flight of scientific fancy, the <i>Time Machine</i>, Mr.
+H.G. Wells has pictured the closing years of the earth in some such
+long-drawn agony as this. He has given us a vision of a desolate beach
+by a salt and almost motionless sea. Foul monsters of crab-like form
+crawl slowly about, beneath a huge<span class='pagenum'><a name="Page_345" id="Page_345">[Pg 345]</a></span> hull of sun, red and fixed in the
+sky. The rocks around are partly coated with an intensely green
+vegetation, like the lichen in caves, or the plants which grow in a
+perpetual twilight. And the air is now of an exceeding thinness.</p>
+
+<p>He dips still further into the future, and thus predicts the final form
+of life:&mdash;</p>
+
+<p>"I saw again the moving thing upon the shoal&mdash;there was no mistake now
+that it was a moving thing&mdash;against the red water of the sea. It was a
+round thing, the size of a football perhaps, or it may be bigger, and
+tentacles trailed down from it; it seemed black against the weltering
+blood-red water, and it was hopping fitfully about."</p>
+
+<p>What a description of the "Heir of all the Ages!"</p>
+
+<p>To picture the end of our world as the result of a cataclysm of some
+kind, is, on the other hand, a form of speculation as intensely dramatic
+as that with which we have just been dealing is unutterably sad.</p>
+
+<p>It is not so many years ago, for instance, that men feared a sudden
+catastrophe from the possible collision of a comet with our earth. The
+unreasoning terror with which the ancients were wont to regard these
+mysterious visitants to our skies had, indeed, been replaced by an
+apprehension of quite another kind. For instance, as we have seen, the
+announcement in 1832 that Biela's Comet, then visible, would cut through
+the orbit of the earth on a certain date threw many persons into a
+veritable panic. They did not stop to find out the real facts of the
+case, namely, that, at the time mentioned, the earth would be nearly a
+month's journey from the point indicated!</p>
+
+<p>It is, indeed, very difficult to say what form of<span class='pagenum'><a name="Page_346" id="Page_346">[Pg 346]</a></span> damage the earth
+would suffer from such a collision. In 1861 it passed, as we have seen,
+through the tail of the comet without any noticeable result. But the
+head of a comet, on the other hand, may, for aught we know, contain
+within it elements of peril for us. A collision with this part might,
+for instance, result in a violent bombardment of meteors. But these
+meteors could not be bodies of any great size, for the masses of comets
+are so very minute that one can hardly suppose them to contain any large
+or dense constituent portions.</p>
+
+<p>The danger, however, from a comet's head might after all be a danger to
+our atmosphere. It might precipitate, into the air, gases which would
+asphyxiate us or cause a general conflagration. It is scarcely necessary
+to point out that dire results would follow upon any interference with
+the balance of our atmosphere. For instance, the well-known French
+astronomer, M. Camille Flammarion,<a name="FNanchor_39_39" id="FNanchor_39_39"></a><a href="#Footnote_39_39" class="fnanchor">[39]</a> has imagined the absorption of
+the nitrogen of the air in this way; and has gone on to picture men and
+animals reduced to breathing only oxygen, first becoming excited, then
+mad, and finally ending in a perfect saturnalia of delirium.</p>
+
+<p>Lastly, though we have no proof that stars eventually become dark and
+cold, for human time has so far been all too short to give us even the
+smallest evidence as to whether heat and light are diminishing in our
+own sun, yet it seems natural to suppose that such bodies must at last
+cease their functions, like<span class='pagenum'><a name="Page_347" id="Page_347">[Pg 347]</a></span> everything else which we know of. We may,
+therefore, reasonably presume that there are dark bodies scattered in
+the depths of space. We have, indeed, a suspicion of at least one,
+though perhaps it partakes rather of a planetary nature, namely, that
+"dark" body which continually eclipses Algol, and so causes the
+temporary diminution of its light. As the sun rushes towards the
+constellation of Lyra such an extinguished sun may chance to find itself
+in his path; just as a derelict hulk may loom up out of the darkness
+right beneath the bows of a vessel sailing the great ocean.</p>
+
+<p>Unfortunately a collision between the sun and a body of this kind could
+not occur with such merciful suddenness. A tedious warning of its
+approach would be given from that region of the heavens whither our
+system is known to be tending. As the dark object would become visible
+only when sufficiently near our sun to be in some degree illuminated by
+his rays, it might run the chance at first of being mistaken for a new
+planet. If such a body were as large, for instance, as our own sun, it
+should, according to Mr. Gore's calculations, reveal itself to the
+telescope some fifteen years before the great catastrophe. Steadily its
+disc would appear to enlarge, so that, about nine years after its
+discovery, it would become visible to the naked eye. At length the
+doomed inhabitants of the earth, paralysed with terror, would see their
+relentless enemy shining like a second moon in the northern skies.
+Rapidly increasing in apparent size, as the gravitational attractions of
+the solar orb and of itself interacted more powerfully with diminishing
+distance, it would at last draw quickly in towards the sun and disappear
+in the glare.</p>
+
+<p><span class='pagenum'><a name="Page_348" id="Page_348">[Pg 348]</a></span></p><p>It is impossible for us to conceive anything more terrible than these
+closing days, for no menace of catastrophe which we can picture could
+bear within it such a certainty of fulfilment. It appears, therefore,
+useless to speculate on the probable actions of men in their now
+terrestrial prison. Hope, which so far had buoyed them up in the direst
+calamities, would here have no place. Humanity, in the fulness of its
+strength, would await a wholesale execution from which there could be no
+chance at all of a reprieve. Observations of the approaching body would
+have enabled astronomers to calculate its path with great exactness, and
+to predict the instant and character of the impact. Eight minutes after
+the moment allotted for the collision the resulting tide of flame would
+surge across the earth's orbit, and our globe would quickly pass away in
+vapour.</p>
+
+<p>And what then?</p>
+
+<p>A nebula, no doubt; and after untold ages the formation possibly from it
+of a new system, rising ph&#339;nix-like from the vast crematorium and
+filling the place of the old one. A new central sun, perhaps, with its
+attendant retinue of planets and satellites. And teeming life,
+perchance, appearing once more in the fulness of time, when temperature
+in one or other of these bodies had fallen within certain limits, and
+other predisposing conditions had supervened.</p>
+
+<p class="poem">
+"The world's great age begins anew,<br />
+<span style="margin-left: 2em;">The golden years return,</span><br />
+The earth doth like a snake renew<br />
+<span style="margin-left: 2em;">Her winter weeds outworn:</span><br />
+Heaven smiles, and faiths and empires gleam<br />
+Like wrecks of a dissolving dream.<br />
+<br />
+<span class='pagenum'><a name="Page_349" id="Page_349">[Pg 349]</a></span>A brighter Hellas rears its mountains<br />
+<span style="margin-left: 2em;">From waves serener far;</span><br />
+A new Peneus rolls his fountains<br />
+<span style="margin-left: 2em;">Against the morning star;</span><br />
+Where fairer Tempes bloom, there sleep<br />
+Young Cyclads on a sunnier deep.<br />
+<br />
+A loftier Argo cleaves the main,<br />
+<span style="margin-left: 2em;">Fraught with a later prize;</span><br />
+Another Orpheus sings again,<br />
+<span style="margin-left: 2em;">And loves, and weeps, and dies;</span><br />
+A new Ulysses leaves once more<br />
+Calypso for his native shore.<br />
+</p>
+
+<hr style='width: 15%;' />
+
+<p class="poem">
+Oh cease! must hate and death return?<br />
+<span style="margin-left: 2em;">Cease! must men kill and die?</span><br />
+Cease! drain not to its dregs the urn<br />
+<span style="margin-left: 2em;">Of bitter prophecy!</span><br />
+The world is weary of the past,&mdash;<br />
+Oh might it die or rest at last!"<br />
+</p>
+
+<div class="footnotes">
+<div class="footnote"><p><a name="Footnote_39_39" id="Footnote_39_39"></a><a href="#FNanchor_39_39"><span class="label">[39]</span></a> See his work, <i>La Fin du Monde</i>, wherein the various ways
+by which our world may come to an end are dealt with at length, and in a
+profoundly interesting manner.<span class='pagenum'><a name="Page_350" id="Page_350">[Pg 350]</a></span></p></div>
+</div>
+
+
+<hr /><p><span class='pagenum'><a name="Page_351" id="Page_351">[Pg 351]</a></span></p>
+<h3><a name="INDEX" id="INDEX"></a>INDEX</h3>
+
+
+<p class="index">
+<span class="smcap">Achromatic</span> telescope, <a href="#Page_115">115</a>, <a href="#Page_116">116</a><br />
+<br />
+Adams, <a href="#Page_24">24</a>, <a href="#Page_236">236</a>, <a href="#Page_243">243</a><br />
+<br />
+Aerial telescopes, <a href="#Page_110">110</a>, <a href="#Page_111">111</a><br />
+<br />
+Agathocles, Eclipse of, <a href="#Page_85">85</a><br />
+<br />
+Agrippa, Camillus, <a href="#Page_44">44</a><br />
+<br />
+Ahaz, dial of, <a href="#Page_85">85</a><br />
+<br />
+Air, <a href="#Page_166">166</a><br />
+<br />
+Airy, Sir G.B., <a href="#Page_92">92</a><br />
+<br />
+Al g&ucirc;l, <a href="#Page_307">307</a><br />
+<br />
+Al Sufi, <a href="#Page_284">284</a>, <a href="#Page_290">290</a>, <a href="#Page_296">296</a>, <a href="#Page_315">315</a><br />
+<br />
+Alcor, <a href="#Page_294">294</a><br />
+<br />
+Alcyone, <a href="#Page_284">284</a><br />
+<br />
+Aldebaran, <a href="#Page_103">103</a>, <a href="#Page_288">288</a>, <a href="#Page_290">290</a>, <a href="#Page_297">297</a><br />
+<br />
+<a name="Algol" id="Algol"></a>Algol, <a href="#Page_307">307</a>, <a href="#Page_309">309&ndash;310</a>, <a href="#Page_312">312</a>, <a href="#Page_323">323</a>, <a href="#Page_347">347</a><br />
+<br />
+Alpha, Centauri, <a href="#Page_52">52&ndash;53</a>, <a href="#Page_280">280</a>, <a href="#Page_298">298&ndash;299</a>, <a href="#Page_304">304</a>, <a href="#Page_320">320</a><br />
+<br />
+Alpha Crucis, <a href="#Page_298">298</a><br />
+<br />
+Alps, Lunar, <a href="#Page_200">200</a><br />
+<br />
+Altair, <a href="#Page_295">295</a><br />
+<br />
+Altitude of objects in sky, <a href="#Page_196">196</a><br />
+<br />
+Aluminium, <a href="#Page_145">145</a><br />
+<br />
+Amos viii. 9, <a href="#Page_85">85</a><br />
+<br />
+Anderson, T.D., <a href="#Page_311">311&ndash;312</a><br />
+<br />
+Andromeda (constellation), <a href="#Page_279">279</a>, <a href="#Page_314">314</a>;<br />
+<span style="margin-left: 1em;">Great Nebula in, <a href="#Page_314">314</a>, <a href="#Page_316">316</a></span><br />
+<br />
+Andromedid meteors, <a href="#Page_272">272</a><br />
+<br />
+Anglo-Saxon Chronicle, <a href="#Page_87">87&ndash;88</a><br />
+<br />
+Anighito meteorite, <a href="#Page_277">277</a><br />
+<br />
+Annular eclipse, <a href="#Page_65">65&ndash;68</a>, <a href="#Page_80">80</a>, <a href="#Page_92">92</a>, <a href="#Page_99">99</a><br />
+<br />
+Annular Nebula in Lyra, <a href="#Page_315">315&ndash;316</a><br />
+<br />
+Annulus, <a href="#Page_68">68</a><br />
+<br />
+Ans&aelig;, <a href="#Page_242">242&ndash;243</a><br />
+<br />
+Anticipation in discovery, <a href="#Page_236">236&ndash;237</a><br />
+<br />
+Apennines, Lunar, <a href="#Page_200">200</a><br />
+<br />
+Aphelion, <a href="#Page_274">274</a><br />
+<br />
+Apparent enlargement of celestial objects, <a href="#Page_192">192&ndash;196</a><br />
+<br />
+Apparent size of celestial objects deceptive, <a href="#Page_196">196</a>, <a href="#Page_294">294</a><br />
+<br />
+Apparent sizes of sun and moon, variations in, <a href="#Page_67">67</a>, <a href="#Page_80">80</a>, <a href="#Page_178">178</a><br />
+<br />
+Aquila (constellation), <a href="#Page_295">295</a><br />
+<br />
+Arabian astronomers, <a href="#Page_107">107</a>, <a href="#Page_307">307</a><br />
+<br />
+Arago, <a href="#Page_92">92</a>, <a href="#Page_257">257</a><br />
+<br />
+Arc, degrees minutes and seconds of, <a href="#Page_60">60</a><br />
+<br />
+Arcturus, <a href="#Page_280">280</a>, <a href="#Page_282">282</a>, <a href="#Page_290">290</a>, <a href="#Page_295">295</a><br />
+<br />
+Argelander, <a href="#Page_290">290</a><br />
+<br />
+Argo (constellation), <a href="#Page_298">298</a><br />
+<br />
+Aristarchus of Samos, <a href="#Page_171">171</a><br />
+<br />
+Aristarchus (lunar crater), <a href="#Page_205">205</a><br />
+<br />
+Aristophanes, <a href="#Page_101">101</a><br />
+<br />
+Aristotle, <a href="#Page_161">161</a>, <a href="#Page_173">173</a>, <a href="#Page_185">185</a><br />
+<br />
+Arrhenius <a href="#Page_222">222</a>, <a href="#Page_253">253&ndash;254</a><br />
+<br />
+Assyrian tablet, <a href="#Page_84">84</a><br />
+<br />
+Asteroidal zone, analogy of, to Saturn's rings, <a href="#Page_238">238</a><br />
+<br />
+<a name="Asteroids" id="Asteroids"></a>Asteroids (or minor planets), <a href="#Page_30">30&ndash;31</a>, <a href="#Page_225">225&ndash;228</a>, <a href="#Page_336">336</a>;<br />
+<span style="margin-left: 1em;">discovery of the, <a href="#Page_23">23</a>, <a href="#Page_244">244</a>;</span><br />
+<span style="margin-left: 1em;">Wolf's method of discovering, <a href="#Page_226">226&ndash;227</a></span><br />
+<br />
+Astrology, <a href="#Page_56">56</a><br />
+<br />
+<i>Astronomical Essays</i>, <a href="#Page_63">63</a>, <a href="#Page_337">337</a><br />
+<br />
+Astronomical Society, Royal, <a href="#Page_144">144</a><br />
+<br />
+<i>Astronomy, Manual of</i>, <a href="#Page_166">166</a><br />
+<br />
+Atlantic Ocean, parallelism of opposite shores, <a href="#Page_340">340&ndash;341</a><br />
+<br />
+Atlas, the Titan, <a href="#Page_18">18</a><br />
+<br />
+Atmosphere, absorption by earth's, <a href="#Page_129">129&ndash;130</a>;<br />
+<span style="margin-left: 1em;">ascertainment of, by spectroscope, <a href="#Page_124">124&ndash;125</a>, <a href="#Page_212">212</a>;</span><br />
+<span style="margin-left: 1em;">height of earth's, <a href="#Page_167">167</a>, <a href="#Page_267">267</a>;</span><br />
+<span style="margin-left: 1em;">of asteroids, <a href="#Page_226">226</a>;</span><br />
+<span style="margin-left: 1em;">of earth, <a href="#Page_129">129</a>, <a href="#Page_130">130</a>, <a href="#Page_166">166&ndash;169</a>, <a href="#Page_218">218</a>, <a href="#Page_222">222</a>, <a href="#Page_267">267</a>, <a href="#Page_346">346</a>;</span><br />
+<span style="margin-left: 1em;">of Mars, <a href="#Page_156">156</a>, <a href="#Page_212">212</a>, <a href="#Page_216">216</a>;</span><br />
+<span style="margin-left: 1em;">of Mercury, <a href="#Page_156">156</a>;</span><br />
+<span style="margin-left: 1em;">of moon, <a href="#Page_70">70&ndash;71</a>, <a href="#Page_156">156</a>, <a href="#Page_201">201&ndash;203</a>;</span><br />
+<span style="margin-left: 1em;">of Jupiter, <a href="#Page_231">231</a>;</span><br />
+<span style="margin-left: 1em;">of planets, <a href="#Page_125">125</a>;</span><br />
+<span style="margin-left: 1em;">of Saturn's rings, <a href="#Page_239">239</a></span><br />
+<br />
+"Atmosphere" of the stars, <a href="#Page_331">331</a><br />
+<br />
+Atmospheric layer and "glass-house" compared, <a href="#Page_167">167</a>, <a href="#Page_203">203</a><br />
+<br />
+August Meteors (Perseids), <a href="#Page_270">270</a><br />
+<br />
+<span class='pagenum'><a name="Page_352" id="Page_352">[Pg 352]</a></span>Auriga (constellation), <a href="#Page_294">294&ndash;296</a>, <a href="#Page_306">306</a>, <a href="#Page_311">311</a>;<br />
+<span style="margin-left: 1em;">New Star in, <a href="#Page_311">311</a></span><br />
+<br />
+Aurig&aelig;, &#946; (Beta), <a href="#Page_294">294</a>, <a href="#Page_297">297</a>, <a href="#Page_304">304</a><br />
+<br />
+Aurora Borealis, <a href="#Page_141">141</a>, <a href="#Page_143">143</a>, <a href="#Page_259">259</a><br />
+<br />
+Australia, suggested origin of, <a href="#Page_340">340</a><br />
+<br />
+Axis, <a href="#Page_29">29&ndash;30</a>;<br />
+<span style="margin-left: 1em;">of earth, <a href="#Page_163">163</a>, <a href="#Page_180">180</a>;</span><br />
+<span style="margin-left: 1em;">small movement of earth's, <a href="#Page_180">180&ndash;181</a></span><br />
+<br />
+<br />
+<span class="smcap">Babylonian</span> tablet, <a href="#Page_84">84</a><br />
+<br />
+Babylonian idea of the moon, <a href="#Page_185">185</a><br />
+<br />
+Bacon, Roger, <a href="#Page_108">108</a><br />
+<br />
+Bacubirito meteorite, <a href="#Page_277">277</a><br />
+<br />
+Bagdad, <a href="#Page_107">107</a><br />
+<br />
+Baily, Francis, <a href="#Page_92">92</a><br />
+<br />
+"Baily's Beads," <a href="#Page_69">69</a>, <a href="#Page_70">70</a>, <a href="#Page_91">91&ndash;92</a>, <a href="#Page_154">154</a><br />
+<br />
+Bailly (lunar crater), <a href="#Page_199">199</a><br />
+<br />
+Ball, Sir Robert, <a href="#Page_271">271</a><br />
+<br />
+Barnard, E.E., <a href="#Page_31">31</a>, <a href="#Page_224">224</a>, <a href="#Page_232">232&ndash;234</a>, <a href="#Page_237">237</a>, <a href="#Page_258">258</a><br />
+<br />
+"Bay of Rainbows," <a href="#Page_197">197</a><br />
+<br />
+Bayer's classification of stars, <a href="#Page_289">289</a>, <a href="#Page_291">291&ndash;292</a><br />
+<br />
+Bayeux Tapestry, <a href="#Page_263">263</a><br />
+<br />
+Bear, Great (constellation). <a href="#Ursa_Major"><i>See</i> Ursa Major</a>;<br />
+<span style="margin-left: 1em;">Little, <a href="#Ursa_minor"><i>see</i> Ursa Minor</a></span><br />
+<br />
+Beehive (Pr&aelig;sepe), <a href="#Page_307">307</a><br />
+<br />
+Beer, <a href="#Page_206">206</a><br />
+<br />
+Belopolsky, <a href="#Page_304">304</a><br />
+<br />
+"Belt" of Orion, <a href="#Page_297">297</a><br />
+<br />
+Belt theory of Milky Way, <a href="#Page_321">321</a><br />
+<br />
+Belts of Jupiter, <a href="#Page_230">230</a><br />
+<br />
+Bergstrand, <a href="#Page_314">314</a><br />
+<br />
+Berlin star chart, <a href="#Page_244">244</a><br />
+<br />
+Bessel, <a href="#Page_173">173</a>, <a href="#Page_280">280</a>, <a href="#Page_305">305</a><br />
+<br />
+Beta (&#946;) Lyr&aelig;, <a href="#Page_307">307</a><br />
+<br />
+Beta (&#946;) Persei. <a href="#Algol"><i>See</i> Algol</a><br />
+<br />
+Betelgeux, <a href="#Page_297">297</a><br />
+<br />
+Bible, eclipses in, <a href="#Page_85">85</a><br />
+<br />
+Biela's Comet, <a href="#Page_256">256&ndash;257</a>, <a href="#Page_272">272&ndash;273</a>, <a href="#Page_345">345</a><br />
+<br />
+Bielids, <a href="#Page_270">270</a>, <a href="#Page_272">272&ndash;273</a><br />
+<br />
+Billion, <a href="#Page_51">51&ndash;52</a><br />
+<br />
+Binary stars, spectroscopic, <a href="#Page_301">301&ndash;306</a>, <a href="#Page_309">309</a>;<br />
+<span style="margin-left: 1em;">visual, <a href="#Page_300">300</a>, <a href="#Page_303">303&ndash;306</a></span><br />
+<br />
+"Black Drop," <a href="#Page_152">152&ndash;154</a><br />
+<br />
+"Black Hour," <a href="#Page_89">89</a><br />
+<br />
+"Black Saturday," <a href="#Page_89">89</a><br />
+<br />
+Blood, moon in eclipse like, <a href="#Page_102">102</a><br />
+<br />
+Blue (rays of light), <a href="#Page_121">121</a>, <a href="#Page_130">130</a><br />
+<br />
+Bode's Law, <a href="#Page_22">22&ndash;23</a>, <a href="#Page_244">244&ndash;245</a><br />
+<br />
+Bolometer, <a href="#Page_127">127</a><br />
+<br />
+Bond, G.P., <a href="#Page_236">236</a>, <a href="#Page_257">257</a><br />
+<br />
+Bonpland, <a href="#Page_270">270</a><br />
+<br />
+Bo&ouml;tes (constellation), <a href="#Page_295">295</a>, <a href="#Page_314">314</a><br />
+<br />
+Bradley, <a href="#Page_111">111</a><br />
+<br />
+Brahe, Tycho, <a href="#Page_290">290</a>, <a href="#Page_311">311</a><br />
+<br />
+Br&eacute;dikhine's theory of comets' tails, <a href="#Page_253">253&ndash;254</a>, <a href="#Page_256">256</a><br />
+<br />
+Bright eclipses of moon, <a href="#Page_65">65</a>, <a href="#Page_102">102</a><br />
+<br />
+British Association for the Advancement of Science, <a href="#Page_318">318</a><br />
+<br />
+<i>British Astronomical Association, Journal of</i>, <a href="#Page_194">194</a><br />
+<br />
+British Museum, <a href="#Page_84">84</a><br />
+<br />
+Bull (constellation). <a href="#Taurus"><i>See</i> Taurus</a>;<br />
+<span style="margin-left: 1em;">"Eye" of the, <a href="#Page_297">297</a>;</span><br />
+<span style="margin-left: 1em;">"Head" of the, <a href="#Page_297">297</a></span><br />
+<br />
+Burgos, <a href="#Page_98">98</a><br />
+<br />
+Busch, <a href="#Page_93">93</a><br />
+<br />
+<br />
+<span class="smcap">C&aelig;sar</span>, Julius, <a href="#Page_85">85</a>, <a href="#Page_110">110</a>, <a href="#Page_180">180</a>, <a href="#Page_259">259</a>, <a href="#Page_262">262</a>, <a href="#Page_291">291</a>, <a href="#Page_293">293</a><br />
+<br />
+Calcium, <a href="#Page_138">138</a>, <a href="#Page_145">145</a><br />
+<br />
+Callisto, <a href="#Page_233">233&ndash;234</a><br />
+<br />
+Cambridge, <a href="#Page_24">24</a>, <a href="#Page_91">91</a>, <a href="#Page_119">119</a>, <a href="#Page_243">243</a><br />
+<br />
+Campbell, <a href="#Page_305">305</a><br />
+<br />
+Canali, <a href="#Page_214">214</a><br />
+<br />
+"Canals" of Mars, <a href="#Page_214">214&ndash;222</a>, <a href="#Page_224">224&ndash;225</a><br />
+<br />
+Cancer (constellation), <a href="#Page_307">307</a><br />
+<br />
+Canes Venatici (constellation), <a href="#Page_306">306</a>, <a href="#Page_314">314</a><br />
+<br />
+<a name="Canis_Major" id="Canis_Major"></a>Canis Major (constellation), <a href="#Page_289">289</a>, <a href="#Page_296">296&ndash;297</a>;<br />
+<span style="margin-left: 1em;"><a name="Canis_Minor" id="Canis_Minor"></a>Minor, <a href="#Page_296">296&ndash;297</a></span><br />
+<br />
+Canopus, <a href="#Page_285">285</a>, <a href="#Page_298">298&ndash;299</a>, <a href="#Page_320">320</a><br />
+<br />
+Capella, <a href="#Page_280">280</a>, <a href="#Page_282">282</a>, <a href="#Page_290">290</a>, <a href="#Page_294">294</a>, <a href="#Page_297">297</a>, <a href="#Page_303">303</a>, <a href="#Page_313">313</a><br />
+<br />
+Carbon, <a href="#Page_145">145</a><br />
+<br />
+Carbon dioxide. <a href="#Carbonic_acid_gas"><i>See</i> Carbonic acid gas</a><br />
+<br />
+<a name="Carbonic_acid_gas" id="Carbonic_acid_gas"></a>Carbonic acid gas, <a href="#Page_166">166</a>, <a href="#Page_213">213</a>, <a href="#Page_221">221&ndash;222</a><br />
+<br />
+Carnegie Institution, Solar Observatory of, <a href="#Page_118">118</a><br />
+<br />
+Cassegrainian telescope, <a href="#Page_114">114</a>, <a href="#Page_118">118</a><br />
+<br />
+Cassini, J.D., <a href="#Page_236">236</a>, <a href="#Page_240">240</a><br />
+<br />
+"Cassini's Division" in Saturn's ring, <a href="#Page_236">236</a>, <a href="#Page_238">238</a><br />
+<br />
+Cassiopeia (constellation), <a href="#Page_279">279</a>, <a href="#Page_294">294</a>, <a href="#Page_311">311</a>, <a href="#Page_314">314</a><br />
+<br />
+Cassiopei&aelig;, &#951; (Eta), <a href="#Page_303">303</a><br />
+<br />
+Cassiopeia's Chair, <a href="#Page_294">294</a><br />
+<br />
+Cassius, Dion, <a href="#Page_86">86</a><br />
+<br />
+Castor, <a href="#Page_282">282</a>, <a href="#Page_297">297</a>, <a href="#Page_304">304</a><br />
+<br />
+<span class='pagenum'><a name="Page_353" id="Page_353">[Pg 353]</a></span>Catalogues of stars, <a href="#Page_106">106</a>, <a href="#Page_290">290&ndash;291</a>, <a href="#Page_311">311</a><br />
+<br />
+Centaur. <a href="#Centaurus"><i>See</i> Centaurus</a><br />
+<br />
+<a name="Centaurus" id="Centaurus"></a>Centaurus (constellation), <a href="#Page_298">298</a>, <a href="#Page_306">306</a><br />
+<br />
+Centre of gravity, <a href="#Page_42">42</a>, <a href="#Page_283">283&ndash;284</a>, <a href="#Page_324">324</a><br />
+<br />
+Ceres, diameter of, <a href="#Page_30">30</a>, <a href="#Page_225">225</a><br />
+<br />
+Ceti, Omicron (or Mira), <a href="#Page_307">307&ndash;308</a><br />
+<br />
+<a name="Cetus" id="Cetus"></a>Cetus, or the Whale (constellation), <a href="#Page_307">307</a><br />
+<br />
+Chaldean astronomers, <a href="#Page_74">74</a>, <a href="#Page_76">76</a><br />
+<br />
+Challis, <a href="#Page_243">243&ndash;244</a><br />
+<br />
+Chamberlin, <a href="#Page_337">337</a><br />
+<br />
+"Chambers of the South," <a href="#Page_299">299</a><br />
+<br />
+Chandler, <a href="#Page_308">308</a><br />
+<br />
+Charles V., <a href="#Page_261">261</a><br />
+<br />
+"Charles' Wain," <a href="#Page_291">291</a><br />
+<br />
+Chemical rays, <a href="#Page_127">127</a><br />
+<br />
+Chinese and eclipses, <a href="#Page_83">83</a><br />
+<br />
+Chloride of sodium, <a href="#Page_122">122</a><br />
+<br />
+Chlorine, <a href="#Page_122">122</a>, <a href="#Page_145">145</a><br />
+<br />
+Christ, Birth of, <a href="#Page_102">102</a><br />
+<br />
+Christian Era, first recorded solar eclipse in, <a href="#Page_85">85</a><br />
+<br />
+Chromatic aberration, <a href="#Page_110">110</a><br />
+<br />
+Chromosphere, <a href="#Page_71">71&ndash;72</a>, <a href="#Page_93">93&ndash;94</a>, <a href="#Page_130">130&ndash;132</a>, <a href="#Page_138">138&ndash;139</a><br />
+<br />
+Circle, <a href="#Page_171">171&ndash;173</a><br />
+<br />
+Clark, Alvan, &amp; Sons, <a href="#Page_117">117&ndash;118</a>, <a href="#Page_303">303</a><br />
+<br />
+Claudius, Emperor, <a href="#Page_86">86</a><br />
+<br />
+Clavius (lunar crater), <a href="#Page_199">199</a><br />
+<br />
+Clerk Maxwell, <a href="#Page_237">237</a><br />
+<br />
+"Clouds" (of Aristophanes), <a href="#Page_101">101</a><br />
+<br />
+Clustering power, <a href="#Page_325">325</a><br />
+<br />
+Clusters of stars, <a href="#Page_300">300</a>, <a href="#Page_306">306</a>, <a href="#Page_314">314</a>, <a href="#Page_328">328</a><br />
+<br />
+Coal Sacks. <a href="#Holes_in_Milky_Way"><i>See</i> Holes in Milky Way</a><br />
+<br />
+C&#339;lostat, <a href="#Page_119">119</a><br />
+<br />
+Coggia's Comet, <a href="#Page_254">254</a><br />
+<br />
+Colour, production of, in telescopes, <a href="#Page_109">109&ndash;111</a>, <a href="#Page_115">115</a>, <a href="#Page_121">121</a><br />
+<br />
+Collision of comet with earth, <a href="#Page_345">345&ndash;346</a>;<br />
+<span style="margin-left: 1em;">of dark star with sun, <a href="#Page_346">346&ndash;348</a>;</span><br />
+<span style="margin-left: 1em;">of stars, <a href="#Page_285">285</a>, <a href="#Page_312">312</a></span><br />
+<br />
+Columbus, <a href="#Page_103">103</a><br />
+<br />
+Coma Berenices (constellation), <a href="#Page_307">307</a>, <a href="#Page_316">316</a><br />
+<br />
+Comet, first discovery of by photography, <a href="#Page_258">258</a>;<br />
+<span style="margin-left: 1em;">first orbit calculated, <a href="#Page_255">255</a>;</span><br />
+<span style="margin-left: 1em;">first photograph of, <a href="#Page_257">257&ndash;258</a>;</span><br />
+<span style="margin-left: 1em;">furthest distance seen, <a href="#Page_258">258</a>;</span><br />
+<span style="margin-left: 1em;">passage of among satellites of Jupiter, <a href="#Page_250">250</a>;</span><br />
+<span style="margin-left: 1em;">passage of earth and moon through tail of, <a href="#Page_257">257</a>, <a href="#Page_346">346</a></span><br />
+<br />
+Comet of 1000 <span class="ampm">A.D.</span>, <a href="#Page_262">262</a>;<br />
+<span style="margin-left: 1em;">1066, <a href="#Page_262">262&ndash;264</a>;</span><br />
+<span style="margin-left: 1em;">1680, <a href="#Page_255">255</a>, <a href="#Page_265">265</a>;</span><br />
+<span style="margin-left: 1em;">1811, <a href="#Page_254">254&ndash;255</a>;</span><br />
+<span style="margin-left: 1em;">1861, <a href="#Page_254">254</a>, <a href="#Page_257">257</a>, <a href="#Page_346">346</a>;</span><br />
+<span style="margin-left: 1em;">1881, <a href="#Page_257">257&ndash;258</a>;</span><br />
+<span style="margin-left: 1em;">1882, <a href="#Page_251">251</a>, <a href="#Page_258">258</a>, <a href="#Page_291">291</a>;</span><br />
+<span style="margin-left: 1em;">1889, <a href="#Page_258">258</a>;</span><br />
+<span style="margin-left: 1em;">1907, <a href="#Page_258">258</a></span><br />
+<br />
+Comets, <a href="#Page_27">27&ndash;28</a>, <a href="#Page_58">58</a>, <a href="#CHAPTER_XIX">Chaps. XIX.</a> and <a href="#CHAPTER_XX">XX.</a>, <a href="#Page_345">345&ndash;346</a>;<br />
+<span style="margin-left: 1em;">ancient view of, <a href="#Page_259">259&ndash;261</a>;</span><br />
+<span style="margin-left: 1em;">captured, <a href="#Page_251">251&ndash;253</a>;</span><br />
+<span style="margin-left: 1em;">Chinese records of, <a href="#Page_83">83&ndash;84</a>;</span><br />
+<span style="margin-left: 1em;">composition of, <a href="#Page_252">252</a>;</span><br />
+<span style="margin-left: 1em;">contrasted with planets, <a href="#Page_247">247</a>;</span><br />
+<span style="margin-left: 1em;">families of, <a href="#Page_251">251&ndash;252</a>, <a href="#Page_256">256</a>;</span><br />
+<span style="margin-left: 1em;">meteor swarms and, <a href="#Page_274">274</a>;</span><br />
+<span style="margin-left: 1em;">revealed by solar eclipses, <a href="#Page_95">95&ndash;96</a>;</span><br />
+<span style="margin-left: 1em;">tails of, <a href="#Page_141">141</a>, <a href="#Page_182">182</a>, <a href="#Page_248">248</a>, <a href="#Page_252">252&ndash;254</a></span><br />
+<br />
+Common, telescopes of Dr. A.A., <a href="#Page_118">118</a><br />
+<br />
+Conjunction, <a href="#Page_209">209</a><br />
+<br />
+Constellations, <a href="#Page_105">105</a>, <a href="#Page_278">278&ndash;279</a>, <a href="#Page_285">285</a>, <a href="#Page_289">289</a><br />
+<br />
+Contraction theory of sun's heat, <a href="#Page_128">128&ndash;129</a>, <a href="#Page_335">335</a><br />
+<br />
+Cook, Captain, <a href="#Page_154">154</a><br />
+<br />
+Cooke, <a href="#Page_118">118</a><br />
+<br />
+Copernican system, <a href="#Page_20">20</a>, <a href="#Page_107">107</a>, <a href="#Page_149">149</a>, <a href="#Page_170">170&ndash;173</a>, <a href="#Page_279">279</a>, <a href="#Page_280">280</a><br />
+<br />
+Copernicus, <a href="#Page_20">20</a>, <a href="#Page_108">108</a>, <a href="#Page_149">149</a>, <a href="#Page_158">158</a>, <a href="#Page_170">170&ndash;172</a>, <a href="#Page_236">236</a><br />
+<br />
+Copernicus (lunar crater), <a href="#Page_200">200</a>, <a href="#Page_204">204</a><br />
+<br />
+Copper, <a href="#Page_145">145</a><br />
+<br />
+Corder, H., <a href="#Page_144">144</a><br />
+<br />
+Corona, <a href="#Page_70">70&ndash;72</a>, <a href="#Page_90">90</a>, <a href="#Page_92">92&ndash;97</a>, <a href="#Page_132">132</a>, <a href="#Page_140">140&ndash;141</a>, <a href="#Page_270">270</a>;<br />
+<span style="margin-left: 1em;">earliest drawing of, <a href="#Page_91">91</a>;</span><br />
+<span style="margin-left: 1em;">earliest employment of term, <a href="#Page_90">90</a>;</span><br />
+<span style="margin-left: 1em;">earliest mention of, <a href="#Page_86">86</a>;</span><br />
+<span style="margin-left: 1em;">earliest photograph of, <a href="#Page_93">93</a>;</span><br />
+<span style="margin-left: 1em;">illumination given by, <a href="#Page_71">71</a>;</span><br />
+<span style="margin-left: 1em;">possible change in shape of during eclipse, <a href="#Page_96">96&ndash;98</a>;</span><br />
+<span style="margin-left: 1em;">structure of, <a href="#Page_142">142&ndash;143</a>;</span><br />
+<span style="margin-left: 1em;">variations in shape of, <a href="#Page_141">141</a></span><br />
+<br />
+Corona Borealis (constellation), <a href="#Page_295">295</a><br />
+<br />
+Coronal matter, <a href="#Page_142">142</a>;<br />
+<span style="margin-left: 1em;">streamers, <a href="#Page_95">95&ndash;96</a>, <a href="#Page_141">141&ndash;143</a></span><br />
+<br />
+Coronium, <a href="#Page_133">133</a>, <a href="#Page_142">142</a>, <a href="#Page_317">317</a><br />
+<br />
+Cotes, <a href="#Page_91">91</a><br />
+<br />
+Coud&eacute;, equatorial, <a href="#Page_119">119</a><br />
+<br />
+Cowell, P.H., <a href="#Page_255">255</a>, <a href="#Page_264">264</a><br />
+<br />
+Crabtree, <a href="#Page_152">152</a><br />
+<br />
+Crape ring of Saturn, <a href="#Page_236">236&ndash;237</a><br />
+<br />
+Craterlets on Mars, <a href="#Page_220">220</a><br />
+<br />
+<a name="Craters" id="Craters"></a>Craters (ring-mountains) on moon, <a href="#Page_197">197&ndash;205</a>, <a href="#Page_214">214</a>, <a href="#Page_340">340</a>;<br />
+<span style="margin-left: 1em;">suggested origin of, <a href="#Page_203">203&ndash;204</a>, <a href="#Page_214">214</a></span><br />
+<br />
+<span class='pagenum'><a name="Page_354" id="Page_354">[Pg 354]</a></span>Crawford, Earl of, <a href="#Page_94">94</a><br />
+<br />
+Crecy, supposed eclipse at battle of, <a href="#Page_88">88&ndash;89</a><br />
+<br />
+Crescent moon, <a href="#Page_183">183</a>, <a href="#Page_185">185</a><br />
+<br />
+Crommelin, A.C.D., <a href="#Page_255">255</a>, <a href="#Page_264">264</a><br />
+<br />
+Crossley Reflector, <a href="#Page_118">118</a>, <a href="#Page_315">315&ndash;316</a><br />
+<br />
+Crown glass, <a href="#Page_115">115</a><br />
+<br />
+Crucifixion, darkness of, <a href="#Page_86">86</a><br />
+<br />
+Crucis, &#945; (Alpha), <a href="#Page_298">298</a><br />
+<br />
+<a name="Crux" id="Crux"></a>Crux, or "Southern Cross" (constellation), <a href="#Page_298">298&ndash;299</a>, <a href="#Page_323">323</a><br />
+<br />
+Cycle, sunspot, <a href="#Page_136">136&ndash;137</a>, <a href="#Page_141">141</a>, <a href="#Page_143">143&ndash;144</a><br />
+<br />
+Cygni, <a href="#Page_61">61</a>, <a href="#Page_173">173</a>, <a href="#Page_280">280</a><br />
+<br />
+<a name="Cygnus" id="Cygnus"></a>Cygnus, or the Swan (constellation), <a href="#Page_295">295</a>, <a href="#Page_325">325</a><br />
+<br />
+<br />
+<span class="smcap">Daniel's</span> Comet of 1897, <a href="#Page_258">258</a><br />
+<br />
+Danzig, <a href="#Page_111">111</a><br />
+<br />
+Dark Ages, <a href="#Page_102">102</a>, <a href="#Page_107">107</a>, <a href="#Page_260">260</a><br />
+<br />
+Dark eclipses of moon, <a href="#Page_65">65</a>, <a href="#Page_102">102&ndash;103</a><br />
+<br />
+Dark matter in space, <a href="#Page_323">323</a><br />
+<br />
+Dark meteors, <a href="#Page_275">275&ndash;276</a><br />
+<br />
+Dark stars, <a href="#Page_309">309&ndash;310</a>, <a href="#Page_312">312</a>, <a href="#Page_323">323</a>, <a href="#Page_346">346&ndash;347</a><br />
+<br />
+"Darkness behind the stars," <a href="#Page_325">325</a><br />
+<br />
+Darwin, Sir G.H., <a href="#Page_339">339</a><br />
+<br />
+Davis, <a href="#Page_94">94</a><br />
+<br />
+Dawes, <a href="#Page_236">236</a><br />
+<br />
+Dearborn Observatory, <a href="#Page_303">303</a><br />
+<br />
+Death from fright at eclipse, <a href="#Page_73">73</a><br />
+<br />
+Debonnaire, Louis le, <a href="#Page_88">88</a>, <a href="#Page_261">261</a><br />
+<br />
+Deimos, <a href="#Page_223">223</a><br />
+<br />
+Deity, symbol of the, <a href="#Page_87">87</a><br />
+<br />
+"Demon star." <a href="#Algol"><i>See</i> Algol</a><br />
+<br />
+Denebola, <a href="#Page_296">296</a><br />
+<br />
+Denning, W.F., <a href="#Page_269">269</a><br />
+<br />
+Densities of sun and planets, <a href="#Page_39">39</a><br />
+<br />
+Density, <a href="#Page_38">38</a><br />
+<br />
+Deslandres, <a href="#Page_140">140</a><br />
+<br />
+Diameters of sun and planets, <a href="#Page_31">31</a><br />
+<br />
+Disappearance of moon in lunar eclipse, <a href="#Page_65">65</a>, <a href="#Page_102">102&ndash;103</a><br />
+<br />
+Disc, <a href="#Page_60">60</a><br />
+<br />
+"Disc" theory. <a href="#Grindstone"><i>See</i> "Grindstone" theory</a><br />
+<br />
+Discoveries, independent, <a href="#Page_236">236</a><br />
+<br />
+Discovery, anticipation in, <a href="#Page_236">236&ndash;237</a>;<br />
+<span style="margin-left: 1em;">indirect methods of, <a href="#Page_120">120</a></span><br />
+<br />
+"Dipper," the, <a href="#Page_291">291</a>;<br />
+<span style="margin-left: 1em;">the "Little," <a href="#Page_294">294</a></span><br />
+<br />
+Distance of a celestial body, how ascertained, <a href="#Page_56">56&ndash;58</a>;<br />
+<span style="margin-left: 1em;">of sun from earth, how determined, <a href="#Page_151">151</a>, <a href="#Page_211">211</a></span><br />
+<br />
+Distances of planets from sun, <a href="#Page_47">47</a><br />
+<br />
+Distances of sun and moon, relative, <a href="#Page_68">68</a><br />
+<br />
+Dog, the Greater. <a href="#Canis_Major"><i>See</i> Canis Major</a>;<br />
+<span style="margin-left: 1em;">the Lesser, <a href="#Canis_Minor"><i>see</i> Canis Minor</a></span><br />
+<br />
+"Dog Star," <a href="#Page_289">289</a>, <a href="#Page_297">297</a><br />
+<br />
+Dollond, John, <a href="#Page_115">115&ndash;116</a><br />
+<br />
+Donati's Comet, <a href="#Page_254">254</a>, <a href="#Page_257">257</a><br />
+<br />
+Doppler's method, <a href="#Page_125">125</a>, <a href="#Page_136">136</a>, <a href="#Page_282">282</a>, <a href="#Page_301">301&ndash;302</a><br />
+<br />
+Dorpat, <a href="#Page_117">117</a><br />
+<br />
+Double canals of Mars, <a href="#Page_214">214&ndash;215</a>, <a href="#Page_218">218&ndash;220</a><br />
+<br />
+Double planet, earth and moon a, <a href="#Page_189">189</a><br />
+<br />
+Double stars, <a href="#Page_300">300</a><br />
+<br />
+Douglass, <a href="#Page_233">233</a><br />
+<br />
+"Dreams, Lake of," <a href="#Page_197">197</a><br />
+<br />
+Dumb-bell Nebula, <a href="#Page_316">316</a><br />
+<br />
+<br />
+<span class="smcap">Earth</span>, <a href="#Page_20">20</a>, <a href="#Page_22">22</a>, <a href="#Page_31">31</a>, <a href="#Page_39">39</a>, <a href="#Page_48">48</a>, <a href="#Page_64">64</a>, <a href="#CHAPTER_XV">Chap. XV.</a>, <a href="#Page_267">267</a>;<br />
+<span style="margin-left: 1em;">cooling of, <a href="#Page_343">343</a>;</span><br />
+<span style="margin-left: 1em;">diameter of, <a href="#Page_31">31</a>;</span><br />
+<span style="margin-left: 1em;">interior of, <a href="#Page_166">166</a>;</span><br />
+<span style="margin-left: 1em;">mean distance of from sun, <a href="#Page_47">47</a>;</span><br />
+<span style="margin-left: 1em;">rigidity of, <a href="#Page_181">181</a>;</span><br />
+<span style="margin-left: 1em;">rotation of, <a href="#Page_30">30</a>, <a href="#Page_33">33</a>, <a href="#Page_161">161&ndash;165</a>, <a href="#Page_170">170</a>;</span><br />
+<span style="margin-left: 1em;">shape of, <a href="#Page_165">165</a>;</span><br />
+<span style="margin-left: 1em;">"tail" to, <a href="#Page_182">182</a></span><br />
+<br />
+"Earthlight," or "Earthshine," <a href="#Page_186">186</a><br />
+<br />
+Earth's axis, Precessional movement of, <a href="#Page_175">175&ndash;177</a>, <a href="#Page_295">295</a>, <a href="#Page_298">298&ndash;299</a><br />
+<br />
+Earth's shadow, circular shape of, <a href="#Page_64">64</a>, <a href="#Page_160">160</a><br />
+<br />
+Eclipse, <a href="#Page_61">61</a><br />
+<br />
+Eclipse knowledge, delay of, <a href="#Page_74">74</a><br />
+<br />
+Eclipse party, work of, <a href="#Page_73">73</a><br />
+<br />
+Eclipse of sun, advance of shadow in total, <a href="#Page_69">69</a>;<br />
+<span style="margin-left: 1em;">animal and plant life during, <a href="#Page_71">71</a>;</span><br />
+<span style="margin-left: 1em;">earliest record of total, <a href="#Page_84">84</a>;</span><br />
+<span style="margin-left: 1em;">description of total, <a href="#Page_69">69&ndash;73</a>;</span><br />
+<span style="margin-left: 1em;">duration of total, <a href="#Page_69">69</a>, <a href="#Page_72">72</a>;</span><br />
+<span style="margin-left: 1em;">importance of total, <a href="#Page_68">68</a></span><br />
+<br />
+Eclipses, ascertainment of dates of past, <a href="#Page_74">74</a>;<br />
+<span style="margin-left: 1em;">experience a necessity in solar, <a href="#Page_73">73&ndash;74</a>;</span><br />
+<span style="margin-left: 1em;">of moon, <a href="#Page_63">63&ndash;65</a>, <a href="#CHAPTER_IX">Chap. IX.</a>, <a href="#Page_203">203</a>;</span><br />
+<span style="margin-left: 1em;">photography in, <a href="#Page_93">93</a>;</span><br />
+<span style="margin-left: 1em;">prediction of future, <a href="#Page_74">74</a>;</span><br />
+<span style="margin-left: 1em;">recurrence of, <a href="#Page_74">74&ndash;80</a></span><br />
+<br />
+<span class='pagenum'><a name="Page_355" id="Page_355">[Pg 355]</a></span>Eclipses of sun, <a href="#Page_25">25</a>, <a href="#Page_65">65&ndash;74</a>, <a href="#CHAPTER_VIII">Chap. VIII.</a>, <a href="#Page_201">201&ndash;202</a>, <a href="#Page_234">234</a>;<br />
+<span style="margin-left: 1em;">1612 <span class="ampm">A.D.</span>, <a href="#Page_90">90</a>;</span><br />
+<span style="margin-left: 1em;">1715, <a href="#Page_88">88</a>, <a href="#Page_91">91</a>;</span><br />
+<span style="margin-left: 1em;">1724, <a href="#Page_88">88</a>, <a href="#Page_91">91</a>;</span><br />
+<span style="margin-left: 1em;">1836, <a href="#Page_92">92</a>;</span><br />
+<span style="margin-left: 1em;">1842, <a href="#Page_92">92&ndash;93</a>;</span><br />
+<span style="margin-left: 1em;">1851, <a href="#Page_81">81</a>, <a href="#Page_93">93</a>;</span><br />
+<span style="margin-left: 1em;">1868, <a href="#Page_93">93</a>;</span><br />
+<span style="margin-left: 1em;">1870, <a href="#Page_94">94</a>;</span><br />
+<span style="margin-left: 1em;">1871, <a href="#Page_94">94</a>;</span><br />
+<span style="margin-left: 1em;">1878, <a href="#Page_95">95</a>;</span><br />
+<span style="margin-left: 1em;">1882, <a href="#Page_95">95</a>;</span><br />
+<span style="margin-left: 1em;">1883, <a href="#Page_95">95&ndash;96</a>;</span><br />
+<span style="margin-left: 1em;">1893, <a href="#Page_95">95&ndash;96</a>;</span><br />
+<span style="margin-left: 1em;">1896, <a href="#Page_96">96</a>, <a href="#Page_99">99</a>;</span><br />
+<span style="margin-left: 1em;">1898, <a href="#Page_96">96</a>, <a href="#Page_98">98</a>;</span><br />
+<span style="margin-left: 1em;">1900, <a href="#Page_97">97</a>;</span><br />
+<span style="margin-left: 1em;">1905, <a href="#Page_75">75&ndash;76</a>, <a href="#Page_80">80&ndash;81</a>, <a href="#Page_97">97&ndash;98</a>;</span><br />
+<span style="margin-left: 1em;">1907, <a href="#Page_98">98</a>;</span><br />
+<span style="margin-left: 1em;">1908, <a href="#Page_98">98</a>;</span><br />
+<span style="margin-left: 1em;">1914, <a href="#Page_99">99</a>;</span><br />
+<span style="margin-left: 1em;">1927, <a href="#Page_92">92</a>, <a href="#Page_99">99&ndash;100</a></span><br />
+<br />
+<i>Eclipses, Past and Future</i>, <a href="#Page_340">340</a><br />
+<br />
+Egenitis, <a href="#Page_272">272</a><br />
+<br />
+Electric furnace, <a href="#Page_128">128</a><br />
+<br />
+Electric light, spectrum of, <a href="#Page_122">122</a><br />
+<br />
+Elements composing sun, <a href="#Page_144">144&ndash;145</a><br />
+<br />
+Ellipses, <a href="#Page_32">32</a>, <a href="#Page_66">66</a>, <a href="#Page_172">172&ndash;173</a>, <a href="#Page_177">177&ndash;178</a><br />
+<br />
+Elliptic orbit, <a href="#Page_66">66</a>, <a href="#Page_177">177</a><br />
+<br />
+Ellipticity, <a href="#Page_32">32</a><br />
+<br />
+Elongation, Eastern, <a href="#Page_147">147</a>, <a href="#Page_149">149</a>;<br />
+<span style="margin-left: 1em;">Western, <a href="#Page_147">147</a>, <a href="#Page_149">149</a></span><br />
+<br />
+Encke's Comet, <a href="#Page_253">253</a>, <a href="#Page_256">256</a><br />
+<br />
+"End of the World," <a href="#Page_342">342</a><br />
+<br />
+England, solar eclipses visible in, <a href="#Page_87">87&ndash;88</a>, <a href="#Page_91">91&ndash;92</a><br />
+<br />
+Epsilon, (&#949;) Lyr&aelig;, <a href="#Page_302">302</a><br />
+<br />
+Equator, <a href="#Page_48">48</a><br />
+<br />
+Equatorial telescope, <a href="#Page_226">226</a><br />
+<br />
+Equinoxes. <a href="#Precession_of_the_Equinoxes"><i>See</i> Precession of</a><br />
+<br />
+Eros, <a href="#Page_210">210&ndash;211</a>, <a href="#Page_223">223</a>, <a href="#Page_226">226&ndash;227</a>;<br />
+<span style="margin-left: 1em;">discovery of, <a href="#Page_24">24</a>, <a href="#Page_210">210</a>, <a href="#Page_227">227</a>;</span><br />
+<span style="margin-left: 1em;">importance of, <a href="#Page_211">211</a>;</span><br />
+<span style="margin-left: 1em;">orbit of, <a href="#Page_32">32</a>, <a href="#Page_37">37</a>, <a href="#Page_210">210</a>, <a href="#Page_336">336</a></span><br />
+<br />
+Eruptive prominences, <a href="#Page_139">139</a><br />
+<br />
+<i>Esclistre</i>, <a href="#Page_89">89</a><br />
+<br />
+Ether, <a href="#Page_322">322&ndash;323</a>, <a href="#Page_331">331&ndash;332</a><br />
+<br />
+Europa, <a href="#Page_233">233</a>, <a href="#Page_235">235</a><br />
+<br />
+Evans, J.E., <a href="#Page_219">219</a><br />
+<br />
+Evening star, <a href="#Page_149">149&ndash;150</a>, <a href="#Page_241">241</a><br />
+<br />
+Everest, Mount, <a href="#Page_200">200</a><br />
+<br />
+Evershed, <a href="#Page_182">182</a><br />
+<br />
+Eye-piece, <a href="#Page_110">110</a><br />
+<br />
+<br />
+<span class="smcap">Fabricius</span>, <a href="#Page_307">307</a><br />
+<br />
+Facul&aelig;, <a href="#Page_136">136</a>, <a href="#Page_143">143</a><br />
+<br />
+Fauth, <a href="#Page_205">205</a><br />
+<br />
+Faye, <a href="#Page_335">335</a><br />
+<br />
+<i>Fin du Monde</i>, <a href="#Page_346">346</a><br />
+<br />
+First quarter, <a href="#Page_183">183</a><br />
+<br />
+"Fixed stars," <a href="#Page_280">280</a><br />
+<br />
+Flagstaff, <a href="#Page_215">215&ndash;216</a>, <a href="#Page_220">220</a><br />
+<br />
+Flammarion, Camille, <a href="#Page_346">346</a><br />
+<br />
+Flamsteed, <a href="#Page_90">90</a><br />
+<br />
+"Flash spectrum," <a href="#Page_137">137</a><br />
+<br />
+"Flat," <a href="#Page_112">112</a><br />
+<br />
+Flint glass, <a href="#Page_115">115</a><br />
+<br />
+Focus, <a href="#Page_66">66</a>, <a href="#Page_177">177</a><br />
+<br />
+"Forty-foot Telescope," <a href="#Page_115">115</a><br />
+<br />
+Foster, <a href="#Page_102">102</a><br />
+<br />
+Fraunhofer, <a href="#Page_117">117</a><br />
+<br />
+French Academy of Sciences, <a href="#Page_115">115</a><br />
+<br />
+Froissart, <a href="#Page_89">89</a><br />
+<br />
+"Full moon" of Laplace, <a href="#Page_190">190</a><br />
+<br />
+<br />
+<span class="smcap">Galaxy</span>. <a href="#Milky_Way"><i>See</i> Milky Way.</a><br />
+<br />
+Galilean telescope, <a href="#Page_109">109</a><br />
+<br />
+Galileo, <a href="#Page_55">55</a>, <a href="#Page_109">109</a>, <a href="#Page_172">172</a>, <a href="#Page_197">197</a>, <a href="#Page_206">206</a>, <a href="#Page_232">232&ndash;235</a>, <a href="#Page_242">242</a><br />
+<br />
+Galle, <a href="#Page_24">24</a>, <a href="#Page_211">211</a>, <a href="#Page_244">244</a><br />
+<br />
+Ganymede, <a href="#Page_233">233&ndash;234</a><br />
+<br />
+Gas light, spectrum of, <a href="#Page_122">122</a><br />
+<br />
+Gegenschein, <a href="#Page_181">181&ndash;182</a><br />
+<br />
+"Gem" of meteor ring, <a href="#Page_271">271</a><br />
+<br />
+<a name="Gemini" id="Gemini"></a>Gemini, or the Twins (constellation), <a href="#Page_22">22</a>, <a href="#Page_296">296&ndash;297</a><br />
+<br />
+Geminorum, &#950; (Zeta), <a href="#Page_304">304</a><br />
+<br />
+Geometrical groupings of stars, <a href="#Page_292">292</a><br />
+<br />
+"Giant" planet, <a href="#Page_230">230</a>, <a href="#Page_238">238&ndash;239</a><br />
+<br />
+Gibbous, <a href="#Page_183">183</a>, <a href="#Page_185">185</a><br />
+<br />
+Gill, Sir David, <a href="#Page_211">211</a>, <a href="#Page_258">258</a>, <a href="#Page_291">291</a>, <a href="#Page_317">317&ndash;318</a><br />
+<br />
+Gold, <a href="#Page_145">145</a><br />
+<br />
+Goodricke, <a href="#Page_307">307</a><br />
+<br />
+Gore, J.E., <a href="#Page_63">63</a>, <a href="#Page_285">285</a>, <a href="#Page_303">303</a>, <a href="#Page_307">307&ndash;308</a>, <a href="#Page_310">310</a>, <a href="#Page_323">323&ndash;324</a>, <a href="#Page_331">331</a>, <a href="#Page_337">337</a>, <a href="#Page_347">347</a><br />
+<br />
+Granulated structure of photosphere, <a href="#Page_134">134</a><br />
+<br />
+Gravitation (or gravity), <a href="#Page_39">39</a>, <a href="#Page_41">41&ndash;45</a>, <a href="#Page_128">128</a>, <a href="#Page_306">306</a><br />
+<br />
+Greek ideas, <a href="#Page_18">18</a>, <a href="#Page_158">158</a>, <a href="#Page_161">161&ndash;162</a>, <a href="#Page_171">171</a>, <a href="#Page_186">186</a>, <a href="#Page_197">197</a><br />
+<br />
+Green (rays of light), <a href="#Page_121">121</a><br />
+<br />
+Greenwich Observatory, <a href="#Page_143">143&ndash;144</a>, <a href="#Page_232">232</a>, <a href="#Page_255">255</a>, <a href="#Page_303">303</a><br />
+<br />
+Gregorian telescope, <a href="#Page_113">113&ndash;114</a><br />
+<br />
+Grimaldi (lunar crater), <a href="#Page_199">199</a><br />
+<br />
+"<a name="Grindstone" id="Grindstone"></a>Grindstone" theory, <a href="#Page_319">319&ndash;322</a><br />
+<br />
+"Groombridge, 1830," <a href="#Page_281">281&ndash;282</a>, <a href="#Page_326">326</a>, <a href="#Page_330">330</a><br />
+<br />
+Groups of stars, <a href="#Page_306">306&ndash;307</a><br />
+<br />
+Grubb, Sir Howard, <a href="#Page_118">118</a><br />
+<br />
+<i>Gulliver's Travels</i>, <a href="#Page_224">224</a><br />
+<br />
+<br />
+<span class='pagenum'><a name="Page_356" id="Page_356">[Pg 356]</a></span><span class="smcap">Hale</span>, G.E., <a href="#Page_119">119</a>, <a href="#Page_140">140</a><br />
+<br />
+Half moon, <a href="#Page_183">183</a>, <a href="#Page_185">185</a><br />
+<br />
+Hall, Asaph, <a href="#Page_223">223</a><br />
+<br />
+Hall, Chester Moor, <a href="#Page_115">115</a><br />
+<br />
+Halley, Edmund, <a href="#Page_91">91</a>, <a href="#Page_255">255</a>, <a href="#Page_264">264&ndash;265</a>, <a href="#Page_306">306</a><br />
+<br />
+Halley's Comet, <a href="#Page_255">255</a>, <a href="#Page_264">264&ndash;265</a><br />
+<br />
+Haraden Hill, <a href="#Page_91">91</a><br />
+<br />
+Harvard, <a href="#Page_118">118</a>, <a href="#Page_302">302</a><br />
+<br />
+Harvest moon, <a href="#Page_190">190&ndash;192</a><br />
+<br />
+Hawaii, <a href="#Page_221">221</a><br />
+<br />
+Heat rays, <a href="#Page_127">127</a><br />
+<br />
+Heidelberg, <a href="#Page_226">226</a>, <a href="#Page_232">232</a><br />
+<br />
+Height of lunar mountains, how determined, <a href="#Page_201">201</a><br />
+<br />
+Height of objects in sky, estimation of, <a href="#Page_196">196</a><br />
+<br />
+Helium, <a href="#Page_138">138</a>, <a href="#Page_145">145</a>, <a href="#Page_182">182</a><br />
+<br />
+Helmholtz, <a href="#Page_128">128</a>, <a href="#Page_335">335</a><br />
+<br />
+Hercules (constellation), <a href="#Page_295">295</a><br />
+<br />
+Herod the Great, <a href="#Page_101">101&ndash;102</a><br />
+<br />
+Herodotus, <a href="#Page_84">84</a><br />
+<br />
+Herschel, A.S., <a href="#Page_269">269</a><br />
+<br />
+Herschel, Sir John, <a href="#Page_92">92</a>, <a href="#Page_322">322</a><br />
+<br />
+Herschel, Sir William, <a href="#Page_22">22</a>, <a href="#Page_36">36</a>, <a href="#Page_114">114&ndash;115</a>, <a href="#Page_204">204</a>, <a href="#Page_213">213</a>, <a href="#Page_235">235</a>, <a href="#Page_283">283</a>, <a href="#Page_292">292</a>, <a href="#Page_308">308</a>,<br />
+<span style="margin-left: 1em;"><a href="#Page_319">319&ndash;320</a>, <a href="#Page_326">326&ndash;328</a></span><br />
+<br />
+Herschelian telescope, <a href="#Page_114">114</a>, <a href="#Page_119">119</a><br />
+<br />
+Hesper, <a href="#Page_109">109</a><br />
+<br />
+Hesperus, <a href="#Page_150">150</a><br />
+<br />
+Hevelius, <a href="#Page_111">111</a><br />
+<br />
+Hezekiah, <a href="#Page_85">85</a><br />
+<br />
+Hi, <a href="#Page_83">83</a><br />
+<br />
+Hindoos, <a href="#Page_18">18</a><br />
+<br />
+Hipparchus, <a href="#Page_106">106</a>, <a href="#Page_177">177</a>, <a href="#Page_290">290</a>, <a href="#Page_311">311</a><br />
+<br />
+Ho, <a href="#Page_83">83</a><br />
+<br />
+<a name="Holes_in_Milky_Way" id="Holes_in_Milky_Way"></a>Holes in Milky Way, <a href="#Page_321">321&ndash;323</a><br />
+<br />
+Holmes, Oliver Wendell, <a href="#Page_213">213</a><br />
+<br />
+Homer, <a href="#Page_223">223</a><br />
+<br />
+Horace, Odes of, <a href="#Page_106">106</a><br />
+<br />
+Horizon, <a href="#Page_159">159</a><br />
+<br />
+Horizontal eclipse, <a href="#Page_169">169</a><br />
+<br />
+Horrox, <a href="#Page_44">44</a>, <a href="#Page_151">151&ndash;152</a><br />
+<br />
+<a name="Hour_Glass_Sea" id="Hour_Glass_Sea"></a>Hour Glass Sea, <a href="#Page_212">212</a><br />
+<br />
+Huggins, Sir William, <a href="#Page_94">94</a>, <a href="#Page_125">125</a>, <a href="#Page_317">317</a><br />
+<br />
+Humboldt, <a href="#Page_270">270</a><br />
+<br />
+"Hunter's moon," <a href="#Page_192">192</a><br />
+<br />
+Huyghens, <a href="#Page_111">111&ndash;112</a>, <a href="#Page_240">240</a>, <a href="#Page_242">242v243</a><br />
+<br />
+Hyades, <a href="#Page_296">296&ndash;297</a>, <a href="#Page_307">307</a><br />
+<br />
+Hydrocarbon gas, <a href="#Page_254">254</a><br />
+<br />
+Hydrogen, <a href="#Page_94">94</a>, <a href="#Page_131">131</a>, <a href="#Page_138">138</a>, <a href="#Page_140">140</a>, <a href="#Page_144">144</a>, <a href="#Page_156">156</a>, <a href="#Page_182">182</a>, <a href="#Page_254">254</a><br />
+<br />
+<br />
+<span class="smcap">Ibrahim</span> ben Ahmed, <a href="#Page_270">270</a><br />
+<br />
+Ice-layer theory:<br />
+<span style="margin-left: 1em;">Mars, <a href="#Page_219">219</a>;</span><br />
+<span style="margin-left: 1em;">moon, <a href="#Page_205">205</a>, <a href="#Page_219">219</a></span><br />
+<br />
+Illusion theory of Martian canals, <a href="#Page_219">219</a><br />
+<br />
+Imbrium, Mare, <a href="#Page_197">197</a><br />
+<br />
+Inclination of orbits, <a href="#Page_36">36&ndash;37</a><br />
+<br />
+Indigo (rays of light), <a href="#Page_121">121</a><br />
+<br />
+Inferior conjunction, <a href="#Page_147">147</a>, <a href="#Page_149">149</a><br />
+<br />
+Inferior planets, <a href="#Page_20">20</a>, <a href="#Page_22">22</a>, <a href="#CHAPTER_XIV">Chap. XIV.</a>, <a href="#Page_229">229</a><br />
+<br />
+Instruments, pre-telescopic, <a href="#Page_106">106&ndash;107</a>, <a href="#Page_172">172</a><br />
+<br />
+International photographic survey of sky, <a href="#Page_290">290&ndash;291</a><br />
+<br />
+Intra-Mercurial planet, <a href="#Page_25">25&ndash;26</a><br />
+<br />
+<i>Introduction to Astronomy</i>, <a href="#Page_31">31</a><br />
+<br />
+Inverted view in astronomical telescope, <a href="#Page_116">116&ndash;117</a><br />
+<br />
+Io, <a href="#Page_233">233&ndash;234</a><br />
+<br />
+Iridum, Sinus, <a href="#Page_197">197</a><br />
+<br />
+Iron, <a href="#Page_145">145</a>, <a href="#Page_254">254</a><br />
+<br />
+<i>Is Mars Habitable?</i> <a href="#Page_221">221</a><br />
+<br />
+<br />
+<span class="smcap">Jansen</span>, <a href="#Page_108">108</a><br />
+<br />
+Janssen, <a href="#Page_94">94</a>, <a href="#Page_236">236</a>, <a href="#Page_258">258</a><br />
+<br />
+Japetus, <a href="#Page_240">240</a><br />
+<br />
+Jessenius, <a href="#Page_89">89</a><br />
+<br />
+Job, Book of, <a href="#Page_299">299</a><br />
+<br />
+Johnson, S.J., <a href="#Page_103">103</a>, <a href="#Page_340">340</a><br />
+<br />
+Josephus, <a href="#Page_101">101</a>, <a href="#Page_262">262</a><br />
+<br />
+Juno, <a href="#Page_225">225</a><br />
+<br />
+Jupiter, <a href="#Page_20">20</a>, <a href="#Page_22">22&ndash;23</a>, <a href="#Page_31">31</a>, <a href="#Page_34">34</a>, <a href="#Page_37">37</a>, <a href="#Page_42">42</a>, <a href="#Page_227">227&ndash;228</a>, <a href="#Page_230">230&ndash;236</a>, <a href="#Page_241">241</a>, <a href="#Page_272">272</a>, <a href="#Page_311">311</a>;<br />
+<span style="margin-left: 1em;">comet family of, <a href="#Page_251">251&ndash;253</a>, <a href="#Page_256">256</a>;</span><br />
+<span style="margin-left: 1em;">discovery of eighth satellite, <a href="#Page_26">26</a>, <a href="#Page_232">232</a>;</span><br />
+<span style="margin-left: 1em;">eclipse of, by satellite, <a href="#Page_234">234</a>;</span><br />
+<span style="margin-left: 1em;">without satellites, <a href="#Page_234">234&ndash;235</a></span><br />
+<br />
+Jupiter, satellites of, <a href="#Page_26">26</a>, <a href="#Page_62">62</a>, <a href="#Page_108">108</a>, <a href="#Page_189">189</a>, <a href="#Page_232">232&ndash;235</a>;<br />
+<span style="margin-left: 1em;">their eclipses, <a href="#Page_234">234&ndash;235</a>;</span><br />
+<span style="margin-left: 1em;">their occultations, <a href="#Page_62">62</a>, <a href="#Page_234">234</a>;</span><br />
+<span style="margin-left: 1em;">their transits, <a href="#Page_62">62</a>, <a href="#Page_234">234</a></span><br />
+<br />
+<br />
+<span class="smcap">Kant</span>, <a href="#Page_334">334</a><br />
+<br />
+Kapteyn, <a href="#Page_284">284</a>, <a href="#Page_313">313</a><br />
+<br />
+Keeler, <a href="#Page_315">315</a>, <a href="#Page_337">337</a><br />
+<br />
+Kelvin, Lord, <a href="#Page_129">129</a><br />
+<br />
+Kepler, <a href="#Page_44">44</a>, <a href="#Page_152">152</a>, <a href="#Page_172">172</a>, <a href="#Page_237">237</a>, <a href="#Page_242">242</a>, <a href="#Page_245">245</a>, <a href="#Page_253">253</a>, <a href="#Page_311">311</a><br />
+<br />
+Kinetic theory, <a href="#Page_156">156</a>, <a href="#Page_202">202</a>, <a href="#Page_212">212</a>, <a href="#Page_226">226</a>, <a href="#Page_231">231</a>, <a href="#Page_239">239</a>, <a href="#Page_336">336</a><br />
+<br />
+King, L.W., <a href="#Page_84">84</a><br />
+<br />
+<i>Knowledge</i>, <a href="#Page_87">87</a><br />
+<br />
+<br />
+<span class='pagenum'><a name="Page_357" id="Page_357">[Pg 357]</a></span><span class="smcap">Labrador</span>, <a href="#Page_97">97</a><br />
+<br />
+Lacus Somniorum, <a href="#Page_197">197</a><br />
+<br />
+"Lake of Dreams," <a href="#Page_197">197</a><br />
+<br />
+Lalande, <a href="#Page_244">244</a>, <a href="#Page_283">283</a><br />
+<br />
+Lampland, <a href="#Page_215">215</a>, <a href="#Page_219">219</a><br />
+<br />
+Langley, <a href="#Page_95">95</a>, <a href="#Page_127">127</a><br />
+<br />
+Laplace, <a href="#Page_190">190</a>, <a href="#Page_333">333</a><br />
+<br />
+Laputa, <a href="#Page_224">224</a><br />
+<br />
+Le Maire, <a href="#Page_115">115</a><br />
+<br />
+Le Verrier, <a href="#Page_24">24</a>, <a href="#Page_236">236</a>, <a href="#Page_243">243&ndash;244</a>, <a href="#Page_275">275</a><br />
+<br />
+Lead, <a href="#Page_145">145</a><br />
+<br />
+Leibnitz Mountains (lunar), <a href="#Page_200">200</a><br />
+<br />
+Leo (constellation), <a href="#Page_270">270</a>, <a href="#Page_295">295&ndash;296</a><br />
+<br />
+Leonids, <a href="#Page_270">270&ndash;272</a>, <a href="#Page_274">274&ndash;275</a><br />
+<br />
+Lescarbault, <a href="#Page_25">25</a><br />
+<br />
+Lewis, T., <a href="#Page_303">303</a><br />
+<br />
+Lexell's Comet, <a href="#Page_250">250</a><br />
+<br />
+Lick Observatory, <a href="#Page_31">31</a>, <a href="#Page_98">98</a>, <a href="#Page_117">117&ndash;118</a>, <a href="#Page_215">215</a>, <a href="#Page_232">232</a>, <a href="#Page_303">303</a>, <a href="#Page_305">305</a>, <a href="#Page_315">315</a>;<br />
+<span style="margin-left: 1em;">Great Telescope of, <a href="#Page_117">117</a>, <a href="#Page_215">215</a>, <a href="#Page_237">237</a></span><br />
+<br />
+"Life" of an eclipse of the moon, <a href="#Page_80">80</a>;<br />
+<span style="margin-left: 1em;">of the sun, <a href="#Page_77">77&ndash;78</a></span><br />
+<br />
+Life on Mars, Lowell's views, <a href="#Page_217">217&ndash;218</a>;<br />
+<span style="margin-left: 1em;">Pickering's, <a href="#Page_221">221</a>;</span><br />
+<span style="margin-left: 1em;">Wallace's, <a href="#Page_221">221&ndash;223</a></span><br />
+<br />
+Light, no extinction of, <a href="#Page_322">322&ndash;324</a>;<br />
+<span style="margin-left: 1em;">rays of, <a href="#Page_127">127</a>;</span><br />
+<span style="margin-left: 1em;">velocity of, <a href="#Page_52">52</a>, <a href="#Page_235">235&ndash;236</a>;</span><br />
+<span style="margin-left: 1em;">white, <a href="#Page_121">121</a></span><br />
+<br />
+"Light year," <a href="#Page_53">53</a>, <a href="#Page_280">280</a><br />
+<br />
+Lindsay, Lord, <a href="#Page_94">94</a><br />
+<br />
+Linn&eacute; (lunar crater), <a href="#Page_205">205</a><br />
+<br />
+Liouville, <a href="#Page_190">190</a><br />
+<br />
+Lippershey, <a href="#Page_108">108</a><br />
+<br />
+Liquid-filled lenses, <a href="#Page_116">116</a><br />
+<br />
+<i>Locksley Hall</i>, <a href="#Page_296">296</a>;<br />
+<span style="margin-left: 1em;"><i>Sixty Years After</i>, <a href="#Page_109">109</a></span><br />
+<br />
+Lockyer, Sir Norman, <a href="#Page_73">73</a>, <a href="#Page_94">94</a>, <a href="#Page_236">236</a>, <a href="#Page_335">335</a><br />
+<br />
+Loewy, <a href="#Page_119">119</a>, <a href="#Page_206">206</a><br />
+<br />
+London, eclipses visible at, <a href="#Page_87">87&ndash;88</a>, <a href="#Page_91">91&ndash;92</a><br />
+<br />
+Longfellow, <a href="#Page_88">88</a><br />
+<br />
+Lowell Observatory, <a href="#Page_215">215</a>, <a href="#Page_219">219</a>, <a href="#Page_233">233&ndash;234</a><br />
+<br />
+Lowell, Percival, <a href="#Page_155">155</a>, <a href="#Page_212">212&ndash;213</a>, <a href="#Page_215">215&ndash;221</a><br />
+<br />
+Lucifer, <a href="#Page_150">150</a><br />
+<br />
+Lynn, W.T., <a href="#Page_219">219</a>, <a href="#Page_263">263</a><br />
+<br />
+Lyra (constellation), <a href="#Page_177">177</a>, <a href="#Page_283">283</a>, <a href="#Page_294">294&ndash;295</a>, <a href="#Page_307">307</a>, <a href="#Page_315">315</a>, <a href="#Page_347">347</a><br />
+<br />
+<br />
+<span class="smcap">M&auml;dler</span>, <a href="#Page_206">206</a>, <a href="#Page_284">284</a><br />
+<br />
+Magellanic Clouds, <a href="#Page_317">317</a><br />
+<br />
+Magnetism, disturbances of terrestrial, <a href="#Page_143">143</a>, <a href="#Page_283">283</a><br />
+<br />
+Magnitudes of stars, <a href="#Page_287">287&ndash;289</a><br />
+<br />
+Major planets, <a href="#Page_229">229&ndash;230</a><br />
+<br />
+"Man in the Moon," <a href="#Page_197">197</a><br />
+<br />
+<i>Manual of Astronomy</i>, <a href="#Page_166">166</a><br />
+<br />
+Maps of the moon, <a href="#Page_206">206</a><br />
+<br />
+Mare Imbrium, <a href="#Page_197">197</a><br />
+<br />
+Mare Serenitatis, <a href="#Page_205">205</a><br />
+<br />
+Mars, <a href="#Page_20">20</a>, <a href="#Page_22">22&ndash;23</a>, <a href="#Page_31">31&ndash;32</a>, <a href="#Page_34">34</a>, <a href="#Page_37">37</a>, <a href="#Page_109">109</a>, <a href="#Page_155">155</a>, <a href="#Page_210">210&ndash;225</a>, <a href="#Page_234">234</a>;<br />
+<span style="margin-left: 1em;">compared with earth and moon, <a href="#Page_221">221</a>, <a href="#Page_225">225</a>;</span><br />
+<span style="margin-left: 1em;">polar caps of, <a href="#Page_212">212&ndash;214</a>, <a href="#Page_216">216</a>;</span><br />
+<span style="margin-left: 1em;">satellites of, <a href="#Page_26">26</a>, <a href="#Page_223">223&ndash;224</a>;</span><br />
+<span style="margin-left: 1em;">temperature of, <a href="#Page_213">213</a>, <a href="#Page_216">216</a>, <a href="#Page_221">221&ndash;222</a></span><br />
+<br />
+Mass, <a href="#Page_38">38</a>;<br />
+<span style="margin-left: 1em;">of a star, how determined, <a href="#Page_305">305</a></span><br />
+<br />
+Masses of celestial bodies, how ascertained, <a href="#Page_42">42</a>;<br />
+<span style="margin-left: 1em;">of earth and moon compared, <a href="#Page_42">42</a>;</span><br />
+<span style="margin-left: 1em;">of sun and planets compared, <a href="#Page_39">39</a></span><br />
+<br />
+Maunder, E.W., <a href="#Page_87">87</a>, <a href="#Page_143">143</a>, <a href="#Page_219">219</a><br />
+<br />
+Maunder, Mrs., E.W., <a href="#Page_96">96</a>, <a href="#Page_144">144</a><br />
+<br />
+Maxwell, Clerk, <a href="#Page_237">237</a><br />
+<br />
+Mayer, Tobias, <a href="#Page_206">206</a>, <a href="#Page_283">283</a><br />
+<br />
+McClean, F.K., <a href="#Page_98">98</a><br />
+<br />
+Mean distance, <a href="#Page_46">46</a><br />
+<br />
+"Medicean Stars," <a href="#Page_232">232</a><br />
+<br />
+Mediterranean, eclipse tracks across, <a href="#Page_94">94</a>, <a href="#Page_97">97</a><br />
+<br />
+Melbourne telescope, <a href="#Page_118">118</a><br />
+<br />
+Melotte, P., <a href="#Page_232">232</a><br />
+<br />
+Mercator's Projection, <a href="#Page_80">80&ndash;81</a><br />
+<br />
+Mercury (the metal), <a href="#Page_145">145</a><br />
+<br />
+Mercury (the planet), <a href="#Page_20">20</a>, <a href="#Page_22">22</a>, <a href="#Page_25">25&ndash;26</a>, <a href="#Page_31">31&ndash;32</a>, <a href="#Page_34">34</a>, <a href="#Page_37">37</a>, <a href="#CHAPTER_XIV">Chap. XIV.</a>;<br />
+<span style="margin-left: 1em;">markings on, <a href="#Page_156">156</a>;</span><br />
+<span style="margin-left: 1em;">possible planets within orbit of, <a href="#Page_25">25&ndash;26</a>;</span><br />
+<span style="margin-left: 1em;">transit of, <a href="#Page_62">62</a>, <a href="#Page_151">151</a>, <a href="#Page_154">154</a></span><br />
+<br />
+Metals in sun, <a href="#Page_145">145</a><br />
+<br />
+Meteor swarms, <a href="#Page_268">268&ndash;269</a>, <a href="#Page_271">271</a>, <a href="#Page_274">274&ndash;275</a><br />
+<br />
+<a name="Meteors" id="Meteors"></a>Meteors, <a href="#Page_28">28</a>, <a href="#Page_56">56</a>, <a href="#Page_167">167</a>, <a href="#Page_259">259</a>, <a href="#CHAPTER_XXI">Chap. XXI.</a><br />
+<br />
+Meteors beyond earth's atmosphere, <a href="#Page_275">275&ndash;276</a><br />
+<br />
+Meteorites, <a href="#Page_276">276&ndash;277</a><br />
+<br />
+Meteoritic Hypothesis, <a href="#Page_335">335</a><br />
+<br />
+Metius, Jacob, <a href="#Page_108">108</a><br />
+<br />
+Michell, <a href="#Page_283">283</a>, <a href="#Page_305">305</a><br />
+<br />
+Middle Ages, <a href="#Page_102">102</a>, <a href="#Page_260">260</a>, <a href="#Page_264">264</a><br />
+<br />
+Middleburgh, <a href="#Page_108">108</a><br />
+<br />
+<span class='pagenum'><a name="Page_358" id="Page_358">[Pg 358]</a></span><a name="Milky_Way" id="Milky_Way"></a>Milky Way (or Galaxy), <a href="#Page_285">285</a>, <a href="#Page_299">299</a>, <a href="#Page_311">311</a>, <a href="#Page_317">317</a>, <a href="#Page_319">319&ndash;327</a>;<br />
+<span style="margin-left: 1em;">penetration of, by photography, <a href="#Page_325">325</a></span><br />
+<br />
+Million, <a href="#Page_47">47</a>, <a href="#Page_51">51&ndash;52</a><br />
+<br />
+Minor planets. <a href="#Asteroids"><i>See</i> Asteroids.</a><br />
+<br />
+Mira Ceti, <a href="#Page_307">307&ndash;308</a><br />
+<br />
+"Mirk Monday," <a href="#Page_89">89</a><br />
+<br />
+Mirror (speculum), <a href="#Page_111">111</a>, <a href="#Page_116">116</a><br />
+<br />
+<a name="Mizar" id="Mizar"></a>Mizar, <a href="#Page_294">294</a>, <a href="#Page_302">302</a><br />
+<br />
+Monck, W.H.S., <a href="#Page_275">275</a><br />
+<br />
+Mongol Emperors of India, <a href="#Page_107">107</a><br />
+<br />
+Moon, <a href="#Page_26">26</a>, <a href="#CHAPTER_XVI">Chap. XVI.</a>;<br />
+<span style="margin-left: 1em;">appearance of, in lunar eclipse, <a href="#Page_65">65</a>, <a href="#Page_102">102&ndash;103</a>;</span><br />
+<span style="margin-left: 1em;">diameter of, <a href="#Page_189">189</a>;</span><br />
+<span style="margin-left: 1em;">distance of, how ascertained, <a href="#Page_58">58</a>;</span><br />
+<span style="margin-left: 1em;">distance of, from earth, <a href="#Page_48">48</a>;</span><br />
+<span style="margin-left: 1em;">full, <a href="#Page_63">63</a>, <a href="#Page_86">86</a>, <a href="#Page_149">149</a>, <a href="#Page_184">184</a>, <a href="#Page_189">189</a>, <a href="#Page_190">190</a>, <a href="#Page_206">206</a>;</span><br />
+<span style="margin-left: 1em;">mass of, <a href="#Page_200">200</a>, <a href="#Page_202">202</a>;</span><br />
+<span style="margin-left: 1em;">mountains on, <a href="#Page_197">197&ndash;205</a>;</span><br />
+<span style="margin-left: 1em;">how their height is determined, <a href="#Page_201">201</a>;</span><br />
+<span style="margin-left: 1em;">movement of, <a href="#Page_40">40&ndash;42</a>;</span><br />
+<span style="margin-left: 1em;">new, <a href="#Page_86">86</a>, <a href="#Page_149">149</a>, <a href="#Page_183">183</a>, <a href="#Page_185">185</a>;</span><br />
+<span style="margin-left: 1em;">origin of, <a href="#Page_339">339&ndash;341</a>;</span><br />
+<span style="margin-left: 1em;">plane of orbit of, <a href="#Page_63">63</a>;</span><br />
+<span style="margin-left: 1em;">possible changes on, <a href="#Page_204">204&ndash;205</a>, <a href="#Page_221">221</a>;</span><br />
+<span style="margin-left: 1em;">"seas" of, <a href="#Page_197">197</a>, <a href="#Page_206">206</a>;</span><br />
+<span style="margin-left: 1em;">smallest detail visible on, <a href="#Page_207">207</a>;</span><br />
+<span style="margin-left: 1em;">volume of, <a href="#Page_200">200</a></span><br />
+<br />
+Morning star, <a href="#Page_149">149&ndash;150</a>, <a href="#Page_241">241</a><br />
+<br />
+Moulton, F.R., <a href="#Page_31">31</a>, <a href="#Page_118">118</a>, <a href="#Page_128">128</a>, <a href="#Page_302">302</a>, <a href="#Page_335">335</a>, <a href="#Page_337">337</a><br />
+<br />
+Moye, <a href="#Page_154">154</a><br />
+<br />
+Multiple stars, <a href="#Page_300">300</a><br />
+<br />
+Musa-ben-Shakir, <a href="#Page_44">44</a><br />
+<br />
+Mythology, <a href="#Page_105">105</a><br />
+<br />
+<br />
+<span class="smcap">Neap-tides</span>, <a href="#Page_179">179</a><br />
+<br />
+Nebul&aelig;, <a href="#Page_314">314&ndash;318</a>, <a href="#Page_328">328</a>, <a href="#Page_335">335</a>, <a href="#Page_345">345</a>;<br />
+<span style="margin-left: 1em;">evolution of stars from, <a href="#Page_317">317&ndash;318</a></span><br />
+<br />
+Nebular Hypothesis of Laplace, <a href="#Page_333">333&ndash;338</a><br />
+<br />
+Nebular hypotheses, <a href="#CHAPTER_XXVII">Chap. XXVII.</a><br />
+<br />
+Nebulium, <a href="#Page_317">317</a><br />
+<br />
+Neison, <a href="#Page_206">206</a><br />
+<br />
+Neptune, <a href="#Page_20">20</a>, <a href="#Page_25">25</a>, <a href="#Page_31">31</a>, <a href="#Page_34">34</a>, <a href="#Page_37">37</a>, <a href="#Page_243">243&ndash;246</a>, <a href="#Page_249">249</a>, <a href="#Page_252">252</a>, <a href="#Page_274">274</a>, <a href="#Page_304">304</a>;<br />
+<span style="margin-left: 1em;">discovery of, <a href="#Page_23">23&ndash;24</a>, <a href="#Page_94">94</a>, <a href="#Page_210">210</a>, <a href="#Page_236">236</a>, <a href="#Page_243">243&ndash;244</a>;</span><br />
+<span style="margin-left: 1em;">Lalande and, <a href="#Page_244">244</a>;</span><br />
+<span style="margin-left: 1em;">possible planets beyond, <a href="#Page_25">25</a>, <a href="#Page_252">252</a>;</span><br />
+<span style="margin-left: 1em;">satellite of, <a href="#Page_26">26</a>, <a href="#Page_245">245</a>;</span><br />
+<span style="margin-left: 1em;">"year" in, <a href="#Page_35">35&ndash;36</a></span><br />
+<br />
+"<a name="New_Stars" id="New_Stars"></a>New" (or temporary), stars, <a href="#Page_310">310&ndash;314</a><br />
+<br />
+Newcomb, Simon, <a href="#Page_181">181</a>, <a href="#Page_267">267</a>, <a href="#Page_281">281</a>, <a href="#Page_324">324</a>, <a href="#Page_326">326&ndash;327</a>, <a href="#Page_329">329</a><br />
+<br />
+Newton, Sir Isaac, <a href="#Page_40">40</a>, <a href="#Page_44">44</a>, <a href="#Page_91">91</a>, <a href="#Page_111">111&ndash;113</a>, <a href="#Page_115">115</a>, <a href="#Page_165">165</a>, <a href="#Page_172">172</a>, <a href="#Page_237">237</a>, <a href="#Page_255">255</a><br />
+<br />
+Newtonian telescope, <a href="#Page_112">112</a>, <a href="#Page_114">114</a>, <a href="#Page_116">116</a>, <a href="#Page_119">119</a><br />
+<br />
+Nineveh Eclipse, <a href="#Page_84">84&ndash;85</a><br />
+<br />
+Nitrogen, <a href="#Page_145">145</a>, <a href="#Page_156">156</a>, <a href="#Page_166">166</a>, <a href="#Page_346">346</a><br />
+<br />
+Northern Crown, <a href="#Page_295">295</a><br />
+<br />
+Nova Aurig&aelig;, <a href="#Page_311">311</a><br />
+<br />
+Nova Persei, <a href="#Page_312">312&ndash;314</a><br />
+<br />
+Nov&aelig;. <a href="#New_Stars"><i>See</i> New (or temporary) stars</a><br />
+<br />
+Nubecul&aelig;, <a href="#Page_317">317</a><br />
+<br />
+<br />
+"<span class="smcap">Oases</span>" of Mars, <a href="#Page_216">216</a>, <a href="#Page_220">220</a><br />
+<br />
+Object-glass, <a href="#Page_109">109</a><br />
+<br />
+Oblate spheroid, <a href="#Page_165">165</a><br />
+<br />
+Occultation, <a href="#Page_61">61&ndash;62</a>, <a href="#Page_202">202</a>, <a href="#Page_296">296</a><br />
+<br />
+<i>Olaf, Saga of King</i>, <a href="#Page_88">88</a><br />
+<br />
+Olbers, <a href="#Page_227">227</a>, <a href="#Page_253">253</a>, <a href="#Page_256">256</a>, <a href="#Page_271">271</a><br />
+<br />
+"Old moon in new moon's arms," <a href="#Page_185">185</a><br />
+<br />
+Olmsted, <a href="#Page_271">271</a><br />
+<br />
+Omicron (or "Mira") Ceti, <a href="#Page_307">307&ndash;308</a><br />
+<br />
+Opposition, <a href="#Page_209">209</a><br />
+<br />
+"Optick tube," <a href="#Page_108">108&ndash;109</a>, <a href="#Page_232">232</a><br />
+<br />
+Orange (rays of light), <a href="#Page_121">121</a><br />
+<br />
+Orbit of moon, plane of, <a href="#Page_63">63</a><br />
+<br />
+Orbits, <a href="#Page_32">32</a>, <a href="#Page_36">36&ndash;37</a>, <a href="#Page_66">66</a>, <a href="#Page_150">150</a>, <a href="#Page_157">157</a><br />
+<br />
+Oriental astronomy, <a href="#Page_107">107</a><br />
+<br />
+Orion (constellation), <a href="#Page_195">195</a>, <a href="#Page_279">279</a>, <a href="#Page_296">296&ndash;297</a>, <a href="#Page_316">316</a>;<br />
+<span style="margin-left: 1em;">Great Nebula in, <a href="#Page_316">316</a>, <a href="#Page_328">328</a></span><br />
+<br />
+Oxford, <a href="#Page_139">139</a><br />
+<br />
+Oxygen, <a href="#Page_145">145</a>, <a href="#Page_156">156</a>, <a href="#Page_166">166</a>, <a href="#Page_346">346</a><br />
+<br />
+<br />
+<span class="smcap">Pacific</span> Ocean, origin of moon in, <a href="#Page_339">339</a><br />
+<br />
+Palitzch, <a href="#Page_255">255</a><br />
+<br />
+Pallas, <a href="#Page_225">225</a>, <a href="#Page_227">227</a><br />
+<br />
+Parallax, <a href="#Page_57">57</a>, <a href="#Page_173">173</a>, <a href="#Page_280">280</a>, <a href="#Page_305">305</a>, <a href="#Page_320">320</a>, <a href="#Page_326">326</a><br />
+<br />
+Par&eacute;, Ambrose, <a href="#Page_264">264&ndash;265</a><br />
+<br />
+Peal, S.E., <a href="#Page_205">205</a><br />
+<br />
+Peary, <a href="#Page_277">277</a><br />
+<br />
+Pegasus (constellation), <a href="#Page_306">306</a><br />
+<br />
+Penumbra of sunspot, <a href="#Page_135">135</a><br />
+<br />
+<span class='pagenum'><a name="Page_359" id="Page_359">[Pg 359]</a></span>Perennial full moon of Laplace, <a href="#Page_190">190</a><br />
+<br />
+Pericles, <a href="#Page_84">84</a><br />
+<br />
+Perrine, C.D., <a href="#Page_232">232&ndash;233</a>, <a href="#Page_315">315</a><br />
+<br />
+Perseids, <a href="#Page_270">270</a>, <a href="#Page_273">273&ndash;275</a><br />
+<br />
+Perseus (constellation), <a href="#Page_273">273</a>, <a href="#Page_279">279</a>, <a href="#Page_307">307</a>, <a href="#Page_312">312</a><br />
+<br />
+Phases of an inferior planet, <a href="#Page_149">149</a>, <a href="#Page_160">160</a>;<br />
+<span style="margin-left: 1em;">of the moon, <a href="#Page_149">149</a>, <a href="#Page_160">160</a>, <a href="#Page_183">183&ndash;185</a></span><br />
+<br />
+Phlegon, Eclipse of, <a href="#Page_85">85&ndash;86</a><br />
+<br />
+Phobos, <a href="#Page_223">223</a><br />
+<br />
+Ph&#339;be, retrograde motion of, <a href="#Page_240">240</a>, <a href="#Page_250">250</a>, <a href="#Page_336">336</a><br />
+<br />
+Phosphorescent glow in sky, <a href="#Page_323">323</a><br />
+<br />
+Phosphorus (Venus), <a href="#Page_150">150</a><br />
+<br />
+Photographic survey of sky, international, <a href="#Page_290">290&ndash;291</a><br />
+<br />
+Photosphere, <a href="#Page_130">130&ndash;131</a>, <a href="#Page_134">134</a><br />
+<br />
+Piazzi, <a href="#Page_23">23</a><br />
+<br />
+Pickering, E.C., <a href="#Page_302">302</a><br />
+<br />
+Pickering, W.H., <a href="#Page_199">199</a>, <a href="#Page_205">205&ndash;206</a>, <a href="#Page_220">220&ndash;221</a>, <a href="#Page_240">240</a>, <a href="#Page_339">339&ndash;341</a><br />
+<br />
+Pictor, "runaway star" in constellation of, <a href="#Page_281">281&ndash;282</a>, <a href="#Page_320">320</a>, <a href="#Page_330">330</a><br />
+<br />
+Plane of orbit, <a href="#Page_36">36</a>, <a href="#Page_150">150</a><br />
+<br />
+Planetary nebul&aelig;, <a href="#Page_245">245</a>, <a href="#Page_315">315</a><br />
+<br />
+<i>Planetary and Stellar Studies</i>, <a href="#Page_331">331</a><br />
+<br />
+Planetesimal hypothesis, <a href="#Page_337">337&ndash;338</a><br />
+<br />
+Planetoids. <a href="#Asteroids"><i>See</i> Asteroids</a><br />
+<br />
+Planets, classification of, <a href="#Page_229">229</a>;<br />
+<span style="margin-left: 1em;">contrasted with comets, <a href="#Page_247">247</a>;</span><br />
+<span style="margin-left: 1em;">in Ptolemaic scheme, <a href="#Page_171">171</a>;</span><br />
+<span style="margin-left: 1em;">relative distances of, from sun, <a href="#Page_31">31&ndash;32</a></span><br />
+<br />
+Plato (lunar crater), <a href="#Page_198">198</a><br />
+<br />
+Pleiades, <a href="#Page_284">284</a>, <a href="#Page_296">296&ndash;297</a>, <a href="#Page_307">307</a><br />
+<br />
+Pliny, <a href="#Page_169">169</a>, <a href="#Page_260">260</a><br />
+<br />
+Plough, <a href="#Page_284">284</a>, <a href="#Page_291">291&ndash;296</a>, <a href="#Page_302">302</a><br />
+<br />
+Plutarch, <a href="#Page_86">86</a>, <a href="#Page_89">89</a>, <a href="#Page_169">169</a>, <a href="#Page_181">181</a><br />
+<br />
+"Pointers," <a href="#Page_292">292</a><br />
+<br />
+Polaris. <a href="#Pole_Star"><i>See</i> Pole Star</a><br />
+<br />
+Pole of earth, Precessional movement of, <a href="#Page_176">176&ndash;177</a>, <a href="#Page_295">295</a>, <a href="#Page_298">298&ndash;299</a><br />
+<br />
+<a name="Pole_Star" id="Pole_Star"></a>Pole Star, <a href="#Page_33">33</a>, <a href="#Page_163">163</a>, <a href="#Page_177">177</a>, <a href="#Page_292">292&ndash;296</a>, <a href="#Page_300">300&ndash;301</a><br />
+<br />
+Poles, <a href="#Page_30">30</a>, <a href="#Page_163">163&ndash;164</a>;<br />
+<span style="margin-left: 1em;">of earth, speed of point at, <a href="#Page_164">164</a></span><br />
+<br />
+Pollux, <a href="#Page_282">282</a>, <a href="#Page_297">297</a><br />
+<br />
+Posidonius, <a href="#Page_186">186</a><br />
+<br />
+Powell, Sir George Baden, <a href="#Page_96">96</a><br />
+<br />
+Pr&aelig;sepe (the Beehive), <a href="#Page_307">307</a><br />
+<br />
+<a name="Precession_of_the_Equinoxes" id="Precession_of_the_Equinoxes"></a>Precession of the Equinoxes, <a href="#Page_177">177</a>, <a href="#Page_295">295</a>, <a href="#Page_298">298&ndash;299</a><br />
+<br />
+Pre-telescopic notions, <a href="#Page_55">55</a><br />
+<br />
+Primaries, <a href="#Page_26">26</a><br />
+<br />
+<i>Princess, The</i> (Tennyson), <a href="#Page_334">334</a><br />
+<br />
+Princeton Observatory, <a href="#Page_258">258</a><br />
+<br />
+Prism, <a href="#Page_121">121</a><br />
+<br />
+Prismatic colours, <a href="#Page_111">111</a>, <a href="#Page_121">121</a><br />
+<br />
+Procyon, <a href="#Page_284">284</a>, <a href="#Page_290">290</a>, <a href="#Page_297">297</a>, <a href="#Page_303">303</a><br />
+<br />
+Prominences, Solar, <a href="#Page_72">72</a>, <a href="#Page_93">93</a>, <a href="#Page_131">131</a>, <a href="#Page_139">139&ndash;140</a>, <a href="#Page_143">143</a>;<br />
+<span style="margin-left: 1em;">first observation of, with spectroscope, <a href="#Page_94">94</a>, <a href="#Page_140">140</a>, <a href="#Page_236">236</a></span><br />
+<br />
+Proper motions of stars, <a href="#Page_126">126</a>, <a href="#Page_281">281&ndash;285</a>, <a href="#Page_326">326</a>, <a href="#Page_329">329&ndash;330</a><br />
+<br />
+Ptolem&aelig;us (lunar crater), <a href="#Page_198">198&ndash;199</a>, <a href="#Page_204">204</a><br />
+<br />
+Ptolemaic idea, <a href="#Page_319">319</a>;<br />
+<span style="margin-left: 1em;">system, <a href="#Page_18">18</a>, <a href="#Page_19">19</a>, <a href="#Page_158">158</a>, <a href="#Page_171">171&ndash;172</a></span><br />
+<br />
+Ptolemy, <a href="#Page_18">18</a>, <a href="#Page_101">101</a>, <a href="#Page_171">171</a>, <a href="#Page_290">290</a>, <a href="#Page_296">296</a><br />
+<br />
+Puiseux, P., <a href="#Page_206">206</a><br />
+<br />
+Pulkowa telescope, <a href="#Page_117">117</a><br />
+<br />
+Puppis, V., <a href="#Page_310">310</a><br />
+<br />
+<br />
+<span class="smcap">Quiescent</span> prominences, <a href="#Page_139">139</a><br />
+<br />
+<br />
+<span class="smcap">Radcliffe</span> Observer, <a href="#Page_139">139</a><br />
+<br />
+"Radiant," or radiant point, <a href="#Page_269">269</a><br />
+<br />
+Radiation from sun, <a href="#Page_130">130</a>, <a href="#Page_134">134</a><br />
+<br />
+Radium, <a href="#Page_129">129</a>, <a href="#Page_138">138</a><br />
+<br />
+Rainbow, <a href="#Page_121">121</a><br />
+<br />
+"Rainbows, Bay of," <a href="#Page_197">197</a><br />
+<br />
+Rambaut, A., <a href="#Page_139">139</a><br />
+<br />
+Ramsay, Sir William, <a href="#Page_138">138</a><br />
+<br />
+Rays (on moon), <a href="#Page_204">204</a><br />
+<br />
+Recurrence of eclipses, <a href="#Page_74">74&ndash;80</a><br />
+<br />
+Red (rays of light), <a href="#Page_121">121</a>, <a href="#Page_125">125</a>, <a href="#Page_127">127</a>, <a href="#Page_130">130</a><br />
+<br />
+Red Spot, the Great, <a href="#Page_230">230</a><br />
+<br />
+<a name="Reflecting_telescope" id="Reflecting_telescope"></a>Reflecting telescope, <a href="#Page_111">111&ndash;116</a>;<br />
+<span style="margin-left: 1em;">future of, <a href="#Page_119">119</a></span><br />
+<br />
+Reflector. <a href="#Reflecting_telescope"><i>See</i> Reflecting telescope</a><br />
+<br />
+Refracting and reflecting telescopes contrasted, <a href="#Page_118">118</a><br />
+<br />
+<a name="Refracting_telescope" id="Refracting_telescope"></a>Refracting telescope, <a href="#Page_109">109&ndash;111</a>, <a href="#Page_115">115&ndash;117</a>;<br />
+<span style="margin-left: 1em;">limits to size of, <a href="#Page_119">119&ndash;120</a></span><br />
+<br />
+Refraction, <a href="#Page_121">121</a>, <a href="#Page_168">168&ndash;169</a><br />
+<br />
+Refractor, <a href="#Refracting_telescope"><i>See</i> Refracting telescope</a><br />
+<br />
+Regulus, <a href="#Page_290">290</a>, <a href="#Page_296">296</a><br />
+<br />
+Retrograde motion of Ph&#339;be, <a href="#Page_240">240</a>, <a href="#Page_250">250</a>, <a href="#Page_336">336</a><br />
+<br />
+<span class='pagenum'><a name="Page_360" id="Page_360">[Pg 360]</a></span>"Reversing Layer," <a href="#Page_94">94</a>, <a href="#Page_130">130</a>, <a href="#Page_132">132</a>, <a href="#Page_137">137&ndash;138</a><br />
+<br />
+Revival of learning, <a href="#Page_107">107</a><br />
+<br />
+Revolution, <a href="#Page_30">30</a>;<br />
+<span style="margin-left: 1em;">of earth around sun, <a href="#Page_170">170&ndash;173</a>;</span><br />
+<span style="margin-left: 1em;">periods of sun and planets, <a href="#Page_35">35</a></span><br />
+<br />
+Riccioli, <a href="#Page_198">198</a><br />
+<br />
+Rice-grain structure of photosphere, <a href="#Page_134">134</a><br />
+<br />
+Rigel, <a href="#Page_285">285</a>, <a href="#Page_297">297</a><br />
+<br />
+Rills (on moon), <a href="#Page_204">204</a><br />
+<br />
+Ring-mountains of moon. <a href="#Craters"><i>See</i> Craters</a><br />
+<br />
+"Ring" nebul&aelig;, <a href="#Page_315">315</a>, <a href="#Page_337">337</a><br />
+<br />
+"Ring with wings," <a href="#Page_87">87</a><br />
+<br />
+Rings of Saturn, <a href="#Page_108">108</a>, <a href="#Page_236">236&ndash;239</a>, <a href="#Page_241">241&ndash;243</a>, <a href="#Page_334">334</a><br />
+<br />
+Ritchey, G.W., <a href="#Page_118">118</a><br />
+<br />
+Roberts, A.W., <a href="#Page_308">308</a>, <a href="#Page_310">310</a><br />
+<br />
+Roberts, Isaac, <a href="#Page_325">325</a><br />
+<br />
+"Roche's limit," <a href="#Page_238">238</a><br />
+<br />
+Roemer, <a href="#Page_235">235</a><br />
+<br />
+Roman history, eclipses in, <a href="#Page_85">85&ndash;86</a><br />
+<br />
+Romulus, <a href="#Page_85">85</a><br />
+<br />
+R&ouml;ntgen, <a href="#Page_120">120</a><br />
+<br />
+Rosse, great telescope of Lord, <a href="#Page_117">117</a>, <a href="#Page_314">314</a><br />
+<br />
+Rotation, <a href="#Page_30">30</a>;<br />
+<span style="margin-left: 1em;">of earth, <a href="#Page_33">33</a>, <a href="#Page_161">161&ndash;165</a>, <a href="#Page_170">170</a>;</span><br />
+<span style="margin-left: 1em;">of sun, <a href="#Page_34">34</a>, <a href="#Page_125">125</a>, <a href="#Page_135">135&ndash;136</a>, <a href="#Page_231">231</a>;</span><br />
+<span style="margin-left: 1em;">periods of sun and planets, <a href="#Page_35">35</a></span><br />
+<br />
+Royal Society of London, <a href="#Page_90">90&ndash;91</a>, <a href="#Page_111">111</a><br />
+<br />
+Rubicon, Passage of the, <a href="#Page_85">85</a><br />
+<br />
+"Runaway" stars, <a href="#Page_281">281</a>, <a href="#Page_326">326</a>, <a href="#Page_330">330</a><br />
+<br />
+<br />
+<span class="smcap">Sagittarius</span> (constellation), <a href="#Page_316">316</a><br />
+<br />
+Salt, spectrum of table, <a href="#Page_122">122</a><br />
+<br />
+Samarcand, <a href="#Page_107">107</a><br />
+<br />
+"Saros," Chaldean, <a href="#Page_76">76&ndash;78</a>, <a href="#Page_84">84</a><br />
+<br />
+Satellites, <a href="#Page_26">26&ndash;27</a>, <a href="#Page_37">37</a><br />
+<br />
+Saturn, <a href="#Page_20">20</a>, <a href="#Page_22">22</a>, <a href="#Page_34">34</a>, <a href="#Page_37">37</a>, <a href="#Page_108">108</a>, <a href="#Page_236">236&ndash;243</a>, <a href="#Page_258">258</a>;<br />
+<span style="margin-left: 1em;">comet family of, <a href="#Page_252">252</a>;</span><br />
+<span style="margin-left: 1em;">a puzzle to the early telescope observers, <a href="#Page_241">241&ndash;243</a>;</span><br />
+<span style="margin-left: 1em;">retrograde motion of satellite Ph&#339;be, <a href="#Page_240">240</a>, <a href="#Page_250">250</a>, <a href="#Page_336">336</a>;</span><br />
+<span style="margin-left: 1em;">ring system of, <a href="#Page_241">241</a>;</span><br />
+<span style="margin-left: 1em;">satellites of, <a href="#Page_36">36</a>, <a href="#Page_239">239&ndash;240</a>;</span><br />
+<span style="margin-left: 1em;">shadows of planet on rings and of rings on planet, <a href="#Page_237">237</a></span><br />
+<br />
+Schaeberle, <a href="#Page_95">95&ndash;96</a>, <a href="#Page_303">303</a>, <a href="#Page_316">316</a><br />
+<br />
+Schiaparelli, <a href="#Page_155">155</a>, <a href="#Page_214">214</a>, <a href="#Page_223">223</a><br />
+<br />
+Schickhard (lunar crater), <a href="#Page_199">199</a><br />
+<br />
+Schmidt, <a href="#Page_206">206</a><br />
+<br />
+Sch&ouml;nfeld, <a href="#Page_290">290</a><br />
+<br />
+Schuster, <a href="#Page_95">95</a><br />
+<br />
+Schwabe, <a href="#Page_136">136</a><br />
+<br />
+Scotland, solar eclipses visible in, <a href="#Page_89">89&ndash;90</a>, <a href="#Page_92">92</a><br />
+<br />
+Sea of Serenity, <a href="#Page_205">205</a><br />
+<br />
+"Sea of Showers," <a href="#Page_197">197</a><br />
+<br />
+"Seas" of moon, <a href="#Page_197">197</a>, <a href="#Page_206">206</a><br />
+<br />
+Seasons on earth, <a href="#Page_174">174&ndash;175</a>;<br />
+<span style="margin-left: 1em;">on Mars, <a href="#Page_211">211</a></span><br />
+<br />
+Secondary bodies, <a href="#Page_26">26</a><br />
+<br />
+Seneca, <a href="#Page_95">95</a>, <a href="#Page_260">260</a><br />
+<br />
+<i>Septentriones</i>, <a href="#Page_291">291</a><br />
+<br />
+Serenitatis, Mare, <a href="#Page_205">205</a><br />
+<br />
+"Seven Stars," <a href="#Page_291">291</a><br />
+<br />
+"Shadow Bands," <a href="#Page_69">69</a><br />
+<br />
+Shadow of earth, circular shape of, <a href="#Page_62">62&ndash;64</a><br />
+<br />
+Shadows on moon, inky blackness of, <a href="#Page_202">202</a><br />
+<br />
+Shakespeare, <a href="#Page_259">259</a>, <a href="#Page_293">293</a><br />
+<br />
+Sheepshanks Telescope, <a href="#Page_119">119</a><br />
+<br />
+"Shining fluid" of Sir W. Herschel, <a href="#Page_328">328</a><br />
+<br />
+"Shooting Stars." <a href="#Meteors"><i>See</i> Meteors</a><br />
+<br />
+Short (of Edinburgh), <a href="#Page_114">114</a><br />
+<br />
+"Showers, Sea of," <a href="#Page_197">197</a><br />
+<br />
+Sickle of Leo, <a href="#Page_270">270&ndash;271</a>, <a href="#Page_296">296</a><br />
+<br />
+Siderostat, <a href="#Page_118">118</a><br />
+<br />
+Silver, <a href="#Page_145">145</a><br />
+<br />
+Silvered mirrors for reflecting telescopes, <a href="#Page_116">116</a><br />
+<br />
+Sinus Iridum, <a href="#Page_197">197</a><br />
+<br />
+Sirius, <a href="#Page_280">280</a>, <a href="#Page_282">282</a>, <a href="#Page_284">284&ndash;285</a>, <a href="#Page_288">288&ndash;290</a>, <a href="#Page_297">297</a>, <a href="#Page_303">303&ndash;304</a>, <a href="#Page_320">320</a>;<br />
+<span style="margin-left: 1em;">companion of, <a href="#Page_303">303</a>;</span><br />
+<span style="margin-left: 1em;">stellar magnitude of, <a href="#Page_289">289</a></span><br />
+<br />
+Size of celestial bodies, how ascertained, <a href="#Page_59">59</a><br />
+<br />
+Skeleton telescopes, <a href="#Page_110">110</a><br />
+<br />
+Sky, international photographic survey of, <a href="#Page_290">290&ndash;291</a>;<br />
+<span style="margin-left: 1em;">light of the, <a href="#Page_323">323</a></span><br />
+<br />
+Slipher, E.C., <a href="#Page_213">213</a>, <a href="#Page_222">222</a><br />
+<br />
+Smithsonian Institution of Washington, <a href="#Page_98">98</a><br />
+<br />
+Snow on Mars, <a href="#Page_213">213</a><br />
+<br />
+Sodium, <a href="#Page_122">122</a>, <a href="#Page_124">124</a>, <a href="#Page_254">254</a><br />
+<br />
+Sohag, <a href="#Page_95">95</a><br />
+<br />
+<span class='pagenum'><a name="Page_361" id="Page_361">[Pg 361]</a></span>Solar system, <a href="#Page_20">20&ndash;21</a>, <a href="#Page_29">29&ndash;31</a>;<br />
+<span style="margin-left: 1em;">centre of gravity of, <a href="#Page_42">42</a>;</span><br />
+<span style="margin-left: 1em;">decay and death of, <a href="#Page_344">344</a></span><br />
+<br />
+Somniorum, Lacus, <a href="#Page_197">197</a><br />
+<br />
+Sound, <a href="#Page_125">125</a>, <a href="#Page_166">166</a>, <a href="#Page_331">331</a><br />
+<br />
+South pole of heavens, <a href="#Page_163">163</a>, <a href="#Page_285">285</a>, <a href="#Page_298">298&ndash;299</a><br />
+<br />
+Southern constellations, <a href="#Page_298">298&ndash;299</a><br />
+<br />
+Southern Cross. <a href="#Crux"><i>See</i> Crux</a><br />
+<br />
+Space, <a href="#Page_328">328</a><br />
+<br />
+Spain, early astronomy in, <a href="#Page_107">107</a>;<br />
+<span style="margin-left: 1em;">eclipse tracks across <a href="#Page_93">93</a>, <a href="#Page_97">97&ndash;98</a></span><br />
+<br />
+Spectroheliograph, <a href="#Page_140">140</a><br />
+<br />
+Spectroscope, <a href="#Page_120">120</a>, <a href="#Page_122">122</a>, <a href="#Page_124">124&ndash;125</a>, <a href="#Page_144">144&ndash;145</a>, <a href="#Page_212">212</a>, <a href="#Page_231">231</a>;<br />
+<span style="margin-left: 1em;">prominences first observed with, <a href="#Page_94">94</a>, <a href="#Page_140">140</a>, <a href="#Page_236">236</a></span><br />
+<br />
+Spectrum of chromosphere, <a href="#Page_132">132&ndash;133</a>;<br />
+<span style="margin-left: 1em;">of corona, <a href="#Page_133">133</a>;</span><br />
+<span style="margin-left: 1em;">of photosphere, <a href="#Page_132">132</a>;</span><br />
+<span style="margin-left: 1em;">of reversing layer, <a href="#Page_132">132</a>, <a href="#Page_137">137</a>;</span><br />
+<span style="margin-left: 1em;">solar, <a href="#Page_122">122&ndash;123</a>, <a href="#Page_127">127</a>, <a href="#Page_132">132</a></span><br />
+<br />
+Speculum, <a href="#Page_111">111</a>, <a href="#Page_116">116</a>;<br />
+<span style="margin-left: 1em;">metal, <a href="#Page_112">112</a></span><br />
+<br />
+Spherical bodies, <a href="#Page_29">29</a><br />
+<br />
+Spherical shape of earth, proofs of, <a href="#Page_158">158&ndash;161</a><br />
+<br />
+Spherical shapes of sun, planets, and satellites, <a href="#Page_160">160</a><br />
+<br />
+Spiral nebul&aelig;, <a href="#Page_314">314&ndash;316</a>, <a href="#Page_337">337&ndash;338</a><br />
+<br />
+Spring balance, <a href="#Page_166">166</a><br />
+<br />
+Spring tides, <a href="#Page_192">192</a><br />
+<br />
+Spy-glass, <a href="#Page_108">108</a><br />
+<br />
+"Square of the distance," <a href="#Page_43">43&ndash;44</a><br />
+<br />
+Stannyan, Captain, <a href="#Page_90">90</a><br />
+<br />
+Star, mass of, how determined, <a href="#Page_305">305</a>;<br />
+<span style="margin-left: 1em;">parallax of, first ascertained, <a href="#Page_173">173</a>, <a href="#Page_280">280</a></span><br />
+<br />
+Stars, the, <a href="#Page_20">20</a>, <a href="#Page_124">124</a>, <a href="#Page_126">126</a>, <a href="#Page_278">278</a> <i>et seq.</i>;<br />
+<span style="margin-left: 1em;">brightness of, <a href="#Page_287">287</a>, <a href="#Page_320">320</a>;</span><br />
+<span style="margin-left: 1em;">distances between, <a href="#Page_326">326&ndash;327</a>;</span><br />
+<span style="margin-left: 1em;">distances of some, <a href="#Page_173">173</a>, <a href="#Page_280">280</a>, <a href="#Page_320">320</a>;</span><br />
+<span style="margin-left: 1em;">diminution of, below twelfth magnitude, <a href="#Page_324">324</a>;</span><br />
+<span style="margin-left: 1em;">evolution of, from nebul&aelig;, <a href="#Page_317">317&ndash;318</a>;</span><br />
+<span style="margin-left: 1em;">faintest magnitude of, <a href="#Page_288">288</a>;</span><br />
+<span style="margin-left: 1em;">number of those visible altogether, <a href="#Page_324">324</a>;</span><br />
+<span style="margin-left: 1em;">number of those visible to naked eye, <a href="#Page_288">288</a></span><br />
+<br />
+"Steam cracks," <a href="#Page_221">221</a><br />
+<br />
+Steinheil, <a href="#Page_118">118</a><br />
+<br />
+Stellar system, estimated extent of, <a href="#Page_325">325&ndash;327</a>;<br />
+<span style="margin-left: 1em;">an organised whole, <a href="#Page_327">327</a>;</span><br />
+<span style="margin-left: 1em;">limited extent of, <a href="#Page_322">322&ndash;328</a>, <a href="#Page_330">330</a>;</span><br />
+<span style="margin-left: 1em;">possible disintegration of, <a href="#Page_329">329</a></span><br />
+<br />
+Stiklastad, eclipse of, <a href="#Page_88">88</a><br />
+<br />
+Stone Age, <a href="#Page_285">285</a><br />
+<br />
+Stoney, G.J., <a href="#Page_202">202</a>, <a href="#Page_222">222</a><br />
+<br />
+Stonyhurst Observatory, <a href="#Page_100">100</a><br />
+<br />
+<i>Story of the Heavens</i>, <a href="#Page_271">271</a><br />
+<br />
+Streams of stars, Kapteyn's two, <a href="#Page_284">284</a><br />
+<br />
+Stroobant, <a href="#Page_196">196</a><br />
+<br />
+Stukeley, <a href="#Page_91">91</a><br />
+<br />
+Sulphur, <a href="#Page_145">145</a><br />
+<br />
+Summer, <a href="#Page_175">175</a>, <a href="#Page_178">178</a><br />
+<br />
+Sun, <a href="#CHAPTER_XII">Chaps XII</a>. and <a href="#CHAPTER_XIII">XIII.</a>;<br />
+<span style="margin-left: 1em;">as a star, <a href="#Page_124">124</a>, <a href="#Page_278">278</a>, <a href="#Page_289">289</a>;</span><br />
+<span style="margin-left: 1em;">as seen from Neptune, <a href="#Page_246">246</a>, <a href="#Page_304">304</a>;</span><br />
+<span style="margin-left: 1em;">chemical composition of, <a href="#Page_144">144&ndash;145</a>;</span><br />
+<span style="margin-left: 1em;">distance of, how ascertained, <a href="#Page_151">151</a>, <a href="#Page_211">211</a>;</span><br />
+<span style="margin-left: 1em;">equator of, <a href="#Page_135">135&ndash;136</a>, <a href="#Page_139">139</a>;</span><br />
+<span style="margin-left: 1em;">gravitation at surface of, <a href="#Page_129">129</a>, <a href="#Page_138">138&ndash;139</a>;</span><br />
+<span style="margin-left: 1em;">growing cold of, <a href="#Page_343">343&ndash;344</a>;</span><br />
+<span style="margin-left: 1em;">mean distance of, from earth, <a href="#Page_47">47</a>, <a href="#Page_211">211</a>;</span><br />
+<span style="margin-left: 1em;">motion of, through space, <a href="#Page_282">282&ndash;286</a>, <a href="#Page_326">326</a>;</span><br />
+<span style="margin-left: 1em;">not a solid body, <a href="#Page_136">136</a>;</span><br />
+<span style="margin-left: 1em;">poles of, <a href="#Page_136">136</a>;</span><br />
+<span style="margin-left: 1em;">radiations from, <a href="#Page_130">130</a>;</span><br />
+<span style="margin-left: 1em;">revolution of earth around, <a href="#Page_170">170&ndash;173</a>;</span><br />
+<span style="margin-left: 1em;">stellar magnitude of, <a href="#Page_288">288&ndash;289</a>;</span><br />
+<span style="margin-left: 1em;">variation in distance of, <a href="#Page_66">66</a>, <a href="#Page_178">178</a></span><br />
+<br />
+Sunspots, <a href="#Page_34">34</a>, <a href="#Page_125">125</a>, <a href="#Page_134">134&ndash;137</a>, <a href="#Page_140">140&ndash;141</a>, <a href="#Page_143">143&ndash;144</a>, <a href="#Page_308">308</a>;<br />
+<span style="margin-left: 1em;">influence of earth on, <a href="#Page_144">144</a></span><br />
+<br />
+Suns and possible systems, <a href="#Page_50">50</a>, <a href="#Page_286">286</a><br />
+<br />
+Superior conjunction, <a href="#Page_147">147&ndash;149</a><br />
+<br />
+Superior planets, <a href="#Page_22">22</a>, <a href="#Page_146">146</a>, <a href="#Page_209">209&ndash;210</a>, <a href="#Page_229">229</a><br />
+<br />
+Swan (constellation). <a href="#Cygnus"><i>See</i> Cygnus</a><br />
+<br />
+Swift, Dean, <a href="#Page_224">224</a><br />
+<br />
+"Sword" of Orion, <a href="#Page_297">297</a>, <a href="#Page_316">316</a><br />
+<br />
+Syrtis Major. <a href="#Hour_Glass_Sea"><i>See</i> Hour Glass Sea</a><br />
+<br />
+"<i>Systematic</i> Parallax," <a href="#Page_326">326</a><br />
+<br />
+Systems, other possible, <a href="#Page_50">50</a>, <a href="#Page_286">286</a><br />
+<br />
+<br />
+<span class="smcap">Tails</span> of comets, <a href="#Page_182">182</a><br />
+<br />
+Tamerlane, <a href="#Page_107">107</a><br />
+<br />
+<a name="Taurus" id="Taurus"></a>Taurus (constellation), <a href="#Page_103">103</a>, <a href="#Page_296">296&ndash;297</a>, <a href="#Page_307">307</a><br />
+<br />
+"Tears of St. Lawrence," <a href="#Page_273">273</a><br />
+<br />
+Tebbutt's Comet, <a href="#Page_257">257&ndash;258</a><br />
+<br />
+Telescope, <a href="#Page_33">33</a>, <a href="#Page_55">55</a>, <a href="#Page_107">107&ndash;108</a>, <a href="#Page_149">149</a>;<br />
+<span style="margin-left: 1em;">first eclipse of moon seen through, <a href="#Page_104">104</a>;</span><br />
+<span style="margin-left: 1em;">of sun, <a href="#Page_90">90</a></span><br />
+<br />
+<span class='pagenum'><a name="Page_362" id="Page_362">[Pg 362]</a></span>Telescopes, direct view reflecting, <a href="#Page_114">114</a>;<br />
+<span style="margin-left: 1em;">gigantic, <a href="#Page_111">111</a>;</span><br />
+<span style="margin-left: 1em;">great constructors of, <a href="#Page_117">117&ndash;118</a>;</span><br />
+<span style="margin-left: 1em;">great modern, <a href="#Page_117">117&ndash;118</a></span><br />
+<br />
+Tempel's Comet, <a href="#Page_274">274</a><br />
+<br />
+Temperature on moon, <a href="#Page_203">203</a>;<br />
+<span style="margin-left: 1em;">of sun, <a href="#Page_128">128</a></span><br />
+<br />
+Temporary (or new) stars, <a href="#Page_310">310&ndash;314</a><br />
+<br />
+Tennyson, Lord, <a href="#Page_109">109</a>, <a href="#Page_296">296</a>, <a href="#Page_334">334</a><br />
+<br />
+Terrestrial planets, <a href="#Page_229">229&ndash;230</a><br />
+<br />
+Terrestrial telescope, <a href="#Page_117">117</a><br />
+<br />
+Thales, Eclipse of, <a href="#Page_84">84</a><br />
+<br />
+Themis, <a href="#Page_240">240</a><br />
+<br />
+"Tidal drag," <a href="#Page_180">180</a>, <a href="#Page_188">188</a>, <a href="#Page_208">208</a>, <a href="#Page_344">344</a><br />
+<br />
+Tide areas, <a href="#Page_179">179&ndash;180</a><br />
+<br />
+Tides, <a href="#Page_178">178&ndash;180</a>, <a href="#Page_338">338&ndash;339</a><br />
+<br />
+<i>Time Machine</i>, <a href="#Page_344">344</a><br />
+<br />
+Tin, <a href="#Page_145">145</a><br />
+<br />
+Titan, <a href="#Page_240">240</a><br />
+<br />
+Titius, <a href="#Page_245">245</a><br />
+<br />
+Total phase, <a href="#Page_71">71&ndash;72</a><br />
+<br />
+Totality, <a href="#Page_72">72</a>;
+<span style="margin-left: 1em;">track of, <a href="#Page_66">66</a></span><br />
+<br />
+Trail of a minor planet, <a href="#Page_226">226&ndash;227</a><br />
+<br />
+Transit, <a href="#Page_62">62</a>, <a href="#Page_150">150&ndash;154</a>;<br />
+<span style="margin-left: 1em;">of Mercury, <a href="#Page_62">62</a>, <a href="#Page_151">151</a>, <a href="#Page_154">154</a>;</span><br />
+<span style="margin-left: 1em;">of Venus, <a href="#Page_62">62</a>, <a href="#Page_151">151&ndash;152</a>, <a href="#Page_154">154</a>, <a href="#Page_211">211</a></span><br />
+<br />
+Trifid Nebula, <a href="#Page_316">316</a><br />
+<br />
+Triple stars, <a href="#Page_300">300</a><br />
+<br />
+Tubeless telescopes, <a href="#Page_110">110&ndash;111</a>, <a href="#Page_243">243</a><br />
+<br />
+Tubes used by ancients, <a href="#Page_110">110</a><br />
+<br />
+Tuttle's Comet, <a href="#Page_274">274</a><br />
+<br />
+Twilight, <a href="#Page_167">167</a>, <a href="#Page_202">202</a><br />
+<br />
+Twinkling of stars, <a href="#Page_168">168</a><br />
+<br />
+Twins (constellation). <a href="#Gemini"><i>See</i> Gemini</a><br />
+<br />
+Tycho Brahe, <a href="#Page_290">290</a>, <a href="#Page_311">311</a><br />
+<br />
+Tycho (lunar crater), <a href="#Page_204">204</a><br />
+<br />
+<br />
+<span class="smcap">Ulugh</span> Beigh, <a href="#Page_107">107</a><br />
+<br />
+Umbra of sunspot, <a href="#Page_134">134&ndash;135</a><br />
+<br />
+Universe, early ideas concerning, <a href="#Page_17">17&ndash;18</a>, <a href="#Page_158">158</a>, <a href="#Page_177">177</a>, <a href="#Page_342">342</a><br />
+<br />
+Universes, possibility of other, <a href="#Page_330">330&ndash;331</a><br />
+<br />
+Uranus, <a href="#Page_22">22&ndash;24</a>, <a href="#Page_31">31</a>, <a href="#Page_210">210</a>, <a href="#Page_243">243</a>, <a href="#Page_245">245</a>, <a href="#Page_275">275</a>;<br />
+<span style="margin-left: 1em;">comet family of, <a href="#Page_252">252</a>;</span><br />
+<span style="margin-left: 1em;">discovery of, <a href="#Page_22">22</a>, <a href="#Page_210">210</a>, <a href="#Page_243">243</a>;</span><br />
+<span style="margin-left: 1em;">rotation period of <a href="#Page_34">34</a>, <a href="#Page_245">245</a>;</span><br />
+<span style="margin-left: 1em;">satellites of, <a href="#Page_26">26</a>, <a href="#Page_245">245</a>;</span><br />
+<span style="margin-left: 1em;">"year" in, <a href="#Page_35">35&ndash;36</a></span><br />
+<br />
+<a name="Ursa_Major" id="Ursa_Major"></a>Ursa Major (constellation), <a href="#Page_279">279</a>, <a href="#Page_281">281</a>, <a href="#Page_291">291</a>, <a href="#Page_295">295</a>, <a href="#Page_314">314</a>;<br />
+<span style="margin-left: 1em;"><a name="Ursa_minor" id="Ursa_minor"></a>minor, <a href="#Page_177">177</a>, <a href="#Page_279">279</a>, <a href="#Page_293">293&ndash;294</a></span><br />
+<br />
+Urs&aelig; Majoris, (&#950;) Zeta. <a href="#Mizar"><i>See</i> Mizar</a><br />
+<br />
+<br />
+<span class="smcap">Variable</span> stars, <a href="#Page_307">307&ndash;310</a><br />
+<br />
+Variations in apparent sizes of sun and moon, <a href="#Page_67">67</a>, <a href="#Page_80">80</a>, <a href="#Page_178">178</a><br />
+<br />
+Vault, shape of the celestial, <a href="#Page_194">194&ndash;196</a><br />
+<br />
+Vega, <a href="#Page_177">177</a>, <a href="#Page_278">278</a>, <a href="#Page_280">280</a>, <a href="#Page_282">282&ndash;283</a>, <a href="#Page_285">285</a>, <a href="#Page_290">290</a>, <a href="#Page_294">294</a>, <a href="#Page_302">302</a>, <a href="#Page_307">307</a>, <a href="#Page_323">323</a><br />
+<br />
+Vegetation on Mars, <a href="#Page_221">221</a>, <a href="#Page_217">217&ndash;218</a>;<br />
+<span style="margin-left: 1em;">on moon, <a href="#Page_205">205</a></span><br />
+<br />
+Venus, <a href="#Page_20">20</a>, <a href="#Page_22">22</a>, <a href="#Page_31">31</a>, <a href="#Page_71">71</a>, <a href="#Page_90">90</a>, <a href="#Page_108">108&ndash;109</a>, <a href="#Page_111">111</a>, <a href="#CHAPTER_XIV">Chap. XIV.</a>, <a href="#Page_246">246</a>, <a href="#Page_311">311</a>;<br />
+<span style="margin-left: 1em;">rotation period of, <a href="#Page_34">34</a>, <a href="#Page_155">155</a></span><br />
+<br />
+Very, F.W., <a href="#Page_314">314</a><br />
+<br />
+Vesta, <a href="#Page_225">225</a>, <a href="#Page_227">227</a><br />
+<br />
+Violet (rays of light), <a href="#Page_121">121&ndash;122</a>, <a href="#Page_125">125</a><br />
+<br />
+Virgil, <a href="#Page_19">19</a><br />
+<br />
+Volcanic theory of lunar craters, <a href="#Page_203">203&ndash;204</a>, <a href="#Page_214">214</a><br />
+<br />
+Volume, <a href="#Page_38">38</a><br />
+<br />
+Volumes of sun and planets compared, <a href="#Page_38">38&ndash;39</a><br />
+<br />
+"Vulcan," <a href="#Page_25">25</a><br />
+<br />
+<br />
+<span class="smcap">Wallace</span>, A.R., on Mars, <a href="#Page_220">220&ndash;223</a><br />
+<br />
+Water, lack of, on moon, <a href="#Page_201">201&ndash;202</a><br />
+<br />
+Water vapour, <a href="#Page_202">202</a>, <a href="#Page_213">213</a>, <a href="#Page_222">222</a><br />
+<br />
+Wargentin, <a href="#Page_103">103</a><br />
+<br />
+Warner and Swasey Co., <a href="#Page_117">117</a><br />
+<br />
+Weather, moon and, <a href="#Page_206">206&ndash;207</a><br />
+<br />
+Weathering, <a href="#Page_202">202</a><br />
+<br />
+Webb, Rev. T.W., <a href="#Page_204">204</a><br />
+<br />
+Weight, <a href="#Page_43">43</a>, <a href="#Page_165">165&ndash;166</a><br />
+<br />
+Wells, H.G., <a href="#Page_344">344</a><br />
+<br />
+Whale (constellation). <a href="#Cetus"><i>See</i> Cetus</a><br />
+<br />
+Whewell, <a href="#Page_190">190</a><br />
+<br />
+Willamette meteorite, <a href="#Page_277">277</a><br />
+<br />
+Wilson, Mount, <a href="#Page_118">118</a><br />
+<br />
+Wilson, W.E., <a href="#Page_313">313</a><br />
+<br />
+"Winged circle" (or "disc"), <a href="#Page_87">87</a><br />
+<br />
+Winter, <a href="#Page_175">175</a>, <a href="#Page_178">178</a><br />
+<br />
+Witt, <a href="#Page_227">227</a><br />
+<br />
+Wolf, Max, <a href="#Page_226">226&ndash;227</a>, <a href="#Page_232">232</a><br />
+<br />
+Wright, Thomas, <a href="#Page_319">319</a>, <a href="#Page_334">334</a><br />
+<br />
+Wybord, <a href="#Page_89">89</a><br />
+<br />
+<br />
+<span class='pagenum'><a name="Page_363" id="Page_363">[Pg 363]</a></span><span class="smcap">Xenophon</span>, <a href="#Page_101">101</a><br />
+<br />
+<br />
+<span class="smcap">Year</span>, <a href="#Page_35">35</a><br />
+<br />
+"Year" in Uranus and Neptune, <a href="#Page_35">35&ndash;36</a><br />
+<br />
+Year, number of eclipses in a, <a href="#Page_68">68</a><br />
+<br />
+"Year of the Stars," <a href="#Page_270">270</a><br />
+<br />
+Yellow (rays of light), <a href="#Page_121">121&ndash;122</a>, <a href="#Page_124">124</a><br />
+<br />
+Yerkes Telescope Great, <a href="#Page_117">117</a>, <a href="#Page_303">303</a><br />
+<br />
+Young, <a href="#Page_94">94</a>, <a href="#Page_137">137</a>, <a href="#Page_166">166</a><br />
+<br />
+<br />
+<span class="smcap">Zenith</span>, <a href="#Page_174">174</a><br />
+<br />
+Zinc, <a href="#Page_145">145</a><br />
+<br />
+Zodiacal light, <a href="#Page_181">181</a><br />
+<br />
+Zone of asteroids, <a href="#Page_30">30&ndash;31</a>, <a href="#Page_227">227</a><br />
+</p>
+
+
+<p class="center"><br /><br /><br /><br /><br />
+THE END<br />
+</p>
+
+<p class="center">Printed by <span class="smcap">Ballantyne, Hanson &amp; Co.</span></p>
+
+<p class="center">Edinburgh &amp; London</p>
+
+
+
+<hr />
+<h3>THE SCIENCE OF TO-DAY SERIES</h3>
+
+<p class="center"><i>With many illustrations. Extra Crown 8vo. 5s. net.</i></p>
+
+<p class="hang"><big>BOTANY OF TO-DAY.</big> A Popular Account of the Evolution of Modern Botany.
+By Prof. <span class="smcap">G.F. Scott Elliot</span>, M.A., B.Sc., Author of "The Romance of Plant
+Life," <i>&amp;c. &amp;c.</i></p>
+
+<div class="blockquot"><p>"One of the books that turn botany from a dryasdust into a
+fascinating study."&mdash;<i>Evening Standard.</i> </p></div>
+
+<p class="hang"><big>AERIAL NAVIGATION OF TO-DAY.</big> A Popular Account of the Evolution of
+Aeronautics. By <span class="smcap">Charles C. Turner</span>.</p>
+
+<div class="blockquot"><p>"Mr. Turner is well qualified to write with authority on the
+subject. The book sets forth the principles of flight in plain
+non-technical language. One is impressed by the complete
+thoroughness with which the subject is treated."&mdash;<i>Daily Graphic.</i> </p></div>
+
+<p class="hang"><big>SCIENTIFIC IDEAS OF TO-DAY.</big> A Popular Account, in Non-technical
+Language, of the Nature of Matter, Electricity, Light, Heat, Electrons,
+&amp;<i>c</i>. &amp;<i>c</i>. By <span class="smcap">Charles R. Gibson</span>, A.I.E.E., Author of "Electricity of
+To-Day," &amp;c.</p>
+
+<div class="blockquot"><p>"Supplies a real need.... Mr. Gibson has a fine gift of
+exposition."&mdash;<i>Birmingham Post.</i> </p></div>
+
+<p class="hang"><big>ASTRONOMY OF TO-DAY.</big> A Popular Introduction in Non-technical Language.
+By <span class="smcap">Cecil G. Dolmage</span>, LL.D., F.R.A.S. With frontispiece in colours, &amp; 45
+other illustrations.</p>
+
+<div class="blockquot"><p>"Dr. Dolmage has absolutely kept to his promise to introduce the
+reader to an acquaintance with the astronomy of to-day in
+non-technical language."&mdash;<i>Saturday Review.</i> </p></div>
+
+<p class="hang"><big>ELECTRICITY OF TO-DAY.</big> Its Work and Mysteries Explained. By <span class="smcap">Charles R.
+Gibson</span>, A.I.E.E.</p>
+
+<div class="blockquot"><p>"Mr. Gibson has given us one of the best examples of popular
+scientific exposition that we remember seeing. His book may be
+strongly commended to all who wish to realise what electricity
+means and does in our daily life."&mdash;<i>The Tribune.</i> </p></div>
+
+<p class="center">
+SEELEY &amp; CO., LIMITED<br />
+</p>
+
+
+<hr />
+<h3>THE SCIENCE OF TO-DAY SERIES</h3>
+
+<p class="center"><i>With many Illustrations. Extra Crown 8vo. 5s. net.</i></p>
+
+
+<p class="hang"><big>AERIAL NAVIGATION OF TO-DAY.</big> A Popular Account of the Evolution of
+Aeronautics. By <span class="smcap">Charles C. Turner</span>.</p>
+
+<div class="blockquot"><p>"If ever the publication of a book was well timed, surely it is the
+case with this book on aviation.... Of the technical chapters we
+need only say that they are so simply written as to present no
+grave difficulties to the beginner who is equipped with an average
+education."&mdash;<i>Globe.</i> </p></div>
+
+
+<p class="hang"><big>BOTANY OF TO-DAY.</big> A Popular Account of the Evolution of Modern Botany.
+By Prof. <span class="smcap">G.F. Scott-Elliot</span>, M.A., B.Sc., Author of "The Romance of Plant
+Life," <i>&amp;c. &amp;c.</i></p>
+
+<div class="blockquot"><p>"This most entertaining and instructive book. It is the fruit of
+wide reading and much patient industry."&mdash;<i>Globe.</i> </p></div>
+
+
+<p class="hang"><big>SCIENTIFIC IDEAS OF TO-DAY.</big> A Popular Account, in Non-technical
+Language, of the Nature of Matter, Electricity, Light, Heat, Electrons,
+<i>&amp;c. &amp;c.</i> By <span class="smcap">Charles R. Gibson</span>, A.I.E.E., Author of "Electricity of
+To-Day," <i>&amp;c.</i></p>
+
+<div class="blockquot"><p>"As a knowledgeable writer, gifted with the power of imparting what
+he knows in a manner intelligible to all, Mr. C.R. Gibson has
+established a well-deserved reputation."&mdash;<i>Field.</i> </p></div>
+
+
+<p class="hang"><big>ASTRONOMY OF TO-DAY.</big> A Popular Introduction in Non-technical Language.
+By <span class="smcap">Cecil G. Dolmage</span>, LL.D., F.R.A.S. With frontispiece in colours, &amp; 45
+other illustrations.</p>
+
+<div class="blockquot"><p>"A lucid exposition much helped by abundant illustrations."&mdash;<i>The
+Times.</i></p>
+
+<p>"From cover to cover the book is readable, and every word is
+intelligible to the layman. Dr. Dolmage displays literary powers of
+a very high order. Those who read it without any previous knowledge
+of astronomy will find that a new interest has been added to their
+lives, and that in a matter of 350 pages they have gained a true
+conception of the meaning of astronomy."&mdash;<i>The Standard.</i> </p></div>
+
+
+<p class="hang"><big>ELECTRICITY OF TO-DAY.</big> Its Work and Mysteries Explained. By <span class="smcap">Charles R.
+Gibson</span>, A.I.E.E.</p>
+
+<div class="blockquot"><p>"Mr. Gibson has given us one of the best examples of popular
+scientific exposition that we remember seeing. His aim has been to
+produce an account of the chief modern applications of electricity
+without using technical language or making any statements which are
+beyond the comprehension of any reader of ordinary intelligence. In
+this he has succeeded to admiration, and his book may be strongly
+commended to all who wish to realise what electricity means and
+does in our daily life."&mdash;<i>The Tribune.</i> </p></div>
+
+<p class="center"><span class="smcap">SEELEY &amp; CO., Ltd., 38 Great Russell Street</span>.</p>
+
+
+
+<hr />
+<h3>THE ROMANCE OF<br /> MODERN ELECTRICITY</h3>
+
+<p class="center"><small>DESCRIBING IN NON-TECHNICAL LANGUAGE WHAT IS KNOWN ABOUT ELECTRICITY &amp;
+MANY OF ITS INTERESTING APPLICATIONS</small></p>
+
+<p class="center"><span class="smcap">By CHARLES R. GIBSON, A.I.E.E.</span></p>
+
+<p class="center">AUTHOR or "ELECTRICITY of TO-DAY," ETC.</p>
+
+<p class="center"><i>Extra Crown 8vo.</i> <i>With 34 Illustrations and 11 Diagrams.</i> 5<i>s.</i></p>
+
+<div class="blockquot"><p>"Everywhere Mr. Charles R. Gibson makes admirable use of simple
+analogies which bespeak the practised lecturer, and bring the
+matter home without technical detail. The attention is further
+sustained by a series of surprises. The description of electric
+units, the volt, the ohm, and especially the ampere, is better than
+we have found in more pretentious works."&mdash;<i>Academy.</i></p>
+
+<p>"Mr. Gibson's style is very unlike the ordinary text-book. It is
+fresh, and is non-technical. Its facts are strictly scientific,
+however, and thoroughly up to date. If we wish to gain a thorough
+knowledge of electricity pleasantly and without too much trouble on
+our own part, we will read Mr. Gibson's 'Romance.'"&mdash;<i>Expository
+Times.</i></p>
+
+<p>"A book which the merest tyro totally unacquainted with elementary
+electrical principles can understand, and should therefore
+especially appeal to the lay reader. Especial interest attaches to
+the chapter on wireless telegraphy, a subject which is apt to
+'floor' the uninitiated. The author reduces the subject to its
+simplest aspect, and describes the fundamental principles
+underlying the action of the coherer in language so simple that
+anyone can grasp them."&mdash;<i>Electricity.</i></p>
+
+<p>"Contains a clear and concise account of the various forms in which
+electricity is used at the present day, and the working of the
+telephone, wireless telegraphy, tramcars, and dynamos is explained
+with the greatest possible lucidity, while the marvels of the
+X-rays and of radium receive their due notice. Now that electricity
+plays such an all-important part in our daily life, such a book as
+this should be in the hands of every boy. Indeed, older people
+would learn much from its pages. For instance, how few people could
+explain the principles of wireless telegraphy in a few words if
+suddenly questioned on the subject. The book is well and
+appropriately illustrated."&mdash;<i>Graphic.</i></p>
+
+<p>"Mr. Gibson sets out to describe in non-technical language the
+marvellous discoveries and adaptation of this pervasive and
+powerful essence, and being a most thorough master of the subject,
+he leads the reader through its mazes with a sure hand. Throughout
+he preserves a clear and authoritative style of exposition which
+will be understood by any intelligent reader."&mdash;<i>Yorkshire
+Observer.</i></p>
+
+<p>"A popular and eminently readable manual for those interested in
+electrical appliances. It describes in simple and non-technical
+language what is known about electricity and many of its
+interesting applications. There are a number of capital
+illustrations and diagrams which will help the reader greatly in
+the study of the book."&mdash;<i>Record.</i> </p></div>
+
+<p class="center"><span class="smcap">SEELEY &amp; CO., Ltd., 38 Great Russell Street.</span></p>
+
+
+
+<hr />
+<h3>THE ROMANCE OF SAVAGE LIFE</h3>
+
+<p class="center"><small>DESCRIBING THE HABITS, CUSTOMS, EVERYDAY LIFE, &amp;c., OF PRIMITIVE MAN</small></p>
+
+<p class="center"><span class="smcap">By Prof. G.F. SCOTT ELLIOT, M.A., B.Sc., &amp;c.</span></p>
+
+<p class="center"><i>With Thirty Illustrations.</i> <i>Extra Crown 8vo.</i> 5<i>s.</i></p>
+
+<div class="blockquot"><p>"Mr. Scott Elliot has hit upon a good idea in this attempt to set
+forth the life of the primitive savage. On the whole, too, he has
+carried it out well and faithfully.... We can recommend the book as
+filling a gap."&mdash;<i>Athen&aelig;um.</i></p>
+
+<p>"A readable contribution to the excellent series of which it forms
+a part. Mr. Scott Elliot writes pleasantly ... he possesses a
+sufficiently vivid imagination to grasp the relation of a savage to
+his environment."&mdash;<i>Nature.</i></p>
+
+<p>"There are things of remarkable interest in this volume, and it
+makes excellent reading and represents much
+research."&mdash;<i>Spectator.</i> </p></div>
+
+
+<h3>THE ROMANCE OF PLANT LIFE</h3>
+
+<p class="center"><small>DESCRIBING THE CURIOUS AND INTERESTING IN THE PLANT WORLD</small></p>
+
+<p class="center"><span class="smcap">By Prof. G.F. SCOTT ELLIOT, M.A., B.Sc., &amp;c.</span></p>
+
+<p class="center"><i>With Thirty-four Illustrations.</i> <i>Extra Crown 8vo.</i> 5<i>s.</i></p>
+
+<div class="blockquot"><p>"The author has worked skilfully into his book details of the facts
+and inferences which form the groundwork of modern Botany. The
+illustrations are striking, and cover a wide field of interest, and
+the style is lively."&mdash;<i>Athen&aelig;um.</i></p>
+
+<p>"In twenty-nine fascinating, well-printed, and well-illustrated
+chapters, Prof. Scott Elliot describes a few of the wonders of
+plant life. A very charming and interesting volume."&mdash;<i>Daily
+Telegraph.</i></p>
+
+<p>"Mr. Scott Elliot is of course a well-known authority on all that
+concerns plants, and the number of facts he has brought together
+will not only surprise but fascinate all his
+readers."&mdash;<i>Westminster Gazette.</i> </p></div>
+
+<p class="center"><span class="smcap">SEELEY &amp; CO., Ltd., 38 Great Russell Street.</span></p>
+
+
+
+<hr />
+<h3>THE ROMANCE OF INSECT LIFE</h3>
+
+<p class="center"><small>DESCRIBING THE CURIOUS &amp; INTERESTING IN THE INSECT WORLD</small></p>
+
+<p class="center"><span class="smcap"><big>By EDMUND SELOUS</big></span></p>
+
+<p class="center"><small>AUTHOR OF "THE ROMANCE OF THE ANIMAL WORLD," ETC.</small></p>
+
+<p class="center"><i>With Sixteen Illustrations.</i> <i>Extra Crown 8vo.</i> 5<i>s.</i></p>
+
+<div class="blockquot"><p>"An entertaining volume, one more of a series which seeks with much
+success to describe the wonders of nature and science in simple,
+attractive form."&mdash;<i>Graphic.</i></p>
+
+<p>"Offers most interesting descriptions of the strange and curious
+inhabitants of the insect world, sure to excite inquiry and to
+foster observation. There are ants white and yellow, locusts and
+cicadas, bees and butterflies, spiders and beetles, scorpions and
+cockroaches&mdash;and especially ants&mdash;with a really scientific
+investigation of their wonderful habits not in dry detail, but in
+free and charming exposition and narrative. An admirable book to
+put in the hands of a boy or girl with a turn for natural
+science&mdash;and whether or not."&mdash;<i>Educational Times.</i></p>
+
+<p>"Both interesting and instructive. Such a work as this is genuinely
+educative. There are numerous illustrations."&mdash;<i>Liverpool Courier.</i></p>
+
+<p>"With beautiful original drawings by Carton Moore Park and Lancelot
+Speed, and effectively bound in dark blue cloth, blazoned with
+scarlet and gold."&mdash;<i>Lady.</i></p>
+
+<p>"Admirably written and handsomely produced. Mr. Selous's volume
+shows careful research, and the illustrations of insects and the
+results of their powers are well done."&mdash;<i>World.</i> </p></div>
+
+
+<h3>THE ROMANCE OF<br /> MODERN MECHANISM</h3>
+
+<p class="center"><small>INTERESTING DESCRIPTIONS IN NON-TECHNICAL LANGUAGE OF WONDERFUL
+MACHINERY, MECHANICAL. DEVICES, &amp; MARVELLOUSLY DELICATE SCIENTIFIC
+INSTRUMENTS</small></p>
+
+<p class="center"><span class="smcap"><big>By ARCHIBALD WILLIAMS, B.A., F.R.G.S.</big></span></p>
+
+<p class="center"><small>AUTHOR OF "THE ROMANCE OF MODERN EXPLORATION," ETC.</small></p>
+
+<p class="center"><i>With Twenty-six Illustrations.</i> <i>Extra Crown 8vo.</i> 5<i>s.</i></p>
+
+<div class="blockquot"><p>"No boy will be able to resist the delights of this book, full to
+the brim of instructive and wonderful matter."&mdash;<i>British Weekly.</i></p>
+
+<p>"This book has kept your reviewer awake when he reasonably expected
+to be otherwise engaged. We do not remember coming across a more
+fascinating volume, even to a somewhat blas&eacute; reader whose business
+it is to read all that comes in his way. The marvels miracles they
+should be called, of the modern workshop are here exploited by Mr.
+Williams for the benefit of readers who have not the opportunity of
+seeing these wonders or the necessary mathematical knowledge to
+understand a scientific treatise on their working. Only the
+simplest language is used and every effort is made, by illustration
+or by analogy, to make sufficiently clear to the non-scientific
+reader how the particular bit of machinery works and what its work
+really is. Delicate instruments, calculating machines, workshop
+machinery, portable tools, the pedrail, motors ashore and afloat,
+fire engines, automatic machines, sculpturing machines&mdash;these are a
+few of the chapters which crowd this splendid
+volume."&mdash;<i>Educational News.</i></p>
+
+<p>"It is difficult to make descriptions of machinery and mechanism
+interesting, but Mr. Williams has the enviable knack of doing so,
+and it is hardly possible to open this book at any page without
+turning up something which you feel you must read; and then you
+cannot stop till you come to the end of the
+chapter."&mdash;<i>Electricity.</i></p>
+
+<p>"This book is full of interest and instruction, and is a welcome
+addition to Messrs. Seeley and Company's Romance Series."&mdash;<i>Leeds
+Mercury.</i></p>
+
+<p>"A book of absorbing interest for the boy with a mechanical turn,
+and indeed for the general reader."&mdash;<i>Educational Times.</i></p>
+
+<p>"An instructive and well-written volume."&mdash;<i>Hobbies.</i> </p></div>
+
+<p class="center"><span class="smcap">SEELEY &amp; CO., Ltd., 88 Great Russell Street.</span></p>
+
+<hr />
+<h3>A Catalogue of Books on Art,<br /> History, and General Literature<br /> Published
+by Seeley, Service &amp; Co<br /> Ltd. 38 Great Russell St. London</h3>
+
+
+<p class="center"><i>Some of the Contents</i></p>
+
+
+<div class='toc'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Catalogue Contents">
+<tr>
+ <td align='left'>Crown Library, The</td>
+ <td align='right'>4</td>
+</tr>
+<tr>
+ <td align='left'>Elzevir Library, The</td>
+ <td align='right'>5</td>
+</tr>
+<tr>
+ <td align='left'>Events of Our Own Times Series</td>
+ <td align='right'>6</td>
+</tr>
+<tr>
+ <td align='left'>Illuminated Series, The</td>
+ <td align='right'>8</td>
+</tr>
+<tr>
+ <td align='left'>Miniature Library of Devotion, The</td>
+ <td align='right'>9</td>
+</tr>
+<tr>
+ <td align='left'>Miniature Portfolio Monographs, The</td>
+ <td align='right'>9</td>
+</tr>
+<tr>
+ <td align='left'>Missions, The Library of</td>
+ <td align='right'>10</td>
+</tr>
+<tr>
+ <td align='left'>New Art Library, The</td>
+ <td align='right'>11</td>
+</tr>
+<tr>
+ <td align='left'>Portfolio Monographs</td>
+ <td align='right'>11</td>
+</tr>
+<tr>
+ <td align='left'>Science of To-Day Series, The</td>
+ <td align='right'>14</td>
+</tr>
+<tr>
+ <td align='left'>Seeley's Illustrated Pocket Library</td>
+ <td align='right'>14</td>
+</tr>
+<tr>
+ <td align='left'>Seeley's Standard Library</td>
+ <td align='right'>15</td>
+</tr>
+<tr>
+ <td align='left'>Story Series, The</td>
+ <td align='right'>15</td>
+</tr>
+<tr>
+ <td align='left'>"Things Seen" Series, The</td>
+ <td align='right'>16</td>
+</tr>
+</table></div>
+
+
+<p class="center"><i>The Publishers will be pleased to post their complete Catalogue or
+their Illustrated Miniature Catalogue on receipt of a post-card</i></p>
+
+
+
+<hr />
+<h2>CATALOGUE OF BOOKS</h2>
+
+<p><i>Arranged alphabetically under the names of Authors and Series</i></p>
+
+
+<p>ABBOTT, Rev. E.A., D.D.</p>
+
+<div class="blockquot"><p class="noin"><b>How to Parse.</b> An English Grammar. Fcap. 8vo, 3s. 6d.</p>
+
+<p class="noin"><b>How to Tell the Parts of Speech.</b> An Introduction to English
+Grammar. Fcap. 8vo, 2s.</p>
+
+<p class="noin"><b>How to Write Clearly.</b> Rules and Exercises on English Composition.
+1s. 6d.</p>
+
+<p class="noin"><b>Latin Gate, The.</b> A First Latin Translation Book. Crown 8vo, 3s. 6d.</p>
+
+<p class="noin"><b>Via Latina.</b> A First Latin Grammar. Crown 8vo, 3s. 6d. </p></div>
+
+
+<p>ABBOTT, Rev. E.A., and Sir J.R. SEELEY.</p>
+
+<div class="blockquot"><p class="noin"><b>English Lessons for English People.</b> Crown 8vo, 4s. 6d.</p>
+
+<p class="noin"><b>ADY, Mrs.</b> <i>See</i> <span class="smcap">Cartwright, Julia</span>. </p></div>
+
+
+<p>&Agrave; KEMPIS, THOMAS.</p>
+
+<div class="blockquot"><p class="noin"><b>Of the Imitation of Christ.</b> With Illuminated Frontispiece and Title
+Page, and Illuminated Sub-Titles to each book. In white or blue
+cloth, with inset miniatures. Gilt top; crown 8vo, 6s. nett; also
+bound in same manner in real classic vellum. Each copy in a box,
+10s. 6d. nett; Antique leather with clasps, 10s. 6d. nett.</p>
+
+<p class="noin">"It may well be questioned whether the great work of Thomas &agrave;
+Kempis has ever been presented to better advantage."&mdash;<i>The
+Guardian.</i> </p></div>
+
+
+<p>ANDERSON, Prof. W.</p>
+
+<div class="blockquot"><p class="noin"><b>Japanese Wood Engravings.</b> Coloured Illustrations. Super-royal 8vo,
+sewed, 2s. 6d. nett; half-linen, 3s. 6d. nett; also small 4to,
+cloth, 2s. nett; lambskin, 3s. nett. </p></div>
+
+
+<p>ARMSTRONG, Sir WALTER.</p>
+
+<div class="blockquot"><p class="noin"><b>The Art of Velazquez.</b> Illustrated. Super-royal 8vo, 3s. 6d. nett.</p>
+
+<p class="noin"><b>The Life of Velazquez.</b> Illustrated. Super-royal 8vo, 3s. 6d. nett.</p>
+
+<p class="noin"><b>Velazquez.</b> A Study of his Life and Art. With Eight Copper Plates
+and many minor Illustrations. Super-royal 8vo, cloth, 9s. nett.</p>
+
+<p class="noin"><b>Thomas Gainsborough.</b> Illustrated. Super-royal 8vo, half-linen, 3s.
+6d. nett. Also new edition small 4to, cloth, 2s. nett; leather, 3s.
+nett and 5s. nett.</p>
+
+<p class="noin"><b>The Peel Collection and the Dutch School of Painting.</b> With
+Illustrations in Photogravure and Half-tone. Super-royal 8vo,
+sewed, 5s. nett; cloth, 7s. nett.</p>
+
+<p class="noin"><b>W.Q. Orchardson.</b> Super-royal 8vo, sewed, 2s. 6d.; half-linen, 3s.
+6d. nett. </p></div>
+
+
+<p>AUGUSTINE, S.</p>
+
+<div class="blockquot"><p class="noin"><b>Confessions of S. Augustine.</b> With Illuminated pages. In white or
+blue cloth, gilt top, crown 8vo, 6s. nett; also in vellum, 10s. 6d.
+nett. </p></div>
+
+
+<p>BAKER, Captain B. GRANVILLE</p>
+
+<div class="blockquot"><p class="noin"><b>The Passing of the Turkish Empire in Europe.</b> With Thirty-two
+Illustrations. Demy 8vo, 16s. nett. </p></div>
+
+
+<p>BARING-GOULD, Rev. S.</p>
+
+<div class="blockquot"><p class="noin"><b>Family Names and their Story.</b> Demy 8vo, 7s. 6d. nett. 5s. nett. </p></div>
+
+
+<p>BEDFORD, Rev. W.K.R.</p>
+
+<div class="blockquot"><p class="noin"><b>Malta and the Knights Hospitallers.</b> Super-royal 8vo, sewed, 2s. 6d.
+nett; half-linen, 3s. 6d. nett. </p></div>
+
+
+<p>BENHAM, Rev. Canon D.D., F.S.A.</p>
+
+<div class="blockquot"><p class="noin"><b>The Tower of London.</b> With Four Plates in Colours and many other
+Illustrations. Super-royal 8vo, sewed, 5s. nett; cloth, 7s. nett.</p>
+
+<p class="noin"><b>Medi&aelig;val London.</b> With a Frontispiece in Photogravure, Four Plates
+in Colour, and many other Illustrations. Super-royal 8vo, sewed,
+5s. nett; cloth, gilt top, 7s. nett. Also extra crown 8vo, 3s. 6d.
+nett.</p>
+
+<p class="noin"><b>Old St. Paul's Cathedral.</b> With a Frontispiece in Photogravure, Four
+Plates printed in Colour, and many other Illustrations. Super-royal
+8vo, sewed, 5s. nett, or cloth, gilt top, 7s. nett. </p></div>
+
+
+<p>BENNETT, EDWARD.</p>
+
+<div class="blockquot"><p class="noin"><b>The Post Office and its Story.</b> An interesting account of the
+activities of a great Government department. With Twenty-five
+Illustrations. Ex. crn. 8vo, 5s. nett. </p></div>
+
+
+<p>BICKERSTETH, Rev. E.</p>
+
+<div class="blockquot"><p class="noin"><b>Family Prayers for Six Weeks.</b> Crown 8vo, 3s. 6d.</p>
+
+<p class="noin"><b>A Companion to the Holy Communion.</b> 32mo, cloth, 1s. </p></div>
+
+
+<p>BINYON, LAURENCE.</p>
+
+<div class="blockquot"><p class="noin"><b>Dutch Etchers of the Seventeenth Century.</b> Illustrated. Super-royal
+8vo, sewed, 2s. 6d.; half-linen, 3s. 6d. nett.</p>
+
+<p class="noin"><b>John Crome and John Sell Cotman.</b> Illustrated. Super-royal 8vo
+sewed, 3s. 6d. nett. </p></div>
+
+
+<p>BIRCH, G.H.</p>
+
+<div class="blockquot"><p class="noin"><b>London on Thames in Bygone Days.</b> With Four Plates printed in Colour
+and many other Illustrations. Super-royal 8vo, sewed, 5s. nett;
+cloth, 7s. nett. </p></div>
+
+
+<p>BRIDGES, Rev. C.</p>
+
+<div class="blockquot"><p class="noin"><b>An Exposition of Psalm CXIX.</b> Crown 8vo, 5s. </p></div>
+
+
+<p>BUTCHER, E.L.</p>
+
+<div class="blockquot"><p class="noin"><b>Things Seen in Egypt.</b> With Fifty Illustrations. Small 4to, cloth,
+2s. nett; lambskin, 3s. nett; velvet leather, in box, 5s. nett.</p>
+
+<p class="noin"><b>Poems</b>, 1s. 6d. nett. </p></div>
+
+
+<p>CACHEMAILLE, Rev. E.P., M.A.</p>
+
+<div class="blockquot"><p class="noin"><b>XXVI Present-Day Papers on Prophecy.</b> An explanation of the visions
+of Daniel and of the Revelation, on the continuous historic system.
+With Maps and Diagrams. 700 pp. 6s. nett. </p></div>
+
+
+<p>CARTWRIGHT, JULIA.</p>
+
+<div class="blockquot"><p class="noin"><b>Jules Bastien-Lepage.</b> Super-royal 8vo, sewed, 2s. 6d.; cloth, 3s.
+6d. nett.</p>
+
+<p class="noin"><b>Sacharissa.</b> Some Account of Dorothy Sidney, Countess of Sunderland,
+her Family and Friends. With Five Portraits. Demy 8vo, 7s. 6d.</p>
+
+<p class="noin"><b>Raphael in Rome.</b> Illustrated. Super-royal 8vo, sewed, 2s. 6d.;
+half-linen, 3s. 6d. nett; also in small 4to, cloth, 2s. nett;
+leather, 3s. nett and 5s. nett.</p>
+
+<p class="noin"><b>The Early Work of Raphael.</b> Illustrated. Super-royal 8vo, sewed 2s.
+6d.; half-linen, 3s. 6d. Also new edition, revised, in small 4to,
+in cloth, 2s. nett; leather, 3s. nett.</p>
+
+<p class="noin"><b>Raphael:</b> A Study of his Life and Work. With Eight Copper Plates and
+many other Illustrations. Super-royal 8vo, 7s. 6d. nett. </p></div>
+
+
+<p>CESARESCO, The Countess MARTINENGO</p>
+
+<div class="blockquot"><p class="noin"><b>The Liberation of Italy.</b> With Portraits on Copper. Crown 8vo, 5s. </p></div>
+
+
+<p>CHATTERTON, E. KEBLE.</p>
+
+<div class="blockquot"><p class="noin"><b>Fore and Aft.</b> The Story of the Fore and Aft Rig from the Earliest
+Times to the Present Day. Sq. ex. royal 8vo. With 150 Illustrations
+and Coloured Frontispiece by C. Dixon, R.I. 16s. nett.</p>
+
+<p class="noin"><b>Through Holland in the "Vivette."</b> The Cruise of a 4&ndash;Tonner from the
+Solent to the Zuyder Zee, through the Dutch Waterways. With Sixty
+Illustrations and Charts, 6s. nett. </p></div>
+
+
+<p>CHITTY, J.R.</p>
+
+<div class="blockquot"><p class="noin"><b>Things Seen in China.</b> With Fifty Illustrations. Small 4to; cloth,
+2s.; leather, 3s.; velvet leather in a box, 5s. nett. </p></div>
+
+
+<p>CHORAL SERVICE-BOOK FOR PARISH CHURCHES, THE.</p>
+
+<div class="blockquot"><p class="noin"><small>Compiled and Edited by J.W. <span class="smcap">Elliott</span>, Organist and Choirmaster of St.
+Mark's, Hamilton Terrace, London. With some Practical Counsels taken by
+permission from "Notes on the Church Service," by Bishop <span class="smcap">Walsham How</span>.</small></p></div>
+
+<p class="poem">
+<small><b>A.</b> Royal 8vo, sewed, 1s.; cloth, 1s. 6d.<br />
+<b>B.</b> 16mo, sewed, 6d.; cloth, 8d.</small><br />
+</p>
+
+
+<p><small><i>The following portions may be had separately:</i>&mdash;</small></p>
+
+<div class="blockquot"><p class="noin"><b>The Ferial and Festal Responses and the Litany.</b> Arranged by J.W.
+<span class="smcap">Elliott</span>. Sewed, 4d.</p>
+
+<p class="noin"><b>The Communion Service, Kyrie, Credo, Sanctus, and Gloria in
+Excelsis.</b> Set to Music by Dr. J. <span class="smcap">Naylor</span>, Organist of York Minster.
+Sewed, 4d. </p></div>
+
+
+<p>CHURCH, Sir ARTHUR H., F.R.S.</p>
+
+<div class="blockquot"><p class="noin"><b>Josiah Wedgwood, Master Potter.</b> With many Illustrations.
+Super-royal 8vo, sewed, 5s. nett; cloth, 7s. nett; also small 4to,
+cloth, 2s. nett; leather, 3s. and 5s. nett.</p>
+
+<p class="noin"><b>The Chemistry of Paints and Painting.</b> Third Edition. Crown 8vo, 6s. </p></div>
+
+
+<p>CHURCH, Rev. A.J.</p>
+
+<div class="blockquot"><p class="noin"><b>Nicias, and the Sicilian Expedition.</b> Crown 8vo, 1s. 6d. </p></div>
+
+<p class="center">
+<small>For other books by Professor <span class="smcap">Church</span> see Complete Catalogue.</small>
+</p>
+
+
+<p>CLARK, J.W., M.A.</p>
+
+<div class="blockquot"><p class="noin"><b>Cambridge.</b> With a coloured Frontispiece and many other
+Illustrations by A. <span class="smcap">Brunet-Debaines</span> and H. <span class="smcap">Toussaint</span> &amp;c. Extra
+crown 8vo, 6s.; also crown 8vo, cloth, 2s. nett; leather, 3s.;
+special leather, in box, 5s. nett. </p></div>
+
+
+<p>CODY, Rev. H.A.</p>
+
+<div class="blockquot"><p class="noin"><b>An Apostle of the North.</b> The Biography of the late Bishop <span class="smcap">Bompas</span>,
+First Bishop of Athabasca, and with an Introduction by the
+<span class="smcap">Archbishop</span> of <span class="smcap">Ruperts-land</span>. With 42 Illustrations. Demy 8vo, 7s.
+6d. nett. 5s. nett. </p></div>
+
+
+<p>CORBIN, T.W.</p>
+
+<div class="blockquot"><p class="noin"><b>Engineering of To-day.</b> With Seventy-three Illustrations and
+Diagrams. Extra crown 8vo, 5s. nett.</p>
+
+<p class="noin"><b>Mechanical Inventions of To-Day.</b> Ex. Crown 8vo; with Ninety-four
+Illustrations, 5s. nett. </p></div>
+
+
+<p>CORNISH, C.J.</p>
+
+<div class="blockquot"><p class="noin"><b>Animals of To-day:</b> Their Life and Conversation. With Illustrations
+from Photographs by C. <span class="smcap">Reid</span> of Wishaw. Crown 8vo, 6s.</p>
+
+<p class="noin"><b>The Isle of Wight.</b> Illustrated. Super-royal 8vo, sewed, 2s. 6d.
+nett; half-linen, 3s. 6d. nett; also a new edition, small 4to,
+cloth, 2s.; leather, 3s. and 5s.</p>
+
+<p class="noin"><b>Life at the Zoo.</b> Notes and Traditions of the Regent's Park Gardens.
+Illustrated from Photographs by <span class="smcap">Gambier Bolton</span>. Fifth Edition.
+Crown 8vo, 6s.</p>
+
+<p class="noin"><b>The Naturalist on the Thames.</b> Many Illustrations. Demy 8vo, 7s. 6d.</p>
+
+<p class="noin"><b>The New Forest.</b> Super-royal 8vo, sewed, 2s. 6d. nett; half-linen,
+3s. 6d. nett; also new edition, small 4to, cloth, 2s.; leather, 3s.
+nett; and special velvet leather, each copy in a box, 5s.</p>
+
+<p class="noin"><b>The New Forest and the Isle of Wight.</b> With Eight Plates and many
+other Illustrations. Super-royal 8vo, 7s. 6d. nett.</p>
+
+<p class="noin"><b>Nights with an Old Gunner,</b> and other Studies of Wild Life. With
+Sixteen Illustrations by <span class="smcap">Lancelot Speed</span>, <span class="smcap">Charles Whymper</span>, and from
+Photographs. Crown 8vo, 6s. </p></div>
+
+<hr style='width: 15%;' />
+
+<h3>THE CROWN LIBRARY</h3>
+
+<p class="center">A series of notable copyright books issued in uniform binding. Extra
+crown 8vo. With many illustrations, 5s. nett.</p>
+
+
+<p class="center"><i>JUST ISSUED. SECOND AND CHEAPER EDITION.</i></p>
+
+
+<p>SWANN, A.J.</p>
+
+<div class="blockquot"><p class="noin"><b>Fighting the Slave Hunters in Central Africa.</b> A Record of
+Twenty-six Years of Travel and Adventure round the Great Lakes, and
+of the overthrow of Tip-pu-Tib, Rumaliza, and other great Slave
+Traders. With 45 Illustrations and a Map, 5s. nett. </p></div>
+
+
+<p class="center"><i>RECENTLY ISSUED.</i></p>
+
+
+<p>GRUBB, W. BARBROOKE.</p>
+
+<div class="blockquot"><p class="noin"><b>An Unknown People in an Unknown Land.</b> An Account of the Life and
+Customs of the Lengua Indians of the Paraguayan Chaco, with
+Adventures and Experiences met with during Twenty Years' Pioneering
+and Exploration amongst them. With Twenty-four Illustrations and a
+Map. Extra crown 8vo, 5s. nett. </p></div>
+
+
+<p>FRASER, Sir A.H.L., K.C.S.I., M.A., LL.D., Litt.D.,
+ex-Lieutenant-Governor of Bengal.</p>
+
+<div class="blockquot"><p class="noin"><b>Among Indian Rajahs and Ryots.</b> A Civil Servants' Recollections and
+Impressions of Thirty-seven Years of Work and Sport in the Central
+Provinces and Bengal. Third Edition, 5s. nett. </p></div>
+
+
+<p>CODY, Rev. H.A.</p>
+
+<div class="blockquot"><p class="noin"><b>An Apostle of the North.</b> The Story of Bishop Bompas's Life amongst
+the Red Indians &amp; Eskimo. Third Edition, 5s. nett. </p></div>
+
+
+<p>PENNELL, T.L., M.D., B.Sc.</p>
+
+<div class="blockquot"><p class="noin"><b>Among the Wild Tribes of the Afghan Frontier.</b> A Record of Sixteen
+Years' close intercourse with the natives of Afghanistan and the
+North-West Frontier. Introduction by EARL ROBERTS. Extra crown 8vo.
+Twenty-six Illustrations and Map. Fifth Edition, 5s. net. </p></div>
+
+<hr style='width: 15%;' />
+
+<p>CUST, LIONEL.</p>
+
+<div class="blockquot"><p class="noin"><b>The Engravings of Albert D&uuml;rer.</b> Illustrated. Super-royal 8vo,
+half-linen, 3s. 6d. nett.</p>
+
+<p class="noin"><b>Paintings and Drawings of Albert D&uuml;rer.</b> Illustrated. Super-royal
+8vo, sewed, 3s. 6d. nett.</p>
+
+<p class="noin"><b>Albrecht D&uuml;rer.</b> A Study of his Life and Work. With Eight Copper
+Plates and many other Illustrations. Super-royal 8vo, 7s. 6d. </p></div>
+
+
+<p>DAVENPORT, CYRIL.</p>
+
+<div class="blockquot"><p class="noin"><b>Cameos.</b> With examples in Colour and many other Illustrations.
+Super-royal 8vo, sewed, 5s. nett; cloth, 7s. nett.</p>
+
+<p class="noin"><b>Royal English Bookbindings.</b> With Coloured Plates and many other
+Illustrations. Super-royal 8vo, sewed, 3s. 6d.; cloth, 4s. 6d. </p></div>
+
+
+<p>DAVIES, RANDALL, F.S.A.</p>
+
+<div class="blockquot"><p class="noin"><b>English Society of the Eighteenth Century in Contemporary Art.</b> With
+Four Coloured and many other Illustrations. Super royal 8vo, sewed,
+5s. nett; cloth, 7s. nett. </p></div>
+
+
+<p>DAWSON, Rev. E.C.</p>
+
+<div class="blockquot"><p class="noin"><b>The Life of Bishop Hannington.</b> Crown 8vo, paper boards, 2s. 6d.; or
+with Map and Illustrations, cloth, 3s. 6d. </p></div>
+
+
+<p>DESTR&Eacute;E, O.G.</p>
+
+<div class="blockquot"><p class="noin"><b>The Renaissance of Sculpture in Belgium.</b> Illustrated. Super-royal
+8vo, sewed, 2s. 6d. nett; half-linen, 3s. 6d. nett. </p></div>
+
+
+<p>DOLMAGE, CECIL G., M.A., D.C.L., LL.D., F.R.A.S.</p>
+
+<div class="blockquot"><p class="noin"><b>Astronomy of To-Day.</b> A popular account in non-technical language.
+With Forty-six Illustrations and Diagrams. Extra crown 8vo, 5s.
+nett. </p></div>
+
+
+<p>DOMVILLE-FIFE, CHARLES W.</p>
+
+<div class="blockquot"><p class="noin"><b>Submarine Engineering of To-Day.</b> Extra crown 8vo, 5s. nett. </p></div>
+
+
+<p>ELZEVIR LIBRARY, THE.</p>
+
+<div class="blockquot"><p class="noin"><b>Selections from the choicest English Writers.</b> Exquisitely
+Illustrated, with Frontispiece and Title-page in Colours by <span class="smcap">H.M.
+Brock,</span> and many other Illustrations. Half bound in cloth, coloured
+top, 1s. nett; full leather, 1s. 6d. nett; velvet leather, gilt
+edges, in a box, 2s. 6d. nett. </p></div>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="English Writers">
+<tr>
+ <td align='left'>Volume I.</td>
+ <td align='left'>Fancy &amp; Humour of Lamb.</td>
+</tr>
+<tr>
+ <td align='left'>Volume II.</td>
+ <td align='left'>Wit &amp; Imagination of Disraeli.</td>
+</tr>
+<tr>
+ <td align='left'>Volume III.</td>
+ <td align='left'>Vignettes from Oliver Goldsmith.</td>
+</tr>
+<tr>
+ <td align='left'>Volume IV.</td>
+ <td align='left'>Wit &amp; Sagacity of Dr. Johnson.</td>
+</tr>
+<tr>
+ <td align='left'>Volume V.</td>
+ <td align='left'>Insight &amp; Imagination of John Ruskin.</td>
+</tr>
+<tr>
+ <td align='left'>Volume VI.</td>
+ <td align='left'>Vignettes of London Life from Dickens.</td>
+</tr>
+<tr>
+ <td align='left'>Volume VII.</td>
+ <td align='left'>XVIIIth Century Vignettes from Thackeray.</td>
+</tr>
+<tr>
+ <td align='left'>Volume VIII.</td>
+ <td align='left'>Vignettes of Country Life from Dickens.</td>
+</tr>
+<tr>
+ <td align='left'>Volume IX.</td>
+ <td align='left'>Wisdom &amp; Humour of Carlyle.</td>
+</tr>
+</table></div>
+
+<p class="center">
+<small>"Decidedly natty and original in get-up."&mdash;<i>The Saturday Review.</i></small>
+</p>
+
+
+<p>EVANS, WILLMOTT, M.D.</p>
+
+<div class="blockquot"><p class="noin"><b>Medical Science of To-Day.</b> Ex. crn. 8vo; 24 Illustrations, 5s.
+nett. </p></div>
+
+
+<p>WILMOT, EARDLEY, Rear-Admiral S.</p>
+
+<div class="blockquot"><p class="noin"><b>Our Fleet To-day and its Development during the last Half Century.</b>
+With many Illustrations. Crown 8vo, 5s. </p></div>
+
+
+<h3>EVENTS OF OUR OWN TIMES</h3>
+
+<p class="center">Crown 8vo. With Illustrations, 5s. each.</p>
+
+<div class="blockquot"><p class="noin"><b>The War in the Crimea.</b> By General Sir E. <span class="smcap">Hamley</span>, K.C.B.</p>
+
+<p class="noin"><b>The Indian Mutiny.</b> By Colonel <span class="smcap">Malleson</span>, C.S.I.</p>
+
+<p class="noin"><b>The Afghan Wars, 1839&ndash;42, and 1878&ndash;80.</b> By <span class="smcap">Archibald Forbes</span>.</p>
+
+<p class="noin"><b>Our Fleet To-Day and its Development during the last Half-Century.</b>
+By Rear-Admiral S. <span class="smcap">Eardley Wilmot</span>.</p>
+
+<p class="noin"><b>The Refounding of the German Empire.</b> By Colonel <span class="smcap">Malleson</span>, C.S.I.</p>
+
+<p class="noin"><b>The Liberation of Italy.</b> By the Countess <span class="smcap">Martinengo Cesaresco</span>.</p>
+
+<p class="noin"><b>Great Britain in Modern Africa.</b> By <span class="smcap">Edgar Sanderson</span>, M.A.</p>
+
+<p class="noin"><b>The War in the Peninsula.</b> By A. <span class="smcap">Innes Shand</span>. </p></div>
+
+
+<p>FLETCHER, W.Y.</p>
+
+<div class="blockquot"><p class="noin"><b>Bookbinding in France.</b> Coloured Plates. Super-royal, sewed, 2s. 6d.
+nett; half-linen, 3s. 6d. nett. </p></div>
+
+
+<p>FORBES, ARCHIBALD.</p>
+
+<div class="blockquot"><p class="noin"><b>The Afghan Wars of 1839&ndash;1842 and 1878&ndash;1880.</b> With Four Portraits on
+Copper, and Maps and Plans. Crown 8vo, 5s. </p></div>
+
+
+<p>FRASER, Sir ANDREW H.L.</p>
+
+<div class="blockquot"><p class="noin"><b>Among Indian Rajahs and Ryots.</b> With 34 Illustrations and a Map.
+Demy 8vo, 18s. nett. Third and Cheaper Edition, 5s. nett. </p></div>
+
+
+<p>FRASER, DONALD.</p>
+
+<div class="blockquot"><p class="noin"><b>Winning a Primitive People.</b> Illustrated. Extra crown 8vo, 5s. nett. </p></div>
+
+
+<p>FRIPP, Sir ALFRED D., K.C.V.O., &amp; R. THOMPSON, F.R.C.S.</p>
+
+<div class="blockquot"><p class="noin"><b>Human Anatomy for Art Students.</b> Profusely Illustrated with
+Photographs and Drawings by <span class="smcap">Innes Fripp</span>, A.R.C.A. Square extra
+crown 8vo, 7s. 6d. nett. </p></div>
+
+
+<p>FROBENIUS, LEO.</p>
+
+<div class="blockquot"><p class="noin"><b>The Childhood of Man.</b> A Popular Account of the Lives and Thoughts
+of Primitive Races. Translated by Prof. A.H. <span class="smcap">Keane</span>, LL.D. With 416
+Illustrations. Demy 8vo, 16s. nett. </p></div>
+
+
+<p>FRY, ROGER.</p>
+
+<div class="blockquot"><p class="noin"><b>Discourses Delivered to the Students of the Royal Academy by Sir
+Joshua Reynolds.</b> With an Introduction and Notes by <span class="smcap">Roger Fry</span>. With
+Thirty-three Illustrations. Square Crown 8vo, 7s. 6d. nett. </p></div>
+
+
+<p>GARDNER, J. STARKIE.</p>
+
+<div class="blockquot"><p class="noin"><b>Armour in England.</b> With Eight Coloured Plates and many other
+Illustrations. Super-royal 8vo, sewed, 3s. 6d. nett.</p>
+
+<p class="noin"><b>Foreign Armour in England.</b> With Eight Coloured Plates and many
+other Illustrations. Super-royal 8vo, sewed, 3s. 6d. nett.</p>
+
+<p class="noin"><b>Armour in England.</b> With Sixteen Coloured Plates and many other
+Illustrations. The two parts in one volume. Super-royal 8vo, cloth,
+gilt top, 9s. nett. </p></div>
+
+
+<p>GARNETT, R., LL.D.</p>
+
+<div class="blockquot"><p class="noin"><b>Richmond on Thames.</b> Illustrated. Super-royal 8vo, sewed, 3s. 6d.
+nett. </p></div>
+
+
+<p>GIBERNE, AGNES.</p>
+
+<div class="blockquot"><p class="noin"><b>Beside the Waters of Comfort.</b> Crown 8vo, 3s. 6d. </p></div>
+
+
+<p>GIBSON, CHARLES R., F.R.S.E.</p>
+
+<div class="blockquot"><p class="noin"><b>Electricity of To-Day.</b> Its Works and Mysteries described in
+non-technical language. With 30 Illustrations. Extra crown 8vo, 5s.
+nett.</p>
+
+<p class="noin"><b>Scientific Ideas of To-day.</b> A Popular Account in non-technical
+language of the Nature of Matter, Electricity, Light, Heat, &amp;c.,
+&amp;c. With 25 Illustrations. Extra crown 8vo, 5s. nett.</p>
+
+<p class="noin"><b>How Telegraphs and Telephones Work.</b> With many Illustrations. Crown
+8vo, 1s. 6d. nett.</p>
+
+<p class="noin"><b>The Autobiography of an Electron.</b> With 8 Illustrations. Long 8vo,
+3s. 6d. nett.</p>
+
+<p class="noin"><b>Wireless Telegraphy.</b> With many Illustrations. Ex. crn. 8vo, 2s.
+nett. </p></div>
+
+
+<p>GODLEY, A.D.</p>
+
+<div class="blockquot"><p class="noin"><b>Socrates and Athenian Society in his Day.</b> Crown 8vo, 4s. 6d.</p>
+
+<p class="noin"><b>Aspects of Modern Oxford.</b> With many Illustrations. Crown 8vo,
+cloth, 2s. nett; lambskin, 3s. nett; velvet leather, in box, 5s.
+nett. </p></div>
+
+
+<p>GOLDEN RECITER (<i>See</i> <span class="smcap">James</span>, Prof. <span class="smcap">Cairns</span>.)</p>
+
+
+<p>GOMES, EDWIN H., M.A.</p>
+
+<div class="blockquot"><p class="noin"><b>Seventeen Years among the Sea Dyaks of Borneo.</b> With 40
+Illustrations and a Map. Demy 8vo, 16s. nett. </p></div>
+
+
+<p>GRAHAME, GEORGE.</p>
+
+<div class="blockquot"><p class="noin"><b>Claude Lorrain.</b> Illustrated. Super-royal 8vo, 2s. 6d. nett;
+half-linen, 3s. 6d. nett. </p></div>
+
+
+<p>GRIFFITH, M.E. HUME.</p>
+
+<div class="blockquot"><p class="noin"><b>Behind the Veil in Persia and Turkish Arabia.</b> An Account of an
+Englishwoman's Eight Years' Residence amongst the Women of the
+East. With 37 Illustrations and a Map. Demy 8vo, 16s. nett. </p></div>
+
+
+<p>GRINDON, LEO.</p>
+
+<div class="blockquot"><p class="noin"><b>Lancashire.</b> Brief Historical and Descriptive Notes. With many
+Illustrations. Crown 8vo, 6s. </p></div>
+
+
+<p>GRUBB, W. BARBROOKE (Pioneer and Explorer of the Chaco).</p>
+
+<div class="blockquot"><p class="noin"><b>An Unknown People in an Unknown Land.</b> With Sixty Illustrations and
+a Map. Demy 8vo, 16s. nett. Third and Cheaper Edition, 5s.</p>
+
+<p class="noin"><b>A Church in the Wilds.</b> Illustrated. Extra crown 8vo, 5s. nett. </p></div>
+
+
+<p>HADOW, W.H.</p>
+
+<div class="blockquot"><p class="noin"><b>A Croatian Composer.</b> Notes toward the Study of Joseph Haydn. Crown
+8vo, 2s. 6d. nett.</p>
+
+<p class="noin"><b>Studies in Modern Music.</b> First Series. Berlioz, Schumann, Wagner.
+With an Essay on Music and Musical Criticism. With Five Portraits.
+Crown 8vo, 7s. 6d.</p>
+
+<p class="noin"><b>Studies in Modern Music.</b> Second Series. Chopin, Dvor&agrave;k, Brahms.
+With an Essay on Musical Form. With Four Portraits. Crown 8vo, 7s.
+6d. </p></div>
+
+
+<p>HAMERTON, P.G.</p>
+
+<div class="blockquot"><p class="noin"><b>The Etchings of Rembrandt, and Dutch Etchers of the Seventeenth
+Century.</b> By P.G. <span class="smcap">Hamerton</span> and <span class="smcap">Laurence Binton</span>. With Eight Copper
+Plates and many other Illustrations. Super-royal 8vo, 7s. 6d. nett.</p>
+
+<p class="noin"><b>The Mount.</b> Narrative of a Visit to the Site of a Gaulish City on
+Mount Beuvray. With a Description of the neighbouring City of
+Autun. Crown 8vo, 3s. 6d.</p>
+
+<p class="noin"><b>Round my House.</b> Notes on Rural Life in Peace and War. Crown 8vo,
+with Illustrations, 2s. 6d. nett. Cheaper edition, 2s. nett.</p>
+
+<p class="noin"><b>Paris.</b> Illustrated. New edition. Cloth, 2s. nett; leather, 3s. nett
+in special leather, full gilt, in box, 5s. nett. </p></div>
+
+
+<p>HAMLEY, Gen. Sir E.</p>
+
+<div class="blockquot"><p class="noin"><b>The War in the Crimea.</b> With Copper Plates and other Illustrations.
+Crown 8vo, 5s. </p></div>
+
+
+<p>HANOUM ZEYNEB (Heroine of Pierre Loti's Novel "Les D&eacute;senchant&eacute;es.")</p>
+
+<div class="blockquot"><p class="noin"><b>A Turkish Woman's European Impressions.</b> Edited by <span class="smcap">Grace Ellison</span>.
+With a portrait by <span class="smcap">Auguste Rodin</span> and 23 other Illustrations from
+photographs. Crown 8vo, 6s. nett. </p></div>
+
+
+<p>HARTLEY, C. GASQUOINE.</p>
+
+<div class="blockquot"><p class="noin"><b>Things Seen in Spain.</b> With Fifty Illustrations. Small 4to, cloth,
+2s.; leather, 3s.; velvet leather in a box, 5s. nett. </p></div>
+
+
+<p>HAYWOOD, Capt. A.H.W.</p>
+
+<div class="blockquot"><p class="noin"><b>Through Timbuctu &amp; Across the Great Sahara.</b> Demy 8vo, with 41
+Illustrations and a Map. 16s. nett. </p></div>
+
+
+<p>HENDERSON, Major PERCY E.</p>
+
+<div class="blockquot"><p class="noin"><b>A British Officer in the Balkans.</b> Through Dalmatia, Montenegro,
+Turkey in Austria, Magyarland, Bosnia and Herzegovina. With 50
+Illustrations and a Map. Gilt top. Demy 8vo, 16s. nett. </p></div>
+
+
+<p>HERBERT, GEORGE.</p>
+
+<div class="blockquot"><p class="noin"><b>The Temple.</b> Sacred Poems and Ejaculations. The Text reprinted from
+the First Edition. With Seventy-six Illustrations after <span class="smcap">Albert
+D&uuml;rer</span>, <span class="smcap">Holbrin</span>, and other Masters. Crown 8vo, cloth, 2s. nett;
+leather, 3s. nett.; velvet leather in box, 5s. nett. </p></div>
+
+
+<p>HOLLAND, CLIVE.</p>
+
+<div class="blockquot"><p class="noin"><b>Things Seen in Japan.</b> With Fifty beautiful illustrations of
+Japanese life in Town and Country. Small 4to, cloth, 2s. nett;
+leather, 3s. nett; velvet leather, in box, 5s. nett. </p></div>
+
+
+<p>HUTCHINSON, Rev. H.N.</p>
+
+<div class="blockquot"><p class="noin"><b>The Story of the Hills.</b> A Popular Account of Mountains and How They
+were Made. With many Illustrations. Crown 8vo, 5s. </p></div>
+
+
+<p>HUTTON, C.A.</p>
+
+<div class="blockquot"><p class="noin"><b>Greek Terracotta Statuettes.</b> With a Preface by A.S. <span class="smcap">Murray</span>, LL.D.
+With Seventeen Examples printed in Colour and Thirty-six printed in
+Monochrome. 5s. nett; or cloth, 7s. nett. </p></div>
+
+
+<p>HUTTON, SAMUEL KING, M.B.</p>
+
+<div class="blockquot"><p class="noin"><b>Among the Eskimos of Labrador.</b> Demy 8vo; with Forty-seven
+Illustrations and a Map. 16s. nett. </p></div>
+
+
+<p>JAMES, CAIRNS.</p>
+
+<div class="blockquot"><p class="noin"><b>The Golden Reciter.</b> With an Introduction by <span class="smcap">Cairns James</span>, Professor
+of Elocution at the Royal Academy of Music, &amp;c. With Selections
+from Rudyard Kipling, Thomas Hardy, R.L. Stevenson, Seton Merriman,
+H.G. Wells, Christina Rossetti, Anthony Hope, Austin Dobson,
+Maurice Hewlett, Conan Doyle, &amp;c. &amp;c. Extra crown 8vo, 704 pp.
+Cloth, 3s. 6d., and thin paper edition in cloth with gilt edges,
+5s. </p></div>
+
+<div class="blockquot"><p class="noin">"A more admirable book of its kind could not well be
+desired."&mdash;<i>Liverpool Courier.</i> </p></div>
+
+<div class="blockquot"><p class="noin"><b>The Golden Humorous Reciter.</b> Edited, and with a Practical
+Introduction, by <span class="smcap">Cairns James</span>, Professor of Elocution at the Royal
+College of Music and the Guildhall School of Music. A volume of
+Recitations and Readings selected from the writings of F. Anstey,
+J.M. Barrie, S.R. Crockett, Jerome K. Jerome, Barry Pain, A.W.
+Pinero, Owen Seaman, G.B. Shaw, &amp;c. &amp;c. Extra crown 8vo, over 700
+pages, cloth, 3s. 6d.; also a thin paper edition, with gilt edges,
+5s. </p></div>
+
+<hr style='width: 15%;' />
+
+<h3>THE ILLUMINATED SERIES</h3>
+
+<p class="center"><span class="smcap">New Binding.</span></p>
+
+<div class="blockquot"><p class="noin">Bound in antique leather with metal clasps. With illuminated
+frontispiece and title-page, and other illuminated pages. Finely
+printed at the Ballantyne Press, Edinburgh. Crown 8vo. Each copy in
+a box, 10s. 6d. nett. Also in real classic vellum. Each copy in a
+box. 10s. 6d. nett. </p></div>
+
+<div class="blockquot"><p class="noin"><b>The Confessions of S. Augustine.</b></p>
+
+<p class="noin"><b>Of the Imitation of Christ.</b> By <span class="smcap">Thomas &agrave; Kempis</span>.</p>
+
+<p class="noin"><b>The Sacred Seasons.</b> By the <span class="smcap">Bishop of Durham</span>. Also cloth, 6s. and
+7s. 6d. nett. </p></div>
+
+
+<p>JOY, BEDFORD.</p>
+
+<div class="blockquot"><p class="noin"><b>A Synopsis of Roman History.</b> Crown 8vo, 2s. </p></div>
+
+
+<p>KEANE, Prof. A.H. (<i>See</i> <span class="smcap">Frobenius</span>.)</p>
+
+
+<p>LANG, ANDREW.</p>
+
+<div class="blockquot"><p class="noin"><b>Oxford.</b> New Edition. With 50 Illustrations by J.H. <span class="smcap">Lorimer</span>, R.S.A.,
+T. <span class="smcap">Hamilton Crawford</span>, R.S.W., J. <span class="smcap">Pennell</span>, A. <span class="smcap">Brunet-Debaines</span>, A.
+<span class="smcap">Toussaint</span>, and R. <span class="smcap">Kent Thomas</span>. Extra crown 8vo, 6s. Also Pocket
+Edition, 2s. nett; leather, 3s. nett. Special yapp leather, full
+gilt, in box, 5s. nett. </p></div>
+
+
+<p>LEE, Sir SIDNEY.</p>
+
+<div class="blockquot"><p class="noin"><b>Stratford-on-Avon.</b> From the Earliest Times to the Death of
+Shakespeare. New revised edition, with additional Illustrations.
+Extra crown 8vo, 6s. Pocket Edition, 2s. nett; leather, 3s. nett;
+and in special yapp leather, full gilt, in box, 5s. nett. </p></div>
+
+
+<p>LEFROY, W. CHAMBERS.</p>
+
+<div class="blockquot"><p class="noin"><b>The Ruined Abbeys of Yorkshire.</b> With many Illustrations by A.
+<span class="smcap">Brunet-Debaines</span> and H. <span class="smcap">Toussaint</span>. Cr. 8vo, Cloth, 2s. nett;
+leather, 3s. &amp; 5s. nett. </p></div>
+
+
+<p>LEYLAND, JOHN.</p>
+
+<div class="blockquot"><p class="noin"><b>The Peak of Derbyshire.</b> With Map, Etchings, and other Illustrations
+by <span class="smcap">Herbert Railton</span> and <span class="smcap">Alfred Dawson</span>. New Edition, Crown 8vo,
+Cloth, 2s.; leather, 3s.; velvet leather, in a box, 5s. nett. </p></div>
+
+
+<p>LOFTIE, Rev. W.J.</p>
+
+<div class="blockquot"><p class="noin"><b>The Inns of Court and Chancery.</b> With many Illustrations, chiefly by
+<span class="smcap">Herbert Railton</span>. Crown 8vo, cloth, 2s. nett; leather, 3s. nett and
+5s. nett.</p>
+
+<p class="noin"><b>Westminster Abbey.</b> With Seventy-four Illustrations, chiefly by
+<span class="smcap">Herbert Railton</span>. Crown 8vo, 2s. 6d. Extra crown 8vo, 6s.</p>
+
+<p class="noin"><b>Whitehall.</b> With many Illustrations. Super-royal 8vo, sewed, 2s. 6d.
+nett; half-linen, 3s. 6d. nett. </p></div>
+
+
+<p>MACKENZIE, Rev. W.B.</p>
+
+<div class="blockquot"><p class="noin"><b>Married Life and the Dwellings of the Righteous.</b> 3s. 6d. </p></div>
+
+
+<p>MALLESON, Colonel G.B., C.S.I.</p>
+
+<div class="blockquot"><p class="noin"><b>The Indian Mutiny.</b> With Copper Plates and other Illustrations.
+Crown 8vo, 5s.</p>
+
+<p class="noin"><b>The Refounding of the German Empire.</b> With Portrait and Plans. Crown
+8vo, 5s. </p></div>
+
+<hr style='width: 15%;' />
+
+<h3>MINIATURE LIBRARY OF DEVOTION</h3>
+
+<div class="blockquot"><p class="hang">Little Volumes of Short Extracts from the Christian Fathers. With
+Decorative Title-page and Photogravure Frontispiece. 32mo, cloth
+extra, each 1s. nett; leather, each 1s. 6d. nett. Also Three
+Volumes in leather in case, 4s. 6d. nett. White vellum with gilt
+edges, each volume in a box, 2s. nett.</p>
+
+<p class="noin">
+<b>1. Saint Augustine.</b><br />
+<b>2. Jeremy Taylor.</b><br />
+<b>3. Saint Chrysostom.</b><br />
+<b>4. Bishop Andrewes.</b><br />
+<b>5. John Keble.</b><br />
+<b>6. Thomas &agrave; Kempis.</b><br />
+<b>7. Canon Liddon.</b><br />
+<b>8. F&eacute;nelon.</b><br />
+<b>9. William Law.</b><br />
+</p></div>
+
+<hr style='width: 15%;' />
+
+<h3>MINIATURE PORTFOLIO MONOGRAPHS</h3>
+
+<div class="blockquot"><p class="hang">A New Edition in 16mo. Most of the Volumes have been carefully
+revised by the Authors. Each Volume profusely Illustrated. Cloth,
+2s. nett; leather, 3s. nett; velvet leather, in box, 5s. nett. </p></div>
+
+
+<div class="blockquot"><p class="noin"><b>Peter Paul Rubens.</b> By R.A.M. <span class="smcap">Stevenson</span>.</p>
+
+<p class="noin"><b>Japanese Wood Engravings.</b> By Professor W. <span class="smcap">Anderson</span>.</p>
+
+<p class="noin"><b>Josiah Wedgwood.</b> By Sir A.H. <span class="smcap">Church</span>, F.R.S., Professor of
+Chemistry, Royal Academy of Arts. New &amp; Revised Edition.</p>
+
+<p class="noin"><b>D.G. Rossetti.</b> By F.G. <span class="smcap">Stephens</span>, One of the Seven Members of the
+Pre-Raphaelite Brotherhood.</p>
+
+<p class="noin"><b>The Early Work of Raphael.</b> By <span class="smcap">Julia Cartwright</span> (Mrs. Ady).</p>
+
+<p class="noin"><b>Fair Women in Painting and Poetry.</b> By <span class="smcap">William Sharp</span> (Fiona
+Macleod).</p>
+
+<p class="noin"><b>Antoine Watteau.</b> By <span class="smcap">Claude Phillips</span>, Keeper of the Wallace
+Collection.</p>
+
+<p class="noin"><b>Raphael in Rome.</b> By <span class="smcap">Julia Cartwright</span> (Mrs. Ady).</p>
+
+<p class="noin"><b>The New Forest.</b> By C.J. <span class="smcap">Cornish</span>, Author of "Life of the Zoo," &amp;c.</p>
+
+<p class="noin"><b>The Isle of Wight.</b> By C.J. <span class="smcap">Cornish</span>.</p>
+
+<p class="noin"><b>Gainsborough.</b> By Sir <span class="smcap">Walter Armstrong</span>, Keeper of the National
+Gallery of Ireland. </p></div>
+
+
+<h3>THE LIBRARY OF MISSIONS</h3>
+
+<p class="center">
+Illustrated. Extra Crown, 5s. 8vo, nett.<br />
+</p>
+
+<div class="blockquot"><p class="noin"><b>A Church in the Wilds.</b> The Remarkable Story of the Establishment of
+the South American Mission amongst the hitherto Savage and
+Intractable Natives of the Paraguayan Chaco. By W. <span class="smcap">Barbrooke Grubb</span>.</p>
+
+<p class="noin"><b>Winning a Primitive People.</b> Sixteen Years' Work among the Warlike
+Tribe of the Ngoni and the Senga and Tumbuka Peoples of Central
+Africa. By the Rev. <span class="smcap">Donald Fraser</span>. </p></div>
+
+<hr style='width: 15%;' />
+
+<p>MITFORD, MARY RUSSELL.</p>
+
+<div class="blockquot"><p class="noin"><b>Country Stories.</b> With 68 Illustrations by <span class="smcap">George Morrow</span>. Crown 8vo,
+cloth, gilt top, 2s. nett; also in leather, 3s. nett; and leather
+yapp, full gilt in box, 5s. nett. </p></div>
+
+
+<p>MOULE, Archdeacon A.E.</p>
+
+<div class="blockquot"><p class="noin"><b>New China and Old.</b> Notes on the Country and People made during a
+Residence of Thirty Years. With Thirty Illustrations. New Edition,
+Revised. Crown 8vo, 5s. </p></div>
+
+
+<p>MOULE, Right Rev. H.C.G., D.D. (Bishop of Durham).</p>
+
+<div class="blockquot"><p class="noin"><b>The Sacred Seasons.</b> Readings for the Sundays and Holy Days of the
+Christian Year. Printed in red and black throughout, and
+illuminated with specially drawn initial letters and ornaments, and
+with 12 illuminated pages printed in three colours and gold after
+illuminations in manuscripts at the British Museum. Extra crown
+8vo, 6s. nett; also white cloth, in box, 7s. 6d. nett; antique
+leather with clasps, 10s. 6d. nett.</p>
+
+<p class="noin"><b>At the Holy Communion.</b> Helps for Preparation and Reception. Cloth,
+1s.; leather, 2s. nett; calf, 4s. 6d.</p>
+
+<p class="noin"><b>Christ's Witness to the Life to Come.</b> Crown 8vo, 3s. 6d.</p>
+
+<p class="noin"><b>Grace and Godliness.</b> Studies in the Epistle to the Ephesians. Crown
+8vo, 2s. 6d.</p>
+
+<p class="noin"><b>In the House of the Pilgrimage.</b> Hymns and Sacred Songs. 2s. 6d.</p>
+
+<p class="noin"><b>Imitations and Translations.</b> Crown 8vo, 2s. 6d. nett.</p>
+
+<p class="noin"><b>Jesus and the Resurrection.</b> Expository Studies on St. John xx. and
+xxi. Third Edition, 2s. 6d.</p>
+
+<p class="noin"><b>Lord's Supper, The.</b> By <span class="smcap">Bishop Ridley</span>. Edited with Notes and a Life
+by the <span class="smcap">Bishop of Durham</span>. Crown 8vo, 5s.</p>
+
+<p class="noin"><b>Our Prayer Book.</b> Short Chapters on the Book of Common Prayer. 16mo,
+1s.; leather, 2s. nett.</p>
+
+<p class="noin"><b>Pledges of His Love, The.</b> Thoughts on the Holy Communion. 16mo,
+1s.; leather, 2s. nett.</p>
+
+<p class="noin"><b>Prayers for the Home.</b> A Month's Cycle of Morning and Evening Family
+Worship, with some Occasional Prayers. Crown 8vo, 3s. 6d.</p>
+
+<p class="noin"><b>Prayers and Promises.</b> Messages from the Holy Scriptures. 16mo, 1s.;
+leather, 2s. nett; calf, 4s. 6d.</p>
+
+<p class="noin"><b>The Secret of the Presence</b>, and other Sermons. Crown 8vo, 3s. 6d.</p>
+
+<p class="noin"><b>Temptation and Escape.</b> Short Chapters for Beginners in the
+Christian Life. 16mo, 1s.; leather, 2s. nett.</p>
+
+<p class="noin"><b>Thoughts on Christian Sanctity.</b> 16mo, cloth, 1s.; leather, 2s.
+nett.</p>
+
+<p class="noin"><b>Thoughts on Secret Prayer.</b> 16mo, cloth, 1s.; leather, 2s. nett;
+calf, 4s. 6d.</p>
+
+<p class="noin"><b>Thoughts on the Spiritual Life.</b> 16mo, cloth, 1s.; leather, 2s.
+nett.</p>
+
+<p class="noin"><b>Thoughts on Union with Christ.</b> 16mo, cloth, 1s.; leather, 2s. nett. </p></div>
+
+
+<p>MURRAY, A.S., LL.D.</p>
+
+<div class="blockquot"><p class="noin"><b>Greek Bronzes.</b> With Four Copper Plates and many other
+Illustrations. Super-royal 8vo, sewed, 3s. 6d. nett; cloth, 4s. 6d.
+nett.</p>
+
+<p class="noin"><b>Greek Bronzes</b>, by Dr. <span class="smcap">Murray</span>, and <b>Greek Terracotta Statuettes</b>, by
+C.A. <span class="smcap">Hutton</span>. With Four Photogravures, Eight Coloured Plates, and
+Seventy-seven other Illustrations. In one Volume. Super-royal 8vo,
+cloth, 10s. 6d. nett. </p></div>
+
+
+<p>NETTLESHIP, J.T.</p>
+
+<div class="blockquot"><p class="noin"><b>Morland, George.</b> With Six Copper Plates and Thirty other
+Illustrations. Super-royal 8vo, sewed, 5s. nett; cloth, 6s. nett. </p></div>
+
+
+<h3>THE NEW ART LIBRARY</h3>
+
+<p class="center">EDITED BY M.H. SPIELMANN, F.S.A., &amp; P.G. KONODY.</p>
+
+<div class="blockquot"><p class="center">"The admirable New Art Library.... Thoroughly practical."&mdash;<i>The
+Connoisseur.</i> </p></div>
+
+
+<p>THE PRACTICE AND SCIENCE OF DRAWING.</p>
+
+<div class="blockquot"><p class="noin">By <span class="smcap">Harold Speed</span>, Associ&eacute; de la Soci&eacute;t&eacute; Nationale des Beaux-Arts;
+Member of the Society of Portrait Painters; Professor of Drawing at
+the Goldsmiths' College, &amp;c. With Ninety-six Illustrations and
+Diagrams. Square ex. crn. 8vo, 6s. nett. </p></div>
+
+
+<p>THE PRACTICE OF OIL PAINTING AND DRAWING.</p>
+
+<div class="blockquot"><p class="noin">By <span class="smcap">Solomon J. Solomon</span>, R.A. With Eighty Illustrations. 6s. nett. </p></div>
+
+
+<p>HUMAN ANATOMY FOR ART STUDENTS.</p>
+
+<div class="blockquot"><p class="noin">By Sir <span class="smcap">Alfred Downing Fripp</span>, K.C.V.O., Lecturer upon Anatomy at
+Guy's Hospital, London, and <span class="smcap">Ralph Thompson</span>, Ch.M., F.R.C.S., with a
+chapter on Comparative Anatomy, and Drawings by <span class="smcap">Harry Dixon</span>. With
+One hundred and fifty-nine Photographs and Drawings. Square extra
+crown 8vo, 7s. 6d. nett. </p></div>
+
+
+<p>MODELLING AND SCULPTURE.</p>
+
+<div class="blockquot"><p class="noin">By <span class="smcap">Albert Toft</span>, A.R.C.A., M.S.B.S. With 119 Photographs and
+Drawings. Square extra crown 8vo, 6s. nett. </p></div>
+
+<hr style='width: 15%;' />
+
+<p>PAGE, J. Ll. WARDEN.</p>
+
+<div class="blockquot"><p class="noin"><b>Exmoor, An Exploration of.</b> With Maps, Etchings, and other
+Illustrations. Cheap Edition, 3s. 6d. </p></div>
+
+
+<p>PENNELL, T.L., M.D., B.Sc., F.R.C.S.</p>
+
+<div class="blockquot"><p class="noin"><b>Among the Wild Tribes of the Afghan Frontier.</b> A Record of Sixteen
+Years' Close Intercourse with the Natives of the Indian Marches.
+With an Introduction by Field-Marshal <span class="smcap">Lord Roberts</span>, V.C. Extra
+Crown 8vo. With 26 Illustrations and a Map. 5s. nett. Fourth and
+Cheaper Edition.</p>
+
+<p class="noin"><b>Things Seen in Northern India.</b> With 50 Illustrations. 2s., 3s., 5s.
+nett. </p></div>
+
+
+<p>PHILLIPS, CLAUDE.</p>
+
+<div class="blockquot"><p class="noin"><b>The Earlier Work of Titian.</b> With many Illustrations. Super-royal
+8vo, sewed, 3s. 6d. nett; cloth, 4s. 6d. nett.</p>
+
+<p class="noin"><b>The Later Work of Titian.</b> With many Illustrations. Super-royal 8vo,
+sewed, 3s. 6d. nett; cloth, 4s. 6d. nett.</p>
+
+<p class="noin"><b>Titian, a Study of his Life and Work.</b> With Eight Copper Plates and
+many other Illustrations. Super-royal 8vo, 9s. nett.</p>
+
+<p class="noin"><b>The Picture Gallery of Charles I.</b> With many Illustrations.
+Super-royal 8vo, sewed, 3s. 6d. nett; cloth, 4s. 6d. nett.</p>
+
+<p class="noin"><b>Frederick Walker.</b> Sup.-roy. 8vo, sewed, 2s. 6d. nett; half-linen,
+3s. 6d. nett.</p>
+
+<p class="noin"><b>Antoine Watteau.</b> Sup.-roy. 8vo, sewed, 2s. 6d. nett; half-linen,
+3s. 6d. nett; also small 4to, cloth, 2s. nett; and 3s. and 5s. nett
+in leather. </p></div>
+
+
+<p>POLLARD, A.W.</p>
+
+<div class="blockquot"><p class="noin"><b>Italian Book Illustrations.</b> Super-royal 8vo, sewed, 2s. 6d. nett;
+half-linen, 3s. 6d. nett. </p></div>
+
+<hr style='width: 15%;' />
+
+<h3>PORTFOLIO MONOGRAPHS ON ARTISTIC SUBJECTS</h3>
+
+<div class="blockquot"><p class="center">"A triumph of magnificent illustration and masterly editing."&mdash;<i>The
+Times.</i> </p></div>
+
+<div class="blockquot"><p class="hang">Many of the Volumes are issued in two forms and at various nett
+prices. Where two prices are given, the first is that of the paper
+cover edition; the second that of the cloth. When only one price is
+given, the Volume is bound in paper only. </p></div>
+
+
+<p>ANDERSON, Prof. W.</p>
+
+<div class="blockquot"><p class="noin"><b>Japanese Wood Engravings.</b> 2s. 6d. and 3s. 6d. </p></div>
+
+
+<p>ARMSTRONG, Sir WALTER.</p>
+
+<div class="blockquot"><p class="noin"><b>The Art of Velazquez.</b> 3s. 6d.</p>
+
+<p class="noin"><b>The Life of Velazquez.</b> 3s. 6d.</p>
+
+<p class="noin"><b>The Peel Collection and the Dutch School of Painting.</b> 5s. and 7s.</p>
+
+<p class="noin"><b>Thomas Gainsborough.</b> Half-linen, 3s. 6d.</p>
+
+<p class="noin"><b>W.Q. Orchardson.</b> 2s. 6d. and 3s. 6d. </p></div>
+
+
+<p>BEDFORD, W.K.R.</p>
+
+<div class="blockquot"><p class="noin"><b>Malta.</b> 2s. 6d. </p></div>
+
+
+<p>BENHAM, Canon, and CHARLES WELCH, F.S.A.</p>
+
+<div class="blockquot"><p class="noin"><b>Medi&aelig;val London.</b> 3s. 6d., 5s., and 7s.</p>
+
+<p class="noin"><b>The Tower of London.</b> 5s. and 7s. </p></div>
+
+
+<p>BENHAM, Canon.</p>
+
+<div class="blockquot"><p class="noin"><b>Old St. Paul's Cathedral.</b> 5s. and 7s. </p></div>
+
+
+<p>BINYON, LAURENCE.</p>
+
+<div class="blockquot"><p class="noin"><b>Dutch Etchers of XVIIth Century.</b> 2s. 6d. and 3s. 6d.</p>
+
+<p class="noin"><b>John Crome and J.S. Cotman.</b> 3s. 6d. </p></div>
+
+
+<p>BIRCH, G.H., F.S.A.</p>
+
+<div class="blockquot"><p class="noin"><b>London on Thames.</b> 5s. and 7s. </p></div>
+
+
+<p>CARTWRIGHT, JULIA (Mrs. ADY).</p>
+
+<div class="blockquot"><p class="noin"><b>Jules Bastien-Lepage.</b> 2s. 6d. and 3s. 6d.</p>
+
+<p class="noin"><b>The Early Work of Raphael.</b> 2s. 6d. and 3s. 6d.</p>
+
+<p class="noin"><b>Raphael in Rome.</b> 2s. 6d. and 3s. 6d. </p></div>
+
+
+<p>CHURCH, A.H., F.R.S.</p>
+
+<div class="blockquot"><p class="noin"><b>Josiah Wedgwood.</b> 5s. and 7s. </p></div>
+
+
+<p>CORNISH, C.J.</p>
+
+<div class="blockquot"><p class="noin"><b>The Isle of Wight.</b> 2s. 6d. &amp; 3s. 6d.</p>
+
+<p class="noin"><b>The New Forest.</b> 2s. 6d. &amp; 3s. 6d. </p></div>
+
+
+<p>CUST, LIONEL, F.S.A.</p>
+
+<div class="blockquot"><p class="noin"><b>The Engravings of Albert D&uuml;rer.</b> 2s. 6d. and 3s. 6d.</p>
+
+<p class="noin"><b>The Paintings and Drawings of Albert D&uuml;rer.</b> 3s. 6d. </p></div>
+
+
+<p>DAVENPORT, CYRIL, F.S.A.</p>
+
+<div class="blockquot"><p class="noin"><b>Royal English Bookbindings.</b> 3s. 6d. &amp; 4s. 6d.</p>
+
+<p class="noin"><b>Cameos.</b> 5s. &amp; 7s. </p></div>
+
+
+<p>DAVIES, RANDALL, F.S.A.</p>
+
+<div class="blockquot"><p class="noin"><b>English Society of the Eighteenth Century in Contemporary Art.</b> </p></div>
+
+
+<p>DESTR&Eacute;E, O.G.</p>
+
+<div class="blockquot"><p class="noin"><b>The Renaissance of Sculpture in Belgium.</b> 2s. 6d. and 3s. 6d. </p></div>
+
+
+<p>FLETCHER, W.Y.</p>
+
+<div class="blockquot"><p class="noin"><b>Bookbinding in France.</b> 2s. 6d. and 3s. 6d. </p></div>
+
+
+<p>GARDNER, J. STARKIE.</p>
+
+<div class="blockquot"><p class="noin"><b>Armour in England.</b> 3s. 6d.</p>
+
+<p class="noin"><b>Foreign Armour in England.</b> 3s. 6d. </p></div>
+
+
+<p>GARNETT, RICHARD, C.B., LL.D.</p>
+
+<div class="blockquot"><p class="noin"><b>Richmond on Thames.</b> 3s. 6d. and 4s. 6d. </p></div>
+
+
+<p>GRAHAME, GEORGE.</p>
+
+<div class="blockquot"><p class="noin"><b>Claude Lorrain.</b> 2s. 6d. and 3s. 6d. </p></div>
+
+
+<p>HAMERTON, P.G.</p>
+
+<div class="blockquot"><p class="noin"><b>The Etchings of Rembrandt.</b> 2s. 6d. and 3s. 6d. </p></div>
+
+
+<p>HUTTON, C.A.</p>
+
+<div class="blockquot"><p class="noin"><b>Greek Terracotta Statuettes.</b> 5s. and 7s. </p></div>
+
+
+<p>LOFTIE, W.J.</p>
+
+<div class="blockquot"><p class="noin"><b>Whitehall.</b> 2s. 6d. and 3s. 6d. </p></div>
+
+
+<p>MURRAY, A.S., LL.D.</p>
+
+<div class="blockquot"><p class="noin"><b>Greek Bronzes.</b> 3s. 6d. and 4s. 6d. </p></div>
+
+
+<p>NETTLESHIP, J.T.</p>
+
+<div class="blockquot"><p class="noin"><b>George Morland.</b> 5s. and 6s. </p></div>
+
+
+<p>PHILLIPS, CLAUDE.</p>
+
+<div class="blockquot"><p class="noin"><b>Frederick Walker.</b> 2s. 6d. and 3s. 6d.</p>
+
+<p class="noin"><b>Antoine Watteau. 2s.</b> 6d. and 3s. 6d.</p>
+
+<p class="noin"><b>The Picture Gallery of Charles I.</b> 3s. 6d.</p>
+
+<p class="noin"><b>The Earlier Work of Titian.</b> 3s. 6d.</p>
+
+<p class="noin"><b>The Later Work of Titian.</b> 3s. 6d. </p></div>
+
+
+<p>POLLARD, ALFRED W.</p>
+
+<div class="blockquot"><p class="noin"><b>Italian Book Illustrations.</b> 2s. 6d. and 3s. 6d. </p></div>
+
+
+<p>PRIOR, E.S., F.S.A.</p>
+
+<div class="blockquot"><p class="noin"><b>The Cathedral Builders in England.</b> 5s. and 7s. </p></div>
+
+
+<p>SHARP, WILLIAM.</p>
+
+<div class="blockquot"><p class="noin"><b>Fair Women.</b> 2s. 6d. and 3s. 6d. </p></div>
+
+
+<p>STEPHENS, F.G.</p>
+
+<div class="blockquot"><p class="noin"><b>Dante Gabriel Rossetti.</b> 2s. 6d. </p></div>
+
+
+<p>STEVENSON, R.A.M.</p>
+
+<div class="blockquot"><p class="noin"><b>Peter Paul Rubens.</b> 3s. 6d. </p></div>
+
+
+<p>WAERN, CECILIA.</p>
+<div class="blockquot"><p class="noin"><b>John La Farge.</b> 3s. 6d. </p></div>
+
+
+<p>WEALE, W.H. JAMES.</p>
+
+<div class="blockquot"><p class="noin"><b>Gerard David, Painter and Illuminator.</b> 2s. 6d. and 3s. 6d. </p></div>
+
+
+<p>PRIOR, Canon E.S., F.S.A.</p>
+
+<div class="blockquot"><p class="noin"><b>The Cathedral Builders of England.</b> Illustrated. Super-royal 8vo,
+sewed, 5s. nett; cloth, extra gilt top, 7s. nett. </p></div>
+
+<hr style='width: 15%;' />
+
+<p>QUILLER COUCH, SIR A.T.</p>
+
+<div class="blockquot"><p class="noin"><b>The Pilgrims' Way.</b> A Little Scrip for Travellers. In Prose and
+Verse. With end papers in colour, and gilt top. Fcap. 8vo, cloth,
+3s. 6d. nett; on thin paper, leather, 5s. nett. </p></div>
+
+
+<p>RAGG, LONSDALE, B.D. (Oxon.), and L.M. RAGG.</p>
+
+<div class="blockquot"><p class="noin"><b>Things Seen in Venice.</b> With Fifty Illustrations. 2s., 3s., 5s.
+nett. </p></div>
+
+
+<p>RANNIE, DOUGLAS (late Government Agent for Queensland.)</p>
+
+<div class="blockquot"><p class="noin"><b>My Adventures Among South Sea Cannibals.</b> Demy 8vo. With Thirty-nine
+Illustrations and a Map, 16s. nett. </p></div>
+
+
+<p>RAWLING, Captain C.G.</p>
+
+<div class="blockquot"><p class="noin"><b>The Land of the New Guinea Pygmies.</b> Demy 8vo. With Forty-eight
+Illustrations and a Map, 16s. nett. </p></div>
+
+
+<p>REYNOLDS, Sir JOSHUA.</p>
+
+<div class="blockquot"><p class="noin"><b>Discourses Delivered to the Students of the Royal Academy.</b> With an
+Introduction and Notes by <span class="smcap">Roger Fry</span>. With Thirty-Three
+Illustrations. Square extra Crown 8vo, 7s. 6d. nett. </p></div>
+
+
+<p>ROCHE, C.E.</p>
+
+<div class="blockquot"><p class="noin"><b>Things Seen in Holland.</b> With Fifty Illustrations. Small 4to, cloth,
+2s. nett; lambskin, 3s. nett; velvet leather, 5s. nett. </p></div>
+
+
+<p>ROPES, A.R.</p>
+
+<div class="blockquot"><p class="noin"><b>Lady Mary Wortley Montagu.</b> With 8 Illustrations. Crown 8vo, cloth,
+2s. 6d. nett. </p></div>
+
+
+<p>RUSSELL, W. CLARK.</p>
+
+<div class="blockquot"><p class="noin"><b>The British Seas.</b> With upwards of Fifty Illustrations. Crown 8vo,
+cloth, 2s.; leather, 3s.; special yapp leather in box, 5s. nett. </p></div>
+
+
+<p>SANDBY, W.</p>
+
+<div class="blockquot"><p class="noin"><b>Thomas and Paul Sandby, Royal Academicians.</b> Their Lives and Works.
+With many Illustrations. Crown 8vo, 7s. 6d. </p></div>
+
+
+<p>SANDERSON, E.</p>
+
+<div class="blockquot"><p class="noin"><b>Great Britain in Modern Africa.</b> With Four Portraits on Copper and a
+Map. Crown 8vo, 5s. </p></div>
+
+
+<p>SCOTT-ELLIOT, Professor G.F., M.A., B.Sc.</p>
+
+<div class="blockquot"><p class="noin"><b>Botany of To-day.</b> With Twenty-seven Illustrations. Ex. cr. 8vo, 5s.
+nett. </p></div>
+
+<hr style='width: 15%;' />
+
+<h4>SCIENCE OF TO-DAY SERIES</h4>
+
+<div class="blockquot"><p class="hang">The volumes of this series give an attractive, lucid, yet at the
+same time scientifically accurate account of various subjects in
+non-technical language. Large crown 8vo, 5s. nett. </p></div>
+
+<div class="blockquot"><p class="noin"><b>Submarine Engineering of To-day.</b> By <span class="smcap">Charles W. Domville-Fife</span>.</p>
+
+<p class="noin"><b>Photography of To-day.</b> By <span class="smcap">H. Chapman Jones</span>, F.I.C., F.C.S.,
+F.R.P.S.</p>
+
+<p class="noin"><b>Aerial Navigation of To-day.</b> By <span class="smcap">Charles C. Turner</span>.</p>
+
+<p class="noin"><b>Astronomy of To-Day.</b> By <span class="smcap">C.G. Dolmage</span>, M.A., LL.D., D.C.L., F.R.A.S.</p>
+
+<p class="noin"><b>Botany of To-day.</b> By Prof. <span class="smcap">G.F. Scott-Elliot</span>, M.A., B.Sc.</p>
+
+<p class="noin"><b>Electricity of To-Day.</b> By <span class="smcap">Charles R. Gibson</span>, F.R.S.E.</p>
+
+<p class="noin"><b>Engineering of To-day.</b> By <span class="smcap">Thomas W. Corbin</span>.</p>
+
+<p class="noin"><b>Mechanical Inventions of To-Day.</b> By <span class="smcap">T.W. Corbin</span>.</p>
+
+<p class="noin"><b>Medical Science of To-Day.</b> By <span class="smcap">Willmott Evans</span>, M.D.</p>
+
+<p class="noin"><b>Scientific Ideas of To-Day.</b> By <span class="smcap">Charles R. Gibson</span>, F.R.S.E. </p></div>
+
+<hr style='width: 15%;' />
+
+<h4>SEELEY'S ILLUSTRATED POCKET LIBRARY</h4>
+
+<p class="center">
+Crown 8vo, cloth, gilt edge, 2s. nett; also in leather, 3s. nett; and<br />
+yapp leather in box at 5s.<br />
+</p>
+
+
+<p>ADDISON and STEELE.</p>
+
+<div class="blockquot"><p class="noin"><b>The Spectator in London.</b> With Fifty-six Illustrations by <span class="smcap">Ralph
+Cleaver</span>, and Headpieces by <span class="smcap">W.A. Atkin Berry</span>, <span class="smcap">Clough Bromley</span>, &amp;c. </p></div>
+
+
+<p>CLARK, J.W., Registrary of the University of Cambridge.</p>
+
+<div class="blockquot"><p class="noin"><b>Cambridge.</b> With many Illustrations. </p></div>
+
+
+<p>GODLEY, A.D.</p>
+
+<div class="blockquot"><p class="noin"><b>Aspects of Modern Oxford.</b> With many Illustrations. </p></div>
+
+
+<p>HAMERTON, P.G.</p>
+
+<div class="blockquot"><p class="noin"><b>Paris.</b> With many Illustrations. </p></div>
+
+
+<p>LEE, Sir SIDNEY.</p>
+
+<div class="blockquot"><p class="noin"><b>Stratford-on-Avon.</b> From the Earliest Times to the Death of
+Shakespeare. With 52 Illustrations by <span class="smcap">Herbert Railton</span> and <span class="smcap">E. Hull</span>. </p></div>
+
+
+<p>MITFORD, MARY RUSSELL.</p>
+
+<div class="blockquot"><p class="noin"><b>Country Stories.</b> With 68 Illustrations by <span class="smcap">George Morrow</span>. </p></div>
+
+
+<p>HERBERT, GEORGE.</p>
+
+<div class="blockquot"><p class="noin"><b>The Temple.</b> Sacred Poems and Ejaculations. The Text reprinted from
+the first edition. With 76 Illustrations after <span class="smcap">D&uuml;rer</span>, <span class="smcap">Holbein</span>, and
+other Masters. </p></div>
+
+
+<p>LANG, ANDREW.</p>
+
+<div class="blockquot"><p class="noin"><b>Oxford.</b> With 40 Illustrations by various artists. </p></div>
+
+
+<p>LEFROY, W. CHAMBERS.</p>
+
+<div class="blockquot"><p class="noin"><b>The Ruined Abbeys of Yorkshire.</b> With many Illustrations. </p></div>
+
+
+<p>LEYLAND, JOHN.</p>
+
+<div class="blockquot"><p class="noin"><b>The Peak of Derbyshire: its Scenery and Antiquities.</b> </p></div>
+
+
+<p>LOFTIE, W.J.</p>
+
+<div class="blockquot"><p class="noin"><b>The Inns of Court.</b> With 60 Illustrations. </p></div>
+
+
+<p>RUSSELL, W. CLARK.</p>
+
+<div class="blockquot"><p class="noin"><b>British Seas.</b> With 50 Illustrations by <span class="smcap">J.C. Hook</span>, R.A., <span class="smcap">Hamilton
+MacCallum</span>, <span class="smcap">Colin Hunter</span>, &amp;c. </p></div>
+
+
+<p>STEVENSON, R.L.</p>
+
+<div class="blockquot"><p class="noin"><b>Edinburgh.</b> With many Illustrations by <span class="smcap">T. Hamilton Crawford</span>, R.S.A.
+(This volume is only to be had in this series in leather, 5s. nett.
+For other editions of this book, see below.) </p></div>
+
+<hr style='width: 15%;' />
+
+<p>SOLOMON, SOLOMON J., R.A.</p>
+
+<div class="blockquot"><p class="noin"><b>The Practice of Oil Painting and Drawing.</b> With 80 Illustrations.
+6s. nett. </p></div>
+
+
+<p>SPEED, HAROLD.</p>
+
+<div class="blockquot"><p class="noin"><b>The Practice and Science of Drawing.</b> With Ninety-six Illustrations
+and Diagrams. Square extra crown 8vo, 6s. </p></div>
+
+<hr style='width: 15%;' />
+
+<h4>THE STANDARD LIBRARY</h4>
+
+<p class="center">
+Extra Crown 8vo, With many Illustrations. Price 2s. 6d. nett.<br />
+</p>
+
+
+<div class="blockquot"><p class="noin"><b>Lady Mary Wortley Montagu.</b> By <span class="smcap">A.R. Ropes</span>.</p>
+
+<p class="noin"><b>Mrs. Thrale.</b> By <span class="smcap">L.B. Seeley</span>.</p>
+
+<p class="noin"><b>Round My House.</b> By <span class="smcap">P.G. Hamerton</span>.</p>
+
+<p class="noin"><b>Fanny Burney &amp; Her Friends.</b> By <span class="smcap">L.B. Seeley</span>. </p></div>
+
+
+<p>STORY SERIES, THE. Extra crown 8vo, 5s. nett.</p>
+
+<div class="blockquot"><p class="noin"><b>The Post Office and its Story.</b> By <span class="smcap">Edward Bennett</span>. With 31 Illus.</p>
+
+<p class="noin"><b>Family Names and their Story.</b> By the Rev. <span class="smcap">S. Baring Gould</span>.</p>
+
+<p class="noin"><b>The Press and its Story.</b> By <span class="smcap">James D. Symon</span>. </p></div>
+
+
+<p>SEELEY, Sir J.R.</p>
+
+<div class="blockquot"><p class="noin"><b>Goethe Reviewed after Sixty Years.</b> With Portrait. Crown 8vo, 3s.
+6d.</p>
+
+<p class="noin"><b>A Short History of Napoleon the First.</b> With Portrait. Crown 8vo,
+5s. </p></div>
+
+
+<p>SEELEY, Sir J.R., and Dr. ABBOTT.</p>
+
+<div class="blockquot"><p class="noin"><b>English Lessons for English People.</b> Crown 8vo, 4s. 6d. </p></div>
+
+
+<p>SEELEY, L.B.</p>
+
+<div class="blockquot"><p class="noin"><b>Mrs. Thrale, afterwards Mrs. Piozzi.</b> With Eight Illustrations.
+Crown 8vo, 2s. 6d nett.</p>
+
+<p class="noin"><b>Fanny Burney and her Friends.</b> With Eight Illustrations. Crown 8vo,
+2s. 6d nett. </p></div>
+
+
+<p>SHAND, A. INNES.</p>
+
+<div class="blockquot"><p class="noin"><b>The War in the Peninsula.</b> With Portraits and Plans. 5s. </p></div>
+
+
+<p>SHARP, WILLIAM.</p>
+
+<div class="blockquot"><p class="noin"><b>Fair Women.</b> Illustrated. Super-royal 8vo, sewed, 2s. 6d. nett;
+half-linen, 3s. 6d. nett. Also new edition, small 4to, cloth, 2s.
+nett; leather, 3s. and 5s. nett. </p></div>
+
+
+<p>SHIPLEY, M.E.</p>
+
+<div class="blockquot"><p class="noin"><b>Daily Help for Daily Need.</b> A Selection of Scripture Verses and
+Poetry for Every Day in the Year. Crown 8vo, 4s. 6d. </p></div>
+
+
+<p>STEPHENS, F.G.</p>
+
+<div class="blockquot"><p class="noin"><b>Rossetti, D.G.</b> Super-royal 8vo, sewed, 2s. 6d. nett; also small
+4to, cloth, 2s. nett; leather, 3s. nett; velvet leather, in a box,
+5s. nett. </p></div>
+
+
+<p>STEVENSON, R.L.</p>
+
+<div class="blockquot"><p class="noin"><b>Edinburgh.</b> Fcap. 8vo, with Frontispiece, gilt top, cloth, 2s. nett;
+leather, 3s. nett. Crown 8vo, Illustrated, cloth, 3s. 6d. Roxburgh,
+gilt top, 5s. Library Edition. Crown 8vo, buckram, dark blue, gilt
+top, Sixteen Full-page Illustrations, 6s. Presentation Edition.
+Extra crown 8vo, with Sixty-four Illustrations, 6s. nett; also
+People's Edition, demy 8vo, 6d. nett. With Twenty-four
+Illustrations in colour, by <span class="smcap">James Heron</span>. Crown 4to. Printed by
+Messrs. T. &amp; A. Constable, of Edinburgh. Ordinary Edition, 12s. 6d.
+nett. <span class="smcap">Edition de Luxe</span>, limited to 385 copies, of which only 375 are
+for sale, printed on unbleached Arnold handmade paper, and bound in
+buckram, with paper label, each copy numbered, 25s. nett. </p></div>
+
+
+<p>STEVENSON, R.A.M.</p>
+
+<div class="blockquot"><p class="noin"><b>Rubens, Peter Paul.</b> Illustrated. Super-royal 8vo, 3s. 6d. nett,
+sewed. Also small 4to, cloth, 2s. nett; leather, 3s. nett and 5s.
+nett. </p></div>
+
+
+<p>STIGAND, Captain C.H., F.R.G.S., F.Z.S.</p>
+
+<div class="blockquot"><p class="noin"><b>To Abyssinia Through an Unknown Land.</b> With Thirty-six Illustrations
+and Two Maps. Demy 8vo, 16s. nett. </p></div>
+
+
+<p>SWANN, ALFRED J.</p>
+
+<div class="blockquot"><p class="noin"><b>Fighting the Slave Hunters in Central Africa.</b> With Forty-five
+Illustrations and a Map. Demy 8vo, 16s. nett. Extra crown 8vo. 5s.
+nett. </p></div>
+
+
+<p>TALBOT, F.A.</p>
+
+<div class="blockquot"><p class="noin"><b>The Makings of a Great Canadian Railway.</b> Demy 8vo. With Forty-one
+Illustrations and a Map. 16s. nett. </p></div>
+
+<hr style='width: 15%;' />
+
+<h4>THE THINGS SEEN SERIES</h4>
+
+<p class="center">
+Each volume with 50 Illustrations. Small 4to, cloth, 2s.; leather, 3s.;<br />
+and velvet leather, in a box, 5s. nett.<br />
+</p>
+
+<div class="blockquot"><p class="noin"><b>Things Seen in Oxford.</b> By <span class="smcap">N.J. Davidson</span>, B.A. (Oxon.)</p>
+
+<p class="noin"><b>Things Seen in Russia.</b> By <span class="smcap">W. Barnes Steveni</span>.</p>
+
+<p class="noin"><b>Things Seen in Palestine.</b> By <span class="smcap">A. Goodrich Freer</span>.</p>
+
+<p class="noin"><b>Things Seen in Japan.</b> By <span class="smcap">Clive Holland</span>.</p>
+
+<p class="noin"><b>Things Seen in China.</b> By <span class="smcap">J.R. Chitty</span>.</p>
+
+<p class="noin"><b>Things Seen in Egypt.</b> By <span class="smcap">E.L. Butcher</span>.</p>
+
+<p class="noin"><b>Things Seen in Holland.</b> By <span class="smcap">C.E. Roche</span>.</p>
+
+<p class="noin"><b>Things Seen in Spain.</b> By <span class="smcap">C. Gasquoine Hartley</span>.</p>
+
+<p class="noin"><b>Things Seen in Northern India.</b> By <span class="smcap">T.L. Pennell</span>, M.D., B.Sc.</p>
+
+<p class="noin"><b>Things Seen in Venice.</b> By <span class="smcap">Lonsdale Ragg</span>, B.D. (Oxon.) </p></div>
+
+
+<p>TOFT, ALBERT, Hon., A.R.C.A., M.S.B.S.</p>
+
+<div class="blockquot"><p class="noin"><b>Modelling and Sculpture.</b> Profusely Illustrated with 119 Photographs
+and Drawings. Square extra crown 8vo, 6s. nett. </p></div>
+
+
+<p>TORDAY, E.</p>
+
+<div class="blockquot"><p class="noin"><b>Camp and Tramp in African Wilds.</b> Demy 8vo. With Forty-five
+Illustrations and a Map, 16s. nett. </p></div>
+
+
+<p>TOWNSHEND, Captain A.T.</p>
+
+<div class="blockquot"><p class="noin"><b>A Military Consul in Turkey.</b> With 29 Illustrations. Demy 8vo, 16s.
+nett. </p></div>
+
+
+<p>TREMEARNE, Major A.J.N.</p>
+
+<div class="blockquot"><p class="noin"><b>The Tailed Head-Hunters of Nigeria.</b> Demy 8vo, with 38 Illustrations
+and a Map. 16s. nett. </p></div>
+
+
+<p>TURNER, CHARLES C.</p>
+
+<div class="blockquot"><p class="noin"><b>Aerial Navigation of To-day.</b> With Seventy Illustrations and
+Diagrams. Extra crown 8vo, 5s. nett. </p></div>
+
+
+<p>WAERN, C.</p>
+
+<div class="blockquot"><p class="noin"><b>John La Farge.</b> Illustrated. Super-royal 8vo, sewed, 3s. 6d. nett. </p></div>
+
+
+<p>WEALE, W.H. JAMES.</p>
+
+<div class="blockquot"><p class="noin"><b>Gerard David, Painter and Illuminator.</b> Illustrated. Super-royal
+8vo, sewed, 2s. 6d. nett; half-linen, 3s. 6d. nett. </p></div>
+
+
+<p>WEEKS, JOHN H.</p>
+
+<div class="blockquot"><p class="noin"><b>Among Congo Cannibals.</b> Demy 8vo. With Fifty-four Illustrations and
+a Map, 16s. nett. </p></div>
+
+
+<p>WELCH, C., and Canon BENHAM.</p>
+
+<div class="blockquot"><p class="noin"><b>Medi&aelig;val London.</b> With a Frontispiece in Photogravure, Four Plates
+in Colour, and many other Illustrations. Super-royal 8vo, sewed,
+5s. nett; cloth, gilt top, 7s. nett. Also extra crown 8vo, 3s. 6d.
+nett. </p></div>
+
+
+<p>WICKS, MARK.</p>
+
+<div class="blockquot"><p class="noin"><b>To Mars via the Moon.</b> An Astronomical Story. With Sixteen
+Illustrations and Diagrams. Extra crown 8vo, 5s. </p></div>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of Astronomy of To-day, by Cecil G. Dolmage
+
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+</body>
+</html>
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+The Project Gutenberg EBook of Astronomy of To-day, by Cecil G. Dolmage
+
+This eBook is for the use of anyone anywhere 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
+
+
+Title: Astronomy of To-day
+       A Popular Introduction in Non-Technical Language
+
+Author: Cecil G. Dolmage
+
+Release Date: April 21, 2009 [EBook #28570]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK ASTRONOMY OF TO-DAY ***
+
+
+
+
+Produced by Brenda Lewis, Scott Marusak, Greg Bergquist
+and the Online Distributed Proofreading Team at
+https://www.pgdp.net (This file was produced from images
+generously made available by The Internet Archive/American
+Libraries.)
+
+
+
+
+
+
+Transcriber's Note
+
+The punctuation and spelling from the original text have been faithfully
+preserved. Only obvious typographical errors have been corrected. The
+advertisement from the beginning of the book has been joined with the
+other advertisements near the end of the book.
+
+Greek words are spelled out and represented as [word]. Greek letters are
+represented as [a] "for alpha".
+
+
+
+
+ASTRONOMY OF TO-DAY
+
+[Illustration: THE TOTAL ECLIPSE OF THE SUN OF AUGUST 30TH, 1905.
+
+The Corona; from a water-colour sketch, made at Burgos, in Spain, during
+the total phase, by the French Artist, Mdlle. ANDREE MOCH.]
+
+
+
+
+  ASTRONOMY OF
+  TO-DAY
+
+  _A POPULAR INTRODUCTION IN
+  NON-TECHNICAL LANGUAGE_
+
+  By
+
+  CECIL G. DOLMAGE, M.A., LL.D., D.C.L.
+
+  Fellow of the Royal Astronomical Society; Member of
+  the British Astronomical Association; Member of
+  the Astronomical Society of the Pacific; Membre
+  de la Societe Astronomique de France;
+  Membre de la Societe Belge
+  d'Astronomie
+
+
+
+  With a Frontispiece in Colour
+  and 45 Illustrations & Diagrams
+
+
+  _THIRD EDITION_
+
+
+  LONDON
+  SEELEY AND CO. LIMITED
+  38 GREAT RUSSELL STREET
+  1910
+
+
+
+
+PREFACE
+
+
+The object of this book is to give an account of the science of
+Astronomy, as it is known at the present day, in a manner acceptable to
+the _general reader_.
+
+It is too often supposed that it is impossible to acquire any useful
+knowledge of Astronomy without much laborious study, and without
+adventuring into quite a new world of thought. The reasoning applied to
+the study of the celestial orbs is, however, of no different order from
+that which is employed in the affairs of everyday life. The science of
+mathematics is perhaps responsible for the idea that some kind of
+difference does exist; but mathematical processes are, in effect, no
+more than ordinary logic in concentrated form, the _shorthand of
+reasoning_, so to speak. I have attempted in the following pages to take
+the main facts and theories of Astronomy out of those mathematical forms
+which repel the general reader, and to present them in the _ordinary
+language of our workaday world_.
+
+The few diagrams introduced are altogether supplementary, and are not
+connected with the text by any wearying cross-references. Each diagram
+is complete in itself, being intended to serve as a pictorial aid, in
+case the wording of the text should not have perfectly conveyed the
+desired meaning. The full page illustrations are also described as
+adequately as possible at the foot of each.
+
+As to the coloured frontispiece, this must be placed in a category by
+itself. It is the work of the _artist_ as distinct from the scientist.
+
+The book itself contains incidentally a good deal of matter concerned
+with the Astronomy of the past, the introduction of which has been found
+necessary in order to make clearer the Astronomy of our time.
+
+It would be quite impossible for me to enumerate here the many sources
+from which information has been drawn. But I acknowledge my especial
+indebtedness to Professor F.R. Moulton's _Introduction to Astronomy_
+(Macmillan, 1906), to the works on Eclipses of the late Rev. S.J.
+Johnson and of Mr. W.T. Lynn, and to the excellent _Journals of the
+British Astronomical Association_. Further, for those grand questions
+concerned with the Stellar Universe at large, I owe a very deep debt to
+the writings of the famous American astronomer, Professor Simon Newcomb,
+and of our own countryman, Mr. John Ellard Gore; to the latter of whom I
+am under an additional obligation for much valuable information
+privately rendered.
+
+In my search for suitable illustrations, I have been greatly aided by
+the kindly advice of Mr. W. H. Wesley, the Assistant Secretary of the
+Royal Astronomical Society. To those who have been so good as to permit
+me to reproduce pictures and photographs, I desire to record my best
+thanks as follows:--To the French Artist, Mdlle. Andree Moch; to the
+Astronomer Royal; to Sir David Gill, K.C.B., LL.D., F.R.S.; to the
+Council of the Royal Astronomical Society; to Professor E.B. Frost,
+Director of the Yerkes Observatory; to M.P. Puiseux, of the Paris
+Observatory; to Dr. Max Wolf, of Heidelberg; to Professor Percival
+Lowell; to the Rev. Theodore E.R. Phillips, M.A., F.R.A.S.; to Mr. W.H.
+Wesley; to the Warner and Swasey Co., of Cleveland, Ohio, U.S.A.; to the
+publishers of _Knowledge_, and to Messrs. Sampson, Low & Co. For
+permission to reproduce the beautiful photograph of the Spiral Nebula in
+Canes Venatici (Plate XXII.), I am indebted to the distinguished
+astronomer, the late Dr. W.E. Wilson, D.Sc., F.R.S., whose untimely
+death, I regret to state, occurred in the early part of this year.
+
+Finally, my best thanks are due to Mr. John Ellard Gore, F.R.A.S.,
+M.R.I.A., to Mr. W.H. Wesley, and to Mr. John Butler Burke, M.A., of
+Cambridge, for their kindness in reading the proof-sheets.
+
+CECIL G. DOLMAGE.
+
+LONDON, S.W.,
+_August 4, 1908._
+
+
+
+
+PREFATORY NOTE TO THE SECOND EDITION
+
+
+The author of this book lived only long enough to hear of the favour
+with which it had been received, and to make a few corrections in view
+of the second edition which it has so soon reached.
+
+_December 1908._
+
+
+
+
+CONTENTS
+
+
+  CHAPTER I
+                                            PAGE
+  THE ANCIENT VIEW                            17
+
+  CHAPTER II
+  THE MODERN VIEW                             20
+
+  CHAPTER III
+  THE SOLAR SYSTEM                            29
+
+  CHAPTER IV
+  CELESTIAL MECHANISM                         38
+
+  CHAPTER V
+  CELESTIAL DISTANCES                         46
+
+  CHAPTER VI
+  CELESTIAL MEASUREMENT                       55
+
+  CHAPTER VII
+  ECLIPSES AND KINDRED PHENOMENA              61
+
+  CHAPTER VIII
+  FAMOUS ECLIPSES OF THE SUN                  83
+
+  CHAPTER IX
+  FAMOUS ECLIPSES OF THE MOON                101
+
+  CHAPTER X
+  THE GROWTH OF OBSERVATION                  105
+
+  CHAPTER XI
+  SPECTRUM ANALYSIS                          121
+
+  CHAPTER XII
+  THE SUN                                    127
+
+  CHAPTER XIII
+  THE SUN--_continued_                       134
+
+  CHAPTER XIV
+  THE INFERIOR PLANETS                       146
+
+  CHAPTER XV
+  THE EARTH                                  158
+
+  CHAPTER XVI
+  THE MOON                                   183
+
+  CHAPTER XVII
+  THE SUPERIOR PLANETS                       209
+
+  CHAPTER XVIII
+  THE SUPERIOR PLANETS--_continued_          229
+
+  CHAPTER XIX
+  COMETS                                     247
+
+  CHAPTER XX
+  REMARKABLE COMETS                          259
+
+  CHAPTER XXI
+  METEORS OR SHOOTING STARS                  266
+
+  CHAPTER XXII
+  THE STARS                                  278
+
+  CHAPTER XXIII
+  THE STARS--_continued_                     287
+
+  CHAPTER XXIV
+  SYSTEMS OF STARS                           300
+
+  CHAPTER XXV
+  THE STELLAR UNIVERSE                       319
+
+  CHAPTER XXVI
+  THE STELLAR UNIVERSE--_continued_          329
+
+  CHAPTER XXVII
+  THE BEGINNING OF THINGS                    333
+
+  CHAPTER XXVIII
+  THE END OF THINGS                          342
+
+  INDEX                                      351
+
+
+
+
+LIST OF ILLUSTRATIONS
+
+
+LIST OF PLATES
+
+  PLATE
+  THE TOTAL ECLIPSE OF THE SUN
+  OF AUGUST 30, 1905                       _Frontispiece_
+
+  I. THE TOTAL ECLIPSE OF THE SUN
+     OF MAY 17, 1882                   _To face page_  96
+
+  II. GREAT TELESCOPE OF HEVELIUS        "    "       110
+
+  III. A TUBELESS, OR "AERIAL" TELESCOPE "    "       112
+
+  IV. THE GREAT YERKES TELESCOPE         "    "       118
+
+  V. THE SUN, SHOWING SEVERAL
+     GROUPS OF SPOTS                     "    "       134
+
+  VI. PHOTOGRAPH OF A SUNSPOT            "    "       136
+
+  VII. FORMS OF THE SOLAR CORONA
+       AT THE EPOCHS OF SUNSPOT
+       MAXIMUM AND SUNSPOT
+       MINIMUM RESPECTIVELY.
+      (A) THE TOTAL ECLIPSE OF
+      THE SUN OF DECEMBER 22, 1870.
+      (B) THE TOTAL ECLIPSE OF
+      THE SUN OF MAY 28, 1900            "    "       142
+
+  VIII. THE MOON                       _To face page_ 196
+
+  IX. MAP OF THE MOON, SHOWING
+      THE PRINCIPAL "CRATERS,"
+      MOUNTAIN RANGES AND
+      "SEAS"                             "    "       198
+
+  X. ONE OF THE MOST INTERESTING
+     REGIONS ON THE MOON                 "    "       200
+
+  XI. THE MOON (SHOWING SYSTEMS
+      OF "RAYS")                         "    "       204
+
+  XII. A MAP OF THE PLANET MARS          "    "       216
+
+  XIII. MINOR PLANET TRAILS              "    "       226
+
+  XIV. THE PLANET JUPITER                "    "       230
+
+  XV. THE PLANET SATURN                  "    "       236
+
+  XVI. EARLY REPRESENTATIONS OF
+       SATURN                            "    "       242
+
+  XVII. DONATI'S COMET                   "    "       256
+
+  XVIII. DANIEL'S COMET OF 1907          "    "       258
+
+  XIX. THE SKY AROUND THE NORTH
+       POLE                              "    "       292
+
+  XX. ORION AND HIS NEIGHBOURS           "    "       296
+
+  XXI. THE GREAT GLOBULAR CLUSTER
+       IN THE SOUTHERN CONSTELLATION
+       OF CENTAURUS                      "    "       306
+
+  XXII. SPIRAL NEBULA IN THE CONSTELLATION
+        OF CANES VENATICI                "    "       314
+
+  XXIII. THE GREAT NEBULA IN THE
+         CONSTELLATION OF ANDROMEDA    _To face page_ 316
+
+  XXIV. THE GREAT NEBULA IN THE
+        CONSTELLATION OF ORION           "    "       318
+
+
+
+
+LIST OF DIAGRAMS
+
+
+  FIG.                                         PAGE
+  1. THE PTOLEMAIC IDEA OF THE UNIVERSE          19
+
+  2. THE COPERNICAN THEORY OF THE SOLAR
+     SYSTEM                                      21
+
+  3. TOTAL AND PARTIAL ECLIPSES OF THE MOON      64
+
+  4. TOTAL AND PARTIAL ECLIPSES OF THE SUN       67
+
+  5. "BAILY'S BEADS"                             70
+
+  6. MAP OF THE WORLD ON MERCATOR'S PROJECTION,
+     SHOWING A PORTION OF THE PROGRESS
+     OF THE TOTAL SOLAR ECLIPSE OF
+     AUGUST 30, 1905, ACROSS THE SURFACE
+     OF THE EARTH                                81
+
+  7. THE "RING WITH WINGS"                       87
+
+  8. THE VARIOUS TYPES OF TELESCOPE             113
+
+  9. THE SOLAR SPECTRUM                         123
+
+  10. A SECTION THROUGH THE SUN, SHOWING HOW
+      THE PROMINENCES RISE FROM THE
+      CHROMOSPHERE                              131
+
+  11. ORBIT AND PHASES OF AN INFERIOR PLANET    148
+
+  12. THE "BLACK DROP"                          153
+
+  13. SUMMER AND WINTER                         176
+
+  14. ORBIT AND PHASES OF THE MOON              184
+
+  15. THE ROTATION OF THE MOON ON HER AXIS      187
+
+  16. LAPLACE'S "PERENNIAL FULL MOON"           191
+
+  17. ILLUSTRATING THE AUTHOR'S EXPLANATION OF
+      THE APPARENT ENLARGEMENT OF CELESTIAL
+      OBJECTS                                   195
+
+  18. SHOWING HOW THE TAIL OF A COMET IS
+      DIRECTED AWAY FROM THE SUN                248
+
+  19. THE COMET OF 1066, AS REPRESENTED IN THE
+      BAYEUX TAPESTRY                           263
+
+  20. PASSAGE OF THE EARTH THROUGH THE THICKEST
+      PORTION OF A METEOR SWARM                 269
+
+
+
+
+ASTRONOMY OF TO-DAY
+
+
+
+
+CHAPTER I
+
+THE ANCIENT VIEW
+
+
+It is never safe, as we know, to judge by appearances, and this is
+perhaps more true of astronomy than of anything else.
+
+For instance, the idea which one would most naturally form of the earth
+and heaven is that the solid earth on which we live and move extends to
+a great distance in every direction, and that the heaven is an immense
+dome upon the inner surface of which the stars are fixed. Such must
+needs have been the idea of the universe held by men in the earliest
+times. In their view the earth was of paramount importance. The sun and
+moon were mere lamps for the day and for the night; and these, if not
+gods themselves, were at any rate under the charge of special deities,
+whose task it was to guide their motions across the vaulted sky.
+
+Little by little, however, this simple estimate of nature began to be
+overturned. Difficult problems agitated the human mind. On what, for
+instance, did the solid earth rest, and what prevented the vaulted
+heaven from falling in upon men and crushing them out of existence?
+Fantastic myths sprang from the vain attempts to solve these riddles.
+The Hindoos, for example, imagined the earth as supported by four
+elephants which stood upon the back of a gigantic tortoise, which, in
+its turn, floated on the surface of an elemental ocean. The early
+Western civilisations conceived the fable of the Titan Atlas, who, as a
+punishment for revolt against the Olympian gods, was condemned to hold
+up the expanse of sky for ever and ever.
+
+Later on glimmerings of the true light began to break in upon men. The
+Greek philosophers, who busied themselves much with such matters,
+gradually became convinced that the earth was spherical in shape, that
+is to say, round like a ball. In this opinion we now know that they were
+right; but in their other important belief, viz. that the earth was
+placed at the centre of all things, they were indeed very far from the
+truth.
+
+By the second century of the Christian era, the ideas of the early
+philosophers had become hardened into a definite theory, which, though
+it appears very incorrect to us to-day, nevertheless demands exceptional
+notice from the fact that it was everywhere accepted as the true
+explanation until so late as some four centuries ago. This theory of the
+universe is known by the name of the Ptolemaic System, because it was
+first set forth in definite terms by one of the most famous of the
+astronomers of antiquity, Claudius Ptolemaeus Pelusinensis (100-170
+A.D.), better known as Ptolemy of Alexandria.
+
+In his system the Earth occupied the centre; while around it circled in
+order outwards the Moon, the planets Mercury and Venus, the Sun, and
+then the planets Mars, Jupiter, and Saturn. Beyond these again revolved
+the background of the heaven, upon which it was believed that the stars
+were fixed--
+
+     "Stellis ardentibus aptum,"
+
+as Virgil puts it (see Fig. 1).
+
+[Illustration: FIG. 1.--The Ptolemaic idea of the Universe.]
+
+The Ptolemaic system persisted unshaken for about fourteen hundred years
+after the death of its author. Clearly men were flattered by the notion
+that their earth was the most important body in nature, that it stood
+still at the centre of the universe, and was the pivot upon which all
+things revolved.
+
+
+
+
+CHAPTER II
+
+THE MODERN VIEW
+
+
+It is still well under four hundred years since the modern, or
+Copernican, theory of the universe supplanted the Ptolemaic, which had
+held sway during so many centuries. In this new theory, propounded
+towards the middle of the sixteenth century by Nicholas Copernicus
+(1473-1543), a Prussian astronomer, the earth was dethroned from its
+central position and considered merely as one of a number of planetary
+bodies which revolve around the sun. As it is not a part of our purpose
+to follow in detail the history of the science, it seems advisable to
+begin by stating in a broad fashion the conception of the universe as
+accepted and believed in to-day.
+
+The Sun, the most important of the celestial bodies so far as we are
+concerned, occupies the central position; not, however, in the whole
+universe, but only in that limited portion which is known as the Solar
+System. Around it, in the following order outwards, circle the planets
+Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
+(see Fig. 2, p. 21). At an immense distance beyond the solar system, and
+scattered irregularly through the depth of space, lie the stars. The two
+first-mentioned members of the solar system, Mercury and Venus, are
+known as the Inferior Planets; and in their courses about the sun, they
+always keep well inside the path along which our earth moves. The
+remaining members (exclusive of the earth) are called Superior Planets,
+and their paths lie all outside that of the earth.
+
+[Illustration: FIG. 2.--The Copernican theory of the Solar System.]
+
+The five planets, Mercury, Venus, Mars, Jupiter, and Saturn, have been
+known from all antiquity. Nothing then can bring home to us more
+strongly the immense advance which has taken place in astronomy during
+modern times than the fact that it is only 127 years since observation
+of the skies first added a planet to that time-honoured number. It was
+indeed on the 13th of March 1781, while engaged in observing the
+constellation of the Twins, that the justly famous Sir William Herschel
+caught sight of an object which he did not recognise as having met with
+before. He at first took it for a comet, but observations of its
+movements during a few days showed it to be a planet. This body, which
+the power of the telescope alone had thus shown to belong to the solar
+family, has since become known to science under the name of Uranus. By
+its discovery the hitherto accepted limits of the solar system were at
+once pushed out to twice their former extent, and the hope naturally
+arose that other planets would quickly reveal themselves in the
+immensities beyond.
+
+For a number of years prior to Herschel's great discovery, it had been
+noticed that the distances at which the then known planets circulated
+appeared to be arranged in a somewhat orderly progression outwards from
+the sun. This seeming plan, known to astronomers by the name of Bode's
+Law, was closely confirmed by the distance of the new planet Uranus.
+There still lay, however, a broad gap between the planets Mars and
+Jupiter. Had another planet indeed circuited there, the solar system
+would have presented an appearance of almost perfect order. But the void
+between Mars and Jupiter was unfilled; the space in which one would
+reasonably expect to find another planet circling was unaccountably
+empty.
+
+On the first day of the nineteenth century the mystery was however
+explained, a body being discovered[1] which revolved in the space that
+had hitherto been considered planetless. But it was a tiny globe hardly
+worthy of the name of planet. In the following year a second body was
+discovered revolving in the same space; but it was even smaller in size
+than the first. During the ensuing five years two more of these little
+planets were discovered. Then came a pause, no more such bodies being
+added to the system until half-way through the century, when suddenly
+the discovery of these so-called "minor planets" began anew. Since then
+additions to this portion of our system have rained thick and fast. The
+small bodies have received the name of Asteroids or Planetoids; and up
+to the present time some six hundred of them are known to exist, all
+revolving in the previously unfilled space between Mars and Jupiter.
+
+In the year 1846 the outer boundary of the solar system was again
+extended by the discovery that a great planet circulated beyond Uranus.
+The new body, which received the name of Neptune, was brought to light
+as the result of calculations made at the same time, though quite
+independently, by the Cambridge mathematician Adams, and the French
+astronomer Le Verrier. The discovery of Neptune differed, however, from
+that of Uranus in the following respect. Uranus was found merely in the
+course of ordinary telescopic survey of the heavens. The position of
+Neptune, on the other hand, was predicted as the result of rigorous
+mathematical investigations undertaken with the object of fixing the
+position of an unseen and still more distant body, the attraction of
+which, in passing by, was disturbing the position of Uranus in its
+revolution around the sun. Adams actually completed his investigation
+first; but a delay at Cambridge in examining that portion of the sky,
+where he announced that the body ought just then to be, allowed France
+to snatch the honour of discovery, and the new planet was found by the
+observer Galle at Berlin, very near the place in the heavens which Le
+Verrier had mathematically predicted for it.
+
+Nearly fifty years later, that is to say, in our own time, another
+important planetary discovery was made. One of the recent additions to
+the numerous and constantly increasing family of the asteroids, a tiny
+body brought to light in 1898, turned out after all not to be
+circulating in the customary space between Mars and Jupiter, but
+actually in that between our earth and Mars. This body is very small,
+not more than about twenty miles across. It has received the name of
+Eros (the Greek equivalent for Cupid), in allusion to its insignificant
+size as compared with the other leading members of the system.
+
+This completes the list of the planets which, so far, have revealed
+themselves to us. Whether others exist time alone will show. Two or
+three have been suspected to revolve beyond the path of Neptune; and it
+has even been asserted, on more than one occasion, that a planet
+circulates nearer to the sun than Mercury. This supposed body, to which
+the name of "Vulcan" was provisionally given, is said to have been
+"discovered" in 1859 by a French doctor named Lescarbault, of Orgeres
+near Orleans; but up to the present there has been no sufficient
+evidence of its existence. The reason why such uncertainty should exist
+upon this point is easy enough to understand, when we consider the
+overpowering glare which fills our atmosphere all around the sun's place
+in the sky. Mercury, the nearest known planet to the sun, is for this
+reason always very difficult to see; and even when, in its course, it
+gets sufficiently far from the sun to be left for a short time above the
+horizon after sunset, it is by no means an easy object to observe on
+account of the mists which usually hang about low down near the earth.
+One opportunity, however, offers itself from time to time to solve the
+riddle of an "intra-Mercurial" planet, that is to say, of a planet which
+circulates within the path followed by Mercury. The opportunity in
+question is furnished by a total eclipse of the sun; for when, during an
+eclipse of that kind, the body of the moon for a few minutes entirely
+hides the sun's face, and the dazzling glare is thus completely cut off,
+astronomers are enabled to give an unimpeded, though all too hurried,
+search to the region close around. A goodly number of total eclipses of
+the sun have, however, come and gone since the days of Lescarbault, and
+no planet, so far, has revealed itself in the intra-Mercurial space. It
+seems, therefore, quite safe to affirm that no globe of sufficient size
+to be seen by means of our modern telescopes circulates nearer to the
+sun than the planet Mercury.
+
+Next in importance to the planets, as permanent members of the solar
+system, come the relatively small and secondary bodies known by the name
+of Satellites. The name _satellite_ is derived from a Latin word
+signifying _an attendant_; for the bodies so-called move along always in
+close proximity to their respective "primaries," as the planets which
+they accompany are technically termed.
+
+The satellites cannot be considered as allotted with any particular
+regularity among the various members of the system; several of the
+planets, for instance, having a goodly number of these bodies
+accompanying them, while others have but one or two, and some again have
+none at all. Taking the planets in their order of distance outward from
+the Sun, we find that neither Mercury nor Venus are provided with
+satellites; the Earth has only one, viz. our neighbour the Moon; while
+Mars has but two tiny ones, so small indeed that one might imagine them
+to be merely asteroids, which had wandered out of their proper region
+and attached themselves to that planet. For the rest, so far as we at
+present know, Jupiter possesses seven,[2] Saturn ten, Uranus four, and
+Neptune one. It is indeed possible, nay more, it is extremely probable,
+that the two last-named planets have a greater number of these secondary
+bodies revolving around them; but, unfortunately, the Uranian and
+Neptunian systems are at such immense distances from us, that even the
+magnificent telescopes of to-day can extract very little information
+concerning them.
+
+From the distribution of the satellites, the reader will notice that the
+planets relatively near to the sun are provided with few or none, while
+the more distant planets are richly endowed. The conclusion, therefore,
+seems to be that nearness to the sun is in some way unfavourable either
+to the production, or to the continued existence, of satellites.
+
+A planet and its satellites form a repetition of the solar system on a
+tiny scale. Just as the planets revolve around the sun, so do these
+secondary bodies revolve around their primaries. When Galileo, in 1610,
+turned his newly invented telescope upon Jupiter, he quickly recognised
+in the four circling moons which met his gaze, a miniature edition of
+the solar system.
+
+Besides the planets and their satellites, there are two other classes of
+bodies which claim membership of the solar system. These are Comets and
+Meteors. Comets differ from the bodies which we have just been
+describing in that they appear filmy and transparent, whereas the others
+are solid and opaque. Again, the paths of the planets around the sun and
+of the satellites around their primaries are not actually circles; they
+are ovals, but their ovalness is not of a marked degree. The paths of
+comets on the other hand are usually _very_ oval; so that in their
+courses many of them pass out as far as the known limits of the solar
+system, and even far beyond. It should be mentioned that nowadays the
+tendency is to consider comets as permanent members of the system,
+though this was formerly not by any means an article of faith with
+astronomers.
+
+Meteors are very small bodies, as a rule perhaps no larger than pebbles,
+which move about unseen in space, and of which we do not become aware
+until they arrive very close to the earth. They are then made visible to
+us for a moment or two in consequence of being heated to a white heat by
+the friction of rushing through the atmosphere, and are thus usually
+turned into ashes and vapour long before they reach the surface of our
+globe. Though occasionally a meteoric body survives the fiery ordeal,
+and reaches the earth more or less in a solid state to bury itself deep
+in the soil, the majority of these celestial visitants constitute no
+source of danger whatever for us. Any one who will take the trouble to
+gaze at the sky for a short time on a clear night, is fairly certain to
+be rewarded with the view of a meteor. The impression received is as if
+one of the stars had suddenly left its accustomed place, and dashed
+across the heavens, leaving in its course a trail of light. It is for
+this reason that meteors are popularly known under the name of "shooting
+stars."
+
+
+[1] By the Italian astronomer, Piazzi, at Palermo.
+
+[2] Probably eight. (See note, page 232.)
+
+
+
+
+CHAPTER III
+
+THE SOLAR SYSTEM
+
+
+We have seen, in the course of the last chapter, that the solar system
+is composed as follows:--there is a central body, the sun, around which
+revolve along stated paths a number of important bodies known as
+planets. Certain of these planets, in their courses, carry along in
+company still smaller bodies called satellites, which revolve around
+them. With regard, however, to the remaining members of the system, viz.
+the comets and the meteors, it is not advisable at this stage to add
+more to what has been said in the preceding chapter. For the time being,
+therefore, we will devote our attention merely to the sun, the planets,
+and the satellites.
+
+Of what shape then are these bodies? Of one shape, and that one alone
+which appears to characterise all solid objects in the celestial spaces:
+they are spherical, which means _round like a ball_.
+
+Each of these spherical bodies rotates; that is to say, turns round and
+round, as a top does when it is spinning. This rotation is said to take
+place "upon an axis," a statement which may be explained as
+follows:--Imagine a ball with a knitting-needle run right through its
+centre. Then imagine this needle held pointing in one fixed direction
+while the ball is turned round and round. Well, it is the same thing
+with the earth. As it journeys about the sun, it keeps turning round and
+round continually as if pivoted upon a mighty knitting needle
+transfixing it from North Pole to South Pole. In reality, however, there
+is no such material axis to regulate the constant direction of the
+rotation, just as there are no actual supports to uphold the earth
+itself in space. The causes which keep the celestial spheres poised, and
+which control their motions, are far more wonderful than any mechanical
+device.
+
+At this juncture it will be well to emphasise the sharp distinction
+between the terms _rotation_ and _revolution_. The term "rotation" is
+invariably used by astronomers to signify the motion which a celestial
+body has upon an axis; the term "revolution," on the other hand, is used
+for the movement of one celestial body around another. Speaking of the
+earth, for instance, we say, that it _rotates_ on its axis, and that it
+_revolves_ around the sun.
+
+So far, nothing has been said about the sizes of the members of our
+system. Is there any stock size, any pattern according to which they may
+be judged? None whatever! They vary enormously. Very much the largest of
+all is the Sun, which is several hundred times larger than all the
+planets and satellites of the system rolled together. Next comes Jupiter
+and afterwards the other planets in the following order of
+size:--Saturn, Uranus, Neptune, the Earth, Venus, Mars, and Mercury.
+Very much smaller than any of these are the asteroids, of which Ceres,
+the largest, is less than 500 miles in diameter. It is, by the way, a
+strange fact that the zone of asteroids should mark the separation of
+the small planets from the giant ones. The following table, giving
+roughly the various diameters of the sun and the principal planets in
+miles, will clearly illustrate the great discrepancy in size which
+prevails in the system.
+
+Sun          866,540 miles
+Mercury        2,765  "
+Venus          7,826  "
+Earth          7,918  "
+Mars           4,332  "
+
+ZONE OF ASTEROIDS
+
+Jupiter       87,380  "
+Saturn        73,125  "
+Uranus[3]     34,900  "
+Neptune[3]    32,900  "
+
+It does not seem possible to arrive at any generalisation from the above
+data, except it be to state that there is a continuous increase in size
+from Mercury to the earth, and a similar decrease in size from Jupiter
+outwards. Were Mars greater than the earth, the planets could then with
+truth be said to increase in size up to Jupiter, and then to decrease.
+But the zone of asteroids, and the relative smallness of Mars, negative
+any attempt to regard the dimensions of the planets as an orderly
+sequence.
+
+Next with respect to relative distance from the sun, Venus circulates
+nearly twice as far from it as Mercury, the earth nearly three times as
+far, and Mars nearly four times. After this, just as we found a sudden
+increase in size, so do we meet with a sudden increase in distance.
+Jupiter, for instance, is about thirteen times as far as Mercury, Saturn
+about twenty-five times, Uranus about forty-nine times, and Neptune
+about seventy-seven. (See Fig. 2, p. 21.)
+
+It will thus be seen how enormously the solar system was enlarged in
+extent by the discovery of the outermost planets. The finding of Uranus
+plainly doubled its breadth; the finding of Neptune made it more than
+half as broad again. Nothing indeed can better show the import of these
+great discoveries than to take a pair of compasses and roughly set out
+the above relative paths in a series of concentric circles upon a large
+sheet of paper, and then to consider that the path of Saturn was the
+supposed boundary of our solar system prior to the year 1781.
+
+We have seen that the usual shape of celestial bodies themselves is
+spherical. Of what form then are their paths, or _orbits_, as these are
+called? One might be inclined at a venture to answer "circular," but
+this is not the case. The orbits of the planets cannot be regarded as
+true circles. They are ovals, or, to speak in technical language,
+"ellipses." Their ovalness or "ellipticity" is, however, in each case
+not by any means of the same degree. Some orbits--for instance, that of
+the earth--differ only slightly from circles; while others--those of
+Mars or Mercury, for example--are markedly elliptic. The orbit of the
+tiny planet Eros is, however, far and away the most elliptic of all, as
+we shall see when we come to deal with that little planet in detail.
+
+It has been stated that the sun and planets are always rotating. The
+various rates at which they do so will, however, be best appreciated by
+a comparison with the rate at which the earth itself rotates.
+
+But first of all, let us see what ground we have, if any, for asserting
+that the earth rotates at all?
+
+If we carefully watch the heavens we notice that the background of the
+sky, with all the brilliant objects which sparkle in it, appears to turn
+once round us in about twenty-four hours; and that the pivot upon which
+this movement takes place is situated somewhere near what is known to us
+as the _Pole Star_. This was one of the earliest facts noted with regard
+to the sky; and to the men of old it therefore seems as if the heavens
+and all therein were always revolving around the earth. It was natural
+enough for them to take this view, for they had not the slightest idea
+of the immense distance of the celestial bodies, and in the absence of
+any knowledge of the kind they were inclined to imagine them
+comparatively near. It was indeed only after the lapse of many
+centuries, when men had at last realised the enormous gulf which
+separated them from even the nearest object in the sky, that a more
+reasonable opinion began to prevail. It was then seen that this
+revolution of the heavens about the earth could be more easily and more
+satisfactorily explained by supposing a mere rotation of the solid earth
+about a fixed axis, pointed in the direction of the polar star. The
+probability of such a rotation on the part of the earth itself was
+further strengthened by the observations made with the telescope. When
+the surfaces of the sun and planets were carefully studied these bodies
+were seen to be rotating. This being the case, there could not surely
+be much hesitation in granting that the earth rotated also; particularly
+when it so simply explained the daily movement of the sky, and saved men
+from the almost inconceivable notion that the whole stupendous vaulted
+heaven was turning about them once in every twenty-four hours.
+
+If the sun be regularly observed through a telescope, it will gradually
+be gathered from the slow displacement of sunspots across its face,
+their disappearance at one edge and their reappearance again at the
+other edge, that it is rotating on an axis in a period of about
+twenty-six days. The movement, too, of various well-known markings on
+the surfaces of the planets Mars, Jupiter, and Saturn proves to us that
+these bodies are rotating in periods, which are about twenty-four hours
+for the first, and about ten hours for each of the other two. With
+regard, however, to Uranus and Neptune there is much more uncertainty,
+as these planets are at such great distances that even our best
+telescopes give but a confused view of the markings which they display;
+still a period of rotation of from ten to twelve hours appears to be
+accepted for them. On the other hand the constant blaze of sunlight in
+the neighbourhood of Mercury and Venus equally hampers astronomers in
+this quest. The older telescopic observers considered that the rotation
+periods of these two planets were about the same as that of the earth;
+but of recent years the opinion has been gaining ground that they turn
+round on their axes in exactly the same time as they revolve about the
+sun. This question is, however, a very doubtful one, and will be again
+referred to later on; but, putting it on one side, it will be seen from
+what we have said above, that the rotation periods of the other planets
+of our system are usually about twenty-four hours, or under. The fact
+that the rotation period of the sun should run into _days_ need not seem
+extraordinary when one considers its enormous size.
+
+The periods taken by the various planets to revolve around the sun is
+the next point which has to be considered. Here, too, it is well to
+start with the earth's period of revolution as the standard, and to see
+how the periods taken by the other planets compare with it.
+
+The earth takes about 365-1/4 days to revolve around the sun. This
+period of time is known to us as a "year." The following table shows in
+days and years the periods taken by each of the other planets to make a
+complete revolution round the sun:--
+
+Mercury  about  88 days.
+Venus     "    226   "
+Mars      "      1 year and 321 days.
+Jupiter   "     11 years and 313 days.
+Saturn    "     29 years and 167 days.
+Uranus    "     84 years and 7 days.
+Neptune   "    164 years and 284 days.
+
+From these periods we gather an important fact, namely, that the nearer
+a planet is to the sun the faster it revolves.
+
+Compared with one of our years what a long time does an Uranian, or
+Neptunian, "year" seem? For instance, if a "year" had commenced in
+Neptune about the middle of the reign of George II., that "year" would
+be only just coming to a close; for the planet is but now arriving back
+to the position, with regard to the sun, which it then occupied. Uranus,
+too, has only completed a little more than 1-1/2 of its "years" since
+Herschel discovered it.
+
+Having accepted the fact that the planets are revolving around the sun,
+the next point to be inquired into is:--What are the positions of their
+orbits, or paths, relatively to each other?
+
+Suppose, for instance, the various planetary orbits to be represented by
+a set of hoops of different sizes, placed one within the other, and the
+sun by a small ball in the middle of the whole; in what positions will
+these hoops have to be arranged so as to imitate exactly the true
+condition of things?
+
+First of all let us suppose the entire arrangement, ball and hoops, to
+be on one level, so to speak. This may be easily compassed by imagining
+the hoops as floating, one surrounding the other, with the ball in the
+middle of all, upon the surface of still water. Such a set of objects
+would be described in astronomical parlance as being _in the same
+plane_. Suppose, on the other hand, that some of these floating hoops
+are tilted with regard to the others, so that one half of a hoop rises
+out of the water and the other half consequently sinks beneath the
+surface. This indeed is the actual case with regard to the planetary
+orbits. They do not by any means lie all exactly in the same plane. Each
+one of them is tilted, or _inclined_, a little with respect to the plane
+of the earth's orbit, which astronomers, for convenience, regard as the
+_level_ of the solar system. This tilting, or "inclination," is, in the
+larger planets, greatest for the orbit of Mercury, least for that of
+Uranus. Mercury's orbit is inclined to that of the earth at an angle of
+about 7 deg., that of Venus at a little over 3 deg., that of Saturn 2-1/2
+deg.; while in those of Mars, Neptune, and Jupiter the inclination is less
+than 2 deg. But greater than any of these is the inclination of the orbit
+of the tiny planet Eros, viz. nearly 11 deg.
+
+The systems of satellites revolving around their respective planets
+being, as we have already pointed out, mere miniature editions of the
+solar system, the considerations so far detailed, which regulate the
+behaviour of the planets in their relations to the sun, will of
+necessity apply to the satellites very closely. In one respect, however,
+a system of satellites differs materially from a system of planets. The
+central body around which planets are in motion is self-luminous,
+whereas the planetary body around which a satellite revolves is not.
+True, planets shine, and shine very brightly too; as, for instance,
+Venus and Jupiter. But they do not give forth any light of their own, as
+the sun does; they merely reflect the sunlight which they receive from
+him. Putting this one fact aside, the analogy between the planetary
+system and a satellite system is remarkable. The satellites are
+spherical in form, and differ markedly in size; they rotate, so far as
+we know, upon their axes in varying times; they revolve around their
+governing planets in orbits, not circular, but elliptic; and these
+orbits, furthermore, do not of necessity lie in the same plane. Last of
+all the satellites revolve around their primaries at rates which are
+directly comparable with those at which the planets revolve around the
+sun, the rule in fact holding good that the nearer a satellite is to its
+primary the faster it revolves.
+
+
+[3] As there seems to be much difference of opinion concerning the
+diameters of Uranus and Neptune, it should here be mentioned that the
+above figures are taken from Professor F.R. Moulton's _Introduction to
+Astronomy_ (1906). They are there stated to be given on the authority of
+"Barnard's many measures at the Lick Observatory."
+
+
+
+
+CHAPTER IV
+
+CELESTIAL MECHANISM
+
+
+As soon as we begin to inquire closely into the actual condition of the
+various members of the solar system we are struck with a certain
+distinction. We find that there are two quite different points of view
+from which these bodies can be regarded. For instance, we may make our
+estimates of them either as regards _volume_--that is to say, the mere
+room which they take up; or as regards _mass_--that is to say, the
+amount of matter which they contain.
+
+Let us imagine two globes of equal volume; in other words, which take up
+an equal amount of space. One of these globes, however, may be composed
+of material much more tightly put together than in the other; or of
+greater _density_, as the term goes. That globe is said to be the
+greater of the two in mass. Were such a pair of globes to be weighed in
+scales, one globe in each pan, we should see at once, by its weighing
+down the other, which of the two was composed of the more tightly packed
+materials; and we should, in astronomical parlance, say of this one that
+it had the greater mass.
+
+Volume being merely another word for size, the order of the members of
+the solar system, with regard to their volumes, will be as follows,
+beginning with the greatest:--the Sun, Jupiter, Saturn, Uranus,
+Neptune, the Earth, Venus, Mars, and Mercury.
+
+With regard to mass the same order strangely enough holds good. The
+actual densities of the bodies in question are, however, very different.
+The densest or closest packed body of all is the Earth, which is about
+five and a half times as dense as if it were composed entirely of water.
+Venus follows next, then Mars, and then Mercury. The remaining bodies,
+on the other hand, are relatively loose in structure. Saturn is the
+least dense of all, less so than water. The density of the Sun is a
+little greater than that of water.
+
+This method of estimating is, however, subject to a qualification. It
+must be remembered that in speaking of the Sun, for instance, as being
+only a little denser than water, we are merely treating the question
+from the point of view of an average. Certain parts of it in fact will
+be ever so much denser than water: those are the parts in the centre.
+Other portions, for instance, the outside portions, will be very much
+less dense. It will easily be understood that in all such bodies the
+densest or most compressed portions are to be found towards the centre;
+while the portions towards the exterior being less pressed upon, will be
+less dense.
+
+We now reach a very important point, the question of Gravitation.
+_Gravitation_, or _gravity_, as it is often called, is the attractive
+force which, for instance, causes objects to fall to the earth. Now it
+seems rather strange that one should say that it is owing to a certain
+force that things fall towards the earth. All things seem to us to fall
+so of their own accord, as if it were quite natural, or rather most
+unnatural if they did not. Why then require a "force" to make them fall?
+
+The story goes that the great Sir Isaac Newton was set a-thinking on
+this subject by seeing an apple fall from a tree to the earth. He then
+carried the train of thought further; and, by studying the movements of
+the moon, he reached the conclusion that a body even so far off as our
+satellite would be drawn towards the earth in the same manner. This
+being the case, one will naturally ask why the moon herself does not
+fall in upon the earth. The answer is indeed found to be that the moon
+is travelling round and round the earth at a certain rapid pace, and it
+is this very same rapid pace which keeps her from falling in upon us.
+Any one can test this simple fact for himself. If we tie a stone to the
+end of a string, and keep whirling it round and round fast enough, there
+will be a strong pull from the stone in an outward direction, and the
+string will remain tight all the time that the stone is being whirled.
+If, however, we gradually slacken the speed at which we are making the
+stone whirl, a moment will come at length when the string will become
+limp, and the stone will fall back towards our hand.
+
+It seems, therefore, that there are two causes which maintain the stone
+at a regular distance all the time it is being steadily whirled. One of
+these is the continual pull inward towards our hand by means of the
+string. The other is the continual pull away from us caused by the rate
+at which the stone is travelling. When the rate of whirling is so
+regulated that these pulls exactly balance each other, the stone travels
+comfortably round and round, and shows no tendency either to fall back
+upon our hand or to break the string and fly away into the air. It is
+indeed precisely similar with regard to the moon. The continual pull of
+the earth's gravitation takes the place of the string. If the moon were
+to go round and round slower than it does, it would tend to fall in
+towards the earth; if, on the other hand, it were to go faster, it would
+tend to rush away into space.
+
+The same kind of pull which the earth exerts upon the objects at its
+surface, or upon its satellite, the moon, exists through space so far as
+we know. Every particle of matter in the universe is found in fact to
+attract every other particle. The moon, for instance, attracts the earth
+also, but the controlling force is on the side of the much greater mass
+of the earth. This force of gravity or attraction of gravitation, as it
+is also called, is perfectly regular in its action. Its power depends
+first of all exactly upon the mass of the body which exerts it. The
+gravitational pull of the sun, for instance, reaches out to an enormous
+distance, controlling perhaps, in their courses, unseen planets circling
+far beyond the orbit of Neptune. Again, the strength with which the
+force of gravity acts depends upon distance in a regularly diminishing
+proportion. Thus, the nearer an object is to the earth, for instance,
+the stronger is the gravitational pull which it gets from it; the
+farther off it is, the weaker is this pull. If then the moon were to be
+brought nearer to the earth, the gravitational pull of the latter would
+become so much stronger that the moon's rate of motion would have also
+to increase in due proportion to prevent her from being drawn into the
+earth. Last of all, the point in a body from which the attraction of
+gravitation acts, is not necessarily the centre of the body, but rather
+what is known as its _centre of gravity_, that is to say, the balancing
+point of all the matter which the body contains.
+
+It should here be noted that the moon does not actually revolve around
+the centre of gravity of the earth. What really happens is that both
+orbs revolve around their _common_ centre of gravity, which is a point
+within the body of the earth, and situated about three thousand miles
+from its centre. In the same manner the planets and the sun revolve
+around the centre of gravity of the solar system, which is a point
+within the body of the sun.
+
+The neatly poised movements of the planets around the sun, and of the
+satellites around their respective planets, will therefore be readily
+understood to result from a nice balance between gravitation and speed
+of motion.
+
+The mass of the earth is ascertained to be about eighty times that of
+the moon. Our knowledge of the mass of a planet is learned from
+comparing the revolutions of its satellite or satellites around it, with
+those of the moon around the earth. We are thus enabled to deduce what
+the mass of such a planet would be compared to the earth's mass; that is
+to say, a study, for instance, of Jupiter's satellite system shows that
+Jupiter must have a mass nearly three hundred and eighteen times that of
+our earth. In the same manner we can argue out the mass of the sun from
+the movements of the planets and other bodies of the system around it.
+With regard, however, to Venus and Mercury, the problem is by no means
+such an easy one, as these bodies have no satellites. For information in
+this latter case we have to rely upon such uncertain evidence as, for
+instance, the slight disturbances caused in the motion of the earth by
+the attraction of these planets when they pass closest to us, or their
+observed effect upon the motions of such comets as may happen to pass
+near to them.
+
+Mass and weight, though often spoken of as one and the same thing, are
+by no means so. Mass, as we have seen, merely means the amount of matter
+which a body contains. The weight of a body, on the other hand, depends
+entirely upon the gravitational pull which it receives. The force of
+gravity at the surface of the earth is, for instance, about six times as
+great as that at the surface of the moon. All bodies, therefore, weigh
+about six times as much on the earth as they would upon the moon; or,
+rather, a body transferred to the moon's surface would weigh only about
+one-sixth of what it did on the terrestrial surface. It will therefore
+be seen that if a body of given _mass_ were to be placed upon planet
+after planet in turn, its _weight_ would regularly alter according to
+the force of gravity at each planet's surface.
+
+Gravitation is indeed one of the greatest mysteries of nature. What it
+is, the means by which it acts, or why such a force should exist at all,
+are questions to which so far we have not had even the merest hint of an
+answer. Its action across space appears to be instantaneous.
+
+The intensity of gravitation is said in mathematical parlance "to vary
+inversely with the square of the distance." This means that at _twice_
+the distance the pull will become only _one-quarter_ as strong, and not
+one-half as otherwise might be expected. At _four_ times the distance,
+therefore, it will be _one-sixteenth_ as strong. At the earth's surface
+a body is pulled by the earth's gravitation, or "falls," as we
+ordinarily term it, through 16 feet in one _second_ of time; whereas at
+the distance of the moon the attraction of the earth is so very much
+weakened that a body would take as long as one _minute_ to fall through
+the same space.
+
+Newton's investigations showed that if a body were to be placed _at
+rest_ in space entirely away from the attraction of any other body it
+would remain always in a motionless condition, because there would
+plainly be no reason why it should move in any one direction rather than
+in another. And, similarly, if a body were to be projected in a certain
+direction and at a certain speed, it would move always in the same
+direction and at the same speed so long as it did not come within the
+gravitational attraction of any other body.
+
+The possibility of an interaction between the celestial orbs had
+occurred to astronomers before the time of Newton; for instance, in the
+ninth century to the Arabian Musa-ben-Shakir, to Camillus Agrippa in
+1553, and to Kepler, who suspected its existence from observation of the
+tides. Horrox also, writing in 1635, spoke of the moon as moved by an
+_emanation_ from the earth. But no one prior to Newton attempted to
+examine the question from a mathematical standpoint.
+
+Notwithstanding the acknowledged truth and far-reaching scope of the law
+of gravitation--for we find its effects exemplified in every portion of
+the universe--there are yet some minor movements which it does not
+account for. For instance, there are small irregularities in the
+movement of Mercury which cannot be explained by the influence of
+possible intra-Mercurial planets, and similarly there are slight
+unaccountable deviations in the motions of our neighbour the Moon.
+
+
+
+
+CHAPTER V
+
+CELESTIAL DISTANCES
+
+
+Up to this we have merely taken a general view of the solar system--a
+bird's-eye view, so to speak, from space.
+
+In the course of our inquiry we noted in a rough way the _relative_
+distances at which the various planets move around the sun. But we have
+not yet stated what these distances _actually_ are, and it were
+therefore well now to turn our attention to this important matter.
+
+Each of us has a fair idea of what a mile is. It is a quarter of an
+hour's sharp walk, for instance; or yonder village or building, we know,
+lies such and such a number of miles away.
+
+The measurements which have already been given of the diameters of the
+various bodies of the solar system appear very great to us, who find
+that a walk of a few miles at a time taxes our strength; but they are a
+mere nothing when we consider the distances from the sun at which the
+various planets revolve in their orbits.
+
+The following table gives these distances in round numbers. As here
+stated they are what are called "mean" distances; for, as the orbits are
+oval, the planets vary in their distances from the sun, and we are
+therefore obliged to strike a kind of average for each case:--
+
+Mercury  about   36,000,000 miles.
+Venus      "     67,200,000  "
+Earth      "     92,900,000  "
+Mars       "    141,500,000  "
+Jupiter    "    483,300,000  "
+Saturn     "    886,000,000  "
+Uranus     "  1,781,900,000  "
+Neptune    "  2,791,600,000  "
+
+From the above it will be seen at a glance that we have entered upon a
+still greater scale of distance than in dealing with the diameters of
+the various bodies of the system. In that case the distances were
+limited to thousands of miles; in this, however, we have to deal with
+millions. A million being ten hundred thousand, it will be noticed that
+even the diameter of the huge sun is well under a million miles.
+
+How indeed are we to get a grasp of such distances, when those to which
+we are ordinarily accustomed--the few miles' walk, the little stretch of
+sea or land which we gaze upon around us--are so utterly minute in
+comparison? The fact is, that though men may think that they can picture
+in their minds such immense distances, they actually can not. In matters
+like these we unconsciously employ a kind of convention, and we estimate
+a thing as being two or three or more times the size of another. More
+than this we are unable to do. For instance, our ordinary experience of
+a mile enables us to judge, in a way, of a stretch of several miles,
+such as one can take in with a glance; but in our estimation of a
+thousand miles, or even of one hundred, we are driven back upon a mental
+trick, so to speak.
+
+In our attempts to realise such immense distances as those in the solar
+system we are obliged to have recourse to analogies; to comparisons with
+other and simpler facts, though this is at the best a mere self-cheating
+device. The analogy which seems most suited to our purpose here, and one
+which has often been employed by writers, is borrowed from the rate at
+which an express train travels.
+
+Let us imagine, for instance, that we possess an express train which is
+capable of running anywhere, never stops, never requires fuel, and
+always goes along at sixty miles an hour. Suppose we commence by
+employing it to gauge the size of our own planet, the earth. Let us send
+it on a trip around the equator, the span of which is about 24,000
+miles. At its sixty-miles-an-hour rate of going, this journey will take
+nearly 17 days. Next let us send it from the earth to the moon. This
+distance, 240,000 miles, being ten times as great as the last, will of
+course take ten times as long to cover, namely, 170 days; that is to
+say, nearly half a year. Again, let us send it still further afield, to
+the sun, for example. Here, however, it enters upon a journey which is
+not to be measured in thousands of miles, as the others were, but in
+millions. The distance from the earth to the sun, as we have seen in the
+foregoing table, is about 93 millions of miles. Our express train would
+take about 178 _years_ to traverse this.
+
+Having arrived at the sun, let us suppose that our train makes a tour
+right round it. This will take more than five years.
+
+Supposing, finally, that our train were started from the sun, and made
+to run straight out to the known boundaries of the solar system, that is
+to say, as far as the orbit of Neptune, it would take over 5000 years to
+traverse the distance.
+
+That sixty miles an hour is a very great speed any one, I think, will
+admit who has stood upon the platform of a country station while one of
+the great mail trains has dashed past. But are not the immensities of
+space appalling to contemplate, when one realises that a body moving
+incessantly at such a rate would take so long as 10,000 years to
+traverse merely the breadth of our solar system? Ten thousand years!
+Just try to conceive it. Why, it is only a little more than half that
+time since the Pyramids were built, and they mark for us the Dawn of
+History. And since then half-a-dozen mighty empires have come and gone!
+
+Having thus concluded our general survey of the appearance and
+dimensions of the solar system, let us next inquire into its position
+and size in relation to what we call the Universe.
+
+A mere glance at the night sky, when it is free from clouds, shows us
+that in every direction there are stars; and this holds good, no matter
+what portion of the globe we visit. The same is really true of the sky
+by day, though in that case we cannot actually see the stars, for their
+light is quite overpowered by the dazzling light of the sun.
+
+We thus reach the conclusion that our earth, that our solar system in
+fact, lies plunged within the midst of a great tangle of stars. What
+position, by the way, do we occupy in this mighty maze? Are we at the
+centre, or anywhere near the centre, or where?
+
+It has been indeed amply proved by astronomical research that the stars
+are bodies giving off a light of their own, just as our sun does; that
+they are in fact suns, and that our sun is merely one, perhaps indeed a
+very unimportant member, of this great universe of stars. Each of these
+stars, or suns, besides, may be the centre of a system similar to what
+we call our solar system, comprising planets and satellites, comets and
+meteors;--or perchance indeed some further variety of attendant bodies
+of which we have no example in our tiny corner of space. But as to
+whether one is right in a conjecture of this kind, there is up to the
+present no proof whatever. No telescope has yet shown a planet in
+attendance upon one of these distant suns; for such bodies, even if they
+do exist, are entirely out of the range of our mightiest instruments. On
+what then can we ground such an assumption? Merely upon analogy; upon
+the common-sense deduction that as the stars have characteristics
+similar to our particular star, the sun, it would seem unlikely that
+ours should be the only such body in the whole of space which is
+attended by a planetary system.
+
+"The Stars," using that expression in its most general sense, do not lie
+at one fixed distance from us, set here and there upon a background of
+sky. There is in fact no background at all. The brilliant orbs are all
+around us in space, at different distances from us and from each other;
+and we can gaze between them out into the blackness of the void which,
+perhaps, continues to extend unceasingly long after the very outposts of
+the stellar universe has been left behind. Shall we then start our
+imaginary express train once more, and send it out towards the nearest
+of the stars? This would, however, be a useless experiment. Our
+express-train method of gauging space would fail miserably in the
+attempt to bring home to us the mighty gulf by which we are now faced.
+Let us therefore halt for a moment and look back upon the orders of
+distance with which we have been dealing. First of all we dealt with
+thousands of miles. Next we saw how they shrank into insignificance when
+we embarked upon millions. We found, indeed, that our sixty-mile-an-hour
+train, rushing along without ceasing, would consume nearly the whole of
+historical time in a journey from the sun to Neptune.
+
+In the spaces beyond the solar system we are faced, however, by a new
+order of distance. From sun to planets is measured in millions of miles,
+but from sun to sun is measured in billions. But does the mere stating
+of this fact convey anything? I fear not. For the word "billion" runs as
+glibly off the tongue as "million," and both are so wholly unrealisable
+by us that the actual difference between them might easily pass
+unnoticed.
+
+Let us, however, make a careful comparison. What is a million? It is a
+thousand thousands. But what is a billion? It is a million millions.
+Consider for a moment! A million of millions. That means a million, each
+unit of which is again a million. In fact every separate "1" in this
+million is itself a million. Here is a way of trying to realise this
+gigantic number. A million seconds make only eleven and a half days and
+nights. But a billion seconds will make actually more than thirty
+thousand years!
+
+Having accepted this, let us try and probe with our express train even a
+little of the new gulf which now lies before us. At our old rate of
+going it took almost two years to cover a million miles. To cover a
+billion miles--that is to say, a million times this distance--would thus
+take of course nearly two million years. Alpha Centauri, the nearest
+star to our earth, is some twenty-five billions of miles away. Our
+express train would thus take about fifty millions of years to reach it!
+
+This shows how useless our illustration, appropriate though it seemed
+for interplanetary space, becomes when applied to the interstellar
+spaces. It merely gives us millions in return for billions; and so the
+mind, driven in upon itself, whirls round and round like a squirrel in
+its revolving cage. There is, however, a useful illustration still left
+us, and it is the one which astronomers usually employ in dealing with
+the distances of the stars. The illustration in question is taken from
+the velocity of light.
+
+Light travels at the tremendous speed of about 186,000 miles a second.
+It therefore takes only about a second and a quarter to come to us from
+the moon. It traverses the 93,000,000 of miles which separate us from
+the sun in about eight minutes. It travels from the sun out to Neptune
+in about four hours, which means that it would cross the solar system
+from end to end in eight. To pass, however, across the distance which
+separates us from Alpha Centauri it would take so long as about four
+and a quarter years!
+
+Astronomers, therefore, agree in estimating the distances of the stars
+from the point of view of the time which light would take to pass from
+them to our earth. They speak of that distance which light takes a year
+to traverse as a "light year." According to this notation, Alpha
+Centauri is spoken of as being about four and a quarter light years
+distant from us.
+
+Now as the rays of light coming from Alpha Centauri to us are chasing
+one another incessantly across the gulf of space, and as each ray left
+that star some four years before it reaches us, our view of the star
+itself must therefore be always some four years old. Were then this star
+to be suddenly removed from the universe at any moment, we should
+continue to see it still in its place in the sky for some four years
+more, after which it would suddenly disappear. The rays which had
+already started upon their journey towards our earth must indeed
+continue travelling, and reaching us in their turn until the last one
+had arrived; after which no more would come.
+
+We have drawn attention to Alpha Centauri as the nearest of the stars.
+The majority of the others indeed are ever so much farther. We can only
+hazard a guess at the time it takes for the rays from many of them to
+reach our globe. Suppose, for instance, we see a sudden change in the
+light of any of these remote stars, we are inclined to ask ourselves
+when that change did actually occur. Was it in the days of Queen
+Elizabeth, or at the time of the Norman Conquest; or was it when Rome
+was at the height of her glory, or perhaps ages before that when the
+Pyramids of Egypt were being built? Even the last of these suppositions
+cannot be treated lightly. We have indeed no real knowledge of the
+distance from us of those stars which our giant telescopes have brought
+into view out of the depths of the celestial spaces.
+
+
+
+
+CHAPTER VI
+
+CELESTIAL MEASUREMENT
+
+
+Had the telescope never been invented our knowledge of astronomy would
+be trifling indeed.
+
+Prior to the year 1610, when Galileo first turned the new instrument
+upon the sky, all that men knew of the starry realms was gathered from
+observation with their own eyes unaided by any artificial means. In such
+researches they had been very much at a disadvantage. The sun and moon,
+in their opinion, were no doubt the largest bodies in the heavens, for
+the mere reason that they looked so! The mighty solar disturbances,
+which are now such common-places to us, were then quite undreamed of.
+The moon displayed a patchy surface, and that was all; her craters and
+ring-mountains were surprises as yet in store for men. Nothing of course
+was known about the surfaces of the planets. These objects had indeed no
+particular characteristics to distinguish them from the great host of
+the stars, except that they continually changed their positions in the
+sky while the rest did not. The stars themselves were considered as
+fixed inalterably upon the vault of heaven. The sun, moon, and planets
+apparently moved about in the intermediate space, supported in their
+courses by strange and fanciful devices. The idea of satellites was as
+yet unknown. Comets were regarded as celestial portents, and meteors as
+small conflagrations taking place in the upper air.
+
+In the entire absence of any knowledge with regard to the actual sizes
+and distances of the various celestial bodies, men naturally considered
+them as small; and, concluding that they were comparatively near,
+assigned to them in consequence a permanent connection with terrestrial
+affairs. Thus arose the quaint and erroneous beliefs of astrology,
+according to which the events which took place upon our earth were
+considered to depend upon the various positions in which the planets,
+for instance, found themselves from time to time.
+
+It must, however, be acknowledged that the study of astrology,
+fallacious though its conclusions were, indirectly performed a great
+service to astronomy by reason of the accurate observations and diligent
+study of the stars which it entailed.
+
+We will now inquire into the means by which the distances and sizes of
+the celestial orbs have been ascertained, and see how it was that the
+ancients were so entirely in the dark in this matter.
+
+There are two distinct methods of finding out the distance at which any
+object happens to be situated from us.
+
+One method is by actual measurement.
+
+The other is by moving oneself a little to the right or left, and
+observing whether the distant object appears in any degree altered in
+position by our own change of place.
+
+One of the best illustrations of this relative change of position which
+objects undergo as a result of our own change of place, is to observe
+the landscape from the window of a moving railway carriage. As we are
+borne rapidly along we notice that the telegraph posts which are set
+close to the line appear to fly past us in the contrary direction; the
+trees, houses, and other things beyond go by too, but not so fast;
+objects a good way off displace slowly; while some spire, or tall
+landmark, in the far distance appears to remain unmoved during a
+comparatively long time.
+
+Actual change of position on our own part is found indeed to be
+invariably accompanied by an apparent displacement of the objects about
+us, such apparent displacement as a result of our own change of position
+being known as "parallax." The dependence between the two is so
+mathematically exact, that if we know the amount of our own change of
+place, and if we observe the amount of the consequent displacement of
+any object, we are enabled to calculate its precise distance from us.
+Thus it comes to pass that distances can be measured without the
+necessity of moving over them; and the breadth of a river, for instance,
+or the distance from us of a ship at sea, can be found merely by such
+means.
+
+It is by the application of this principle to the wider field of the sky
+that we are able to ascertain the distance of celestial bodies. We have
+noted that it requires a goodly change of place on our own part to shift
+the position in which some object in the far distance is seen by us. To
+two persons separated by, say, a few hundred yards, a ship upon the
+horizon will appear pretty much in the same direction. They would
+require, in fact, to be much farther apart in order to displace it
+sufficiently for the purpose of estimating their distance from it. It
+is the same with regard to the moon. Two observers, standing upon our
+earth, will require to be some thousands of miles apart in order to see
+the position of our satellite sufficiently altered with regard to the
+starry background, to give the necessary data upon which to ground their
+calculations.
+
+The change of position thus offered by one side of the earth's surface
+at a time is, however, not sufficient to displace any but the nearest
+celestial bodies. When we have occasion to go farther afield we have to
+seek a greater change of place. This we can get as a consequence of the
+earth's movement around the sun. Observations, taken several days apart,
+will show the effect of the earth's change of place during the interval
+upon the positions of the other bodies of our system. But when we desire
+to sound the depths of space beyond, and to reach out to measure the
+distance of the nearest star, we find ourselves at once thrown upon the
+greatest change of place which we can possibly hope for; and this we get
+during the long journey of many millions of miles which our earth
+performs around the sun during the course of each year. But even this
+last change of place, great as it seems in comparison with terrestrial
+measurements, is insufficient to show anything more than the tiniest
+displacements in a paltry forty-three out of the entire host of the
+stars.
+
+We can thus realise at what a disadvantage the ancients were. The
+measuring instruments at their command were utterly inadequate to detect
+such small displacements. It was reserved for the telescope to reveal
+them; and even then it required the great telescopes of recent times to
+show the slight changes in the position of the nearer stars, which were
+caused by the earth's being at one time at one end of its orbit, and
+some six months later at the other end--stations separated from each
+other by a gulf of about one hundred and eighty-six millions of miles.
+
+The actual distances of certain celestial bodies being thus
+ascertainable, it becomes a matter of no great difficulty to determine
+the actual sizes of the measurable ones. It is a matter of everyday
+experience that the size which any object appears to have, depends
+exactly upon the distance it is from us. The farther off it is the
+smaller it looks; the nearer it is the bigger. If, then, an object which
+lies at a known distance from us looks such and such a size, we can of
+course ascertain its real dimensions. Take the moon, for instance. As we
+have already shown, we are able to ascertain its distance. We observe
+also that it looks a certain size. It is therefore only a matter of
+calculation to find what its actual dimensions should be, in order that
+it may look that size at that distance away. Similarly we can ascertain
+the real dimensions of the sun. The planets, appearing to us as points
+of light, seem at first to offer a difficulty; but, by means of the
+telescope, we can bring them, as it were, so much nearer to us, that
+their broad expanses may be seen. We fail, however, signally with regard
+to the stars; for they are so very distant, and therefore such tiny
+points of light, that our mightiest telescopes cannot magnify them
+sufficiently to show any breadth of surface.
+
+Instead of saying that an object looks a certain breadth across, such
+as a yard or a foot, a statement which would really mean nothing,
+astronomers speak of it as measuring a certain angle. Such angles are
+estimated in what are called "degrees of arc"; each degree being divided
+into sixty minutes, and each minute again into sixty seconds. Popularly
+considered the moon and sun _look_ about the same size, or, as an
+astronomer would put it, they measure about the same angle. This is an
+angle, roughly, of thirty-two minutes of arc; that is to say, slightly
+more than half a degree. The broad expanse of surface which a celestial
+body shows to us, whether to the naked eye, as in the case of the sun
+and moon, or in the telescope, as in the case of other members of our
+system, is technically known as its "disc."
+
+
+
+
+CHAPTER VII
+
+ECLIPSES AND KINDRED PHENOMENA
+
+
+Since some members of the solar system are nearer to us than others, and
+all are again much nearer than any of the stars, it must often happen
+that one celestial body will pass between us and another, and thus
+intercept its light for a while. The moon, being the nearest object in
+the universe, will, of course, during its motion across the sky,
+temporarily blot out every one of the others which happen to lie in its
+path. When it passes in this manner across the face of the sun, it is
+said to _eclipse_ it. When it thus hides a planet or star, it is said to
+_occult_ it. The reason why a separate term is used for what is merely a
+case of obscuring light in exactly the same way, will be plain when one
+considers that the disc of the sun is almost of the same apparent size
+as that of the moon, and so the complete hiding of the sun can last but
+a few minutes at the most; whereas a planet or a star looks so very
+small in comparison, that it is always _entirely swallowed up for some
+time_ when it passes behind the body of our satellite.
+
+The sun, of course, occults planets and stars in exactly the same manner
+as the moon does, but we cannot see these occultations on account of the
+blaze of sunlight.
+
+By reason of the small size which the planets look when viewed with the
+naked eye, we are not able to note them in the act of passing over stars
+and so blotting them out; but such occurrences may be seen in the
+telescope, for the planetary bodies then display broad discs.
+
+There is yet another occurrence of the same class which is known as a
+_transit_. This takes place when an apparently small body passes across
+the face of an apparently large one, the phenomenon being in fact the
+exact reverse of an occultation. As there is no appreciable body nearer
+to us than the moon, we can never see anything in transit across her
+disc. But since the planets Venus and Mercury are both nearer to us than
+the sun, they will occasionally be seen to pass across his face, and
+thus we get the well-known phenomena called Transits of Venus and
+Transits of Mercury.
+
+As the satellites of Jupiter are continually revolving around him, they
+will often pass behind or across his disc. Such occultations and
+transits of satellites can be well observed in the telescope.
+
+There is, however, a way in which the light of a celestial body may be
+obscured without the necessity of its being hidden from us by one
+nearer. It will no doubt be granted that any opaque object casts a
+shadow when a strong light falls directly upon it. Thus the earth, under
+the powerful light which is directed upon it from the sun, casts an
+extensive shadow, though we are not aware of the existence of this
+shadow until it falls upon something. The shadow which the earth casts
+is indeed not noticeable to us until some celestial body passes into it.
+As the sun is very large, and the earth in comparison very small, the
+shadow thrown by the earth is comparatively short, and reaches out in
+space for only about a million miles. There is no visible object except
+the moon, which circulates within that distance from our globe, and
+therefore she is the only body which can pass into this shadow. Whenever
+such a thing happens, her surface at once becomes dark, for the reason
+that she never emits any light of her own, but merely reflects that of
+the sun. As the moon is continually revolving around the earth, one
+would be inclined to imagine that once in every month, namely at what is
+called _full moon_, when she is on the other side of the earth with
+respect to the sun, she ought to pass through the shadow in question.
+But this does not occur every time, because the moon's orbit is not
+quite _upon the same plane_ with the earth's. It thus happens that time
+after time the moon passes clear of the earth's shadow, sometimes above
+it, and sometimes below it. It is indeed only at intervals of about six
+months that the moon can be thus obscured. This darkening of her light
+is known as an _eclipse of the moon_. It seems a great pity that custom
+should oblige us to employ the one term "eclipse" for this and also for
+the quite different occurrence, an eclipse of the sun; in which the
+sun's face is hidden as a consequence of the moon's body coming directly
+_between_ it and our eyes.
+
+The popular mind seems always to have found it more difficult to grasp
+the causes of an eclipse of the moon than an eclipse of the sun. As Mr.
+J.E. Gore[4] puts it: "The darkening of the sun's light by the
+interposition of the moon's body seems more obvious than the passing of
+the moon through the earth's shadow."
+
+Eclipses of the moon furnish striking spectacles, but really add little
+to our knowledge. They exhibit, however, one of the most remarkable
+evidences of the globular shape of our earth; for the outline of its
+shadow when seen creeping over the moon's surface is always circular.
+
+[Illustration: FIG. 3.--Total and Partial Eclipses of the Moon. The Moon
+is here shown in two positions; i.e. _entirely_ plunged in the earth's
+shadow and therefore totally eclipsed, and only _partly_ plunged in it
+or partially eclipsed.]
+
+_Eclipses of the Moon_, or Lunar Eclipses, as they are also called, are
+of two kinds--_Total_, and _Partial_. In a total lunar eclipse the moon
+passes entirely into the earth's shadow, and the whole of her surface is
+consequently darkened. This darkening lasts for about two hours. In a
+partial lunar eclipse, a portion only of the moon passes through the
+shadow, and so only _part_ of her surface is darkened (see Fig. 3). A
+very striking phenomenon during a total eclipse of the moon, is that the
+darkening of the lunar surface is usually by no means so intense as one
+would expect, when one considers that the sunlight at that time should
+be _wholly_ cut off from it. The occasions indeed upon which the moon
+has completely disappeared from view during the progress of a total
+lunar eclipse are very rare. On the majority of these occasions she has
+appeared of a coppery-red colour, while sometimes she has assumed an
+ashen hue. The explanations of these variations of colour is to be found
+in the then state of the atmosphere which surrounds our earth. When
+those portions of our earth's atmosphere through which the sun's rays
+have to filter on their way towards the moon are free from watery
+vapour, the lunar surface will be tinged with a reddish light, such as
+we ordinarily experience at sunset when our air is dry. The ashen colour
+is the result of our atmosphere being laden with watery vapour, and is
+similar to what we see at sunset when rain is about. Lastly, when the
+air around the earth is thickly charged with cloud, no light at all can
+pass; and on such occasions the moon disappears altogether for the time
+being from the night sky.
+
+_Eclipses of the Sun_, otherwise known as Solar Eclipses, are divided
+into _Total_, _Partial_, and _Annular_. A total eclipse of the sun takes
+place when the moon comes between the sun and the earth, in such a
+manner that it cuts off the sunlight _entirely_ for the time being from
+a _portion_ of the earth's surface. A person situated in the region in
+question will, therefore, at that moment find the sun temporarily
+blotted out from his view by the body of the moon. Since the moon is a
+very much smaller body than the sun, and also very much the nearer to us
+of the two, it will readily be understood that the portion of the earth
+from which the sun is seen thus totally eclipsed will be of small
+extent. In places not very distant from this region, the moon will
+appear so much shifted in the sky that the sun will be seen only
+partially eclipsed. The moon being in constant movement round the earth,
+the portion of the earth's surface from which an eclipse is seen as
+total will be always a comparatively narrow band lying roughly from west
+to east. This band, known as the _track of totality_, can, at the
+utmost, never be more than about 165 miles in width, and as a rule is
+very much less. For about 2000 miles on either side of it the sun is
+seen partially eclipsed. Outside these limits no eclipse of any kind is
+visible, as from such regions the moon is not seen to come in the way of
+the sun (see Fig. 4 (i.), p. 67).
+
+It may occur to the reader that eclipses can also take place in the
+course of which the positions, where the eclipse would ordinarily be
+seen as total, will lie outside the surface of the earth. Such an
+eclipse is thus not dignified with the name of total eclipse, but is
+called a partial eclipse, because from the earth's surface the sun is
+only seen _partly eclipsed at the utmost_ (see Fig. 4 (ii.), p. 67).
+
+[Illustration: (i.) Total Eclipse of the Sun.]
+
+[Illustration: (ii.) Partial Eclipse of the Sun.
+
+FIG. 4.--Total and Partial Eclipses of the Sun. From the position A the
+Sun cannot be seen, as it is entirely blotted out by the Moon. From B it
+is seen partially blotted out, because the Moon is to a certain degree
+in the way. From C no eclipse is seen, because the Moon does not come in
+the way.
+
+It is to be noted that in a Partial Eclipse of the Sun, the position A
+lies _outside_ the surface of the Earth.]
+
+An _Annular eclipse_ is an eclipse which just fails to become total for
+yet another reason. We have pointed out that the orbits of the various
+members of the solar system are not circular, but oval. Such oval
+figures, it will be remembered, are technically known as ellipses. In an
+elliptic orbit the controlling body is situated not in the middle of the
+figure, but rather towards one of the ends; the actual point which it
+occupies being known as the _focus_. The sun being at the focus of the
+earth's orbit, it follows that the earth is, at times, a little nearer
+to him than at others. The sun will therefore appear to us to vary a
+little in size, looking sometimes slightly larger than at other times.
+It is so, too, with the moon, at the focus of whose orbit the earth is
+situated. She therefore also appears to us at times to vary slightly in
+size. The result is that when the sun is eclipsed by the moon, and the
+moon at the time appears the larger of the two, she is able to blot out
+the sun completely, and so we can get a total eclipse. But when, on the
+other hand, the sun appears the larger, the eclipse will not be quite
+total, for a portion of the sun's disc will be seen protruding all
+around the moon like a ring of light. This is what is known as an
+annular eclipse, from the Latin word _annulus_, which means a ring. The
+term is consecrated by long usage, but it seems an unfortunate one on
+account of its similarity to the word "annual." The Germans speak of
+this kind of eclipse as "ring-formed," which is certainly much more to
+the point.
+
+There can never be a year without an eclipse of the sun. Indeed there
+must be always two such eclipses _at least_ during that period, though
+there need be no eclipse of the moon at all. On the other hand, the
+greatest number of eclipses which can ever take place during a year are
+seven; that is to say, either five solar eclipses and two lunar, or four
+solar and three lunar. This general statement refers merely to eclipses
+in their broadest significance, and informs us in no way whether they
+will be total or partial.
+
+Of all the phenomena which arise from the hiding of any celestial body
+by one nearer coming in the way, a total eclipse of the sun is far the
+most important. It is, indeed, interesting to consider how much poorer
+modern astronomy would be but for the extraordinary coincidence which
+makes a total solar eclipse just possible. The sun is about 400 times
+farther off from us than the moon, and enormously greater than her in
+bulk. Yet the two are relatively so distanced from us as to look about
+the same size. The result of this is that the moon, as has been seen,
+can often blot out the sun entirely from our view for a short time. When
+this takes place the great blaze of sunlight which ordinarily dazzles
+our eyes is completely cut off, and we are thus enabled, unimpeded, to
+note what is going on in the immediate vicinity of the sun itself.
+
+In a total solar eclipse, the time which elapses from the moment when
+the moon's disc first begins to impinge upon that of the sun at his
+western edge until the eclipse becomes total, lasts about an hour.
+During all this time the black lunar disc may be watched making its way
+steadily across the solar face. Notwithstanding the gradual obscuration
+of the sun, one does not notice much diminution of light until about
+three-quarters of his disc are covered. Then a wan, unearthly appearance
+begins to pervade all things, the temperature falls noticeably, and
+nature seems to halt in expectation of the coming of something unusual.
+The decreasing portion of sun becomes more and more narrow, until at
+length it is reduced to a crescent-shaped strip of exceeding fineness.
+Strange, ill-defined, flickering shadows (known as "Shadow Bands") may
+at this moment be seen chasing each other across any white expanse such
+as a wall, a building, or a sheet stretched upon the ground. The western
+side of the sky has now assumed an appearance dark and lowering, as if a
+rainstorm of great violence were approaching. This is caused by the
+mighty mass of the lunar shadow sweeping rapidly along. It flies onward
+at the terrific velocity of about half a mile a second.
+
+If the gradually diminishing crescent of sun be now watched through a
+telescope, the observer will notice that it does not eventually vanish
+all at once, as he might have expected. Rather, it breaks up first of
+all along its length into a series of brilliant dots, known as "Baily's
+Beads." The reason of this phenomenon is perhaps not entirely agreed
+upon, but the majority of astronomers incline to the opinion that the
+so-called "beads" are merely the last remnants of sunlight peeping
+between those lunar mountain peaks which happen at the moment to fringe
+the advancing edge of the moon. The beads are no sooner formed than they
+rapidly disappear one after the other, after which no portion of the
+solar surface is left to view, and the eclipse is now total (see Fig.
+5).
+
+[Illustration: _In a total Eclipse_ _In an annular Eclipse_
+
+FIG. 5.--"Baily's Beads."]
+
+But with the disappearance of the sun there springs into view a new and
+strange appearance, ordinarily unseen because of the blaze of sunlight.
+It is a kind of aureole, or halo, pearly white in colour, which is seen
+to surround the black disc of the moon. This white radiance is none
+other than the celebrated phenomenon widely known as the _Solar Corona_.
+It was once upon a time thought to belong to the moon, and to be perhaps
+a lunar atmosphere illuminated by the sunlight shining through it from
+behind. But the suddenness with which the moon always blots out stars
+when occulting them, has amply proved that she possesses no atmosphere
+worth speaking about. It is now, however, satisfactorily determined that
+the corona belongs to the sun, for during the short time that it remains
+in view the black body of the moon can be seen creeping across it.
+
+All the time that the _total phase_ (as it is called) lasts, the corona
+glows with its pale unearthly light, shedding upon the earth's surface
+an illumination somewhat akin to full moonlight. Usually the planet
+Venus and a few stars shine out the while in the darkened heaven.
+Meantime around the observer animal and plant life behave as at
+nightfall. Birds go to roost, bats fly out, worms come to the surface of
+the ground, flowers close up. In the Norwegian eclipse of 1896 fish were
+seen rising to the surface of the water. When the total phase at length
+is over, and the moon in her progress across the sky has allowed the
+brilliant disc of the sun to spring into view once more at the other
+side, the corona disappears.
+
+There is another famous accompaniment of the sun which partly reveals
+itself during total solar eclipses. This is a layer of red flame which
+closely envelops the body of the sun and lies between it and the corona.
+This layer is known by the name of the _Chromosphere_. Just as at
+ordinary times we cannot see the corona on account of the blaze of
+sunlight, so are we likewise unable to see the chromosphere because of
+the dazzling white light which shines through from the body of the sun
+underneath and completely overpowers it. When, however, during a solar
+eclipse, the lunar disc has entirely hidden the brilliant face of the
+sun, we are still able for a few moments to see an edgewise portion of
+the chromosphere in the form of a narrow red strip, fringing the
+advancing border of the moon. Later on, just before the moon begins to
+uncover the face of the sun from the other side, we may again get a view
+of a strip of chromosphere.
+
+The outer surface of the chromosphere is not by any means even. It is
+rough and billowy, like the surface of a storm-tossed sea. Portions of
+it, indeed, rise at times to such heights that they may be seen standing
+out like blood-red points around the black disc of the moon, and remain
+thus during a good part of the total phase. These projections are known
+as the _Solar Prominences_. In the same way as the corona, the
+chromosphere and prominences were for a time supposed to belong to the
+moon. This, however, was soon found not to be the case, for the lunar
+disc was noticed to creep slowly across them also.
+
+The total phase, or "totality," as it is also called, lasts for
+different lengths of time in different eclipses. It is usually of about
+two or three minutes' duration, and at the utmost it can never last
+longer than about eight minutes.
+
+When totality is over and the corona has faded away, the moon's disc
+creeps little by little from the face of the sun, light and heat returns
+once more to the earth, and nature recovers gradually from the gloom in
+which she has been plunged. About an hour after totality, the last
+remnant of moon draws away from the solar disc, and the eclipse is
+entirely at an end.
+
+The corona, the chromosphere, and the prominences are the most important
+of these accompaniments of the sun which a total eclipse reveals to us.
+Our further consideration of them must, however, be reserved for a
+subsequent chapter, in which the sun will be treated of at length.
+
+Every one who has had the good fortune to see a total eclipse of the sun
+will, the writer feels sure, agree with the verdict of Sir Norman
+Lockyer that it is at once one of the "grandest and most awe-inspiring
+sights" which man can witness. Needless to say, such an occurrence used
+to cause great consternation in less civilised ages; and that it has not
+in modern times quite parted with its terrors for some persons, is shown
+by the fact that in Iowa, in the United States, a woman died from fright
+during the eclipse of 1869.
+
+To the serious observer of a total solar eclipse every instant is
+extremely precious. Many distinct observations have to be crowded into a
+time all too limited, and this in an eclipse-party necessitates constant
+rehearsals in order that not a moment may be wasted when the longed-for
+totality arrives. Such preparation is very necessary; for the rarity and
+uncommon nature of a total eclipse of the sun, coupled with its
+exceeding short duration, tends to flurry the mind, and to render it
+slow to seize upon salient points of detail. And, even after every
+precaution has been taken, weather possibilities remain to be reckoned
+with, so that success is rather a lottery.
+
+Above all things, therefore, a total solar eclipse is an occurrence for
+the proper utilisation of which personal experience is of absolute
+necessity. It was manifestly out of the question that such experience
+could be gained by any individual in early times, as the imperfection
+of astronomical theory and geographical knowledge rendered the
+predicting of the exact position of the track of totality well-nigh
+impossible. Thus chance alone would have enabled one in those days to
+witness a total phase, and the probabilities, of course, were much
+against a second such experience in the span of a life-time. And even in
+more modern times, when the celestial motions had come to be better
+understood, the difficulties of foreign travel still were in the way;
+for it is, indeed, a notable fact that during many years following the
+invention of the telescope the tracks were placed for the most part in
+far-off regions of the earth, and Europe was visited by singularly few
+total solar eclipses. Thus it came to pass that the building up of a
+body of organised knowledge upon this subject was greatly delayed.
+
+Nothing perhaps better shows the soundness of modern astronomical theory
+than the almost exact agreement of the time predicted for an eclipse
+with its actual occurrence. Similarly, by calculating backwards,
+astronomers have discovered the times and seasons at which many ancient
+eclipses took place, and valuable opportunities have thus arisen for
+checking certain disputed dates in history.
+
+It should not be omitted here that the ancients were actually able, _in
+a rough way_, to predict eclipses. The Chaldean astronomers had indeed
+noticed very early a curious circumstance, _i.e._ that eclipses tend to
+repeat themselves after a lapse of slightly more than eighteen years.
+
+In this connection it must, however, be pointed out, in the first
+instance, that the eclipses which occur in any particular year are in
+no way associated with those which occurred in the previous year. In
+other words, the mere fact that an eclipse takes place upon a certain
+day this year will not bring about a repetition of it at the same time
+next year. However, the nicely balanced behaviour of the solar system,
+an equilibrium resulting from aeons of orbital ebb and flow, naturally
+tends to make the members which compose that family repeat their ancient
+combinations again and again; so that after definite lapses of time the
+same order of things will _almost exactly_ recur. Thus, as a consequence
+of their beautifully poised motions, the sun, the moon, and the earth
+tend, after a period of 18 years and 10-1/3 days,[5] to occupy very
+nearly the same positions with regard to each other. The result of this
+is that, during each recurring period, the eclipses comprised within it
+will be repeated in their order.
+
+To give examples:--
+
+The total solar eclipse of August 30, 1905, was a repetition of that of
+August 19, 1887.
+
+The partial solar eclipse of February 23, 1906, corresponded to that
+which took place on February 11, 1888.
+
+The annular eclipse of July 10, 1907, was a recurrence of that of June
+28, 1889.
+
+In this way we can go on until the eighteen year cycle has run out, and
+we come upon a total solar eclipse predicted for September 10, 1923,
+which will repeat the above-mentioned ones of 1905 and 1887; and so on
+too with the others.
+
+From mere observation alone, extending no doubt over many ages, those
+time-honoured watchers of the sky, the early Chaldeans, had arrived at
+this remarkable generalisation; and they used it for the rough
+prediction of eclipses. To the period of recurrence they give the name
+of "Saros."
+
+And here we find ourselves led into one of the most interesting and
+fascinating by-paths in astronomy, to which writers, as a rule, pay all
+too little heed.
+
+In order not to complicate matters unduly, the recurrence of solar
+eclipses alone will first be dealt with. This limitation will, however,
+not affect the arguments in the slightest, and it will be all the more
+easy in consequence to show their application to the case of eclipses of
+the moon.
+
+The reader will perhaps have noticed that, with regard to the repetition
+of an eclipse, it has been stated that the conditions which bring it on
+at each recurrence are reproduced _almost exactly_. Here, then, lies the
+_crux_ of the situation. For it is quite evident that were the
+conditions _exactly_ reproduced, the recurrences of each eclipse would
+go on for an indefinite period. For instance, if the lapse of a saros
+period found the sun, moon, and earth again in the precise relative
+situations which they had previously occupied, the recurrences of a
+solar eclipse would tend to duplicate its forerunner with regard to the
+position of the shadow upon the terrestrial surface. But the conditions
+_not_ being exactly reproduced, the shadow-track does not pass across
+the earth in quite the same regions. It is shifted a little, so to
+speak; and each time the eclipse comes round it is found to be shifted a
+little farther. Every solar eclipse has therefore a definite "life" of
+its own upon the earth, lasting about 1150 years, or 64 saros returns,
+and working its way little by little across our globe from north to
+south, or from south to north, as the case may be. Let us take an
+imaginary example. A _partial_ eclipse occurs, say, somewhere near the
+North Pole, the edge of the "partial" shadow just grazing the earth, and
+the "track of totality" being as yet cast into space. Here we have the
+beginning of a series. At each saros recurrence the partial shadow
+encroaches upon a greater extent of earth-surface. At length, in its
+turn, the track of totality begins to impinge upon the earth. This track
+streaks across our globe at each return of the eclipse, repeating itself
+every time in a slightly more southerly latitude. South and south it
+moves, passing in turn the Tropic of Cancer, the Equator, the Tropic of
+Capricorn, until it reaches the South Pole; after which it touches the
+earth no longer, but is cast into space. The rear portion of the partial
+shadow, in its turn, grows less and less in extent; and it too in time
+finally passes off. Our imaginary eclipse series is now no more--its
+"life" has ended.
+
+We have taken, as an example, an eclipse series moving from north to
+south. We might have taken one moving from south to north, for they
+progress in either direction.
+
+From the description just given the reader might suppose that, if the
+tracks of totality of an eclipse series were plotted upon a chart of the
+world, they would lie one beneath another like a set of steps. This is,
+however, _not_ the case, and the reason is easily found. It depends upon
+the fact that the saros does not comprise an exact number of days, but
+includes, as we have seen, one-third of a day in addition.
+
+It will be granted, of course, that if the number of days was exact, the
+_same_ parts of the earth would always be brought round by the axial
+rotation _to front the sun_ at the moment of the recurrence of the
+eclipse. But as there is still one-third of a day to complete the saros
+period, the earth has yet to make one-third of a rotation upon its axis
+before the eclipse takes place. Thus at every recurrence the track of
+totality finds itself placed one-third of the earth's circumference to
+the _westward_. Three of the recurrences will, of course, complete the
+circuit of the globe; and so the fourth recurrence will duplicate the
+one which preceded it, three saros returns, or 54 years and 1 month
+before. This duplication, as we have already seen, will, however, be
+situated in a latitude to the south or north of its predecessor,
+according as the eclipse series is progressing in a southerly or
+northerly direction.
+
+Lastly, every eclipse series, after working its way across the earth,
+will return again to go through the same process after some 12,000
+years; so that, at the end of that great lapse of time, the entire
+"life" of every eclipse should repeat itself, provided that the
+conditions of the solar system have not altered appreciably during the
+interval.
+
+We are now in a position to consider this gradual southerly or
+northerly progress of eclipse recurrences in its application to the case
+of eclipses of the moon. It should be evident that, just as in solar
+eclipses the lunar shadow is lowered or raised (as the case may be) each
+time it strikes the terrestrial surface, so in lunar eclipses will the
+body of the moon shift its place at each recurrence relatively to the
+position of the earth's shadow. Every lunar eclipse, therefore, will
+commence on our satellite's disc as a partial eclipse at the northern or
+southern extremity, as the case may be. Let us take, as an example, an
+imaginary series of eclipses of the moon progressing from north to
+south. At each recurrence the partial phase will grow greater, its
+boundary encroaching more and more to the southward, until eventually
+the whole disc is enveloped by the shadow, and the eclipse becomes
+total. It will then repeat itself as total during a number of
+recurrences, until the entire breadth of the shadow has been passed
+through, and the northern edge of the moon at length springs out into
+sunlight. This illuminated portion will grow more and more extensive at
+each succeeding return, the edge of the shadow appearing to recede from
+it until it finally passes off at the south. Similarly, when a lunar
+eclipse commences as partial at the south of the moon, the edge of the
+shadow at each subsequent recurrence finds itself more and more to the
+northward. In due course the total phase will supervene, and will
+persist during a number of recurrences until the southerly trend of the
+moon results in the uncovering of the lunar surface at the south. Thus,
+as the boundary of the shadow is left more and more to the northward,
+the illuminated portion on the southern side of the moon becomes at each
+recurrence greater and the darkened portion on the northern side less,
+until the shadow eventually passes off at the north.
+
+The "life" of an eclipse of the moon happens, for certain reasons, to be
+much shorter than that of an eclipse of the sun. It lasts during only
+about 860 years, or 48 saros returns.
+
+Fig. 6, p. 81, is a map of the world on Mercator's Projection, showing a
+portion of the march of the total solar eclipse of August 30, 1905,
+across the surface of the earth. The projection in question has been
+employed because it is the one with which people are most familiar. This
+eclipse began by striking the neighbourhood of the North Pole in the
+guise of a partial eclipse during the latter part of the reign of Queen
+Elizabeth, and became total on the earth for the first time on the 24th
+of June 1797. Its next appearance was on the 6th of July 1815. It has
+not been possible to show the tracks of totality of these two early
+visitations on account of the distortion of the polar regions consequent
+on the _fiction_ of Mercator's Projection. It is therefore made to
+commence with the track of its third appearance, viz. on July 17, 1833.
+In consequence of those variations in the apparent sizes of the sun and
+moon, which result, as we have seen, from the variations in their
+distances from the earth, this eclipse will change from a total into an
+annular eclipse towards the end of the twenty-first century. By that
+time the track will have passed to the southern side of the equator. The
+track will eventually leave the earth near the South Pole about the
+beginning of the twenty-sixth century, and the rear portion of the
+partial shadow will in its turn be clear of the terrestrial surface by
+about 2700 A.D., when the series comes to an end.
+
+[Illustration: FIG. 6.--Map of the World on Mercator's Projection,
+showing a portion of the progress of the Total Solar Eclipse of August
+30, 1905, across the surface of the earth.]
+
+
+[4] Astronomical Essays (p. 40), London, 1907.
+
+[5] In some cases the periods between the dates of the corresponding
+eclipses _appear_ to include a greater number of days than ten; but this
+is easily explained when allowance is made for intervening _leap_ years
+(in each of which an _extra_ day has of course been added), and also for
+variations in local time.
+
+
+
+
+CHAPTER VIII
+
+FAMOUS ECLIPSES OF THE SUN
+
+
+What is thought to be the earliest reference to an eclipse comes down to
+us from the ancient Chinese records, and is over four thousand years
+old. The eclipse in question was a solar one, and occurred, so far as
+can be ascertained, during the twenty-second century B.C. The story runs
+that the two state astronomers, Ho and Hi by name, being exceedingly
+intoxicated, were unable to perform their required duties, which
+consisted in superintending the customary rites of beating drums,
+shooting arrows, and the like, in order to frighten away the mighty
+dragon which it was believed was about to swallow up the Lord of Day.
+This eclipse seems to have been only partial; nevertheless a great
+turmoil ensued, and the two astronomers were put to death, no doubt with
+the usual _celestial_ cruelty.
+
+The next eclipse mentioned in the Chinese annals is also a solar
+eclipse, and appears to have taken place more than a thousand years
+later, namely in 776 B.C. Records of similar eclipses follow from the
+same source; but as they are mere notes of the events, and do not enter
+into any detail, they are of little interest. Curiously enough the
+Chinese have taken practically no notice of eclipses of the moon, but
+have left us a comparatively careful record of comets, which has been
+of value to modern astronomy.
+
+The earliest mention of a _total_ eclipse of the sun (for it should be
+noted that the ancient Chinese eclipse above-mentioned was merely
+partial) was deciphered in 1905, on a very ancient Babylonian tablet, by
+Mr. L.W. King of the British Museum. This eclipse took place in the year
+1063 B.C.
+
+Assyrian tablets record three solar eclipses which occurred between
+three and four hundred years later than this. The first of these was in
+763 B.C.; the total phase being visible near Nineveh.
+
+The next record of an eclipse of the sun comes to us from a Grecian
+source. This eclipse took place in 585 B.C., and has been the subject of
+much investigation. Herodotus, to whom we are indebted for the account,
+tells us that it occurred during a battle in a war which had been waging
+for some years between the Lydians and Medes. The sudden coming on of
+darkness led to a termination of the contest, and peace was afterwards
+made between the combatants. The historian goes on to state that the
+eclipse had been foretold by Thales, who is looked upon as the Founder
+of Grecian astronomy. This eclipse is in consequence known as the
+"Eclipse of Thales." It would seem as if that philosopher were
+acquainted with the Chaldean saros.
+
+The next solar eclipse worthy of note was an annular one, and occurred
+in 431 B.C., the first year of the Peloponnesian War. Plutarch relates
+that the pilot of the ship, which was about to convey Pericles to the
+Peloponnesus, was very much frightened by it; but Pericles calmed him by
+holding up a cloak before his eyes, and saying that the only difference
+between this and the eclipse was that something larger than the cloak
+prevented his seeing the sun for the time being.
+
+An eclipse of great historical interest is that known as the "Eclipse of
+Agathocles," which occurred on the morning of the 15th of August, 310
+B.C. Agathocles, Tyrant of Syracuse, had been blockaded in the harbour
+of that town by the Carthaginian fleet, but effected the escape of his
+squadron under cover of night, and sailed for Africa in order to invade
+the enemy's territory. During the following day he and his vessels
+experienced a total eclipse, in which "day wholly put on the appearance
+of night, and the stars were seen in all parts of the sky."
+
+A few solar eclipses are supposed to be referred to in early Roman
+history, but their identity is very doubtful in comparison with those
+which the Greeks have recorded. Additional doubt is cast upon them by
+the fact that they are usually associated with famous events. The birth
+and death of Romulus, and the Passage of the Rubicon by Julius Caesar,
+are stated indeed to have been accompanied by these marks of the
+approval or disapproval of the gods!
+
+Reference to our subject in the Bible is scanty. Amos viii. 9 is thought
+to refer to the Nineveh eclipse of 763 B.C., to which allusion has
+already been made; while the famous episode of Hezekiah and the shadow
+on the dial of Ahaz has been connected with an eclipse which was partial
+at Jerusalem in 689 B.C.
+
+The first solar eclipse, recorded during the Christian Era, is known as
+the "Eclipse of Phlegon," from the fact that we are indebted for the
+account to a pagan writer of that name. This eclipse took place in A.D.
+29, and the total phase was visible a little to the north of Palestine.
+It has sometimes been confounded with the "darkness of the Crucifixion,"
+which event took place near the date in question; but it is sufficient
+here to say that the Crucifixion is well known to have occurred during
+the Passover of the Jews, which is always celebrated at the _full_ moon,
+whereas an eclipse of the sun can only take place at _new_ moon.
+
+Dion Cassius, commenting on the Emperor Claudius about the year A.D. 45,
+writes as follows:--
+
+"As there was going to be an eclipse on his birthday, through fear of a
+disturbance, as there had been other prodigies, he put forth a public
+notice, not only that the obscuration would take place, and about the
+time and magnitude of it, but also about the causes that produce such an
+event."
+
+This is a remarkable piece of information; for the Romans, an
+essentially military nation, appear hitherto to have troubled themselves
+very little about astronomical matters, and were content, as we have
+seen, to look upon phenomena, like eclipses, as mere celestial
+prodigies.
+
+What is thought to be the first definite mention of the solar corona
+occurs in a passage of Plutarch. The eclipse to which he refers is
+probably one which took place in A.D. 71. He says that the obscuration
+caused by the moon "has no time to last and no extensiveness, but some
+light shows itself round the sun's circumference, which does not allow
+the darkness to become deep and complete." No further reference to this
+phenomenon occurs until near the end of the sixteenth century. It
+should, however, be here mentioned that Mr. E.W. Maunder has pointed
+out the probability[6] that we have a very ancient symbolic
+representation of the corona in the "winged circle," "winged disc," or
+"ring with wings," as it is variously called, which appears so often
+upon Assyrian and Egyptian monuments, as the symbol of the Deity (Fig.
+7).
+
+[Illustration: FIG. 7.--The "Ring with Wings." The upper is the Assyrian
+form of the symbol, the lower the Egyptian. (From _Knowledge_.) Compare
+the form of the corona on Plate VII. (B), p. 142.]
+
+The first solar eclipse recorded to have been seen in England is that of
+A.D. 538, mention of which is found in the _Anglo-Saxon Chronicle_. The
+track of totality did not, however, come near our islands, for only
+two-thirds of the sun's disc were eclipsed at London.
+
+In 840 a great eclipse took place in Europe, which was total for more
+than five minutes across what is now Bavaria. Terror at this eclipse is
+said to have hastened the death of Louis le Debonnaire, Emperor of the
+West, who lay ill at Worms.
+
+In 878--_temp._ King Alfred--an eclipse of the sun took place which was
+total at London. From this until 1715 no other eclipse was total at
+London itself; though this does not apply to other portions of England.
+
+An eclipse, generally known as the "Eclipse of Stiklastad," is said to
+have taken place in 1030, during the sea-fight in which Olaf of Norway
+is supposed to have been slain. Longfellow, in his _Saga of King Olaf_,
+has it that
+
+"The Sun hung red
+As a drop of blood,"
+
+but, as in the case of most poets, the dramatic value of an eclipse
+seems to have escaped his notice.
+
+In the year 1140 there occurred a total eclipse of the sun, the last to
+be visible in England for more than five centuries. Indeed there have
+been only two such since--namely, those of 1715 and 1724, to which we
+shall allude in due course. The eclipse of 1140 took place on the 20th
+March, and is thus referred to in the _Anglo-Saxon Chronicle_:--
+
+"In the Lent, the sun and the day darkened, about the noon-tide of the
+day, when men were eating, and they lighted candles to eat by. That was
+the 13th day before the calends of April. Men were very much struck with
+wonder."
+
+Several of the older historians speak of a "fearful eclipse" as having
+taken place on the morning of the Battle of Crecy, 1346. Lingard, for
+instance, in his _History of England_, has as follows:--
+
+"Never, perhaps, were preparations for battle made under circumstances
+so truly awful. On that very day the sun suffered a partial eclipse:
+birds, in clouds, the precursors of a storm, flew screaming over the two
+armies, and the rain fell in torrents, accompanied by incessant thunder
+and lightning. About five in the afternoon the weather cleared up; the
+sun in full splendour darted his rays in the eyes of the enemy."
+
+Calculations, however, show that no eclipse of the sun took place in
+Europe during that year. This error is found to have arisen from the
+mistranslation of an obsolete French word _esclistre_ (lightning), which
+is employed by Froissart in his description of the battle.
+
+In 1598 an eclipse was total over Scotland and part of North Germany. It
+was observed at Torgau by Jessenius, an Hungarian physician, who noticed
+a bright light around the moon during the time of totality. This is said
+to be the first reference to the corona since that of Plutarch, to which
+we have already drawn attention.
+
+Mention of Scotland recalls the fact that an unusual number of eclipses
+happen to have been visible in that country, and the occult bent natural
+to the Scottish character has traditionalised a few of them in such
+terms as the "Black Hour" (an eclipse of 1433), "Black Saturday" (the
+eclipse of 1598 which has been alluded to above), and "Mirk Monday"
+(1652). The track of the last-named also passed over Carrickfergus in
+Ireland, where it was observed by a certain Dr. Wybord, in whose account
+the term "corona" is first employed. This eclipse is the last which has
+been total in Scotland, and it is calculated that there will not be
+another eclipse seen as total there until the twenty-second century.
+
+An eclipse of the sun which took place on May 30, 1612, is recorded as
+having been seen "through a tube." This probably refers to the then
+recent invention--the telescope.
+
+The eclipses which we have been describing are chiefly interesting from
+an historical point of view. The old mystery and confusion to the
+beholders seem to have lingered even into comparatively enlightened
+times, for we see how late it is before the corona attracts definite
+attention for the sake of itself alone.
+
+It is not a far cry from notice of the corona to that of other
+accompaniments of a solar eclipse. Thus the eclipse of 1706, the total
+phase of which was visible in Switzerland, is of great interest; for it
+was on this occasion that the famous red prominences seem first to have
+been noted. A certain Captain Stannyan observed this eclipse from Berne
+in Switzerland, and described it in a letter to Flamsteed, the then
+Astronomer Royal. He says the sun's "getting out of his eclipse was
+preceded by a blood-red streak of light from its left limb, which
+continued not longer than six or seven seconds of time; then part of the
+Sun's disc appeared all of a sudden, as bright as Venus was ever seen in
+the night, nay brighter; and in that very instant gave a Light and
+Shadow to things as strong as Moonlight uses to do." How little was then
+expected of the sun is, however, shown by Flamsteed's words, when
+communicating this information to the Royal Society:--
+
+"The Captain is the first man I ever heard of that took notice of a Red
+Streak of Light preceding the Emersion of the Sun's body from a total
+Eclipse. And I take notice of it to you because it infers that _the Moon
+has an atmosphere_; and its short continuance of only six or seven
+seconds of time, tells us that _its height is not more than the five or
+six hundredth part of her diameter_."
+
+What a change has since come over the ideas of men! The sun has proved a
+veritable mine of discovery, while the moon has yielded up nothing new.
+
+The eclipse of 1715, the first total at London since that of 878, was
+observed by the famous astronomer, Edmund Halley, from the rooms of the
+Royal Society, then in Crane Court, Fleet Street. On this occasion both
+the corona and a red projection were noted. Halley further makes
+allusion to that curious phenomenon, which later on became celebrated
+under the name of "Baily's beads." It was also on the occasion of this
+eclipse that the _earliest recorded drawings of the corona_ were made.
+Cambridge happened to be within the track of totality; and a certain
+Professor Cotes of that University, who is responsible for one of the
+drawings in question, forwarded them to Sir Isaac Newton together with a
+letter describing his observations.
+
+In 1724 there occurred an eclipse, the total phase of which was visible
+from the south-west of England, but not from London. The weather was
+unfavourable, and the eclipse consequently appears to have been seen by
+only one person, a certain Dr. Stukeley, who observed it from Haraden
+Hill near Salisbury Plain. This is the last eclipse of which the total
+phase was seen in any part of England. The next will not be until June
+29, 1927, and will be visible along a line across North Wales and
+Lancashire. The discs of the sun and moon will just then be almost of
+the same apparent size, and so totality will be of extremely short
+duration; in fact only a few seconds. London itself will not see a
+totality until the year 2151--a circumstance which need hardly distress
+any of us personally!
+
+It is only from the early part of the nineteenth century that serious
+scientific attention to eclipses of the sun can be dated. An _annular_
+eclipse, visible in 1836 in the south of Scotland, drew the careful
+notice of Francis Baily of Jedburgh in Roxburghshire to that curious
+phenomenon which we have already described, and which has ever since
+been known by the name of "Baily's beads." Spurred by his observation,
+the leading astronomers of the day determined to pay particular
+attention to a total eclipse, which in the year 1842 was to be visible
+in the south of France and the north of Italy. The public interest
+aroused on this occasion was also very great, for the region across
+which the track of totality was to pass was very populous, and inhabited
+by races of a high degree of culture.
+
+This eclipse occurred on the morning of the 8th July, and from it may be
+dated that great enthusiasm with which total eclipses of the sun have
+ever since been received. Airy, our then Astronomer Royal, observed it
+from Turin; Arago, the celebrated director of the Paris Observatory,
+from Perpignan in the south of France; Francis Baily from Pavia; and Sir
+John Herschel from Milan. The corona and three large red prominences
+were not only well observed by the astronomers, but drew tremendous
+applause from the watching multitudes.
+
+The success of the observations made during this eclipse prompted
+astronomers to pay similar attention to that of July 28, 1851, the total
+phase of which was to be visible in the south of Norway and Sweden, and
+across the east of Prussia. This eclipse was also a success, and it was
+now ascertained that the red prominences belonged to the sun and not to
+the moon; for the lunar disc, as it moved onward, was seen to cover and
+to uncover them in turn. It was also noted that these prominences were
+merely uprushes from a layer of glowing gaseous matter, which was seen
+closely to envelop the sun.
+
+The total eclipse of July 18, 1860, was observed in Spain, and
+photography was for the first time _systematically_ employed in its
+observation.[7] In the photographs taken the stationary appearance of
+both the corona and prominences with respect to the moving moon,
+definitely confirmed the view already put forward that they were actual
+appendages of the sun.
+
+The eclipse of August 18, 1868, the total phase of which lasted nearly
+six minutes, was visible in India, and drew thither a large concourse of
+astronomers. In this eclipse the spectroscope came to the front, and
+showed that both the prominences, and the chromospheric layer from which
+they rise, are composed of glowing vapours--chief among which is the
+vapour of hydrogen. The direct result of the observations made on this
+occasion was the spectroscopic method of examining prominences at any
+time in full daylight, and without a total eclipse. This method, which
+has given such an immense impetus to the study of the sun, was the
+outcome of independent and simultaneous investigation on the part of the
+French astronomer, the late M. Janssen, and the English astronomer,
+Professor (now Sir Norman) Lockyer, a circumstance strangely reminiscent
+of the discovery of Neptune. The principles on which the method was
+founded seem, however, to have occurred to Dr. (now Sir William) Huggins
+some time previously.
+
+The eclipse of December 22, 1870, was total for a little more than two
+minutes, and its track passed across the Mediterranean. M. Janssen, of
+whom mention has just been made, escaped in a balloon from then besieged
+Paris, taking his instruments with him, and made his way to Oran, in
+Algeria, in order to observe it; but his expectations were disappointed
+by cloudy weather. The expedition sent out from England had the
+misfortune to be shipwrecked off the coast of Sicily. But the occasion
+was redeemed by a memorable observation made by the American astronomer,
+the late Professor Young, which revealed the existence of what is now
+known as the "Reversing Layer." This is a shallow layer of gases which
+lies immediately beneath the chromosphere. An illustration of the
+corona, as it was seen during the above eclipse, will be found on Plate
+VII. (A), p. 142.
+
+In the eclipse of December 12, 1871, total across Southern India, the
+photographs of the corona obtained by Mr. Davis, assistant to Lord
+Lindsay (now the Earl of Crawford), displayed a wealth of detail
+hitherto unapproached.
+
+The eclipse of July 29, 1878, total across the western states of North
+America, was a remarkable success, and a magnificent view of the corona
+was obtained by the well-known American astronomer and physicist, the
+late Professor Langley, from the summit of Pike's Peak, Colorado, over
+14,000 feet above the level of the sea. The coronal streamers were seen
+to extend to a much greater distance at this altitude than at points
+less elevated, and the corona itself remained visible during more than
+four minutes after the end of totality. It was, however, not entirely a
+question of altitude; the coronal streamers were actually very much
+longer on this occasion than in most of the eclipses which had
+previously been observed.
+
+The eclipse of May 17, 1882, observed in Upper Egypt, is notable from
+the fact that, in one of the photographs taken by Dr. Schuster at Sohag,
+a bright comet appeared near the outer limit of the corona (see Plate
+I., p. 96). The comet in question had not been seen before the eclipse,
+and was never seen afterwards. This is the third occasion on which
+attention has been drawn to a comet _merely_ by a total eclipse. The
+first is mentioned by Seneca; and the second by Philostorgius, in an
+account of an eclipse observed at Constantinople in A.D. 418. A fourth
+case of the kind occurred in 1893, when faint evidences of one of these
+filmy objects were found on photographs of the corona taken by the
+American astronomer, Professor Schaeberle, during the total eclipse of
+April 16 of that year.
+
+The eclipse of May 6, 1883, had a totality of over five minutes, but
+the central track unfortunately passed across the Pacific Ocean, and the
+sole point of land available for observing it from was one of the
+Marquesas Group, Caroline Island, a coral atoll seven and a half miles
+long by one and a half broad. Nevertheless astronomers did not hesitate
+to take up their posts upon that little spot, and were rewarded with
+good weather.
+
+The next eclipse of importance was that of April 16, 1893. It stretched
+from Chili across South America and the Atlantic Ocean to the West Coast
+of Africa, and, as the weather was fine, many good results were
+obtained. Photographs were taken at both ends of the track, and these
+showed that the appearance of the corona remained unchanged during the
+interval of time occupied by the passage of the shadow across the earth.
+It was on the occasion of this eclipse that Professor Schaeberle found
+upon his photographs those traces of the presence of a comet, to which
+allusion has already been made.
+
+Extensive preparations were made to observe the eclipse of August 9,
+1896. Totality lasted from two to three minutes, and the track stretched
+from Norway to Japan. Bad weather disappointed the observers, with the
+exception of those taken to Nova Zembla by Sir George Baden Powell in
+his yacht _Otaria_.
+
+The eclipse of January 22, 1898, across India _via_ Bombay and Benares,
+was favoured with good weather, and is notable for a photograph obtained
+by Mrs. E.W. Maunder, which showed a ray of the corona extending to a
+most unusual distance.
+
+[Illustration: PLATE I. THE TOTAL ECLIPSE OF THE SUN OF MAY 17TH, 1882
+
+A comet is here shown in the immediate neighbourhood of the corona.
+
+Drawn by Mr. W.H. Wesley from the photographs.
+
+(Page 95)]
+
+Of very great influence in the growth of our knowledge with regard to
+the sun, is the remarkable piece of good fortune by which the countries
+around the Mediterranean, so easy of access, have been favoured with a
+comparatively large number of total eclipses during the past sixty
+years. Tracks of totality have, for instance, traversed the Spanish
+peninsula on no less than five occasions during that period. Two of
+these are among the most notable eclipses of recent years, namely, those
+of May 28, 1900, and of August 30, 1905. In the former the track of
+totality stretched from the western seaboard of Mexico, through the
+Southern States of America, and across the Atlantic Ocean, after which
+it passed over Portugal and Spain into North Africa. The total phase
+lasted for about a minute and a half, and the eclipse was well observed
+from a great many points along the line. A representation of the corona,
+as it appeared on this occasion, will be found on Plate VII. (B), p.
+142.
+
+The track of the other eclipse to which we have alluded, _i.e._ that of
+August 30, 1905, crossed Spain about 200 miles to the northward of that
+of 1900. It stretched from Winnipeg in Canada, through Labrador, and
+over the Atlantic; then traversing Spain, it passed across the Balearic
+Islands, North Africa, and Egypt, and ended in Arabia (see Fig. 6, p.
+81). Much was to be expected from a comparison between the photographs
+taken in Labrador and Egypt on the question as to whether the corona
+would show any alteration in shape during the time that the shadow was
+traversing the intervening space--some 6000 miles. The duration of the
+total phase in this eclipse was nearly four minutes. Bad weather,
+however, interfered a good deal with the observations. It was not
+possible, for instance, to do anything at all in Labrador. In Spain the
+weather conditions were by no means favourable; though at Burgos, where
+an immense number of people had assembled, the total phase was,
+fortunately, well seen. On the whole, the best results were obtained at
+Guelma in Algeria. The corona on the occasion of this eclipse was a very
+fine one, and some magnificent groups of prominences were plainly
+visible to the naked eye (see the Frontispiece).
+
+The next total eclipse after that of 1905 was one which occurred on
+January 14, 1907. It passed across Central Asia and Siberia, and had a
+totality lasting two and a half minutes at most; but it was not observed
+as the weather was extremely bad, a circumstance not surprising with
+regard to those regions at that time of year.
+
+The eclipse of January 3, 1908, passed across the Pacific Ocean. Only
+two small coral islands--Hull Island in the Phoenix Group, and Flint
+Island about 400 miles north of Tahiti--lay in the track. Two
+expeditions set out to observe it, _i.e._ a combined American party from
+the Lick Observatory and the Smithsonian Institution of Washington, and
+a private one from England under Mr. F.K. McClean. As Hull Island
+afforded few facilities, both parties installed their instruments on
+Flint Island, although it was very little better. The duration of the
+total phase was fairly long--about four minutes, and the sun very
+favourably placed, being nearly overhead. Heavy rain and clouds,
+however, marred observation during the first minute of totality, but the
+remaining three minutes were successfully utilised, good photographs of
+the corona being obtained.
+
+The next few years to come are unfortunately by no means favourable
+from the point of view of the eclipse observer. An eclipse will take
+place on June 17, 1909, the track stretching from Greenland across the
+North Polar regions into Siberia. The geographical situation is,
+however, a very awkward one, and totality will be extremely short--only
+six seconds in Greenland and twenty-three seconds in Siberia.
+
+The eclipse of May 9, 1910, will be visible in Tasmania. Totality will
+last so long as four minutes, but the sun will be at the time much too
+low in the sky for good observation.
+
+The eclipse of the following year, April 28, 1911, will also be
+confined, roughly speaking, to the same quarter of the earth, the track
+passing across the old convict settlement of Norfolk Island, and then
+out into the Pacific.
+
+The eclipse of April 17, 1912, will stretch from Portugal, through
+France and Belgium into North Germany. It will, however, be of
+practically no service to astronomy. Totality, for instance, will last
+for only three seconds in Portugal; and, though Paris lies in the
+central track, the eclipse, which begins as barely total, will have
+changed into an _annular_ one by the time it passes over that city.
+
+The first really favourable eclipse in the near future will be that of
+August 21, 1914. Its track will stretch from Greenland across Norway,
+Sweden, and Russia. This eclipse is a return, after one saros, of the
+eclipse of August 9, 1896.
+
+The last solar eclipse which we will touch upon is that predicted for
+June 29, 1927. It has been already alluded to as the first of those in
+the future to be _total_ in England. The central line will stretch from
+Wales in a north-easterly direction. Stonyhurst Observatory, in
+Lancashire, will lie in the track; but totality there will be very
+short, only about twenty seconds in duration.
+
+
+[6] _Knowledge_, vol. xx. p. 9, January 1897.
+
+[7] The _first photographic representation of the corona_ had, however,
+been made during the eclipse of 1851. This was a daguerreotype taken by
+Dr. Busch at Koenigsberg in Prussia.
+
+
+
+
+CHAPTER IX
+
+FAMOUS ECLIPSES OF THE MOON
+
+
+The earliest lunar eclipse, of which we have any trustworthy
+information, was a total one which took place on the 19th March, 721
+B.C., and was observed from Babylon. For our knowledge of this eclipse
+we are indebted to Ptolemy, the astronomer, who copied it, along with
+two others, from the records of the reign of the Chaldean king,
+Merodach-Baladan.
+
+The next eclipse of the moon worth noting was a total one, which took
+place some three hundred years later, namely, in 425 B.C. This eclipse
+was observed at Athens, and is mentioned by Aristophanes in his play,
+_The Clouds_.
+
+Plutarch relates that a total eclipse of the moon, which occurred in 413
+B.C., so greatly frightened Nicias, the general of the Athenians, then
+warring in Sicily, as to cause a delay in his retreat from Syracuse
+which led to the destruction of his whole army.
+
+Seven years later--namely, in 406 B.C., the twenty-sixth year of the
+Peloponnesian War--there took place another total lunar eclipse of which
+mention is made by Xenophon.
+
+Omitting a number of other eclipses alluded to by ancient writers, we
+come to one recorded by Josephus as having occurred a little before the
+death of Herod the Great. It is probable that the eclipse in question
+was the total lunar one, which calculation shows to have taken place on
+the 15th September 5 B.C., and to have been visible in Western Asia.
+This is very important, for we are thus enabled to fix that year as the
+date of the birth of Christ, for Herod is known to have died in the
+early part of the year following the Nativity.
+
+In those accounts of total lunar eclipses, which have come down to us
+from the Dark and Middle Ages, the colour of the moon is nearly always
+likened to "blood." On the other hand, in an account of the eclipse of
+January 23, A.D. 753, our satellite is described as "covered with a
+horrid black shield." We thus have examples of the two distinct
+appearances alluded to in Chapter VII., _i.e._ when the moon appears of
+a coppery-red colour, and when it is entirely darkened.
+
+It appears, indeed, that, in the majority of lunar eclipses on record,
+the moon has appeared of a ruddy, or rather of a coppery hue, and the
+details on its surface have been thus rendered visible. One of the best
+examples of a _bright_ eclipse of this kind is that of the 19th March
+1848, when the illumination of our satellite was so great that many
+persons could not believe that an eclipse was actually taking place. A
+certain Mr. Foster, who observed this eclipse from Bruges, states that
+the markings on the lunar disc were almost as visible as on an "ordinary
+dull moonlight night." He goes on to say that the British Consul at
+Ghent, not knowing that there had been any eclipse, wrote to him for an
+explanation of the red colour of the moon on that evening.
+
+Out of the _dark_ eclipses recorded, perhaps the best example is that
+of May 18, 1761, observed by Wargentin at Stockholm. On this occasion
+the lunar disc is said to have disappeared so completely, that it could
+not be discovered even with the telescope. Another such instance is the
+eclipse of June 10, 1816, observed from London. The summer of that year
+was particularly wet--a point worthy of notice in connection with the
+theory that these different appearances are due to the varying state of
+our earth's atmosphere.
+
+Sometimes, indeed, it has happened that an eclipse of the moon has
+partaken of both appearances, part of the disc being visible and part
+invisible. An instance of this occurred in the eclipse of July 12, 1870,
+when the late Rev. S.J. Johnson, one of the leading authorities on
+eclipses, who observed it, states that he found one-half the moon's
+surface quite invisible, both with the naked eye and with the telescope.
+
+In addition to the examples given above, there are three total lunar
+eclipses which deserve especial mention.
+
+1. A.D. 755, November 23. During the progress of this eclipse the moon
+occulted the star Aldebaran in the constellation of Taurus.
+
+2. A.D. 1493, April 2. This is the celebrated eclipse which is said to
+have so well served the purposes of Christopher Columbus. Certain
+natives having refused to supply him with provisions when in sore
+straits, he announced to them that the moon would be darkened as a sign
+of the anger of heaven. When the event duly came to pass, the savages
+were so terrified that they brought him provisions as much as he needed.
+
+3. A.D. 1610, July 6. The eclipse in question is notable as having been
+seen through the telescope, then a recent invention. It was without
+doubt the first so observed, but unfortunately the name of the observer
+has not come down to us.
+
+
+
+
+CHAPTER X
+
+THE GROWTH OF OBSERVATION
+
+
+The earliest astronomical observations must have been made in the Dawn
+of Historic Time by the men who tended their flocks upon the great
+plains. As they watched the clear night sky they no doubt soon noticed
+that, with the exception of the moon and those brilliant wandering
+objects known to us as the planets, the individual stars in the heaven
+remained apparently fixed with reference to each other. These seemingly
+changeless points of light came in time to be regarded as sign-posts to
+guide the wanderer across the trackless desert, or the voyager upon the
+wide sea.
+
+Just as when looking into the red coals of a fire, or when watching the
+clouds, our imagination conjures up strange and grotesque forms, so did
+the men of old see in the grouping of the stars the outlines of weird
+and curious shapes. Fed with mythological lore, they imagined these to
+be rough representations of ancient heroes and fabled beasts, whom they
+supposed to have been elevated to the heavens as a reward for great
+deeds done upon the earth. We know these groupings of stars to-day under
+the name of the Constellations. Looking up at them we find it extremely
+difficult to fit in the majority with the figures which the ancients
+believed them to represent. Nevertheless, astronomy has accepted the
+arrangement, for want of a better method of fixing the leading stars in
+the memory.
+
+Our early ancestors lived the greater part of their lives in the open
+air, and so came to pay more attention in general to the heavenly orbs
+than we do. Their clock and their calendar was, so to speak, in the
+celestial vault. They regulated their hours, their days, and their
+nights by the changing positions of the sun, the moon, and the stars;
+and recognised the periods of seed-time and harvest, of calm and stormy
+weather, by the rising or setting of certain well-known constellations.
+Students of the classics will recall many allusions to this, especially
+in the Odes of Horace.
+
+As time went on and civilisation progressed, men soon devised measuring
+instruments, by means of which they could note the positions of the
+celestial bodies in the sky with respect to each other; and, from
+observations thus made, they constructed charts of the stars. The
+earliest complete survey of this kind, of which we have a record, is the
+great Catalogue of stars which was made, in the second century B.C., by
+the celebrated Greek astronomer, Hipparchus, and in which he is said to
+have noted down about 1080 stars.
+
+It is unnecessary to follow in detail the tedious progress of
+astronomical discovery prior to the advent of the telescope. Certain it
+is that, as time went on, the measuring instruments to which we have
+alluded had become greatly improved; but, had they even been perfect,
+they would have been utterly inadequate to reveal those minute
+displacements, from which we have learned the actual distance of the
+nearest of the celestial orbs. From the early times, therefore, until
+the mediaeval period of our own era, astronomy grew up upon a faulty
+basis, for the earth ever seemed so much the largest body in the
+universe, that it continued from century to century to be regarded as
+the very centre of things.
+
+To the Arabians is due the credit of having kept alive the study of the
+stars during the dark ages of European history. They erected some fine
+observatories, notably in Spain and in the neighbourhood of Bagdad.
+Following them, some of the Oriental peoples embraced the science in
+earnest; Ulugh Beigh, grandson of the famous Tamerlane, founding, for
+instance, a great observatory at Samarcand in Central Asia. The Mongol
+emperors of India also established large astronomical instruments in the
+chief cities of their empire. When the revival of learning took place in
+the West, the Europeans came to the front once more in science, and
+rapidly forged ahead of those who had so assiduously kept alight the
+lamp of knowledge through the long centuries.
+
+The dethronement of the older theories by the Copernican system, in
+which the earth was relegated to its true place, was fortunately soon
+followed by an invention of immense import, the invention of the
+Telescope. It is to this instrument, indeed, that we are indebted for
+our knowledge of the actual scale of the celestial distances. It
+penetrated the depths of space; it brought the distant orbs so near,
+that men could note the detail on the planets, or measure the small
+changes in their positions in the sky which resulted from the movement
+of our own globe.
+
+It was in the year 1609 that the telescope was first constructed. A
+year or so previous to this a spectacle-maker of Middleburgh in Holland,
+one Hans Lippershey, had, it appears, hit upon the fact that distant
+objects, when viewed through certain glass lenses suitably arranged,
+looked nearer.[8] News of this discovery reached the ears of Galileo
+Galilei, of Florence, the foremost philosopher of the day, and he at
+once applied his great scientific attainments to the construction of an
+instrument based upon this principle. The result was what was called an
+"optick tube," which magnified distant objects some few times. It was
+not much larger than what we nowadays contemptuously refer to as a
+"spy-glass," yet its employment upon the leading celestial objects
+instantly sent astronomical science onward with a bound. In rapid
+succession Galileo announced world-moving discoveries; large spots upon
+the face of the sun; crater-like mountains upon the moon; four
+subordinate bodies, or satellites, circling around the planet Jupiter;
+and a strange appearance in connection with Saturn, which later
+telescopic observers found to be a broad flat ring encircling that
+planet. And more important still, the magnified image of Venus showed
+itself in the telescope at certain periods in crescent and other forms;
+a result which Copernicus is said to have announced should of necessity
+follow if his system were the true one.
+
+The discoveries made with the telescope produced, as time went on, a
+great alteration in the notions of men with regard to the universe at
+large. It must have been, indeed, a revelation to find that those points
+of light which they called the planets, were, after all, globes of a
+size comparable with the earth, and peopled perchance with sentient
+beings. Even to us, who have been accustomed since our early youth to
+such an idea, it still requires a certain stretch of imagination to
+enlarge, say, the Bright Star of Eve, into a body similar in size to our
+earth. The reader will perhaps recollect Tennyson's allusion to this in
+_Locksley Hall, Sixty Years After_:--
+
+"Hesper--Venus--were we native to that splendour or in Mars,
+We should see the Globe we groan in, fairest of their evening stars.
+
+"Could we dream of wars and carnage, craft and madness, lust and spite,
+Roaring London, raving Paris, in that point of peaceful light?"
+
+The form of instrument as devised by Galileo is called the Refracting
+Telescope, or "Refractor." As we know it to-day it is the same in
+principle as his "optick tube," but it is not quite the same in
+construction. The early _object-glass_, or large glass at the end, was a
+single convex lens (see Fig. 8, p. 113, "Galilean"); the modern one is,
+on the other hand, composed of two lenses fitted together. The attempts
+to construct large telescopes of the Galilean type met in course of time
+with a great difficulty. The magnified image of the object observed was
+not quite pure; its edges, indeed, were fringed with rainbow-like
+colours. This defect was found to be aggravated with increase in the
+size of object-glasses. A method was, however, discovered of
+diminishing this colouration, or _chromatic aberration_ as it is called
+from the Greek word [chroma] (_chroma_), which means colour, viz. by
+making telescopes of great length and only a few inches in width. But
+the remedy was, in a way, worse than the disease; for telescopes thus
+became of such huge proportions as to be too unwieldy for use. Attempts
+were made to evade this unwieldiness by constructing them with skeleton
+tubes (see Plate II., p. 110), or, indeed, even without tubes at all;
+the object-glass in the tubeless or "aerial" telescope being fixed at
+the top of a high post, and the _eye-piece_, that small lens or
+combination of lenses, which the eye looks directly into, being kept in
+line with it by means of a string and manoeuvred about near the ground
+(Plate III., p. 112). The idea of a telescope without a tube may appear
+a contradiction in terms; but it is not really so, for the tube adds
+nothing to the magnifying power of the instrument, and is, in fact, no
+more than a mere device for keeping the object-glass and eye-piece in a
+straight line, and for preventing the observer from being hindered by
+stray lights in his neighbourhood. It goes without saying, of course,
+that the image of a celestial object will be more clear and defined when
+examined in the darkness of a tube.
+
+The ancients, though they knew nothing of telescopes, had, however,
+found out the merit of a tube in this respect; for they employed simple
+tubes, blackened on the inside, in order to obtain a clearer view of
+distant objects. It is said that Julius Caesar, before crossing the
+Channel, surveyed the opposite coast of Britain through a tube of this
+kind.
+
+[Illustration: PLATE II. GREAT TELESCOPE OF HEVELIUS
+
+This instrument, 150 feet in length, with a _skeleton_ tube, was
+constructed by the celebrated seventeenth century astronomer, Hevelius
+of Danzig. From an illustration in the _Machina Celestis_.
+
+(Page 110)]
+
+A few of the most famous of the immensely long telescopes above alluded
+to are worthy of mention. One of these, 123 feet in length, was
+presented to the Royal Society of London by the Dutch astronomer
+Huyghens. Hevelius of Danzig constructed a skeleton one of 150 feet in
+length (see Plate II., p. 110). Bradley used a tubeless one 212 feet
+long to measure the diameter of Venus in 1722; while one of 600 feet is
+said to have been constructed, but to have proved quite unworkable!
+
+Such difficulties, however, produced their natural result. They set men
+at work to devise another kind of telescope. In the new form, called the
+Reflecting Telescope, or "Reflector," the light coming from the object
+under observation was _reflected_ into the eye-piece from the surface of
+a highly polished concave metallic mirror, or _speculum_, as it was
+called. It is to Sir Isaac Newton that the world is indebted for the
+reflecting telescope in its best form. That philosopher had set himself
+to investigate the causes of the rainbow-like, or prismatic colours
+which for a long time had been such a source of annoyance to telescopic
+observers; and he pointed out that, as the colours were produced in the
+passage of the rays of light _through_ the glass, they would be entirely
+absent if the light were reflected from the _surface_ of a mirror
+instead.
+
+The reflecting telescope, however, had in turn certain drawbacks of its
+own. A mirror, for instance, can plainly never be polished to such a
+high degree as to reflect as much light as a piece of transparent glass
+will let through. Further, the position of the eye-piece is by no means
+so convenient. It cannot, of course, be pointed directly towards the
+mirror, for the observer would then have to place his head right in the
+way of the light coming from the celestial object, and would thus, of
+course, cut it off. In order to obviate this difficulty, the following
+device was employed by Newton in his telescope, of which he constructed
+his first example in 1668. A small, flat mirror was fixed by thin wires
+in the centre of the tube of the telescope, and near to its open end. It
+was set slant-wise, so that it reflected the rays of light directly into
+the eye-piece, which was screwed into a hole at the side of the tube
+(see Fig. 8, p. 113, "Newtonian").
+
+Although the Newtonian form of telescope had the immense advantage of
+doing away with the prismatic colours, yet it wasted a great deal of
+light; for the objection in this respect with regard to loss of light by
+reflection from the large mirror applied, of course, to the small mirror
+also. In addition, the position of the "flat," as the small mirror is
+called, had the further effect of excluding from the great mirror a
+certain proportion of light. But the reflector had the advantage, on the
+other hand, of costing less to make than the refractor, as it was not
+necessary to procure flawless glass for the purpose. A disc of a certain
+metallic composition, an alloy of copper and tin, known in consequence
+as _speculum metal_, had merely to be cast; and this had to be ground
+and polished _upon one side only_, whereas a lens has to be thus treated
+_upon both its sides_. It was, therefore, possible to make a much larger
+instrument at a great deal less labour and expense.
+
+[Illustration: PLATE III. A TUBELESS, OR "AERIAL" TELESCOPE
+
+From an illustration in the _Opera Varia_ of Christian Huyghens.
+
+(Page 110)]
+
+[Illustration: FIG. 8.--The various types of Telescope. All the above
+telescopes are _pointed_ in the same direction; that is to say, the rays
+of light from the object are coming from the left-hand side.]
+
+We have given the Newtonian form as an example of the principle of the
+reflecting telescope. A somewhat similar instrument had, however, been
+projected, though not actually constructed, by James Gregory a few years
+earlier than Newton's, _i.e._ in 1663. In this form of reflector, known
+as the "Gregorian" telescope, a hole was made in the big concave mirror;
+and a small mirror, also concave, which faced it at a certain distance,
+received the reflected rays, and reflected them back again through the
+hole in question into the eye-piece, which was fixed just behind (see
+Fig. 8, p. 113, "Gregorian"). The Gregorian had thus the sentimental
+advantage of being _pointed directly at the object_. The hole in the big
+mirror did not cause any loss of light, for the central portion in which
+it was made was anyway unable to receive light through the small mirror
+being directly in front of it. An adaptation of the Gregorian was the
+"Cassegrainian" telescope, devised by Cassegrain in 1672, which differed
+from it chiefly in the small mirror being convex instead of concave (see
+Fig. 8, p. 113, "Cassegrainian"). These _direct-view_ forms of the
+reflecting telescope were much in vogue about the middle of the
+eighteenth century, when many beautiful examples of Gregorians were made
+by the famous optician, James Short, of Edinburgh.
+
+An adaptation of the Newtonian type of telescope is known as the
+"Herschelian," from being the kind favoured by Sir William Herschel. It
+is, however, only suitable in immense instruments, such as Herschel was
+in the habit of employing. In this form the object-glass is set at a
+slight slant, so that the light coming from the object is reflected
+straight into the eye-piece, which is fixed facing it in the side of the
+tube (see Fig. 8, p. 113, "Herschelian"). This telescope has an
+advantage over the other forms of reflector through the saving of light
+consequent on doing away with the _second_ reflection. There is,
+however, the objection that the slant of the object-glass is productive
+of some distortion in the appearance of the object observed; but this
+slant is of necessity slight when the length of the telescope is very
+great.
+
+The principle of this type of telescope had been described to the
+French Academy of Sciences as early as 1728 by Le Maire, but no one
+availed himself of the idea until 1776, when Herschel tried it. At
+first, however, he rejected it; but in 1786 he seems to have found that
+it suited the huge instruments which he was then making. Herschel's
+largest telescope, constructed in 1789, was about four feet in diameter
+and forty feet in length. It is generally spoken of as the "Forty-foot
+Telescope," though all other instruments have been known by their
+_diameters_, rather than by their lengths.
+
+To return to the refracting telescope. A solution of the colour
+difficulty was arrived at in 1729 (two years after Newton's death) by an
+Essex gentleman named Chester Moor Hall. He discovered that by making a
+double object-glass, composed of an outer convex lens and an inner
+concave lens, made respectively of different kinds of glass, _i.e._
+_crown_ glass and _flint_ glass, the troublesome colour effects could
+be, _to a very great extent_, removed. Hall's investigations appear to
+have been rather of an academic nature; and, although he is believed to
+have constructed a small telescope upon these lines, yet he seems to
+have kept the matter so much to himself that it was not until the year
+1758 that the first example of the new instrument was given to the
+world. This was done by John Dollond, founder of the well-known optical
+firm of Dollond, of Ludgate Hill, London, who had, quite independently,
+re-discovered the principle.
+
+This "Achromatic" telescope, or telescope "free from colour effects," is
+the kind ordinarily in use at present, whether for astronomical or for
+terrestrial purposes (see Fig. 8, p. 113, "Achromatic"). The expense of
+making large instruments of this type is very great, for, in the
+object-glass alone, no less than _four_ surfaces have to be ground and
+polished to the required curves; and, usually, the two lenses of which
+it is composed have to fit quite close together.
+
+With the object of evading the expense referred to, and of securing
+_complete_ freedom from colour effects, telescopes have even been made,
+the object-glasses of which were composed of various transparent liquids
+placed between thin lenses; but leakages, and currents set up within
+them by changes of temperature, have defeated the ingenuity of those who
+devised these substitutes.
+
+The solution of the colour difficulty by means of Dollond's achromatic
+refractor has not, however, ousted the reflecting telescope in its best,
+or Newtonian form, for which great concave mirrors made of glass,
+covered with a thin coating of silver and highly polished, have been
+used since about 1870 instead of metal mirrors. They are very much
+lighter in weight and cheaper to make than the old specula; and though
+the silvering, needless to say, deteriorates with time, it can be
+renewed at a comparatively trifling cost. Also these mirrors reflect
+much more light, and give a clearer view, than did the old metallic
+ones.
+
+When an object is viewed through the type of astronomical telescope
+ordinarily in use, it is seen _upside down_. This is, however, a matter
+of very small moment in dealing with celestial objects; for, as they are
+usually round, it is really not of much consequence which part we regard
+as top and which as bottom. Such an inversion would, of course, be most
+inconvenient when viewing terrestrial objects. In order to observe the
+latter we therefore employ what is called a terrestrial telescope, which
+is merely a refractor with some extra lenses added in the eye portion
+for the purpose of turning the inverted image the right way up again.
+These extra lenses, needless to say, absorb a certain amount of light;
+wherefore it is better in astronomical observation to save light by
+doing away with them, and putting up with the slight inconvenience of
+seeing the object inverted.
+
+This inversion of images by the astronomical telescope must be specially
+borne in mind with regard to the photographs of the moon in Chapter XVI.
+
+In the year 1825 the largest achromatic refractor in existence was one
+of nine and a half inches in diameter constructed by Fraunhofer for the
+Observatory of Dorpat in Russia. The largest refractors in the world
+to-day are in the United States, _i.e._ the forty-inch of the Yerkes
+Observatory (see Plate IV., p. 118), and the thirty-six inch of the
+Lick. The object-glasses of these and of the thirty-inch telescope of
+the Observatory of Pulkowa, in Russia, were made by the great optical
+house of Alvan Clark & Sons, of Cambridge, Massachusetts, U.S.A. The
+tubes and other portions of the Yerkes and Lick telescopes were,
+however, constructed by the Warner and Swasey Co., of Cleveland, Ohio.
+
+The largest reflector, and so the largest telescope in the world, is
+still the six-foot erected by the late Lord Rosse at Parsonstown in
+Ireland, and completed in the year 1845. It is about fifty-six feet in
+length. Next come two of five feet, with mirrors of silver on glass;
+one of them made by the late Dr. Common, of Ealing, and the other by the
+American astronomer, Professor G.W. Ritchey. The latter of these is
+installed in the Solar Observatory belonging to Carnegie Institution of
+Washington, which is situated on Mount Wilson in California. The former
+is now at the Harvard College Observatory, and is considered by
+Professor Moulton to be probably the most efficient reflector in use at
+present. Another large reflector is the three-foot made by Dr. Common.
+It came into the possession of Mr. Crossley of Halifax, who presented it
+to the Lick Observatory, where it is now known as the "Crossley
+Reflector."
+
+Although to the house of Clark belongs, as we have seen, the credit of
+constructing the object-glasses of the largest refracting telescopes of
+our time, it has nevertheless keen competitors in Sir Howard Grubb, of
+Dublin, and such well-known firms as Cooke of York and Steinheil of
+Munich. In the four-foot reflector, made in 1870 for the Observatory of
+Melbourne by the firm of Grubb, the Cassegrainian principle was
+employed.
+
+With regard to the various merits of refractors and reflectors much
+might be said. Each kind of instrument has, indeed, its special
+advantages; though perhaps, on the whole, the most perfect type of
+telescope is the achromatic refractor.
+
+[Illustration: PLATE IV. THE GREAT YERKES TELESCOPE
+
+Great telescope at the Yerkes Observatory of the University of Chicago,
+Williams Bay, Wisconsin, U.S.A. It was erected in 1896-7, and is the
+largest refracting telescope in the world. Diameter of object-glass, 40
+inches; length of telescope, about 60 feet. The object-glass was made by
+the firm of Alvan Clark and Sons, of Cambridge, Massachusetts; the other
+portions of the instrument by the Warner and Swasey Co., of Cleveland,
+Ohio.
+
+(Page 117)]
+
+In connection with telescopes certain devices have from time to time
+been introduced, but these merely aim at the _convenience_ of the
+observer and do not supplant the broad principles upon which are based
+the various types of instrument above described. Such, for instance, are
+the "Siderostat," and another form of it called the "Coelostat," in
+which a plane mirror is made to revolve in a certain manner, so as to
+reflect those portions of the sky which are to be observed, into the
+tube of a telescope kept fixed. Such too are the "Equatorial Coude" of
+the late M. Loewy, Director of the Paris Observatory, and the
+"Sheepshanks Telescope" of the Observatory of Cambridge, in which a
+telescope is separated into two portions, the eye-piece portion being
+fixed upon a downward slant, and the object-glass portion jointed to it
+at an angle and pointed up at the sky. In these two instruments (which,
+by the way, differ materially) an arrangement of slanting mirrors in the
+tubes directs the journey of the rays of light from the object-glass to
+the eye-piece. The observer can thus sit at the eye-end of his telescope
+in the warmth and comfort of his room, and observe the stars in the same
+unconstrained manner as if he were merely looking down into a
+microscope.
+
+Needless to say, devices such as these are subject to the drawback that
+the mirrors employed sap a certain proportion of the rays of light. It
+will be remembered that we made allusion to loss of light in this way,
+when pointing out the advantage in light grasp of the Herschelian form
+of telescope, where only _one_ reflection takes place, over the
+Newtonian in which there are _two_.
+
+It is an interesting question as to whether telescopes can be made much
+larger. The American astronomer, Professor G.E. Hale, concludes that the
+limit of refractors is about five feet in diameter, but he thinks that
+reflectors as large as nine feet in diameter might now be made. As
+regards refractors there are several strong reasons against augmenting
+their proportions. First of all comes the great cost. Secondly, since
+the lenses are held in position merely round their rims, they will bend
+by their weight in the centres if they are made much larger. On the
+other hand, attempts to obviate this, by making the lenses thicker,
+would cause a decrease in the amount of light let through.
+
+But perhaps the greatest stumbling-block to the construction of larger
+telescopes is the fact that the unsteadiness of the air will be
+increasingly magnified. And further, the larger the tubes become, the
+more difficult will it be to keep the air within them at one constant
+temperature throughout their lengths.
+
+It would, indeed, seem as if telescopes are not destined greatly to
+increase in size, but that the means of observation will break out in
+some new direction, as it has already done in the case of photography
+and the spectroscope. The direct use of the eye is gradually giving
+place to indirect methods. We are, in fact, now _feeling_ rather than
+seeing our way about the universe. Up to the present, for instance, we
+have not the slightest proof that life exists elsewhere than upon our
+earth. But who shall say that the twentieth century has not that in
+store for us, by which the presence of life in other orbs may be
+perceived through some form of vibration transmitted across illimitable
+space? There is no use speaking of the impossible or the inconceivable.
+After the extraordinary revelations of the spectroscope--nay, after the
+astounding discovery of Roentgen--the word impossible should be cast
+aside, and inconceivability cease to be regarded as any criterion.
+
+
+[8] The principle upon which the telescope is based appears to have been
+known _theoretically_ for a long time previous to this. The monk Roger
+Bacon, who lived in the thirteenth century, describes it very clearly;
+and several writers of the sixteenth century have also dealt with the
+idea. Even Lippershey's claims to a practical solution of the question
+were hotly contested at the time by two of his own countrymen, _i.e._ a
+certain Jacob Metius, and another spectacle-maker of Middleburgh, named
+Jansen.
+
+
+
+
+CHAPTER XI
+
+SPECTRUM ANALYSIS
+
+
+If white light (that of the sun, for instance) be passed through a glass
+prism, namely, a piece of glass of triangular shape, it will issue from
+it in rainbow-tinted colours. It is a common experience with any of us
+to notice this when the sunlight shines through cut-glass, as in the
+pendant of a chandelier, or in the stopper of a wine-decanter.
+
+The same effect may be produced when light passes through water. The
+Rainbow, which we all know so well, is merely the result of the sunlight
+passing through drops of falling rain.
+
+White light is composed of rays of various colours. Red, orange, yellow,
+green, blue, indigo, and violet, taken all together, go, in fact, to
+make up that effect which we call white.
+
+It is in the course of the _refraction_, or bending of a beam of light,
+when it passes in certain conditions through a transparent and denser
+medium, such as glass or water, that the constituent rays are sorted out
+and spread in a row according to their various colours. This production
+of colour takes place usually near the edges of a lens; and, as will be
+recollected, proved very obnoxious to the users of the old form of
+refracting telescope.
+
+It is, indeed, a strange irony of fate that this very same production
+of colour, which so hindered astronomy in the past, should have aided it
+in recent years to a remarkable degree. If sunlight, for instance, be
+admitted through a narrow slit before it falls upon a glass prism, it
+will issue from the latter in the form of a band of variegated colour,
+each colour blending insensibly with the next. The colours arrange
+themselves always in the order which we have mentioned. This seeming
+band is, in reality, an array of countless coloured images of the
+original slit ranged side by side; the colour of each image being the
+slightest possible shade different from that next to it. This strip of
+colour when produced by sunlight is called the "Solar Spectrum" (see
+Fig. 9, p. 123). A similar strip, or _spectrum_, will be produced by any
+other light; but the appearance of the strip, with regard to
+preponderance of particular colours, will depend upon the character of
+that light. Electric light and gas light yield spectra not unlike that
+of sunlight; but that of gas is less rich in blue and violet than that
+of the sun.
+
+The Spectroscope, an instrument devised for the examination of spectra,
+is, in its simplest form, composed of a small tube with a narrow slit
+and prism at one end, and an eye-piece at the other. If we drop ordinary
+table salt into the flame of a gas light, the flame becomes strongly
+yellow. If, then, we observe this yellow flame with the spectroscope, we
+find that its spectrum consists almost entirely of two bright yellow
+transverse lines. Chemically considered ordinary table salt is sodium
+chloride; that is to say, a compound of the metal sodium and the gas
+chlorine. Now if other compounds of sodium be experimented with in the
+same manner, it will soon be found that these two yellow lines are
+characteristic of sodium when turned into vapour by great heat. In the
+same manner it can be ascertained that every element, when heated to a
+condition of vapour, gives as its spectrum a set of lines peculiar to
+itself. Thus the spectroscope enables us to find out the composition of
+substances when they are reduced to vapour in the laboratory.
+
+[Illustration: FIG. 9.--The Solar Spectrum.]
+
+In order to increase the power of a spectroscope, it is necessary to
+add to the number of prisms. Each extra prism has the effect of
+lengthening the coloured strip still more, so that lines, which at first
+appeared to be single merely through being crowded together, are
+eventually drawn apart and become separately distinguishable.
+
+On this principle it has gradually been determined that the sun is
+composed of elements similar to those which go to make up our earth.
+Further, the composition of the stars can be ascertained in the same
+manner; and we find them formed on a like pattern, though with certain
+elements in greater or less proportion as the case may be. It is in
+consequence of our thus definitely ascertaining that the stars are
+self-luminous, and of a sun-like character, that we are enabled to speak
+of them as _suns_, or to call the sun a _star_.
+
+In endeavouring to discover the elements of which the planets and
+satellites of our system are composed, we, however, find ourselves
+baffled, for the simple reason that these bodies emit no real light of
+their own. The light which reaches us from them, being merely reflected
+sunlight, gives only the ordinary solar spectrum when examined with the
+spectroscope. But in certain cases we find that the solar spectrum thus
+viewed shows traces of being weakened, or rather of suffering
+absorption; and it is concluded that this may be due to the sunlight
+having had to pass through an atmosphere on its way to and from the
+surface of the planet from which it is reflected to us.
+
+Since the sun is found to be composed of elements similar to those which
+go to make up our earth, we need not be disheartened at this failure of
+the spectroscope to inform us of the composition of the planets and
+satellites. We are justified, indeed, in assuming that more or less the
+same constituents run through our solar system; and that the elements of
+which these bodies are composed are similar to those which are found
+upon our earth and in the sun.
+
+The spectroscope supplies us with even more information. It tells us,
+indeed, whether the sun-like body which we are observing is moving away
+from us or towards us. A certain slight shifting of the lines towards
+the red or violet end of the spectrum respectively, is found to follow
+such movement. This method of observation is known by the name of
+_Doppler's Method_,[9] and by it we are enabled to confirm the evidence
+which the sunspots give us of the rotation of the sun; for we find thus
+that one edge of that body is continually approaching us, and the other
+edge is continually receding from us. Also, we can ascertain in the same
+manner that certain of the stars are moving towards us, and certain of
+them away from us.
+
+
+[9] The idea, initiated by Christian Doppler at Prague in 1842, was
+originally applied to sound. The approach or recession of a source from
+which sound is coming is invariably accompanied by alterations of pitch,
+as the reader has no doubt noticed when a whistling railway-engine has
+approached him or receded from him. It is to Sir William Huggins,
+however, that we are indebted for the application of the principle to
+spectroscopy. This he gave experimental proof of in the year 1868.
+
+
+
+
+CHAPTER XII
+
+THE SUN
+
+
+The sun is the chief member of our system. It controls the motions of
+the planets by its immense gravitative power. Besides this it is the
+most important body in the entire universe, so far as we are concerned;
+for it pours out continually that flood of light and heat, without which
+life, as we know it, would quickly become extinct upon our globe.
+
+Light and heat, though not precisely the same thing, may be regarded,
+however, as next-door neighbours. The light rays are those which
+directly affect the eye and are comprised in the visible spectrum. We
+_feel_ the heat rays, the chief of which are beyond the red portion of
+the spectrum. They may be investigated with the _bolometer_, an
+instrument invented by the late Professor Langley. Chemical rays--for
+instance, those radiations which affect the photographic plate--are for
+the most part also outside the visible spectrum. They are, however, at
+the other end of it, namely, beyond the violet.
+
+Such a scale of radiations may be compared to the keyboard of an
+imaginary piano, the sound from only one of whose octaves is audible to
+us.
+
+The brightest light we know on the earth is dull compared with the light
+of the sun. It would, indeed, look quite dark if held up against it.
+
+It is extremely difficult to arrive at a precise notion of the
+temperature of the body of the sun. However, it is far in excess of any
+temperature which we can obtain here, even in the most powerful electric
+furnace.
+
+A rough idea of the solar heat may be gathered from the calculation that
+if the sun's surface were coated all over with a layer of ice 4000 feet
+thick, it would melt through this completely in one hour.
+
+The sun cannot be a hot body merely cooling; for the rate at which it is
+at present giving off heat could not in such circumstances be kept up,
+according to Professor Moulton, for more than 3000 years. Further, it is
+not a mere burning mass, like a coal fire, for instance; as in that case
+about a thousand years would show a certain drop in temperature. No
+perceptible diminution of solar heat having taken place within historic
+experience, so far as can be ascertained, we are driven to seek some
+more abstruse explanation.
+
+The theory which seems to have received most acceptance is that put
+forward by Helmholtz in 1854. His idea was that gravitation produces
+continual contraction, or falling in of the outer parts of the sun; and
+that this falling in, in its turn, generates enough heat to compensate
+for what is being given off. The calculations of Helmholtz showed that a
+contraction of about 100 feet a year from the surface towards the centre
+would suffice for the purpose. In recent years, however, this estimate
+has been extended to about 180 feet. Nevertheless, even with this
+increased figure, the shrinkage required is so slight in comparison with
+the immense girth of the sun, that it would take a continual
+contraction at this rate for about 6000 years, to show even in our
+finest telescopes that any change in the size of that body was taking
+place at all. Upon this assumption of continuous contraction, a time
+should, however, eventually be reached when the sun will have shrunk to
+such a degree of solidity, that it will not be able to shrink any
+further. Then, the loss of heat not being made up for any longer, the
+body of the sun should begin to grow cold. But we need not be distressed
+on this account; for it will take some 10,000,000 years, according to
+the above theory, before the solar orb becomes too cold to support life
+upon our earth.
+
+Since the discovery of radium it has, on the other hand, been suggested,
+and not unreasonably, that radio-active matter may possibly play an
+important part in keeping up the heat of the sun. But the body of
+scientific opinion appears to consider the theory of contraction as a
+result of gravitation, which has been outlined above, to be of itself
+quite a sound explanation. Indeed, the late Lord Kelvin is said to have
+held to the last that it was amply sufficient to account for the
+underground heat of the earth, the heat of the sun, and that of all the
+stars in the universe.
+
+One great difficulty in forming theories with regard to the sun, is the
+fact that the temperature and gravitation there are enormously in excess
+of anything we meet with upon our earth. The force of gravity at the
+sun's surface is, indeed, about twenty-seven times that at the surface
+of our globe.
+
+The earth's atmosphere appears to absorb about one-half of the
+radiations which come to us from the sun. This absorptive effect is very
+noticeable when the solar orb is low down in our sky, for its light and
+heat are then clearly much reduced. Of the light rays, the blue ones are
+the most easily absorbed in this way; which explains why the sun looks
+red when near the horizon. It has then, of course, to shine through a
+much greater thickness of atmosphere than when high up in the heavens.
+
+What astonishes one most about the solar radiation, is the immense
+amount of it that is apparently wasted into space in comparison with
+what falls directly upon the bodies of the solar system. Only about the
+one-hundred-millionth is caught by all the planets together. What
+becomes of the rest we cannot tell.
+
+That brilliant white body of the sun, which we see, is enveloped by
+several layers of gases and vaporous matter, in the same manner as our
+globe is enveloped by its atmosphere (see Fig. 10, p. 131). These are
+transparent, just as our atmosphere is transparent; and so we see the
+white bright body of the sun right through them.
+
+This white bright portion is called the _Photosphere_. From it comes
+most of that light and heat which we see and feel. We do not know what
+lies under the photosphere, but, no doubt, the more solid portions of
+the sun are there situated. Just above the photosphere, and lying close
+upon it, is a veil of smoke-like haze.
+
+Next upon this is what is known as the _Reversing Layer_, which is
+between 500 and 1000 miles in thickness. It is cooler than the
+underlying photosphere, and is composed of glowing gases. Many of the
+elements which go to make up our earth are present in the reversing
+layer in the form of vapour.
+
+The _Chromosphere_, of which especial mention has already been made in
+dealing with eclipses of the sun, is another layer lying immediately
+upon the last one. It is between 5000 and 10,000 miles in thickness.
+Like the reversing layer, it is composed of glowing gases, chief among
+which is the vapour of hydrogen. The colour of the chromosphere is, in
+reality, a brilliant scarlet; but, as we have already said, the
+intensely white light of the photosphere shines through it from behind,
+and entirely overpowers its redness. The upper portion of the
+chromosphere is in violent agitation, like the waves of a stormy sea,
+and from it rise those red prominences which, it will be recollected,
+are such a notable feature in total solar eclipses.
+
+[Illustration: FIG. 10.--A section through the Sun, showing how the
+prominences rise from the chromosphere.]
+
+The _Corona_ lies next in order outside the chromosphere, and is, so
+far as we know, the outermost of the accompaniments of the sun. This
+halo of pearly-white light is irregular in outline, and fades away into
+the surrounding sky. It extends outwards from the sun to several
+millions of miles. As has been stated, we can never see the corona
+unless, when during a total solar eclipse, the moon has, for the time
+being, hidden the brilliant photosphere completely from our view.
+
+The solar spectrum is really composed of three separate spectra
+commingled, _i.e._ those of the photosphere, of the reversing layer, and
+of the chromosphere respectively.
+
+If, therefore, the photosphere could be entirely removed, or covered up,
+we should see only the spectra of those layers which lie upon it. Such a
+state of things actually occurs in a total eclipse of the sun. When the
+moon's body has crept across the solar disc, and hidden the last piece
+of photosphere, the solar spectrum suddenly becomes what is technically
+called "reversed,"--the dark lines crossing it changing into bright
+lines. This occurs because a strip of those layers which lie immediately
+upon the photosphere remains still uncovered. The lower of these layers
+has therefore been called the "reversing layer," for want of a better
+name. After a second or two this reversed spectrum mostly vanishes, and
+an altered spectrum is left to view. Taking into consideration the rate
+at which the moon is moving across the face of the sun, and the very
+short time during which the spectrum of the reversing layer lasts, the
+thickness of that layer is estimated to be not more than a few hundred
+miles. In the same way the last of the three spectra--namely, that of
+the chromosphere--remains visible for such a time as allows us to
+estimate its depth at about ten times that of the reversing layer, or
+several thousand miles.
+
+When the chromosphere, in its turn during a total eclipse, has been
+covered by the moon, the corona alone is left. This has a distinct
+spectrum of its own also; wherein is seen a strange line in the green
+portion, which does not tally with that of any element we are acquainted
+with upon the earth. This unknown element has received for the time
+being the name of "Coronium."
+
+
+
+
+CHAPTER XIII
+
+THE SUN--_continued_
+
+
+The various parts of the Sun will now be treated of in detail.
+
+
+I. PHOTOSPHERE.
+
+The photosphere, or "light-sphere," from the Greek [phos] (_phos_),
+which means _light_, is, as we have already said, the innermost portion
+of the sun which can be seen. Examined through a good telescope it shows
+a finely mottled structure, as of brilliant granules, somewhat like rice
+grains, with small dark spaces lying in between them. It has been
+supposed that we have here the process of some system of circulation by
+which the sun keeps sending forth its radiations. In the bright granules
+we perhaps see masses of intensely heated matter, rising from the
+interior of the sun. The dark interspaces may represent matter which has
+become cooled and darkened through having parted with its heat and
+light, and is falling back again into the solar furnace.
+
+The _sun spots_, so familiar to every one nowadays, are dark patches
+which are often seen to break out in the photosphere (see Plate V., p.
+134). They last during various periods of time; sometimes only for a few
+days, sometimes so long as a month or more. A spot is usually composed
+of a dark central portion called the _umbra_, and a less dark fringe
+around this called the _penumbra_ (see Plate VI., p. 136). The umbra
+ordinarily has the appearance of a deep hole in the photosphere; but,
+that it is a hole at all, has by no means been definitely proved.
+
+[Illustration: PLATE V. THE SUN, SHOWING SEVERAL GROUPS OF SPOTS
+
+From a photograph taken at the Royal Observatory, Greenwich. The
+cross-lines seen on the disc are in no way connected with the Sun, but
+belong to the telescope through which the photograph was taken.
+
+(Page 134)]
+
+Sun spots are, as a rule, some thousands of miles across. The umbra of
+a good-sized spot could indeed engulf at once many bodies the size of
+our earth.
+
+Sun spots do not usually appear singly, but in groups. The total area of
+a group of this kind may be of immense extent; even so great as to cover
+the one-hundredth part of the whole surface of the sun. Very large
+spots, when such are present, may be seen without any telescope; either
+through a piece of smoked glass, or merely with the naked eye when the
+air is misty, or the sun low on the horizon.
+
+The umbra of a spot is not actually dark. It only appears so in contrast
+with the brilliant photosphere around.
+
+Spots form, grow to a large size in comparatively short periods of time,
+and then quickly disappear. They seem to shrink away as a consequence of
+the photosphere closing in upon them.
+
+That the sun is rotating upon an axis, is shown by the continual change
+of position of all spots in one constant direction across his disc. The
+time in which a spot is carried completely round depends, however, upon
+the position which it occupies upon the sun's surface. A spot situated
+near the equator of the sun goes round once in about twenty-five days.
+The further a spot is situated from this equator, the longer it takes.
+About twenty-seven days is the time taken by a spot situated midway
+between the equator and the solar poles. Spots occur to the north of
+the sun's equator, as well as to the south; though, since regular
+observations have been made--that is to say, during the past fifty years
+or so--they appear to have broken out a little more frequently in the
+southern parts.
+
+From these considerations it will be seen that the sun does not rotate
+as the earth does, but that different portions appear to move at
+different speeds. Whether in the neighbourhood of the solar poles the
+time of rotation exceeds twenty-seven days we are unable to ascertain,
+for spots are not seen in those regions. No explanation has yet been
+given of this peculiar rotation; and the most we can say on the subject
+is that the sun is not by any means a solid body.
+
+_Faculae_ (Latin, little torches) are brilliant patches which appear here
+and there upon the sun's surface, and are in some way associated with
+spots. Their displacement, too, across the solar face confirms the
+evidence which the spots give us of the sun's rotation.
+
+Our proofs of this rotation are still further strengthened by the
+Doppler spectroscopic method of observation alluded to in Chapter XI. As
+was then stated, one edge of the sun is thus found to be continually
+approaching us, and the other side continually receding from us. The
+varying rates of rotation, which the spots and faculae give us, are duly
+confirmed by this method.
+
+[Illustration: PLATE VI. PHOTOGRAPH OF A SUNSPOT
+
+This fine picture was taken by the late M. Janssen. The granular
+structure of the Sun's surface is here well represented. (From
+_Knowledge_.)
+
+(Page 135)]
+
+The first attempt to bring some regularity into the question of
+sunspots was the discovery by Schwabe, in 1852, that they were subject
+to a regular variation. As a matter of fact they wax and wane in their
+number, and the total area which they cover, in the course of a period,
+or cycle, of on an average about 11-1/4 years; being at one part of this
+period large and abundant, and at another few and small. This period of
+11-1/4 years is known as the sun spot cycle. No explanation has yet been
+given of the curious round of change, but the period in question seems
+to govern most of the phenomena connected with the sun.
+
+
+II. REVERSING LAYER.
+
+This is a layer of relatively cool gases lying immediately upon the
+photosphere. We never see it directly; and the only proof we have of its
+presence is that remarkable reversal of the spectrum already described,
+when during an instant or two in a total eclipse, the advancing edge of
+the moon, having just hidden the brilliant photosphere, is moving across
+the fine strip which the layer then presents edgewise towards us. The
+fleeting moments during which this reversed spectrum lasts, informs us
+that the layer is comparatively shallow; little more indeed than about
+500 miles in depth.
+
+The spectrum of the reversing layer, or "flash spectrum," as it is
+sometimes called on account of the instantaneous character with which
+the change takes place, was, as we have seen, first noticed by Young in
+1870; and has been successfully photographed since then during several
+eclipses. The layer itself appears to be in a fairly quiescent state; a
+marked contrast to the seething photosphere beneath, and the agitated
+chromosphere above.
+
+
+III. THE CHROMOSPHERE.
+
+The Chromosphere--so called from the Greek [chroma] (_chroma_), which
+signifies _colour_--is a layer of gases lying immediately upon the
+preceding one. Its thickness is, however, plainly much the greater of
+the two; for whereas the reversing layer is only revealed to us
+_indirectly_ by the spectroscope, a portion of the chromosphere may
+clearly be _seen_ in a total eclipse in the form of a strip of scarlet
+light. The time which the moon's edge takes to traverse it tells us that
+it must be about ten times as deep as the reversing layer, namely, from
+5000 to 10,000 miles in depth. Its spectrum shows that it is composed
+chiefly of hydrogen, calcium and helium, in the state of vapour. Its red
+colour is mainly due to glowing hydrogen. The element helium, which it
+also contains, has received its appellation from [helios] (_helios_),
+the Greek name for the sun; because, at the time when it first attracted
+attention, there appeared to be no element corresponding to it upon our
+earth, and it was consequently imagined to be confined to the sun alone.
+Sir William Ramsay, however, discovered it to be also a terrestrial
+element in 1895, and since then it has come into much prominence as one
+of the products given off by radium.
+
+Taking into consideration the excessive force of gravity on the sun, one
+would expect to find the chromosphere and reversing layer growing
+gradually thicker in the direction of the photosphere. This, however, is
+not the case. Both these layers are strangely enough of the same
+densities all through; which makes it suspected that, in these regions,
+the force of gravity may be counteracted by some other force or forces,
+exerting a powerful pressure outwards from the sun.
+
+
+IV. THE PROMINENCES.
+
+We have already seen, in dealing with total eclipses, that the exterior
+surface of the chromosphere is agitated like a stormy sea, and from it
+billows of flame are tossed up to gigantic heights. These flaming jets
+are known under the name of prominences, because they were first noticed
+in the form of brilliant points projecting from behind the rim of the
+moon when the sun was totally eclipsed. Prominences are of two kinds,
+_eruptive_ and _quiescent_. The eruptive prominences spurt up directly
+from the chromosphere with immense speeds, and change their shape with
+great rapidity. Quiescent prominences, on the other hand, have a form
+somewhat like trees, and alter their shape but slowly. In the eruptive
+prominences glowing masses of gas are shot up to altitudes sometimes as
+high as 300,000 miles,[10] with velocities even so great as from 500 to
+600 miles a second. It has been noticed that the eruptive prominences
+are mostly found in those portions of the sun where spots usually
+appear, namely, in the regions near the solar equator. The quiescent
+prominences, on the other hand, are confined, as a rule, to the
+neighbourhood of the sun's poles.
+
+Prominences were at first never visible except during total eclipses of
+the sun. But in the year 1868, as we have already seen, a method of
+employing the spectroscope was devised, by means of which they could be
+observed and studied at any time, without the necessity of waiting for
+an eclipse.
+
+A still further development of the spectroscope, the
+_Spectroheliograph_, an instrument invented almost simultaneously by
+Professor Hale and the French astronomer, M. Deslandres, permits of
+photographs being taken of the sun, with the light emanating from _only
+one_ of its glowing gases at a time. For instance, we can thus obtain a
+record of what the glowing hydrogen alone is doing on the solar body at
+any particular moment. With this instrument it is also possible to
+obtain a series of photographs, showing what is taking place upon the
+sun at various levels. This is very useful in connection with the study
+of the spots; for we are, in consequence, enabled to gather more
+evidence on the subject of their actual form than is given us by their
+highly foreshortened appearances when observed directly in the
+telescope.
+
+
+V. CORONA. (Latin, _a Crown_.)
+
+This marvellous halo of pearly-white light, which displays itself to our
+view only during the total phase of an eclipse of the sun, is by no
+means a layer like those other envelopments of the sun of which we have
+just been treating. It appears, on the other hand, to be composed of
+filmy matter, radiating outwards in every direction, and fading away
+gradually into space. Its structure is noted to bear a strong
+resemblance to the tails of comets, or the streamers of the aurora
+borealis.
+
+Our knowledge concerning the corona has, however, advanced very slowly.
+We have not, so far, been as fortunate with regard to it as with regard
+to the prominences; and, for all we can gather concerning it, we are
+still entirely dependent upon the changes and chances of total solar
+eclipses. All attempts, in fact, to apply the spectroscopic method, so
+as to observe the corona at leisure in full sunlight in the way in which
+the prominences can be observed, have up to the present met with
+failure.
+
+The general form under which the corona appears to our eyes varies
+markedly at different eclipses. Sometimes its streamers are many, and
+radiate all round; at other times they are confined only to the middle
+portions of the sun, and are very elongated, with short feathery-looking
+wisps adorning the solar poles. It is noticed that this change of shape
+varies in close accordance with that 11-1/4 year period during which the
+sun spots wax and wane; the many-streamered regular type corresponding
+to the time of great sunspot activity, while the irregular type with the
+long streamers is present only when the spots are few (see Plate VII.,
+p. 142). Streamers have often been noted to issue from those regions of
+the sun where active prominences are at the moment in existence; but it
+cannot be laid down that this is always the case.
+
+No hypothesis has yet been formulated which will account for the
+structure of the corona, or for its variation in shape. The great
+difficulty with regard to theorising upon this subject, is the fact
+that we see so much of the corona under conditions of marked
+foreshortening. Assuming, what indeed seems natural, that the rays of
+which it is composed issue in every direction from the solar body, in a
+manner which may be roughly imitated by sticking pins all over a ball;
+it is plainly impossible to form any definite idea concerning streamers,
+which actually may owe most of the shape they present to us, to the
+mixing up of multitudes of rays at all kinds of angles to the line of
+sight. In a word, we have to try and form an opinion concerning an
+arrangement which, broadly speaking, is _spherical_, but which, on
+account of its distance, must needs appear to us as absolutely _flat_.
+
+The most known about the composition of the corona is that it is made up
+of particles of matter, mingled with a glowing gas. It is an element in
+the composition of this gas which, as has been stated, is not found to
+tally with any known terrestrial element, and has, therefore, received
+the name of coronium for want of a better designation.
+
+One definite conclusion appears to be reached with regard to the corona,
+_i.e._ that the matter of which it is composed, must be exceedingly
+rarefied; as it is not found, for instance, to retard appreciably the
+speed of comets, on occasions when these bodies pass very close to the
+sun. A calculation has indeed been made which would tend to show that
+the particles composing the coronal matter, are separated from each
+other by a distance of perhaps between two and three yards! The density
+of the corona is found not to increase inwards towards the sun. This is
+what has already been noted with regard to the layers lying beneath it.
+Powerful forces, acting in opposition to gravity, must hold sway here
+also.
+
+[Illustration: (A.) THE TOTAL ECLIPSE OF THE SUN OF DECEMBER 22ND, 1870
+
+Drawn by Mr. W.H. Wesley from a photograph taken at Syracuse by Mr.
+Brothers. This is the type of corona seen at the time of _greatest_
+sunspot activity. The coronas of 1882 (Plate I., p. 96) and of 1905
+(Frontispiece) are of the same type.
+
+(B.) THE TOTAL ECLIPSE OF THE SUN OF MAY 28TH, 1900
+
+Drawn by Mr. W.H. Wesley from photographs taken by Mr. E.W. Maunder.
+This is the type of corona seen when the sunspots are _least_ active.
+Compare the "Ring with Wings," Fig. 7, p. 87.
+
+PLATE VII. FORMS OF THE SOLAR CORONA AT THE EPOCHS OF SUNSPOT MAXIMUM
+AND SUNSPOT MINIMUM, RESPECTIVELY
+
+(Page 141)]
+
+The 11-1/4 year period, during which the sun spots vary in number and
+size, appears to govern the activities of the sun much in the same way
+that our year does the changing seasonal conditions of our earth. Not
+only, as we have seen, does the corona vary its shape in accordance with
+the said period, but the activity of the prominences, and of the faculae,
+follow suit. Further, this constant round of ebb and flow is not
+confined to the sun itself, but, strangely enough, affects the earth
+also. The displays of the aurora borealis, which we experience here,
+coincide closely with it, as does also the varying state of the earth's
+magnetism. The connection may be still better appreciated when a great
+spot, or group of spots, has made its appearance upon the sun. It has,
+for example, often been noted that when the solar rotation carries a
+spot, or group of spots, across the middle of the visible surface of the
+sun, our magnetic and electrical arrangements are disturbed for the time
+being. The magnetic needles in our observatories are, for instance, seen
+to oscillate violently, telegraphic communication is for a while upset,
+and magnificent displays of the aurora borealis illumine our night
+skies. Mr. E.W. Maunder, of Greenwich Observatory, who has made a very
+careful investigation of this subject, suspects that, when elongated
+coronal streamers are whirled round in our direction by the solar
+rotation, powerful magnetic impulses may be projected upon us at the
+moments when such streamers are pointing towards the earth.
+
+Some interesting investigations with regard to sunspots have recently
+been published by Mrs. E.W. Maunder. In an able paper, communicated to
+the Royal Astronomical Society on May 10, 1907, she reviews the
+Greenwich Observatory statistics dealing with the number and extent of
+the spots which have appeared during the period from 1889 to 1901--a
+whole sunspot cycle. From a detailed study of the dates in question, she
+finds that the number of those spots which are formed on the side of the
+sun turned away from us, and die out upon the side turned towards us, is
+much greater than the number of those which are formed on the side
+turned towards us and die out upon the side turned away. It used, for
+instance, to be considered that the influence of a planet might
+_produce_ sunspots; but these investigations make it look rather as if
+some influence on the part of the earth tends, on the contrary, to
+_extinguish_ them. Mrs. Maunder, so far, prefers to call the influence
+thus traced an _apparent_ influence only, for, as she very fairly points
+out, it seems difficult to attribute a real influence in this matter to
+the earth, which is so small a thing in comparison not only with the
+sun, but even with many individual spots.
+
+The above investigation was to a certain degree anticipated by Mr. Henry
+Corder in 1895; but Mrs. Maunder's researches cover a much longer
+period, and the conclusions deduced are of a wider and more defined
+nature.
+
+With regard to its chemical composition, the spectroscope shows us that
+thirty-nine of the elements which are found upon our earth are also to
+be found in the sun. Of these the best known are hydrogen, oxygen,
+helium, carbon, calcium, aluminium, iron, copper, zinc, silver, tin, and
+lead. Some elements of the metallic order have, however, not been found
+there, as, for instance, gold and mercury; while a few of the other
+class of element, such as nitrogen, chlorine, and sulphur, are also
+absent. It must not, indeed, be concluded that the elements apparently
+missing do not exist at all in the solar body. Gold and mercury have, in
+consequence of their great atomic weight, perhaps sunk away into the
+centre. Again, the fact that we cannot find traces of certain other
+elements, is no real proof of their entire absence. Some of them may,
+for instance, be resolved into even simpler forms, under the unusual
+conditions which exist in the sun; and so we are unable to trace them
+with the spectroscope, the experience of which rests on laboratory
+experiments conducted, at best, in conditions which obtain upon the
+earth.
+
+
+[10] On November 15, 1907, Dr. A. Rambaut, Radcliffe Observer at Oxford
+University, noted a prominence which rose to a height of 324,600 miles.
+
+
+
+
+CHAPTER XIV
+
+THE INFERIOR PLANETS
+
+
+Starting from the centre of the solar system, the first body we meet
+with is the planet Mercury. It circulates at an average distance from
+the sun of about thirty-six millions of miles. The next body to it is
+the planet Venus, at about sixty-seven millions of miles, namely, about
+double the distance of Mercury from the sun. Since our earth comes next
+again, astronomers call those planets which circulate within its orbit,
+_i.e._ Mercury and Venus, the Inferior Planets, while those which
+circulate outside it they call the Superior Planets.[11]
+
+In studying the inferior planets, the circumstances in which we make our
+observations are so very similar with regard to each, that it is best to
+take them together. Let us begin by considering the various positions of
+an inferior planet, as seen from the earth, during the course of its
+journeys round the sun. When furthest from us it is at the other side of
+the sun, and cannot then be seen owing to the blaze of light. As it
+continues its journey it passes to the left of the sun, and is then
+sufficiently away from the glare to be plainly seen. It next draws in
+again towards the sun, and is once more lost to view in the blaze at
+the time of its passing nearest to us. Then it gradually comes out to
+view on the right hand, separates from the sun up to a certain distance
+as before, and again recedes beyond the sun, and is for the time being
+once more lost to view.
+
+To these various positions technical names are given. When the inferior
+planet is on the far side of the sun from us, it is said to be in
+_Superior Conjunction_. When it has drawn as far as it can to the left
+hand, and is then as east as possible of the sun, it is said to be at
+its _Greatest Eastern Elongation_. Again, when it is passing nearest to
+us, it is said to be in _Inferior Conjunction_; and, finally, when it
+has drawn as far as it can to the right hand, it is spoken of as being
+at its _Greatest Western Elongation_ (see Fig. 11, p. 148).
+
+The continual variation in the distance of an interior planet from us,
+during its revolution around the sun, will of course be productive of
+great alterations in its apparent size. At superior conjunction it
+ought, being then farthest away, to show the smallest disc; while at
+inferior conjunction, being the nearest, it should look much larger.
+When at greatest elongation, whether eastern or western, it should
+naturally present an appearance midway in size between the two.
+
+[Illustration: Various positions, and illumination by the Sun, of an
+Inferior Planet in the course of its orbit.
+
+Corresponding views of the same situations of an Inferior Planet as seen
+from the Earth, showing consequent phases and alterations in apparent
+size.
+
+FIG. 11.--Orbit and Phases of an Inferior Planet.]
+
+From the above considerations one would be inclined to assume that the
+best time for studying the surface of an interior planet with the
+telescope is when it is at inferior conjunction, or, nearest to us. But
+that this is not the case will at once appear if we consider that the
+sunlight is then falling upon the side away from us, leaving the side
+which is towards us unillumined. In superior conjunction, on the other
+hand, the light falls full upon the side of the planet facing us; but
+the disc is then so small-looking, and our view besides is so dazzled by
+the proximity of the sun, that observations are of little avail. In the
+elongations, however, the sunlight comes from the side, and so we see
+one half of the planet lit up; the right half at eastern elongation, and
+the left half at western elongation. Piecing together the results given
+us at these more favourable views, we are enabled, bit by bit, to gather
+some small knowledge concerning the surface of an inferior planet.
+
+From these considerations it will be seen at once that the inferior
+planets show various phases comparable to the waxing and waning of our
+moon in its monthly round. Superior conjunction is, in fact, similar to
+full moon, and inferior conjunction to new moon; while the eastern and
+western elongations may be compared respectively to the moon's first and
+last quarters. It will be recollected how, when these phases were first
+seen by the early telescopic observers, the Copernican theory was felt
+to be immensely strengthened; for it had been pointed out that if this
+system were the correct one, the planets Venus and Mercury, were it
+possible to see them more distinctly, would of necessity present phases
+like these when viewed from the earth. It should here be noted that the
+telescope was not invented until nearly seventy years after the death of
+Copernicus.
+
+The apparent swing of an inferior planet from side to side of the sun,
+at one time on the east side, then passing into and lost in the sun's
+rays to appear once more on the west side, is the explanation of what is
+meant when we speak of an _evening_ or a _morning star_. An inferior
+planet is called an evening star when it is at its eastern elongation,
+that is to say, on the left-hand of the sun; for, being then on the
+eastern side, it will set after the sun sets, as both sink in their turn
+below the western horizon at the close of day. Similarly, when such a
+planet is at its western elongation, that is to say, to the right-hand
+of the sun, it will go in advance of him, and so will rise above the
+eastern horizon before the sun rises, receiving therefore the
+designation of morning star. In very early times, however, before any
+definite ideas had been come to with regard to the celestial motions, it
+was generally believed that the morning and evening stars were quite
+distinct bodies. Thus Venus, when a morning star, was known to the
+ancients under the name of Phosphorus, or Lucifer; whereas they called
+it Hesperus when it was an evening star.
+
+Since an inferior planet circulates between us and the sun, one would be
+inclined to expect that such a body, each time it passed on the side
+nearest to the earth, should be seen as a black spot against the bright
+solar disc. Now this would most certainly be the case were the orbit of
+an inferior planet in the same plane with the orbit of the earth. But we
+have already seen how the orbits in the solar system, whether those of
+planets or of satellites, are by no means in the one plane; and that it
+is for this very reason that the moon is able to pass time after time in
+the direction of the sun, at the epoch known as new moon, and yet not to
+eclipse him save after the lapse of several such passages. Transits,
+then, as the passages of an inferior planet across the sun's disc are
+called, take place, for the same reason, only after certain regular
+lapses of time; and, as regards the circumstances of their occurrence,
+are on a par with eclipses of the sun. The latter, however, happen much
+more frequently, because the moon passes in the neighbourhood of the
+sun, roughly speaking, once a month, whereas Venus comes to each
+inferior conjunction at intervals so long apart as a year and a half,
+and Mercury only about every four months. From this it will be further
+gathered that transits of Mercury take place much oftener than transits
+of Venus.
+
+Until recent years _Transits of Venus_ were phenomena of great
+importance to astronomers, for they furnished the best means then
+available of calculating the distance of the sun from the earth. This
+was arrived at through comparing the amount of apparent displacement in
+the planet's path across the solar disc, when the transit was observed
+from widely separated stations on the earth's surface. The last transit
+of Venus took place in 1882, and there will not be another until the
+year 2004.
+
+_Transits of Mercury_, on the other hand, are not of much scientific
+importance. They are of no interest as a popular spectacle; for the
+dimensions of the planet are so small, that it can be seen only with the
+aid of a telescope when it is in the act of crossing the sun's disc. The
+last transit of Mercury took place on November 14, 1907, and there will
+be another on November 6, 1914.
+
+The first person known to have observed a transit of an inferior planet
+was the celebrated French philosopher, Gassendi. This was the transit of
+Mercury which took place on the 7th of December 1631.
+
+The first time a transit of Venus was ever seen, so far as is known, was
+on the 24th of November 1639. The observer was a certain Jeremiah
+Horrox, curate of Hoole, near Preston, in Lancashire. The transit in
+question commenced shortly before sunset, and his observations in
+consequence were limited to only about half-an-hour. Horrox happened to
+have a great friend, one William Crabtree, of Manchester, whom he had
+advised by letter to be on the look out for the phenomenon. The weather
+in Crabtree's neighbourhood was cloudy, with the result that he only got
+a view of the transit for about ten minutes before the sun set.
+
+That this transit was observed at all is due entirely to the remarkable
+ability of Horrox. According to the calculations of the great Kepler, no
+transit could take place that year (1639), as the planet would just pass
+clear of the lower edge of the sun. Horrox, however, not being satisfied
+with this, worked the question out for himself, and came to the
+conclusion that the planet would _actually_ traverse the lower portion
+of the sun's disc. The event, as we have seen, proved him to be quite in
+the right. Horrox is said to have been a veritable prodigy of
+astronomical skill; and had he lived longer would, no doubt, have become
+very famous. Unfortunately he died about two years after his celebrated
+transit, in his _twenty-second_ year only, according to the accounts.
+His friend Crabtree, who was then also a young man, is said to have been
+killed at the battle of Naseby in 1645.
+
+There is an interesting phenomenon in connection with transits which is
+known as the "Black Drop." When an inferior planet has just made its way
+on to the face of the sun, it is usually seen to remain for a short time
+as if attached to the sun's edge by what looks like a dark ligament (see
+Fig. 12, p. 153). This gives to the planet for the time being an
+elongated appearance, something like that of a pear; but when the
+ligament, which all the while keeps getting thinner and thinner, has at
+last broken, the black body of the planet is seen to stand out round
+against the solar disc.
+
+[Illustration: FIG. 12.--The "Black Drop."]
+
+This appearance may be roughly compared to the manner in which a drop of
+liquid (or, preferably, of some glutinous substance) tends for a while
+to adhere to an object from which it is falling.
+
+When the planet is in turn making its way off the face of the sun, the
+ligament is again seen to form and to attach it to the sun's edge before
+its due time.
+
+The phenomenon of the black drop, or ligament, is entirely an illusion,
+and, broadly speaking, of an optical origin. Something very similar will
+be noticed if one brings one's thumb and forefinger _slowly_ together
+against a very bright background.
+
+This peculiar phenomenon has proved one of the greatest drawbacks to the
+proper observation of transits, for it is quite impossible to note the
+exact instant of the planet's entrance upon and departure from the solar
+disc in conditions such as these.
+
+The black drop seems to bear a family resemblance, so to speak, to the
+phenomenon of Baily's beads. In the latter instance the lunar peaks, as
+they approach the sun's edge, appear to lengthen out in a similar manner
+and bridge the intervening space before their time, thus giving
+prominence to an effect which otherwise should scarcely be noticeable.
+
+The last transit of Mercury, which, as has been already stated, took
+place on November 14, 1907, was not successfully observed by astronomers
+in England, on account of the cloudiness of the weather. In France,
+however, Professor Moye, of Montpellier, saw it under good conditions,
+and mentions that the black drop remained very conspicuous for fully a
+minute. The transit was also observed in the United States, the reports
+from which speak of the black drop as very "troublesome."
+
+Before leaving the subject of transits it should be mentioned that it
+was in the capacity of commander of an expedition to Otaheite, in the
+Pacific, to observe the transit of Venus of June 3, 1769, that Captain
+Cook embarked upon the first of his celebrated voyages.
+
+In studying the surfaces of Venus and Mercury with the telescope,
+observers are, needless to say, very much hindered by the proximity of
+the sun. Venus, when at the greatest elongations, certainly draws some
+distance out of the glare; but her surface is, even then, so dazzlingly
+bright, that the markings upon it are difficult to see. Mercury, on the
+other hand, is much duller in contrast, but the disc it shows in the
+telescope is exceedingly small; and, in addition, when that planet is
+left above the horizon for a short time after sunset, as necessarily
+happens after certain intervals, the mists near the earth's surface
+render observation of it very difficult.
+
+Until about twenty-five years ago, it was generally believed that both
+these planets rotated on their axes in about twenty-four hours, a
+notion, no doubt, originally founded upon an unconscious desire to bring
+them into some conformity with our earth. But Schiaparelli, observing in
+Italy, and Percival Lowell, in the clear skies of Arizona and Mexico,
+have lately come to the conclusion that both planets rotate upon their
+axes in the same time as they revolve in their orbits,[12] the result
+being that they turn one face ever towards the sun in the same manner
+that the moon turns one face ever towards the earth--a curious state of
+things, which will be dealt with more fully when we come to treat of our
+satellite.
+
+The marked difference in the brightness between the two planets has
+already been alluded to. The surface of Venus is, indeed, about five
+times as bright as that of Mercury. The actual brightness of Mercury is
+about equivalent to that of our moon, and astronomers are, therefore,
+inclined to think that it may resemble her in having a very rugged
+surface and practically no atmosphere. This probable lack of atmosphere
+is further corroborated by two circumstances. One of these is that when
+Mercury is just about to transit the face of the sun, no ring of
+diffused light is seen to encircle its disc as would be the case if it
+possessed an atmosphere. Such a lack of atmosphere is, indeed, only to
+be expected from what is known as the _Kinetic Theory of Gases_.
+According to this theory, which is based upon the behaviour of various
+kinds of gas, it is found that these elements tend to escape into space
+from the surface of bodies whose force of gravitation is weak. Hydrogen
+gas, for example, tends to fly away from our earth, as any one may see
+for himself when a balloon rises into the air. The gravitation of the
+earth seems, however, powerful enough to hold down other gases, as, for
+instance, those of which the air is chiefly composed, namely, oxygen and
+nitrogen. In due accordance with the Kinetic theory, we find the moon
+and Mercury, which are much about the same size, destitute of
+atmospheres. Mars, too, whose diameter is only about double that of the
+moon, has very little atmosphere. We find, on the other hand, that
+Venus, which is about the same size as our earth, clearly possesses an
+atmosphere, as just before the planet is in transit across the sun, the
+outline of its dark body is seen to be surrounded by a bright ring of
+light.
+
+The results of telescopic observation show that more markings are
+visible on Mercury than on Venus. The intense brilliancy of Venus is,
+indeed, about the same as that of our white clouds when the sun is
+shining directly upon them. It has, therefore, been supposed that the
+planet is thickly enveloped in cloud, and that we do not ever see any
+part of its surface, except perchance the summit of some lofty mountain
+projecting through the fleecy mass.
+
+With regard to the great brilliancy of Venus, it may be mentioned that
+she has frequently been seen in England, with the naked eye in full
+sunshine, when at the time of her greatest brightness. The writer has
+seen her thus at noonday. Needless to say, the sky at the moment was
+intensely blue and clear.
+
+The orbit of Mercury is very oval, and much more so than that of any
+other planet. The consequence is that, when Mercury is nearest to the
+sun, the heat which it receives is twice as great as when it is farthest
+away. The orbit of Venus, on the other hand, is in marked contrast with
+that of Mercury, and is, besides, more nearly of a circular shape than
+that of any of the other planets. Venus, therefore, always keeps about
+the same distance from the sun, and so the heat which she receives
+during the course of her year can only be subject to very slight
+variations.
+
+
+[11] In employing the terms Inferior and Superior the writer bows to
+astronomical custom, though he cannot help feeling that, in the
+circumstances, Interior and Exterior would be much more appropriate.
+
+[12] This question is, however, uncertain, for some very recent
+spectroscopic observations of Venus seem to show a rotation period of
+about twenty-four hours.
+
+
+
+
+CHAPTER XV
+
+THE EARTH
+
+
+We have already seen (in Chapter I.) how, in very early times, men
+naturally enough considered the earth to be a flat plane extending to a
+very great distance in every direction; but that, as years went on,
+certain of the Greek philosophers suspected it to be a sphere. One or
+two of the latter are, indeed, said to have further believed in its
+rotation about an axis, and even in its revolution around the sun; but,
+as the ideas in question were founded upon fancy, rather than upon any
+direct evidence, they did not generally attract attention. The small
+effect, therefore, which these theories had upon astronomy, may well be
+gathered from the fact that in the Ptolemaic system the earth was
+considered as fixed and at the centre of things; and this belief, as we
+have seen, continued unaltered down to the days of Copernicus. It was,
+indeed, quite impossible to be certain of the real shape of the earth or
+the reality of its motions until knowledge became more extended and
+scientific instruments much greater in precision.
+
+We will now consider in detail a few of the more obvious arguments which
+can be put forward to show that our earth is a sphere.
+
+If, for instance, the earth were a plane surface, a ship sailing away
+from us over the sea would appear to grow smaller and smaller as it
+receded into the distance, becoming eventually a tiny speck, and fading
+gradually from our view. This, however, is not at all what actually
+takes place. As we watch a vessel receding, its hull appears bit by bit
+to slip gently down over the horizon, leaving the masts alone visible.
+Then, in their turn, the masts are seen to slip down in the same manner,
+until eventually every trace of the vessel is gone. On the other hand,
+when a ship comes into view, the masts are the first portions to appear.
+They gradually rise up from below the horizon, and the hull follows in
+its turn, until the whole vessel is visible. Again, when one is upon a
+ship at sea, a set of masts will often be seen sticking up alone above
+the horizon, and these may shorten and gradually disappear from view
+without the body of the ship to which they belong becoming visible at
+all. Since one knows from experience that there is no _edge_ at the
+horizon over which a vessel can drop down, the appearance which we have
+been describing can only be explained by supposing that the surface of
+the earth is always curving gradually in every direction.
+
+The distance at which what is known as the _horizon_ lies away from us
+depends entirely upon the height above the earth's surface where we
+happen at the moment to be. A ship which has appeared to sink below the
+horizon for a person standing on the beach, will be found to come back
+again into view if he at once ascends a high hill. Experiment shows that
+the horizon line lies at about three miles away for a person standing at
+the water's edge. The curving of the earth's surface is found, indeed,
+to be at the rate of eight inches in every mile. Now it can be
+ascertained, by calculation, that a body curving at this rate in every
+direction must be a globe about 8000 miles in diameter.
+
+Again, the fact that, if not stopped by such insuperable obstacles as
+the polar ice and snow, those who travel continually in any one
+direction upon the earth's surface always find themselves back again at
+the regions from which they originally set out, is additional ground for
+concluding that the earth is a globe.
+
+We can find still further evidence. For instance, in an eclipse of the
+moon the earth's shadow, when seen creeping across the moon's face, is
+noted to be _always_ circular in shape. One cannot imagine how such a
+thing could take place unless the earth were a sphere.
+
+Also, it is found from observation that the sun, the planets, and the
+satellites are, all of them, round. This roundness cannot be the
+roundness of a flat plate, for instance, for then the objects in
+question would sometimes present their thin sides to our view. It
+happens, also, that upon the discs which these bodies show, we see
+certain markings shifting along continually in one direction, to
+disappear at one side and to reappear again at the other. Such bodies
+must, indeed, be spheres in rotation.
+
+The crescent and other phases, shown by the moon and the inferior
+planets, should further impress the truth of the matter upon us, as such
+appearances can only be caused by the sunlight falling from various
+directions upon the surfaces of spherical bodies.
+
+Another proof, perhaps indeed the weightiest of all, is the continuous
+manner in which the stars overhead give place to others as one travels
+about the surface of the earth. When in northern regions the Pole Star
+and its neighbours--the stars composing the Plough, for instance--are
+over our heads. As one journeys south these gradually sink towards the
+northern horizon, while other stars take their place, and yet others are
+uncovered to view from the south. The regularity with which these
+changes occur shows that every point on the earth's surface faces a
+different direction of the sky, and such an arrangement would only be
+possible if the earth were a sphere. The celebrated Greek philosopher,
+Aristotle, is known to have believed in the globular shape of the earth,
+and it was by this very argument that he had convinced himself that it
+was so.
+
+The idea of the sphericity of the earth does not appear, however, to
+have been generally accepted until the voyages of the great navigators
+showed that it could be sailed round.
+
+The next point we have to consider is the rotation of the earth about
+its axis. From the earliest times men noticed that the sky and
+everything in it appeared to revolve around the earth in one fixed
+direction, namely, towards what is called the West, and that it made one
+complete revolution in the period of time which we know as twenty-four
+hours. The stars were seen to come up, one after another, from below the
+eastern horizon, to mount the sky, and then to sink in turn below the
+western horizon. The sun was seen to perform exactly the same journey,
+and the moon, too, whenever she was visible. One or two of the ancient
+Greek philosophers perceived that this might be explained, either by a
+movement of the entire heavens around the earth, or by a turning motion
+on the part of the earth itself. Of these diverse explanations, that
+which supposed an actual movement of the heavens appealed to them the
+most, for they could hardly conceive that the earth should continually
+rotate and men not be aware of its movement. The question may be
+compared to what we experience when borne along in a railway train. We
+see the telegraph posts and the trees and buildings near the line fly
+past us one after another in the contrary direction. Either these must
+be moving, or we must be moving; and as we happen to _know_ that it is,
+indeed, we who are moving, there can be no question therefore about the
+matter. But it would not be at all so easy to be sure of this movement
+were one unable to see the objects close at hand displacing themselves.
+For instance, if one is shut up in a railway carriage at night with the
+blinds down, there is really nothing to show that one is moving, except
+the jolting of the train. And even then it is hard to be sure in which
+direction one is actually travelling.
+
+The way we are situated upon the earth is therefore as follows. There
+are no other bodies sufficiently near to be seen flying past us in turn;
+our earth spins without a jolt; we and all things around us, including
+the atmosphere itself, are borne along together with precisely the same
+impetus, just as all the objects scattered about a railway carriage
+share in the forward movement of the train. Such being the case, what
+wonder that we are unconscious of the earth's rotation, of which we
+should know nothing at all, were it not for that slow displacement of
+the distant objects in the heavens, as we are borne past them in turn.
+
+If the night sky be watched, it will be soon found that its apparent
+turning movement seems to take place around a certain point, which
+appears as if fixed. This point is known as the north pole of the
+heavens; and a rather bright star, which is situated very close to this
+hub of movement, is in consequence called the Pole Star. For the
+dwellers in southern latitudes there is also a point in their sky which
+appears to remain similarly fixed, and this is known as the south pole
+of the heavens. Since, however, the heavens do not turn round at all,
+but the earth does, it will easily be seen that these apparently
+stationary regions in the sky are really the points towards which the
+axis of the earth is directed. The positions on the earth's surface
+itself, known as the North and South Poles, are merely the places where
+the earth's axis, if there were actually such a thing, would be expected
+to jut out. The north pole of the earth will thus be situated exactly
+beneath the north pole of the heavens, and the south pole of the earth
+exactly beneath the south pole of the heavens.
+
+We have seen that the earth rotates upon its imaginary axis once in
+about every twenty-four hours. This means that everything upon the
+surface of the earth is carried round once during that time. The
+measurement around the earth's equator is about 24,000 miles; and,
+therefore, an object situated at the equator must be carried round
+through a distance of about 24,000 miles in each twenty-four hours.
+Everything at the equator is thus moving along at the rapid rate of
+about 1000 miles an hour, or between sixteen and seventeen times as
+fast as an express train. If, however, one were to take measurements
+around the earth parallel to the equator, one would find these
+measurements becoming less and less, according as the poles were
+approached. It is plain, therefore, that the speed with which any point
+moves, in consequence of the earth's rotation, will be greatest at the
+equator, and less and less in the direction of the poles; while at the
+poles themselves there will be practically no movement, and objects
+there situated will merely turn round.
+
+The considerations above set forth, with regard to the different speeds
+at which different portions of a rotating globe will necessarily be
+moving, is the foundation of an interesting experiment, which gives us
+further evidence of the rotation of our earth. The measurement around
+the earth at any distance below the surface, say, for instance, at the
+depth of a mile, will clearly be less than a similar measurement at the
+surface itself. The speed of a point at the bottom of a mine, which
+results from the actual rotation of the earth, must therefore be less
+than the speed of a point at the surface overhead. This can be
+definitely proved by dropping a heavy object down a mine shaft. The
+object, which starts with the greater speed of the surface, will, when
+it reaches the bottom of the mine, be found, as might be indeed
+expected, to be a little ahead (_i.e._ to the east) of the point which
+originally lay exactly underneath it. The distance by which the object
+gains upon this point is, however, very small. In our latitudes it
+amounts to about an inch in a fall of 500 feet.
+
+The great speed at which, as we have seen, the equatorial regions of
+the earth are moving, should result in giving to the matter there
+situated a certain tendency to fly outwards. Sir Isaac Newton was the
+first to appreciate this point, and he concluded from it that the earth
+must be _bulged_ a little all round the equator. This is, indeed, found
+to be the case, the diameter at the equator being nearly twenty-seven
+miles greater than it is from pole to pole. The reader will, no doubt,
+be here reminded of the familiar comparison in geographies between the
+shape of the earth and that of an orange.
+
+In this connection it is interesting to consider that, were the earth to
+rotate seventeen times as fast as it does (_i.e._ in one hour
+twenty-five minutes, instead of twenty-four hours), bodies at the
+equator would have such a strong tendency to fly outwards that the force
+of terrestrial gravity acting upon them would just be counterpoised, and
+they would virtually have _no weight_. And, further, were the earth to
+rotate a little faster still, objects lying loose upon its surface would
+be shot off into space.
+
+The earth is, therefore, what is technically known as an _oblate
+spheroid_; that is, a body of spherical shape flattened at the poles. It
+follows of course from this, that objects at the polar regions are
+slightly nearer to the earth's centre than objects at the equatorial
+regions. We have already seen that gravitation acts from the central
+parts of a body, and that its force is greater the nearer are those
+central parts. The result of this upon our earth will plainly be that
+objects in the polar regions will be pulled with a slightly stronger
+pull, and will therefore _weigh_ a trifle more than objects in the
+equatorial regions. This is, indeed, found by actual experiment to be
+the case. As an example of the difference in question, Professor Young,
+in his _Manual of Astronomy_, points out that a man who weighs 190
+pounds at the equator would weigh 191 at the pole. In such an experiment
+the weighing would, however, have to be made with a _spring balance_,
+and _not with scales_; for, in the latter case, the "weights" used would
+alter in their weight in exactly the same degree as the objects to be
+weighed.
+
+It used to be thought that the earth was composed of a relatively thin
+crust, with a molten interior. Scientific men now believe, on the other
+hand, that such a condition cannot after all prevail, and that the earth
+must be more or less solid all through, except perhaps in certain
+isolated places where collections of molten matter may exist.
+
+The _atmosphere_, or air which we breathe, is in the form of a layer of
+limited depth which closely envelops the earth. Actually, it is a
+mixture of several gases, the most important being nitrogen and oxygen,
+which between them practically make up the air, for the proportion of
+the other gases, the chief of which is carbonic acid gas, is exceedingly
+small.
+
+It is hard to picture our earth, as we know it, without this atmosphere.
+Deprived of it, men at once would die; but even if they could be made to
+go on living without it by any miraculous means, they would be like unto
+deaf beings, for they would never hear any sound. What we call _sounds_
+are merely vibrations set up in the air, which travel along and strike
+upon the drum of the ear.
+
+The atmosphere is densest near the surface of the earth, and becomes
+less and less dense away from it, as a result of diminishing pressure of
+air from above. The greater portion of it is accumulated within four or
+five miles of the earth's surface.
+
+It is impossible to determine exactly at what distance from the earth's
+surface the air ceases altogether, for it grows continually more and
+more rarefied. There are, however, two distinct methods of ascertaining
+the distance beyond which it can be said practically not to exist. One
+of these methods we get from twilight. Twilight is, in fact, merely
+light reflected to us from those upper regions of the air, which still
+continue to be illuminated by the sun after it has disappeared from our
+view below the horizon. The time during which twilight lasts, shows us
+that the atmosphere must be at least fifty miles high.
+
+But the most satisfactory method of ascertaining the height to which the
+atmosphere extends is from the observation of meteors. It is found that
+these bodies become ignited, by the friction of passing into the
+atmosphere, at a height of about 100 miles above the surface of the
+earth. We thus gather that the atmosphere has a certain degree of
+density even at this height. It may, indeed, extend as far as about 150
+miles.
+
+The layer of atmosphere surrounding our earth acts somewhat in the
+manner of the glass covering of a greenhouse, bottling in the sun's
+rays, and thus storing up their warmth for our benefit. Were this not
+so, the heat which we get from the sun would, after falling upon the
+earth, be quickly radiated again into space.
+
+It is owing to the unsteadiness of the air that stars are seen to
+twinkle. A night when this takes place, though it may please the average
+person, is worse than useless to the astronomer, for the unsteadiness is
+greatly magnified in the telescope. This twinkling is, no doubt, in a
+great measure responsible for the conventional "points" with which Art
+has elected to embellish stars, and which, of course, have no existence
+in fact.
+
+The phenomena of _Refraction_,[13] namely, that bending which rays of
+light undergo, when passing _slant-wise_ from a rare into a dense
+transparent medium, are very marked with regard to the atmosphere. The
+denser the medium into which such rays pass, the greater is this bending
+found to be. Since the layer of air around us becomes denser and denser
+towards the surface of the earth, it will readily be granted that the
+rays of light reaching our eyes from a celestial object, will suffer the
+greater bending the lower the object happens to be in the sky. Celestial
+objects, unless situated directly overhead, are thus not seen in their
+true places, and when nearest to the horizon are most out of place. The
+bending alluded to is upwards. Thus the sun and the moon, for instance,
+when we see them resting upon the horizon, are actually _entirely_
+beneath it.
+
+When the sun, too, is sinking towards the horizon, the lower edge of its
+disc will, for the above reason, look somewhat more raised than the
+upper. The result is a certain appearance of flattening; which may
+plainly be seen by any one who watches the orb at setting.
+
+In observations to determine the exact positions of celestial objects
+correction has to be made for the effects of refraction, according to
+the apparent elevation of these objects in the sky. Such effects are
+least when the objects in question are directly overhead, for then the
+rays of light, coming from them to the eye, enter the atmosphere
+perpendicularly, and not at any slant.
+
+A very curious effect, due to refraction, has occasionally been observed
+during a total eclipse of the moon. To produce an eclipse of this kind,
+_the earth must, of course, lie directly between the sun and the moon_.
+Therefore, when we see the shadow creeping over the moon's surface, the
+sun should actually be well below the horizon. But when a lunar eclipse
+happens to come on just about sunset, the sun, although really sunk
+below the horizon, appears still above it through refraction, and the
+eclipsed moon, situated, of course, exactly opposite to it in the sky,
+is also lifted up above the horizon by the same cause. Pliny, writing in
+the first century of the Christian era, describes an eclipse of this
+kind, and refers to it as a "prodigy." The phenomenon is known as a
+"horizontal eclipse." It was, no doubt, partly owing to it that the
+ancients took so long to decide that an eclipse of the moon was really
+caused by the shadow cast by the earth. Plutarch, indeed, remarks that
+it was easy enough to understand that a solar eclipse was caused by the
+interposition of the moon, but that one could not imagine by the
+interposition _of what body_ the moon itself could be eclipsed.
+
+In that apparent movement of the heavens about the earth, which men now
+know to be caused by the mere rotation of the earth itself, a slight
+change is observed to be continually taking place. The stars, indeed,
+are always found to be gradually drawing westward, _i.e._ towards the
+sun, and losing themselves one after the other in the blaze of his
+light, only to reappear, however, on the other side of him after a
+certain lapse of time. This is equivalent to saying that the sun itself
+seems always creeping slowly _eastward_ in the heaven. The rate at which
+this appears to take place is such that the sun finds itself back again
+to its original position, with regard to the starry background, at the
+end of a year's time. In other words, the sun seems to make a complete
+tour of the heavens in the course of a year. Here, however, we have
+another illusion, just as the daily movement of the sky around the earth
+was an illusion. The truth indeed is, that this apparent movement of the
+sun eastward among the stars during a year, arises merely from a
+_continuous displacement of his position_ caused by an actual motion of
+the earth itself around him in that very time. In a word, it is the
+earth which really moves around the sun, and not the sun around the
+earth.
+
+The stress laid upon this fundamental point by Copernicus, marks the
+separation of the modern from the ancient view. Not that Copernicus,
+indeed, had obtained any real proof that the earth is merely a planet
+revolving around the sun; but it seemed to his profound intellect that a
+movement of this kind on the part of our globe was the more likely
+explanation of the celestial riddle. The idea was not new; for, as we
+have already seen, certain of the ancient Greeks (Aristarchus of Samos,
+for example) had held such a view; but their notions on the subject were
+very fanciful, and unsupported by any good argument.
+
+What Copernicus, however, really seems to have done was to _insist_ upon
+the idea that the sun occupied the _centre_, as being more consonant
+with common sense. No doubt, he was led to take up this position by the
+fact that the sun appeared entirely of a different character from the
+other members of the system. The one body in the scheme, which performed
+the important function of dispenser of light and heat, would indeed be
+more likely to occupy a position apart from the rest; and what position
+more appropriate for its purposes than the centre!
+
+But here Copernicus only partially solved the difficult question. He
+unfortunately still clung to an ancient belief, which as yet remained
+unquestioned; _i.e._ the great virtue, one might almost say, the
+_divineness_, of circular motion. The ancients had been hag-ridden, so
+to speak, by the circle; and it appeared to them that such a perfectly
+formed curve was alone fitted for the celestial motions. Ptolemy
+employed it throughout his system. According to him the "planets" (which
+included, under the ancient view, both the sun and the moon), moved
+around the earth in circles; but, as their changing positions in the sky
+could not be altogether accounted for in this way, it was further
+supposed that they performed additional circular movements, around
+peculiarly placed centres, during the course of their orbital
+revolutions. Thus the Ptolemaic system grew to be extremely
+complicated; for astronomers did not hesitate to add new circular
+movements whenever the celestial positions calculated for the planets
+were found not to tally with the positions observed. In this manner,
+indeed, they succeeded in doctoring the theory, so that it fairly
+satisfied the observations made with the rough instruments of
+pre-telescopic times.
+
+Although Copernicus performed the immense service to astronomy of boldly
+directing general attention to the central position of the sun, he
+unfortunately took over for the new scheme the circular machinery of the
+Ptolemaic system. It therefore remained for the famous Kepler, who lived
+about a century after him, to find the complete solution. Just as
+Copernicus, for instance, had broken free from tradition with regard to
+the place of the sun; so did Kepler, in turn, break free from the spell
+of circular motion, and thus set the coping-stone to the new
+astronomical edifice. This astronomer showed, in fact, that if the paths
+of the planets around the sun, and of the moon around the earth, were
+not circles, but _ellipses_, the movements of these bodies about the sky
+could be correctly accounted for. The extreme simplicity of such an
+arrangement was far more acceptable than the bewildering intricacy of
+movement required by the Ptolemaic theory. The Copernican system, as
+amended by Kepler, therefore carried the day; and was further
+strengthened, as we have already seen, by the telescopic observations of
+Galileo and the researches of Newton into the effects of gravitation.
+
+And here a word on the circle, now fallen from its high estate. The
+ancients were in error in supposing that it stood entirely apart--the
+curve of curves. As a matter of fact it is merely _a special kind of
+ellipse_. To put it paradoxically, it is an ellipse which has no
+ellipticity, an oval without any ovalness!
+
+Notwithstanding all this, astronomy had to wait yet a long time for a
+definite proof of the revolution of the earth around the sun. The
+leading argument advanced by Aristotle, against the reality of any
+movement of the earth, still held good up to about seventy years ago.
+That philosopher had pointed out that the earth could not move about in
+space to any great extent, or the stars would be found to alter their
+apparent places in the sky, a thing which had never been observed to
+happen. Centuries ran on, and instruments became more and more perfect,
+yet no displacements of stars were noted. In accepting the Copernican
+theory men were therefore obliged to suppose these objects as
+immeasurably distant. At length, however, between the years 1835 and
+1840, it was discovered by the Prussian astronomer, Bessel, that a star
+known as 61 Cygni--that is to say, the star marked in celestial atlases
+as No. 61 in the constellation of the Swan--appeared, during the course
+of a year, to perform a tiny circle in the heavens, such as would result
+from a movement on our own part around the sun. Since then about
+forty-three stars have been found to show minute displacements of a
+similar kind, which cannot be accounted for upon any other supposition
+than that of a continuous revolution of the earth around the sun. The
+triumph of the Copernican system is now at last supreme.
+
+If the axis of the earth stood "straight up," so to speak, while the
+earth revolved in its orbit, the sun would plainly keep always on a
+level with the equator. This is equivalent to stating that, in such
+circumstances, a person at the equator would see it rise each morning
+exactly in the east, pass through the _zenith_, that is, the point
+directly overhead of him, at midday, and set in the evening due in the
+west. As this would go on unchangingly at the equator every day
+throughout the year, it should be clear that, at any particular place
+upon the earth, the sun would in these conditions always be seen to move
+in an unvarying manner across the sky at a certain altitude depending
+upon the latitude of the place. Thus the more north one went upon the
+earth's surface, the more southerly in the sky would the sun's path lie;
+while at the north pole itself, the sun would always run round and round
+the horizon. Similarly, the more south one went from the equator the
+more northerly would the path of the sun lie, while at the south pole it
+would be seen to skirt the horizon in the same manner as at the north
+pole. The result of such an arrangement would be, that each place upon
+the earth would always have one unvarying climate; in which case there
+would not exist any of those beneficial changes of season to which we
+owe so much.
+
+The changes of season, which we fortunately experience, are due,
+however, to the fact that the sun does not appear to move across the sky
+each day at one unvarying altitude, but is continually altering the
+position of its path; so that at one period of the year it passes across
+the sky low down, and remains above the horizon for a short time only,
+while at another it moves high up across the heavens, and is above the
+horizon for a much longer time. Actually, the sun seems little by little
+to creep up the sky during one half of the year, namely, from mid-winter
+to mid-summer, and then, just as gradually, to slip down it again during
+the other half, namely, from mid-summer to mid-winter. It will therefore
+be clear that every region of the earth is much more thoroughly warmed
+during one portion of the year than during another, _i.e._ when the
+sun's path is high in the heavens than when it is low down.
+
+Once more we find appearances exactly the contrary from the truth. The
+earth is in this case the real cause of the deception, just as it was in
+the other cases. The sun does not actually creep slowly up the sky, and
+then slowly dip down it again, but, owing to the earth's axis being set
+aslant, different regions of the earth's surface are presented to the
+sun at different times. Thus, in one portion of its orbit, the northerly
+regions of the earth are presented to the sun, and in the other portion
+the southerly. It follows of course from this, that when it is summer in
+the northern hemisphere it is winter in the southern, and _vice versa_
+(see Fig. 13, p. 176).
+
+[Illustration: FIG. 13.--Summer and Winter.]
+
+The fact that, in consequence of this slant of the earth's axis, the sun
+is for part of the year on the north side of the equator and part of the
+year on the south side, leads to a very peculiar result. The path of the
+moon around the earth is nearly on the same plane with the earth's path
+around the sun. The moon, therefore, always keeps to the same regions of
+the sky as the sun. The slant of the earth's axis thus regularly
+displaces the position of both the sun and the moon to the north and
+south sides of the equator respectively in the manner we have been
+describing. Were the earth, however, a perfect sphere, such change of
+position would not produce any effect. We have shown, however, that the
+earth is not a perfect sphere, but that it is bulged out all round the
+equator. The result is that this bulged-out portion swings slowly under
+the pulls of solar and lunar gravitation, in response to the
+displacements of the sun and moon to the north and to the south of it.
+This slow swing of the equatorial regions results, of course, in a
+certain slow change of the direction of the earth's axis, so that the
+north pole does not go on pointing continually to the same region of the
+sky. The change in the direction of the axis is, however, so extremely
+slight, that it shows up only after the lapse of ages. The north pole of
+the heavens, that is, the region of the sky towards which the north pole
+of the earth's axis points, displaces therefore extremely slowly,
+tracing out a wide circle, and arriving back again to the same position
+in the sky only after a period of about 25,000 years. At present the
+north pole of the heavens is quite close to a bright star in the tail of
+the constellation of the Little Bear, which is consequently known as the
+Pole Star; but in early Greek times it was at least ten times as far
+away from this star as it is now. After some 12,000 years the pole will
+point to the constellation of Lyra, and Vega, the most brilliant star in
+that constellation, will then be considered as the pole star. This slow
+twisting of the earth's axis is technically known as _Precession_, or
+the _Precession of the Equinoxes_ (see Plate XIX., p. 292).
+
+The slow displacement of the celestial pole appears to have attracted
+the attention of men in very early times, but it was not until the
+second century B.C. that precession was established as a fact by the
+celebrated Greek astronomer, Hipparchus. For the ancients this strange
+cyclical movement had a mystic significance; and they looked towards the
+end of the period as the end, so to speak, of a "dispensation," after
+which the life of the universe would begin anew:--
+
+"Magnus ab integro saeclorum nascitur ordo.
+Jam redit et Virgo, redeunt Saturnia regna;
+   .      .      .      .      .      .
+Alter erit tum Tiphys, et altera quae vehat Argo
+Delectos heroas; erunt etiam altera bella,
+Atque iterum ad Trojam magnus mittetur Achilles."
+
+We have seen that the orbit of the earth is an ellipse, and that the sun
+is situated at what is called the _focus_, a point not in the middle of
+the ellipse, but rather towards one of its ends. Therefore, during the
+course of the year the distance of the earth from the sun varies. The
+sun, in consequence of this, is about 3,000,000 miles _nearer_ to us in
+our northern _winter_ than it is in our northern summer, a statement
+which sounds somewhat paradoxical. This variation in distance, large as
+it appears in figures, can, however, not be productive of much
+alteration in the amount of solar heat which we receive, for during the
+first week in January, when the distance is least, the sun only looks
+about _one-eighteenth_ broader than at the commencement of July, when
+the distance is greatest. The great disparity in temperature between
+winter and summer depends, as we have seen, upon causes of quite another
+kind, and varies between such wide limits that the effects of this
+slight alteration in the distance of the sun from the earth may be
+neglected for practical purposes.
+
+The Tides are caused by the gravitational pull of the sun and moon upon
+the water of the earth's surface. Of the two, the moon, being so much
+the nearer, exerts the stronger pull, and therefore may be regarded as
+the chief cause of the tides. This pull always draws that portion of the
+water, which happens to be right underneath the moon at the time, into a
+heap; and there is also a _second_ heaping of water at the same moment
+_at the contrary side of the earth_, the reasons for which can be shown
+mathematically, but cannot be conveniently dealt with here.
+
+As the earth rotates on its axis each portion of its surface passes
+beneath the moon, and is swelled up by this pull; the watery portions
+being, however, the only ones to yield visibly. A similar swelling up,
+as we have seen, takes place at the point exactly away from the moon.
+Thus each portion of our globe is borne by the rotation through two
+"tide-areas" every day, and this is the reason why there are two tides
+during every twenty-four hours.
+
+The crest of the watery swelling is known as high tide. The journey of
+the moon around the earth takes about a month, and this brings her past
+each place in turn by about fifty minutes later each day, which is the
+reason why high tide is usually about twenty-five minutes later each
+time.
+
+The moon is, however, not the sole cause of the tides, but the sun, as
+we have said, has a part in the matter also. When it is new moon the
+gravitational attractions of both sun and moon are clearly acting
+together from precisely the same direction, and, therefore, the tide
+will be pulled up higher than at other times. At full moon, too, the
+same thing happens; for, although the bodies are now acting from
+opposite directions, they do not neutralise each other's pulls as one
+might imagine, since the sun, in the same manner as the moon, produces a
+tide both under it and also at the opposite side of the earth. Thus both
+these tides are actually increased in height. The exceptionally high
+tides which we experience at new and full moons are known as _Spring
+Tides_, in contradistinction to the minimum high tides, which are known
+as _Neap Tides_.
+
+The ancients appear to have had some idea of the cause of the tides. It
+is said that as early as 1000 B.C. the Chinese noticed that the moon
+exerted an influence upon the waters of the sea. The Greeks and Romans,
+too, had noticed the same thing; and Caesar tells us that when he was
+embarking his troops for Britain the tide was high _because_ the moon
+was full. Pliny went even further than this, in recognising a similar
+connection between the waters and the sun.
+
+From casual observation one is inclined to suppose that the high tide
+always rises many feet. But that this is not the case is evidenced by
+the fact that the tides in the midst of the great oceans are only from
+three to four feet high. However, in the seas and straits around our
+Isles, for instance, the tides rise very many feet indeed, but this is
+merely owing to the extra heaping up which the large volumes of water
+undergo in forcing their passage through narrow channels.
+
+As the earth, in rotating, is continually passing through these
+tide-areas, one might expect that the friction thus set up would tend to
+slow down the rotation itself. Such a slowing down, or "tidal drag," as
+it is called, is indeed continually going on; but the effects produced
+are so exceedingly minute that it will take many millions of years to
+make the rotation appreciably slower, and so to lengthen the day.
+
+Recently it has been proved that the axis of the earth is subject to a
+very small displacement, or rather, "wobbling," in the course of a
+period of somewhat over a year. As a consequence of this, the pole
+shifts its place through a circle of, roughly, a few yards in width
+during the time in question. This movement is, perhaps, the combined
+result of two causes. One of these is the change of place during the
+year of large masses of material upon our earth; such as occurs, for
+instance, when ice and snow melt, or when atmospheric and ocean
+currents transport from place to place great bodies of air and water.
+The other cause is supposed to be the fact that the earth is not
+absolutely rigid, and so yields to certain strains upon it. In the
+course of investigation of this latter point the interesting conclusion
+has been reached by the famous American astronomer, Professor Simon
+Newcomb, that our globe as a whole is _a little more rigid than steel_.
+
+We will bring this chapter to a close by alluding briefly to two strange
+appearances which are sometimes seen in our night skies. These are known
+respectively as the Zodiacal Light and the Gegenschein.
+
+The _Zodiacal Light_ is a faint cone-shaped illumination which is seen
+to extend upwards from the western horizon after evening twilight has
+ended, and from the eastern horizon before morning twilight has begun.
+It appears to rise into the sky from about the position where the sun
+would be at that time. The proper season of the year for observing it
+during the evening is in the spring, while in autumn it is best seen in
+the early morning. In our latitudes its light is not strong enough to
+render it visible when the moon is full, but in the tropics it is
+reported to be very bright, and easily seen in full moonlight. One
+theory regards it as the reflection of light from swarms of meteors
+revolving round the sun; another supposes it to be a very rarefied
+extension of the corona.
+
+The _Gegenschein_ (German for "counter-glow") is a faint oval patch of
+light, seen in the sky exactly opposite to the place of the sun. It is
+usually treated of in connection with the zodiacal light, and one theory
+regards it similarly as of meteoric origin. Another theory,
+however--that of Mr. Evershed--considers it a sort of _tail_ to the
+earth (like a comet's tail) composed of hydrogen and helium--the two
+_lightest_ gases we know--driven off from our planet in the direction
+contrary to the sun.
+
+
+[13] Every one knows the simple experiment in which a coin lying at the
+bottom of an empty basin, and hidden from the eye by its side, becomes
+visible when a certain quantity of water has been poured in. This is an
+example of refraction. The rays of light coming from the coin ought
+_not_ to reach the eye, on account of the basin's side being in the way;
+yet by the action of the water they are _refracted_, or bent over its
+edge, in such a manner that they do.
+
+
+
+
+CHAPTER XVI
+
+THE MOON
+
+
+What we call the moon's "phases" are merely the various ways in which we
+see the sun shining upon her surface during the course of her monthly
+revolutions around the earth (see Fig. 14, p. 184). When she passes in
+the neighbourhood of the sun all his light falls upon that side which is
+turned away from us, and so the side which is turned towards us is
+unillumined, and therefore invisible. When in this position the moon is
+spoken of as _new_.
+
+As she continues her motion around the earth, she draws gradually to the
+east of the sun's place in the sky. The sunlight then comes somewhat
+from the side; and so we see a small portion of the right side of the
+lunar disc illuminated. This is the phase known as the _crescent_ moon.
+
+As she moves on in her orbit more and more of her illuminated surface is
+brought into view; and so the crescent of light becomes broader and
+broader, until we get what is called half-moon, or _first quarter_, when
+we see exactly one-half of her surface lit up by the sun's rays. As she
+draws still further round yet more of her illuminated surface is brought
+into view, until three-quarters of the disc appear lighted up. She is
+then said to be _gibbous_.
+
+Eventually she moves round so that she faces the sun completely, and
+the whole of her disc appears illuminated. She is then spoken of as
+_full_. When in this position it is clear that she is on the contrary
+side of the earth to the sun, and therefore rises about the same time
+that he is setting. She is now, in fact, at her furthest from the sun.
+
+[Illustration: Direction from which the sun's rays are coming.
+
+Various positions and illumination of the mooon by the sun during her
+revolution around the earth.
+
+The corresponding positions as viewed from the earth, showing the
+consequent phases.
+
+FIG. 14.--Orbit and Phases of the Moon.]
+
+After this, the motion of the moon in her orbit carries her on back
+again in the direction of the sun. She thus goes through her phases as
+before, only these of course are _in the reverse order_. The full phase
+is seen to give place to the gibbous, and this in turn to the half-moon
+and to the crescent; after which her motion carries her into the
+neighbourhood of the sun, and she is once more new, and lost to our
+sight in the solar glare. Following this she draws away to the east of
+the sun again, and the old order of phases repeat themselves as before.
+
+The early Babylonians imagined that the moon had a bright and a dark
+side, and that her phases were caused by the bright side coming more and
+more into view during her movement around the sky. The Greeks, notably
+Aristotle, set to work to examine the question from a mathematical
+standpoint, and came to the conclusion that the crescent and other
+appearances were such as would necessarily result if the moon were a
+dark body of spherical shape illumined merely by the light of the sun.
+
+Although the true explanation of the moon's phases has thus been known
+for centuries, it is unfortunately not unusual to see
+pictures--advertisement posters, for instance--in which stars appear
+_within_ the horns of a crescent moon! Can it be that there are to-day
+educated persons who believe that the moon is a thing which _grows_ to a
+certain size and then wastes away again; who, in fact, do not know that
+the entire body of the moon is there all the while?
+
+When the moon shows a very thin crescent, we are able dimly to see her
+still dark portion standing out against the sky. This appearance is
+popularly known as the "old moon in the new moon's arms." The dark part
+of her surface must, indeed, be to some degree illumined, or we should
+not be able to see it at all. Whence then comes the light which
+illumines it, since it clearly cannot come from the sun? The riddle is
+easily solved, if we consider what kind of view of our earth an observer
+situated on this darkened part of the moon would at that moment get. He
+would, as a matter of fact, just then see nearly the whole disc of the
+earth brightly lit up by sunlight. The lunar landscape all around would,
+therefore, be bathed in what to _him_ would be "earthlight," which of
+course takes the place there of what _we_ call moonlight. If, then, we
+recollect how much greater in size the earth is than the moon, it should
+not surprise us that this earthlight will be many times brighter than
+moonlight. It is considered, indeed, to be some twenty times brighter.
+It is thus not at all astonishing that we can see the dark portion of
+the moon illumined merely by sunlight reflected upon it from our earth.
+
+The ancients were greatly exercised in their minds to account for this
+"earthlight," or "earthshine," as it is also called. Posidonius (135-51
+B.C.) tried to explain it by supposing that the moon was partially
+transparent, and that some sunlight consequently filtered through from
+the other side. It was not, however, until the fifteenth century that
+the correct solution was arrived at.
+
+[Illustration: One side of the moon only is ever presented to the
+earth. This side is here indicated by the letters S.F.E. (side facing
+earth).
+
+By placing the above positions in a row, we can see at once that the
+moon makes one complete rotation on her axis in exactly the same time as
+she revolves around the earth.
+
+FIG. 15.--The Rotation of the Moon on her Axis.]
+
+Perhaps the most remarkable thing which one notices about the moon is
+that she always turns the same side towards us, and so we never see her
+other side. One might be led from this to jump to the conclusion that
+she does not rotate upon an axis, as do the other bodies which we see;
+but, paradoxical as it may appear, the fact that she turns one face
+always towards the earth, is actually a proof that she _does_ rotate
+upon an axis. The rotation, however, takes place with such slowness,
+that she turns round but once during the time in which she revolves
+around the earth (see Fig. 15). In order to understand the matter
+clearly, let the reader place an object in the centre of a room and walk
+around it once, _keeping his face turned towards it the whole time_,
+While he is doing this, it is evident that he will face every one of the
+four walls of the room in succession. Now in order to face each of the
+four walls of a room in succession one would be obliged _to turn oneself
+entirely round_. Therefore, during the act of walking round an object
+with his face turned directly towards it, a person at the same time
+turns his body once entirely round.
+
+In the long, long past the moon must have turned round much faster than
+this. Her rate of rotation has no doubt been slowed down by the action
+of some force. It will be recollected how, in the course of the previous
+chapter, we found that the tides were tending, though exceedingly
+gradually, to slow down the rotation of the earth upon its axis. But, on
+account of the earth's much greater mass, the force of gravitation
+exercised by it upon the surface of the moon is, of course, much more
+powerful than that which the moon exercises upon the surface of the
+earth. The tendency to tidal action on the moon itself must, therefore,
+be much in excess of anything which we here experience. It is, in
+consequence, probable that such a tidal drag, extending over a very long
+period of time, has resulted in slowing down the moon's rotation to its
+present rate.
+
+The fact that we never see but one side of the moon has given rise from
+time to time to fantastic speculations with regard to the other side.
+Some, indeed, have wished to imagine that our satellite is shaped like
+an egg, the more pointed end being directed away from us. We are here,
+of course, faced with a riddle, which is all the more tantalising from
+its appearing for ever insoluble to men, chained as they are to the
+earth. However, it seems going too far to suppose that any abnormal
+conditions necessarily exist at the other side of the moon. As a matter
+of fact, indeed, small portions of that side are brought into our view
+from time to time in consequence of slight irregularities in the moon's
+movement; and these portions differ in no way from those which we
+ordinarily see. On the whole, we obtain a view of about 60 per cent. of
+the entire lunar surface; that is to say, a good deal more than
+one-half.
+
+The actual diameter of the moon is about 2163 miles, which is somewhat
+more than one-quarter the diameter of the earth. For a satellite,
+therefore, she seems very large compared with her primary, the earth;
+when we consider that Jupiter's greatest satellite, although nearly
+twice as broad as our moon, has a diameter only one twenty-fifth that of
+Jupiter. Furthermore, the moon moves around the earth comparatively
+slowly, making only about thirteen revolutions during the entire year.
+Seen from space, therefore, she would not give the impression of a
+circling body, as other satellites do. Her revolutions are, indeed,
+relatively so very slow that she would appear rather like a smaller
+planet accompanying the earth in its orbit. In view of all this, some
+astronomers are inclined to regard the earth and moon rather as a
+"double planet" than as a system of planet and satellite.
+
+When the moon is full she attracts more attention perhaps than in any of
+her other phases. The moon, in order to be full, must needs be in that
+region of the heavens exactly opposite to the sun. The sun _appears_ to
+go once entirely round the sky in the course of a year, and the moon
+performs the same journey in the space of about a month. The moon, when
+full, having got half-way round this journey, occupies, therefore, that
+region of the sky which the sun itself will occupy half a year later.
+Thus in winter the full moon will be found roughly to occupy the sun's
+summer position in the sky, and in summer the sun's winter position. It
+therefore follows that the full moon in winter time is high up in the
+heavens, while in summer time it is low down. We thus get the greatest
+amount of full moonlight when it is the most needed.
+
+The great French astronomer, Laplace, being struck by the fact that the
+"lesser light" did not rule the night to anything like the same extent
+that the "greater light" ruled the day, set to work to examine the
+conditions under which it might have been made to do so. The result of
+his speculations showed that if the moon were removed to such a distance
+that she took a year instead of a month to revolve around the earth; and
+if she were started off in her orbit at full moon, she would always
+continue to remain full--a great advantage for us. Whewell, however,
+pointed out that in order to get the moon to move with the requisite
+degree of slowness, she would have to revolve so far from the earth that
+she would only look one-sixteenth as large as she does at present, which
+rather militates against the advantage Laplace had in mind! Finally,
+however, it was shown by M. Liouville, in 1845, that the position of a
+_perennial full moon_, such as Laplace dreamed of, would be
+unstable--that is to say, the body in question could not for long remain
+undisturbed in the situation suggested (see Fig. 16, p. 191).
+
+[Illustration: Various positions of Laplace's "Moon" with regard to the
+earth and sun during the course of a year.
+
+The same positions of Laplace's "Moon," arranged around the earth, show
+that it would make only one revolution in a year.
+
+FIG. 16.--Laplace's "Perennial Full Moon."]
+
+There is a well-known phenomenon called _harvest moon_, concerning the
+nature of which there seems to be much popular confusion. An idea in
+fact appears to prevail among a good many people that the moon is a
+harvest moon when, at rising, it looks bigger and redder than usual.
+Such an appearance has, however, nothing at all to say to the matter;
+for the moon always _looks_ larger when low down in the sky, and,
+furthermore, it usually looks red in the later months of the year, when
+there is more mist and fog about than there is in summer. What
+astronomers actually term the harvest moon is, indeed, something
+entirely different from this. About the month of September the slant at
+which the full moon comes up from below the horizon happens to be such
+that, during several evenings together, she _rises almost at the same
+hour_, instead of some fifty minutes later, as is usually the case. As
+the harvest is being gathered in about that time, it has come to be
+popularly considered that this is a provision of nature, according to
+which the sunlight is, during several evenings, replaced without delay
+by more or less full-moonlight, in order that harvesters may continue
+their work straight on into the night, and not be obliged to break off
+after sunset to wait until the moon rises. The same phenomenon is almost
+exactly repeated a month later, but by reason of the pursuits then
+carried on it is known as the "hunter's moon."
+
+In this connection should be mentioned that curious phenomenon above
+alluded to, and which seems to attract universal notice, namely, that
+the moon _looks much larger when near the horizon_--at its rising, for
+instance, than when higher up in the sky. This seeming enlargement is,
+however, by no means confined to the moon. That the sun also looks much
+larger when low down in the sky than when high up, seems to strike even
+the most casual watcher of a sunset. The same kind of effect will,
+indeed, be noted if close attention be paid to the stars themselves. A
+constellation, for instance, appears more spread out when low down in
+the sky than when high up. This enlargement of celestial objects when in
+the neighbourhood of the horizon is, however, only _apparent_ and not
+real. It must be entirely an _illusion_; for the most careful
+measurements of the discs of the sun and of the moon fail to show that
+the bodies are any larger when near the horizon than when high up in the
+sky. In fact, if there be any difference in measurements with regard to
+the moon, it will be found to be the other way round; for her disc, when
+carefully measured, is actually the slightest degree _greater_ when
+_high_ in the sky, than when low down. The reason for this is that, on
+account of the rotundity of the earth's surface, she is a trifle nearer
+the observer when overhead of him.
+
+This apparent enlargement of celestial objects, when low down in the
+sky, is granted on all sides to be an illusion; but although the
+question has been discussed with animation time out of mind, none of the
+explanations proposed can be said to have received unreserved
+acceptance. The one which usually figures in text-books is that we
+unconsciously compare the sun and moon, when low down in the sky, with
+the terrestrial objects in the same field of view, and are therefore
+inclined to exaggerate the size of these orbs. Some persons, on the
+other hand, imagine the illusion to have its source in the structure of
+the human eye; while others, again, put it down to the atmosphere,
+maintaining that the celestial objects in question _loom_ large in the
+thickened air near the horizon, in the same way that they do when viewed
+through fog or mist.
+
+The writer[14] ventures, however, to think that the illusion has its
+origin in our notion of the shape of the celestial vault. One would be
+inclined, indeed, to suppose that this vault ought to appear to us as
+the half of a hollow sphere; but he maintains that it does not so
+appear, as a consequence of the manner in which the eyes of men are set
+quite close together in their heads. If one looks, for instance, high up
+in the sky, the horizon cannot come within the field of view, and so
+there is nothing to make one think that the expanse then gazed upon is
+other than quite _flat_--let us say like the ceiling of a room. But, as
+the eyes are lowered, a portion of the _encircling_ horizon comes
+gradually into the field of view, and the region of the sky then gazed
+upon tends in consequence to assume a _hollowed-out_ form. From this it
+would seem that our idea of the shape of the celestial vault is, that it
+is _flattened down over our heads and hollowed out all around in the
+neighbourhood of the horizon_ (see Fig. 17, p. 195). Now, as a
+consequence of their very great distance, all the objects in the heavens
+necessarily appear to us to move as if they were placed on the
+background of the vault; the result being that the mind is obliged to
+conceive them as expanded or contracted, in its unconscious attempts to
+make them always fill their due proportion of space in the various parts
+of this abnormally shaped sky.
+
+From such considerations the writer concludes that the apparent
+enlargement in question is merely the natural consequence of the idea we
+have of the shape of the celestial vault--an idea gradually built up in
+childhood, to become later on what is called "second nature." And in
+support of this contention, he would point to the fact that the
+enlargement is not by any means confined to the sun and moon, but is
+every whit as marked in the case of the constellations. To one who has
+not noticed this before, it is really quite a revelation to compare the
+appearance of one of the large constellations (Orion, for instance) when
+high up in the sky and when low down. The widening apart of the various
+stars composing the group, when in the latter position, is very
+noticeable indeed.
+
+[Illustration: FIG. 17.--Illustrating the author's explanation of the
+apparent enlargement of celestial objects.]
+
+Further, if a person were to stand in the centre of a large dome, he
+would be exactly situated as if he were beneath the vaulted heaven, and
+one would consequently expect him to suffer the same illusion as to the
+shape of the dome. Objects fixed upon its background would therefore
+appear to him under the same conditions as objects in the sky, and the
+illusions as to their apparent enlargement should hold good here also.
+
+Some years ago a Belgian astronomer, M. Stroobant, in an investigation
+of the matter at issue, chanced to make a series of experiments under
+the very conditions just detailed. To various portions of the inner
+surface of a large dome he attached pairs of electric lights; and on
+placing himself at the centre of the building, he noticed that, in every
+case, those pairs which were high up appeared closer together than those
+which were low down! He does not, however, seem to have sought for the
+cause in the vaulted expanse. On the contrary, he attributed the effect
+to something connected with our upright stature, to some physiological
+reason which regularly makes us estimate objects as larger when in front
+than when overhead.
+
+In connection with this matter, it may be noted that it always appears
+extremely difficult to estimate with the eye the exact height above the
+horizon at which any object (say a star) happens to be. Even skilled
+observers find themselves in error in attempting to do so. This seems to
+bear out the writer's contention that the form under which the celestial
+vault really appears to us is a peculiar one, and tends to give rise to
+false judgments.
+
+Before leaving this question, it should also be mentioned that nothing
+perhaps is more deceptive than the size which objects in the sky appear
+to present. The full moon looks so like a huge plate, that it astonishes
+one to find that a threepenny bit held at arm's length will a long way
+more than cover its disc.
+
+[Illustration: PLATE VIII. THE MOON
+
+From a photograph taken at the Paris Observatory by M.P. Puiseux.
+
+(Page 197)]
+
+The moon is just too far off to allow us to see the actual detail on
+her surface with the naked eye. When thus viewed she merely displays a
+patchy appearance,[15] and the imaginary forms which her darker markings
+suggest to the fancy are popularly expressed by the term "Man in the
+Moon." An examination of her surface with very moderate optical aid is,
+however, quite a revelation, and the view we then get is not easily
+comparable to what we see with the unaided eye.
+
+Even with an ordinary opera-glass, an observer will be able to note a
+good deal of detail upon the lunar disc. If it be his first observation
+of the kind, he cannot fail to be struck by the fact to which we have
+just made allusion, namely, the great change which the moon appears to
+undergo when viewed with magnifying power. "Cain and his Dog," the "Man
+in the Moon gathering sticks," or whatever indeed his fancy was wont to
+conjure up from the lights and shades upon the shining surface, have now
+completely disappeared; and he sees instead a silvery globe marked here
+and there with extensive dark areas, and pitted all over with
+crater-like formations (see Plate VIII., p. 196). The dark areas retain
+even to the present day their ancient name of "seas," for Galileo and
+the early telescopic observers believed them to be such, and they are
+still catalogued under the mystic appellations given to them in the long
+ago; as, for instance, "Sea of Showers," "Bay of Rainbows," "Lake of
+Dreams."[16] The improved telescopes of later times showed, however,
+that they were not really seas (there is no water on the moon), but
+merely areas of darker material.
+
+The crater-like formations above alluded to are the "lunar mountains." A
+person examining the moon for the first time with telescopic aid, will
+perhaps not at once grasp the fact that his view of lunar mountains must
+needs be what is called a "bird's-eye" one, namely, a view from above,
+like that from a balloon and that he cannot, of course, expect to see
+them from the side, as he does the mountains upon the earth. But once he
+has realised this novel point of view, he will no doubt marvel at the
+formations which lie scattered as it were at his feet. The type of lunar
+mountain is indeed in striking contrast to the terrestrial type. On our
+earth the range-formation is supreme; on the moon the crater-formation
+is the rule, and is so-called from analogy to our volcanoes. A typical
+lunar crater may be described as a circular wall, enclosing a central
+plain, or "floor," which is often much depressed below the level of the
+surface outside. These so-called "craters," or "ring-mountains," as they
+are also termed, are often of gigantic proportions. For instance, the
+central plain of one of them, known as Ptolemaeus,[17] is about 115 miles
+across, while that of Plato is about 60. The walls of craters often rise
+to great heights; which, in proportion to the small size of the moon,
+are very much in excess of our highest terrestrial elevations.
+Nevertheless, a person posted at the centre of one of the larger craters
+might be surprised to find that he could not see the encompassing
+crater-walls, which would in every direction be below his horizon. This
+would arise not alone from the great breadth of the crater itself, but
+also from the fact that the curving of the moon's surface is very sharp
+compared with that of our earth.
+
+[Illustration: PLATE IX. MAP OF THE MOON, SHOWING THE PRINCIPAL
+"CRATERS," MOUNTAIN RANGES, AND "SEAS"
+
+In this, as in the other plates of the Moon, the _South_ will be found
+at the top of the picture; such being the view given by the ordinary
+astronomical telescope, in which all objects are seen _inverted_.
+
+(Page 199)]
+
+We have mentioned Ptolemaeus as among the very large craters, or
+ring-mountains, on the moon. Its encompassing walls rise to nearly
+13,000 feet, and it has the further distinction of being almost in the
+centre of the lunar disc. There are, however, several others much wider,
+but they are by no means in such a conspicuous position. For instance,
+Schickard, close to the south-eastern border, is nearly 130 miles in
+diameter, and its wall rises in one point to over 10,000 feet. Grimaldi,
+almost exactly at the east point, is nearly as large as Schickard.
+Another crater, Clavius, situated near the south point, is about 140
+miles across; while its neighbour Bailly--named after a famous French
+astronomer of the eighteenth century--is 180, and the largest of those
+which we can see (see Plate IX., p. 198).
+
+Many of the lunar craters encroach upon one another; in fact there is
+not really room for them all upon the visible hemisphere of the moon.
+About 30,000 have been mapped; but this is only a small portion, for
+according to the American astronomer, Professor W.H. Pickering, there
+are more than 200,000 in all.
+
+Notwithstanding the fact that the crater is the type of mountain
+associated in the mind with the moon, it must not be imagined that upon
+our satellite there are no mountains at all of the terrestrial type.
+There are indeed many isolated peaks, but strangely enough they are
+nearly always to be found in the centres of craters. Some of these peaks
+are of great altitude, that in the centre of the crater Copernicus being
+over 11,000 feet high. A few mountain ranges also exist; the best known
+of which are styled, the Lunar Alps and Lunar Apennines (see Plate X.,
+p. 200).
+
+Since the _mass_ of the moon is only about one-eightieth that of the
+earth, it will be understood that the force of gravity which she
+exercises is much less. It is calculated that, at her surface, this is
+only about one-sixth of what we experience. A man transported to the
+moon would thus be able to jump _six times as high_ as he can here. A
+building could therefore be six times as tall as upon our earth, without
+causing any more strain upon its foundations. It should not, then, be
+any subject for wonder, that the highest peaks in the Lunar Apennines
+attain to such heights as 22,000 feet. Such a height, upon a
+comparatively small body like the moon, for her _volume_ is only
+one-fiftieth that of the earth, is relatively very much in excess of the
+29,000 feet of Himalayan structure, Mount Everest, the boast of our
+planet, 8000 miles across!
+
+High as are the Lunar Apennines, the highest peaks on the moon are yet
+not found among them. There is, for instance, on the extreme southern
+edge of the lunar disc, a range known as the Leibnitz Mountains; several
+peaks of which rise to a height of nearly 30,000 feet, one peak in
+particular being said to attain to 36,000 feet (see Plate IX., p. 198).
+
+[Illustration: PLATE X. ONE OF THE MOST INTERESTING REGIONS ON THE MOON
+
+We have here (see "Map," Plate IX., p. 198) the mountain ranges of the
+Apennines, the Caucasus and the Alps; also the craters Plato, Aristotle,
+Eudoxus, Cassini, Aristillus, Autolycus, Archimedes and Linne. The
+crater Linne is the very bright spot in the dark area at the upper left
+hand side of the picture. From a photograph taken at the Paris
+Observatory by M.M. Loewy and Puiseux.
+
+(Page 200)]
+
+But the reader will surely ask the question: "How is it possible to
+determine the actual height of a lunar mountain, if one cannot go upon
+the moon to measure it?" The answer is, that we can calculate its height
+from noting the length of the shadow which it casts. Any one will allow
+that the length of a shadow cast by the sun depends upon two things:
+firstly, upon the height of the object which causes the shadow, and
+secondly, upon the elevation of the sun at the moment in the sky. The
+most casual observer of nature upon our earth can scarcely have failed
+to notice that shadows are shortest at noonday, when the sun is at its
+highest in the sky; and that they lengthen out as the sun declines
+towards its setting. Here, then, we have the clue. To ascertain,
+therefore, the height of a lunar mountain, we have first to consider at
+what elevation the sun is at that moment above the horizon of the place
+where the mountain in question is situated. Then, having measured the
+actual length in miles of the shadow extended before us, all that is
+left is to ask ourselves the question: "What height must an object be
+whose shadow cast by the sun, when at that elevation in the sky, will
+extend to this length?"
+
+There is no trace whatever of water upon the moon. The opinion, indeed,
+which seems generally held, is that water has never existed upon its
+surface. Erosions, sedimentary deposits, and all those marks which point
+to a former occupation by water are notably absent.
+
+Similarly there appears to be no atmosphere on the moon; or, at any
+rate, such an excessively rare one, as to be quite inappreciable. Of
+this there are several proofs. For instance, in a solar eclipse the
+moon's disc always stands out quite clear-cut against that of the sun.
+Again during occultations, stars disappear behind the moon with a
+suddenness, which could not be the case were there any appreciable
+atmosphere. Lastly, we see no traces of twilight upon the lunar surface,
+nor any softening at the edges of shadows; both which effects would be
+apparent if there were an atmosphere.
+
+The moon's surface is rough and rocky, and displays no marks of the
+"weathering" that would necessarily follow, had it possessed anything of
+an atmosphere in the past. This makes us rather inclined to doubt that
+it ever had one at all. Supposing, however, that it did possess an
+atmosphere in the past, it is interesting to inquire what may have
+become of it. In the first place it might have gradually disappeared, in
+consequence of the gases which composed it uniting chemically with the
+materials of which the lunar body is constructed; or, again, its
+constituent gases may have escaped into space, in accordance with the
+principles of that kinetic theory of which we have already spoken. The
+latter solution seems, indeed, the most reasonable of the two, for the
+force of gravity at the lunar surface appears too weak to hold down any
+known gases. This argument seems also to dispose of the question of
+absence of water; for Dr. George Johnstone Stoney, in a careful
+investigation of the subject, has shown that the liquid in question,
+when in the form of vapour, will escape from a planet if its mass is
+less than _one-fourth_ that of our earth. And the mass of the moon is
+very much less than this; indeed only the _one-eightieth_, as we have
+already stated.
+
+In consequence of this lack of atmosphere, the condition of things upon
+the moon will be in marked contrast to what we experience upon the
+earth. The atmosphere here performs a double service in shielding us
+from the direct rays of the sun, and in bottling the heat as a
+glass-house does. On the moon, however, the sun beats down in the
+day-time with a merciless force; but its rays are reflected away from
+the surface as quickly as they are received, and so the cold of the
+lunar night is excessive. It has been calculated that the day
+temperature on the moon may, indeed, be as high as our boiling-point,
+while the night temperature may be more than twice as low as the
+greatest cold known in our arctic regions.
+
+That a certain amount of solar heat is reflected to us from the moon is
+shown by the sharp drop in temperature which certain heat-measuring
+instruments record when the moon becomes obscured in a lunar eclipse.
+The solar heat which is thus reflected to us by the moon is, however, on
+the whole extremely small; more light and heat, indeed, reach us
+_direct_ from the sun in half a minute than we get by _reflection_ from
+the moon during the entire course of the year.
+
+With regard to the origin of the lunar craters there has been much
+discussion. Some have considered them to be evidence of violent volcanic
+action in the dim past; others, again, as the result of the impact of
+meteorites upon the lunar surface, when the moon was still in a plastic
+condition; while a third theory holds that they were formed by the
+bursting of huge bubbles during the escape into space of gases from the
+interior. The question is, indeed, a very difficult one. Though
+volcanic action, such as would result in craters of the size of
+Ptolemaeus, is hard for us to picture, and though the lone peaks which
+adorn the centres of many craters have nothing reminiscent of them in
+our terrestrial volcanoes, nevertheless the volcanic theory seems to
+receive more favour than the others.
+
+In addition to the craters there are two more features which demand
+notice, namely, what are known as _rays_ and _rills_. The rays are long,
+light-coloured streaks which radiate from several of the large craters,
+and extend to a distance of some hundreds of miles. That they are mere
+markings on the surface is proved by the fact that they cast no shadows
+of any kind. One theory is, that they were originally great cracks which
+have been filled with lighter coloured material, welling up from
+beneath. The rills, on the other hand, are actually fissures, about a
+mile or so in width and about a quarter of a mile in depth.
+
+The rays are seen to the best advantage in connection with the craters
+Tycho and Copernicus (see Plate XI., p. 204). In consequence of its
+fairly forward position on the lunar disc, and of the remarkable system
+of rays which issue from it like spokes from the axle of a wheel, Tycho
+commands especial attention. The late Rev. T.W. Webb, a famous observer,
+christened it, very happily, the "metropolitan crater of the moon."
+
+[Illustration: PLATE XI. THE MOON
+
+The systems of rays from the craters Tycho, Copernicus and Kepler are
+well shown here. From a photograph taken at the Paris Observatory by
+M.P. Puiseux.
+
+(Page 204)]
+
+A great deal of attention is, and has been, paid by certain astronomers
+to the moon, in the hope of finding out if any changes are actually in
+progress at present upon her surface. Sir William Herschel, indeed, once
+thought that he saw a lunar volcano in eruption, but this proved to be
+merely the effect of the sunlight striking the top of the crater
+Aristarchus, while the region around it was still in shadow--sunrise
+upon Aristarchus, in fact! No change of any real importance has,
+however, been noted, although it is suspected that some minor
+alterations have from time to time taken place. For instance, slight
+variations of tint have been noticed in certain areas of the lunar
+surface. Professor W.H. Pickering puts forward the conjecture that these
+may be caused by the growth and decay of some low form of vegetation,
+brought into existence by vapours of water, or carbonic acid gas, making
+their way out from the interior through cracks near at hand.
+
+Again, during the last hundred years one small crater known as Linne
+(Linnaeus), situated in the Mare Serenitatis (Sea of Serenity), has
+appeared to undergo slight changes, and is even said to have been
+invisible for a while (see Plate X., p. 200). It is, however, believed
+that the changes in question may be due to the varying angles at which
+the sunlight falls upon the crater; for it is an understood fact that
+the irregularities of the moon's motion give us views of her surface
+which always differ slightly.
+
+The suggestion has more than once been put forward that the surface of
+the moon is covered with a thick layer of ice. This is generally
+considered improbable, and consequently the idea has received very
+little support. It first originated with the late Mr. S.E. Peal, an
+English observer of the moon, and has recently been resuscitated by the
+German observer, Herr Fauth.
+
+The most unfavourable time for telescopic study of the moon is when she
+is full. The sunlight is then falling directly upon her visible
+hemisphere, and so the mountains cast no shadows. We thus do not get
+that impression of hill and hollow which is so very noticeable in the
+other phases.
+
+The first map of the moon was constructed by Galileo. Tobias Mayer
+published another in 1775; while during the nineteenth century greatly
+improved ones were made by Beer and Maedler, Schmidt, Neison and others.
+In 1903, Professor W.H. Pickering brought out a complete photographic
+lunar atlas; and a similar publication has recently appeared, the work
+of MM. Loewy and Puiseux of the Observatory of Paris.
+
+The so-called "seas" of the moon are, as we have seen, merely dark
+areas, and there appears to be no proof that they were ever occupied by
+any liquid. They are for the most part found in the _northern_ portion
+of the moon; a striking contrast to our seas and oceans, which take up
+so much of the _southern_ hemisphere of the earth.
+
+There are many erroneous ideas popularly held with regard to certain
+influences which the moon is supposed to exercise upon the earth. For
+instance, a change in the weather is widely believed to depend upon a
+change in the moon. But the word "change" as here used is meaningless,
+for the moon is continually changing her phase during the whole of her
+monthly round. Besides, the moon is visible over a great portion of the
+earth _at the same moment_, and certainly all the places from which it
+can then be seen do not get the same weather! Further, careful
+observations, and records extending over the past one hundred years and
+more, fail to show any reliable connection between the phases of the
+moon and the condition of the weather.
+
+It has been stated, on very good authority, that no telescope ever shows
+the surface of the moon as clearly as we could see it with the naked eye
+were it only 240 miles distant from us.
+
+Supposing, then, that we were able to approach our satellite, and view
+it without optical aid at such comparatively close quarters, it is
+interesting to consider what would be the smallest detail which our eye
+could take in. The question of the limit of what can be appreciated with
+the naked eye is somewhat uncertain, but it appears safe to say that at
+a distance of 240 miles the _minutest speck_ visible would have to be
+_at least_ some 60 yards across.
+
+Atmosphere and liquid both wanting, the lunar surface must be the seat
+of an eternal calm; where no sound breaks the stillness and where
+change, as we know it, does not exist. The sun beats down upon the arid
+rocks, and inky shadows lie athwart the valleys. There is no mellowing
+of the harsh contrasts.
+
+We cannot indeed absolutely affirm that Life has no place at all upon
+this airless and waterless globe, since we know not under what strange
+conditions it may manifest its presence; and our most powerful
+telescopes, besides, do not bring the lunar surface sufficiently near to
+us to disprove the existence there of even such large creatures as
+disport themselves upon our planet. Still, we find it hard to rid
+ourselves of the feeling that we are in the presence of a dead world. On
+she swings around the earth month after month, with one face ever
+turned towards us, leaving a certain mystery to hang around that hidden
+side, the greater part of which men can never hope to see. The rotation
+of the moon upon her axis--the lunar day--has become, as we have seen,
+equal to her revolution around the earth. An epoch may likewise
+eventually be reached in the history of our own planet, when the length
+of the terrestrial day has been so slowed down by tidal friction that it
+will be equal to the year. Then will the earth revolve around the
+central orb, with one side plunged in eternal night and the other in
+eternal sunshine. But such a vista need not immediately distress us. It
+is millions of years forward in time.
+
+
+[14] _Journal of the British Astronomical Association_, vol. x.
+(1899-1900), Nos. 1 and 3.
+
+[15] Certain of the ancient Greeks thought the markings on the moon to
+be merely the reflection of the seas and lands of our earth, as in a
+badly polished mirror.
+
+[16] Mare Imbrium, Sinus Iridum, Lacus Somniorum.
+
+[17] The lunar craters have, as a rule, received their names from
+celebrated persons, usually men of science. This system of nomenclature
+was originated by Riccioli, in 1651.
+
+
+
+
+CHAPTER XVII
+
+THE SUPERIOR PLANETS
+
+
+Having, in a previous chapter, noted the various aspects which an
+inferior planet presents to our view, in consequence of its orbit being
+nearer to the sun than the orbit of the earth, it will be well here to
+consider in the same way the case of a superior planet, and to mark
+carefully the difference.
+
+To begin with, it should be quite evident that we cannot ever have a
+transit of a superior planet. The orbit of such a body being entirely
+_outside_ that of the earth, the body itself can, of course, never pass
+between us and the sun.
+
+A superior planet will be at its greatest distance from us when on the
+far side of the sun. It is said then to be in _conjunction_. As it comes
+round in its orbit it eventually passes, so to speak, at the _back_ of
+us. It is then at its nearest, or in _opposition_, as this is
+technically termed, and therefore in the most favourable position for
+telescopic observation of its surface. Being, besides, seen by us at
+that time in the direction of the heavens exactly opposite to where the
+sun is, it will thus at midnight be high up in the south side of the
+sky, a further advantage to the observer.
+
+Last of all, a superior planet cannot show crescent shapes like an
+interior; for whether it be on the far side of the sun, or behind us,
+or again to our right or left, the sunlight must needs appear to fall
+more or less full upon its face.
+
+
+THE PLANETOID EROS
+
+The nearest to us of the superior planets is the tiny body, Eros, which,
+as has been already stated, was discovered so late as the year 1898. In
+point of view, however, of its small size, it can hardly be considered
+as a true planet, and the name "planetoid" seems much more appropriate
+to it.
+
+Eros was not discovered, like Uranus, in the course of telescopic
+examination of the heavens, nor yet, like Neptune, as the direct result
+of difficult calculations, but was revealed by the impress of its light
+upon a photographic plate, which had been exposed for some length of
+time to the starry sky. Since many of the more recent additions to the
+asteroids have been discovered in the same manner, we shall have
+somewhat more to say about this special employment of photography when
+we come to deal with those bodies later on.
+
+The path of Eros around the sun is so very elliptical, or, to use the
+exact technical term, so very "eccentric," that the planetoid does not
+keep all the time entirely in the space between our orbit and that of
+Mars, which latter happens to be the next body in the order of planetary
+succession outwards. In portions of its journey Eros, indeed, actually
+goes outside the Martian orbit. The paths of the planetoid and of Mars
+are, however, _not upon the same plane_, so the bodies always pass clear
+of each other, and there is thus as little chance of their dashing
+together as there would be of trains which run across a bridge at an
+upper level, colliding with those which pass beneath it at a lower
+level.
+
+When Eros is in opposition, it comes within about 13-1/2 million miles
+of our earth, and, after the moon, is therefore by a long way our
+nearest neighbour in space. It is, however, extremely small, not more,
+perhaps, than twenty miles in diameter, and is subject to marked
+variations in brightness, which do not appear up to the present to meet
+with a satisfactory explanation. But, insignificant as is this little
+body, it is of great importance to astronomy; for it happens to furnish
+the best method known of calculating the sun's distance from our
+earth--a method which Galle, in 1872, and Sir David Gill, in 1877,
+suggested that asteroids might be employed for, and which has in
+consequence supplanted the old one founded upon transits of Venus. The
+sun's distance is now an ascertained fact to within 100,000 miles, or
+less than half the distance of the moon.
+
+
+THE PLANET MARS
+
+We next come to the planet Mars. This body rotates in a period of
+slightly more than twenty-four hours. The inclination, or slant, of its
+axis is about the same as that of the earth, so that, putting aside its
+greater distance from the sun, the variations of season which it
+experiences ought to be very much like ours.
+
+The first marking detected upon Mars was the notable one called the
+Syrtis Major, also known, on account of its shape, as the Hour-Glass
+Sea. This observation was made by the famous Huyghens in 1659; and, from
+the movement of the marking in question across the disc, he inferred
+that the planet rotated on its axis in a period of about twenty-four
+hours.
+
+There appears to be very little atmosphere upon Mars, the result being
+that we almost always obtain a clear view of the detail on its surface.
+Indeed, it is only to be expected from the kinetic theory that Mars
+could not retain much of an atmosphere, as the force of gravity at its
+surface is less than one-half of what we experience upon the earth. It
+should here be mentioned that recent researches with the spectroscope
+seem to show that, whatever atmosphere there may be upon Mars, its
+density at the surface of the planet cannot be more than the one-fourth
+part of the density of the air at the surface of the earth. Professor
+Lowell, indeed, thinks it may be more rarefied than that upon our
+highest mountain-tops.
+
+Seen with the naked eye, Mars appears of a red colour. Viewed in the
+telescope, its surface is found to be in general of a ruddy hue, varied
+here and there with darker patches of a bluish-green colour. These
+markings are permanent, and were supposed by the early telescopic
+observers to imply a distribution of the planet's surface into land and
+water, the ruddy portions being considered as continental areas (perhaps
+sandy deserts), and the bluish-green as seas. The similarity to our
+earth thus suggested was further heightened by the fact that broad white
+caps, situated at the poles, were seen to vary with the planet's
+seasons, diminishing greatly in extent during the Martian summer (the
+southern cap in 1894 even disappearing altogether), and developing again
+in the Martian winter.[18] Readers of Oliver Wendell Holmes will no
+doubt recollect that poet's striking lines:--
+
+"The snows that glittered on the disc of Mars
+Have melted, and the planet's fiery orb
+Rolls in the crimson summer of its year."
+
+A state of things so strongly analogous to what we experience here,
+naturally fired the imaginations of men, and caused them to look on Mars
+as a world like ours, only upon a much smaller scale. Being smaller, it
+was concluded to have cooled quicker, and to be now long past its prime;
+and its "inhabitants" were, therefore, pictured as at a later stage of
+development than the inhabitants of our earth.
+
+Notwithstanding the strong temptation to assume that the whiteness of
+the Martian polar caps is due to fallen snow, such a solution is,
+however, by no means so simple as it looks. The deposition of water in
+the form of snow, or even of hoar frost, would at least imply that the
+atmosphere of Mars should now and then display traces of aqueous vapour,
+which it does not appear to do.[19] It has, indeed, been suggested that
+the whiteness may not after all be due to this cause, but to carbonic
+acid gas (carbon dioxide), which is known to freeze at a _very low_
+temperature. The suggestion is plainly based upon the assumption that,
+as Mars is so much further from the sun than we are, it would receive
+much less heat, and that the little thus received would be quickly
+radiated away into space through lack of atmosphere to bottle it in.
+
+We now come to those well-known markings, popularly known as the
+"canals" of Mars, which have been the subject of so much discussion
+since their discovery thirty years ago.
+
+It was, in fact, in the year 1877, when Mars was in opposition, and thus
+at its nearest to us, that the famous Italian astronomer, Schiaparelli,
+announced to the world that he had found that the ruddy areas, thought
+to be continents, were intersected by a network of straight dark lines.
+These lines, he reported, appeared in many cases to be of great length,
+so long, indeed, as several thousands of miles, and from about twenty to
+sixty miles in width. He christened the lines _channels_, the Italian
+word for which, "canali," was unfortunately translated into English as
+"canals." The analogy, thus accidentally suggested, gave rise to the
+idea that they might be actual waterways.[20]
+
+In the winter of 1881-1882, when Mars was again in opposition,
+Schiaparelli further announced that he had found some of these lines
+doubled; that is to say, certain of them were accompanied by similar
+lines running exactly parallel at no great distance away. There was at
+first a good deal of scepticism on the subject of Schiaparelli's
+discoveries, but gradually other observers found themselves seeing both
+the lines and their doublings. We have in this a good example of a
+curious circumstance in astronomical observation, namely, the fact that
+when fine detail has once been noted by a competent observer, it is not
+long before other observers see the same detail with ease.
+
+An immense amount of close attention has been paid to the planet Mars
+during recent years by the American observer, Professor Percival Lowell,
+at his famous observatory, 7300 feet above the sea, near the town of
+Flagstaff, Arizona, U.S.A. His observations have not, like those of most
+astronomers, been confined merely to "oppositions," but he has
+systematically kept the planet in view, so far as possible, since the
+year 1894.
+
+The instrumental equipment of his observatory is of the very best, and
+the "seeing" at Flagstaff is described as excellent. In support of the
+latter statement, Mr. Lampland, of the Lowell Observatory, maintains
+that the faintest stars shown on charts made at the Lick Observatory
+with the 36-inch telescope there, are _perfectly visible_ with the
+24-inch telescope at Flagstaff.
+
+Professor Lowell is, indeed, generally at issue with the other observers
+of Mars. He finds the canals extremely narrow and sharply defined, and
+he attributes the blurred and hazy appearance, which they have presented
+to other astronomers, to the unsteady and imperfect atmospheric
+conditions in which their observations have been made. He assigns to the
+thinnest a width of two or three miles, and from fifteen to twenty to
+the larger. Relatively to their width, however, he finds their length
+enormous. Many of them are 2000 miles long, while one is even as much
+as 3540. Such lengths as these are very great in comparison with the
+smallness of the planet. He considers that the canals stand in some
+peculiar relation to the polar cap, for they crowd together in its
+neighbourhood. In place, too, of ill-defined condensations, he sees
+sharp black spots where the canals meet and intersect, and to these he
+gives the name of "Oases." He further lays particular stress upon a dark
+band of a blue tint, which is always seen closely to surround the edges
+of the polar caps all the time that they are disappearing; and this he
+takes to be a proof that the white material is something which actually
+_melts_. Of all substances which we know, water alone, he affirms, would
+act in such a manner.
+
+The question of melting at all may seem strange in a planet which is
+situated so far from the sun, and possesses such a rarefied atmosphere.
+But Professor Lowell considers that this very thinness of the atmosphere
+allows the direct solar rays to fall with great intensity upon the
+planet's surface, and that this heating effect is accentuated by the
+great length of the Martian summer. In consequence he concludes that,
+although the general climate of Mars is decidedly cold, it is above the
+freezing point of water.
+
+The observations at Flagstaff appear to do away with the old idea that
+the darkish areas are seas, for numerous lines belonging to the
+so-called "canal system" are seen to traverse them. Again, there is no
+star-like image of the sun reflected from them, as there would be, of
+course, from the surface of a great sheet of water. Lastly, they are
+observed to vary in tone and colour with the changing Martian seasons,
+the blue-green changing into ochre, and later on back again into
+blue-green. Professor Lowell regards these areas as great tracts of
+vegetation, which are brought into activity as the liquid reaches them
+from the melting snows.
+
+[Illustration: PLATE XII. A MAP OF THE PLANET MARS
+
+We see here the Syrtis Major (or "Hour-Glass Sea"), the polar caps,
+several "oases," and a large number of "canals," some of which are
+double. The South is at the top of the picture, in accordance with the
+_inverted_ view given by an astronomical telescope. From a drawing by
+Professor Percival Lowell.
+
+(Page 216)]
+
+With respect to the canals, the Lowell observations further inform us
+that these are invisible during the Martian winter, but begin to appear
+in the spring when the polar cap is disappearing. Professor Lowell,
+therefore, inclines to the view that in the middle of the so-called
+canals there exist actual waterways which serve the purposes of
+irrigation, and that what we see is not the waterways themselves, for
+they are too narrow, but the fringe of vegetation which springs up along
+the banks as the liquid is borne through them from the melting of the
+polar snows. He supports this by his observation that the canals begin
+to appear in the neighbourhood of the polar caps, and gradually grow, as
+it were, in the direction of the planet's equator.
+
+It is the idea of life on Mars which has given this planet such a
+fascination in the eyes of men. A great deal of nonsense has, however,
+been written in newspapers upon the subject, and many persons have thus
+been led to think that we have obtained some actual evidence of the
+existence of living beings upon Mars. It must be clearly understood,
+however, that Professor Lowell's advocacy of the existence of life upon
+that planet is by no means of this wild order. At the best he merely
+indulges in such theories as his remarkable observations naturally call
+forth. His views are as follows:--He considers that the planet has
+reached a time when "water" has become so scarce that the "inhabitants"
+are obliged to employ their utmost skill to make their scanty supply
+suffice for purposes of irrigation. The changes of tone and colour upon
+the Martian surface, as the irrigation produces its effects, are similar
+to what a telescopic observer--say, upon Venus--would notice on our
+earth when the harvest ripens over huge tracts of country; that is, of
+course, if the earth's atmosphere allowed a clear view of the
+terrestrial surface--a very doubtful point indeed. Professor Lowell
+thinks that the perfect straightness of the lines, and the geometrical
+manner in which they are arranged, are clear evidences of artificiality.
+On a globe, too, there is plainly no reason why the liquid which results
+from the melting of the polar caps should trend at all in the direction
+of the equator. Upon our earth, for instance, the transference of water,
+as in rivers, merely follows the slope of the ground, and nothing else.
+The Lowell observations show, however, that the Martian liquid is
+apparently carried from one pole towards the equator, and then past it
+to the other pole, where it once more freezes, only to melt again in due
+season, and to reverse the process towards and across the equator as
+before. Professor Lowell therefore holds, and it seems a strong point in
+favour of his theory, that the liquid must, in some artificial manner,
+as by pumping, for instance, be _helped_ in its passage across the
+surface of the planet.
+
+A number of attempts have been made to explain the _doubling_ of the
+canals merely as effects of refraction or reflection; and it has even
+been suggested that it may arise from the telescope not being accurately
+focussed.
+
+The actual doubling of the canals once having been doubted, it was an
+easy step to the casting of doubt on the reality of the canals
+themselves. The idea, indeed, was put forward that the human eye, in
+dealing with detail so very close to the limit of visibility, may
+unconsciously treat as an actual line several point-like markings which
+merely happen to lie in a line. In order to test this theory,
+experiments were carried out in 1902 by Mr. E.W. Maunder of Greenwich
+Observatory, and Mr. J.E. Evans of the Royal Hospital School at
+Greenwich, in which certain schoolboys were set to make drawings of a
+white disc with some faint markings upon it. The boys were placed at
+various distances from the disc in question; and it was found that the
+drawings made by those who were just too far off to see distinctly, bore
+out the above theory in a remarkable manner. Recently, however, the
+plausibility of the _illusion_ view has been shaken by photographs of
+Mars taken during the opposition of 1905 by Mr. Lampland at the Lowell
+Observatory, in which a number of the more prominent canals come out as
+straight dark lines. Further still, in some photographs made there quite
+lately, several canals are said to appear visibly double.
+
+Following up the idea alluded to in Chapter XVI., that the moon may be
+covered with a layer of ice, Mr. W.T. Lynn has recently suggested that
+this may be the case on Mars; and that, at certain seasons, the water
+may break through along definite lines, and even along lines parallel to
+these. This, he maintains, would account for the canals becoming
+gradually visible across the disc, without the necessity of Professor
+Lowell's "pumping" theory.
+
+And now for the views of Professor Lowell himself with regard to the
+doubling of the canals. From his observations, he considers that no
+pairs of railway lines could apparently be laid down with greater
+parallelism. He draws attention to the fact that the doubling does not
+take place by any means in every canal; indeed, out of 400 canals seen
+at Flagstaff, only fifty-one--or, roughly, one-eighth--have at any time
+been seen double. He lays great stress upon this, which he considers
+points strongly against the duplication being an optical phenomenon. He
+finds that the distance separating pairs of canals is much less in some
+doubles than in others, and varies on the whole from 75 to 200 miles.
+According to him, the double canals appear to be confined to within 40
+degrees of the equator: or, to quote his own words, they are "an
+equatorial feature of the planet, confined to the tropic and temperate
+belts." Finally, he points out that they seem to _avoid_ the blue-green
+areas. But, strangely enough, Professor Lowell does not so far attempt
+to fit in the doubling with his body of theory. He makes the obvious
+remark that they may be "channels and return channels," and with that he
+leaves us.
+
+The conclusions of Professor Lowell have recently been subjected to
+strenuous criticism by Professor W.H. Pickering and Dr. Alfred Russel
+Wallace. It was Professor Pickering who discovered the "oases," and who
+originated the idea that we did not see the so-called "canals"
+themselves, but only the growth of vegetation along their borders. He
+holds that the oases are craterlets, and that the canals are cracks
+which radiate from them, as do the rifts and streaks from craters upon
+the moon. He goes on to suggest that vapours of water, or of carbonic
+acid gas, escaping from the interior, find their way out through these
+cracks, and promote the growth of a low form of vegetation on either
+side of them. In support of this view he draws attention to the
+existence of long "steam-cracks," bordered by vegetation, in the deserts
+of the highly volcanic island of Hawaii. We have already seen, in an
+earlier chapter, how he has applied this idea to the explanation of
+certain changes which are suspected to be taking place upon the moon.
+
+In dealing with the Lowell canal system, Professor Pickering points out
+that under such a slight atmospheric pressure as exists on Mars, the
+evaporation of the polar caps--supposing them to be formed of
+snow--would take place with such extraordinary rapidity that the
+resulting water could never be made to travel along open channels, but
+that a system of gigantic tubes or water-mains would have to be
+employed!
+
+As will be gathered from his theories regarding vegetation, Professor
+Pickering does not deny the existence of a form of life upon Mars. But
+he will not hear of civilisation, or of anything even approaching it. He
+thinks, however, that as Mars is intermediate physically between the
+moon and earth, the form of life which it supports may be higher than
+that on the moon and lower than that on the earth.
+
+In a small book published in the latter part of 1907, and entitled _Is
+Mars Habitable?_ Dr. Alfred Russel Wallace sets himself, among other
+things, to combat the idea of a comparatively high temperature, such as
+Professor Lowell has allotted to Mars. He shows the immense service
+which the water-vapour in our atmosphere exercises, through keeping the
+solar heat from escaping from the earth's surface. He then draws
+attention to the fact that there is no spectroscopic evidence of
+water-vapour on Mars[21]; and points out that its absence is only to be
+expected, as Dr. George Johnstone Stoney has shown that it will escape
+from a body whose mass is less than one-quarter the mass of the earth.
+The mass of Mars is, in fact, much less than this, _i.e._ only
+one-ninth. Dr. Wallace considers, therefore, that the temperature of
+Mars ought to be extremely low, unless the constitution of its
+atmosphere is very different from ours. With regard to the latter
+statement, it should be mentioned that the Swedish physicist, Arrhenius,
+has recently shown that the carbonic acid gas in our atmosphere has an
+important influence upon climate. The amount of it in our air is, as we
+have seen, extremely small; but Arrhenius shows that, if it were
+doubled, the temperature would be more uniform and much higher. We thus
+see how futile it is, with our present knowledge, to dogmatise on the
+existence or non-existence of life in other celestial orbs.
+
+As to the canals Dr. Wallace puts forward a theory of his own. He
+contends that after Mars had cooled to a state of solidity, a great
+swarm of meteorites and small asteroids fell in upon it, with the result
+that a thin molten layer was formed all over the planet. As this layer
+cooled, the imprisoned gases escaped, producing vents or craterlets; and
+as it attempted to contract further upon the solid interior, it split in
+fissures radiating from points of weakness, such, for instance, as the
+craterlets. And he goes on to suggest that the two tiny Martian
+satellites, with which we shall deal next, are the last survivors of his
+hypothetical swarm. Finally, with regard to the habitability of Mars,
+Dr. Wallace not only denies it, but asserts that the planet is
+"absolutely uninhabitable."
+
+For a long time it was supposed that Mars did not possess any
+satellites. In 1877, however, during that famous opposition in which
+Schiaparelli first saw the canals, two tiny satellites were discovered
+at the Washington Observatory by an American astronomer, Professor Asaph
+Hall. These satellites are so minute, and so near to the planet, that
+they can only be seen with very large telescopes; and even then the
+bright disc of the planet must be shielded off. They have been
+christened Phobos and Deimos (Fear and Dread); these being the names of
+the two subordinate deities who, according to Homer, attended upon Mars,
+the god of war.
+
+It is impossible to measure the exact sizes of these satellites, as they
+are too small to show any discs, but an estimate has been formed from
+their brightness. The diameter of Phobos was at first thought to be six
+miles, and that of Deimos, seven. As later estimates, however,
+considerably exceed this, it will, perhaps, be not far from the truth to
+state that they are each roughly about the size of the planetoid Eros.
+Phobos revolves around Mars in about 7-1/2 hours, at a distance of about
+only 4000 miles from the planet's surface, and Deimos in about 30 hours,
+at a distance of about 12,000 miles. As Mars rotates on its axis in
+about 24 hours, it will be seen that Phobos makes more than three
+revolutions while the planet is rotating once--a very interesting
+condition of things.
+
+A strange foreshadowing of the discovery of the satellites of Mars will
+be familiar to readers of _Gulliver's Travels_. According to Dean
+Swift's hero, the astronomers on the Flying Island of Laputa had found
+two tiny satellites to Mars, one of which revolved around the planet in
+ten hours. The correctness of this guess is extraordinarily close,
+though at best it is, of course, nothing more than a pure coincidence.
+
+It need not be at all surprising that much uncertainty should exist with
+regard to the actual condition of the surface of Mars. The circumstances
+in which we are able to see that planet at the best are, indeed, hardly
+sufficient to warrant us in propounding any hard and fast theories. One
+of the most experienced of living observers, the American astronomer,
+Professor E.E. Barnard, considers that the view we get of Mars with the
+best telescope may be fairly compared with our naked eye view of the
+moon. Since we have seen that a view with quite a small telescope
+entirely alters our original idea of the lunar surface, a slight
+magnification revealing features of whose existence we had not
+previously the slightest conception, it does not seem too much to say
+that a further improvement in optical power might entirely subvert the
+present notions with regard to the Martian canals. Therefore, until we
+get a still nearer view of these strange markings, it seems somewhat
+futile to theorise. The lines which we see are perhaps, indeed, a
+foreshortened and all too dim view of some type of formation entirely
+novel to us, and possibly peculiar to Mars. Differences of gravity and
+other conditions, such as obtain upon different planets, may perhaps
+produce very diverse results. The earth, the moon, and Mars differ
+greatly from one another in size, gravitation, and other such
+characteristics. Mountain-ranges so far appear typical of our globe, and
+ring-mountains typical of the moon. May not the so-called "canals" be
+merely some special formation peculiar to Mars, though quite a natural
+result of its particular conditions and of its past history?
+
+
+THE ASTEROIDS (OR MINOR PLANETS)
+
+We now come to that belt of small planets which are known by the name of
+asteroids. In the general survey of the solar system given in Chapter
+II., we saw how it was long ago noticed that the distances of the
+planetary orbits from the sun would have presented a marked appearance
+of orderly sequence, were it not for a gap between the orbits of Mars
+and Jupiter where no large planet was known to circulate. The suspicion
+thus aroused that some planet might, after all, be moving in this
+seemingly empty space, gave rise to the gradual discovery of a great
+number of small bodies; the largest of which, Ceres, is less than 500
+miles in diameter. Up to the present day some 600 of these bodies have
+been discovered; the four leading ones, in order of size, being named
+Ceres, Pallas, Juno, and Vesta. All the asteroids are invisible to the
+naked eye, with the exception of Vesta, which, though by no means the
+largest, happens to be the brightest. It is, however, only just visible
+to the eye under favourable conditions. No trace of an atmosphere has
+been noted upon any of the asteroids, but such a state of things is only
+to be expected from the kinetic theory.
+
+For a good many years the discoveries of asteroids were made by means of
+the telescope. When, in the course of searching the heavens, an object
+was noticed which did not appear upon any of the recognised star charts,
+it was kept under observation for several nights to see whether it
+changed its place in the sky. Since asteroids move around the sun in
+orbits, just as planets do, they, of course, quickly reveal themselves
+by their change of position against the starry background.
+
+The year 1891 started a new era in the discovery of asteroids. It
+occurred to the Heidelberg observer, Dr. Max Wolf, one of the most
+famous of the hunters of these tiny planets, that photography might be
+employed in the quest with success. This photographic method, to which
+allusion has already been made in dealing with Eros, is an extremely
+simple one. If a photograph of a portion of the heavens be taken through
+an "equatorial"--that is, a telescope, moving by machinery, so as to
+keep the stars, at which it is pointed, always exactly in the field of
+view during their apparent movement across the sky--the images of these
+stars will naturally come out in the photograph as sharply defined
+points. If, however, there happens to be an asteroid, or other planetary
+body, in the same field of view, its image will come out as a short
+white streak; because the body has a comparatively rapid motion of its
+own, and will, during the period of exposure, have moved sufficiently
+against the background of the stars to leave a short trail, instead of a
+dot, upon the photographic plate. By this method Wolf himself has
+succeeded in discovering more than a hundred asteroids (see Plate XIII.,
+p. 226). It was, indeed, a little streak of this kind, appearing upon a
+photograph taken by the astronomer Witt, at Berlin, in 1898, which first
+informed the world of the existence of Eros.
+
+[Illustration: PLATE XIII. MINOR PLANET TRAILS
+
+Two trails of minor planets (asteroids) imprinted _at the same time_
+upon one photographic plate. In the white streak on the left-hand side
+of the picture we witness the _discovery_ of a new minor planet. The
+streak on the right was made by a body already known--the minor planet
+"Fiducia." This photograph was taken by Dr. Max Wolf, at Heidelberg, on
+the 4th of November, 1901, with the aid of a 16-inch telescope. The time
+of exposure was two hours.
+
+(Page 227)]
+
+It has been calculated that the total mass of the asteroids must be
+much less than one-quarter that of the earth. They circulate as a rule
+within a space of some 30,000,000 miles in breadth, lying about midway
+between the paths of Mars and Jupiter. Two or three, however, of the
+most recently discovered of these small bodies have been found to pass
+quite close to Jupiter. The orbits of the asteroids are by no means in
+the one plane, that of Pallas being the most inclined to the plane of
+the earth's orbit. It is actually three times as much inclined as that
+of Eros.
+
+Two notable theories have been put forward to account for the origin of
+the asteroids. The first is that of the celebrated German astronomer,
+Olbers, who was the discoverer of Pallas and Vesta. He suggested that
+they were the fragments of an exploded planet. This theory was for a
+time generally accepted, but has now been abandoned in consequence of
+certain definite objections. The most important of these objections is
+that, in accordance with the theory of gravitation, the orbits of such
+fragments would all have to pass through the place where the explosion
+originally occurred. But the wide area over which the asteroids are
+spread points rather against the notion that they all set out originally
+from one particular spot. Another objection is that it does not appear
+possible that, within a planet already formed, forces could originate
+sufficiently powerful to tear the body asunder.
+
+The second theory is that for some reason a planet here failed in the
+making. Possibly the powerful gravitational action of the huge body of
+Jupiter hard by, disturbed this region so much that the matter
+distributed through it was never able to collect itself into a single
+mass.
+
+
+[18] Sir William Herschel was the first to note these polar changes.
+
+[19] Quite recently, however, Professor Lowell has announced that his
+observer, Mr. E.C. Slipher, finds with the spectroscope faint traces of
+water vapour in the Martian atmosphere.
+
+[20] In a somewhat similar manner the term "crater," as applied to the
+ring-mountain formation on the moon, has evidently given a bias in
+favour of the volcanic theory as an explanation of that peculiar
+structure.
+
+[21] Mr. Slipher's results (see note 2, page 213) were not then known.
+
+
+
+
+CHAPTER XVIII
+
+THE SUPERIOR PLANETS--_continued_
+
+
+The planets, so far, have been divided into inferior and superior. Such
+a division, however, refers merely to the situation of their orbits with
+regard to that of our earth. There is, indeed, another manner in which
+they are often classed, namely, according to size. On this principle
+they are divided into two groups; one group called the _Terrestrial
+Planets_, or those which have characteristics like our earth, and the
+other called the _Major Planets_, because they are all of very great
+size. The terrestrial planets are Mercury, Venus, the earth, and Mars.
+The major planets are the remainder, namely, Jupiter, Saturn, Uranus,
+and Neptune. As the earth's orbit is the boundary which separates the
+inferior from the superior planets, so does the asteroidal belt divide
+the terrestrial from the major planets. We found the division into
+inferior and superior useful for emphasising the marked difference in
+aspect which those two classes present as seen from our earth; the
+inferior planets showing phases like the moon when viewed in the
+telescope, whereas the superior planets do not. But the division into
+terrestrial and major planets is the more far-reaching classification of
+the two, for it includes the whole number of planets, whereas the other
+arrangement necessarily excludes the earth. The members of each of
+these classes have many definite characteristics in common. The
+terrestrial planets are all of them relatively small in size,
+comparatively near together, and have few or no satellites. They are,
+moreover, rather dense in structure. The major planets, on the other
+hand, are huge bodies, circulating at great distances from each other,
+and are, as a rule, provided with a number of satellites. With respect
+to structure, they may be fairly described as being loosely put
+together. Further, the markings on the surfaces of the terrestrial
+planets are permanent, whereas those on the major planets are
+continually shifting.
+
+
+THE PLANET JUPITER
+
+Jupiter is the greatest of the major planets. It has been justly called
+the "Giant" planet, for both in volume and in mass it exceeds all the
+other planets put together. When seen through the telescope it exhibits
+a surface plentifully covered with markings, the most remarkable being a
+series of broad parallel belts. The chief belt lies in the central parts
+of the planet, and is at present about 10,000 miles wide. It is bounded
+on either side by a reddish brown belt of about the same width. Bright
+spots also appear upon the surface of the planet, last for a while, and
+then disappear. The most notable of the latter is one known as the
+"Great Red Spot." This is situated a little beneath the southern red
+belt, and appeared for the first time about thirty years ago. It has
+undergone a good many changes in colour and brightness, and is still
+faintly visible. This spot is the most permanent marking which has yet
+been seen upon Jupiter. In general, the markings change so often that
+the surface which we see is evidently not solid, but of a fleeting
+nature akin to cloud (see Plate XIV., p. 230).
+
+[Illustration: PLATE XIV. THE PLANET JUPITER
+
+The Giant Planet as seen at 11.30 p.m., on the 11th of January, 1908,
+with a 12-1/2-inch reflecting telescope. The extensive oval marking in
+the upper portion of the disc is the "Great Red Spot." The South is at
+the top of the picture, the view being the _inverted_ one given by an
+astronomical telescope. From a drawing by the Rev. Theodore E.R.
+Phillips, M.A., F.R.A.S., Director of the Jupiter Section of the British
+Astronomical Association.
+
+(Page 231)]
+
+Observations of Jupiter's markings show that on an average the planet
+rotates on its axis in a period of about 9 hours 54 minutes. The mention
+here of _an average_ with reference to the rotation will, no doubt,
+recall to the reader's mind the similar case of the sun, the different
+portions of which rotate with different velocities. The parts of Jupiter
+which move quickest take 9 hours 50 minutes to go round, while those
+which move slowest take 9 hours 57 minutes. The middle portions rotate
+the fastest, a phenomenon which the reader will recollect was also the
+case with regard to the sun.
+
+Jupiter is a very loosely packed body. Its density is on an average only
+about 1-1/2 times that of water, or about one-fourth the density of the
+earth; but its bulk is so great that the gravitation at that surface
+which we see is about 2-1/2 times what it is on the surface of the
+earth. In accordance, therefore, with the kinetic theory, we may expect
+the planet to retain an extensive layer of gases around it; and this is
+confirmed by the spectroscope, which gives evidence of the presence of a
+dense atmosphere.
+
+All things considered, it may be safely inferred that the interior of
+Jupiter is very hot, and that what we call its surface is not the actual
+body of the planet, but a voluminous layer of clouds and vapours driven
+upwards from the heated mass underneath. The planet was indeed formerly
+thought to be self-luminous; but this can hardly be the case, for those
+portions of the surface which happen to lie at any moment in the
+shadows cast by the satellites appear to be quite black. Again, when a
+satellite passes into the great shadow cast by the planet it becomes
+entirely invisible, which would not be the case did the planet emit any
+perceptible light of its own.
+
+In its revolutions around the sun, Jupiter is attended, so far as we
+know, by seven[22] satellites. Four of these were among the first
+celestial objects which Galileo discovered with his "optick tube," and
+he named them the "Medicean Stars" in honour of his patron, Cosmo de
+Medici. Being comparatively large bodies they might indeed just be seen
+with the naked eye, were it not for the overpowering glare of the
+planet.
+
+It was only in quite recent times, namely, in 1892, that a fifth
+satellite was added to the system of Jupiter. This body, discovered by
+Professor E.E. Barnard, is very small. It circulates nearer to the
+planet than the innermost of Galileo's moons; and, on account of the
+glare, is a most difficult object to obtain a glimpse of, even in the
+best of telescopes. In December 1904 and January 1905 respectively, two
+more moons were added to the system, these being found by _photography_,
+by the American astronomer, Professor C.D. Perrine. Both the bodies in
+question revolve at a greater distance from the planet than the
+outermost of the older known satellites.
+
+Galileo's moons, though the largest bodies of Jupiter's satellite
+system, are, as we have already pointed out, very small indeed when
+compared with the planet itself. The diameters of two of them, Europa
+and Io, are, however, about the same as that of our moon, while those of
+the other two, Callisto and Ganymede, are more than half as large again.
+The recently discovered satellites are, on the other hand,
+insignificant; that found by Barnard, for example, being only about 100
+miles in diameter.
+
+Of the four original satellites Io is the nearest to Jupiter, and, seen
+from the planet, it would show a disc somewhat larger than that of our
+moon. The others would appear somewhat smaller. However, on account of
+the great distance of the sun, the entire light reflected to Jupiter by
+all the satellites should be very much less than what we get from our
+moon.
+
+Barnard's satellite circles around Jupiter at a distance less than our
+moon is from us, and in a period of about 12 hours. Galileo's four
+satellites revolve in periods of about 2, 3-1/2, 7, and 16-1/2 days
+respectively, at distances lying roughly between a quarter of a million
+and one million miles. Perrine's two satellites are at a distance of
+about seven million miles, and take about nine months to complete their
+revolutions.
+
+The larger satellites, when viewed in the telescope, exhibit certain
+defined markings; but the bodies are so far away from us, that only
+those details which are of great extent can be seen. The satellite Io,
+according to Professor Barnard, shows a darkish disc, with a broad white
+belt across its middle regions. Mr. Douglass, one of the observers at
+the Lowell Observatory, has noted upon Ganymede a number of markings
+somewhat resembling those seen on Mars, and he concludes, from their
+movement, that this satellite rotates on its axis in about seven days.
+Professor Barnard, on the other hand, does not corroborate this, though
+he claims to have discovered bright polar caps on both Ganymede and
+Callisto.
+
+In an earlier chapter we dealt at length with eclipses, occultations,
+and transits, and endeavoured to make clear the distinction between
+them. The system of Jupiter's satellites furnishes excellent examples of
+all these phenomena. The planet casts a very extensive shadow, and the
+satellites are constantly undergoing obscuration by passing through it.
+Such occurrences are plainly comparable to our lunar eclipses. Again,
+the satellites may, at one time, be occulted by the huge disc of the
+planet, and at another time seen in transit over its face. A fourth
+phenomenon is what is known as an _eclipse of the planet by a
+satellite_, which is the exact equivalent of what we style on the earth
+an eclipse of the sun. In this last case the shadow, cast by the
+satellite, appears as a round black spot in movement across the planet's
+surface.
+
+In the passages of these attendant bodies behind the planet, into its
+shadow, or across its face, respectively, it occasionally happens that
+Galileo's four satellites all disappear from view, and the planet is
+then seen for a while in the unusual condition of being apparently
+without its customary attendants. An instance of this phenomenon took
+place on the 3rd of October 1907. On that occasion, the satellites known
+as I. and III. (_i.e._ Io and Ganymede) were eclipsed, that is to say,
+obscured by passing into the planet's shadow; Satellite IV. (Callisto)
+was occulted by the planet's disc; while Satellite II. (Europa), being
+at the same moment in transit across the planet's face, was invisible
+against that brilliant background. A number of instances of this kind of
+occurrence are on record. Galileo, for example, noted one on the 15th of
+March 1611, while Herschel observed another on the 23rd of May 1802.
+
+It was indirectly to Jupiter's satellites that the world was first
+indebted for its knowledge of the velocity of light. When the periods of
+revolution of the satellites were originally determined, Jupiter
+happened, at the time, to be at his nearest to us. From the periods thus
+found tables were made for the prediction of the moments at which the
+eclipses and other phenomena of the satellites should take place. As
+Jupiter, in the course of his orbit, drew further away from the earth,
+it was noticed that the disappearances of the satellites into the shadow
+of the planet occurred regularly later than the time predicted. In the
+year 1675, Roemer, a Danish astronomer, inferred from this, not that the
+predictions were faulty, but that light did not travel instantaneously.
+It appeared, in fact, to take longer to reach us, the greater the
+distance it had to traverse. Thus, when the planet was far from the
+earth, the last ray given out by the satellite, before its passage into
+the shadow, took a longer time to cross the intervening space, than when
+the planet was near. Modern experiments in physics have quite confirmed
+this, and have proved for us that light does not travel across space in
+the twinkling of an eye, as might hastily be supposed, but actually
+moves, as has been already stated, at the rate of about 186,000 miles
+per second.
+
+
+THE PLANET SATURN
+
+Seen in the telescope the planet Saturn is a wonderful and very
+beautiful object. It is distinguished from all the other planets, in
+fact from all known celestial bodies, through being girt around its
+equator by what looks like a broad, flat ring of exceeding thinness.
+This, however, upon closer examination, is found to be actually composed
+of three concentric rings. The outermost of these is nearly of the same
+brightness as the body of the planet itself. The ring which comes
+immediately within it is also bright, and is separated from the outer
+one all the way round by a relatively narrow space, known as "Cassini's
+division," because it was discovered by the celebrated French
+astronomer, J.D. Cassini, in the year 1675. Inside the second ring, and
+merging insensibly into it, is a third one, known as the "crape ring,"
+because it is darker in hue than the others and partly transparent, the
+body of Saturn being visible through it. The inner boundary of this
+third and last ring does not adjoin the planet, but is everywhere
+separated from it by a definite space. This ring was discovered
+_independently_[23] in 1850 by Bond in America and Dawes in England.
+
+[Illustration: PLATE XV. THE PLANET SATURN
+
+From a drawing made by Professor Barnard with the Great Lick Telescope.
+The black band fringing the outer ring, where it crosses the disc, is
+portion of the _shadow which the rings cast upon the planet_. The black
+wedge-shaped mark, where the rings disappear behind the disc at the
+left-hand side, is portion of the _shadow which the planet casts upon
+the rings_.
+
+(Page 237)]
+
+As distinguished from the crape ring, the bright rings must have a
+considerable closeness of texture; for the shadow of the planet may be
+seen projected upon them, and their shadows in turn projected upon the
+surface of the planet (see Plate XV., p. 236).
+
+According to Professor Barnard, the entire breadth of the ring system,
+that is to say, from one side to the other of the outer ring, is 172,310
+miles, or somewhat more than double the planet's diameter.
+
+In the varying views which we get of Saturn, the system of the rings is
+presented to us at very different angles. Sometimes we are enabled to
+gaze upon its broad expanse; at other times, however, its thin edge is
+turned exactly towards us, an occurrence which takes place after
+intervals of about fifteen years. When this happened in 1892 the rings
+are said to have disappeared entirely from view in the great Lick
+telescope. We thus get an idea of their small degree of thickness, which
+would appear to be only about 50 miles. The last time the system of
+rings was exactly edgewise to the earth was on the 3rd of October 1907.
+
+The question of the composition of these rings has given rise to a good
+deal of speculation. It was formerly supposed that they were either
+solid or liquid, but in 1857 it was proved by Clerk Maxwell that a
+structure of this kind would not be able to stand. He showed, however,
+that they could be fully explained by supposing them to consist of an
+immense number of separate solid particles, or, as one might otherwise
+put it, extremely small satellites, circling in dense swarms around the
+middle portions of the planet. It is therefore believed that we have
+here the materials ready for the formation of a satellite or satellites;
+but that the powerful gravitative action, arising through the planet's
+being so near at hand, is too great ever to allow these materials to
+aggregate themselves into a solid mass. There is, as a matter of fact, a
+minimum distance from the body of any planet within which it can be
+shown that a satellite will be unable to form on account of
+gravitational stress. This is known as "Roche's limit," from the name of
+a French astronomer who specially investigated the question.
+
+There thus appears to be a certain degree of analogy between Saturn's
+rings and the asteroids. Empty spaces, too, exist in the asteroidal
+zone, the relative position of one of which bears a striking resemblance
+to that of "Cassini's division." It is suggested, indeed, that this
+division had its origin in gravitational disturbances produced by the
+attraction of the larger satellites, just as the empty spaces in the
+asteroidal zone are supposed to be the result of perturbations caused by
+the Giant Planet hard by.
+
+It has long been understood that the system of the rings must be
+rotating around Saturn, for if they were not in motion his intense
+gravitational attraction would quickly tear them in pieces. This was at
+length proved to be the fact by the late Professor Keeler, Director of
+the Lick Observatory, who from spectroscopic observations found that
+those portions of the rings situated near to the planet rotated faster
+than those farther from it. This directly supports the view that the
+rings are composed of satellites; for, as we have already seen, the
+nearer a satellite is to its primary the faster it will revolve. On the
+other hand, were the rings solid, their outer portions would move the
+fastest; as we have seen takes place in the body of the earth, for
+example. The mass of the ring system, however, must be exceedingly
+small, for it does not appear to produce any disturbances in the
+movements of Saturn's satellites. From the kinetic theory, therefore,
+one would not expect to find any atmosphere on the rings, and the
+absence of it is duly shown by spectroscopic observations.
+
+The diameter of Saturn, roughly speaking, is about one-fifth less than
+that of Jupiter. The planet is very flattened at the poles, this
+flattening being quite noticeable in a good telescope. For instance, the
+diameter across the equator is about 76,470 miles, while from pole to
+pole it is much less, namely, 69,770.
+
+The surface of Saturn bears a strong resemblance to that of Jupiter. Its
+markings, though not so well defined, are of the same belt-like
+description; and from observation of them it appears that the planet
+rotates _on an average_ in a little over ten hours. The rotation is in
+fact of the same peculiar kind as that of the sun and Jupiter; but the
+difference of speed at which the various portions of Saturn go round are
+even more marked than in the case of the Giant Planet. The density of
+Saturn is less than that of Jupiter; so that it must be largely in a
+condition of vapour, and in all probability at a still earlier stage of
+planetary evolution.
+
+Up to the present we know of as many as ten satellites circling around
+Saturn, which is more than any other planet of the solar system can lay
+claim to. Two of these, however, are very recent discoveries; one,
+Phoebe, having been found by photography in August 1898, and the other,
+Themis, in 1904, also by the same means. For both of these we are
+indebted to Professor W.H. Pickering. Themis is said to be _the faintest
+object in the solar system_. It cannot be _seen_, even with the largest
+telescope in existence; a fact which should hardly fail to impress upon
+one the great advantage the photographic plate possesses in these
+researches over the human eye.
+
+The most important of the whole Saturnian family of satellites are the
+two known as Titan and Japetus. These were discovered respectively by
+Huyghens in 1655 and by Cassini in 1671. Japetus is about the same size
+as our moon; while the diameter of Titan, the largest of the satellites,
+is about half as much again. Titan takes about sixteen days to revolve
+around Saturn, while Japetus takes more than two months and a half. The
+former is about three-quarters of a million miles distant from the
+planet, and the latter about two and a quarter millions. To Sir William
+Herschel we are indebted for the discovery of two more satellites, one
+of which he found on the evening that he used his celebrated 40-foot
+telescope for the first time. The ninth satellite, Phoebe, one of the
+two discovered by Professor Pickering, is perhaps the most remarkable
+body in the solar system, for all the other known members of that system
+perform their revolutions in one fixed direction, whereas this satellite
+revolves in the _contrary_ direction.
+
+In consequence of the great distance of Saturn, the sun, as seen from
+the planet, would appear so small that it would scarcely show any disc.
+The planet, indeed, only receives from the sun about one-ninetieth of
+the heat and light which the earth receives. Owing to this diminished
+intensity of illumination, the combined light reflected to Saturn by the
+whole of its satellites must be very small.
+
+With the sole exception of Jupiter, not one of the planets circulating
+nearer to the sun could be seen from Saturn, as they would be entirely
+lost in the solar glare. For an observer upon Saturn, Jupiter would,
+therefore, fill much the same position as Venus does for us, regularly
+displaying phases and being alternately a morning and an evening star.
+
+It is rather interesting to consider the appearances which would be
+produced in our skies were the earth embellished with a system of rings
+similar to those of Saturn. In consequence of the curving of the
+terrestrial surface, they would not be seen at all from within the
+Arctic or Antarctic circles, as they would be always below the horizon.
+From the equator they would be continually seen edgewise, and so would
+appear merely as line of light stretching right across the heaven and
+passing through the zenith. But the dwellers in the remaining regions
+would find them very objectionable, for they would cut off the light of
+the sun during lengthy periods of time.
+
+Saturn was a sore puzzle to the early telescopic observers. They did not
+for a long time grasp the fact that it was surrounded by a ring--so slow
+is the human mind to seek for explanations out of the ordinary course of
+things. The protrusions of the ring on either side of the planet, at
+first looked to Galileo like two minor globes placed on opposite sides
+of it, and slightly overlapping the disc. He therefore informed Kepler
+that "Saturn consists of three stars in contact with one another." Yet
+he was genuinely puzzled by the fact that the two attendant bodies (as
+he thought them) always retained the same position with regard to the
+planet's disc, and did not appear to revolve around it, nor to be in any
+wise shifted as a consequence of the movements of our earth.
+
+About a year and a half elapsed before he again examined Saturn; and, if
+he was previously puzzled, he was now thoroughly amazed. It happened
+just then to be one of those periods when the ring is edgewise towards
+the earth, and of course he only saw a round disc like that of Jupiter.
+What, indeed, had become of the attendant orbs? Was some demon mocking
+him? Had Saturn devoured his own children? He was, however, fated to be
+still more puzzled, for soon the minor orbs reappeared, and, becoming
+larger and larger as time went on, they ended by losing their globular
+appearance and became like two pairs of arms clasping the planet from
+each side! (see Plate XVI., p. 242).
+
+Galileo went to his grave with the riddle still unsolved, and it
+remained for the famous Dutch astronomer, Huyghens, to clear up the
+matter. It was, however, some little time before he hit upon the real
+explanation. Having noticed that there were dark spaces between the
+strange appendages and the body of the planet, he imagined Saturn to be
+a globe fitted with handles at each side; "ansae" these came to be
+called, from the Latin _ansa_, which means a handle. At length, in the
+year 1656, he solved the problem, and this he did by means of that
+123-foot tubeless telescope, of which mention has already been made. The
+ring happened then to be at its edgewise period, and a careful study of
+the behaviour of the ansae when disappearing and reappearing soon
+revealed to Huyghens the true explanation.
+
+[Illustration: PLATE XVI. EARLY REPRESENTATIONS OF SATURN
+
+From an illustration in the _Systema Saturnium_ of Christian Huyghens.
+
+(Page 242)]
+
+
+THE PLANETS URANUS AND NEPTUNE
+
+We have already explained (in Chapter II.) the circumstances in which
+both Uranus and Neptune were discovered. It should, however, be added
+that after the discovery of Uranus, that planet was found to have been
+already noted upon several occasions by different observers, but always
+without the least suspicion that it was other than a mere faint star.
+Again, with reference to the discovery of Neptune, it may here be
+mentioned that the apparent amount by which that planet had pulled
+Uranus out of its place upon the starry background was exceedingly
+small--so small, indeed, that no eye could have detected it without the
+aid of a telescope!
+
+Of the two predictions of the place of Neptune in the sky, that of Le
+Verrier was the nearer. Indeed, the position calculated by Adams was
+more than twice as far out. But Adams was by a long way the first in the
+field with his results, and only for unfortunate delays the prize would
+certainly have fallen to him. For instance, there was no star-map at
+Cambridge, and Professor Challis, the director of the observatory there,
+was in consequence obliged to make a laborious examination of the stars
+in the suspected region. On the other hand, all that Galle had to do was
+to compare that part of the sky where Le Verrier told him to look with
+the Berlin star-chart which he had by him. This he did on September 23,
+1846, with the result that he quickly noted an eighth magnitude star
+which did not figure in that chart. By the next night this star had
+altered its position in the sky, thus disclosing the fact that it was
+really a planet.
+
+Six days later Professor Challis succeeded in finding the planet, but of
+course he was now too late. On reviewing his labours he ascertained that
+he had actually noted down its place early in August, and had he only
+been able to sift his observations as he made them, the discovery would
+have been made then.
+
+Later on it was found that Neptune had only just missed being discovered
+about fifty years earlier. In certain observations made during 1795, the
+famous French astronomer, Lalande, found that a star, which he had
+mapped in a certain position on the 8th of May of that year, was in a
+different position two days later. The idea of a planet does not appear
+to have entered his mind, and he merely treated the first observation as
+an error!
+
+The reader will, no doubt, recollect how the discovery of the asteroids
+was due in effect to an apparent break in the seemingly regular sequence
+of the planetary orbits outwards from the sun. This curious sequence of
+relative distances is usually known as "Bode's Law," because it was
+first brought into general notice by an astronomer of that name. It had,
+however, previously been investigated mathematically by Titius in 1772.
+Long before this, indeed, the unnecessarily wide space between the
+orbits of Mars and Jupiter had attracted the attention of the great
+Kepler to such a degree, that he predicted that a planet would some day
+be found to fill the void. Notwithstanding the service which the
+so-called Law of Bode has indirectly rendered to astronomy, it has
+strangely enough been found after all not to rest upon any scientific
+foundation. It will not account for the distance from the sun of the
+orbit of Neptune, and the very sequence seems on the whole to be in the
+nature of a mere coincidence.
+
+Neptune is invisible to the naked eye; Uranus is just at the limit of
+visibility. Both planets are, however, so far from us that we can get
+but the poorest knowledge of their condition and surroundings. Uranus,
+up to the present, is known to be attended by four satellites, and
+Neptune by one. The planets themselves are about equal in size; their
+diameters, roughly speaking, being about one-half that of Saturn. Some
+markings have, indeed, been seen upon the disc of Uranus, but they are
+very indistinct and fleeting. From observation of them, it is assumed
+that the planet rotates on its axis in a period of some ten to twelve
+hours. No definite markings have as yet been seen upon Neptune, which
+body is described by several observers as resembling a faint planetary
+nebula.
+
+With regard to their physical condition, the most that can be said about
+these two planets is that they are probably in much the same vaporous
+state as Jupiter and Saturn. On account of their great distance from the
+sun they can receive but little solar heat and light. Seen from
+Neptune, in fact, the sun would appear only about the size of Venus at
+her best, though of a brightness sufficiently intense to illumine the
+Neptunian landscape with about seven hundred times our full moonlight.
+
+
+[22] Mr. P. Melotte, of Greenwich Observatory, while examining a
+photograph taken there on February 28, 1908, discovered upon it a very
+faint object which it is firmly believed will prove to be an _eighth_
+satellite of Jupiter. This object was afterwards found on plates exposed
+as far back as January 27. It has since been photographed several times
+at Greenwich, and also at Heidelberg (by Dr. Max Wolf) and at the Lick
+Observatory. Its movement is probably _retrograde_, like that of Phoebe
+(p. 240).
+
+[23] In the history of astronomy two salient points stand out.
+
+The first of these is the number of "independent" discoveries which have
+taken place; such, for instance, as in the cases of Le Verrier and Adams
+with regard to Neptune, and of Lockyer and Janssen in the matter of the
+spectroscopic method of observing solar prominences.
+
+The other is the great amount of "anticipation." Copernicus, as we have
+seen, was anticipated by the Greeks; Kepler was not actually the first
+who thought of elliptic orbits; others before Newton had imagined an
+attractive force.
+
+Both these points furnish much food for thought!
+
+
+
+
+CHAPTER XIX
+
+COMETS
+
+
+The reader has, no doubt, been struck by the marked uniformity which
+exists among those members of the solar system with which we have dealt
+up to the present. The sun, the planets, and their satellites are all
+what we call solid bodies. The planets move around the sun, and the
+satellites around the planets, in orbits which, though strictly
+speaking, ellipses, are yet not in any instance of a very oval form. Two
+results naturally follow from these considerations. Firstly, the bodies
+in question hide the light coming to us from those further off, when
+they pass in front of them. Secondly, the planets never get so far from
+the sun that we lose sight of them altogether.
+
+With the objects known as Comets it is, however, quite the contrary.
+These objects do not conform to our notions of solidity. They are so
+transparent that they can pass across the smallest star without dimming
+its light in the slightest degree. Again, they are only visible to us
+during a portion of their orbits. A comet may be briefly described as an
+illuminated filmy-looking object, made up usually of three portions--a
+head, a nucleus, or brighter central portion within this head, and a
+tail. The heads of comets vary greatly in size; some, indeed, appear
+quite small, like stars, while others look even as large as the moon.
+Occasionally the nucleus is wanting, and sometimes the tail also.
+
+[Illustration: FIG. 18.--Showing how the Tail of a Comet is directed
+away from the Sun.]
+
+These mysterious visitors to our skies come up into view out of the
+immensities beyond, move towards the sun at a rapidly increasing speed,
+and, having gone around it, dash away again into the depths of space. As
+a comet approaches the sun, its body appears to grow smaller and
+smaller, while, at the same time, it gradually throws out behind it an
+appendage like a tail. As the comet moves round the central orb this
+tail is always directed _away_ from the sun; and when it departs again
+into space the tail goes in advance. As the comet's distance from the
+sun increases, the tail gradually shrinks away and the head once more
+grows in size (see Fig. 18). In consequence of these changes, and of the
+fact that we lose sight of comets comparatively quickly, one is much
+inclined to wonder what further changes may take place after the bodies
+have passed beyond our ken.
+
+The orbits of comets are, as we have seen, very elliptic. In some
+instances this ellipticity is so great as to take the bodies out into
+space to nearly six times the distance of Neptune from the sun. For a
+long time, indeed, it was considered that comets were of two kinds,
+namely, those which actually _belonged_ to the solar system, and those
+which were merely _visitors_ to it for the first and only time--rushing
+in from the depths of space, rapidly circuiting the sun, and finally
+dashing away into space again, never to return. On the contrary,
+nowadays, astronomers are generally inclined to regard comets as
+permanent members of the solar system.
+
+The difficulty, however, of deciding absolutely whether the orbits of
+comets are really always _closed_ curves, that is to say, curves which
+must sooner or later bring the bodies back again towards the sun, is,
+indeed, very great. Comets, in the first place, are always so diffuse,
+that it is impossible to determine their exact position, or, rather, the
+exact position of that important point within them, known as the centre
+of gravity. Secondly, that stretch of its orbit along which we can
+follow a comet, is such a very small portion of the whole path, that the
+slightest errors of observation which we make will result in
+considerably altering our estimate of the actual shape of the orbit.
+
+Comets have been described as so transparent that they can pass across
+the sky without dimming the lustre of the smallest stars, which the
+thinnest fog or mist would do. This is, indeed, true of every portion
+of a comet except the nucleus, which is, as its name implies, the
+densest part. And yet, in contrast to this ghostlike character, is the
+strange fact that when comets are of a certain brightness they may
+actually be seen in full daylight.
+
+As might be gathered from their extreme tenuity, comets are so
+exceedingly small in mass that they do not appear to exert any
+gravitational attraction upon the other bodies of our system. It is,
+indeed, a known fact that in the year 1886 a comet passed right amidst
+the satellites of Jupiter without disturbing them in the slightest
+degree. The attraction of the planet, on the other hand, so altered the
+comet's orbit, as to cause it to revolve around the sun in a period of
+seven years, instead of twenty-seven, as had previously been the case.
+Also, in 1779, the comet known as Lexell's passed quite close to
+Jupiter, and its orbit was so changed by that planet's attraction that
+it has never been seen since. The density of comets must, as a rule, be
+very much less than the one-thousandth part of that of the air at the
+surface of our globe; for, if the density of the comet were even so
+small as this, its mass would _not_ be inappreciable.
+
+If comets are really undoubted members of the solar system, the
+circumstances in which they were evolved must have been different from
+those which produced the planets and satellites. The axial rotations of
+both the latter, and also their revolutions, take place in one certain
+direction;[24] their orbits, too, are ellipses which do not differ much
+from circles, and which, furthermore, are situated fairly in the one
+plane. Comets, on the other hand, do not necessarily travel round the
+sun in the same fixed direction as the planets. Their orbits, besides,
+are exceedingly elliptic; and, far from keeping to one plane, or even
+near it, they approach the sun from all directions.
+
+Broadly speaking, comets may be divided into two distinct classes, or
+"families." In the first class, the same orbit appears to be shared in
+common by a series of comets which travel along it, one following the
+other. The comets which appeared in the years 1668, 1843, 1880, 1882,
+and 1887 are instances of a number of different bodies pursuing the same
+path around the sun. The members of a comet family of this kind are
+observed to have similar characteristics. The idea is that such comets
+are merely portions of one much larger cometary body, which became
+broken up by the gravitational action of other bodies in the system, or
+through violent encounter with the sun's surroundings.
+
+The second class is composed of comets which are supposed to have been
+seized by the gravitative action of certain planets, and thus forced to
+revolve in short ellipses around the sun, well within the limits of the
+solar system. These comets are, in consequence, spoken of as "captures."
+They move around the sun in the same direction as the planets do.
+Jupiter has a fairly large comet family of this kind attached to him. As
+a result of his overpowering gravitation, it is imagined that during the
+ages he must have attracted a large number of these bodies on his own
+account, and, perhaps, have robbed other planets of their captures. His
+family at present numbers about thirty. Of the other planets, so far as
+we know, Saturn possesses a comet family of two, Uranus three, and
+Neptune six. There are, indeed, a few comets which appear as if under
+the influence of some force situated outside the known bounds of the
+solar system, a circumstance which goes to strengthen the idea that
+other planets may revolve beyond the orbit of Neptune. The terrestrial
+planets, on the other hand, cannot have comet families; because the
+enormous gravitative action of the sun in their vicinity entirely
+overpowers the attractive force which they exert upon those comets which
+pass close to them. Besides this, a comet, when in the inner regions of
+the solar system, moves with such rapidity, that the gravitational pull
+of the planets there situated is not powerful enough to deflect it to
+any extent. It must not be presumed, however, that a comet once captured
+should always remain a prisoner. Further disturbing causes might
+unsettle its newly acquired orbit, and send it out again into the
+celestial spaces.
+
+With regard to the matter of which comets are composed, the spectroscope
+shows the presence in them of hydrocarbon compounds (a notable
+characteristic of these bodies), and at times, also, of sodium and iron.
+Some of the light which we get from comets is, however, merely reflected
+sunlight.
+
+The fact that the tails of comets are always directed away from the sun,
+has given rise to the idea that this is caused by some repelling action
+emanating from the sun itself, which is continually driving off the
+smallest particles. Two leading theories have been formulated to account
+for the tails themselves upon the above assumption. One of these, first
+suggested by Olbers in 1812, and now associated with the name of the
+Russian astronomer, the late Professor Bredikhine, who carefully worked
+it out, presumes an electrical action emanating from the sun; the other,
+that of Arrhenius, supposes a pressure exerted by the solar light in its
+radiation outwards into space. It is possible, indeed, that repelling
+forces of both these kinds may be at work together. Minute particles are
+probably being continually produced by friction and collisions among the
+more solid parts in the heads of comets. Supposing that such particles
+are driven off altogether, one may therefore assume that the so-called
+captured comets are disintegrating at a comparatively rapid rate. Kepler
+long ago maintained that "comets die," and this actually appears to be
+the case. The ordinary periodic ones, such, for instance, as Encke's
+Comet, are very faint, and becoming fainter at each return. Certain of
+these comets have, indeed, failed altogether to reappear. It is notable
+that the members of Jupiter's comet family are not very conspicuous
+objects. They have small tails, and even in some cases have none at all.
+The family, too, does not contain many members, and yet one cannot but
+suppose that Jupiter, on account of his great mass, has had many
+opportunities for making captures adown the ages.
+
+Of the two theories to which allusion has above been made, that of
+Bredikhine has been worked out so carefully, and with such a show of
+plausibility, that it here calls for a detailed description. It appears
+besides to explain the phenomena of comets' tails so much more
+satisfactorily than that of Arrhenius, that astronomers are inclined to
+accept it the more readily of the two. According to Bredikhine's theory
+the electrical repulsive force, which he assumes for the purposes of his
+argument, will drive the minutest particles of the comet in a direction
+away from the sun much more readily than the gravitative action of that
+body will pull them towards it. This may be compared to the ease with
+which fine dust may be blown upwards, although the earth's gravitation
+is acting upon it all the time.
+
+The researches of Bredikhine, which began seriously with his
+investigation of Coggia's Comet of 1874, led him to classify the tails
+of comets in _three types_. Presuming that the repulsive force emanating
+from the sun did not vary, he came to the conclusion that the different
+forms assumed by cometary tails must be ascribed to the special action
+of this force upon the various elements which happen to be present in
+the comet. The tails which he classes as of the first type, are those
+which are long and straight and point directly away from the sun.
+Examples of such tails are found in the comets of 1811, 1843, and 1861.
+Tails of this kind, he thinks, are in all probability formed of
+_hydrogen_. His second type comprises those which are pointed away from
+the sun, but at the same time are considerably curved, as was seen in
+the comets of Donati and Coggia. These tails are formed of _hydrocarbon
+gas_. The third type of tail is short, brush-like, and strongly bent,
+and is formed of the _vapour of iron_, mixed with that of sodium and
+other elements. It should, however, be noted that comets have
+occasionally been seen which possess several tails of these various
+types.
+
+We will now touch upon a few of the best known comets of modern times.
+
+The comet of 1680 was the first whose orbit was calculated according to
+the laws of gravitation. This was accomplished by Newton, and he found
+that the comet in question completed its journey round the sun in a
+period of about 600 years.
+
+In 1682 there appeared a great comet, which has become famous under the
+name of Halley's Comet, in consequence of the profound investigations
+made into its motion by the great astronomer, Edmund Halley. He fixed
+its period of revolution around the sun at about seventy-five years, and
+predicted that it would reappear in the early part of 1759. He did not,
+however, live to see this fulfilled, but the comet duly returned--_the
+first body of the kind to verify such a prediction_--and was detected on
+Christmas Day, 1758, by George Palitzch, an amateur observer living near
+Dresden. Halley also investigated the past history of the comet, and
+traced it back to the year 1456. The orbit of Halley's comet passes out
+slightly beyond the orbit of Neptune. At its last visit in 1835, this
+comet passed comparatively close to us, namely, within five million
+miles of the earth. According to the calculations of Messrs P.H. Cowell
+and A.C.D. Crommelin of Greenwich Observatory, its next return will be
+in the spring of 1910; the nearest approach to the earth taking place
+about May 12.
+
+On the 26th of March, 1811, a great comet appeared, which remained
+visible for nearly a year and a half. It was a magnificent object; the
+tail being about 100 millions of miles in length, and the head about
+127,000 miles in diameter. A detailed study which he gave to this comet
+prompted Olbers to put forward that theory of electrical repulsion
+which, as we have seen, has since been so carefully worked out by
+Bredikhine. Olbers had noticed that the particles expelled from the head
+appeared to travel to the end of the tail in about eleven minutes, thus
+showing a velocity per second very similar to that of light.
+
+The discovery in 1819 of the comet known as Encke's, because its orbit
+was determined by an astronomer of that name, drew attention for the
+first time to Jupiter's comet family, and, indeed, to short-period
+comets in general. This comet revolves around the sun in the shortest
+known period of any of these bodies, namely, 3-1/3 years. Encke
+predicted that it would return in 1822. This duly occurred, the comet
+passing at its nearest to the sun within three hours of the time
+indicated; being thus the second instance of the fulfilment of a
+prediction of the kind. A certain degree of irregularity which Encke's
+Comet displays in the dates of its returns to the sun, has been supposed
+to indicate that it passes in the course of its orbit through some
+retarding medium, but no definite conclusions have so far been arrived
+at in this matter.
+
+A comet, which appeared in 1826, goes by the name of Biela's Comet,
+because of its discovery by an Austrian military officer, Wilhelm von
+Biela. This comet was found to have a period of between six and seven
+years. Certain calculations made by Olbers showed that, at its return in
+1832, it would pass _through the earth's orbit_. The announcement of
+this gave rise to a panic; for people did not wait to inquire whether
+the earth would be anywhere near that part of its orbit when the comet
+passed. The panic, however, subsided when the French astronomer, Arago,
+showed that at the moment in question the earth would be some 50
+millions of miles away from the point indicated!
+
+[Illustration: PLATE XVII. DONATI'S COMET
+
+From a drawing made on October 9th, 1858, by G.P. Bond, of Harvard
+College Observatory, U.S.A. A good illustration of Bredikhine's theory:
+note the straight tails of his _first_ type, and the curved tail of his
+_second_.
+
+(Page 257)]
+
+In 1846, shortly after one of its returns, Biela's Comet divided into
+two portions. At its next appearance (1852) these portions had separated
+to a distance of about 1-1/2 millions of miles from each other. This
+comet, or rather its constituents, have never since been seen.
+
+Perhaps the most remarkable comet of recent times was that of 1858,
+known as Donati's, it having been discovered at Florence by the Italian
+astronomer, G.B. Donati. This comet, a magnificent object, was visible
+for more than three months with the naked eye. Its tail was then 54
+millions of miles in length. It was found to revolve around the sun in a
+period of over 2000 years, and to go out in its journey to about 5-1/2
+times the distance of Neptune. Its motion is retrograde, that is to say,
+in the contrary direction to the usual movement in the solar system. A
+number of beautiful drawings of Donati's Comet were made by the American
+astronomer, G.P. Bond. One of the best of these is reproduced on Plate
+XVII., p. 256.
+
+In 1861 there appeared a great comet. On the 30th of June of that year
+the earth and moon actually passed through its tail; but no effects were
+noticed, other than a peculiar luminosity in the sky.
+
+In the year 1881 there appeared another large comet, known as Tebbutt's
+Comet, from the name of its discoverer. This was the _first comet of
+which a satisfactory photograph was obtained_. The photograph in
+question was taken by the late M. Janssen.
+
+The comet of 1882 was of vast size and brilliance. It approached so
+close to the sun that it passed through some 100,000 miles of the solar
+corona. Though its orbit was not found to have been altered by this
+experience, its nucleus displayed signs of breaking up. Some very fine
+photographs of this comet were obtained at the Cape of Good Hope by Mr.
+(now Sir David) Gill.
+
+The comet of 1889 was followed with the telescope nearly up to the orbit
+of Saturn, which seems to be the greatest distance at which a comet has
+ever been seen.
+
+The _first discovery of a comet by photographic means_[25] was made by
+Professor Barnard in 1892; and, since then, photography has been
+employed with marked success in the detection of small periodic comets.
+
+The best comet seen in the Northern hemisphere since that of 1882,
+appears to have been Daniel's Comet of 1907 (see Plate XVIII., p. 258).
+This comet was discovered on June 9, 1907, by Mr. Z. Daniel, at
+Princeton Observatory, New Jersey, U.S.A. It became visible to the naked
+eye about mid-July of that year, and reached its greatest brilliancy
+about the end of August. It did not, however, attract much popular
+attention, as its position in the sky allowed it to be seen only just
+before dawn.
+
+
+[24] With the exception, of course, of such an anomaly as the retrograde
+motion of the ninth satellite of Saturn.
+
+[25] If we except the case of the comet which was photographed near the
+solar corona in the eclipse of 1882.
+
+[Illustration: PLATE XVIII. DANIEL'S COMET OF 1907
+
+From a photograph taken, on August 11th, 1907, by Dr. Max Wolf, at the
+Astrophysical Observatory, Heidelberg. The instrument used was a 28-inch
+reflecting telescope, and the time of exposure was fifteen minutes. As
+the telescope was guided to follow the moving comet, the stars have
+imprinted themselves upon the photographic plate as short trails. This
+is clearly the opposite to what is depicted on Plate XIII.
+
+(Page 258)]
+
+
+
+
+CHAPTER XX
+
+REMARKABLE COMETS
+
+
+If eclipses were a cause of terror in past ages, comets appear to have
+been doubly so. Their much longer continuance in the sight of men had no
+doubt something to say to this, and also the fact that they arrived
+without warning; it not being then possible to give even a rough
+prediction of their return, as in the case of eclipses. As both these
+phenomena were occasional, and out of the ordinary course of things,
+they drew exceptional attention as unusual events always do; for it must
+be allowed that quite as wonderful things exist, but they pass unnoticed
+merely because men have grown accustomed to them.
+
+For some reason the ancients elected to class comets along with meteors,
+the aurora borealis, and other phenomena of the atmosphere, rather than
+with the planets and the bodies of the spaces beyond. The sudden
+appearance of these objects led them to be regarded as signs sent by the
+gods to announce remarkable events, chief among these being the deaths
+of monarchs. Shakespeare has reminded us of this in those celebrated
+lines in _Julius Caesar_:--
+
+"When beggars die there are no comets seen,
+The heavens themselves blaze forth the death of princes."
+
+Numbed by fear, the men of old blindly accepted these presages of fate;
+and did not too closely question whether the threatened danger was to
+their own nation or to some other, to their ruler or to his enemy. Now
+and then, as in the case of the Roman Emperor Vespasian, there was a
+cynical attempt to apply some reasoning to the portent. That emperor, in
+alluding to the comet of A.D. 79, is reported to have said: "This hairy
+star does not concern me; it menaces rather the King of the Parthians,
+for he is hairy and I am bald." Vespasian, all the same, died shortly
+afterwards!
+
+Pliny, in his natural history, gives several instances of the terrible
+significance which the ancients attached to comets. "A comet," he says,
+"is ordinarily a very fearful star; it announces no small effusion of
+blood. We have seen an example of this during the civil commotion of
+Octavius."
+
+A very brilliant comet appeared in 371 B.C., and about the same time an
+earthquake caused Helice and Bura, two towns in Achaia, to be swallowed
+up by the sea. The following remark made by Seneca concerning it shows
+that the ancients did not consider comets merely as precursors, but even
+as actual _causes_ of fatal events: "This comet, so anxiously observed
+by every one, _because of the great catastrophe which it produced as
+soon as it appeared_, the submersion of Bura and Helice."
+
+Comets are by no means rare visitors to our skies, and very few years
+have elapsed in historical times without such objects making their
+appearance. In the Dark and Middle Ages, when Europe was split up into
+many small kingdoms and principalities, it was, of course, hardly
+possible for a comet to appear without the death of some ruler occurring
+near the time. Critical situations, too, were continually arising in
+those disturbed days. The end of Louis le Debonnaire was hastened, as
+the reader will, no doubt, recollect, by the great eclipse of 840; but
+it was firmly believed that a comet which had appeared a year or two
+previously presaged his death. The comet of 1556 is reported to have
+_influenced_ the abdication of the Emperor Charles V.; but curiously
+enough, this event had already taken place before the comet made its
+appearance! Such beliefs, no doubt, had a very real effect upon rulers
+of a superstitious nature, or in a weak state of health. For instance,
+Gian Galeazzo Visconti, Duke of Milan, was sick when the comet of 1402
+appeared. After seeing it, he is said to have exclaimed: "I render
+thanks to God for having decreed that my death should be announced to
+men by this celestial sign." His malady then became worse, and he died
+shortly afterwards.
+
+It is indeed not improbable that such superstitious fears in monarchs
+were fanned by those who would profit by their deaths, and yet did not
+wish to stain their own hands with blood.
+
+Evil though its effects may have been, this morbid interest which past
+ages took in comets has proved of the greatest service to our science.
+Had it not been believed that the appearance of these objects was
+attended with far-reaching effects, it is very doubtful whether the old
+chroniclers would have given themselves the trouble of alluding to them
+at all; and thus the modern investigators of cometary orbits would have
+lacked a great deal of important material.
+
+We will now mention a few of the most notable comets which historians
+have recorded.
+
+A comet which appeared in 344 B.C. was thought to betoken the success
+of the expedition undertaken in that year by Timoleon of Corinth against
+Sicily. "The gods by an extraordinary prodigy announced his success and
+future greatness: a burning torch appeared in the heavens throughout the
+night and preceded the fleet of Timoleon until it arrived off the coast
+of Sicily."
+
+The comet of 43 B.C. was generally believed to be the soul of Caesar on
+its way to heaven.
+
+Josephus tells us that in A.D. 69 several prodigies, and amongst them a
+comet in the shape of a sword, announced the destruction of Jerusalem.
+This comet is said to have remained over the city for the space of a
+year!
+
+A comet which appeared in A.D. 336 was considered to have announced the
+death of the Emperor Constantine.
+
+But perhaps the most celebrated comet of early times was the one which
+appeared in A.D. 1000. That year was, in more than one way, big with
+portent, for there had long been a firm belief that the Christian era
+could not possibly run into four figures. Men, indeed, steadfastly
+believed that when the thousand years had ended, the millennium would
+immediately begin. Therefore they did not reap neither did they sow,
+they toiled not, neither did they spin, and the appearance of the comet
+strengthened their convictions. The fateful year, however, passed by
+without anything remarkable taking place; but the neglect of husbandry
+brought great famine and pestilence over Europe in the years which
+followed.
+
+In April 1066, that year fraught with such immense consequences for
+England, a comet appeared. No one doubted but that it was a presage of
+the success of the Conquest, and perhaps, indeed, it had its due weight
+in determining the minds and actions of the men who took part in the
+expedition. _Nova stella, novus rex_ ("a new star, a new sovereign") was
+a favourite proverb of the time. The chroniclers, with one accord, have
+delighted to relate that the Normans, "guided by a comet," invaded
+England. A representation of this object appears in the Bayeux Tapestry
+(see Fig. 19, p. 263).[26]
+
+[Illustration: FIG. 19.--The comet of 1066, as represented in the Bayeux
+Tapestry.
+
+(From the _World of Comets_.)]
+
+We have mentioned Halley's Comet of 1682, and how it revisits the
+neighbourhood of the earth at intervals of seventy-six years. The comet
+of 1066 has for many years been supposed to be Halley's Comet on one of
+its visits. The identity of these two, however, was only quite recently
+placed beyond all doubt by the investigations of Messrs Cowell and
+Crommelin. This comet appeared also in 1456, when John Huniades was
+defending Belgrade against the Turks led by Mahomet II., the conqueror
+of Constantinople, and is said to have paralysed both armies with fear.
+
+The Middle Ages have left us descriptions of comets, which show only too
+well how the imagination will run riot under the stimulus of terror. For
+instance, the historian, Nicetas, thus describes the comet of the year
+1182: "After the Romans were driven from Constantinople a prognostic was
+seen of the excesses and crimes to which Andronicus was to abandon
+himself. A comet appeared in the heavens similar to a writhing serpent;
+sometimes it extended itself, sometimes it drew itself in; sometimes, to
+the great terror of the spectators, it opened a huge mouth; it seemed
+that, as if thirsting for human blood, it was upon the point of
+satiating itself." And, again, the celebrated Ambrose Pare, the father
+of surgery, has left us the following account of the comet of 1528,
+which appeared in his own time: "This comet," said he, "was so horrible,
+so frightful, and it produced such great terror in the vulgar, that some
+died of fear, and others fell sick. It appeared to be of excessive
+length, and was of the colour of blood. At the summit of it was seen the
+figure of a bent arm, holding in its hand a great sword, as if about to
+strike. At the end of the point there were three stars. On both sides of
+the rays of this comet were seen a great number of axes, knives,
+blood-coloured swords, among which were a great number of hideous human
+faces, with beards and bristling hair." Pare, it is true, was no
+astronomer; yet this shows the effect of the phenomenon, even upon a man
+of great learning, as undoubtedly he was. It should here be mentioned
+that nothing very remarkable happened at or near the year 1528.
+
+Concerning the comet of 1680, the extraordinary story got about that, at
+Rome, a hen had laid an egg on which appeared a representation of the
+comet!
+
+But the superstitions with regard to comets were now nearing their end.
+The last blow was given by Halley, who definitely proved that they
+obeyed the laws of gravitation, and circulated around the sun as planets
+do; and further announced that the comet of 1682 had a period of
+seventy-six years, which would cause it to reappear in the year 1759. We
+have seen how this prediction was duly verified. We have seen, too, how
+this comet appeared again in 1835, and how it is due to return in the
+early part of 1910.
+
+
+[26] With regard to the words "Isti mirant stella" in the figure, Mr.
+W.T. Lynn suggests that they may not, after all, be the grammatically
+bad Latin which they appear, but that the legend is really "Isti
+mirantur stellam," the missing letters being supposed to be hidden by
+the building and the comet.
+
+
+
+
+CHAPTER XXI
+
+METEORS OR SHOOTING STARS
+
+
+Any one who happens to gaze at the sky for a short time on a clear night
+is pretty certain to be rewarded with a view of what is popularly known
+as a "shooting star." Such an object, however, is not a star at all, but
+has received its appellation from an analogy; for the phenomenon gives
+to the inexperienced in these matters an impression as if one of the
+many points of light, which glitter in the vaulted heaven, had suddenly
+become loosened from its place, and was falling towards the earth. In
+its passage across the sky the moving object leaves behind a trail of
+light which usually lasts for a few moments. Shooting stars, or meteors,
+as they are technically termed, are for the most part very small bodies,
+perhaps no larger than peas or pebbles, which, dashing towards our earth
+from space beyond, are heated to a white heat, and reduced to powder by
+the friction resulting from their rapid passage into our atmosphere.
+This they enter at various degrees of speed, in some cases so great as
+45 miles a second. The speed, of course, will depend greatly upon
+whether the earth and the meteors are rushing towards each other, or
+whether the latter are merely overtaking the earth. In the first of
+these cases the meteors will naturally collide with the atmosphere with
+great force; in the other case they will plainly come into it with much
+less rapidity. As has been already stated, it is from observations of
+such bodies that we are enabled to estimate, though very imperfectly,
+the height at which the air around our globe practically ceases, and
+this height is imagined to be somewhere about 100 miles. Fortunate,
+indeed, is it for us that there is a goodly layer of atmosphere over our
+heads, for, were this not so, these visitors from space would strike
+upon the surface of our earth night and day, and render existence still
+more unendurable than many persons choose to consider it. To what a
+bombardment must the moon be continually subject, destitute as she is of
+such an atmospheric shield!
+
+It is only in the moment of their dissolution that we really learn
+anything about meteors, for these bodies are much too small to be seen
+before they enter our atmosphere. The debris arising from their
+destruction is wafted over the earth, and, settling down eventually upon
+its surface, goes to augment the accumulation of that humble domestic
+commodity which men call dust. This continual addition of material
+tends, of course, to increase the mass of the earth, though the effect
+thus produced will be on an exceedingly small scale.
+
+The total number of meteors moving about in space must be practically
+countless. The number which actually dash into the earth's atmosphere
+during each year is, indeed, very great. Professor Simon Newcomb, the
+well-known American astronomer, has estimated that, of the latter, those
+large enough to be seen with the naked eye cannot be in all less than
+146,000,000,000 per annum. Ten times more numerous still are thought to
+be those insignificant ones which are seen to pass like mere sparks of
+light across the field of an observer's telescope.
+
+Until comparatively recent times, perhaps up to about a hundred years
+ago, it was thought that meteors were purely terrestrial phenomena which
+had their origin in the upper regions of the air. It, however, began to
+be noticed that at certain periods of the year these moving objects
+appeared to come from definite areas of the sky. Considerations,
+therefore, respecting their observed velocities, directions, and
+altitudes, gave rise to the theory that they are swarms of small bodies
+travelling around the sun in elongated elliptical orbits, all along the
+length of which they are scattered, and that the earth, in its annual
+revolution, rushing through the midst of such swarms at the same epoch
+each year, naturally entangles many of them in its atmospheric net.
+
+The dates at which the earth is expected to pass through the principal
+meteor-swarms are now pretty well known. These swarms are distinguished
+from one another by the direction of the sky from which the meteors seem
+to arrive. Many of the swarms are so wide that the earth takes days, and
+even weeks, to pass through them. In some of these swarms, or streams,
+as they are also called, the meteors are distributed with fair evenness
+along the entire length of their orbits, so that the earth is greeted
+with a somewhat similar shower at each yearly encounter. In others, the
+chief portions are bunched together, so that, in certain years, the
+display is exceptional (see Fig. 20, p. 269). That part of the heavens
+from which a shower of meteors is seen to emanate is called the
+"radiant," or radiant point, because the foreshortened view we get of
+the streaks of light makes it appear as if they radiated outwards from
+this point. In observations of these bodies the attention of astronomers
+is directed to registering the path and speed of each meteor, and to
+ascertaining the position of the radiant. It is from data such as these
+that computations concerning the swarms and their orbits are made.
+
+[Illustration: FIG. 20.--Passage of the Earth through the thickest
+portion of a Meteor Swarm. The Earth and the Meteors are here
+represented as approaching each other from opposite directions.]
+
+For the present state of knowledge concerning meteors, astronomy is
+largely indebted to the researches of Mr. W.F. Denning, of Bristol, and
+of the late Professor A.S. Herschel.
+
+During the course of each year the earth encounters a goodly number of
+meteor-swarms. Three of these, giving rise to fine displays, are very
+well known--the "Perseids," or August Meteors, and the "Leonids" and
+"Bielids," which appear in November.
+
+Of the above three the _Leonid_ display is by far the most important,
+and the high degree of attention paid to it has laid the foundation of
+meteoric astronomy in much the same way that the study of the
+fascinating corona has given such an impetus to our knowledge of the
+sun. The history of this shower of meteors may be traced back as far as
+A.D. 902, which was known as the "Year of the Stars." It is related that
+in that year, on the night of October 12th--the shower now comes about a
+month later--whilst the Moorish King, Ibrahim Ben Ahmed, lay dying
+before Cosenza, in Calabria, "a multitude of falling stars scattered
+themselves across the sky like rain," and the beholders shuddered at
+what they considered a dread celestial portent. We have, however, little
+knowledge of the subsequent history of the Leonids until 1698, since
+which time the maximum shower has appeared with considerable regularity
+at intervals of about thirty-three years. But it was not until 1799 that
+they sprang into especial notice. On the 11th November in that year a
+splendid display was witnessed at Cumana, in South America, by the
+celebrated travellers, Humboldt and Bonpland. Finer still, and
+surpassing all displays of the kind ever seen, was that of November 12,
+1833, when the meteors fell thick as snowflakes, 240,000 being estimated
+to have appeared during seven hours. Some of them were even so bright as
+to be seen in full daylight. The radiant from which the meteors seem to
+diverge was ascertained to be situated in the head of the constellation
+of the Lion, or "Sickle of Leo," as it is popularly termed, whence
+their name--Leonids. It was from a discussion of the observations then
+made that the American astronomer, Olmsted, concluded that these meteors
+sprang upon us from interplanetary space, and were not, as had been
+hitherto thought, born of our atmosphere. Later on, in 1837, Olbers
+formulated the theory that the bodies in question travelled around the
+sun in an elliptical orbit, and at the same time he established the
+periodicity of the maximum shower.
+
+The periodic time of recurrence of this maximum, namely, about
+thirty-three years, led to eager expectancy as 1866 drew near. Hopes
+were then fulfilled, and another splendid display took place, of which
+Sir Robert Ball, who observed it, has given a graphic description in his
+_Story of the Heavens_. The display was repeated upon a smaller scale in
+the two following years. The Leonids were henceforth deemed to hold an
+anomalous position among meteor swarms. According to theory the earth
+cut through their orbit at about the same date each year, and so a
+certain number were then seen to issue from the radiant. But, in
+addition, after intervals of thirty-three years, as has been seen, an
+exceptional display always took place; and this state of things was not
+limited to one year alone, but was repeated at each meeting for about
+three years running. The further assumption was, therefore, made that
+the swarm was much denser in one portion of the orbit than
+elsewhere,[27] and that this congested part was drawn out to such an
+extent that the earth could pass through the crossing place during
+several annual meetings, and still find it going by like a long
+procession (see Fig. 20, p. 269).
+
+In accordance with this ascertained period of thirty-three years, the
+recurrence of the great Leonid shower was timed to take place on the
+15th of November 1899. But there was disappointment then, and the
+displays which occurred during the few years following were not of much
+importance. A good deal of comment was made at the time, and theories
+were accordingly put forward to account for the failure of the great
+shower. The most probable explanation seems to be, that the attraction
+of one of the larger planets--Jupiter perhaps--has diverted the orbit
+somewhat from its old position, and the earth does not in consequence
+cut through the swarm in the same manner as it used to do.
+
+The other November display alluded to takes place between the 23rd and
+27th of that month. It is called the _Andromedid_ Shower, because the
+meteors appear to issue from the direction of the constellation of
+Andromeda, which at that period of the year is well overhead during the
+early hours of the night. These meteors are also known by the name of
+_Bielids_, from a connection which the orbit assigned to them appears to
+have with that of the well-known comet of Biela.
+
+M. Egenitis, Director of the Observatory of Athens, accords to the
+Bielids a high antiquity. He traces the shower back to the days of the
+Emperor Justinian. Theophanes, the Chronicler of that epoch, writing of
+the famous revolt of Nika in the year A.D. 532, says:--"During the same
+year a great fall of stars came from the evening till the dawn." M.
+Egenitis notes another early reference to these meteors in A.D. 752,
+during the reign of the Eastern Emperor, Constantine Copronymous.
+Writing of that year, Nicephorus, a Patriarch of Constantinople, has as
+follows:--"All the stars appeared to be detached from the sky, and to
+fall upon the earth."
+
+The Bielids, however, do not seem to have attracted particular notice
+until the nineteenth century. Attention first began to be riveted upon
+them on account of their suspected connection with Biela's comet. It
+appeared that the same orbit was shared both by that comet and the
+Bielid swarm. It will be remembered that the comet in question was not
+seen after its appearance in 1852. Since that date, however, the Bielid
+shower has shown an increased activity; which was further noticed to be
+especially great in those years in which the comet, had it still
+existed, would be due to pass near the earth.
+
+The third of these great showers to which allusion has above been made,
+namely, the _Perseids_, strikes the earth about the 10th of August; for
+which reason it is known on the Continent under the name of the "tears
+of St. Lawrence," the day in question being sacred to that Saint. This
+shower is traceable back many centuries, even as far as the year A.D.
+811. The name given to these meteors, "Perseids," arises from the fact
+that their radiant point is situated in the constellation of Perseus.
+This shower is, however, not by any means limited to the particular
+night of August 10th, for meteors belonging to the swarm may be observed
+to fall in more or less varying quantities from about July 8th to August
+22nd. The Perseid meteors sometimes fall at the rate of about sixty per
+hour. They are noted for their great rapidity of motion, and their
+trails besides often persist for a minute or two before being
+disseminated. Unlike the other well-known showers, the radiants of which
+are stationary, that of the Perseids shifts each night a little in an
+easterly direction.
+
+The orbit of the Perseids cuts that of the earth almost perpendicularly.
+The bodies are generally supposed to be the result of the disintegration
+of an ancient comet which travelled in the same orbit. Tuttle's Comet,
+which passed close to the earth in 1862, also belongs to this orbit; and
+its period of revolution is calculated to be 131 years. The Perseids
+appear to be disseminated all along this great orbit, for we meet them
+in considerable quantities each year. The bodies in question are in
+general particularly small. The swarm has, however, like most others, a
+somewhat denser portion, and through this the earth passed in 1848. The
+_aphelion_, or point where the far end of the orbit turns back again
+towards the sun, is situated right away beyond the path of Neptune, at a
+distance of forty-eight times that of the earth from the sun. The comet
+of 1532 also belongs to the Perseid orbit. It revisited the
+neighbourhood of the earth in 1661, and should have returned in 1789.
+But we have no record of it in that year; for which omission the then
+politically disturbed state of Europe may account. If not already
+disintegrated, this comet is due to return in 1919.
+
+This supposed connection between comets and meteor-swarms must be also
+extended to the case of the Leonids. These meteors appear to travel
+along the same track as Tempel's Comet of 1866.
+
+It is considered that the attractions of the various bodies of the
+solar system upon a meteor swarm must eventually result in breaking up
+the "bunched" portion, so that in time the individual meteors should
+become distributed along the whole length of the orbit. Upon this
+assumption the Perseid swarm, in which the meteors are fairly well
+scattered along its path, should be of greater age than the Leonid. As
+to the Leonid swarm itself, Le Verrier held that it was first brought
+into the solar system in A.D. 126, having been captured from outer space
+by the gravitative action of the planet Uranus.
+
+The acknowledged theory of meteor swarms has naturally given rise to an
+idea, that the sunlight shining upon such a large collection of
+particles ought to render a swarm visible before its collision with the
+earth's atmosphere. Several attempts have therefore been made to search
+for approaching swarms by photography, but, so far, it appears without
+success. It has also been proposed, by Mr. W.H.S. Monck, that the stars
+in those regions from which swarms are due, should be carefully watched,
+to see if their light exhibits such temporary diminutions as would be
+likely to arise from the momentary interposition of a cloud of moving
+particles.
+
+Between ten and fifteen years ago it happened that several well-known
+observers, employed in telescopic examination of the sun and moon,
+reported that from time to time they had seen small dark bodies,
+sometimes singly, sometimes in numbers, in passage across the discs of
+the luminaries. It was concluded that these were meteors moving in space
+beyond the atmosphere of the earth. The bodies were called "dark
+meteors," to emphasise the fact that they were seen in their natural
+condition, and not in that momentary one in which they had hitherto been
+always seen; _i.e._ when heated to white heat, and rapidly vaporised, in
+the course of their passage through the upper regions of our air. This
+"discovery" gave promise of such assistance to meteor theories, that
+calculations were made from the directions in which they had been seen
+to travel, and the speeds at which they had moved, in the hope that some
+information concerning their orbits might be revealed. But after a while
+some doubt began to be thrown upon their being really meteors, and
+eventually an Australian observer solved the mystery. He found that they
+were merely tiny particles of dust, or of the black coating on the inner
+part of the tube of the telescope, becoming detached from the sides of
+the eye-piece and falling across the field of view. He was led to this
+conclusion by having noted that a gentle tapping of his instrument
+produced the "dark" bodies in great numbers! Thus the opportunity of
+observing meteors beyond our atmosphere had once more failed.
+
+_Meteorites_, also known as aerolites and fireballs, are usually placed
+in quite a separate category from meteors. They greatly exceed the
+latter in size, are comparatively rare, and do not appear in any way
+connected with the various showers of meteors. The friction of their
+passage through the atmosphere causes them to shine with a great light;
+and if not shattered to pieces by internal explosions, they reach the
+ground to bury themselves deep in it with a great rushing and noise.
+When found by uncivilised peoples, or savages, they are, on account of
+their celestial origin, usually regarded as objects of wonder and of
+worship, and thus have arisen many mythological legends and deifications
+of blackened stones. On the other hand, when they get into the
+possession of the civilised, they are subjected to careful examinations
+and tests in chemical laboratories. The bodies are, as a rule, composed
+of stone, in conjunction with iron, nickel, and such elements as exist
+in abundance upon our earth; though occasionally specimens are found
+which are practically pure metal. In the museums of the great capitals
+of both Continents are to be seen some fine collections of meteorites.
+Several countries--Greenland and Mexico, for instance--contain in the
+soil much meteoric iron, often in masses so large as to baffle all
+attempts at removal. Blocks of this kind have been known to furnish the
+natives in their vicinity for many years with sources of workable iron.
+
+The largest meteorite in the world is one known as the Anighito
+meteorite. It was brought to the United States by the explorer Peary,
+who found it at Cape York in Greenland. He estimates its weight at from
+90 to 100 tons. One found in Mexico, called the Bacubirito, comes next,
+with an estimated weight of 27-1/2 tons. The third in size is the
+Willamette meteorite, found at Willamette in Oregon in 1902. It measures
+10 x 6-1/2 x 4-1/2 feet, and weighs about 15-1/2 tons.
+
+
+[27] The "gem" of the meteor ring, as it has been termed.
+
+
+
+
+CHAPTER XXII
+
+THE STARS
+
+
+In the foregoing chapters we have dealt at length with those celestial
+bodies whose nearness to us brings them into our especial notice. The
+entire room, however, taken up by these bodies, is as a mere point in
+the immensities of star-filled space. The sun, too, is but an ordinary
+star; perhaps quite an insignificant one[28] in comparison with the
+majority of those which stud that background of sky against which the
+planets are seen to perform their wandering courses.
+
+Dropping our earth and the solar system behind, let us go afield and
+explore the depths of space.
+
+We have seen how, in very early times, men portioned out the great mass
+of the so-called "fixed stars" into divisions known as constellations.
+The various arrangements, into which the brilliant points of light fell
+as a result of perspective, were noticed and roughly compared with such
+forms as were familiar to men upon the earth. Imagination quickly saw in
+them the semblances of heroes and of mighty fabled beasts; and, around
+these monstrous shapes, legends were woven, which told how the great
+deeds done in the misty dawn of historical time had been enshrined by
+the gods in the sky as an example and a memorial for men. Though the
+centuries have long outlived such fantasies, yet the constellation
+figures and their ancient names have been retained to this day, pretty
+well unaltered for want of any better arrangement. The Great and Little
+Bears, Cassiopeia, Perseus, and Andromeda, Orion and the rest, glitter
+in our night skies just as they did centuries and centuries ago.
+
+Many persons seem to despair of gaining any real knowledge of astronomy,
+merely because they are not versed in recognising the constellations.
+For instance, they will say:--"What is the use of my reading anything
+about the subject? Why, I believe I couldn't even point out the Great
+Bear, were I asked to do so!" But if such persons will only consider for
+a moment that what we call the Great Bear has no existence in fact, they
+need not be at all disheartened. Could we but view this familiar
+constellation from a different position in space, we should perhaps be
+quite unable to recognise it. Mountain masses, for instance, when seen
+from new directions, are often unrecognisable.
+
+It took, as we have seen, a very long time for men to acknowledge the
+immense distances of the stars from our earth. Their seeming
+unchangeableness of position was, as we have seen, largely responsible
+for the idea that the earth was immovable in space. It is a wonder that
+the Copernican system ever gained the day in the face of this apparent
+fixity of the stars. As time went on, it became indeed necessary to
+accord to these objects an almost inconceivable distance, in order to
+account for the fact that they remained apparently quite undisplaced,
+notwithstanding the journey of millions of miles which the earth was now
+acknowledged to make each year around the sun. In the face of the
+gradual and immense improvement in telescopes, this apparent immobility
+of the stars was, however, not destined to last. The first ascertained
+displacement of a star, namely that of 61 Cygni, noted by Bessel in the
+year 1838, definitely proved to men the truth of the Copernican system.
+Since then some forty more stars have been found to show similar tiny
+displacements. We are, therefore, in possession of the fact, that the
+actual distances of a few out of the great host can be calculated.
+
+To mention some of these. The nearest star to the earth, so far as we
+yet know, is Alpha Centauri, which is distant from us about 25 billions
+of miles. The light from this star, travelling at the stupendous rate of
+about 186,000 miles per second, takes about 4-1/4 years to reach our
+earth, or, to speak astronomically, Alpha Centauri is about 4-1/4 "light
+years" distant from us. Sirius--the brightest star in the whole sky--is
+at twice this distance, _i.e._ about 8-1/2 light years. Vega is about 30
+light years distant from us, Capella about 32, and Arcturus about 100.
+
+The displacements, consequent on the earth's movement, have, however,
+plainly nothing to say to any real movements on the part of the stars
+themselves. The old idea was that the stars were absolutely fixed; hence
+arose the term "fixed stars"--a term which, though inaccurate, has not
+yet been entirely banished from the astronomical vocabulary. But careful
+observations extending over a number of years have shown slight changes
+of position among these bodies; and such alterations cannot be ascribed
+to the revolution of the earth in its orbit, for they appear to take
+place in every direction. These evidences of movement are known as
+"proper motions," that is to say, actual motions in space proper to the
+stars themselves. Stars which are comparatively near to us show, as a
+rule, greater proper motions than those which are farther off. It must
+not, however, be concluded that these proper motions are of any very
+noticeable amounts. They are, as a matter of fact, merely upon the same
+apparently minute scale as other changes in the heavens; and would
+largely remain unnoticed were it not for the great precision of modern
+astronomical instruments.
+
+One of the swiftest moving of the stars is a star of the sixth magnitude
+in the constellation of the Great Bear; which is known as "1830
+Groombridge," because this was the number assigned to it in a catalogue
+of stars made by an astronomer of that name. It is popularly known as
+the "Runaway Star," a name given to it by Professor Newcomb. Its speed
+is estimated to be at least 138 miles per second. It may be actually
+moving at a much greater rate, for it is possible that we see its path
+somewhat foreshortened.
+
+A still greater proper motion--the greatest, in fact, known--is that of
+an eighth magnitude star in the southern hemisphere, in the
+constellation of Pictor. Nothing, indeed, better shows the enormous
+distance of the stars from us, and the consequent inability of even such
+rapid movements to alter the appearance of the sky during the course of
+ages, than the fact that it would take more than two centuries for the
+star in question to change its position in the sky by a space equal to
+the apparent diameter of the moon; a statement which is equivalent to
+saying that, were it possible to see this star with the naked eye, which
+it is not, at least twenty-five years would have to elapse before one
+would notice that it had changed its place at all!
+
+Both the stars just mentioned are very faint. That in Pictor is, as has
+been said, not visible to the naked eye. It appears besides to be a very
+small body, for Sir David Gill finds a parallax which makes it only as
+far off from us as Sirius. The Groombridge star, too, is just about the
+limit of ordinary visibility. It is, indeed, a curious fact that the
+fainter stars seem, on the average, to be moving more rapidly than the
+brighter.
+
+Investigations into proper motions lead us to think that every one of
+the stars must be moving in space in some particular direction. To take
+a few of the best known. Sirius and Vega are both approaching our system
+at a rate of about 10 miles per second, Arcturus at about 5 miles per
+second, while Capella is receding from us at about 15 miles per second.
+Of the twin brethren, Castor and Pollux, Castor is moving away from us
+at about 4-1/2 miles per second, while Pollux is coming towards us at
+about 33 miles per second.
+
+Much of our knowledge of proper motions has been obtained indirectly by
+means of the spectroscope, on the Doppler principle already treated of,
+by which we are enabled to ascertain whether a source from which light
+is coming is approaching or receding.
+
+The sun being, after all, a mere star, it will appear only natural for
+it also to have a proper motion of its own. This is indeed the case; and
+it is rushing along in space at a rate of between ten and twelve miles
+per second, carrying with it its whole family of planets and satellites,
+of comets and meteors. The direction in which it is advancing is towards
+a point in the constellation of Lyra, not far from its chief star Vega.
+This is shown by the fact that the stars about the region in question
+appear to be opening out slightly, while those in the contrary portion
+of the sky appear similarly to be closing together.
+
+Sir William Herschel was the first to discover this motion of the sun
+through space; though in the idea that such a movement might take place
+he seems to have been anticipated by Mayer in 1760, by Michell in 1767,
+and by Lalande in 1776.
+
+A suggestion has been made that our solar system, in its motion through
+the celestial spaces, may occasionally pass through regions where
+abnormal magnetic conditions prevail, in consequence of which
+disturbances may manifest themselves throughout the system at the same
+instant. Thus the sun may be getting the credit of _producing_ what it
+merely reacts to in common with the rest of its family. But this
+suggestion, plausible though it may seem, will not explain why the
+magnetic disturbances experienced upon our earth show a certain
+dependence upon such purely local facts, as the period of the sun's
+rotation, for instance.
+
+One would very much like to know whether the movement of the sun is
+along a straight line, or in an enormous orbit around some centre. The
+idea has been put forward that it may be moving around the centre of
+gravity of the whole visible stellar universe. Maedler, indeed,
+propounded the notion that Alcyone--the chief star in the group known as
+the Pleiades--occupied this centre, and that everything revolved around
+it. He went even further to proclaim that here was the Place of the
+Almighty, the Mansion of the Eternal! But Maedler's ideas upon this point
+have long been shelved.
+
+To return to the general question of the proper motion of stars.
+
+In several instances these motions appear to take place in groups, as if
+certain stars were in some way associated together. For example, a large
+number of the stars composing the Pleiades appear to be moving through
+space in the same direction. Also, of the seven stars composing the
+Plough, all but two--the star at the end of its "handle," and that one
+of the "pointers," as they are called, which is the nearer to the pole
+star--have a common proper motion, _i.e._ are moving in the same
+direction and nearly at the same rate.
+
+Further still, the well-known Dutch astronomer, Professor Kapteyn, of
+Groningen, has lately reached the astonishing conclusion that a great
+part of the visible universe is occupied by two vast streams of stars
+travelling in opposite directions. In both these great streams, the
+individual bodies are found, besides, to be alike in design, alike in
+chemical constitution, and alike in the stage of their development.
+
+A fable related by the Persian astronomer, Al Sufi (tenth century, A.D.)
+shows well the changes in the face of the sky which proper motions are
+bound to produce after great lapses of time. According to this fable the
+stars Sirius and Procyon were the sisters of the star Canopus. Canopus
+married Rigel (another star,) but, having murdered her, he fled towards
+the South Pole, fearing the anger of his sisters. The fable goes on to
+relate, among other things, that Sirius followed him across the Milky
+Way. Mr. J. E. Gore, in commenting on the story, thinks that it may be
+based upon a tradition of Sirius having been seen by the men of the
+Stone Age on the opposite side of the Milky Way to that on which it now
+is.
+
+Sirius is in that portion of the heavens _from_ which the sun is
+advancing. Its proper motion is such that it is gaining upon the earth
+at the rate of about ten miles per second, and so it must overtake the
+sun after the lapse of great ages. Vega, on the other hand, is coming
+towards us from that part of the sky _towards_ which the sun is
+travelling. It should be about half a million years before the sun and
+Vega pass by one another. Those who have specially investigated this
+question say that, as regards the probability of a near approach, it is
+much more likely that Vega will be then so far to one side of the sun,
+that her brightness will not be much greater than it is at this moment.
+
+Considerations like these call up the chances of stellar collisions.
+Such possibilities need not, however, give rise to alarm; for the stars,
+as a rule, are at such great distances from each other, that the
+probability of relatively near approaches is slight.
+
+We thus see that the constellations do not in effect exist, and that
+there is in truth no real background to the sky. We find further that
+the stars are strewn through space at immense distances from each other,
+and are moving in various directions hither and thither. The sun, which
+is merely one of them, is moving also in a certain direction, carrying
+the solar system along with it. It seems, therefore, but natural to
+suppose that many a star may be surrounded by some planetary system in a
+way similar to ours, which accompanies it through space in the course of
+its celestial journeyings.
+
+
+[28] Vega, for instance, shines one hundred times more brightly than the
+sun would do, were it to be removed to the distance at which that star
+is from us.
+
+
+
+
+CHAPTER XXIII
+
+THE STARS--_continued_
+
+
+The stars appear to us to be scattered about the sky without any orderly
+arrangement. Further, they are of varying degrees of brightness; some
+being extremely brilliant, whilst others can but barely be seen. The
+brightness of a star may arise from either of two causes. On the one
+hand, the body may be really very bright in itself; on the other hand,
+it may be situated comparatively near to us. Sometimes, indeed, both
+these circumstances may come into play together.
+
+Since variation in brightness is the most noticeable characteristic of
+the stars, men have agreed to class them in divisions called
+"magnitudes." This term, it must be distinctly understood, is employed
+in such classification without any reference whatever to actual size,
+being merely taken to designate roughly the amount of light which we
+receive from a star. The twenty brightest stars in the sky are usually
+classed in the first magnitude. In descending the scale, each magnitude
+will be noticed to contain, broadly speaking, three times as many stars
+as the one immediately above it. Thus the second magnitude contains 65,
+the third 190, the fourth 425, the fifth 1100, and the sixth 3200. The
+last of these magnitudes is about the limit of the stars which we are
+able to see with the naked eye. Adding, therefore, the above numbers
+together, we find that, without the aid of the telescope, we cannot see
+more than about 5000 stars in the entire sky--northern and southern
+hemispheres included. Quite a small telescope will, however, allow us to
+see down to the ninth magnitude, so that the total number of stars
+visible to us with such very moderate instrumental means will be well
+over 100,000.
+
+It must not, however, be supposed that the stars included within each
+magnitude are all of exactly the same brightness. In fact, it would be
+difficult to say if there exist in the whole sky two stars which send us
+precisely the same amount of light. In arranging the magnitudes, all
+that was done was to make certain broad divisions, and to class within
+them such stars as were much on a par with regard to brightness. It may
+here be noted that a standard star of the first magnitude gives us about
+one hundred times as much light as a star of the sixth magnitude, and
+about one million times as much as one of the sixteenth magnitude--which
+is near the limit of what we can see with the very best telescope.
+
+Though the first twenty stars in the sky are popularly considered as
+being of the first magnitude, yet several of them are much brighter than
+an average first magnitude star would be. For instance, Sirius--the
+brightest star in the whole sky--is equal to about eleven first
+magnitude stars, like, say, Aldebaran. In consequence of such
+differences, astronomers are agreed in classifying the brightest of them
+as _brighter_ than the standard first magnitude star. On this principle
+Sirius would be about two and a half magnitudes _above_ the first. This
+notation is usefully employed in making comparisons between the amount
+of light which we receive from the sun, and that which we get from an
+individual star. Thus the sun will be about twenty-seven and a half
+magnitudes _above_ the first magnitude. The range, therefore, between
+the light which we receive from the sun (considered merely as a very
+bright star) and the first magnitude stars is very much greater than
+that between the latter and the faintest star which can be seen with the
+telescope, or even registered upon the photographic plate.
+
+To classify stars merely by their magnitudes, without some definite note
+of their relative position in the sky, would be indeed of little avail.
+We must have some simple method of locating them in the memory, and the
+constellations of the ancients here happily come to our aid. A system
+combining magnitudes with constellations was introduced by Bayer in
+1603, and is still adhered to. According to this the stars in each
+constellation, beginning with the brightest star, are designated by the
+letters of the Greek alphabet taken in their usual order. For example,
+in the constellation of Canis Major, or the Greater Dog, the brightest
+star is the well-known Sirius, called by the ancients the "Dog Star";
+and this star, in accordance with Bayer's method, has received the Greek
+letter [a] (alpha), and is consequently known as Alpha Canis
+Majoris.[29] As soon as the Greek letters are used up in this way the
+Roman alphabet is brought into requisition, after which recourse is had
+to ordinary numbers.
+
+Notwithstanding this convenient arrangement, some of the brightest
+stars are nearly always referred to by certain proper names given to
+them in old times. For instance, it is more usual to speak of Sirius,
+Arcturus, Vega, Capella, Procyon, Aldebaran, Regulus, and so on, than of
+[a] Canis Majoris, [a] Booetis, [a] Lyrae, [a] Aurigae, [a] Canis Minoris,
+[a] Tauri, [a] Leonis, &c. &c.
+
+In order that future generations might be able to ascertain what changes
+were taking place in the face of the sky, astronomers have from time to
+time drawn up catalogues of stars. These lists have included stars of a
+certain degree of brightness, their positions in the sky being noted
+with the utmost accuracy possible at the period. The earliest known
+catalogue of this kind was made, as we have seen, by the celebrated
+Greek astronomer, Hipparchus, about the year 125 B.C. It contained 1080
+stars. It was revised and brought up to date by Ptolemy in A.D. 150.
+Another celebrated list was that drawn up by the Persian astronomer, Al
+Sufi, about the year A.D. 964. In it 1022 stars were noted down. A
+catalogue of 1005 stars was made in 1580 by the famous Danish
+astronomer, Tycho Brahe. Among modern catalogues that of Argelander
+(1799-1875) contained as many as 324,198 stars. It was extended by
+Schoenfeld so as to include a portion of the Southern Hemisphere, in
+which way 133,659 more stars were added.
+
+In recent years a project was placed on foot of making a photographic
+survey of the sky, the work to be portioned out among various nations. A
+great part of this work has already been brought to a conclusion. About
+15,000,000 stars will appear upon the plates; but, so far, it has been
+proposed to catalogue only about a million and a quarter of the
+brightest of them. This idea of surveying the face of the sky by
+photography sprang indirectly from the fine photographs which Sir David
+Gill took, when at the Cape of Good Hope, of the Comet of 1882. The
+immense number of star-images which had appeared upon his plates
+suggested the idea that photography could be very usefully employed to
+register the relative positions of the stars.
+
+The arrangement of seven stars known as the "Plough" is perhaps the most
+familiar configuration in the sky (see Plate XIX., p. 292). In the
+United States it is called the "Dipper," on account of its likeness to
+the outline of a saucepan, or ladle. "Charles' Wain" was the old English
+name for it, and readers of Caesar will recollect it under
+_Septentriones_, or the "Seven Stars," a term which that writer uses as
+a synonym for the North. Though identified in most persons' minds with
+_Ursa Major_, or the Great Bear, the Plough is actually only a small
+portion of that famous constellation. Six out of the seven stars which
+go to make up the well-known figure are of the second magnitude, while
+the remaining one, which is the middle star of the group, is of the
+third.
+
+The Greek letters, as borne by the individual stars of the Plough, are a
+plain transgression of Bayer's method as above described, for they have
+certainly not been allotted here in accordance with the proper order of
+brightness. For instance, the third magnitude star, just alluded to as
+being in the middle of the group, has been marked with the Greek letter
+[d] (Delta); and so is made to take rank _before_ the stars composing
+what is called the "handle" of the Plough, which are all of the second
+magnitude. Sir William Herschel long ago drew attention to the irregular
+manner in which Bayer's system had been applied. It is, indeed, a great
+pity that this notation was not originally worked out with greater care
+and correctness; for, were it only reliable, it would afford great
+assistance to astronomers in judging of what changes in relative
+brightness have taken place among the stars.
+
+Though we may speak of using the constellations as a method of finding
+our way about the sky, it is, however, to certain marked groupings in
+them of the brighter stars that we look for our sign-posts.
+
+Most of the constellations contain a group or so of noticeable stars,
+whose accidental arrangement dimly recalls the outline of some familiar
+geometrical figure and thus arrests the attention.[30] For instance, in
+an almost exact line with the two front stars of the Plough, or
+"pointers" as they are called,[31] and at a distance about five times as
+far away as the interval between them, there will be found a third star
+of the second magnitude. This is known as Polaris, or the Pole Star, for
+it very nearly occupies that point of the heaven towards which the north
+pole of the earth's axis is _at present_ directed (see Plate XIX., p.
+292). Thus during the apparently daily rotation of the heavens, this
+star looks always practically stationary. It will, no doubt, be
+remembered how Shakespeare has put into the mouth of Julius Caesar these
+memorable words:--
+
+"But I am constant as the northern star,
+Of whose true-fix'd and resting quality
+There is no fellow in the firmament."
+
+[Illustration: PLATE XIX. THE SKY AROUND THE NORTH POLE
+
+We see here the Plough, the Pole Star, Ursa Minor, Auriga, Cassiopeia's
+Chair, and Lyra. Also the Circle of Precession, along which the Pole
+makes a complete revolution in a period of 25,868 years, and the
+Temporary Star discovered by Tycho Brahe in the year 1572.
+
+(Page 291)]
+
+On account of the curvature of the earth's surface, the height at which
+the Pole Star is seen above the horizon at any place depends regularly
+upon the latitude; that is to say, the distance of the place in question
+from the equator. For instance, at the north pole of the earth, where
+the latitude is greatest, namely, 90 deg., the Pole Star will appear
+directly overhead; whereas in England, where the latitude is about 50
+deg., it will be seen a little more than half way up the northern sky.
+At the equator, where the latitude is _nil_, the Pole Star will be on
+the horizon due north.
+
+In consequence of its unique position, the Pole Star is of very great
+service in the study of the constellations. It is a kind of centre
+around which to hang our celestial ideas--a starting point, so to speak,
+in our voyages about the sky.
+
+According to the constellation figures, the Pole Star is in _Ursa
+Minor_, or the Little Bear, and is situated at the end of the tail of
+that imaginary figure (see Plate XIX., p. 292). The chief stars of this
+constellation form a group not unlike the Plough, except that the
+"handle" is turned in the contrary direction. The Americans, in
+consequence, speak of it as the "Little Dipper."
+
+Before leaving this region of the sky, it will be well to draw attention
+to the second magnitude star [z] in the Great Bear (Zeta Ursae Majoris),
+which is the middle star in the "handle" of the Plough. This star is
+usually known as Mizar, a name given to it by the Arabians. A person
+with good eyesight can see quite near to it a fifth magnitude star,
+known under the name of Alcor. We have here a very good example of that
+deception in the estimation of objects in the sky, which has been
+alluded to in an earlier chapter. Alcor is indeed distant from Mizar by
+about one-third the apparent diameter of the moon, yet no one would
+think so!
+
+On the other side of Polaris from the Plough, and at about an equal
+apparent distance, will be found a figure in the form of an irregular
+"W", made up of second and third magnitude stars. This is the well-known
+"Cassiopeia's Chair"--portion of the constellation of _Cassiopeia_ (see
+Plate XIX., p. 292).
+
+On either side of the Pole Star, about midway between the Plough and
+Cassiopeia's Chair, but a little further off from it than these, are the
+constellations of _Auriga_ and _Lyra_ (see Plate XIX., p. 292). The
+former constellation will be easily recognised, because its chief
+features are a brilliant yellowish first magnitude star, with one of the
+second magnitude not far from it. The first magnitude star is Capella,
+the other is [b] Aurigae. Lyra contains only one first magnitude
+star--Vega, pale blue in colour. This star has a certain interest for us
+from the fact that, as a consequence of that slow shift of direction of
+the earth's axis known as Precession, it will be very near the north
+pole of the heavens in some 12,000 years, and so will then be considered
+the pole star (see Plate XIX., p. 292). The constellation of Lyra
+itself, it must also be borne in mind, occupies that region of the
+heavens towards which the solar system is travelling.
+
+The handle of the Plough points roughly towards the constellation of
+_Booetes_, in which is the brilliant first magnitude star Arcturus. This
+star is of an orange tint.
+
+Between Booetes and Lyra lie the constellations of _Corona Borealis_ (or
+the Northern Crown) and _Hercules_. The chief feature of Corona
+Borealis, which is a small constellation, is a semicircle of six small
+stars, the brightest of which is of the second magnitude. The
+constellation of Hercules is very extensive, but contains no star
+brighter than the third magnitude.
+
+Near to Lyra, on the side away from Hercules, are the constellations of
+_Cygnus_ and _Aquila_. Of the two, the former is the nearer to the Pole
+Star, and will be recognised by an arrangement of stars widely set in
+the form of a cross, or perhaps indeed more like the framework of a
+boy's kite. The position of Aquila will be found through the fact that
+three of its brightest stars are almost in a line and close together.
+The middle of these is Altair, a yellowish star of the first magnitude.
+
+At a little distance from Ursa Major, on the side away from the Pole
+Star, is the constellation of _Leo_, or the Lion. Its chief feature is a
+series of seven stars, supposed to form the head of that animal. The
+arrangement of these stars is, however, much more like a sickle,
+wherefore this portion of the constellation is usually known as the
+"Sickle of Leo." At the end of the handle of the sickle is a white first
+magnitude star--Regulus.
+
+The reader will, no doubt, recollect that it is from a point in the
+Sickle of Leo that the Leonid meteors appear to radiate.
+
+The star second in brightness in the constellation of Leo is known as
+Denebola. This star, now below the second magnitude, seems to have been
+very much brighter in the past. It is noted, indeed, as a brilliant
+first magnitude star by Al Sufi, that famous Persian astronomer who
+lived, as we have seen, in the tenth century. Ptolemy also notes it as
+of the first magnitude.
+
+In the neighbourhood of Auriga, and further than it from the Pole Star,
+are several remarkable constellations--Taurus, Orion, Gemini, Canis
+Minor, and Canis Major (see Plate XX., p. 296).
+
+The first of these, _Taurus_ (or the Bull), contains two conspicuous
+star groups--the Pleiades and the Hyades. The Pleiades are six or seven
+small stars quite close together, the majority of which are of the
+fourth magnitude. This group is sometimes occulted by the moon. The way
+in which the stars composing it are arranged is somewhat similar to that
+in the Plough, though of course on a scale ever so much smaller. The
+impression which the group itself gives to the casual glance is thus
+admirably pictured in Tennyson's _Locksley Hall_:--
+
+"Many a night I saw the Pleiads, rising through the mellow shade,
+Glitter like a swarm of fire-flies tangled in a silver braid."
+
+[Illustration: PLATE XX. ORION AND HIS NEIGHBOURS
+
+We see here that magnificent region of the sky which contains the
+brightest star of all--Sirius. Note also especially the Milky Way, the
+Pleiades, the Hyades, and the "Belt" and "Sword" of Orion.
+
+(Page 296)]
+
+The group of the Hyades occupies the "head" of the Bull, and is much
+more spread out than that of the Pleiades. It is composed besides of
+brighter stars, the brightest being one of the first magnitude,
+Aldebaran. This star is of a red colour, and is sometimes known as the
+"Eye of the Bull."
+
+The constellation of _Orion_ is easily recognised as an irregular
+quadrilateral formed of four bright stars, two of which, Betelgeux
+(reddish) and Rigel (brilliant white), are of the first magnitude. In
+the middle of the quadrilateral is a row of three second magnitude
+stars, known as the "Belt" of Orion. Jutting off from this is another
+row of stars called the "Sword" of Orion.
+
+The constellation of _Gemini_, or the Twins, contains two bright
+stars--Castor and Pollux--close to each other. Pollux, though marked
+with the Greek letter [b], is the brighter of the two, and nearly of the
+standard first magnitude.
+
+Just further from the Pole than Gemini, is the constellation of _Canis
+Minor_, or the Lesser Dog. Its chief star is a white first magnitude
+one--Procyon.
+
+Still further again from the Pole than Canis Minor is the constellation
+of _Canis Major_, or the Greater Dog. It contains the brightest star in
+the whole sky, the first magnitude star Sirius, bluish-white in colour,
+also known as the "Dog Star." This star is almost in line with the stars
+forming the Belt of Orion, and is not far from that constellation.
+
+Taken in the following order, the stars Capella, [b] Aurigae, Castor,
+Pollux, Procyon, and Sirius, when they are all above the horizon at the
+same time, form a beautiful curve stretching across the heaven.
+
+The groups of stars visible in the southern skies have by no means the
+same fascination for us as those in the northern. The ancients were in
+general unacquainted with the regions beyond the equator, and so their
+scheme of constellations did not include the sky around the South Pole
+of the heavens. In modern times, however, this part of the celestial
+expanse was also portioned out into constellations for the purpose of
+easy reference; but these groupings plainly lack that simplicity of
+conception and legendary interest which are so characteristic of the
+older ones.
+
+The brightest star in the southern skies is found in the constellation
+of _Argo_, and is known as Canopus. In brightness it comes next to
+Sirius, and so is second in that respect in the entire heaven. It does
+not, however, rise above the English horizon.
+
+Of the other southern constellations, two call for especial notice, and
+these adjoin each other. One is _Centaurus_ (or the Centaur), which
+contains the two first magnitude stars, [a] and [b] Centauri. The first
+of these, Alpha Centauri, comes next in brightness to Canopus, and is
+notable as being the nearest of all the stars to our earth. The other
+constellation is called _Crux_, and contains five stars set in the form
+of a rough cross, known as the "Southern Cross." The brightest of these,
+[a] Crucis, is of the first magnitude.
+
+Owing to the Precession of the Equinoxes, which, as we have seen,
+gradually shifts the position of the Pole among the stars, certain
+constellations used to be visible in ancient times in more northerly
+latitudes than at present. For instance, some five thousand years ago
+the Southern Cross rose above the English horizon, and was just visible
+in the latitude of London. It has, however, long ago even ceased to be
+seen in the South of Europe. The constellation of Crux happens to be
+situated in that remarkable region of the southern skies, in which are
+found the stars Canopus and Alpha Centauri, and also the most brilliant
+portion of the Milky Way. It is believed to be to this grand celestial
+region that allusion is made in the Book of Job (ix. 9), under the title
+of the "Chambers of the South." The "Cross" must have been still a
+notable feature in the sky of Palestine in the days when that ancient
+poem was written.
+
+There is no star near enough to the southern pole of the heavens to earn
+the distinction of South Polar Star.
+
+The Galaxy, or _Milky Way_ (see Plate XX., p. 296), is a broad band of
+diffused light which is seen to stretch right around the sky. The
+telescope, however, shows it to be actually composed of a great host of
+very faint stars--too faint, indeed, to be separately distinguished with
+the naked eye. Along a goodly stretch of its length it is cleft in two;
+while near the south pole of the heavens it is entirely cut across by a
+dark streak.
+
+In this rapid survey of the face of the sky, we have not been able to do
+more than touch in the broadest manner upon some of the most noticeable
+star groups and a few of the most remarkable stars. To go any further is
+not a part of our purpose; our object being to deal with celestial
+bodies as they actually are, and not in those groupings under which they
+display themselves to us as a mere result of perspective.
+
+
+[29] Attention must here be drawn to the fact that the name of the
+constellation is always put in the genitive case.
+
+[30] The early peoples, as we have seen, appear to have been attracted
+by those groupings of the stars which reminded them in a way of the
+figures of men and animals. We moderns, on the other hand, seek almost
+instinctively for geometrical arrangements. This is, perhaps,
+symptomatic of the evolution of the race. In the growth of the
+individual we find, for example, something analogous. A child, who has
+been given pencil and paper, is almost certain to produce grotesque
+drawings of men and animals; whereas the idle and half-conscious
+scribblings which a man may make upon his blotting-paper are usually of
+a geometrical character.
+
+[31] Because the line joining them _points_ in the direction of the Pole
+Star.
+
+
+
+
+CHAPTER XXIV
+
+SYSTEMS OF STARS
+
+
+Many stars are seen comparatively close together. This may plainly arise
+from two reasons. Firstly, the stars may happen to be almost in the same
+line of sight; that is to say, seen in nearly the same direction; and
+though one star may be ever so much nearer to us than the other, the
+result will give all the appearance of a related pair. A seeming
+arrangement of two stars in this way is known as a "double," or double
+star; or, indeed, to be very precise, an "optical double." Secondly, in
+a pair of stars, both bodies may be about the same distance from us, and
+actually connected as a system like, for instance, the moon and the
+earth. A pairing of stars in this way, though often casually alluded to
+as a double star, is properly termed a "binary," or binary system.
+
+But collocations of stars are by no means limited to two. We find,
+indeed, all over the sky such arrangements in which there are three or
+more stars; and these are technically known as "triple" or "multiple"
+stars respectively. Further, groups are found in which a great number of
+stars are closely massed together, such a massing together of stars
+being known as a "cluster."
+
+The Pole Star (Polaris) is a double star, one of the components being of
+a little below the second magnitude, and the other a little below the
+ninth. They are so close together that they appear as one star to the
+naked eye, but they may be seen separate with a moderately sized
+telescope. The brighter star is yellowish, and the faint one white. This
+brighter star is found _by means of the spectroscope_ to be actually
+composed of three stars so very close together that they cannot be seen
+separately even with a telescope. It is thus a triple star, and the
+three bodies of which it is composed are in circulation about each
+other. Two of them are darker than the third.
+
+The method of detecting binary stars by means of the spectroscope is an
+application of Doppler's principle. It will, no doubt, be remembered
+that, according to the principle in question, we are enabled, from
+certain shiftings of the lines in the spectrum of a luminous body, to
+ascertain whether that body is approaching us or receding from us. Now
+there are certain stars which always appear single even in the largest
+telescopes, but when the spectroscope is directed to them a spectrum
+_with two sets of lines_ is seen. Such stars must, therefore, be double.
+Further, if the shiftings of the lines, in a spectrum like this, tell us
+that the component stars are making small movements to and from us which
+go on continuously, we are therefore justified in concluding that these
+are the orbital revolutions of a binary system greatly compressed by
+distance. Such connected pairs of stars, since they cannot be seen
+separately by means of any telescope, no matter how large, are known as
+"spectroscopic binaries."
+
+In observations of spectroscopic binaries we do not always get a double
+spectrum. Indeed, if one of the components be below a certain
+magnitude, its spectrum will not appear at all; and so we are left in
+the strange uncertainty as to whether this component is merely faint or
+actually dark. It is, however, from the shiftings of the lines in the
+spectrum of the other component that we see that an orbital movement is
+going on, and are thus enabled to conclude that two bodies are here
+connected into a system, although one of these bodies resolutely refuses
+directly to reveal itself even to the all-conquering spectroscope.
+
+Mizar, that star in the handle of the Plough to which we have already
+drawn attention, will be found with a small telescope to be a fine
+double, one of the components being white and the other greenish.
+Actually, however, as the American astronomer, Professor F.R. Moulton,
+points out, these stars are so far from each other that if we could be
+transferred to one of them we should see the other merely as an ordinary
+bright star. The spectroscope shows that the brighter of these stars is
+again a binary system of two huge suns, the components revolving around
+each other in a period of about twenty days. This discovery made by
+Professor E.C. Pickering, the _first_ of the kind by means of the
+spectroscope, was announced in 1889 from the Harvard Observatory in the
+United States.
+
+A star close to Vega, known as [e] (Epsilon) Lyrae (see Plate XIX., p.
+292), is a double, the components of which may be seen separately with
+the naked eye by persons with very keen eyesight. If this star, however,
+be viewed with the telescope, the two companions will be seen far apart;
+and it will be noticed that each of them is again a double.
+
+By means of the spectroscope Capella is shown to be really composed of
+two stars (one about twice as bright as the other) situated very close
+together and forming a binary system. Sirius is also a binary system;
+but it is what is called a "visual" one, for its component stars may be
+_seen_ separately in very large telescopes. Its double, or rather
+binary, nature, was discovered in 1862 by the celebrated optician Alvan
+G. Clark, while in the act of testing the 18-inch refracting telescope,
+then just constructed by his firm, and now at the Dearborn Observatory,
+Illinois, U.S.A. The companion is only of the tenth magnitude, and
+revolves around Sirius in a period of about fifty years, at a mean
+distance equal to about that of Uranus from the sun. Seen from Sirius,
+it would shine only something like our full moon. It must be
+self-luminous and not a mere planet; for Mr. Gore has shown that if it
+shone only by the light reflected from Sirius, it would be quite
+invisible even in the Great Yerkes Telescope.
+
+Procyon is also a binary, its companion having been discovered by
+Professor J.M. Schaeberle at the Lick Observatory in 1896. The period of
+revolution in this system is about forty years. Observations by Mr. T.
+Lewis of Greenwich seem, however, to point to the companion being a
+small nebula rather than a star.
+
+The star [e] (Eta) Cassiopeiae (see Plate XIX., p. 292), is easily seen
+as a fine double in telescopes of moderate size. It is a binary system,
+the component bodies revolving around their common centre of gravity in
+a period of about two hundred years. This system is comparatively near
+to us, _i.e._ about nine light years, or a little further off than
+Sirius.
+
+In a small telescope the star Castor will be found double, the
+components, one of which is brighter than the other, forming a binary
+system. The fainter of these was found by Belopolsky, with the
+spectroscope, to be composed of a system of two stars, one bright and
+the other either dark or not so bright, revolving around each other in a
+period of about three days. The brighter component of Castor is also a
+spectroscopic binary, with a period of about nine days; so that the
+whole of what we see with the naked eye as Castor, is in reality a
+remarkable system of four stars in mutual orbital movement.
+
+Alpha Centauri--the nearest star to the earth--is a visual binary, the
+component bodies revolving around each other in a period of about
+eighty-one years. The extent of this system is about the same as that of
+Sirius. Viewed from each other, the bodies would shine only like our sun
+as seen from Neptune.
+
+Among the numerous binary stars the orbits of some fifty have been
+satisfactorily determined. Many double stars, for which this has not yet
+been done, are, however, believed to be, without doubt, binary. In some
+cases a parallax has been found; so that we are enabled to estimate in
+miles the actual extent of such systems, and the masses of the bodies in
+terms of the sun's mass.
+
+Most of the spectroscopic binaries appear to be upon a smaller scale
+than the telescopic ones. Some are, indeed, comparatively speaking,
+quite small. For instance, the component stars forming [b] Aurigae are
+about eight million miles apart, while in [z] Geminorum, the distance
+between the bodies is only a little more than a million miles.
+
+Spectroscopic binaries are probably very numerous. Professor W.W.
+Campbell, Director of the Lick Observatory, estimates, for instance,
+that, out of about every half-a-dozen stars, one is a spectroscopic
+binary.
+
+It is only in the case of binary systems that we can discover the masses
+of stars at all. These are ascertained from their movements with regard
+to each other under the influence of their mutual gravitative
+attractions. In the case of simple stars we have clearly nothing of the
+kind to judge by; though, if we can obtain a parallax, we may hazard a
+guess from their brightness.
+
+Binary stars were incidentally discovered by Sir William Herschel. In
+his researches to get a stellar parallax he had selected a number of
+double stars for test purposes, on the assumption that, if one of such a
+pair were much nearer than the other, it might show a displacement with
+regard to its neighbour as a direct consequence of the earth's orbital
+movement around the sun. He, however, failed entirely to obtain any
+parallaxes, the triumph in this being, as we have seen, reserved for
+Bessel. But in some of the double stars which he had selected, he found
+certain alterations in the relative positions of the bodies, which
+plainly were not a consequence of the earth's motion, but showed rather
+that there was an actual circling movement of the bodies themselves
+under their mutual attractions. It is to be noted that the existence of
+such connected pairs had been foretold as probable by the Rev. John
+Michell, who lived a short time before Herschel.
+
+The researches into binary systems--both those which can be seen with
+the eye and those which can be observed by means of the spectroscope,
+ought to impress upon us very forcibly the wide sway of the law of
+gravitation.
+
+Of star clusters about 100 are known, and such systems often contain
+several thousand stars. They usually cover an area of sky somewhat
+smaller than the moon appears to fill. In most clusters the stars are
+very faint, and, as a rule, are between the twelfth and sixteenth
+magnitudes. It is difficult to say whether these are actually small
+bodies, or whether their faintness is due merely to their great distance
+from us, since they are much too far off to show any appreciable
+parallactic displacement. Mr. Gore, however, thinks there is good
+evidence to show that the stars in clusters are really close, and that
+the clusters themselves fill a comparatively small space.
+
+One of the finest examples of a cluster is the great globular one, in
+the constellation of Hercules, discovered by Halley in 1714. It contains
+over 5000 stars, and upon a clear, dark night is visible to the naked
+eye as a patch of light. In the telescope, however, it is a wonderful
+object. There are also fine clusters in the constellations of Auriga,
+Pegasus, and Canes Venatici. In the southern heavens there are some
+magnificent examples of globular clusters. This hemisphere seems,
+indeed, to be richer in such objects than the northern. For instance,
+there is a great one in the constellation of the Centaur, containing
+some 6000 stars (see Plate XXI., p. 306).
+
+[Illustration: PLATE XXI. THE GREAT GLOBULAR CLUSTER IN THE SOUTHERN
+CONSTELLATION OF CENTAURUS
+
+From a photograph taken at the Cape Observatory, on May 24th, 1903. Time
+of exposure, 1 hour.
+
+(Page 306)]
+
+Certain remarkable groups of stars, of a nature similar to clusters,
+though not containing such faint or densely packed stars as those we
+have just alluded to, call for a mention in this connection. The best
+example of such star groups are the Pleiades and the Hyades (see Plate
+XX., p. 296), Coma Berenices, and Praesepe (or the Beehive), the
+last-named being in the constellation of Cancer.
+
+Stars which alter in their brightness are called _Variable Stars_, or
+"variables." The first star whose variability attracted attention is
+that known as Omicron Ceti, namely, the star marked with the Greek
+letter [o] (Omicron) in the constellation of Cetus, or the Whale, a
+constellation situated not far from Taurus. This star, the variability
+of which was discovered by Fabricius in 1596, is also known as Mira, or
+the "Wonderful," on account of the extraordinary manner in which its
+light varies from time to time. The star known by the name of Algol,[32]
+popularly called the "Demon Star"--whose astronomical designation is [b]
+(Beta) Persei, or the star second in brightness in the constellation of
+Perseus--was discovered by Goodricke, in the year 1783, to be a variable
+star. In the following year [b] Lyrae, the star in Lyra next in order of
+brightness after Vega, was also found by the same observer to be a
+variable. It may be of interest to the reader to know that Goodricke was
+deaf and dumb, and that he died in 1786 at the early age of twenty-one
+years!
+
+It was not, however, until the close of the nineteenth century that much
+attention was paid to variable stars. Now several hundreds of these are
+known, thanks chiefly to the observations of, amongst others, Professor
+S.C. Chandler of Boston, U.S.A., Mr. John Ellard Gore of Dublin, and Dr.
+A.W. Roberts of South Africa. This branch of astronomy has not, indeed,
+attracted as much popular attention as it deserves, no doubt because the
+nature of the work required does not call for the glamour of an
+observatory or a large telescope.
+
+The chief discoveries with regard to variable stars have been made by
+the naked eye, or with a small binocular. The amount of variation is
+estimated by a comparison with other stars. As in many other branches of
+astronomy, photography is now employed in this quest with marked
+success; and lately many variable stars have been found to exist in
+clusters and nebulae.
+
+It was at one time considered that a variable star was in all
+probability a body, a portion of whose surface had been relatively
+darkened in some manner akin to that in which sun spots mar the face of
+the sun; and that when its axial rotation brought the less illuminated
+portions in turn towards us, we witnessed a consequent diminution in the
+star's general brightness. Herschel, indeed, inclined to this
+explanation, for his belief was that all the stars bore spots like those
+of the sun. It appears preferably thought nowadays that disturbances
+take place periodically in the atmosphere or surroundings of certain
+stars, perhaps through the escape of imprisoned gases, and that this may
+be a fruitful cause of changes of brilliancy. The theory in question
+will, however, apparently account for only one class of variable star,
+namely, that of which Mira Ceti is the best-known example. The scale on
+which it varies in brightness is very great, for it changes from the
+second to the ninth magnitude. For the other leading type of variable
+star, Algol, of which mention has already been made, is the best
+instance. The shortness of the period in which the changes of brightness
+in such stars go their round, is the chief characteristic of this latter
+class. The period of Algol is a little under three days. This star when
+at its brightest is of about the second magnitude, and when least bright
+is reduced to below the third magnitude; from which it follows that its
+light, when at the minimum, is only about one-third of what it is when
+at the maximum. It seems definitely proved by means of the spectroscope
+that variables of this kind are merely binary stars, too close to be
+separated by the telescope, which, as a consequence of their orbits
+chancing to be edgewise towards us, eclipse each other in turn time
+after time. If, for instance, both components of such a pair are bright,
+then when one of them is right behind the other, we will not, of course,
+get the same amount of light as when they are side by side. If, on the
+other hand, one of the components happens to be dark or less luminous
+and the other bright, the manner in which the light of the bright star
+will be diminished when the darker star crosses its face should easily
+be understood. It is to the second of these types that Algol is supposed
+to belong. The Algol system appears to be composed of a body about as
+broad as our sun, which regularly eclipses a brighter body which has a
+diameter about half as great again.
+
+Since the companion of Algol is often spoken of as a _dark_ body, it
+were well here to point out that we have no evidence at all that it is
+entirely devoid of light. We have already found, in dealing with
+spectroscopic binaries, that when one of the component stars is below a
+certain magnitude[33] its spectrum will not be seen; so one is left in
+the glorious uncertainty as to whether the body in question is
+absolutely dark, or darkish, or faint, or indeed only just out of range
+of the spectroscope.
+
+It is thought probable by good authorities that the companion of Algol
+is not quite dark, but has some inherent light of its own. It is, of
+course, much too near Algol to be seen with the largest telescope. There
+is in fact a distance of only from two to three millions of miles
+between the bodies, from which Mr. Gore infers that they would probably
+remain unseparated even in the largest telescope which could ever be
+constructed by man.
+
+The number of known variables of the Algol type is, so far, small; not
+much indeed over thirty. In some of them the components are believed to
+revolve touching each other, or nearly so. An extreme example of this is
+found in the remarkable star V. Puppis, an Algol variable of the
+southern hemisphere. Both its components are bright, and the period of
+light variation is about one and a half days. Dr. A. W. Roberts finds
+that the bodies are revolving around each other in actual contact.
+
+_Temporary stars_ are stars which have suddenly blazed out in regions of
+the sky where no star was previously seen, and have faded away more or
+less gradually.
+
+It was the appearance of such a star, in the year 134 B.C., which
+prompted Hipparchus to make his celebrated catalogue, with the object of
+leaving a record by which future observers could note celestial changes.
+In 1572 another star of this kind flashed out in the constellation of
+Cassiopeia (see Plate XIX., p. 292), and was detected by Tycho Brahe. It
+became as bright as the planet Venus, and eventually was visible in the
+day-time. Two years later, however, it disappeared, and has never since
+been seen. In 1604 Kepler recorded a similar star in the constellation
+of Ophiuchus which grew to be as bright as Jupiter. It also lasted for
+about two years, and then faded away, leaving no trace behind. It is
+rarely, however, that temporary stars attain to such a brilliance; and
+so possibly in former times a number of them may have appeared, but not
+have risen to a sufficient magnitude to attract attention. Even now,
+unless such a star becomes clearly visible to the naked eye, it runs a
+good chance of not being detected. A curious point, worth noting, with
+regard to temporary stars is that the majority of them have appeared in
+the Milky Way.
+
+These sudden visitations have in our day received the name of _Novae_;
+that is to say, "New" Stars. Two, in recent years, attracted a good deal
+of attention. The first of these, known as Nova Aurigae, or the New Star
+in the constellation of Auriga, was discovered by Dr. T.D. Anderson at
+Edinburgh in January 1892. At its greatest brightness it attained to
+about the fourth magnitude. By April it had sunk to the twelfth, but
+during August it recovered to the ninth magnitude. After this last
+flare-up it gradually faded away.
+
+The startling suddenness with which temporary stars usually spring into
+being is the groundwork upon which theories to account for their origin
+have been erected. That numbers of dark stars, extinguished suns, so to
+speak, may exist in space, there is a strong suspicion; and it is just
+possible that we have an instance of one dark stellar body in the
+companion of Algol. That such dark stars might be in rapid motion is
+reasonable to assume from the already known movements of bright stars.
+Two dark bodies might, indeed, collide together, or a collision might
+take place between a dark star and a star too faint to be seen even with
+the most powerful telescope. The conflagration produced by the impact
+would thus appear where nothing had been seen previously. Again, a
+similar effect might be produced by a dark body, or a star too faint to
+be seen, being heated to incandescence by plunging in its course through
+a nebulous mass of matter, of which there are many examples lying about
+in space.
+
+The last explanation, which is strongly reminiscent of what takes place
+in shooting stars, appears more probable than the collision theory. The
+flare-up of new stars continues, indeed, only for a comparatively short
+time; whereas a collision between two bodies would, on the other hand,
+produce an enormous nebula which might take even millions of years to
+cool down. We have, indeed, no record of any such sudden appearance of a
+lasting nebula.
+
+The other temporary star, known as Nova Persei, or the new star in the
+constellation of Perseus, was discovered early in the morning of
+February 22, 1901, also by Dr. Anderson. A day later it had grown to be
+brighter than Capella. Photographs which had been taken, some three days
+previous to its discovery, of the very region of the sky in which it had
+burst forth, were carefully examined, and it was not found in these. At
+the end of two days after its discovery Nova Persei had lost one-third
+of its light. During the ensuing six months it passed through a series
+of remarkable fluctuations, varying in brightness between the third and
+fifth magnitudes. In the month of August it was seen to be surrounded by
+luminous matter in the form of a nebula, which appeared to be gradually
+spreading to some distance around. Taking into consideration the great
+way off at which all this was taking place, it looked as if the new star
+had ejected matter which was travelling outward with a velocity
+equivalent to that of light. The remarkable theory was, however, put
+forward by Professor Kapteyn and the late Dr. W.E. Wilson that there
+might be after all no actual transmission of matter; but that perhaps
+the real explanation was the gradual _illumination_ of hitherto
+invisible nebulous matter, as a consequence of the flare-up which had
+taken place about six months before. It was, therefore, imagined that
+some dark body moving through space at a very rapid rate had plunged
+through a mass of invisible nebulous matter, and had consequently become
+heated to incandescence in its passage, very much like what happens to a
+meteor when moving through our atmosphere. The illumination thus set up
+temporarily in one point, being transmitted through the nebulous wastes
+around with the ordinary velocity of light, had gradually rendered this
+surrounding matter visible. On the assumptions required to fit in with
+such a theory, it was shown that Nova Persei must be at a distance from
+which light would take about three hundred years in coming to us. The
+actual outburst of illumination, which gave rise to this temporary star,
+would therefore have taken place about the beginning of the reign of
+James I.
+
+Some recent investigations with regard to Nova Persei have, however,
+greatly narrowed down the above estimate of its distance from us. For
+instance, Bergstrand proposes a distance of about ninety-nine light
+years; while the conclusions of Mr. F.W. Very would bring it still
+nearer, _i.e._ about sixty-five light years.
+
+The last celestial objects with which we have here to deal are the
+_Nebulae_. These are masses of diffused shining matter scattered here and
+there through the depths of space. Nebulae are of several kinds, and have
+been classified under the various headings of Spiral, Planetary, Ring,
+and Irregular.
+
+A typical _spiral_ nebula is composed of a disc-shaped central portion,
+with long curved arms projecting from opposite sides of it, which give
+an impression of rapid rotatory movement.
+
+The discovery of spiral nebulae was made by Lord Rosse with his great
+6-foot reflector. Two good examples of these objects will be found in
+Ursa Major, while there is another fine one in Canes Venatici (see Plate
+XXII., p. 314), a constellation which lies between Ursa Major and
+Booetes. But the finest spiral of all, perhaps the most remarkable nebula
+known to us, is the Great Nebula in the constellation of Andromeda, (see
+Plate XXIII., p. 316)--a constellation just further from the pole than
+Cassiopeia. When the moon is absent and the night clear this nebula can
+be easily seen with the naked eye as a small patch of hazy light. It is
+referred to by Al Sufi.
+
+[Illustration: PLATE XXII. SPIRAL NEBULA IN THE CONSTELLATION OF CANES
+VENATICI
+
+From a photograph by the late Dr. W.E. Wilson, D.Sc., F.R.S.
+
+(Page 314)]
+
+Spiral nebulae are white in colour, whereas the other kinds of nebula
+have a greenish tinge. They are also by far the most numerous; and the
+late Professor Keeler, who considered this the normal type of nebula,
+estimated that there were at least 120,000 of such spirals within the
+reach of the Crossley reflector of the Lick Observatory. Professor
+Perrine has indeed lately raised this estimate to half a million, and
+thinks that with more sensitive photographic plates and longer exposures
+the number of spirals would exceed a million. The majority of these
+objects are very small, and appear to be distributed over the sky in a
+fairly uniform manner.
+
+_Planetary_ nebulae are small faint roundish objects which, when seen in
+the telescope, recall the appearance of a planet, hence their name. One
+of these nebulae, known astronomically as G.C. 4373, has recently been
+found to be rushing through space towards the earth at a rate of between
+thirty and forty miles per second. It seems strange, indeed, that any
+gaseous mass should move at such a speed!
+
+What are known as _ring_ nebulae were until recently believed to form a
+special class. These objects have the appearance of mere rings of
+nebulous matter. Much doubt has, however, been thrown upon their being
+rings at all; and the best authorities regard them merely as spiral
+nebulae, of which we happen to get a foreshortened view. Very few
+examples are known, the most famous being one in the constellation of
+Lyra, usually known as the Annular Nebula in Lyra. This object is so
+remote from us as to be entirely invisible to the naked eye. It contains
+a star of the fifteenth magnitude near to its centre. From photographs
+taken with the Crossley reflector, Professor Schaeberle finds in this
+nebula evidences of spiral structure. It may here be mentioned that the
+Great Nebula in Andromeda, which has now turned out to be a spiral, had
+in earlier photographs the appearance of a ring.
+
+There also exist nebulae of _irregular_ form, the most notable being the
+Great Nebula in the constellation of Orion (see Plate XXIV., p. 318). It
+is situated in the centre of the "Sword" of Orion (see Plate XX., p.
+296). In large telescopes it appears as a magnificent object, and in
+actual dimensions it must be much on the same scale as the Andromeda
+Nebula. The spectroscope tells us that it is a mass of glowing gas.
+
+The Trifid Nebula, situated in the constellation of Sagittarius, is an
+object of very strange shape. Three dark clefts radiate from its centre,
+giving it an appearance as if it had been torn into shreds.
+
+The Dumb-bell Nebula, a celebrated object, so called from its likeness
+to a dumb-bell, turns out, from recent photographs taken by Professor
+Schaeberle, which bring additional detail into view, to be after all a
+great spiral.
+
+There is a nest, or rather a cluster of nebulae in the constellation of
+Coma Berenices; over a hundred of these objects being here gathered into
+a space of sky about the size of our full moon.
+
+[Illustration: PLATE XXIII. THE GREAT NEBULA IN THE CONSTELLATION OF
+ANDROMEDA
+
+From a photograph taken at the Yerkes Observatory.
+
+(Page 314)]
+
+The spectroscope informs us that spiral nebulae are composed of
+partially-cooled matter. Their colour, as we have seen, is white. Nebulae
+of a greenish tint are, on the other hand, found to be entirely in a
+gaseous condition. Just as the solar corona contains an unknown element,
+which for the time being has been called "Coronium," so do the gaseous
+nebulae give evidence of the presence of another unknown element. To this
+Sir William Huggins has given the provisional name of "Nebulium."
+
+The _Magellanic Clouds_ are two patches of nebulous-looking light, more
+or less circular in form, which are situated in the southern hemisphere
+of the sky. They bear a certain resemblance to portions of the Milky
+Way, but are, however, not connected with it. They have received their
+name from the celebrated navigator, Magellan, who seems to have been one
+of the first persons to draw attention to them. "Nubeculae" is another
+name by which they are known, the larger cloud being styled _nubecula
+major_ and the smaller one _nubecula minor_. They contain within them
+stars, clusters, and gaseous nebulae. No parallax has yet been found for
+any object which forms part of the nubeculae, so it is very difficult to
+estimate at what distance from us they may lie. They are, however,
+considered to be well within our stellar universe.
+
+Having thus brought to a conclusion our all too brief review of the
+stars and the nebulae--of the leading objects in fine which the celestial
+spaces have revealed to man--we will close this chapter with a recent
+summation by Sir David Gill of the relations which appear to obtain
+between these various bodies. "Huggins's spectroscope," he says, "has
+shown that many nebulae are not stars at all; that many well-condensed
+nebulae, as well as vast patches of nebulous light in the sky, are but
+inchoate masses of luminous gas. Evidence upon evidence has accumulated
+to show that such nebulae consist of the matter out of which stars
+(_i.e._ suns) have been and are being evolved. The different types of
+star spectra form such a complete and gradual sequence (from simple
+spectra resembling those of nebulae onwards through types of gradually
+increasing complexity) as to suggest that we have before us, written in
+the cryptograms of these spectra, the complete story of the evolution of
+suns from the inchoate nebula onwards to the most active sun (like our
+own), and then downward to the almost heatless and invisible ball. The
+period during which human life has existed upon our globe is probably
+too short--even if our first parents had begun the work--to afford
+observational proof of such a cycle of change in any particular star;
+but the fact of such evolution, with the evidence before us, can hardly
+be doubted."[34]
+
+[32] The name Al gul, meaning the Demon, was what the old Arabian
+astronomers called it, which looks very much as if they had already
+noticed its rapid fluctuations in brightness.
+
+[33] Mr. Gore thinks that the companion of Algol may be a star of the
+sixth magnitude.
+
+[34] Presidential Address to the British Association for the Advancement
+of Science (Leicester, 1907), by Sir David Gill, K.C.B., LL.D., F.R.S.,
+&c. &c.
+
+[Illustration: PLATE XXIV. THE GREAT NEBULA IN THE CONSTELLATION OF
+ORION
+
+From a photograph taken at the Yerkes Observatory.
+
+(Page 316)]
+
+
+
+
+CHAPTER XXV
+
+THE STELLAR UNIVERSE
+
+
+The stars appear fairly evenly distributed all around us, except in one
+portion of the sky where they seem very crowded, and so give one an
+impression of being very distant. This portion, known as the Milky Way,
+stretches, as we have already said, in the form of a broad band right
+round the entire heavens. In those regions of the sky most distant from
+the Milky Way the stars appear to be thinly sown, but become more and
+more closely massed together as the Milky Way is approached.
+
+This apparent distribution of the stars in space has given rise to a
+theory which was much favoured by Sir William Herschel, and which is
+usually credited to him, although it was really suggested by one Thomas
+Wright of Durham in 1750; that is to say, some thirty years or more
+before Herschel propounded it. According to this, which is known as the
+"Disc" or "Grindstone" Theory, the stars are considered as arranged in
+space somewhat in the form of a thick disc, or grindstone, close to the
+_central_ parts of which our solar system is situated.[35] Thus we
+should see a greater number of stars when we looked out through the
+_length_ of such a disc in any direction, than when we looked out
+through its _breadth_. This theory was, for a time, supposed to account
+quite reasonably for the Milky Way, and for the gradual increase in the
+number of stars in its vicinity.
+
+It is quite impossible to verify directly such a theory, for we know the
+actual distance of only about forty-three stars. We are unable,
+therefore, definitely to assure ourselves whether, as the grindstone
+theory presupposes, the stellar universe actually reaches out very much
+further from us in the direction of the Milky Way than in the other
+parts of the sky. The theory is clearly founded upon the supposition
+that the stars are more or less equal in size, and are scattered through
+space at fairly regular distances from each other.
+
+Brightness, therefore, had been taken as implying nearness to us, and
+faintness great distance. But we know to-day that this is not the case,
+and that the stars around us are, on the other hand, of various degrees
+of brightness and of all orders of size. Some of the faint stars--for
+instance, the galloping star in Pictor--are indeed nearer to us than
+many of the brighter ones. Sirius, on the other hand, is twice as far
+off from us as [a] Centauri, and yet it is very much brighter; while
+Canopus, which in brightness is second only to Sirius out of the whole
+sky, is too far off for its distance to be ascertained! It must be
+remembered that no parallax had yet been found for any star in the days
+of Herschel, and so his estimations of stellar distances were
+necessarily of a very circumstantial kind. He did not, however, continue
+always to build upon such uncertain ground; but, after some further
+examination of the Milky Way, he gave up his idea that the stars were
+equally disposed in space, and eventually abandoned the grindstone
+theory.
+
+Since we have no means of satisfactorily testing the matter, through
+finding out the various distances from us at which the stars are really
+placed, one might just as well go to the other extreme, and assume that
+the thickening of stars in the region of the Milky Way is not an effect
+of perspective at all, but that the stars in that part of the sky are
+actually more crowded together than elsewhere--a thing which astronomers
+now believe to be the case. Looked at in this way, the shape of the
+stellar universe might be that of a globe-shaped aggregation of stars,
+in which the individuals are set at fairly regular distances from each
+other; the whole being closely encircled by a belt of densely packed
+stars. It must, however, be allowed that the gradual increase in the
+number of stars towards the Milky Way appears a strong argument in
+favour of the grindstone theory; yet the belt theory, as above detailed,
+seems to meet with more acceptance.
+
+There is, in fact, one marked circumstance which is remarkably difficult
+of explanation by means of the grindstone theory. This is the existence
+of vacant spaces--holes, so to speak, in the groundwork of the Milky
+Way. For instance, there is a cleft running for a good distance along
+its length, and there is also a starless gap in its southern portion. It
+seems rather improbable that such a great number of stars could have
+arranged themselves so conveniently, as to give us a clear view right
+out into empty space through such a system in its greatest thickness;
+as if, in fact, holes had been bored, and clefts made, from the boundary
+of the disc clean up to where our solar system lies. Sir John Herschel
+long ago drew attention to this point very forcibly. It is plain that
+such vacant spaces can, on the other hand, be more simply explained as
+mere holes in a belt; and the best authorities maintain that the
+appearance of the Milky Way confirms a view of this kind.
+
+Whichever theory be indeed the correct one, it appears at any rate that
+the stars do not stretch out in every direction to an infinite distance;
+but that _the stellar system is of limited extent_, and has in fact a
+boundary.
+
+In the first place, Science has no grounds for supposing that light is
+in any way absorbed or destroyed merely by its passage through the
+"ether," that imponderable medium which is believed to transmit the
+luminous radiations through space. This of course is tantamount to
+saying that all the direct light from all the stars should reach us,
+excepting that little which is absorbed in its passage through our own
+atmosphere. If stars, and stars, and stars existed in every direction
+outwards without end, it can be proved mathematically that in such
+circumstances there could not remain the tiniest space in the sky
+without a star to fill it, and that therefore the heavens would always
+blaze with light, and the night would be as bright as the noonday.[36]
+How very far indeed this is from being the case, may be gathered from an
+estimate which has been made of the general amount of light which we
+receive from the stars. According to this estimate the sky is considered
+as more or less dark, the combined illumination sent to us by all the
+stars being only about the one-hundreth part of what we get from the
+full moon.[37]
+
+Secondly, it has been suggested that although light may not suffer any
+extinction or diminution from the ether itself, still a great deal of
+illumination may be prevented from reaching us through myriads of
+extinguished suns, or dark meteoric matter lying about in space. The
+idea of such extinguished suns, dark stars in fact, seems however to be
+merely founded upon the sole instance of the invisible companion of
+Algol; but, as we have seen, there is no proof whatever that it is a
+dark body. Again, some astronomers have thought that the dark holes in
+the Milky Way, "Coal Sacks," as they are called, are due to masses of
+cool, or partially cooled matter, which cuts off the light of the stars
+beyond. The most remarkable of these holes is one in the neighbourhood
+of the Southern Cross, known as the "Coal Sack in Crux." But Mr. Gore
+thinks that the cause of the holes is to be sought for rather in what
+Sir William Herschel termed "clustering power," _i.e._ a tendency on the
+part of stars to accumulate in certain places, thus leaving others
+vacant; and the fact that globular and other clusters are to be found
+very near to such holes certainly seems corroborative of this theory. In
+summing up the whole question, Professor Newcomb maintains that there
+does not appear any evidence of the light from the Milky Way stars,
+which are apparently the furthest bodies we see, being intercepted by
+dark bodies or dark matter. As far as our telescopes can penetrate, he
+holds that we see the stars _just as they are_.
+
+Also, if there did exist an infinite number of stars, one would expect
+to find evidence in some direction of an overpoweringly great
+force,--the centre of gravity of all these bodies.
+
+It is noticed, too, that although the stars increase in number with
+decrease in magnitude, so that as we descend in the scale we find three
+times as many stars in each magnitude as in the one immediately above
+it, yet this progression does not go on after a while. There is, in
+fact, a rapid falling off in numbers below the twelfth magnitude; which
+looks as if, at a certain distance from us, the stellar universe were
+beginning to _thin out_.
+
+Again, it is estimated, by Mr. Gore and others, that only about 100
+millions of stars are to be seen in the whole of the sky with the best
+optical aids. This shows well the limited extent of the stellar system,
+for the number is not really great. For instance, there are from fifteen
+to sixteen times as many persons alive upon the earth at this moment!
+
+Last of all, there appears to be strong photographic evidence that our
+sidereal system is limited in extent. Two photographs taken by the late
+Dr. Isaac Roberts of a region rich in stellar objects in the
+constellation of Cygnus, clearly show what has been so eloquently called
+the "darkness behind the stars." One of these photographs was taken in
+1895, and the other in 1898. On both occasions the state of the
+atmosphere was practically the same, and the sensitiveness of the films
+was of the same degree. The exposure in the first case was only one
+hour; in the second it was about two hours and a half. And yet both
+photographs show _exactly the same stars, even down to the faintest_.
+From this one would gather that the region in question, which is one of
+the most thickly star-strewn in the Milky Way, is _penetrable right
+through_ with the means at our command. Dr. Roberts himself in
+commenting upon the matter drew attention to the fact, that many
+astronomers seemed to have tacitly adopted the assumption that the stars
+extend indefinitely through space.
+
+From considerations such as these the foremost astronomical authorities
+of our time consider themselves justified in believing that the
+collection of stars around us is _finite_; and that although our best
+telescopes may not yet be powerful enough to penetrate to the final
+stars, still the rapid decrease in numbers as space is sounded with
+increasing telescopic power, points strongly to the conclusion that the
+boundaries of the stellar system may not lie very far beyond the
+uttermost to which we can at present see.
+
+Is it possible then to make an estimate of the extent of this stellar
+system?
+
+Whatever estimates we may attempt to form cannot however be regarded as
+at all exact, for we know the actual distances of such a very few only
+of the nearest of the stars. But our knowledge of the distances even of
+these few, permits us to assume that the stars close around us may be
+situated, on an average, at about eight light-years from each other; and
+that this holds good of the stellar spaces, with the exception of the
+encircling girdle of the Milky Way, where the stars seem actually to be
+more closely packed together. This girdle further appears to contain the
+greater number of the stars. Arguing along these lines, Professor
+Newcomb reaches the conclusion that the farthest stellar bodies which we
+see are situated at about between 3000 and 4000 light-years from us.
+
+Starting our inquiry from another direction, we can try to form an
+estimate by considering the question of proper motions.
+
+It will be noticed that such motions do not depend entirely upon the
+actual speed of the stars themselves, but that some of the apparent
+movement arises indirectly from the speed of our own sun. The part in a
+proper motion which can be ascribed to the movement of our solar system
+through space is clearly a displacement in the nature of a parallax--Sir
+William Herschel called it "_Systematic_ Parallax"; so that knowing the
+distance which we move over in a certain lapse of time, we are able to
+hazard a guess at the distances of a good many of the stars. An inquiry
+upon such lines must needs be very rough, and is plainly based upon the
+assumption that the stars whose distances we attempt to estimate are
+moving at an average speed much like that of our own sun, and that they
+are not "runaway stars" of the 1830 Groombridge order. Be that as it
+may, the results arrived at by Professor Newcomb from this method of
+reasoning are curiously enough very much on a par with those founded on
+the few parallaxes which we are really certain about; with the exception
+that they point to somewhat closer intervals between the individual
+stars, and so tend to narrow down our previous estimate of the extent of
+the stellar system.
+
+Thus far we get, and no farther. Our solar system appears to lie
+somewhere near the centre of a great collection of stars, separated each
+one from the other, on an average, by some 40 billions of miles; the
+whole being arranged in the form of a mighty globular cluster. Light
+from the nearest of these stars takes some four years to come to us. It
+takes about 1000 times as long to reach us from the confines of the
+system. This globe of stars is wrapt around closely by a stellar girdle,
+the individual stars in which are set together more densely than those
+in the globe itself. The entire arrangement appears to be constructed
+upon a very regular plan. Here and there, as Professor Newcomb points
+out, the aspect of the heavens differs in small detail; but generally it
+may be laid down that the opposite portions of the sky, whether in the
+Milky Way itself, or in those regions distant from it, show a marked
+degree of symmetry. The proper motions of stars in corresponding
+portions of the sky reveal the same kind of harmony, a harmony which may
+even be extended to the various colours of the stars. The stellar
+system, which we see disposed all around us, appears in fine to bear all
+the marks of an _organised whole_.
+
+The older astronomers, to take Sir William Herschel as an example,
+supposed some of the nebulae to be distant "universes." Sir William was
+led to this conclusion by the idea he had formed that, when his
+telescopes failed to show the separate stars of which he imagined these
+objects to be composed, he must put down the failure to their stupendous
+distance from us. For instance, he thought the Orion Nebula, which is
+now known to be made up of glowing gas, to be an external stellar
+system. Later on, however, he changed his mind upon this point, and came
+to the conclusion that "shining fluid" would better account both for
+this nebula, and for others which his telescopes had failed to separate
+into component stars.
+
+The old ideas with regard to external systems and distant universes have
+been shelved as a consequence of recent research. All known clusters and
+nebulae are now firmly believed to lie _within_ our stellar system.
+
+This view of the universe of stars as a sort of island in the
+immensities, does not, however, give us the least idea about the actual
+extent of space itself. Whether what is called space is really infinite,
+that is to say, stretches out unendingly in every direction, or whether
+it has eventually a boundary somewhere, are alike questions which the
+human mind seems utterly unable to picture to itself.
+
+
+[35] The Ptolemaic idea dies hard!
+
+[36] Even the Milky Way itself is far from being a blaze of light, which
+shows that the stars composing it do not extend outwards indefinitely.
+
+[37] Mr. Gore has recently made some remarkable deductions, with regard
+to the amount of light which we get from the stars. He considers that
+most of this light comes from stars below the sixth magnitude; and
+consequently, if all the stars visible to the naked eye were to be
+blotted out, the glow of the night sky would remain practically the same
+as it is at present. Going to the other end of the scale, he thinks also
+that the combined light which we get from all the stars below the
+seventeenth magnitude is so very small, that it may be neglected in such
+an estimation. He finds, indeed, that if there are stars so low as the
+twentieth magnitude, one hundred millions of them would only be equal in
+brightness to a single first-magnitude star like Vega. On the other
+hand, it is possible that the light of the sky at night is not entirely
+due to starlight, but that some of it may be caused by phosphorescent
+glow.
+
+
+
+
+CHAPTER XXVI
+
+THE STELLAR UNIVERSE--_continued_
+
+
+It is very interesting to consider the proper motions of stars with
+reference to such an isolated stellar system as has been pictured in the
+previous chapter. These proper motions are so minute as a rule, that we
+are quite unable to determine whether the stars which show them are
+moving along in straight lines, or in orbits of immense extent. It
+would, in fact, take thousands of years of careful observation to
+determine whether the paths in question showed any degree of curving. In
+the case of the more distant stars, the accurate observations which have
+been conducted during the last hundred years have not so far revealed
+any proper motions with regard to them; but one cannot escape the
+conclusion that these stars move as the others do.
+
+If space outside our stellar system is infinite in extent, and if all
+the stars within that system are moving unchecked in every conceivable
+direction, the result must happen that after immense ages these stars
+will have drawn apart to such a distance from each other, that the
+system will have entirely disintegrated, and will cease to exist as a
+connected whole. Eventually, indeed, as Professor Newcomb points out,
+the stars will have separated so far from each other that each will be
+left by itself in the midst of a black and starless sky. If, however, a
+certain proportion of stars have a speed sufficiently slow, they will
+tend under mutual attraction to be brought to rest by collisions, or
+forced to move in orbits around each other. But those stars which move
+at excessive speeds, such, for instance, as 1830 Groombridge, or the
+star in the southern constellation of Pictor, seem utterly incapable of
+being held back in their courses by even the entire gravitative force of
+our stellar system acting as a whole. These stars must, therefore, move
+eventually right through the system and pass out again into the empty
+spaces beyond. Add to this; certain investigations, made into the speed
+of 1830 Groombridge, furnish a remarkable result. It is calculated,
+indeed, that had this star been _falling through infinite space for
+ever_, pulled towards us by the combined gravitative force of our entire
+system of stars, it could not have gathered up anything like the speed
+with which it is at present moving. No force, therefore, which we can
+conjure out of our visible universe, seems powerful enough either to
+have impressed upon this runaway star the motion which it now has, or to
+stay it in its wild course. What an astounding condition of things!
+
+Speculations like this call up a suspicion that there may yet exist
+other universes, other centres of force, notwithstanding the apparent
+solitude of our stellar system in space. It will be recollected that the
+idea of this isolation is founded upon such facts as, that the heavens
+do not blaze with light, and that the stars gradually appear to thin out
+as we penetrate the system with increasing telescopic power. But
+perchance there is something which hinders us from seeing out into space
+beyond our cluster of stars; which prevents light, in fact, from
+reaching us from other possible systems scattered through the depths
+beyond. It has, indeed, been suggested by Mr. Gore[38] that the
+light-transmitting ether may be after all merely a kind of "atmosphere"
+of the stars; and that it may, therefore, thin off and cease a little
+beyond the confines of our stellar system, just as the air thins off and
+practically ceases at a comparatively short distance from the earth. A
+clashing together of solid bodies outside our atmosphere could plainly
+send us no sound, for there is no air extending the whole way to bear to
+our ears the vibrations thus set up; so light emitted from any body
+lying beyond our system of stars, would not be able to come to us if the
+ether, whose function it is to convey the rays of light, ceased at or
+near the confines of that system.
+
+Perchance we have in this suggestion the key to the mystery of how our
+sun and the other stellar bodies maintain their functions of temperature
+and illumination. The radiations of heat and light arriving at the
+limits of this ether, and unable to pass any further, may be thrown back
+again into the system in some altered form of energy.
+
+But these, at best, are mere airy and fascinating speculations. We have,
+indeed, no evidence whatever that the luminiferous ether ceases at the
+boundary of the stellar system. If, therefore, it extends outwards
+infinitely in every direction, and if it has no absorbing or weakening
+effect on the vibrations which it transmits, we cannot escape from the
+conclusion that practically all the rays of light ever emitted by all
+the stars must chase one another eternally through the never-ending
+abysses of space.
+
+
+[38] _Planetary and Stellar Studies_, by John Ellard Gore, F.R.A.S.,
+M.R.I.A., London, 1888.
+
+
+
+
+CHAPTER XXVII
+
+THE BEGINNING OF THINGS
+
+
+LAPLACE'S NEBULAR HYPOTHESIS
+
+Dwelling upon the fact that all the motions of revolution and rotation
+in the solar system, as known in his day, took place in the same
+direction and nearly in the same plane, the great French astronomer,
+Laplace, about the year 1796, put forward a theory to account for the
+origin and evolution of that system. He conceived that it had come into
+being as a result of the gradual contraction, through cooling, of an
+intensely heated gaseous lens-shaped mass, which had originally occupied
+its place, and had extended outwards beyond the orbit of the furthest
+planet. He did not, however, attempt to explain how such a mass might
+have originated! He went on to suppose that this mass, _in some manner_,
+perhaps by mutual gravitation among its parts, had acquired a motion of
+rotation in the same direction as the planets now revolve. As this
+nebulous mass parted with its heat by radiation, it contracted towards
+the centre. Becoming smaller and smaller, it was obliged to rotate
+faster and faster in order to preserve its equilibrium. Meanwhile, in
+the course of contraction, rings of matter became separated from the
+nucleus of the mass, and were left behind at various intervals. These
+rings were swept up into subordinate masses similar to the original
+nebula. These subordinate masses also contracted in the same manner,
+leaving rings behind them which, in turn, were swept up to form
+satellites. Saturn's ring was considered, by Laplace, as the only
+portion of the system left which still showed traces of this
+evolutionary process. It is even probable that it may have suggested the
+whole of the idea to him.
+
+Laplace was, however, not the first philosopher who had speculated along
+these lines concerning the origin of the world.
+
+Nearly fifty years before, in 1750 to be exact, Thomas Wright, of
+Durham, had put forward a theory to account for the origin of the whole
+sidereal universe. In his theory, however, the birth of our solar system
+was treated merely as an incident. Shortly afterwards the subject was
+taken up by the famous German philosopher, Kant, who dealt with the
+question in a still more ambitious manner, and endeavoured to account in
+detail for the origin of the solar system as well as of the sidereal
+universe. Something of the trend of such theories may be gathered from
+the remarkable lines in Tennyson's _Princess_:--
+
+"This world was once a fluid haze of light,
+Till toward the centre set the starry tides,
+And eddied into suns, that wheeling cast
+The planets."
+
+The theory, as worked out by Kant, was, however, at the best merely a
+_tour de force_ of philosophy. Laplace's conception was much less
+ambitious, for it did not attempt to explain the origin of the entire
+universe, but only of the solar system. Being thus reasonably limited in
+its scope, it more easily obtained credence. The arguments of Laplace
+were further founded upon a mathematical basis. The great place which
+he occupied among the astronomers of that time caused his theory to
+exert a preponderating influence on scientific thought during the
+century which followed.
+
+A modification of Laplace's theory is the Meteoritic Hypothesis of Sir
+Norman Lockyer. According to the views of that astronomer, the material
+of which the original nebula was composed is presumed to have been in
+the meteoric, rather than in the gaseous, state. Sir Norman Lockyer
+holds, indeed, that nebulae are, in reality, vast swarms of meteors, and
+the light they emit results from continual collisions between the
+constituent particles. The French astronomer, Faye, also proposed to
+modify Laplace's theory by assuming that the nebula broke up into rings
+all at once, and not in detail, as Laplace had wished to suppose.
+
+The hypothesis of Laplace fits in remarkably well with the theory put
+forward in later times by Helmholtz, that the heat of the sun is kept up
+by the continual contraction of its mass. It could thus have only
+contracted to its present size from one very much larger.
+
+Plausible, however, as Laplace's great hypothesis appears on the
+surface, closer examination shows several vital objections, a few of
+those set forth by Professor Moulton being here enumerated--
+
+Although Laplace held that the orbits of the planets were sufficiently
+near to being in the one plane to support his views, yet later
+investigators consider that their very deviations from this plane are a
+strong argument against the hypothesis.
+
+Again, it is thought that if the theory were the correct explanation,
+the various orbits of the planets would be much more nearly circular
+than they are.
+
+It is also thought that such interlaced paths, as those in which the
+asteroids and the little planet Eros move, are most unlikely to have
+been produced as a result of Laplace's nebula.
+
+Further, while each of the rings was sweeping up its matter into a body
+of respectable dimensions, its gravitative power would have been for the
+time being so weak, through being thus spread out, that any lighter
+elements, as, for instance, those of the gaseous order, would have
+escaped into space in accordance with the principles of the kinetic
+theory.
+
+_The idea that rings would at all be left behind at certain intervals
+during the contraction of the nebula is, perhaps, one of the weakest
+points in Laplace's hypothesis._
+
+Mathematical investigation does not go to show that the rings, presuming
+they could be left behind during the contraction of the mass, would have
+aggregated into planetary bodies. Indeed, it rather points to the
+reverse.
+
+Lastly, such a discovery as that the ninth satellite of Saturn revolves
+in a _retrograde_ direction--that is to say, in a direction contrary to
+the other revolutions and rotations in our solar system--appears
+directly to contradict the hypothesis.
+
+Although Laplace's hypothesis seems to break down under the keen
+criticism to which it has been subjected, yet astronomers have not
+relinquished the idea that our solar system has probably had its origin
+from a nebulous mass. But the apparent failure of the Laplacian theory
+is emphasised by the fact, that _not a single example of a nebula, in
+the course of breaking up into concentric rings, is known to exist in
+the entire heaven_. Indeed, as we saw in Chapter XXIV., there seems to
+be no reliable example of even a "ring" nebula at all. Mr. Gore has
+pointed this out very succinctly in his recently published work,
+_Astronomical Essays_, where he says:--"To any one who still persists in
+maintaining the hypothesis of ring formation in nebulae, it may be said
+that the whole heavens are against him."
+
+The conclusions of Keeler already alluded to, that the spiral is the
+normal type of nebula, has led during the past few years to a new theory
+by the American astronomers, Professors Chamberlin and Moulton. In the
+detailed account of it which they have set forth, they show that those
+anomalies which were stumbling-blocks to Laplace's theory do not
+contradict theirs. To deal at length with this theory, to which the name
+of "Planetesimal Hypothesis" has been given, would not be possible in a
+book of this kind. But it may be of interest to mention that the authors
+of the theory in question remount the stream of time still further than
+did Laplace, and seek to explain the _origin_ of the spiral nebulae
+themselves in the following manner:--
+
+Having begun by assuming that the stars are moving apparently in every
+direction with great velocities, they proceed to point out that sooner
+or later, although the lapse of time may be extraordinarily long,
+collisions or near approaches between stars are bound to occur. In the
+case of collisions the chances are against the bodies striking together
+centrally, it being very much more likely that they will hit each other
+rather towards the side. The nebulous mass formed as a result of the
+disintegration of the bodies through their furious impact would thus
+come into being with a spinning movement, and a spiral would ensue.
+Again, the stars may not actually collide, but merely approach near to
+each other. If very close, the interaction of gravitation will give rise
+to intense strains, or tides, which will entirely disintegrate the
+bodies, and a spiral nebula will similarly result. As happens upon our
+earth, two such tides would rise opposite to each other; and,
+consequently, it is a noticeable fact that spiral nebulae have almost
+invariably two opposite branches (see Plate XXII., p 314). Even if not
+so close, the gravitational strains set up would produce tremendous
+eruptions of matter; and in this case, a spiral movement would also be
+generated. On such an assumption the various bodies of the solar system
+may be regarded as having been ejected from parent masses.
+
+The acceptance of the Planetesimal Hypothesis in the place of the
+Hypothesis of Laplace will not, as we have seen, by any means do away
+with the probability that our solar system, and similar systems, have
+originated from a nebulous mass. On the contrary it puts that idea on a
+firmer footing than before. The spiral nebulae which we see in the
+heavens are on a vast scale, and may represent the formation of stellar
+systems and globular clusters. Our solar system may have arisen from a
+small spiral.
+
+We will close these speculations concerning the origin of things with a
+short sketch of certain investigations made in recent years by Sir
+George H. Darwin, of Cambridge University, into the question of the
+probable birth of our moon. He comes to the conclusion that at least
+fifty-four millions of years ago the earth and moon formed one body,
+which had a diameter of a little over 8000 miles. This body rotated on
+an axis in about five hours, namely, about five times as fast as it does
+at present. The rapidity of the rotation caused such a tremendous strain
+that the mass was in a condition of, what is called, unstable
+equilibrium; very little more, in fact, being required to rend it
+asunder. The gravitational pull of the sun, which, as we have already
+seen, is in part the cause of our ordinary tides, supplied this extra
+strain, and a portion of the mass consequently broke off, which receded
+gradually from the rest and became what we now know as the moon. Sir
+George Darwin holds that the gravitational action of the sun will in
+time succeed in also disturbing the present apparent harmony of the
+earth-moon system, and will eventually bring the moon back towards the
+earth, so that after the lapse of great ages they will re-unite once
+again.
+
+In support of this theory of the terrestrial origin of the moon,
+Professor W.H. Pickering has put forward a bold hypothesis that our
+satellite had its origin in the great basin of the Pacific. This ocean
+is roughly circular, and contains no large land masses, except the
+Australian Continent. He supposes that, prior to the moon's birth, our
+globe was already covered with a slight crust. In the tearing away of
+that portion which was afterwards destined to become the moon the
+remaining area of the crust was rent in twain by the shock; and thus
+were formed the two great continental masses of the Old and New Worlds.
+These masses floated apart across the fiery ocean, and at last settled
+in the positions which they now occupy. In this way Professor Pickering
+explains the remarkable parallelism which exists between the opposite
+shores of the Atlantic. The fact of this parallelism had, however, been
+noticed before; as, for example, by the late Rev. S.J. Johnson, in his
+book _Eclipses, Past and Future_, where we find the following passage:--
+
+"If we look at our maps we shall see the parts of one Continent that jut
+out agree with the indented portions of another. The prominent coast of
+Africa would fit in the opposite opening between North and South
+America, and so in numerous other instances. A general rending asunder
+of the World would seem to have taken place when the foundations of the
+great deep were broken up."
+
+Although Professor Pickering's theory is to a certain degree anticipated
+in the above words, still he has worked out the idea much more fully,
+and given it an additional fascination by connecting it with the birth
+of the moon. He points out, in fact, that there is a remarkable
+similarity between the lunar volcanoes and those in the immediate
+neighbourhood of the Pacific Ocean. He goes even further to suggest that
+Australia is another portion of the primal crust which was detached out
+of the region now occupied by the Indian Ocean, where it was originally
+connected with the south of India or the east of Africa.
+
+Certain objections to the theory have been put forward, one of which is
+that the parallelism noticed between the opposite shores of the Atlantic
+is almost too perfect to have remained through some sixty millions of
+years down to our own day, in the face of all those geological movements
+of upheaval and submergence, which are perpetually at work upon our
+globe. Professor Pickering, however, replies to this objection by
+stating that many geologists believe that the main divisions of land and
+water on the earth are permanent, and that the geological alterations
+which have taken place since these were formed have been merely of a
+temporary and superficial nature.
+
+
+
+
+CHAPTER XXVIII
+
+THE END OF THINGS
+
+
+We have been trying to picture the beginning of things. We will now try
+to picture the end.
+
+In attempting this, we find that our theories must of necessity be
+limited to the earth, or at most to the solar system. The time-honoured
+expression "End of the World" really applies to very little beyond the
+end of our own earth. To the people of past ages it, of course, meant
+very much more. For them, as we have seen, the earth was the centre of
+everything; and the heavens and all around were merely a kind of minor
+accompaniment, created, as they no doubt thought, for their especial
+benefit. In the ancient view, therefore, the beginning of the earth
+meant the beginning of the universe, and the end of the earth the
+extinction of all things. The belief, too, was general that this end
+would be accomplished through fire. In the modern view, however, the
+birth and death of the earth, or indeed of the solar system, might pass
+as incidents almost unnoticed in space. They would be but mere links in
+the chain of cosmic happenings.
+
+A number of theories have been forward from time to time prognosticating
+the end of the earth, and consequently of human life. We will conclude
+with a recital of a few of them, though which, if any, is the true one,
+the Last Men alone can know.
+
+Just as a living creature may at any moment die in the fulness of
+strength through sudden malady or accident, or, on the other hand, may
+meet with death as a mere consequence of old age, so may our globe be
+destroyed by some sudden cataclysm, or end in slow processes of decay.
+Barring accidents, therefore, it would seem probable that the growing
+cold of the earth, or the gradual extinction of the sun, should after
+many millions of years close the chapter of life, as we know it. On the
+former of these suppositions, the decrease of temperature on our globe
+might perhaps be accelerated by the thinning of the atmosphere, through
+the slow escape into space of its constituent gases, or their gradual
+chemical combination with the materials of the earth. The subterranean
+heat entirely radiated away, there would no longer remain any of those
+volcanic elevating forces which so far have counteracted the slow
+wearing down of the land surface of our planet, and thus what water
+remained would in time wash over all. If this preceded the growing cold
+of the sun, certain strange evolutions of marine forms of life would be
+the last to endure, but these, too, would have to go in the end.
+
+Should, however, the actual process be the reverse of this, and the sun
+cool down the quicker, then man would, as a consequence of his
+scientific knowledge, tend in all probability to outlive the other forms
+of terrestrial life. In such a vista we can picture the regions of the
+earth towards the north and south becoming gradually more and more
+uninhabitable through cold, and human beings withdrawing before the
+slow march of the icy boundary, until the only regions capable of
+habitation would lie within the tropics. In such a struggle between man
+and destiny science would be pressed to the uttermost, in the devising
+of means to counteract the slow diminution of the solar heat and the
+gradual disappearance of air and water. By that time the axial rotation
+of our globe might possibly have been slowed down to such an extent that
+one side alone of its surface would be turned ever towards the fast
+dying sun. And the mind's eye can picture the last survivors of the
+human race, huddled together for warmth in a glass-house somewhere on
+the equator, waiting for the end to come.
+
+The mere idea of the decay and death of the solar system almost brings
+to one a cold shudder. All that sun's light and heat, which means so
+much to us, entirely a thing of the past. A dark, cold ball rushing
+along in space, accompanied by several dark, cold balls circling
+ceaselessly around it. One of these a mere cemetery, in which there
+would be no longer any recollection of the mighty empires, the loves and
+hates, and all that teeming play of life which we call History.
+Tombstones of men and of deeds, whirling along forgotten in the darkness
+and silence. _Sic transit gloria mundi._
+
+In that brilliant flight of scientific fancy, the _Time Machine_, Mr.
+H.G. Wells has pictured the closing years of the earth in some such
+long-drawn agony as this. He has given us a vision of a desolate beach
+by a salt and almost motionless sea. Foul monsters of crab-like form
+crawl slowly about, beneath a huge hull of sun, red and fixed in the
+sky. The rocks around are partly coated with an intensely green
+vegetation, like the lichen in caves, or the plants which grow in a
+perpetual twilight. And the air is now of an exceeding thinness.
+
+He dips still further into the future, and thus predicts the final form
+of life:--
+
+"I saw again the moving thing upon the shoal--there was no mistake now
+that it was a moving thing--against the red water of the sea. It was a
+round thing, the size of a football perhaps, or it may be bigger, and
+tentacles trailed down from it; it seemed black against the weltering
+blood-red water, and it was hopping fitfully about."
+
+What a description of the "Heir of all the Ages!"
+
+To picture the end of our world as the result of a cataclysm of some
+kind, is, on the other hand, a form of speculation as intensely dramatic
+as that with which we have just been dealing is unutterably sad.
+
+It is not so many years ago, for instance, that men feared a sudden
+catastrophe from the possible collision of a comet with our earth. The
+unreasoning terror with which the ancients were wont to regard these
+mysterious visitants to our skies had, indeed, been replaced by an
+apprehension of quite another kind. For instance, as we have seen, the
+announcement in 1832 that Biela's Comet, then visible, would cut through
+the orbit of the earth on a certain date threw many persons into a
+veritable panic. They did not stop to find out the real facts of the
+case, namely, that, at the time mentioned, the earth would be nearly a
+month's journey from the point indicated!
+
+It is, indeed, very difficult to say what form of damage the earth
+would suffer from such a collision. In 1861 it passed, as we have seen,
+through the tail of the comet without any noticeable result. But the
+head of a comet, on the other hand, may, for aught we know, contain
+within it elements of peril for us. A collision with this part might,
+for instance, result in a violent bombardment of meteors. But these
+meteors could not be bodies of any great size, for the masses of comets
+are so very minute that one can hardly suppose them to contain any large
+or dense constituent portions.
+
+The danger, however, from a comet's head might after all be a danger to
+our atmosphere. It might precipitate, into the air, gases which would
+asphyxiate us or cause a general conflagration. It is scarcely necessary
+to point out that dire results would follow upon any interference with
+the balance of our atmosphere. For instance, the well-known French
+astronomer, M. Camille Flammarion,[39] has imagined the absorption of
+the nitrogen of the air in this way; and has gone on to picture men and
+animals reduced to breathing only oxygen, first becoming excited, then
+mad, and finally ending in a perfect saturnalia of delirium.
+
+Lastly, though we have no proof that stars eventually become dark and
+cold, for human time has so far been all too short to give us even the
+smallest evidence as to whether heat and light are diminishing in our
+own sun, yet it seems natural to suppose that such bodies must at last
+cease their functions, like everything else which we know of. We may,
+therefore, reasonably presume that there are dark bodies scattered in
+the depths of space. We have, indeed, a suspicion of at least one,
+though perhaps it partakes rather of a planetary nature, namely, that
+"dark" body which continually eclipses Algol, and so causes the
+temporary diminution of its light. As the sun rushes towards the
+constellation of Lyra such an extinguished sun may chance to find itself
+in his path; just as a derelict hulk may loom up out of the darkness
+right beneath the bows of a vessel sailing the great ocean.
+
+Unfortunately a collision between the sun and a body of this kind could
+not occur with such merciful suddenness. A tedious warning of its
+approach would be given from that region of the heavens whither our
+system is known to be tending. As the dark object would become visible
+only when sufficiently near our sun to be in some degree illuminated by
+his rays, it might run the chance at first of being mistaken for a new
+planet. If such a body were as large, for instance, as our own sun, it
+should, according to Mr. Gore's calculations, reveal itself to the
+telescope some fifteen years before the great catastrophe. Steadily its
+disc would appear to enlarge, so that, about nine years after its
+discovery, it would become visible to the naked eye. At length the
+doomed inhabitants of the earth, paralysed with terror, would see their
+relentless enemy shining like a second moon in the northern skies.
+Rapidly increasing in apparent size, as the gravitational attractions of
+the solar orb and of itself interacted more powerfully with diminishing
+distance, it would at last draw quickly in towards the sun and disappear
+in the glare.
+
+It is impossible for us to conceive anything more terrible than these
+closing days, for no menace of catastrophe which we can picture could
+bear within it such a certainty of fulfilment. It appears, therefore,
+useless to speculate on the probable actions of men in their now
+terrestrial prison. Hope, which so far had buoyed them up in the direst
+calamities, would here have no place. Humanity, in the fulness of its
+strength, would await a wholesale execution from which there could be no
+chance at all of a reprieve. Observations of the approaching body would
+have enabled astronomers to calculate its path with great exactness, and
+to predict the instant and character of the impact. Eight minutes after
+the moment allotted for the collision the resulting tide of flame would
+surge across the earth's orbit, and our globe would quickly pass away in
+vapour.
+
+And what then?
+
+A nebula, no doubt; and after untold ages the formation possibly from it
+of a new system, rising phoenix-like from the vast crematorium and
+filling the place of the old one. A new central sun, perhaps, with its
+attendant retinue of planets and satellites. And teeming life,
+perchance, appearing once more in the fulness of time, when temperature
+in one or other of these bodies had fallen within certain limits, and
+other predisposing conditions had supervened.
+
+  "The world's great age begins anew,
+      The golden years return,
+  The earth doth like a snake renew
+      Her winter weeds outworn:
+  Heaven smiles, and faiths and empires gleam
+  Like wrecks of a dissolving dream.
+
+  A brighter Hellas rears its mountains
+      From waves serener far;
+  A new Peneus rolls his fountains
+      Against the morning star;
+  Where fairer Tempes bloom, there sleep
+  Young Cyclads on a sunnier deep.
+
+  A loftier Argo cleaves the main,
+      Fraught with a later prize;
+  Another Orpheus sings again,
+      And loves, and weeps, and dies;
+  A new Ulysses leaves once more
+  Calypso for his native shore.
+
+       *       *       *       *       *
+
+Oh cease! must hate and death return?
+    Cease! must men kill and die?
+Cease! drain not to its dregs the urn
+    Of bitter prophecy!
+The world is weary of the past,--
+Oh might it die or rest at last!"
+
+
+[39] See his work, _La Fin du Monde_, wherein the various ways by which
+our world may come to an end are dealt with at length, and in a
+profoundly interesting manner.
+
+
+
+
+INDEX
+
+
+  Achromatic telescope, 115, 116
+
+  Adams, 24, 236, 243
+
+  Aerial telescopes, 110, 111
+
+  Agathocles, Eclipse of, 85
+
+  Agrippa, Camillus, 44
+
+  Ahaz, dial of, 85
+
+  Air, 166
+
+  Airy, Sir G.B., 92
+
+  Al gul, 307
+
+  Al Sufi, 284, 290, 296, 315
+
+  Alcor, 294
+
+  Alcyone, 284
+
+  Aldebaran, 103, 288, 290, 297
+
+  Algol, 307, 309-310, 312, 323, 347
+
+  Alpha, Centauri, 52-53, 280, 298-299, 304, 320
+
+  Alpha Crucis, 298
+
+  Alps, Lunar, 200
+
+  Altair, 295
+
+  Altitude of objects in sky, 196
+
+  Aluminium, 145
+
+  Amos viii. 9, 85
+
+  Anderson, T.D., 311-312
+
+  Andromeda (constellation), 279, 314;
+    Great Nebula in, 314, 316
+
+  Andromedid meteors, 272
+
+  Anglo-Saxon Chronicle, 87-88
+
+  Anighito meteorite, 277
+
+  Annular eclipse, 65-68, 80, 92, 99
+
+  Annular Nebula in Lyra, 315-316
+
+  Annulus, 68
+
+  Ansae, 242-243
+
+  Anticipation in discovery, 236-237
+
+  Apennines, Lunar, 200
+
+  Aphelion, 274
+
+  Apparent enlargement of celestial objects, 192-196
+
+  Apparent size of celestial objects deceptive, 196, 294
+
+  Apparent sizes of sun and moon, variations in, 67, 80, 178
+
+  Aquila (constellation), 295
+
+  Arabian astronomers, 107, 307
+
+  Arago, 92, 257
+
+  Arc, degrees minutes and seconds of, 60
+
+  Arcturus, 280, 282, 290, 295
+
+  Argelander, 290
+
+  Argo (constellation), 298
+
+  Aristarchus of Samos, 171
+
+  Aristarchus (lunar crater), 205
+
+  Aristophanes, 101
+
+  Aristotle, 161, 173, 185
+
+  Arrhenius 222, 253-254
+
+  Assyrian tablet, 84
+
+  Asteroidal zone, analogy of, to Saturn's rings, 238
+
+  Asteroids (or minor planets), 30-31, 225-228, 336;
+    discovery of the, 23, 244;
+    Wolf's method of discovering, 226-227
+
+  Astrology, 56
+
+  _Astronomical Essays_, 63, 337
+
+  Astronomical Society, Royal, 144
+
+  _Astronomy, Manual of_, 166
+
+  Atlantic Ocean, parallelism of opposite shores, 340-341
+
+  Atlas, the Titan, 18
+
+  Atmosphere, absorption by earth's, 129-130;
+    ascertainment of, by spectroscope, 124-125, 212;
+    height of earth's, 167, 267;
+    of asteroids, 226;
+    of earth, 129, 130, 166-169, 218, 222, 267, 346;
+    of Mars, 156, 212, 216;
+    of Mercury, 156;
+    of moon, 70-71, 156, 201-203;
+    of Jupiter, 231;
+    of planets, 125;
+    of Saturn's rings, 239
+
+  "Atmosphere" of the stars, 331
+
+  Atmospheric layer and "glass-house" compared, 167, 203
+
+  August Meteors (Perseids), 270
+
+  Auriga (constellation), 294-296, 306, 311;
+    New Star in, 311
+
+  Aurigae, [b] (Beta), 294, 297, 304
+
+  Aurora Borealis, 141, 143, 259
+
+  Australia, suggested origin of, 340
+
+  Axis, 29-30;
+    of earth, 163, 180;
+    small movement of earth's, 180-181
+
+
+  Babylonian tablet, 84
+
+  Babylonian idea of the moon, 185
+
+  Bacon, Roger, 108
+
+  Bacubirito meteorite, 277
+
+  Bagdad, 107
+
+  Baily, Francis, 92
+
+  "Baily's Beads," 69, 70, 91-92, 154
+
+  Bailly (lunar crater), 199
+
+  Ball, Sir Robert, 271
+
+  Barnard, E.E., 31, 224, 232-234, 237, 258
+
+  "Bay of Rainbows," 197
+
+  Bayer's classification of stars, 289, 291-292
+
+  Bayeux Tapestry, 263
+
+  Bear, Great (constellation). _See_ Ursa Major;
+    Little, _see_ Ursa Minor
+
+  Beehive (Praesepe), 307
+
+  Beer, 206
+
+  Belopolsky, 304
+
+  "Belt" of Orion, 297
+
+  Belt theory of Milky Way, 321
+
+  Belts of Jupiter, 230
+
+  Bergstrand, 314
+
+  Berlin star chart, 244
+
+  Bessel, 173, 280, 305
+
+  Beta ([b]) Lyrae, 307
+
+  Beta ([b]) Persei. _See_ Algol
+
+  Betelgeux, 297
+
+  Bible, eclipses in, 85
+
+  Biela's Comet, 256-257, 272-273, 345
+
+  Bielids, 270, 272-273
+
+  Billion, 51-52
+
+  Binary stars, spectroscopic, 301-306, 309;
+    visual, 300, 303-306
+
+  "Black Drop," 152-154
+
+  "Black Hour," 89
+
+  "Black Saturday," 89
+
+  Blood, moon in eclipse like, 102
+
+  Blue (rays of light), 121, 130
+
+  Bode's Law, 22-23, 244-245
+
+  Bolometer, 127
+
+  Bond, G.P., 236, 257
+
+  Bonpland, 270
+
+  Booetes (constellation), 295, 314
+
+  Bradley, 111
+
+  Brahe, Tycho, 290, 311
+
+  Bredikhine's theory of comets' tails, 253-254, 256
+
+  Bright eclipses of moon, 65, 102
+
+  British Association for the Advancement of Science, 318
+
+  _British Astronomical Association, Journal of_, 194
+
+  British Museum, 84
+
+  Bull (constellation). _See_ Taurus;
+    "Eye" of the, 297;
+    "Head" of the, 297
+
+  Burgos, 98
+
+  Busch, 93
+
+
+  Caesar, Julius, 85, 110, 180, 259, 262, 291, 293
+
+  Calcium, 138, 145
+
+  Callisto, 233-234
+
+  Cambridge, 24, 91, 119, 243
+
+  Campbell, 305
+
+  Canali, 214
+
+  "Canals" of Mars, 214-222, 224-225
+
+  Cancer (constellation), 307
+
+  Canes Venatici (constellation), 306, 314
+
+  Canis Major (constellation), 289, 296-297;
+    Minor, 296-297
+
+  Canopus, 285, 298-299, 320
+
+  Capella, 280, 282, 290, 294, 297, 303, 313
+
+  Carbon, 145
+
+  Carbon dioxide. _See_ Carbonic acid gas
+
+  Carbonic acid gas, 166, 213, 221-222
+
+  Carnegie Institution, Solar Observatory of, 118
+
+  Cassegrainian telescope, 114, 118
+
+  Cassini, J.D., 236, 240
+
+  "Cassini's Division" in Saturn's ring, 236, 238
+
+  Cassiopeia (constellation), 279, 294, 311, 314
+
+  Cassiopeiae, [e] (Eta), 303
+
+  Cassiopeia's Chair, 294
+
+  Cassius, Dion, 86
+
+  Castor, 282, 297, 304
+
+  Catalogues of stars, 106, 290-291, 311
+
+  Centaur. _See_ Centaurus
+
+  Centaurus (constellation), 298, 306
+
+  Centre of gravity, 42, 283-284, 324
+
+  Ceres, diameter of, 30, 225
+
+  Ceti, Omicron (or Mira), 307-308
+
+  Cetus, or the Whale (constellation), 307
+
+  Chaldean astronomers, 74, 76
+
+  Challis, 243-244
+
+  Chamberlin, 337
+
+  "Chambers of the South," 299
+
+  Chandler, 308
+
+  Charles V., 261
+
+  "Charles' Wain," 291
+
+  Chemical rays, 127
+
+  Chinese and eclipses, 83
+
+  Chloride of sodium, 122
+
+  Chlorine, 122, 145
+
+  Christ, Birth of, 102
+
+  Christian Era, first recorded solar eclipse in, 85
+
+  Chromatic aberration, 110
+
+  Chromosphere, 71-72, 93-94, 130-132, 138-139
+
+  Circle, 171-173
+
+  Clark, Alvan, & Sons, 117-118, 303
+
+  Claudius, Emperor, 86
+
+  Clavius (lunar crater), 199
+
+  Clerk Maxwell, 237
+
+  "Clouds" (of Aristophanes), 101
+
+  Clustering power, 325
+
+  Clusters of stars, 300, 306, 314, 328
+
+  Coal Sacks. _See_ Holes in Milky Way
+
+  Coelostat, 119
+
+  Coggia's Comet, 254
+
+  Colour, production of, in telescopes, 109-111, 115, 121
+
+  Collision of comet with earth, 345-346;
+    of dark star with sun, 346-348;
+    of stars, 285, 312
+
+  Columbus, 103
+
+  Coma Berenices (constellation), 307, 316
+
+  Comet, first discovery of by photography, 258;
+    first orbit calculated, 255;
+    first photograph of, 257-258;
+    furthest distance seen, 258;
+    passage of among satellites of Jupiter, 250;
+    passage of earth and moon through tail of, 257, 346
+
+  Comet of 1000 A.D., 262;
+    1066, 262-264;
+    1680, 255, 265;
+    1811, 254-255;
+    1861, 254, 257, 346;
+    1881, 257-258;
+    1882, 251, 258, 291;
+    1889, 258;
+    1907, 258
+
+  Comets, 27-28, 58, Chaps. XIX. and XX., 345-346;
+    ancient view of, 259-261;
+    captured, 251-253;
+    Chinese records of, 83-84;
+    composition of, 252;
+    contrasted with planets, 247;
+    families of, 251-252, 256;
+    meteor swarms and, 274;
+    revealed by solar eclipses, 95-96;
+    tails of, 141, 182, 248, 252-254
+
+  Common, telescopes of Dr. A.A., 118
+
+  Conjunction, 209
+
+  Constellations, 105, 278-279, 285, 289
+
+  Contraction theory of sun's heat, 128-129, 335
+
+  Cook, Captain, 154
+
+  Cooke, 118
+
+  Copernican system, 20, 107, 149, 170-173, 279, 280
+
+  Copernicus, 20, 108, 149, 158, 170-172, 236
+
+  Copernicus (lunar crater), 200, 204
+
+  Copper, 145
+
+  Corder, H., 144
+
+  Corona, 70-72, 90, 92-97, 132, 140-141, 270;
+    earliest drawing of, 91;
+    earliest employment of term, 90;
+    earliest mention of, 86;
+    earliest photograph of, 93;
+    illumination given by, 71;
+    possible change in shape of during eclipse, 96-98;
+    structure of, 142-143;
+    variations in shape of, 141
+
+  Corona Borealis (constellation), 295
+
+  Coronal matter, 142;
+    streamers, 95-96, 141-143
+
+  Coronium, 133, 142, 317
+
+  Cotes, 91
+
+  Coude, equatorial, 119
+
+  Cowell, P.H., 255, 264
+
+  Crabtree, 152
+
+  Crape ring of Saturn, 236-237
+
+  Craterlets on Mars, 220
+
+  Craters (ring-mountains) on moon, 197-205, 214, 340;
+    suggested origin of, 203-204, 214
+
+  Crawford, Earl of, 94
+
+  Crecy, supposed eclipse at battle of, 88-89
+
+  Crescent moon, 183, 185
+
+  Crommelin, A.C.D., 255, 264
+
+  Crossley Reflector, 118, 315-316
+
+  Crown glass, 115
+
+  Crucifixion, darkness of, 86
+
+  Crucis, [a] (Alpha), 298
+
+  Crux, or "Southern Cross" (constellation), 298-299, 323
+
+  Cycle, sunspot, 136-137, 141, 143-144
+
+  Cygni, 61, 173, 280
+
+  Cygnus, or the Swan (constellation), 295, 325
+
+
+  Daniel's Comet of 1897, 258
+
+  Danzig, 111
+
+  Dark Ages, 102, 107, 260
+
+  Dark eclipses of moon, 65, 102-103
+
+  Dark matter in space, 323
+
+  Dark meteors, 275-276
+
+  Dark stars, 309-310, 312, 323, 346-347
+
+  "Darkness behind the stars," 325
+
+  Darwin, Sir G.H., 339
+
+  Davis, 94
+
+  Dawes, 236
+
+  Dearborn Observatory, 303
+
+  Death from fright at eclipse, 73
+
+  Debonnaire, Louis le, 88, 261
+
+  Deimos, 223
+
+  Deity, symbol of the, 87
+
+  "Demon star." _See_ Algol
+
+  Denebola, 296
+
+  Denning, W.F., 269
+
+  Densities of sun and planets, 39
+
+  Density, 38
+
+  Deslandres, 140
+
+  Diameters of sun and planets, 31
+
+  Disappearance of moon in lunar eclipse, 65, 102-103
+
+  Disc, 60
+
+  "Disc" theory. _See_ "Grindstone" theory
+
+  Discoveries, independent, 236
+
+  Discovery, anticipation in, 236-237;
+    indirect methods of, 120
+
+  "Dipper," the, 291;
+    the "Little," 294
+
+  Distance of a celestial body, how ascertained, 56-58;
+    of sun from earth, how determined, 151, 211
+
+  Distances of planets from sun, 47
+
+  Distances of sun and moon, relative, 68
+
+  Dog, the Greater. _See_ Canis Major;
+    the Lesser, _see_ Canis Minor
+
+  "Dog Star," 289, 297
+
+  Dollond, John, 115-116
+
+  Donati's Comet, 254, 257
+
+  Doppler's method, 125, 136, 282, 301-302
+
+  Dorpat, 117
+
+  Double canals of Mars, 214-215, 218-220
+
+  Double planet, earth and moon a, 189
+
+  Double stars, 300
+
+  Douglass, 233
+
+  "Dreams, Lake of," 197
+
+  Dumb-bell Nebula, 316
+
+
+  Earth, 20, 22, 31, 39, 48, 64, Chap. XV., 267;
+    cooling of, 343;
+    diameter of, 31;
+    interior of, 166;
+    mean distance of from sun, 47;
+    rigidity of, 181;
+    rotation of, 30, 33, 161-165, 170;
+    shape of, 165;
+    "tail" to, 182
+
+  "Earthlight," or "Earthshine," 186
+
+  Earth's axis, Precessional movement of, 175-177, 295, 298-299
+
+  Earth's shadow, circular shape of, 64, 160
+
+  Eclipse, 61
+
+  Eclipse knowledge, delay of, 74
+
+  Eclipse party, work of, 73
+
+  Eclipse of sun, advance of shadow in total, 69;
+    animal and plant life during, 71;
+    earliest record of total, 84;
+    description of total, 69-73;
+    duration of total, 69, 72;
+    importance of total, 68
+
+  Eclipses, ascertainment of dates of past, 74;
+    experience a necessity in solar, 73-74;
+    of moon, 63-65, Chap. IX., 203;
+    photography in, 93;
+    prediction of future, 74;
+    recurrence of, 74-80
+
+  Eclipses of sun, 25, 65-74, Chap. VIII., 201-202, 234;
+    1612 A.D., 90;
+    1715, 88, 91;
+    1724, 88, 91;
+    1836, 92;
+    1842, 92-93;
+    1851, 81, 93;
+    1868, 93;
+    1870, 94;
+    1871, 94;
+    1878, 95;
+    1882, 95;
+    1883, 95-96;
+    1893, 95-96;
+    1896, 96, 99;
+    1898, 96, 98;
+    1900, 97;
+    1905, 75-76, 80-81, 97-98;
+    1907, 98;
+    1908, 98;
+    1914, 99;
+    1927, 92, 99-100
+
+  _Eclipses, Past and Future_, 340
+
+  Egenitis, 272
+
+  Electric furnace, 128
+
+  Electric light, spectrum of, 122
+
+  Elements composing sun, 144-145
+
+  Ellipses, 32, 66, 172-173, 177-178
+
+  Elliptic orbit, 66, 177
+
+  Ellipticity, 32
+
+  Elongation, Eastern, 147, 149;
+    Western, 147, 149
+
+  Encke's Comet, 253, 256
+
+  "End of the World," 342
+
+  England, solar eclipses visible in, 87-88, 91-92
+
+  Epsilon, ([e]) Lyrae, 302
+
+  Equator, 48
+
+  Equatorial telescope, 226
+
+  Equinoxes. _See_ Precession of
+
+  Eros, 210-211, 223, 226-227;
+    discovery of, 24, 210, 227;
+    importance of, 211;
+    orbit of, 32, 37, 210, 336
+
+  Eruptive prominences, 139
+
+  _Esclistre_, 89
+
+  Ether, 322-323, 331-332
+
+  Europa, 233, 235
+
+  Evans, J.E., 219
+
+  Evening star, 149-150, 241
+
+  Everest, Mount, 200
+
+  Evershed, 182
+
+  Eye-piece, 110
+
+
+  Fabricius, 307
+
+  Faculae, 136, 143
+
+  Fauth, 205
+
+  Faye, 335
+
+  _Fin du Monde_, 346
+
+  First quarter, 183
+
+  "Fixed stars," 280
+
+  Flagstaff, 215-216, 220
+
+  Flammarion, Camille, 346
+
+  Flamsteed, 90
+
+  "Flash spectrum," 137
+
+  "Flat," 112
+
+  Flint glass, 115
+
+  Focus, 66, 177
+
+  "Forty-foot Telescope," 115
+
+  Foster, 102
+
+  Fraunhofer, 117
+
+  French Academy of Sciences, 115
+
+  Froissart, 89
+
+  "Full moon" of Laplace, 190
+
+
+  Galaxy. _See_ Milky Way.
+
+  Galilean telescope, 109
+
+  Galileo, 55, 109, 172, 197, 206, 232-235, 242
+
+  Galle, 24, 211, 244
+
+  Ganymede, 233-234
+
+  Gas light, spectrum of, 122
+
+  Gegenschein, 181-182
+
+  "Gem" of meteor ring, 271
+
+  Gemini, or the Twins (constellation), 22, 296-297
+
+  Geminorum, [z] (Zeta), 304
+
+  Geometrical groupings of stars, 292
+
+  "Giant" planet, 230, 238-239
+
+  Gibbous, 183, 185
+
+  Gill, Sir David, 211, 258, 291, 317-318
+
+  Gold, 145
+
+  Goodricke, 307
+
+  Gore, J.E., 63, 285, 303, 307-308, 310, 323-324, 331, 337, 347
+
+  Granulated structure of photosphere, 134
+
+  Gravitation (or gravity), 39, 41-45, 128, 306
+
+  Greek ideas, 18, 158, 161-162, 171, 186, 197
+
+  Green (rays of light), 121
+
+  Greenwich Observatory, 143-144, 232, 255, 303
+
+  Gregorian telescope, 113-114
+
+  Grimaldi (lunar crater), 199
+
+  "Grindstone" theory, 319-322
+
+  "Groombridge, 1830," 281-282, 326, 330
+
+  Groups of stars, 306-307
+
+  Grubb, Sir Howard, 118
+
+  _Gulliver's Travels_, 224
+
+
+  Hale, G.E., 119, 140
+
+  Half moon, 183, 185
+
+  Hall, Asaph, 223
+
+  Hall, Chester Moor, 115
+
+  Halley, Edmund, 91, 255, 264-265, 306
+
+  Halley's Comet, 255, 264-265
+
+  Haraden Hill, 91
+
+  Harvard, 118, 302
+
+  Harvest moon, 190-192
+
+  Hawaii, 221
+
+  Heat rays, 127
+
+  Heidelberg, 226, 232
+
+  Height of lunar mountains, how determined, 201
+
+  Height of objects in sky, estimation of, 196
+
+  Helium, 138, 145, 182
+
+  Helmholtz, 128, 335
+
+  Hercules (constellation), 295
+
+  Herod the Great, 101-102
+
+  Herodotus, 84
+
+  Herschel, A.S., 269
+
+  Herschel, Sir John, 92, 322
+
+  Herschel, Sir William, 22, 36, 114-115, 204, 213, 235, 283, 292, 308,
+    319-320, 326-328
+
+  Herschelian telescope, 114, 119
+
+  Hesper, 109
+
+  Hesperus, 150
+
+  Hevelius, 111
+
+  Hezekiah, 85
+
+  Hi, 83
+
+  Hindoos, 18
+
+  Hipparchus, 106, 177, 290, 311
+
+  Ho, 83
+
+  Holes in Milky Way, 321-323
+
+  Holmes, Oliver Wendell, 213
+
+  Homer, 223
+
+  Horace, Odes of, 106
+
+  Horizon, 159
+
+  Horizontal eclipse, 169
+
+  Horrox, 44, 151-152
+
+  Hour Glass Sea, 212
+
+  Huggins, Sir William, 94, 125, 317
+
+  Humboldt, 270
+
+  "Hunter's moon," 192
+
+  Huyghens, 111-112, 240, 242-243
+
+  Hyades, 296-297, 307
+
+  Hydrocarbon gas, 254
+
+  Hydrogen, 94, 131, 138, 140, 144, 156, 182, 254
+
+
+  Ibrahim ben Ahmed, 270
+
+  Ice-layer theory:
+    Mars, 219;
+    moon, 205, 219
+
+  Illusion theory of Martian canals, 219
+
+  Imbrium, Mare, 197
+
+  Inclination of orbits, 36-37
+
+  Indigo (rays of light), 121
+
+  Inferior conjunction, 147, 149
+
+  Inferior planets, 20, 22, Chap. XIV., 229
+
+  Instruments, pre-telescopic, 106-107, 172
+
+  International photographic survey of sky, 290-291
+
+  Intra-Mercurial planet, 25-26
+
+  _Introduction to Astronomy_, 31
+
+  Inverted view in astronomical telescope, 116-117
+
+  Io, 233-234
+
+  Iridum, Sinus, 197
+
+  Iron, 145, 254
+
+  _Is Mars Habitable?_ 221
+
+
+  Jansen, 108
+
+  Janssen, 94, 236, 258
+
+  Japetus, 240
+
+  Jessenius, 89
+
+  Job, Book of, 299
+
+  Johnson, S.J., 103, 340
+
+  Josephus, 101, 262
+
+  Juno, 225
+
+  Jupiter, 20, 22-23, 31, 34, 37, 42, 227-228, 230-236, 241, 272, 311;
+    comet family of, 251-253, 256;
+    discovery of eighth satellite, 26, 232;
+    eclipse of, by satellite, 234;
+    without satellites, 234-235
+
+  Jupiter, satellites of, 26, 62, 108, 189, 232-235;
+    their eclipses, 234-235;
+    their occultations, 62, 234;
+    their transits, 62, 234
+
+
+  Kant, 334
+
+  Kapteyn, 284, 313
+
+  Keeler, 315, 337
+
+  Kelvin, Lord, 129
+
+  Kepler, 44, 152, 172, 237, 242, 245, 253, 311
+
+  Kinetic theory, 156, 202, 212, 226, 231, 239, 336
+
+  King, L.W., 84
+
+  _Knowledge_, 87
+
+
+  Labrador, 97
+
+  Lacus Somniorum, 197
+
+  "Lake of Dreams," 197
+
+  Lalande, 244, 283
+
+  Lampland, 215, 219
+
+  Langley, 95, 127
+
+  Laplace, 190, 333
+
+  Laputa, 224
+
+  Le Maire, 115
+
+  Le Verrier, 24, 236, 243-244, 275
+
+  Lead, 145
+
+  Leibnitz Mountains (lunar), 200
+
+  Leo (constellation), 270, 295-296
+
+  Leonids, 270-272, 274-275
+
+  Lescarbault, 25
+
+  Lewis, T., 303
+
+  Lexell's Comet, 250
+
+  Lick Observatory, 31, 98, 117-118, 215, 232, 303, 305, 315;
+    Great Telescope of, 117, 215, 237
+
+  "Life" of an eclipse of the moon, 80;
+    of the sun, 77-78
+
+  Life on Mars, Lowell's views, 217-218;
+    Pickering's, 221;
+    Wallace's, 221-223
+
+  Light, no extinction of, 322-324;
+    rays of, 127;
+    velocity of, 52, 235-236;
+    white, 121
+
+  "Light year," 53, 280
+
+  Lindsay, Lord, 94
+
+  Linne (lunar crater), 205
+
+  Liouville, 190
+
+  Lippershey, 108
+
+  Liquid-filled lenses, 116
+
+  _Locksley Hall_, 296;
+    _Sixty Years After_, 109
+
+  Lockyer, Sir Norman, 73, 94, 236, 335
+
+  Loewy, 119, 206
+
+  London, eclipses visible at, 87-88, 91-92
+
+  Longfellow, 88
+
+  Lowell Observatory, 215, 219, 233-234
+
+  Lowell, Percival, 155, 212-213, 215-221
+
+  Lucifer, 150
+
+  Lynn, W.T., 219, 263
+
+  Lyra (constellation), 177, 283, 294-295, 307, 315, 347
+
+
+  Maedler, 206, 284
+
+  Magellanic Clouds, 317
+
+  Magnetism, disturbances of terrestrial, 143, 283
+
+  Magnitudes of stars, 287-289
+
+  Major planets, 229-230
+
+  "Man in the Moon," 197
+
+  _Manual of Astronomy_, 166
+
+  Maps of the moon, 206
+
+  Mare Imbrium, 197
+
+  Mare Serenitatis, 205
+
+  Mars, 20, 22-23, 31-32, 34, 37, 109, 155, 210-225, 234;
+    compared with earth and moon, 221, 225;
+    polar caps of, 212-214, 216;
+    satellites of, 26, 223-224;
+    temperature of, 213, 216, 221-222
+
+  Mass, 38;
+    of a star, how determined, 305
+
+  Masses of celestial bodies, how ascertained, 42;
+    of earth and moon compared, 42;
+    of sun and planets compared, 39
+
+  Maunder, E.W., 87, 143, 219
+
+  Maunder, Mrs., E.W., 96, 144
+
+  Maxwell, Clerk, 237
+
+  Mayer, Tobias, 206, 283
+
+  McClean, F.K., 98
+
+  Mean distance, 46
+
+  "Medicean Stars," 232
+
+  Mediterranean, eclipse tracks across, 94, 97
+
+  Melbourne telescope, 118
+
+  Melotte, P., 232
+
+  Mercator's Projection, 80-81
+
+  Mercury (the metal), 145
+
+  Mercury (the planet), 20, 22, 25-26, 31-32, 34, 37, Chap. XIV.;
+    markings on, 156;
+    possible planets within orbit of, 25-26;
+    transit of, 62, 151, 154
+
+  Metals in sun, 145
+
+  Meteor swarms, 268-269, 271, 274-275
+
+  Meteors, 28, 56, 167, 259, Chap. XXI.
+
+  Meteors beyond earth's atmosphere, 275-276
+
+  Meteorites, 276-277
+
+  Meteoritic Hypothesis, 335
+
+  Metius, Jacob, 108
+
+  Michell, 283, 305
+
+  Middle Ages, 102, 260, 264
+
+  Middleburgh, 108
+
+  Milky Way (or Galaxy), 285, 299, 311, 317, 319-327;
+    penetration of, by photography, 325
+
+  Million, 47, 51-52
+
+  Minor planets. _See_ Asteroids.
+
+  Mira Ceti, 307-308
+
+  "Mirk Monday," 89
+
+  Mirror (speculum), 111, 116
+
+  Mizar, 294, 302
+
+  Monck, W.H.S., 275
+
+  Mongol Emperors of India, 107
+
+  Moon, 26, Chap. XVI.;
+    appearance of, in lunar eclipse, 65, 102-103;
+    diameter of, 189;
+    distance of, how ascertained, 58;
+    distance of, from earth, 48;
+    full, 63, 86, 149, 184, 189, 190, 206;
+    mass of, 200, 202;
+    mountains on, 197-205;
+    how their height is determined, 201;
+    movement of, 40-42;
+    new, 86, 149, 183, 185;
+    origin of, 339-341;
+    plane of orbit of, 63;
+    possible changes on, 204-205, 221;
+    "seas" of, 197, 206;
+    smallest detail visible on, 207;
+    volume of, 200
+
+  Morning star, 149-150, 241
+
+  Moulton, F.R., 31, 118, 128, 302, 335, 337
+
+  Moye, 154
+
+  Multiple stars, 300
+
+  Musa-ben-Shakir, 44
+
+  Mythology, 105
+
+
+  Neap-tides, 179
+
+  Nebulae, 314-318, 328, 335, 345;
+    evolution of stars from, 317-318
+
+  Nebular Hypothesis of Laplace, 333-338
+
+  Nebular hypotheses, Chap. XXVII.
+
+  Nebulium, 317
+
+  Neison, 206
+
+  Neptune, 20, 25, 31, 34, 37, 243-246, 249, 252, 274, 304;
+    discovery of, 23-24, 94, 210, 236, 243-244;
+    Lalande and, 244;
+    possible planets beyond, 25, 252;
+    satellite of, 26, 245;
+    "year" in, 35-36
+
+  "New" (or temporary), stars, 310-314
+
+  Newcomb, Simon, 181, 267, 281, 324, 326-327, 329
+
+  Newton, Sir Isaac, 40, 44, 91, 111-113, 115, 165, 172, 237, 255
+
+  Newtonian telescope, 112, 114, 116, 119
+
+  Nineveh Eclipse, 84-85
+
+  Nitrogen, 145, 156, 166, 346
+
+  Northern Crown, 295
+
+  Nova Aurigae, 311
+
+  Nova Persei, 312-314
+
+  Novae. _See_ New (or temporary) stars
+
+  Nubeculae, 317
+
+
+  "Oases" of Mars, 216, 220
+
+  Object-glass, 109
+
+  Oblate spheroid, 165
+
+  Occultation, 61-62, 202, 296
+
+  _Olaf, Saga of King_, 88
+
+  Olbers, 227, 253, 256, 271
+
+  "Old moon in new moon's arms," 185
+
+  Olmsted, 271
+
+  Omicron (or "Mira") Ceti, 307-308
+
+  Opposition, 209
+
+  "Optick tube," 108-109, 232
+
+  Orange (rays of light), 121
+
+  Orbit of moon, plane of, 63
+
+  Orbits, 32, 36-37, 66, 150, 157
+
+  Oriental astronomy, 107
+
+  Orion (constellation), 195, 279, 296-297, 316;
+    Great Nebula in, 316, 328
+
+  Oxford, 139
+
+  Oxygen, 145, 156, 166, 346
+
+
+  Pacific Ocean, origin of moon in, 339
+
+  Palitzch, 255
+
+  Pallas, 225, 227
+
+  Parallax, 57, 173, 280, 305, 320, 326
+
+  Pare, Ambrose, 264-265
+
+  Peal, S.E., 205
+
+  Peary, 277
+
+  Pegasus (constellation), 306
+
+  Penumbra of sunspot, 135
+
+  Perennial full moon of Laplace, 190
+
+  Pericles, 84
+
+  Perrine, C.D., 232-233, 315
+
+  Perseids, 270, 273-275
+
+  Perseus (constellation), 273, 279, 307, 312
+
+  Phases of an inferior planet, 149, 160;
+    of the moon, 149, 160, 183-185
+
+  Phlegon, Eclipse of, 85-86
+
+  Phobos, 223
+
+  Phoebe, retrograde motion of, 240, 250, 336
+
+  Phosphorescent glow in sky, 323
+
+  Phosphorus (Venus), 150
+
+  Photographic survey of sky, international, 290-291
+
+  Photosphere, 130-131, 134
+
+  Piazzi, 23
+
+  Pickering, E.C., 302
+
+  Pickering, W.H., 199, 205-206, 220-221, 240, 339-341
+
+  Pictor, "runaway star" in constellation of, 281-282, 320, 330
+
+  Plane of orbit, 36, 150
+
+  Planetary nebulae, 245, 315
+
+  _Planetary and Stellar Studies_, 331
+
+  Planetesimal hypothesis, 337-338
+
+  Planetoids. _See_ Asteroids
+
+  Planets, classification of, 229;
+    contrasted with comets, 247;
+    in Ptolemaic scheme, 171;
+    relative distances of, from sun, 31-32
+
+  Plato (lunar crater), 198
+
+  Pleiades, 284, 296-297, 307
+
+  Pliny, 169, 260
+
+  Plough, 284, 291-296, 302
+
+  Plutarch, 86, 89, 169, 181
+
+  "Pointers," 292
+
+  Polaris. _See_ Pole Star
+
+  Pole of earth, Precessional movement of, 176-177, 295, 298-299
+
+  Pole Star, 33, 163, 177, 292-296, 300-301
+
+  Poles, 30, 163-164;
+    of earth, speed of point at, 164
+
+  Pollux, 282, 297
+
+  Posidonius, 186
+
+  Powell, Sir George Baden, 96
+
+  Praesepe (the Beehive), 307
+
+  Precession of the Equinoxes, 177, 295, 298-299
+
+  Pre-telescopic notions, 55
+
+  Primaries, 26
+
+  _Princess, The_ (Tennyson), 334
+
+  Princeton Observatory, 258
+
+  Prism, 121
+
+  Prismatic colours, 111, 121
+
+  Procyon, 284, 290, 297, 303
+
+  Prominences, Solar, 72, 93, 131, 139-140, 143;
+    first observation of, with spectroscope, 94, 140, 236
+
+  Proper motions of stars, 126, 281-285, 326, 329-330
+
+  Ptolemaeus (lunar crater), 198-199, 204
+
+  Ptolemaic idea, 319;
+    system, 18, 19, 158, 171-172
+
+  Ptolemy, 18, 101, 171, 290, 296
+
+  Puiseux, P., 206
+
+  Pulkowa telescope, 117
+
+  Puppis, V., 310
+
+
+  Quiescent prominences, 139
+
+
+  Radcliffe Observer, 139
+
+  "Radiant," or radiant point, 269
+
+  Radiation from sun, 130, 134
+
+  Radium, 129, 138
+
+  Rainbow, 121
+
+  "Rainbows, Bay of," 197
+
+  Rambaut, A., 139
+
+  Ramsay, Sir William, 138
+
+  Rays (on moon), 204
+
+  Recurrence of eclipses, 74-80
+
+  Red (rays of light), 121, 125, 127, 130
+
+  Red Spot, the Great, 230
+
+  Reflecting telescope, 111-116;
+    future of, 119
+
+  Reflector. _See_ Reflecting telescope
+
+  Refracting and reflecting telescopes contrasted, 118
+
+  Refracting telescope, 109-111, 115-117;
+    limits to size of, 119-120
+
+  Refraction, 121, 168-169
+
+  Refractor, _See_ Refracting telescope
+
+  Regulus, 290, 296
+
+  Retrograde motion of Phoebe, 240, 250, 336
+
+  "Reversing Layer," 94, 130, 132, 137-138
+
+  Revival of learning, 107
+
+  Revolution, 30;
+    of earth around sun, 170-173;
+    periods of sun and planets, 35
+
+  Riccioli, 198
+
+  Rice-grain structure of photosphere, 134
+
+  Rigel, 285, 297
+
+  Rills (on moon), 204
+
+  Ring-mountains of moon. _See_ Craters
+
+  "Ring" nebulae, 315, 337
+
+  "Ring with wings," 87
+
+  Rings of Saturn, 108, 236-239, 241-243, 334
+
+  Ritchey, G.W., 118
+
+  Roberts, A.W., 308, 310
+
+  Roberts, Isaac, 325
+
+  "Roche's limit," 238
+
+  Roemer, 235
+
+  Roman history, eclipses in, 85-86
+
+  Romulus, 85
+
+  Roentgen, 120
+
+  Rosse, great telescope of Lord, 117, 314
+
+  Rotation, 30;
+    of earth, 33, 161-165, 170;
+    of sun, 34, 125, 135-136, 231;
+    periods of sun and planets, 35
+
+  Royal Society of London, 90-91, 111
+
+  Rubicon, Passage of the, 85
+
+  "Runaway" stars, 281, 326, 330
+
+
+  Sagittarius (constellation), 316
+
+  Salt, spectrum of table, 122
+
+  Samarcand, 107
+
+  "Saros," Chaldean, 76-78, 84
+
+  Satellites, 26-27, 37
+
+  Saturn, 20, 22, 34, 37, 108, 236-243, 258;
+    comet family of, 252;
+    a puzzle to the early telescope observers, 241-243;
+    retrograde motion of satellite Phoebe, 240, 250, 336;
+    ring system of, 241;
+    satellites of, 36, 239-240;
+    shadows of planet on rings and of rings on planet, 237
+
+  Schaeberle, 95-96, 303, 316
+
+  Schiaparelli, 155, 214, 223
+
+  Schickhard (lunar crater), 199
+
+  Schmidt, 206
+
+  Schoenfeld, 290
+
+  Schuster, 95
+
+  Schwabe, 136
+
+  Scotland, solar eclipses visible in, 89-90, 92
+
+  Sea of Serenity, 205
+
+  "Sea of Showers," 197
+
+  "Seas" of moon, 197, 206
+
+  Seasons on earth, 174-175;
+    on Mars, 211
+
+  Secondary bodies, 26
+
+  Seneca, 95, 260
+
+  _Septentriones_, 291
+
+  Serenitatis, Mare, 205
+
+  "Seven Stars," 291
+
+  "Shadow Bands," 69
+
+  Shadow of earth, circular shape of, 62-64
+
+  Shadows on moon, inky blackness of, 202
+
+  Shakespeare, 259, 293
+
+  Sheepshanks Telescope, 119
+
+  "Shining fluid" of Sir W. Herschel, 328
+
+  "Shooting Stars." _See_ Meteors
+
+  Short (of Edinburgh), 114
+
+  "Showers, Sea of," 197
+
+  Sickle of Leo, 270-271, 296
+
+  Siderostat, 118
+
+  Silver, 145
+
+  Silvered mirrors for reflecting telescopes, 116
+
+  Sinus Iridum, 197
+
+  Sirius, 280, 282, 284-285, 288-290, 297, 303-304, 320;
+    companion of, 303;
+    stellar magnitude of, 289
+
+  Size of celestial bodies, how ascertained, 59
+
+  Skeleton telescopes, 110
+
+  Sky, international photographic survey of, 290-291;
+    light of the, 323
+
+  Slipher, E.C., 213, 222
+
+  Smithsonian Institution of Washington, 98
+
+  Snow on Mars, 213
+
+  Sodium, 122, 124, 254
+
+  Sohag, 95
+
+  Solar system, 20-21, 29-31;
+    centre of gravity of, 42;
+    decay and death of, 344
+
+  Somniorum, Lacus, 197
+
+  Sound, 125, 166, 331
+
+  South pole of heavens, 163, 285, 298-299
+
+  Southern constellations, 298-299
+
+  Southern Cross. _See_ Crux
+
+  Space, 328
+
+  Spain, early astronomy in, 107;
+    eclipse tracks across 93, 97-98
+
+  Spectroheliograph, 140
+
+  Spectroscope, 120, 122, 124-125, 144-145, 212, 231;
+    prominences first observed with, 94, 140, 236
+
+  Spectrum of chromosphere, 132-133;
+    of corona, 133;
+    of photosphere, 132;
+    of reversing layer, 132, 137;
+    solar, 122-123, 127, 132
+
+  Speculum, 111, 116;
+    metal, 112
+
+  Spherical bodies, 29
+
+  Spherical shape of earth, proofs of, 158-161
+
+  Spherical shapes of sun, planets, and satellites, 160
+
+  Spiral nebulae, 314-316, 337-338
+
+  Spring balance, 166
+
+  Spring tides, 192
+
+  Spy-glass, 108
+
+  "Square of the distance," 43-44
+
+  Stannyan, Captain, 90
+
+  Star, mass of, how determined, 305;
+    parallax of, first ascertained, 173, 280
+
+  Stars, the, 20, 124, 126, 278 _et seq._;
+    brightness of, 287, 320;
+    distances between, 326-327;
+    distances of some, 173, 280, 320;
+    diminution of, below twelfth magnitude, 324;
+    evolution of, from nebulae, 317-318;
+    faintest magnitude of, 288;
+    number of those visible altogether, 324;
+    number of those visible to naked eye, 288
+
+  "Steam cracks," 221
+
+  Steinheil, 118
+
+  Stellar system, estimated extent of, 325-327;
+    an organised whole, 327;
+    limited extent of, 322-328, 330;
+    possible disintegration of, 329
+
+  Stiklastad, eclipse of, 88
+
+  Stone Age, 285
+
+  Stoney, G.J., 202, 222
+
+  Stonyhurst Observatory, 100
+
+  _Story of the Heavens_, 271
+
+  Streams of stars, Kapteyn's two, 284
+
+  Stroobant, 196
+
+  Stukeley, 91
+
+  Sulphur, 145
+
+  Summer, 175, 178
+
+  Sun, Chaps XII. and XIII.;
+    as a star, 124, 278, 289;
+    as seen from Neptune, 246, 304;
+    chemical composition of, 144-145;
+    distance of, how ascertained, 151, 211;
+    equator of, 135-136, 139;
+    gravitation at surface of, 129, 138-139;
+    growing cold of, 343-344;
+    mean distance of, from earth, 47, 211;
+    motion of, through space, 282-286, 326;
+    not a solid body, 136;
+    poles of, 136;
+    radiations from, 130;
+    revolution of earth around, 170-173;
+    stellar magnitude of, 288-289;
+    variation in distance of, 66, 178
+
+  Sunspots, 34, 125, 134-137, 140-141, 143-144, 308;
+    influence of earth on, 144
+
+  Suns and possible systems, 50, 286
+
+  Superior conjunction, 147-149
+
+  Superior planets, 22, 146, 209-210, 229
+
+  Swan (constellation). _See_ Cygnus
+
+  Swift, Dean, 224
+
+  "Sword" of Orion, 297, 316
+
+  Syrtis Major. _See_ Hour Glass Sea
+
+  "_Systematic_ Parallax," 326
+
+  Systems, other possible, 50, 286
+
+
+  Tails of comets, 182
+
+  Tamerlane, 107
+
+  Taurus (constellation), 103, 296-297, 307
+
+  "Tears of St. Lawrence," 273
+
+  Tebbutt's Comet, 257-258
+
+  Telescope, 33, 55, 107-108, 149;
+    first eclipse of moon seen through, 104;
+    of sun, 90
+
+  Telescopes, direct view reflecting, 114;
+    gigantic, 111;
+    great constructors of, 117-118;
+    great modern, 117-118
+
+  Tempel's Comet, 274
+
+  Temperature on moon, 203;
+    of sun, 128
+
+  Temporary (or new) stars, 310-314
+
+  Tennyson, Lord, 109, 296, 334
+
+  Terrestrial planets, 229-230
+
+  Terrestrial telescope, 117
+
+  Thales, Eclipse of, 84
+
+  Themis, 240
+
+  "Tidal drag," 180, 188, 208, 344
+
+  Tide areas, 179-180
+
+  Tides, 178-180, 338-339
+
+  _Time Machine_, 344
+
+  Tin, 145
+
+  Titan, 240
+
+  Titius, 245
+
+  Total phase, 71-72
+
+  Totality, 72;
+    track of, 66
+
+  Trail of a minor planet, 226-227
+
+  Transit, 62, 150-154;
+    of Mercury, 62, 151, 154;
+    of Venus, 62, 151-152, 154, 211
+
+  Trifid Nebula, 316
+
+  Triple stars, 300
+
+  Tubeless telescopes, 110-111, 243
+
+  Tubes used by ancients, 110
+
+  Tuttle's Comet, 274
+
+  Twilight, 167, 202
+
+  Twinkling of stars, 168
+
+  Twins (constellation). _See_ Gemini
+
+  Tycho Brahe, 290, 311
+
+  Tycho (lunar crater), 204
+
+
+  Ulugh Beigh, 107
+
+  Umbra of sunspot, 134-135
+
+  Universe, early ideas concerning, 17-18, 158, 177, 342
+
+  Universes, possibility of other, 330-331
+
+  Uranus, 22-24, 31, 210, 243, 245, 275;
+    comet family of, 252;
+    discovery of, 22, 210, 243;
+    rotation period of 34, 245;
+    satellites of, 26, 245;
+    "year" in, 35-36
+
+  Ursa Major (constellation), 279, 281, 291, 295, 314;
+    minor, 177, 279, 293-294
+
+  Ursae Majoris, ([z]) Zeta. _See_ Mizar
+
+
+  Variable stars, 307-310
+
+  Variations in apparent sizes of sun and moon, 67, 80, 178
+
+  Vault, shape of the celestial, 194-196
+
+  Vega, 177, 278, 280, 282-283, 285, 290, 294, 302, 307, 323
+
+  Vegetation on Mars, 221, 217-218;
+    on moon, 205
+
+  Venus, 20, 22, 31, 71, 90, 108-109, 111, Chap. XIV., 246, 311;
+    rotation period of, 34, 155
+
+  Very, F.W., 314
+
+  Vesta, 225, 227
+
+  Violet (rays of light), 121-122, 125
+
+  Virgil, 19
+
+  Volcanic theory of lunar craters, 203-204, 214
+
+  Volume, 38
+
+  Volumes of sun and planets compared, 38-39
+
+  "Vulcan," 25
+
+
+  Wallace, A.R., on Mars, 220-223
+
+  Water, lack of, on moon, 201-202
+
+  Water vapour, 202, 213, 222
+
+  Wargentin, 103
+
+  Warner and Swasey Co., 117
+
+  Weather, moon and, 206-207
+
+  Weathering, 202
+
+  Webb, Rev. T.W., 204
+
+  Weight, 43, 165-166
+
+  Wells, H.G., 344
+
+  Whale (constellation). _See_ Cetus
+
+  Whewell, 190
+
+  Willamette meteorite, 277
+
+  Wilson, Mount, 118
+
+  Wilson, W.E., 313
+
+  "Winged circle" (or "disc"), 87
+
+  Winter, 175, 178
+
+  Witt, 227
+
+  Wolf, Max, 226-227, 232
+
+  Wright, Thomas, 319, 334
+
+  Wybord, 89
+
+
+  Xenophon, 101
+
+
+  Year, 35
+
+  "Year" in Uranus and Neptune, 35-36
+
+  Year, number of eclipses in a, 68
+
+  "Year of the Stars," 270
+
+  Yellow (rays of light), 121-122, 124
+
+  Yerkes Telescope Great, 117, 303
+
+  Young, 94, 137, 166
+
+
+  Zenith, 174
+
+  Zinc, 145
+
+  Zodiacal light, 181
+
+  Zone of asteroids, 30-31, 227
+
+
+THE END
+
+Printed by BALLANTYNE, HANSON & CO.
+
+Edinburgh & London
+
+
+
+
+THE SCIENCE OF TO-DAY SERIES
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+BOTANY OF TO-DAY. A Popular Account of the Evolution of Modern Botany.
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+AERIAL NAVIGATION OF TO-DAY. A Popular Account of the Evolution of
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+SCIENTIFIC IDEAS OF TO-DAY. A Popular Account, in Non-technical
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+ELECTRICITY OF TO-DAY. Its Work and Mysteries Explained. By CHARLES R.
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+BOTANY OF TO-DAY. A Popular Account of the Evolution of Modern Botany.
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+ASTRONOMY OF TO-DAY. A Popular Introduction in Non-technical Language.
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+other illustrations.
+
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+
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+
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+ELECTRICITY OF TO-DAY. Its Work and Mysteries Explained. By CHARLES R.
+GIBSON, A.I.E.E.
+
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+
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+
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+
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+
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+     Times._
+
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+
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+
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+     will be understood by any intelligent reader."--_Yorkshire
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+
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+
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+
+
+THE ROMANCE OF MODERN MECHANISM
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+INTERESTING DESCRIPTIONS IN NON-TECHNICAL LANGUAGE OF WONDERFUL
+MACHINERY, MECHANICAL. DEVICES, & MARVELLOUSLY DELICATE SCIENTIFIC
+INSTRUMENTS
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+BY ARCHIBALD WILLIAMS, B.A., F.R.G.S.
+
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+
+_With Twenty-six Illustrations._ _Extra Crown 8vo._ 5_s._
+
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+
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+     fascinating volume, even to a somewhat blase reader whose business
+     it is to read all that comes in his way. The marvels miracles they
+     should be called, of the modern workshop are here exploited by Mr.
+     Williams for the benefit of readers who have not the opportunity of
+     seeing these wonders or the necessary mathematical knowledge to
+     understand a scientific treatise on their working. Only the
+     simplest language is used and every effort is made, by illustration
+     or by analogy, to make sufficiently clear to the non-scientific
+     reader how the particular bit of machinery works and what its work
+     really is. Delicate instruments, calculating machines, workshop
+     machinery, portable tools, the pedrail, motors ashore and afloat,
+     fire engines, automatic machines, sculpturing machines--these are a
+     few of the chapters which crowd this splendid
+     volume."--_Educational News._
+
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+     interesting, but Mr. Williams has the enviable knack of doing so,
+     and it is hardly possible to open this book at any page without
+     turning up something which you feel you must read; and then you
+     cannot stop till you come to the end of the
+     chapter."--_Electricity._
+
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+     addition to Messrs. Seeley and Company's Romance Series."--_Leeds
+     Mercury._
+
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+
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+
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+
+A Catalogue of Books on Art, History, and General Literature Published
+by Seeley, Service & Co Ltd. 38 Great Russell St. London
+
+
+_Some of the Contents_
+
+Crown Library, The                       4
+
+Elzevir Library, The                     5
+
+Events of Our Own Times Series           6
+
+Illuminated Series, The                  8
+
+Miniature Library of Devotion, The       9
+
+Miniature Portfolio Monographs, The      9
+
+Missions, The Library of                10
+
+New Art Library, The                    11
+
+Portfolio Monographs                    11
+
+Science of To-Day Series, The           14
+
+Seeley's Illustrated Pocket Library     14
+
+Seeley's Standard Library               15
+
+Story Series, The                       15
+
+"Things Seen" Series, The               16
+
+
+_The Publishers will be pleased to post their complete Catalogue or
+their Illustrated Miniature Catalogue on receipt of a post-card_
+
+
+
+
+CATALOGUE OF BOOKS
+
+_Arranged alphabetically under the names of Authors and Series_
+
+
+ABBOTT, Rev. E.A., D.D.
+
+     How to Parse. An English Grammar. Fcap. 8vo, 3s. 6d.
+
+     How to Tell the Parts of Speech. An Introduction to English
+     Grammar. Fcap. 8vo, 2s.
+
+     How to Write Clearly. Rules and Exercises on English Composition.
+     1s. 6d.
+
+     Latin Gate, The. A First Latin Translation Book. Crown 8vo, 3s. 6d.
+
+     Via Latina. A First Latin Grammar. Crown 8vo, 3s. 6d.
+
+
+ABBOTT, Rev. E.A., and Sir J.R. SEELEY.
+
+     English Lessons for English People. Crown 8vo, 4s. 6d.
+
+     ADY, Mrs. _See_ CARTWRIGHT, JULIA.
+
+
+A KEMPIS, THOMAS.
+
+     Of the Imitation of Christ. With Illuminated Frontispiece and Title
+     Page, and Illuminated Sub-Titles to each book. In white or blue
+     cloth, with inset miniatures. Gilt top; crown 8vo, 6s. nett; also
+     bound in same manner in real classic vellum. Each copy in a box,
+     10s. 6d. nett; Antique leather with clasps, 10s. 6d. nett.
+
+     "It may well be questioned whether the great work of Thomas a
+     Kempis has ever been presented to better advantage."--_The
+     Guardian._
+
+
+ANDERSON, Prof. W.
+
+     Japanese Wood Engravings. Coloured Illustrations. Super-royal 8vo,
+     sewed, 2s. 6d. nett; half-linen, 3s. 6d. nett; also small 4to,
+     cloth, 2s. nett; lambskin, 3s. nett.
+
+
+ARMSTRONG, Sir WALTER.
+
+     The Art of Velazquez. Illustrated. Super-royal 8vo, 3s. 6d. nett.
+
+     The Life of Velazquez. Illustrated. Super-royal 8vo, 3s. 6d. nett.
+
+     Velazquez. A Study of his Life and Art. With Eight Copper Plates
+     and many minor Illustrations. Super-royal 8vo, cloth, 9s. nett.
+
+     Thomas Gainsborough. Illustrated. Super-royal 8vo, half-linen, 3s.
+     6d. nett. Also new edition small 4to, cloth, 2s. nett; leather, 3s.
+     nett and 5s. nett.
+
+     The Peel Collection and the Dutch School of Painting. With
+     Illustrations in Photogravure and Half-tone. Super-royal 8vo,
+     sewed, 5s. nett; cloth, 7s. nett.
+
+     W.Q. Orchardson. Super-royal 8vo, sewed, 2s. 6d.; half-linen, 3s.
+     6d. nett.
+
+
+AUGUSTINE, S.
+
+     Confessions of S. Augustine. With Illuminated pages. In white or
+     blue cloth, gilt top, crown 8vo, 6s. nett; also in vellum, 10s. 6d.
+     nett.
+
+
+BAKER, Captain B. GRANVILLE
+
+     The Passing of the Turkish Empire in Europe. With Thirty-two
+     Illustrations. Demy 8vo, 16s. nett.
+
+
+BARING-GOULD, Rev. S.
+
+     Family Names and their Story. Demy 8vo, 7s. 6d. nett. 5s. nett.
+
+
+BEDFORD, Rev. W.K.R.
+
+     Malta and the Knights Hospitallers. Super-royal 8vo, sewed, 2s. 6d.
+     nett; half-linen, 3s. 6d. nett.
+
+
+BENHAM, Rev. Canon D.D., F.S.A.
+
+     The Tower of London. With Four Plates in Colours and many other
+     Illustrations. Super-royal 8vo, sewed, 5s. nett; cloth, 7s. nett.
+
+     Mediaeval London. With a Frontispiece in Photogravure, Four Plates
+     in Colour, and many other Illustrations. Super-royal 8vo, sewed,
+     5s. nett; cloth, gilt top, 7s. nett. Also extra crown 8vo, 3s. 6d.
+     nett.
+
+     Old St. Paul's Cathedral. With a Frontispiece in Photogravure, Four
+     Plates printed in Colour, and many other Illustrations. Super-royal
+     8vo, sewed, 5s. nett, or cloth, gilt top, 7s. nett.
+
+
+BENNETT, EDWARD.
+
+     The Post Office and its Story. An interesting account of the
+     activities of a great Government department. With Twenty-five
+     Illustrations. Ex. crn. 8vo, 5s. nett.
+
+
+BICKERSTETH, Rev. E.
+
+     Family Prayers for Six Weeks. Crown 8vo, 3s. 6d.
+
+     A Companion to the Holy Communion. 32mo, cloth, 1s.
+
+
+BINYON, LAURENCE.
+
+     Dutch Etchers of the Seventeenth Century. Illustrated. Super-royal
+     8vo, sewed, 2s. 6d.; half-linen, 3s. 6d. nett.
+
+     John Crome and John Sell Cotman. Illustrated. Super-royal 8vo
+     sewed, 3s. 6d. nett.
+
+
+BIRCH, G.H.
+
+     London on Thames in Bygone Days. With Four Plates printed in Colour
+     and many other Illustrations. Super-royal 8vo, sewed, 5s. nett;
+     cloth, 7s. nett.
+
+
+BRIDGES, Rev. C.
+
+     An Exposition of Psalm CXIX. Crown 8vo, 5s.
+
+
+BUTCHER, E.L.
+
+     Things Seen in Egypt. With Fifty Illustrations. Small 4to, cloth,
+     2s. nett; lambskin, 3s. nett; velvet leather, in box, 5s. nett.
+
+     Poems, 1s. 6d. nett.
+
+
+CACHEMAILLE, Rev. E.P., M.A.
+
+     XXVI Present-Day Papers on Prophecy. An explanation of the visions
+     of Daniel and of the Revelation, on the continuous historic system.
+     With Maps and Diagrams. 700 pp. 6s. nett.
+
+
+CARTWRIGHT, JULIA.
+
+     Jules Bastien-Lepage. Super-royal 8vo, sewed, 2s. 6d.; cloth, 3s.
+     6d. nett.
+
+     Sacharissa. Some Account of Dorothy Sidney, Countess of Sunderland,
+     her Family and Friends. With Five Portraits. Demy 8vo, 7s. 6d.
+
+     Raphael in Rome. Illustrated. Super-royal 8vo, sewed, 2s. 6d.;
+     half-linen, 3s. 6d. nett; also in small 4to, cloth, 2s. nett;
+     leather, 3s. nett and 5s. nett.
+
+     The Early Work of Raphael. Illustrated. Super-royal 8vo, sewed 2s.
+     6d.; half-linen, 3s. 6d. Also new edition, revised, in small 4to,
+     in cloth, 2s. nett; leather, 3s. nett.
+
+     Raphael: A Study of his Life and Work. With Eight Copper Plates and
+     many other Illustrations. Super-royal 8vo, 7s. 6d. nett.
+
+
+CESARESCO, The Countess MARTINENGO
+
+     The Liberation of Italy. With Portraits on Copper. Crown 8vo, 5s.
+
+
+CHATTERTON, E. KEBLE.
+
+     Fore and Aft. The Story of the Fore and Aft Rig from the Earliest
+     Times to the Present Day. Sq. ex. royal 8vo. With 150 Illustrations
+     and Coloured Frontispiece by C. Dixon, R.I. 16s. nett.
+
+     Through Holland in the "Vivette." The Cruise of a 4-Tonner from the
+     Solent to the Zuyder Zee, through the Dutch Waterways. With Sixty
+     Illustrations and Charts, 6s. nett.
+
+
+CHITTY, J.R.
+
+     Things Seen in China. With Fifty Illustrations. Small 4to; cloth,
+     2s.; leather, 3s.; velvet leather in a box, 5s. nett.
+
+
+CHORAL SERVICE-BOOK FOR PARISH CHURCHES, THE.
+
+Compiled and Edited by J.W. ELLIOTT, Organist and Choirmaster of St.
+Mark's, Hamilton Terrace, London. With some Practical Counsels taken by
+permission from "Notes on the Church Service," by Bishop WALSHAM HOW.
+
+A. Royal 8vo, sewed, 1s.; cloth, 1s. 6d.
+B. 16mo, sewed, 6d.; cloth, 8d.
+
+
+_The following portions may be had separately:_--
+
+     The Ferial and Festal Responses and the Litany. Arranged by J.W.
+     ELLIOTT. Sewed, 4d.
+
+     The Communion Service, Kyrie, Credo, Sanctus, and Gloria in
+     Excelsis. Set to Music by Dr. J. NAYLOR, Organist of York Minster.
+     Sewed, 4d.
+
+
+CHURCH, Sir ARTHUR H., F.R.S.
+
+     Josiah Wedgwood, Master Potter. With many Illustrations.
+     Super-royal 8vo, sewed, 5s. nett; cloth, 7s. nett; also small 4to,
+     cloth, 2s. nett; leather, 3s. and 5s. nett.
+
+     The Chemistry of Paints and Painting. Third Edition. Crown 8vo, 6s.
+
+
+CHURCH, Rev. A.J.
+
+     Nicias, and the Sicilian Expedition. Crown 8vo, 1s. 6d.
+
+For other books by Professor CHURCH see Complete Catalogue.
+
+
+CLARK, J.W., M.A.
+
+     Cambridge. With a coloured Frontispiece and many other
+     Illustrations by A. BRUNET-DEBAINES and H. TOUSSAINT &c. Extra
+     crown 8vo, 6s.; also crown 8vo, cloth, 2s. nett; leather, 3s.;
+     special leather, in box, 5s. nett.
+
+
+CODY, Rev. H.A.
+
+     An Apostle of the North. The Biography of the late Bishop BOMPAS,
+     First Bishop of Athabasca, and with an Introduction by the
+     ARCHBISHOP of RUPERTS-LAND. With 42 Illustrations. Demy 8vo, 7s.
+     6d. nett. 5s. nett.
+
+
+CORBIN, T.W.
+
+     Engineering of To-day. With Seventy-three Illustrations and
+     Diagrams. Extra crown 8vo, 5s. nett.
+
+     Mechanical Inventions of To-Day. Ex. Crown 8vo; with Ninety-four
+     Illustrations, 5s. nett.
+
+
+CORNISH, C.J.
+
+     Animals of To-day: Their Life and Conversation. With Illustrations
+     from Photographs by C. REID of Wishaw. Crown 8vo, 6s.
+
+     The Isle of Wight. Illustrated. Super-royal 8vo, sewed, 2s. 6d.
+     nett; half-linen, 3s. 6d. nett; also a new edition, small 4to,
+     cloth, 2s.; leather, 3s. and 5s.
+
+     Life at the Zoo. Notes and Traditions of the Regent's Park Gardens.
+     Illustrated from Photographs by GAMBIER BOLTON. Fifth Edition.
+     Crown 8vo, 6s.
+
+     The Naturalist on the Thames. Many Illustrations. Demy 8vo, 7s. 6d.
+
+     The New Forest. Super-royal 8vo, sewed, 2s. 6d. nett; half-linen,
+     3s. 6d. nett; also new edition, small 4to, cloth, 2s.; leather, 3s.
+     nett; and special velvet leather, each copy in a box, 5s.
+
+     The New Forest and the Isle of Wight. With Eight Plates and many
+     other Illustrations. Super-royal 8vo, 7s. 6d. nett.
+
+     Nights with an Old Gunner, and other Studies of Wild Life. With
+     Sixteen Illustrations by LANCELOT SPEED, CHARLES WHYMPER, and from
+     Photographs. Crown 8vo, 6s.
+
+       *       *       *       *       *
+
+THE CROWN LIBRARY
+
+A series of notable copyright books issued in uniform binding. Extra
+crown 8vo. With many illustrations, 5s. nett.
+
+
+_JUST ISSUED. SECOND AND CHEAPER EDITION._
+
+
+SWANN, A.J.
+
+     Fighting the Slave Hunters in Central Africa. A Record of
+     Twenty-six Years of Travel and Adventure round the Great Lakes, and
+     of the overthrow of Tip-pu-Tib, Rumaliza, and other great Slave
+     Traders. With 45 Illustrations and a Map, 5s. nett.
+
+
+_RECENTLY ISSUED._
+
+
+GRUBB, W. BARBROOKE.
+
+     An Unknown People in an Unknown Land. An Account of the Life and
+     Customs of the Lengua Indians of the Paraguayan Chaco, with
+     Adventures and Experiences met with during Twenty Years' Pioneering
+     and Exploration amongst them. With Twenty-four Illustrations and a
+     Map. Extra crown 8vo, 5s. nett.
+
+
+FRASER, Sir A.H.L., K.C.S.I., M.A., LL.D., Litt.D.,
+ex-Lieutenant-Governor of Bengal.
+
+     Among Indian Rajahs and Ryots. A Civil Servants' Recollections and
+     Impressions of Thirty-seven Years of Work and Sport in the Central
+     Provinces and Bengal. Third Edition, 5s. nett.
+
+
+CODY, Rev. H.A.
+
+     An Apostle of the North. The Story of Bishop Bompas's Life amongst
+     the Red Indians & Eskimo. Third Edition, 5s. nett.
+
+
+PENNELL, T.L., M.D., B.Sc.
+
+     Among the Wild Tribes of the Afghan Frontier. A Record of Sixteen
+     Years' close intercourse with the natives of Afghanistan and the
+     North-West Frontier. Introduction by EARL ROBERTS. Extra crown 8vo.
+     Twenty-six Illustrations and Map. Fifth Edition, 5s. net.
+
+       *       *       *       *       *
+
+CUST, LIONEL.
+
+     The Engravings of Albert Duerer. Illustrated. Super-royal 8vo,
+     half-linen, 3s. 6d. nett.
+
+     Paintings and Drawings of Albert Duerer. Illustrated. Super-royal
+     8vo, sewed, 3s. 6d. nett.
+
+     Albrecht Duerer. A Study of his Life and Work. With Eight Copper
+     Plates and many other Illustrations. Super-royal 8vo, 7s. 6d.
+
+
+DAVENPORT, CYRIL.
+
+     Cameos. With examples in Colour and many other Illustrations.
+     Super-royal 8vo, sewed, 5s. nett; cloth, 7s. nett.
+
+     Royal English Bookbindings. With Coloured Plates and many other
+     Illustrations. Super-royal 8vo, sewed, 3s. 6d.; cloth, 4s. 6d.
+
+
+DAVIES, RANDALL, F.S.A.
+
+     English Society of the Eighteenth Century in Contemporary Art. With
+     Four Coloured and many other Illustrations. Super royal 8vo, sewed,
+     5s. nett; cloth, 7s. nett.
+
+
+DAWSON, Rev. E.C.
+
+     The Life of Bishop Hannington. Crown 8vo, paper boards, 2s. 6d.; or
+     with Map and Illustrations, cloth, 3s. 6d.
+
+
+DESTREE, O.G.
+
+     The Renaissance of Sculpture in Belgium. Illustrated. Super-royal
+     8vo, sewed, 2s. 6d. nett; half-linen, 3s. 6d. nett.
+
+
+DOLMAGE, CECIL G., M.A., D.C.L., LL.D., F.R.A.S.
+
+     Astronomy of To-Day. A popular account in non-technical language.
+     With Forty-six Illustrations and Diagrams. Extra crown 8vo, 5s.
+     nett.
+
+
+DOMVILLE-FIFE, CHARLES W.
+
+     Submarine Engineering of To-Day. Extra crown 8vo, 5s. nett.
+
+
+ELZEVIR LIBRARY, THE.
+
+     Selections from the choicest English Writers. Exquisitely
+     Illustrated, with Frontispiece and Title-page in Colours by H.M.
+     BROCK, and many other Illustrations. Half bound in cloth, coloured
+     top, 1s. nett; full leather, 1s. 6d. nett; velvet leather, gilt
+     edges, in a box, 2s. 6d. nett.
+
+Volume I. Fancy & Humour of Lamb.
+  "   II. Wit & Imagination of Disraeli.
+  "  III. Vignettes from Oliver Goldsmith.
+  "   IV. Wit & Sagacity of Dr. Johnson.
+  "    V. Insight & Imagination of John Ruskin.
+  "   VI. Vignettes of London Life from Dickens.
+  "  VII. XVIIIth Century Vignettes from Thackeray.
+  " VIII. Vignettes of Country Life from Dickens.
+  "   IX. Wisdom & Humour of Carlyle.
+
+"Decidedly natty and original in get-up."--_The Saturday Review._
+
+
+EVANS, WILLMOTT, M.D.
+
+     Medical Science of To-Day. Ex. crn. 8vo; 24 Illustrations, 5s.
+     nett.
+
+
+WILMOT, EARDLEY, Rear-Admiral S.
+
+     Our Fleet To-day and its Development during the last Half Century.
+     With many Illustrations. Crown 8vo, 5s.
+
+
+EVENTS OF OUR OWN TIMES
+
+Crown 8vo. With Illustrations, 5s. each.
+
+     The War in the Crimea. By General Sir E. HAMLEY, K.C.B.
+
+     The Indian Mutiny. By Colonel MALLESON, C.S.I.
+
+     The Afghan Wars, 1839-42, and 1878-80. By ARCHIBALD FORBES.
+
+     Our Fleet To-Day and its Development during the last Half-Century.
+     By Rear-Admiral S. EARDLEY WILMOT.
+
+     The Refounding of the German Empire. By Colonel MALLESON, C.S.I.
+
+     The Liberation of Italy. By the Countess MARTINENGO CESARESCO.
+
+     Great Britain in Modern Africa. By EDGAR SANDERSON, M.A.
+
+     The War in the Peninsula. By A. INNES SHAND.
+
+
+FLETCHER, W.Y.
+
+     Bookbinding in France. Coloured Plates. Super-royal, sewed, 2s. 6d.
+     nett; half-linen, 3s. 6d. nett.
+
+
+FORBES, ARCHIBALD.
+
+     The Afghan Wars of 1839-1842 and 1878-1880. With Four Portraits on
+     Copper, and Maps and Plans. Crown 8vo, 5s.
+
+
+FRASER, Sir ANDREW H.L.
+
+     Among Indian Rajahs and Ryots. With 34 Illustrations and a Map.
+     Demy 8vo, 18s. nett. Third and Cheaper Edition, 5s. nett.
+
+
+FRASER, DONALD.
+
+     Winning a Primitive People. Illustrated. Extra crown 8vo, 5s. nett.
+
+
+FRIPP, Sir ALFRED D., K.C.V.O., & R. THOMPSON, F.R.C.S.
+
+     Human Anatomy for Art Students. Profusely Illustrated with
+     Photographs and Drawings by INNES FRIPP, A.R.C.A. Square extra
+     crown 8vo, 7s. 6d. nett.
+
+
+FROBENIUS, LEO.
+
+     The Childhood of Man. A Popular Account of the Lives and Thoughts
+     of Primitive Races. Translated by Prof. A.H. KEANE, LL.D. With 416
+     Illustrations. Demy 8vo, 16s. nett.
+
+
+FRY, ROGER.
+
+     Discourses Delivered to the Students of the Royal Academy by Sir
+     Joshua Reynolds. With an Introduction and Notes by ROGER FRY. With
+     Thirty-three Illustrations. Square Crown 8vo, 7s. 6d. nett.
+
+
+GARDNER, J. STARKIE.
+
+     Armour in England. With Eight Coloured Plates and many other
+     Illustrations. Super-royal 8vo, sewed, 3s. 6d. nett.
+
+     Foreign Armour in England. With Eight Coloured Plates and many
+     other Illustrations. Super-royal 8vo, sewed, 3s. 6d. nett.
+
+     Armour in England. With Sixteen Coloured Plates and many other
+     Illustrations. The two parts in one volume. Super-royal 8vo, cloth,
+     gilt top, 9s. nett.
+
+
+GARNETT, R., LL.D.
+
+     Richmond on Thames. Illustrated. Super-royal 8vo, sewed, 3s. 6d.
+     nett.
+
+
+GIBERNE, AGNES.
+
+     Beside the Waters of Comfort. Crown 8vo, 3s. 6d.
+
+
+GIBSON, CHARLES R., F.R.S.E.
+
+     Electricity of To-Day. Its Works and Mysteries described in
+     non-technical language. With 30 Illustrations. Extra crown 8vo, 5s.
+     nett.
+
+     Scientific Ideas of To-day. A Popular Account in non-technical
+     language of the Nature of Matter, Electricity, Light, Heat, &c.,
+     &c. With 25 Illustrations. Extra crown 8vo, 5s. nett.
+
+     How Telegraphs and Telephones Work. With many Illustrations. Crown
+     8vo, 1s. 6d. nett.
+
+     The Autobiography of an Electron. With 8 Illustrations. Long 8vo,
+     3s. 6d. nett.
+
+     Wireless Telegraphy. With many Illustrations. Ex. crn. 8vo, 2s.
+     nett.
+
+
+GODLEY, A.D.
+
+     Socrates and Athenian Society in his Day. Crown 8vo, 4s. 6d.
+
+     Aspects of Modern Oxford. With many Illustrations. Crown 8vo,
+     cloth, 2s. nett; lambskin, 3s. nett; velvet leather, in box, 5s.
+     nett.
+
+
+GOLDEN RECITER (_See_ JAMES, Prof. CAIRNS.)
+
+
+GOMES, EDWIN H., M.A.
+
+     Seventeen Years among the Sea Dyaks of Borneo. With 40
+     Illustrations and a Map. Demy 8vo, 16s. nett.
+
+
+GRAHAME, GEORGE.
+
+     Claude Lorrain. Illustrated. Super-royal 8vo, 2s. 6d. nett;
+     half-linen, 3s. 6d. nett.
+
+
+GRIFFITH, M.E. HUME.
+
+     Behind the Veil in Persia and Turkish Arabia. An Account of an
+     Englishwoman's Eight Years' Residence amongst the Women of the
+     East. With 37 Illustrations and a Map. Demy 8vo, 16s. nett.
+
+
+GRINDON, LEO.
+
+     Lancashire. Brief Historical and Descriptive Notes. With many
+     Illustrations. Crown 8vo, 6s.
+
+
+GRUBB, W. BARBROOKE (Pioneer and Explorer of the Chaco).
+
+     An Unknown People in an Unknown Land. With Sixty Illustrations and
+     a Map. Demy 8vo, 16s. nett. Third and Cheaper Edition, 5s.
+
+     A Church in the Wilds. Illustrated. Extra crown 8vo, 5s. nett.
+
+
+HADOW, W.H.
+
+     A Croatian Composer. Notes toward the Study of Joseph Haydn. Crown
+     8vo, 2s. 6d. nett.
+
+     Studies in Modern Music. First Series. Berlioz, Schumann, Wagner.
+     With an Essay on Music and Musical Criticism. With Five Portraits.
+     Crown 8vo, 7s. 6d.
+
+     Studies in Modern Music. Second Series. Chopin, Dvorak, Brahms.
+     With an Essay on Musical Form. With Four Portraits. Crown 8vo, 7s.
+     6d.
+
+
+HAMERTON, P.G.
+
+     The Etchings of Rembrandt, and Dutch Etchers of the Seventeenth
+     Century. By P.G. HAMERTON and LAURENCE BINTON. With Eight Copper
+     Plates and many other Illustrations. Super-royal 8vo, 7s. 6d. nett.
+
+     The Mount. Narrative of a Visit to the Site of a Gaulish City on
+     Mount Beuvray. With a Description of the neighbouring City of
+     Autun. Crown 8vo, 3s. 6d.
+
+     Round my House. Notes on Rural Life in Peace and War. Crown 8vo,
+     with Illustrations, 2s. 6d. nett. Cheaper edition, 2s. nett.
+
+     Paris. Illustrated. New edition. Cloth, 2s. nett; leather, 3s. nett
+     in special leather, full gilt, in box, 5s. nett.
+
+
+HAMLEY, Gen. Sir E.
+
+     The War in the Crimea. With Copper Plates and other Illustrations.
+     Crown 8vo, 5s.
+
+
+HANOUM ZEYNEB (Heroine of Pierre Loti's Novel "Les Desenchantees.")
+
+     A Turkish Woman's European Impressions. Edited by GRACE ELLISON.
+     With a portrait by AUGUSTE RODIN and 23 other Illustrations from
+     photographs. Crown 8vo, 6s. nett.
+
+
+HARTLEY, C. GASQUOINE.
+
+     Things Seen in Spain. With Fifty Illustrations. Small 4to, cloth,
+     2s.; leather, 3s.; velvet leather in a box, 5s. nett.
+
+
+HAYWOOD, Capt. A.H.W.
+
+     Through Timbuctu & Across the Great Sahara. Demy 8vo, with 41
+     Illustrations and a Map. 16s. nett.
+
+
+HENDERSON, Major PERCY E.
+
+     A British Officer in the Balkans. Through Dalmatia, Montenegro,
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+     Illustrations and a Map. Gilt top. Demy 8vo, 16s. nett.
+
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+HERBERT, GEORGE.
+
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+HUTCHINSON, Rev. H.N.
+
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+
+
+HUTTON, C.A.
+
+     Greek Terracotta Statuettes. With a Preface by A.S. MURRAY, LL.D.
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+HUTTON, SAMUEL KING, M.B.
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+
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+JAMES, CAIRNS.
+
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+       *       *       *       *       *
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+THE ILLUMINATED SERIES
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+
+LANG, ANDREW.
+
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+
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+
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+
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+
+     The Early Work of Raphael. By JULIA CARTWRIGHT (Mrs. Ady).
+
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+
+     Winning a Primitive People. Sixteen Years' Work among the Warlike
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+     Africa. By the Rev. DONALD FRASER.
+
+       *       *       *       *       *
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+
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+MOULE, Right Rev. H.C.G., D.D. (Bishop of Durham).
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+MURRAY, A.S., LL.D.
+
+     Greek Bronzes. With Four Copper Plates and many other
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+     cloth, 10s. 6d. nett.
+
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+NETTLESHIP, J.T.
+
+     Morland, George. With Six Copper Plates and Thirty other
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+
+
+THE NEW ART LIBRARY
+
+EDITED BY M.H. SPIELMANN, F.S.A., & P.G. KONODY.
+
+     "The admirable New Art Library.... Thoroughly practical."--_The
+     Connoisseur._
+
+
+THE PRACTICE AND SCIENCE OF DRAWING.
+
+     By HAROLD SPEED, Associe de la Societe Nationale des Beaux-Arts;
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+     Diagrams. Square ex. crn. 8vo, 6s. nett.
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+THE PRACTICE OF OIL PAINTING AND DRAWING.
+
+     By SOLOMON J. SOLOMON, R.A. With Eighty Illustrations. 6s. nett.
+
+
+HUMAN ANATOMY FOR ART STUDENTS.
+
+     By Sir ALFRED DOWNING FRIPP, K.C.V.O., Lecturer upon Anatomy at
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+
+
+MODELLING AND SCULPTURE.
+
+     By ALBERT TOFT, A.R.C.A., M.S.B.S. With 119 Photographs and
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+
+       *       *       *       *       *
+
+PAGE, J. Ll. WARDEN.
+
+     Exmoor, An Exploration of. With Maps, Etchings, and other
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+
+
+PENNELL, T.L., M.D., B.Sc., F.R.C.S.
+
+     Among the Wild Tribes of the Afghan Frontier. A Record of Sixteen
+     Years' Close Intercourse with the Natives of the Indian Marches.
+     With an Introduction by Field-Marshal LORD ROBERTS, V.C. Extra
+     Crown 8vo. With 26 Illustrations and a Map. 5s. nett. Fourth and
+     Cheaper Edition.
+
+     Things Seen in Northern India. With 50 Illustrations. 2s., 3s., 5s.
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+
+
+PHILLIPS, CLAUDE.
+
+     The Earlier Work of Titian. With many Illustrations. Super-royal
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+
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+
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+POLLARD, A.W.
+
+     Italian Book Illustrations. Super-royal 8vo, sewed, 2s. 6d. nett;
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+
+       *       *       *       *       *
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+       *       *       *       *       *
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+       *       *       *       *       *
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+     Mechanical Inventions of To-Day. By T.W. CORBIN.
+
+     Medical Science of To-Day. By WILLMOTT EVANS, M.D.
+
+     Scientific Ideas of To-Day. By CHARLES R. GIBSON, F.R.S.E.
+
+       *       *       *       *       *
+
+SEELEY'S ILLUSTRATED POCKET LIBRARY
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+Crown 8vo, cloth, gilt edge, 2s. nett; also in leather, 3s. nett; and
+yapp leather in box at 5s.
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+     CLEAVER, and Headpieces by W.A. ATKIN BERRY, CLOUGH BROMLEY, &c.
+
+
+CLARK, J.W., Registrary of the University of Cambridge.
+
+     Cambridge. With many Illustrations.
+
+
+GODLEY, A.D.
+
+     Aspects of Modern Oxford. With many Illustrations.
+
+
+HAMERTON, P.G.
+
+     Paris. With many Illustrations.
+
+
+LEE, Sir SIDNEY.
+
+     Stratford-on-Avon. From the Earliest Times to the Death of
+     Shakespeare. With 52 Illustrations by HERBERT RAILTON and E. HULL.
+
+
+MITFORD, MARY RUSSELL.
+
+     Country Stories. With 68 Illustrations by GEORGE MORROW.
+
+
+HERBERT, GEORGE.
+
+     The Temple. Sacred Poems and Ejaculations. The Text reprinted from
+     the first edition. With 76 Illustrations after DUeRER, HOLBEIN, and
+     other Masters.
+
+
+LANG, ANDREW.
+
+     Oxford. With 40 Illustrations by various artists.
+
+
+LEFROY, W. CHAMBERS.
+
+     The Ruined Abbeys of Yorkshire. With many Illustrations.
+
+
+LEYLAND, JOHN.
+
+     The Peak of Derbyshire: its Scenery and Antiquities.
+
+
+LOFTIE, W.J.
+
+     The Inns of Court. With 60 Illustrations.
+
+
+RUSSELL, W. CLARK.
+
+     British Seas. With 50 Illustrations by J.C. HOOK, R.A., HAMILTON
+     MACCALLUM, COLIN HUNTER, &c.
+
+
+STEVENSON, R.L.
+
+     Edinburgh. With many Illustrations by T. HAMILTON CRAWFORD, R.S.A.
+     (This volume is only to be had in this series in leather, 5s. nett.
+     For other editions of this book, see below.)
+
+       *       *       *       *       *
+
+SOLOMON, SOLOMON J., R.A.
+
+     The Practice of Oil Painting and Drawing. With 80 Illustrations.
+     6s. nett.
+
+
+SPEED, HAROLD.
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+     The Practice and Science of Drawing. With Ninety-six Illustrations
+     and Diagrams. Square extra crown 8vo, 6s.
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+       *       *       *       *       *
+
+THE STANDARD LIBRARY
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+Extra Crown 8vo, With many Illustrations. Price 2s. 6d. nett.
+
+
+     Lady Mary Wortley Montagu. By A.R. ROPES.
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+     Family Names and their Story. By the Rev. S. BARING GOULD.
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+     The Press and its Story. By JAMES D. SYMON.
+
+
+SEELEY, Sir J.R.
+
+     Goethe Reviewed after Sixty Years. With Portrait. Crown 8vo, 3s.
+     6d.
+
+     A Short History of Napoleon the First. With Portrait. Crown 8vo,
+     5s.
+
+
+SEELEY, Sir J.R., and Dr. ABBOTT.
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+     English Lessons for English People. Crown 8vo, 4s. 6d.
+
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+SEELEY, L.B.
+
+     Mrs. Thrale, afterwards Mrs. Piozzi. With Eight Illustrations.
+     Crown 8vo, 2s. 6d nett.
+
+     Fanny Burney and her Friends. With Eight Illustrations. Crown 8vo,
+     2s. 6d nett.
+
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+SHAND, A. INNES.
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+     The War in the Peninsula. With Portraits and Plans. 5s.
+
+
+SHARP, WILLIAM.
+
+     Fair Women. Illustrated. Super-royal 8vo, sewed, 2s. 6d. nett;
+     half-linen, 3s. 6d. nett. Also new edition, small 4to, cloth, 2s.
+     nett; leather, 3s. and 5s. nett.
+
+
+SHIPLEY, M.E.
+
+     Daily Help for Daily Need. A Selection of Scripture Verses and
+     Poetry for Every Day in the Year. Crown 8vo, 4s. 6d.
+
+
+STEPHENS, F.G.
+
+     Rossetti, D.G. Super-royal 8vo, sewed, 2s. 6d. nett; also small
+     4to, cloth, 2s. nett; leather, 3s. nett; velvet leather, in a box,
+     5s. nett.
+
+
+STEVENSON, R.L.
+
+     Edinburgh. Fcap. 8vo, with Frontispiece, gilt top, cloth, 2s. nett;
+     leather, 3s. nett. Crown 8vo, Illustrated, cloth, 3s. 6d. Roxburgh,
+     gilt top, 5s. Library Edition. Crown 8vo, buckram, dark blue, gilt
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+STEVENSON, R.A.M.
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+     Rubens, Peter Paul. Illustrated. Super-royal 8vo, 3s. 6d. nett,
+     sewed. Also small 4to, cloth, 2s. nett; leather, 3s. nett and 5s.
+     nett.
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+STIGAND, Captain C.H., F.R.G.S., F.Z.S.
+
+     To Abyssinia Through an Unknown Land. With Thirty-six Illustrations
+     and Two Maps. Demy 8vo, 16s. nett.
+
+
+SWANN, ALFRED J.
+
+     Fighting the Slave Hunters in Central Africa. With Forty-five
+     Illustrations and a Map. Demy 8vo, 16s. nett. Extra crown 8vo. 5s.
+     nett.
+
+
+TALBOT, F.A.
+
+     The Makings of a Great Canadian Railway. Demy 8vo. With Forty-one
+     Illustrations and a Map. 16s. nett.
+
+       *       *       *       *       *
+
+THE THINGS SEEN SERIES
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+Each volume with 50 Illustrations. Small 4to, cloth, 2s.; leather, 3s.;
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+     Things Seen in Oxford. By N.J. DAVIDSON, B.A. (Oxon.)
+
+     Things Seen in Russia. By W. BARNES STEVENI.
+
+     Things Seen in Palestine. By A. GOODRICH FREER.
+
+     Things Seen in Japan. By CLIVE HOLLAND.
+
+     Things Seen in China. By J.R. CHITTY.
+
+     Things Seen in Egypt. By E.L. BUTCHER.
+
+     Things Seen in Holland. By C.E. ROCHE.
+
+     Things Seen in Spain. By C. GASQUOINE HARTLEY.
+
+     Things Seen in Northern India. By T.L. PENNELL, M.D., B.Sc.
+
+     Things Seen in Venice. By LONSDALE RAGG, B.D. (Oxon.)
+
+
+TOFT, ALBERT, Hon., A.R.C.A., M.S.B.S.
+
+     Modelling and Sculpture. Profusely Illustrated with 119 Photographs
+     and Drawings. Square extra crown 8vo, 6s. nett.
+
+
+TORDAY, E.
+
+     Camp and Tramp in African Wilds. Demy 8vo. With Forty-five
+     Illustrations and a Map, 16s. nett.
+
+
+TOWNSHEND, Captain A.T.
+
+     A Military Consul in Turkey. With 29 Illustrations. Demy 8vo, 16s.
+     nett.
+
+
+TREMEARNE, Major A.J.N.
+
+     The Tailed Head-Hunters of Nigeria. Demy 8vo, with 38 Illustrations
+     and a Map. 16s. nett.
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+
+TURNER, CHARLES C.
+
+     Aerial Navigation of To-day. With Seventy Illustrations and
+     Diagrams. Extra crown 8vo, 5s. nett.
+
+
+WAERN, C.
+
+     John La Farge. Illustrated. Super-royal 8vo, sewed, 3s. 6d. nett.
+
+
+WEALE, W.H. JAMES.
+
+     Gerard David, Painter and Illuminator. Illustrated. Super-royal
+     8vo, sewed, 2s. 6d. nett; half-linen, 3s. 6d. nett.
+
+
+WEEKS, JOHN H.
+
+     Among Congo Cannibals. Demy 8vo. With Fifty-four Illustrations and
+     a Map, 16s. nett.
+
+
+WELCH, C., and Canon BENHAM.
+
+     Mediaeval London. With a Frontispiece in Photogravure, Four Plates
+     in Colour, and many other Illustrations. Super-royal 8vo, sewed,
+     5s. nett; cloth, gilt top, 7s. nett. Also extra crown 8vo, 3s. 6d.
+     nett.
+
+
+WICKS, MARK.
+
+     To Mars via the Moon. An Astronomical Story. With Sixteen
+     Illustrations and Diagrams. Extra crown 8vo, 5s.
+
+
+
+
+
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